Cardiovascular and hormonal changes with different angles of head-up tilt in men

The purpose of this study was to assess the endocrine status, thoracic impe dance, blood concentration, and hemodynamic dose-responses using different angles of passive head-up tilt (HUT) ranging from 12 degrees to 70 degrees in the same subjects. Measurements were performed during 20 min supine posi tion (pre-HUT), 30 min upright (HUT12, HUT30, HUT53, or HUT70), and 20 min supine (post-HUT); subjects 70 min in the supine position only (HUT0) serve d as resting controls. Norepinephrine increased above resting control value s by 19, 44, 80, and 102 %; epinephrine by 30, 41, 64, and 68 %; aldosteron e by 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, an d 16 %; heart rate n. s., 5, 26, and 45 %, and mean arterial pressure n. s. , 5, 7, and 10 %; at min 27 of HUT12, HUT30, HUT53, and HUT70, respectively . Pulse pressure decreased with HUT53 and HUT70 by 4 and 10 %. Hematocrit i ncreased by 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 HUT53 and HUT70 . After finishing HUT, heart rate fell to values which stayed below pre-HUT , and also below resting control levels for greater than or equal to 5 min ("post-orthostatic bradycardia") even after the lowest orthostatic load (HU T12). Thoracic impedance and arterial pressure remained increased after ter minating HUT30, HUT53, and HUT70. in conclusion, passive orthostatic loadin g of different extent produces specific dose-responses of different magnitu de in the endocrine system, blood composition, thoracic impedance, and hemo dynamic variables. The heart rate is depressed even after HUT12, while arte rial blood pressure and thoracic impedance exceed pre-stimulus levels after greater head-up tilt, indicating altered cardiovascular response after pas sive orthostasis.