Abstract
The effects of acute inhalation of hypoxic gas mixtures on minute ventilation (VE), respiratory frequency (fR) and heart rate (HR) were studied in healthy subjects executing constant-load 100 W and 150 W hindlimb exercises (protocol 1) or unloaded (0 W) cycling (protocol 2). Attention was focussed on early changes in variables during phase I of constant load exercise, a period where neurogenic afferents from working muscles play a key role in adaptative cardiorespiratory response as they did also during 0 W cycling. In protocol 1, a 15 % O 2 gas mixture was used while in protocol 2, 15 % and 10 % O 2 mixtures were tested. Compared to the variations of cardiorespiratory variables measured during room air breathing (normoxia), hypoxemia significantly and markedly depressed the rates of VE and fR changes during phase I exercise but did not affect the changes in HR. Reduced phase I ventilatory response was not accompanied by significant variations in rest values of PaCO 2 and pHa associated with the response to hypoxia. The cardiorespiratory response to 0 W cycling was also lowered under hypoxemic conditions, the magnitude of VE and HR changes being inversely proportional to the fall in PaO 2 level. Based on electrophysiological animal observations, the present results may be interpreted in terms of inhibitory influences of hypoxemia on proprioceptive muscle afferents.