Right bundle-branch block unmasking a myocardial infarction

A patient with an acute anteroseptal myocardial infarction and intermittent right bundle-branch block is presented. Abnormal Q waves in right precordial leads were only observed during right bundle-branch block. In the absence of right bundle-branch block, depolarization of the right ventricular free wall concealed the abnormal Q waves typical of an anteroseptal infarct. Right bundle-branch block, by preventing early activation of the right ventricular free wall, facilitated the correct diagnosis and localization of the infarction.