Right bundle branch block-induced Q waves simulating anterior myocardial infarction extension

A concept generally accepted in clinical electrocardiography is the assumption that a right bundle branch block (RBBB) does not alter significantly the initial portion of the QRS complex and because the left bundle branch is intact, the septum is activated normally in a left-to-right direction. We report a woman with an acute anterior myocardial infarction (MI) in which a small R wave was present in leads V1 and V4, but with the development of RBBB associated with PR-interval prolongation, these R waves were replaced by Q waves. Subsequently, the electrocardiographic features of anterior MI disappeared concomitantly with the loss of RBBB. The clinical and electrophysiologic implications of these findings are discussed.