The effects of breathing a helium–oxygen gas mixture on maximal pulmonary ventilation and maximal oxygen consumption during exercise in acute moderate hypobaric hypoxia

Abstract

To test the hypothesis that maximal exercise pulmonary ventilation ($\dot{V}{\text{E}}_{ \max }$) is a limiting factor affecting maximal oxygen uptake ($\dot{V}{\text{O}}_{{ 2 {\text{max}}}}$) in moderate hypobaric hypoxia (H), we examined the effect of breathing a helium–oxygen gas mixture (He–O₂; 20.9% O₂), which would reduce air density and would be expected to increase $\dot{V}{\text{E}}_{ \max }$. Fourteen healthy young male subjects performed incremental treadmill running tests to exhaustion in normobaric normoxia (N; sea level) and in H (atmospheric pressure equivalent to 2,500 m above sea level). These exercise tests were carried out under three conditions [H with He–O₂, H with normal air and N] in random order. $\dot{V}{\text{O}}_{{ 2 {\text{max}}}}$ and arterial oxy-hemoglobin saturation (SaO₂) were, respectively, 15.2, 7.5 and 4.0% higher (all p < 0.05) with He–O₂ than with normal air ($\dot{V}{\text{E}}_{ \max }$ _, 171.9 ± 16.1 vs. 150.1 ± 16.9 L/min; $\dot{V}{\text{O}}_{{ 2 {\text{max}}}}$, 52.50 ± 9.13 vs. 48.72 ± 5.35 mL/kg/min; arterial oxyhemoglobin saturation (SaO₂), 79 ± 3 vs. 76 ± 3%). There was a linear relationship between the increment in $\dot{V}{\text{E}}_{ \max }$ and the increment in $\dot{V}{\text{O}}_{{ 2 {\text{max}}}}$ in H (r = 0.77; p < 0.05). When subjects were divided into two groups based on their $\dot{V}{\text{O}}_{{ 2 {\text{max}}}}$, both groups showed increased $\dot{V}{\text{E}}_{ \max }$ and SaO₂ in H with He–O₂, but $\dot{V}{\text{O}}_{{ 2 {\text{max}}}}$ was increased only in the high $\dot{V}{\text{O}}_{{ 2 {\text{max}}}}$ group. These findings suggest that in acute moderate hypobaric hypoxia, air-flow resistance can be a limiting factor affecting $\dot{V}{\text{E}}_{ \max }$; consequently, $\dot{V}{\text{O}}_{{ 2 {\text{max}}}}$ is limited in part by $\dot{V}{\text{E}}_{ \max }$, especially in subjects with high $\dot{V}{\text{O}}_{{ 2 {\text{max}}}}$.