Cardiovascular and humoral readjustment after different levels of head-up tilt in humans

Purpose: To get a more complete picture of cardiovascular regulation after postural changes, this investigation directly monitored volume-related, hem odynamic, and endocrine variables during and after 30 min of passive head-u p tilt (HUT) of various degrees. It was hypothesized that the return of var iables to pre-tilt control level is of system-specific duration and differe nt from what is found after lower body negative pressure (LBNP). Design: We tested 7 persons on 5 different days using, in random order, no (HUT0) or different intensity (12 degrees, 30 degrees, 53 degrees, and 70 degrees) of passive orthostasis (HUT12, HUT30, HUT53, HUT70). Data were collected befo re (supine), during, and after (supine) HUT and compared with synchronous d ata from HUT0. Results: There was graded alteration with the sine of till a ngle for all hormones and directly volume-related variables. The effects of HUT70 were of the same magnitude as previously documented by others. After HUT, hemodynamic variables and catecholamines returned to control levels m ost rapidly. Heart rate depression, as observed in a companion LBNP study i n the same subjects, did not occur. Vasopressin, PRA, plasma volume and Z(0 ) returned to nominal values more slowly. Plasma aldosterone was still elev ated 50 min after reassuming supine posture. Conclusion: Besides specific d ose-responses within hemodynamic, volume-dependent, and hormonal variables after orthostatic loading of different degree, the return to control levels after HUT occurs with distinctly different time-courses, which are not ide ntical with those seen after LBNP-simulated orthostasis.