Changes in blood pressure and body fluid volumes during diuretic therapy in patients with essential hypertension who receive enalapril

We evaluated the effect of additional chlorthalidone therapy on blood pressure and body fluid volumes in 10 patients with essential hypertension who did not respond to chronic converting enzyme inhibition with enalapril. Values assessed after 3 days and 6 weeks of combined enalapril and chlorthalidone therapy were compared with initial values during enalapril monotherapy. After 3 days the mean arterial pressure (MAP), plasma volume (PV), blood volume (BV), and extracellular fluid volume (ECFV) decreased. There was a positive correlation between the percentage decreases in MAP and BV. After 6 weeks the MAP decreased further, but the decreases in PV, BV, and ECFV were less pronounced. At this time there was a positive correlation between the percentage decreases in MAP and ECFV. Our results support the hypothesis that contraction of the ECFV is an antihypertensive mechanism of diuretics. The antihypertensive effect of diuretics is enhanced during converting enzyme inhibition, while the body remains protected against volume deficits, possibly by the lower blood pressure itself. Clinical Pharmacology and Therapeutics (1986) 39, 60–64; doi:10.1038/clpt.1986.11