Oxygen delivery and oxygen return in humans exercising in acute normobaric hypoxia

Abstract.

At a given steady O₂ consumption ($\dot V{\rm O}_{\rm 2}$ ) in normoxia, cardiac output ($\dot Q$ ) is inversely proportional to arterial O₂ concentration (CaO₂), so that O₂ delivery ($\dot Q{\rm aO}_{\rm 2} {\rm = }\dot Q\,C{\rm aO}_{\rm 2}$ ) is kept constant and adapted to $\dot V{\rm O}_{\rm 2}$ . The matching between $\dot Q{\rm aO}_{\rm 2}$ and $\dot V{\rm O}_{\rm 2}$ keeps O₂ return ($\dot Q\overline {\rm v} {\rm O}_{\rm 2} = \dot Q{\rm aO}_{\rm 2} - \dot V{\rm O}_{\rm 2}$ ) constant and independent of $\dot V{\rm O}_{\rm 2}$ and haemoglobin concentration ([Hb]). This may not be so in hypoxia: in order for $\dot Q\overline {\rm v} {\rm O}_{\rm 2}$ to be independent of the inspired O₂ fractions (FIO₂), the slopes of the $\dot Q$ versus $\dot V{\rm O}_{\rm 2}$ lines should be greater the lower the CaO₂, which may not be the case. Thus, we tested the hypothesis of constant $\dot Q\overline {\rm v} {\rm O}_{\rm 2}$ by determining $\dot Q{\rm aO}_{\rm 2}$ and $\dot Q\overline {\rm v} {\rm O}_{\rm 2}$ in acute hypoxia. Thirteen subjects performed steady-state submaximal exercise on the cycle ergometer at 30, 60, 90 and 120 W breathing FIO₂ of 0.21, 0.16, 0.13, 0.11 and 0.09. $\dot V{\rm O}_{\rm 2}$ was measured by a metabolic cart, $\dot Q$ by CO₂ rebreathing, [Hb] by a photometric technique and arterial O₂ saturation (SaO₂) by infrared oximetry. CaO₂ was calculated from [Hb], SaO₂ and the O₂ binding coefficient of haemoglobin. The $\dot V{\rm O}_{\rm 2}$ versus power relation was independent of FIO₂. The relations between $\dot Q$ and $\dot V{\rm O}_{\rm 2}$ were displaced upward and had higher slopes in hypoxia than in normoxia. However, the $\dot Q$ changes did not compensate for those in CaO₂. The slopes of the $\dot Q{\rm aO}_{\rm 2}$ versus $\dot V{\rm O}_{\rm 2}$ lines tended to decrease in hypoxia. $\dot Q\overline {\rm v} {\rm O}_{\rm 2}$ was lower the lower the FIO₂. A significant relationship was found between $\dot Q\overline {\rm v} {\rm O}_{\rm 2}$ and SaO₂ ($\dot Q\overline {\rm v} {\rm O}_{\rm 2}$ =1.442 SaO₂+0.107, r=0.871, n=24, P<10^–7), which confutes the hypothesis of constant $\dot Q\overline {\rm v} {\rm O}_{\rm 2}$ in hypoxia.