A history of methemoglobinemia may or may not be known at the time of surgery.
Clinical features
(1) cyanosis (when methemoglobin is 5-15%)
(2) decrease in oxygen saturation on pulse oximetry
(3) decrease in oxygen saturation on pulse oximetry persisting after increase in inspired oxygen concentration
(4) normal PaO2 and adequate ventilation
(5) elevated methemoglobin on co-oximetry
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Methemoglobin Concentration
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Clinical Findings
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> 15%
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brownish discoloration of the blood
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30 - 40%
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headache, weakness, dyspnea, tachycardia, dizziness
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> 50%
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altered mental consciousness, seizures and circulatory collapse
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Anesthetic agents which may precipitate the reaction:
(1) benzocaine and related local anesthetics (contain nitrates)
(2) nitrous oxide
Other medications:
(1) aspirin and acetaminophen
(2) lidocaine
(3) fentanyl
(4) phenobarbital
(5) nitroglycerine
(6) antibiotics (TMP-SMX, ciprofloxacin, others)
Management:
(1) Discontinue the inciting drug.
(2) For a patient without glucose 6-phosphate dehydrogenase deficiency, infuse methylene blue 1 mg/kg IV over 5 minutes. This can be repeated every hour if cyanosis persists, with the maximum cumulative dose 7 mg/kg.
