The hemodynamic effects of repeated bed rest exposure.

Hemodynamic changes were measured during stepwise exposure to lower-body negative pressure (LBNP) (5 min, -20, -30, and -40 mm Hg) in a group of seven physically active subjects before and after consecutive exposure to three 2-week bed rest periods. Bed rest exposures were separated by 3-week periods of ambulatory recovery. Dynamic exercise (68% max O2, 30 min each day) and isometric exercise (21% max leg extension, 30 min each day) performed during bed rest and reambulation failed to prevent deconditioning or accelerate the recovery process between bed rest exposures. Heart rate (HR) and end-diastolic volume index (EDVI) proved to be parameters showing greatest changes during LBNP. Heart rate increases at -40 mm Hg LBNP (compared to respective pre-LBNP levels) were 13.3%, 35.1%, and 51.0% for each of the pre-bed rest exposures, while respective changes after bed rest were 57.8%, 57.2%, and 75.5%. The significantly elevated HR responses during subsequent pre-bed rest (control) periods indicated incomplete recovery despite mild exercise and ambulation. Comparison of EDVI and HR revealed a similar linear regression relationship during LBNP before and after bed rest so that EDVI = 112.5-0.85 x HR, r = -0.97. We conclude from these findings that cardiovascular deconditioning for physically active individuals involves factors other than simple loss of plasma volume, requires at least 3 weeks or longer to return to the pre-bed rest state, and is not counteracted by the levels of aerobic and/or isometric exercise used in the present study.