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 space flight (SF) and head-down-tilt bed rest (HDT). Six subjects were studied before, during and after 120 days of -6 degrees HDT, and four subjects were studied before, during (two subjects) and after 179-389 days of SF. Subjects performed a sustained handgrip (SHG) at a force equivalent to 30% of maximum contraction force for 2 min, and heart-rate (HR) and pressor (mean arterial pressure, deltaMAP) responses were recorded. At the same relative force, both deltaHR and deltaMAP were significantly reduced during the first days after HDT (-54%, P<0.05 and -43%, P<0.05). In two subjects studied within 24 h after their return from SF, deltaMAP was practically absent (-79%, P<0.05) whereas in four subjects studied 1-4 days after return from SF, deltaMAP was reduced by 35% (P<0.05). deltaHR was not significantly changed. Our finding of attenuated pressor responses to SHG after HDT and SF supports the notion of impairments at both the neurocirculatory control and effector organ levels.