If the methemoglobin concentration is <= 30%, discontinuation of causative exposure is usually sufficient. The goal of therapy with congenital methemoglobinemia may be cosmesis.
Slower acting agents that may be useful for noncritical exposures:
(1) oral ascorbic acid: 300 - 500 mg per day
(2) oral riboflavin: 20 mg per day
(3) oral methylene blue: 100 - 300 mg per day
Therapy with intravenous methylene blue infusion is indicated in
(1) symptomatic patients with methemoglobin concentration > 30%
(2) symptomatic patients with methemoglobin concentrations <= 30% and comorbid conditions affecting coronary or cerebral perfusion
Therapy with methylene blue is not indicated in patients with:
(1) glucose 6-phosphate dehydrogenase deficiency, since NADPH is required for the reductase involved.
(2) NADPH methemoglobin reductase deficiency
(3) hypersensitivity to methylene blue
dose of methylene blue in mg =
= (1 to 2 mg/kg) * (body weight in kilograms)
maximum cumulative dose of methylene blue =
= (7 mg/kg) * (body weight in kilograms
volume of intravenous infusion in mL as 1% solution =
= (mg methylene blue) / 1000 * 100
volume of intravenous infusion in mL as 2% solution =
= (mg methylene blue) / 1000 * 50
where:
• Probably the ideal body weight should be used than actual body weight since that better correlates with blood volume.
• The infusion should be in normal saline.
• The infusion is administered over 5-10 minutes.
• Above 7 mg/kg methylene blue may act as an oxidant and can cause hemolytic anemia, cyanosis, dyspnea, precordial pain, and other symptoms.
The expected response is that a methemoglobin measurement at 1 hour after infusion should be at least a 50% reduction from the pretreatment level. If the response is considered inadequate, then repeat infusions may be given up to the maximum cumulative dose.
If exposure to the inciting agent continues (for example, following an ingestion), then continued monitoring and repeat therapy may be required.
In severe methemoglobinemia exchange transfusion may be considered but usually is not required.
