Hydralazine relaxes arteriolar smooth muscle, causing dilation of precapillary resistance vessels via increased cGMP.
Intraoperative hypertension is usually con-trolled with an intravenous dose of 5–20 mg of hydralazine. The onset of action is within 15 min, and the antihypertensive effect usually lasts 2–4 hr. Hydralazine can be used to control pregnancy-induced hypertension.Hydralazine undergoes acetylation and hydrox-ylation in the liver.
The lowering of peripheral vascular resistance causes a drop in arterial blood pressure. The body reacts to a hydralazine-induced fall inblood pressure by increasing heart rate, myocardial contractility, and cardiac output. These compensa-tory responses can be detrimental to patients with coronary artery disease and are minimized by the concurrent administration of a β-adrenergic antagonist. Conversely, the decline in afterload often proves beneficial to patients in congestive heart failure.
Hydralazine is a potent cerebral vasodilator and inhibitor of cerebral blood flow autoregulation. Unless blood pressure is markedly reduced, cerebral blood flow and intracranial pressure will rise.
Renal blood flow is usually maintained or increased by hydralazine.