Hemodynamic effects of head-down tilt in normal subjects and sustained hypertensive patients.

Central and forearm arterial and venous hemodynamics, arterial baroreflex sensitivity, plasma renin activity, and catecholamines were studied in supine position and after -10 degrees head-down tilt in 29 patients with sustained essential hypertension and in 29 normotensive controls of the same age and sex. In both populations, blood pressure, heart rate, and arterial baroreflex sensitivity did not change during the maneuver. Head-down tilt induced a similar increase in cardiopulmonary blood volume in controls and hypertensives, whereas the increase in central venous pressure, cardiac output, and forearm blood flow was higher in hypertensives. Forearm venous tone decreased in controls (from 16.6 +/- 0.8 to 13.8 +/- 0.9 mmHg X ml-1 X 100 g-1; P less than 0.01) but did not change in hypertensive patients (24.9 +/- 1.6 vs. 25.1 +/- 1.9. The magnitude of forearm vascular resistance changes induced by head-down tilt were significantly related to the basal venous tone in the overall population (P less than 0.01). The decrease in plasma renin activity and plasma catecholamines was similar in the two groups. The study provides the evidence that the higher increase in cardiac output and local flow observed in head-down tilt in hypertensive patients is probably due to a higher change in central venous pressure related to a decrease in venous distensibility.