Quantitative measurement of electrical instability as a function of myocardial infarct size in the dog.

To investigate the relation between electrical instability and myocardial infarct size, 20 foxhounds were studied in the awake state 3 to 5 days after closed chest coronary occlusion. Programmed right ventricular stimulation was performed with use of an epicardial electrode. After six paced beats at 10 percent greater than control rate, single and then double extrastimuli were introduced, scanning from late diastole to ventricular refractoriness in steps of 10 to 20 ms. Abnormal responses observed after this provocation were repetitive ventricular response, unsustained ventricular tachycardia, sustained ventricular tachycardia and ventricular fibrillation. Scores for electrical instability were determined for each dog, with higher scores assigned for more hazardous tachyarrhythmias (ventricular fibrillation greater than sustained ventricular tachycardia greater than unsustained ventricular tachycardia greater than repetitive ventricular response) and for those provokable later in diastole. An electrical instability index derived from these scores correlated well with infarct size measured with tetrazolium staining (r = 0.94). When scores were given only for the type of abnormal response elicited, excluding the effect of diastolic timing and the number of extrastimuli or vice versa, there was no significant difference in correlation with infarct size (r = 0.85 versus 0.92). Thus the results demonstrate that inducible electrical instability early after infarction is directly related to infarct size. Further, these data demonstrate the usefulness of an electrical instability index derived from the results of programmed right ventricular stimulation in assessing the severity of ischemic damage to the heart.