I. Pathophysiology & diagnosis
- 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.
- 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
- Degree of MetHb is expressed in % of total Hb
- 1-2%= normal
- >15% get sx (DOE, HR, sl. HTN, HA
- > 35% also get met. acidosis, BP, agitation, sz, coma, death
- 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:
- Local anesthetics
III. Tx = methylene blue (accelerates reduction of MetHb to Hb), but can causes severe hemolysis in pts w/G6PD!