The effects of breathing He-O-2 mixtures on maximal oxygen consumption in normoxic and hypoxic men

1. The hypothesis that the ventilatory resistance to O-2 flow (R-V) does limit maximal O-2 consumption (V over dot(O2.max)) in hypoxia, but not in normoxia, at least in non-athletic subjects, was tested. R, was reduced by using He-O-2 mixtures. 2. V over dot(O2.max) was measured during graded cyclo-ergometric exercise in eight men (aged 30 +/- 3 years) who breathed N-2-O-2 and He-O-2 mixtures in normoxia (inspired oxygen fraction (F-I,F-O2) = 0.21) and hypoxia (F-I,F-O2 = 0.11). O-2 consumption, expired and alveolar ventilations (V over dot(E) and V over dot(A), respectively), blood lactate and haemoglobin concentrations, heart rate and arterial oxygen saturation (S-a,S-O2) were determined at the steady state of each work load. Arterial O-2 and CO2 partial pressures (P-a,P-O2 and P-a,P-CO2, respectively) were measured at rest and at the end of the highest work load. 3. Maximal V over dot(E) and V over dot were significantly increased by He-O-2 breathing in normoxia (+27 and +18%, respectively), without significant changes in P-a,P-O2, S-a,S-O2 and V over dot(O2,max). In hypoxia, V over dot(E) and V over dot(A) increased (+31 and +24%, respectively), together with P-a,P-O2 (+17%) S-a,S-O2(+6%) and V over dot(O2,max)(+14%). 4. The results support the hypothesis that the role of R-V in limiting V over dot(O2,max) is negligible in normoxia. In hypoxia, the finding that higher V over dot(E) and V over dot(A) values during He-O-2 breathing led to higher V over dot(O2,max) values suggests a greater role of R-V as a limiting factor. It is unclear whether the finding that the V over dot values were the same during He-O-2 and N-2-O-2 breathing in normoxia is due to a non-linear response of the O-2 transfer system, as previously proposed.