What is the definition of unstable angina?

Definition of Unstable Angina

Unstable angina is defined as typical chest pain or other ischemic symptoms occurring at rest or with minimal exertion, presumed to be related to an acutely active coronary plaque, without evidence of myocardial infarction (no elevation of cardiac troponins). 1

Clinical Presentations

Unstable angina can present in three principal ways:

1. Rest angina: Angina occurring at rest and prolonged, usually greater than 20 minutes 1
2. New-onset angina: New-onset severe angina (at least Canadian Cardiovascular Society Class III severity) 1
3. Increasing angina: Previously diagnosed angina that has become distinctly more frequent, longer in duration, or lower in threshold (crescendo pattern) 1

Clinical Characteristics

• In contrast to stable angina, unstable angina is often described as severe and as a frank pain 1
• May be new in onset or occur in a crescendo pattern in a patient with a previous stable pattern of angina 1
• Pain is typically prolonged (>20 minutes) and occurs at rest or with minimal exertion 1
• The definition is largely based on clinical presentation rather than laboratory findings 1

Electrocardiographic Findings

Unstable angina may be associated with new electrocardiographic changes such as:

• Transient ST-segment elevation
• ST-segment depression (>0.5 mm)
• T-wave inversion
• Bundle-branch block (new or presumed new)

However, it may also present without any electrocardiographic changes 1

Distinction from NSTEMI

The key distinction between unstable angina and NSTEMI is:

• In unstable angina, by definition, there is no evidence of myocardial infarction (no elevation of cardiac troponins) 1, 2
• Both conditions may have similar presentations and underlying pathophysiology (disrupted or eroded atherosclerotic plaque), but unstable angina does not result in detectable myocardial necrosis 1

High-Risk Features

High-risk features for adverse outcomes in unstable angina include:

• Accelerating tempo of anginal symptoms in preceding 48 hours
• Prolonged ongoing (>20 minutes) rest pain
• Age >75 years
• Pulmonary edema likely due to ischemia
• New or worsening mitral regurgitation murmur
• Hypotension, bradycardia, or tachycardia
• Transient ST-segment deviation >0.5 mm
• Bundle-branch block, new or presumed new
• Sustained ventricular tachycardia 1

Pathophysiology

The underlying pathophysiology of unstable angina typically involves:

• An acutely active coronary plaque with potential thrombus formation
• Subtotally occlusive thrombus on pre-existing plaque
• Dynamic obstruction (coronary spasm or vasoconstriction)
• Microembolization of platelet aggregates 1

Clinical Importance

Understanding the definition of unstable angina is crucial for proper risk stratification and management, as patients with unstable angina:

• Have a lower risk of death compared to NSTEMI patients 1
• Appear to derive less benefit from intensified antiplatelet therapy and early invasive strategy compared to NSTEMI patients 1
• Require prompt evaluation and appropriate treatment to prevent progression to myocardial infarction 2

The diagnosis of unstable angina has become less common with the introduction of high-sensitivity cardiac troponin assays, as many patients previously classified as having unstable angina are now diagnosed with NSTEMI due to detection of minor elevations in troponin levels 1.