Which class of antihypertensive agents causes a reflex tachycardia that can be blunted by concurrent use of a beta-blocker?

Explanation

## Reflex Tachycardia and Direct Vasodilators **Key Point:** Direct vasodilators (hydralazine and minoxidil) cause potent peripheral vasodilation, which triggers baroreceptor-mediated reflex sympathetic activation, resulting in tachycardia and increased cardiac output. ### Mechanism of Reflex Tachycardia ```mermaid flowchart TD A[Direct vasodilator administered]:::action --> B[Peripheral vasodilation]:::action B --> C[↓ Systemic vascular resistance]:::outcome C --> D[↓ Blood pressure]:::outcome D --> E{Baroreceptor reflex}:::decision E --> F[↑ Sympathetic activity]:::action F --> G[↑ Heart rate + ↑ Contractility]:::urgent G --> H[↑ Cardiac output]:::outcome H --> I[Counteracts BP reduction]:::outcome I --> J[Add beta-blocker to blunt]:::action ``` ### Why Beta-Blockers Are Used Concurrently | Agent | Mechanism | Reflex Tachycardia | Solution | | --- | --- | --- | --- | | **Hydralazine** | Direct smooth muscle relaxation | Marked | β-blocker + diuretic | | **Minoxidil** | Opens K^+^ channels → hyperpolarization | Severe | β-blocker mandatory | | **Calcium channel blockers** | L-type Ca^2+^ channel blockade | Minimal (some agents like nifedipine cause mild reflex tachycardia) | Not always needed | | **Alpha-2 agonists** | Central sympathetic inhibition | **Bradycardia** (opposite) | Not indicated | | **Thiazides** | Mild vasodilation + volume depletion | Minimal | Not needed | **High-Yield:** Hydralazine + isosorbide dinitrate + beta-blocker is the classic triple combination used in hypertensive emergencies and in African Americans with heart failure (per ACC/AHA guidelines). **Clinical Pearl:** Minoxidil causes such severe reflex tachycardia and fluid retention that it is reserved for refractory hypertension and is almost always given with a beta-blocker and a loop diuretic. **Warning:** Alpha-2 agonists (clonidine, methyldopa) cause **bradycardia**, not tachycardia, because they reduce central sympathetic outflow. This is a common exam trap.