Differential Effect of Bed Rest on Leg and Arm Venous Properties

Veins play an important role in cardiovascular homeostasis by both passively (conduit function) and actively (venomotor function) facilitating venous return which determines cardiac filling pressure. In bipeds such as human beings, gravitational forces create dissimilar pressure gradients in legs and arms during upright posture. Therefore the effect of bed rest and thus microgravity on the capacitance beds of arm and leg may differ. PURPOSE: To investigate the effect of 18 days of strict head down tilt bed rest on leg and arm venous properties. METHODS: Eleven healthy subjects (10 men, 1 woman), mean age 24 ± 6 years participated in the study. Before and after the 18 days bed rest period, mercury strain gauge venous occlusion plethysmography was used to assess leg and arm venous vascular characteristics. RESULTS: Leg venous compliance was significantly decreased after 18 days of bed rest deconditioning (pre: 0.048 ± 0.007 ml/100ml/mmHg, post: 0.033 ± 0.007 ml/100ml/mmHg, p < 0.01) whereas no changes in venous compliance could be observed in the arm (pre: 0.065 ± 0.014 ml/100ml/mmHg, post: 0.064 ± 0.016 ml/100ml/mmHg). Venous flow resistance followed basically the same pattern, i.e. venous flow resistance in the leg increased significantly after bed rest (pre: 1.73 ± 1.08 mmHg/ml/100ml/min, post 3.10 ± 1.00 mmHg/ml/100ml/min, post 1.26 ± 0.64 mmHg/ml/100ml/min). CONCLUSION: Leg venous vascular properties were significantly altered after 18 days of strict bed rest deconditioning with no changes in the arm. We speculate that the combination of differential changes in both activity patterns and hydrostatic gradients of arms and legs during bed rest may alter both functional and structural venous vascular properties after microgravity exposure.