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

Abstract

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 were studied. Studies were performed in 20 adult mongrel dogs during cardiopulmonary bypass, using direct endocardial extracellular bipolar recordings of activation 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), 7 were in the distal portion and 6 were diffusely delayed throughout. In 9 dogs subjected to premature right ventricular stimulation, 9 sites of retrograde conduction delay or block were noted: 8 were in the distal portion of the RBB and 1 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 2 dogs, who had 2:1 (or greater) block between activation of the ventricular muscle and RBB. 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.