The atrioventricular junctions are the regions where atrial myocardium, valvar attachments, fibrous tissues, and ventricular myocardium meet. They are not equivalent to simple circular valve rings. The right and left junctions differ in geometry and in the extent of fibrous insulation, and both contain epicardial vascular and fatty tissues. These features explain the distribution of accessory atrioventricular connections and define the anatomical limits of mapping and ablation. [1-3]

1  General organisation

Each atrioventricular junction has an atrial vestibule supporting a valvar hinge, an insulating fibrous component, and underlying ventricular myocardium. The valvar orifices are dynamic and non-planar. Joining selected hinge points may create useful annular planes for imaging, but these planes do not reproduce the complete anatomical junction. [1,2]

The atrial and ventricular myocardial masses are normally insulated by fibrous tissue, except at the penetrating atrioventricular conduction axis. Accessory pathways represent additional muscular connections that bridge this insulation.

2  Right atrioventricular junction

The tricuspid junction is related to the right atrial vestibule and the inlet of the right ventricle. Its septal portion is displaced apically relative to the mitral hinge. The inferior junction extends from the paraseptal region across the cavotricuspid isthmus towards the lateral free wall. Epicardially, the right coronary artery runs within the atrioventricular groove. [1]

The junction is not uniformly fibrous. Thin fibro-fatty tissues may separate atrial and ventricular myocardium, and muscular continuity can occur in accessory pathways. The inferior and rightward pathways are approached across a region whose endocardial thickness and trabeculation vary considerably.

3  Left atrioventricular junction

The mitral junction is supported anteriorly by fibrous continuity with the aortic root and posteriorly by a more variable fibro-fatty groove. The posterior leaflet hinge follows a curved junction adjacent to the coronary sinus and circumflex coronary artery. The atrial vestibule forms the smooth muscular tissue immediately proximal to the leaflet attachments. [1,3]

Unlike the simplified concept of a complete fibrous mitral annulus, the left junction contains discontinuous fibrous tissues whose prominence varies regionally. This arrangement is relevant both to accessory pathways and to the concept of mitral annular disjunction, which must be assessed by location and extent rather than treated as a uniform abnormality.

4  Septal and paraseptal junctions

The offset between the tricuspid and mitral hinges creates the atrioventricular muscular septum. The true membranous septum has atrioventricular and interventricular components according to the attachment of the septal tricuspid leaflet. The triangle of Koch overlies the inferior pyramidal space and contains the atrioventricular node and its inferior extensions. [1,2]

Many sites described clinically as septal are more accurately paraseptal because the atrial and ventricular chambers are not separated by a true septal partition at those points. Attitudinally appropriate terminology helps distinguish inferior, superior, rightward, and leftward paraseptal locations.

5  Epicardial grooves and coronary vessels

The external atrioventricular grooves contain adipose tissue, coronary arteries, veins, and autonomic nerves. The coronary sinus occupies the inferior left atrioventricular groove and approaches the right atrium near the crux. Its myocardial sleeve and tributaries can provide recording or ablation sites for pathways close to the epicardial junction.

The right coronary and circumflex arteries may lie close to endocardial or coronary venous targets. Their distance from the valvar hinges is variable, so the apparent annular position of a catheter does not reliably establish vascular safety.

6  Electrophysiological relevance

Accessory atrioventricular pathways can occur around most of the junctional circumference, but their anatomy differs by region. Free-wall pathways bridge atrial vestibular and ventricular myocardium across the groove, whereas paraseptal pathways may relate to the central fibrous body, coronary sinus, middle cardiac vein, or inferior pyramidal space. [1]

Successful mapping requires integration of chamber geometry, valvar hinge points, local electrograms, coronary venous anatomy, and nearby arteries. This chapter establishes the structural framework; pathway classification and ablation technique should be developed separately.