Haemodynamic dose-response effects of intravenous labetalol in acute myocardial infarction.

The haemodynamic consequences of combined alpha- and beta-adrenoceptor blockade with intravenous labetalol were evaluated in 21 males with either high (n = 6), normal (n = 9) or low (n = 6) systemic arterial pressure, 4-17 hours after acute myocardial infarction without heart failure. Labetalol 190 +/- 18 mg (range 62-300 mg) infusion resulted in dose-related falls in systolic (P less than 0.01) and diastolic (P less than 0.05) arterial pressure in all patients. The rate of pressure reduction per mg of infused labetalol was directly proportional to the control systolic arterial pressure. Cardiac output (thermodilution) was reduced (P less than 0.05) with low dose labetalol (39 +/- 5 mg) without change on continued infusion. Left heart filling pressure (pulmonary artery occluded pressure) was unchanged throughout infusion in all patients. In conclusion, the vasodilating properties of labetalol appeared to offset the beta-blocker induced depression of cardiac function. The increased sensitivity to the effects of labetalol in those patients with hypertension may be related to excessive alpha-adrenoceptor mediated vasoconstriction in this patient group.