Changes in plasma volume and extracellular fluid volume after addition of prazosin to propranolol treatment in patients with hypertension.

In fifteen patients with hypertension, inadequately controlled during treatment with propranolol alone (mean dosage 333 mg/day), plasma volume (PV) and extracellular fluid volume (ECV) were determined. After addition of prazosin for 3 months (mean dosage 9 mg/day) there was a significant increase in PV and ECV, on average 8 and 5%, respectively. The decrease in supine blood pressure, systolic as well as diastolic, was very modest, on average 11 mmHg (SD +/- 12) and 4 mmHg (SD +/- 7), respectively. The changes in standing blood pressure were more pronounced. It is assumed that the expansion of PV and ECV contributes to the inadequate blood pressure response found in the present study.