Vasodilator

A vasodilator is a substance or drug that causes vasodilation, the widening of blood vessels. By relaxing vascular smooth muscle, vasodilators decrease vascular resistance and can increase blood flow to tissues. They can act directly on the vessel wall or indirectly through neurohumoral pathways, and their effects may be systemic or targeted to particular circulations such as the systemic or pulmonary circuits.

Mechanisms and classifications vary. Direct-acting arterial vasodilators, including hydralazine, minoxidil, and diazoxide, primarily dilate small arteries and reduce afterload. Sodium nitroprusside dilates both arteries and veins by releasing nitric oxide, lowering both preload and afterload. Nitrates (for example, nitroglycerin and isosorbide dinitrate) act as nitric oxide donors with prominent venodilation and preload reduction. Calcium channel blockers cause vasodilation by inhibiting calcium influx into vascular smooth muscle; dihydropyridines preferentially affect arteries, while non-dihydropyridines affect both arteries and veins. Agents such as ACE inhibitors and ARBs promote vasodilation by blocking pathways that constrict vessels (reducing angiotensin II activity). Prostacyclin analogs and endothelin receptor antagonists dilate vessels, with particular use in pulmonary arterial hypertension. Phosphodiesterase-5 inhibitors enhance cyclic GMP–mediated relaxation and are used in select settings.

Clinical uses and contexts vary. Vasodilators treat hypertension and angina, aid in certain heart failure regimens by reducing afterload or preload, and are central to management of pulmonary arterial hypertension. In acute care, intravenous vasodilators such as nitroprusside, nitroglycerin, or clevidipine are used for hypertensive emergencies or severe heart failure. Combination regimens, such as hydralazine with isosorbide dinitrate, are recommended in specific patient populations.

Adverse effects include hypotension, reflex tachycardia, dizziness, headache, edema, and flushing. Drug interactions and contraindications (for example with PDE-5 inhibitors or in certain hypotensive states) require careful consideration.