Nitrovasodilators

These drugs are the most frequently used vasodilators in the ICU (Anderson —— 199.4). Although sodium nitroprusside has mainly arterial vasodilating properties, nitroglycerin and analogous compounds predominantly dilate veins (Anderson eL..M 1994). At higher doses, they also have an arterial dilating effect and lower arterial blood pressure. The compounds release nitric oxide (NO) which stimulates production of cyclic guanosine monophosphate in the vessel wall, leading to relaxation

(Anderson... eLal, 1994). When administered intravenously, they have a rapid onset of action which also rapidly disappears after discontinuation of the drug. The major limitation is the development of nitrate tolerance, but this can be circumvented by nitrate-free intervals (asymmetric dosing). Indications for administration include severe forms of hypertension (nitroprusside), acute left heart failure with pulmonary edema (nitroglycerin), and unstable angina pectoris (nitroglycerin) ( Anderson et a| 1994).

The drugs are occasionally used as adjunctive therapy in the treatment of shock, particularly if caused by a low cardiac output (e.g. in cardiogenic shock, after cardiopulmonary surgery, etc.). Similarly, intravenous nitroglycerin has been used to protect patients from the excessive vasoconstriction used in the treatment of esophageal variceal bleeding. The major side-effect of nitroprusside is thiocyanate and cyanide toxicity, a complication that is usually avoided if treatment is limited to 3 days at doses below 3 pg/kg/min with normal renal function. Signs of toxicity include central nervous system symptoms. Thus monitoring of thiocyanate levels in blood is mandatory during prolonged treatment with nitroprusside. The treatment of choice for toxicity is intravenous hydroxycobalamine (5 mg). Another potential specific side-effect of nitrovasodilators is methemoglobinemia. Continuous intravenous infusion is preferred.

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