Marked exacerbation of orthostatic intolerance after long- vs. short-duration spaceflight in veteran astronauts

Objective: The incidence of postflight orthostatic intolerance after short- duration spaceflight is about 20%. However, the incidence after long-durati on spaceflight was unknown. The purpose of this study was to test the hypot hesis that orthostatic intolerance is more severe after long-duration than after short-duration flight. Methods: We performed tilt tests on six astron auts before and after long-duration (129-190 days) spaceflights and compare d these data with data obtained during stand tests before and after previou s short-duration missions. Results: Five of the six astronauts studied beca me presyncopal during tilt testing after long-duration flights. Only one ha d become presyncopal during stand testing after short-duration flights. We also compared the long-duration flight tilt test data to tilt test data fro m 20 different astronauts who flow on the short-duration Shuttle missions t hat delivered and recovered the astronauts to and from the Mir Space Statio n. Five of these 20 astronauts became presyncopal on landing day. Heart rat e responses to tilt were no different between astronauts on long-duration f lights and astronauts on short-duration flights, but long-duration subjects had lower stroke volumes and cardiac outputs than short-duration presyncop al subjects, suggesting a possible decrease in cardiac contractile function . One subject had subnormal norepinephrine release with upright posture aft er the long flight but not after the short flight. Plasma volume losses wer e not greater after long flights. Conclusion: Long-duration spaceflight mar kedly increases orthostatic intolerance, probably with multiple contributin g factors.