Cardiovascular deconditioning produced by 20 hours of bedrest with head-down tilt (−5°) in middle-aged healthy men

Abstract Cardiovascular deconditioning after prolonged bedrest has been attributed to inactivity. To examine the role of the altered distribution of body fluids, 5 healthy men, aged 41 to 48 years, were studied before, during and after a 20-hour period of bedrest with head-down tilt (−5°). This intervention produces a marked central shift of intravascular and interstitial fluid, but the short duration minimizes the effects of inactivity. Central venous pressure, cardiac output and stroke volume all increased significantly (p 2 O, cardiac output from 6.9 to 7.9 liters/min, and stroke volume from 104 to 113 ml after 15 minutes of tilt, but all values returned to baseline within 20 hours. Supine central venous pressure after tilt was 7.4 cm H 2 O, cardiac output 5.7 liters/min and stroke volume 84 ml. Blood volume decreased 0.51 liters. After tilt, orthostatic stress produced a higher heart rate (90 ± 18 vs 68 ± 12 beats/min). Maximal oxygen consumption decreased (2.36 ± 0.41 vs 2.62 ± 0.48 liters/min), mainly owing to reduced stroke volume (87 ± 22 vs 107 ± 18 ml, p