12-Lead STEMI Recognition

Why this matters

Get a 12-lead early. The longer a coronary artery stays closed, the more heart muscle dies. The 12-lead you skip on scene is the one the emergency department gets after registration. Look-alike patterns make crews afraid to call anything: early repolarization, pericarditis, left bundle-branch block (LBBB), a paced rhythm, high potassium, and left-ventricular hypertrophy (LVH) with strain. You do not have to be a specialist. Name a concerning pattern, transmit it, and choose the right hospital. Do not call a STEMI from Lead II alone.

STEMI means ST-elevation myocardial infarction. ACS means acute coronary syndrome, a heart-attack workup. PCI means percutaneous coronary intervention, the cath lab.

Clinical sequence

1. Protect the airway, breathing, and circulation, and confirm a pulse. Cardiac arrest is not a 12-lead problem first.
2. If ACS is possible — pressure, pain that radiates, sweating, unexplained shortness of breath, syncope, or ROSC (pulses back) — get a 12-lead as soon as it will not delay life-saving care. Acquire and interpret it within about 10 minutes of first medical contact.
3. If symptoms continue or change, get another 12-lead. One normal tracing does not close the case.
4. Transmit the tracing and use your STEMI-alert language. Tell the hospital what you see. Do not say only “chest pain.”
5. Give aspirin and other drugs only per protocol. See the aspirin lesson. Give oxygen if the sat is under about 90%. Do not put a non-rebreather on a patient whose sat is already near 98%. When criteria are met, go to a PCI-capable (cath-lab) hospital.

What to look for

• ST elevation in one territory, with reciprocal change, is the classic teaching picture. Know your system’s millimeter and lead rules.
• For inferior patterns, watch the blood pressure. Get a right-sided tracing before nitroglycerin if your protocol includes it.
• Mimics exist. When you are unsure, transmit and say, “possible STEMI versus mimic. We need a cath-lab capable evaluation.”
• LBBB, paced rhythms, and Sgarbossa rules (concordant ST elevation in LBBB or a paced rhythm) are advanced. Do not let uncertainty delay transport or a STEMI-alert call.

Common errors

• Waiting for a “better tracing” while the patient sits on the couch.
• Calling a STEMI from Lead II alone.
• Skipping transmission because the hospital will repeat it.

Practice

Related: Aspirin in ACS, ROSC & Post-Arrest Care.

Sources & Further Study

Named guidelines for further study. They are not clinical orders and do not replace your protocol or medical direction.

• 2025 ACC/AHA/ACEP/NAEMSP/SCAI Guideline for the Management of Patients With Acute Coronary Syndromes
• NASEMSO National Model EMS Clinical Guidelines (2022)
• Acquire and interpret a 12-lead within 10 minutes of first medical contact when ACS is suspected. Transmit and use a PCI-capable destination per protocol. This site does not certify STEMI diagnosis.