We investigated the anatomic substrate of the slow pathway of the atrio-ventricular (AV) node in a dog with AV nodal reentrant tachycardia. Sustained AV nodal reentrant tachycardia was repeatedly induced in a mongrel dog after putting cryolesions on the anterior atrial septum for 1 month. A posterior perinodal dissection was performed for ablation of the slow pathway and cure of the tachycardia. After the operation, AV nodal reentrant tachycardia and discontinuity of the AV conduction were no longer demonstrated. A histologic examination of the AV junction revealed that the dissection injured mainly in two areas: (1) the superficial atrial fibers of the subendocardium over the compact AV node; and (2) the inferior portion of the posterior atrial inputs near the tricuspid annulus. The compact AV node and its transitional cells remained intact. In conclusion, our results suggested that (1) the anatomic circuit of AV nodal reentrant tachycardia does involve some extranodal atrial tissues; and (2) the superficial right atrial fibers over the AV node and/or the inferior portion of the posterior atrial inputs, contribute to part of the slow AV nodal pathway.
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