A trial of intravenous and oral mexiletine in acute myocardial infarction.

Intravenous and oral mexiletine prophylaxis was compared with lignocaine supplemented placebo in a single blind trial in 240 high-risk patients with acute myocardial infarction. Although atrial fibrillation, supraventricular tachycardia and ventricular extrasystoles occurred less frequently in the mexiletine treated patients, ventricular tachycardia and primary ventricular fibrillation were not prevented. Mortality at 6 weeks was less in the mexiletine group (19%) than placebo (27%) but not significantly so (0.2 greater than p greater than 0.1). An 80% chance of showing a significant difference would require 860 high-risk patients. Low plasma mexiletine levels after 3 h treatment were due to diamorphine and may explain failure to prevent major arrhythmias. Pretreatment with intravenous metoclopramide tended to reverse this effect of diamorphine.