1. The right atrioventricular junction

Right free-wall accessory pathways are strands of working myocardium that cross the parietal component of the right atrioventricular junction. They connect the right atrial vestibule with the inlet myocardium of the right ventricle, bypassing the normal insulating plane. The tricuspid “annulus” should not be imagined as a uniform fibrous ring. It is a three-dimensional junction composed of the hinge lines of the tricuspid leaflets, variable fibro-fatty tissue, atrial vestibular myocardium, and the basal ventricular wall. Fibrous insulation is less developed and more frequently discontinuous than at the left atrioventricular junction [1–5].

The tricuspid junction is displaced apically relative to the mitral junction. Its leaflets are hinged obliquely and angle acutely toward the ventricular cavity. A catheter located at the apparent annular contour on fluoroscopy may therefore lie on atrial vestibular myocardium, against a leaflet, or beneath the leaflet on the ventricular aspect. These relationships affect both electrogram interpretation and stability. The anatomical target is the accessory myocardial connection or one of its insertions, not an assumed circular annular line [2,3,5].

2. Attitudinal sectors and pathway course

When the heart is described relative to the body coordinates, the parietal right atrioventricular junction can be divided into right superior, right superoanterior, right anterior, right anteroinferior, and right inferior sectors. The traditional term right lateral corresponds largely to an anteriorly located sector of the junction. Likewise, what was formerly termed right anterior is more accurately superior. Legacy labels may remain useful when interpreting older electrocardiographic algorithms, but the final anatomical description should use attitudinal coordinates [1,2,4].

A pathway may cross the junction perpendicularly, but many have an oblique course. The atrial and ventricular insertions can consequently be separated along the junction. Some connections have broad or branching atrial insertions, broad ventricular insertions, or both. The earliest atrial activation during orthodromic atrioventricular reciprocating tachycardia may therefore not coincide with the earliest ventricular activation during manifest pre-excitation. Describing a single annular point can conceal the true extent and direction of the pathway [5,6].

3. Epicardial groove and right coronary artery

The right coronary artery runs within the epicardial right atrioventricular groove, accompanied by fat and small coronary veins. Its distance from the endocardial surface and from the tricuspid hinge varies with the sector and with individual anatomy. The inferior and anteroinferior portions of the junction merge with the cavotricuspid isthmus, where wall thickness, recesses, trabeculations, and the amount of intervening adipose tissue are also variable. These features can alter lesion depth and may explain why an apparently appropriate endocardial site fails to interrupt a deeper connection [3].

Coronary arterial injury is uncommon but anatomically plausible when energy is delivered deeply near the atrioventricular groove. The risk cannot be inferred from the catheter projection alone. When a pathway appears epicardial, when repeated endocardial applications have failed, or when the intended lesion is close to the coronary artery, coronary imaging can clarify the relationship. An early epicardial signal does not by itself prove that energy can be delivered safely from that surface [3,5].

4. Mapping and catheter stability

Mapping should compare the atrial and ventricular aspects of the tricuspid junction. During manifest pre-excitation, localization emphasizes the earliest ventricular activation relative to the delta wave and the degree of atrioventricular electrogram fusion. During orthodromic tachycardia or ventricular pacing, the sequence of retrograde atrial activation identifies the atrial insertion. A discrete pathway potential can be valuable when present, but no isolated electrogram criterion replaces confirmation of catheter position, pathway participation, and loss of accessory conduction [6].

Right free-wall pathways are technically demanding because the broad tricuspid orifice, leaflet mobility, acute leaflet-to-ventricle angulation, and irregular right ventricular endocardium can reduce catheter stability. A long sheath, a superior venous trajectory, a catheter positioned beneath the leaflet, or imaging guidance may improve contact in selected cases. These are alternative anatomical solutions rather than universally interchangeable techniques. Intracardiac echocardiography and three-dimensional mapping can show whether the catheter is above or below the hinge and whether contact is maintained during the cardiac cycle.

The ventricular insertion may sometimes be reached more securely from beneath the tricuspid leaflet, whereas a predominantly atrial or branching insertion may require mapping farther from the apparent hinge. Recurrence after an initially successful application should prompt reassessment of stability, pathway width, and the possibility of separate insertions rather than automatic repetition at the same geometric point. Lesions should remain limited to tissue shown to participate in the connection while avoiding unnecessary injury to the tricuspid leaflet and right coronary artery.

5. Distinction from atriofascicular pathways

Not every pathway with an atrial insertion along the right free wall is a typical atrioventricular connection. Atriofascicular pathways usually originate from the right atrial free-wall region but extend distally to the right ventricular conduction system, often with decremental antegrade conduction. Nodoventricular and nodofascicular connections involve the atrioventricular node or its extensions. Their physiology, ventricular insertion, and role in tachycardia differ from those of a direct atrial-to-ventricular myocardial bridge. A descriptive diagnosis should therefore specify both ends of the pathway whenever they can be established [7].