An Unusual Response to Para‐Hisian Pacing: What is the Mechanism?

A 68-year-old woman with history of hypertension and diabetes mellitus presented to our electrophysiology (EP) laboratory for EP study and catheter ablation of symptomatic supraventricular tachycardia (SVT). At baseline, 12-lead ECG showed normal sinus rhythm with normal PR interval and no manifest ventricular pre-excitation. The atrio-Hisian interval was 69 milliseconds and Hisventricular (HV) interval was 42 milliseconds. With incremental atrial pacing, the atrio-ventricular (AV) nodal conduction was decremental. Typical right bundle branch block appeared with atrial pacing at cycle length (CL) faster than 500 milliseconds, without HV prolongation and persisted throughout the case. Atrial extra-stimuli failed to demonstrate dual AV nodal physiology. Apical right ventricular pacing showed no retrograde conduction to the atrium at 600 milliseconds. Ventricular para-Hisian pacing, however, did result in retrograde atrial conduction as shown in Figure 1. A stimulus-to-atrial activation time (HRA) of 114–116 milliseconds was noted with both narrow and wide paced QRSs. Another stimulus-to-atrial activation time (HRA) of 196 milliseconds was measured with a wide paced QRS. More rapid atrial pacing at 420 milliseconds resulted in 2:1 infra-Hisian AV block. Single atrial extrastimuli (baseline CL of 500 milliseconds and extrastimulus CL of 330 milliseconds) also resulted in infra-Hisian block. An SVT with a CL of 350 milliseconds was then induced, with 2:1 infra-Hisian AV block (Fig. 2A) and spontaneous termination with an atrial depolarization. What is the explanation of the para-Hisian pacing findings and what is the SVT diagnosis?