Methemoglobinemia
Background
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MetHb = oxidized form of Hb (heme iron configuration changed from ferrous (Fe2+) to ferric (Fe3+) state)
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doesn't bind O2 --> can't deliver O2 to tissues
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causes left shift of Hb O2 dissociation curve --> further ↓ in O2 delivery
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Can be congenital or acquired
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Congenital - ↓ enzymatic reduction of MetHb back to Hb
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may appear cyanotic but generally asymptomatic
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Acquired - from drugs that can oxidize Hb to MetHb
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↑ risk in infants & heterozygous CYB5R3 (cytochrome b5 reductase) mutations, G6PD deficiency
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can be fatal
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Drugs that can cause MetHb:
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local anesthetics: prilocaine, lidocaine, benzocaine
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NTG, sodium nitroprusside
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Inhaled nitric oxide
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Phenytoin
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Sulfonamides
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Metoclopramide
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Diagnosis
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symptoms of hypoxia that don't improve w/ O2
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blood may have "chocolate brown" colour
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[MetHb] on blood gas (co-oximetry)
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Nl [MetHb] <1%
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>10%: cyanosis
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<20%: asymptomatic or headache, fatigue, lethargy
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>20%: resp depression, altered LOC, seizures
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>40%: life threatening
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PaO2 usually normal or high
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SpO2 may read 85% w/ no improvement w/ O2
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Management
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D/C precipitating agents
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Rule out other causes of cyanosis
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Supportive care - IV fluids, intubation/ventilation prn, antiseizure Rx prn
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Methylene blue if concerning symptoms and/or [MetHb] >20-30%
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accelerates reduction of MetHb
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fast-acting; resolution w/in 20-60 mins
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Contraindications:
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G6PD deficiency --> can precipitate hemolysis
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Caution if pt on serotonergic Rx --> can precipitate serotonin syndrome
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Ascorbic acid if methylene blue contraindicated
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Consider blood or exchange transfusion & hyperbaric in severe/refractory dz
References
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Ludlow JT, Wilkerson RG, Nappe TM. Methemoglobinemia. [Updated 2022 Aug 29]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK537317/