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 during exercise attenuates the degree of decline in hypoxic ventilatory sensitivity which occurs during sustained hypoxia at rest. The acute ventilatory response to hypoxia (AHVR) was used as a measure of the hypoxic ventilatory chemoreflex sensitivity. Seven subjects undertook three protocols. Protocol A was designed to assess the reduction in AHVR as a result of 20 min of isocapnic hypoxia (end-tidal PO2 50 mm Hg) at rest. The first AHVR (control) was measured on exposure to the hypoxia, and the second AHVR (test) measured 6 min after the end of the hypoxic period. Protocols B and C were designed to assess the reduction in 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 PO2 55 mm Hg) exercise. In protocol C, the AHVR (control) as measured at rest, 6 min after the end of 20 min of euoxic (end-tidal PO2 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 hypoxic exercise was significantly less than following hypoxia at rest (p < 0.05; paired t test). We conclude that the decline in hypoxic chemoreflex sensitivity which occurs during sustained hypoxia at rest is genuinely attenuated as a result of exercise.