Pathogenesis of Left Bundle Branch Block.

In 1909 Eppinger and Rothberger showed that characteristic changes in the electrocardiogram arose after section of the right and left branches of the bundle of His in dogs, and the following year these experimental results were applied to human pathology, so that bundle branch block became a clinical concept. Since that time the electrocardiogram originally designated right branch block has come to be regarded as indicative of a leftsided lesion, and vice versa, and this new conception must be considered as well-founded (Rasmussen, 1942). Apart from this correction, current opinion holds as in 1910, that the bundle branch block cardiogram is an indication of local damage to the right or left branch of the bundle of His. However, in the last ten years especially, the great similarity between the so-called electrocardiogram of left ventricular hypertrophy and that of left bundle branch block has often been emphasized, and it has been suggested that the latter, like the former, may be due to great enlargement of the left ventricle, without any local lesion of the branch being present. In an investigation of the electrocardiogram in essential hypertension, Rasmussen and Thingstad (1939) concluded that great enlargement of the left ventricle was the common cause of the left bundle branch block cardiogram. In cases of hypertension with gross cardiac enlargement they found the left bundle branch block cardiogram as a sequence of the left ventricular hypertrophy curve. This view has been confirmed by further investigations. In a series of 100 patients with aortic insufficiency, a similar concordance between the type of cardiogram and cardiac enlargement was found as in hypertension (Rasmussen, 1944). By producing dilatation of the right and of the left heart in experiments on dogs it was possible to produce typical bundle branch block electrocardiograms (Rasmussen, 1942). On re-examination of a group of hypertensive patients, it was found that the cardiogram of left ventricular hypertrophy and of left bundle branch block developed gradually, pari passu with increase in the size of the heart (Rasmussen and B0e, 1945). It was also shown that