title: Cardiovascular adaptations to long-duration head-down bed rest.
abstract: BACKGROUND
  Orthostatic hypotension is a serious risk for crewmembers returning from spaceflight. Numerous cardiovascular mechanisms have been proposed to account for this problem, including vascular and cardiac dysfunction, which we studied during bed rest.
  
  
  METHODS
  Thirteen subjects were studied before and during bed rest. Statistical analysis was limited to the first 49-60 d of bed rest and compared to pre-bed rest data. Ultrasound data were collected on vascular and cardiac structure and function. Tilt testing was conducted for 30 min or until presyncopal symptoms intervened.
  
  
  RESULTS
  Plasma volume was significantly reduced (15%) by day 7 of bed rest. Flow-mediated dilation in the leg was significantly increased at bed rest day 49 (6% from pre-bed rest). Arterial responses to nitroglycerin differed in the arm and leg, but did not change as a result of bed rest. Anterior tibial artery intimal-medial thickness markedly decreased at bed rest days 21 (21%), 35 (22%), and 49 (19%). Several cardiac functional parameters, including isovolumic relaxation time (73 ms to 85 ms at day 7) and myocardial performance index, were significantly increased (0.41 to 0.49 by day 7 of bed rest; indicating a decrease in cardiac function) during bed rest. There was a trend for decreased orthostatic tolerance following 60 d of bed rest (P = 0.1).
  
  
  CONCLUSIONS
  Our data suggest that bed rest altered cardiovascular structure and function in a pattern similar to short-duration spaceflight. Additionally, the vascular alterations were primarily seen in the lower body, while vessels of the upper body were unaffected.
authors.affiliation: NASA Johnson Space Center, Mail Code SK, 2101 NASA Parkway, Houston, TX 77058, USA. steven.platts-1@nasa.gov
authors.forename: Steven H
authors.initials: SH
authors.lastname: Platts
authors.forename: David S
authors.initials: DS
authors.lastname: Martin
authors.forename: Michael B
authors.initials: MB
authors.lastname: Stenger
authors.forename: Sondra A
authors.initials: SA
authors.lastname: Perez
authors.forename: L Christine
authors.initials: LC
authors.lastname: Ribeiro
authors.forename: Richard
authors.initials: R
authors.lastname: Summers
authors.forename: Janice V
authors.initials: JV
authors.lastname: Meck
mesh_terms: D000222:Adaptation, Physiological; D000328:Adult; D001510:Bed Rest; D002320:Cardiovascular Physiological Phenomena; D002319:Cardiovascular System / Q000000981:diagnostic imaging*; D005260:Female; D018475:Head-Down Tilt; D006439:Hemodynamics; D006801:Humans; D007024:Hypotension, Orthostatic; D008297:Male; D010953:Plasma Volume; D011446:Prospective Studies; D013026:Space Flight; D013997:Time Factors; D014463:Ultrasonography; D014893:Weightlessness