What are the key differences between stable and unstable angina?

Key Differences Between Stable and Unstable Angina

The correct answers are: (1) Unstable angina reflects a higher risk of myocardial infarction, (2) Stable angina results primarily from exertion, and (3) Unstable angina occurs at rest and with exertion.

Unstable Angina and MI Risk

Unstable angina is definitively associated with a higher risk of myocardial infarction and represents a critical phase of coronary heart disease with variable prognosis. 1, 2 The ACC/AHA defines unstable angina as "clear evidence of important reversible myocardial ischemia" and considers it more ominous when occurring without stimuli to increased oxygen consumption. 3

• Unstable angina represents an acute coronary syndrome with underlying pathogenic substrate of unstable coronary plaque with overlying intracoronary thrombus. 1
• Early studies showed acute infarction rates under 15.5% and death rates under 2%, though high-risk patients (those with persistent pain despite bed rest) have substantially worse outcomes. 2
• The European Society of Cardiology notes that compared to NSTEMI patients, those with unstable angina have substantially lower risk of death but still require urgent evaluation. 3

Stable Angina and Exertion

Stable angina is characterized by symptoms that occur predictably with exertion and are relieved by rest and/or sublingual/transdermal medications. 3

• The ACC/AHA defines stable angina as "angina pectoris without a recent change in frequency or pattern" that is "relieved by rest and/or sublingual/transdermal medications." 3
• According to the Canadian Cardiovascular Society classification, stable angina occurs with varying levels of physical activity depending on severity (Class I through IV), but follows a predictable pattern related to exertion. 3
• Class I stable angina occurs only with "strenuous or prolonged exertion at work or recreation" while ordinary physical activity does not cause symptoms. 3

Unstable Angina Occurrence Pattern

Unstable angina characteristically occurs both at rest and with minimal exertion, distinguishing it fundamentally from stable angina. 3, 4

• The ACC/AHA specifically defines unstable angina/NSTEMI as including "symptoms at rest and prolonged, usually ≥20 min" as well as "new-onset symptoms of CCS grade III or grade IV severity." 3
• Praxis Medical Insights (citing the American College of Cardiology) defines unstable angina as "dolor torácico típico u otros síntomas isquémicos que ocurren en reposo o con mínimo esfuerzo." 4
• The ACC/AHA guidelines note that unstable angina includes "severe and persisting angina on presentation" as well as "recurring or prolonged (>15 min) severe angina within 10 days of presentation." 3
• Unstable angina may present as: (1) rest angina lasting up to 20 minutes, (2) rapidly increasing crescendo angina, or (3) new onset severe angina with marked limitation within 2 months. 3

Degree of Coronary Blockage

The statement that stable angina results from a higher degree of coronary arterial blockage than unstable angina is FALSE. The degree of coronary stenosis does not reliably distinguish stable from unstable angina; rather, the distinction relates to plaque stability and thrombotic activity.

• Unstable angina results from "unstable coronary plaque with an overlying intracoronary thrombus" rather than simply the degree of stenosis. 1
• The ACC/AHA notes that unstable angina "does not appear to be the consequence of a particular anatomic pattern of coronary artery disease." 2
• Patients with unstable angina may occasionally have "non-obstructive coronary atherosclerosis or no angiographic evidence of CAD, particularly in women" (5-10% of cases). 3
• The pathophysiology involves plaque instability and acute thrombotic processes rather than fixed stenosis severity. 1, 2

Clinical Pitfalls

• Do not assume normal ECG excludes unstable angina: Approximately 5% of patients with unstable angina may have completely normal ECG even during symptoms. 5
• Recognize atypical presentations: Elderly patients, diabetics, and women are more likely to present with atypical symptoms and non-diagnostic ECGs. 5
• Distinguish from Type 2 MI: Type 2 MI involves supply-demand mismatch from conditions like hypotension or tachyarrhythmias, not primary plaque instability. 3
• Serial evaluation is critical: When unstable angina is suspected but initial ECG is not diagnostic, serial ECGs should be performed at 15-30 minute intervals. 5