What is Acute Coronary Syndrome (ACS)?

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What is Acute Coronary Syndrome (ACS)?

Acute Coronary Syndrome (ACS) is a spectrum of acute life-threatening conditions caused by sudden reduction in coronary blood flow to the myocardium, encompassing three related clinical entities: unstable angina (UA), non-ST-segment elevation myocardial infarction (NSTEMI), and ST-segment elevation myocardial infarction (STEMI). 1

Underlying Pathophysiology

ACS typically results from disruption (rupture or erosion) of an unstable atherosclerotic plaque in the coronary arteries, with subsequent thrombosis and/or microemboli formation that diminishes myocardial blood flow and causes ischemia 1. The mechanism involves:

  • Progressive lipid accumulation and inflammation within an atherosclerotic plaque leading to plaque instability 2
  • Plaque rupture or erosion exposing the highly procoagulant contents of the atheroma core to circulating platelets and coagulation proteins 2
  • Intracoronary thrombus formation that partially or completely occludes the coronary artery 1
  • Resulting myocardial ischemia and potential myonecrosis depending on the degree and duration of obstruction 1

The Three Clinical Entities

Unstable Angina (UA)

  • Defined by transient myocardial ischemia with diminished coronary flow in the absence of significant myonecrosis 1
  • Cardiac troponin levels remain below the 99th percentile upper reference limit 3
  • Severe chest pain at rest typically lasting more than 20 minutes 3
  • Becoming increasingly rare with high-sensitivity troponin assays 3

Non-ST-Segment Elevation Myocardial Infarction (NSTEMI)

  • Characterized by myocardial ischemia with elevated cardiac troponin (at least one value above the 99th percentile) 1, 3
  • ECG shows no persistent ST-segment elevation but may demonstrate ST-segment depression ≥0.5 mm, T-wave inversion >1 mm in ≥2 contiguous leads, or transient ST-elevation 1
  • Typically results from partially occluded coronary artery causing subendocardial ischemia 1
  • Accounts for approximately 70% of ACS cases 4

ST-Segment Elevation Myocardial Infarction (STEMI)

  • Defined by persistent ST-segment elevation (≥1 mm in ≥2 anatomically contiguous leads, or ≥2 mm in men ≥40 years, ≥2.5 mm in men <40 years in leads V2-V3) 1
  • Reflects complete coronary artery occlusion leading to transmural myocardial ischemia and infarction 1
  • Elevated cardiac troponin levels present 3
  • Accounts for approximately 30% of ACS cases 4

Diagnostic Approach

The initial diagnosis and classification of ACS is based on three key elements 1, 2:

  1. Clinical history and symptomatology: Chest discomfort affects approximately 79% of men and 74% of women, though 40% of men and 48% of women present with atypical symptoms (dyspnea, diaphoresis, nausea) 3, 4

  2. 12-lead electrocardiogram (ECG): Should be obtained and interpreted within 10 minutes of presentation to distinguish STEMI from non-ST-elevation ACS 2, 4

  3. Cardiac troponin assessment: High-sensitivity cardiac troponin (hs-cTn) T or I with at least one value above the 99th percentile distinguishes NSTEMI from unstable angina 2, 3

Critical Clinical Considerations

The pathophysiology of ACS is dynamic, and patients can rapidly progress from one clinical condition (unstable angina, NSTEMI, STEMI) to another during presentation and initial evaluation 1, 2. This underscores the necessity for continuous monitoring and serial assessments.

A normal ECG does not exclude ACS—many patients with NSTE-ACS have either nonspecific ST-segment or T-wave changes or a completely normal ECG at presentation, requiring serial troponin measurements and continued clinical observation 2.

Less Common Causes

While most ACS results from atherosclerotic plaque disruption, other mechanisms include 1:

  • Coronary artery spasm
  • Coronary embolism
  • Spontaneous coronary artery dissection
  • Microvascular coronary dysfunction

Approximately 5-10% of ACS cases occur without obstructive coronary artery disease (termed MINOCA—myocardial infarction with non-obstructive coronary arteries), particularly in women 3.

References

1
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

2
Guideline

Acute Coronary Syndrome Diagnosis and Management

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

3
Guideline

Acute Coronary Syndrome Diagnosis and Management

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2026

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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