Clinical Exercise Testing
This is an excerpt from Cardiopulmonary Exercise Testing in Children and Adolescents by Thomas Rowland, American College of Sports Medicine & North American Society for Pediatric Exercise Medicine (NASPEM).
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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