Table 2 Symptoms observed at increasing levels of methemoglobin

< 3% (Normal)

None

3–15%

Possibly asymptomatic, low saturations on pulse oximeter, gray skin

15–20%

“Chocolate-brown” blood, cyanosis

20–50%

Dizziness, syncope, headache, dyspnea, exercise intolerance, weakness

50–70%

Central nervous system depression, dysrhythmias, acidosis, seizures, coma

> 70%

Profound hypoxia, death