Acute Coronary Syndrome: Definition, Types, and Diagnosis
Acute coronary syndrome (ACS) is caused by disruption (rupture or erosion) of an unstable coronary artery atherosclerotic plaque with associated thrombosis, resulting in diminished myocardial blood flow and ischemia, and encompasses three conditions along a continuum: unstable angina, NSTEMI, and STEMI 1.
Definition and Pathophysiology
ACS results from atherosclerotic plaque instability leading to rupture or erosion, which exposes plaque contents to circulation and triggers the coagulation cascade. The resulting thrombus compromises coronary blood flow, causing myocardial ischemia and potential myonecrosis 1. Less common causes include coronary artery spasm, embolism, and dissection 1.
The pathophysiology is dynamic—patients can rapidly progress from one ACS type to another during initial presentation and evaluation 1.
Three Types of ACS
1. Unstable Angina
- Transient myocardial ischemia with diminished coronary flow
- No significant myonecrosis (normal cardiac troponin)
- No biomarker elevation 1
2. Non-ST-Segment Elevation Myocardial Infarction (NSTEMI)
- Partially occluded coronary artery causing subendocardial ischemia
- Elevated cardiac troponin indicating myonecrosis
- Accounts for approximately 70% of ACS 2
3. ST-Segment Elevation Myocardial Infarction (STEMI)
- Completely occluded vessel causing transmural myocardial ischemia and infarction
- Elevated cardiac troponin with characteristic ECG changes
- Accounts for approximately 30% of ACS 2
Diagnostic Approach
The diagnosis and classification of ACS must be based on three components: clinical history/symptomatology, ECG interpretation, and cardiac troponin assessment 1.
Clinical Presentation
- Most predictive symptom: Substernal chest discomfort spreading to arms or jaw (present in 79% of men, 74% of women) 2
- Common pitfall: 40% of men and 48% of women present with nonspecific symptoms like dyspnea, either isolated or combined with chest pain 2
- Chest pain reproducible with palpation or varying with breathing/position is less likely to be ACS 3
ECG Criteria (Perform Within 10 Minutes of Presentation)
For STEMI 1:
- New ST-elevation ≥1 mm in ≥2 anatomically contiguous leads (measured at J-point) in all leads except V2-V3
- In leads V2-V3: ≥2 mm in men ≥40 years, ≥2.5 mm in men <40 years, ≥1.5 mm in women regardless of age
- Obtain posterior leads (V7-V9) in suspected left circumflex occlusion, particularly with isolated ST-depression ≥0.5 mm in V1-V3
For NSTE-ACS 1:
- New horizontal or down-sloping ST-depression ≥0.5 mm in ≥2 contiguous leads
- T-wave inversion >1 mm in ≥2 contiguous leads with prominent R wave or R/S ratio >1
- Transient ST-elevation
- Critical caveat: Many NSTE-ACS patients have nonspecific ST/T-wave changes or normal ECG—absence of ECG changes does not exclude ACS 1
Cardiac Biomarkers
- High-sensitivity cardiac troponin is the preferred test for detecting myonecrosis 4, 3, 2
- Elevated troponin without ST-elevation suggests NSTE-ACS 3
- Normal troponin with ischemic symptoms indicates unstable angina 1
ECG Mimics to Avoid
ST-segment changes may occur in other conditions that can obscure true ACS: acute pericarditis, left ventricular hypertrophy, LBBB, Brugada syndrome, right ventricular pacing, Takotsubo syndrome, and early repolarization 1.
Risk Factors Increasing ACS Likelihood
Age ≥65 years, current smoking, hypertension, diabetes mellitus, hyperlipidemia, BMI >25 kg/m², family history of premature CAD, or prior abnormal cardiac stress test 3.