Acute hypoxemia depresses the cardiorespiratory response during phase I constant load exercise and unloaded cycling

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% O2 gas mixture was used while in protocol 2, 15% and 10% O2 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 PaCO2 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 PaO2 level. Based on electrophysiological animal observations, the present results may be interpreted in terms of inhibitory influences of hypoxemia on proprioceptive muscle afferents.