Ablation of Atrio-ventricular Nodal Reentrant Tachycardia
Introduction
Catheter ablation is a highly effective treatment for atrio-ventricular nodal reentrant tachycardia (AVNRT). While ablation of the fast pathway (FP) was initially performed, slow pathway (SP) ablation is preferred because it 1) carries a lower risk of causing atrio-ventricular (AV) block, 2) does not prolong the PR interval, and 3) treats atypical forms of AVNRT (e.g., slow-slow) that do not use the FP.1,2,3,4,5,6,7,8
The purpose of this chapter is to:
Describe the anatomy of the AV node and its atrial approaches (SP and FP).
Discuss the technique of SP ablation.
Define endpoints for ablation of AVNRT.
ANATOMY OF THE AV NODE
The compact AV node and its perinodal structures are contained within the triangle of Koch, which is defined by the 1) tendon of Todaro, 2) septal leaflet of the tricuspid valve, and 3) coronary sinus ostium (CS os) (base of the triangle).9,10 The central fibrous body and penetrating His bundle lie at the apex of the triangle. The compact AV node is located along the right interatrial septum posterior and inferior to the His bundle and superior to the CS os. The SP (right inferior extension) is located at the level of the CS os, while the FP (superior extensions) lies superior to the His bundle above the tendon of Todaro and outside of the triangle of Koch.11 Left atrio-nodal inputs (left inferior extensions, inferolateral inputs) course along the CS.
ABLATION OF THE SLOW PATHWAY (RIGHT INFERIOR EXTENSION)
Properly positioned His bundle and CS catheters are useful landmarks to identify the triangle of Koch and the His bundle during ablation. The His bundle catheter, however, does not define the entire location of the His bundle, and electro-anatomic mapping of the His bundle (His bundle cloud) identifies additional sites that should be avoided during ablation. Additionally, the CS catheter positioned from the femoral versus internal jugular vein outlines the CS differently (roof versus floor of the CS, respectively).
The baseline PR interval should be analyzed prior to ablation. While SP ablation can be successful in patients with first-degree AV block, PR prolongation (especially >300 ms) suggests impaired or absent FP conduction where ablation of the SP in the latter can cause acute or delayed AV block.12,13,14
SINUS RHYTHM MAPPING OF THE SLOW PATHWAY
Anatomic SP ablation targeting the right inferior extension treats the majority of AVNRT (slow-fast, fast-slow, slow-slow, and even AVNRT using left atrio-nodal inputs).15,16 The ablation catheter is advanced across the tricuspid valve into the right ventricle. With clockwise torque, it is slowly pulled back along the posteroseptum until a small atrial and large ventricular electrogram is recorded inferior to the His bundle and slightly anterior to the CS os. Target site criteria for the SP are 1) a low-amplitude (far-field) atrial electrogram; 2) a late, sharp, high-frequency (near-field) SP potential; 3) a moderate to large ventricular electrogram (AV ratio ≤0.5); and 4) an absent His bundle potential (Figs. 8-1, 8-2, 8-3, 8-4, 8-5 and 8-6).1,2 Sharp “SP-type” potentials, however, can be found at multiple different sites within the triangle of Koch.17 In the absence of a discrete SP potential, the atrial signal is often a low-amplitude, multicomponent electrogram. Because the compact AV node is posterior and inferior to the His bundle, the roof of the CS and sites with large atrial electrograms should be avoided for ablation.
RETROGRADE MAPPING OF THE SLOW PATHWAY
The SP can also be mapped when it conducts retrogradely during ventricular pacing or atypical AVNRT (fast-slow, slow-slow) by identifying the earliest site of atrial activation (Figs. 8-7, 8-8, 8-9 and 8-10). Because retrograde SP conduction precedes
atrial activation, the order of the high frequency (near-field) SP potential relative to the low-amplitude (far-field) atrial electrogram is reversed compared to sinus rhythm.1
atrial activation, the order of the high frequency (near-field) SP potential relative to the low-amplitude (far-field) atrial electrogram is reversed compared to sinus rhythm.1
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