In the pathophysiology of unstable angina and NSTEMI, which mechanism is responsible for the acute reduction in coronary blood flow?

Explanation

## Pathophysiology of Unstable Angina and NSTEMI **Key Point:** Both unstable angina and NSTEMI are acute coronary syndromes caused by acute reduction in coronary blood flow due to plaque rupture and thrombosis, not chronic progressive narrowing. ### The Acute Plaque Rupture Mechanism The pathophysiologic sequence in ACS: 1. **Plaque Destabilization** — Rupture of a lipid-rich, thin-capped atherosclerotic plaque 2. **Thrombosis** — Exposure of tissue factor and collagen triggers platelet aggregation and coagulation cascade 3. **Acute Stenosis** — Thrombus partially or completely obstructs the coronary lumen 4. **Ischemia** — Acute reduction in myocardial blood flow **High-Yield:** Plaque rupture + thrombosis = the common pathologic substrate for both unstable angina and NSTEMI. The difference is the degree of myocardial necrosis (troponin elevation). ### Distinction from Stable Angina | Feature | Stable Angina | Unstable Angina / NSTEMI | |---------|---------------|-------------------------| | Mechanism | Fixed atherosclerotic narrowing | Acute plaque rupture + thrombosis | | Onset | Predictable, with exertion | Sudden, at rest or minimal exertion | | Plaque morphology | Fibrotic, stable | Lipid-rich, thin-capped, vulnerable | | Pathology | Chronic narrowing | Acute obstruction | **Clinical Pearl:** Angiographically, unstable angina and NSTEMI often show only mild-to-moderate stenosis (< 70%) because the culprit lesion is a ruptured plaque with overlying thrombus, not a fixed atherosclerotic narrowing. ### Why Other Mechanisms Are Secondary ```mermaid flowchart TD A[Acute Coronary Syndrome]:::outcome --> B[Primary mechanism]:::action B --> C[Plaque rupture + thrombosis]:::action C --> D{Degree of obstruction?}:::decision D -->|Complete| E[STEMI]:::outcome D -->|Partial| F[NSTEMI or Unstable Angina]:::outcome G[Secondary mechanisms]:::action --> H[Vasospasm] G --> I[Increased demand] H --> J[May worsen ischemia]:::action I --> J ``` **Mnemonic:** **TRAP** — Thrombosis, Rupture, Acute obstruction, Partial flow = unstable angina/NSTEMI.