Learning Objective:
Understand the mechanisms, clinical uses, and key adverse effects of vasodilator drugs that act via nitric oxide or potassium channel modulation, and recognize their role in managing hypertension and related conditions.
1. Drugs Acting Through Nitric Oxide
Hydralazine
- Mechanism: Arteriolar dilation → ↓ total peripheral resistance (TPR)
- Uses: Moderate-to-severe hypertension
- Side Effects:
-
- SLE-like syndrome (especially in slow acetylators)
- Edema
- Reflex tachycardia
Nitroprusside
- Mechanism: Dilation of both arterioles and venules → ↓ TPR
- Uses: Hypertensive emergencies (IV administration)
- Side Effects:
- Cyanide toxicity
2. Drugs Acting to Open Potassium Channels
Minoxidil & Diazoxide
- Mechanism: Open K⁺ channels → hyperpolarization of vascular smooth muscle → arteriolar vasodilation
- Clinical Uses:
- Minoxidil: Severe hypertension, baldness (topical)
- Diazoxide: Insulinoma, severe hypertension
- Side Effects:
- Hypertrichosis (minoxidil)
- Hyperglycemia (diazoxide, via ↓ insulin release)
- Edema
- Reflex tachycardia
Clinical Correlation
A hypertensive emergency is characterized by severe hypertension with end-organ damage. First-line therapy often includes:
- Nitroprusside (IV)
- Labetalol (IV)
- Fenoldopam (D1 agonist, IV)
These agents rapidly reduce blood pressure while preserving organ perfusion.
