#EMConf: Methemoglobinemia


  • O2 binds iron portiono of Hemoglobin in Ferrous (Fe2+) state
  • Methemoglobinemia is when iron exists in Ferric (Fe3+) state; cannot bind O2


  • Heriditary:
    • Can happen naturally but Cytochrome B5 converts back to Fe 2+
    • Deficiency in Cytochrome B5 reductase enzyme
    • Hemoglobin M: Favors Fe3+ (ferric) state
  • Acquired:
    • Dapsone, Chloroquine 
    • Topical Anesthetics (benzocaine)
    • Ingested Nitrites & Nitrates (well water)
    • Antifreeze
    • Aniline Dyes
    • Poppers (inhaled nitrites used to enhance sexual actvity that induces Methemoglobinemia when ingested)

Clinical: symptoms secondary to Hypoxia

  • Cyanosis
  • Non-specific symptoms: headache, malaise, lightheadedness, lethargy
  • Dyspnea
  • Seizure, Coma, Shock
  • Hypoxia that does not improve with Supplemental Oxygen
    • usually around 85% on Pulxe Oximetry
    • Methemoglobinemia absorbs same wave length as both oxygenated and deoxygenated Hemoglobin 
  • Chocolate brown blood

Diagnosis: can send serum level 


  • Methylene Blue
    • Mechanism: Converts iron from Fe3+ to Fe2+ allowing oxygen to bind hemoglobin again 
    • Indications:
      • >25% Methemoglobin level 
      • Symptomatic 
    • Contraindications: 
      • G6PD Deficiency: can precipitate hemolysis
      • Relative: use of serotonergic agents as Methylene Blue can inhibit MAO and can increase risk for Serotonin Syndrome