Role of the conduction system in the endocardial excitation spread in the right ventricle.

The role of the Purkinje network in the excitation sequence in the endocardial surface of the right ventricle was studied using an isolated perfused canine preparation. The preparation was electrically stimulated at the proximal right bundle branch, and the activation time was mapped using contiguous bipolar electrodes or a microelectrode, or both. The earliest activation of muscle was observed at the junction between the ventricular septum and the free wall in front of the anterior papillary muscle. After the initial activation, the spread of ventricular muscle excitation in the free wall was essentially radial at a mean (+/- standard deviation) conduction velocity of 1.67 +/- 0.20 m/s. The activation of muscle in this area was almost always preceded by the activation of Purkinje fibers by 2 to 6 ms. Thus the Purkinje system was considered indispensable for the excitation spread in this area. In the lower third of the septum, the excitation sequence was essentially similar to that of the free wall, indicating a possible contribution of the fast conducting Purkinje system. In contrast, in the upper two thirds of the septum the activation of ventricular muscle spread from apex to base with a significantly delayed conduction velocity of 0.41 +/- 0.88 m/s, and it was not preceded by activation of Purkinje fibers, thus indicating the lack of involvement of the Purkinje system. Functional distribution of the anterior, posterior and lateral branches of the right bundle to the excitation spread of the right ventricular endocardial muscle was confirmed by the selective transection of one of these special conducting fibers, which suggested the etiologic significance of injury to each branch of the right bundle as a cause of various electrocardiographic patterns of incomplete right bundle branch block.