Diagnostics & Procedures · Topic 2 of 16

ECG Ischemia & STEMI Equivalents

Diagnostics · ECG in acute coronary syndrome

ECG Ischemia & STEMI Equivalents

Not every occluded artery meets STEMI millimeters: posterior MI, hyperacute T waves, de Winter and Sgarbossa-positive LBBB go to reperfusion too.

Key numbers

ST elevation in 2 contiguous leads≥ 1 mm (V2–V3 higher)

Posterior MI: confirm with V7–V9V7–V9 STE ≥ 0.5 mm

LBBB or paced: Smith-modified SgarbossaST/S ≥ 25%

01 · Defined byST elevation in 2 contiguous leads

STEMI = new J-point ST elevation ≥ 1 mm in 2 contiguous leads; in V2–V3 it must be ≥ 2 mm (men ≥ 40), ≥ 2.5 mm (men < 40) or ≥ 1.5 mm (women).

02 · Defined byPosterior MI: confirm with V7–V9

Horizontal ST depression in V1–V3 with prominent R waves and upright T waves suggests posterior MI (circumflex); ST elevation ≥ 0.5 mm in V7–V9 (≥ 1 mm in men < 40) confirms it.

03 · Defined byLBBB or paced: Smith-modified Sgarbossa

In LBBB or ventricular pacing, any lead with concordant ST elevation ≥ 1 mm, concordant ST depression ≥ 1 mm in V1–V3, or discordant ST elevation ≥ 25% of the S-wave depth means acute MI.

Presentation

What you'll see

Hyperacute T waves: the earliest STEMI signHyperacute T waves — broad-based, asymmetric, tall relative to the QRS — can appear within 5 minutes of occlusion and evolve into ST elevation within about 30 minutes; treat as a STEMI equivalent.

aVR elevation with diffuse ST depressionST elevation > 0.5 mm in aVR with widespread ST depression (more than 2 regions) suggests left main or multivessel ischemia: high risk, not a STEMI equivalent; PCI or CABG preferred over fibrinolysis.

Diagonal occlusion: ST elevation in aVL and V2Occlusion of the first diagonal branch gives non-contiguous ST elevation in aVL and V2 with ST depression in III, aVF, V4–V5 — a large territory that warrants emergent reperfusion.

Diagnosis & monitoring

Work-up

Map ST elevation to the coronary territory must knowTerritories: II, III, aVF = inferior (right coronary or dominant circumflex); V1–V4 = anteroseptal (LAD); I, aVL, V5–V6 = lateral (circumflex or a diagonal). Reciprocal ST depression supports infarction.

de Winter T waves: a STEMI equivalent must knowde Winter pattern: upsloping J-point ST depression rising into tall, symmetric precordial T waves, no precordial ST elevation, often 0.5–1 mm ST elevation in aVR — proximal LAD occlusion needing emergent reperfusion.

Wellens: deep or biphasic T waves V2–V3Wellens syndrome: deeply inverted symmetric (about 75%) or biphasic (about 25%) T waves in V2–V3, isoelectric or < 1 mm ST elevation, no Q waves, normal or slightly raised troponin — often recorded pain-free.

NSTE-ACS: ST depression ≥ 0.5 mmIschemic changes short of STEMI: new horizontal or downsloping ST depression ≥ 0.5 mm in 2 contiguous leads, T-wave inversion > 1 mm in leads with a prominent R wave, or transient ST elevation.

Do right now

In this order

First moves

ST elevation in 2 contiguous leads≥ 1 mm (V2–V3 higher)

→

Map ST elevation to the coronary territory

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Posterior MI: confirm with V7–V9V7–V9 STE ≥ 0.5 mm

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Repeat the ECG when symptoms persist

Why each step

ST elevation in 2 contiguous leadsSTEMI = new J-point ST elevation ≥ 1 mm in 2 contiguous leads; in V2–V3 it must be ≥ 2 mm (men ≥ 40), ≥ 2.5 mm (men < 40) or ≥ 1.5 mm (women).

Map ST elevation to the coronary territoryTerritories: II, III, aVF = inferior (right coronary or dominant circumflex); V1–V4 = anteroseptal (LAD); I, aVL, V5–V6 = lateral (circumflex or a diagonal). Reciprocal ST depression supports infarction.

Posterior MI: confirm with V7–V9Horizontal ST depression in V1–V3 with prominent R waves and upright T waves suggests posterior MI (circumflex); ST elevation ≥ 0.5 mm in V7–V9 (≥ 1 mm in men < 40) confirms it.

Repeat the ECG when symptoms persistA single normal or nondiagnostic ECG does not exclude ACS: repeat it with ongoing or recurrent symptoms; ST–T changes that resolve or worsen on serial tracings make ischemia very likely.

Management

Orders

Inferior STEMI: record right-sided leadsWith inferior ST elevation, record right-sided leads: ST elevation in V4R (the most sensitive lead) means right ventricular infarction — give fluid and avoid nitrates and diuretics.

Wellens: early angiography, never a stress testWellens signals a critical proximal LAD lesion: it is a high-risk NSTE-ACS, not a STEMI equivalent — admit for early coronary angiography and do not order stress testing.

Red flags & don'ts

Avoid harm

New LBBB alone is not a STEMI equivalent avoidA new or presumed-new LBBB is not a STEMI equivalent in isolation: with suggestive symptoms, suspect MI and apply the Sgarbossa criteria rather than activating reperfusion on the bundle branch block alone.

de Winter or Wellens?

Commonly confused

Precordial T waves	Tall, symmetric, after upsloping J-point ST depression
Pain at the time of ECG	Ongoing — acutely ill
Culprit	Proximal LAD occlusion
STEMI equivalent?	Yes
Next step	Emergent reperfusion

Commonly confused

Tell apart

Pericarditis: diffuse ST elevation, PR depressionPericarditis mimics STEMI with diffuse concave ST elevation and PR depression but no reciprocal ST depression outside aVR and V1; other mimics are early repolarization, LVH, LV aneurysm and hyperkalemia.

More detail

Reference

Show 6 reference facts

Original weighted Sgarbossa: concordant ST elevation ≥ 1 mm (5 points), ST depression ≥ 1 mm in V1–V3 (3 points), discordant ST elevation ≥ 5 mm (2 points); ≥ 3 points is positive.

A 2025 meta-analysis found the Smith-modified Sgarbossa criteria far more sensitive than the original rule in LBBB (84% vs 40%) with similar specificity (93% vs 97%).

With typical symptoms, ST elevation is highly specific (98–99%) but only 35–56% sensitive for MI — most infarcts lack STEMI criteria on the first ECG.

About 20–25% of posterior infarctions found by ST elevation in V7–V9 show no anterior ST depression on the 12-lead ECG.

The de Winter pattern is uncommon — about 2% of anterior MIs in the largest series — and usually progresses to typical anterior STEMI within minutes to hours.

Pathologic Q waves develop over hours to days as ST elevation falls; they mark infarcted myocardium but are not required to diagnose STEMI.

Pearl

Anterior ST depression with upright T waves is a posterior STEMI until V7–V9 say otherwise.

Sources · Merck Manual Professional — Acute Myocardial Infarction (MI) · Brady WJ, Muck AE, Moak JH. High-risk electrocardiogram presentations in the ACS patient — beyond STEMI. Turk J Emerg Med 2025;25:1–9 (summarizes the 2022 ACC chest-pain pathway STEMI equivalents) · Essential Electrocardiography (University of Miami Gordon Center) — Planes and axis (lead groups) · Meyers HP et al. Accuracy of OMI ECG findings versus STEMI criteria for diagnosis of acute coronary occlusion MI. J Am Heart Assoc 2021 · Evidence for ECG patterns identifying acute coronary occlusion in non-ST-elevation ACS: a scoping review. Acad Emerg Med 2026 · Merck Manual Professional — Overview of Acute Coronary Syndromes (cites the 2025 ACC/AHA ACS guideline) · Merck Manual Professional — Electrocardiography (right-sided and posterior leads) · Merck Manual Professional — Pericarditis · Merck Manual Professional — Complications of Acute Coronary Syndromes (right ventricular infarction) · SAEM CDEM M3 Curriculum — STEMI (territories and occluded arteries) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).