Ablation of Posteroseptal Accessory Pathways

Anatomy

Accessory pathways (APs) are located around the tricuspid or mitral atrioventricular (AV) valves with the exception of the aorto-mitral continuity in the left anterior septal aspect of the heart. Posteroseptal APs are the second most common variety of AV connection encountered clinically after left free wall APs. In general, they exhibit somewhat negative delta waves in the frontal plane electrocardiogram (ECG) leads and an early transition on the precordium (often in V ₂ ). Neighbors of the posterior septal AP include left posterior free wall APs along the mitral annulus (MA), true midseptal APs along the tricuspid annulus (TA), and right posterior APs along the TA. Posteroseptal APs exhibit very complex anatomy consisting of several subtypes. Although the term posteroseptal is firmly entrenched in the electrophysiologic vocabulary, cardiac anatomists have long protested that these pathways are not truly septal in location. Furthermore from a true anatomic perspective, this region could be considered inferior rather than posterior. As opposed to most left free wall APs, which anatomically lie epicardial to a well-defined annulus fibrosis, tricuspid and posteroseptal APs lack the same robust annulus fibrosis as shown in Fig. 24.1 . Like other APs, they are now understood to course obliquely across the AV annulus rather than transversely, and this is appreciated in Fig. 24.1 with the atrial ventricular aspects of the AP residing in different sections ( A and C , respectively). Fig. 24.2A illustrates relevant right atrial endocardial anatomic relationships of posterior septal APs. The triangle of Koch is defined by the tendon of Todaro (TT), the septal leaflet of the tricuspid valve, and the coronary sinus (CS) orifice (see Fig. 24.2A ). The His bundle resides in the apex near the central fibrous body ( asterisk ) and the compact AV node outlined in yellow according to its typical location. Posterior septal pathways lie inferior (termed somewhat inaccurately as posterior) to the roof of the CS or within the CS ostium or proximal extent (about 2 cm), or along the posterior (inferior) septal aspect of the MA. It can easily be appreciated that these pathways are challenging in view of their proximity to the normal conduction system. Moreover, differentiating atrioventricular reentrant tachycardia (AVRT) caused by a posteroseptal AP from AV node reentry, wherein the earliest A can be mapped to a very similar area, may pose additional challenges. Patients, especially those with congenital anomalies, exhibit considerable variation in Koch’s triangle and location of the AV node. Fig. 24.2B illustrates this variation with a large CS orifice and small triangle of Koch. Fig. 24.2C–E further shows sections through the septal region demonstrating the AV node giving rise to the bundle of His and the relationships of the AV nodal artery. Because the TA lies slightly more apical than the MA and the interatrial septum slightly leftward to the interventricular septum, one must recall that the right atrium is anatomically juxtaposed with the left ventricle (LV) in this region ( Fig. 24.3 ). Fig. 24.4 exhibits progressively deeper dissection into the posterior septal region or inferior pyramidal space. Fig. 24.5 illustrates this inferior pyramidal space by computed tomography scan showing it is not true septal territory and illustrating the anatomic relationships including CS and AV nodal artery.

Epicardial accessory AV connections are an important subgroup of posteroseptal APs. The various endocardial as well as epicardial pathways are illustrated in Fig. 24.6 . Pathway type 1 (see Fig. 24.6 ) is an endocardial connection between the right atrium and right ventricle. Pathway type 2 is right atrial to left ventricular; this portion of the LV is termed the posterior superior process . Pathway 3 refers to an endocardial left atrial (LA) to left ventricular AV connection (see Fig. 24.6 ). Pathway type 4, an epicardial one, depicts a muscular connection between the LV and CS musculature in the middle cardiac vein; similar pathways can occur in slightly more distal coronary venous branches such as the posterior cardiac vein. Lastly, pathway type 5, another epicardial one, portrays a CS diverticulum, an anatomic anomaly, electrically connecting the LV and the CS musculature of the diverticulum. An additional schematic, Fig. 24.7 , delves further into the proposed anatomy of epicardial posteroseptal APs.