Fig. 4.

Internal views of the right atrium. Three inter-nodal routes arise from the sinus node (red circle) to the atrioventricular (AV) node (yellow circle). (A) The anterior inter-nodal route (#1) is from the sinus node (red circle), running along the medial wall of the right atrium in front of the opening of the superior vena cava (asterisk), and then giving rise to three branches. Branch #1-1 travels anteriorly and descends to the AV node (yellow circle) in a short pathway after joining #2-1. Branch #1-2 spans the anterior wall of the left and right atrial chambers to form Bachmann’s bundle. Branch #1-3 runs around the tricuspid annulus and joins branch #3-1 to approach the posterior part of the AV node in the long pathway. (B) The middle inter-nodal route (#2) runs along the posterior wall of the sinus venarum and descends through the top of the limbus of the oval fossa (FO) (#2-1) to approach in a short pathway to the anterior aspect of the AV node. This route also supplies the left atrial wall between the upper and lower pulmonary veins (#2-2, #2-3) (Fig. 3A, C). The posterior inter-nodal route (#3) is the terminal crest and approaches the AV node through the Eustachian ridge in front of the inferior vena cava (IVC) (#3-1). Route #3-1 joins route #1-3 to approach the posterior part of the AV node in a long pathway. (C) Three anatomical circles are drawn in the same picture as in Fig. 4B. The circle around the FO starts from the anterior part of the AV node and shares #2-1 and #3-1. The large circle uses the root of #1 and #3 and their branches, #1-1 and #3-1. The circle at the tricuspid annulus runs through #1-1 and #1-3. The re-entry circuits in these circles may be in clockwise or counterclockwise directions. The interventional cut at the cavo-tricuspid isthmus (CTI: red lines) is blocking #1-3 and #3-1. Fig. 4A and B are modified from the authors’ previous publication [25]. STL, septal leaflet of the tricuspid valve; ATL, anterior leaflet of the tricuspid valve; CS, coronary sinus.