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

We analysed the importance of systemic and peripheral arteriovenous O2 difference ( a - v ¯ O 2 difference and a-vf O2 difference, respectively) and O2 extraction fraction for maximal oxygen uptake ( V ˙ O 2max ). Fick law of diffusion and the Piiper and Scheid model were applied to investigate whether diffusion versus perfusion limitations vary with V ˙ O 2max . Articles (n = 17) publishing individual data (n = 154) on V ˙ O 2max , maximal cardiac output ( Q ˙ max ; indicator-dilution or the Fick method), a - 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-vf O2 difference and O2 extraction fraction (arterial and femoral venous catheters) were obtained. Q ˙ max and two-LBF increased linearly by 4.9-6.0 L · min-1 per 1 L · min-1 increase in V ˙ O 2max (R2  = .73 and R2  = .67, respectively; both P < .001). The a - v ¯ O 2 difference increased from 118-168 mL · L-1 from a V ˙ O 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 V ˙ O 2max (R2  = .17; P < .001), systemic O2 extraction fraction increased up to ~90% ( V ˙ O 2max : 4.5 L · min-1 ) with no further change (exponential decay model: R2  = .42). Likewise, leg O2 extraction fraction increased with V ˙ O 2max to approach a maximal value of ~90-95% (R2  = .83). Muscle O2 diffusing capacity and the equilibration index Y increased linearly with V ˙ O 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 V ˙ O 2max , enhanced O2 extraction fraction (≥90%) contributes to the remarkably high V ˙ O 2max in endurance-trained individuals.