The decrease of maximal oxygen consumption during hypoxia in man: A mirror image of the oxygen equilibrium curve

1. Endurance athletes (E) undergo a marked reduction of arterial O2 saturation (S(a,O2) at maximal exercise in normoxia, which disappears when they breathe hyperoxic mixtures. In addition, at a given level of hypoxia, the drop in maximal O2 consumption (V̇(O2)) is positively related to the individual normoxic V̇(O2,max). 2. These data suggest that the curve relating V̇(O2,max) to P(I,O2) may be steeper and perhaps less curved in E than in sedentary subjects (S) with low V̇(O2,max) values because of the greater hypoxaemia in the latter, whence the hypotheses that (i) the relationship between V̇(O2,max) and P(I,O2) may be set by the shape of the oxygen equilibrium curve; and (ii) the differences between E and S may be due to the different position on the oxygen equilibrium curve on which these subjects operate. These hypotheses have been tested by performing a systematic comparison of the V̇(O2,max) or S(a,O2) vs P(I,O2) relationships in E and S. 3. On ten subjects (five S and five E), V̇(O2,max) was measured by standard procedure during cyclo-ergometric exercise. S(a,O2) was measured by finger-tip infrared oximetry. Arterialized blood P(O2) (P(a,O2)) and P(CO2) (P(a,CO2)) were determined in 80 μl blood samples from an ear lobe. The subjects breathed ambient air or a N2-O2 mixture with an inspired O2 fraction (F(I,O2)) of 0.30) 0.18, 016, 013 and 0.11, respectivey. V̇(O2,max) was normalized with respect to that obtained at the highest F(I,O2). 4. The relationships between S(a,O2) or normalized V̇(O2,max) and F(I,O2) (or P(I,O2)) had similar shapes, the data for E being systematically below and significantly different from those for S. Linear relationships between S(a,O2) and normalized V̇(O2,max), statistically equal between E and S, were found. 5. We conclude that the relationships between either V̇(O2,max) or S(a,O2) and F(I,O2) (or P(a,O2)) may indeed be the mirror images of one another, implying a strict link between the decrease of V̇(O2,max) in hypoxia and the shape of the oxygen equilibrium curve, as hypothesized.