Sex difference in orthostatic tolerance during simulated partial-gravity head-up tilts and efficacy of a lower body compression garment countermeasure.

Abstract

After adaptation to weightlessness, astronauts will experience head-to-foot accelerations (G) during descent to and ascent from the lunar surface that may provoke symptoms of orthostatic intolerance (OI). There were no reports consistent with OI among Apollo astronauts who experienced up to 10 minutes of 0.4-0.6-G while standing without OI countermeasures during dynamic flight phases, although heart rate (HR) was elevated. The likelihood of developing OI in future Artemis crews, which will include females, may be greater given observations of increased postflight OI in females and potentially greater acceleration requirements. We assessed orthostatic tolerance and cardiovascular responses in 34 healthy individuals (16 females) during 20 minutes of 30, 50, and 80° head-up tilt (HUT), simulating sustained accelerations (0.50, 0.75, and 1.00-G) after pharmacologically induced hypovolemia (∼10% plasma volume loss) as a short-duration spaceflight model. On a separate day using the same model, we evaluated the efficacy of a lower body orthostatic intolerance garment (OIG) to prevent OI. Tilt tolerance time without OIG was 4.6 minutes greater in males than females during 80° HUT (P = 0.003). Without OIG, hypovolemic females demonstrated lower mean arterial pressures before and during HUT at 80° compared to males (P = 0.038). OIG use increased tolerance by 4.2 (P < 0.001) and 1.6 min (P = 0.007) for 80 and 50° HUT, respectively, and attenuated the increase in HR and decrease in stroke volume, which was greater in females (P < 0.001). Hypovolemic females demonstrated an increased cardiovascular response to HUT contributing to differences in OI throughout 20-min exposures, and the OIG demonstrates protection in all participants.NEW & NOTEWORTHY We report here that females were more likely to become presyncopal earlier than males during 20 min of 80° and 50° head-up tilt tests (HUT), representing 1.00-G and 0.75-G, respectively, but not during 30° HUT, representing 0.50-G. Additionally, wearing lower body compression garments protected against symptoms of orthostatic intolerance in both sexes. These results inform the spaceflight community for planning acceleration limits and countermeasure use during future exploration missions to the Moon and Mars.