The effects of anterior infarction complicated by bundle branch block upon the form of the QRS complex of the canine electrocardiogram

Abstract

In dogs, myocardial infarcts induced by ligating the anterior descending coronary artery in its middle third do not usually modify the QRS complexes of the standard limb leads in a characteristic manner when bundle branch block is present.When such infarcts are complicated by right bundle branch block, the QRS complexes of unipolar leads from the right side of the precordium display a large, initial Q deflection, followed by an R wave which attains its summit late in the long QRS interval. The first component is due to potential variations transmitted from the epicardial surface of the infarcted region, and the second to potential variations transmitted from the epicardial surface of the free wall of the right ventricle. Leads from that part of the precordium overlying the infarct present large, broad QS deflections, often conspicuously slurred or notched.When left branch block is present, infarction of the kind in question does not give rise to characteristic changes in the QRS complexes of the precordial leads because the potential of the left ventricular cavity and, therefore, of the epicardial surface of the infarcted region is positive during the earliest part of the QRS interval. In direct leads from the epicardial surface of the infarct the QRS complex consists of an initial R deflection of variable size, followed by an S component of like or greater voltage. In the case of very large lesions, QRS complexes of this kind probably occur in leads from precordial points overlying the part of the left ventricular wall which is affected, but cannot be considered reliable evidence of infarction.