Atrioventricular node and input pathways: A correlated gross anatomical and histological study of the canine atrioventricular junctional region

To determine the architecture of the atrioventricular (AV) junctional region, structures in atrial preparations were correlated to those in serial sections made either parallel or perpendicular to the long axis of the AV node (AVN)/AV bundle complex. The results demonstrated the following for the first time: 1) A right medial atrial wall (MAW) extends anteriorly from the interatrial septum, superior to the interventricular septum (IVS). 2) An atrial interventricular septum (A-IVS) groove is located between the base of the MAW and the crest of the IVS. 3) Three atrionodal bundles converge to form a proximal AV bundle (PAVB), which in turn is contiguous with the AVN. The atrionodal bundles are associated with the MAW or the superomedial and inferolateral margins of the coronory sinus. Terminal portions of the atrionodal bundles and the PAVB reside within the A-IVS groove. The AV bundle was termed distal (DAVB) to avoid confusion. 4) The location of the AVN/DAVB complex topographically is deep to the apex of the septal cusp of the tricuspid valve subjacent to the MAW. Intracardially, the AVN/DAVB complex is within the central fibrous body. Significantly, this study resulted in the first unequivocal demonstration of discrete bundles of myocardial fibers associated with the atrial end of the AV node. Moreover, it appears likely that the atrionodal AV bundles are continuous with the sinoatrial nodal extensions, thereby forming internodal tracts.