Vasodilator Pharmacology
Mechanisms and clinical uses of arterial, venous, and mixed vasodilators in hypertensive emergencies, heart failure, and peripheral vascular disease.
## Overview
Vasodilators relax vascular smooth muscle, reducing peripheral resistance (arterial dilators), preload (venous dilators), or both (mixed dilators). They are used across cardiovascular medicine for hypertensive crises, heart failure, angina, and peripheral vascular disease. The hemodynamic profile differs by agent and determines clinical application.
## Nitric Oxide Donors
Sodium nitroprusside releases NO directly, producing balanced arterial and venous dilation. It is the most potent IV vasodilator for hypertensive emergencies, with an onset of seconds and offset within 1-2 minutes of stopping infusion. Prolonged use risks cyanide toxicity from its metabolism, requiring monitoring of thiocyanate levels. Nitroglycerin preferentially dilates veins at low doses, reducing preload, with arterial effects at higher doses.
## Direct Arterial Vasodilators
Hydralazine directly relaxes arteriolar smooth muscle via mechanisms that may involve interference with calcium signaling. It reduces afterload and is used in heart failure (combined with isosorbide dinitrate) and in pregnancy-related hypertension where ACE inhibitors and ARBs are contraindicated. Side effects include reflex tachycardia, fluid retention, and a lupus-like syndrome with chronic high-dose use.
Minoxidil opens ATP-sensitive potassium channels (KATP) in vascular smooth muscle, causing potent arterial vasodilation. Reserved for severe refractory hypertension. Causes marked fluid retention (requiring loop diuretics) and reflex tachycardia (requiring beta-blockers). Hypertrichosis (excessive hair growth) limits cosmetic acceptability but led to its topical use for alopecia.
## Endothelin Receptor Antagonists
Bosentan, ambrisentan, and macitentan block endothelin receptors (ETA and/or ETB) on vascular smooth muscle. Endothelin-1 is one of the most potent endogenous vasoconstrictors. These agents are specifically used for pulmonary arterial hypertension, not systemic hypertension. Hepatotoxicity monitoring is required for bosentan.
## Phosphodiesterase-5 Inhibitors
Sildenafil and tadalafil inhibit PDE5, increasing cGMP and enhancing NO-mediated vasodilation. In addition to erectile dysfunction, both are approved for pulmonary arterial hypertension. They are contraindicated with nitrates due to the risk of profound, potentially fatal hypotension.
## Clinical Application by Hemodynamic Effect
Venous dilators (nitrates) reduce preload and are preferred for pulmonary congestion. Arterial dilators (hydralazine, minoxidil) reduce afterload and are used for systemic hypertension. Mixed dilators (nitroprusside) address both and are used in hypertensive emergencies with acute heart failure. Agent selection depends on the dominant hemodynamic abnormality.
## Key Takeaways
- Vasodilators differ in arterial versus venous selectivity
- Nitroprusside is the most potent IV agent but risks cyanide toxicity
- Hydralazine plus isosorbide dinitrate is an alternative to ACE inhibitors in HF
- PDE5 inhibitors are used for PAH but are absolutely contraindicated with nitrates