The Effect of Exercise on the Development of Respiratory Depression during Sustained Isocapnic Hypoxia in Humans

Abstract

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) was 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.

Controversies in the physiological basis of the ‘anaerobic threshold’ and their implications for clinical cardiopulmonary exercise testing

“…In cardiology, it has been recognised that recovery of heart rate after exercise is an independent predictor of survival [81], and respiratory measurements made in the post‐exercise period are insightful of control mechanisms in physiology [82]. However, it remains to be explored whether gas exchange kinetics in this phase might predict postoperative outcomes [83].…”

Prior sustained hypoxia attenuates interaction between hypoxia and exercise as ventilatory stimuli in humans

“…In itself, this is an incomplete characterization of the strength of interaction because it is not possible to calculate values for Φ from these data alone. Furthermore, values obtained for the increment in G p with exercise have differed between studies, with percentage increases of 59% (Pandit & Robbins, 1997), 68% (Pandit & Robbins, 1991), 146% (Regensteiner et al 1988) and 245% (Martin et al 1978) reported for work rates broadly similar to those used here. The increase of 88% reported in the present study is well within this range.…”