Hemoglobins with Altered Oxygen Affinity (Ref. Hari. 18th ed., Pg - 857)
- High-affinity hemoglobins [e.g., Hb Yakima bind oxygen more readily but deliver less O2 to tissues at normal capillary PO2 levels.
- Low-affinity hemoglobins [e.g., Hb Kansas bind sufficient oxygen in the lungs, despite their lower oxygen affinity, to achieve nearly full saturation.
- At capillary oxygen tensions, they lose sufficient amounts of oxygen to maintain homeostasis at a low hematocrit (pseudoanemia).
- Capillary hemoglobin desaturation can also be sufficient to produce clinically apparent cyanosis.
- Despite these findings, patients usually require no specific treatment.
- The two most important acquired hemoglobinopathies are carbon monoxide poisoning and methemoglobinemia. Carbon monoxide has a higher affinity for hemoglobin than does oxygen.
- It can replace oxygen and diminish O2 delivery. Chronic elevation of carboxyhemoglobin levels to 10 or 15%, as occurs in smokers, can lead to secondary polycythemia.
- Carboxyhemoglobin is cherry red in color and masks the development of cyanosis usually associated with poor O2 delivery to tissues.
- The superconducting quantum-interference device (SQUID) is accurate at measuring hepatic iron
- Methemoglobin should be suspected in patients with hypoxic symptoms who appear cyanotic but have a PaO2 sufficiently high that hemoglobin should be fully saturated with oxygen.
- A history of nitrite or other oxidant ingestions may be available. The characteristic muddy appearance of freshly drawn blood can be a critical clue.
- The best diagnostic test is methemoglobin assay.
- Methemoglobinemia often causes symptoms of cerebral ischemia at levels >15%; levels >60% are usually lethal. Intravenous injection of 1 mg/kg of methylene blue is effective emergency therapy.
- Milder cases can be treated orally with methylene blue or ascorbic acid.