Vectorcardiographic and epicardial activation studies on experimentally-induced subdivision block of the left bundle...

The functional significance of the septal Purkinje network (SEP) in the left ventricular conduction system was examined by measuring changes of the activation sequence and vectorcardiogram (VCG) after inducing experimental blocks of the left anterior subdivision (LA) and the SEP. A discrete block of the SEP (10 dogs) resulted in a slight delay of the epicardial activation in the apical area. In lead of the McFee system of VCG, the antero-posterior ratio was increased significantly, and in 3 of 10 cases (30%) major QRS loops were deviated anteriorly on the horizontal plane. In the discrete block of the LA (10 dogs), epicardial mapping revealed a slight activation delay in the anterior basal area. There were no significant changes on VCG, except for a superior and leftward deviation of the terminal QRS vector. In the combined block of the LA + SEP (9 dogs), an activation delay of the greater area and magnitude was observed from the anterior basal area to the apical area, and the direction of the maximum QRS vector on the frontal plane showed a significant displacement to the left and superiorly. These results suggest that the SEP block itself may produce an electrocardiographic change and that this network also plays an important role in the so-called hemiblock, producing axis changes on the frontal plane of a VCG, in association with the blocks of the left anterior and posterior subdivisions.