Irregular Long‐RP Supraventricular Tachycardia Terminated by Double Ventricular Scan Pacing: What Is the Mechanism?

PresentationA woman in her 50s was referred to our hospital because of symptomatic palpitations.A surface 12-lead electrocardiogram showed a narrow QRS tachycardia with a long RP interval.At baseline, the atrio-His (AH) and His-ventricular (HV) intervals were 82 and 46 ms, respectively.A narrow QRS tachycardia was induced by ventricular burst pacing (Figure 1 ).The earliest atrial activation site was recorded at the proximal coronary sinus (CS).The tachycardia cycle length (TCL) was irregular and ranged from 360 to 400 ms.Ventricular overdrive pacing terminated the tachycardia.Ventricular single scan pacing did not terminate the tachycardia at a minimum coupling interval of 200 ms.Ventricular double scan pacing with a fixed sense-S1 interval of 220 ms reproducibly terminated the tachycardia as the S1-S2 coupling interval was progressively shortened in 10-ms steps (Figure 2 ).During scan pacing with progressive S1-S2 shortening, the AA interval immediately after the S1 stimulus was unstable, ranging from 360 to 390 ms.What is the mechanism of this tachycardia?