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Cardiopulmonary Exercise Testing in Children and Adolescents
Edited by Thomas W. Rowland, American College of Sports Medicine and North American Society for Pediatric Exercise Medicine (NASPEM)
288 Pages
Exercise testing plays an increasingly important role in the diagnosis and assessment of heart disease and lung disease in children and adolescents. In Cardiopulmonary Exercise Testing in Children and Adolescents, leading expert Thomas W. Rowland, backed by the American College of Sports Medicine (ACSM) and the North American Society for Pediatric Exercise Medicine (NASPEM), compiles the latest evidence-based research to provide guidance for clinical exercise physiologists, cardiologists, pulmonologists, and students of exercise physiology who conduct exercise stress testing for young patients.
The core objective of the book is to clarify the differences between clinical exercise testing for children and testing for adults. Because of obvious differences between the two populations, test protocols must be modified based on the patient's age, size, level of physical fitness, body composition, intellectual and emotional maturity, and state of cardiac and pulmonary health.
Part I provides an introduction to pediatric exercise testing. Part II examines exercise testing methodologies and discusses blood pressure, cardiac output, electrocardiography, oxygen uptake, and pulmonary function. Part III focuses on specific clinical issues addressed by exercise testing, guiding readers through protocols for diagnosis, evaluation, and exercise testing. Part IV explores testing in special populations and focuses on topics such as childhood obesity, neuromuscular disease, and intellectual disabilities.
Where applicable, sample forms and checklists provide practitioners with practical materials to use during exercise testing. Sidebars offer readers insight into considerations such as the presence of parents during testing and adjustments of cardiac measures for youth body dimensions.
This book serves as a means of focusing and unifying approaches to performing pediatric exercise testing in order to lay the foundation for new and innovative approaches to exercise testing in the health care of children and adolescents.
Part I. Introduction
Chapter 1. Clinical Applicability of the Pediatric Exercise Test
Thomas Rowland
Chapter 2. Conducting the Pediatric Exercise Test
Amy Lynne Taylor
Part II. Exercise Testing Methodology
Chapter 3. Exercise Testing Protocols
Richard J. Sabath III, David A. White, and Kelli M. Teson
Chapter 4. Normal Cardiovascular Responses to Progressive Exercise
Thomas Rowland
Chapter 5. Exercise Electrocardiography
Thomas Rowland
Chapter 6. Blood Pressure Response to Dynamic Exercise
Bruce Alpert and Ranjit Philip
Chapter 7. Maximal Oxygen Uptake
Ali M. McManus and Neil Armstrong
Chapter 8. Other Measures of Aerobic Fitness
Robert P. Garofano
Chapter 9. Cardiac Output Measurement Techniques
Darren E.R. Warburton and Shannon S.D. Bredin
Chapter 10. Assessing Myocardial Function
Thomas Rowland
Chapter 11. Pulmonary Function
Patricia A. Nixon
Part III. Exertion-Based Applications
Chapter 12. Congenital and Acquired Heart Disease
Michael G. McBride and Stephen M. Paridon
Chapter 13. Exercise-Induced Dyspnea
Steven R. Boas
Chapter 14. Chest Pain With Exercise
Julie Brothers
Chapter 15. Presyncope and Syncope With Exercise
Julie Brothers
Chapter 16. Exercise Fatigue
Thomas Rowland
Part IV. Testing Special Populations
Chapter 17. Pectus Excavatum
Thomas Rowland
Chapter 18. Obesity
Laura Banks and Brian W. McCrindle
Chapter 19. Intellectual Disability
Bo Fernhall and Tracy Baynard
Chapter 20. Neuromuscular Disease
Olaf Verschuren, Janke de Groot, and Tim Takken
Thomas W. Rowland, MD, is a pediatric cardiologist at Baystate Medical Center in Springfield, Massachusetts, and a professor of pediatrics at Tufts University School of Medicine. A graduate of the University of Michigan Medical School, Rowland is board certified in pediatrics and pediatric cardiology by the American Board of Pediatrics.
Rowland, who has had more than 150 journal articles published, is the author of four books: Biologic Regulation of Physical Activity; Children’s Exercise Physiology, Second Edition; Tennisology: Inside the Science of Serves, Nerves, and On-Court Dominance; and The Athlete’s Clock. He has served as editor of the journal Pediatric Exercise Science and as president of the North American Society for Pediatric Exercise Medicine (NASPEM) and was on the board of trustees of the American College of Sports Medicine (ACSM). He is past president of the New England chapter of the ACSM and received the Honor Award from that organization in 1993.
Rowland is a competitive tennis player and distance runner. He and his wife, Margot, reside in Longmeadow, Massachusetts.
The American College of Sports Medicine (ACSM), founded in 1954, is the largest sports medicine and exercise science organization in the world. With more than 50,000 members and certified professionals worldwide, ACSM is dedicated to improving health through science, education, and medicine. ACSM members work in a wide range of medical specialties, allied health professions, and scientific disciplines. Members are committed to the diagnosis, treatment, and prevention of sport-related injuries and the advancement of the science of exercise.
The ACSM promotes and integrates scientific research, education, and practical applications of sports medicine and exercise science to maintain and enhance physical performance, fitness, health, and quality of life.
The North American Society for Pediatric Exercise Medicine (NASPEM), founded in 1985, is a professional organization whose membership is composed of medical doctors, researchers, educators, and students interested in pediatric exercise. NASPEM is dedicated to the mission of promoting exercise science, physical activity, and fitness in the health and medical care of children and adolescents. That mission is accomplished in part through scientific meetings, a scholarly journal (Pediatric Exercise Science), collaborative research, student aid in the form of grants and awards, and a training program database.
Asthma
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms.
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms. It is believed that the majority of children with asthma exhibit EIB, which may affect exercise tolerance as well as participation in physical activity. As reviewed by Welsh et al., studies examining aerobic fitness in children provide mixed results, with some reporting comparable fitness and others reporting reduced fitness in asthmatic children compared to their nonasthmatic peers. The inconsistencies may reflect differences in measurement of fitness (e.g., laboratory vs. field tests), differences in levels of severity of the children's asthma, the relatively small sample, selection bias in choice of nonasthmatic children for comparison, use of bronchodilator therapy prior to testing, and perhaps inappropriate statistical analyses. Some studies suggest that fitness is comparable as long as levels of physical activity are similar. Pharmacologic advances such as inhaled corticosteroid therapy have reduced the number of hospitalizations and emergency department visits, suggesting that the severity of disease has lessened. Consequently, the results of studies conducted prior to these advances may not accurately reflect the current impact of the disease on exercise tolerance.
In the past two decades, surprisingly few studies of children report ventilatory parameters assessed during exercise testing. Santuz et al. reported lower E at peak exercise in sedentary and regularly active children when compared to healthy controls (matched on age, height, weight, and habitual level of physical activity), but no difference in E was observed in asthmatic children who participated in organized or competitive sports compared to their peers. During submaximal exercise, the asthmatic and nonasthmatic children had a similar E, but it was achieved by a lower f b and a higher VT in the asthmatic children, which would lessen dead space breathing and improve alveolar ventilation. In contrast, Moraes et al. reported no differences among newly diagnosed adolescents with mild intermittent asthma (n = 20), mild persistent asthma (n = 13), and their healthy peers (n = 36) for O2, E, E/O2, or ventilatory reserve at maximal exercise. Berntsen et al. likewise found no difference in O2 or E at peak exercise in a group of 86 13-year-old adolescents with asthma compared to 76 same-aged peers without asthma. Resting lung function was slightly lower in the asthmatic children but generally within normal limits. In a multiple regression model that included skinfold thickness, resting pulmonary function, and physical activity, analysis indicated that skinfold thickness and vigorous physical activity participation (in asthmatics only) were the primary independent predictors of O2peak in the study.
Expiratory flow limitations after exercise are well documented in children with asthma, and a recent study reported postexercise inspiratory flow limitation as well. However, no study has reported measurement of exercise tidal loops to assess ventilatory mechanics in children with asthma during exercise. Consequently, the results of one study of adults are presented. Exercise tolerance and pulmonary mechanics during exercise were compared between eight adults with asthma and six adults without asthma. Asthmatic adults had similar larger airway function but reduced smaller airway function at rest, and O2peak was slightly lower (but not significantly) in asthmatic compared to control subjects (104% vs. 130% of predicted). At maximal exercise, the groups attained similar E, but the asthmatic subjects had higher mean values for E/CO2 and lower PETCO2 than the control subjects. Superimposing the exercise tidal loop on the resting maximal F-V loops produced similarities to figure 11.3, revealing increases in EELV with the expiratory flow of the tidal loop encroaching on the resting F-V loop during exercise, indicative of expiratory flow limitation. Interestingly, the authors noted variability in EELV and in the degree of exercise flow limitation during interval exercise (intensity switching from 60% to 40% of O2peak), and they suggested that the variability reflected changes in bronchomotor tone (i.e., bronchodilation vs. bronchoconstriction ) at different exercise intensities.
Recent studies reporting measurements of arterial blood gases are rare in children with asthma. One recent study collected expired gases and earlobe capillary blood samples in eight adolescents with mild intermittent asthma (MIA), eight with mild persistent asthma (MPA), and 12 nonasthmatic control subjects to examine gas exchange during exercise and to estimate PaO2 before and after maximal exercise stress. Before exercise, the children with MPA had significantly lower PaO2 (75.1 ± 6.6 mmHg) than those with MIA (81.7 ± 6.7 mmHg) and control (83.3 ± 4.9 mmHg) subjects, suggesting hypoxemia. Postexercise PaO2 did not differ among groups, but this was attributed to a decrease in the control subjects because the PaO2 of the children with MPA did not change significantly. Older studies report gas exchange abnormalities (elevated PA - aO2 difference) at rest that improved during exercise, which the authors attributed to better A/ matching. A more recent study of habitually active 18- to 45-year-old men and women with mild-to-moderate asthma examined arterial blood gases during exercise. Eight of 21 subjects exhibited oxyhemoglobin desaturation (SaO2 ≤94%) during prolonged exercise and were classified as the Lo-SaO2 group, and those whose SaO2 remained above 94% were classified as the Hi-SaO2 group. Despite similar fitness levels, the PaCO2 of the Hi-SaO2 group fell from rest to 34.0 ± 2.7 mmHg at exhaustion, whereas that of the Lo-SaO2 group fell during the first minute of exercise but then rose progressively, reaching 39.8 ± 4 mmHg at exhaustion, suggesting hypoventilation and impaired gas exchange.
Given the prevalence of asthma in children, as well as reports of reduced fitness, more studies examining ventilatory mechanics and gas exchange parameters during exercise are warranted in children with asthma to determine their potential contributions to reduced fitness and exercise intolerance.
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Clinical Exercise Testing
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function.
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function. Karlman Wasserman and coworkers at UCLA demonstrated, too, how the acquisition of gas exchange variables measured during exercise could further delineate and differentiate abnormal cardiac and pulmonary responses to exercise.
The use of exercise testing in pediatric populations, whose members do not normally suffer from coronary artery disease, initially developed in the shadow of this story about adult patients. Early exercise studies in youth were performed in the research setting. They were designed to examine physiological differences that separate children from adults. Sid Robinson provided the first such treadmill-derived data in the Harvard Fatigue Laboratory in Boston in the 1930s, demonstrating the progressive changes in metabolic and physiological responses that normally occurred between the ages of 6 and 91. Similar exercise data in healthy children were subsequently provided by other investigators in the middle of the 20th century, including Per-Olof …strand in Sweden, Simon Godfrey in Great Britain, and Gordon Cumming in Canada. When Oded Bar-Or published his landmark book Pediatric Sports Medicine for the Practitioner in 1983, he was able to accumulate a large base of normative data from these earlier studies to outline aspects of physiological responses to exercise in youth and how these developed during the growing years.
While such research was designed to reveal the normal development of physiological responses to exercise in children, this information also served as normative data for those who developed exercise testing for the clinical assessment of heart and lung disease in pediatric patients. As studies dealt, for the most part, with those with congenital heart disease, early exercise stress testing in young patients involved a more diverse approach than that of the traditional adult laboratory focused on the detection of coronary artery disease. The assessment of ischemic changes on the ECG was still an issue, particularly in assessing the severity of aortic outflow obstruction, but exercise testing in young patients also involved a wider range of information, such as blood pressure responses (in coarctation of the aorta, systemic hypertension), endurance capacity (postoperative cyanotic heart disease), and rhythm responses (complete heart block).
Useful clinical testing methodologies and clinical findings were described by a number of key early pioneers, such as Fred James at Cincinnati Children's Hospital, David Driscoll at the Mayo Clinic, Bruce Alpert and William Strong at the Medical College of Georgia, Rolf Mocellin in Germany, and Tony Reybrouck and Dirk Matthys in Belgium. The importance of gas exchange measures, including O2max and O2 kinetics, was highlighted by the early reports of exercise testing in patients with congenital heart disease by Hans Wessel at Children's Memorial Hospital in Chicago. During this time, too, exercise testing became established in both children and adults as a useful means of assessing bronchospasm and lung function in patients with asthma and other respiratory diseases (particularly via the early experience reported by Hans Stoboy, Gerd Cropp, and Svein Oseid).
In many cases, clinical exercise testing in children was performed in adult laboratories, using protocols, exercise equipment, and monitoring systems (ECG, blood pressure) traditionally used to test adults with suspected coronary artery disease. A number of developments have now expanded the role of exercise testing in youth and have identified the need for more specific approaches to exercise testing for this population of patients.
- Perhaps most importantly, the past several decades have witnessed a dramatic expansion in the scope and nature of patients cared for by pediatric cardiologists. Children with complex forms of congenital heart disease, particularly those characterized by marked unilateral ventricular hypoplasia (hypoplastic left heart syndrome, tricuspid valve atresia), once had little hope for long-term survival. Now, thanks to remarkable progress in surgical techniques, these young patients not only often survive but also live productive and fulfilling lives. The physicians caring for these survivors as they grow toward the adult years are confronted with new issues, such as myocardial dysfunction, stubborn tachyarrhythmia, hypoxemia, and pulmonary hypertension. These problems have required new diagnostic and therapeutic approaches, often using information available in the exercise stress-testing laboratory. Similarly, in patients with lung diseases such as cystic fibrosis, improvements in patient care have successfully extended survival and have at the same time introduced new clinical questions that can be assessed through exercise testing. It is likely that the future will continue to bring steady improvements in the survival of young patients with both cardiac and pulmonary disease that will be paralleled by expanded indications for clinical exercise testing.
- A growing understanding of the pathophysiology of cardiac malformations and factors influencing risk stratification have created a need to expand the information obtained during exercise stress testing in young patients. It is true that many issues can be adequately examined by a limited study involving a traditional bout of progressive exercise accompanied by electrocardiographic monitoring and measurement of blood pressure. Assessment of possible ischemic changes in a child after Kawasaki disease, for example, or determination of blood pressure responses after medical treatment of a hypertensive young wrestler could be adequately performed using this approach.
However, the clinical insights gained from exercise testing can be improved by the measurement of gas exchange variables, which are now readily obtained with user-friendly commercial metabolic systems. The changes in the oxygen and carbon dioxide content of expired air during exercise reflects similar gas exchange dynamics at the cellular level. With this approach, for example, the measurement of O2max provides an objective physiological assessment of aerobic fitness, and the determination of ventilatory variables (minute ventilation, CO2) offers insights into pulmonary responses as well. As will be outlined in the chapters that follow, it is often the calculation of relationships between these variables that provides clues into the relative importance of cardiac and pulmonary etiologies of exercise limitation.
Termed an "integrative cardiopulmonary test" by Wasserman et al., this approach expands the utility of exercise testing by providing data that can help to answer questions about cardiac and pulmonary issues in youth. In a teenage boy with moderate aortic valve insufficiency, does his cardiac disease explain the shortness of breath that limits his ability to exercise? What mechanisms lie behind a star athlete's inability to perform well after an extended viral illness? Is syncope of an anxious child during running related to hyperventilation? Is breathlessness during exercise in a markedly obese child caused by excess body fat, exercise-induced bronchoconstriction, or cardiac dysfunction? These types of issues are best addressed by a full examination of gas exchange variables during clinical exercise testing.
- A growing recognition of the effects of exercise on electrophysiological function has created new roles for exercise testing in youth. Assessment of changes in ventricular ectopy during exercise is a traditional indication for exercise testing. Newer indications include the use of responses of rate of ventricular repolarization (QT interval) and conduction down accessory pathways (WPW syndrome) during exercise as means of patient risk stratification.
- The increased use of pediatric exercise testing has also been stimulated by the concerns of parents, coaches, and physical education instructors over the occurrence of symptoms of chest pain, dizziness, syncope, or palpitations in young people during exercise. Such concerns have been fueled by the tragic occurrences of sudden unexpected death of young, presumably healthy athletes during sports training or competition. While such symptoms are highly unlikely to reflect the rare diseases that pose a risk of sudden death, the youngster with occult hypertrophic cardiomyopathy, coronary artery anomalies, or repolarization abnormalities that can predispose to fatal dysrhythmias can present with such complaints. Findings on exercise testing have thus become part of the assessment of symptomatic children and athletes to rule out these anomalies.
- A normal exercise test can provide clearance for sports play in young patients with heart disease or in those who have suffered illnesses such as viral myocarditis. Exercise testing also plays a role in assessing risk and exercise capacities in young patients who are enrolled in cardiac and pulmonary rehabilitation programs.
We can expect that clinical exercise testing in children and adolescents will continue to expand as the value of exercise is recognized in the assessment of not only heart and lung disease but also metabolic and musculoskeletal disorders. Such trends will undoubtedly follow improvements in medical and surgical treatment of these patients. We can also expect to see new techniques for performing exercise tests (miniaturization of metabolic systems permitting field testing, for example) and assessing their results (three-dimensional echocardiography, myocardial strain Doppler studies).
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Determining Functional Capacity
In summary, our quest is to determine children’s functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise.
In summary, our quest is to determine children's functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise. Exercise testing is essential to achieve this goal. Simple measurement of workload on a cycle ergometer will provide information on how close we are to "normal." Peak workload on a cycle ergometer is a measure of very vigorous exercise. The calibration of the cycle is often very good, and the workloads are very consistent over fast and slow pedaling. It is therefore a very useful measure of functional capacity for many subjects of different sizes and levels of physical conditioning. Within-subject reliability is also very good, so we can compare the child's responses over time accurately. Work in watts measured on the cycle ergometer can be used in a calculation to predict the oxygen consumption, again giving us a measure of functional capacity.
Current research indicates that one can predict the peak O2 from the O2 at VAT with good accuracy in healthy children as well as in those with lung disease. Caution should be used when predicting peak O2 from the O2 at VAT in children with congenital heart disease. Metabolic measurements need to be available to calculate the VAT, which adds to the cost and resources needed to predict the O2 from this method. The measurement of VAT becomes more useful when a submaximal effort has limited the exercise test. Age, comprehension, size, motivation, and protocol all factor into a submaximal response. The caveat of the method is when we assume that the O2 at VAT is about 60% of the peak O2. Physical conditioning and specifically a lack of physical fitness influence this method greatly, and this may lead to over- or underestimation of peak O2. We know that physical deconditioning, either from being sedentary or as the result of a disease process, lowers the O2 at the VAT.
OUES is a useful submaximal measure of aerobic fitness that is effort independent. This method also relies on the availability of metabolic measurements. It has been shown to be reproducible and does not suffer the intra-observer or inter-observer discrepancies often encountered with VAT. The OUES is easy to calculate and is determined by many more data points than VAT, peak work, or peak heart rate. The consensus of the research data indicates that it is most useful at the higher percentages of the peak O2 (that is, cut points of OUES 80% and OUES 90%). OUES below 45% to 50% of peak O2 is less reliable and does not correlate as well with peak O2. There is conflicting evidence regarding whether OUES is equally useful in predicting peak O2 in healthy subjects and in those with chronic heart or lung disease. If a subject can achieve 80% or 90% of predicted peak O2, many of us would consider that an adequate level of exertion. If the subject achieves 80% or 90% of peak O2, we do not know if the OUES slope can be abnormal. My guess is probably not. The slope of the line that represents the VE and O2 relationship is strongly influenced by the onset of anaerobic metabolism. The V-slope method of VAT detection is based on the change in that slope. Therefore the choice of the cut points will influence the OUES slope. Another factor that will influence the OUES is the choice of ergometer. We know that the peak O2 on the cycle ergometer is about 10% to 15% less than what is achieved on a treadmill. Therefore percent cut points will actually represent different amounts of O2. This could be problematic if we did not have equations for both the treadmill and cycle ergometer.
The six-minute walk is a simple test that is easy to administer and has good reproducibility. There are two versions, encouraged and unencouraged. The unencouraged version may reduce variability among test administrators. Distance and dyspnea score are recorded. The words and phrases of the Borg scale may not be completely understood by young children. Other scales with pictures have been used to assess the child's perceived exertion. The six-minute walk test may be more useful in underserved areas where metabolic equipment is not available. Also, large groups of children can be tested at very little cost, and over time increases in distance walked and a lower rate of perceived exertion may relate to improved fitness. This topic has been covered in greater detail in chapter 3.
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Case Presentation
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago.
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago. She describes the dizziness as "spinning," and it occurs not during running but just after she has finished. It usually occurs after running at a high-tempo pace for several miles and usually in the summer months. She denies frank syncope, chest pain, shortness of breath, or palpitations. There was a documented blood pressure of 62/40 at one time when paramedics were called while the event was occurring. She has been seen in the emergency room twice for the complaint; the most recent was 2 d ago. In the emergency room, she had a normal heart rate and blood pressure, normal physical examination, and normal laboratory values for anemia and thyroid. Her chest radiograph and electrocardiogram (ECG) were unremarkable. She has been restricted from exercise until she is seen by you. She is requesting exercise clearance.
Turning back to our case study, on further questioning it is revealed that Rebecca drinks 20 oz of water daily and one cup of coffee in the morning. She often skips breakfast as well because she does not have time to eat. She sometimes experiences dizziness in the morning when she stands up while getting out of bed. Rebecca's physical examination included orthostatic vital signs. Her heart rate increased from 52 bpm supine to 90 bpm in the upright position. Her blood pressure decreased from 116/68 to 108/64 from supine to upright position, respectively. Her physical examination was otherwise unremarkable.
Rebecca had an ECG performed, which was normal. Although her symptoms were classic for dehydration as well as a vasovagal component, because she was a competitive athlete, the cardiologist opted for an echocardiogram, which was normal, and an exercise stress test to try to induce her symptoms.
Rebecca performed a maximal treadmill exercise stress test. She had a high-normal aerobic capacity. There were no arrhythmias or ischemic changes. There was a normal heart rate and blood pressure response to exercise. She developed dizziness in early recovery with a 30 mmHg drop in her blood pressure. After she drank a cup of water and was placed in the supine position, her blood pressure improved within 2 min.
For Rebecca, her history, physical examination, and ECG were reassuring that this was unlikely to be cardiac-related exertional syncope. Her exercise stress test was able to document dizziness associated with a drop in blood pressure that responded to fluids and supine positioning. She was counseled on improving her hydration and salt intake and not to skip meals. She returned for a follow-up visit 3 mo later and had not experienced any further dizziness or syncope.
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Unique Features of Exercise Testing in Children
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults.
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults. One need only consider the various approaches needed to perform a satisfactory exercise test first in a 15-year-old cross country runner who experienced precordial chest pain in her last race, followed by a test looking for heart rate response in a 5-year-old youngster with complete heart block, and then a 12-year-old obese boy with a dilated cardiomyopathy and progressively worsening shortness of breath with exercise.
The pediatric exercise testing laboratory must accommodate wide variations in patient age, size, and fitness, and that means that testing protocols and equipment must be similarly adjusted. The indications for testing are much broader than those in the adult lab, so the questions to be answered must be carefully considered before the exercise begins.
Most children can be easily motivated to give exhaustive efforts during exercise testing, but it requires charismatic skill from staff members experienced with the emotional and physical responses of children during treadmill or cycle exercise. It has been said that perhaps the single most important factor in a successful exercise test in the pediatric laboratory is the staff administering the test.
Pediatric exercise testing, then, is distinguished by the need for a creative approach to each patient. The staff must know what information is needed to address the clinical question being asked, the proper modality - cycle or treadmill - to obtain that answer, and the optimal protocol for the subject's age and fitness level.
The physiological mechanisms underlying the cardiac and pulmonary responses to a bout of progressive exercise are no different in children (at least those over age 6) and adults. Nonetheless, certain quantitative measurements (heart rate, blood pressure, endurance time) are different in children, and these must be recognized in the testing of immature subjects.
For example, resting and maximal heart rates in an exhaustive exercise test are greater in children than in adults. As will be discussed in chapter 5, peak heart rate depends on testing modality and protocol, and there is considerable variability between individuals. It is important to recognize that there is generally no specific "target heart rate" for an exercise test. Importantly, too, the maximal heart rate during exercise testing in a given subject does not change over the course of childhood. Only at about age 16 does this value begin to decline. Thus, age-related formulae for predicting a maximal heart rate, such as "220 minus age," do not apply to youths.
The concept of metabolic equivalents, or METs, as a measure of energy expenditure during exercise is commonly used with adult subjects but is fraught with difficulty in children and adolescents, and it is best avoided in the pediatric exercise laboratory. METs is a means of expressing the oxygen requirement of a physical activity relative to an assumed resting value. One MET, or resting O2, in an adult is considered to be 3.5 ml ∙ kg-1 ∙ min-1; thus, when walking on a treadmill at a certain speed and slope that is expected to demand 17.5 ml ∙ kg-1 ∙ min-1, a patient is exercising at a level of 5 METs.
The difficulty with this concept in youth is that resting energy expenditure is not constant but evolves throughout childhood during physical growth. As would be expected, absolute values of resting O2 rise with the accrual of body mass. When adjusted for body mass, or body surface area, basal or resting values of energy expenditure decline progressively during the pediatric years. When expressed as calories per meter of square body surface area per hour, the basal metabolic rate declines by about 20% between the ages of six and the mid-teen years.
In considering a mass-relative definition of a MET in children, the story is more exaggerated. Harrell et al. reported that the resting O2 per kg in a group of 8- to 12-year-old children was almost 50% higher than that of 16- to 18-year-old subjects and 70% higher than that expected in adults. The use of the MET as defined in adults as a "currency" or multiplier of energy expenditure in children, then, clearly would introduce large errors in defining O2 levels during exercise - and the extent of the error would be different depending on the age and size of the child.
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Asthma
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms.
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms. It is believed that the majority of children with asthma exhibit EIB, which may affect exercise tolerance as well as participation in physical activity. As reviewed by Welsh et al., studies examining aerobic fitness in children provide mixed results, with some reporting comparable fitness and others reporting reduced fitness in asthmatic children compared to their nonasthmatic peers. The inconsistencies may reflect differences in measurement of fitness (e.g., laboratory vs. field tests), differences in levels of severity of the children's asthma, the relatively small sample, selection bias in choice of nonasthmatic children for comparison, use of bronchodilator therapy prior to testing, and perhaps inappropriate statistical analyses. Some studies suggest that fitness is comparable as long as levels of physical activity are similar. Pharmacologic advances such as inhaled corticosteroid therapy have reduced the number of hospitalizations and emergency department visits, suggesting that the severity of disease has lessened. Consequently, the results of studies conducted prior to these advances may not accurately reflect the current impact of the disease on exercise tolerance.
In the past two decades, surprisingly few studies of children report ventilatory parameters assessed during exercise testing. Santuz et al. reported lower E at peak exercise in sedentary and regularly active children when compared to healthy controls (matched on age, height, weight, and habitual level of physical activity), but no difference in E was observed in asthmatic children who participated in organized or competitive sports compared to their peers. During submaximal exercise, the asthmatic and nonasthmatic children had a similar E, but it was achieved by a lower f b and a higher VT in the asthmatic children, which would lessen dead space breathing and improve alveolar ventilation. In contrast, Moraes et al. reported no differences among newly diagnosed adolescents with mild intermittent asthma (n = 20), mild persistent asthma (n = 13), and their healthy peers (n = 36) for O2, E, E/O2, or ventilatory reserve at maximal exercise. Berntsen et al. likewise found no difference in O2 or E at peak exercise in a group of 86 13-year-old adolescents with asthma compared to 76 same-aged peers without asthma. Resting lung function was slightly lower in the asthmatic children but generally within normal limits. In a multiple regression model that included skinfold thickness, resting pulmonary function, and physical activity, analysis indicated that skinfold thickness and vigorous physical activity participation (in asthmatics only) were the primary independent predictors of O2peak in the study.
Expiratory flow limitations after exercise are well documented in children with asthma, and a recent study reported postexercise inspiratory flow limitation as well. However, no study has reported measurement of exercise tidal loops to assess ventilatory mechanics in children with asthma during exercise. Consequently, the results of one study of adults are presented. Exercise tolerance and pulmonary mechanics during exercise were compared between eight adults with asthma and six adults without asthma. Asthmatic adults had similar larger airway function but reduced smaller airway function at rest, and O2peak was slightly lower (but not significantly) in asthmatic compared to control subjects (104% vs. 130% of predicted). At maximal exercise, the groups attained similar E, but the asthmatic subjects had higher mean values for E/CO2 and lower PETCO2 than the control subjects. Superimposing the exercise tidal loop on the resting maximal F-V loops produced similarities to figure 11.3, revealing increases in EELV with the expiratory flow of the tidal loop encroaching on the resting F-V loop during exercise, indicative of expiratory flow limitation. Interestingly, the authors noted variability in EELV and in the degree of exercise flow limitation during interval exercise (intensity switching from 60% to 40% of O2peak), and they suggested that the variability reflected changes in bronchomotor tone (i.e., bronchodilation vs. bronchoconstriction ) at different exercise intensities.
Recent studies reporting measurements of arterial blood gases are rare in children with asthma. One recent study collected expired gases and earlobe capillary blood samples in eight adolescents with mild intermittent asthma (MIA), eight with mild persistent asthma (MPA), and 12 nonasthmatic control subjects to examine gas exchange during exercise and to estimate PaO2 before and after maximal exercise stress. Before exercise, the children with MPA had significantly lower PaO2 (75.1 ± 6.6 mmHg) than those with MIA (81.7 ± 6.7 mmHg) and control (83.3 ± 4.9 mmHg) subjects, suggesting hypoxemia. Postexercise PaO2 did not differ among groups, but this was attributed to a decrease in the control subjects because the PaO2 of the children with MPA did not change significantly. Older studies report gas exchange abnormalities (elevated PA - aO2 difference) at rest that improved during exercise, which the authors attributed to better A/ matching. A more recent study of habitually active 18- to 45-year-old men and women with mild-to-moderate asthma examined arterial blood gases during exercise. Eight of 21 subjects exhibited oxyhemoglobin desaturation (SaO2 ≤94%) during prolonged exercise and were classified as the Lo-SaO2 group, and those whose SaO2 remained above 94% were classified as the Hi-SaO2 group. Despite similar fitness levels, the PaCO2 of the Hi-SaO2 group fell from rest to 34.0 ± 2.7 mmHg at exhaustion, whereas that of the Lo-SaO2 group fell during the first minute of exercise but then rose progressively, reaching 39.8 ± 4 mmHg at exhaustion, suggesting hypoventilation and impaired gas exchange.
Given the prevalence of asthma in children, as well as reports of reduced fitness, more studies examining ventilatory mechanics and gas exchange parameters during exercise are warranted in children with asthma to determine their potential contributions to reduced fitness and exercise intolerance.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Clinical Exercise Testing
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function.
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function. Karlman Wasserman and coworkers at UCLA demonstrated, too, how the acquisition of gas exchange variables measured during exercise could further delineate and differentiate abnormal cardiac and pulmonary responses to exercise.
The use of exercise testing in pediatric populations, whose members do not normally suffer from coronary artery disease, initially developed in the shadow of this story about adult patients. Early exercise studies in youth were performed in the research setting. They were designed to examine physiological differences that separate children from adults. Sid Robinson provided the first such treadmill-derived data in the Harvard Fatigue Laboratory in Boston in the 1930s, demonstrating the progressive changes in metabolic and physiological responses that normally occurred between the ages of 6 and 91. Similar exercise data in healthy children were subsequently provided by other investigators in the middle of the 20th century, including Per-Olof …strand in Sweden, Simon Godfrey in Great Britain, and Gordon Cumming in Canada. When Oded Bar-Or published his landmark book Pediatric Sports Medicine for the Practitioner in 1983, he was able to accumulate a large base of normative data from these earlier studies to outline aspects of physiological responses to exercise in youth and how these developed during the growing years.
While such research was designed to reveal the normal development of physiological responses to exercise in children, this information also served as normative data for those who developed exercise testing for the clinical assessment of heart and lung disease in pediatric patients. As studies dealt, for the most part, with those with congenital heart disease, early exercise stress testing in young patients involved a more diverse approach than that of the traditional adult laboratory focused on the detection of coronary artery disease. The assessment of ischemic changes on the ECG was still an issue, particularly in assessing the severity of aortic outflow obstruction, but exercise testing in young patients also involved a wider range of information, such as blood pressure responses (in coarctation of the aorta, systemic hypertension), endurance capacity (postoperative cyanotic heart disease), and rhythm responses (complete heart block).
Useful clinical testing methodologies and clinical findings were described by a number of key early pioneers, such as Fred James at Cincinnati Children's Hospital, David Driscoll at the Mayo Clinic, Bruce Alpert and William Strong at the Medical College of Georgia, Rolf Mocellin in Germany, and Tony Reybrouck and Dirk Matthys in Belgium. The importance of gas exchange measures, including O2max and O2 kinetics, was highlighted by the early reports of exercise testing in patients with congenital heart disease by Hans Wessel at Children's Memorial Hospital in Chicago. During this time, too, exercise testing became established in both children and adults as a useful means of assessing bronchospasm and lung function in patients with asthma and other respiratory diseases (particularly via the early experience reported by Hans Stoboy, Gerd Cropp, and Svein Oseid).
In many cases, clinical exercise testing in children was performed in adult laboratories, using protocols, exercise equipment, and monitoring systems (ECG, blood pressure) traditionally used to test adults with suspected coronary artery disease. A number of developments have now expanded the role of exercise testing in youth and have identified the need for more specific approaches to exercise testing for this population of patients.
- Perhaps most importantly, the past several decades have witnessed a dramatic expansion in the scope and nature of patients cared for by pediatric cardiologists. Children with complex forms of congenital heart disease, particularly those characterized by marked unilateral ventricular hypoplasia (hypoplastic left heart syndrome, tricuspid valve atresia), once had little hope for long-term survival. Now, thanks to remarkable progress in surgical techniques, these young patients not only often survive but also live productive and fulfilling lives. The physicians caring for these survivors as they grow toward the adult years are confronted with new issues, such as myocardial dysfunction, stubborn tachyarrhythmia, hypoxemia, and pulmonary hypertension. These problems have required new diagnostic and therapeutic approaches, often using information available in the exercise stress-testing laboratory. Similarly, in patients with lung diseases such as cystic fibrosis, improvements in patient care have successfully extended survival and have at the same time introduced new clinical questions that can be assessed through exercise testing. It is likely that the future will continue to bring steady improvements in the survival of young patients with both cardiac and pulmonary disease that will be paralleled by expanded indications for clinical exercise testing.
- A growing understanding of the pathophysiology of cardiac malformations and factors influencing risk stratification have created a need to expand the information obtained during exercise stress testing in young patients. It is true that many issues can be adequately examined by a limited study involving a traditional bout of progressive exercise accompanied by electrocardiographic monitoring and measurement of blood pressure. Assessment of possible ischemic changes in a child after Kawasaki disease, for example, or determination of blood pressure responses after medical treatment of a hypertensive young wrestler could be adequately performed using this approach.
However, the clinical insights gained from exercise testing can be improved by the measurement of gas exchange variables, which are now readily obtained with user-friendly commercial metabolic systems. The changes in the oxygen and carbon dioxide content of expired air during exercise reflects similar gas exchange dynamics at the cellular level. With this approach, for example, the measurement of O2max provides an objective physiological assessment of aerobic fitness, and the determination of ventilatory variables (minute ventilation, CO2) offers insights into pulmonary responses as well. As will be outlined in the chapters that follow, it is often the calculation of relationships between these variables that provides clues into the relative importance of cardiac and pulmonary etiologies of exercise limitation.
Termed an "integrative cardiopulmonary test" by Wasserman et al., this approach expands the utility of exercise testing by providing data that can help to answer questions about cardiac and pulmonary issues in youth. In a teenage boy with moderate aortic valve insufficiency, does his cardiac disease explain the shortness of breath that limits his ability to exercise? What mechanisms lie behind a star athlete's inability to perform well after an extended viral illness? Is syncope of an anxious child during running related to hyperventilation? Is breathlessness during exercise in a markedly obese child caused by excess body fat, exercise-induced bronchoconstriction, or cardiac dysfunction? These types of issues are best addressed by a full examination of gas exchange variables during clinical exercise testing.
- A growing recognition of the effects of exercise on electrophysiological function has created new roles for exercise testing in youth. Assessment of changes in ventricular ectopy during exercise is a traditional indication for exercise testing. Newer indications include the use of responses of rate of ventricular repolarization (QT interval) and conduction down accessory pathways (WPW syndrome) during exercise as means of patient risk stratification.
- The increased use of pediatric exercise testing has also been stimulated by the concerns of parents, coaches, and physical education instructors over the occurrence of symptoms of chest pain, dizziness, syncope, or palpitations in young people during exercise. Such concerns have been fueled by the tragic occurrences of sudden unexpected death of young, presumably healthy athletes during sports training or competition. While such symptoms are highly unlikely to reflect the rare diseases that pose a risk of sudden death, the youngster with occult hypertrophic cardiomyopathy, coronary artery anomalies, or repolarization abnormalities that can predispose to fatal dysrhythmias can present with such complaints. Findings on exercise testing have thus become part of the assessment of symptomatic children and athletes to rule out these anomalies.
- A normal exercise test can provide clearance for sports play in young patients with heart disease or in those who have suffered illnesses such as viral myocarditis. Exercise testing also plays a role in assessing risk and exercise capacities in young patients who are enrolled in cardiac and pulmonary rehabilitation programs.
We can expect that clinical exercise testing in children and adolescents will continue to expand as the value of exercise is recognized in the assessment of not only heart and lung disease but also metabolic and musculoskeletal disorders. Such trends will undoubtedly follow improvements in medical and surgical treatment of these patients. We can also expect to see new techniques for performing exercise tests (miniaturization of metabolic systems permitting field testing, for example) and assessing their results (three-dimensional echocardiography, myocardial strain Doppler studies).
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Determining Functional Capacity
In summary, our quest is to determine children’s functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise.
In summary, our quest is to determine children's functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise. Exercise testing is essential to achieve this goal. Simple measurement of workload on a cycle ergometer will provide information on how close we are to "normal." Peak workload on a cycle ergometer is a measure of very vigorous exercise. The calibration of the cycle is often very good, and the workloads are very consistent over fast and slow pedaling. It is therefore a very useful measure of functional capacity for many subjects of different sizes and levels of physical conditioning. Within-subject reliability is also very good, so we can compare the child's responses over time accurately. Work in watts measured on the cycle ergometer can be used in a calculation to predict the oxygen consumption, again giving us a measure of functional capacity.
Current research indicates that one can predict the peak O2 from the O2 at VAT with good accuracy in healthy children as well as in those with lung disease. Caution should be used when predicting peak O2 from the O2 at VAT in children with congenital heart disease. Metabolic measurements need to be available to calculate the VAT, which adds to the cost and resources needed to predict the O2 from this method. The measurement of VAT becomes more useful when a submaximal effort has limited the exercise test. Age, comprehension, size, motivation, and protocol all factor into a submaximal response. The caveat of the method is when we assume that the O2 at VAT is about 60% of the peak O2. Physical conditioning and specifically a lack of physical fitness influence this method greatly, and this may lead to over- or underestimation of peak O2. We know that physical deconditioning, either from being sedentary or as the result of a disease process, lowers the O2 at the VAT.
OUES is a useful submaximal measure of aerobic fitness that is effort independent. This method also relies on the availability of metabolic measurements. It has been shown to be reproducible and does not suffer the intra-observer or inter-observer discrepancies often encountered with VAT. The OUES is easy to calculate and is determined by many more data points than VAT, peak work, or peak heart rate. The consensus of the research data indicates that it is most useful at the higher percentages of the peak O2 (that is, cut points of OUES 80% and OUES 90%). OUES below 45% to 50% of peak O2 is less reliable and does not correlate as well with peak O2. There is conflicting evidence regarding whether OUES is equally useful in predicting peak O2 in healthy subjects and in those with chronic heart or lung disease. If a subject can achieve 80% or 90% of predicted peak O2, many of us would consider that an adequate level of exertion. If the subject achieves 80% or 90% of peak O2, we do not know if the OUES slope can be abnormal. My guess is probably not. The slope of the line that represents the VE and O2 relationship is strongly influenced by the onset of anaerobic metabolism. The V-slope method of VAT detection is based on the change in that slope. Therefore the choice of the cut points will influence the OUES slope. Another factor that will influence the OUES is the choice of ergometer. We know that the peak O2 on the cycle ergometer is about 10% to 15% less than what is achieved on a treadmill. Therefore percent cut points will actually represent different amounts of O2. This could be problematic if we did not have equations for both the treadmill and cycle ergometer.
The six-minute walk is a simple test that is easy to administer and has good reproducibility. There are two versions, encouraged and unencouraged. The unencouraged version may reduce variability among test administrators. Distance and dyspnea score are recorded. The words and phrases of the Borg scale may not be completely understood by young children. Other scales with pictures have been used to assess the child's perceived exertion. The six-minute walk test may be more useful in underserved areas where metabolic equipment is not available. Also, large groups of children can be tested at very little cost, and over time increases in distance walked and a lower rate of perceived exertion may relate to improved fitness. This topic has been covered in greater detail in chapter 3.
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Case Presentation
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago.
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago. She describes the dizziness as "spinning," and it occurs not during running but just after she has finished. It usually occurs after running at a high-tempo pace for several miles and usually in the summer months. She denies frank syncope, chest pain, shortness of breath, or palpitations. There was a documented blood pressure of 62/40 at one time when paramedics were called while the event was occurring. She has been seen in the emergency room twice for the complaint; the most recent was 2 d ago. In the emergency room, she had a normal heart rate and blood pressure, normal physical examination, and normal laboratory values for anemia and thyroid. Her chest radiograph and electrocardiogram (ECG) were unremarkable. She has been restricted from exercise until she is seen by you. She is requesting exercise clearance.
Turning back to our case study, on further questioning it is revealed that Rebecca drinks 20 oz of water daily and one cup of coffee in the morning. She often skips breakfast as well because she does not have time to eat. She sometimes experiences dizziness in the morning when she stands up while getting out of bed. Rebecca's physical examination included orthostatic vital signs. Her heart rate increased from 52 bpm supine to 90 bpm in the upright position. Her blood pressure decreased from 116/68 to 108/64 from supine to upright position, respectively. Her physical examination was otherwise unremarkable.
Rebecca had an ECG performed, which was normal. Although her symptoms were classic for dehydration as well as a vasovagal component, because she was a competitive athlete, the cardiologist opted for an echocardiogram, which was normal, and an exercise stress test to try to induce her symptoms.
Rebecca performed a maximal treadmill exercise stress test. She had a high-normal aerobic capacity. There were no arrhythmias or ischemic changes. There was a normal heart rate and blood pressure response to exercise. She developed dizziness in early recovery with a 30 mmHg drop in her blood pressure. After she drank a cup of water and was placed in the supine position, her blood pressure improved within 2 min.
For Rebecca, her history, physical examination, and ECG were reassuring that this was unlikely to be cardiac-related exertional syncope. Her exercise stress test was able to document dizziness associated with a drop in blood pressure that responded to fluids and supine positioning. She was counseled on improving her hydration and salt intake and not to skip meals. She returned for a follow-up visit 3 mo later and had not experienced any further dizziness or syncope.
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Unique Features of Exercise Testing in Children
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults.
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults. One need only consider the various approaches needed to perform a satisfactory exercise test first in a 15-year-old cross country runner who experienced precordial chest pain in her last race, followed by a test looking for heart rate response in a 5-year-old youngster with complete heart block, and then a 12-year-old obese boy with a dilated cardiomyopathy and progressively worsening shortness of breath with exercise.
The pediatric exercise testing laboratory must accommodate wide variations in patient age, size, and fitness, and that means that testing protocols and equipment must be similarly adjusted. The indications for testing are much broader than those in the adult lab, so the questions to be answered must be carefully considered before the exercise begins.
Most children can be easily motivated to give exhaustive efforts during exercise testing, but it requires charismatic skill from staff members experienced with the emotional and physical responses of children during treadmill or cycle exercise. It has been said that perhaps the single most important factor in a successful exercise test in the pediatric laboratory is the staff administering the test.
Pediatric exercise testing, then, is distinguished by the need for a creative approach to each patient. The staff must know what information is needed to address the clinical question being asked, the proper modality - cycle or treadmill - to obtain that answer, and the optimal protocol for the subject's age and fitness level.
The physiological mechanisms underlying the cardiac and pulmonary responses to a bout of progressive exercise are no different in children (at least those over age 6) and adults. Nonetheless, certain quantitative measurements (heart rate, blood pressure, endurance time) are different in children, and these must be recognized in the testing of immature subjects.
For example, resting and maximal heart rates in an exhaustive exercise test are greater in children than in adults. As will be discussed in chapter 5, peak heart rate depends on testing modality and protocol, and there is considerable variability between individuals. It is important to recognize that there is generally no specific "target heart rate" for an exercise test. Importantly, too, the maximal heart rate during exercise testing in a given subject does not change over the course of childhood. Only at about age 16 does this value begin to decline. Thus, age-related formulae for predicting a maximal heart rate, such as "220 minus age," do not apply to youths.
The concept of metabolic equivalents, or METs, as a measure of energy expenditure during exercise is commonly used with adult subjects but is fraught with difficulty in children and adolescents, and it is best avoided in the pediatric exercise laboratory. METs is a means of expressing the oxygen requirement of a physical activity relative to an assumed resting value. One MET, or resting O2, in an adult is considered to be 3.5 ml ∙ kg-1 ∙ min-1; thus, when walking on a treadmill at a certain speed and slope that is expected to demand 17.5 ml ∙ kg-1 ∙ min-1, a patient is exercising at a level of 5 METs.
The difficulty with this concept in youth is that resting energy expenditure is not constant but evolves throughout childhood during physical growth. As would be expected, absolute values of resting O2 rise with the accrual of body mass. When adjusted for body mass, or body surface area, basal or resting values of energy expenditure decline progressively during the pediatric years. When expressed as calories per meter of square body surface area per hour, the basal metabolic rate declines by about 20% between the ages of six and the mid-teen years.
In considering a mass-relative definition of a MET in children, the story is more exaggerated. Harrell et al. reported that the resting O2 per kg in a group of 8- to 12-year-old children was almost 50% higher than that of 16- to 18-year-old subjects and 70% higher than that expected in adults. The use of the MET as defined in adults as a "currency" or multiplier of energy expenditure in children, then, clearly would introduce large errors in defining O2 levels during exercise - and the extent of the error would be different depending on the age and size of the child.
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Asthma
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms.
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms. It is believed that the majority of children with asthma exhibit EIB, which may affect exercise tolerance as well as participation in physical activity. As reviewed by Welsh et al., studies examining aerobic fitness in children provide mixed results, with some reporting comparable fitness and others reporting reduced fitness in asthmatic children compared to their nonasthmatic peers. The inconsistencies may reflect differences in measurement of fitness (e.g., laboratory vs. field tests), differences in levels of severity of the children's asthma, the relatively small sample, selection bias in choice of nonasthmatic children for comparison, use of bronchodilator therapy prior to testing, and perhaps inappropriate statistical analyses. Some studies suggest that fitness is comparable as long as levels of physical activity are similar. Pharmacologic advances such as inhaled corticosteroid therapy have reduced the number of hospitalizations and emergency department visits, suggesting that the severity of disease has lessened. Consequently, the results of studies conducted prior to these advances may not accurately reflect the current impact of the disease on exercise tolerance.
In the past two decades, surprisingly few studies of children report ventilatory parameters assessed during exercise testing. Santuz et al. reported lower E at peak exercise in sedentary and regularly active children when compared to healthy controls (matched on age, height, weight, and habitual level of physical activity), but no difference in E was observed in asthmatic children who participated in organized or competitive sports compared to their peers. During submaximal exercise, the asthmatic and nonasthmatic children had a similar E, but it was achieved by a lower f b and a higher VT in the asthmatic children, which would lessen dead space breathing and improve alveolar ventilation. In contrast, Moraes et al. reported no differences among newly diagnosed adolescents with mild intermittent asthma (n = 20), mild persistent asthma (n = 13), and their healthy peers (n = 36) for O2, E, E/O2, or ventilatory reserve at maximal exercise. Berntsen et al. likewise found no difference in O2 or E at peak exercise in a group of 86 13-year-old adolescents with asthma compared to 76 same-aged peers without asthma. Resting lung function was slightly lower in the asthmatic children but generally within normal limits. In a multiple regression model that included skinfold thickness, resting pulmonary function, and physical activity, analysis indicated that skinfold thickness and vigorous physical activity participation (in asthmatics only) were the primary independent predictors of O2peak in the study.
Expiratory flow limitations after exercise are well documented in children with asthma, and a recent study reported postexercise inspiratory flow limitation as well. However, no study has reported measurement of exercise tidal loops to assess ventilatory mechanics in children with asthma during exercise. Consequently, the results of one study of adults are presented. Exercise tolerance and pulmonary mechanics during exercise were compared between eight adults with asthma and six adults without asthma. Asthmatic adults had similar larger airway function but reduced smaller airway function at rest, and O2peak was slightly lower (but not significantly) in asthmatic compared to control subjects (104% vs. 130% of predicted). At maximal exercise, the groups attained similar E, but the asthmatic subjects had higher mean values for E/CO2 and lower PETCO2 than the control subjects. Superimposing the exercise tidal loop on the resting maximal F-V loops produced similarities to figure 11.3, revealing increases in EELV with the expiratory flow of the tidal loop encroaching on the resting F-V loop during exercise, indicative of expiratory flow limitation. Interestingly, the authors noted variability in EELV and in the degree of exercise flow limitation during interval exercise (intensity switching from 60% to 40% of O2peak), and they suggested that the variability reflected changes in bronchomotor tone (i.e., bronchodilation vs. bronchoconstriction ) at different exercise intensities.
Recent studies reporting measurements of arterial blood gases are rare in children with asthma. One recent study collected expired gases and earlobe capillary blood samples in eight adolescents with mild intermittent asthma (MIA), eight with mild persistent asthma (MPA), and 12 nonasthmatic control subjects to examine gas exchange during exercise and to estimate PaO2 before and after maximal exercise stress. Before exercise, the children with MPA had significantly lower PaO2 (75.1 ± 6.6 mmHg) than those with MIA (81.7 ± 6.7 mmHg) and control (83.3 ± 4.9 mmHg) subjects, suggesting hypoxemia. Postexercise PaO2 did not differ among groups, but this was attributed to a decrease in the control subjects because the PaO2 of the children with MPA did not change significantly. Older studies report gas exchange abnormalities (elevated PA - aO2 difference) at rest that improved during exercise, which the authors attributed to better A/ matching. A more recent study of habitually active 18- to 45-year-old men and women with mild-to-moderate asthma examined arterial blood gases during exercise. Eight of 21 subjects exhibited oxyhemoglobin desaturation (SaO2 ≤94%) during prolonged exercise and were classified as the Lo-SaO2 group, and those whose SaO2 remained above 94% were classified as the Hi-SaO2 group. Despite similar fitness levels, the PaCO2 of the Hi-SaO2 group fell from rest to 34.0 ± 2.7 mmHg at exhaustion, whereas that of the Lo-SaO2 group fell during the first minute of exercise but then rose progressively, reaching 39.8 ± 4 mmHg at exhaustion, suggesting hypoventilation and impaired gas exchange.
Given the prevalence of asthma in children, as well as reports of reduced fitness, more studies examining ventilatory mechanics and gas exchange parameters during exercise are warranted in children with asthma to determine their potential contributions to reduced fitness and exercise intolerance.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Clinical Exercise Testing
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function.
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function. Karlman Wasserman and coworkers at UCLA demonstrated, too, how the acquisition of gas exchange variables measured during exercise could further delineate and differentiate abnormal cardiac and pulmonary responses to exercise.
The use of exercise testing in pediatric populations, whose members do not normally suffer from coronary artery disease, initially developed in the shadow of this story about adult patients. Early exercise studies in youth were performed in the research setting. They were designed to examine physiological differences that separate children from adults. Sid Robinson provided the first such treadmill-derived data in the Harvard Fatigue Laboratory in Boston in the 1930s, demonstrating the progressive changes in metabolic and physiological responses that normally occurred between the ages of 6 and 91. Similar exercise data in healthy children were subsequently provided by other investigators in the middle of the 20th century, including Per-Olof …strand in Sweden, Simon Godfrey in Great Britain, and Gordon Cumming in Canada. When Oded Bar-Or published his landmark book Pediatric Sports Medicine for the Practitioner in 1983, he was able to accumulate a large base of normative data from these earlier studies to outline aspects of physiological responses to exercise in youth and how these developed during the growing years.
While such research was designed to reveal the normal development of physiological responses to exercise in children, this information also served as normative data for those who developed exercise testing for the clinical assessment of heart and lung disease in pediatric patients. As studies dealt, for the most part, with those with congenital heart disease, early exercise stress testing in young patients involved a more diverse approach than that of the traditional adult laboratory focused on the detection of coronary artery disease. The assessment of ischemic changes on the ECG was still an issue, particularly in assessing the severity of aortic outflow obstruction, but exercise testing in young patients also involved a wider range of information, such as blood pressure responses (in coarctation of the aorta, systemic hypertension), endurance capacity (postoperative cyanotic heart disease), and rhythm responses (complete heart block).
Useful clinical testing methodologies and clinical findings were described by a number of key early pioneers, such as Fred James at Cincinnati Children's Hospital, David Driscoll at the Mayo Clinic, Bruce Alpert and William Strong at the Medical College of Georgia, Rolf Mocellin in Germany, and Tony Reybrouck and Dirk Matthys in Belgium. The importance of gas exchange measures, including O2max and O2 kinetics, was highlighted by the early reports of exercise testing in patients with congenital heart disease by Hans Wessel at Children's Memorial Hospital in Chicago. During this time, too, exercise testing became established in both children and adults as a useful means of assessing bronchospasm and lung function in patients with asthma and other respiratory diseases (particularly via the early experience reported by Hans Stoboy, Gerd Cropp, and Svein Oseid).
In many cases, clinical exercise testing in children was performed in adult laboratories, using protocols, exercise equipment, and monitoring systems (ECG, blood pressure) traditionally used to test adults with suspected coronary artery disease. A number of developments have now expanded the role of exercise testing in youth and have identified the need for more specific approaches to exercise testing for this population of patients.
- Perhaps most importantly, the past several decades have witnessed a dramatic expansion in the scope and nature of patients cared for by pediatric cardiologists. Children with complex forms of congenital heart disease, particularly those characterized by marked unilateral ventricular hypoplasia (hypoplastic left heart syndrome, tricuspid valve atresia), once had little hope for long-term survival. Now, thanks to remarkable progress in surgical techniques, these young patients not only often survive but also live productive and fulfilling lives. The physicians caring for these survivors as they grow toward the adult years are confronted with new issues, such as myocardial dysfunction, stubborn tachyarrhythmia, hypoxemia, and pulmonary hypertension. These problems have required new diagnostic and therapeutic approaches, often using information available in the exercise stress-testing laboratory. Similarly, in patients with lung diseases such as cystic fibrosis, improvements in patient care have successfully extended survival and have at the same time introduced new clinical questions that can be assessed through exercise testing. It is likely that the future will continue to bring steady improvements in the survival of young patients with both cardiac and pulmonary disease that will be paralleled by expanded indications for clinical exercise testing.
- A growing understanding of the pathophysiology of cardiac malformations and factors influencing risk stratification have created a need to expand the information obtained during exercise stress testing in young patients. It is true that many issues can be adequately examined by a limited study involving a traditional bout of progressive exercise accompanied by electrocardiographic monitoring and measurement of blood pressure. Assessment of possible ischemic changes in a child after Kawasaki disease, for example, or determination of blood pressure responses after medical treatment of a hypertensive young wrestler could be adequately performed using this approach.
However, the clinical insights gained from exercise testing can be improved by the measurement of gas exchange variables, which are now readily obtained with user-friendly commercial metabolic systems. The changes in the oxygen and carbon dioxide content of expired air during exercise reflects similar gas exchange dynamics at the cellular level. With this approach, for example, the measurement of O2max provides an objective physiological assessment of aerobic fitness, and the determination of ventilatory variables (minute ventilation, CO2) offers insights into pulmonary responses as well. As will be outlined in the chapters that follow, it is often the calculation of relationships between these variables that provides clues into the relative importance of cardiac and pulmonary etiologies of exercise limitation.
Termed an "integrative cardiopulmonary test" by Wasserman et al., this approach expands the utility of exercise testing by providing data that can help to answer questions about cardiac and pulmonary issues in youth. In a teenage boy with moderate aortic valve insufficiency, does his cardiac disease explain the shortness of breath that limits his ability to exercise? What mechanisms lie behind a star athlete's inability to perform well after an extended viral illness? Is syncope of an anxious child during running related to hyperventilation? Is breathlessness during exercise in a markedly obese child caused by excess body fat, exercise-induced bronchoconstriction, or cardiac dysfunction? These types of issues are best addressed by a full examination of gas exchange variables during clinical exercise testing.
- A growing recognition of the effects of exercise on electrophysiological function has created new roles for exercise testing in youth. Assessment of changes in ventricular ectopy during exercise is a traditional indication for exercise testing. Newer indications include the use of responses of rate of ventricular repolarization (QT interval) and conduction down accessory pathways (WPW syndrome) during exercise as means of patient risk stratification.
- The increased use of pediatric exercise testing has also been stimulated by the concerns of parents, coaches, and physical education instructors over the occurrence of symptoms of chest pain, dizziness, syncope, or palpitations in young people during exercise. Such concerns have been fueled by the tragic occurrences of sudden unexpected death of young, presumably healthy athletes during sports training or competition. While such symptoms are highly unlikely to reflect the rare diseases that pose a risk of sudden death, the youngster with occult hypertrophic cardiomyopathy, coronary artery anomalies, or repolarization abnormalities that can predispose to fatal dysrhythmias can present with such complaints. Findings on exercise testing have thus become part of the assessment of symptomatic children and athletes to rule out these anomalies.
- A normal exercise test can provide clearance for sports play in young patients with heart disease or in those who have suffered illnesses such as viral myocarditis. Exercise testing also plays a role in assessing risk and exercise capacities in young patients who are enrolled in cardiac and pulmonary rehabilitation programs.
We can expect that clinical exercise testing in children and adolescents will continue to expand as the value of exercise is recognized in the assessment of not only heart and lung disease but also metabolic and musculoskeletal disorders. Such trends will undoubtedly follow improvements in medical and surgical treatment of these patients. We can also expect to see new techniques for performing exercise tests (miniaturization of metabolic systems permitting field testing, for example) and assessing their results (three-dimensional echocardiography, myocardial strain Doppler studies).
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Determining Functional Capacity
In summary, our quest is to determine children’s functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise.
In summary, our quest is to determine children's functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise. Exercise testing is essential to achieve this goal. Simple measurement of workload on a cycle ergometer will provide information on how close we are to "normal." Peak workload on a cycle ergometer is a measure of very vigorous exercise. The calibration of the cycle is often very good, and the workloads are very consistent over fast and slow pedaling. It is therefore a very useful measure of functional capacity for many subjects of different sizes and levels of physical conditioning. Within-subject reliability is also very good, so we can compare the child's responses over time accurately. Work in watts measured on the cycle ergometer can be used in a calculation to predict the oxygen consumption, again giving us a measure of functional capacity.
Current research indicates that one can predict the peak O2 from the O2 at VAT with good accuracy in healthy children as well as in those with lung disease. Caution should be used when predicting peak O2 from the O2 at VAT in children with congenital heart disease. Metabolic measurements need to be available to calculate the VAT, which adds to the cost and resources needed to predict the O2 from this method. The measurement of VAT becomes more useful when a submaximal effort has limited the exercise test. Age, comprehension, size, motivation, and protocol all factor into a submaximal response. The caveat of the method is when we assume that the O2 at VAT is about 60% of the peak O2. Physical conditioning and specifically a lack of physical fitness influence this method greatly, and this may lead to over- or underestimation of peak O2. We know that physical deconditioning, either from being sedentary or as the result of a disease process, lowers the O2 at the VAT.
OUES is a useful submaximal measure of aerobic fitness that is effort independent. This method also relies on the availability of metabolic measurements. It has been shown to be reproducible and does not suffer the intra-observer or inter-observer discrepancies often encountered with VAT. The OUES is easy to calculate and is determined by many more data points than VAT, peak work, or peak heart rate. The consensus of the research data indicates that it is most useful at the higher percentages of the peak O2 (that is, cut points of OUES 80% and OUES 90%). OUES below 45% to 50% of peak O2 is less reliable and does not correlate as well with peak O2. There is conflicting evidence regarding whether OUES is equally useful in predicting peak O2 in healthy subjects and in those with chronic heart or lung disease. If a subject can achieve 80% or 90% of predicted peak O2, many of us would consider that an adequate level of exertion. If the subject achieves 80% or 90% of peak O2, we do not know if the OUES slope can be abnormal. My guess is probably not. The slope of the line that represents the VE and O2 relationship is strongly influenced by the onset of anaerobic metabolism. The V-slope method of VAT detection is based on the change in that slope. Therefore the choice of the cut points will influence the OUES slope. Another factor that will influence the OUES is the choice of ergometer. We know that the peak O2 on the cycle ergometer is about 10% to 15% less than what is achieved on a treadmill. Therefore percent cut points will actually represent different amounts of O2. This could be problematic if we did not have equations for both the treadmill and cycle ergometer.
The six-minute walk is a simple test that is easy to administer and has good reproducibility. There are two versions, encouraged and unencouraged. The unencouraged version may reduce variability among test administrators. Distance and dyspnea score are recorded. The words and phrases of the Borg scale may not be completely understood by young children. Other scales with pictures have been used to assess the child's perceived exertion. The six-minute walk test may be more useful in underserved areas where metabolic equipment is not available. Also, large groups of children can be tested at very little cost, and over time increases in distance walked and a lower rate of perceived exertion may relate to improved fitness. This topic has been covered in greater detail in chapter 3.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Case Presentation
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago.
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago. She describes the dizziness as "spinning," and it occurs not during running but just after she has finished. It usually occurs after running at a high-tempo pace for several miles and usually in the summer months. She denies frank syncope, chest pain, shortness of breath, or palpitations. There was a documented blood pressure of 62/40 at one time when paramedics were called while the event was occurring. She has been seen in the emergency room twice for the complaint; the most recent was 2 d ago. In the emergency room, she had a normal heart rate and blood pressure, normal physical examination, and normal laboratory values for anemia and thyroid. Her chest radiograph and electrocardiogram (ECG) were unremarkable. She has been restricted from exercise until she is seen by you. She is requesting exercise clearance.
Turning back to our case study, on further questioning it is revealed that Rebecca drinks 20 oz of water daily and one cup of coffee in the morning. She often skips breakfast as well because she does not have time to eat. She sometimes experiences dizziness in the morning when she stands up while getting out of bed. Rebecca's physical examination included orthostatic vital signs. Her heart rate increased from 52 bpm supine to 90 bpm in the upright position. Her blood pressure decreased from 116/68 to 108/64 from supine to upright position, respectively. Her physical examination was otherwise unremarkable.
Rebecca had an ECG performed, which was normal. Although her symptoms were classic for dehydration as well as a vasovagal component, because she was a competitive athlete, the cardiologist opted for an echocardiogram, which was normal, and an exercise stress test to try to induce her symptoms.
Rebecca performed a maximal treadmill exercise stress test. She had a high-normal aerobic capacity. There were no arrhythmias or ischemic changes. There was a normal heart rate and blood pressure response to exercise. She developed dizziness in early recovery with a 30 mmHg drop in her blood pressure. After she drank a cup of water and was placed in the supine position, her blood pressure improved within 2 min.
For Rebecca, her history, physical examination, and ECG were reassuring that this was unlikely to be cardiac-related exertional syncope. Her exercise stress test was able to document dizziness associated with a drop in blood pressure that responded to fluids and supine positioning. She was counseled on improving her hydration and salt intake and not to skip meals. She returned for a follow-up visit 3 mo later and had not experienced any further dizziness or syncope.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Unique Features of Exercise Testing in Children
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults.
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults. One need only consider the various approaches needed to perform a satisfactory exercise test first in a 15-year-old cross country runner who experienced precordial chest pain in her last race, followed by a test looking for heart rate response in a 5-year-old youngster with complete heart block, and then a 12-year-old obese boy with a dilated cardiomyopathy and progressively worsening shortness of breath with exercise.
The pediatric exercise testing laboratory must accommodate wide variations in patient age, size, and fitness, and that means that testing protocols and equipment must be similarly adjusted. The indications for testing are much broader than those in the adult lab, so the questions to be answered must be carefully considered before the exercise begins.
Most children can be easily motivated to give exhaustive efforts during exercise testing, but it requires charismatic skill from staff members experienced with the emotional and physical responses of children during treadmill or cycle exercise. It has been said that perhaps the single most important factor in a successful exercise test in the pediatric laboratory is the staff administering the test.
Pediatric exercise testing, then, is distinguished by the need for a creative approach to each patient. The staff must know what information is needed to address the clinical question being asked, the proper modality - cycle or treadmill - to obtain that answer, and the optimal protocol for the subject's age and fitness level.
The physiological mechanisms underlying the cardiac and pulmonary responses to a bout of progressive exercise are no different in children (at least those over age 6) and adults. Nonetheless, certain quantitative measurements (heart rate, blood pressure, endurance time) are different in children, and these must be recognized in the testing of immature subjects.
For example, resting and maximal heart rates in an exhaustive exercise test are greater in children than in adults. As will be discussed in chapter 5, peak heart rate depends on testing modality and protocol, and there is considerable variability between individuals. It is important to recognize that there is generally no specific "target heart rate" for an exercise test. Importantly, too, the maximal heart rate during exercise testing in a given subject does not change over the course of childhood. Only at about age 16 does this value begin to decline. Thus, age-related formulae for predicting a maximal heart rate, such as "220 minus age," do not apply to youths.
The concept of metabolic equivalents, or METs, as a measure of energy expenditure during exercise is commonly used with adult subjects but is fraught with difficulty in children and adolescents, and it is best avoided in the pediatric exercise laboratory. METs is a means of expressing the oxygen requirement of a physical activity relative to an assumed resting value. One MET, or resting O2, in an adult is considered to be 3.5 ml ∙ kg-1 ∙ min-1; thus, when walking on a treadmill at a certain speed and slope that is expected to demand 17.5 ml ∙ kg-1 ∙ min-1, a patient is exercising at a level of 5 METs.
The difficulty with this concept in youth is that resting energy expenditure is not constant but evolves throughout childhood during physical growth. As would be expected, absolute values of resting O2 rise with the accrual of body mass. When adjusted for body mass, or body surface area, basal or resting values of energy expenditure decline progressively during the pediatric years. When expressed as calories per meter of square body surface area per hour, the basal metabolic rate declines by about 20% between the ages of six and the mid-teen years.
In considering a mass-relative definition of a MET in children, the story is more exaggerated. Harrell et al. reported that the resting O2 per kg in a group of 8- to 12-year-old children was almost 50% higher than that of 16- to 18-year-old subjects and 70% higher than that expected in adults. The use of the MET as defined in adults as a "currency" or multiplier of energy expenditure in children, then, clearly would introduce large errors in defining O2 levels during exercise - and the extent of the error would be different depending on the age and size of the child.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Asthma
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms.
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms. It is believed that the majority of children with asthma exhibit EIB, which may affect exercise tolerance as well as participation in physical activity. As reviewed by Welsh et al., studies examining aerobic fitness in children provide mixed results, with some reporting comparable fitness and others reporting reduced fitness in asthmatic children compared to their nonasthmatic peers. The inconsistencies may reflect differences in measurement of fitness (e.g., laboratory vs. field tests), differences in levels of severity of the children's asthma, the relatively small sample, selection bias in choice of nonasthmatic children for comparison, use of bronchodilator therapy prior to testing, and perhaps inappropriate statistical analyses. Some studies suggest that fitness is comparable as long as levels of physical activity are similar. Pharmacologic advances such as inhaled corticosteroid therapy have reduced the number of hospitalizations and emergency department visits, suggesting that the severity of disease has lessened. Consequently, the results of studies conducted prior to these advances may not accurately reflect the current impact of the disease on exercise tolerance.
In the past two decades, surprisingly few studies of children report ventilatory parameters assessed during exercise testing. Santuz et al. reported lower E at peak exercise in sedentary and regularly active children when compared to healthy controls (matched on age, height, weight, and habitual level of physical activity), but no difference in E was observed in asthmatic children who participated in organized or competitive sports compared to their peers. During submaximal exercise, the asthmatic and nonasthmatic children had a similar E, but it was achieved by a lower f b and a higher VT in the asthmatic children, which would lessen dead space breathing and improve alveolar ventilation. In contrast, Moraes et al. reported no differences among newly diagnosed adolescents with mild intermittent asthma (n = 20), mild persistent asthma (n = 13), and their healthy peers (n = 36) for O2, E, E/O2, or ventilatory reserve at maximal exercise. Berntsen et al. likewise found no difference in O2 or E at peak exercise in a group of 86 13-year-old adolescents with asthma compared to 76 same-aged peers without asthma. Resting lung function was slightly lower in the asthmatic children but generally within normal limits. In a multiple regression model that included skinfold thickness, resting pulmonary function, and physical activity, analysis indicated that skinfold thickness and vigorous physical activity participation (in asthmatics only) were the primary independent predictors of O2peak in the study.
Expiratory flow limitations after exercise are well documented in children with asthma, and a recent study reported postexercise inspiratory flow limitation as well. However, no study has reported measurement of exercise tidal loops to assess ventilatory mechanics in children with asthma during exercise. Consequently, the results of one study of adults are presented. Exercise tolerance and pulmonary mechanics during exercise were compared between eight adults with asthma and six adults without asthma. Asthmatic adults had similar larger airway function but reduced smaller airway function at rest, and O2peak was slightly lower (but not significantly) in asthmatic compared to control subjects (104% vs. 130% of predicted). At maximal exercise, the groups attained similar E, but the asthmatic subjects had higher mean values for E/CO2 and lower PETCO2 than the control subjects. Superimposing the exercise tidal loop on the resting maximal F-V loops produced similarities to figure 11.3, revealing increases in EELV with the expiratory flow of the tidal loop encroaching on the resting F-V loop during exercise, indicative of expiratory flow limitation. Interestingly, the authors noted variability in EELV and in the degree of exercise flow limitation during interval exercise (intensity switching from 60% to 40% of O2peak), and they suggested that the variability reflected changes in bronchomotor tone (i.e., bronchodilation vs. bronchoconstriction ) at different exercise intensities.
Recent studies reporting measurements of arterial blood gases are rare in children with asthma. One recent study collected expired gases and earlobe capillary blood samples in eight adolescents with mild intermittent asthma (MIA), eight with mild persistent asthma (MPA), and 12 nonasthmatic control subjects to examine gas exchange during exercise and to estimate PaO2 before and after maximal exercise stress. Before exercise, the children with MPA had significantly lower PaO2 (75.1 ± 6.6 mmHg) than those with MIA (81.7 ± 6.7 mmHg) and control (83.3 ± 4.9 mmHg) subjects, suggesting hypoxemia. Postexercise PaO2 did not differ among groups, but this was attributed to a decrease in the control subjects because the PaO2 of the children with MPA did not change significantly. Older studies report gas exchange abnormalities (elevated PA - aO2 difference) at rest that improved during exercise, which the authors attributed to better A/ matching. A more recent study of habitually active 18- to 45-year-old men and women with mild-to-moderate asthma examined arterial blood gases during exercise. Eight of 21 subjects exhibited oxyhemoglobin desaturation (SaO2 ≤94%) during prolonged exercise and were classified as the Lo-SaO2 group, and those whose SaO2 remained above 94% were classified as the Hi-SaO2 group. Despite similar fitness levels, the PaCO2 of the Hi-SaO2 group fell from rest to 34.0 ± 2.7 mmHg at exhaustion, whereas that of the Lo-SaO2 group fell during the first minute of exercise but then rose progressively, reaching 39.8 ± 4 mmHg at exhaustion, suggesting hypoventilation and impaired gas exchange.
Given the prevalence of asthma in children, as well as reports of reduced fitness, more studies examining ventilatory mechanics and gas exchange parameters during exercise are warranted in children with asthma to determine their potential contributions to reduced fitness and exercise intolerance.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Clinical Exercise Testing
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function.
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function. Karlman Wasserman and coworkers at UCLA demonstrated, too, how the acquisition of gas exchange variables measured during exercise could further delineate and differentiate abnormal cardiac and pulmonary responses to exercise.
The use of exercise testing in pediatric populations, whose members do not normally suffer from coronary artery disease, initially developed in the shadow of this story about adult patients. Early exercise studies in youth were performed in the research setting. They were designed to examine physiological differences that separate children from adults. Sid Robinson provided the first such treadmill-derived data in the Harvard Fatigue Laboratory in Boston in the 1930s, demonstrating the progressive changes in metabolic and physiological responses that normally occurred between the ages of 6 and 91. Similar exercise data in healthy children were subsequently provided by other investigators in the middle of the 20th century, including Per-Olof …strand in Sweden, Simon Godfrey in Great Britain, and Gordon Cumming in Canada. When Oded Bar-Or published his landmark book Pediatric Sports Medicine for the Practitioner in 1983, he was able to accumulate a large base of normative data from these earlier studies to outline aspects of physiological responses to exercise in youth and how these developed during the growing years.
While such research was designed to reveal the normal development of physiological responses to exercise in children, this information also served as normative data for those who developed exercise testing for the clinical assessment of heart and lung disease in pediatric patients. As studies dealt, for the most part, with those with congenital heart disease, early exercise stress testing in young patients involved a more diverse approach than that of the traditional adult laboratory focused on the detection of coronary artery disease. The assessment of ischemic changes on the ECG was still an issue, particularly in assessing the severity of aortic outflow obstruction, but exercise testing in young patients also involved a wider range of information, such as blood pressure responses (in coarctation of the aorta, systemic hypertension), endurance capacity (postoperative cyanotic heart disease), and rhythm responses (complete heart block).
Useful clinical testing methodologies and clinical findings were described by a number of key early pioneers, such as Fred James at Cincinnati Children's Hospital, David Driscoll at the Mayo Clinic, Bruce Alpert and William Strong at the Medical College of Georgia, Rolf Mocellin in Germany, and Tony Reybrouck and Dirk Matthys in Belgium. The importance of gas exchange measures, including O2max and O2 kinetics, was highlighted by the early reports of exercise testing in patients with congenital heart disease by Hans Wessel at Children's Memorial Hospital in Chicago. During this time, too, exercise testing became established in both children and adults as a useful means of assessing bronchospasm and lung function in patients with asthma and other respiratory diseases (particularly via the early experience reported by Hans Stoboy, Gerd Cropp, and Svein Oseid).
In many cases, clinical exercise testing in children was performed in adult laboratories, using protocols, exercise equipment, and monitoring systems (ECG, blood pressure) traditionally used to test adults with suspected coronary artery disease. A number of developments have now expanded the role of exercise testing in youth and have identified the need for more specific approaches to exercise testing for this population of patients.
- Perhaps most importantly, the past several decades have witnessed a dramatic expansion in the scope and nature of patients cared for by pediatric cardiologists. Children with complex forms of congenital heart disease, particularly those characterized by marked unilateral ventricular hypoplasia (hypoplastic left heart syndrome, tricuspid valve atresia), once had little hope for long-term survival. Now, thanks to remarkable progress in surgical techniques, these young patients not only often survive but also live productive and fulfilling lives. The physicians caring for these survivors as they grow toward the adult years are confronted with new issues, such as myocardial dysfunction, stubborn tachyarrhythmia, hypoxemia, and pulmonary hypertension. These problems have required new diagnostic and therapeutic approaches, often using information available in the exercise stress-testing laboratory. Similarly, in patients with lung diseases such as cystic fibrosis, improvements in patient care have successfully extended survival and have at the same time introduced new clinical questions that can be assessed through exercise testing. It is likely that the future will continue to bring steady improvements in the survival of young patients with both cardiac and pulmonary disease that will be paralleled by expanded indications for clinical exercise testing.
- A growing understanding of the pathophysiology of cardiac malformations and factors influencing risk stratification have created a need to expand the information obtained during exercise stress testing in young patients. It is true that many issues can be adequately examined by a limited study involving a traditional bout of progressive exercise accompanied by electrocardiographic monitoring and measurement of blood pressure. Assessment of possible ischemic changes in a child after Kawasaki disease, for example, or determination of blood pressure responses after medical treatment of a hypertensive young wrestler could be adequately performed using this approach.
However, the clinical insights gained from exercise testing can be improved by the measurement of gas exchange variables, which are now readily obtained with user-friendly commercial metabolic systems. The changes in the oxygen and carbon dioxide content of expired air during exercise reflects similar gas exchange dynamics at the cellular level. With this approach, for example, the measurement of O2max provides an objective physiological assessment of aerobic fitness, and the determination of ventilatory variables (minute ventilation, CO2) offers insights into pulmonary responses as well. As will be outlined in the chapters that follow, it is often the calculation of relationships between these variables that provides clues into the relative importance of cardiac and pulmonary etiologies of exercise limitation.
Termed an "integrative cardiopulmonary test" by Wasserman et al., this approach expands the utility of exercise testing by providing data that can help to answer questions about cardiac and pulmonary issues in youth. In a teenage boy with moderate aortic valve insufficiency, does his cardiac disease explain the shortness of breath that limits his ability to exercise? What mechanisms lie behind a star athlete's inability to perform well after an extended viral illness? Is syncope of an anxious child during running related to hyperventilation? Is breathlessness during exercise in a markedly obese child caused by excess body fat, exercise-induced bronchoconstriction, or cardiac dysfunction? These types of issues are best addressed by a full examination of gas exchange variables during clinical exercise testing.
- A growing recognition of the effects of exercise on electrophysiological function has created new roles for exercise testing in youth. Assessment of changes in ventricular ectopy during exercise is a traditional indication for exercise testing. Newer indications include the use of responses of rate of ventricular repolarization (QT interval) and conduction down accessory pathways (WPW syndrome) during exercise as means of patient risk stratification.
- The increased use of pediatric exercise testing has also been stimulated by the concerns of parents, coaches, and physical education instructors over the occurrence of symptoms of chest pain, dizziness, syncope, or palpitations in young people during exercise. Such concerns have been fueled by the tragic occurrences of sudden unexpected death of young, presumably healthy athletes during sports training or competition. While such symptoms are highly unlikely to reflect the rare diseases that pose a risk of sudden death, the youngster with occult hypertrophic cardiomyopathy, coronary artery anomalies, or repolarization abnormalities that can predispose to fatal dysrhythmias can present with such complaints. Findings on exercise testing have thus become part of the assessment of symptomatic children and athletes to rule out these anomalies.
- A normal exercise test can provide clearance for sports play in young patients with heart disease or in those who have suffered illnesses such as viral myocarditis. Exercise testing also plays a role in assessing risk and exercise capacities in young patients who are enrolled in cardiac and pulmonary rehabilitation programs.
We can expect that clinical exercise testing in children and adolescents will continue to expand as the value of exercise is recognized in the assessment of not only heart and lung disease but also metabolic and musculoskeletal disorders. Such trends will undoubtedly follow improvements in medical and surgical treatment of these patients. We can also expect to see new techniques for performing exercise tests (miniaturization of metabolic systems permitting field testing, for example) and assessing their results (three-dimensional echocardiography, myocardial strain Doppler studies).
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Determining Functional Capacity
In summary, our quest is to determine children’s functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise.
In summary, our quest is to determine children's functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise. Exercise testing is essential to achieve this goal. Simple measurement of workload on a cycle ergometer will provide information on how close we are to "normal." Peak workload on a cycle ergometer is a measure of very vigorous exercise. The calibration of the cycle is often very good, and the workloads are very consistent over fast and slow pedaling. It is therefore a very useful measure of functional capacity for many subjects of different sizes and levels of physical conditioning. Within-subject reliability is also very good, so we can compare the child's responses over time accurately. Work in watts measured on the cycle ergometer can be used in a calculation to predict the oxygen consumption, again giving us a measure of functional capacity.
Current research indicates that one can predict the peak O2 from the O2 at VAT with good accuracy in healthy children as well as in those with lung disease. Caution should be used when predicting peak O2 from the O2 at VAT in children with congenital heart disease. Metabolic measurements need to be available to calculate the VAT, which adds to the cost and resources needed to predict the O2 from this method. The measurement of VAT becomes more useful when a submaximal effort has limited the exercise test. Age, comprehension, size, motivation, and protocol all factor into a submaximal response. The caveat of the method is when we assume that the O2 at VAT is about 60% of the peak O2. Physical conditioning and specifically a lack of physical fitness influence this method greatly, and this may lead to over- or underestimation of peak O2. We know that physical deconditioning, either from being sedentary or as the result of a disease process, lowers the O2 at the VAT.
OUES is a useful submaximal measure of aerobic fitness that is effort independent. This method also relies on the availability of metabolic measurements. It has been shown to be reproducible and does not suffer the intra-observer or inter-observer discrepancies often encountered with VAT. The OUES is easy to calculate and is determined by many more data points than VAT, peak work, or peak heart rate. The consensus of the research data indicates that it is most useful at the higher percentages of the peak O2 (that is, cut points of OUES 80% and OUES 90%). OUES below 45% to 50% of peak O2 is less reliable and does not correlate as well with peak O2. There is conflicting evidence regarding whether OUES is equally useful in predicting peak O2 in healthy subjects and in those with chronic heart or lung disease. If a subject can achieve 80% or 90% of predicted peak O2, many of us would consider that an adequate level of exertion. If the subject achieves 80% or 90% of peak O2, we do not know if the OUES slope can be abnormal. My guess is probably not. The slope of the line that represents the VE and O2 relationship is strongly influenced by the onset of anaerobic metabolism. The V-slope method of VAT detection is based on the change in that slope. Therefore the choice of the cut points will influence the OUES slope. Another factor that will influence the OUES is the choice of ergometer. We know that the peak O2 on the cycle ergometer is about 10% to 15% less than what is achieved on a treadmill. Therefore percent cut points will actually represent different amounts of O2. This could be problematic if we did not have equations for both the treadmill and cycle ergometer.
The six-minute walk is a simple test that is easy to administer and has good reproducibility. There are two versions, encouraged and unencouraged. The unencouraged version may reduce variability among test administrators. Distance and dyspnea score are recorded. The words and phrases of the Borg scale may not be completely understood by young children. Other scales with pictures have been used to assess the child's perceived exertion. The six-minute walk test may be more useful in underserved areas where metabolic equipment is not available. Also, large groups of children can be tested at very little cost, and over time increases in distance walked and a lower rate of perceived exertion may relate to improved fitness. This topic has been covered in greater detail in chapter 3.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Case Presentation
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago.
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago. She describes the dizziness as "spinning," and it occurs not during running but just after she has finished. It usually occurs after running at a high-tempo pace for several miles and usually in the summer months. She denies frank syncope, chest pain, shortness of breath, or palpitations. There was a documented blood pressure of 62/40 at one time when paramedics were called while the event was occurring. She has been seen in the emergency room twice for the complaint; the most recent was 2 d ago. In the emergency room, she had a normal heart rate and blood pressure, normal physical examination, and normal laboratory values for anemia and thyroid. Her chest radiograph and electrocardiogram (ECG) were unremarkable. She has been restricted from exercise until she is seen by you. She is requesting exercise clearance.
Turning back to our case study, on further questioning it is revealed that Rebecca drinks 20 oz of water daily and one cup of coffee in the morning. She often skips breakfast as well because she does not have time to eat. She sometimes experiences dizziness in the morning when she stands up while getting out of bed. Rebecca's physical examination included orthostatic vital signs. Her heart rate increased from 52 bpm supine to 90 bpm in the upright position. Her blood pressure decreased from 116/68 to 108/64 from supine to upright position, respectively. Her physical examination was otherwise unremarkable.
Rebecca had an ECG performed, which was normal. Although her symptoms were classic for dehydration as well as a vasovagal component, because she was a competitive athlete, the cardiologist opted for an echocardiogram, which was normal, and an exercise stress test to try to induce her symptoms.
Rebecca performed a maximal treadmill exercise stress test. She had a high-normal aerobic capacity. There were no arrhythmias or ischemic changes. There was a normal heart rate and blood pressure response to exercise. She developed dizziness in early recovery with a 30 mmHg drop in her blood pressure. After she drank a cup of water and was placed in the supine position, her blood pressure improved within 2 min.
For Rebecca, her history, physical examination, and ECG were reassuring that this was unlikely to be cardiac-related exertional syncope. Her exercise stress test was able to document dizziness associated with a drop in blood pressure that responded to fluids and supine positioning. She was counseled on improving her hydration and salt intake and not to skip meals. She returned for a follow-up visit 3 mo later and had not experienced any further dizziness or syncope.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Unique Features of Exercise Testing in Children
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults.
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults. One need only consider the various approaches needed to perform a satisfactory exercise test first in a 15-year-old cross country runner who experienced precordial chest pain in her last race, followed by a test looking for heart rate response in a 5-year-old youngster with complete heart block, and then a 12-year-old obese boy with a dilated cardiomyopathy and progressively worsening shortness of breath with exercise.
The pediatric exercise testing laboratory must accommodate wide variations in patient age, size, and fitness, and that means that testing protocols and equipment must be similarly adjusted. The indications for testing are much broader than those in the adult lab, so the questions to be answered must be carefully considered before the exercise begins.
Most children can be easily motivated to give exhaustive efforts during exercise testing, but it requires charismatic skill from staff members experienced with the emotional and physical responses of children during treadmill or cycle exercise. It has been said that perhaps the single most important factor in a successful exercise test in the pediatric laboratory is the staff administering the test.
Pediatric exercise testing, then, is distinguished by the need for a creative approach to each patient. The staff must know what information is needed to address the clinical question being asked, the proper modality - cycle or treadmill - to obtain that answer, and the optimal protocol for the subject's age and fitness level.
The physiological mechanisms underlying the cardiac and pulmonary responses to a bout of progressive exercise are no different in children (at least those over age 6) and adults. Nonetheless, certain quantitative measurements (heart rate, blood pressure, endurance time) are different in children, and these must be recognized in the testing of immature subjects.
For example, resting and maximal heart rates in an exhaustive exercise test are greater in children than in adults. As will be discussed in chapter 5, peak heart rate depends on testing modality and protocol, and there is considerable variability between individuals. It is important to recognize that there is generally no specific "target heart rate" for an exercise test. Importantly, too, the maximal heart rate during exercise testing in a given subject does not change over the course of childhood. Only at about age 16 does this value begin to decline. Thus, age-related formulae for predicting a maximal heart rate, such as "220 minus age," do not apply to youths.
The concept of metabolic equivalents, or METs, as a measure of energy expenditure during exercise is commonly used with adult subjects but is fraught with difficulty in children and adolescents, and it is best avoided in the pediatric exercise laboratory. METs is a means of expressing the oxygen requirement of a physical activity relative to an assumed resting value. One MET, or resting O2, in an adult is considered to be 3.5 ml ∙ kg-1 ∙ min-1; thus, when walking on a treadmill at a certain speed and slope that is expected to demand 17.5 ml ∙ kg-1 ∙ min-1, a patient is exercising at a level of 5 METs.
The difficulty with this concept in youth is that resting energy expenditure is not constant but evolves throughout childhood during physical growth. As would be expected, absolute values of resting O2 rise with the accrual of body mass. When adjusted for body mass, or body surface area, basal or resting values of energy expenditure decline progressively during the pediatric years. When expressed as calories per meter of square body surface area per hour, the basal metabolic rate declines by about 20% between the ages of six and the mid-teen years.
In considering a mass-relative definition of a MET in children, the story is more exaggerated. Harrell et al. reported that the resting O2 per kg in a group of 8- to 12-year-old children was almost 50% higher than that of 16- to 18-year-old subjects and 70% higher than that expected in adults. The use of the MET as defined in adults as a "currency" or multiplier of energy expenditure in children, then, clearly would introduce large errors in defining O2 levels during exercise - and the extent of the error would be different depending on the age and size of the child.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Asthma
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms.
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms. It is believed that the majority of children with asthma exhibit EIB, which may affect exercise tolerance as well as participation in physical activity. As reviewed by Welsh et al., studies examining aerobic fitness in children provide mixed results, with some reporting comparable fitness and others reporting reduced fitness in asthmatic children compared to their nonasthmatic peers. The inconsistencies may reflect differences in measurement of fitness (e.g., laboratory vs. field tests), differences in levels of severity of the children's asthma, the relatively small sample, selection bias in choice of nonasthmatic children for comparison, use of bronchodilator therapy prior to testing, and perhaps inappropriate statistical analyses. Some studies suggest that fitness is comparable as long as levels of physical activity are similar. Pharmacologic advances such as inhaled corticosteroid therapy have reduced the number of hospitalizations and emergency department visits, suggesting that the severity of disease has lessened. Consequently, the results of studies conducted prior to these advances may not accurately reflect the current impact of the disease on exercise tolerance.
In the past two decades, surprisingly few studies of children report ventilatory parameters assessed during exercise testing. Santuz et al. reported lower E at peak exercise in sedentary and regularly active children when compared to healthy controls (matched on age, height, weight, and habitual level of physical activity), but no difference in E was observed in asthmatic children who participated in organized or competitive sports compared to their peers. During submaximal exercise, the asthmatic and nonasthmatic children had a similar E, but it was achieved by a lower f b and a higher VT in the asthmatic children, which would lessen dead space breathing and improve alveolar ventilation. In contrast, Moraes et al. reported no differences among newly diagnosed adolescents with mild intermittent asthma (n = 20), mild persistent asthma (n = 13), and their healthy peers (n = 36) for O2, E, E/O2, or ventilatory reserve at maximal exercise. Berntsen et al. likewise found no difference in O2 or E at peak exercise in a group of 86 13-year-old adolescents with asthma compared to 76 same-aged peers without asthma. Resting lung function was slightly lower in the asthmatic children but generally within normal limits. In a multiple regression model that included skinfold thickness, resting pulmonary function, and physical activity, analysis indicated that skinfold thickness and vigorous physical activity participation (in asthmatics only) were the primary independent predictors of O2peak in the study.
Expiratory flow limitations after exercise are well documented in children with asthma, and a recent study reported postexercise inspiratory flow limitation as well. However, no study has reported measurement of exercise tidal loops to assess ventilatory mechanics in children with asthma during exercise. Consequently, the results of one study of adults are presented. Exercise tolerance and pulmonary mechanics during exercise were compared between eight adults with asthma and six adults without asthma. Asthmatic adults had similar larger airway function but reduced smaller airway function at rest, and O2peak was slightly lower (but not significantly) in asthmatic compared to control subjects (104% vs. 130% of predicted). At maximal exercise, the groups attained similar E, but the asthmatic subjects had higher mean values for E/CO2 and lower PETCO2 than the control subjects. Superimposing the exercise tidal loop on the resting maximal F-V loops produced similarities to figure 11.3, revealing increases in EELV with the expiratory flow of the tidal loop encroaching on the resting F-V loop during exercise, indicative of expiratory flow limitation. Interestingly, the authors noted variability in EELV and in the degree of exercise flow limitation during interval exercise (intensity switching from 60% to 40% of O2peak), and they suggested that the variability reflected changes in bronchomotor tone (i.e., bronchodilation vs. bronchoconstriction ) at different exercise intensities.
Recent studies reporting measurements of arterial blood gases are rare in children with asthma. One recent study collected expired gases and earlobe capillary blood samples in eight adolescents with mild intermittent asthma (MIA), eight with mild persistent asthma (MPA), and 12 nonasthmatic control subjects to examine gas exchange during exercise and to estimate PaO2 before and after maximal exercise stress. Before exercise, the children with MPA had significantly lower PaO2 (75.1 ± 6.6 mmHg) than those with MIA (81.7 ± 6.7 mmHg) and control (83.3 ± 4.9 mmHg) subjects, suggesting hypoxemia. Postexercise PaO2 did not differ among groups, but this was attributed to a decrease in the control subjects because the PaO2 of the children with MPA did not change significantly. Older studies report gas exchange abnormalities (elevated PA - aO2 difference) at rest that improved during exercise, which the authors attributed to better A/ matching. A more recent study of habitually active 18- to 45-year-old men and women with mild-to-moderate asthma examined arterial blood gases during exercise. Eight of 21 subjects exhibited oxyhemoglobin desaturation (SaO2 ≤94%) during prolonged exercise and were classified as the Lo-SaO2 group, and those whose SaO2 remained above 94% were classified as the Hi-SaO2 group. Despite similar fitness levels, the PaCO2 of the Hi-SaO2 group fell from rest to 34.0 ± 2.7 mmHg at exhaustion, whereas that of the Lo-SaO2 group fell during the first minute of exercise but then rose progressively, reaching 39.8 ± 4 mmHg at exhaustion, suggesting hypoventilation and impaired gas exchange.
Given the prevalence of asthma in children, as well as reports of reduced fitness, more studies examining ventilatory mechanics and gas exchange parameters during exercise are warranted in children with asthma to determine their potential contributions to reduced fitness and exercise intolerance.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Clinical Exercise Testing
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function.
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function. Karlman Wasserman and coworkers at UCLA demonstrated, too, how the acquisition of gas exchange variables measured during exercise could further delineate and differentiate abnormal cardiac and pulmonary responses to exercise.
The use of exercise testing in pediatric populations, whose members do not normally suffer from coronary artery disease, initially developed in the shadow of this story about adult patients. Early exercise studies in youth were performed in the research setting. They were designed to examine physiological differences that separate children from adults. Sid Robinson provided the first such treadmill-derived data in the Harvard Fatigue Laboratory in Boston in the 1930s, demonstrating the progressive changes in metabolic and physiological responses that normally occurred between the ages of 6 and 91. Similar exercise data in healthy children were subsequently provided by other investigators in the middle of the 20th century, including Per-Olof …strand in Sweden, Simon Godfrey in Great Britain, and Gordon Cumming in Canada. When Oded Bar-Or published his landmark book Pediatric Sports Medicine for the Practitioner in 1983, he was able to accumulate a large base of normative data from these earlier studies to outline aspects of physiological responses to exercise in youth and how these developed during the growing years.
While such research was designed to reveal the normal development of physiological responses to exercise in children, this information also served as normative data for those who developed exercise testing for the clinical assessment of heart and lung disease in pediatric patients. As studies dealt, for the most part, with those with congenital heart disease, early exercise stress testing in young patients involved a more diverse approach than that of the traditional adult laboratory focused on the detection of coronary artery disease. The assessment of ischemic changes on the ECG was still an issue, particularly in assessing the severity of aortic outflow obstruction, but exercise testing in young patients also involved a wider range of information, such as blood pressure responses (in coarctation of the aorta, systemic hypertension), endurance capacity (postoperative cyanotic heart disease), and rhythm responses (complete heart block).
Useful clinical testing methodologies and clinical findings were described by a number of key early pioneers, such as Fred James at Cincinnati Children's Hospital, David Driscoll at the Mayo Clinic, Bruce Alpert and William Strong at the Medical College of Georgia, Rolf Mocellin in Germany, and Tony Reybrouck and Dirk Matthys in Belgium. The importance of gas exchange measures, including O2max and O2 kinetics, was highlighted by the early reports of exercise testing in patients with congenital heart disease by Hans Wessel at Children's Memorial Hospital in Chicago. During this time, too, exercise testing became established in both children and adults as a useful means of assessing bronchospasm and lung function in patients with asthma and other respiratory diseases (particularly via the early experience reported by Hans Stoboy, Gerd Cropp, and Svein Oseid).
In many cases, clinical exercise testing in children was performed in adult laboratories, using protocols, exercise equipment, and monitoring systems (ECG, blood pressure) traditionally used to test adults with suspected coronary artery disease. A number of developments have now expanded the role of exercise testing in youth and have identified the need for more specific approaches to exercise testing for this population of patients.
- Perhaps most importantly, the past several decades have witnessed a dramatic expansion in the scope and nature of patients cared for by pediatric cardiologists. Children with complex forms of congenital heart disease, particularly those characterized by marked unilateral ventricular hypoplasia (hypoplastic left heart syndrome, tricuspid valve atresia), once had little hope for long-term survival. Now, thanks to remarkable progress in surgical techniques, these young patients not only often survive but also live productive and fulfilling lives. The physicians caring for these survivors as they grow toward the adult years are confronted with new issues, such as myocardial dysfunction, stubborn tachyarrhythmia, hypoxemia, and pulmonary hypertension. These problems have required new diagnostic and therapeutic approaches, often using information available in the exercise stress-testing laboratory. Similarly, in patients with lung diseases such as cystic fibrosis, improvements in patient care have successfully extended survival and have at the same time introduced new clinical questions that can be assessed through exercise testing. It is likely that the future will continue to bring steady improvements in the survival of young patients with both cardiac and pulmonary disease that will be paralleled by expanded indications for clinical exercise testing.
- A growing understanding of the pathophysiology of cardiac malformations and factors influencing risk stratification have created a need to expand the information obtained during exercise stress testing in young patients. It is true that many issues can be adequately examined by a limited study involving a traditional bout of progressive exercise accompanied by electrocardiographic monitoring and measurement of blood pressure. Assessment of possible ischemic changes in a child after Kawasaki disease, for example, or determination of blood pressure responses after medical treatment of a hypertensive young wrestler could be adequately performed using this approach.
However, the clinical insights gained from exercise testing can be improved by the measurement of gas exchange variables, which are now readily obtained with user-friendly commercial metabolic systems. The changes in the oxygen and carbon dioxide content of expired air during exercise reflects similar gas exchange dynamics at the cellular level. With this approach, for example, the measurement of O2max provides an objective physiological assessment of aerobic fitness, and the determination of ventilatory variables (minute ventilation, CO2) offers insights into pulmonary responses as well. As will be outlined in the chapters that follow, it is often the calculation of relationships between these variables that provides clues into the relative importance of cardiac and pulmonary etiologies of exercise limitation.
Termed an "integrative cardiopulmonary test" by Wasserman et al., this approach expands the utility of exercise testing by providing data that can help to answer questions about cardiac and pulmonary issues in youth. In a teenage boy with moderate aortic valve insufficiency, does his cardiac disease explain the shortness of breath that limits his ability to exercise? What mechanisms lie behind a star athlete's inability to perform well after an extended viral illness? Is syncope of an anxious child during running related to hyperventilation? Is breathlessness during exercise in a markedly obese child caused by excess body fat, exercise-induced bronchoconstriction, or cardiac dysfunction? These types of issues are best addressed by a full examination of gas exchange variables during clinical exercise testing.
- A growing recognition of the effects of exercise on electrophysiological function has created new roles for exercise testing in youth. Assessment of changes in ventricular ectopy during exercise is a traditional indication for exercise testing. Newer indications include the use of responses of rate of ventricular repolarization (QT interval) and conduction down accessory pathways (WPW syndrome) during exercise as means of patient risk stratification.
- The increased use of pediatric exercise testing has also been stimulated by the concerns of parents, coaches, and physical education instructors over the occurrence of symptoms of chest pain, dizziness, syncope, or palpitations in young people during exercise. Such concerns have been fueled by the tragic occurrences of sudden unexpected death of young, presumably healthy athletes during sports training or competition. While such symptoms are highly unlikely to reflect the rare diseases that pose a risk of sudden death, the youngster with occult hypertrophic cardiomyopathy, coronary artery anomalies, or repolarization abnormalities that can predispose to fatal dysrhythmias can present with such complaints. Findings on exercise testing have thus become part of the assessment of symptomatic children and athletes to rule out these anomalies.
- A normal exercise test can provide clearance for sports play in young patients with heart disease or in those who have suffered illnesses such as viral myocarditis. Exercise testing also plays a role in assessing risk and exercise capacities in young patients who are enrolled in cardiac and pulmonary rehabilitation programs.
We can expect that clinical exercise testing in children and adolescents will continue to expand as the value of exercise is recognized in the assessment of not only heart and lung disease but also metabolic and musculoskeletal disorders. Such trends will undoubtedly follow improvements in medical and surgical treatment of these patients. We can also expect to see new techniques for performing exercise tests (miniaturization of metabolic systems permitting field testing, for example) and assessing their results (three-dimensional echocardiography, myocardial strain Doppler studies).
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Determining Functional Capacity
In summary, our quest is to determine children’s functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise.
In summary, our quest is to determine children's functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise. Exercise testing is essential to achieve this goal. Simple measurement of workload on a cycle ergometer will provide information on how close we are to "normal." Peak workload on a cycle ergometer is a measure of very vigorous exercise. The calibration of the cycle is often very good, and the workloads are very consistent over fast and slow pedaling. It is therefore a very useful measure of functional capacity for many subjects of different sizes and levels of physical conditioning. Within-subject reliability is also very good, so we can compare the child's responses over time accurately. Work in watts measured on the cycle ergometer can be used in a calculation to predict the oxygen consumption, again giving us a measure of functional capacity.
Current research indicates that one can predict the peak O2 from the O2 at VAT with good accuracy in healthy children as well as in those with lung disease. Caution should be used when predicting peak O2 from the O2 at VAT in children with congenital heart disease. Metabolic measurements need to be available to calculate the VAT, which adds to the cost and resources needed to predict the O2 from this method. The measurement of VAT becomes more useful when a submaximal effort has limited the exercise test. Age, comprehension, size, motivation, and protocol all factor into a submaximal response. The caveat of the method is when we assume that the O2 at VAT is about 60% of the peak O2. Physical conditioning and specifically a lack of physical fitness influence this method greatly, and this may lead to over- or underestimation of peak O2. We know that physical deconditioning, either from being sedentary or as the result of a disease process, lowers the O2 at the VAT.
OUES is a useful submaximal measure of aerobic fitness that is effort independent. This method also relies on the availability of metabolic measurements. It has been shown to be reproducible and does not suffer the intra-observer or inter-observer discrepancies often encountered with VAT. The OUES is easy to calculate and is determined by many more data points than VAT, peak work, or peak heart rate. The consensus of the research data indicates that it is most useful at the higher percentages of the peak O2 (that is, cut points of OUES 80% and OUES 90%). OUES below 45% to 50% of peak O2 is less reliable and does not correlate as well with peak O2. There is conflicting evidence regarding whether OUES is equally useful in predicting peak O2 in healthy subjects and in those with chronic heart or lung disease. If a subject can achieve 80% or 90% of predicted peak O2, many of us would consider that an adequate level of exertion. If the subject achieves 80% or 90% of peak O2, we do not know if the OUES slope can be abnormal. My guess is probably not. The slope of the line that represents the VE and O2 relationship is strongly influenced by the onset of anaerobic metabolism. The V-slope method of VAT detection is based on the change in that slope. Therefore the choice of the cut points will influence the OUES slope. Another factor that will influence the OUES is the choice of ergometer. We know that the peak O2 on the cycle ergometer is about 10% to 15% less than what is achieved on a treadmill. Therefore percent cut points will actually represent different amounts of O2. This could be problematic if we did not have equations for both the treadmill and cycle ergometer.
The six-minute walk is a simple test that is easy to administer and has good reproducibility. There are two versions, encouraged and unencouraged. The unencouraged version may reduce variability among test administrators. Distance and dyspnea score are recorded. The words and phrases of the Borg scale may not be completely understood by young children. Other scales with pictures have been used to assess the child's perceived exertion. The six-minute walk test may be more useful in underserved areas where metabolic equipment is not available. Also, large groups of children can be tested at very little cost, and over time increases in distance walked and a lower rate of perceived exertion may relate to improved fitness. This topic has been covered in greater detail in chapter 3.
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Case Presentation
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago.
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago. She describes the dizziness as "spinning," and it occurs not during running but just after she has finished. It usually occurs after running at a high-tempo pace for several miles and usually in the summer months. She denies frank syncope, chest pain, shortness of breath, or palpitations. There was a documented blood pressure of 62/40 at one time when paramedics were called while the event was occurring. She has been seen in the emergency room twice for the complaint; the most recent was 2 d ago. In the emergency room, she had a normal heart rate and blood pressure, normal physical examination, and normal laboratory values for anemia and thyroid. Her chest radiograph and electrocardiogram (ECG) were unremarkable. She has been restricted from exercise until she is seen by you. She is requesting exercise clearance.
Turning back to our case study, on further questioning it is revealed that Rebecca drinks 20 oz of water daily and one cup of coffee in the morning. She often skips breakfast as well because she does not have time to eat. She sometimes experiences dizziness in the morning when she stands up while getting out of bed. Rebecca's physical examination included orthostatic vital signs. Her heart rate increased from 52 bpm supine to 90 bpm in the upright position. Her blood pressure decreased from 116/68 to 108/64 from supine to upright position, respectively. Her physical examination was otherwise unremarkable.
Rebecca had an ECG performed, which was normal. Although her symptoms were classic for dehydration as well as a vasovagal component, because she was a competitive athlete, the cardiologist opted for an echocardiogram, which was normal, and an exercise stress test to try to induce her symptoms.
Rebecca performed a maximal treadmill exercise stress test. She had a high-normal aerobic capacity. There were no arrhythmias or ischemic changes. There was a normal heart rate and blood pressure response to exercise. She developed dizziness in early recovery with a 30 mmHg drop in her blood pressure. After she drank a cup of water and was placed in the supine position, her blood pressure improved within 2 min.
For Rebecca, her history, physical examination, and ECG were reassuring that this was unlikely to be cardiac-related exertional syncope. Her exercise stress test was able to document dizziness associated with a drop in blood pressure that responded to fluids and supine positioning. She was counseled on improving her hydration and salt intake and not to skip meals. She returned for a follow-up visit 3 mo later and had not experienced any further dizziness or syncope.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Unique Features of Exercise Testing in Children
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults.
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults. One need only consider the various approaches needed to perform a satisfactory exercise test first in a 15-year-old cross country runner who experienced precordial chest pain in her last race, followed by a test looking for heart rate response in a 5-year-old youngster with complete heart block, and then a 12-year-old obese boy with a dilated cardiomyopathy and progressively worsening shortness of breath with exercise.
The pediatric exercise testing laboratory must accommodate wide variations in patient age, size, and fitness, and that means that testing protocols and equipment must be similarly adjusted. The indications for testing are much broader than those in the adult lab, so the questions to be answered must be carefully considered before the exercise begins.
Most children can be easily motivated to give exhaustive efforts during exercise testing, but it requires charismatic skill from staff members experienced with the emotional and physical responses of children during treadmill or cycle exercise. It has been said that perhaps the single most important factor in a successful exercise test in the pediatric laboratory is the staff administering the test.
Pediatric exercise testing, then, is distinguished by the need for a creative approach to each patient. The staff must know what information is needed to address the clinical question being asked, the proper modality - cycle or treadmill - to obtain that answer, and the optimal protocol for the subject's age and fitness level.
The physiological mechanisms underlying the cardiac and pulmonary responses to a bout of progressive exercise are no different in children (at least those over age 6) and adults. Nonetheless, certain quantitative measurements (heart rate, blood pressure, endurance time) are different in children, and these must be recognized in the testing of immature subjects.
For example, resting and maximal heart rates in an exhaustive exercise test are greater in children than in adults. As will be discussed in chapter 5, peak heart rate depends on testing modality and protocol, and there is considerable variability between individuals. It is important to recognize that there is generally no specific "target heart rate" for an exercise test. Importantly, too, the maximal heart rate during exercise testing in a given subject does not change over the course of childhood. Only at about age 16 does this value begin to decline. Thus, age-related formulae for predicting a maximal heart rate, such as "220 minus age," do not apply to youths.
The concept of metabolic equivalents, or METs, as a measure of energy expenditure during exercise is commonly used with adult subjects but is fraught with difficulty in children and adolescents, and it is best avoided in the pediatric exercise laboratory. METs is a means of expressing the oxygen requirement of a physical activity relative to an assumed resting value. One MET, or resting O2, in an adult is considered to be 3.5 ml ∙ kg-1 ∙ min-1; thus, when walking on a treadmill at a certain speed and slope that is expected to demand 17.5 ml ∙ kg-1 ∙ min-1, a patient is exercising at a level of 5 METs.
The difficulty with this concept in youth is that resting energy expenditure is not constant but evolves throughout childhood during physical growth. As would be expected, absolute values of resting O2 rise with the accrual of body mass. When adjusted for body mass, or body surface area, basal or resting values of energy expenditure decline progressively during the pediatric years. When expressed as calories per meter of square body surface area per hour, the basal metabolic rate declines by about 20% between the ages of six and the mid-teen years.
In considering a mass-relative definition of a MET in children, the story is more exaggerated. Harrell et al. reported that the resting O2 per kg in a group of 8- to 12-year-old children was almost 50% higher than that of 16- to 18-year-old subjects and 70% higher than that expected in adults. The use of the MET as defined in adults as a "currency" or multiplier of energy expenditure in children, then, clearly would introduce large errors in defining O2 levels during exercise - and the extent of the error would be different depending on the age and size of the child.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Asthma
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms.
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms. It is believed that the majority of children with asthma exhibit EIB, which may affect exercise tolerance as well as participation in physical activity. As reviewed by Welsh et al., studies examining aerobic fitness in children provide mixed results, with some reporting comparable fitness and others reporting reduced fitness in asthmatic children compared to their nonasthmatic peers. The inconsistencies may reflect differences in measurement of fitness (e.g., laboratory vs. field tests), differences in levels of severity of the children's asthma, the relatively small sample, selection bias in choice of nonasthmatic children for comparison, use of bronchodilator therapy prior to testing, and perhaps inappropriate statistical analyses. Some studies suggest that fitness is comparable as long as levels of physical activity are similar. Pharmacologic advances such as inhaled corticosteroid therapy have reduced the number of hospitalizations and emergency department visits, suggesting that the severity of disease has lessened. Consequently, the results of studies conducted prior to these advances may not accurately reflect the current impact of the disease on exercise tolerance.
In the past two decades, surprisingly few studies of children report ventilatory parameters assessed during exercise testing. Santuz et al. reported lower E at peak exercise in sedentary and regularly active children when compared to healthy controls (matched on age, height, weight, and habitual level of physical activity), but no difference in E was observed in asthmatic children who participated in organized or competitive sports compared to their peers. During submaximal exercise, the asthmatic and nonasthmatic children had a similar E, but it was achieved by a lower f b and a higher VT in the asthmatic children, which would lessen dead space breathing and improve alveolar ventilation. In contrast, Moraes et al. reported no differences among newly diagnosed adolescents with mild intermittent asthma (n = 20), mild persistent asthma (n = 13), and their healthy peers (n = 36) for O2, E, E/O2, or ventilatory reserve at maximal exercise. Berntsen et al. likewise found no difference in O2 or E at peak exercise in a group of 86 13-year-old adolescents with asthma compared to 76 same-aged peers without asthma. Resting lung function was slightly lower in the asthmatic children but generally within normal limits. In a multiple regression model that included skinfold thickness, resting pulmonary function, and physical activity, analysis indicated that skinfold thickness and vigorous physical activity participation (in asthmatics only) were the primary independent predictors of O2peak in the study.
Expiratory flow limitations after exercise are well documented in children with asthma, and a recent study reported postexercise inspiratory flow limitation as well. However, no study has reported measurement of exercise tidal loops to assess ventilatory mechanics in children with asthma during exercise. Consequently, the results of one study of adults are presented. Exercise tolerance and pulmonary mechanics during exercise were compared between eight adults with asthma and six adults without asthma. Asthmatic adults had similar larger airway function but reduced smaller airway function at rest, and O2peak was slightly lower (but not significantly) in asthmatic compared to control subjects (104% vs. 130% of predicted). At maximal exercise, the groups attained similar E, but the asthmatic subjects had higher mean values for E/CO2 and lower PETCO2 than the control subjects. Superimposing the exercise tidal loop on the resting maximal F-V loops produced similarities to figure 11.3, revealing increases in EELV with the expiratory flow of the tidal loop encroaching on the resting F-V loop during exercise, indicative of expiratory flow limitation. Interestingly, the authors noted variability in EELV and in the degree of exercise flow limitation during interval exercise (intensity switching from 60% to 40% of O2peak), and they suggested that the variability reflected changes in bronchomotor tone (i.e., bronchodilation vs. bronchoconstriction ) at different exercise intensities.
Recent studies reporting measurements of arterial blood gases are rare in children with asthma. One recent study collected expired gases and earlobe capillary blood samples in eight adolescents with mild intermittent asthma (MIA), eight with mild persistent asthma (MPA), and 12 nonasthmatic control subjects to examine gas exchange during exercise and to estimate PaO2 before and after maximal exercise stress. Before exercise, the children with MPA had significantly lower PaO2 (75.1 ± 6.6 mmHg) than those with MIA (81.7 ± 6.7 mmHg) and control (83.3 ± 4.9 mmHg) subjects, suggesting hypoxemia. Postexercise PaO2 did not differ among groups, but this was attributed to a decrease in the control subjects because the PaO2 of the children with MPA did not change significantly. Older studies report gas exchange abnormalities (elevated PA - aO2 difference) at rest that improved during exercise, which the authors attributed to better A/ matching. A more recent study of habitually active 18- to 45-year-old men and women with mild-to-moderate asthma examined arterial blood gases during exercise. Eight of 21 subjects exhibited oxyhemoglobin desaturation (SaO2 ≤94%) during prolonged exercise and were classified as the Lo-SaO2 group, and those whose SaO2 remained above 94% were classified as the Hi-SaO2 group. Despite similar fitness levels, the PaCO2 of the Hi-SaO2 group fell from rest to 34.0 ± 2.7 mmHg at exhaustion, whereas that of the Lo-SaO2 group fell during the first minute of exercise but then rose progressively, reaching 39.8 ± 4 mmHg at exhaustion, suggesting hypoventilation and impaired gas exchange.
Given the prevalence of asthma in children, as well as reports of reduced fitness, more studies examining ventilatory mechanics and gas exchange parameters during exercise are warranted in children with asthma to determine their potential contributions to reduced fitness and exercise intolerance.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Clinical Exercise Testing
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function.
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function. Karlman Wasserman and coworkers at UCLA demonstrated, too, how the acquisition of gas exchange variables measured during exercise could further delineate and differentiate abnormal cardiac and pulmonary responses to exercise.
The use of exercise testing in pediatric populations, whose members do not normally suffer from coronary artery disease, initially developed in the shadow of this story about adult patients. Early exercise studies in youth were performed in the research setting. They were designed to examine physiological differences that separate children from adults. Sid Robinson provided the first such treadmill-derived data in the Harvard Fatigue Laboratory in Boston in the 1930s, demonstrating the progressive changes in metabolic and physiological responses that normally occurred between the ages of 6 and 91. Similar exercise data in healthy children were subsequently provided by other investigators in the middle of the 20th century, including Per-Olof …strand in Sweden, Simon Godfrey in Great Britain, and Gordon Cumming in Canada. When Oded Bar-Or published his landmark book Pediatric Sports Medicine for the Practitioner in 1983, he was able to accumulate a large base of normative data from these earlier studies to outline aspects of physiological responses to exercise in youth and how these developed during the growing years.
While such research was designed to reveal the normal development of physiological responses to exercise in children, this information also served as normative data for those who developed exercise testing for the clinical assessment of heart and lung disease in pediatric patients. As studies dealt, for the most part, with those with congenital heart disease, early exercise stress testing in young patients involved a more diverse approach than that of the traditional adult laboratory focused on the detection of coronary artery disease. The assessment of ischemic changes on the ECG was still an issue, particularly in assessing the severity of aortic outflow obstruction, but exercise testing in young patients also involved a wider range of information, such as blood pressure responses (in coarctation of the aorta, systemic hypertension), endurance capacity (postoperative cyanotic heart disease), and rhythm responses (complete heart block).
Useful clinical testing methodologies and clinical findings were described by a number of key early pioneers, such as Fred James at Cincinnati Children's Hospital, David Driscoll at the Mayo Clinic, Bruce Alpert and William Strong at the Medical College of Georgia, Rolf Mocellin in Germany, and Tony Reybrouck and Dirk Matthys in Belgium. The importance of gas exchange measures, including O2max and O2 kinetics, was highlighted by the early reports of exercise testing in patients with congenital heart disease by Hans Wessel at Children's Memorial Hospital in Chicago. During this time, too, exercise testing became established in both children and adults as a useful means of assessing bronchospasm and lung function in patients with asthma and other respiratory diseases (particularly via the early experience reported by Hans Stoboy, Gerd Cropp, and Svein Oseid).
In many cases, clinical exercise testing in children was performed in adult laboratories, using protocols, exercise equipment, and monitoring systems (ECG, blood pressure) traditionally used to test adults with suspected coronary artery disease. A number of developments have now expanded the role of exercise testing in youth and have identified the need for more specific approaches to exercise testing for this population of patients.
- Perhaps most importantly, the past several decades have witnessed a dramatic expansion in the scope and nature of patients cared for by pediatric cardiologists. Children with complex forms of congenital heart disease, particularly those characterized by marked unilateral ventricular hypoplasia (hypoplastic left heart syndrome, tricuspid valve atresia), once had little hope for long-term survival. Now, thanks to remarkable progress in surgical techniques, these young patients not only often survive but also live productive and fulfilling lives. The physicians caring for these survivors as they grow toward the adult years are confronted with new issues, such as myocardial dysfunction, stubborn tachyarrhythmia, hypoxemia, and pulmonary hypertension. These problems have required new diagnostic and therapeutic approaches, often using information available in the exercise stress-testing laboratory. Similarly, in patients with lung diseases such as cystic fibrosis, improvements in patient care have successfully extended survival and have at the same time introduced new clinical questions that can be assessed through exercise testing. It is likely that the future will continue to bring steady improvements in the survival of young patients with both cardiac and pulmonary disease that will be paralleled by expanded indications for clinical exercise testing.
- A growing understanding of the pathophysiology of cardiac malformations and factors influencing risk stratification have created a need to expand the information obtained during exercise stress testing in young patients. It is true that many issues can be adequately examined by a limited study involving a traditional bout of progressive exercise accompanied by electrocardiographic monitoring and measurement of blood pressure. Assessment of possible ischemic changes in a child after Kawasaki disease, for example, or determination of blood pressure responses after medical treatment of a hypertensive young wrestler could be adequately performed using this approach.
However, the clinical insights gained from exercise testing can be improved by the measurement of gas exchange variables, which are now readily obtained with user-friendly commercial metabolic systems. The changes in the oxygen and carbon dioxide content of expired air during exercise reflects similar gas exchange dynamics at the cellular level. With this approach, for example, the measurement of O2max provides an objective physiological assessment of aerobic fitness, and the determination of ventilatory variables (minute ventilation, CO2) offers insights into pulmonary responses as well. As will be outlined in the chapters that follow, it is often the calculation of relationships between these variables that provides clues into the relative importance of cardiac and pulmonary etiologies of exercise limitation.
Termed an "integrative cardiopulmonary test" by Wasserman et al., this approach expands the utility of exercise testing by providing data that can help to answer questions about cardiac and pulmonary issues in youth. In a teenage boy with moderate aortic valve insufficiency, does his cardiac disease explain the shortness of breath that limits his ability to exercise? What mechanisms lie behind a star athlete's inability to perform well after an extended viral illness? Is syncope of an anxious child during running related to hyperventilation? Is breathlessness during exercise in a markedly obese child caused by excess body fat, exercise-induced bronchoconstriction, or cardiac dysfunction? These types of issues are best addressed by a full examination of gas exchange variables during clinical exercise testing.
- A growing recognition of the effects of exercise on electrophysiological function has created new roles for exercise testing in youth. Assessment of changes in ventricular ectopy during exercise is a traditional indication for exercise testing. Newer indications include the use of responses of rate of ventricular repolarization (QT interval) and conduction down accessory pathways (WPW syndrome) during exercise as means of patient risk stratification.
- The increased use of pediatric exercise testing has also been stimulated by the concerns of parents, coaches, and physical education instructors over the occurrence of symptoms of chest pain, dizziness, syncope, or palpitations in young people during exercise. Such concerns have been fueled by the tragic occurrences of sudden unexpected death of young, presumably healthy athletes during sports training or competition. While such symptoms are highly unlikely to reflect the rare diseases that pose a risk of sudden death, the youngster with occult hypertrophic cardiomyopathy, coronary artery anomalies, or repolarization abnormalities that can predispose to fatal dysrhythmias can present with such complaints. Findings on exercise testing have thus become part of the assessment of symptomatic children and athletes to rule out these anomalies.
- A normal exercise test can provide clearance for sports play in young patients with heart disease or in those who have suffered illnesses such as viral myocarditis. Exercise testing also plays a role in assessing risk and exercise capacities in young patients who are enrolled in cardiac and pulmonary rehabilitation programs.
We can expect that clinical exercise testing in children and adolescents will continue to expand as the value of exercise is recognized in the assessment of not only heart and lung disease but also metabolic and musculoskeletal disorders. Such trends will undoubtedly follow improvements in medical and surgical treatment of these patients. We can also expect to see new techniques for performing exercise tests (miniaturization of metabolic systems permitting field testing, for example) and assessing their results (three-dimensional echocardiography, myocardial strain Doppler studies).
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Determining Functional Capacity
In summary, our quest is to determine children’s functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise.
In summary, our quest is to determine children's functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise. Exercise testing is essential to achieve this goal. Simple measurement of workload on a cycle ergometer will provide information on how close we are to "normal." Peak workload on a cycle ergometer is a measure of very vigorous exercise. The calibration of the cycle is often very good, and the workloads are very consistent over fast and slow pedaling. It is therefore a very useful measure of functional capacity for many subjects of different sizes and levels of physical conditioning. Within-subject reliability is also very good, so we can compare the child's responses over time accurately. Work in watts measured on the cycle ergometer can be used in a calculation to predict the oxygen consumption, again giving us a measure of functional capacity.
Current research indicates that one can predict the peak O2 from the O2 at VAT with good accuracy in healthy children as well as in those with lung disease. Caution should be used when predicting peak O2 from the O2 at VAT in children with congenital heart disease. Metabolic measurements need to be available to calculate the VAT, which adds to the cost and resources needed to predict the O2 from this method. The measurement of VAT becomes more useful when a submaximal effort has limited the exercise test. Age, comprehension, size, motivation, and protocol all factor into a submaximal response. The caveat of the method is when we assume that the O2 at VAT is about 60% of the peak O2. Physical conditioning and specifically a lack of physical fitness influence this method greatly, and this may lead to over- or underestimation of peak O2. We know that physical deconditioning, either from being sedentary or as the result of a disease process, lowers the O2 at the VAT.
OUES is a useful submaximal measure of aerobic fitness that is effort independent. This method also relies on the availability of metabolic measurements. It has been shown to be reproducible and does not suffer the intra-observer or inter-observer discrepancies often encountered with VAT. The OUES is easy to calculate and is determined by many more data points than VAT, peak work, or peak heart rate. The consensus of the research data indicates that it is most useful at the higher percentages of the peak O2 (that is, cut points of OUES 80% and OUES 90%). OUES below 45% to 50% of peak O2 is less reliable and does not correlate as well with peak O2. There is conflicting evidence regarding whether OUES is equally useful in predicting peak O2 in healthy subjects and in those with chronic heart or lung disease. If a subject can achieve 80% or 90% of predicted peak O2, many of us would consider that an adequate level of exertion. If the subject achieves 80% or 90% of peak O2, we do not know if the OUES slope can be abnormal. My guess is probably not. The slope of the line that represents the VE and O2 relationship is strongly influenced by the onset of anaerobic metabolism. The V-slope method of VAT detection is based on the change in that slope. Therefore the choice of the cut points will influence the OUES slope. Another factor that will influence the OUES is the choice of ergometer. We know that the peak O2 on the cycle ergometer is about 10% to 15% less than what is achieved on a treadmill. Therefore percent cut points will actually represent different amounts of O2. This could be problematic if we did not have equations for both the treadmill and cycle ergometer.
The six-minute walk is a simple test that is easy to administer and has good reproducibility. There are two versions, encouraged and unencouraged. The unencouraged version may reduce variability among test administrators. Distance and dyspnea score are recorded. The words and phrases of the Borg scale may not be completely understood by young children. Other scales with pictures have been used to assess the child's perceived exertion. The six-minute walk test may be more useful in underserved areas where metabolic equipment is not available. Also, large groups of children can be tested at very little cost, and over time increases in distance walked and a lower rate of perceived exertion may relate to improved fitness. This topic has been covered in greater detail in chapter 3.
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Case Presentation
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago.
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago. She describes the dizziness as "spinning," and it occurs not during running but just after she has finished. It usually occurs after running at a high-tempo pace for several miles and usually in the summer months. She denies frank syncope, chest pain, shortness of breath, or palpitations. There was a documented blood pressure of 62/40 at one time when paramedics were called while the event was occurring. She has been seen in the emergency room twice for the complaint; the most recent was 2 d ago. In the emergency room, she had a normal heart rate and blood pressure, normal physical examination, and normal laboratory values for anemia and thyroid. Her chest radiograph and electrocardiogram (ECG) were unremarkable. She has been restricted from exercise until she is seen by you. She is requesting exercise clearance.
Turning back to our case study, on further questioning it is revealed that Rebecca drinks 20 oz of water daily and one cup of coffee in the morning. She often skips breakfast as well because she does not have time to eat. She sometimes experiences dizziness in the morning when she stands up while getting out of bed. Rebecca's physical examination included orthostatic vital signs. Her heart rate increased from 52 bpm supine to 90 bpm in the upright position. Her blood pressure decreased from 116/68 to 108/64 from supine to upright position, respectively. Her physical examination was otherwise unremarkable.
Rebecca had an ECG performed, which was normal. Although her symptoms were classic for dehydration as well as a vasovagal component, because she was a competitive athlete, the cardiologist opted for an echocardiogram, which was normal, and an exercise stress test to try to induce her symptoms.
Rebecca performed a maximal treadmill exercise stress test. She had a high-normal aerobic capacity. There were no arrhythmias or ischemic changes. There was a normal heart rate and blood pressure response to exercise. She developed dizziness in early recovery with a 30 mmHg drop in her blood pressure. After she drank a cup of water and was placed in the supine position, her blood pressure improved within 2 min.
For Rebecca, her history, physical examination, and ECG were reassuring that this was unlikely to be cardiac-related exertional syncope. Her exercise stress test was able to document dizziness associated with a drop in blood pressure that responded to fluids and supine positioning. She was counseled on improving her hydration and salt intake and not to skip meals. She returned for a follow-up visit 3 mo later and had not experienced any further dizziness or syncope.
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Unique Features of Exercise Testing in Children
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults.
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults. One need only consider the various approaches needed to perform a satisfactory exercise test first in a 15-year-old cross country runner who experienced precordial chest pain in her last race, followed by a test looking for heart rate response in a 5-year-old youngster with complete heart block, and then a 12-year-old obese boy with a dilated cardiomyopathy and progressively worsening shortness of breath with exercise.
The pediatric exercise testing laboratory must accommodate wide variations in patient age, size, and fitness, and that means that testing protocols and equipment must be similarly adjusted. The indications for testing are much broader than those in the adult lab, so the questions to be answered must be carefully considered before the exercise begins.
Most children can be easily motivated to give exhaustive efforts during exercise testing, but it requires charismatic skill from staff members experienced with the emotional and physical responses of children during treadmill or cycle exercise. It has been said that perhaps the single most important factor in a successful exercise test in the pediatric laboratory is the staff administering the test.
Pediatric exercise testing, then, is distinguished by the need for a creative approach to each patient. The staff must know what information is needed to address the clinical question being asked, the proper modality - cycle or treadmill - to obtain that answer, and the optimal protocol for the subject's age and fitness level.
The physiological mechanisms underlying the cardiac and pulmonary responses to a bout of progressive exercise are no different in children (at least those over age 6) and adults. Nonetheless, certain quantitative measurements (heart rate, blood pressure, endurance time) are different in children, and these must be recognized in the testing of immature subjects.
For example, resting and maximal heart rates in an exhaustive exercise test are greater in children than in adults. As will be discussed in chapter 5, peak heart rate depends on testing modality and protocol, and there is considerable variability between individuals. It is important to recognize that there is generally no specific "target heart rate" for an exercise test. Importantly, too, the maximal heart rate during exercise testing in a given subject does not change over the course of childhood. Only at about age 16 does this value begin to decline. Thus, age-related formulae for predicting a maximal heart rate, such as "220 minus age," do not apply to youths.
The concept of metabolic equivalents, or METs, as a measure of energy expenditure during exercise is commonly used with adult subjects but is fraught with difficulty in children and adolescents, and it is best avoided in the pediatric exercise laboratory. METs is a means of expressing the oxygen requirement of a physical activity relative to an assumed resting value. One MET, or resting O2, in an adult is considered to be 3.5 ml ∙ kg-1 ∙ min-1; thus, when walking on a treadmill at a certain speed and slope that is expected to demand 17.5 ml ∙ kg-1 ∙ min-1, a patient is exercising at a level of 5 METs.
The difficulty with this concept in youth is that resting energy expenditure is not constant but evolves throughout childhood during physical growth. As would be expected, absolute values of resting O2 rise with the accrual of body mass. When adjusted for body mass, or body surface area, basal or resting values of energy expenditure decline progressively during the pediatric years. When expressed as calories per meter of square body surface area per hour, the basal metabolic rate declines by about 20% between the ages of six and the mid-teen years.
In considering a mass-relative definition of a MET in children, the story is more exaggerated. Harrell et al. reported that the resting O2 per kg in a group of 8- to 12-year-old children was almost 50% higher than that of 16- to 18-year-old subjects and 70% higher than that expected in adults. The use of the MET as defined in adults as a "currency" or multiplier of energy expenditure in children, then, clearly would introduce large errors in defining O2 levels during exercise - and the extent of the error would be different depending on the age and size of the child.
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Asthma
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms.
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms. It is believed that the majority of children with asthma exhibit EIB, which may affect exercise tolerance as well as participation in physical activity. As reviewed by Welsh et al., studies examining aerobic fitness in children provide mixed results, with some reporting comparable fitness and others reporting reduced fitness in asthmatic children compared to their nonasthmatic peers. The inconsistencies may reflect differences in measurement of fitness (e.g., laboratory vs. field tests), differences in levels of severity of the children's asthma, the relatively small sample, selection bias in choice of nonasthmatic children for comparison, use of bronchodilator therapy prior to testing, and perhaps inappropriate statistical analyses. Some studies suggest that fitness is comparable as long as levels of physical activity are similar. Pharmacologic advances such as inhaled corticosteroid therapy have reduced the number of hospitalizations and emergency department visits, suggesting that the severity of disease has lessened. Consequently, the results of studies conducted prior to these advances may not accurately reflect the current impact of the disease on exercise tolerance.
In the past two decades, surprisingly few studies of children report ventilatory parameters assessed during exercise testing. Santuz et al. reported lower E at peak exercise in sedentary and regularly active children when compared to healthy controls (matched on age, height, weight, and habitual level of physical activity), but no difference in E was observed in asthmatic children who participated in organized or competitive sports compared to their peers. During submaximal exercise, the asthmatic and nonasthmatic children had a similar E, but it was achieved by a lower f b and a higher VT in the asthmatic children, which would lessen dead space breathing and improve alveolar ventilation. In contrast, Moraes et al. reported no differences among newly diagnosed adolescents with mild intermittent asthma (n = 20), mild persistent asthma (n = 13), and their healthy peers (n = 36) for O2, E, E/O2, or ventilatory reserve at maximal exercise. Berntsen et al. likewise found no difference in O2 or E at peak exercise in a group of 86 13-year-old adolescents with asthma compared to 76 same-aged peers without asthma. Resting lung function was slightly lower in the asthmatic children but generally within normal limits. In a multiple regression model that included skinfold thickness, resting pulmonary function, and physical activity, analysis indicated that skinfold thickness and vigorous physical activity participation (in asthmatics only) were the primary independent predictors of O2peak in the study.
Expiratory flow limitations after exercise are well documented in children with asthma, and a recent study reported postexercise inspiratory flow limitation as well. However, no study has reported measurement of exercise tidal loops to assess ventilatory mechanics in children with asthma during exercise. Consequently, the results of one study of adults are presented. Exercise tolerance and pulmonary mechanics during exercise were compared between eight adults with asthma and six adults without asthma. Asthmatic adults had similar larger airway function but reduced smaller airway function at rest, and O2peak was slightly lower (but not significantly) in asthmatic compared to control subjects (104% vs. 130% of predicted). At maximal exercise, the groups attained similar E, but the asthmatic subjects had higher mean values for E/CO2 and lower PETCO2 than the control subjects. Superimposing the exercise tidal loop on the resting maximal F-V loops produced similarities to figure 11.3, revealing increases in EELV with the expiratory flow of the tidal loop encroaching on the resting F-V loop during exercise, indicative of expiratory flow limitation. Interestingly, the authors noted variability in EELV and in the degree of exercise flow limitation during interval exercise (intensity switching from 60% to 40% of O2peak), and they suggested that the variability reflected changes in bronchomotor tone (i.e., bronchodilation vs. bronchoconstriction ) at different exercise intensities.
Recent studies reporting measurements of arterial blood gases are rare in children with asthma. One recent study collected expired gases and earlobe capillary blood samples in eight adolescents with mild intermittent asthma (MIA), eight with mild persistent asthma (MPA), and 12 nonasthmatic control subjects to examine gas exchange during exercise and to estimate PaO2 before and after maximal exercise stress. Before exercise, the children with MPA had significantly lower PaO2 (75.1 ± 6.6 mmHg) than those with MIA (81.7 ± 6.7 mmHg) and control (83.3 ± 4.9 mmHg) subjects, suggesting hypoxemia. Postexercise PaO2 did not differ among groups, but this was attributed to a decrease in the control subjects because the PaO2 of the children with MPA did not change significantly. Older studies report gas exchange abnormalities (elevated PA - aO2 difference) at rest that improved during exercise, which the authors attributed to better A/ matching. A more recent study of habitually active 18- to 45-year-old men and women with mild-to-moderate asthma examined arterial blood gases during exercise. Eight of 21 subjects exhibited oxyhemoglobin desaturation (SaO2 ≤94%) during prolonged exercise and were classified as the Lo-SaO2 group, and those whose SaO2 remained above 94% were classified as the Hi-SaO2 group. Despite similar fitness levels, the PaCO2 of the Hi-SaO2 group fell from rest to 34.0 ± 2.7 mmHg at exhaustion, whereas that of the Lo-SaO2 group fell during the first minute of exercise but then rose progressively, reaching 39.8 ± 4 mmHg at exhaustion, suggesting hypoventilation and impaired gas exchange.
Given the prevalence of asthma in children, as well as reports of reduced fitness, more studies examining ventilatory mechanics and gas exchange parameters during exercise are warranted in children with asthma to determine their potential contributions to reduced fitness and exercise intolerance.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Clinical Exercise Testing
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function.
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function. Karlman Wasserman and coworkers at UCLA demonstrated, too, how the acquisition of gas exchange variables measured during exercise could further delineate and differentiate abnormal cardiac and pulmonary responses to exercise.
The use of exercise testing in pediatric populations, whose members do not normally suffer from coronary artery disease, initially developed in the shadow of this story about adult patients. Early exercise studies in youth were performed in the research setting. They were designed to examine physiological differences that separate children from adults. Sid Robinson provided the first such treadmill-derived data in the Harvard Fatigue Laboratory in Boston in the 1930s, demonstrating the progressive changes in metabolic and physiological responses that normally occurred between the ages of 6 and 91. Similar exercise data in healthy children were subsequently provided by other investigators in the middle of the 20th century, including Per-Olof …strand in Sweden, Simon Godfrey in Great Britain, and Gordon Cumming in Canada. When Oded Bar-Or published his landmark book Pediatric Sports Medicine for the Practitioner in 1983, he was able to accumulate a large base of normative data from these earlier studies to outline aspects of physiological responses to exercise in youth and how these developed during the growing years.
While such research was designed to reveal the normal development of physiological responses to exercise in children, this information also served as normative data for those who developed exercise testing for the clinical assessment of heart and lung disease in pediatric patients. As studies dealt, for the most part, with those with congenital heart disease, early exercise stress testing in young patients involved a more diverse approach than that of the traditional adult laboratory focused on the detection of coronary artery disease. The assessment of ischemic changes on the ECG was still an issue, particularly in assessing the severity of aortic outflow obstruction, but exercise testing in young patients also involved a wider range of information, such as blood pressure responses (in coarctation of the aorta, systemic hypertension), endurance capacity (postoperative cyanotic heart disease), and rhythm responses (complete heart block).
Useful clinical testing methodologies and clinical findings were described by a number of key early pioneers, such as Fred James at Cincinnati Children's Hospital, David Driscoll at the Mayo Clinic, Bruce Alpert and William Strong at the Medical College of Georgia, Rolf Mocellin in Germany, and Tony Reybrouck and Dirk Matthys in Belgium. The importance of gas exchange measures, including O2max and O2 kinetics, was highlighted by the early reports of exercise testing in patients with congenital heart disease by Hans Wessel at Children's Memorial Hospital in Chicago. During this time, too, exercise testing became established in both children and adults as a useful means of assessing bronchospasm and lung function in patients with asthma and other respiratory diseases (particularly via the early experience reported by Hans Stoboy, Gerd Cropp, and Svein Oseid).
In many cases, clinical exercise testing in children was performed in adult laboratories, using protocols, exercise equipment, and monitoring systems (ECG, blood pressure) traditionally used to test adults with suspected coronary artery disease. A number of developments have now expanded the role of exercise testing in youth and have identified the need for more specific approaches to exercise testing for this population of patients.
- Perhaps most importantly, the past several decades have witnessed a dramatic expansion in the scope and nature of patients cared for by pediatric cardiologists. Children with complex forms of congenital heart disease, particularly those characterized by marked unilateral ventricular hypoplasia (hypoplastic left heart syndrome, tricuspid valve atresia), once had little hope for long-term survival. Now, thanks to remarkable progress in surgical techniques, these young patients not only often survive but also live productive and fulfilling lives. The physicians caring for these survivors as they grow toward the adult years are confronted with new issues, such as myocardial dysfunction, stubborn tachyarrhythmia, hypoxemia, and pulmonary hypertension. These problems have required new diagnostic and therapeutic approaches, often using information available in the exercise stress-testing laboratory. Similarly, in patients with lung diseases such as cystic fibrosis, improvements in patient care have successfully extended survival and have at the same time introduced new clinical questions that can be assessed through exercise testing. It is likely that the future will continue to bring steady improvements in the survival of young patients with both cardiac and pulmonary disease that will be paralleled by expanded indications for clinical exercise testing.
- A growing understanding of the pathophysiology of cardiac malformations and factors influencing risk stratification have created a need to expand the information obtained during exercise stress testing in young patients. It is true that many issues can be adequately examined by a limited study involving a traditional bout of progressive exercise accompanied by electrocardiographic monitoring and measurement of blood pressure. Assessment of possible ischemic changes in a child after Kawasaki disease, for example, or determination of blood pressure responses after medical treatment of a hypertensive young wrestler could be adequately performed using this approach.
However, the clinical insights gained from exercise testing can be improved by the measurement of gas exchange variables, which are now readily obtained with user-friendly commercial metabolic systems. The changes in the oxygen and carbon dioxide content of expired air during exercise reflects similar gas exchange dynamics at the cellular level. With this approach, for example, the measurement of O2max provides an objective physiological assessment of aerobic fitness, and the determination of ventilatory variables (minute ventilation, CO2) offers insights into pulmonary responses as well. As will be outlined in the chapters that follow, it is often the calculation of relationships between these variables that provides clues into the relative importance of cardiac and pulmonary etiologies of exercise limitation.
Termed an "integrative cardiopulmonary test" by Wasserman et al., this approach expands the utility of exercise testing by providing data that can help to answer questions about cardiac and pulmonary issues in youth. In a teenage boy with moderate aortic valve insufficiency, does his cardiac disease explain the shortness of breath that limits his ability to exercise? What mechanisms lie behind a star athlete's inability to perform well after an extended viral illness? Is syncope of an anxious child during running related to hyperventilation? Is breathlessness during exercise in a markedly obese child caused by excess body fat, exercise-induced bronchoconstriction, or cardiac dysfunction? These types of issues are best addressed by a full examination of gas exchange variables during clinical exercise testing.
- A growing recognition of the effects of exercise on electrophysiological function has created new roles for exercise testing in youth. Assessment of changes in ventricular ectopy during exercise is a traditional indication for exercise testing. Newer indications include the use of responses of rate of ventricular repolarization (QT interval) and conduction down accessory pathways (WPW syndrome) during exercise as means of patient risk stratification.
- The increased use of pediatric exercise testing has also been stimulated by the concerns of parents, coaches, and physical education instructors over the occurrence of symptoms of chest pain, dizziness, syncope, or palpitations in young people during exercise. Such concerns have been fueled by the tragic occurrences of sudden unexpected death of young, presumably healthy athletes during sports training or competition. While such symptoms are highly unlikely to reflect the rare diseases that pose a risk of sudden death, the youngster with occult hypertrophic cardiomyopathy, coronary artery anomalies, or repolarization abnormalities that can predispose to fatal dysrhythmias can present with such complaints. Findings on exercise testing have thus become part of the assessment of symptomatic children and athletes to rule out these anomalies.
- A normal exercise test can provide clearance for sports play in young patients with heart disease or in those who have suffered illnesses such as viral myocarditis. Exercise testing also plays a role in assessing risk and exercise capacities in young patients who are enrolled in cardiac and pulmonary rehabilitation programs.
We can expect that clinical exercise testing in children and adolescents will continue to expand as the value of exercise is recognized in the assessment of not only heart and lung disease but also metabolic and musculoskeletal disorders. Such trends will undoubtedly follow improvements in medical and surgical treatment of these patients. We can also expect to see new techniques for performing exercise tests (miniaturization of metabolic systems permitting field testing, for example) and assessing their results (three-dimensional echocardiography, myocardial strain Doppler studies).
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Determining Functional Capacity
In summary, our quest is to determine children’s functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise.
In summary, our quest is to determine children's functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise. Exercise testing is essential to achieve this goal. Simple measurement of workload on a cycle ergometer will provide information on how close we are to "normal." Peak workload on a cycle ergometer is a measure of very vigorous exercise. The calibration of the cycle is often very good, and the workloads are very consistent over fast and slow pedaling. It is therefore a very useful measure of functional capacity for many subjects of different sizes and levels of physical conditioning. Within-subject reliability is also very good, so we can compare the child's responses over time accurately. Work in watts measured on the cycle ergometer can be used in a calculation to predict the oxygen consumption, again giving us a measure of functional capacity.
Current research indicates that one can predict the peak O2 from the O2 at VAT with good accuracy in healthy children as well as in those with lung disease. Caution should be used when predicting peak O2 from the O2 at VAT in children with congenital heart disease. Metabolic measurements need to be available to calculate the VAT, which adds to the cost and resources needed to predict the O2 from this method. The measurement of VAT becomes more useful when a submaximal effort has limited the exercise test. Age, comprehension, size, motivation, and protocol all factor into a submaximal response. The caveat of the method is when we assume that the O2 at VAT is about 60% of the peak O2. Physical conditioning and specifically a lack of physical fitness influence this method greatly, and this may lead to over- or underestimation of peak O2. We know that physical deconditioning, either from being sedentary or as the result of a disease process, lowers the O2 at the VAT.
OUES is a useful submaximal measure of aerobic fitness that is effort independent. This method also relies on the availability of metabolic measurements. It has been shown to be reproducible and does not suffer the intra-observer or inter-observer discrepancies often encountered with VAT. The OUES is easy to calculate and is determined by many more data points than VAT, peak work, or peak heart rate. The consensus of the research data indicates that it is most useful at the higher percentages of the peak O2 (that is, cut points of OUES 80% and OUES 90%). OUES below 45% to 50% of peak O2 is less reliable and does not correlate as well with peak O2. There is conflicting evidence regarding whether OUES is equally useful in predicting peak O2 in healthy subjects and in those with chronic heart or lung disease. If a subject can achieve 80% or 90% of predicted peak O2, many of us would consider that an adequate level of exertion. If the subject achieves 80% or 90% of peak O2, we do not know if the OUES slope can be abnormal. My guess is probably not. The slope of the line that represents the VE and O2 relationship is strongly influenced by the onset of anaerobic metabolism. The V-slope method of VAT detection is based on the change in that slope. Therefore the choice of the cut points will influence the OUES slope. Another factor that will influence the OUES is the choice of ergometer. We know that the peak O2 on the cycle ergometer is about 10% to 15% less than what is achieved on a treadmill. Therefore percent cut points will actually represent different amounts of O2. This could be problematic if we did not have equations for both the treadmill and cycle ergometer.
The six-minute walk is a simple test that is easy to administer and has good reproducibility. There are two versions, encouraged and unencouraged. The unencouraged version may reduce variability among test administrators. Distance and dyspnea score are recorded. The words and phrases of the Borg scale may not be completely understood by young children. Other scales with pictures have been used to assess the child's perceived exertion. The six-minute walk test may be more useful in underserved areas where metabolic equipment is not available. Also, large groups of children can be tested at very little cost, and over time increases in distance walked and a lower rate of perceived exertion may relate to improved fitness. This topic has been covered in greater detail in chapter 3.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Case Presentation
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago.
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago. She describes the dizziness as "spinning," and it occurs not during running but just after she has finished. It usually occurs after running at a high-tempo pace for several miles and usually in the summer months. She denies frank syncope, chest pain, shortness of breath, or palpitations. There was a documented blood pressure of 62/40 at one time when paramedics were called while the event was occurring. She has been seen in the emergency room twice for the complaint; the most recent was 2 d ago. In the emergency room, she had a normal heart rate and blood pressure, normal physical examination, and normal laboratory values for anemia and thyroid. Her chest radiograph and electrocardiogram (ECG) were unremarkable. She has been restricted from exercise until she is seen by you. She is requesting exercise clearance.
Turning back to our case study, on further questioning it is revealed that Rebecca drinks 20 oz of water daily and one cup of coffee in the morning. She often skips breakfast as well because she does not have time to eat. She sometimes experiences dizziness in the morning when she stands up while getting out of bed. Rebecca's physical examination included orthostatic vital signs. Her heart rate increased from 52 bpm supine to 90 bpm in the upright position. Her blood pressure decreased from 116/68 to 108/64 from supine to upright position, respectively. Her physical examination was otherwise unremarkable.
Rebecca had an ECG performed, which was normal. Although her symptoms were classic for dehydration as well as a vasovagal component, because she was a competitive athlete, the cardiologist opted for an echocardiogram, which was normal, and an exercise stress test to try to induce her symptoms.
Rebecca performed a maximal treadmill exercise stress test. She had a high-normal aerobic capacity. There were no arrhythmias or ischemic changes. There was a normal heart rate and blood pressure response to exercise. She developed dizziness in early recovery with a 30 mmHg drop in her blood pressure. After she drank a cup of water and was placed in the supine position, her blood pressure improved within 2 min.
For Rebecca, her history, physical examination, and ECG were reassuring that this was unlikely to be cardiac-related exertional syncope. Her exercise stress test was able to document dizziness associated with a drop in blood pressure that responded to fluids and supine positioning. She was counseled on improving her hydration and salt intake and not to skip meals. She returned for a follow-up visit 3 mo later and had not experienced any further dizziness or syncope.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Unique Features of Exercise Testing in Children
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults.
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults. One need only consider the various approaches needed to perform a satisfactory exercise test first in a 15-year-old cross country runner who experienced precordial chest pain in her last race, followed by a test looking for heart rate response in a 5-year-old youngster with complete heart block, and then a 12-year-old obese boy with a dilated cardiomyopathy and progressively worsening shortness of breath with exercise.
The pediatric exercise testing laboratory must accommodate wide variations in patient age, size, and fitness, and that means that testing protocols and equipment must be similarly adjusted. The indications for testing are much broader than those in the adult lab, so the questions to be answered must be carefully considered before the exercise begins.
Most children can be easily motivated to give exhaustive efforts during exercise testing, but it requires charismatic skill from staff members experienced with the emotional and physical responses of children during treadmill or cycle exercise. It has been said that perhaps the single most important factor in a successful exercise test in the pediatric laboratory is the staff administering the test.
Pediatric exercise testing, then, is distinguished by the need for a creative approach to each patient. The staff must know what information is needed to address the clinical question being asked, the proper modality - cycle or treadmill - to obtain that answer, and the optimal protocol for the subject's age and fitness level.
The physiological mechanisms underlying the cardiac and pulmonary responses to a bout of progressive exercise are no different in children (at least those over age 6) and adults. Nonetheless, certain quantitative measurements (heart rate, blood pressure, endurance time) are different in children, and these must be recognized in the testing of immature subjects.
For example, resting and maximal heart rates in an exhaustive exercise test are greater in children than in adults. As will be discussed in chapter 5, peak heart rate depends on testing modality and protocol, and there is considerable variability between individuals. It is important to recognize that there is generally no specific "target heart rate" for an exercise test. Importantly, too, the maximal heart rate during exercise testing in a given subject does not change over the course of childhood. Only at about age 16 does this value begin to decline. Thus, age-related formulae for predicting a maximal heart rate, such as "220 minus age," do not apply to youths.
The concept of metabolic equivalents, or METs, as a measure of energy expenditure during exercise is commonly used with adult subjects but is fraught with difficulty in children and adolescents, and it is best avoided in the pediatric exercise laboratory. METs is a means of expressing the oxygen requirement of a physical activity relative to an assumed resting value. One MET, or resting O2, in an adult is considered to be 3.5 ml ∙ kg-1 ∙ min-1; thus, when walking on a treadmill at a certain speed and slope that is expected to demand 17.5 ml ∙ kg-1 ∙ min-1, a patient is exercising at a level of 5 METs.
The difficulty with this concept in youth is that resting energy expenditure is not constant but evolves throughout childhood during physical growth. As would be expected, absolute values of resting O2 rise with the accrual of body mass. When adjusted for body mass, or body surface area, basal or resting values of energy expenditure decline progressively during the pediatric years. When expressed as calories per meter of square body surface area per hour, the basal metabolic rate declines by about 20% between the ages of six and the mid-teen years.
In considering a mass-relative definition of a MET in children, the story is more exaggerated. Harrell et al. reported that the resting O2 per kg in a group of 8- to 12-year-old children was almost 50% higher than that of 16- to 18-year-old subjects and 70% higher than that expected in adults. The use of the MET as defined in adults as a "currency" or multiplier of energy expenditure in children, then, clearly would introduce large errors in defining O2 levels during exercise - and the extent of the error would be different depending on the age and size of the child.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Asthma
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms.
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms. It is believed that the majority of children with asthma exhibit EIB, which may affect exercise tolerance as well as participation in physical activity. As reviewed by Welsh et al., studies examining aerobic fitness in children provide mixed results, with some reporting comparable fitness and others reporting reduced fitness in asthmatic children compared to their nonasthmatic peers. The inconsistencies may reflect differences in measurement of fitness (e.g., laboratory vs. field tests), differences in levels of severity of the children's asthma, the relatively small sample, selection bias in choice of nonasthmatic children for comparison, use of bronchodilator therapy prior to testing, and perhaps inappropriate statistical analyses. Some studies suggest that fitness is comparable as long as levels of physical activity are similar. Pharmacologic advances such as inhaled corticosteroid therapy have reduced the number of hospitalizations and emergency department visits, suggesting that the severity of disease has lessened. Consequently, the results of studies conducted prior to these advances may not accurately reflect the current impact of the disease on exercise tolerance.
In the past two decades, surprisingly few studies of children report ventilatory parameters assessed during exercise testing. Santuz et al. reported lower E at peak exercise in sedentary and regularly active children when compared to healthy controls (matched on age, height, weight, and habitual level of physical activity), but no difference in E was observed in asthmatic children who participated in organized or competitive sports compared to their peers. During submaximal exercise, the asthmatic and nonasthmatic children had a similar E, but it was achieved by a lower f b and a higher VT in the asthmatic children, which would lessen dead space breathing and improve alveolar ventilation. In contrast, Moraes et al. reported no differences among newly diagnosed adolescents with mild intermittent asthma (n = 20), mild persistent asthma (n = 13), and their healthy peers (n = 36) for O2, E, E/O2, or ventilatory reserve at maximal exercise. Berntsen et al. likewise found no difference in O2 or E at peak exercise in a group of 86 13-year-old adolescents with asthma compared to 76 same-aged peers without asthma. Resting lung function was slightly lower in the asthmatic children but generally within normal limits. In a multiple regression model that included skinfold thickness, resting pulmonary function, and physical activity, analysis indicated that skinfold thickness and vigorous physical activity participation (in asthmatics only) were the primary independent predictors of O2peak in the study.
Expiratory flow limitations after exercise are well documented in children with asthma, and a recent study reported postexercise inspiratory flow limitation as well. However, no study has reported measurement of exercise tidal loops to assess ventilatory mechanics in children with asthma during exercise. Consequently, the results of one study of adults are presented. Exercise tolerance and pulmonary mechanics during exercise were compared between eight adults with asthma and six adults without asthma. Asthmatic adults had similar larger airway function but reduced smaller airway function at rest, and O2peak was slightly lower (but not significantly) in asthmatic compared to control subjects (104% vs. 130% of predicted). At maximal exercise, the groups attained similar E, but the asthmatic subjects had higher mean values for E/CO2 and lower PETCO2 than the control subjects. Superimposing the exercise tidal loop on the resting maximal F-V loops produced similarities to figure 11.3, revealing increases in EELV with the expiratory flow of the tidal loop encroaching on the resting F-V loop during exercise, indicative of expiratory flow limitation. Interestingly, the authors noted variability in EELV and in the degree of exercise flow limitation during interval exercise (intensity switching from 60% to 40% of O2peak), and they suggested that the variability reflected changes in bronchomotor tone (i.e., bronchodilation vs. bronchoconstriction ) at different exercise intensities.
Recent studies reporting measurements of arterial blood gases are rare in children with asthma. One recent study collected expired gases and earlobe capillary blood samples in eight adolescents with mild intermittent asthma (MIA), eight with mild persistent asthma (MPA), and 12 nonasthmatic control subjects to examine gas exchange during exercise and to estimate PaO2 before and after maximal exercise stress. Before exercise, the children with MPA had significantly lower PaO2 (75.1 ± 6.6 mmHg) than those with MIA (81.7 ± 6.7 mmHg) and control (83.3 ± 4.9 mmHg) subjects, suggesting hypoxemia. Postexercise PaO2 did not differ among groups, but this was attributed to a decrease in the control subjects because the PaO2 of the children with MPA did not change significantly. Older studies report gas exchange abnormalities (elevated PA - aO2 difference) at rest that improved during exercise, which the authors attributed to better A/ matching. A more recent study of habitually active 18- to 45-year-old men and women with mild-to-moderate asthma examined arterial blood gases during exercise. Eight of 21 subjects exhibited oxyhemoglobin desaturation (SaO2 ≤94%) during prolonged exercise and were classified as the Lo-SaO2 group, and those whose SaO2 remained above 94% were classified as the Hi-SaO2 group. Despite similar fitness levels, the PaCO2 of the Hi-SaO2 group fell from rest to 34.0 ± 2.7 mmHg at exhaustion, whereas that of the Lo-SaO2 group fell during the first minute of exercise but then rose progressively, reaching 39.8 ± 4 mmHg at exhaustion, suggesting hypoventilation and impaired gas exchange.
Given the prevalence of asthma in children, as well as reports of reduced fitness, more studies examining ventilatory mechanics and gas exchange parameters during exercise are warranted in children with asthma to determine their potential contributions to reduced fitness and exercise intolerance.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Clinical Exercise Testing
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function.
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function. Karlman Wasserman and coworkers at UCLA demonstrated, too, how the acquisition of gas exchange variables measured during exercise could further delineate and differentiate abnormal cardiac and pulmonary responses to exercise.
The use of exercise testing in pediatric populations, whose members do not normally suffer from coronary artery disease, initially developed in the shadow of this story about adult patients. Early exercise studies in youth were performed in the research setting. They were designed to examine physiological differences that separate children from adults. Sid Robinson provided the first such treadmill-derived data in the Harvard Fatigue Laboratory in Boston in the 1930s, demonstrating the progressive changes in metabolic and physiological responses that normally occurred between the ages of 6 and 91. Similar exercise data in healthy children were subsequently provided by other investigators in the middle of the 20th century, including Per-Olof …strand in Sweden, Simon Godfrey in Great Britain, and Gordon Cumming in Canada. When Oded Bar-Or published his landmark book Pediatric Sports Medicine for the Practitioner in 1983, he was able to accumulate a large base of normative data from these earlier studies to outline aspects of physiological responses to exercise in youth and how these developed during the growing years.
While such research was designed to reveal the normal development of physiological responses to exercise in children, this information also served as normative data for those who developed exercise testing for the clinical assessment of heart and lung disease in pediatric patients. As studies dealt, for the most part, with those with congenital heart disease, early exercise stress testing in young patients involved a more diverse approach than that of the traditional adult laboratory focused on the detection of coronary artery disease. The assessment of ischemic changes on the ECG was still an issue, particularly in assessing the severity of aortic outflow obstruction, but exercise testing in young patients also involved a wider range of information, such as blood pressure responses (in coarctation of the aorta, systemic hypertension), endurance capacity (postoperative cyanotic heart disease), and rhythm responses (complete heart block).
Useful clinical testing methodologies and clinical findings were described by a number of key early pioneers, such as Fred James at Cincinnati Children's Hospital, David Driscoll at the Mayo Clinic, Bruce Alpert and William Strong at the Medical College of Georgia, Rolf Mocellin in Germany, and Tony Reybrouck and Dirk Matthys in Belgium. The importance of gas exchange measures, including O2max and O2 kinetics, was highlighted by the early reports of exercise testing in patients with congenital heart disease by Hans Wessel at Children's Memorial Hospital in Chicago. During this time, too, exercise testing became established in both children and adults as a useful means of assessing bronchospasm and lung function in patients with asthma and other respiratory diseases (particularly via the early experience reported by Hans Stoboy, Gerd Cropp, and Svein Oseid).
In many cases, clinical exercise testing in children was performed in adult laboratories, using protocols, exercise equipment, and monitoring systems (ECG, blood pressure) traditionally used to test adults with suspected coronary artery disease. A number of developments have now expanded the role of exercise testing in youth and have identified the need for more specific approaches to exercise testing for this population of patients.
- Perhaps most importantly, the past several decades have witnessed a dramatic expansion in the scope and nature of patients cared for by pediatric cardiologists. Children with complex forms of congenital heart disease, particularly those characterized by marked unilateral ventricular hypoplasia (hypoplastic left heart syndrome, tricuspid valve atresia), once had little hope for long-term survival. Now, thanks to remarkable progress in surgical techniques, these young patients not only often survive but also live productive and fulfilling lives. The physicians caring for these survivors as they grow toward the adult years are confronted with new issues, such as myocardial dysfunction, stubborn tachyarrhythmia, hypoxemia, and pulmonary hypertension. These problems have required new diagnostic and therapeutic approaches, often using information available in the exercise stress-testing laboratory. Similarly, in patients with lung diseases such as cystic fibrosis, improvements in patient care have successfully extended survival and have at the same time introduced new clinical questions that can be assessed through exercise testing. It is likely that the future will continue to bring steady improvements in the survival of young patients with both cardiac and pulmonary disease that will be paralleled by expanded indications for clinical exercise testing.
- A growing understanding of the pathophysiology of cardiac malformations and factors influencing risk stratification have created a need to expand the information obtained during exercise stress testing in young patients. It is true that many issues can be adequately examined by a limited study involving a traditional bout of progressive exercise accompanied by electrocardiographic monitoring and measurement of blood pressure. Assessment of possible ischemic changes in a child after Kawasaki disease, for example, or determination of blood pressure responses after medical treatment of a hypertensive young wrestler could be adequately performed using this approach.
However, the clinical insights gained from exercise testing can be improved by the measurement of gas exchange variables, which are now readily obtained with user-friendly commercial metabolic systems. The changes in the oxygen and carbon dioxide content of expired air during exercise reflects similar gas exchange dynamics at the cellular level. With this approach, for example, the measurement of O2max provides an objective physiological assessment of aerobic fitness, and the determination of ventilatory variables (minute ventilation, CO2) offers insights into pulmonary responses as well. As will be outlined in the chapters that follow, it is often the calculation of relationships between these variables that provides clues into the relative importance of cardiac and pulmonary etiologies of exercise limitation.
Termed an "integrative cardiopulmonary test" by Wasserman et al., this approach expands the utility of exercise testing by providing data that can help to answer questions about cardiac and pulmonary issues in youth. In a teenage boy with moderate aortic valve insufficiency, does his cardiac disease explain the shortness of breath that limits his ability to exercise? What mechanisms lie behind a star athlete's inability to perform well after an extended viral illness? Is syncope of an anxious child during running related to hyperventilation? Is breathlessness during exercise in a markedly obese child caused by excess body fat, exercise-induced bronchoconstriction, or cardiac dysfunction? These types of issues are best addressed by a full examination of gas exchange variables during clinical exercise testing.
- A growing recognition of the effects of exercise on electrophysiological function has created new roles for exercise testing in youth. Assessment of changes in ventricular ectopy during exercise is a traditional indication for exercise testing. Newer indications include the use of responses of rate of ventricular repolarization (QT interval) and conduction down accessory pathways (WPW syndrome) during exercise as means of patient risk stratification.
- The increased use of pediatric exercise testing has also been stimulated by the concerns of parents, coaches, and physical education instructors over the occurrence of symptoms of chest pain, dizziness, syncope, or palpitations in young people during exercise. Such concerns have been fueled by the tragic occurrences of sudden unexpected death of young, presumably healthy athletes during sports training or competition. While such symptoms are highly unlikely to reflect the rare diseases that pose a risk of sudden death, the youngster with occult hypertrophic cardiomyopathy, coronary artery anomalies, or repolarization abnormalities that can predispose to fatal dysrhythmias can present with such complaints. Findings on exercise testing have thus become part of the assessment of symptomatic children and athletes to rule out these anomalies.
- A normal exercise test can provide clearance for sports play in young patients with heart disease or in those who have suffered illnesses such as viral myocarditis. Exercise testing also plays a role in assessing risk and exercise capacities in young patients who are enrolled in cardiac and pulmonary rehabilitation programs.
We can expect that clinical exercise testing in children and adolescents will continue to expand as the value of exercise is recognized in the assessment of not only heart and lung disease but also metabolic and musculoskeletal disorders. Such trends will undoubtedly follow improvements in medical and surgical treatment of these patients. We can also expect to see new techniques for performing exercise tests (miniaturization of metabolic systems permitting field testing, for example) and assessing their results (three-dimensional echocardiography, myocardial strain Doppler studies).
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Determining Functional Capacity
In summary, our quest is to determine children’s functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise.
In summary, our quest is to determine children's functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise. Exercise testing is essential to achieve this goal. Simple measurement of workload on a cycle ergometer will provide information on how close we are to "normal." Peak workload on a cycle ergometer is a measure of very vigorous exercise. The calibration of the cycle is often very good, and the workloads are very consistent over fast and slow pedaling. It is therefore a very useful measure of functional capacity for many subjects of different sizes and levels of physical conditioning. Within-subject reliability is also very good, so we can compare the child's responses over time accurately. Work in watts measured on the cycle ergometer can be used in a calculation to predict the oxygen consumption, again giving us a measure of functional capacity.
Current research indicates that one can predict the peak O2 from the O2 at VAT with good accuracy in healthy children as well as in those with lung disease. Caution should be used when predicting peak O2 from the O2 at VAT in children with congenital heart disease. Metabolic measurements need to be available to calculate the VAT, which adds to the cost and resources needed to predict the O2 from this method. The measurement of VAT becomes more useful when a submaximal effort has limited the exercise test. Age, comprehension, size, motivation, and protocol all factor into a submaximal response. The caveat of the method is when we assume that the O2 at VAT is about 60% of the peak O2. Physical conditioning and specifically a lack of physical fitness influence this method greatly, and this may lead to over- or underestimation of peak O2. We know that physical deconditioning, either from being sedentary or as the result of a disease process, lowers the O2 at the VAT.
OUES is a useful submaximal measure of aerobic fitness that is effort independent. This method also relies on the availability of metabolic measurements. It has been shown to be reproducible and does not suffer the intra-observer or inter-observer discrepancies often encountered with VAT. The OUES is easy to calculate and is determined by many more data points than VAT, peak work, or peak heart rate. The consensus of the research data indicates that it is most useful at the higher percentages of the peak O2 (that is, cut points of OUES 80% and OUES 90%). OUES below 45% to 50% of peak O2 is less reliable and does not correlate as well with peak O2. There is conflicting evidence regarding whether OUES is equally useful in predicting peak O2 in healthy subjects and in those with chronic heart or lung disease. If a subject can achieve 80% or 90% of predicted peak O2, many of us would consider that an adequate level of exertion. If the subject achieves 80% or 90% of peak O2, we do not know if the OUES slope can be abnormal. My guess is probably not. The slope of the line that represents the VE and O2 relationship is strongly influenced by the onset of anaerobic metabolism. The V-slope method of VAT detection is based on the change in that slope. Therefore the choice of the cut points will influence the OUES slope. Another factor that will influence the OUES is the choice of ergometer. We know that the peak O2 on the cycle ergometer is about 10% to 15% less than what is achieved on a treadmill. Therefore percent cut points will actually represent different amounts of O2. This could be problematic if we did not have equations for both the treadmill and cycle ergometer.
The six-minute walk is a simple test that is easy to administer and has good reproducibility. There are two versions, encouraged and unencouraged. The unencouraged version may reduce variability among test administrators. Distance and dyspnea score are recorded. The words and phrases of the Borg scale may not be completely understood by young children. Other scales with pictures have been used to assess the child's perceived exertion. The six-minute walk test may be more useful in underserved areas where metabolic equipment is not available. Also, large groups of children can be tested at very little cost, and over time increases in distance walked and a lower rate of perceived exertion may relate to improved fitness. This topic has been covered in greater detail in chapter 3.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Case Presentation
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago.
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago. She describes the dizziness as "spinning," and it occurs not during running but just after she has finished. It usually occurs after running at a high-tempo pace for several miles and usually in the summer months. She denies frank syncope, chest pain, shortness of breath, or palpitations. There was a documented blood pressure of 62/40 at one time when paramedics were called while the event was occurring. She has been seen in the emergency room twice for the complaint; the most recent was 2 d ago. In the emergency room, she had a normal heart rate and blood pressure, normal physical examination, and normal laboratory values for anemia and thyroid. Her chest radiograph and electrocardiogram (ECG) were unremarkable. She has been restricted from exercise until she is seen by you. She is requesting exercise clearance.
Turning back to our case study, on further questioning it is revealed that Rebecca drinks 20 oz of water daily and one cup of coffee in the morning. She often skips breakfast as well because she does not have time to eat. She sometimes experiences dizziness in the morning when she stands up while getting out of bed. Rebecca's physical examination included orthostatic vital signs. Her heart rate increased from 52 bpm supine to 90 bpm in the upright position. Her blood pressure decreased from 116/68 to 108/64 from supine to upright position, respectively. Her physical examination was otherwise unremarkable.
Rebecca had an ECG performed, which was normal. Although her symptoms were classic for dehydration as well as a vasovagal component, because she was a competitive athlete, the cardiologist opted for an echocardiogram, which was normal, and an exercise stress test to try to induce her symptoms.
Rebecca performed a maximal treadmill exercise stress test. She had a high-normal aerobic capacity. There were no arrhythmias or ischemic changes. There was a normal heart rate and blood pressure response to exercise. She developed dizziness in early recovery with a 30 mmHg drop in her blood pressure. After she drank a cup of water and was placed in the supine position, her blood pressure improved within 2 min.
For Rebecca, her history, physical examination, and ECG were reassuring that this was unlikely to be cardiac-related exertional syncope. Her exercise stress test was able to document dizziness associated with a drop in blood pressure that responded to fluids and supine positioning. She was counseled on improving her hydration and salt intake and not to skip meals. She returned for a follow-up visit 3 mo later and had not experienced any further dizziness or syncope.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Unique Features of Exercise Testing in Children
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults.
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults. One need only consider the various approaches needed to perform a satisfactory exercise test first in a 15-year-old cross country runner who experienced precordial chest pain in her last race, followed by a test looking for heart rate response in a 5-year-old youngster with complete heart block, and then a 12-year-old obese boy with a dilated cardiomyopathy and progressively worsening shortness of breath with exercise.
The pediatric exercise testing laboratory must accommodate wide variations in patient age, size, and fitness, and that means that testing protocols and equipment must be similarly adjusted. The indications for testing are much broader than those in the adult lab, so the questions to be answered must be carefully considered before the exercise begins.
Most children can be easily motivated to give exhaustive efforts during exercise testing, but it requires charismatic skill from staff members experienced with the emotional and physical responses of children during treadmill or cycle exercise. It has been said that perhaps the single most important factor in a successful exercise test in the pediatric laboratory is the staff administering the test.
Pediatric exercise testing, then, is distinguished by the need for a creative approach to each patient. The staff must know what information is needed to address the clinical question being asked, the proper modality - cycle or treadmill - to obtain that answer, and the optimal protocol for the subject's age and fitness level.
The physiological mechanisms underlying the cardiac and pulmonary responses to a bout of progressive exercise are no different in children (at least those over age 6) and adults. Nonetheless, certain quantitative measurements (heart rate, blood pressure, endurance time) are different in children, and these must be recognized in the testing of immature subjects.
For example, resting and maximal heart rates in an exhaustive exercise test are greater in children than in adults. As will be discussed in chapter 5, peak heart rate depends on testing modality and protocol, and there is considerable variability between individuals. It is important to recognize that there is generally no specific "target heart rate" for an exercise test. Importantly, too, the maximal heart rate during exercise testing in a given subject does not change over the course of childhood. Only at about age 16 does this value begin to decline. Thus, age-related formulae for predicting a maximal heart rate, such as "220 minus age," do not apply to youths.
The concept of metabolic equivalents, or METs, as a measure of energy expenditure during exercise is commonly used with adult subjects but is fraught with difficulty in children and adolescents, and it is best avoided in the pediatric exercise laboratory. METs is a means of expressing the oxygen requirement of a physical activity relative to an assumed resting value. One MET, or resting O2, in an adult is considered to be 3.5 ml ∙ kg-1 ∙ min-1; thus, when walking on a treadmill at a certain speed and slope that is expected to demand 17.5 ml ∙ kg-1 ∙ min-1, a patient is exercising at a level of 5 METs.
The difficulty with this concept in youth is that resting energy expenditure is not constant but evolves throughout childhood during physical growth. As would be expected, absolute values of resting O2 rise with the accrual of body mass. When adjusted for body mass, or body surface area, basal or resting values of energy expenditure decline progressively during the pediatric years. When expressed as calories per meter of square body surface area per hour, the basal metabolic rate declines by about 20% between the ages of six and the mid-teen years.
In considering a mass-relative definition of a MET in children, the story is more exaggerated. Harrell et al. reported that the resting O2 per kg in a group of 8- to 12-year-old children was almost 50% higher than that of 16- to 18-year-old subjects and 70% higher than that expected in adults. The use of the MET as defined in adults as a "currency" or multiplier of energy expenditure in children, then, clearly would introduce large errors in defining O2 levels during exercise - and the extent of the error would be different depending on the age and size of the child.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Asthma
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms.
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms. It is believed that the majority of children with asthma exhibit EIB, which may affect exercise tolerance as well as participation in physical activity. As reviewed by Welsh et al., studies examining aerobic fitness in children provide mixed results, with some reporting comparable fitness and others reporting reduced fitness in asthmatic children compared to their nonasthmatic peers. The inconsistencies may reflect differences in measurement of fitness (e.g., laboratory vs. field tests), differences in levels of severity of the children's asthma, the relatively small sample, selection bias in choice of nonasthmatic children for comparison, use of bronchodilator therapy prior to testing, and perhaps inappropriate statistical analyses. Some studies suggest that fitness is comparable as long as levels of physical activity are similar. Pharmacologic advances such as inhaled corticosteroid therapy have reduced the number of hospitalizations and emergency department visits, suggesting that the severity of disease has lessened. Consequently, the results of studies conducted prior to these advances may not accurately reflect the current impact of the disease on exercise tolerance.
In the past two decades, surprisingly few studies of children report ventilatory parameters assessed during exercise testing. Santuz et al. reported lower E at peak exercise in sedentary and regularly active children when compared to healthy controls (matched on age, height, weight, and habitual level of physical activity), but no difference in E was observed in asthmatic children who participated in organized or competitive sports compared to their peers. During submaximal exercise, the asthmatic and nonasthmatic children had a similar E, but it was achieved by a lower f b and a higher VT in the asthmatic children, which would lessen dead space breathing and improve alveolar ventilation. In contrast, Moraes et al. reported no differences among newly diagnosed adolescents with mild intermittent asthma (n = 20), mild persistent asthma (n = 13), and their healthy peers (n = 36) for O2, E, E/O2, or ventilatory reserve at maximal exercise. Berntsen et al. likewise found no difference in O2 or E at peak exercise in a group of 86 13-year-old adolescents with asthma compared to 76 same-aged peers without asthma. Resting lung function was slightly lower in the asthmatic children but generally within normal limits. In a multiple regression model that included skinfold thickness, resting pulmonary function, and physical activity, analysis indicated that skinfold thickness and vigorous physical activity participation (in asthmatics only) were the primary independent predictors of O2peak in the study.
Expiratory flow limitations after exercise are well documented in children with asthma, and a recent study reported postexercise inspiratory flow limitation as well. However, no study has reported measurement of exercise tidal loops to assess ventilatory mechanics in children with asthma during exercise. Consequently, the results of one study of adults are presented. Exercise tolerance and pulmonary mechanics during exercise were compared between eight adults with asthma and six adults without asthma. Asthmatic adults had similar larger airway function but reduced smaller airway function at rest, and O2peak was slightly lower (but not significantly) in asthmatic compared to control subjects (104% vs. 130% of predicted). At maximal exercise, the groups attained similar E, but the asthmatic subjects had higher mean values for E/CO2 and lower PETCO2 than the control subjects. Superimposing the exercise tidal loop on the resting maximal F-V loops produced similarities to figure 11.3, revealing increases in EELV with the expiratory flow of the tidal loop encroaching on the resting F-V loop during exercise, indicative of expiratory flow limitation. Interestingly, the authors noted variability in EELV and in the degree of exercise flow limitation during interval exercise (intensity switching from 60% to 40% of O2peak), and they suggested that the variability reflected changes in bronchomotor tone (i.e., bronchodilation vs. bronchoconstriction ) at different exercise intensities.
Recent studies reporting measurements of arterial blood gases are rare in children with asthma. One recent study collected expired gases and earlobe capillary blood samples in eight adolescents with mild intermittent asthma (MIA), eight with mild persistent asthma (MPA), and 12 nonasthmatic control subjects to examine gas exchange during exercise and to estimate PaO2 before and after maximal exercise stress. Before exercise, the children with MPA had significantly lower PaO2 (75.1 ± 6.6 mmHg) than those with MIA (81.7 ± 6.7 mmHg) and control (83.3 ± 4.9 mmHg) subjects, suggesting hypoxemia. Postexercise PaO2 did not differ among groups, but this was attributed to a decrease in the control subjects because the PaO2 of the children with MPA did not change significantly. Older studies report gas exchange abnormalities (elevated PA - aO2 difference) at rest that improved during exercise, which the authors attributed to better A/ matching. A more recent study of habitually active 18- to 45-year-old men and women with mild-to-moderate asthma examined arterial blood gases during exercise. Eight of 21 subjects exhibited oxyhemoglobin desaturation (SaO2 ≤94%) during prolonged exercise and were classified as the Lo-SaO2 group, and those whose SaO2 remained above 94% were classified as the Hi-SaO2 group. Despite similar fitness levels, the PaCO2 of the Hi-SaO2 group fell from rest to 34.0 ± 2.7 mmHg at exhaustion, whereas that of the Lo-SaO2 group fell during the first minute of exercise but then rose progressively, reaching 39.8 ± 4 mmHg at exhaustion, suggesting hypoventilation and impaired gas exchange.
Given the prevalence of asthma in children, as well as reports of reduced fitness, more studies examining ventilatory mechanics and gas exchange parameters during exercise are warranted in children with asthma to determine their potential contributions to reduced fitness and exercise intolerance.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Clinical Exercise Testing
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function.
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function. Karlman Wasserman and coworkers at UCLA demonstrated, too, how the acquisition of gas exchange variables measured during exercise could further delineate and differentiate abnormal cardiac and pulmonary responses to exercise.
The use of exercise testing in pediatric populations, whose members do not normally suffer from coronary artery disease, initially developed in the shadow of this story about adult patients. Early exercise studies in youth were performed in the research setting. They were designed to examine physiological differences that separate children from adults. Sid Robinson provided the first such treadmill-derived data in the Harvard Fatigue Laboratory in Boston in the 1930s, demonstrating the progressive changes in metabolic and physiological responses that normally occurred between the ages of 6 and 91. Similar exercise data in healthy children were subsequently provided by other investigators in the middle of the 20th century, including Per-Olof …strand in Sweden, Simon Godfrey in Great Britain, and Gordon Cumming in Canada. When Oded Bar-Or published his landmark book Pediatric Sports Medicine for the Practitioner in 1983, he was able to accumulate a large base of normative data from these earlier studies to outline aspects of physiological responses to exercise in youth and how these developed during the growing years.
While such research was designed to reveal the normal development of physiological responses to exercise in children, this information also served as normative data for those who developed exercise testing for the clinical assessment of heart and lung disease in pediatric patients. As studies dealt, for the most part, with those with congenital heart disease, early exercise stress testing in young patients involved a more diverse approach than that of the traditional adult laboratory focused on the detection of coronary artery disease. The assessment of ischemic changes on the ECG was still an issue, particularly in assessing the severity of aortic outflow obstruction, but exercise testing in young patients also involved a wider range of information, such as blood pressure responses (in coarctation of the aorta, systemic hypertension), endurance capacity (postoperative cyanotic heart disease), and rhythm responses (complete heart block).
Useful clinical testing methodologies and clinical findings were described by a number of key early pioneers, such as Fred James at Cincinnati Children's Hospital, David Driscoll at the Mayo Clinic, Bruce Alpert and William Strong at the Medical College of Georgia, Rolf Mocellin in Germany, and Tony Reybrouck and Dirk Matthys in Belgium. The importance of gas exchange measures, including O2max and O2 kinetics, was highlighted by the early reports of exercise testing in patients with congenital heart disease by Hans Wessel at Children's Memorial Hospital in Chicago. During this time, too, exercise testing became established in both children and adults as a useful means of assessing bronchospasm and lung function in patients with asthma and other respiratory diseases (particularly via the early experience reported by Hans Stoboy, Gerd Cropp, and Svein Oseid).
In many cases, clinical exercise testing in children was performed in adult laboratories, using protocols, exercise equipment, and monitoring systems (ECG, blood pressure) traditionally used to test adults with suspected coronary artery disease. A number of developments have now expanded the role of exercise testing in youth and have identified the need for more specific approaches to exercise testing for this population of patients.
- Perhaps most importantly, the past several decades have witnessed a dramatic expansion in the scope and nature of patients cared for by pediatric cardiologists. Children with complex forms of congenital heart disease, particularly those characterized by marked unilateral ventricular hypoplasia (hypoplastic left heart syndrome, tricuspid valve atresia), once had little hope for long-term survival. Now, thanks to remarkable progress in surgical techniques, these young patients not only often survive but also live productive and fulfilling lives. The physicians caring for these survivors as they grow toward the adult years are confronted with new issues, such as myocardial dysfunction, stubborn tachyarrhythmia, hypoxemia, and pulmonary hypertension. These problems have required new diagnostic and therapeutic approaches, often using information available in the exercise stress-testing laboratory. Similarly, in patients with lung diseases such as cystic fibrosis, improvements in patient care have successfully extended survival and have at the same time introduced new clinical questions that can be assessed through exercise testing. It is likely that the future will continue to bring steady improvements in the survival of young patients with both cardiac and pulmonary disease that will be paralleled by expanded indications for clinical exercise testing.
- A growing understanding of the pathophysiology of cardiac malformations and factors influencing risk stratification have created a need to expand the information obtained during exercise stress testing in young patients. It is true that many issues can be adequately examined by a limited study involving a traditional bout of progressive exercise accompanied by electrocardiographic monitoring and measurement of blood pressure. Assessment of possible ischemic changes in a child after Kawasaki disease, for example, or determination of blood pressure responses after medical treatment of a hypertensive young wrestler could be adequately performed using this approach.
However, the clinical insights gained from exercise testing can be improved by the measurement of gas exchange variables, which are now readily obtained with user-friendly commercial metabolic systems. The changes in the oxygen and carbon dioxide content of expired air during exercise reflects similar gas exchange dynamics at the cellular level. With this approach, for example, the measurement of O2max provides an objective physiological assessment of aerobic fitness, and the determination of ventilatory variables (minute ventilation, CO2) offers insights into pulmonary responses as well. As will be outlined in the chapters that follow, it is often the calculation of relationships between these variables that provides clues into the relative importance of cardiac and pulmonary etiologies of exercise limitation.
Termed an "integrative cardiopulmonary test" by Wasserman et al., this approach expands the utility of exercise testing by providing data that can help to answer questions about cardiac and pulmonary issues in youth. In a teenage boy with moderate aortic valve insufficiency, does his cardiac disease explain the shortness of breath that limits his ability to exercise? What mechanisms lie behind a star athlete's inability to perform well after an extended viral illness? Is syncope of an anxious child during running related to hyperventilation? Is breathlessness during exercise in a markedly obese child caused by excess body fat, exercise-induced bronchoconstriction, or cardiac dysfunction? These types of issues are best addressed by a full examination of gas exchange variables during clinical exercise testing.
- A growing recognition of the effects of exercise on electrophysiological function has created new roles for exercise testing in youth. Assessment of changes in ventricular ectopy during exercise is a traditional indication for exercise testing. Newer indications include the use of responses of rate of ventricular repolarization (QT interval) and conduction down accessory pathways (WPW syndrome) during exercise as means of patient risk stratification.
- The increased use of pediatric exercise testing has also been stimulated by the concerns of parents, coaches, and physical education instructors over the occurrence of symptoms of chest pain, dizziness, syncope, or palpitations in young people during exercise. Such concerns have been fueled by the tragic occurrences of sudden unexpected death of young, presumably healthy athletes during sports training or competition. While such symptoms are highly unlikely to reflect the rare diseases that pose a risk of sudden death, the youngster with occult hypertrophic cardiomyopathy, coronary artery anomalies, or repolarization abnormalities that can predispose to fatal dysrhythmias can present with such complaints. Findings on exercise testing have thus become part of the assessment of symptomatic children and athletes to rule out these anomalies.
- A normal exercise test can provide clearance for sports play in young patients with heart disease or in those who have suffered illnesses such as viral myocarditis. Exercise testing also plays a role in assessing risk and exercise capacities in young patients who are enrolled in cardiac and pulmonary rehabilitation programs.
We can expect that clinical exercise testing in children and adolescents will continue to expand as the value of exercise is recognized in the assessment of not only heart and lung disease but also metabolic and musculoskeletal disorders. Such trends will undoubtedly follow improvements in medical and surgical treatment of these patients. We can also expect to see new techniques for performing exercise tests (miniaturization of metabolic systems permitting field testing, for example) and assessing their results (three-dimensional echocardiography, myocardial strain Doppler studies).
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Determining Functional Capacity
In summary, our quest is to determine children’s functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise.
In summary, our quest is to determine children's functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise. Exercise testing is essential to achieve this goal. Simple measurement of workload on a cycle ergometer will provide information on how close we are to "normal." Peak workload on a cycle ergometer is a measure of very vigorous exercise. The calibration of the cycle is often very good, and the workloads are very consistent over fast and slow pedaling. It is therefore a very useful measure of functional capacity for many subjects of different sizes and levels of physical conditioning. Within-subject reliability is also very good, so we can compare the child's responses over time accurately. Work in watts measured on the cycle ergometer can be used in a calculation to predict the oxygen consumption, again giving us a measure of functional capacity.
Current research indicates that one can predict the peak O2 from the O2 at VAT with good accuracy in healthy children as well as in those with lung disease. Caution should be used when predicting peak O2 from the O2 at VAT in children with congenital heart disease. Metabolic measurements need to be available to calculate the VAT, which adds to the cost and resources needed to predict the O2 from this method. The measurement of VAT becomes more useful when a submaximal effort has limited the exercise test. Age, comprehension, size, motivation, and protocol all factor into a submaximal response. The caveat of the method is when we assume that the O2 at VAT is about 60% of the peak O2. Physical conditioning and specifically a lack of physical fitness influence this method greatly, and this may lead to over- or underestimation of peak O2. We know that physical deconditioning, either from being sedentary or as the result of a disease process, lowers the O2 at the VAT.
OUES is a useful submaximal measure of aerobic fitness that is effort independent. This method also relies on the availability of metabolic measurements. It has been shown to be reproducible and does not suffer the intra-observer or inter-observer discrepancies often encountered with VAT. The OUES is easy to calculate and is determined by many more data points than VAT, peak work, or peak heart rate. The consensus of the research data indicates that it is most useful at the higher percentages of the peak O2 (that is, cut points of OUES 80% and OUES 90%). OUES below 45% to 50% of peak O2 is less reliable and does not correlate as well with peak O2. There is conflicting evidence regarding whether OUES is equally useful in predicting peak O2 in healthy subjects and in those with chronic heart or lung disease. If a subject can achieve 80% or 90% of predicted peak O2, many of us would consider that an adequate level of exertion. If the subject achieves 80% or 90% of peak O2, we do not know if the OUES slope can be abnormal. My guess is probably not. The slope of the line that represents the VE and O2 relationship is strongly influenced by the onset of anaerobic metabolism. The V-slope method of VAT detection is based on the change in that slope. Therefore the choice of the cut points will influence the OUES slope. Another factor that will influence the OUES is the choice of ergometer. We know that the peak O2 on the cycle ergometer is about 10% to 15% less than what is achieved on a treadmill. Therefore percent cut points will actually represent different amounts of O2. This could be problematic if we did not have equations for both the treadmill and cycle ergometer.
The six-minute walk is a simple test that is easy to administer and has good reproducibility. There are two versions, encouraged and unencouraged. The unencouraged version may reduce variability among test administrators. Distance and dyspnea score are recorded. The words and phrases of the Borg scale may not be completely understood by young children. Other scales with pictures have been used to assess the child's perceived exertion. The six-minute walk test may be more useful in underserved areas where metabolic equipment is not available. Also, large groups of children can be tested at very little cost, and over time increases in distance walked and a lower rate of perceived exertion may relate to improved fitness. This topic has been covered in greater detail in chapter 3.
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Case Presentation
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago.
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago. She describes the dizziness as "spinning," and it occurs not during running but just after she has finished. It usually occurs after running at a high-tempo pace for several miles and usually in the summer months. She denies frank syncope, chest pain, shortness of breath, or palpitations. There was a documented blood pressure of 62/40 at one time when paramedics were called while the event was occurring. She has been seen in the emergency room twice for the complaint; the most recent was 2 d ago. In the emergency room, she had a normal heart rate and blood pressure, normal physical examination, and normal laboratory values for anemia and thyroid. Her chest radiograph and electrocardiogram (ECG) were unremarkable. She has been restricted from exercise until she is seen by you. She is requesting exercise clearance.
Turning back to our case study, on further questioning it is revealed that Rebecca drinks 20 oz of water daily and one cup of coffee in the morning. She often skips breakfast as well because she does not have time to eat. She sometimes experiences dizziness in the morning when she stands up while getting out of bed. Rebecca's physical examination included orthostatic vital signs. Her heart rate increased from 52 bpm supine to 90 bpm in the upright position. Her blood pressure decreased from 116/68 to 108/64 from supine to upright position, respectively. Her physical examination was otherwise unremarkable.
Rebecca had an ECG performed, which was normal. Although her symptoms were classic for dehydration as well as a vasovagal component, because she was a competitive athlete, the cardiologist opted for an echocardiogram, which was normal, and an exercise stress test to try to induce her symptoms.
Rebecca performed a maximal treadmill exercise stress test. She had a high-normal aerobic capacity. There were no arrhythmias or ischemic changes. There was a normal heart rate and blood pressure response to exercise. She developed dizziness in early recovery with a 30 mmHg drop in her blood pressure. After she drank a cup of water and was placed in the supine position, her blood pressure improved within 2 min.
For Rebecca, her history, physical examination, and ECG were reassuring that this was unlikely to be cardiac-related exertional syncope. Her exercise stress test was able to document dizziness associated with a drop in blood pressure that responded to fluids and supine positioning. She was counseled on improving her hydration and salt intake and not to skip meals. She returned for a follow-up visit 3 mo later and had not experienced any further dizziness or syncope.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Unique Features of Exercise Testing in Children
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults.
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults. One need only consider the various approaches needed to perform a satisfactory exercise test first in a 15-year-old cross country runner who experienced precordial chest pain in her last race, followed by a test looking for heart rate response in a 5-year-old youngster with complete heart block, and then a 12-year-old obese boy with a dilated cardiomyopathy and progressively worsening shortness of breath with exercise.
The pediatric exercise testing laboratory must accommodate wide variations in patient age, size, and fitness, and that means that testing protocols and equipment must be similarly adjusted. The indications for testing are much broader than those in the adult lab, so the questions to be answered must be carefully considered before the exercise begins.
Most children can be easily motivated to give exhaustive efforts during exercise testing, but it requires charismatic skill from staff members experienced with the emotional and physical responses of children during treadmill or cycle exercise. It has been said that perhaps the single most important factor in a successful exercise test in the pediatric laboratory is the staff administering the test.
Pediatric exercise testing, then, is distinguished by the need for a creative approach to each patient. The staff must know what information is needed to address the clinical question being asked, the proper modality - cycle or treadmill - to obtain that answer, and the optimal protocol for the subject's age and fitness level.
The physiological mechanisms underlying the cardiac and pulmonary responses to a bout of progressive exercise are no different in children (at least those over age 6) and adults. Nonetheless, certain quantitative measurements (heart rate, blood pressure, endurance time) are different in children, and these must be recognized in the testing of immature subjects.
For example, resting and maximal heart rates in an exhaustive exercise test are greater in children than in adults. As will be discussed in chapter 5, peak heart rate depends on testing modality and protocol, and there is considerable variability between individuals. It is important to recognize that there is generally no specific "target heart rate" for an exercise test. Importantly, too, the maximal heart rate during exercise testing in a given subject does not change over the course of childhood. Only at about age 16 does this value begin to decline. Thus, age-related formulae for predicting a maximal heart rate, such as "220 minus age," do not apply to youths.
The concept of metabolic equivalents, or METs, as a measure of energy expenditure during exercise is commonly used with adult subjects but is fraught with difficulty in children and adolescents, and it is best avoided in the pediatric exercise laboratory. METs is a means of expressing the oxygen requirement of a physical activity relative to an assumed resting value. One MET, or resting O2, in an adult is considered to be 3.5 ml ∙ kg-1 ∙ min-1; thus, when walking on a treadmill at a certain speed and slope that is expected to demand 17.5 ml ∙ kg-1 ∙ min-1, a patient is exercising at a level of 5 METs.
The difficulty with this concept in youth is that resting energy expenditure is not constant but evolves throughout childhood during physical growth. As would be expected, absolute values of resting O2 rise with the accrual of body mass. When adjusted for body mass, or body surface area, basal or resting values of energy expenditure decline progressively during the pediatric years. When expressed as calories per meter of square body surface area per hour, the basal metabolic rate declines by about 20% between the ages of six and the mid-teen years.
In considering a mass-relative definition of a MET in children, the story is more exaggerated. Harrell et al. reported that the resting O2 per kg in a group of 8- to 12-year-old children was almost 50% higher than that of 16- to 18-year-old subjects and 70% higher than that expected in adults. The use of the MET as defined in adults as a "currency" or multiplier of energy expenditure in children, then, clearly would introduce large errors in defining O2 levels during exercise - and the extent of the error would be different depending on the age and size of the child.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Asthma
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms.
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms. It is believed that the majority of children with asthma exhibit EIB, which may affect exercise tolerance as well as participation in physical activity. As reviewed by Welsh et al., studies examining aerobic fitness in children provide mixed results, with some reporting comparable fitness and others reporting reduced fitness in asthmatic children compared to their nonasthmatic peers. The inconsistencies may reflect differences in measurement of fitness (e.g., laboratory vs. field tests), differences in levels of severity of the children's asthma, the relatively small sample, selection bias in choice of nonasthmatic children for comparison, use of bronchodilator therapy prior to testing, and perhaps inappropriate statistical analyses. Some studies suggest that fitness is comparable as long as levels of physical activity are similar. Pharmacologic advances such as inhaled corticosteroid therapy have reduced the number of hospitalizations and emergency department visits, suggesting that the severity of disease has lessened. Consequently, the results of studies conducted prior to these advances may not accurately reflect the current impact of the disease on exercise tolerance.
In the past two decades, surprisingly few studies of children report ventilatory parameters assessed during exercise testing. Santuz et al. reported lower E at peak exercise in sedentary and regularly active children when compared to healthy controls (matched on age, height, weight, and habitual level of physical activity), but no difference in E was observed in asthmatic children who participated in organized or competitive sports compared to their peers. During submaximal exercise, the asthmatic and nonasthmatic children had a similar E, but it was achieved by a lower f b and a higher VT in the asthmatic children, which would lessen dead space breathing and improve alveolar ventilation. In contrast, Moraes et al. reported no differences among newly diagnosed adolescents with mild intermittent asthma (n = 20), mild persistent asthma (n = 13), and their healthy peers (n = 36) for O2, E, E/O2, or ventilatory reserve at maximal exercise. Berntsen et al. likewise found no difference in O2 or E at peak exercise in a group of 86 13-year-old adolescents with asthma compared to 76 same-aged peers without asthma. Resting lung function was slightly lower in the asthmatic children but generally within normal limits. In a multiple regression model that included skinfold thickness, resting pulmonary function, and physical activity, analysis indicated that skinfold thickness and vigorous physical activity participation (in asthmatics only) were the primary independent predictors of O2peak in the study.
Expiratory flow limitations after exercise are well documented in children with asthma, and a recent study reported postexercise inspiratory flow limitation as well. However, no study has reported measurement of exercise tidal loops to assess ventilatory mechanics in children with asthma during exercise. Consequently, the results of one study of adults are presented. Exercise tolerance and pulmonary mechanics during exercise were compared between eight adults with asthma and six adults without asthma. Asthmatic adults had similar larger airway function but reduced smaller airway function at rest, and O2peak was slightly lower (but not significantly) in asthmatic compared to control subjects (104% vs. 130% of predicted). At maximal exercise, the groups attained similar E, but the asthmatic subjects had higher mean values for E/CO2 and lower PETCO2 than the control subjects. Superimposing the exercise tidal loop on the resting maximal F-V loops produced similarities to figure 11.3, revealing increases in EELV with the expiratory flow of the tidal loop encroaching on the resting F-V loop during exercise, indicative of expiratory flow limitation. Interestingly, the authors noted variability in EELV and in the degree of exercise flow limitation during interval exercise (intensity switching from 60% to 40% of O2peak), and they suggested that the variability reflected changes in bronchomotor tone (i.e., bronchodilation vs. bronchoconstriction ) at different exercise intensities.
Recent studies reporting measurements of arterial blood gases are rare in children with asthma. One recent study collected expired gases and earlobe capillary blood samples in eight adolescents with mild intermittent asthma (MIA), eight with mild persistent asthma (MPA), and 12 nonasthmatic control subjects to examine gas exchange during exercise and to estimate PaO2 before and after maximal exercise stress. Before exercise, the children with MPA had significantly lower PaO2 (75.1 ± 6.6 mmHg) than those with MIA (81.7 ± 6.7 mmHg) and control (83.3 ± 4.9 mmHg) subjects, suggesting hypoxemia. Postexercise PaO2 did not differ among groups, but this was attributed to a decrease in the control subjects because the PaO2 of the children with MPA did not change significantly. Older studies report gas exchange abnormalities (elevated PA - aO2 difference) at rest that improved during exercise, which the authors attributed to better A/ matching. A more recent study of habitually active 18- to 45-year-old men and women with mild-to-moderate asthma examined arterial blood gases during exercise. Eight of 21 subjects exhibited oxyhemoglobin desaturation (SaO2 ≤94%) during prolonged exercise and were classified as the Lo-SaO2 group, and those whose SaO2 remained above 94% were classified as the Hi-SaO2 group. Despite similar fitness levels, the PaCO2 of the Hi-SaO2 group fell from rest to 34.0 ± 2.7 mmHg at exhaustion, whereas that of the Lo-SaO2 group fell during the first minute of exercise but then rose progressively, reaching 39.8 ± 4 mmHg at exhaustion, suggesting hypoventilation and impaired gas exchange.
Given the prevalence of asthma in children, as well as reports of reduced fitness, more studies examining ventilatory mechanics and gas exchange parameters during exercise are warranted in children with asthma to determine their potential contributions to reduced fitness and exercise intolerance.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Clinical Exercise Testing
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function.
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function. Karlman Wasserman and coworkers at UCLA demonstrated, too, how the acquisition of gas exchange variables measured during exercise could further delineate and differentiate abnormal cardiac and pulmonary responses to exercise.
The use of exercise testing in pediatric populations, whose members do not normally suffer from coronary artery disease, initially developed in the shadow of this story about adult patients. Early exercise studies in youth were performed in the research setting. They were designed to examine physiological differences that separate children from adults. Sid Robinson provided the first such treadmill-derived data in the Harvard Fatigue Laboratory in Boston in the 1930s, demonstrating the progressive changes in metabolic and physiological responses that normally occurred between the ages of 6 and 91. Similar exercise data in healthy children were subsequently provided by other investigators in the middle of the 20th century, including Per-Olof …strand in Sweden, Simon Godfrey in Great Britain, and Gordon Cumming in Canada. When Oded Bar-Or published his landmark book Pediatric Sports Medicine for the Practitioner in 1983, he was able to accumulate a large base of normative data from these earlier studies to outline aspects of physiological responses to exercise in youth and how these developed during the growing years.
While such research was designed to reveal the normal development of physiological responses to exercise in children, this information also served as normative data for those who developed exercise testing for the clinical assessment of heart and lung disease in pediatric patients. As studies dealt, for the most part, with those with congenital heart disease, early exercise stress testing in young patients involved a more diverse approach than that of the traditional adult laboratory focused on the detection of coronary artery disease. The assessment of ischemic changes on the ECG was still an issue, particularly in assessing the severity of aortic outflow obstruction, but exercise testing in young patients also involved a wider range of information, such as blood pressure responses (in coarctation of the aorta, systemic hypertension), endurance capacity (postoperative cyanotic heart disease), and rhythm responses (complete heart block).
Useful clinical testing methodologies and clinical findings were described by a number of key early pioneers, such as Fred James at Cincinnati Children's Hospital, David Driscoll at the Mayo Clinic, Bruce Alpert and William Strong at the Medical College of Georgia, Rolf Mocellin in Germany, and Tony Reybrouck and Dirk Matthys in Belgium. The importance of gas exchange measures, including O2max and O2 kinetics, was highlighted by the early reports of exercise testing in patients with congenital heart disease by Hans Wessel at Children's Memorial Hospital in Chicago. During this time, too, exercise testing became established in both children and adults as a useful means of assessing bronchospasm and lung function in patients with asthma and other respiratory diseases (particularly via the early experience reported by Hans Stoboy, Gerd Cropp, and Svein Oseid).
In many cases, clinical exercise testing in children was performed in adult laboratories, using protocols, exercise equipment, and monitoring systems (ECG, blood pressure) traditionally used to test adults with suspected coronary artery disease. A number of developments have now expanded the role of exercise testing in youth and have identified the need for more specific approaches to exercise testing for this population of patients.
- Perhaps most importantly, the past several decades have witnessed a dramatic expansion in the scope and nature of patients cared for by pediatric cardiologists. Children with complex forms of congenital heart disease, particularly those characterized by marked unilateral ventricular hypoplasia (hypoplastic left heart syndrome, tricuspid valve atresia), once had little hope for long-term survival. Now, thanks to remarkable progress in surgical techniques, these young patients not only often survive but also live productive and fulfilling lives. The physicians caring for these survivors as they grow toward the adult years are confronted with new issues, such as myocardial dysfunction, stubborn tachyarrhythmia, hypoxemia, and pulmonary hypertension. These problems have required new diagnostic and therapeutic approaches, often using information available in the exercise stress-testing laboratory. Similarly, in patients with lung diseases such as cystic fibrosis, improvements in patient care have successfully extended survival and have at the same time introduced new clinical questions that can be assessed through exercise testing. It is likely that the future will continue to bring steady improvements in the survival of young patients with both cardiac and pulmonary disease that will be paralleled by expanded indications for clinical exercise testing.
- A growing understanding of the pathophysiology of cardiac malformations and factors influencing risk stratification have created a need to expand the information obtained during exercise stress testing in young patients. It is true that many issues can be adequately examined by a limited study involving a traditional bout of progressive exercise accompanied by electrocardiographic monitoring and measurement of blood pressure. Assessment of possible ischemic changes in a child after Kawasaki disease, for example, or determination of blood pressure responses after medical treatment of a hypertensive young wrestler could be adequately performed using this approach.
However, the clinical insights gained from exercise testing can be improved by the measurement of gas exchange variables, which are now readily obtained with user-friendly commercial metabolic systems. The changes in the oxygen and carbon dioxide content of expired air during exercise reflects similar gas exchange dynamics at the cellular level. With this approach, for example, the measurement of O2max provides an objective physiological assessment of aerobic fitness, and the determination of ventilatory variables (minute ventilation, CO2) offers insights into pulmonary responses as well. As will be outlined in the chapters that follow, it is often the calculation of relationships between these variables that provides clues into the relative importance of cardiac and pulmonary etiologies of exercise limitation.
Termed an "integrative cardiopulmonary test" by Wasserman et al., this approach expands the utility of exercise testing by providing data that can help to answer questions about cardiac and pulmonary issues in youth. In a teenage boy with moderate aortic valve insufficiency, does his cardiac disease explain the shortness of breath that limits his ability to exercise? What mechanisms lie behind a star athlete's inability to perform well after an extended viral illness? Is syncope of an anxious child during running related to hyperventilation? Is breathlessness during exercise in a markedly obese child caused by excess body fat, exercise-induced bronchoconstriction, or cardiac dysfunction? These types of issues are best addressed by a full examination of gas exchange variables during clinical exercise testing.
- A growing recognition of the effects of exercise on electrophysiological function has created new roles for exercise testing in youth. Assessment of changes in ventricular ectopy during exercise is a traditional indication for exercise testing. Newer indications include the use of responses of rate of ventricular repolarization (QT interval) and conduction down accessory pathways (WPW syndrome) during exercise as means of patient risk stratification.
- The increased use of pediatric exercise testing has also been stimulated by the concerns of parents, coaches, and physical education instructors over the occurrence of symptoms of chest pain, dizziness, syncope, or palpitations in young people during exercise. Such concerns have been fueled by the tragic occurrences of sudden unexpected death of young, presumably healthy athletes during sports training or competition. While such symptoms are highly unlikely to reflect the rare diseases that pose a risk of sudden death, the youngster with occult hypertrophic cardiomyopathy, coronary artery anomalies, or repolarization abnormalities that can predispose to fatal dysrhythmias can present with such complaints. Findings on exercise testing have thus become part of the assessment of symptomatic children and athletes to rule out these anomalies.
- A normal exercise test can provide clearance for sports play in young patients with heart disease or in those who have suffered illnesses such as viral myocarditis. Exercise testing also plays a role in assessing risk and exercise capacities in young patients who are enrolled in cardiac and pulmonary rehabilitation programs.
We can expect that clinical exercise testing in children and adolescents will continue to expand as the value of exercise is recognized in the assessment of not only heart and lung disease but also metabolic and musculoskeletal disorders. Such trends will undoubtedly follow improvements in medical and surgical treatment of these patients. We can also expect to see new techniques for performing exercise tests (miniaturization of metabolic systems permitting field testing, for example) and assessing their results (three-dimensional echocardiography, myocardial strain Doppler studies).
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Determining Functional Capacity
In summary, our quest is to determine children’s functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise.
In summary, our quest is to determine children's functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise. Exercise testing is essential to achieve this goal. Simple measurement of workload on a cycle ergometer will provide information on how close we are to "normal." Peak workload on a cycle ergometer is a measure of very vigorous exercise. The calibration of the cycle is often very good, and the workloads are very consistent over fast and slow pedaling. It is therefore a very useful measure of functional capacity for many subjects of different sizes and levels of physical conditioning. Within-subject reliability is also very good, so we can compare the child's responses over time accurately. Work in watts measured on the cycle ergometer can be used in a calculation to predict the oxygen consumption, again giving us a measure of functional capacity.
Current research indicates that one can predict the peak O2 from the O2 at VAT with good accuracy in healthy children as well as in those with lung disease. Caution should be used when predicting peak O2 from the O2 at VAT in children with congenital heart disease. Metabolic measurements need to be available to calculate the VAT, which adds to the cost and resources needed to predict the O2 from this method. The measurement of VAT becomes more useful when a submaximal effort has limited the exercise test. Age, comprehension, size, motivation, and protocol all factor into a submaximal response. The caveat of the method is when we assume that the O2 at VAT is about 60% of the peak O2. Physical conditioning and specifically a lack of physical fitness influence this method greatly, and this may lead to over- or underestimation of peak O2. We know that physical deconditioning, either from being sedentary or as the result of a disease process, lowers the O2 at the VAT.
OUES is a useful submaximal measure of aerobic fitness that is effort independent. This method also relies on the availability of metabolic measurements. It has been shown to be reproducible and does not suffer the intra-observer or inter-observer discrepancies often encountered with VAT. The OUES is easy to calculate and is determined by many more data points than VAT, peak work, or peak heart rate. The consensus of the research data indicates that it is most useful at the higher percentages of the peak O2 (that is, cut points of OUES 80% and OUES 90%). OUES below 45% to 50% of peak O2 is less reliable and does not correlate as well with peak O2. There is conflicting evidence regarding whether OUES is equally useful in predicting peak O2 in healthy subjects and in those with chronic heart or lung disease. If a subject can achieve 80% or 90% of predicted peak O2, many of us would consider that an adequate level of exertion. If the subject achieves 80% or 90% of peak O2, we do not know if the OUES slope can be abnormal. My guess is probably not. The slope of the line that represents the VE and O2 relationship is strongly influenced by the onset of anaerobic metabolism. The V-slope method of VAT detection is based on the change in that slope. Therefore the choice of the cut points will influence the OUES slope. Another factor that will influence the OUES is the choice of ergometer. We know that the peak O2 on the cycle ergometer is about 10% to 15% less than what is achieved on a treadmill. Therefore percent cut points will actually represent different amounts of O2. This could be problematic if we did not have equations for both the treadmill and cycle ergometer.
The six-minute walk is a simple test that is easy to administer and has good reproducibility. There are two versions, encouraged and unencouraged. The unencouraged version may reduce variability among test administrators. Distance and dyspnea score are recorded. The words and phrases of the Borg scale may not be completely understood by young children. Other scales with pictures have been used to assess the child's perceived exertion. The six-minute walk test may be more useful in underserved areas where metabolic equipment is not available. Also, large groups of children can be tested at very little cost, and over time increases in distance walked and a lower rate of perceived exertion may relate to improved fitness. This topic has been covered in greater detail in chapter 3.
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Case Presentation
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago.
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago. She describes the dizziness as "spinning," and it occurs not during running but just after she has finished. It usually occurs after running at a high-tempo pace for several miles and usually in the summer months. She denies frank syncope, chest pain, shortness of breath, or palpitations. There was a documented blood pressure of 62/40 at one time when paramedics were called while the event was occurring. She has been seen in the emergency room twice for the complaint; the most recent was 2 d ago. In the emergency room, she had a normal heart rate and blood pressure, normal physical examination, and normal laboratory values for anemia and thyroid. Her chest radiograph and electrocardiogram (ECG) were unremarkable. She has been restricted from exercise until she is seen by you. She is requesting exercise clearance.
Turning back to our case study, on further questioning it is revealed that Rebecca drinks 20 oz of water daily and one cup of coffee in the morning. She often skips breakfast as well because she does not have time to eat. She sometimes experiences dizziness in the morning when she stands up while getting out of bed. Rebecca's physical examination included orthostatic vital signs. Her heart rate increased from 52 bpm supine to 90 bpm in the upright position. Her blood pressure decreased from 116/68 to 108/64 from supine to upright position, respectively. Her physical examination was otherwise unremarkable.
Rebecca had an ECG performed, which was normal. Although her symptoms were classic for dehydration as well as a vasovagal component, because she was a competitive athlete, the cardiologist opted for an echocardiogram, which was normal, and an exercise stress test to try to induce her symptoms.
Rebecca performed a maximal treadmill exercise stress test. She had a high-normal aerobic capacity. There were no arrhythmias or ischemic changes. There was a normal heart rate and blood pressure response to exercise. She developed dizziness in early recovery with a 30 mmHg drop in her blood pressure. After she drank a cup of water and was placed in the supine position, her blood pressure improved within 2 min.
For Rebecca, her history, physical examination, and ECG were reassuring that this was unlikely to be cardiac-related exertional syncope. Her exercise stress test was able to document dizziness associated with a drop in blood pressure that responded to fluids and supine positioning. She was counseled on improving her hydration and salt intake and not to skip meals. She returned for a follow-up visit 3 mo later and had not experienced any further dizziness or syncope.
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Unique Features of Exercise Testing in Children
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults.
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults. One need only consider the various approaches needed to perform a satisfactory exercise test first in a 15-year-old cross country runner who experienced precordial chest pain in her last race, followed by a test looking for heart rate response in a 5-year-old youngster with complete heart block, and then a 12-year-old obese boy with a dilated cardiomyopathy and progressively worsening shortness of breath with exercise.
The pediatric exercise testing laboratory must accommodate wide variations in patient age, size, and fitness, and that means that testing protocols and equipment must be similarly adjusted. The indications for testing are much broader than those in the adult lab, so the questions to be answered must be carefully considered before the exercise begins.
Most children can be easily motivated to give exhaustive efforts during exercise testing, but it requires charismatic skill from staff members experienced with the emotional and physical responses of children during treadmill or cycle exercise. It has been said that perhaps the single most important factor in a successful exercise test in the pediatric laboratory is the staff administering the test.
Pediatric exercise testing, then, is distinguished by the need for a creative approach to each patient. The staff must know what information is needed to address the clinical question being asked, the proper modality - cycle or treadmill - to obtain that answer, and the optimal protocol for the subject's age and fitness level.
The physiological mechanisms underlying the cardiac and pulmonary responses to a bout of progressive exercise are no different in children (at least those over age 6) and adults. Nonetheless, certain quantitative measurements (heart rate, blood pressure, endurance time) are different in children, and these must be recognized in the testing of immature subjects.
For example, resting and maximal heart rates in an exhaustive exercise test are greater in children than in adults. As will be discussed in chapter 5, peak heart rate depends on testing modality and protocol, and there is considerable variability between individuals. It is important to recognize that there is generally no specific "target heart rate" for an exercise test. Importantly, too, the maximal heart rate during exercise testing in a given subject does not change over the course of childhood. Only at about age 16 does this value begin to decline. Thus, age-related formulae for predicting a maximal heart rate, such as "220 minus age," do not apply to youths.
The concept of metabolic equivalents, or METs, as a measure of energy expenditure during exercise is commonly used with adult subjects but is fraught with difficulty in children and adolescents, and it is best avoided in the pediatric exercise laboratory. METs is a means of expressing the oxygen requirement of a physical activity relative to an assumed resting value. One MET, or resting O2, in an adult is considered to be 3.5 ml ∙ kg-1 ∙ min-1; thus, when walking on a treadmill at a certain speed and slope that is expected to demand 17.5 ml ∙ kg-1 ∙ min-1, a patient is exercising at a level of 5 METs.
The difficulty with this concept in youth is that resting energy expenditure is not constant but evolves throughout childhood during physical growth. As would be expected, absolute values of resting O2 rise with the accrual of body mass. When adjusted for body mass, or body surface area, basal or resting values of energy expenditure decline progressively during the pediatric years. When expressed as calories per meter of square body surface area per hour, the basal metabolic rate declines by about 20% between the ages of six and the mid-teen years.
In considering a mass-relative definition of a MET in children, the story is more exaggerated. Harrell et al. reported that the resting O2 per kg in a group of 8- to 12-year-old children was almost 50% higher than that of 16- to 18-year-old subjects and 70% higher than that expected in adults. The use of the MET as defined in adults as a "currency" or multiplier of energy expenditure in children, then, clearly would introduce large errors in defining O2 levels during exercise - and the extent of the error would be different depending on the age and size of the child.
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Asthma
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms.
Asthma is the most common chronic disease in children, affecting nearly 7 million (9.3% of) U.S. children. It is characterized by chronic inflammation of the airways, which contributes to airway hyperreactivity, and by episodic or in more severe cases, persistent airway obstruction and respiratory symptoms. It is believed that the majority of children with asthma exhibit EIB, which may affect exercise tolerance as well as participation in physical activity. As reviewed by Welsh et al., studies examining aerobic fitness in children provide mixed results, with some reporting comparable fitness and others reporting reduced fitness in asthmatic children compared to their nonasthmatic peers. The inconsistencies may reflect differences in measurement of fitness (e.g., laboratory vs. field tests), differences in levels of severity of the children's asthma, the relatively small sample, selection bias in choice of nonasthmatic children for comparison, use of bronchodilator therapy prior to testing, and perhaps inappropriate statistical analyses. Some studies suggest that fitness is comparable as long as levels of physical activity are similar. Pharmacologic advances such as inhaled corticosteroid therapy have reduced the number of hospitalizations and emergency department visits, suggesting that the severity of disease has lessened. Consequently, the results of studies conducted prior to these advances may not accurately reflect the current impact of the disease on exercise tolerance.
In the past two decades, surprisingly few studies of children report ventilatory parameters assessed during exercise testing. Santuz et al. reported lower E at peak exercise in sedentary and regularly active children when compared to healthy controls (matched on age, height, weight, and habitual level of physical activity), but no difference in E was observed in asthmatic children who participated in organized or competitive sports compared to their peers. During submaximal exercise, the asthmatic and nonasthmatic children had a similar E, but it was achieved by a lower f b and a higher VT in the asthmatic children, which would lessen dead space breathing and improve alveolar ventilation. In contrast, Moraes et al. reported no differences among newly diagnosed adolescents with mild intermittent asthma (n = 20), mild persistent asthma (n = 13), and their healthy peers (n = 36) for O2, E, E/O2, or ventilatory reserve at maximal exercise. Berntsen et al. likewise found no difference in O2 or E at peak exercise in a group of 86 13-year-old adolescents with asthma compared to 76 same-aged peers without asthma. Resting lung function was slightly lower in the asthmatic children but generally within normal limits. In a multiple regression model that included skinfold thickness, resting pulmonary function, and physical activity, analysis indicated that skinfold thickness and vigorous physical activity participation (in asthmatics only) were the primary independent predictors of O2peak in the study.
Expiratory flow limitations after exercise are well documented in children with asthma, and a recent study reported postexercise inspiratory flow limitation as well. However, no study has reported measurement of exercise tidal loops to assess ventilatory mechanics in children with asthma during exercise. Consequently, the results of one study of adults are presented. Exercise tolerance and pulmonary mechanics during exercise were compared between eight adults with asthma and six adults without asthma. Asthmatic adults had similar larger airway function but reduced smaller airway function at rest, and O2peak was slightly lower (but not significantly) in asthmatic compared to control subjects (104% vs. 130% of predicted). At maximal exercise, the groups attained similar E, but the asthmatic subjects had higher mean values for E/CO2 and lower PETCO2 than the control subjects. Superimposing the exercise tidal loop on the resting maximal F-V loops produced similarities to figure 11.3, revealing increases in EELV with the expiratory flow of the tidal loop encroaching on the resting F-V loop during exercise, indicative of expiratory flow limitation. Interestingly, the authors noted variability in EELV and in the degree of exercise flow limitation during interval exercise (intensity switching from 60% to 40% of O2peak), and they suggested that the variability reflected changes in bronchomotor tone (i.e., bronchodilation vs. bronchoconstriction ) at different exercise intensities.
Recent studies reporting measurements of arterial blood gases are rare in children with asthma. One recent study collected expired gases and earlobe capillary blood samples in eight adolescents with mild intermittent asthma (MIA), eight with mild persistent asthma (MPA), and 12 nonasthmatic control subjects to examine gas exchange during exercise and to estimate PaO2 before and after maximal exercise stress. Before exercise, the children with MPA had significantly lower PaO2 (75.1 ± 6.6 mmHg) than those with MIA (81.7 ± 6.7 mmHg) and control (83.3 ± 4.9 mmHg) subjects, suggesting hypoxemia. Postexercise PaO2 did not differ among groups, but this was attributed to a decrease in the control subjects because the PaO2 of the children with MPA did not change significantly. Older studies report gas exchange abnormalities (elevated PA - aO2 difference) at rest that improved during exercise, which the authors attributed to better A/ matching. A more recent study of habitually active 18- to 45-year-old men and women with mild-to-moderate asthma examined arterial blood gases during exercise. Eight of 21 subjects exhibited oxyhemoglobin desaturation (SaO2 ≤94%) during prolonged exercise and were classified as the Lo-SaO2 group, and those whose SaO2 remained above 94% were classified as the Hi-SaO2 group. Despite similar fitness levels, the PaCO2 of the Hi-SaO2 group fell from rest to 34.0 ± 2.7 mmHg at exhaustion, whereas that of the Lo-SaO2 group fell during the first minute of exercise but then rose progressively, reaching 39.8 ± 4 mmHg at exhaustion, suggesting hypoventilation and impaired gas exchange.
Given the prevalence of asthma in children, as well as reports of reduced fitness, more studies examining ventilatory mechanics and gas exchange parameters during exercise are warranted in children with asthma to determine their potential contributions to reduced fitness and exercise intolerance.
Learn more about Cardiopulmonary Exercise Testing in Children and Adolescents.
Clinical Exercise Testing
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function.
Exercise stress testing in adults, eventually supplemented by radionuclide angiography and postexercise echocardiography, rapidly became accepted as a standard component of the diagnostic armamentarium, not only for coronary artery disease but also for an assortment of other clinical issues surrounding dysrhythmias, hypertension, and cardiac function. Karlman Wasserman and coworkers at UCLA demonstrated, too, how the acquisition of gas exchange variables measured during exercise could further delineate and differentiate abnormal cardiac and pulmonary responses to exercise.
The use of exercise testing in pediatric populations, whose members do not normally suffer from coronary artery disease, initially developed in the shadow of this story about adult patients. Early exercise studies in youth were performed in the research setting. They were designed to examine physiological differences that separate children from adults. Sid Robinson provided the first such treadmill-derived data in the Harvard Fatigue Laboratory in Boston in the 1930s, demonstrating the progressive changes in metabolic and physiological responses that normally occurred between the ages of 6 and 91. Similar exercise data in healthy children were subsequently provided by other investigators in the middle of the 20th century, including Per-Olof …strand in Sweden, Simon Godfrey in Great Britain, and Gordon Cumming in Canada. When Oded Bar-Or published his landmark book Pediatric Sports Medicine for the Practitioner in 1983, he was able to accumulate a large base of normative data from these earlier studies to outline aspects of physiological responses to exercise in youth and how these developed during the growing years.
While such research was designed to reveal the normal development of physiological responses to exercise in children, this information also served as normative data for those who developed exercise testing for the clinical assessment of heart and lung disease in pediatric patients. As studies dealt, for the most part, with those with congenital heart disease, early exercise stress testing in young patients involved a more diverse approach than that of the traditional adult laboratory focused on the detection of coronary artery disease. The assessment of ischemic changes on the ECG was still an issue, particularly in assessing the severity of aortic outflow obstruction, but exercise testing in young patients also involved a wider range of information, such as blood pressure responses (in coarctation of the aorta, systemic hypertension), endurance capacity (postoperative cyanotic heart disease), and rhythm responses (complete heart block).
Useful clinical testing methodologies and clinical findings were described by a number of key early pioneers, such as Fred James at Cincinnati Children's Hospital, David Driscoll at the Mayo Clinic, Bruce Alpert and William Strong at the Medical College of Georgia, Rolf Mocellin in Germany, and Tony Reybrouck and Dirk Matthys in Belgium. The importance of gas exchange measures, including O2max and O2 kinetics, was highlighted by the early reports of exercise testing in patients with congenital heart disease by Hans Wessel at Children's Memorial Hospital in Chicago. During this time, too, exercise testing became established in both children and adults as a useful means of assessing bronchospasm and lung function in patients with asthma and other respiratory diseases (particularly via the early experience reported by Hans Stoboy, Gerd Cropp, and Svein Oseid).
In many cases, clinical exercise testing in children was performed in adult laboratories, using protocols, exercise equipment, and monitoring systems (ECG, blood pressure) traditionally used to test adults with suspected coronary artery disease. A number of developments have now expanded the role of exercise testing in youth and have identified the need for more specific approaches to exercise testing for this population of patients.
- Perhaps most importantly, the past several decades have witnessed a dramatic expansion in the scope and nature of patients cared for by pediatric cardiologists. Children with complex forms of congenital heart disease, particularly those characterized by marked unilateral ventricular hypoplasia (hypoplastic left heart syndrome, tricuspid valve atresia), once had little hope for long-term survival. Now, thanks to remarkable progress in surgical techniques, these young patients not only often survive but also live productive and fulfilling lives. The physicians caring for these survivors as they grow toward the adult years are confronted with new issues, such as myocardial dysfunction, stubborn tachyarrhythmia, hypoxemia, and pulmonary hypertension. These problems have required new diagnostic and therapeutic approaches, often using information available in the exercise stress-testing laboratory. Similarly, in patients with lung diseases such as cystic fibrosis, improvements in patient care have successfully extended survival and have at the same time introduced new clinical questions that can be assessed through exercise testing. It is likely that the future will continue to bring steady improvements in the survival of young patients with both cardiac and pulmonary disease that will be paralleled by expanded indications for clinical exercise testing.
- A growing understanding of the pathophysiology of cardiac malformations and factors influencing risk stratification have created a need to expand the information obtained during exercise stress testing in young patients. It is true that many issues can be adequately examined by a limited study involving a traditional bout of progressive exercise accompanied by electrocardiographic monitoring and measurement of blood pressure. Assessment of possible ischemic changes in a child after Kawasaki disease, for example, or determination of blood pressure responses after medical treatment of a hypertensive young wrestler could be adequately performed using this approach.
However, the clinical insights gained from exercise testing can be improved by the measurement of gas exchange variables, which are now readily obtained with user-friendly commercial metabolic systems. The changes in the oxygen and carbon dioxide content of expired air during exercise reflects similar gas exchange dynamics at the cellular level. With this approach, for example, the measurement of O2max provides an objective physiological assessment of aerobic fitness, and the determination of ventilatory variables (minute ventilation, CO2) offers insights into pulmonary responses as well. As will be outlined in the chapters that follow, it is often the calculation of relationships between these variables that provides clues into the relative importance of cardiac and pulmonary etiologies of exercise limitation.
Termed an "integrative cardiopulmonary test" by Wasserman et al., this approach expands the utility of exercise testing by providing data that can help to answer questions about cardiac and pulmonary issues in youth. In a teenage boy with moderate aortic valve insufficiency, does his cardiac disease explain the shortness of breath that limits his ability to exercise? What mechanisms lie behind a star athlete's inability to perform well after an extended viral illness? Is syncope of an anxious child during running related to hyperventilation? Is breathlessness during exercise in a markedly obese child caused by excess body fat, exercise-induced bronchoconstriction, or cardiac dysfunction? These types of issues are best addressed by a full examination of gas exchange variables during clinical exercise testing.
- A growing recognition of the effects of exercise on electrophysiological function has created new roles for exercise testing in youth. Assessment of changes in ventricular ectopy during exercise is a traditional indication for exercise testing. Newer indications include the use of responses of rate of ventricular repolarization (QT interval) and conduction down accessory pathways (WPW syndrome) during exercise as means of patient risk stratification.
- The increased use of pediatric exercise testing has also been stimulated by the concerns of parents, coaches, and physical education instructors over the occurrence of symptoms of chest pain, dizziness, syncope, or palpitations in young people during exercise. Such concerns have been fueled by the tragic occurrences of sudden unexpected death of young, presumably healthy athletes during sports training or competition. While such symptoms are highly unlikely to reflect the rare diseases that pose a risk of sudden death, the youngster with occult hypertrophic cardiomyopathy, coronary artery anomalies, or repolarization abnormalities that can predispose to fatal dysrhythmias can present with such complaints. Findings on exercise testing have thus become part of the assessment of symptomatic children and athletes to rule out these anomalies.
- A normal exercise test can provide clearance for sports play in young patients with heart disease or in those who have suffered illnesses such as viral myocarditis. Exercise testing also plays a role in assessing risk and exercise capacities in young patients who are enrolled in cardiac and pulmonary rehabilitation programs.
We can expect that clinical exercise testing in children and adolescents will continue to expand as the value of exercise is recognized in the assessment of not only heart and lung disease but also metabolic and musculoskeletal disorders. Such trends will undoubtedly follow improvements in medical and surgical treatment of these patients. We can also expect to see new techniques for performing exercise tests (miniaturization of metabolic systems permitting field testing, for example) and assessing their results (three-dimensional echocardiography, myocardial strain Doppler studies).
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Determining Functional Capacity
In summary, our quest is to determine children’s functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise.
In summary, our quest is to determine children's functional capacity so they can engage in physical activity for all of the benefits exercise provides. We want to be safe and prudent in our recommendation for increasing activity, certainly when a child has a condition that may limit the level of exercise. Exercise testing is essential to achieve this goal. Simple measurement of workload on a cycle ergometer will provide information on how close we are to "normal." Peak workload on a cycle ergometer is a measure of very vigorous exercise. The calibration of the cycle is often very good, and the workloads are very consistent over fast and slow pedaling. It is therefore a very useful measure of functional capacity for many subjects of different sizes and levels of physical conditioning. Within-subject reliability is also very good, so we can compare the child's responses over time accurately. Work in watts measured on the cycle ergometer can be used in a calculation to predict the oxygen consumption, again giving us a measure of functional capacity.
Current research indicates that one can predict the peak O2 from the O2 at VAT with good accuracy in healthy children as well as in those with lung disease. Caution should be used when predicting peak O2 from the O2 at VAT in children with congenital heart disease. Metabolic measurements need to be available to calculate the VAT, which adds to the cost and resources needed to predict the O2 from this method. The measurement of VAT becomes more useful when a submaximal effort has limited the exercise test. Age, comprehension, size, motivation, and protocol all factor into a submaximal response. The caveat of the method is when we assume that the O2 at VAT is about 60% of the peak O2. Physical conditioning and specifically a lack of physical fitness influence this method greatly, and this may lead to over- or underestimation of peak O2. We know that physical deconditioning, either from being sedentary or as the result of a disease process, lowers the O2 at the VAT.
OUES is a useful submaximal measure of aerobic fitness that is effort independent. This method also relies on the availability of metabolic measurements. It has been shown to be reproducible and does not suffer the intra-observer or inter-observer discrepancies often encountered with VAT. The OUES is easy to calculate and is determined by many more data points than VAT, peak work, or peak heart rate. The consensus of the research data indicates that it is most useful at the higher percentages of the peak O2 (that is, cut points of OUES 80% and OUES 90%). OUES below 45% to 50% of peak O2 is less reliable and does not correlate as well with peak O2. There is conflicting evidence regarding whether OUES is equally useful in predicting peak O2 in healthy subjects and in those with chronic heart or lung disease. If a subject can achieve 80% or 90% of predicted peak O2, many of us would consider that an adequate level of exertion. If the subject achieves 80% or 90% of peak O2, we do not know if the OUES slope can be abnormal. My guess is probably not. The slope of the line that represents the VE and O2 relationship is strongly influenced by the onset of anaerobic metabolism. The V-slope method of VAT detection is based on the change in that slope. Therefore the choice of the cut points will influence the OUES slope. Another factor that will influence the OUES is the choice of ergometer. We know that the peak O2 on the cycle ergometer is about 10% to 15% less than what is achieved on a treadmill. Therefore percent cut points will actually represent different amounts of O2. This could be problematic if we did not have equations for both the treadmill and cycle ergometer.
The six-minute walk is a simple test that is easy to administer and has good reproducibility. There are two versions, encouraged and unencouraged. The unencouraged version may reduce variability among test administrators. Distance and dyspnea score are recorded. The words and phrases of the Borg scale may not be completely understood by young children. Other scales with pictures have been used to assess the child's perceived exertion. The six-minute walk test may be more useful in underserved areas where metabolic equipment is not available. Also, large groups of children can be tested at very little cost, and over time increases in distance walked and a lower rate of perceived exertion may relate to improved fitness. This topic has been covered in greater detail in chapter 3.
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Case Presentation
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago.
Rebecca is a 17-year-old varsity cross country runner whom you have followed in your pediatric practice since she was a young child. She presents to you today with complaints of multiple episodes of dizziness with exertion, the first event having started approximately 1 yr ago. She describes the dizziness as "spinning," and it occurs not during running but just after she has finished. It usually occurs after running at a high-tempo pace for several miles and usually in the summer months. She denies frank syncope, chest pain, shortness of breath, or palpitations. There was a documented blood pressure of 62/40 at one time when paramedics were called while the event was occurring. She has been seen in the emergency room twice for the complaint; the most recent was 2 d ago. In the emergency room, she had a normal heart rate and blood pressure, normal physical examination, and normal laboratory values for anemia and thyroid. Her chest radiograph and electrocardiogram (ECG) were unremarkable. She has been restricted from exercise until she is seen by you. She is requesting exercise clearance.
Turning back to our case study, on further questioning it is revealed that Rebecca drinks 20 oz of water daily and one cup of coffee in the morning. She often skips breakfast as well because she does not have time to eat. She sometimes experiences dizziness in the morning when she stands up while getting out of bed. Rebecca's physical examination included orthostatic vital signs. Her heart rate increased from 52 bpm supine to 90 bpm in the upright position. Her blood pressure decreased from 116/68 to 108/64 from supine to upright position, respectively. Her physical examination was otherwise unremarkable.
Rebecca had an ECG performed, which was normal. Although her symptoms were classic for dehydration as well as a vasovagal component, because she was a competitive athlete, the cardiologist opted for an echocardiogram, which was normal, and an exercise stress test to try to induce her symptoms.
Rebecca performed a maximal treadmill exercise stress test. She had a high-normal aerobic capacity. There were no arrhythmias or ischemic changes. There was a normal heart rate and blood pressure response to exercise. She developed dizziness in early recovery with a 30 mmHg drop in her blood pressure. After she drank a cup of water and was placed in the supine position, her blood pressure improved within 2 min.
For Rebecca, her history, physical examination, and ECG were reassuring that this was unlikely to be cardiac-related exertional syncope. Her exercise stress test was able to document dizziness associated with a drop in blood pressure that responded to fluids and supine positioning. She was counseled on improving her hydration and salt intake and not to skip meals. She returned for a follow-up visit 3 mo later and had not experienced any further dizziness or syncope.
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Unique Features of Exercise Testing in Children
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults.
The approach to clinical exercise testing of children and adolescents differs from that of the laboratory dedicated to testing adults. One need only consider the various approaches needed to perform a satisfactory exercise test first in a 15-year-old cross country runner who experienced precordial chest pain in her last race, followed by a test looking for heart rate response in a 5-year-old youngster with complete heart block, and then a 12-year-old obese boy with a dilated cardiomyopathy and progressively worsening shortness of breath with exercise.
The pediatric exercise testing laboratory must accommodate wide variations in patient age, size, and fitness, and that means that testing protocols and equipment must be similarly adjusted. The indications for testing are much broader than those in the adult lab, so the questions to be answered must be carefully considered before the exercise begins.
Most children can be easily motivated to give exhaustive efforts during exercise testing, but it requires charismatic skill from staff members experienced with the emotional and physical responses of children during treadmill or cycle exercise. It has been said that perhaps the single most important factor in a successful exercise test in the pediatric laboratory is the staff administering the test.
Pediatric exercise testing, then, is distinguished by the need for a creative approach to each patient. The staff must know what information is needed to address the clinical question being asked, the proper modality - cycle or treadmill - to obtain that answer, and the optimal protocol for the subject's age and fitness level.
The physiological mechanisms underlying the cardiac and pulmonary responses to a bout of progressive exercise are no different in children (at least those over age 6) and adults. Nonetheless, certain quantitative measurements (heart rate, blood pressure, endurance time) are different in children, and these must be recognized in the testing of immature subjects.
For example, resting and maximal heart rates in an exhaustive exercise test are greater in children than in adults. As will be discussed in chapter 5, peak heart rate depends on testing modality and protocol, and there is considerable variability between individuals. It is important to recognize that there is generally no specific "target heart rate" for an exercise test. Importantly, too, the maximal heart rate during exercise testing in a given subject does not change over the course of childhood. Only at about age 16 does this value begin to decline. Thus, age-related formulae for predicting a maximal heart rate, such as "220 minus age," do not apply to youths.
The concept of metabolic equivalents, or METs, as a measure of energy expenditure during exercise is commonly used with adult subjects but is fraught with difficulty in children and adolescents, and it is best avoided in the pediatric exercise laboratory. METs is a means of expressing the oxygen requirement of a physical activity relative to an assumed resting value. One MET, or resting O2, in an adult is considered to be 3.5 ml ∙ kg-1 ∙ min-1; thus, when walking on a treadmill at a certain speed and slope that is expected to demand 17.5 ml ∙ kg-1 ∙ min-1, a patient is exercising at a level of 5 METs.
The difficulty with this concept in youth is that resting energy expenditure is not constant but evolves throughout childhood during physical growth. As would be expected, absolute values of resting O2 rise with the accrual of body mass. When adjusted for body mass, or body surface area, basal or resting values of energy expenditure decline progressively during the pediatric years. When expressed as calories per meter of square body surface area per hour, the basal metabolic rate declines by about 20% between the ages of six and the mid-teen years.
In considering a mass-relative definition of a MET in children, the story is more exaggerated. Harrell et al. reported that the resting O2 per kg in a group of 8- to 12-year-old children was almost 50% higher than that of 16- to 18-year-old subjects and 70% higher than that expected in adults. The use of the MET as defined in adults as a "currency" or multiplier of energy expenditure in children, then, clearly would introduce large errors in defining O2 levels during exercise - and the extent of the error would be different depending on the age and size of the child.
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