Variant forms of AV and VA conduction in the canine heart

In 19 pentobarbital anesthetized dogs subjected to right thoracotomy, plunge wire and catheter electrodes were positioned to record and pace from the high right atrium, coronary sinus os, aortic root (for His bundle electrograms), and the right ventricular outflow tract and apex. A deflectable-tip catheter electrode was placed under the septal tricuspid leaflet for pacing the right ventricular septal crest. Electrocardiographic leads II and aVR were also recorded during atrial and ventricular pacing in the control state and during low- and high-intensity left vagosympathetic trunk stimulation. Pacing from each A or V site in 12 dogs showed normal atrioventricular (AV) and ventriculoatrial (VA) conduction over a wide range of heart rates (120-360 beats/min) with progressive AV and VA delay at each heart rate until Wenckebach cycles occurred (AV Wenckebach cycle averaged 325 +/- 44 beats/min; VA Wenckebach cycle averaged 246 +/- 52 beats/min). In two dogs, pacing from the coronary sinus os showed a 30-beat/min difference in the Wenckebach cycle rate compared to high right atrium pacing, and the AH intervals for the former were relatively insensitive to vagosympathetic trunk stimulation compared with the latter. In five other dogs, retrograde VA and HA intervals showed little, if any, decrement over a wide range of heart rates, with no immediate response to low-intensity vagal stimulation. After 7-22 seconds of continuous vagal stimulation, VA intervals abruptly prolonged and/or blocked. In all, second-degree heart block occurred at ventricular pacing rates > or = 300 beats/min in the form of atypical Wenckebach cycles. In all cases, the earliest atrial activation, as measured by a close bipolar probing electrode, was in the posterior septum adjacent to the coronary sinus os. In one dog, VA block occurred as a Mobitz type II with paroxysmal complete VA block (sinus escape rhythm) during continuous ventricular pacing at 330 beats/min. After radiofrequency ablation of the earliest atrial activation site typical VA Wenckebach cycles, immediately responsive to vagal stimulation, was then observed at a ventricular pacing rate of 150 beats/min. These data suggest that AV and VA connections in some canine hearts show varying degrees of bypass of the AV node, similar to cases reported in the clinical literature.