STEMI ECG Interpretation

An ST-elevation MI is a can't-miss diagnosis. Learn the ST-elevation criteria, how to localise the culprit vessel, and the mimics that fool people.

What counts as STEMI

A STEMI is diagnosed from ST elevation in two or more contiguous leads (leads viewing the same wall), in the right clinical context. General thresholds are ≥1 mm in the limb leads and precordial leads, with higher cut-offs in V2–V3. New ST elevation with reciprocal depression is highly suggestive of acute coronary occlusion.

Localising the culprit artery

Reciprocal changes and posterior MI

ST elevation in one territory is frequently mirrored by reciprocal ST depression in the opposite leads, which strengthens the diagnosis. A true posterior MI is subtle: look for tall R waves and ST depression in V1–V2 (the mirror image of posterior elevation), and confirm with posterior leads V7–V9.

STEMI mimics to rule out

• Pericarditis — diffuse (not territorial) ST elevation with PR depression.
• Benign early repolarization — concave ST elevation, often young, healthy patients.
• Left bundle branch block / paced rhythm — use the Sgarbossa criteria.
• Left ventricular hypertrophy — high voltage with secondary ST-T changes.

Why speed matters

STEMI is an occluded coronary artery — time is muscle. Recognition triggers activation of the cath lab for primary PCI, with a goal door-to-balloon time of 90 minutes or less. See how this plays out in the lab in our PCI overview.

Summary

• STEMI = ST elevation in ≥2 contiguous leads in context.
• Localise by lead group → wall → artery.
• Reciprocal depression supports the diagnosis; posterior MI hides in V1–V2.
• Rule out pericarditis, early repolarization, LBBB, and LVH.
• Recognition triggers emergent reperfusion.

Frequently asked questions

What is the ECG criteria for STEMI?

New ST-segment elevation in two or more contiguous leads (typically ≥1 mm, with higher thresholds in V2–V3), in the right clinical context, often with reciprocal ST depression.

How do you localise a STEMI?

By the lead group showing ST elevation: inferior (II, III, aVF; RCA), septal/anterior (V1–V4; LAD), or lateral (I, aVL, V5–V6; circumflex).

What are common STEMI mimics?

Pericarditis, benign early repolarization, left bundle branch block or paced rhythm (use Sgarbossa criteria), and left ventricular hypertrophy.

Why is a STEMI an emergency?

It reflects an acutely occluded coronary artery; rapid reperfusion by primary PCI (goal door-to-balloon ≤90 minutes) limits heart-muscle loss.