- 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%,
- 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.
- This leads to an overall reduced ability of the red blood cell to release oxygen to tissues, with the associated oxygenâ€“hemoglobin dissociation curve therefore shifted to the left.
- When methemoglobin concentration is elevated in red blood cells, tissue hypoxia can occur.
- Acquired methemoglobinemia
- Bismuth nitrates
- Antibiotics (trimethoprim, sulfonamides and dapsone),
- Local Anesthetics (especially articaine and prilocaine),
- Aniline dyes,
- Signs and symptoms of methemoglobinemia (methemoglobin >1%) include shortness of breath, cyanosis, mental status changes, headache, fatigue, exercise intolerance, dizziness and loss of consciousness.
- Arterial blood with elevated methemoglobin levels has a characteristic chocolate-brown color (LQ 2012) as compared to normal bright red oxygen-containing arterial blood.
- In advances cases hypotension, dysrhythmias, seizures, coma and death can occur.
Methemoglobinemia can be treated with supplemental oxygen and methylene blue