• G Kim Prisk, Janelle M Fine, Ann R Elliott, and John B West.
• Department of Medicine, University of California, San Diego, La Jolla, 92093-0931, USA. kprisk@ucsd.edu
• Aviat Space Envir Md. 2002 Jan 1; 73 (1): 8-16.

BackgroundHead-down tilt (HDT) of 6 degrees is a commonly used model of weightlessness, but there are few comparisons with actual microgravity.HypothesisOur study was designed to prove that the changes in cardiopulmonary function seen in HDT would be similar to those seen in microgravity.MethodsWe compared measurements of cardiovascular and pulmonary function from three separate spaceflights of 14 to 17 d duration, with data collected during a 17-d period of HDT.ResultsHDT proved a good model of the cardiovascular response to microgravity, resulting in increases in cardiac output and stroke volume of a similar magnitude to those seen in microgravity, with a concomitant reduction in heart rate. By contrast, HDT was a poor model of the effects of microgravity on pulmonary ventilation and gas exchange.ConclusionPulmonary function in HDT approximated the changes seen in the 1-G supine posture, while in microgravity this was much closer to that seen in the 1-G upright position. The differences probably reflect the fact that changes in cardiovascular function result primarily from fluid shifts within the entire body, whereas changes in pulmonary ventilation are primarily a result of mechanical influences on the lung and chest and abdominal wall.

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