STEMI Recognition: Contiguous Leads, Reciprocal Changes, and Field Decisions
Which leads group together, what reciprocal depression proves, and how EMS STEMI activation changes patient outcomes.
December 3, 2026 · 8 min read
A field STEMI activation can cut door-to-balloon time nearly in half. Recognizing it reliably comes down to knowing lead groups and trusting reciprocal changes.
Lead groups
- Septal: V1–V2
- Anterior: V3–V4
- Lateral: V5–V6, I, aVL
- Inferior: II, III, aVF
- Posterior: tall R and ST depression in V1–V3
Reciprocal changes
Injury current pointing toward one lead points away from the opposite lead. Inferior elevation with depression in I and aVL is highly specific for a true inferior MI. If you see elevation with no reciprocal depression anywhere, consider pericarditis, benign early repolarization, or left ventricular hypertrophy.
Inferior MI and nitroglycerin
Inferior MIs frequently involve the right ventricle, which is preload dependent. Nitroglycerin can crash the pressure. Get a right-sided ECG when you can, and be ready with fluids.
Field priorities
- 12-lead within 10 minutes of patient contact
- Aspirin 324 mg chewed unless contraindicated
- Transmit or announce the STEMI early
- Two IVs, avoid the right wrist if radial access is likely
- Bypass to a PCI-capable center per protocol
On the exam, the correct answer nearly always includes early activation and transport to a PCI center rather than another round of on-scene interventions.
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