Intermittent Right and Left Bundle Branch Blocks and 2:1 Narrow QRS in a Patient with Uncommon Atrioventicular Nodal Reentrant Tachycardia

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The development of bundle branch block, especially left bundle branch block and 2:1 narrow QRS morphology, during atrioventricular nodal reentrant tachycardia (AVNRT) is unusual. We report a case of AVNRT patient with a short tachycardia cycle length, in whom transient wide QRS tachycardia with right and left bundle branch blocks (RBBB and LBBB). The patient was a 28 year-old-male whose chief complaint was palpitation with short of breath and pre-syncope for past 18 years. There was no structural organic heart disease on the echocardiogram. An electrophysiological study was performed without any anti-arrhythmic drugs. Uncommon (fast/slow) AVNRT was induced by atrial burst pacing at 300 ms and atrial programmed extra-stimuli without jump up phenomenon. Spontaneous conversion of QRS morphology from narrow QRS to RBBB and LBBB was observed during tachycardia. There was no change in atrial cycle length by a single premature ventricular stimulus during tachycardia. The wide QRS tachycardia with LBBB was transformed to narrow QRS tachycardia by the introduction of a single premature ventricular stimulus without changing the tachycardia cycle length. Radiofrequency catheter ablation targeting anatomical slow-pathway area was performed and successfully eliminated tachycardia. We speculated that enhanced AV nodal conduction could be closely related with the bundle branch block during AVNRT in this patient.