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 vent ilation (V over dot E), respiratory frequency (fR) and heart rate (HR) were studied in healthy subjects executing constant-load 100 W and 15 0 W hindlimb exercises (protocol 1) or unloaded (0 W) cycling (protoco l 2). Attention was focussed on early changes in variables during phas e I of constant load exercise, a period where neurogenic afferents fro m tool-king muscles play a key role in adaptative cardiorespiratory re sponse 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 w ere tested. Compared to the variations of cardiorespiratory variables measured during room air breathing (normoxia), hypoxemia significantly and markedly depressed the rates of V over dot E and fR changes durin g 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 hypoxi a. The cardiorespiratory response to 0 TY cycling was also lowered und er hypoxemic conditions, the magnitude of V over dot E and HR changes being inversely proportional to the fall in PaO2 level. Based on elect rophysiological animal observations, the present results may be interp reted in terms of inhibitory influences of hypoxemia on proprioceptive muscle afferents.