Differences Between Unstable Angina, NSTEMI, and STEMI
The key difference between unstable angina, NSTEMI, and STEMI lies in the presence of myocardial necrosis and ECG findings: unstable angina has no biomarker elevation, NSTEMI shows elevated cardiac biomarkers without ST-elevation, while STEMI presents with ST-segment elevation and biomarker elevation. 1, 2
Pathophysiology and Definition
Unstable Angina
- Characterized by myocardial ischemia at rest or minimal exertion
- No evidence of cardiomyocyte injury/necrosis (normal cardiac biomarkers)
- Caused by reduced myocardial perfusion from partially obstructive thrombus
- ECG may show ischemic changes or be normal
- Better short-term prognosis than NSTEMI 1, 2
NSTEMI (Non-ST-Elevation Myocardial Infarction)
- Positive cardiac biomarkers (troponin or CK-MB above 99th percentile)
- No persistent ST-segment elevation on ECG
- Usually results from subtotally occlusive thrombus or microembolization
- ECG may show ST-segment depression, T-wave inversion, or be normal
- Sufficient ischemia to cause irreversible myocardial damage 1, 2
STEMI (ST-Elevation Myocardial Infarction)
- Positive cardiac biomarkers
- New ST-segment elevation on ECG diagnostic of acute myocardial infarction
- Typically results from complete coronary artery occlusion
- Accounts for approximately 30% of acute coronary syndromes
- Requires immediate reperfusion therapy 1, 3
Diagnostic Criteria
| Feature | Unstable Angina | NSTEMI | STEMI |
|---|---|---|---|
| Cardiac Biomarkers | Normal | Elevated | Elevated |
| ECG Changes | May show ischemic changes or be normal | ST depression, T-wave inversion, or normal | ST-segment elevation |
| Coronary Occlusion | Partial/transient | Partial/subtotal | Complete |
| Myocardial Necrosis | Absent | Present | Present |
Clinical Presentation
All three conditions may present with similar symptoms:
- Chest discomfort/pain (typically at rest, anginal in character)
- Pain radiating to arms, back, or jaw
- Associated symptoms: dyspnea, nausea, diaphoresis
- Duration often >20 minutes 2, 4
Management Approach
Unstable Angina
- Anti-ischemic therapy (nitrates, beta-blockers, calcium channel blockers)
- Antiplatelet therapy with aspirin
- Risk stratification to determine need for invasive strategy 2
NSTEMI
- Dual antiplatelet therapy (DAPT) with aspirin plus P2Y12 inhibitor
- Early invasive approach for high-risk patients (coronary angiography within 24-72 hours)
- Cardiac rhythm monitoring >24 hours for high-risk patients 1, 2
STEMI
- Immediate reperfusion therapy is critical
- Primary PCI within 120 minutes (preferred) or fibrinolytic therapy if PCI not available within timeframe
- Reduces mortality from 9% to 7% 3
Prognosis
- Unstable angina: Better short-term prognosis than NSTEMI but risk of progression to MI if underlying coronary disease not addressed
- NSTEMI: Worse long-term prognosis than STEMI due to higher burden of comorbidities and older age
- STEMI: Higher short-term mortality but better long-term prognosis with successful reperfusion 2
Clinical Pitfalls to Avoid
- Don't rely solely on ECG for diagnosis - cardiac biomarkers are essential for distinguishing between unstable angina and NSTEMI
- Don't delay treatment while awaiting biomarker results in suspected ACS
- Remember that approximately 40% of men and 48% of women present with nonspecific symptoms 3
- Don't forget that high-sensitivity troponin assays have changed the landscape - many cases previously classified as unstable angina are now diagnosed as NSTEMI 1
The spectrum of acute coronary syndromes represents a continuum of the same pathophysiological process with varying degrees of severity, with unstable angina and NSTEMI often sharing similar management approaches but differing in the presence of myocardial necrosis.