What conditions fall under Acute Coronary Syndrome (ACS)?

What Conditions Fall 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, 2

Pathophysiology of ACS

ACS is typically caused by disruption (rupture or erosion) of an unstable coronary artery atherosclerotic plaque with associated partial or complete coronary artery thrombosis and/or microemboli, resulting in diminished blood flow to the myocardium and subsequent myocardial ischemia 1.

These conditions exist along a continuum of severity:

1. Unstable Angina (UA):

   • Characterized by transient myocardial ischemia
   • Diminished coronary flow without significant myonecrosis
   • Normal cardiac troponin levels
   • May show transient ST-segment depression or T-wave changes on ECG 1, 2

2. Non-ST-Segment Elevation Myocardial Infarction (NSTEMI):

   • Partially occluded coronary artery leading to subendocardial ischemia
   • Elevated cardiac biomarkers (troponin) indicating myocardial necrosis
   • ECG may show ST-segment depression, T-wave inversion, or may be normal 1, 2

3. ST-Segment Elevation Myocardial Infarction (STEMI):

   • Typically involves a completely occluded vessel
   • Leads to transmural myocardial ischemia and infarction
   • Elevated cardiac biomarkers
   • ECG shows persistent ST-segment elevation or new left bundle branch block 1, 2

Key Distinctions Between ACS Conditions

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
Mortality Risk	Lower	Intermediate	Higher

Clinical Significance

The classification of ACS into these three conditions is critical for:

• Determining appropriate treatment strategies
• Guiding the need for immediate reperfusion therapy
• Selection of antiplatelet and anticoagulant regimens
• Timing of invasive coronary angiography
• Long-term secondary prevention strategies 2

Evolving Nomenclature

Recent guidelines have proposed a new binary classification system of "myocardial ischemic syndromes" with subcategories of "acute myocardial ischemic syndromes" (AMIS) and "non-acute myocardial ischemic syndromes" (NAMIS) 1. Within this framework, ACS remains a critically important subcategory of AMIS, specifically referring to cases presumed to result from epicardial coronary artery plaque rupture or erosion, for which revascularization is of proven benefit 1.

Important Considerations

• The pathophysiology of ACS can be dynamic, with patients potentially progressing rapidly from one condition to another during presentation and initial evaluation 1
• Less common causes of myocardial ischemia include coronary artery spasm, embolism, and dissection 1
• Under the Universal Definition of MI, these ACS events would be classified as type 1 MI events, distinguishing them from type 2 MI which results from oxygen supply/demand imbalance without direct coronary artery pathology 1