The primary role of hemoglobin in blood circulation is to deliver oxygen from lungs to tissues. Hemoglobin molecules comprise four polypeptide chains, e.g. two α and two β chains in hemoglobin A, with a heme group attached to each chain. Each heme group contains one iron atom, which can bind one oxygen molecule. Hemoglobin carrying oxygen molecule, known as oxyhemoglobin, gives out oxygen to tissue cells, and becomes deoxyhemoglobin, which can then load oxygen again. It is critical for the iron atom to remain in reduced state as ferrous (Fe2+) to bind to oxygen. Under oxidative stress, one or more iron atoms are oxidized to ferric (Fe3+) to form a brown color pigment, known as methemoglobin. Methemoglobin is unable to bind oxygen, and elevated level of methemoglobin or methemoglobinemia, can lead to cyanosis and be life-threatening.
A small amount of Fe2+ oxidizes to Fe3+ everyday but at the same time, methemoglobin converts back to hemoglobin through reducing activity of cytochrome b5-reductase. This maintains blood methemoglobin below 1% in normal circumstance. Methemoglobinemia can occur as inherited, mainly due to genetic defects of cytochrome b5-reductase, or as acquired, due to insufficient cytochrome b5-reductase activity under induced oxidative stress. Inherited methemoglobinemia is rare and patient may present mildly cyanotic and asymptomatic. Acquired methemoglobinemia can present acutely, more severe, and is most commonly encountered as a result of exposure to drugs and oxidant chemicals, such as local anesthetics, phenacetin, dapsone, and nitrites. FDA has released multiple warnings on the use of benzocaine-containing products, for its risk of causing serious, life-threatening methemoglobinemia. Benzocaine was found to be the causative agent of local anesthetic-related methemoglobinemia in two-thirds of cases.
Methemoglobin can be detected and quantified in clinical laboratories with co-oximetry, which is capable of measuring absorbance at multiple wavelengths. The dual-wavelength pulse oximetry only measures oxyhemoglobin and deoxyhemoglobin and is unreliable in the setting of methemoglobinemia. The oxygen saturation calculated from partial pressure of oxygen is also unreliable, because it assumes a normal oxygen dissociation curve.
A case of an 88 year old male who underwent neurosurgery was found to have hypoxemic respiratory failure and subsequently developed acute pulmonary embolism. A blood sample was delivered to the laboratory for blood gas analysis and was soon noticed by a medical student that the blood has a dark brown color. The abnormal observation was discussed with the clinical team and a methemoglobin test was added on the specimen. Result of methemoglobin was as high as 48.6%, while oxygen saturation was falsely normal. Immediate treatment of methylene blue was initiated, and the treatment worked effectively and rapidly. A few hours after treatment, methemoglobin level was reduced to 3.3%. Upon reviewing patient’s history, it was noted that patient was given lidocaine for 3 days post-surgery, which may be the cause of methemoglobinemia. It is important to note that methemoglobinemia is a serious condition and can be fatal. Clinical diagnosis based on symptoms and history, as well as laboratory confirmation, are critical for timely management of methemoglobinemia.
-Xin Yi, PhD, DABCC, FACB, is a board-certified clinical chemist, currently serving as the Co-director of Clinical Chemistry at Houston Methodist Hospital in Houston, TX and an Assistant Professor of Clinical Pathology and Laboratory Medicine at Weill Cornell Medical College.