Methemoglobinemia: from diagnosis to treatment

Methemoglobin is the oxidized form of hemoglobin, which does not bind oxygen and increases the affinity of oxygen for the partially oxidized portion of hemoglobin. Increased levels of methemoglobin in the blood are secondary to congenital changes and exposure to several chemical agents, resulting in a disorder with several differential diagnoses, which it can lead to death if it is not treated. The objective of this report was to review this subject, emphasizing relevant information for the clinical management of patients with methemoglobinemia.

When the concentration of methemoglobin in the blood is above 1.5%, the patient develops cyanosis, the main characteristic of this disorder. The color of the arterial blood changes to dark brown with normal PaO2. One should suspect the diagnosis in patients with cyanosis and low saturation (SpO2) without significant cardiopulmonary dysfunction. Co-oximetry is the gold standard and defines the diagnosis. Treatment should be based on whether the syndrome is acute or chronic (etiology) and on the severity of symptoms. Blood levels of methemoglobin are important, especially in acute cases. Basic treatment includes removal of the agent responsible for the disorder, administration of oxygen, and observation. Severe cases should be treated with the specific antidote, methylene blue, which is not effective in some situations.

Methemoglobinemia is a potentially severe disorder, whose diagnosis depends on a high degree of suspicion. In general, anesthesiologists are the first to detect the problem in the preoperative period and should lead the treatment.