Curing the 'nitrate blues'.

Methemoglobinemia should always be considered in the differential diagnosis of cyanosis in patients with normal arterial oxygen tension. Cyanosis that is brownish and not relieved by oxygen administration should lead to consideration of the diagnosis. A normal or elevated PaO2 value further suggests the presence of methemoglobinemia. Mild cases can be treated by removing the offending agent. In more severe cases, intravenous methylene blue and, if necessary, packed red cells or exchange transfusions may be given.