title: Effect of 6 degrees head-down tilt on cardiopulmonary function: comparison with microgravity.
abstract: BACKGROUND
  Head-down tilt (HDT) of 6 degrees is a commonly used model of weightlessness, but there are few comparisons with actual microgravity.
  
  
  OBJECTIVE
  Our study was designed to prove that the changes in cardiopulmonary function seen in HDT would be similar to those seen in microgravity.
  
  
  METHODS
  We 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.
  
  
  RESULTS
  HDT 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.
  
  
  CONCLUSIONS
  Pulmonary 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.
authors.affiliation: Department of Medicine, University of California, San Diego, La Jolla, 92093-0931, USA. kprisk@ucsd.edu
authors.forename: G Kim
authors.initials: GK
authors.lastname: Prisk
authors.forename: Janelle M
authors.initials: JM
authors.lastname: Fine
authors.forename: Ann R
authors.initials: AR
authors.lastname: Elliott
authors.forename: John B
authors.initials: JB
authors.lastname: West
mesh_terms: D000328:Adult; D002302:Cardiac Output; D005260:Female; D018475:Head-Down Tilt; D006321:Heart / Q000502:physiology*; D006801:Humans; D008168:Lung / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D011659:Pulmonary Gas Exchange; D012129:Respiratory Function Tests; D014893:Weightlessness; D018474:Weightlessness Simulation