Spontaneous Conversion of a Long RP to Short RP Tachycardia: What Is the Mechanism?

A 43-year-old woman was referred for electrophysiologic study and ablation for drug refractory palpitations. Her electrocardiogram showed intermittent pre-excitation with positive delta wave in II, AVF and negative delta wave in lead III in the 12-lead ECG (Figure 1A). Patient had normal echocardiogram. Multipolar catheters were positioned at the right atrium, His bundle, right ventricle (RV), and within the coronary sinus. Baseline Atrio-His and His- ventricular intervals measured 84 and 37 ms respectively. The retrograde atrial activation was eccentric with earliest at the lateral wall of the right atrium with VA jump shown in Figure 2A. Retrograde conduction during ventricular pacing and early coupled ventricular extrastimuli showed VA interval variation (from 140 to 400 ms) with a sudden shortening of V pacing interval (from 230 to 220 ms) and without the change of atrial activation sequence (Figure 2A–C). Antegrade conduction showed dual AV node property (Figure 2D showing pattern of response to atrial extrastimuli). Supraventricular tachycardia (SVT) was induced with atrial burst pacing with a pacing cycle length of 500 ms. Single extra stimulation and burst pacing from the RV apex reproducibly induced her clinical tachycardia. During SVT, there is a spontaneous conversion of a long RP to short RP tachycardia with the same cycle length (Figure 1B,C). What is the mechanism? The differential diagnosis of the variable VA and AV conduction with same cycle length includes multiform atrioventricular nodal re-entrant tachycardia, atrial tachycardia and orthodromic reciprocating tachycardia (ORT) using a decremental right lateral (RL) accessory pathway. The earliest retrograde atrial activation at the lateral wall of the right atrium excluded the (AVNRT) atrioventricular (AV) nodal re-entrant tachycardia. Ventricular premature complex during his refractory period could reset the tachycardia and VAV response, ruling out atrial tachycardia (Figure 1D). There is also a possibility of two closely spaced pathways, each with own conduction time but as mapping of the tricuspid annulus showed atrial insertion of accessory pathway localized to lateral part of the tricuspid annulus and delivery of radiofrequency ablation at this site with 4 mm ablation catheter eliminated accessory pathway. Therefore, this has ruled out two closely spaced accessory pathways. So finally, the condition which can have a varying VA interval, is a bypass tract with decremental property and potentially longitudinal dissociation. The longitudinal dissociation of bypass tract can exhibit a short and long conduction time. Discrete “jumps” in the accessory pathway conduction time with similar activation sequence had been observed with alternation of AV and VA intervals during orthodromic atrioventricular reciprocating tachycardia [1]. The criteria for longitudinal dissociation of bypass tract in our case is cycle length alternation because of changes in VA interval and an abrupt prolongation in the VA interval using the accessory pathway during ventricular pacing after minimal changes in pacing rate or premature beat interval without a change in the retrograde atria activation sequence [2]. The VA interval is varying as there is conduction through the dissociated longitudinal AP, the fast component of the retrograde bypass tract (effective refractory period [ERP] ∼ 225 ms) buffered with antegrade slow pathway of the AV node (ERP ∼ 330 ms). Meanwhile, the slow component of bypass tract (ERP ∼ 220 ms) buffered with antegrade fast AV node (ERP ∼ 360 ms) to cause spontaneous varying AV and VA interval with same tachycardia cycle length. Hence this unique feature leads to maintenance of ORT using longitudinal dissociation as compared to the previous report in which the dual AV node could inhibit tachycardia initiation by bradycardia related retrograde block in a concealed accessory pathway [3]. The bypass tract was successfully ablated and the supraventricular tachycardia was not inducible with extrastimuli and intravenous infusion of isoproterenol. This case highlights the easy initiation and maintenance of SVT with the combination of antegrade dual AV node and retrograde longitudinal dissociation of atrioventricular accessory pathway with utilization of conduction curve for its diagnosis. The authors declare no conflicts of interest. Data available on request from the authors. The data that support the findings of this study are available from the corresponding author upon reasonable request.