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Reference — Cardiac

STEMI Activation Criteria Reference

Quick reference for STEMI identification criteria — ST elevation thresholds by lead group, contiguous lead definitions, LBBB and posterior MI as STEMI equivalents, and immediate nursing actions.

Educational use only. STEMI activation is an institutional protocol decision. Criteria may vary slightly by institution. This reference supports learning and NCLEX preparation. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.

Primary ECG Criteria for STEMI

ST elevation in ≥2 contiguous leads at the J-point:

Lead GroupST Elevation ThresholdTerritory
V2–V3 (men <40 yr)≥2.5 mmAnterior (LAD)
V2–V3 (men ≥40 yr)≥2.0 mmAnterior (LAD)
V2–V3 (women)≥1.5 mmAnterior (LAD)
All other leads≥1.0 mmInferior (RCA), Lateral (LCx), aVR, etc.

Measured at the J-point (junction of QRS complex and ST segment).

Contiguous Lead Groups

Contiguous leads are anatomically adjacent leads that view the same cardiac territory. STEMI requires ST elevation in ≥2 contiguous leads.

TerritoryContiguous Lead GroupArteryClinical Notes
InferiorII, III, aVFRCA (right-dominant)Check for RV involvement; right-sided ECG; avoid NTG if RV infarct
AnteriorV1, V2, V3, V4LAD (proximal)Largest territory; highest mortality; watch for LBBB, complete block
SeptalV1, V2LAD (septal perforators)Bundle branch blocks common
Apical / anteriorV3, V4LAD (distal or diagonal)Medium-sized territory
Lateral (high)I, aVLLCx or diagonalLook for reciprocal changes in II, III, aVF
Lateral (low)V5, V6LCx or LAD (diagonal)Often with anterior or high lateral changes
Extensive anteriorI, aVL, V1–V6LAD or LMCACatastrophic — large territory; cardiogenic shock risk

New LBBB — STEMI Equivalent

New or presumed new left bundle branch block (LBBB) in the context of ACS symptoms is treated as a STEMI equivalent — activating the cath lab protocol. LBBB masks the ST changes of anterior STEMI, so ECG-based STEMI criteria cannot be applied in the standard way.

LBBB ECG Features:
  • ›Wide QRS (≥120 ms or ≥0.12 sec)
  • ›Broad, notched R in lateral leads (I, aVL, V5–V6) — 'M' pattern
  • ›Broad S wave or QS in V1 — 'W' pattern
  • ›Discordant ST changes (T-wave opposite to QRS deflection) — these are expected with LBBB and do not indicate ischemia
  • ›New LBBB: no prior ECG showing LBBB on file; or patient with new symptoms + LBBB = activate protocol
Sgarbossa Criteria: A scoring system to identify ischemia in LBBB. Concordant ST elevation ≥1 mm (5 pts), concordant ST depression ≥1 mm in V1–V3 (3 pts), discordant ST elevation ≥5 mm (2 pts). Score ≥3 is specific for MI.

Posterior MI — Recognition Clues

Posterior MI does not produce ST elevation on a standard 12-lead — it produces reciprocal changes that must be recognized. Posterior MI is frequently missed if posterior leads are not used.

Posterior MI Signs on Standard 12-Lead (Reciprocal Changes):

  • !ST depression in V1–V3 (mirror image of posterior ST elevation)
  • !Tall, broad R wave in V1–V2 (mirror of posterior Q waves)
  • !Upright, prominent T waves in V1–V3

Confirm with Posterior Leads (V7–V9):

  • ›V7 — left posterior axillary line, same level as V4–V6
  • ›V8 — left midscapular line
  • ›V9 — left paraspinal area
  • ›ST elevation ≥0.5 mm in V7–V9 = posterior STEMI confirmed

Immediate Actions Upon STEMI Identification

1
Activate STEMI protocolNotify charge nurse, provider, and cath lab per institution protocol. Time = myocardium.
2
12-lead ECG within 10 minutesConfirm ST elevation. Right-sided ECG if inferior STEMI suspected (V4R) — look for RV involvement before giving NTG.
3
Aspirin 325 mg chewedNon-enteric coated, chewed immediately — unless contraindicated or already given.
4
IV access × 2 and stat labsLarge-bore IV access. Troponin, BMP, CBC, PT/INR, type and screen. Do NOT delay cath lab activation for lab results.
5
Continuous monitoringCardiac telemetry, SpO₂. Supplemental O₂ only if SpO₂ <90%.
6
Medications per orderP2Y12 inhibitor loading dose, heparin per PCI protocol, NTG for pain if BP permits (not in inferior STEMI + RV involvement).
7
Transport to cath labDoor-to-balloon goal ≤90 minutes. Keep patient, family, and cath lab team informed. Monitor during transport.

NCLEX Pearls

  • ›STEMI = ST elevation ≥1 mm in ≥2 contiguous leads (≥1.5 mm women in V2–V3; ≥2 mm men in V2–V3).
  • ›New LBBB + ACS symptoms = STEMI equivalent — activate cath lab protocol.
  • ›Posterior MI shows ST DEPRESSION in V1–V3 — obtain posterior leads (V7–V9) to confirm ST elevation.
  • ›Inferior STEMI (II, III, aVF): always check right-sided ECG for RV infarct before giving nitroglycerin.
  • ›Do NOT wait for troponin results to activate STEMI — ECG drives the decision.
  • ›Door-to-balloon ≤90 minutes for PCI; ≤30 minutes for thrombolytics if no PCI available.

Related Resources

STEMI vs NSTEMI GuideECG differences, troponin findings, treatment priorities, nursing implications
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Coronary Artery Anatomy ReferenceLeft Main, LAD, Circumflex, RCA — territories, ECG leads, infarct locations
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Coronary Artery Territories ChartArtery, supplies, ECG leads, and common complications side-by-side
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ACS Medications ReferenceAspirin, nitroglycerin, heparin, P2Y12 inhibitors, beta-blockers, statins
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ECG Interpretation Fundamentals GuideSystematic ECG reading: intervals, bundle branch blocks, ST analysis, AV blocks
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Standards & sources

Fact-checked Jun 20, 2026

This page is written to align with American Heart Association (AHA) · American College of Cardiology (ACC) · AHA ACLS Guidelines. It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →