Labetalol in Acute Myocardial Infarction It reduces total systemic resistance

Labetalol has al-selective antagonistic properties and Ihintrinsic sympathomimetic activity and seems to decrease pulmonary wedge pressure (PWP) in patients with heart failure, although its mechanism is not understood.

18 patients (mean age 57 years) with acute myocardial infarction (for less than 48 hours) and systemic hypertension (systolic BP greater than 150mm Hg for over 6 hours) were given labetalol 0.5 mg/min IV increased gradually until systolic BP was less than 130mm Hg, and continued at the optimal dose (0.8-4.8 mg/min) for 60 min. The patients were divided into two groups with initial PWP greater than 15mm Hg (Group 1 - mean PWP 22mm Hg) and PWP less than 12mm Hg (Group 2 - mean PWP 7mm Hg). Initial right atrial pressure was also significantly higher in Group 1, and initial stroke index and left ventricular stroke work index (LVSWI) were significantly lower than in Group 2.

After labetalol infusion, the mean systemic BP was very significantly decreased, by 33% in Group 1 and 23% in Group 2. Total systemic resistance and cardiac index decreased more significantly in Group 1 (by 22% and 17%) than in Group 2 (13% and 12%). There were significant decreases in heart rate (17%), LVSWI (27-30%) and double product (43-46"10) in both groups, but no significant change in stroke index, total pulmonary resistance or right atrial pressure. In Group 1 there were significant decreases in mean pulmonary pressure (23%) and PWP (32%), but slight, although not significant increases in Group 2. Side effects were transient nausea, weakness, headache, flushing and dizziness (each in 1 patient) and in 1 patient the dose was reduced.

Labetalol seemed effective in reducing hypertension in acute myocardial infarction, producing significant decreases in total systemic resistance and cardiac index. The significant decrease in PWP in Group 1 is unlikely to be due to the a-blocking action of labetalol, as vasodilation was seen in both groups, and a decrease of PWP by fj·blockers has not consistently been demonstrated. This effect could be related to changes in ventricular relaxation and compliance. Renard, M. et al.: Journal of Cardiovascular Pharmacology 6: 90 (Jan·Feb 1984)

16 INPHARMA eJ 26 May 1984 0156-2703/84/0526-0016/ 0$01.00/0 © ADIS Press