Ventricular Echo Beats and Retrograde Atrioventricular Nodal Exits in the Dog Heart: Multiplicity in Their Electrophysiologic and Anatomic Characteristics

INTRODUCTION: A single ventricular echo beat frequently is induced in the dog heart by ventricular pacing, but it has not been investigated using a concomitant ablative technique. We studied the effects of ablating the anterior atrial input to the AV node on ventricular echo beats induced in the dog heart to evaluate their electrophysiologic characteristics, the anatomic reentrant circuit, and the retrograde AV nodal exits. METHODS AND RESULTS: In 20 dogs, an epicardial radiofrequency current was applied to the right anterior septum in an attempt to ablate the anterior input to the AV node. Ventricular programmed stimulation was performed to evaluate the ventricular echo beat and the retrograde AV nodal exit before and after ablation. The AV junction was examined with light microscopy. Seventeen dogs in which the PR interval was prolonged significantly from 108+/-17 msec to 153+/-19 msec (P < 0.001) were selected for ventricular echo evaluation; 3 dogs in which persistent second- or third-degree AV block was induced by ablation were excluded. Ventricular echo beats, which were induced in 13 of 17 dogs, were classified into the anterior type (n = 6) or posterior type (n = 7) according to the earliest atrial activation site during the echo beat. The retrograde AV nodal exit site showed anterior-exit only (n = 10), posterior-exit only (n = 2), and dual-exit (n = 5) patterns. After ablation, the anterior-type ventricular echo beat was noninducible in all 6 dogs, whereas the posterior-type ventricular echo beat was noninducible in only 3 of 7 dogs. In 17 dogs, VA conduction was not demonstrated after ablation in 3 dogs, all of which showed the anterior-exit only pattern. CONCLUSION: The effect of ablation on the ventricular echo beats and retrograde AV nodal exit site suggests multiplicity in their electrophysiologic and anatomic characteristics in the dog heart.