STEMI and its territories, and STEMI mimics

What it is

ST-segment elevation myocardial infarction (STEMI) is the ECG of a coronary artery that has closed completely, usually by thrombus on a ruptured plaque, so that the full thickness of the wall it supplies is injured. Injured muscle shifts the ST segment towards the leads that face it, and the leads with elevation map the territory and the culprit artery: inferior (II, III, aVF) is usually the right coronary artery and sometimes the circumflex; anterior and septal (V1–V4) is the left anterior descending artery; lateral (I, aVL, V5–V6) is the circumflex or a diagonal branch. The criterion is new ST elevation at the J point in at least two contiguous leads: 1 mm or more in every lead except V2–V3, where the threshold is 2 mm in men aged 40 or over, 2.5 mm in men under 40 and 1.5 mm in women. Reciprocal ST depression in the opposite leads (for example in I and aVL with an inferior infarct) makes a true infarct much more likely. The tracing evolves: hyperacute, broad and tall T waves in the first minutes, then ST elevation, then pathological Q waves and T-wave inversion over hours to days. Some occlusions do not meet the classic criteria and are treated as STEMI equivalents: posterior infarction (ST depression in V1–V3 with tall R waves, confirmed by ST elevation in V7–V9), new or presumed new left bundle branch block or a paced rhythm with Sgarbossa criteria, and the de Winter pattern. Wellens' pattern warns of a critical proximal LAD stenosis in a patient who is pain-free. The other half of the skill is the mimics: pericarditis, benign early repolarisation, left ventricular hypertrophy, bundle branch block, Brugada pattern, hyperkalaemia, left ventricular aneurysm and Takotsubo cardiomyopathy all raise the ST segment without an acute occlusion.

How to recognise it

• J-point ST elevation in 2 or more contiguous leads: 1 mm or more in all leads except V2–V3 (2 mm men 40 or over, 2.5 mm men under 40, 1.5 mm women).
• Inferior (II, III, aVF): ST elevation greater in III than in II plus ST depression in I and aVL points to the right coronary artery.
• Anterior (V1–V4, septal V1–V2): LAD; with I and aVL too, a proximal LAD lesion; extensive anterior with V5–V6.
• Lateral (I, aVL, V5–V6): circumflex or diagonal; a high lateral infarct may show only in I and aVL.
• Right ventricle: ST elevation of 1 mm or more in V4R, usually with an inferior infarct.
• Posterior: horizontal ST depression in V1–V3, tall broad R waves and upright T; ST elevation of 0.5 mm or more in V7–V9 confirms it.
• Evolution: hyperacute T waves, then ST elevation, then Q waves (40 ms or wider) and T inversion; persistent ST elevation weeks later suggests aneurysm.
• Equivalents: de Winter (upsloping ST depression of 1 mm or more at the J point in V1–V6 with tall symmetrical T waves); LBBB or pacing with Sgarbossa criteria.

Why it happens

• Full-thickness (transmural) injury makes the injured zone electrically different from healthy muscle during the plateau, so the ST segment rises in leads facing it.
• Leads looking at the injured wall from the opposite side record the same vector inverted: reciprocal ST depression.
• Necrosis leaves an electrical window: leads over it see the cavity, and Q waves appear.
• Posterior leads V1–V3 look at the back wall from the front, so posterior injury appears mirrored as ST depression and tall R waves.
• Subendocardial ischaemia (NSTEMI, demand ischaemia) usually depresses the ST segment and does not localise the artery.

Step by step

1. Record and read the ECG within 10 minutes of arrival with chest pain — every minute of occlusion loses muscle.
2. Measure ST at the J point against the PR segment, lead by lead — the baseline before the QRS is the reference.
3. Group the leads with elevation into a territory — two contiguous leads make it a regional, arterial pattern.
4. Look for reciprocal depression in the opposite leads — it supports an infarct over a mimic.
5. In inferior STEMI record V4R; with ST depression in V1–V3 record V7–V9 — right and posterior infarcts hide on the standard leads.
6. With LBBB or a paced rhythm apply Sgarbossa criteria — ordinary ST criteria do not work there.
7. Compare with an old ECG and repeat every 15 to 30 minutes if symptoms persist — occlusion evolves, and the first tracing may be early.

Territories, leads and usual culprit artery

Differential and pitfalls

• Pericarditis: diffuse concave ST elevation across territories, PR depression, ST depression only in aVR and V1, no Q waves.
• Benign early repolarisation: concave ST elevation mainly in V2–V5 with J-point notching, stable over time, in young people.
• LVH and LBBB: ST elevation discordant to a deep S wave in V1–V3 is expected; use Sgarbossa criteria in LBBB.
• Concordant ST elevation of 1 mm or more, or concordant ST depression of 1 mm or more in V1–V3, in LBBB suggests acute occlusion.
• Other mimics: Brugada pattern, hyperkalaemia, left ventricular aneurysm, Takotsubo cardiomyopathy, myocarditis, paced rhythm.
• Pitfalls: a first ECG that looks normal (repeat it), a missed posterior infarct, and Wellens' T waves that disappear when pain returns.
• Wellens: deep symmetrical or biphasic T waves in V2–V3 in a pain-free patient; a critical LAD lesion, not a case for a stress test.

Next step

• STEMI or equivalent: activate the reperfusion pathway at once and call cardiology; do not wait for troponin.
• Primary PCI is the preferred reperfusion when it can be done promptly; otherwise fibrinolysis without delay if there is no contraindication.
• Give antiplatelet and anticoagulant treatment as the local protocol sets; avoid nitrates when right ventricular infarction or hypotension is present.
• Monitor continuously with a defibrillator at hand: ventricular fibrillation and AV block are early complications.
• Non-diagnostic ECG with ongoing pain: serial ECGs and high-sensitivity troponin; echo if a mimic or complication is suspected.

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Elsewhere in DurgiPedia

• The ECG in ischemia and infarction
• ECG basics: waves, intervals and leads
• The cardiac electrical axis
• Chest pain: ischemic, pericardial, pleural and other patterns
• Recording a 12-lead ECG

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Acute ST-Segment Elevation Myocardial Infarction (STEMI) (StatPearls (NCBI Bookshelf), 2024).

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