Case Report
A 27-year-old woman having frequent exercise-induced palpitation without ECG documentation underwent electrophysiology study. Slightly shortened H-V interval (21ms) with mild preexcitation was seen at baseline (Figure 1A). Programmed atrial extrastimuli and atrial pacing were associated with inapparent surface delta wave, while intracardiac ventricular signals were still preexcited given the activation at basal right ventricle (HIS) remaining earlier than apex (Figure 1B). The H-V interval was constant (32ms) when A-H prolonged during programmed atrial extrastimuli, consistent with an extra fasciculoventricular AP. Ventricular pacing and extrastimuli showed poor retrograde conduction solely over the AV node. When high-dose isoproterenol was given, the preexcitation progressively became prominent (Figure 2A). A wide QRS tachycardia demonstrating 1:1 V-A relationship was then initiated by ventricular pacing without visible His before QRS. Instead, a presumable retrograde His could be observed after ventricular electrogram on HISd with an H-A interval of around 100ms. Entrainment from atrium was associated with unchanged QRS morphology and an A-V-A response following overdrive cessation, which excluded ventricular tachycardia and confirmed antidromic AVRT utilizing an isoproterenol-sensitive atrioventricular AP (Figure 2B).
Mapping of the pathway was performed during sinus rhythm with low isoproterenol infusion rate to avoid catheter instability. Mild preexcitation caused by atrioventricular pathway conduction was present in this setting. When the ablation catheter was positioned near the earliest ventricular activation site at 12 o’clock site of tricuspid annulus, 15 millimeters away from the nearest His, split potentials with an interval of 30ms were recorded by the proximal electrode pair (ABLp) between atrial and ventricular signals (Figure 3A). The potentials were significantly later than all atrial signals which made it highly unlikely to be an atrial component. The second one was also visible on distal ABL according to the nearly identical timing, followed by an early local ventricular electrogram with QS unipolar pattern. The interval between the double potentials and QRS onset/local V was variable with spontaneous change of preexcitation (Figure 3B), suggesting that the potentials were also irrelevant to the ventricle, and thus could both be considered as AP potentials, demonstrating an example of intra-pathway delay. Gently withdrawing the catheter made the split potentials shifting to ABLd (Figure 3C) and caused frequent ectopy showing QRS with maximal preexcitation pattern identical to tachycardia, indicating pathway automaticity. The atrial signals were not followed by the split potential when V-A dissociation occurred, further confirming the identity AP potentials. Radiofrequency application at this site was also associated with accelerated pathway rhythm (Supplemental Figure).
The H-V interval was back to 32ms after ablation, which could not be changed by atrial pacing or isoproterenol infusion, suggesting the block of atrioventricular AP conduction. The patient has been free of arrhythmia for 6 months.