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South African flag sign: a teaching tool for easier ECG recognition of high lateral infarct

Published:October 15, 2015DOI:https://doi.org/10.1016/j.ajem.2015.10.022
      We read with great interest the case report by Drs Durant and Singh on the peculiar pattern of ST elevation caused by acute occlusion of the first diagonal branch of the left anterior descending coronary artery (LAD-D) [
      • Durant E.
      • Singh A.
      Acute first diagonal artery occlusion: a characteristic pattern of ST elevation in noncontiguous leads.
      ]. In our experience too, such high lateral infarcts (STEMIs) are frequently missed even by experienced emergency medicine physicians because of the apparently noncontiguous nature of ST elevation in the electrocardiogram (ECG). We have developed a simple educational tool that can help conceptualize and recall the typical ST changes seen in high lateral STEMI. We use a 3-step approach.
      • 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.
        Figure thumbnail gr1
        Fig. 1The ST vector in high lateral ST elevation myocardial infarction due to occlusion of the first diagonal branch of the LAD. The ST vector points to leads I, aVL, V2, and to the mirror image of III and aVF. The ST vector abnormality should therefore be considered to be in contiguous leads.
      • 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.
        Figure thumbnail gr2
        Fig. 2The “South African flag sign.” With the most common 4×3 display of the 12-lead ECG, the location of the most impressive ST deviations resembles the shape of the South African flag.
      • 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.
        Figure thumbnail gr3
        Fig. 3A more subtle form of the South African flag sign.
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      Reference

        • Durant E.
        • Singh A.
        Acute first diagonal artery occlusion: a characteristic pattern of ST elevation in noncontiguous leads.
        Am J Emerg Med. 2015; 33: 1326.e3-5