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.