title: Bed rest effects on human calf hemodynamics and orthostatic intolerance: a model-based analysis.
abstract: BACKGROUND
  Microgravity-induced orthostatic intolerance continues to be a primary problem after space missions. Its etiology remains uncertain despite significant research efforts over the past years. We hypothesized that calf hemodynamic parameters (compliance and resistance) are significantly affected by 14 to 16-d head-down bed rest (simulated microgravity), and their alterations play a role in the pathogenesis of orthostatic intolerance (OI) following bed rest.
  
  
  METHODS
  To estimate these parameters, we developed a model-based approach to quantitatively simulate calf vascular response to venous occlusion, which only necessitates measurement of plethysmography data. In this study, plethysmography data were obtained from 29 subjects before and after 14-16 d of head-down bed rest. The subjects also underwent a tilt/stand test before and after bed rest.
  
  
  RESULTS
  Statistical analyses demonstrated an increase in calf compliance (1.87 +/- 0.08, mean +/- SE, pre-bed rest; 2.16 +/- 0.10, end-bed rest) but no significant change in vascular resistance following bed rest. Compared with the tilt-intolerant subjects, those who were tilt-tolerant before bed rest had significantly higher calf compliance [2.00 +/- 0.09 (tolerant); 1.58 +/- 0.09 (intolerant)] and higher vascular resistance [7.79 +/- 0.18 (tolerant); 6.91 +/- 0.40 (intolerant)]. After bed rest, no such difference was detected.
  
  
  CONCLUSIONS
  Based on these results, we validated the hypothesis that, instead of causing orthostatic intolerance, higher calf compliance before bed rest leads to recruitment of compensatory mechanisms (validated by the enhanced vascular resistance during venous occlusion) for a better toleration of orthostatic stress. With the absence of orthostatic challenge during bed rest, the difference in calf hemodynamic parameters is attenuated between the tilt-tolerant and tilt-intolerant groups.
authors.affiliation: Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, USA. xiao@mit.edu
authors.forename: Xinshu
authors.initials: X
authors.lastname: Xiao
authors.forename: S Marlene
authors.initials: SM
authors.lastname: Grenon
authors.forename: Christine
authors.initials: C
authors.lastname: Kim
authors.forename: Natalie
authors.initials: N
authors.lastname: Sheynberg
authors.forename: Shelly
authors.initials: S
authors.lastname: Hurwitz
authors.forename: Gordon H
authors.initials: GH
authors.lastname: Williams
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Cohen
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D000328:Adult; D001510:Bed Rest; D018475:Head-Down Tilt / Q000502:physiology; D006439:Hemodynamics / Q000502:physiology*; D006801:Humans; D007866:Leg / Q000098:blood supply*; D008297:Male; D008954:Models, Biological; D010991:Plethysmography; D012039:Regional Blood Flow / Q000502:physiology; D013026:Space Flight; D013575:Syncope / Q000503:physiopathology*; D018667:Tilt-Table Test; D014655:Vascular Resistance / Q000502:physiology