17.6: Acute Coronary Syndrome I: Introduction

Acute Coronary Syndrome (ACS) encompasses a spectrum of heart conditions caused by sudden obstruction of coronary arteries, typically resulting from the rupture of an atherosclerotic plaque and subsequent thrombus (blood clot) formation. This obstruction can lead to partial or complete blockage of blood flow, causing varying degrees of myocardial ischemia or infarction.

ACS includes the following clinical entities:

1. Unstable Angina (UA)

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

3. ST-Elevation Myocardial Infarction (STEMI)

1. Unstable Angina (UA)

Unstable angina is characterized by chest pain or discomfort that:

• Occurs at rest or with minimal exertion.

• It is more severe, prolonged, or unpredictable compared to stable angina.

• It may not be relieved by rest or nitroglycerin.

Key Features:

• ECG Findings:

  • Transient ST-segment depression.

  • ST-segment elevation or T-wave inversion may be observed but is reversible.

• Cardiac Biomarkers:

  • There is no significant elevation of cardiac markers like CK-MB or Troponins, as there is no myocardial necrosis.

  • In some cases, high-sensitivity troponin (hs-Tn) levels may slightly increase due to minor ischemic injury.

Unstable angina represents a pre-infarction state, where urgent evaluation and intervention can prevent progression to myocardial infarction.

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

NSTEMI is a form of myocardial infarction involving partial or temporary blockage of a coronary artery, leading to ischemic injury confined to the subendocardium, the innermost layer of the heart muscle.

Key Features:

• ECG Findings:

  • ST-segment depression.

  • T-wave inversion, or a combination of both.

  • No persistent ST-segment elevation.

• Cardiac Biomarkers:

  • Elevated levels of Troponin I or Troponin T.

  • Elevated CK-MB levels indicate myocardial injury.

• Pathophysiology:

  • Partial blockage leads to an oxygen supply-demand mismatch in the heart muscle, causing ischemia and infarction.

NSTEMI requires urgent medical therapy to restore coronary blood flow and prevent progression to transmural infarction.

3. ST-Elevation Myocardial Infarction (STEMI)

STEMI is the most severe form of ACS, involving transmural (full-thickness) myocardial infarction due to a complete and prolonged blockage of a coronary artery.

Key Features:

• ECG Findings:

  • Persistent ST-segment elevation in two or more contiguous leads is a hallmark sign.

  • The presence of a new left bundle branch block (LBBB) may also indicate STEMI.

• Cardiac Biomarkers:

  • Significantly elevated levels of Troponin I, Troponin T, and CK-MB.

• Pathophysiology:

  • The complete obstruction causes irreversible myocardial necrosis across the entire thickness of the heart wall (transmural infarction).

STEMI is a medical emergency requiring immediate reperfusion therapy, such as percutaneous coronary intervention (PCI) or thrombolysis, to restore blood flow and minimize cardiac damage.

Diagnostic Summary:

Clinical Implications of ACS

• Prompt Diagnosis: Differentiating between UA, NSTEMI, and STEMI is critical for determining the appropriate treatment strategy.

• Treatment Strategies:

  • UA/NSTEMI: Antiplatelet therapy (aspirin, P2Y12 inhibitors), anticoagulation, and possible coronary angiography.

  • STEMI: Immediate reperfusion therapy (PCI preferred) or thrombolysis if PCI is unavailable.

• Risk Stratification: Utilizing tools like the TIMI risk score helps guide the urgency of interventions.

Early recognition and management of ACS can significantly reduce mortality and improve long-term outcomes.