Sodium nitroprusside and cyanide release: reasons for re-appraisal

The standard method for estimating cyanide liberated from sodium nitroprusside (SNP) has been colorimetric. Using a cyanide ion-selective electrode technique, it was found that SNP remained unchanged in the presence of whole blood, plasma and washed erythrocytes, no cyanide being detected. Previously reported releases of cyanide from SNP in vivo and in vitro are probably inaccurate. Values obtained could have arisen not from reaction with blood, but from photo-decomposition during the lengthy analysis required by the colorimetric method or during infusion, if the solution was left uncovered. Because of the supposed cyanide hazard, low doses of SNP have been recommended; it may now be possible to revise these.