Metabolism of sodium nitroprusside and cyanide in the dog

Blood cyanide (HCN) and thiocyanate (SCN) concentrations were measured at intervals in anaesthetized dogs given bolus doses of sodium nitroprusside (SNP) 1 mg kg-1 or potassium cyanide 1.07 mg kg-1 and in animals infused with SNP 1.5 mg kg-1 for 1 h. Cyanide appeared rapidly in the red cells to give peak concentrations which accounted for more than 90% of the total blood HCN. A delay between the peak plasma and red cell HCN concentrations confirmed that some of the SNP was degraded in the plasma. Comparison of HCN and SCN concentrations with those measured previously in patients receiving an infusion of SNP suggests that the degradation of SNP and detoxication of HCN may be more rapid in the dog. The various pathways of HCN detoxication are discussed in relation to the reduced formation of SCN in dogs receiving SNP compared with those receiving KCN.