Hide and seek: Masking of preexcitation from a slow‐conducting, decremental right lateral accessory pathway due to preexcitation via a fasciculoventricular pathway

Masking of preexcitation from a slow-conducting, decremental AP due to preexcitation via an FV pathway. Multiple accessory pathways (APs) have been reported in 8% of pediatric patients without structural heart disease.1 Fasciculoventricular (FV) pathways are a rare cause of preexcitation but are often associated with additional APs and atrioventricular nodal reentrant tachycardia (AVNRT).2 In a large series (n = 1437), 1.8% patients who underwent an electrophysiology (EP) study for ventricular preexcitation had an FV pathway.3 Approximately 20% of these patients also had an additional AP.3 Other studies have also reported a higher prevalence of additional APs in patients with FV pathways.2 However, to the best of our knowledge, masking of preexcitation from a slow-conducting, decremental AP due to preexcitation via an FV pathway has not been reported. A 13-year-old healthy girl with a structurally normal heart presented with preexcited narrow QRS tachycardia (Figure 1A) that had required an emergency room visit and intravenous adenosine for termination. Patient's baseline electrocardiogram showed subtle preexcitation (Figure 1B). She underwent an EP study, where pertinent findings included subtle preexcitation with a His to ventricular (H–V) interval of 33 msec (Figure 2A). Of note, His to local ventricular activation timing in the distal His pole measured only 15 msec (Figure 2A). Upon decremental atrial pacing close to atrioventricular (AV) Wenckebach cycle length, a completely different pattern of progressive preexcitation, suggestive of the presence of a right lateral AP, was noted (Figure 2B). Adenosine administration resulted in a complete AV and ventriculoatrial (VA) block without any change in QRS morphology. Dual AV nodal physiology, single and double short ventriculoatrial (VA) echoes, and a short nonsustained run of a narrow QRS 2:1 tachycardia was noted with aggressive triple atrial extra stimulus testing (Figure 2C). The patient had several brief self-limited episodes of atrial fibrillation and atrial flutter during the procedure; however, the clinical tachycardia could not be induced. Mapping was performed during atrial pacing at a cycle length of 380 msec (close to AV nodal Wenckebach cycle length) to bring out the preexcitation via the right lateral pathway. As the ablation catheter was moved to the ventricular end of the lateral tricuspid valve annulus, abrupt prolongation of local AV interval was noted (Figure 2D,E) due to mechanical bumping of the pathway. Cryoablation followed by radiofrequency (RF) ablation (Freezor Xtra 6 mm tip cryoablation catheter, Medtronic, MN, USA and TactiCath D/F curve irrigated tip ablation catheter, 35–40 W, 45°C, Abbott Labs, IL, USA) was carried out at this site (Figure 2F). Following ablation, the second pattern of preexcitation (via right lateral AP) was eliminated. However, the patient continued to have subtle preexcitation with borderline short H–V interval (32 msec) and a short interval between local His and ventricular activation in the distal His catheter. In addition, decremental atrial pacing resulted in a progressive increase in AH interval but no change in HV interval. Prior to ablation, VA block cycle length off and on 1 mcg/min isoproterenol measured 420 and less than 250 msec, respectively, with decremental atrial activation. Postablation, VA block cycle length measured 290 msec with decremental concentric atrial activation. With aggressive atrial pacing and single, double, and triple atrial extrastimulus testing on and off isoproterenol, no echos or AVNRT could be induced. After discussion with the family, a shared decision was made to not carry out slow pathway modification. At 8-month follow-up, the patient continues to do well and has not had any further episodes of tachycardia. Her 12-lead electrocardiogram continues to show subtle preexcitation due to antegrade conduction via the FV pathway. This 13-year-old girl with a history of preexcited narrow QRS tachycardia underwent an EP study that showed two APs—an FV pathway and a right lateral AP. The right lateral AP that was "hidden" was slow conducting, had decremental conduction properties, and was adenosine sensitive. Interestingly, preexcitation via the FV pathway was either leading to retrograde concealment in the adenosine-sensitive, slow conducting, and decremental right lateral AP or antegrade conduction via the fasciculoventricular pathway was significantly more rapid than antegrade conduction via the adenosine-sensitive, slow conducting, and decremental right lateral AP. As AV nodal Wenckebach was achieved with decremental atrial pacing, thereby abolishing conduction across the FV pathway, manifest preexcitation was noted through the right lateral AP, which permitted us to map the pathway. Of note, atrial pacing close to lateral tricuspid annulus did not bring out preexcitation via the right lateral AP until the AV node was blocked, suggesting that retrograde concealment was more likely the cause of masking of AP conduction. At the site of successful ablation, AV signals that were tight but not fused were noted (Figure 2D,E). Moreover, the onset of the local ventricular signal was later than the onset of ventricular activation on the 12-lead electrocardiogram (Figure 2D,E). These findings suggest that the right lateral AP was slow conducting and the insertion of this pathway was deeper in the ventricle and not at the lateral tricuspid annulus. Upon mechanically bumping the pathway at the ventricular end of the lateral tricuspid annulus, marked prolongation of the local AV interval was noted, guiding us toward the site of successful ablation. Given the lack of inducibility of clinical tachycardia under anesthesia, lack of inducibility of double echos or tachycardia after ablation of the right lateral AP, and a small risk of AV nodal block with slow pathway modification, a shared decision was made with the family to not carry out slow pathway modification. As per the current PACES/HRS guidelines, ablation of FV pathways, which are typically not associated with a risk of supraventricular tachycardia or sudden death, is a class III indication with a greater risk than benefit and was therefore not considered.4 To conclude, preexcitation via an FV pathway masked preexcitation due to a slow-conducting decremental AP in our patient which is likely due to retrograde concealment in the AP. To the best of our knowledge, these findings have previously not been reported in a patient with an FV pathway. They inform the need for a careful EP study with close attention to subtle changes in QRS morphology in patients with FV pathway who often have additional APs. The study was supported by an internal grant funded by the Department of Pediatrics at WVU Children's Hospital. All authors have reported that they have no relationships relevant to the contents of this paper to disclose. The study complies with ethical principles as outlined in Declaration of Helsinki. IRB approval is not required to report deidentified single patients at WVU. Written informed consent was obtained from the patient's mother for publishing this report. Not applicable. Most of the data associated with this study are provided in text and figures. Additional data will be made available upon request.