Left Anterior Arborization Block Combined with Right Bundle Branch Block in Canine and Primate Hearts

Interruption of anterior fibers of the left bundle branch system together with right bundle branch block was accomplished experimentally in canine and primate hearts. In both species the electrocardiographic effects included a major alteration of the mean electrical axis to a superior and anterior direction. Epicardial excitation was markedly delayed anteriorly, causing widening of the QRS complex in standard electrocardiographic leads. Normal outward intramural spread of excitation in anterior regions of the left ventricle was reversed after left anterior arborization block alone. After addition of right bundle branch block, intramural spread of excitation was again directed outwardly but was markedly delayed in endocardial onset. Electrical effects of these blocks could be "corrected" individually or in combination by introducing synchronized electrical stimuli distal to each lesion. A scheme is proposed by which various forms of intraventricular conduction disturbance can be defined in terms of block of one or more divisions of a three-pronged system of rapid ventricular excitation.