Functional importance of the left septal Purkinje network in the left ventricular conduction system

Changes of the activation sequence and vectorcardiogram (VCG) resulting from experimental block of specific sites of the left bundle branch were studied to determine the functional significance of the septal Purkinje network. Discrete block of the septal Purkinje network (11 dogs) resulted in slight delay of epicardial activation in the apical area. In lead Z of McFee system of VCG, the antero-posterior ratio was increased from 1.4 to 2.6, and the QRS loop tended to be displaced anteriorly. Discrete block of the left posterior division (9 dogs) resulted in slight delay of activation in the posterior basal area. Block of the left posterior division plus the septal Purkinje network (11 dogs) resulted in marked delay in activation of a wider area extending from the posterior basal area to the apical area. In lead X, the S wave became evident and the direction of the maximum QRS vector in the frontal plane showed significant right axis deviation. Discrete block of the left posterior division did not change the epicardial activation sequence or the direction of the maximum QRS vector, because there is extensive interconnection of the left ventricular conduction system. These results suggest that block of the septal Purkinje network itself may produce an electrocardiographic change and that this network may also be important in left anterior and posterior hemiblocks producing axis changes in the frontal plane of VCG.