Tetramethylthionine Chloride (Methylene Blue, Urolene Blue)

Anesthesia Implications

Classification: Monoamine Oxidase Inhibitor (MAOI)
Therapeutic Effects: Injectable dye, Vasoconstriction

Contraindications

Patients with G6PD deficiency (may cause hemolysis)

Primary Considerations

This drug is most often used as a dye to detect damage to unintended (usually urinary) structures during surgery.

Methylene blue is also used to fill the ETT cuff.  This allows early detection of a ruptured cuff in cases where lasers are being used in close proximity to the ETT.

IV injection of this drug results in a significant but transient drop in SpO2 (pulse oximeter).  This is NOT a true drop – the dye simply interferes with the absorption of infrared light – which alters the ability of the pulse oximeter to correctly read the SpO2.

Can be used to treat methemoglobinemia, cyanide toxicity, and vasoplegic syndrome (second-line treatment)

This drug can theoretically precipitate serotonin toxicity or serotonin syndrome if the patient is has also received SSRI’s. The doses typically have to exceed 5 mg/kg in order to see these problems. As you can see, the doses for typical treatment are well below those thresholds, but it is something to rule out if signs or symptoms should appear.

IV push dose

Treatment of Methemoglobinemia: 1-2 mg/kg (over 5 minutes)
Treatment of Cyanide Toxicity: 1-2 mg/kg (over 5 minutes)
Treatment of Vasoplegic Syndrome: 1-2 mg/kg bolus (over 5 minutes) followed by an infusion of 0.25 mg/kg/hr for 48-72 hours.
Max dose: 7mg/kg

Method of Action

Believed to interfere with the nitric oxide cyclic guanylate monophosphate (cGMP) pathway – which inhibits its vasorelaxant affect on smooth muscle