Cardiopulmonary Bypass, Hemolysis, and Nitroprusside-Induced Cyanide Production

BACKGROUND: Cyanide toxicity is a complication of sodium nitroprusside administration. Cardiac surgery may increase the risk of cyanide toxicity, because hemolysis during cardiopulmonary bypass (CPB) may catalyze the release of free cyanide from sodium nitroprusside. METHODS: We obtained serial blood specimens from 25 cardiac surgical patients during CPB. Plasma specimens were analyzed for free hemoglobin concentration and ability to generate free cyanide anion upon exposure to sodium nitroprusside. RESULTS: Hemolysis based on plasma-free hemoglobin concentration increased over time during CPB at an average rate of 0.27 mg x dL(-1) x min(-1) (P < 0.001). The concentration of free cyanide generated by the addition of sodium nitroprusside to the plasma samples was directly related to the plasma-free hemoglobin concentration (P < 0.001). CONCLUSION: CPB-associated hemolysis and free hemoglobin release accelerated the immediate release of free cyanide from sodium nitroprusside. These in vitro findings suggest that cardiac surgical patients may be at increased risk of cyanide toxicity in response to the perioperative administration of sodium nitroprusside.