IRBBB pattern after incising a subdivision of the right bundle branch.

The genesis of IRBBB pattern was studied by incising subdivisions of the right bundle branch in 23 anesthetized dogs. The excised heart, perfused with Tyrode's solution using Langendorff's technique, was placed within a simplified model of the human torso. This procedure greatly improves vision, offering a completely unimpeded direct view of the right ventricular cavity during the incising procedure and provides a superior method for recording electroand vectorcardiograms. The QRS duration was not significantly prolonged by incising any of the subdivisions. After incising the lateral subdivision, the QRS pattern showing a prominent R' wave in Lead V1 occurred in 35% of the cases; the QRS pattern showing rather deep or deep S waves in almost all the precordial leads occurred in 47%; and in 18% there were no characteristic pattern changes. The midportion of the basal region of the right ventricle was confirmed to be the region which exhibits a significant delay in activation after incising the lateral subdivision. This experimental study suggests that damage to the lateral subdivision of the right bundle branch, (i.e., so-called false tendons), may be an underlying factor in the production of IRBBB pattern.