Abstract
AN ABNORMALLY INCREASED concentration of methemoglobin in the blood (methemoglobinemia) occurs whenever there is an imbalance between oxidation and reduction of heme iron. Methemoglobin is a form of hemoglobin (the oxygen-carrying pigment of the red blood cells) in which heme iron has been oxidized to the ferric state, making it unable to transport oxygen (Table 1). Normally, methemoglobin is continuously formed at a slow rate by the oxidation of heme iron to the ferric state and is then enzymatically reduced back to hemoglobin. The enzyme NADH-methemoglobin reductase accounts for nearly all methemoglobin reduction. Other mechanisms for methemoglobin reduction include antioxidants such as vitamin C and glutathione. Methemoglobinemia occurs when more than 1 percent of hemoglobin is oxidized to methemoglobin; cyanosis occurs with methemoglobin levels of approximately 1.5 gm/dl.1 Table 2 shows the normal methemoglobin concentrations for infants of various ages.
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