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Hemoglobins with Altered Oxygen Affinity (Ref. Hari. 18th ed., Pg - 857)

  1. High-affinity hemoglobins [e.g., Hb Yakima bind oxygen more readily but deliver less O2 to tissues at normal capillary PO2 levels.
  2. Low-affinity hemoglobins [e.g., Hb Kansas bind sufficient oxygen in the lungs, despite their lower oxygen affinity, to achieve nearly full saturation.
    1. At capillary oxygen tensions, they lose sufficient amounts of oxygen to maintain homeostasis at a low hematocrit (pseudoanemia).
    2. Capillary hemoglobin desaturation can also be sufficient to produce clinically apparent cyanosis.
    3. Despite these findings, patients usually require no specific treatment.
Acquired hemoglobinopathies
  1. The two most important acquired hemoglobinopathies are carbon monoxide poisoning and methemoglobinemia. Carbon monoxide has a higher affinity for hemoglobin than does oxygen.
  2. 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.
  3. Carboxyhemoglobin is cherry red in color and masks the development of cyanosis usually associated with poor O2 delivery to tissues.
  4. The superconducting quantum-interference device (SQUID) is accurate at measuring hepatic iron
  5. 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.
  6. A history of nitrite or other oxidant ingestions may be available. The characteristic muddy appearance of freshly drawn blood can be a critical clue.
  7. The best diagnostic test is methemoglobin assay.
  8. 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.
  9. Milder cases can be treated orally with methylene blue or ascorbic acid.

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