Left bundle branch block on flecainide in Wolff-Parkinson-white syndrome.

A 40-year-old male presented with recurrent paroxysmal palpitations, with one episode of pre-excited atrial fibrillation, and was put on Flecainide 50 mg twice a day elsewhere. Because of recurrent symptoms, he was keen on an electrophysiological study (EPS) while on drug therapy. The electrocardiogram (ECG) before drug therapy and the presentation ECG on drug therapy had right and left bundle block morphology, respectively. He had a structurally normal heart without any specific cardiac comorbidities.