1. The left atrioventricular junction

Left free-wall accessory pathways are myocardial connections crossing the mural portion of the left atrioventricular junction. They link the left atrial vestibule to the basal left ventricular myocardium, bypassing the insulating tissues that normally separate the two myocardial masses. The mitral “annulus” is not a uniform circular cord. It is a complex junction formed by the hinge of the mitral leaflets, variable fibrous tissue, atrial vestibular myocardium, and the basal ventricular wall. The anterior limit of the free wall is the zone of aortic–mitral continuity, where fibrous continuity normally prevents an atrioventricular myocardial connection [1–4].

Along the mural junction, the amount and continuity of fibrous tissue vary. Basal ventricular myocardial fibres may extend toward the leaflet hinge, while atrial myocardium approaches from the vestibule. An accessory strand bridges the intervening plane. The pathway may be short and perpendicular or may run obliquely within the groove, producing spatially separated atrial and ventricular insertions. This morphology explains why the optimal site during retrograde conduction can differ from the earliest ventricular site during antegrade pre-excitation [2,3].

2. Attitudinal sectors of the left free wall

The left atrioventricular junction runs obliquely in the thorax. When viewed in left anterior oblique orientation, its mural arc can be divided into left inferior, inferoposterior, posterior, posterosuperior, and superior sectors. Traditional terms such as left posterior, lateral, and anterior reflect the surgically exposed or Valentine-position heart and may misrepresent body coordinates. In particular, many pathways formerly called left lateral are situated posteriorly when the heart is described in its attitudinal position [2,4].

A clock-face description can assist communication within a laboratory, but it depends on the chosen projection and should be anchored to named anatomical sectors. Three-dimensional mapping, computed tomography, and fluoroscopy are most coherent when they share the same body-based coordinates. The pathway location should therefore identify both the side of the junction and its attitudinal sector, together with the chamber and surface from which successful interruption was achieved [2,4].

3. The coronary sinus and circumflex artery

The coronary sinus courses epicardially in the left atrioventricular groove and is an important mapping landmark, but it does not mark the mitral hinge with fixed precision. Its distance from the hinge and from the electrical atrioventricular junction varies along the mural arc and between individuals. A signal recorded in the coronary sinus may reflect adjacent left atrial myocardium, ventricular myocardium, a pathway potential, or activation conducted through the venous myocardial sleeve. The catheter position must therefore be interpreted in relation to the endocardial surfaces rather than used as a substitute for them [3].

The circumflex coronary artery also occupies the left atrioventricular groove, often between the coronary sinus and the mitral junction, although their relationship is variable. This becomes especially important when an apparently epicardial pathway is targeted from the coronary venous system. Coronary arterial proximity, venous calibre, and tissue thickness can limit energy delivery. Coronary imaging should be considered when ablation within a vein or in close epicardial proximity to an artery is contemplated. The potential benefit of a short venous-to-pathway distance must be weighed against arterial injury, venous damage, and limited catheter stability [3,5].

4. Endocardial approaches and mapping strategy

The left free wall can be approached from the left atrium after transseptal access or from the left ventricle using a retrograde aortic route. The atrial approach places the catheter near the vestibular insertion and the mitral leaflet hinge. The ventricular approach reaches the basal ventricular insertion beneath the leaflet. Neither is universally superior: pathway obliquity, signal quality, valve anatomy, prior ablation, and procedural access determine which surface offers the most direct and stable contact.

During manifest pre-excitation, the ventricular insertion is sought at the earliest local ventricular activation relative to the delta wave, while during orthodromic reciprocating tachycardia or ventricular pacing the atrial insertion is defined by the earliest atrial activation. A discrete pathway potential, fusion of atrial and ventricular electrograms, and appropriate pacing responses can strengthen localization, but each observation must be interpreted in context. Broad or branching insertions may require mapping over a segment rather than treating the pathway as a geometric point [5,6].

The mitral leaflets and subvalvar apparatus create practical constraints. A catheter that appears annular fluoroscopically may be on a leaflet, within the ventricle, or above the hinge in the atrium. Intracardiac echocardiography and three-dimensional reconstruction can clarify tissue contact and reduce reliance on projection alone. The intended lesion should interrupt the myocardial bridge at an insertion while avoiding unnecessary injury to the leaflet, circumflex artery, coronary sinus, and adjacent atrial or ventricular myocardium.