Methemoglobinemia

Background

• Methemoglobin refers to ferric form of Hb (Fe3+) that is unable to bind O2
• Suspect in any patient with cyanosis not responsive to supplemental O2
• Caused by exposure to oxidizing agent
  • Benzocaine
  • Dapsone
  • CN kit
  • Antimalarials

Clinical Features

• Depends on level of metHb
  • 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 fatal

Diagnosis

• Co-oximetry
  • Measures relative levels of oxyhemoglobin, carboxyhemoglobin, methemoglobin, reduced hemoglobin
  • Usually performed on arterial blood sample, but fingertip CO-oximeter devices are available
• SpO2 will initially be falsely elevated
  • Once MetHb >30% → SpO2 will fall to 80-85%
• "Chocolate brown" color of blood

Management

• Methylene Blue
  • Treat if symptomatic OR asymptomatic w/ MetHb >25%
  • 1-2mg/kg Methylene Blue IV over 5min; repeat dose if no effect
  • Improvement seen within 20min

See Also

• Toxidromes