To elucidate the architecture of the left bundle branch, activation of Purkinje (P) and myocardial (M) cells of the canine left ventricular endocardial surface was mapped in vitro before and after lesions were placed in the proximal left bundle branch. Close bipolar electrodes or microelectrodes were employed to record P and M potentials by standard techniques. The left bundle branch has conspicuous and straight border fibers that continue as bridging strands to the papillary muscles. In addition, there is a less conspicuous network of serpentine interwoven fibers in the interior of the septal portion of the bundle that has interconnections with the border fibers. Below the mid-septum and over the free wall the P fibers form a densely interwoven network. The interior septal fibers are activated nearly simultaneously with equidistant points on the border fibers. The apparent conduction velocity of the border fibers was 1.8 m/sec as opposed to 1.45 m/sec in the interior network. The bridging strands provide a shortcut across the chamber to upper portions of the free wall and septum. The pattern of activation of M cells is represented by concentric rings with an island of earliest activation (at 15 to 25 msec) in the central septum. The M cells latest to be activated are at the tips of the papillary muscles and the upper septum below the valves. Interruption of one group of border fibers may delay by 5 to 8 msec activation of P and M cells of the upper paraseptal free wall and upper septum. Lesions in the interior network delay by 2 to 4 msec activation of the P and M cells of comparatively small areas of the septum. Activation of the mid-papillary free walls, the bases of the papillary muscles and the lower septum is not affected by proximal lesions partially interrupting the bundle branch. Transection of both groups of border fibers does not produce complete block. The left bundle branch is best represented as a fan-shaped network of interwoven fibers whose diverging border fibers are specialized for rapid, distant transport.
ASJC Scopus subject areas
- Cardiology and Cardiovascular Medicine