In anterior STEMI, when the maximal ST-segment elevation is in leads V2 to V4, the culprit artery is almost exclusively the LAD. In a minority of cases, the LM coronary artery, the LD, the LCx, or the RCA may be the culprit artery. In anterior STEMI, both the level of LAD occlusion and the size of the artery modify the ECG pattern. When the occlusion is proximal to the first LD branch, the anterolateral segment of the LV will be involved in the ischemic process, and ST-segment elevation will be present in lead aVL. Typically, reciprocal ST-segment depression is seen in lead III. However, if the LAD is large, wrapping around the LV apex, ischemia of two anatomically opposite regions, the anterolateral and the inferior, results in a cancellation of injury vectors, with resultant attenuation of the ST-segment elevations. If the occlusion is distal to the first diagonal branch, the size of the artery will determine the ECG pattern. In small LADs, the ST segments in the extremity leads will be isoelectric (anteroseptal ischemia), whereas in large LADs, there will be ST-segment elevation in the inferior leads II, III, and aVF, with reciprocal ST-segment depression in lead aVL (see Fig. 4–5). In some cases with anterior STEMI with proximal occlusion of the LAD, there is no ST-segment elevation in lead aVL. Instead, the ECG shows ST-segment elevation in lead aVR and ST-segment depression in lead V6.33