ECG criteria
- ST-segment elevation in lead I
- ST-segment elevation in lead aVL
- ST-segment elevation in leads V1–V2 (anteroseptal/parasternal region)
- Frequently reciprocal ST-segment depression in lead III, sometimes involving II and aVF
- The orientation of the ST vectors visually resembles the diagonal bands and wedge of the South African flag when plotted on a lead diagram
- Clinical presentation of acute coronary syndrome (anginal chest pain)
- This sign is descriptive and lacks a strict, validated quantitative formula; variants with a less pronounced or absent elevation in V1 may occur
- The pattern is associated with likely proximal LAD occlusion, often proximal to the first diagonal branch; exact localization is confirmed by coronary angiography
What it looks like on ECG
The South African flag sign is not an independent, isolated ECG element, but rather a combination of ST-segment shift directions across multiple leads that is figuratively compared to the flag of the Republic of South Africa (a combination of horizontal and diagonal colored bands).
Key components:
| Leads | ST changes |
|---|---|
| I, aVL | elevation |
| V1–V2 | elevation |
| III (sometimes with II, aVF) | depression (reciprocal) |
Classically described as ST elevation in leads I, aVL, and V1–V2 with reciprocal depression predominantly in lead III. Variants exist where the elevation in V1 is less prominent than in V2 or virtually absent—there is no single rigid formula in the literature requiring the participation of every specific lead, and the sign itself should be viewed as descriptive rather than a strictly validated algorithm.
When plotting ST displacement vectors on a hexaxial reference system or a schematic diagram of the thorax, the elevation lines in leads I, aVL, V1–V2 and the depression line in lead III visually form a pattern reminiscent of the South African flag stripes—hence the name. This is a mnemonic, figurative name rather than a quantitative criterion.
For formal diagnosis of STEMI, standardized J-point ST-elevation thresholds in two contiguous leads are used; these thresholds depend on the lead, sex, and age—specifically, leads V2–V3 have separate values (higher for men under 40 and men ≥ 40 years, and different values for women), whereas in most other leads the threshold is at least 1 mm (0.1 mV). The South African flag sign itself has no such established mandatory millimeter threshold: it is a qualitative pattern of ST displacement distribution, and the amplitude of changes in individual leads may not reach formal STEMI criteria, yet the configuration itself raises suspicion for proximal LAD ischemia.
This sign belongs to the group of so-called equivalent patterns of classic STEMI and occlusion myocardial infarction (OMI), which raise the probability of a specific occlusion site based on curve morphology even before the full picture of ST elevation appears in all anterior leads.
Mechanism
Proximal LAD occlusion, especially proximal to the first diagonal branch, can lead to ischemia of the anterolateral wall and septal region. This can produce ST elevation in leads I, aVL (lateral vector) and V1–V2 (anteroseptal vector), with reciprocal ST lowering predominantly in lead III—the lead electrically opposite to the zone of injury. The direction and magnitude of the ST vectors are variable and depend on the extent and distribution of ischemia, individual coronary anatomy, and concomitant myocardial changes. The combination of multidirectional ST displacements produces a characteristic ECG pattern that has been given this figurative name.
Causes
- Acute thrombotic occlusion of the proximal LAD, often proximal to the first diagonal branch
- Acute coronary syndrome with coronary artery occlusion in this region
- Less commonly, pronounced vasospasm in this coronary bed territory
Differential diagnosis
- Anterior/anterolateral MI (ST elevation may extend to V1–V6, as well as I and aVL in various combinations)—the presence or absence of reciprocal depression in lead III is variable; the South African flag sign is a descriptive variant of change distribution within the spectrum of anterior/anterolateral MIs, rather than an independent anatomical classification or a reliable method to precisely determine occlusion proximity from a single ECG
- High lateral MI (ST elevation in leads I, aVL with reciprocal depression in lead III)—without significant elevation in leads V1–V2
- ST elevation in lead aVR in multivessel disease or left main coronary artery occlusion—characterized by diffuse ST depression in most leads and elevation specifically in lead aVR rather than aVL
- Inferior MI with reciprocal depression in leads I and aVL—the direction of changes is reversed: elevation in inferior leads, depression in lateral leads
- Pericarditis—diffuse, typically concave ST elevation, often with ST depression in leads aVR and V1, lacking the localized pattern of reciprocal changes characteristic of acute coronary injury
Clinical significance and management
The South African flag sign is not a formal diagnostic criterion for STEMI and does not prove acute coronary occlusion on its own. It is considered a concerning potential pattern of occlusion myocardial infarction (OMI), associated with a likely proximal LAD lesion location and a potentially large volume of anterior wall and septal ischemia.
When this combination of ST displacements is identified in the setting of ongoing ischemic chest pain, immediate patient evaluation and activation of the occlusion/reperfusion protocol are required: urgent consultation for primary percutaneous coronary intervention. The decision regarding thrombolytic therapy is made based on standard STEMI criteria and contraindications rather than this single descriptive sign alone—especially if the displacement amplitude in individual leads does not reach formal STEMI thresholds. The sign helps avoid missing a pattern of proximal LAD ischemia when focusing solely on the anterior chest leads, but diagnosis and anatomical localization require clinical, serial (repeat ECGs, troponin), and typically angiographic correlation.
Common interpretation errors
- Evaluating only chest leads V1–V4 without paying attention to I, aVL, and III, resulting in failure to recognize the pattern as a whole
- Interpreting ST depression in lead III (and potentially II, aVF) as isolated inferior wall ischemia rather than a reciprocal change
- Ignoring the sign due to a small amplitude of ST displacements that does not reach formal millimeter STEMI thresholds in each individual lead
- Automatically interpreting the pattern as confirmed STEMI and an indication for thrombolysis without considering the clinical picture, serial changes, and standard criteria
- Confusion with left main coronary artery occlusion pattern (elevation in lead aVR) due to superficial resemblance of "diffuse" changes
1. Which combination of ST changes is most characteristic of the South African flag sign?
The combination of ST elevation in leads I, aVL, and V1–V2 with reciprocal depression predominantly in lead III creates a characteristic pattern resembling the South African flag.
2. Occlusion of which segment of the coronary bed may this sign indicate?
The pattern is associated with proximal LAD ischemia, but the exact level of occlusion relative to a specific branch cannot be reliably determined from a single ECG and is confirmed by coronary angiography.
3. What is the physician's management strategy when identifying this sign in a patient with ongoing anginal pain?
The pattern itself does not equal a confirmed STEMI: immediate evaluation prioritizing urgent coronary angiography and PCI is required, while the decision on fibrinolysis is made based on standard criteria and contraindications.
Frequently asked questions
What does the South African flag sign mean on an ECG?
This is a descriptive term for the combination of ST elevation in leads I, aVL, and V1–V2, frequently with reciprocal ST depression in lead III. This combination increases the probability of proximal LAD occlusion, but definitive diagnosis and localization require clinical and angiographic correlation.
Is the South African flag sign dangerous?
This is a concerning potential occlusion myocardial infarction (OMI) pattern associated with a proximal LAD lesion and a risk of extensive anterior wall and septal damage. With ongoing ischemia, immediate evaluation and usually an urgent PCI strategy are required; thrombolysis is permissible only if standard STEMI criteria are met and PCI is unavailable within the required timeframe.
How does the South African flag sign differ from a standard anterior myocardial infarction?
An anterior or anterolateral MI may present with ST-segment elevation in various combinations of precordial and lateral leads, accompanied by variable reciprocal changes. The South African flag sign is a descriptive variant of this distribution with characteristic involvement of I, aVL, V1–V2 and depression in III, which may be associated with a proximal LAD and a large ischemic burden, but it is not a precise independent anatomical marker of the occlusion level.
Do you need to look for this sign separately when interpreting an ECG?
There is no need to actively search for it specifically—it is identified during a careful, systematic analysis of ST segments across all 12 leads, which is already part of the standard interpretation protocol when acute coronary syndrome is suspected.
Is this term found in official ESC or AHA guidelines?
This is a descriptive figurative name for the pattern used in educational and expert literature on ECG diagnostics of occlusion myocardial infarctions, rather than a formal term in official ESC or AHA guidelines.
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Related terms
More in the section "Ischemia and Myocardial Infarction"
Sources
- Goldberger's Clinical Electrocardiography, latest edition
- ESC Guidelines for the management of acute coronary syndromes, 2023
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.