What is acute coronary syndrome?

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Last updated: February 16, 2026 • View editorial policy

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What is Acute Coronary Syndrome?

Acute coronary syndrome (ACS) is a spectrum of three life-threatening clinical conditions caused by sudden reduction in coronary blood flow due to atherosclerotic plaque disruption with thrombosis: unstable angina, non-ST-segment elevation myocardial infarction (NSTEMI), and ST-segment elevation myocardial infarction (STEMI). 1, 2

Core Pathophysiology

ACS results from progressive lipid accumulation and inflammation within an atherosclerotic plaque leading to plaque instability, followed by rupture or erosion that exposes the highly procoagulant plaque contents to circulating blood, culminating in intracoronary thrombus formation and reduced myocardial oxygen supply. 1, 2

  • The degree of coronary occlusion determines which ACS entity develops: partial occlusion produces unstable angina or NSTEMI, while complete occlusion causes STEMI. 1, 3
  • Progressive thrombus extension can convert unstable angina to NSTEMI and NSTEMI to STEMI during the initial presentation, making ACS a dynamic process requiring continuous monitoring. 1, 2

The Three Clinical Entities

Unstable Angina

  • Transient myocardial ischemia with reduced coronary flow but without myocardial necrosis (no cell death). 2
  • Cardiac troponin remains below the 99th percentile upper reference limit despite ischemic symptoms. 1, 3
  • Incidence has declined substantially with widespread use of high-sensitivity troponin assays that detect smaller amounts of myocardial injury. 1, 3

Non-ST-Segment Elevation Myocardial Infarction (NSTEMI)

  • Myocardial ischemia with necrosis evidenced by cardiac troponin elevation above the 99th percentile, but without persistent ST-segment elevation on ECG. 1, 2
  • Typically results from a partially occluded coronary artery producing subendocardial ischemia. 1, 2
  • ECG may show ST-segment depression (31% of cases), T-wave inversion (12%), both (16%), or be completely normal (41% of cases). 4, 3

ST-Segment Elevation Myocardial Infarction (STEMI)

  • Myocardial ischemia with necrosis (elevated troponin) plus persistent ST-segment elevation lasting more than 20 minutes on ECG. 1, 2, 3
  • Results from a completely occluded coronary vessel causing transmural myocardial ischemia and infarction. 1, 2
  • Accounts for approximately 30% of all ACS presentations. 4

Clinical Presentation

Typical Symptoms

  • Chest discomfort (pain, pressure, tightness, or burning) is the leading symptom, present in approximately 79% of men and 74% of women with ACS. 1, 3, 4
  • Anginal equivalents include dyspnea, epigastric pain, left-arm radiation, and diaphoresis. 3
  • Symptoms that worsen with physical exertion and improve with rest or nitrates increase diagnostic confidence. 3

Atypical Presentations

  • Approximately 40% of men and 48% of women present with atypical symptoms such as isolated dyspnea, nausea, or diaphoresis without classic chest pain. 3, 4
  • Atypical presentations are particularly common in elderly patients, women, and those with diabetes or chronic renal failure. 3

Diagnostic Approach

Immediate ECG (Within 10 Minutes)

  • A 12-lead ECG must be obtained and interpreted within 10 minutes of presentation to differentiate STEMI from non-ST-elevation ACS. 1, 3, 4
  • Persistent ST-segment elevation (≥1 mm in ≥2 contiguous leads, lasting >20 minutes) or new left bundle branch block defines STEMI and mandates immediate reperfusion therapy. 1, 3
  • Non-ST-elevation ACS may show transient ST elevation, ST depression, T-wave abnormalities, or a completely normal ECG. 1, 3

Critical ECG Pitfall

  • A normal ECG does not exclude ACS—up to 41% of NSTE-ACS patients have a normal initial ECG. 3, 4
  • Ischemia in the circumflex or isolated right-ventricular territory often escapes detection on standard leads. 3
  • Record posterior leads (V7-V9) for suspected posterior MI and right-ventricular leads (V3R-V4R) for inferior MI to improve diagnostic yield. 3

High-Sensitivity Cardiac Troponin Testing

  • High-sensitivity cardiac troponin (hs-cTn T or I) is the preferred biomarker to distinguish NSTEMI from unstable angina. 1, 3
  • A rise and/or fall in troponin with at least one value above the 99th percentile upper reference limit confirms myocardial injury. 1, 3
  • Serial measurements at 3 hours, 6-9 hours, and 24 hours after presentation are essential for accurate classification. 3

Additional Diagnostic Criteria

When troponin is elevated, at least one of the following must be present to diagnose MI: 1, 3

  • Clinical symptoms of myocardial ischemia
  • New ischemic ECG changes or left bundle branch block
  • Development of pathological Q waves
  • Imaging evidence of new loss of viable myocardium or regional wall motion abnormality
  • Intracoronary thrombus detected on angiography or autopsy

Important Clinical Considerations

Non-Obstructive Coronary Disease

  • Approximately 5-20% of ACS cases occur without obstructive coronary artery disease on angiography, particularly in women. 1, 3
  • Causes include coronary artery spasm, spontaneous coronary dissection, coronary embolism, and microvascular dysfunction. 1, 3
  • A normal angiogram does not rule out ACS, as thrombus may have lysed spontaneously before catheterization. 3

Type 1 vs Type 2 MI Classification

  • All three ACS entities (unstable angina, NSTEMI, STEMI) correspond to Type 1 MI, reflecting primary coronary atherothrombotic events. 1, 2
  • Type 2 MI involves myocardial necrosis from oxygen supply-demand mismatch due to conditions like coronary spasm, tachyarrhythmias, anemia, respiratory failure, or severe hypertension, rather than plaque rupture. 1

Risk Stratification Tools

Validated multivariable risk scores provide superior prediction of major adverse cardiac events compared with any single clinical element: 3

  • HEART score (History, ECG, Age, Risk factors, Troponin) for emergency department evaluation
  • TIMI score (Thrombolysis In Myocardial Infarction)
  • GRACE score (Global Registry of Acute Cardiac Events)

References

1
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

2
Guideline

Acute Coronary Syndrome: Core Definitions and Diagnostic Framework

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2026

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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