Response of the systemic pulmonary circulation to labetalol at rest and during exercise.

1 Labetalol was administered to 18 hypertensive patients for an average duration of 2.44 weeks, with an average final daily dose of 1.65 g (principally to study its short-term haemodynamic effects). 2 Labetalol decreased resting heart rate by 16% and maximal exercise heart rate by 21%; the phenylephrine-induced increase in systolic brachial arterial pressure was reduced by 36%. 3 During labetalol treatment brachial arterial pressure was decreased by 29/15 mmHg in the recumbent position, by 41/23 mmHg at rest sitting and by 53/23 mmHg at maximal exercise; total peripheral resistance was not significantly affected at rest recumbent but was reduced at sitting and at exercise; cardiac output was decreased in all conditions. 4 Labetalol reduced mean pulmonary arterial and capillary wedge pressures only in the sitting position. Pulmonary vascular resistance remained unchanged. 5 The drug produced significant decreases in plasma renin activity and in plasma aldosterone concentration.