Effects Of Face-down -6°Head-down Tilt Microgravity Simulation On Fluid Redistribution And Cardiovascular Variables

Abstract

When exposed to space, the microgravity environment results in fluid redistribution, with a higher relative accumulation in the upper body and fluid loss leading to facial swelling, bird like legs, and increased urine excretion. The face-up (FU) -6° head-down tilt (HDT) methodology has been used as a standard microgravity simulation tool for over 5 decades. PURPOSE: To examine the effects of a -6° face-down (FD) HDT vs. the FU HDT microgravity simulation on cardiovascular variables, fluid redistribution, facial swelling, and reported side effects. METHODS: 18 healthy individuals (n = 11 females and 7 males) were recruited (21 ± 1.7 years; 165.5 ± 24.5 cm; 71.1 ± 15.9 kg; mean ± SD). Subjects participated in two randomly assigned 2.5 hour testing sessions which included a 1.5 hour simulated microgravity trial; one at -6° FD HDT and one at -6° FU HDT. Pre-tests consisted of emptying the bladder (urine collection), obtaining a 3D body scan and then recording baseline heart rate and blood pressures while lying horizontally for 5 min. The table was brought to a -6° incline by raising the foot end of the table. Participants remained in this position for 1.5 hours, with cardiovascular variables recorded every 3 minutes for the first 15 minutes and every 5 minutes thereafter, 3D body scans were redone immediately after the intervention and urine was again collected and analyzed. Lastly, side effects were recorded throughout whenever a subject indicated any. RESULTS: There were no differences between FU and FD HDT on heart rate (p > 0.05) or systolic and diastolic blood pressures (p > 0.05). Urine production was not statistically different between the FU and FD conditions (295 ± 204 ml vs. 233 ± 144 ml, p = 0.28). Preliminary observations indicated that subjects had more facial swelling in the FD trial when compared to the FU condition. More participants subjectively reported an increased number of side effects and/ or more discomfort in the FD trial (e.g. drowsiness, inability to focus, nausea). CONCLUSION: This position change (FD) during an acute simulated microgravity test appears to alter the fluid redistribution as seen in the facial puffiness. The -6° FD HDT model may be a better simulation to study some of the acute effects of microgravity exposure on fluid redistribution and possible counter-measures.Supported by Acadia HSRA.