Myocardium is usually supplied by three coronary arteries, although there are several variations in the number, origin, course and distribution of coronary arteries. Major contribution to left ventricular myocardial blood flow is by left anterior descending coronary artery (LAD) (50%), rest is equally contributed by right coronary artery (RCA) and left circumflex artery (LCx). In addition, most of the right ventricle is supplied by RCA. The need for a rapid reperfusion therapy is largely determined by how close the occlusion site is to the origin of the coronary artery, which corresponds to the area of ischaemic myocardium.Each artery contributes its blood supply to specific regional areas in the heart. These areas are topographically represented by the following groups of leads: 1 (Table 1). MYOCARDIAL DISTRIBUTION OF THREE MAIN CORONARY ARTERIES Left anterior descending arteryThe LAD travels along the anterior interventricular groove towards the apex of the heart. The major branches of LAD are septal and diagonal branches.The septal branches arise perpendicularly from the LAD and pass into the interventricular septum. The diagonal branches of the LAD course over the anterolateral aspect of the heart. Considerable variations exist in the number and size of the diagonal branches. In most (80%) patients, the LAD courses around the apex of the left ventricle and terminates along the diaphragmatic aspect of the left ventricle. In the remaining patients, the LAD terminates either at or before the cardiac apex. In these patients, the left ventricular apical portion is supplied by the posterior descending branch (PDA) of the RCA or LCx, which is larger and longer than usual. Left circumflex arteryThe LCx artery passes within the left atrioventricular groove toward the inferior interventricular groove. The LCx artery is the dominant vessel in 15% of patients, supplying the left PDA from the distal continuation of the LCx. In the remaining patients, the distal LCx varies in size and length, depending on the number of posterolateral branches supplied by the distal RCA. The major ABSTRACTThe electrocardiogram (ECG) remains a crucial tool in the identification and management of acute myocardial infarction (MI). A detailed analysis of patterns of ST-segment elevation may influence decisions regarding the use of reperfusion therapy. The early and accurate identification of the infarct-related artery on the ECG can help predict the amount of myocardium at risk and guide decisions regarding the urgency of revascularization. The specificity of the ECG in acute MI is limited by individual variations in coronary anatomy as well as by the presence of preexisting coronary artery disease, particularly in patients with a previous MI, collateral circulation, or previous coronary-artery bypass surgery. The ECG is also limited by its inadequate representation of the posterior, lateral, and apical walls of the left ventricle. Despite these limitations, the electrocardiogram can help in identifying proximal occlusion of the coronary arte...