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A 70-year-old gentleman was referred for radiofrequency catheter ablation of wide QRS tachycardia that was not terminated with intravenous adenosine. Twelve-lead ECG was normal during sinus rhythm and transthoracic echocardiography demonstrated no evidence of structural heart disease. The electrophysiological study showed a normal AH interval of 104 m and HV interval of 45 m during sinus rhythm. Atrial burst pacing reproducibly induced wide QRS tachycardia with left bundle branch block (LBBB) morphology with left axis (Panel A). An atrial premature beat (APB) was delivered during the tachycardia (Panel B). What is mechanism of the tachycardia? (see Fig. 1)
The differential diagnosis for wide QRS tachycardia with 1:1 AV relationship includes supraventricular tachycardia (SVT) with aberrancy, ventricular tachycardia (VT) [myocardial VT and bundle branch re-entry VT] with 1:1 retrograde conduction and preexcited tachycardia (SVT with bystander accessory pathway activation, antidromic AV re-entrant tachycardia (AVRT) and preexcited tachycardia with pathway to pathway conduction).
The wide QRS tachycardia has LBBB morphology with left axis (Panel A). His bundle catheter was unstable and His bundle recording was not recordable during tachycardia. Hence To delineate the mechanism of tachycardia, an APB was delivered during the tachycardia . This APB resulted in advancement of ventricular electrogram without any change in the QRS morphology (Panel B) excluding myocardial VT. The ventricular advancement following the APB was preceded by advancement of atrial electrogram in the His region (HBED) suggesting that antegrade conduction has happened through the node. The atrial activation sequence prior and after the APB are the same. The VA interval prior and after the APB are different, which suggests absence of VA linking excluding atrioventricular nodal reentrant tachycardia (AVNRT) and AVRT. Importantly APB did not change the timing of next A despite changing the VV timing. More over the extra stimulus captures peri-AV nodal atrium as well as coronary sinus (left atrial analogs), and yet does not reset the tachycardia, it must be an atrial tachycardia that doesn't involve either of these structures (thus not left atrial or septal tachycardia) and must therefore be of right atrial origin.
From the above observations it could be concluded that the wide QRS tachycardia with LBBB morphology is more likely an atrial tachycardia (AT) with LBBB aberrancy. The focal AT was mapped to posterior right atrial free wall region.
The authors have no competing interests, funding or financial relationships to disclose.
Peer review under responsibility of Indian Heart Rhythm Society.