Reassessment of the subdivision block of the left bundle branch.

Abstract

There still exist many questions to the human intraventricular conduction system, especially morphology of left ventricular (LV) conduction system. In the present study, LV conduction system was examined by serial histological examinations, and based on these results experimental and clinical subdivision block of the left bundle branch (LBB) were studied electrophysiologically and histopathologically. 1 ) Reconstruction of the human LBB: Human LV conduction system of 14 hearts showed much variability. LV conduction system was a diffuse fan like structure broadly distributed over the left septal surface and always covered diffusely mid-septal area. 2) Experimental subdivision block of the LBB: In the discrete block of the septal Purkinje network (11 dogs), the activation time in the apical area prolonged from 20.0±4.9 msec to 24.5±4.9 msec (p<0.05), and the QRS loop tended to be displaced anteriorly. In the discrete block of the left posterior division (9 dogs), the activation time showed slight prolongation in the posterior basal area, but the QRS waves of the scalar ECG did not change significantly. In the block of the left posterior division and septal Purkinje network ( 11 dogs), epicardial activation delay was observed in a relatively wider area from the apex to the posterior basal area. The maximum QRS vector in the frontal plane shifted rightward (p<0.05). 3) Prominent anterior QRS force (PAF) in clinical electrocardiogram: PAF was observed frequently in patients with ischemic heart disease (12.4%) and diabetes mellitus (8.5%). Serial histological examination of a patient with PAF showed marked fibrosis in the mid-septal fibers in association with fibrosis of the right bundle branch and posterior part of the LBB. Intermittent PAF was observed in supraventricular premature beats suggesting that PAF might be due to intraventricular conduction disturbances as well as left anterior or posterior hemiblock. This intermittency also supports the existence of this new type of divisional block of the LBB. New concepts of intraventricular conduction disturbances must be established including the conduction disturbances of the mid-septal fibers of the LV conduction system.