Two LBBB‐Like Tachycardias with Identical QRS Morphology: What Is the Mechanism?

Abstract

Case Presentation A 17-year-old boy was referred for an electrophysiological (EP) study because of recurrent palpitations due to a left bundle branch block (LBBB) shaped wide QRS tachycardia. The baseline ECG showed a normal QRS complex during sinus rhythm, but intermittently broad QRS complexes were noted on telemetric monitoring before the study. Three diagnostic quadripolar catheters (Biosense Webster, USA) were introduced and placed in the high right atrium (HRA), His bundle region (His), and right ventricular apex (RVa), respectively. A decapolar catheter (St. Jude Medical, USA) was inserted into the coronary sinus (CS). Two kinds of tachycardia with identical LBBB-shaped QRS morphology (width 120 ms) but different cycle lengths were easily induced by programmed atrial stimulation. Tachycardia 1 (Fig. 1, left panel), having a heart rate of 150 beats per minute (bpm), was similar to the clinical tachycardia and, according to the patient, duplicated his symptom. Tachycardia 2 (Fig. 1, right panel) was even faster, with a heart rate of 190 bpm. Both tachycardias showed a superior frontal plane axis (−60 ◦ ). No definite P waves could be identified. Overdrive HRA pacing during tachycardia 1 led to a shift to the faster tachycardia 2 (Fig. 2) and vice versa. What was the mechanism of these two tachycardias? The EP Study and Differential Diagnosis Although a negative concordant pattern was present in the precordial leads, the initiation of the tachycardia by atrial pacing, the width, and configuration of the QRS complex in lead V1, V2, and the clearly 1:1 V/A relationship made a ventricular origin unlikely. Both tachycardias in this case showed same QRS morphology and same concentric VA ac