AVRT & WPW Syndrome

Atrioventricular reentrant tachycardia (AVRT) is a macroreentrant tachycardia that utilizes an accessory pathway (AP) — an anomalous muscular connection bridging the atrial and ventricular myocardium across the atrioventricular groove, bypassing the normal AV node–His-Purkinje conduction system. These pathways are congenital remnants of the fetal AV ring tissue that normally regresses during cardiac development.

In orthodromic AVRT (the most common form, ~95%), the impulse travels antegradely down the AV node and His-Purkinje system to activate the ventricles, then returns retrogradely up the accessory pathway to activate the atria, completing the circuit. The QRS is narrow because ventricular depolarization occurs via the normal conduction system. In antidromic AVRT (~5%), the circuit reverses: antegrade conduction travels down the accessory pathway (producing a maximally pre-excited, wide QRS) and returns retrogradely up the AV node. Antidromic AVRT can be difficult to distinguish from ventricular tachycardia on the surface ECG.

The critical danger in WPW syndrome is the development of atrial fibrillation with rapid conduction over the accessory pathway. Unlike the AV node, which has decremental conduction and limits ventricular rates, many accessory pathways conduct in a non-decremental (all-or-none) fashion. If the AP has a short antegrade refractory period, atrial fibrillation can conduct at rates exceeding 300 bpm, producing pre-excited AF — an irregular wide-complex tachycardia that can degenerate into ventricular fibrillation and sudden cardiac death.

Sinus Rhythm (Pre-excitation)

The classic WPW triad on the resting ECG consists of: (1) short PR interval (<120 ms), reflecting bypass of the normal AV nodal delay; (2) delta wave — a slurred upstroke of the QRS representing early ventricular activation via the AP; and (3) wide QRS (>120 ms) due to the fusion of pre-excited and normally conducted ventricular activation. The degree of pre-excitation depends on the relative contribution of AP vs AV node conduction and the distance of the AP from the sinus node.

Accessory Pathway Localization

The delta wave polarity on the 12-lead ECG provides localization of the AP around the mitral and tricuspid annuli. The Arruda algorithm is the most widely used stepwise approach, analyzing delta wave polarity in specific leads to classify pathways into ~10 anatomic locations.

Baseline Assessment

In manifest WPW, the baseline intracardiac recordings show a short or negative HV interval (normal: 35–55 ms). The HV is short because the ventricle is pre-excited via the AP before the His bundle activates through the normal conduction system. In maximal pre-excitation, the HV may be negative (V before H on the His catheter). The AH interval is typically normal, reflecting intact AV nodal conduction.

Key Diagnostic Maneuvers

Para-Hisian pacing is the gold standard for proving the presence of a septal AP. Pacing near the His bundle at high output captures both myocardium and the His bundle, while at low output only myocardium is captured. With an AP, retrograde atrial activation sequence and timing remain the same regardless of His capture (because the impulse reaches the atrium via the AP, not the His-AV node). Without an AP, loss of His capture produces a longer stimulus-to-A interval and the activation sequence changes to concentric (retrograde through the AV node).

Additional maneuvers include the PVC during His refractoriness test: during SVT, a PVC delivered when the His bundle is refractory that advances or resets the next atrial electrogram proves the existence of an AP (the PVC can only reach the atrium via the AP since the His-AV node pathway is refractory). This is the single most definitive maneuver to confirm AVRT over AVNRT.

Catheter ablation of the accessory pathway is the definitive treatment for AVRT/WPW, with overall success rates of approximately 95% and a major complication rate of 1–3%. The approach depends on the AP location around the AV groove.

Left-Sided Pathways

Left-sided APs (left lateral, left posterior, left posteroseptal) are the most common (~50–60%). Access is achieved via retrograde aortic approach (catheter advanced retrograde through the aortic valve and curled under the mitral annulus) or transseptal approach (crossing the interatrial septum to reach the left atrium and map the mitral annulus from the atrial side). The target is the earliest ventricular activation during sinus rhythm or the earliest atrial activation during AVRT at the AV groove. Ablation at the mitral annulus typically achieves excellent tissue contact and high success rates (>95%).

Right-Sided Pathways

Right free wall pathways are ablated along the tricuspid annulus. These pathways have lower success rates (~90%) and higher recurrence rates compared to left-sided pathways, partly due to unstable catheter contact along the tricuspid valve and the thinner tissue at the right AV groove. A steerable sheath often improves stability.

Septal and Para-Hisian Pathways

Anteroseptal and para-Hisian pathways pose the greatest challenge because of their proximity to the compact AV node and His bundle. The risk of AV block is 2–5% for anteroseptal pathways. Cryoablation is strongly preferred in this location, allowing cryomapping at −30°C to confirm AP elimination without permanent damage before creating a definitive lesion at −80°C.

Posteroseptal and Epicardial Pathways

Posteroseptal APs may have epicardial connections via the coronary sinus (CS) or its branches, including coronary sinus diverticula. If ablation from the endocardial surface at the CS os and posteroseptal tricuspid annulus is unsuccessful, mapping within the CS or its branches (particularly the middle cardiac vein) is required. A CS venogram can identify diverticula. Ablation within the CS carries a risk of coronary artery injury (circumflex artery courses along the CS) and cardiac tamponade.