Intravenous Nitroglycerin

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ACTIONS AND PHARMACOLOGY This agent acts by causing both arteriolar dilation and venodilation, with a greater effect on the venous system than on the arterial vasculature. Onset of action is almost immediate when nitroglycerin is given intravenously, and the half-life is 4 min. The mechanism of action is thought to involve the reduction of sulfhydryl groups at a smooth muscle nitrate receptor, resulting in an increase in cyclic GMP. Nitroglycerin is hepatically metabolized. Cardiac output usually remains unchanged but may decrease slightly.

INDICATIONS The main indication for nitroglycerin is in the setting of myocardial ischemia, because it is a better vasodilator of the coronary vessels than is nitroprusside; therefore, it is the agent of choice for moderate hypertension complicating unstable angina, myocardial infarction, or pulmonary edema. It also has a less harmful effect on pulmonary gas exchange than does nitroprusside.

USE The initial infusion rate should be 5 to 20 pg/min, with 5-pg/min incremental increases every 5 min until symptoms are improved or adverse effects necessitate stopping the infusion.

SIDE EFFECTS AND CONTRAINDICATIONS The most common side effects include headache, tachycardia, nausea, vomiting, hypoxia, and hypotension. Hydralazine

ACTIONS AND PHARMACOLOGY This agent acts as a direct arteriolar dilator, with onset of action within 10 min when given intravenously and a duration of action of 4 to 6 h. The onset of action increases to 20 min when hydralazine is given intramuscularly and to 30 min after an oral dose. The plasma half-life is 2 to 4 h, but the antihypertensive affect may outlast this time interval. Its mechanisms of action are not well understood, but it is known to directly relax vascular smooth muscle, resulting in vasodilation. Hydralazine is metabolized by acetylation in the liver and gut walls by ring hydroxylation and conjugation. In patients who are "slow acetylators" (50 percent of the US population), there is a higher incidence of hypotension and toxic complications. Within 24 h, 80 percent of hydralazine and its metabolites are excreted in the urine. Doses need to be decreased in patients with renal insufficiency.

INDICATIONS The main indication for use of intravenous hydralazine is pregnancy-induced hypertension. 19 The drug can be used orally and in combination with other drugs.

USE In cases of eclampsia, hydralazine can be given as either a 10- to 20-mg intravenous dose or a 10- to 50-mg intramuscular dose. This dose can be repeated in 30 min.

SIDE EFFECTS AND CONTRAINDICATIONS Hydralazine should not be used in patients with aortic dissection or a history of coronary artery disease, because it causes reflex tachycardia and increases plasma renin and catecholamines. It also causes sodium and water retention and can cause headaches, nausea, tachycardia, lethargy, and postural hypotension. A lupuslike syndrome can result from chronic oral use.

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