Gas exchange during exercise in habitually active asthmatic subjects

Abstract

We determined the relations among gas exchange, breathing mechanics, and airway inflammation during moderate- to maximum-intensity exercise in asthmatic subjects. Twenty-one habitually active (48.2 ± 7.0 ml·kg−1·min−1 maximal O2 uptake) mildly to moderately asthmatic subjects (94 ± 13% predicted forced expiratory volume in 1.0 s) performed treadmill exercise to exhaustion (11.2 ± 0.15 min) at ∼90% of maximal O2 uptake. Arterial O2 saturation decreased to ≤94% during the exercise in 8 of 21 subjects, in large part as a result of a decrease in arterial Po2 (PaO2): from 93.0 ± 7.7 to 79.7 ± 4.0 Torr. A widened alveolar-to-arterial Po2 difference and the magnitude of the ventilatory response contributed approximately equally to the decrease in PaO2 during exercise. Airflow limitation and airway inflammation at baseline did not correlate with exercise gas exchange, but an exercise-induced increase in sputum histamine levels correlated with exercise PaO2 (negatively) and alveolar-to-arterial Po2 difference (positively). Mean pulmonary resistance was high during exercise (3.4 ± 1.2 cmH2O·l−1·s) and did not increase throughout exercise. Expiratory flow limitation occurred in 19 of 21 subjects, averaging 43 ± 35% of tidal volume near end exercise, and end-expiratory lung volume rose progressively to 0.25 ± 0.47 liter greater than resting end-expiratory lung volume at exhaustion. These mechanical constraints to ventilation contributed to a heterogeneous and frequently insufficient ventilatory response; arterial Pco2 was 30–47 Torr at end exercise. Thus pulmonary gas exchange is impaired during high-intensity exercise in a significant number of habitually active asthmatic subjects because of high airway resistance and, possibly, a deleterious effect of exercise-induced airway inflammation on gas exchange efficiency.