Acute Moderate Hypoxia Reduces One-Legged Cycling Performance Despite Compensatory Increase in Peak Cardiac Output: A Pilot Study

In severe hypoxia, single-leg peak oxygen uptake (VO2peak) is reduced mainly due to the inability to increase cardiac output (CO). Whether moderate altitude allows CO to increase during single-leg cycling, thereby restoring VO2peak, has not been extensively investigated. Five healthy subjects performed an incremental, maximal, two-legged cycle ergometer test, and on separate days a maximal incremental one-leg cycling test in normoxia and in moderate hypoxia (fraction of inspired oxygen (FiO2) = 15%). Oxygen uptake, heart rate, blood pressure responses, power output, and CO (PhysioFlow) were measured during all tests. Moderate hypoxia lowered single-leg peak power output (154 ± 31 vs. 128 ± 26 watts, p = 0.03) and oxygen uptake (VO2) (36.8 ± 6.6 vs. 33.9 ± 6.9 mL/min/kg, p = 0.04), despite higher peak CO (16.83 ± 3.10 vs. 18.96 ± 3.59 L/min, p = 0.04) and systemic oxygen (O2) delivery (3.37 ± 0.84 vs. 3.47 ± 0.89 L/min, p = 0.04) in hypoxia compared to normoxia. Arterial–venous O2 difference (a–vDO2) was lower in hypoxia (137 ± 21 vs. 112 ± 19 mL/l, p = 0.03). The increases in peak CO from normoxia to hypoxia were negatively correlated with changes in mean arterial pressure (MABP) (p < 0.05). These preliminary data indicate that the rise in CO was not sufficient to prevent single-leg performance loss at moderate altitude and that enhanced baroreceptor activity might limit CO increases in acute hypoxia, likely by reducing sympathetic activation. Since the systemic O2 delivery was enhanced and the calculated a–vDO2 reduced in moderate hypoxia, a potential diffusion limitation cannot be excluded.