Clinical Manifestations of Methemoglobinemia The clinical manifestations associated with methemoglobinemia usually correlated with the methemogobin concentration and the percentage of total hemoglobin. Initially, patients typically develop a grayish-brown or blue cyanosis, which usually develops when methemoglobin concentrations exceed 10 to 15%. Patients characteristically develop brownish lips and mucous membranes and are often referred to as having "chocolate cyanosis." In patients with no underlying lung disease or co-morbid conditions, additional symptoms of methemoglobinemia generally arise only when the concentration reaches 20 to 30%, though symptoms can develop at lower levels, particularly in anemic patients, since their oxygen carrying capacity is already compromised. Prominent symptoms of methemoglobinemia, such as headache, tachycardia, fatigue, weakness, dizziness, and dyspnea, are usually noted at concentrations around 20 to 45%. Levels greater than 45% lead to acidosis, cardiac dysrhythmias, heart failure, seizures, and coma. As methemoglobin concentrations increase, intravascular hemolysis and jaundice may also develop. Mortality rates become high when levels surpass 70%. Typically, the more rapid the methemoglobin levels rise, the more severe the symptoms. Diagnosis of Methemoglobinemia Several key findings point toward the diagnosis of methemoglobinemia. First, the diagnosis of methemoglobinemia should be suspected in a patient who appears cyanotic but has a normal PaO2 on arterial blood gas. Second, the blood of a patient with methemoglobinemia will typically appear dark brown or chocolate-colored and will not turn red on exposure to air; normal oxygenated blood is red, deoxygenated blood blue, and blood that contains abnormal amounts of methemoglobin is a dark red-to brown color. Third, although pulse oximetry is not reliable for monitoring oxygen saturation in patients with methemoglobinemia, the diagnosis should be suspected in any patient with a "saturation gap" (an oxygen saturation that is significantly less on pulse oximetry than that calculated from the PaO2 on the arterial blood gas). Fourth, oxygen saturation on pulse oximetry generally decreases to a plateau around 85% in patients with methemoglobinemia, even with severe tissue hypoxemia. Levels of methemoglobin can be measured by co-oximetry; the use of co-oximetry is an accurate way to diagnose methemoglobinemia because co-oximetry measures light at 4 different wavelengths and can determine the concentration of methemoglobin compared to hemoglobin. Fresh specimens of blood should be used for this analysis since levels of methemoglobin often increase with storage. Management of Methemoglobinemia Patients with methemoglobinemia do not respond to high concentrations of oxygen because methemoglobin is an ineffective oxygen transporter. The mainstay of treatment is to discontinue the offending agent and administer intravenous methylene blue 1 to 2 mg/kg (as a 1% solution) for one dose given over five minutes, which may be repeated after 1 hour if symptoms persist. Methylene blue acts as a cofactor for the enzyme NADPH-methemoglobin reductase. The drug accepts an electron from NADPH, which converts it to leukomethylene blue, which then donates an electron to methemoglobin, thus reducing it back to hemoglobin. Treatment with methylene blue is usually considered if methemoglobin levels are above 30% in an asymptomatic patient or above 20% in a symptomatic patient. If, however, the patient has concurrent anemia and is symptomatic at concentrations lower than 20%, treatment is warranted. Adverse Effects of Methylene Blue Potential side effects of methylene blue include dyspnea, chest pain, hemolysis, nausea, diarrhea, oral dysesthesias, restlessness, tremor, apprehension, pre-cordial pain, and a bluish-gray discoloration of the skin. Methylene blue can cause paradoxical worsening of hemolytic anemia or methemoglobinemia in patients with G6PD deficiency (these patients do not have adequate levels of NADPH to reduce methylene blue to leukomethylene blue). For this reason, reports have warned that methylene blue may be ineffective or dangerous in patients with G6PD deficiency, and some experts recommend avoiding the use of methylene blue in patients with G6PD deficiency. Response to Methylene Blue Generally, patients have a rapid improvement in symptoms and a decrease in methemoglobin concentration. After methylene blue administration the methemoglobin concentration should be rechecked and methylene blue therapy discontinued once the methemoglobin concentration is less than 10% and the patient is asymptomatic. In addition, administering dextrose may be important since glycolysis is the major source of NADPH in erythrocytes, and NADPH is necessary for methylene blue to be effective. Hypoglycemic patients and even euglycemic patients should probably receive dextrose infusions to ensure that NADPH levels are sufficient. Although no clear recommendations exist for the management of methemoglobinemia refractory to methylene blue, possible strategies include use of exchange transfusion, hyperbaric oxygen therapy, and cimetidine
Posted on: Wed, 04 Sep 2013 06:14:14 +0000
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