Reversal of bedrest-induced orthostatic intolerance by lower body negative pressure and saline

Six healthy male volunteers underwent two 1-week periods of bedrest, each preceded and followed by 2-week control and recovery periods. The daily metabolic diet contained 150 mEq of sodium. Following one 7-day bedrest period, each man was subjected to LBNP at a level of -30 mm Hg for 4 hr while consuming 1000 ml of beef bouillon containing 154 mEq of sodium. After the other bedrest period, each man simply consumed the bouillon without LBNP treatment during 4 hr of continued bedrest. Measurements of plasma volume and orthostatic tolerance were made before and after each treatment period. After combined LBNP and saline therapy, plasma volume and response to LBNP testing showed a return to pre-bedrest levels. Saline consumption alone had a lesser effect. With continuation of bedrest in three subjects, the beneficial effects of these measures appeared to be largely gone after 18 hr.