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Catheter Ablation of Superoparaseptal (Anteroseptal) and Midseptal Accessory Pathways


Key Points

  • Diagnosis of superoparaseptal ( anteroseptal ) and midseptal accessory pathways (APs) is made on the basis of an electrocardiographic pattern (if overt preexcitation is present) and evidence of midseptal/anteroseptal AP insertions during intracardiac mapping studies.

  • Orthodromic supraventricular tachycardia using a midseptal AP must be differentiated from atrioventricular (AV) nodal reentry, and septal or parahissian atrial tachycardias.

  • Mapping of superoparaseptal APs is used to locate the site with the earliest anterograde ventricular activation near or anterior to the His bundle recording, the earliest anterograde ventricular activation-to-delta wave interval (15–40 ms pre-delta), and earliest retrograde atrial activation in the region of the His bundle recording. Recording a discrete AP potential is very helpful but not always achievable and must be distinguished from the His potential.

  • Mapping of midseptal APs is used to locate the site of the earliest anterograde ventricular activation between the coronary sinus (CS) ostium and the His recording location, the earliest anterograde ventricular activation-to-delta wave interval (15–25 ms pre-delta), and the earliest retrograde atrial activation between the CS ostium and the His recording location. Left midseptal connections are rare.

  • The ablation target can be the site of earliest anterograde ventricular activation or retrograde atrial activation on the AV annulus. Recording the AP potential can verify the correct target. Pathways typically traverse the AV groove at a slant, with atrial insertions lateral to ventricular insertion sites.

  • The use of preformed vascular sheaths may be helpful; catheter navigation systems are often useful to tag sites of interest, and cryoablation may be useful. Cooled/irrigated radiofrequency ablation is rarely needed and is possibly contraindicated.

  • Sources of difficulty include lack of catheter stability, proximity to elements of the normal conduction system with risk of heart block, catheter-induced mechanical block of pathway conduction, and accelerated junctional rhythm (narrow QRS) during ablation that can be mistaken for elimination of preexcitation.

Atrioventricular (AV) accessory pathways (APs) are thin fibers, usually composed of typical myocardial cells, which allow electrical communication between atrium and ventricle extrinsic to the normal AV node-His bundle axis. The clinical expression of these pathways ranges from simply causing an abnormal electrocardiogram to forming an integral component of a macroreentrant circuit incorporating atrial and ventricular myocardium, AV node, His bundle, and the AP (AV reciprocating supraventricular tachycardia [SVT]), or functioning as an alternative pathway for transmission of rapid atrial tachyarrhythmias such as flutter and fibrillation to the ventricles. Symptoms may range from none to occasional mild palpitations, severe palpitations accompanied by dyspnea, chest discomfort, lightheadedness, and even syncope or cardiac arrest from rapidly conducted atrial fibrillation.

Since its introduction in the late 1980s, catheter ablation of APs has become a relatively routine matter in most electrophysiology laboratories. However, ablation of pathways in the so-called anterior and midseptal locations can be a challenge for even experienced operators because of the proximity of these pathways to the normal cardiac conduction system (AV node and His bundle). Inadvertent injury to these structures resulting in the need for permanent pacing, especially in a young patient, is a significant adverse outcome. Fortunately, techniques have been developed to decrease the likelihood of this complication. This chapter discusses the relevant anatomy of these pathways and the use of techniques to achieve optimal outcomes.

Anatomy and Nomenclature

Older nomenclature of septal pathways has undergone modification. A reexamination of the anatomy of the AV junctions has suggested that the terminology used in the original descriptions of AP locations was anatomically inaccurate, and in some cases, frankly misleading. Most electrophysiology trainees have had the experience of asking, “Why is my attending telling me to move the catheter anteriorly when I see it moving toward the head?!” A reclassification of cardiac electrophysiologic anatomy has been developed to try to correct these antiquated but ingrained terms. In addition, a more complete understanding of the anatomy of the atrial and ventricular septa has resulted in a shrinking of the atrial septum; most trainees conceive of the atrial septum as a relatively large disk comprising the intersection of two spheres (atria) compressed together. In fact, the true muscular atrial septum is much smaller, consisting of a relatively thin rim of atrial tissue surrounding the fossa ovalis. This has implications for how precisely one must position a needle and catheter to safely puncture the septum for left atrial access and also for evaluation and ablation of the pathways under consideration in this chapter.

In the old vernacular, anteroseptal pathways were regarded as being located in the apex of Koch’s triangle, connecting the atrial and ventricular septa in the region of the His bundle. In the anatomically accurate nomenclature, these pathways are more properly regarded as superoparaseptal, because there is no atrial septum in the region anterior to the His recording location (atrial walls are separated here by the aortic root; Fig. 25.1 ). These connections are thus right free wall, paraseptal pathways. Posteriorly, pathways in the region of the ostium of the coronary sinus (CS), which previously were called posteroseptal , are in fact posterior paraseptal, because the CS itself is, by definition, entirely posterior to the atrial septum. Pathways located between these two boundaries of the septum have been called midseptal or intermediate septal , but because they are the only truly septal interconnections, they may simply be called septal. Further complicating the situation is the fact that the AV valves are not isoplanar; the tricuspid valve is slightly inferiorly displaced relative to the mitral valve such that a portion of the medial right atrium is juxtaposed to subaortic left ventricular muscle rather than the right ventricle.

Fig. 25.1, View of atrioventricular (AV) groove from above with most of the atrial muscle removed; the right atrial rim has been rendered semitransparent to reveal structures beneath. Note the small dimensions of the actual atrial septum ( dashed line ). True septal ( midseptal ) pathways have an atrial insertion on the right or left side in this region, in which the AV node resides. Pathways in the region of the His bundle, previously called anteroseptal, in fact have free wall and not septal atrial insertions (hence superoparaseptal pathways). APs , accessory pathways; RV , right ventricle.

In the discussion that follows, we will consider that superoparaseptal (anteroseptal) APs are located in the apex of Koch’s triangle at a site from which a small His potential can usually be recorded. True septal or midseptal APs are located in the floor of Koch’s triangle, between the His recording location and the anterior portion of the CS ostium.

Diagnosis and Differential Diagnosis

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