Case presentation
A 39-year-old woman with frequent palpitations was referred for radiofrequency ablation. A baseline 12-lead electrocardiogram showed normal sinus rhythm. At baseline, atrio-His (AH) and His-ventricular (HV) intervals were 80 and 45 ms, respectively. Antegrade atrioventricular (AV) nodal Wenckebach-type ventriculoatrial (VA) block was observed at a paced cycle length (CL) of 430 ms, and dual AV nodal physiology was observed during programmed atrial extrastimulation. The earliest retrograde atrial activation during ventricular pacing was recorded at the coronary sinus (CS) ostium. Ventriculoatrial conduction was intermittent at a paced CL of 545 ms. Para-Hisian pacing showed AV nodal pattern. A narrow QRS supraventricular tachycardia (SVT) with HV interval of 45 ms dependent on a jump-up in the AH interval was induced by atrial extra stimulation (Figure 1). During SVT, intermittent AV dissociation was observed, and the atrial activation sequence at transient VA conduction was identical to that during ventricular pacing. Figure 2 shows that the H-H interval fluctuated during SVT. Based on these observations, what is the mechanism of this tachycardia?