Bit · Cardio
Acute Coronary Syndromes — STEMI vs NSTEMI vs Unstable Angina
Three points on the same spectrum of coronary thrombosis. The pivot is the troponin and the ECG.
Mechanism
All three result from coronary plaque rupture and thrombosis. The differences come down to (1) how complete the occlusion is and (2) how much myocardium has died:
- STEMI (ST-elevation MI) — complete occlusion of a coronary artery → full-thickness infarction. ECG: ST elevation in contiguous leads (or new LBBB). Troponin elevated. EMERGENT reperfusion within 90 min (PCI) or 30 min (fibrinolytics if PCI > 120 min away).
- NSTEMI (non-ST-elevation MI) — incomplete occlusion or complete occlusion with collateral flow → subendocardial infarction. ECG: ST depression, T-wave inversion, or no changes. Troponin elevated. Treat: anti-platelet (aspirin + P2Y12), anticoagulation, β-blocker, statin, angiography within 24–72 h.
- Unstable angina (UA) — same plaque-rupture mechanism, but no myocardial necrosis (yet). New-onset, rest, or accelerating angina. ECG: similar to NSTEMI but troponin negative. Treatment is identical to NSTEMI.
Differentiator Table
| STEMI | NSTEMI | Unstable Angina | |
| Occlusion | Complete | Partial (or complete + collaterals) | Plaque rupture, no necrosis |
| ECG | ST elevation (contiguous leads) OR new LBBB | ST depression / T inversion / normal | ST depression / T inversion / normal |
| Troponin | ELEVATED | ELEVATED | NEGATIVE |
| Infarct depth | Transmural | Subendocardial | None (yet) |
| Treatment | EMERGENT reperfusion (PCI within 90 min) | Anti-thrombotic; angiography within 24–72 h | Anti-thrombotic; angiography within 24–72 h |
| MONA-BASH | All ACS: Morphine (if pain), Oxygen if SaO₂<90, Nitrates, Aspirin, β-blocker, ACEi, Statin, Heparin | Same |
The Pivot
Two questions:
- ST elevation on ECG? → STEMI. Call cath lab.
- If no ST elevation — is troponin positive? Yes → NSTEMI. No → unstable angina.
Treatment of NSTEMI and UA is identical; only the diagnosis label differs based on troponin.
NBME-Style Stem
A 64-year-old man with hypertension and a 40-pack-year smoking history presents with 2 hours of crushing substernal chest pain radiating to the left jaw. ECG shows 3-mm ST elevation in leads II, III, and aVF. Troponin is pending. Which of the following is the most appropriate next step?
Concept Anchor
All three are coronary plaque rupture with thrombosis. STEMI is complete and the cath lab opens now. NSTEMI is partial with dead cells. Unstable angina is partial without dead cells. Troponin and ECG decide which one.