Introduction: The complex electrophysiological phenomena related to the atrioventricular node (AVN) are due to its complex anatomical structures. Aside from the inferior nodal extension (INE), other node-like tissues, such as the retroaortic node (RN), have been less described and may also share the mechanism of normal conduction and abnormal conduction in AVN re-entrant tachycardia (AVNRT). Methods: High-density sections of the entire AVN were obtained from rats and rabbits. Fibrosis was analyzed by Masson’s trichrome staining. Connexin (Cx43, Cx40, and Cx45) and ion channel (Nav1.5, Cav3.1, and HCN4) proteins were immunohistochemically labeled for the analysis of tissue features. Three-dimensional (3D) reconstruction of the AV junction was performed to clarify the relationships among different structures. Results: The RN expressed the same connexin isoforms as the compact node (CN) and INE. Nav1.5 labeling was present at a low level in the CN, RN and INE, where Cav3.1 and HCN4 were expressed. The CN connected with the RN in a narrow strip pattern at the level of the start of the CN. The RN presented as a shuttle shape and was the only tissue directly connected with the atrium in the anterior septum. Conclusion: The RN connects with the AVN anatomically, suggesting that there is direct electrical conduction between them. The entrance of the atria into the AVN is the distal part of the RN, which may form the fast pathway of the AVN.

Background: Multiple atrial tachycardias (ATs) in one patient usually require more complex ablation procedures. Despite the superior accuracy and understanding of conduction features provided by high-resolution mapping, Multiple ATs are still associated with high recurrence rates, and other mechanisms may play a role. Therefore, we aimed to uncover the substrates maintaining these multiple reentrant circuits and the probable mechanisms for the high occurrence of arrhythmia. Methods: Mapping via the Carto system was carried out in 8 patients with more than two types of reentrant circuits during ablation. Functional conduction block (FCB) regions were marked and further analyzed. Results: Twenty sustained ATs were mapped in the 8 patients. Five of these patients exhibited a potential FCB region that changed between different ATs. The potentials of these regions converted between double potentials (DPs), fractionated potentials (FPs) and normal potential due to the different ATs. The FCB regions were the main obstacles and the center of the reentrant circuit in 8 of 14 ATs, and in the other ATs, these regions played a role in reorganizing the conduction pathway. In the activation mapping, the FCB areas were never the target ablation site. Conclusion: The potential FCB region is common in ATs with more than two types of reentrant circuits, especially in scar-related localized reentry. The convertibility of FCB regions provide one of the critical substrates in maintaining multiple ATs. The changefulness of this substrate may be one of the important causes of the high recurrence of related ATs