Unstable Angina Definition
Unstable angina is defined as angina pectoris (or anginal equivalent) characterized by one of three principal presentations: (1) rest angina that is prolonged, usually greater than 20 minutes; (2) new-onset severe angina of at least Canadian Cardiovascular Society (CCS) class III severity within the past 2 months; or (3) crescendo angina—previously stable angina that has become distinctly more frequent, longer in duration, or lower in threshold, increasing by at least 1 CCS class to at least CCS class III severity. 1, 2
Core Clinical Features
The defining characteristic that distinguishes unstable angina from stable angina is the pattern of instability rather than just symptom severity. The 2025 ACC/AHA guideline emphasizes that unstable angina represents transient myocardial ischemia leading to diminished coronary flow without significant myonecrosis detected by circulating troponin 3. This is the critical distinction from NSTEMI—both may present identically, but troponin elevation indicates myocardial injury has occurred.
The Three Presentations in Detail
1. Rest Angina
- Angina occurring at rest
- Prolonged duration: typically >20 minutes
- Most concerning presentation as it suggests severe flow limitation without provocation 1, 4
2. New-Onset Severe Angina
- Recent onset (within 2 months)
- Must be at least CCS class III severity at presentation
- Represents marked limitation where walking 1-2 blocks on level ground or climbing 1 flight of stairs at normal pace triggers symptoms 1, 2
3. Crescendo (Increasing) Angina
- Previously diagnosed stable angina that has accelerated
- Must show increase of at least 1 CCS class
- Must reach at least CCS class III severity
- Progression occurs over a short period (≤4 weeks) 4
Pathophysiologic Mechanism
The 2025 ACC/AHA guideline clarifies that unstable angina results from atherosclerotic plaque rupture or erosion with associated partial coronary artery thrombosis and/or microemboli, resulting in diminished myocardial blood flow 3. The European Society of Cardiology guidelines note this may be associated with plaque rupture and places patients at significantly higher risk of acute coronary events in the short term 4.
Critical Diagnostic Distinctions
Unstable angina is distinguished from NSTEMI solely by biomarkers: both present with similar symptoms and ECG changes (ST-segment depression, T-wave inversion, or even normal ECG), but unstable angina has negative cardiac troponin while NSTEMI has elevated troponin indicating myonecrosis 3, 2.
The diagnosis requires:
- Clinical presentation matching one of the three patterns above
- ECG findings may show transient ST-segment elevation, ST-segment depression, T-wave inversion, or be normal
- Negative biomarkers for myocardial necrosis (troponin, CK-MB) 1, 5
Common Pitfalls
The 2011 ACC/AHA guidelines emphasize that secondary unstable angina must be considered—this occurs when an extrinsic condition precipitates ischemia in patients with underlying coronary disease: fever, tachycardia, thyrotoxicosis (increased oxygen demand), hypotension (reduced coronary flow), or anemia/hypoxemia (reduced oxygen delivery) 1. These patients often have chronic stable angina at baseline.
The most recent 2025 guideline stresses that ACS pathophysiology is dynamic—patients can rapidly progress from unstable angina to NSTEMI to STEMI during initial evaluation, requiring serial troponin measurements and continuous ECG monitoring 3.