A 52-year-old woman with chronic stable angina is prescribed a beta-blocker as first-line therapy. Which is the most common mechanism by which beta-blockers exert their antianginal effect?

Explanation

## Beta-Blockers: Mechanism of Antianginal Action ### Primary Mechanism: Reduction of Myocardial Oxygen Demand **Key Point:** Beta-blockers relieve angina primarily by **decreasing myocardial oxygen consumption (MVO₂)**, not by increasing coronary blood flow. This is the fundamental distinction from nitrates and calcium channel blockers. ### The Oxygen Supply-Demand Balance $$MVO_2 = Heart\ Rate \times Contractility \times Blood\ Pressure$$ Beta-blockers reduce all three determinants: | Parameter | Mechanism | Effect on MVO₂ | |-----------|-----------|----------------| | **Heart rate** | β₁-blockade → negative chronotropic effect | ↓↓ (most significant) | | **Contractility** | β₁-blockade → negative inotropic effect | ↓ | | **Blood pressure** | β₁-blockade → reduced cardiac output; β₂-blockade → mild vasoconstriction (offset by reduced CO) | ↓ | | **Diastolic filling time** | Slower HR → longer diastole | ↑ coronary perfusion | **Clinical Pearl:** The reduction in heart rate is the **single most important** contributor to decreased MVO₂. A 20% reduction in heart rate can decrease MVO₂ by ~20–25%. ### Why This Matters in Angina In stable angina, the coronary stenosis is **fixed**. The goal is not to dilate the stenotic vessel (which cannot be done pharmacologically) but to **reduce the demand** on the heart so that available coronary flow meets the reduced oxygen requirement. **High-Yield:** Beta-blockers are particularly effective in **demand-induced angina** (effort-related angina) because they prevent the heart rate and contractility surge that occurs with exertion. ### Secondary Benefit: Diastolic Perfusion By slowing the heart rate, beta-blockers **lengthen diastole**, during which coronary perfusion occurs. This provides a modest additional benefit to coronary blood flow. **Mnemonic: BETA-BLOCK** — **B**lood pressure ↓, **E**xertional demand ↓, **T**achycardia prevention, **A**ngina relief, **B**lood flow (diastolic) ↑, **L**ower MVO₂, **O**xygen demand ↓, **C**ontractility ↓, **K**eep rate slow. ### Comparison with Other Antianginals | Drug Class | Primary Mechanism | Secondary Mechanism | |------------|-------------------|---------------------| | **Beta-blockers** | ↓ MVO₂ (demand) | ↑ Diastolic perfusion time | | **Nitrates** | ↑ Coronary flow (supply) | ↓ Preload → ↓ MVO₂ | | **Calcium channel blockers** | ↑ Coronary flow (supply) | ↓ MVO₂ (via ↓ HR, contractility) | [cite:Harrison 21e Ch 297]