Intravenous labetalol in severe hypertension. Effects on blood pressure, plasma renin activity and catecholamines.

2-Hydroxy-(1-hydroxy-[(1-methyl-3-phenylpropyl)amino]-2-ethyl)-5-benzamide (labetalol), a new alpha- and beta-adrenergic blocking agent, was employed in 21 patients with severe hypertension by slow (6 patients) and rapid (15 patients) i.v. infusion. A marked and significant fall of blood pressure was observed in both groups, though more gradual in patients treated by slow infusion. A rapid blood pressure fall with cardiac output decrease was observed by passing from supine to standing position in the first hours after infusion. Therefore, it is advisable to keep a supine position for a few hours. Plasma renin activity decreased after labetalol infusion, but basal plasma renin levels were not related to hypotensive effect of labetalol. In slow infusion patients, plasma noradrenaline levels increased and no changes of plasma adrenaline levels were observed during infusion. The most likely explanation of these variations is an increase of sympathetic activity secondary to hypotension, caused by labetalol, whereas a technical interference seems to be excluded. The simultaneous blockade of alpha- and beta-receptors can inhibit, in this case, the pressor effects of the sympathetic reflex.