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1 National Space Biomedical Research Institute, Baylor College of Medicine, Houston, Texas 77030; 2 Department of Medicine, University of California, San Diego, California 92103; and 3 Space Life Sciences Research Laboratories, National Aeronautics and Space Administration Johnson Space Center, Houston, Texas 77058
About 20% of astronauts suffer
postspaceflight presyncope. We studied pre- to postflight (5- to
16-day missions) cardiovascular responses to standing in 35 astronauts
to determine differences between 1) men and women and
2) presyncopal and nonpresyncopal groups. The groups were
presyncopal women, presyncopal men, and nonpresyncopal men based on
their ability to stand for 10 min postflight. Preflight, women and
presyncopal men had low vascular resistance, with the women having the
lowest. Postflight, women experienced higher rates of presyncope (100 vs. 20%; P = 0.001) and greater losses of plasma
volume (20 vs. 7%; P < 0.05) than men. Also,
presyncopal subjects had lower standing mean arterial pressure (P 
0.001) and vascular resistance (P < 0.05), smaller increases in norepinephrine (P 0.058) and greater
increases in epinephrine (P 0.058) than nonpresyncopal
subjects. Presyncopal subjects had a strong dependence on
plasma volume to maintain standing stroke volume. These findings
suggest that postflight presyncope is greatest in women, and this can
be ascribed to a combination of inherently low-resistance responses, a
strong dependence on volume status, and relative hypoadrenergic
responses. Conversely, high vascular resistance and postflight
hyperadrenergic responses prevent presyncope.
microgravity; gender; sympathetic; plasma volume; vascular resistance
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