The comparative permanence of electrocardiographic abnormalities, especially bundle branch block, once established is an admitted maxim. The more promising possibility of the transient nature of such disturbances is usually not considered. In experimental work on conduction in the heart of the dog, Wilson and I1 were impressed by the fact that relatively light pressure exerted over one of the main branches of the His'bundle would produce bundle branch block from which there was complete recovery within a few minutes and a return to an absolutely normal intraventricular conduction time which was recorded.Lewis had observed a patient with transient bundle branch block as early as 1913. There have been 9 other instances recorded in the literature which at one examination presented defective conduction while at another later examination the normal intraventricular conduction was recorded and vice versa. In none of these instances, however, was there any record of a transition of a sudden nature. In fact, in m...