The 12-lead electrocardiogram (EKG), a powerful tool used in evaluating the chest pain patient, has its shortcomings. One such failing is encountered in a patient with one of the following electrocardiographic patterns: left bundle branch block (LBBB), ventricular paced rhythm (VPR), and left ventricular hypertrophy (LVH). These patterns reduce the ability of the EKG to detect acute coronary ischemic change and acute myocardial infarction (AMI). Several strategies are available to assist in the correct interpretation of these complicated electrocardiographic patterns, including a knowledge of the ST segment-T wave changes associated with these confounding patterns, performance of serial EKGs, and comparison with previous EKGs if available. This article suggests guidelines and interpretive tools for diagnosing AMI on EKG in patients with these confounding patterns.