Sex differences in exercise-induced flow limitation in prepubescent children: prevalence and implications

Abstract

In comparison to adults and adolescents, relatively little is known about ventilatory responses of prepubescent children to exercise. Children have smaller airways relative to lung size than adults and ventilate "out of proportion" to metabolic demands of exercise which may render them more susceptible to ventilatory limitations during exercise. It is also not known if there are any sex differences in ventilatory limitations during incremental exercise in children. Therefore, the purpose of this study was to determine the prevalence of ventilatory constraints (expiratory flow limitation, EFL) during incremental exercise to exhaustion in prepubescent (Tanner stage 1) boys and girls. Forty healthy, prepubescent boys (n = 20) and girls (n = 20) with no history of asthma completed baseline pulmonary function and lung volume tests. Subjects then completed an incremental cycle VO[subscript]2max test where workload was increased 10W every 90 sec until exhaustion. RPE, dyspnea ratings, and % EFL were recorded at the end of each exercise stage. EFL was determined by placing the exercising tidal volume loop inside a post-exercise maximal flow volume envelope. Ventilatory and metabolic data were recorded on a breath by breath basis throughout exercise via a metabolic cart. Arterial oxygen saturation was determined via pulse oximetry. Body composition was determined using dual-energy x-ray absorptiometry. Following 15 minutes of recovery, subjects exercised at 105% of their VO[subscript]2max workload until exhaustion to provide confident in the VO[subscript]2max measurement. There were no differences (p>0.05) in anthropometric measures (height, weight) or body composition (lean body mass, percent body fat) measures between boys and girls. At rest, boys had significantly higher lung volumes (TLC, boys = 2.6 + 0.5 L, girls = 2.1 + 0.5 L; FRC, boys = 0.9 + 0.3 L, girls = 0.7 + 0.3 L) and maximal flows (FVC, boys = 2.2 + 0.3 L, girls = 1.9 + 0.4 L; PEF, boys = 3.6 + 0.7 L/sec, girls = 2.9 + 0.6 L/sec; FEV1, boys = 1.9 + 0.2 L/sec, girls = 1.6 + 0.3 L/sec). At maximal exercise, boys had significantly higher VO[subscript]2max (boys = 35.4 + 7.5 ml/kg/min, girls = 29.5 + 6.6 ml/kg/min; boys = 1.2 + 0.2 L/min, girls = 1.0 + 0.2 L/min), VE (boys = 49.8 + 8.8 L/min, girls = 41.2 U+U 8.3 L/min), and VCO[subscript]2 (boys = 1.2 + 0.2 L/min, girls = 0.9 + 0.2 L/min) compared to girls. There were no differences (p>0.05) in VE/VCO2 (boys = 41.1 + 3.9, girls = 43.4 + 5.5), PETCO2 (boys = 35.5 + 2.5 mmHg, girls = 35.7 + 3.2 mmHg) maximal HR (boys = 174.4 + 23.1 bpm; girls = 183.4 + 16.6 bpm), RER (boys = 1.04 + 0.05, 1.03 + 0.08), or SaO2 (boys = 96.7 + 3.4%, girls = 97.7 + 1.3%) which was maintained within 3% of baseline throughout exercise for all subjects. EFL during exercise was present in 19 of 20 boys and 18 of 20 girls. Severity of EFL at VO2max, as judged by % overlap of tidal volume with maximal flow volume envelope, was not different between genders at any time during exercise (at VO[subscript]2max: boys = 58 + 7%, girls = 43 + 8%). There was no significant association between % EFL at VO[subscript]2max and aerobic capacity or total lung volume. A significant relationship existed between % EFL at VO[subscript]2max and the change in end-expiratory lung volume from rest to maximal exercise in boys (r = 0.77) and girls (r = 0.75). In summary, our data suggests that ventilatory constraints in the form of expiratory flow limitation are highly and equally prevalent in prepubescent boys and girls from moderate to maximal exercise which likely leads to an increased work of breathing, but not to decreases in arterial oxygen saturation.