[Right bundle branch block during right ventricular pacing]

We describe the case of a 78-year-old patient who presented for ablation of a wide complex tachycardia with right bundle branch block (RBBB) morphology. A pacemaker spike at the QRS onset indicated supraventricular tachycardia with AV synchronous ventricular pacemaker activation.Correct positioning of the ventricular lead in the right ventricular apex was confirmed by fluoroscopy and echocardiography, excluding malpositioning of the right ventricular lead.In the electrophysiological study we diagnosed atrial tachycardia with 1:1 AV stimulation by the pacemaker. The ECG, however, presented negative concordance in the precordial leads. Only after shifting the precordial leads V1 and V2 from the 4th to the 2nd intercostal space were all 12 ECG leads in accordance with the clinical tachycardia.Thus, it is suspected that malpositioning of the ECG electrodes generated an apparent RBBB morphology of the clinical tachycardia. Malpositioning of ECG electrodes switches the lead characteristics from horizontal (anterior-posterior) to frontal (cranial-caudal) plane properties. In this situation, the precordial leads V1 and V2 with positive vector in V1 and V2 imitate aVL (V2) and aVR (V1) and can produce an apparent RBBB morphology.