BED-REST (BR) ENHANCES CALF VASODILATOR RESPONSE TO SALINE INFUSION (SI) INDUCED CENTRAL HYPERVOLEMIA 1031

Abstract

Orthostatic intolerance following spaceflight or bedrest (BR) deconditioning may be due to inadequate vasoconstriction or inappropriate reflex vasodilation. The aim of this study was to test the hypothesis that the calf vasodilator response to increases in pulmonary capillary wedge pressure(PCWP) and right atrial pressure (RAP) produced by SI (100ml/min) is potentiated following BR. Eleven volunteers (age: 24±2 yrs, Wt:78±3kg, Ht:176±3cm) completed the experiments prior to (Pre) and following (Post) 2-weeks of -6° head-down tilt BR in which cardiac filling pressure (Swan-Ganz), calf blood flow (CBF, plethysmography) and mean arterial pressure (MAP, Finapres) were measured. BR significantly decreased baseline PCWP (pre vs post: 13.2 ± 1.0 vs 10.5 ± 1.1mmHg), RAP(7.9 ± 0.8 vs 6.2 ± 0.8 mmHg), CBF (2.3 ± 0.2 vs 1.7± 0.1 ml/dl/min), and increased MAP (85 ± 2 vs 91 ± 3 mmHg) and calf vascular resistance (CVR, 37 ± 4 vs 51 ± 3 PRU) calculated from the ratio (MAP-RAP)/CBF, SI with 15 or 30 ml/kg significantly increased PCWP, RAP, CBF, and decreased CVR both pre- and post-BR (seetable). MAP was not significantly changed by SI pre- or post-BR. In terms of per unit ▵PCWP or ▵RAP, ▵CVR was significantly greater in post- than pre-BR (-2.4 ± 0.0 or -3.9 ± 0.1 vs -2.0 ± 0.1 or-2.9 ± 0.2 PRU/mmHg). Conclusion: peripheral vasodilation in response to central hypervolemia appears to be augmented following BR deconditioning.