This is an excerpt from Cardiopulmonary Exercise Testing in Children and Adolescents.
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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