Impaired pressor response after spaceflight and bed rest: evidence for cardiovascular dysfunction

We hypothesized that impaired cardiovascular responses to isometric muscle action contribute to the cardiovascular deconditioning that occurs after sp ace flight (SF) and head-down-tilt bed rest (HDT). Six subjects were studie d before, during and after 120 days of -6 degrees HDT, and four subjects we re studied before, during (two subjects) and after 179-389 days of SF. Subj ects performed a sustained handgrip (SHG) at a force equivalent to 30% of m aximum contraction force for 2 min, and heart-rate (HR) and pressor (mean a rterial pressure, Delta MAP) responses were recorded. At the same relative force, both Delta HR and Delta MAP were significantly reduced during the fi rst days after HDT (-54%, P<0.05 and -43%, P<0.05). In two subjects studied within 24 h after their return from SF, Delta MAP was practically absent ( -79%, P<0.05) whereas in four subjects studied 1-4 days after return from S F, <Delta>MAP was reduced by 35% (P<0.05). <Delta>HR was not significantly changed. Our finding of attenuated pressor responses to SHG after HDT and S F supports the notion of impairments at both the neurocirculatory control a nd effector organ levels.