From the Guidelines
Methemoglobinemia should be treated with intravenous methylene blue as the first-line treatment, at a dose of 1-2 mg/kg administered over 5 minutes, which can be repeated after one hour if necessary, as recommended by the 2023 American Heart Association guidelines 1. Methemoglobinemia is a blood disorder characterized by abnormally high levels of methemoglobin, a form of hemoglobin that cannot effectively carry oxygen.
Key Considerations
- The condition can be acquired through exposure to certain medications (like dapsone, local anesthetics, or nitrates) or chemicals, or it can be congenital due to enzyme deficiencies.
- Symptoms range from cyanosis and headache to severe hypoxia, confusion, and even death in severe cases.
- Prevention involves avoiding triggering agents in susceptible individuals and proper handling of chemicals known to cause the condition.
Treatment Options
- Intravenous methylene blue is the primary treatment for decreasing methemoglobin levels, acting as a cofactor to reduce methemoglobin to hemoglobin in erythrocytes 1.
- Alternative treatments for patients who cannot receive methylene blue include exchange transfusion, which may be necessary in life-threatening cases unresponsive to methylene blue 1.
- Hyperbaric oxygen therapy may also be considered in cases where methylene blue is not effective 1.
Important Notes
- Ascorbic acid is not recommended as a treatment for methemoglobinemia, according to the 2023 American Heart Association guidelines 1.
- N-acetylcysteine is also not recommended as a treatment for methemoglobinemia 1.
- Patients should be monitored for response to treatment, with improvement typically occurring within 30-60 minutes 1.
From the Research
Definition and Causes of Methemoglobinemia
- Methemoglobinemia is a condition characterized by the presence of methemoglobin, the oxidized form of hemoglobin, in the blood 2, 3.
- Methemoglobin is formed by the oxidation of ferrous (FeII) haem to the ferric (FeIII) state, and the mechanisms by which this occurs are complex 4.
- The main causes of methemoglobinemia include:
Clinical Characteristics and Diagnosis
- The main clinical characteristic of methemoglobinemia is cyanosis, which is unresponsive to oxygen therapy 2, 3.
- Other clinical characteristics include:
- Blood that is chocolate-colored when drawn
- Low saturation (SpO2) without significant cardiopulmonary dysfunction 2
- Co-oximetry is the gold standard for diagnosis, but arterial blood gas paired with pulse oximetry and serum methemoglobin levels can also confirm the diagnosis clinically 2, 3.
- Pulse oximetry is unreliable in the presence of methemoglobinemia, and CO-oximetry is superior to standard pulse oximetry in detecting methemoglobinemia 4, 5.
Treatment and Management
- Treatment is aimed at removal of the offending agent, if medication-induced, and is directed at aggressive oxygen therapy and treatment with the antidote, methylene blue 4, 3.
- Methylene blue is effective in reducing methemoglobin levels, but its efficacy is reduced in the presence of glucose-6-phosphate dehydrogenase deficiency or haemolysis 4.
- Ascorbic acid has been proposed as an alternative treatment for methemoglobinemia, and has been shown to be effective in reducing methemoglobin levels in some cases 6, 5.
- Other treatments, such as riboflavin, may also be effective in reducing methemoglobin levels 5.
- Exchange transfusion may have a role in the management of severe haemolysis or in G-6-P-D deficiency associated with life-threatening methemoglobinemia where methylene blue is relatively contraindicated 4.