Sodium nitroprusside (SNP)

Sodium nitroprusside is an inorganic complex and functions as a prodrug.

Presentation

It is presented in vials as a lyophilized reddish-brown powder containing 50 mg SNP. When reconstituted in 5% dextrose it produces a light orange- or straw-coloured solution with pH 4.5. If exposed to sunlight it will turn dark brown or blue because of liberation of cyanide (CN−) ions at which point the solution should be discarded. Infusions may be protected from sunlight by aluminium foil or opaque syringes and giving sets.

Uses

Sodium nitroprusside is usually administered as a 0.005–0.02% (50–200 μg.ml−1) intravenous infusion, the dose of 0.5–6 μg.kg−1.min−1 being titrated to effect. The onset of action is within 3 minutes and because of its rapid breakdown its effects are short-lived. Various dose regimes are recommended and are all designed to avoid CN− toxicity and thiocyanate (SCN) levels exceeding 100 μg.ml−1. Up to 4 μg.kg−1.min−1 may be used chronically while no more than 1.5 μg.kg−1.min−1 is recommended during anaesthesia. It is not available orally.

Mechanism of action

Sodium nitroprusside vasodilates arteries and veins by the production of NO. This activates the enzyme guanylate cyclase leading to increased levels of intracellular cyclic GMP.