Observations on changes in ventricular complexes produced by bundle branch block with special reference to the hypothesis of electrical axis and the concept of dextrocardiogram and levocardiogram

Abstract 1. 1. Five cases of transient right bundle branch block are reported which showed that the main deflections of the QRS complex in limb leads were very slightly altered as a result of right bundle branch block, although all cases showed the broad terminal deflection characteristic of that condition. The T waves of limb leads likewise showed only slight alterations during bundle branch block. Precordial leads made with the exploring electrode placed over the right side of the precordium showed marked changes in both QRS complexes and T waves during bundle branch block. In some cases, however, there was no change in the earliest part of the QRS complex. Leads made with the exploring electrode placed over the left side of the precordium showed little change except for the broad terminal deflection of the QRS complex. 2. 2. The development of left bundle branch block produced radical changes in the ventricular complexes of the same leads that were little changed by right bundle branch block. However, in the leads most radically changed by right bundle branch block (namely, those made with an exploring electrode placed over the right side of the precordium) an early deflection in the QRS complex was preserved, although somewhat modified in certain of its characteristics. 3. 3. The changes in the ventricular complex that occur in bundle branch block indicate, provided current views as to the mechanism of bundle branch block are sound and also provided the right ventricle is not enlarged, that (1) the left ventricle and the part of the septum normally activated via the left ventricle are the source of electrical activity mainly responsible for the main deflections of the QRS complexes and also the T waves of limb leads, as well as leads made with the exploring electrode placed over the left side of the precordium when intraventricular conduction is of the normal type, and (2) the right ventricle is mainly, although not completely, responsible for differences of potential in leads made with the exploring electrode placed over the right side of the precordium. These findings are not in accord with views widely held regarding the parts played by the two ventricles in the hypothesis of electrical axis. They indicate that, except in leads made with an electrode placed near the right ventricle, the levocardiogram tends to contribute far more to the electrocardiogram than the dextrocardiogram. This, considered in connection with evidence that (1) certain parts of the left ventricle are electrocardiographically silent as far as “axis” is concerned and (2) by certain methods of pairing electrodes the pattern of differences of potential obtained with an exploring electrode placed just to the left of the precordium closely resembles that obtained by moving the exploring electrode to the left arm, appears to expose the hypothesis of electrical axis to the charge of having been a hindrance to progress in understanding the nature of the mechanisms concerned in the formation of electrocardiograms.