Introduction
The atrioventricular node (AVN) lies within the triangle of Koch and is bound by the coronary sinus, tendon of Todaro and tricuspid valve. The AVN functions by conducting action potentials at an appropriate conduction velocity from the atria to the ventricles. The AVN also acts as a backup pacemaker in the case of sinus node failure. On the other hand, the AVN is part of the underlying circuitry that causes AVN reentrant tachycardia (AVNRT).1Inferior nodal extensions (INEs) are part of the AVN,1-5 and characteristics of these structures are conserved among various species, including humans,2rats,3 mice,4rabbits5 and dogs. Moreover, it is thought that these extensions may be involved in slow-pathway conduction and are part of the underlying circuitry that causes AVNRT, which is supported by electrophysiological studies.6,7 The use of catheter or surgical techniques targeting injuries in this region is thought to be the best method for curing these forms of tachycardia.7 However, there are still some electrophysiological phenomena that cannot be explained by relying only on these anatomical structures, as the exact circuitry responsible for reentrant tachycardias in different types of AVNRT remains unknown. Complex anatomical structures and functional longitudinal dissociations are considered important in understanding the behavior of these tachycardias.1 Other conduction tissues may also participate in the real mechanism of AVNRT. The retroaortic node (RN) has been described as a heap of node-like tissue located on the right side of the atrium neighboring the aorta.8 Previous studies have described the anatomical features of this region and its relationship with the compact node (CN) in little detail. The potential electrophysiological function of these node-like tissues also remains unknown.
The aim of this study was to comprehensively analyze the detailed anatomical structure and histological features of this node-like tissue and better understand its electrophysiological behavior in normal AV conduction and AVNRT.