Hyperoxia and moderate hypoxia fail to affect inspiratory muscle fatigue in humans

Normal human subjects (n = 7) breathing 21% O₂ (normoxia), 13% O₂ (hypoxia), or 100% O₂ (hyperoxia) performed repeated maximal inspiratory maneuvers (inspiratory duration = 1.5 s, total breath duration = 3.5 s) on an 'isoflow' system, which delivered a constant mouth flow (1.25 or 1 l/s) while maintaining normocapnia (5.5% end-tidal CO₂). Respective mean arterial O₂ saturation values (ear lobe oximetry) were 98 ± 1, 91 ± 4 (P ≤ 0.01) and 99 ± 1% (NS). Maximal mouth pressure (Pm) was measured during inspirations at rest and during a 10-min fatigue trial, and the Pm measurements obtained during the fatigue trials were fit to an exponential equation. The parameters of the equation included the time constant (τ), which describes the rate of decay of Pm from the initial pressure (Pi) to the asymptote, or 'sustainable' pressure (Ps). The mean fraction of Pm remaining at the end of the fatigue trials (Ps/Pi) was 63 ± 5%. No significant differences in Pi, Ps, or τ were observed between O₂ treatments. This suggests that fatigue of the inspiratory muscles in normal humans occurs by a mechanism that is insensitive to changes in blood O₂ content that occur during inspiration of O₂ in the range of 13-100%.