ECG criteria
- ST-segment elevation ≥ 1 mm (0.1 mV) in two contiguous limb leads—typically I and aVL—measured at the J-point. Isolated elevation in only one of these leads does not meet formal STEMI criteria, but in combination with reciprocal changes in lead III, it may indicate occlusive MI and warrants immediate expert evaluation.
- Reciprocal ST-segment depression in lead III (often the most pronounced), and occasionally in aVF and II. Such depression, especially when dynamic and concordant with the clinical presentation, increases the specificity of the finding for acute ischemia/occlusion; however, the magnitude of depression does not allow for quantitative estimation of occlusion probability on its own, as this depends on coronary anatomy, collaterals, time from symptom onset, and trace quality.
- Clinical presentation of ischemia: anginal chest pain, dynamic evolution on serial ECGs
- South African flag sign: ST elevation in leads I, aVL, and V2 with reciprocal depression in lead III, typically seen in occlusion of the first diagonal branch. If concomitant posterior MI is suspected (ST depression in V1–V3 separate from this sign), recording leads V7–V9 should be considered; ST elevation ≥ 0.5 mm in two contiguous posterior leads supports the diagnosis, though thresholds and interpretation depend on the clinical context.
- A pathological Q wave may appear in leads I and aVL during the subacute phase, but is frequently absent due to the relatively small myocardial mass involved.
- In isolated high lateral involvement, changes are most prominent in leads I and aVL, though changes may also appear in V2–V6 and inferior leads; the absence of changes in chest leads does not rule out MI.
- Hyperacute, peaked T waves in leads I and aVL may be the earliest sign preceding overt ST elevation.
What it looks like on ECG
The most pronounced direct changes occur in leads I and aVL, which view the high (basal) lateral LV wall. These leads show ST-segment elevation, sometimes accompanied by peaked, "hyperacute" T waves in the earliest stage. The formal STEMI criterion requires ST elevation ≥ 1 mm in two contiguous leads; leads I and aVL are contiguous, but isolated elevation in only one of them does not meet this criterion. However, isolated elevation in aVL with reciprocal depression in lead III is a well-known pattern of occlusive myocardial infarction and requires urgent clinical evaluation regardless of formal classification.
Reciprocal changes include ST depression in lead III (and frequently in aVF, II). This is the typical reciprocal change to elevation in aVL: its presence, especially when dynamic and accompanied by ischemic symptoms, increases the specificity of the finding for acute occlusion, though the magnitude of depression does not provide a reliable quantitative estimate of probability, as it depends on coronary anatomy, collateral circulation, time from symptom onset, and heart position.
The South African flag sign is described separately: ST elevation in I and aVL (horizontal stripe), reciprocal ST depression in lead III (vertical stripe), and ST elevation in V2 (flag corner), typical for occlusion of the first diagonal branch. ST depression in V1–V3 should not be confused with this sign—if concomitant posterior ischemia is suspected, recording leads V7–V9 is indicated.
| Lead | Parameter |
|---|---|
| I, aVL | ST elevation, possible hyperacute T wave |
| III | reciprocal ST depression (frequent finding) |
| II, aVF | mild reciprocal ST depression (variable) |
| V2 | possible ST elevation (flag sign) |
| V1–V3 | possible ST depression — suspect posterior MI, acquire V7–V9 |
| V3–V6 | changes may occur with involvement of the diagonal/intermediate branch or anterolateral zone |
A pathological Q wave in leads I and aVL does not always form — the infarction zone may be small, and the Q wave here is low-amplitude and short even under normal conditions, so it must be interpreted with caution.
Mechanism
The high lateral LV walls are supplied by the first diagonal branch of the LAD, the obtuse marginal branch of the circumflex artery, or the ramus intermedius — a separate intermediate branch arising from the bifurcation or trifurcation of the left main coronary artery between the LAD and circumflex, rather than branching from the latter. The size of the injury zone depends on the caliber and supply basin of the specific artery and can range from small to extensive; a large diagonal or marginal branch, or the ramus intermedius, can produce an extensive anterolateral or lateral MI. The injury vector is directed toward the electrodes of leads I and aVL, producing ST elevation in them, while the opposite pole of this vector forms reciprocal depression in lead III. When the affected myocardial volume is small, the amplitude of changes is often modest, and the MI is easily missed, especially if relying solely on the classic criterion of ST elevation in two contiguous chest leads.
Causes
- Occlusion of the first diagonal branch of the LAD
- Occlusion of the circumflex branch (obtuse marginal branch)
- Occlusion of the ramus intermedius
- Part of a more extensive anterolateral or inferolateral infarction in proximal disease
- Multivessel disease involving the lateral wall
Differential diagnosis
- Benign ST elevation (normal variant, early repolarization syndrome) — no reciprocal depression in lead III, no dynamic evolution, no ischemic symptoms
- LV hypertrophy with ST-T changes — diffuse changes, high QRS voltage, deep S in V1–V2, ST-T direction opposite to the main QRS wave, no acute evolution
- Pericarditis — diffuse, usually concave ST elevation and PR depression, without reciprocal ST depression in most leads; exceptions are aVR and sometimes V1
- Inferior myocardial infarction — ST elevation is identified in at least two contiguous leads among II, III, aVF, often maximal in lead III, with reciprocal depression in leads I and aVL (inverse pattern)
- Wellens' syndrome and other NSTE-ACS patterns — no ST elevation, changes involve the T wave in chest leads
Clinical significance and management
Formal STEMI classification requires ST elevation ≥ 1 mm in two contiguous limb leads — typically I and aVL. Isolated elevation in aVL alone with reciprocal depression in lead III does not automatically meet this criterion, but it is recognized as a pattern suspicious for occlusive myocardial infarction (OMI). In the presence of persistent ischemic symptoms and a convincing dynamic ECG pattern, the ACS algorithm for suspected occlusion should be activated immediately, and the patient urgently referred for coronary angiography / primary PCI. ST elevation in leads I/aVL must not be dismissed as insignificant due to low amplitude — the clinical presentation and reciprocal depression in lead III, which increases the specificity of the finding, are critical. Serial ECGs should be acquired; if posterior ischemia is suspected, additional leads V7–V9 should be recorded, and serial troponin evaluated without delaying the reperfusion strategy when a clear clinical-ECG presentation is present. Thrombolysis is used only when STEMI criteria are met, there are no contraindications, and timely PCI is unavailable.
Common interpretation errors
- Ignoring ST elevation in aVL due to low amplitude (1–1.5 mm) — combined with depression in lead III, this is often the only early sign of occlusion
- Interpreting ST depression in lead III as isolated inferior wall ischemia rather than a reciprocal change
- Automatically assigning a "STEMI" diagnosis to isolated elevation in aVL alone without evaluating the contiguous lead I and the clinical picture
- Missing the South African flag sign due to focusing solely on limb leads without evaluating V2
- Expecting a pathological Q wave in leads I/aVL as a mandatory criterion — it may be entirely absent
- Dismissing the diagnosis of MI in the absence of changes in chest leads or, conversely, ignoring involvement of V2–V6 in more extensive disease
South African flag sign: ST elevation in leads I–aVL and V2 + reciprocal depression in lead III.
1. Which leads show direct changes in high lateral infarction?
The high lateral LV walls are represented in leads I and aVL, where ST elevation is recorded.
2. Which change is considered reciprocal in high lateral infarction?
ST depression in lead III is a frequent reciprocal change to ST elevation in aVL.
3. What does the South African flag sign include?
This is a combination of ST elevation in leads I, aVL, and V2 with reciprocal depression in lead III, resembling the South African flag in its spatial arrangement.
Frequently asked questions
What constitutes ST elevation in high lateral infarction?
A formal criterion is an ST-segment elevation of ≥ 1 mm (0.1 mV) at the J point in two contiguous limb leads, usually I and aVL. The amplitude is often small, making reciprocal ST depression in lead III and the clinical picture crucial; isolated elevation in aVL alone with depression in III warrants prompt evaluation even without strictly meeting the formal criterion.
Why is high lateral infarction easy to miss?
Changes are frequently concentrated in leads I and aVL, the ST-elevation amplitude is typically low, and the strict "two contiguous leads" criterion is often unmet due to the finding being isolated to aVL. Leads I, aVL, and III must be purposefully correlated, without ruling out potential involvement of the precordial leads.
What is the South African flag sign?
This combination of ST elevation in I, aVL, and V2 with reciprocal ST depression in III is typical of occlusion of the first diagonal branch. The name derives from the visual similarity of these changes' layout to the South African flag. ST depression in V1–V3 is not considered part of this sign—it separately points to possible posterior ischemia.
How to differentiate high lateral infarction from inferior myocardial infarction?
In inferior MI, ST elevation is identified in at least two of the contiguous leads II, III, and aVF (often maximal in III), with reciprocal depression in I and aVL. In high lateral infarction, the picture is reversed: elevation in I and aVL, reciprocal depression in III.
Is emergency reperfusion necessary for isolated ST elevation in aVL?
With persistent ischemic symptoms and convincing ECG dynamics, such a pattern requires immediate activation of the ACS protocol with suspected occlusion and urgent referral for coronary angiography/primary PCI. The formal classification of STEMI depends on elevation in two contiguous leads, so thrombolysis is used only when STEMI criteria are fully met, there are no contraindications, and timely PCI is not feasible.
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Related terms
More in the section "Ischemia and Myocardial Infarction"
Sources
- Goldberger's Clinical Electrocardiography, current edition
- Hampton. The ECG in Practice
- Surawicz, Knilans. Chou's Electrocardiography in Clinical Practice
- ESC Guidelines for the management of acute coronary syndromes, 2023
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.