Direct vasodilators act on arteries and veins. They dilate arteriolar smooth muscle by direct relaxation, reducing systolic and diastolic blood pressure while increasing heart rate and CO.Direct vasodilators include diazoxide, hydralazine hydrochloride, minoxidil, and nitroprusside.

Indications and Contraindications

A physician prescribes diazoxide and nitroprusside to treat patients in hypertensive crisis when an urgent decrease in diastolic blood pressure is needed. Oral hydralazine is used to treat patients with primary hypertension; parenteral hydralazine is used in patients with severe primary hypertension and heart failure. Minoxidil is prescribed when severe hypertension is unresponsive to other therapy.

Don’t administer hydralazine to patients with CAD or rheumatic heart disease. Do not use minoxidil in patients with acute MI, dissecting aortic aneurysm, or pheochromocytoma. And do not administer nitroprusside to patients with compensatory hypertension.

Don’t use diazoxide in patients with hypersensitivity to thiazides or sulfonamide or in patients whose hypertension is caused by coarctation of the aorta, dissecting aortic aneurysm, atrioventricular shunt, or pheochromocytoma. And use it cautiously in patients with tachycardia, fluid and electrolyte imbalances, or impaired cerebral or cardiac circulation.

Adverse Effects and Interactions

Direct vasodilators commonly produce adverse effects related to reflex activation of the sympathetic nervous system, such as palpitations, angina, tachycardia, ECG changes, edema, rash, breast tenderness, fatigue, and headache. Severe pericardial effusions can develop. And alkaline phosphatase, BUN, and creatinine levels may increase.

Diazoxide commonly causes headache, anorexia, nausea, and diaphoresis. It can also cause excessive hypotension, and in diabetic patients, it may cause hyperglycemia. If more serious effects occur, such as rash, urticaria, polyneuritis, GI hemorrhage, anemia, and pancytopenia, diazoxide should be discontinued.

Hydralazine commonly causes headache, diarrhea, constipation, dizziness, orthostatic hypotension, facial flushing, shortness of breath, nasal congestion, urinary hesitancy, edema, tremors, and muscle cramps. It may also cause impotence.

Minoxidil commonly produces hair growth on the face, arms, and back. It also causes reflex tachycardia and fluid retention. When minoxidil is taken with guanethidine, orthostatic hypotension can occur.

Nitroprusside causes headache, dizziness, nausea, vomiting, abdominal pain, and thiocyanate or cyanide toxicity . Severe hypotension occurs when nitroprusside is administered with ganglionic blockers, volatile liquid anesthetics, halothane, enflurane, and circulatory depressants. Nitroprusside is incompatible with any drug in syringe or solution.

When diazoxide is administered with a thiazide diuretic, another antihypertensive drug, warfarin, guanethidine, or a sympathomimetic, its effects increase. It’s incompatible with other drugs in a syringe or solution. Hyperglycemia and hyperuricemia can result when diazoxide is combined with thiazides and other diuretics. The effects of both diazoxide and sulfonylureas decrease when the drugs are given together.

When hydralazine is used with epinephrine or norepinephrine, tachycardia and angina increase. Hydralazine increases the effects of beta-blockers. And it is incompatible with aminophylline, ampicillin, edetate calcium disodium, chlorothiazide, ethacrynic acid, hydrocortisone, mephentermine, methohexital sodium, nitroglycerin, phenobarbital, verapamil, fructose 10%, dextrose 10%, and lactated Ringer’s solution.