Abstract
Safe and successful catheter ablation of superoparaseptal (previously called anterior septal ) and midseptal accessory pathways (APs)—those for which ablation poses the highest risk of damaging the normal conduction system—requires a thorough knowledge of anatomy and relationships between structures. Selection of proper ablation targets, including AP potentials, requires careful techniques and analysis especially because pathway potentials can resemble and be in locations similar to the His potential. When preexcitation is present in sinus rhythm, accelerated junctional rhythm during ablation should be recognized as a warning sign rather than an indicator of successful ablation (no preexcitation). Concealed APs in this region present special challenges because retrograde atrial activation patterns can be nearly identical with conduction over the atrioventricular node or septal/paraseptal APs. After ablation energy delivery, assessment of success can be made by showing absence of any retrograde conduction, or His-dependence of retrograde atrial activation.
Keywords
accessory pathway, anteroseptal pathway, parahissian pathway, catheter ablation, superoparaseptal pathway
Key Points
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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.
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Orthodromic supraventricular tachycardia using a midseptal AP must be differentiated from atrioventricular (AV) nodal reentry, and septal or parahissian atrial tachycardias.
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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.
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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.
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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.
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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.
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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.
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
Superoparaseptal Accessory Pathways
Superoparaseptal APs comprise 6% to 7% of all APs in most large series. About 80% of these APs exhibit anterograde conduction, and 20% are retrograde-only conducting (concealed); only about 5% conduct exclusively in the anterograde direction. Because these pathways connect the right atrial and right ventricular paraseptal free walls in a region that is cephalad, or superior, as well as anterior to most of the rest of the ventricular mass, an anterograde conducting pathway manifests positive delta waves in the inferior leads (II, III, and aVF) and the lateral precordial leads (V3 through V6); negative delta waves are present in lead V 1 ( Fig. 25.2 ) and often in V 2 . Leads I and aVL have positive delta waves (negative in aVR). During orthodromic SVT, the P wave is typically situated in the early portion of the ST segment; although retrograde, it is usually positive in the inferior leads, because much of the atrial mass is located caudad from the atrial insertion ( Fig. 25.3 ).
Midseptal Accessory Pathways
Midseptal pathways account for 5% or less of all APs in most series. Approximately 85% of midseptal APs show anterograde conduction (15% are retrograde only), with only about 4% conducting anterograde only. These APs connect atrium and ventricle in a complex region that can give rise to slightly different delta wave polarities in different individuals. A typical preexcitation pattern has predominantly positive delta waves in leads I, II, aVL, and V 2 through V 6 , with leads III and aVF usually having predominantly negative delta waves, and aVR and V 1 having isoelectric delta waves. Variations in this pattern, especially in the inferior leads and in V 2 , have been reported. During SVT, because of the more posterior location of the pathway’s atrial insertion (near the compact AV node), the P wave is usually inverted in the inferior leads. Multiple APs are present in up to 25% of patients with midseptal APs.
Electrophysiologic Testing
The electrophysiologic diagnosis of SVT incorporating a superoparaseptal or midseptal pathway is usually relatively straightforward ( Table 25.1 ); however, the atrial activation sequence during SVT may resemble that of normal AV nodal output (unlike the situation with left or right free wall APs, in which the atrial activation sequence during SVT is eccentric). This similarity of atrial activation can provide a diagnostic challenge in some cases. Standard diagnostic techniques, including introduction of ventricular premature extrastimuli during His refractoriness in an episode SVT, should be used to define the tachycardia mechanism. In some cases, such as when the ventricular-to-atrial interval is very short, or SVT is nonsustained or noninducible, other techniques must be used to establish the presence of an AP. These include parahissian pacing, differential site ventricular pacing, comparison of His-to-atrial (HA) intervals, use of single ventricular extrastimuli, and ventricular overdrive pacing during (entrainment of) SVT, including inspection of phenomena observed during initiation of SVT and of pacing during SVT.
| Surface ECG: Sinus Rhythm (in Presence of Preexcitation) | |
| Superoparaseptal APs |
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| Midseptal APs |
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| Surface ECG: Orthodromic SVT | |
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| Intracardiac Recordings and Electrophysiologic Testing | |
| General principles |
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| Superoparaseptal APs |
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| Midseptal APs |
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Pacing from a location near the His bundle (parahissian pacing) can distinguish conduction over an AP from that over the AV node, as follows: at low pacing outputs ventricular capture occurs, whereas at higher outputs the His bundle/proximal right bundle branch are also captured with the adjacent ventricular myocardium, resulting in a narrower QRS. If a septal AP is present, the stimulus-to-atrial interval will be identical regardless of whether the His is captured, because these APs typically conduct more rapidly than the AV node does. If there is no AP, the stimulus-to-atrial interval is longer with ventricular-only capture than when the His is captured. With ventricular pacing the impulse must travel some distance using relatively slow myocyte conduction before it encounters elements of the His-Purkinje network to begin activating it retrogradely. The impulse need only traverse the AV node to activate the atrium with His capture ( Fig. 25.4 ). This technique can be used to assess whether the AP has been successfully ablated ( Fig 25.5 ).
Using a principle similar to that of parahissian pacing, comparison of the stimulus-to-atrial intervals observed with right ventricular apical versus basal stimulation can demonstrate the presence of an AP. The apex, although physically more distant from the atrium than the ventricular base, is nonetheless electrically closer because of the proximity of the distal right bundle branch to the pacing site. Entry into the rapidly-conducting His-Purkinje system allows a shorter stimulus-to-atrial interval during pacing from the apex than from the base. From the base, the impulse must again travel relatively slowly for some distance through ventricular myocardium before it engages the His-Purkinje system. However, in the presence of an extranodal AP, pacing from the base results in a shorter stimulus-to-atrial interval compared with apical stimulation because the AP makes the base electrically closer to the atrium as well as physically ( Fig. 25.6 ). Fixed-rate pacing or extrastimuli during SVT can be used. This technique can also be used to confirm successful AP ablation.
