Consequently, iron is assigned an oxidation state of 2+. The iron center has a diamagnetic low-spin d6 electron configuration, although a paramagnetic long-lived metastable state has been observed by EPR spectroscopy.The chemical reactions of sodium nitroprusside are mainly associated with the NO ligand. For example, addition of S2− ion to [Fe(CN)5(NO)]2− produces the violet colour[Fe(CN)5(NOS)]4− ion, which is the basis for a sensitive test for S2− ions. An analogous reaction also exists with OH− ions, giving [Fe(CN)5(NO2)]4−. Roussins red salt (K2[Fe2S2(NO)4]) and Roussins black salt (NaFe4S3(NO)7) are related iron nitrosyl complexes. The former was first prepared by treating nitroprusside with sulfur.
Its use in hepatically impaired individuals is also not recommended, as is its use in cases of pre-existing hypothyroidism.Its use in pregnant women is advised against, although the available evidence suggests it may be safe, provided maternal pH and cyanide levels are closely monitored. Some evidence suggests sodium nitroprusside use in critically ill children may be safe, even without monitoring of cyanide level. Interactions
The only known drug interactions are pharmacodynamic in nature, that is it is possible for other antihypertensive drugs to reduce the threshold for dangerous hypotensive effects to be seen. Overdose
Due to its cyanogenic nature, overdose may be particularly dangerous. Treatment of sodium nitroprusside overdose includes the following:
Discontinuing sodium nitroprusside administration
Buffering the cyanide by using sodium nitrite to convert haemoglobin to methaemoglobin as much as the patient can safely tolerate
Infusing sodium thiosulfate to convert the cyanide to thiocyanate.Haemodialysis is ineffective for removing cyanide from the body but it can be used to remove most of the thiocyanate produced from the above procedure. Toxicology
The cyanide can be detoxified by reaction with a sulfur-donor such as thiosulfate, catalysed by the enzyme rhodanese. In the absence of sufficient thiosulfate, cyanide ions can quickly reach toxic levels. Hydroxocobalamin can be administered to reduce the risk of thiocyanate toxicity induced by nitroprusside. Mechanism of action
As a result of its breakdown to nitric oxide (NO), sodium nitroprusside has potent vasodilating effects on arterioles and venules (arterial more than venous), whereas other nitrates exhibit more selectivity for veins (e.g. nitroglycerin).Sodium nitroprusside breaks down in circulation to release nitric oxide (NO).