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  1. Methemoglobinemia is a disorder characterized by the presence of a higher than normal level of methemoglobin (metHb, i.e., ferric [Fe3+] rather than ferrous [Fe2+] haemoglobin) in the blood.  Normally, methemoglobin levels are <1%, 
  2. Methemoglobin is an oxidized form of hemoglobin that has a decreased affinity for oxygen, resulting in an increased affinity of oxygen to other heme sites (that are still ferrous) within the same red blood cell. 
  3. This leads to an overall reduced ability of the red blood cell to release oxygen to tissues, with the associated oxygen–hemoglobin dissociation curve therefore shifted to the left. 
  4. When methemoglobin concentration is elevated in red blood cells, tissue hypoxia can occur.
  1. Congenital
  2. Acquired methemoglobinemia
    1. Dapsone          
    2. Bismuth nitrates          
    3. Antibiotics (trimethoprim, sulfonamides and dapsone), 
    4. Local Anesthetics (especially articaine and prilocaine),      
    5. Aniline dyes,            
    6. Metoclopramide 


  1. Signs and symptoms of methemoglobinemia (methemoglobin >1%) include shortness of breath, cyanosis, mental status changes, headache, fatigue, exercise intolerance, dizziness and loss of consciousness.
  2. Arterial blood with elevated methemoglobin levels has a characteristic chocolate-brown color (LQ 2012) as compared to normal bright red oxygen-containing arterial blood.
  3. In advances cases hypotension, dysrhythmias, seizures, coma and death can occur.
Methemoglobinemia can be treated with supplemental oxygen and methylene blue

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