Cyanide and thiocyanate concentrations following sodium nitroprusside infusion in man

Twenty-six patients, receiving an infusion of sodium nitroprusside (SNP) during surgery, had considerable increases in both red cell and plasma cyanide concentration, but only small changes in plasma thiocyanate concentration. There was a linear relationship between both plasma and RBC cyanide concentrations and the total dose of SNP. The expired cyanide concentration followed the changes in the plasma. We believe that the development of metabolic acidosis, and the recent fatalities involving SNP, are attributable to histotoxic hypoxia as a result of excessive plasma concentrations of cyanide. On the basis of our results, we recommend that the total dose of SNP should not exceed 1.5 mg/kg during short-term infusions and that the plasma cyanide should not exceed 300 nmol%. Plasma thiocyanate concentrations are, in general, an unreliable indication of extent of exposure to cyanide, although they may become important during long-term infusions.