Orthostatic intolerance during a 13-day bed rest does not result from increased leg compliance

Increased leg compliance (LC) has been proposed as a mechanism for orthostatic intolerance after spaceflight or bed rest. Using venous occlusion plethysmography with mercury-in-Silastic strain gauge, we evaluated LC before, during, and after a 13-day head-down (-6 degrees) bed rest in 10 men. LC was measured by the relationship between the increased calf areas (in cm2) at thigh cuff occlusions of 20, 30, 50, 70, and 80 mmHg. Orthostatic tolerance was evaluated by a presyncopal-limited lower body negative pressure test (PSL-LBNP) before and after bed rest. The 10 subjects were divided into TOL (n = 5) and INT (n = 5) groups for which the orthostatic tolerance was similar and lower after bed rest, respectively. For TOL (INT) before bed rest, calf area increases were 2.2 +/- 0.5 (SE) (1.3 +/- 0.4), 3.5 +/- 0.7 (2.3 +/- 0.5), 5.0 +/- 0.9 (3.5 +/- 0.6), 5.6 +/- 0.9 (4.4 +/- 0.6), and 6.4 +/- 1.1 (4.7 +/- 0.6) cm2 for thigh occlusion pressures of 20, 30, 50, 70, and 80 mmHg, respectively. Neither for INT nor for TOL were these results significantly changed by bed rest. These results suggest that other mechanisms than increased LC have to be taken into account to explain the decreased orthostatic tolerance induced by this 13-day bed rest.