What is Acute Coronary Syndrome (ACS)?
Acute coronary syndrome (ACS) is a constellation of life-threatening clinical conditions caused by the sudden disruption (rupture or erosion) of an unstable coronary artery atherosclerotic plaque with associated partial or complete coronary artery thrombosis, resulting in diminished blood flow to the myocardium and subsequent myocardial ischemia. 1
Clinical Spectrum
ACS encompasses three related clinical conditions that exist along a continuum of severity 1:
Unstable Angina (UA): Transient myocardial ischemia leading to diminished coronary flow in the absence of significant myonecrosis detected by circulating troponin 1
Non-ST-Segment Elevation Myocardial Infarction (NSTEMI): More prolonged or severe myocardial ischemia with elevated biomarkers of myonecrosis, typically caused by a partially occluded coronary artery leading to subendocardial ischemia 1
ST-Segment Elevation Myocardial Infarction (STEMI): Acute total or subtotal coronary occlusion with persistent ST-segment elevation (>20 minutes), typically caused by a completely occluded vessel leading to transmural myocardial ischemia and infarction 1
Pathophysiology
The underlying mechanism involves progressive lipid accumulation and inflammation within an atherosclerotic plaque leading to plaque instability 1. When the plaque ruptures or erodes, the highly procoagulant contents of the atheroma core are exposed to circulating platelets and coagulation proteins, culminating in intracoronary thrombus formation 1. This thrombus compromises flow to the myocardium, causing ischemia and eventual myonecrosis 1.
Less common causes of ACS include coronary artery spasm, embolism, and dissection 1.
Initial Diagnosis and Classification
The initial diagnosis should be based on three key elements 1:
Clinical history and symptomatology: Acute chest discomfort described as pain, pressure, tightness, or burning is the leading symptom, though chest pain-equivalent symptoms may include dyspnoea, epigastric pain, and pain in the left arm 1
12-lead electrocardiogram (ECG): Should be obtained and interpreted within 10 minutes of presentation 1. The ECG distinguishes between STEMI (persistent ST-elevation) and non-ST-elevation ACS (NSTE-ACS), which may show ST-segment depression, T-wave inversion, flat T waves, pseudo-normalization of T waves, or may be normal 1
Cardiac troponin assessment: High-sensitivity cardiac troponin (hs-cTn) T or I with at least one value above the 99th percentile of the upper reference limit distinguishes NSTEMI from unstable angina 1
Clinical Importance
The pathophysiology of ACS is dynamic, and patients can rapidly progress from one clinical condition (unstable angina, NSTEMI, STEMI) to another during the course of their presentation and initial evaluation 1. This underscores the critical importance of continuous monitoring and serial assessments.
Approximately 70% of ACS presentations are NSTE-ACS, while STEMI accounts for approximately 30% 2. Each year, more than 7 million people worldwide are diagnosed with ACS, including more than 1 million patients hospitalized in the United States 2.
Key Clinical Pitfall
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 1. The absence of electrocardiographic evidence of ischemia requires serial troponin measurements and continued clinical observation 1.