STEMI vs NSTEMI: How to Tell Them Apart
STEMI and NSTEMI are both acute coronary syndromes caused by acute plaque rupture or erosion with thrombus formation on a vulnerable coronary plaque. Both cause myocardial injury with troponin elevation. The core axis that separates them is the degree of coronary occlusion, which is reflected in the ECG pattern: STEMI shows ST elevation from complete occlusion and mandates emergent reperfusion, while NSTEMI shows ST depression, T-wave inversion, or non-specific changes and is managed by risk stratification.
How to tell them apart
| Feature | STEMI (ST-Elevation Myocardial Infarction) | NSTEMI (Non-ST-Elevation Myocardial Infarction) |
|---|---|---|
| Degree of coronary occlusion | Complete (total) occlusion of the coronary artery | Partial or incomplete occlusion (subtotal thrombus) |
| Infarct extent | Typically transmural (full-thickness) myocardial necrosis | Typically subendocardial (partial-thickness) necrosis |
| ECG pattern | ST elevation ≥1 mm in two contiguous leads, with special thresholds in V2-V3 (≥2 mm in men ≥40, ≥2.5 mm in men <40, ≥1.5 mm in women); often with reciprocal ST depression | ST depression ≥0.5 mm, T-wave inversion ≥1 mm, or normal/non-specific changes — no ST elevation meeting criteria |
| ECG evolution | Evolves over time: hyperacute T waves → ST elevation → Q waves → T-wave inversion | No characteristic ST-elevation evolution; changes limited to ST depression or T-wave inversion |
| Reperfusion strategy | Emergent reperfusion is mandatory: PCI preferred, fibrinolysis if PCI cannot be achieved in time | No mandatory immediate reperfusion; management guided by risk stratification with immediate, early, or delayed invasive strategy |
| Role of risk stratification | Does not drive timing — immediate reperfusion is required regardless | Central to management; TIMI or GRACE scores determine timing of invasive strategy |
| Role of fibrinolysis | An option when timely PCI is unavailable | Not indicated |
The reasoning
Once the clinical picture suggests acute coronary syndrome (substernal pressure, radiation, diaphoresis, risk factors), the ECG is the immediate arbiter between STEMI and NSTEMI. The single decision that separates them is whether ST elevation meets criteria in two contiguous leads. If it does, call STEMI and activate emergent reperfusion at once — PCI if achievable in time, otherwise fibrinolysis — without waiting for troponin results, since any delay costs myocardium. Troponin confirms infarction retrospectively but is not required to trigger STEMI reperfusion. If the ECG shows only ST depression, T-wave inversion, or non-specific changes and troponin is elevated, this is NSTEMI: stabilize medically and use risk stratification (TIMI, GRACE) to decide the timing of the invasive strategy.
Key tests
- ECG: the primary discriminating test. STEMI shows ST elevation meeting criteria in contiguous leads (with reciprocal depression); NSTEMI shows ST depression, T-wave inversion, or non-specific/normal findings.
- Troponin: elevated in both STEMI and NSTEMI, confirming myocardial injury but not distinguishing between them. A normal troponin instead points toward unstable angina rather than either infarction — and reperfusion for STEMI should never wait on troponin results.
- Coronary angiography: defines the culprit lesion. In STEMI it guides emergent PCI; in NSTEMI its timing (immediate, early, or delayed) is determined by risk stratification.
What they share
- Acute coronary syndrome caused by vulnerable plaque rupture or erosion with platelet activation and thrombus formation
- Elevated troponin confirming myocardial injury (distinguishing both from unstable angina)
- Classic presentation of substernal chest pressure, often radiating to the left arm, with diaphoresis
- Shared antiplatelet therapy: aspirin plus a P2Y12 inhibitor (clopidogrel, ticagrelor, or prasugrel)
- Shared anticoagulation with heparin (UFH, LMWH, fondaparinux, or bivalirudin)
- Shared secondary prevention: aspirin, P2Y12 inhibitor, high-intensity statin, beta-blocker, and ACE inhibitor/ARB, with nitrates reserved for relief of ongoing ischemic symptoms
Pitfalls
- Missing a posterior STEMI: ST depression in V1-V3 with tall R waves and tall upright T waves is the mirror image of posterior ST elevation. This is a STEMI equivalent requiring emergent reperfusion — do not mislabel it as NSTEMI.
- Forgetting that a normal or non-specific ECG does not exclude NSTEMI; the diagnosis rests on troponin elevation, so obtain serial ECGs and troponins when initial findings are equivocal.
- Overlooking right ventricular involvement in inferior STEMI: check right-sided leads (V4R). These patients are preload-dependent, so nitrates and diuretics can precipitate severe hypotension.
- Giving thrombolytics without excluding aortic dissection — dissection can extend into the coronary ostia and mimic STEMI, and fibrinolytics are catastrophic in that setting.
- Confusing infarction with unstable angina: unstable angina shares the ischemic ECG changes of NSTEMI but has no troponin elevation.
- Delaying STEMI reperfusion to await troponin results — the STEMI diagnosis and reperfusion decision are made on clinical presentation plus ECG alone.
Practice this the way the exam tests it — on branching cases where your decisions shape the patient.