Oxidative hemolysis of RBCs can result from exposure to a number of drugs that cause the formation of methemoglobin. These drugs oxidize ferrous hemoglobin (+2) to ferric hemoglobin (+3), which is methemoglobin. Methemoglobin cannot bind oxygen, so the oxygen-carrying capacity of the blood is decreased. A large number of commonly used drugs can cause methemoglobinemia, but not at therapeutic doses (Tab.!e,214-3). Toxic methemoglobinemia occurs when more than 10 percent of the hemoglobin has been oxidized to the ferric form. Clinically, methemoglobinemia should be suspected in patients who are cyanotic without cardiopulmonary disease. This cyanosis is not relieved by oxygen. The venous blood appears chocolate brown. The arterial blood gas will reflect a normal Pa o2, but decreased measured oxygen saturation. Tabje,214:4 shows the clinical effects of acute methemoglobinemia. Levels of methemoglobin greater than 20 to 30 percent of the total hemoglobin should be treated. Methylene blue is given intravenously at a dose of 1 to 2 mg/kg in a 1% solution over 5 min. Methylene blue reduces methemoglobin back to oxygen-carrying hemoglobin through a series of reactions.
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