The Anatomic Substrate of Complete Left Bundle Branch Block

The conduction systems of eight cases diagnosed clinically as having complete left bundle branch block (LBBB) were serially sectioned. Left axis deviation was present in six and normal axis in two. In all cases there was evidence of ischemia of the myocardium and left bundle branch (LBB) in various parts of the coronary circulation. In all cases, the LBB showed pathologic change at its junction point with the bundle of His, with disruption, complete or incomplete, in six, and with recent and old lesions in two. There was no difference in lesions of the LBB between cases with and without left axis deviation. There is thus complete correlation between the electrocardiographic abnormality LBBB and lesions of the LBB in these cases. The pathogenesis of the LBB lesions in these cases is probably both ischemic and mechanical.