Determinants of maximal oxygen uptake in moderate acute hypoxia in endurance athletes

The factors determining maximal oxygen consumption were explored in eight endurance trained subjects (TS) and eight untrained subjects (US) exposed to moderate acute normobaric hypoxia. Subjects performed maximal incremental tests at sea level and simulated altitudes (1,000, 2,500, 4,500 m). Heart rate (HR), stroke volume (SV), cardiac output \({(\dot{{Q}})},\) arterialized oxygen saturation \({(\hbox{Sa}^{\prime}\hbox{O}_2)},\) oxygen uptake \({(\dot{{V}}\hbox{O}_{\rm 2max})},\) ventilation (\({\dot{{V}}{E}},\) expressed in normobaric conditions) were measured. At maximal exercise, ventilatory equivalent \({(\dot{{V}}{E}/\dot{{V}}\hbox{O}_{\rm 2max}),\, \hbox{O}_2}\) transport \({(\dot{{Q}}\hbox{aO}_{\rm 2max})}\) and O₂ extraction (O₂ER_max) were calculated. In TS, \({\dot{{Q}}_{\rm max}}\) remained unchanged despite a significant reduction in \({\hbox{HR}_{\rm max}}\) at 4,500 m. SV_max remained unchanged. \({\dot{{V}}{E}_{\rm max}}\) decreased in TS at 4,500 m, \({\dot{{V}}{E}/\dot{{V}}\hbox{O}_{\rm 2max}}\) was lower in TS and greater at 4,500 m vs. sea level in both groups. Sa′O_2max decreased at and above 1,000 m in TS and 2,500 m in US, O₂ER_max increased at 4,500 m in both groups. \({\dot{{Q}}\hbox{aO}_{\rm 2max}}\) decreased with altitude and was greater in TS than US up to 2,500 m but not at 4,500 m. \({\dot{{V}}\hbox{O}_{\rm 2max}}\) decreased with altitude but the decrement \({(\Delta \dot{{V}}\hbox{O}_{\rm 2max})}\) was larger in TS at 4,500 m. In both groups \({\Delta \dot{{V}}\hbox{O}_{\rm 2max}}\) in moderate hypoxia was correlated with \({\Delta \dot{{Q}}\hbox{aO}_{\rm 2max}}.\) Several differences between the two groups are probably responsible for the greater \({\Delta \dot{{V}}\hbox{O}_{\rm 2max}}\) in TS at 4,500 m : (1) the relative hypoventilation in TS as shown by the decrement in \({\dot{{V}}{E}_{\rm max}}\) at 4,500 m (2) the greater \({\dot{{Q}}\hbox{aO}_{\rm 2max}}\) decrement in TS due to a lower Sa′O_2max and unchanged \({\dot{{Q}}_{\rm max}}\) 3) the smaller increase in O₂ER_max in TS, insufficient to compensate the decrease in \({\dot{{Q}}a\hbox{O}_{\rm 2max}}.\)