Two distinct ventriculo-atrial relationships during narrow QRS complex tachycardia: What is the mechanism?

A 65-year-old woman underwent ablation for a short RP narrow complex tachycardia (NCT). EP study showed dual AVN phenomenon and two inducible narrow complex tachycardias showing 1:1 VA relationship, concentric atrial activation but different H-A intervals. RV overdrive pacing failed to entrain the first long RP tachycardia (NCT1) but transformed it to another A-on-V tachycardia (NCT2), excluding the diagnosis of AVRT. Premature atrial contractions (PACs) were delivered for further differential diagnosis. During NCT1, a late PAC was able to advance the following His without resetting of the tachycardia. An early PAC terminated the tachycardia with anterograde slow pathway conduction to the His. Both were inconsistent with fast-slow AVNRT and supported AT as the diagnosis. NCT2 was likely to be diagnosed as AVNRT given the A-on-V pattern and the presence of slow pathway. However, His-synchronous PAC advanced the next H over the slow pathway but failed to advance the following A as well as the subsequent H i.e., tachycardia was not reset. All responses to PACs were repeatable, indicating the mechanism of NCT2 was the same AT with anterograde slow pathway conduction to His. The tachycardia was proved to be para-His AT and ablated without difficulty.