Changes in VA Intervals With Bundle Branch Block Aberration During Supraventricular Tachycardia: Wenckebach or Coumel Phenomenon?

A 28-year-old male patient was hospitalized due to frequent episodes of palpitations and documented NCTs. The PVS induced an NCT with intermittent BBB aberrancy (Figure 1) and repeatedly terminated with an atrial potential (Figure 2). Based on the findings within Figure 1, what is the mechanism of the tachycardia and VA changes with the development of the right BBB aberrancy? The approach to SVT diagnosis has mainly been described for tachycardias with a stable cycle length. However, extrapolation of criteria derived from regular tachycardias can also be performed with caution in regularly irregular tachycardias [1]. In the current case, the tachycardia was initiated by a VAV response from PVS (Figure 2A). We also noted that changes in the A-A interval during tachycardia resulted in subsequent changes in the V-V interval. Indeed, this finding occurs in both AT and AVNRT. Despite the use of multiple criteria, sometimes distinguishing AT arising from the peri–AV nodal area and AVNRT might be extremely difficult in clinical practice. The targeting of the earliest atrial activation is generally successful in such cases, even when the diagnosis is not certain (Video S1). In the current case, Figure 1B shows tachycardia continuing after block to the V, ruling out AVRT. The AV and VA intervals were each somewhat variable and interdependent (prolongation of AV resulting in shortening of next VA, which in turn changes next AV), behavior most consistent with AVNRT [2-9]. AVNRT with Wenckebach conduction in the lower common pathway is rare, and differentiating this condition from AT presents a significant diagnostic challenge [10]. The reproducible termination by atrial extrastimuli also strongly supported AVNRT rather than AT (because the fortuitous occurrence of both termination of AT and AV block is highly unlikely) (Figure 2). Furthermore, the exclusive induction of tachycardia by ventricular pacing is rarely observed in AT. For these reasons, the differential diagnosis is narrowed down to AVNRT and NVRT/NFRT. VA interval prolongation during an orthodromic AVRT with the appearance of a BBB ipsilateral to an atrioventricular AV-AP is well known as Coumel's law [5, 11]. It reflects His and ipsilateral bundle participation in tachycardia and is unique to orthodromic AVRT [12, 13]. At first glance, Figure 1A shows that right BBB causes prolongation of both the VA interval (Coumel's law), indicating that the right bundle is an integral part of the tachycardia circuit, as occurs with NFRT or right-sided AVRT [14-17]. However, it should be kept in mind that they cannot be applied in tachycardias with variable VA intervals, as it is in the current case. In the presence of the alternating cycle length during SVT, it is difficult to demonstrate the retrograde limb of the tachycardia to establish a specific mechanism; however, Figure 1B demonstrates progressive prolongation of the AH intervals, culminating in a single non-conducted V beat [18]. Despite this, another possible explanation for the irregular pattern of the tachycardia may be involvement of multiple functional pathways in the AV nodal reentrant circuit mimicking Wenckebach periodicity [19-22]; therefore, we postulate that the noted irregularity of the tachycardia may be attributable to Wenckebach AV block in the lower common pathway [23, 24]. This tracing represents a case that underscores the importance of evaluating electrophysiological mechanisms as a whole and illustrates how the Coumel and Wenckebach phenomena can be easily mistaken for one another at first glance. Radiofrequency ablation of the slow pathway near the coronary sinus ostium successfully eliminated the tachycardia (Video S2). Written informed consent was obtained from the patient for publication of this case report and any accompanying images. A copy of the consent form is available for review by the journal's editorial office. The authors declare no conflicts of interest. No. The data that support the findings of this study are available from the corresponding author upon reasonable request Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.