Recognizing STEMI: A Step-by-Step Guide to ST Elevation

Time is myocardium. Every 30 minutes of delayed reperfusion in STEMI corresponds to roughly 7.5% relative mortality. Which means recognizing ST elevation accurately — and fast — is one of the most consequential skills in all of medicine.

But recognition requires more than just seeing elevation. You need to interpret it in context: the lead grouping, the morphology, the reciprocal changes, and the mimics that can fool even experienced clinicians.

Step 1: Measure the ST Elevation Correctly

The ST segment is measured at the J point — the junction where the QRS complex ends and the ST segment begins. Compare the J point to the isoelectric baseline (the TP segment between beats).

Standard criteria for significant ST elevation (adults, 2018 ACC/AHA):

• ≥1 mm elevation in two contiguous limb leads (I, II, III, aVL, aVF, V4–V6)
• ≥2 mm elevation in men ≥40 years in V2–V3
• ≥2.5 mm elevation in men <40 years in V2–V3
• ≥1.5 mm elevation in women in V2–V3

Step 2: Identify the Lead Grouping

Lead groupings tell you which coronary artery is occluded and which wall of the heart is affected:

Step 3: Look for Reciprocal Changes

Reciprocal ST depression is nearly pathognomonic for true STEMI. If you see ST elevation in the inferior leads (II, III, aVF), look for depression in aVL or I. If you see elevation in the anterior leads, look for depression in II, III, or aVF.

Reciprocal changes help distinguish true STEMI from its mimics. Pericarditis causes diffuse elevation without reciprocal depression. Early repolarization shows no reciprocal changes either. When in doubt — look for the mirror.

Step 4: Beware the STEMI Mimics

Pericarditis

Diffuse ST elevation in nearly all leads. PR depression (especially in lead II). No reciprocal changes. Usually concave-up morphology rather than the convex "tombstone" pattern of STEMI.

Early Repolarization

Often seen in young, healthy patients. Notching at the J point ("fish hook"). Typically involves the precordial leads. ST segment appears elevated but concave up, with tall upright T waves. No dynamic changes on serial ECGs.

Left Bundle Branch Block

LBBB obscures STEMI detection. Use the Sgarbossa criteria — concordant ST elevation ≥1 mm in a lead with positive QRS, concordant ST depression ≥1 mm in V1–V3, or discordant ST elevation ≥5 mm in a lead with negative QRS.

Left Ventricular Hypertrophy

Strain pattern can cause ST elevation in V1–V3. Typically accompanied by tall R waves in lateral leads and the classic "strain" pattern with asymmetric T wave inversion.

Takotsubo (Stress) Cardiomyopathy

Can exactly mimic anterior STEMI. Often triggered by emotional or physical stress. Apical ballooning on echo. Only diagnosed definitively at cath when no culprit lesion is found.

Practicing in CardioLens

The CardioLens ECG Library includes every classic STEMI territory with paired rhythm strips showing reciprocal changes. The Clinical Cases feature includes a full MI series with 60 cases across all severity levels — from subtle early changes to massive anterior STEMIs. The AI Scanner will also flag ST elevation it detects in any uploaded strip with territory-specific commentary.