cardiac output, working capacity and aerobic capacity can be preserved by several mechanisms Exercising muscles can extract a higher percentage of delivered oxygen, widening the arteriovenous gradient of oxygen A higher amount of work can also be performed anaerobically Assessment of the increase in cardiac output with exercise may identify and help to better define the degree of limited cardiovascular reserve in the presence of these compensatory mechanisms Data on the response of cardiac output in healthy children are less available than that on aerobic capacity Most healthy children increase resting cardiac output approximately fourfold during progressive exercise The maximal cardiac output with exercise is less in preadolescents compared with adolescents and adults when normalized for body surface area,32,42 primarily because of lower stroke volume Noninvasive measurement of cardiac output is one of the less reliable measurements routinely made in a pediatric exercise laboratory Variations of at least 10% from test to test have been found in individual subjects.42 The reasons for such variation are multiple and reflect inherent inaccuracies in the methodology, technical difficulties, and variation from test to test within individual patients Normal values for exercise cardiac output are presented in Table 23.4 Table 23.4 Cardiac Output With Exercise Age 11–13 years 13–14 years 32–58 years, men supine 32–58 years, men sitting Resting Cardiac Index 3.9 L/min/m2 5.25 L/min/m2 3.5 L/min/m2 2.8 L/min/m2 Exercise Cardiac Index 12.5 L/min/m2 17.4 L/min/m2 7.7 L/min/m2 7.3 L/min/m2 While pediatric data are at maximal exercise, adult exercise data reflect values during effortful work on a cycle ergometer sustained for 4 to 6 minutes (average work rate ~100W, HR 128/146 beats/min for supine/sitting, respectively).77 Modified from Eriksson BO, Koch G Cardiac output and intraarterial blood pressure at rest and during submaximal and maximal exercise in 11- to 13-year-old boys before and after physical training In: Bar-Or O, Natanya I, eds, Pediatric Work Physiology Netanya: Wingate Institute; 1973:139–150; and Eriksson BO, Grimby G, Saltin B Cardiac output and arterial blood gasses during exercise in pubertal boys J Appl Physiol 1971;31:348–352 Pulmonary Function Measurements of pulmonary function with exercise are often abnormal in children and adults with structural heart defects.67,68,78,79 The causes of these abnormalities are not always entirely clear and are often multifactorial Defects involving abnormalities of the pulmonary vasculature, such as seen in patients with tetralogy of Fallot, appear to be more commonly associated with abnormal pulmonary mechanics In these patients, a restrictive pattern of breathing, with decreased forced vital capacity and tidal volume, is commonly observed on resting spirometry.67 This compels a higher frequency of breathing during exercise to maintain adequate minute ventilation Multiple thoracotomies resulting in formation of scar tissue may be one cause of these findings These patterns have also been observed in children who have had a single surgical intervention.67 Residual lesions, such as significant pulmonary regurgitation, appear to be associated with increased ventilatory abnormalities In some cases, there is evidence that developmental abnormalities of the airway may be an intrinsic part of this particular cardiac abnormality.67,80 Although the key mechanisms remain unclear, low forced vital capacity appears to be independently associated with lower aerobic exercise capacity and even mortality.81–84 Hyperresponsive airways (obstructive spirometry) are not especially common among patients with congenital heart disease, with the exception of tetralogy of Fallot and pulmonary atresia There are age-related changes that should be considered in the assessment of the patterns of ventilation of children and adolescents Younger children normally have more inefficient patterns, with lower tidal volumes and higher frequencies of breathing than adolescents and adults.85 Thus children tend to overventilate or hyperventilate relative to adults This inefficiency results in a higher ratio of physiologic dead space to tidal volume and should not be misinterpreted as evidence of a pulmonary abnormality Most pulmonary abnormalities do not limit exercise performance in children with congenitally malformed hearts In many cases the pulmonary abnormalities are relatively mild, and the exercising subject compensates by changing the pattern of breathing In addition, many children with congenitally malformed hearts have impairment in their cardiovascular system that limits their exercise capacity before decreased pulmonary reserve becomes a factor in exercise performance Occasionally, a child is limited by pulmonary performance Treatment directed at improving their pulmonary function may then result in improved exercise performance Response of Blood Pressure Systolic blood pressure rises during exercise Diastolic blood pressure generally falls or is unchanged Age, gender, and racial differences in blood pressure are found in the general population Therefore using appropriate normal values is essential when assessing blood pressure response As a general rule, with the exception of larger adolescent males, it is uncommon for systolic blood pressure to exceed 200 mm Hg in healthy children In the evaluation of children with suspected hypertension and a structurally normal heart, systolic blood pressure more than 250 mm Hg is usually grounds for terminating the exercise test.32 Abnormal blood pressure responses during exercise are seen with certain cardiac lesions A blunted rise or a fall in systolic blood pressure may indicate severe obstruction of the left ventricular outflow tract, or pulmonary vascular disease.32 Elevated systolic blood pressure has been observed with residual coarctation of the aorta and with abnormalities of the sympathetic-adrenal axis following successful repair of coarctation.86 Electrocardiographic Data Complete electrocardiographic data are routinely obtained during testing of patients with structural cardiac disease Evidence of chronotropic impairment, exercise-induced arrhythmias, and changes suggestive of myocardial ischemia are also routinely assessed We will discuss the latter in more detail in the subsequent section Impairment of the response of the heart rate to exercise limits augmentation of cardiac output Chronotropic impairment is common after operative repairs that require transection or partial disruption of the great arteries such the atrial switch and Ross operation as complex operations such as the atrial switch, Fontan operations68,78 as well as following surgical repair of tetralogy of Fallot.67 Often, it is difficult to determine the effect that mild chronotropic impairment will have on aerobic performance given the potential for compensation via stroke volume or increased arteriovenous oxygen content difference Marked chronotropic impairment invariably results in decreased aerobic capacity Evaluations of arrhythmias include identification of the source, be it supraventricular or ventricular, and the response of the disturbance to increasing exercise effort The occurrence of new or potentially malignant arrhythmias, such as premature ventricular depolarizations that degenerate into ventricular