I Triangle of Koch and atrioventricular node

Triangle of Koch is a triangular area demarcated by the tendon of Todaro, the septal leaflet of the tricuspid valve and the orifice of the coronary sinus (Figure 1.3). This area has major electrophysiological significance because it contains the atrioventricular node. The atrioventricular node is located in the anterosuperior aspect or in the apical portion of the triangle (close to the central fibrous body). The compact node is located subendocardially. The mean length, width and thickness are the following: 5.1 mm (range 3.2-6.2 mm), 5.2 mm (range 3.1-7.2 mm) and 0.8 mm (range 0.3-1.2 mm), respectively [59]. There is some variation in the location of the node in the triangle of Koch. In the study by Sanchez-Quintana et al., the compact node was located medially at the mid-level of the triangle of Koch in 82% of specimens; in the remaining 18% the node was located closer to the

Fig. 1.3. a Schematic drawing of the region of the triangle of Koch with localization of the "fast" and "slow" pathway. AVN atrio-ventricular node, CFB central fibrous body, CRT crista ter-minalis, C5 coronary sinus, FO fossa ovalis, IVC inferior vena cava, PB penetrating bundle, TT tendon of Todaro. Modified from Mazgalev et al. [49] with permission. b Schematic drawing illustrating the course of the artery to the AV node through

The inferior pyramidal space and toward the central fibrous body (dark blue). The AV node and the His bundle is colored in yellow. C5O coronary sinus os. Modified from Sanchez-Quin-tana et al. [59] with permission. c A surgeon's view shows the landmarks of the triangle of Koch. The triangle is clearly visible, limited by the septal tricuspid leaflet and the tendon of Todaro (reproduced with permission from Hurst et al. [33])

Hinge of the tricuspid valve. In 12% of specimens, the inferior parts of the node extended up to the mouth of the coronary sinus [59]. Inferiorly, the node bifurcates into rightward and leftward extensions in the majority of specimens. The atrioventricular node artery originates from the right coronary artery in 83% of specimens and from the circumflex artery in 17% of specimens. Its length from origin to the triangle of Koch is 14 to 28 mm (mean

20.5 mm). The artery ascends subendocardially from the inferior-posterior septum to the central fibrous body being closer to the coronary sinus ostium than the tricuspid valve (Figure 1.3B) [59]. The so-called open node (or AN part; A stands for atrium and N for node) consists of transitional cells intermingled with working myocardial cells. Transitional cells extend from the compact node towards the eustachian ridge, the anterior superior rim of the fossa ovalis and the left atrial aspect of the septum. It is believed that these transitional cells, due to differences in the conduction velocity and refractory periods, constitute the electrophysiological basis for discontinuous atrioventricular conduction (fast and slow pathways) in patients with atrioventricular nodal reentrant tachycardia. However, despite electrophysiological evidence for the existence of fast and slow pathways, histological markers have never been reported. The compact node continues with the penetrating bundle of His which is located at the apical part of the triangle of Koch.

The triangle of Koch is a frequent target for ablation mostly for the treatment of atrioventricular nodal reentrant tachycardia and atrioventricular conduction modification in patients with atrial fibrillation. The putative location of fast and slow pathways is shown in Figure 1.3 a. Slow pathway ablation is mostly used in the treatment of atrioventricular nodal reentrant tachycardia. In the EP laboratory, the site of recording the His bundle activity (apical region) and the os of the coronary sinus are anatomic markers for location of the triangle of Koch and for guiding the delivery of ablative energy.

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