Selective impairment of excitatory pressor responses after prolonged simulated microgravity in humans

The haemodynamic and autonomic effects of prolonged exposure to simulated m icrogravity were assessed non-invasively in seven healthy volunteers comple ting a 42-day -6 degrees head down tilt. Before, during and after head down tilt, subjects were exposed to moderate excitatory stimuli (mental arithme tic and static handgrip) to gauge possible progressive impairment of presso r responses. Before and after head down tilt, subjects were also exposed to orthostatic stress, to assess influences of simulated microgravity on orth ostatic defence. Simple haemodynamics (heart rate and systolic arterial blo od pressure), linear (i.e., oscillatory) components of beat-by-beat variabi lity, non-linear properties (i.e., corrected conditional entropy (CCE)) of RR interval variability, and baroreflex slope furnished a non-invasive eval uation of autonomic regulatory mechanisms. Pressor responses to mental arit hmetic and to handgrip were markedly impaired after 42 days head down tilt, whereas responses in markers of autonomic regulation were not modified. St anding, performed 8 days after head down tilt to limit the risk of syncope, still induced a variable degree of hypotension, with signs of progressivel y greater sympathetic activation than before head down tilt, Simulated micr ogravity-induced reduction of pressor responses, in spite of largely mainta ined autonomic activation, favours the hypothesis of a peripheral impairmen t of cardiovascular homeostasis. (C) 2001 Elsevier Science B.V. All rights reserved.