What Falls Under Acute Coronary Syndrome (ACS)
Acute Coronary Syndrome (ACS) includes three distinct clinical conditions: unstable angina (UA), non-ST-segment elevation myocardial infarction (NSTEMI), and ST-segment elevation myocardial infarction (STEMI). 1
ACS Components and Their Characteristics
Unstable Angina (UA)
- Characterized by rest angina, new-onset severe angina, or increasing pattern of previously stable angina
- ECG shows transient ST-segment depression or T-wave changes
- Normal cardiac biomarkers (no elevation in troponin)
- Pathophysiology: Non-occlusive thrombus without myocardial necrosis
- Lower mortality risk compared to NSTEMI and STEMI 1
Non-ST-Segment Elevation Myocardial Infarction (NSTEMI)
- Characterized by prolonged chest pain/discomfort (often >20 minutes)
- ECG shows ST-segment depression, T-wave inversion, or may be normal
- Elevated cardiac biomarkers (troponin)
- Pathophysiology: Partially occlusive thrombus with subendocardial necrosis
- Intermediate mortality risk 1, 2
ST-Segment Elevation Myocardial Infarction (STEMI)
- Characterized by prolonged chest pain/discomfort (often >20 minutes)
- ECG shows persistent ST-segment elevation or new left bundle branch block
- Elevated cardiac biomarkers (troponin)
- Pathophysiology: Completely occlusive thrombus with transmural necrosis
- Higher mortality risk compared to UA and NSTEMI 1, 2
Common Pathophysiology
All three ACS conditions share a common pathophysiological substrate:
- Atherosclerotic plaque rupture or erosion
- Exposure of plaque contents to circulation
- Activation of coagulation cascade
- Thrombus formation
- Compromised coronary blood flow
- Myocardial ischemia 1
Less common causes of ACS include:
- Coronary artery spasm
- Coronary embolism
- Coronary dissection 1
Diagnostic Differences Between ACS Types
| Feature | Unstable Angina | NSTEMI | STEMI |
|---|---|---|---|
| ECG Changes | Transient ST depression or T-wave changes | ST depression, T-wave inversion | ST-segment elevation |
| Cardiac Biomarkers | Normal | Elevated | Elevated |
| Coronary Occlusion | Non-occlusive thrombus | Partially occlusive thrombus | Completely occlusive thrombus |
| Myocardial Damage | No myonecrosis | Subendocardial necrosis | Transmural necrosis |
Clinical Importance of Classification
The classification of ACS into these three distinct entities is clinically important because it:
- Guides appropriate risk stratification
- Determines need for immediate reperfusion therapy (essential for STEMI)
- Informs selection of antiplatelet and anticoagulant regimens
- Helps determine timing of invasive coronary angiography
- Guides long-term secondary prevention strategies 1
Treatment Implications
- STEMI requires immediate reperfusion therapy, ideally with primary percutaneous coronary intervention (PCI) within 120 minutes of presentation 2
- High-risk NSTE-ACS patients benefit from an early invasive strategy with coronary angiography and revascularization within 24-48 hours 2
- All ACS patients require dual antiplatelet therapy, with clopidogrel being indicated for all types of ACS to reduce the rate of myocardial infarction and stroke 3
Understanding the differences between these three conditions allows for appropriate triage, risk stratification, and evidence-based management strategies aimed at reducing morbidity and mortality in patients with ACS.