In this episode, I’ll discuss the dose of methylene blue for the treatment of acquired methemoglobinemia.
Acquired methemoglobinemia is typically a reaction to medications. The ferric hemes of methemoglobin are unable to bind oxygen therefore causing a functional anemia.
Suspect methemoglobinemia when the following occur:
1. Sudden cyanosis after ingestion of a drug that may cause methemoglobinemia
2. Hypoxia that does not improve with increasing amounts of oxygen
3. Chocolate brown or otherwise discolored blood during phlebotomy
If the methemoglobin levels are 20% or higher or the patient has symptoms of impaired oxygen delivery including tachycardia, confusion, tachypnea, seizure, or coma, this is a medical emergency that requires prompt treatment. When in doubt, bring the methylene blue to the bedside immediately when methemoglobinemia is suspected so that it will be available if needed.
Methylene blue accelerates the conversion of methemoglobin to hemoglobin effectively reversing the functional anemia caused by methemoglobinemia.
1 to 2 mg/kg IV over 5 minutes of methylene blue is given immediately if the methemoglobin levels are 20% or higher or the patient is symptomatic. The dose may be repeated in 1 hour if needed.
Remember that administration of methylene blue will render standard detection of methemoglobin inaccurate. Most patients will improve rapidly and not require any further treatment.
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