STEMI vs NSTEMI: the reperfusion decision
Same crushing chest pain, two different ECGs. One sends the patient for reperfusion within minutes; the other starts a risk assessment. Here is how to tell them apart and what to do first.
Chapters (13)
Using ECG and troponin findings, separate STEMI from NSTE-ACS and choose the initial strategy: primary PCI or fibrinolysis-then-transfer for STEMI, and antithrombotic therapy with risk-timed angiography for NSTE-ACS.
The short version
- ST elevation of at least 1 mm in two anatomically contiguous leads (higher cut-offs in V2–V3) means STEMI, and STEMI means immediate reperfusion.
- For symptoms under 12 hours, the goal for primary PCI is ≤90 minutes from first medical contact to device, or ≤120 minutes if the patient needs a transfer.
- If PCI can't happen within 120 minutes of first medical contact and symptoms began less than 12 hours ago, give a fibrinolytic unless contraindicated, then transfer straight away for angiography within 2–24 hours, or rescue PCI if it fails.
- Fibrinolysis is harmful in NSTE-ACS: with ST depression alone, unless a true posterior STEMI is suspected, it only adds bleeding risk.
- NSTE-ACS gets antithrombotic therapy, beginning with aspirin, and risk-timed angiography: immediate for refractory angina or hemodynamic or electrical instability.
STEMI vs NSTEMI: the culprit artery
The difference between STEMI vs NSTEMI starts in the culprit artery. In both conditions, a plaque has ruptured or eroded and a clot has formed on it.[1]
When the clot completely blocks the artery, the muscle downstream loses its blood supply, and the ECG usually shows ST elevation. When the clot only narrows the artery, the result is non-ST-elevation acute coronary syndrome (NSTE-ACS): unstable angina, or an NSTEMI if troponin rises.[3]
So the ECG is the fork in the road. In suspected ACS it is acquired and read within 10 minutes of arrival.[1][2]
STEMI ECG criteria
An ST-elevation myocardial infarction shows new ST elevation, measured at the J point, of at least 1 mm in two anatomically contiguous leads. Leads V2 and V3 have higher cut-offs. These thresholds apply in the absence of left ventricular hypertrophy or left bundle branch block.[1]
| Leads | Group | New ST elevation |
|---|---|---|
| Leads other than V2–V3 | All patients | ≥1 mm in ≥2 contiguous leads |
| V2–V3 | Men ≥40 years | ≥2 mm |
| V2–V3 | Men <40 years | ≥2.5 mm |
| V2–V3 | Women | ≥1.5 mm |
Once the ECG meets these criteria, the diagnosis is made. Reperfusion does not wait for a troponin result.[1]
STEMI reperfusion: primary PCI or fibrinolysis
The answer to STEMI is immediate reperfusion: open the artery. Primary PCI is preferred when it can happen quickly.[1]
- Can primary PCI happen in time?For symptoms under 12 hours, the 2025 guideline goal is ≤90 minutes from first medical contact to device, or ≤120 minutes if the patient needs a transfer.[1]
- If not, give a fibrinolyticIf PCI can't happen within 120 minutes of first medical contact and symptoms began less than 12 hours ago, give a fibrinolytic drug, as long as there is no contraindication.[1]
- Then transfer straight awayAfter fibrinolysis, transfer the patient to a PCI centre immediately, for angiography within 2–24 hours, or rescue PCI if the drug fails.[1]
Check the absolute contraindications to fibrinolysis first. Examples include:
- any previous intracranial hemorrhage
- ischemic stroke within 3 months
- active bleeding
- suspected aortic dissection
These are examples, not the full list (the guideline's Table 14 gives it).[1]
NSTEMI and NSTE-ACS: what the ECG can show
Non-ST-elevation ACS has no single ECG signature. The tracing can show ST depression, T-wave inversion, brief (transient) ST elevation, or nothing at all: a completely normal ECG does not exclude it.[1]
NSTEMI management: antithrombotic therapy and risk-timed angiography
Instead of fibrinolysis, start antithrombotic therapy, beginning with aspirin, and let risk decide when to do angiography.[1]
The common misconception is that an NSTEMI means no cath lab. It does not: the question is only when.[1]
| Patient | Invasive strategy |
|---|---|
| Refractory angina, or hemodynamic or electrical instability | Immediate |
| High risk | Angiography within 24 hours is reasonable |
| Intermediate risk | During the admission |
| Low risk | Routine or selective invasive approach |
STEMI vs NSTE-ACS at a glance
| STEMI | NSTE-ACS (unstable angina or NSTEMI) | |
|---|---|---|
| Artery (teaching model) | Usually complete occlusion | Narrowed, not fully blocked |
| ECG | New ST elevation ≥1 mm in ≥2 contiguous leads | ST depression, T-wave inversion, transient ST elevation, or normal |
| First strategy | Immediate reperfusion: primary PCI, or fibrinolysis then transfer | Antithrombotic therapy, beginning with aspirin |
| Fibrinolysis | If PCI can't happen within 120 min, symptoms <12 h, no contraindication | Harmful (ST depression alone, unless true posterior STEMI suspected) |
| Angiography | 2–24 h after fibrinolysis, or rescue PCI | Timed by risk: immediate to during the admission |
Practice question
Original · USMLE styleA 59-year-old man is brought to the emergency department of a rural hospital 2 hours after the onset of crushing substernal chest pain with nausea and sweating. The hospital has no cardiac catheterization laboratory; the nearest PCI-capable center is a 3-hour transfer away. He has hypertension and type 2 diabetes mellitus. He has no history of stroke, bleeding, recent surgery or head trauma. Pulse is 92/min, respirations are 18/min, and blood pressure is 138/84 mm Hg, equal in both arms. Oxygen saturation is 96% on room air. The lungs are clear. An ECG obtained 6 minutes after arrival shows 3-mm ST-segment elevation in leads V1 through V4. He has received aspirin.
Common questions
What is the difference between STEMI and NSTEMI?
Both start with a clot on a ruptured or eroded plaque. In the usual teaching model, the clot in STEMI blocks the artery completely and the ECG shows new ST elevation; in NSTEMI the clot usually narrows the artery without blocking it, the ECG does not meet STEMI criteria, and troponin rises.
What is the difference between NSTEMI and unstable angina?
Both are non-ST-elevation ACS. Troponin separates them: in NSTEMI it rises and/or falls with at least one value above the 99th percentile; in unstable angina it stays normal.[3]
What is the target time for primary PCI in STEMI?
For symptoms under 12 hours, the 2025 guideline goal is 90 minutes or less from first medical contact to device, or 120 minutes or less if the patient needs a transfer.
When is fibrinolysis given instead of primary PCI?
When PCI can't happen within 120 minutes of first medical contact, symptoms began less than 12 hours ago, and there is no contraindication. The patient is then transferred straight away to a PCI centre.
Does an NSTEMI need angiography?
Usually, and the timing depends on risk: immediately for refractory angina or hemodynamic or electrical instability, within 24 hours for high-risk patients, and during the admission for intermediate-risk patients. Low-risk patients can take a routine invasive approach or a selective one, with angiography only if ischemia is shown.
Transcript
Read the full narration, by chapter684 words
0:00Same pain, two ECGs
Same patient, same crushing chest pain, and two possible ECGs. On the first, the ST segments rise in leads V1 to V4. On the second, they dip down in the lateral leads. One of those ECGs sends the patient for reperfusion within minutes. The other starts a risk assessment. By the end of this lesson, you'll separate an ST elevation heart attack from non-ST elevation acute coronary syndrome, and choose the first reperfusion or invasive strategy.
0:33The culprit artery: occlusive vs non-occlusive clot
Think about the culprit artery. In both conditions, a plaque has ruptured or eroded, and a clot has formed on it. When the clot completely blocks the artery, the muscle downstream loses its blood supply, and the ECG usually shows ST elevation. When the clot only narrows it, you get non-ST elevation ACS: unstable angina, or an NSTEMI if troponin rises. So the ECG is the fork in the road, and you read it within 10 minutes of arrival.
1:05STEMI ECG criteria
An ST elevation myocardial infarction, or STEMI, shows new ST elevation of at least 1 mm in two anatomically contiguous leads, with higher cut-offs in V2 and V3.
1:18Open the artery: primary PCI within 90 or 120 minutes
The answer to STEMI is immediate reperfusion: open the artery. Primary PCI is preferred when it can happen quickly. For symptoms under 12 hours, the 2025 guideline sets a goal of 90 minutes or less from first medical contact to device, or 120 minutes or less if the patient needs a transfer.
1:40PCI too far: fibrinolysis, then transfer
If PCI can't happen within 120 minutes, and symptoms began less than 12 hours ago, give a fibrinolytic drug, as long as there's no contraindication. Then transfer the patient to a PCI centre straight away, for angiography within 2 to 24 hours, or rescue PCI if the drug fails.
2:01Contraindications and shock
Check the absolute contraindications first, like any previous brain bleed, a recent ischemic stroke, active bleeding, or a suspected aortic dissection. And in shock or hemodynamic instability, emergency revascularization is indicated however long ago the pain began.
2:21NSTE-ACS: what the ECG can show
Now the other branch. Non-ST elevation ACS can show ST depression, T wave inversion, brief ST elevation, or a completely normal tracing.
2:33Why fibrinolysis harms in NSTE-ACS
Fibrinolysis is harmful here. With ST depression alone, unless you suspect a true posterior STEMI, it adds the risk of bleeding, including hemorrhagic stroke.
2:45Antithrombotic therapy, risk-timed angiography, oxygen
Instead, start antithrombotic therapy, beginning with aspirin, and let risk decide when to do angiography. And here's the misconception: an NSTEMI doesn't mean no cath lab. Refractory angina, or hemodynamic or electrical instability, calls for an immediate invasive strategy. High-risk patients reasonably get angiography within 24 hours, and intermediate-risk patients during the admission. Low-risk patients can take either a routine or a selective invasive approach. On both branches, give oxygen only if the saturation is below 90%.
3:25Practice question
Your turn. Pause if you want to work it out. A 59-year-old man comes to a rural hospital with no cath lab, 2 hours after crushing chest pain began. His ECG shows 3 mm of ST elevation in leads V1 to V4, and his blood pressure is 138/84. The nearest PCI centre is 3 hours away, and he has no contraindications to fibrinolysis. What's the most appropriate next step?
3:55Answer and explanation
The answer is A: give a fibrinolytic drug now, then transfer him to the PCI centre. It's a STEMI, it's under 12 hours, and the delay to PCI is well over 120 minutes. B, transfer for primary PCI without fibrinolysis, would miss that window. C, waiting for a repeat troponin, delays reperfusion when the ECG has already made the diagnosis. D, heparin and angiography within 24 hours, is a plan for high-risk non-ST elevation ACS, not for a STEMI. And E, CT angiography of the aorta, isn't needed, because nothing here suggests a dissection.
4:39Recap
Quick recap. One: ST elevation means immediate reperfusion, with primary PCI within 90 minutes, or 120 with a transfer. Two: if that can't happen and symptoms are under 12 hours, give fibrinolysis unless it's contraindicated, then transfer. Three: non-ST elevation ACS gets antithrombotic therapy and risk-timed angiography, immediately if the patient is unstable or the pain is refractory.
5:10Next: chest pain case
Next, we put it all together in a full chest pain case, with one twist that changes the answer.
Sources
Numbers in the text, e.g. [1], point here- Rao SV, O'Donoghue ML, Ruel M, et al. 2025 ACC/AHA/ACEP/NAEMSP/SCAI Guideline for the Management of Patients With Acute Coronary Syndromes. Circulation. 2025;151(13):e771–e862. ahajournals.org/doi/10.1161/CIR.0000000000001309
- Gulati M, Levy PD, Mukherjee D, et al. 2021 AHA/ACC/ASE/CHEST/SAEM/SCCT/SCMR Guideline for the Evaluation and Diagnosis of Chest Pain. Circulation. 2021;144:e368–e454. ahajournals.org/doi/10.1161/CIR.0000000000001029
- Thygesen K, Alpert JS, Jaffe AS, et al. Fourth Universal Definition of Myocardial Infarction (2018). Circulation. 2018;138:e618–e651. ahajournals.org/doi/10.1161/CIR.0000000000000617
Guideline basis: US guidelines, 2025 ACC/AHA ACS guideline (reviewed 22 Oct 2026). UK (PLAB) note: NICE offers primary PCI if it can be delivered within 120 minutes of when fibrinolysis could have been given; NSTEMI/unstable angina at intermediate or higher risk has angiography within 72 hours (sooner if unstable); oxygen if SpO₂ is below 94%.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined, resized): atherothrombosis, myocardial infarction (anteroseptal infarct), stent and balloon catheter. Hospital and ambulance pictograms: Health Icons (healthicons.org), CC0. Schematic ECGs (25 mm/s, 10 mm/mV grid), charts, clock and all handwriting are drawn in code (original). The ECGs are teaching schematics, not patient tracings.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-CK-EMER-003.
For exam study only, not medical advice. Read the medical disclaimer.