New Irregular Rhythm in a Patient with Baseline Left Bundle Branch Block Marwan M. Refaat, MD a , Byron K. Lee, MD, MAS b, * CLINICAL PRESENTATION A 63-year-old man with a history of Reiter syndrome, nonischemic idiopathic cardiomyop- athy, and baseline left bundle branch block (LBBB) had a syncopal episode and episodes of nonsustained ventricular tachycardia (VT). He underwent biventricular implantable cardioverter- defibrillator (ICD) placement. Four months later, the patient presented with fatigue and episodes of palpitations. He was admitted for further management. Twelve-lead electrocardiograms (ECGs) recorded during his initial presentation are shown in Fig. 1. CLINICAL QUESTION What is the rhythm in this patient with baseline LBBB pattern? ELECTROPHYSIOLOGY STUDY AND CLINICAL COURSE The patient had an irregular tachycardia at 138 beats/min with QRS complexes much narrower than his baseline LBBB QRS. The QRS duration was generally 105 milliseconds, but there were some occasional wide complex beats. P waves could not be clearly identified on the baseline ECG. The rhythm on the initial 12-lead ECG was diagnosed as atrial fibrillation with occasional aberrant beats or premature ventricular complexes (PVCs). This diagnosis was later proved to be wrong. The true diagnosis was VT, which was deter- mined from the ICD interrogation that clearly showed atrioventricular dissociation (Fig. 2). The atrial beats march out at about 65 beats/min, whereas the ventricular beats are occurring at Disclosures: None of the authors have any conflicts to disclose relevant to this article. a Section of Cardiac Electrophysiology, Division of Cardiology, Department of Medicine, University of California, San Francisco, 500 Parnassus Avenue, San Francisco, CA 94143, USA; b Section of Cardiac Electrophysiology, Division of Cardiology, Department of Medicine, University of California, San Francisco, 500 Parnassus Avenue, Box 1354, MU 429, San Francisco, CA 94143, USA * Corresponding author. E-mail address: leeb@medicine.ucsf.edu KEYWORDS  Bundle branch block  Implantable cardioverter-defibrillator  Ventricular tachycardia KEY POINTS  Transition from wide to narrow QRS complex has several mechanisms such as heart rate change, “peel-back” refractoriness, rate-dependent progressive shortening of bundle branch refractori- ness, gap phenomena, supernormal conduction, loss of preexcitation, premature ventricular complex, or ventricular tachycardia (VT) ipsilateral to the bundle branch block and equal conduction delay in both of the bundle branches.  The QRS complexes can be normal or near normal in width when the VT originates from the ventric- ular conduction system or near the ventricular conduction system. Card Electrophysiol Clin 4 (2012) 655–657 http://dx.doi.org/10.1016/j.ccep.2012.08.027 1877-9182/12/$ – see front matter Ó 2012 Elsevier Inc. All rights reserved. cardiacEP.theclinics.com