Case report
A 70-year-old man with a history of hypertension was referred to our hospital for catheter ablation of a narrow QRS tachycardia. After obtaining informed consent, an electrophysiological study was performed. Atrial extrastimuli delineated a smooth AV decremental conduction curve. Ventricular extrastimuli also delineated a smooth ventriculoatrial (VA) decremental conduction curve with the earliest activation mainly seen in the His bundle (HB) region (Figure 1A), but occasionally in the distal coronary sinus (CS) (Suppl. Figure 1). A clinical supraventricular tachycardia (SVT1) with a tachycardia cycle length (TCL) of 390 ms and His-atrial (HA) interval of 180 ms was induced without an atrio-His (AH) jump by decreasing the coupling interval of the atrial extrastimuli. The earliest atrial activation was in the HB region and it preceded the EAA in the CS by 15 msec. Ventricular overdrive pacing showed VA dissociation (Figure 1B), excluding ORT with any accessory pathways. Differential atrial overdrive pacing from the proximal CS and lateral right atrium showed the presence of VA linking, which was diagnostic of AVNRT excluding AT (Figure C-1 and C-2). A 3D activation map of the right atrium showed that the EAA was in the HB region, suggestive of fast-slow AVNRT with an SSP as the retrograde limb. A radiofrequency (RF) application targeting the antegrade slow pathway region up to the level of the CS roof failed to terminate the tachycardia. The ablation catheter was advanced to the non-coronary cusp (NCC) to avoid the risk of atrioventricular block. An RF application at the site with the atrial potential preceding that in the HB region by 12ms successfully rendered SVT1 noninducible (Figure 2B and C).
After a successful ablation of SVT1, SVT2 with a TCL of 390-400 and HA interval of 180 ms (same as SVT1) was induced. The earliest atrial activation was observed in the distal CS. Ventricular overdrive pacing showed VA dissociation (Figure 3B), excluding a diagnosis of ORT. Further, repeated RV overdrive pacing successfully entrained the atrium exhibiting a V-A-V response and the post-pacing interval minus the tachycardia cycle length (PPI-TCL) of 301ms (>115ms) (Figure 3C), indicated AVNRT. A bolus injection of adenosine-triphosphate (ATP) with a dose of 10 mg terminated this tachycardia without atrial activation, suggesting ATP sensitivity in a retrograde limb of SVT2 (Figure 3D). The AH interval during the SVT2 was almost the same as in SVT1, suggesting that the antegrade limb of the SVT2 was the fast pathway. These findings diagnosed SVT2 as a fast-slow AVNRT using an ILA-SP. A couple of the RF applications at the earliest atrial site within the CS failed to terminate the SVT2. An RF application in the LA where the local atrial potential preceded the CS potential by 20 ms (Figure 2F) rendered the SVT2 non-inducible, and which a junctional rhythm was observed during the RF application (Suppl. Fig 2).