Sex Difference in Orthostatic Tolerance During Simulated Partial Gravity Head-Up Tilts and Efficacy of a Lower Body Compression Garment Countermeasure.

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, though 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 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, 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 prior to and during HUT at 80° compared to males (p=0.038). OIG use increased tolerance by 4.2 (P<0.001) and 1.6 minutes (P=0.007) for 80 and 50° HUT, respectively, and attenuated the increase in HR and decrease in SV, which was greater in females (P<0.001). Hypovolemic females demonstrated an increased cardiovascular response to HUT contributing to differences in OI throughout 20-minute exposures and the OIG demonstrates protection in all participants.