Mouse AV Nodal Maturation. Introduction: Dual AV nodal physiology is characterized by discontinuous conduction from the atrium to His bundle during programmed atrial extrastimulus testing (A₂V₂ conduction curves), AV nodal echo beats, and induction of AV nodal reentry tachycardia (AVNRT). The purpose of this study was to characterize in vivo murine maturational AV nodal conduction properties and determine the frequency of dual AV nodal physiology and inducible AVNRT.

Methods and Results: A complete transvenous in vivo electrophysiologic study was performed on 30 immature and 19 mature mice. Assessment of AV nodal conduction included (1) surface ECG and intracardiac atrial and ventricular electrograms; (2) decremental atrial pacing to the point of Wenckebach block and 2:1 conduction; and (3) programmed premature atrial extrastimuli to determine AV effective refractory periods (AVERP), construct A₂V₂ conduction curves, and attempt arrhythmia induction. The mean Wenckebach block interval was 73 ± 12 msec, 2:1 block pacing cycle length was 61 ± 11 msec, and mean AVERP₁₀₀ was 54 ± 11 msec. The frequency of dual AV nodal physiology increased with chronologic age, with discontinuous A₂V₂, conduction curves or AV nodal echo heats in 27% of young mice < 8 weeks and 58% in adult mice (P = 0.03).

Conclusion: These data suggest that mice, similar to humans, have maturation of AV nodal physiology, hut they do not have inducible AVNRT. Characterization of murine electrophysiology may be of value in studying genetically modified animals with AV conduction abnormalities. Furthermore, extrapolation to humans may help explain the relative rarity of AVNRT in the younger pediatric population.