Right ventricular enlargement mimicking electrocardiographic left ventricular pacing.

Electrocardiographic right bundle branch block morphology during cardiac pacing is occasionally the result of accidental placement of pacemaker or defibrillator leads into the left ventricle. Inadvertent lead placement in the left heart is associated with a risk of systemic embolism. Previous authors have attempted to define safe (right ventricular origin) and unsafe (left ventricular origin) patterns of right bundle branch block during pacing. We report a case of a patient with severe dilated cardiomyopathy and a correctly positioned pacemaker-defibrillator lead in the right ventricular apex, who meets electrocardiographic criteria for lead implantation into the left ventricle.