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?