Why is thiosulfate mixed in with nitroprusside in some formularies?

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At our institution’s pharmacy, thiosulfate is mixed in with nitroprusside infusions. However, I know that this isn’t done by all institutions. Why is this done and how is dosing of the thiosulfate determined?

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Nitroprusside is quickly metabolized into cyanide. The limiting factor in the clearance of cyanide is a sulfur donor. Thiosulfate is a sulfur donor available in the body but not at quantities sufficient enough to compensate for the doses of nitroprusside that are used [Clinical pharmacokinetics of nitroprusside, cyanide, thiosulphate and thiocyanate.]. Therefore, thiosulfate is exogenously administered.

The body will naturally breakdown cyanide at about 1 microgram/kg/min. This corresponds to a nitroprusside infusion rate of just over 2mcg/kg/min. Therefore, a 10:1 ratio of thiosulfate:nitroprusside is used to supply enough sulfur donors without affecting the efficacy of the nitroprusside [Sodium nitroprusside-induced cyanide intoxication and prevention with sodium thiosulfate prophylaxis.] [Stability of Sodium Nitroprusside and Sodium Thiosulfate 1:10 Intravenous Admixture.].

Other references:
Nitroprusside Sodium Drug Information, Professional

Nitroprusside Sodium Drug Information, Professional - general dosing section has a bunch of references

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