Contribution of oxygen extraction fraction to maximal oxygen uptake in healthy young men

Abstract

We analysed the importance of systemic and peripheral arteriovenous O₂ difference ( $a\text{-}\overline{v}{O}_2$ difference and a-v_f O₂ difference, respectively) and O₂ extraction fraction for maximal oxygen uptake ( ${\dot{V}O}_{\text{2max}}$ ). Fick law of diffusion and the Piiper and Scheid model were applied to investigate whether diffusion versus perfusion limitations vary with ${\dot{V}O}_{\text{2max}}$ . Articles (n = 17) publishing individual data (n = 154) on ${\dot{V}O}_{\text{2max}}$ , maximal cardiac output ( ${\dot{Q}}_{max}$ ; indicator-dilution or the Fick method), $a\text{-}\overline{v}{O}_2$ difference (catheters or the Fick equation) and systemic O₂ extraction fraction were identified. For the peripheral responses, group-mean data (articles: n = 27; subjects: n = 234) on leg blood flow (LBF; thermodilution), a-v_f O₂ difference and O₂ extraction fraction (arterial and femoral venous catheters) were obtained. ${\dot{Q}}_{max}$ and two-LBF increased linearly by 4.9-6.0 L · min^-1 per 1 L · min^-1 increase in ${\dot{V}O}_{\text{2max}}$ (R² = .73 and R² = .67, respectively; both P < .001). The $a\text{-}\overline{v}{O}_2$ difference increased from 118-168 mL · L^-1 from a ${\dot{V}O}_{\text{2max}}$ of 2-4.5 L · min^-1 followed by a reduction (second-order polynomial: R² = .27). After accounting for a hypoxemia-induced decrease in arterial O₂ content with increasing ${\dot{V}O}_{\text{2max}}$ (R² = .17; P < .001), systemic O₂ extraction fraction increased up to ~90% ( ${\dot{V}O}_{\text{2max}}$ : 4.5 L · min^-1 ) with no further change (exponential decay model: R² = .42). Likewise, leg O₂ extraction fraction increased with ${\dot{V}O}_{\text{2max}}$ to approach a maximal value of ~90-95% (R² = .83). Muscle O₂ diffusing capacity and the equilibration index Y increased linearly with ${\dot{V}O}_{\text{2max}}$ (R² = .77 and R² = .31, respectively; both P < .01), reflecting decreasing O₂ diffusional limitations and accentuating O₂ delivery limitations. In conclusion, although O₂ delivery is the main limiting factor to ${\dot{V}O}_{\text{2max}}$ , enhanced O₂ extraction fraction (≥90%) contributes to the remarkably high ${\dot{V}O}_{\text{2max}}$ in endurance-trained individuals.

Keywords: arteriovenous oxygen difference; cardiac output; exercise; leg blood flow; limiting factors; maximal oxygen uptake; oxygen diffusion; stroke volume.