Cyanosis can be seen in patients with methemoglobinemia when methemoglobin makes up approximately 10% of total hemoglobin. Patients with oxygen saturation in the 80s (measured by pulse oximetry) are not typically cyanotic due to the oxygen saturation gap theory.
Respiratory distress can be seen in patients with methemoglobinemia when methemoglobin makes up approximately 30-50% of total hemoglobin.
Headache and other hypoxic symptoms can be seen in methemoglobinemia patients when methemoglobin makes up approximately 20% of total hemoglobin.
Lethargy or loss of consciousness can be seen in methemoglobinemia patients when methemoglobin makes up approximately 30-50% of total hemoglobin. If it is near 50%, severe symptoms may occur, such as arrhythmias, seizures, metabolic acidosis, and coma. Mortality increases as binding approaches 70%.
Normally, the color of blood is red due to the presence of oxygen bound to hemoglobin. Patients with methemoglobinemia have a reduced oxygen-carrying capacity in their blood. This will cause the patient's blood to appear dark red to brown. This can be easily seen if the blood is dripped on a piece of white gauze.
Pulse oximetry in patients with methemoglobinemia will erroneously reveal a number around 85%. This occurs due to the high absorption of light by methemoglobin. Under normal conditions, pulse oximetry measures the ratio of oxygenated and deoxygenated hemoglobin by measuring the absorbance of light. Methemoglobin, however, absorbs light differently than both oxygenated and deoxygenated hemoglobin. Thus, the pulse oximeter will reveal a falsely elevated oxygen saturation despite the actual oxygen saturation. For example, a methemoglobin level of 5% and 40% will both present on pulse oximetry as an O2 saturation of ~85%. Oxygen saturation on blood gas analysis can be falsely high as it is based on the PaO2, which reflects plasma oxygen content. Because of this, patients can present with a saturation gap. This is also referred to as "refractory hypoxemia."
Normal PaO2 is commonly seen in patients' blood gas analysis despite cyanosis. This finding suggests methemoglobinemia.
The first step in treating methemoglobinemia is to identify triggers and stop the inciting agent. Potential triggers include dietary nitrates and drugs such as nitroglycerin, dapsone, and lidocaine.
Methylene Blue is a drug that reduces methemoglobin via NADPH. It will convert methemoglobin to hemoglobin. It is used as the primary emergency treatment for symptomatic methemoglobinemia and should be used with caution in patients with G6PD deficiency since it depletes NADPH reserves and may cause oxidative crisis in G6PD deficiency patients.
Vitamin C can be used for the treatment of methemoglobinemia. It is indicated if methylene blue is not available, ineffective, or not recommended.
An exchange transfusion can also be initiated If methylene blue does not work or is not recommended in a patient. It works by replacing abnormal methemoglobin with normal hemoglobin.
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