Reassessment of AV and VA conduction and AV junctional reentry in the normal dog heart: the role of altered autonomic tone.

The effects of isoproterenol, atropine, and metoprolol on atrioventricular (AV) and ventriculoatrial (VA) conduction were studied in 30 normal dogs under pentobarbital anesthesia using percutaneously introduced catheters. The inducibility of AV junctional reentry was also assessed before and after drug administration. Resting AV conduction was normal in all dogs, but VA conduction was present in only 57%. Isoproterenol facilitated both antegrade and retrograde conduction, with a preferential effect on retrograde conduction. VA conduction was demonstrated after isoproterenol in 91% of dogs. After testing all drugs, VA conduction was demonstrable in at least one study in 97% of dogs. Atropine had less effect than isoproterenol, suggesting that basal vagal tone was not high in this model. Dual AV nodal pathways were detectable in the antegrade direction in four (13%) dogs, and in the retrograde direction in an additional four (13%) dogs. Single AV junctional echoes were inducible with atrial stimulation in one dog with dual antegrade pathways, but were inducible with ventricular stimulation in at least one study in 83% of dogs with intact retrograde conduction. Sustained AV junctional reentry was never induced before or after drug administration. In conclusion, VA electrical continuity is almost always intact in the normal dog, but its demonstration is significantly modified by the autonomic nervous system. Isoproterenol has preferential effects on retrograde conduction and may have selective influence on distal AV nodal conduction. Twenty-six percent of normal dogs have evidence of dual AV nodal pathways. Single AV junctional echoes are inducible with ventricular stimulation in the majority of dogs and are a normal finding. Sustained AV junctional reentry is not inducible in the normal intact dog heart.