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1 Department of Physiology, University Medical Centre Nijmegen, Nijmegen, The Netherlands
2 Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas and the University of Texas Medical Center, Dallas, Texas, USA
* To whom correspondence should be addressed. E-mail: M.Bleeker{at}fysiol.umcn.nl.
Venous function may be altered by bed rest deconditioning. Yet the contribution of altered venous compliance to the orthostatic intolerance observed after bed rest is uncertain. The purpose of this study was to assess the effect of 18 days of bed rest on leg and arm (respectively large and small change in gravitational gradients and use patterns) venous properties. We hypothesized that the magnitude of these venous changes would be related to orthostatic intolerance. Eleven healthy subjects (10 men, 1 woman) participated in the study. Before and after 18 days of 6° head-down tilt bed rest, strain gauge venous occlusion plethysmography was used to assess limb venous vascular characteristics. Leg venous compliance was significantly decreased after bed rest (pre: 0.048 ± 0.007 ml/100ml/mmHg, post: 0.033 ± 0.007 ml/100ml/mmHg, p<0.01), while arm compliance did not change. Leg venous flow resistance increased significantly after bed rest (pre: 1.73 ± 1.08 mmHg/ml/100ml/min, post: 3.10 ± 1.00 mmHg/ml/100ml/min, p<0.05). Maximal LBNP tolerance, which was expressed as cumulative stress index (pressure . time), decreased in all subjects after bed rest (pre: 932 mmHg.min, post: 747 mmHg.min). The decrease in orthostatic tolerance was not related to changes in leg venous compliance. In conclusion, this study demonstrates that after bed rest, leg venous compliance is reduced and leg venous outflow resistance is enhanced. However these changes are not related to measures of orthostatic tolerance; therefore alterations in venous compliance do not to play a major role in orthostatic intolerance after 18 days of head-down tilt bed rest.
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