Effects of early beta-blockade with metoprolol on cardiac function in patients with acute myocardial infarction

Twenty patients with acute myocardial infarction were treated with a combination of intravenous metoprolol (15 mg divided in three doses at five-minutes intervals) and oral metoprolol (100 mg twice daily). Blood pressure, heart rate, P-R interval duration, and systolic time intervals were measured prior to and 5, 10, 15, and 60 minutes and 24 hours after the onset of treatment. Blood pressure and heart rate fell on average 10% and 13%, respectively, from their control values, but no patient dropped out due to symptomatic bradycardia or hypotension. Although atrioventricular conduction time lengthened during the trial, P-R interval did not exceed 0.21 seconds in any case. Ejection and pre-ejection times were prolonged in almost all patients, particularly in those who received antiadrenergic drugs before admission. While electromechanical systole shortened at the last stage, pre-ejection time remained prolonged. Despite this sign of ventricular failure, pre-ejection/ejection times ratio and ejection fraction remained normal throughout the study. One patient died of myocardial rupture during the trial. The data did not show severe deleterious effects on blood pressure, heart rate, P-R interval, or left ventricular function assessed by systolic time intervals, indicating that this intravenous early-entry type of beta-blockade could be done safely in patients with uncomplicated myocardial infarction, and even in those with previous abnormal ventricular performance.