Pharmacology for Anaesthesia and Intensive Care

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Vasodilators

Sodium nitroprusside


Nitrates


Potassium channel activators


Calcium channel antagonsis


Miscellaneous

Sodium nitroprusside (SNP)

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

Presentation

Itis 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% (50–200μg.ml−^1 )

intravenous infusion, the dose of 0.5–6μg.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 100μg.ml−^1 .Upto

4 μg.kg−^1 .min−^1 may be used chronically while no more than 1.5μg.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 intracellu-

lar cyclic GMP. Although Ca^2 +influx into vascular smooth muscle in inhibited, its

uptake into smooth endoplasmic reticulum is enhanced so that cytoplasmic levels

fall, resulting in vasodilation.