Acute Coronary Syndrome Overview
Acute coronary syndrome (ACS) is a spectrum of acute myocardial ischemic conditions caused primarily by atherosclerotic plaque rupture or erosion with superimposed thrombosis, encompassing three distinct clinical entities: ST-elevation myocardial infarction (STEMI), non-ST-elevation myocardial infarction (NSTEMI), and unstable angina (UA). 1, 2
Definition and Classification
ACS represents a continuum of disease severity sharing a common pathophysiological mechanism:
STEMI accounts for approximately 30% of ACS cases and reflects acute total coronary occlusion with persistent ST-segment elevation (>20 minutes) on ECG 3, 2, 4
NSTEMI represents approximately 70% of ACS cases, involves myocardial necrosis with elevated cardiac biomarkers (troponin above the 99th percentile) but without persistent ST-elevation, indicating partial or intermittent arterial occlusion 1, 2, 4
Unstable angina presents with ischemic symptoms and ECG changes but without biomarker elevation indicating myocardial necrosis—though with increasingly sensitive troponin assays, biomarker-negative ACS is becoming rarer 3, 2
Pathophysiology
The underlying mechanism involves a dynamic cascade of events:
Plaque disruption occurs through atherosclerotic plaque rupture or erosion, exposing the highly procoagulant lipid core to circulating platelets and coagulation proteins 3, 1
Plaques prone to rupture have characteristic features: large lipid core, thin fibrous cap, high macrophage density, and low smooth muscle cell density 3
Thrombosis formation with varying degrees of coronary obstruction and distal embolization creates the acute imbalance between myocardial oxygen supply and demand 3
The pathophysiology is dynamic—patients can rapidly progress from one clinical condition to another during initial presentation, requiring continuous monitoring 1
Clinical Presentation
Chest discomfort at rest is the cardinal symptom, affecting approximately 79% of men and 74% of women, typically described as substernal pressure, tightness, or burning that may radiate to the left arm, jaw, or back. 2, 4
Atypical presentations are common and include:
- Isolated dyspnea without chest pain (approximately 40% of men and 48% of women present with nonspecific symptoms) 2, 4
- Epigastric pain mimicking gastrointestinal pathology 2
- Syncope or presyncope 2
- These atypical presentations occur more frequently in elderly patients, diabetics, and women 2
Critical pitfall to avoid:
Chest pain that is reproducible with palpation or varies with breathing or position is less likely to represent ACS, but a normal ECG does not exclude ACS—many patients with NSTE-ACS have either nonspecific changes or a completely normal ECG at presentation 3, 1, 5
Diagnostic Approach
The initial diagnosis must be based on three key elements obtained rapidly:
12-lead ECG within 10 minutes of presentation to distinguish STEMI from NSTE-ACS 3, 1, 4
High-sensitivity cardiac troponin (hs-cTn) T or I with at least one value above the 99th percentile distinguishes NSTEMI from unstable angina 1
- Serial troponin measurements at 6-12 hour intervals are required for patients with initial negative results 2
Clinical history and symptomatology focusing on symptom characteristics, risk factors, and prior cardiac history 1
Risk Stratification
Two critical questions guide management decisions:
- What is the likelihood that symptoms represent ACS? 3
- What is the likelihood of adverse clinical outcomes (death, MI, recurrent ischemia)? 3
Common risk factors include age ≥65 years, current smoking, hypertension, diabetes mellitus, hyperlipidemia, BMI >25 kg/m², family history of premature coronary artery disease, and prior abnormal cardiac stress test 5
The median age at ACS presentation in the United States is 68 years (interquartile range 56-79), with a male-to-female ratio of approximately 3:2 3