11.2 Catheter Ablation of AVNRT Slow Pathway & Accessory Pathways (WPW)

11.2 Catheter Ablation of AVNRT Slow Pathway & Accessory Pathways (WPW)

Catheter ablation of supraventricular tachycardias (SVT) represents one of the most successful and definitive curative interventions in interventional cardiac electrophysiology. The two most prevalent forms of paroxysmal SVT encountered in the clinical electrophysiology laboratory are atrioventricular nodal reentrant tachycardia (AVNRT) and atrioventricular reentrant tachycardia (AVRT) mediated by accessory pathways in Wolff-Parkinson-White (WPW) syndrome or concealed bypass tracts.

Achieving high procedural success rates exceeding 95% to 98% while maintaining complication rates (such as complete heart block) below 0.5% requires deep comprehension of cardiac regional anatomy, fluoroscopic projections, intracardiac electrogram signatures, real-time physiological monitoring, and rigorous post-ablation endpoints.

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AVNRT Slow Pathway Ablation

Pathophysiological Substrate & Dual AV Nodal Physiology

Typical AVNRT (slow-fast) requires dual atrioventricular nodal pathways exhibiting distinct electrophysiological properties:

1. Fast Pathway: Characterized by rapid conduction velocity and a relatively long effective refractory period ($ERP$). Anatomically, it courses anteriorly and superiorly along the tendon of Todaro near the apex of Koch's triangle, inserting into the compact AV node.
2. Slow Pathway: Characterized by slower conduction velocity and a relatively short $ERP$. Anatomically, it is composed of transitional atrial tissue and inferior nodal extensions running along the tricuspid valve annulus toward the coronary sinus ostium.

During sinus rhythm, impulses conduct preferentially down the fast pathway due to its rapid conduction velocity. A premature atrial complex (PAC) occurring at a critical coupling interval can find the fast pathway refractory, deflecting antegrade conduction down the slow pathway (manifesting as a sudden jump in the $AH$ interval by $\ge 50\text{ ms}$ for a $10\text{ ms}$ decrement in pacing coupling interval). By the time the impulse reaches the lower turnaround point in the compact node/His bundle, the fast pathway has recovered excitability, allowing retrograde conduction back to the atria and initiating sustained reentry.

                    [Atrial Wavefront / PAC]
                               |
            +------------------+------------------+
            |                                     |
     [Fast Pathway]                        [Slow Pathway]
   (Long ERP, Fast Cond)                 (Short ERP, Slow Cond)
            |                                     |
    Blocks Antegradely!                    Conducts Antegradely
            |                                 (AH Jump >=50 ms)
            |                                     |
            |                              [Lower Turnaround]
            |                                     |
            +<======= Retrograde Flow <===========+

The Triangle of Koch: Anatomical Boundaries & Zonal Mapping

The anatomical arena for slow pathway ablation is the Triangle of Koch, situated on the endocardial surface of the low right atrial interatrial septum. The boundaries of this vital triangle comprise:

• Apex: The central fibrous body, where the compact atrioventricular node transitions into the penetrating bundle of His.
• Anterior / Superior Border: The attachment (hinge line) of the septal leaflet of the tricuspid valve along the tricuspid annulus.
• Posterior / Inferior Border: The Tendon of Todaro, a fibrous subendocardial cord running anteriorly from the Eustachian valve of the inferior vena cava (IVC) through the sinus septum to insert into the central fibrous body.
• Base: The ostium of the coronary sinus (CS os) and the Eustachian ridge.

                              [Apex: Compact AV Node & His Bundle]
                                            /     \
                                           /       \
                                          /         \
         [Anterior: Septal TV Annulus]   /  Zone A   \   [Posterior: Tendon of Todaro]
                                        /             \
                                       /   Zone M1/M2  \
                                      /   (Mid Septum)  \
                                     /                   \
                                    /     Zone P1/P2      \
                                   /  (Inferior Isthmus)   \
                                  +-------------------------+
                                 [Base: Coronary Sinus Ostium]

To minimize the risk of inadvertent damage to the compact AV node and penetrating His bundle, ablation is approached in a stepwise, inferior-to-superior fashion across defined anatomical zones:

• Zone P (Posterior / Inferior): Located at the base of the triangle, bounded between the anterior lip of the coronary sinus os and the tricuspid annulus (divided into $P_1$ deep within or near the CS os floor and $P_2$ along the anterior CS margin). This is the primary, safest target site for initial radiofrequency application.
• Zone M (Mid-Septal): Located midway between the CS os and the His bundle recording site ($M_1$ and $M_2$). Ablation is advanced here only if applications in Zone P fail to eliminate slow pathway conduction.
• Zone A (Anterior / Superior): Located in close proximity to the apex of Koch's triangle where the compact AV node and His bundle reside. Ablation in Zone A is strictly avoided due to an unacceptable risk of permanent high-grade or complete atrioventricular block.

Electrogram Signatures at the Target Site

Precise positioning of the ablation catheter in Zone P requires continuous analysis of bipolar intracardiac electrograms recorded from the catheter tip (distal bipole $1-2$):

1. Atrial-to-Ventricular Ratio ($A:V$ Ratio): At the His bundle recording site at the apex of Koch's triangle, the $A:V$ ratio is typically $1:1$ to $1:2$ with a sharp His deflection. As the ablation catheter is manipulated into the inferior triangle of Koch for slow pathway ablation, the local atrial electrogram becomes significantly smaller than the ventricular electrogram. The optimal slow pathway target displays a small atrial ($A$) to large ventricular ($V$) electrogram ratio between $1:3$ and $1:5$ (and up to $1:10$). An $A:V$ ratio $>1:1$ indicates the catheter is too superior or too high in the atrium, dangerously close to the compact AV node.
2. Slow Pathway Potentials: At successful target sites, a distinct, multicomponent, fragmented, or sharp high-frequency electrogram is recorded between the local atrial and ventricular signals. This "slow pathway potential" (often occurring 30 to 50 ms after the onset of low right atrial activation) reflects delayed activation of transitional nodal tissue and inferior extensions.

Real-Time Monitoring During Radiofrequency (RF) Delivery

Radiofrequency energy for slow pathway ablation is typically delivered using a 4 mm non-irrigated catheter (at 20 to 35 Watts, temperature limit 55°C to 60°C) or an irrigated catheter (at 25 to 35 Watts, flow rate 17 to 30 mL/min, power-controlled mode).

During energy delivery, the electrophysiology specialist must monitor specific electrographic hallmarks:

• Junctional Rhythm (Accelerated Junctional Rhythm): The development of an accelerated junctional rhythm (rate 60 to 110 bpm) during RF delivery is the clinical hallmark of successful slow pathway heating. It is produced by thermal automaticity in the nodal extensions. If no junctional rhythm appears within 15 to 20 seconds of RF delivery, the catheter is unlikely to be in the effective zone, and energy delivery should be aborted to reposition the catheter.
• Mandatory 1:1 Retrograde $V-A$ Conduction: It is paramount that every junctional beat conducts backward to the atria in a 1:1 fashion. This is confirmed by observing inverted P waves following each QRS on the surface ECG (leads II, III, aVF) and sharp retrograde atrial electrograms on the high right atrium (HRA) and coronary sinus recordings.

RF Red Flags & Immediate Cessation Criteria

Energy delivery must be terminated immediately (<1 second) upon the occurrence of any of the following warning signs to avoid permanent complete heart block:

1. Junctional Rhythm with $V-A$ Block: Loss of retrograde ventriculoatrial conduction during junctional rhythm (a junctional QRS without a following retrograde atrial potential) indicates thermal injury spreading toward the compact AV node and is the most critical warning sign of impending complete heart block.
2. Fast Junctional Rhythm (>120 to 140 bpm): Paroxysmal, irregular, or rapid junctional tachycardia indicates excessive heating or direct thermal irritation of the compact node itself.
3. Prolongation of the PR Interval or AH Interval: On sinus beats preceding or during energy delivery, any lengthening of the baseline PR or AH interval signifies fast pathway compromise.
4. Sudden Conduction Block or "Jump": Any dropped P wave, sudden occurrence of second-degree AV block, or catheter dislodgement superiorly toward the His bundle.
5. Impedance Rise: An abrupt rise in impedance ($>10-15\text{ }Ω$ above baseline) indicates coagulum formation (charring) on the catheter tip, requiring immediate termination and catheter inspection.

Procedural Endpoints for AVNRT Ablation

Following lesion delivery, the EP specialist conducts comprehensive post-ablation programmed electrical stimulation under baseline conditions and during isoproterenol infusion (1 to 4 mcg/min, titrated to increase baseline sinus rate by $\ge 20-30%$):

1. Complete Elimination of Slow Pathway Conduction: Demonstrated by the complete absence of an AH jump on single atrial extrastimulus testing ($S_1-S_2$) and incremental atrial pacing.
2. Non-Inducibility of AVNRT: Inability to induce sustained or non-sustained AVNRT using single, double, or triple atrial extrastimuli across multiple drive cycle lengths (e.g., 600, 500, 400 ms) and rapid burst pacing down to the atrial effective refractory period ($AERP$).
3. Single Echo Beat (The Acceptable Endpoint): In clinical practice, complete elimination of dual AV nodal physiology is not always achievable without risking compact nodal injury. The presence of an AH jump with at most a single AV nodal echo beat without tachycardia induction is considered a clinically successful and durable endpoint with low recurrence rates (<2-3%). Persistent multiple echo beats or inducible AVNRT requires further mapping and ablation.

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Accessory Pathway Ablation (WPW & Concealed AVRT)

Anatomical Classification & Conduction Properties

Accessory atrioventricular pathways (Kent bundles) are abnormal muscular connections bridging atrial and ventricular myocardium across the fibrous annuli of the mitral or tricuspid valves.

• Manifest Pathways (Wolff-Parkinson-White Syndrome): Conduct antegradely from atrium to ventricle, producing ventricular pre-excitation (shortened PR interval <120 ms, slurred QRS upstroke or delta wave, and widened QRS complex). Most manifest pathways also conduct retrogradely.
• Concealed Pathways: Conduct exclusively in the retrograde direction (ventricle to atrium). They exhibit a normal 12-lead ECG in sinus rhythm but serve as the retrograde limb in orthodromic atrioventricular reentrant tachycardia (AVRT).

Unlike the atrioventricular node, accessory pathways typically exhibit non-decremental conduction (all-or-none fixed conduction velocity independent of pacing rate) and shorter refractory periods, making them conduits for life-threatening rapid ventricular rates during atrial fibrillation.

12-Lead ECG Localization Algorithms

The spatial orientation of the accessory pathway along the atrioventricular annuli can be predicted non-invasively using 12-lead surface ECG algorithms (such as the Arruda and St. George's algorithms) during maximal pre-excitation:

Intracardiac Mapping Techniques

Definitive localization requires endocardial electroanatomic mapping:

1. Antegrade Pre-excitation Mapping (Manifest Pathways): Mapping is performed during sinus rhythm or atrial pacing (atrial pacing close to the pathway maximizes pre-excitation). The catheter is moved along the annular circumference to identify the site of earliest local ventricular activation ($V$) relative to the onset of the surface delta wave. At the optimal target, local $V$ precedes the delta wave by 10 to 30 ms.
2. Retrograde Mapping (Concealed or Manifest Pathways): Mapping is performed during right ventricular pacing (apex or RVOT) or during orthodromic AVRT. The catheter is navigated along the atrial aspect of the annulus to identify the site of earliest retrograde atrial activation ($A$) relative to the onset of the surface retrograde P wave or ventricular electrogram.
3. Accessory Pathway Potential (Kent Potential): At the precise insertion site, a discrete, sharp, high-frequency deflection (representing depolarization of the accessory pathway fibers) is recorded bridging the local atrial and ventricular signals. Fused, continuous electrical activity spanning from the local $A$ wave into the local $V$ wave with no isoelectric baseline indicates direct pathway contact.

Normal Annular Site:         (A) -------------- [Isoelectric Baseline] -------------- (V)
                                                                                       |
Successful Target Site:       (A) ======= [Fused Kent Potential] ======= (V)            |
                              |                                         |              |
                              +--- Precedes Delta Wave by 15-30 ms -----+              |
                                                                                       |
Surface ECG:                                   [Delta Wave] === [Wide QRS Complex] ===+

Left-Sided Accessory Pathways

Left-sided pathways account for 50% to 60% of all accessory connections (most frequently left lateral and left posterior):

• Access Routes:
  1. Transseptal Approach: The preferred, most common modern approach. A steerable sheath (e.g., Agilis) and ablation catheter are advanced across the fossa ovalis into the left atrium. The catheter rests on the atrial aspect of the mitral valve annulus, offering stable contact, excellent maneuverability, and lower systemic embolic risks.
  2. Retrograde Transaortic Approach: The catheter is advanced from the femoral artery, across the aortic valve, into the left ventricle, and curved under the posterior or lateral leaflet of the mitral valve to contact the ventricular aspect of the mitral annulus. Requires careful anticoagulation and carries risks of aortic valve trauma, coronary ostial cannulation, and left ventricular ectopy.
• Electrogram Targets: Simultaneous recording of coronary sinus (CS) electrograms guides localization. For example, earliest retrograde activation at CS 1-2 indicates a left lateral pathway, while earliest activation at CS 7-8 indicates a left posteroseptal pathway.

Right-Sided & Septal Pathways

Right-sided pathways (accounting for 20% to 30% of pathways) lie along the tricuspid annulus:

• Right Free-Wall Pathways: Often associated with low-amplitude local electrograms and challenging catheter stability due to annular motion. Long guiding sheaths (e.g., Mullins or steerable sheaths) are required for mechanical backup.
• Posteroseptal Pathways: Located in the vicinity of the coronary sinus ostium, the middle cardiac vein (MCV), and the posterior pyramidal space. Successful ablation may require RF delivery inside the proximal CS or MCV (using lower power, e.g., 20 to 25 Watts, with coronary angiography to rule out proximity to the posterior descending coronary artery).
• Anteroseptal and Midseptal Pathways: Course near the compact AV node and His bundle. Intracardiac electrograms at these sites often display a prominent His potential alongside the pre-excited ventricular deflection. Ablation carries an exceptionally high risk of complete AV block. Cryoablation is frequently chosen over RF energy due to cryomapping (cooling to -30°C to confirm pathway loss without permanent tissue damage) before freezing to -80°C.

Ablation Endpoints for Accessory Pathways

Successful elimination of an accessory pathway requires meeting strict, verifiable electrophysiological endpoints:

1. Immediate Loss of Pre-excitation: During radiofrequency application in sinus rhythm, antegrade conduction across the pathway should cease within 3 to 5 seconds of RF onset, manifested by immediate disappearance of the delta wave, prolongation of the PR interval, and normalization of the QRS complex. If pre-excitation does not disappear within 5 to 10 seconds, RF delivery must be aborted and the catheter repositioned.
2. Concentric, Decremental Retrograde AV Nodal Conduction: During ventricular pacing post-ablation, retrograde atrial activation must convert from eccentric (earliest at the pathway site) to concentric (earliest at the His bundle and proximal CS) and demonstrate decremental properties (progressive lengthening of the $VA$ interval in response to decremental pacing cycle lengths), proving that conduction is traveling exclusively through the normal AV node.
3. Adenosine Challenge: Intravenous administration of adenosine (6 to 12 mg bolus) produces transient atrioventricular nodal block. During sinus rhythm, adenosine should produce transient AV block without unmasking antegrade pre-excitation. During ventricular pacing, adenosine should produce transient ventriculoatrial (VA) block without retrograde pathway conduction, confirming the absence of dormant accessory pathway conduction.
4. Post-Ablation Observation Period: A mandatory 30-minute waiting period following the last RF delivery is observed. During this time, programmed stimulation and isoproterenol challenges are repeated to ensure that transient tissue edema has not created false-positive acute success.

Procedural Endpoints Summary Table