The Effects of Breathing He–O2 Mixtures on Maximal Oxygen Consumption in Normoxic and Hypoxic Men

Abstract

1 The hypothesis that the ventilatory resistance to O₂ flow (R_V) does limit maximal O₂ consumption (V̇_{O2 max}) in hypoxia, but not in normoxia, at least in non‐athletic subjects, was tested. R_V was reduced by using He–O₂ mixtures. 2 V̇_{O2, max} was measured during graded cyclo‐ergometric exercise in eight men (aged 30 ± 3 years) who breathed N₂–O₂ and He–O₂ mixtures in normoxia (inspired oxygen fraction (FI,O₂= 0.21) and hypoxia (FI,O₂= 0.11). O₂ consumption, expired and alveolar ventilations (V̇_E and V̇_A, respectively), blood lactate and haemoglobin concentrations, heart rate and arterial oxygen saturation (Sa,O₂) were determined at the steady state of each work load. Arterial O₂ and CO₂ partial pressures (Pa,O₂) and Pa,CO₂, respectively) were measured at rest and at the end of the highest work load. 3 Maximal V̇_E and V̇_A were significantly increased by He—O₂ breathing in normoxia (+27 and +18%, respectively), without significant changes in Pa, O₂, Sa, O₂V̇O_{2, max} In hypoxia, V̇_E and V̇_A increased (+31 and +24%, respectively), together with Pa,O₂ (+17%), Sa, O₂ (+6%) and V̇_{O2, max} (+14%). 4 The results support the hypothesis that the role of R_V in limiting V̇_O2,max is negligible in normoxia. In hypoxia, the finding that higher V̇_E and V̇_A values during He–O₂ breathing led to higher V̇_O2,max values suggests a greater role of R_V as a limiting factor. It is unclear whether the finding that the V̇_{O2 max} values were the same during He–O₂ and N₂–O₂ breathing in normoxia is due to a non‐linear response of the O₂ transfer system, as previously proposed.