Methemoglobinemia

Background

• Methemoglobin refers to ferric form of Hb (Fe3+) that is unable to bind O2
• Caused by exposure to oxidizing agent (benzocaine, CN kit, antimalarials)
• Suspect in any pt whose cyanosis does not respond to supplemental O2

Clinical Features

• Levels
  • Pts with anemia or preexisting cardiopulmonary disease will show symptoms sooner
  • <20%: asymptomatic, gray-blue cyanosis
  • 20-30%: anxiety, HA, weakness, light-headedness, tachycardia
  • 50-60%: myocardial ischemia, dysrhythmias, depressed mental status, seizure
  • >70%: usually lethal

Diagnosis

• Co-oximetry
• SpO2 will initially be falsely elevated until MetHb >30% (SpO2 will then fall to 80-85%)
• "Chocolate brown" color of blood

Treatment

• Methylene Blue
  • Reserved for symptomatic pts or asymptomatic pts w/ MetHb >25%
  • 1-2mg/kg Methylene Blue IV over 5min; repeat dose if needed
  • Improvement seen w/in 20min

See Also

• Toxidromes