methemoglobinemia and sulfhemoglobinemia

15 Mar

When hemoglobin loses an electron and becomes oxidized, it is converted to the ferric state (Fe3+) or methemoglobin.

Methemoglobin lacks the electron that is needed to form a bond with oxygen and, thus, is incapable of oxygen transport.

Because red blood cells are continuously exposed to various oxidant stresses, blood normally contains approximately 1% methemoglobin levels.


Early symptoms include:

  • headache
  • fatigue
  • dyspnea
  • lethargy
  • cyanosis

Symptoms at higher methemoglobin levels:

  • respiratory depression
  • altered consciousness
  • shock
  • seizures
  • death (the final “symptom”)

Acquired methemoglobinemia is life threatening when methemoglobin comprises more than 30 percent of total hemoglobin.

Methemoglobinemia may be clinically suspected by the presence of clinical “cyanosis” in the presence of a normal arterial pO2 (PaO2)

An asymptomatic patient with a methemoglobin level <20 percent my only need supportive treatment.

If the patient is symptomatic, or if the methemoglobin level is >20 percent:

  • treatment = methylene blue


A lack of response to methylene blue should prompt consideration of sulfhemoglobinemia. Methemoglobinemia and sulfhemoglobinemia are difficult to distinguish, although the former occurs much more frequently than the latter.

sulfhemoglobinemia = rare blood condition that occurs when a sulfur atom is incorporated into the hemoglobin molecule.

  • usually drug induced
  • no specific treatment for sulfhemoglobinemia except the removal of the inciting agent.



Submitted by H. Reed-Day


References:  Clinicopathologic Conference: A 28-year-old Woman with Perioral Cyanosis and Low Oxygen Saturation, ACADEMIC EMERGENCY MEDICINE, Volume 15, Issue 4, April 2008, Pages: 363–367, Jeffrey Sankoff and Andrew D. Louie; medline plus; medscape reference; picture

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