Chapter 51: METHEMOGLOBINEMIA

YOUR PATIENT

A 4-year-old undergoes an uneventful esophagogastroduodenoscopy (EGD) but is noted to be cyanotic in the postanesthesia care unit 1 hour later. He appears agitated and tachypneic, and has perioral cyanosis. Despite being placed on 100% oxygen by face mask, his SaO₂ is 86%. His lungs are clear, and his stat chest x-ray is normal. An arterial blood sample is chocolate-colored. PaO₂ and calculated oxygen saturation are normal. Cetacaine spray (14% benzocaine, 2% butamben, 2% tetracaine) was used for topicalization.

PREOPERATIVE CONSIDERATIONS

Patients at higher risk of symptomatic methemoglobinemia include children less than 4 months of age; patients with methemoglobin reductase deficiency (more likely in Native Americans of Alaska or Inuit descent), G6PD deficiency, or cardiopulmonary disease; and elderly and anemic patients.

There should be high suspicion for methemoglobinemia in a cyanotic patient who has been exposed to any of the following: nitrates (nitroglycerin, nitric oxide, nitroprusside, silver nitrate, metoclopramide), local anesthetics (benzocaine, prilocaine, lidocaine, EMLA cream), sulfonamides (antimalarials, primaquine, chloroquine), dapsone, and acetaminophen.

Manifestations vary by level of methemoglobin (metHb): cyanosis at 10%; headache, shortness of breath, weakness, fatigue, confusion, tachypnea, and tachycardia at 30%-50%; coma, seizures, arrhythmia, and acidosis at >50%; and fatality at >70%.

ANESTHETIC MANAGEMENT

• Rule out other causes of cyanosis, including airway, pulmonary, and cardiac.

• The pulse oximeter reading plateaus at around 85% when metHb levels are 30% or greater. MetHb absorbs the two wavelengths 660 nm (red) and 940 nm (infrared) in equal amounts, and this results in the calculated saturation of 85%, although this number does not reflect the true oxygen saturation.

• Check arterial blood gases. The PaO₂ should be high or normal because metHb does not affect the amount of oxygen dissolved in the blood. For the same reason, calculated oxygen saturation is reported as normal because this is calculated from the PaO₂.

• Check a true oxygen saturation using co-oximetry. Co-oximetry uses four wavelengths and is able to differentiate metHb, carboxyHb, oxyHb, and deoxyHb.

• MetHb levels >20% should prompt treatment including elimination of the causative agent, maximizing oxygen delivery, and giving methylene blue 1-2 mg/kg IV over 5 minutes (can repeat in 1 hour if the patient is still symptomatic; maximum dose 7 mg/kg).

• Methylene blue also leads to artifacts in the pulse oximetry reading (as low as 65%) due to its absorbance of light in the red range.

• Resolution of methemoglobinemia usually occurs within 20-60 minutes after administration of methylene blue.

POSTOPERATIVE CONSIDERATIONS

Rebound methemoglobinemia can occur up to 20 hours after treatment. Therefore, close observation is warranted and overnight admission should be considered.