vasodilator
A vasodilator widens blood vessels, like loosening a kink in a hose so water flows more freely at lower pressure. Wider vessels offer less resistance to flow, so the heart can push blood through against a smaller push-back. Depending on the drug, it may relax arteries, veins or both.
Vasodilators relax vascular smooth muscle, but they do so by many different routes, which makes vasodilator an umbrella term rather than a single class. Some act through nitric oxide and cyclic GMP (nitrates, hydralazine in part), some block calcium entry (dihydropyridine calcium channel blockers), some interrupt the renin–angiotensin system (ACE inhibitors and angiotensin receptor blockers), and some open potassium channels (minoxidil). Whether a drug mainly affects arteries (reducing afterload) or veins (reducing preload) shapes its clinical use.
Because dropping vascular resistance lowers blood pressure, the body often responds with a reflex faster heartbeat and fluid retention, which can blunt the benefit; this is why direct arterial vasodilators like hydralazine and minoxidil are usually paired with a beta-blocker and a diuretic. Other typical effects are headache, flushing, dizziness on standing, and ankle swelling. Powerful intravenous vasodilators such as sodium nitroprusside are reserved for hypertensive emergencies under close monitoring.
In a hypertensive emergency, an intravenous infusion of sodium nitroprusside, a fast-acting arterial and venous vasodilator, lowers blood pressure in a controlled, minute-by-minute fashion.
Titratable intravenous vasodilators let clinicians steer blood pressure precisely in a crisis.
Minoxidil's vasodilator side effect of excess hair growth led to its reformulation as a topical hair-loss treatment, a vivid reminder that mechanisms and side effects travel together.