THE EFFECT OF EXERCISE ON THE DEVELOPMENT OF RESPIRATORY DEPRESSION DURING SUSTAINED ISOCAPNIC HYPOXIA IN HUMANS

The purpose of this study was to examine whether sustained hypoxia dur ing exercise attenuates the degree of decline in hypoxic ventilatory s ensitivity which occurs during sustained hypoxia at rest. The acute ve ntilatory response to hypoxia (AHVR) was used as a measure of the hypo xic ventilatory chemoreflex sensitivity. Seven subjects undertook thre e protocols. Protocol A was designed to assess the reduction in AHVR a s a result of 20 min of isocapnic hypoxia (end-tidal P-O?2 50 mm Hg) a t rest. The first AHVR (control) was measured on exposure to the hypox ia, and the second AHVR (test) measured 6 min after the end of the hyp oxic period. Protocols B and C were designed to assess the reduction i n AHVR as a result of 20 min of isocapnic hypoxic exercise (70 W). In protocol B, the AHVR (test) was measured at rest, 6 min after the end of 20 min of isocapnic hypoxic (end-tidal P-O?2 55 mm Hg) exercise. In protocol C, the AHVR (control) was measured at rest, 6 min after the end of 20 min of euoxic (end-tidal P-O?2 100 mm-Hg) isocapnic exercise . There was a 30 +/- 5% decline (mean +/- SEM) in the magnitude of the AHVR after the period of sustained hypoxia at rest. There was an 11 /- 7% decline in the magnitude of the resting AHVR after the period of sustained hypoxic exercise. The percentage change in AHVR following hy poxic exercise was significantly less than following hypoxia at rest ( p < 0.05; paired t test). We conclude that the decline in hypoxic chem oreflex sensitivity which occurs during sustained hypoxia at rest is g enuinely attenuated as a result of exercise.