AVNRT and anatomical structural features of the AV node
We consider the structure we described above to be the fast pathway of the AVN and to participate in the slow-fast type of AVNRT. During AVNRT, the earliest action site is not always as concentrated as that in atrial tachycardias (ATs),18 which could be explained by the dispersed connection pattern between the RN and atrium providing multiple exits in the retrograde region (Figure 6). Retrograde atrial activation most often occurs on the left (up to 53%) rather than on the right side of the septum during AVNRT mapping,17 which could also be explained by the dispersed distribution of the RN in the atrial septum mentioned above. A study of isolated perfused canine hearts found that an anterograde conduction block in the fast pathway occurred near the TT. As the atrial pacing rate was increased, the block occurred at progressively more proximal locations, that is, closer to the anterior limbus of the fossa ovalis and further from the TT.19 This corresponds to the electrophysiological characteristics of node-like tissue, which we considered part of the RN. The site of the anterograde block in the fast pathway, outside the triangle of Koch, may prevent retrograde invasion of the slow pathway, and this may facilitate the initiation of “AV nodal” reentry. While the volume of the CN is not large at all, the connection with the RN is rather narrow, and the rate dependence of conduction slowing and blocking occurs at this natural isthmus in the slow-fast type of AVNRT (Figure 6).
While high-density sectioning and 3D reconstruction of the whole AV junction were used, it seems that the anatomical structure of the RN region is not related to the electrophysiological phenomena of other types of AVNRT described in previous studies.