Vasodilators decrease blood pressure by dose-related effects on vascular smooth muscle independent of αor βreceptors (Table 15-7: Doses and Sites of Action of Vasodilators). These drugs may evoke baroreceptor-mediated increases in heart rate. Combination with a β-antagonist may be necessary to offset this reflex tachycardia (maintain heart rate <100 beats/min).
- Hydralazine (5–10 mg IV every 10–20 minutes) is useful to control perioperative hypertension.
- Nitroprusside is administered as a continuous infusion (0.25–0.5 µg/kg/min IV) using an infusion pump and continuous monitoring of blood pressure. The dose is increased slowly as needed to control hypertension or to produce controlled hypotension. Rarely is more than 3 to 5 µg/kg/min of nitroprusside required in an anesthetized patient. Acute hypertensive responses can be treated with single IV doses of 50 to 100 µg.
- The hypotensive effect of nitroprusside reflects direct relaxation of arterial and venous smooth muscle, causing decreases in preload and afterload. Hypotensive effects of nitroprusside are potentiated by volatile anesthetics and blood loss.
- The ferrous iron of nitroprusside reacts with sulfhydryl groups in red blood cells and releases cyanide, which is reduced to thiocyanate in the liver. High doses of nitroprusside (>10 µg/kg/min IV) may result in cyanide toxicity. There is no evidence that renal or hepatic diseases increase the likelihood of cyanide toxicity.
- Tachyphylaxis, increased venous oxygen tension, and metabolic acidosis signal the development of cyanide toxicity (cyanide binds to cytochrome oxidase, causing cellular hypoxia) and the need to discontinue the infusion of nitroprusside immediately.
- Treatment of cyanide toxicity is with sodium thiosulfate (150 mg/kg IV in 50 mL of water) administered over 15 minutes to speed the conversion of cyanide to thiocyanate.
- Nitroglycerin is administered as a continuous infusion (0.25–3.0 µg/kg/min IV) to treat myocardial ischemia. Its predominant action is on venules, causing increased venous capacitance and decreased venous return.
- Control of hypertension with nitroglycerin is less reliable than with nitroprusside, emphasizing the minimal effect of this drug on arterial smooth muscle.
- Unlike nitroprusside, nitroglycerin poses no risk of cyanide toxicity. For this reason, nitroglycerin may be chosen over nitroprusside to control hypertension associated with pregnancy-induced hypertension.
- Nesiritide is a recombinant form of human B-type natriuretic peptide that produces beneficial hemodynamic effects by venous and arterial vasodilation, including coronary vasodilation.