Retrograde P Wave With Short RP Interval During Long RP' Tachycardia: What Is the Mechanism?

An 80-year-old female was referred for catheter ablation of narrow QRS regular tachycardia. During the clinical tachycardia which exhibited long RP' tachycardia, retrograde P wave with short RP interval was repetitively observed (Figure 1). What is the mechanism? At baseline, atrio-His (AH) and His-ventricular (HV) interval was 132 ms and 40 ms, respectively. The earliest atrial activation during ventricular pacing was recorded at the proximal coronary sinus (CS), and the ventriculoatrial (VA) conduction curve revealed decremental property. Para-Hisian pacing showed atrioventricular (AV) nodal pattern with prolongation of VA interval during wide QRS complex compared with during narrow QRS complex without the change of atrial signal sequence. VA conduction was considered to be via retrograde slow pathway. The clinical tachycardia was induced by atrial double extra stimulus pacing without AH jump after administration of isoproterenol. Atrial signal sequence during the tachycardia was identical to that during ventricular pacing. After ventricular overdrive pacing, the tachycardia continued with a V-A-V pattern, and the difference between corrected post pacing interval and tachycardia cycle length was > 110 ms. His-refractory ventricular extra stimulation did not perturb next atrial signal. The clinical tachycardia was diagnosed with fast-slow type atrioventricular nodal reentrant tachycardia (AVNRT). Although the clinical tachycardia basically exhibited long RP' tachycardia, retrograde P wave with short RP interval was repetitively observed as shown in Figure 1, and this phenomenon was reproduced in the electrophysiology lab as well (Figure 2). For the P wave with short RP interval, the earliest atrial activation site was shifted from the proximal CS to the His region (Figure 2). One possible explanation for different timing of the P wave would be premature atrial contraction (PAC) during the tachycardia. However, the P wave with short RP interval was reproducibly identified in the same manner; after two beats of retrograde slow pathway conduction as shown in Figure 2. Another explanation would be retrograde limb of the circuit was converted from slow pathway to fast pathway. Otomo previously reported "fast-fast" type AVNRT which accounted for 0.5% of 950 AVNRT cases [1]. In fast-fast type AVNRT cases with shorter tachycardia cycle length (260 ± 55 ms), AH and HA interval was less than 220 ms and 120 ms, respectively with the earliest atrial activation recorded at His region [1]. In the current case, HA interval with short RP sequence was 132 ms and tachycardia cycle length was 330 ms, both of which were longer than those in previously reported "fast-fast" type AVNRT cases. Interestingly, ventricular extra stimulus pacing with coupling interval less than 500 ms revealed not only retrograde slow pathway conduction but also VA conduction with different atrial signal sequence which was identical to that observed during the tachycardia with short RP interval (Figure 3). This retrograde conduction with the earliest activation site of His region showed decremental property, and one ventricular echo beat was observed during ventricular extra stimulus pacing (Figure 3). Kaneko et al previously reported superior slow pathway which served as a retrograde limb of atypical AVNRT [2]. Among eight patients with fast-slow type AVNRT including a superior slow pathway they reported, double atrial response via retrograde fast pathway and retrograde superior slow pathway was observed during ventricular pacing in three patients. They also reported in another study that the V-A-A-V sequence on ventricular induction and/or on entrainment was frequently (8/9 patients) observed in fast-slow type AVNRT incorporating a superior slow pathway as a retrograde limb, most of which was due to double atrial response [3]. In our case, the retrograde P wave with short RP interval during the clinical long RP' tachycardia was considered to result from retrograde conduction over superior slow pathway from two beats before the immediate previous ventricular activation (double atrial response) (Figure 2) since the atrial signal sequence was identical to that of the second atrial signal during ventricular extra stimulus pacing. As shown in Figure 2, retrograde conduction via superior slow pathway was considered to exhibit Wenckebach block, and short RP interval was observed only when it could conduct to the atrium. For catheter ablation, we applied radiofrequency (RF) energy at the proximal CS targeting the left inferior extension of "typical" slow pathway [4], which terminated the tachycardia 2.6 s after RF application. After RF energy application to the proximal coronary sinus, residual retrograde conduction was confined to the His region and double atrial response was still observed via retrograde fast pathway and retrograde superior slow pathway. Since no tachycardia including AVNRT using superior slow pathway was inducible after elimination of retrograde typical slow pathway, we ended the session and the patient has been free from palpitation since then. The data that support the findings of this study are available from the corresponding author upon reasonable request.