Nitroprusside

Nitroprusside, or sodium nitroprusside dihydrate, is a potent, fast-acting intravenous vasodilator used to rapidly reduce blood pressure in hypertensive emergencies and to provide controlled hypotension during certain surgical procedures. It is an inorganic nitroso complex with the formula Na2[Fe(CN)5NO]·2H2O that releases nitric oxide as its active moiety.

Mechanism and pharmacodynamics: Nitroprusside acts as an NO donor, releasing nitric oxide which activates soluble guanylate cyclase in vascular smooth muscle. This increases cyclic GMP, causing relaxation of smooth muscle and dilation of both arteries and veins. The net effect is a decrease in systemic vascular resistance and venous return, lowering blood pressure. The onset is rapid with IV administration, and effects dissipate quickly after the infusion is stopped, requiring continuous monitoring.

Administration and monitoring: Nitroprusside is given by continuous IV infusion, typically in an intensive care unit with arterial pressure monitoring. Dosing is titrated to reach the desired blood pressure. The solution is light-sensitive and is usually protected from light; it is intended for short-term use and requires careful handling and monitoring due to its potent hemodynamic effects.

Metabolism, toxicity, and safety: Nitroprusside is metabolized to cyanide and thiocyanate. Cyanide must be detoxified by hepatic rhodanese; toxicity can occur with prolonged infusions, high doses, or hepatic/renal impairment. Monitor thiocyanate levels during extended therapy. Adverse effects include hypotension, reflex tachycardia, headaches, flushing, and potential increases in intracranial pressure. Cyanide or thiocyanate toxicity is treated by stopping the infusion and administering antidotes such as thiosulfate or hydroxocobalamin. It is contraindicated in known cyanide poisoning and used with caution in patients with severe hepatic or renal dysfunction.