Pediatric emergency medicine trisk 2199 2199

TABLE 93.4
SYMPTOMS ASSOCIATED WITH MHB LEVELS
Methemoglobin % Total
concentration hemoglobin
(g/dL)

Symptoms (assuming Hb 15 g/dL)

<1.5

<10

None

1.5–3

10–20

Cyanosis

3–4.5

20–30

Anxiety, light-headedness, headache, tachycardia

4.5–7.5

30–50

Fatigue, confusion, dizziness, tachypnea, increased

tachycardia

7.5–10.5

50–70

Coma, seizures, arrhythmias, acidosis

>10.5

>70

Death

Reprinted with permission from Wright RO, Lewander WJ, Woolf AD. Methemoglobinemia: etiology, pharmacology,
and clinical management. Ann Emerg Med 1999;34:646–656.

Management/Diagnostic Testing
The treatment of methemoglobinemia depends on the clinical severity. In all cases,
attempt to identify and remove the causative oxidant stress. If symptoms are mild after
oxidant exposure, therapy may be unnecessary. RBCs with normal metabolism will
reduce the MHb in several hours. In general, treat patients with MHb level >20% with 1
to 2 mg/kg of methylene blue as a 1% solution in saline infused intravenously over 5
minutes. Administer a second dose if symptoms are still present 1 hour later. Patients
with a significant concurrent medical condition, especially cardiopulmonary conditions,
should be considered for treatment at MHb levels starting at 10%. Methylene blue is an
oxidant at high dosages, so total dosage should not exceed 7 mg/kg to avoid paradoxical
methemoglobinemia.
Use methylene blue with extreme caution in patients with G6PD due to the risk of
hemolytic anemia. The mechanism of action of methylene blue relies on NADPH. These

patients may not produce sufficient quantities of NADPH to respond to this therapy;
however, some patients have partial enzyme activity. Methylene blue at lower doses (0.3
to 0.5 mg/kg/dose) may lower MHb levels without causing significant hemolytic
anemia. The addition of ascorbic acid 5 to 8 mg/kg/day may benefit G6PD patients.
Consider exchange transfusion in patients who fail methylene blue treatment or have
absent G6PD activity.
Clinical Indications for Discharge or Admission
Even if treatment with methylene blue or ascorbic acid in the ED is successful, admit
any child with symptomatic methemoglobinemia to the hospital for close observation,
especially if the etiology is unknown. Some oxidizing agents such as dapsone and