Wide QRS Tachycardia With Bundle Branch Block Transition Without Cycle Length Change

RA, right atrium; LA, left atrium; AP, accessory pathway; RV, right ventricle; LV, left ventricle; RBBB, right bundle branch block; HPS; His-Purkinje System. A 28-year-old man with recurrent palpitations and no structural heart disease was referred for an electrophysiological study. The patient's resting electrocardiogram (ECG) showed a delta wave, compatible with a right anterior accessory pathway (AP). Programmed atrial stimulation (PAS) revealed an antegrade jump with a wide QRS complex tachycardia (WCT) alternating left (LBBB) and right bundle branch block (RBBB) tachycardias (Figure 1). What is the mechanism of the tachycardia? The first step in understanding tachycardia mechanisms begins with careful assessment of the HV interval and His sequence during WCT 1 2. Several additional features need to be scrutinized carefully. First, the retrograde activation sequence was unchanged throughout the tracing, suggesting retrograde conduction over a lateral AP. Second, the onset could realistically only be compatible with a preexcited tachycardia or VT. Third, the cycle length (CL) is identical for the LBBB tachycardia as per the RBBB tachycardia, most compatible with the same circuit for each morphology, with and without bystander AP participation. This essentially rules out ventricular tachycardia which would have to fortuitously appear with an identical CL. The first beat of the LBBB tachycardia is narrower than the subsequent beats, that is, it is fused between normal and AP conduction (Figure 1). The remaining two cycles have pure antegrade pre-excitation. The first beat of LBBB tachycardia suggests simultaneous conduction over the atriofascicular AP and the normal AV conduction system (the latter exiting via the left bundle). One might expect that the first beat of LBBB tachycardia should be fully preexcited to facilitate the onset of duodromic tachycardia in subsequent beats. It is noted that the CL of the LBBB tachycardia and the RBBB tachycardia are identical, which is the most compatible with the same circuit for each morphology with and without bystander AP participation. One observes that RBBB is not observed on the sinus beat after pacing or tachycardia. On the other hand, RBBB is observed during orthodromic tachycardia. This is well explained as resting RBBB during normal conduction which is masked when right AP conduction occurs, that is, the AP when present provides a “pseudonormalization” during normal sinus rhythm. That is, the RBBB is not obviously evident in sinus rhythm because the AP excites the right ventricle concealing the RBBB during normal rhythm. While PAS from the right atrial electrograms (Figure 2) demonstrated fused QRS complexes and the initiation of the RBBB-morphology WCT by an AH jump, PAS from the distal coronary sinus electrograms (Figure 3) showed only orthodromic activation without fusion, exhibiting an RBBB morphology. This leaves only bystander AP participation with the original atrioventricular reentrant tachycardia as the only viable explanation. The morphology and the ventricular activation (basal pattern) of the pre-excited rhythm are more suggestive of anteroseptal or paraseptal pre-excitation or an atriofascicular AP 3-5. The left-sided AP was mapped to the 3 o'clock of the mitral annulus and was successfully ablated (Video S1). After ablation of the left-sided AP, tachycardia was no longer inducible; therefore, active participation of the right-sided AP in the tachycardia could not be confirmed. Nevertheless, the right-sided AP was also successfully eliminated. This tracings serves as an illustrative example underscoring the diagnostic significance of differentiating active from bystander participation in the setting of multiple APs. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request. Video S1. demonstrates the open-window map, showing breakthrough signals across the right atrioventricular ring into the right ventricle during atrial pacing, and across the left atrioventricular ring into the left atrium during ventricular pacing. 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.