What is Acute Coronary Syndrome (ACS)?

Acute Coronary Syndrome (ACS): Definition and Classification

Acute Coronary Syndrome (ACS) refers to a constellation of clinical symptoms caused by acute myocardial ischemia, typically resulting from disruption (rupture or erosion) of an unstable coronary artery atherosclerotic plaque with associated partial or complete coronary artery thrombosis, leading to diminished blood flow to the myocardium and subsequent myocardial ischemia. 1

Pathophysiology

The pathophysiology of ACS involves:

• Progressive lipid accumulation and inflammation within atherosclerotic plaque leading to plaque instability
• Rupture or erosion of the atherosclerotic plaque exposing plaque contents to circulation
• Activation of the coagulation cascade and subsequent thrombosis
• Compromise of blood flow to the myocardium, causing ischemia and potential myonecrosis 1

This process typically occurs in the epicardial vessels of the coronary circulation. The pathophysiology is dynamic, and patients can rapidly progress from one clinical condition to another during presentation and initial evaluation. 1

Classification of ACS

ACS includes three related clinical conditions that exist along a continuum of severity:

1. Unstable Angina (UA):

   • Transient myocardial ischemia with diminished flow
   • No significant myonecrosis (normal cardiac troponin)
   • May show ECG changes but no biomarker elevation 1, 2

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

   • Partial coronary artery occlusion leading to subendocardial ischemia
   • Elevated biomarkers of myonecrosis (troponin)
   • May show ST-segment depression or T-wave inversion on ECG 1, 2

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

   • Complete coronary artery occlusion leading to transmural myocardial ischemia
   • Elevated biomarkers of myonecrosis
   • ST-segment elevation on ECG 1, 2

Diagnostic Criteria

The initial diagnosis and classification of ACS should be based on:

1. Clinical history and symptoms:

   • Chest discomfort/pain (present in ~79% of men and 74% of women)
   • Associated symptoms: dyspnea, radiation to arms/jaw/back, diaphoresis, nausea 3

2. ECG findings:

   • NSTE-ACS: New or presumed new horizontal or down-sloping ST-segment depression ≥0.5 mm in ≥2 contiguous leads and/or T-wave inversion >1 mm in ≥2 contiguous leads
   • STEMI: New or presumed new ST-elevation of ≥1 mm in ≥2 anatomically contiguous leads (measured at the J-point) in all leads other than V2-V3, and ≥2 mm in men ≥40 years, ≥2.5 mm in men <40 years, and ≥1.5 mm in women regardless of age in leads V2-V3 1

3. Cardiac biomarkers:

   • Cardiac troponin (cTn) is the preferred biomarker
   • Serial measurements may be necessary 1, 3

Comparative Features of ACS Subtypes

Feature	Unstable Angina	NSTEMI	STEMI
Symptoms	Rest pain, new-onset, or increasing pattern	Prolonged chest pain	Prolonged chest pain
Duration	Often >20 minutes	>20 minutes	>20 minutes
ECG	May show transient ST depression or T-wave changes	ST depression, T-wave inversion	ST-segment elevation
Biomarkers	Normal	Elevated	Elevated
Pathophysiology	Plaque disruption with non-occlusive thrombus	Partial occlusion with myocardial necrosis	Complete occlusion
Mortality Risk	Lower	Intermediate	Higher

Clinical Implications and Management Considerations

• The diagnosis of ACS should be made promptly as it has significant implications for morbidity and mortality 3
• Early risk stratification is essential for determining appropriate management strategies 4
• Patients with STEMI require immediate reperfusion therapy (ideally within 120 minutes) 3
• High-risk patients with NSTE-ACS benefit from an early invasive strategy with coronary angiography within 24-48 hours 3
• Pharmacological therapy typically includes:
  • Dual antiplatelet therapy (aspirin plus P2Y12 inhibitor like clopidogrel) 5
  • Anticoagulation
  • Consideration of adjunctive therapies (beta-blockers, ACE inhibitors, statins) 4

Important Considerations

• A normal ECG does not exclude ACS; approximately 40% of patients with NSTE-ACS may have neither ST-segment depression nor T-wave inversion 1, 2
• Women, elderly patients, and those with diabetes may present with atypical symptoms 2
• Less common causes of myocardial ischemia include coronary artery spasm, embolism, and dissection 1
• The pathophysiology of ACS is dynamic, and patients can rapidly progress from one clinical condition to another during evaluation and treatment 1

Understanding the spectrum of ACS is crucial for prompt diagnosis and appropriate management to reduce morbidity and mortality associated with these conditions.