Leg venous compliance in orthostatic intolerance before and after 14-day head-down bed rest

To test the hypothesis that increased leg venous compliance (LVC) is one of the contributory factors to orthostatic intolerance (OI) after simulated microgravity, 28 healthy young males were exposed to a 14-day head-down bed rest, and LVC was measured by venous occlusion plethysmography. Orthostatic tolerance was evaluated by a 60 degree head-up tilt (HUT) for 15 min. Sixteen subjects suffered from OI after the bed rest. They were then divided into orthostatic tolerance (non-fainters, n=12) and intolerance (fainters, n=16) groups. We found that fainters had significantly larger LVC before bed rest (0.055 +/- 0.003 vs. 0.065 +/- 0.002 ml 100 ml-1 mm Hg-1, non-fainter vs. fainter; P < 0.05). After bed rest, LVC markedly increased in both groups. In all the subjects calf circumference was reduced on average by 4.7% and the percent change in LVC was negatively correlated with the percent change in calf circumference when all subjects' data were combined after bed rest (r = -0.42, P < 0.05). Our results did not support the hypothesis that increased LVC is the contributory factor to OI after a 14-day bed rest; however, the mechanisms behind the large LVC in the fainters before bed rest are unclear, and the initial LVC might be a predictive indicator for OI after microgravity exposure.