Mechanisms of spontaneous and induced ventricular arrhythmias in the 24-hour infarcted dog heart.

The incidence and types of ventricular arrhythmias were evaluated in 14 dogs, 24 hours after occlusion of the left anterior descending coronary artery. After induction of anesthesia and left thoracotomy, standard electrocardiographic leads and electrograms from the His bundle and the left ventricular endocardium and epicardium (both infarct and normal zones) were recorded. Spontaneous ventricular tachycardia presumably due to abnormal automaticity was consistently observed (average rate 154 +/- 26 beats/min). These arrhythmias were irregular and multiform. In this same group of dogs 3 ventricular paced beats at rates above 300 beats/min induced rapid and uniform reentrant ventricular tachycardias (average rate 345 +/- 17 beats/min) that were difficult to terminate by premature beats or overdrive pacing. During this sustained ventricular tachycardia, continuous electrical activity was recorded from composite electrodes on the epicardial surface overlying the infarcted zone. Such interectopic activity was not observed during the spontaneous, automatic arrhythmias. Two of the 14 dogs showed only ventricular fibrillation in response to the provocative stimuli. These data confirm previous findings that 24 hours after acute myocardial infarction in the dog heart multiform ventricular rhythms can result from enhanced automaticity. In addition, it was found that ventricular tachycardia can be induced in the same dog heart by a standard ventricular pacing procedure. Continuous electrical activity bridging the interectopic intervals only during the latter tachycardia provides strong evidence for the reentrant basis of these induced arrhythmias. These experimental findings are comparable to the 2 different forms of ventricular arrhythmias described clinically, at the same stage of myocardial infarction. The direct myocardial recordings also provide new insights into the phenomenon of entrainment of tachyarrhythmias by overdrive pacing.