Step 1: understand the ST vector in high lateral STEMI (Fig. 1). In high lateral infarct, the ST vector is pointing towards the axilla and is usually directed from 0° to −90° in the frontal plane. There are actually 6 ECG leads clustered in this area that can pick up a high lateral ST vector. 0° corresponds to lead I, -30° corresponds to aVL, -60° corresponds to the mirror image of lead III, and -90° corresponds to the mirror image of aVF. Being to the left and higher than the center of the heart, the frontal plane projection of lead V2 too is pointing towards the axilla. The morphology of the complexes in V2, therefore, frequently resembles the morphology in lead aVL. Based on the cartoon displayed in Fig. 1, it is easy to understand why LAD-D STEMI is characterized by ST elevation in leads I, aVL and V2, and by ST depression in leads III and aVF.
Step 2: recognize the electrocardiographic “South African flag sign” (Fig. 2). Most electrocardiographs display the 12-lead ECG in a 4x3 lead format. With such a display, the arrangement of ST-segment deviation resembles the pattern of the South African flag with ST elevation in the upper left panel (lead I) and in the two middle panels in the second and third columns (leads aVL and V2, respectively), and ST depression in the bottom left panel (lead III). Fig. 2 displays a case where the STEMI is unmistakable and therefore, it helps memorize the South African flag pattern. It also demonstrates that the marked ST elevation seen in V2 must be a frontal plane rather than a horizontal plane reflection of the ST vector because there is absolutely no corresponding ST elevation in leads V1 and V3, the two neighboring chest leads.
Step 3: recognize a more subtle high lateral STEMI (Fig. 3). In our experience, LAD-D STEMIs are frequently missed because the ST depression seen in leads III and aVF is mistaken for inferior ischemia. It should be taught, however, that ST depression in any lead can be a reflection of ST elevation in mirror image leads. Even when the ST elevation is quite subtle, if it is localized to leads I, aVL and V2, ST depression in the inferior leads should be considered to be probably reciprocal.
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- Acute first diagonal artery occlusion: a characteristic pattern of ST elevation in noncontiguous leads.Am J Emerg Med. 2015; 33: 1326.e3-5