Effect of hypoxia and hyperoxia on human +Gz duration tolerance

To determine the effects of varying inspired O2 on positive radial acceleration (+Gz; i.e., head-to-foot inertial load) duration tolerance, seven men were exposed to the +4.5- to +7.0-Gz simulated aerial combat maneuver (SACM) by use of the Armstrong Laboratory (Brooks Air Force Base) centrifuge. Exposures were repeated on different days while subjects breathed gas mixtures of fractional concentration of O2 in inspired air (FIO2) between 0.12 and 0.6. SACM duration tolerance was positively related to inspired O2 of FIO2 between 0.12 and 0.2 but was unchanged at FIO2 between 0.2 and 0.6. SACM exposure decreased arterial O2 saturation and increased heart rates; SACM-induced changes were additive to FIO2 effects. The positive relationship between blood lactate and SACM duration tolerance at all FIO2 indicated an anaerobic component. It is concluded that SACM duration tolerance is limited by reduced FIO2 but not enhanced by hyperoxia. Thus the aerobic component of +4.5- to +7.0-Gz SACM duration tolerance is much greater than previously believed.