Sodium nitroprusside is infused for hypertensive emergency in a patient with renal failure and ileus. After 48 hours the patient develops metabolic acidosis, confusion, and a rising lactate. What is happening and what is the antidote?

• A Cyanide intoxication treated with sodium thiosulfate ✓
• B Thiocyanate accumulation treated with dialysis
• C Methemoglobinemia treated with methylene blue
• D Thiocyanate accumulation treated with sodium thiosulfate

Explanation

Each nitroprusside molecule releases five cyanide moieties before conversion to thiocyanate. Cyanide accumulates when the rate of infusion exceeds hepatic rhodanese capacity, particularly with prolonged infusion, liver disease, or high doses above 2 microgram/kg/min, causing anaerobic metabolism, lactic acidosis and arrhythmias. Sodium thiosulfate donates sulfur for rhodanese to convert cyanide to thiocyanate. Thiocyanate accumulation causes a different picture, mainly psychosis and hypothyroidism, and is managed with dialysis in renal failure.

Reference: Katzung Basic and Clinical Pharmacology, 16th ed.

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