Methemoglobin (metHb) is an abnormal form of hemoglobin in which the iron molecule is in the ferric (3+) state rather than the normal ferrous (2+) state. When iron in hemoglobin is switched to the ferric state, it is no longer capable of carrying oxygen or carbon dioxide. In addition, the oxy-hemoglobin dissociation curve is shifted to the left, impairing the delivery of oxygen at the tissue level. RBCs are continuously subjected to oxidative stressors that result in the formation of metHg. The erythrocytes have 2 mechanisms that work to keep the abnormal hemoglobin percent down to generally less than 1%. One reduces the oxidizing compounds before they can change the hemoglobin; the other changes the abnormal hemoglobin back into working hemoglobin by way of 2 enzyme pathways.

Methemoglobinemia may be inherited or acquired. In the inherited variety, the hemoglobin can be congenitally abnormal, or the person may have a deficiency of an enzyme (G6PD) needed to return hemoglobin to the ferrous form. These individuals can tolerate a metHb between 10-50% with no health risk. Acquired metHg can be the result of exposure to chemicals or drugs that increase the rate of metHg formation to a level that exceeds the ability of the body to change back to hemoglobin.

Topical anesthetics (Benzocaine, Lidocaine), silver nitrate, NTG, nipride, sulfonamides, pyridium, quinones, and dapsone are some of the drugs that can lead to methemoglobinemia. Chemicals that can cause the change in iron ions are: aniline compounds found in inks, polishes, paints & varnishes; organic nitrates.

ABG determination will show a normal PaO2, with a SaO2 lower than expected for the given PaO2. The arterial blood will take on a characteristic "chocolate brown" color. The blood gas lab will further report the make up of the other possible forms of hemoglobin: reduced, carboxyhemoglobin & methemoglobin. A pulse oximeter cannot distinguish between normal & abnormal hemoglobin and will report a combination of saturated hemoglobin, carboxyhemoglobin & metHg. With a metHg > 5%, a pulse oximeter may frequently report a SpO2 of about 85%, despite a much lower actual arterial saturation.

Treatment is not always necessary. Once the causative agent is removed, the methemoglobin will convert back to hemoglobin within 2-3 days. Methylene blue can be used in the treatment of symptomatic methemoglobinemia. This dye will reduce methemoglobin back to hemoglobin. Administration of 1 to 2 mg/kg of a 1% solution intravenously over 5 minutes will generally reduce the amount of metHg by 50% within an hour. Oral doses can then be started for more long-term coverage. Methylene blue is contraindicated in the patient with a G6PD deficiency and in patients with severe renal impairment.