Recognition of the atrioventricular node anatomical structure: Connection between the retroaortic node and the compact node

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 described less extensively and may also share the mechanism of normal conduction and abnormal conduction in AVN re-entrant tachycardia.

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 (Na_v 1.5, Ca_v 3.1, 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. Na_v 1.5 labeling was observed at low levels in the CN, RN, and INE, where Ca_v 3.1 and HCN4 were expressed. The CN connected with the RN in a narrow strip pattern at 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 direct electrical conduction occurs between them. The entrance of the atria into the AVN is distal to the RN, which may form the fast AVN pathway.