Unstable Angina Definition
Unstable angina is defined as myocardial ischemia at rest or with minimal exertion in the absence of myocardial necrosis (no elevated cardiac troponin). 1
Core Diagnostic Criteria
Unstable angina represents transient myocardial ischemia leading to diminished coronary blood flow without significant myonecrosis detected by circulating troponin. 1 The diagnosis requires:
- Cardiac ischemic symptoms (chest discomfort or anginal equivalent) occurring at rest or with minimal exertion 1
- Absence of elevated cardiac biomarkers (troponin must remain below the 99th percentile upper reference limit on at least 2 samples collected ≥6 hours apart) 1
- Presumed acute coronary plaque activity (rupture, erosion, or thrombosis) as the underlying mechanism 1, 2
Clinical Presentations
Unstable angina manifests in three principal patterns: 1, 3
- Rest angina: Prolonged anginal pain (typically ≥20 minutes) occurring at rest 1
- New-onset severe angina: Recent development (within 2 months) of angina with at least Canadian Cardiovascular Society Class III severity (marked limitation of ordinary physical activity) 1, 3
- Crescendo angina: Previously stable angina that becomes distinctly more frequent, longer in duration, or lower in threshold (increased by ≥1 CCS class to at least CCS Class III) 1, 3
Key Distinguishing Features from NSTEMI
The only difference between unstable angina and NSTEMI is the presence or absence of detectable myocardial necrosis: 1
- Both conditions present identically at initial evaluation with similar symptoms and ECG findings 1
- Unstable angina: No troponin elevation (no myocardial necrosis) 1
- NSTEMI: Elevated troponin (myocardial necrosis present) 1
- Patients can progress from unstable angina to NSTEMI during their acute presentation 1
ECG Findings
Electrocardiographic changes in unstable angina may include: 1, 4
- ST-segment depression (≥0.5 mm in two or more contiguous leads) 4
- T-wave inversion (≥1 mm in leads with predominant R waves) 4
- Transient ST-segment elevation (between 0.6-1 mm) 1
- Normal ECG: Approximately 5% of patients with unstable angina have completely normal ECG even during symptoms 3, 4
Risk Stratification Context
Compared to NSTEMI patients, those with unstable angina have: 1
- Substantially lower risk of death 1
- Less benefit from intensified antiplatelet therapy 1
- Less benefit from early invasive strategy 1
However, unstable angina remains a critical phase of coronary disease requiring urgent evaluation, as it represents acute coronary plaque instability with risk of progression to MI or death. 3, 5
Impact of High-Sensitivity Troponin
The introduction of high-sensitivity cardiac troponin assays has resulted in: 1
- 4% absolute increase (20% relative increase) in MI detection 1
- Reciprocal decrease in unstable angina diagnoses 1
- Many patients previously classified as unstable angina are now diagnosed with NSTEMI due to detection of minimal myocardial necrosis 1
Common Pitfalls
- Do not assume normal ECG excludes unstable angina: 5% of patients have normal tracings even during symptoms 3, 4
- Serial troponin measurements are mandatory: Single negative troponin is insufficient; obtain at least 2 samples ≥6 hours apart 1
- Women may present atypically: Non-obstructive coronary disease or completely normal angiography occurs more frequently in women with unstable angina 3
- Atypical presentations are common in elderly patients, diabetics, and those with chronic kidney disease 1