Methemoglobinemia

Background

  • Methemoglobin refers to ferric form of hemoglobin (Fe3+) that is unable to bind O2
  • Suspect in any patient with cyanosis not responsive to supplemental O2
    • Consider in pediatric patient (<6 months) with diarrhe and low oxygen saturation

Causes

  • Rarely congenital, NADH reductase deficiency - unable to reduce ferric to ferrous iron (Fe3+ → Fe2+)
  • Exposure to oxidizing agent[1][2]

Clinical Features

Cyanosis from methemoglobinemia.
Cyanosis resolved after methylene blue.
Severe methemoglobinemia.
Chocolate-brown blood due to methemoglobinemia.
MetHb level (%) Signs & Symptoms^
<3 None
3-15 Blue/slate-gray skin
15-30 Cyanosis, chocolate-brown blood
30-50 Breathlessness, headache, dizziness, syncope
50-70 Tachypnea, myocardial ischemia, lactic acidosis, arrythmia, seizure, CNS depression, coma
>70 Death

^Patients with anemia or preexisting cardiopulmonary disease will show symptoms sooner

Differential Diagnosis

  • Consider rare entity of sulfhemoglobinemia
    • Sulfonamides, sulfasalazine, phenazopyridine
    • Occupational exposure
    • Conventional co-oximerty misidentifies sulfhemoglobin as methemoglobin so diagnosis is suspected when patient does not respond to methylene blue
    • Typically requires no treatment other than removal of offending agent
    • Blue-green discoloration of blood

Acute dyspnea

Emergent

Non-Emergent

Evaluation

Chocolate-brown blood due to methemoglobinemia.
Color chart for the detection of the amount of methemoglobin in the blood.
  • Diagnosis requires high index of suspicion; lack of improvement in oxygen saturation with high-flow oxygen plus discrepancy in oxygen saturation >5% between measurements by ABG vs pulse oximetry ("saturation gap") should raise suspicion. Diagnosis is then confirmed by measuring level of methemoglobin in blood
  • Co-oximetry
    • Measures relative levels of oxyhemoglobin, carboxyhemoglobin, methemoglobin, reduced hemoglobin
    • Usually performed on arterial blood sample, but fingertip CO-oximeter devices are available
  • Normal PaO2
  • SpO2 will initially be falsely elevated
    • Once MetHb >30% → SpO2 will fall to 80-85%
    • Mid-80s SpO2 due to light absorption of both oxyHb and deoxyHb
  • "Chocolate brown" color of blood

Management

  • High-Flow Oxygen
    • All patients warrant supplemental oxygen. Symptoms may improve with a non-rebreather but the pulse oximeter reading will remain unchanged and patients may remain cyanotic. This is a diagnostic clue.
  • Methylene blue
    • Treat if symptomatic OR asymptomatic with MetHb >25%
    • 1-2mg/kg Methylene blue IV over 5min; repeat dose if no effect
    • Improvement seen within 20min
    • Contraindicated in G6PD deficiency
      • May induce acute hemolytic anemia
      • Instead use moderate dose of ascorbic acid (vitamin C), 300 to 1000 mg/day orally in divided doses
  • Other treatment modalities[5]
    • Exchange transfusion for symptomatic methemoglobinemia in patient with G6PD deficiency
    • Hyperbaric O2 when methylene blue ineffective or contraindicated
    • IV hydration and bicarbonate for metabolic acidosis


Medication Dosing

Methylene blue 1-2mg/kg over 5min; may repeat IV — Contraindicated in G6PD deficiency; improvement seen within 20min

Disposition

  • Will vary depending on response to the antidote and persistence of symptoms. Discuss with toxicologist/poison control

See Also

External Links

References

  1. ↑ Fernandez-Frackelton M, Bocock J: Cyanosis, in Marx JA, Hockberger RS, Walls RM, et al (eds): Rosen’s Emergency Medicine: Concepts and Clinical Practice, ed 7. St. Louis, Mosby, Inc., 2010, (Ch) 29:p 211-216.
  2. ↑ Mody, A., Silverman, B.: Problems in the Early Neonatal Period, In Fleisher and Ludwig's Textbook of Pediatric Emergency Medicine, Lippincott, WIlliams and Wilkins 2010, Page 995.
  3. ↑ Jeffery WH, Zelicoff AP, Hardy WR. Acquired methemoglobinemia and hemolytic anemia after usual doses of phenazopyridine. Drug Intell Clin Pharm. 1982 Feb;16(2):157-9.
  4. ↑ Denshaw-Burke et al. Methemoglobinemia: Practice Essentials. Jan 4, 2016. http://emedicine.medscape.com/article/204178-overview
  5. ↑ Denshaw-Burke et al. Methemoglobinemia: Practice Essentials. Jan 4, 2016. http://emedicine.medscape.com/article/204178-overview
This page is issued from WikEM. The text is available under an unknown license unless otherwise noted. Additional terms may apply for the media files.