COMPARISON OF A 4-DAY CONFINEMENT AND HEAD-DOWN TILT ON ENDOCRINE RESPONSE AND CARDIOVASCULAR VARIABILITY IN HUMANS

The aim of this study was to determine the effects of a 4-day head-dow n tilt (HDT; - 6 degrees) and 4-day confinement on several indicators that might reflect a state of cardiovascular deconditioning on eight m ale subjects. Measurements were made of endocrine responses, heart rat e variability and spontaneous baroreflex response (SBR) slope before, during and after each intervention. Plasma volume decreased by 10% aft er the 4-day HDT. The concentration of active renin was increased and that of urinary atrial natriuretic peptide decreased during the 4-day experiment in both groups. Plasma arginine vasopressin concentration d ecreased significantly only after 4-day confinement. After the 4-day H DT, one of the spectrum analysis parameters was statistically changed: the parasympathetic indicator decreased significantly (P < 0.05) wher eas the sympathetic indicator and the total power spectrum were unalte red. After 4-day confinement spectrum analysis parameters were not sta tistically altered. A significant decrease of SBR (P < 0.05) was notic ed only after the 4-day HDT. These data would suggest that exposure to a 4-day HDT was sufficient to induce a cardiovascular deconditioning which may have been induced by confinement and inactivity.