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Bed rest effects on human calf hemodynamics and orthostatic intolerance: a model-based analysis
Xinshu Xiao1, S Marlene Grenon, Christine Kim
1Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, USA. xiao@mit.edu
Aviation, Space, and Environmental Medicine
|December 1, 2005
Summary
Higher calf compliance before bed rest may improve orthostatic tolerance by activating compensatory mechanisms. This effect diminishes after simulated microgravity, suggesting a role for calf hemodynamics in spaceflight adaptation.
Area of Science:
- Cardiovascular Physiology
- Space Medicine
- Hemodynamics
Background:
- Orthostatic intolerance (OI) is a significant challenge post-spaceflight, with unclear causes.
- Head-down bed rest (HDBR) simulates microgravity's effects on the body.
- Calf hemodynamic parameters' role in OI requires further investigation.
Purpose of the Study:
- To test the hypothesis that calf hemodynamic parameters (compliance and resistance) are altered by HDBR.
- To determine if these alterations contribute to OI after simulated microgravity.
Main Methods:
- A model-based approach using plethysmography to assess calf vascular response to venous occlusion.
- Measurements taken from 29 subjects before and after 14-16 days of HDBR.
- Subjects also underwent tilt/stand tests pre- and post-HDBR.
Main Results:
- Calf compliance increased significantly after HDBR (pre: 1.87±0.08, post: 2.16±0.10).
- No significant change in calf vascular resistance was observed post-HDBR.
- Pre-HDBR, tilt-tolerant subjects had higher calf compliance and vascular resistance compared to intolerant subjects; this difference was absent post-HDBR.
Conclusions:
- Higher pre-HDBR calf compliance is associated with better orthostatic tolerance, likely via compensatory mechanisms.
- HDBR attenuates differences in calf hemodynamic parameters between tolerant and intolerant individuals.
- Calf hemodynamics may play a role in adaptation to and recovery from microgravity-induced OI.