## Mechanism of Antianginal Action: Nitrates vs Beta-Blockers ### Nitrates (e.g., GTN, ISDN) **Key Point:** Nitrates work primarily through **vasodilation** — they cause: - Preload reduction (venous dilation → decreased LV end-diastolic pressure) - Afterload reduction (arterial dilation) - **Redistribution of coronary blood flow** to the subendocardial region (where ischemia begins) - Reduced myocardial oxygen demand indirectly ### Beta-Blockers (e.g., metoprolol, atenolol) **Key Point:** Beta-blockers work through **metabolic reduction** — they cause: - **Negative chronotropic effect** (↓ heart rate) - **Negative inotropic effect** (↓ contractility) - **Reduced myocardial oxygen consumption** (MVO₂ = HR × BP × contractility) - Increased diastolic filling time → improved coronary perfusion ### Comparative Table | Feature | Nitrates | Beta-Blockers | | --- | --- | --- | | **Primary mechanism** | Vasodilation (preload ↓) | Reduced MVO₂ demand | | **Heart rate** | ↑ (reflex tachycardia) | ↓ (direct effect) | | **Contractility** | ↑ (reflex) | ↓ (direct effect) | | **Onset** | 1–3 min (sublingual) | Days to weeks | | **Use in acute attack** | Yes (first-line) | No | | **Prophylaxis** | Yes (long-acting) | Yes (standard) | **High-Yield:** The **discriminating feature** is the **mechanism of action**: nitrates reduce preload and improve perfusion; beta-blockers reduce oxygen *demand*. This explains why nitrates work acutely and why beta-blockers are prophylactic. **Clinical Pearl:** In acute angina, GTN acts within 1–3 minutes because it immediately reduces preload; a beta-blocker would take days to weeks to show benefit and cannot abort an acute attack. [cite:KD Tripathi 8e Ch 31]
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