[Cardiovascular and humoral adaptation with passive orthostasis in men]

The aim of our study was to quantitate, in the same test subjects, hormonal, thoracic bioimpedance, blood composition, and hemodynamic dose-responses during different angles of passive orthostatic loading (head-up tilt, HUT) ranging from 12 degrees to 70 degrees. Measurements were performed with 20 min supine (pre-HUT), 30 min upright (HUT-12 degrees, HUT-30 degrees, HUT-53 degrees, or HUT-70 degrees), and 20 min supine (post-HUT); or supine only (HUT-0 degree, rest control). Norepinephrine increased above rest control by 19, 44, 80 and 102%; epinephrine 30, 41, 64, and 68%; aldosterone 29, 62, 139, and 165%; plasma renin activity n. s., 41, 91, and 89%; vasopressin n. s., 27, 47, and 59%; thoracic bioimpedance n. s., 8, 13, and 16%; heart rate n. s., 5, 26, and 45%, and mean arterial pressure n. s., 5, 7, and 10%; respectively, at min 27 of HUT-12 degrees, -30 degrees, -53 degrees, and -70 degrees. Pulse pressure narrowed with HUT-53 degrees and -70 degrees by 4 and 10%. Hematocrit increased 0.2, 1.7, 6.3, and 7.2%, respectively. Blood density increased by 2.3 and 3.0 g/l, plasma density by 1.7 and 1.8 g/l with HUT-53 degrees and -70 degrees. After finishing HUT, heart rate fell to values which stayed below pre-HUT, and also below rest control levels for > or = 5 min ("post-orthostatic bradycardia") even after the lowest orthostatic load (HUT-12 degrees). Thoracic bioimpedance and arterial blood pressure stayed increased after finishing HUT-30 degrees, -53 degrees, and -70 degrees. In summary, passive orthostasis of different degree produces specific dose-responses of different magnitude within endocrine, blood concentration, thoracic bioimpedance, and hemodynamic variables. Heart rate is depressed even after HUT-12 degrees, while arterial blood pressure and thoracic bioimpedance exceed pre-stimulus levels after HUT of higher degree, indicating altered cardiovascular state after passive orthostasis.