A patient on nitroprusside infusion develops desaturation to 85% with stable exam after several days. The most likely cause is:

A patient on nitroprusside infusion develops desaturation to 85% with stable exam after several days. The most likely cause is:

• A. Cyanide toxicity
• B. Thiocyanate toxicity
• C. O2 toxicity
• D. Methemoglobinemia

Answer: (D)

Methemoglobinemia from nitroprusside is the most likely explanation. Nitroprusside can release nitrite, which oxidizes iron in hemoglobin from Fe2+ to Fe3+, forming methemoglobin. Methemoglobin cannot carry oxygen, and its presence also shifts the remaining hemoglobin's oxygen affinity, reducing overall oxygen delivery. A hallmark is a desaturation that sits around 85% on pulse oximetry, often despite adequate supplemental oxygen and a normal-looking exam—precisely the pattern seen after several days of nitroprusside exposure. This contrasts with cyanide toxicity, which causes tissue hypoxia signs and often lactic acidosis; thiocyanate toxicity presents more with neuro or thyroid symptoms after longer-term use; and oxygen toxicity isn’t the mechanism here. If suspected, stop the drug and consider treatment with methylene blue if methemoglobin levels are high or the patient is symptomatic.

Methemoglobinemia from nitroprusside is the most likely explanation. Nitroprusside can release nitrite, which oxidizes iron in hemoglobin from Fe2+ to Fe3+, forming methemoglobin. Methemoglobin cannot carry oxygen, and its presence also shifts the remaining hemoglobin's oxygen affinity, reducing overall oxygen delivery. A hallmark is a desaturation that sits around 85% on pulse oximetry, often despite adequate supplemental oxygen and a normal-looking exam—precisely the pattern seen after several days of nitroprusside exposure. This contrasts with cyanide toxicity, which causes tissue hypoxia signs and often lactic acidosis; thiocyanate toxicity presents more with neuro or thyroid symptoms after longer-term use; and oxygen toxicity isn’t the mechanism here. If suspected, stop the drug and consider treatment with methylene blue if methemoglobin levels are high or the patient is symptomatic.