Key result
Fluid loading raises mean arterial pressure by ~10 mmHg during head-down tilt versus no fluid.
Why the study?
Up to 64% of astronauts experience postflight orthostatic intolerance, and the optimal timing for fluid loading as a countermeasure prior to re-entry was unknown.
Does fluid loading improve cardiovascular responses during head-down tilt in healthy subjects?
Does fluid loading improve cardiovascular responses during head-down tilt in healthy subjects?
Mean Difference: 10
Fluid loading 1 to 3 hours prior to re-entry may be the optimal time window to improve orthostatic tolerance in astronauts, though individual variability is high.
May guide pre-reentry fluid loading timing in astronauts; hypothesis-generating given high variability and Level 5 evidence.
BACKGROUND: Susceptibility of healthy astronauts to orthostatic hypotension and presyncope is exacerbated upon return from spaceflight. Up to 64% of astronauts experience postflight orthostatic intolerance. A promising countermeasure for postflight orthostatic intolerance is fluid loading by giving crew salt tablets and water prior to re-entry. The primary goal of the current study was to determine the optimal time window prior to re-entry when individual crew-persons would initiate fluid loading. METHODS: There were 16 subjects who were given two 6-h exposures, with and without fluid loading (conditions), to head-down tilt (HDT) to simulate the effects of microgravity. Pre- and post-HDT stand tests of orthostatic tolerance were given. Physiological measurements recorded included heart rate, blood pressure, peripheral blood volume, total peripheral resistance, and impedance cardiography. Echocardiography measures of stroke volume and cardiac output were also recorded. RESULTS: Data were analyzed with three-way repeated measures ANOVA (gender × condition × time). Only the condition × time interaction was significant for mean arterial pressure. Post hoc multiple comparison tests revealed significant increases in mean arterial pressure occurred between hours 1 and 3 of HDT after fluid loading (10 mmHg higher than no fluid). DISCUSSION: These findings indicate that the optimal time for crew to begin fluid loading is within 1 to 3 h prior to re-entry. Nonsignificant trends of multiple cardiovascular responses showed similar time profiles. The large amount of individual variability suggests that fluid loading alone may be an inadequate countermeasure for all crewmembers. Further research is needed on possible adjunct methods of tailoring countermeasures for individuals.
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Cowings et al. (2015) studied Orthostatic intolerance (simulated microgravity) (n=16). Fluid loading (salt tablets and water) vs. No fluid loading was evaluated on Mean arterial pressure (10 mmHg higher). Fluid loading increased mean arterial pressure by 10 mmHg between hours 1 and 3 of head-down tilt compared to no fluid loading.
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