Site of antegrade and retrograde functional right bundle branch block in the intact canine heart.

The purpose of this study was to determine the site of conduction delay or block in situ during antegrade and retrograde functional right bundle branch block (FRBBB) and whether reentrant excitation can occur in both directions in the blocked bundle branch, depending on whether the site of block occurred proximally or distally. Studies were performed in 20 adult mongrel dogs during cardiopulmonary bypass, using direct endocardial extracellular bipolar recordings of activaation in the His-Purkinje system. In 20 dogs subjected to premature atrial stimulation, 30 sites of antegrade conduction delay or block were documented: 17 were in the proximal portion of the right bundle branch (RBB), seven were in the distal portion and six were diffusely delayed throughout. In nine dogs subjected to premature right ventricular stimulation, nine sites of retrograde conduction delay or block were noted: eight were in the distal portion of the RBB and one was in the proximal portion of the RBB. When antegrade FRBBB occurred, the impulse.traveled to the ventricle over the left bundle branch (LBB) and reentered retrogradely the previously blocked RBB. When retrograde FRBBB occurred, the impulse traveled retrogradely over the LBB and reentered antegradely the previously blocked RBB. Both proximal and distal block and both types of reentry could be produced in the same.dog. Ventricular tachycardia developed in two dogs, who had 2: 1 (or greater) block between activation of the ventricular muscle and RBB. We conclude that antegrade FRBBB usually occurs in the proximal RBB and retrograde FRBBB usually in the distal RBB, but may occur at other sites as well. Reentry in the RBB can occur in either direction, depending on the site of the block. Sustained bundle branch reentry involving the RBB was not a likely cause for sustained ventricular tachycardia in this dog model.