After 72 hours of high-dose sodium nitroprusside infusion in a hypertensive emergency, a patient with renal insufficiency develops metabolic acidosis, tachyphylaxis to the infusion, confusion, and a raised anion gap. The immediate corrective step is:

• A Stop the infusion and administer intravenous thiosulfate ✓
• B Switch directly to oral captopril without interruption
• C Give hydroxocobalamin orally over the next week
• D Continue the infusion at half the dose with close monitoring

Explanation

Nitroprusside metabolism releases cyanide, which is normally converted to thiocyanate by rhodanese using sulfur donors. With prolonged infusion beyond 24 to 48 hours, high doses, or renal impairment, cyanide accumulates, producing lactic acidosis with a raised anion gap, CNS signs, and loss of antihypertensive response. Management is immediate discontinuation plus intravenous thiosulfate to regenerate the substrate pool. Continuing at any dose risks fatal cyanide poisoning, and hydroxocobalamin is given intravenously, not orally, in severe cases.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.