Effects of fractional inspired O ₂ on the O ₂ pathway during submaximal and maximal exercise in male endurance athletes

We utilized non-invasive methods and novel computational approaches to examine the effects of acutely varying fractional inspired O₂ (FIO₂) on convective and diffusive steps of O₂ transport and muscle tissue (de)oxygenation during incremental cycling to exhaustion in 10 Tier 3 and 4 endurance athletes breathing either 0.152, 0.209, or 0.298 FIO₂. At submaximal work rates (100-275 W) in hypoxia, higher cardiac output compensated for lower arterial O₂ content. At maximal work rate, convective O₂ transport was lower in hypoxia (mean [95%CI]: 5.37 [5.14-5.59] L/min, q < 0.0001) and higher in hyperoxia (6.84 [6.50-7.18] L/min, q = 0.043) compared to normoxia (6.56 [6.16-6.95] L/min), whereas O₂ diffusive conductance did not differ between conditions (94 [82-106], 98 [83-112], 98[87-109] mL/min/mmHg for hypoxia, normoxia and hyperoxia, respectively, p = 0.490). Consequently, maximal O₂ uptake (V̇O_2max) was lower in hypoxia (4.06 [3.85-4.27] L/min, q < 0.0001) and higher in hyperoxia (5.02 [4.85-5.19] L/min, q = 0.003) compared to normoxia (4.83 [4.63-5.02] L/min). In hypoxia, muscle tissue saturation index was 1%-4%-units lower compared to normoxia and hyperoxia during submaximal cycling but similar at maximal work rate. In summary, central and peripheral compensatory mechanisms maintained O₂ uptake despite altered FIO₂ at submaximal work rates. At maximal work rate the effects of hypoxia and hyperoxia on V̇O_2max were mediated through convective O₂ transport.