Methemoglobin (METHB)

I.    DESCRIPTION:
Methemoglobin (METHB) is a condition by which oxygen molecules cannot bind to hemoglobin thus the patient presents with a distinct cyanosis.  The cyanosis occurs due to an  altered hemoglobin ion from the reduced form to the oxidized ferric molecule.  This molecule is not capable of binding to or releasing oxygen.

The  distinct cyanosis described above is a brownish coloration .  This is referred to as “chocolate cyanosis” and it does not respond to increased oxygen ventilation of the cardiopulmonary bypass oxygenator or a increased FiO2 if the patient is in the ICU.  This finding should alert the medical staff to perform a confirmation laboratory test for methemoglobinemia.

Under normal circumstances, methemoglobin in the body does not exceed 1% of the total circulating hemoglobin.  Clinically significant METHB is in excess of 5-10% of the total circulating hemoglobin.  Levels of 1.5 gm/dl of METHB usually present with visible cyanosis.  METHB greater than 20% of the total hemoglobin is associated in the awake patient with headache, dyspnea, and fatigue.

II.    TREATMENT:

A.    Methylene Blue:  1-2 mg/Kg, administered in a 1% solution.  It acts as an electron-transporting substance that induces a 10 fold increase in erythrocyte oxygen consumption.  In the presence of glucose, methylene blue converts methemoglobin rapidly to active hemoglobin.   The clinical response to methylene blue is usually rapid and complete.  Care must be taken not to exceed 7 mg/Kg in patient dosage.

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