Methemoglobinemia may pose problems during anesthesia, especially if the diagnosis is unsuspected. Hemoglobin contains iron in the ferric state, and hemoglobin is unable to bind oxygen. In addition, there is a shift in the oxyhemoglobin dissociation curve with reduced release of oxygen in the tissues,


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


Methemoglobin Concentration

Clinical Findings

> 15%

brownish discoloration of the blood

30 - 40%

headache, weakness, dyspnea, tachycardia, dizziness

> 50%

altered mental consciousness, seizures and circulatory collapse


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)



(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.


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