HEMODYNAMIC, RENAL, AND HORMONAL RESPONSES TO LOWER-BODY POSITIVE PRESSURE IN HUMAN-SUBJECTS

Studies in healthy human subjects subjected to lower body positive pre ssure (LBPP) have failed to elucidate many of the physiologic effects of this maneuver. In 7 healthy, well-hydrated men we studied the follo wing responses to LBPP (35 mm Hg, 1 hour, supine position): systemic a nd renal hemodynamics; urine volume (UV), urine osmolality (Uosm), and urine sodium level (UNaV); free water (CH20) and osmolar (Cosm) clear ances; plasma renin activity (PRA); levels of aldosterone (PA), cortis ol (CORT), norepinephrine (NE), atrial natriuretic peptide (ANP), and vasopressin (AVP); osmolality (Posm); and serum sodium revel. Subjects were restudied on a control day with zero trouser pressure. The recor ded changes (p < 0.05) when comparing the LBPP day with the control da y were as follows: fractional Na+ reabsorption increased (98.7% +/- 0. 2% to 99.3% +/- 0.1%) and UNaV decreased (0.19 +/- 0.03 mEq/min to 0.1 0 +/- 0.01 mEq/min), with concomitant increases in PRA (1.7 +/- 0.2 ng /ml/90 min to 4.5 +/- 1.8 ng/ml/90 min), PA (7.7 +/- 0.7 ng/dl to 9.3 +/- 1.5 ng/dl), and CORT (13.0 +/- 2.6 mg/dl to 19.2 +/- 3 mg/dl); the increase in blood pressure with LBPP (96 +/- 3 mm Hg to 112 +/- 4 mm Hg) was greater than that during control conditions. Renal plasma flow tended to display an interactive pattern across days, with ct slight decline during LBPP (5%) and a slight elevation under control conditio ns (9%). On the LBPP day only, filtered Na+ declined (15 +/- 1 mEq/min to 12 +/- 1 mEq/min) as a function of reduced glomerular filtration r ate (112 +/- 5 ml/min to 91 +/- 7 ml/min), blood volume decreased (by 2.7% +/- 0.7%), CO decreased (5.5 +/- 0.3 L/min to 4.7 +/- 0.3 L/min), and stroke volume declined (101 +/- 6 ml to 84 +/- 3 ml). On both day s, NE increased (control, 221 +/- 23 pg/ml to 340 +/- 33 pg/ml; L8PP, 236 +/- 17 pg/ml to 369 +/- 31 pg/mD and ANP increased (control, 47 +/ - 7 pg/ml to 97 +/- 21 pg/ml; LBPP, 49 +/- 10 pg/ml to 104 +/- 30 pg/m l). We concluded that LBPP reduces renal sodium excretion. The mechani sm for this reduction is not known, although it did occur in associati on with an increase in plasma renin activity, which in turn results fr om mechanical reduction of renal perfusion, stress-related CORT stimul ation, a reflex-based elevation in peripheral vascular resistance lead ing to a reflex increase in plasma renin activity, or a combination of these.