METHEMOGLOBINEMIA


I. Pathophysiology & diagnosis

  1. MetHb is produced when the ferrous (2+) iron of Hb is oxidized to the ferric (3+) state: produces a functional anemia b/c can't carry O2; also causes O2-Hb dissociation curve to shift to L.
  2. MetHb is dark-brown & causes cyanosis without abnormal decrease in arterial pO2 (except with severe anemia); dx is on this basis & other signs of O2 delivery
  3. Degree of MetHb is expressed in % of total Hb
  1. Critically ill pts do worse with MetHb b/c already have decreased O2 delivery due to secondary anemia, hypoxemia, peripheral vascular disease, etc.

II. Drugs which can cause MetHb:

  1. Nitroglycerin
  2. Sulfonamides
  3. Local anesthetics
  4. Dapsone
  5. Pyridium
  6. Antimalarials

III. Tx = methylene blue (accelerates reduction of MetHb to Hb), but can causes severe hemolysis in pts w/G6PD!