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Methemoglobinemia

Description: Benzocaine-induced methemoglobinemia is a rare complication associated with topical anesthesia and can be life-threatening.  Acquired Methemoglobinemia is characterized by cyanosis, low pulse oximetry readings, and normal arterial PO2 values. 


Treatments: Although studies have shown rare spontaneously resolving cases, it will normally require Methylene Blue for treatment at a dose of 1-2 mg/kg IV (up to a total of 50 mg in adults, adolescents, and older children) as a 1% solution in saline over 3-5 minutes.


Unfortunately ASHP and other articles do not truly recommend alternative medications.  Exchange transfusion (which replaces abnormal hemoglobin with normal hemoglobin) is the preferred second-line treatment for severely symptomatic patients for whom methylene blue is contraindicated or ineffective.  


Intravenous ascorbic acid has been used for methemoglobinemia when methylene blue is unavailable or contraindicated, such as in pregnant patients or those with G6PD deficiency.  However dosing is not standardized, and AHA recommends against its use in life-threatening methemoglobinemia, noting that most case reports describe its administration in combination with other treatments and that its onset of action is slow; multiple doses over several hours may be necessary for it have any significant effect.


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