Methemoglobinemia: Clinical Essentials
Methemoglobinemia occurs when the iron in heme is oxidized from the ferrous state (Fe2+) to the ferric state (Fe3+). This form of hemoglobin cannot bind oxygen effectively and increases the affinity of the remaining heme sites for oxygen, shifting the curve to the left and causing tissue hypoxia.
| Feature | Description |
|---|---|
| Classic Presentation | “Chocolate-colored” blood; cyanosis that does not improve with supplemental oxygen. |
| Common Triggers | Nitrates/nitrites, local anesthetics (e.g., benzocaine), dapsone, sulfonamides. |
| Treatment | Methylene blue (or Vitamin C as an alternative). |
High-Yield Core Realities:
- Pulse Oximetry Caveat: SpO2 readings are often falsely “stuck” near 85% because methemoglobin absorbs light at both 660 nm and 940 nm.
- Biochemical Note: Individuals with G6PD deficiency are at higher risk for oxidative stress-induced methemoglobinemia because they lack the necessary NADPH to reduce methemoglobin back to hemoglobin.