Bundle Branch Block

The difficulties experienced in differentiating between simple left axis deviation of the heart and beginning right bundle branch block are known to all clinicians interested in cardiovascular pathology. Observers of wide experience have noted from time to time that what appeared to be a simple left axis deviation of the heart at one electrocardiographic examination might subsequently and within a short time prove to be a true right bundle branch block. During the past five years, we have observed a group of patients whose electrocardiograms showed certain common characteristics. In some of these patients complete right bundle branch block developed; in one of them, the condition started with what appeared to be a right axis deviation of the heart, and at death the electrocardiogram showed a left bundle branch disturbance. A consideration of the earliest manifestations of bundle branch block forms the basis of this paper. The cases selected were