Symptoms – Symptoms typically include sudden-onset of hypoxia and cyanosis that is resistant to high-flow oxygen in the absence of cardiac or pulmonary disease. Cyanosis without a cardiorespiratory change is the cardinal sign.

SpO2 – In patients with methemoglobinemia, the pulse oximeter (SpO2) will read 80-85%, irrespective of the SaO2.

Clinical Presentation:
15% methemoglobin: A grey-brown appearance will appear
20-30% methemoglobin: fatigue, headache, tachycardia, dizziness, and weakness
45% methemoglobin: dyspnea, bradycardia, hypoxia, acidosis, siezures, coma, cardiac arrhythmias.
70% methemoglobin: rapid fatality

Check blood levels – Suspected patients should have blood levels checked promptly after the draw as methemoglobin levels are unstable in the drawn sample and can exhibit falsely low levels if too much time has passed.

Treatment – 100% O2. IV methylene blue 1% solution (1–2 mg/kg) over 5 minutes. If no response over 1 hour, the dose may be repeated. Caution: greater than 7 mg/kg can, by itself, cause methemoglobinemia.

Other treatments – Ascorbic acid, IV vitamin C, Blood transfusion

This problem is VERY RARE

There are 4 types of hemoglobin:
1. Oxyhemoglobin (oxygenated hemoglobin)
2. Deoxyhemoglobin (hemoglobin without oxygen)
3. Carbaminohemoglobin (hemoglobin transporting carbon dioxide)
4. Oxidized/methemoglobin (oxidized hemoglobin)

Most hemoglobin is in the ferrous state (Fe+2). Oxidized hemoglobin is in the ferric state (Fe+3). This ferric state (oxidized) makes hemoglobin incapable of carrying oxygen.

Normally, this ferric state makes up less than 1% of the total blood concentration of hemoglobin. Methemoglobin reductase maintains this balance. Therefore, a disruption in this system will cause an imbalanced level of oxidized hemoglobin.

May be caused by:
Prilocaine metabolites (most common – usually appearing after a dose of greater than or equal to 8 mg/kg)
Nitric oxide at higher concentrations for prolonged periods of time
Nitroglycerin (especially in patients with reductase deficiency)
Lidocaine
Benzocaine – typically when applied to infants as an ointment, as a rectal suppository, or in the perineal area. Also implied as a causative factor after lubricant of the ET tube, bronchoscopic tubes, and esophageal tubes.
There is an enormous list of drugs that have been associated with this condition, but those most notable have been listed above.

The inherited form is when a patient is born with a deficiency in methemoglobin reductase.

The acquired form comes as a result of exposure to a chemical which oxidizes ferrous iron in the hemoglobin at a rate that overwhelms the balancing affects of methemoglobin reductase.

In either form, oxygen carrying capacity of the blood is severely reduced due to high levels of methemoglobin in the blood.