Response of the Systemic and Pulmonary Circulation

Labetalol (L), a drug with both aand fl-adrenoceptor blocking properties, was administered to 18 hypertensive patients for an average duration of 2.44 weeks, with an average final daily dose of 1.65 g. L decreased resting heart rate (HR) by 16% and maximal exercise HR by 21%; the phenylephrine-induced rise of brachial artery pressure (BAP) was reduced by 30-40%, and the rise of systemic vascular resistance (SVR) by 50%. L lowered BAP by 29/15 mm Hg in the recumbent position (RR), by 41/23 mm Hg at rest sitting (RS), and by 53/23 mm Hg at maximal exercise; SVR was not significantly affected at RR but was reduced at RS and at exercise; cardiac output (CO) decreased in all conditions. L reduced mean pulmonary artery and capillary wedge pressures only at RS. These hemodynamic observations suggest that the antihypertensive action of L is based mainly on its f-receptor blocking properties at RR, and on both its aand f-receptor blocking effects sitting and at exercise. Finally pulmonary vascular resistance (PVR) was not influenced by L, and the phenylephrine-induced increase of PVR was unaffected; the pulmonary arterioles thus seem to react differently to L than the systemic arterioles.