Unstable Angina Definition
Unstable angina is defined as transient myocardial ischemia leading to diminished coronary blood flow in the absence of significant myonecrosis (no elevated cardiac troponin), typically caused by atherosclerotic plaque rupture or erosion with associated partial coronary artery thrombosis. 1, 2
Clinical Presentation
Unstable angina presents in three principal patterns 3, 4, 5:
- Rest angina: Angina occurring at rest and prolonged, usually >20 minutes
- New-onset severe angina: New symptoms of at least CCS Class III severity (marked limitation of ordinary physical activity)
- Crescendo angina: Previously stable angina that has recently accelerated, with increased frequency, duration, or lower threshold—increasing by ≥1 CCS class to at least CCS Class III severity
Key Distinguishing Features
The critical distinction from NSTEMI is the absence of elevated cardiac biomarkers (troponin remains negative), while the clinical presentation may be identical 1, 2. Both conditions exist on the acute coronary syndrome spectrum and share the same underlying pathophysiology of plaque disruption with partial thrombosis.
Characteristics of Ischemic Discomfort
The discomfort is typically 4:
- Location: Deep, poorly localized chest or arm discomfort; may also present as jaw, neck, shoulder, back, or epigastric discomfort
- Character: Pressure, tightness, heaviness, strangling, constricting, or burning
- Duration: More severe and prolonged than stable angina episodes
- Triggers: Occurs at rest or precipitated by less exertion than previously
- Relief: May not respond promptly to nitroglycerin (unlike stable angina which relieves in <5 minutes)
Anginal Equivalents
Some patients present without chest discomfort but with 4:
- Isolated unexplained dyspnea (most common equivalent, especially in older adults)
- Nausea and vomiting
- Diaphoresis
- Unexplained fatigue
These symptoms should be considered equivalent to angina if they have a clear relationship to exertion/stress or are relieved by nitroglycerin.
Pathophysiology
The underlying mechanism involves 6, 1:
- Rupture or erosion of an unstable atherosclerotic coronary plaque
- Exposure of plaque contents to circulation
- Activation of coagulation cascade with partial coronary artery thrombosis and/or microemboli
- Resulting in diminished but not completely absent myocardial blood flow
- Transient ischemia without sufficient duration/severity to cause detectable myonecrosis
ECG Findings
Unstable angina may be associated with 3, 6:
- New or dynamic horizontal/down-sloping ST-segment depression ≥0.5 mm in ≥2 contiguous leads
- T-wave inversion >1 mm in ≥2 contiguous leads with prominent R wave
- Transient ST-segment elevation (particularly in variant/Prinzmetal angina)
- Normal ECG (absence of ECG changes does not exclude unstable angina)
Critical Clinical Caveat
The 2000 definition that divided unstable angina into troponin-positive and troponin-negative groups is now obsolete 7. With contemporary high-sensitivity troponin assays, troponin-positive patients are classified as NSTEMI under current definitions. The 1989 clinical definition based on symptom patterns remains the most appropriate description of unstable angina as a distinct entity 7.
High-Risk Features
Patients with unstable angina demonstrating the following require urgent intervention 6:
- Accelerating tempo of symptoms in preceding 48 hours
- Prolonged ongoing rest pain (>20 minutes)
- Pulmonary edema likely due to ischemia
- New/worsening mitral regurgitation murmur
- Hemodynamic instability (hypotension, new heart failure signs)
- Age >75 years
- Dynamic ECG changes as described above
These high-risk patients should receive intensive antiplatelet therapy (aspirin, P2Y12 inhibitor, often GP IIb/IIIa inhibitors), anticoagulation (heparin or fondaparinux), and early invasive strategy with coronary angiography 4, 5.