Hematology: Practice Question

A 62 year old alcoholic passed out on his couch and was rescued from a house fire by the fire department. He was unconscious, but did not suffer any burns. Which of the following is most likely true regarding his blood?
  1. It has an increased affinity for oxygen.
  2. It is green colored and has Heinz bodies.
  3. Ferric iron (Fe 3+) is replacing normal ferrous iron (2+)
  4. It contains arsenic
  5. It has decreased erythropoietin
Show Answer and Explanations

It has an increased affinity for oxygen. — Correct answer

Correct. Carbon monoxide greatly increases the affinity of hemoglobin for oxygen, left shifting the oxygen affinity curve, so that oxygen is not appropriately delivered to the tissues. It acts even more strongly on Hbg F than Hgb A, so infants and patients with increased Hbg F (beta thalassemia, hereditary persistence of fetal hemoglobin etc.), are particularly prone its toxic effect. Patients may be a cherry red color.

It is green colored and has Heinz bodies.

Incorrect. Green-colored blood and Heinz bodies are not features of carbon monoxide poisoning. Heinz bodies are denatured hemoglobin precipitates seen in oxidative hemolysis (such as G6PD deficiency) and sulfhemoglobinemia. Green-colored blood suggests sulfhemoglobinemia or other rare hemoglobin abnormalities. In CO poisoning, patients may appear cherry red due to carboxyhemoglobin, not cyanotic or green.

Ferric iron (Fe 3+) is replacing normal ferrous iron (2+)

Incorrect. Methemoglobinemia involves oxidation of hemoglobin iron from Fe²⁺ (ferrous) to Fe³⁺ (ferric), which cannot bind oxygen. This is distinct from carbon monoxide poisoning, where CO binds to normal ferrous hemoglobin (Fe²⁺). Methemoglobinemia is caused by oxidizing agents including nitrates, antibiotics (trimethoprim, dapsone, sulfonamides), quinones, and local anesthetics (benzocaine, lidocaine). Infants are particularly susceptible due to lower levels of methemoglobin reductase (NADH-cytochrome b5 reductase). In this house fire scenario, CO poisoning is the correct diagnosis, not methemoglobinemia.

It contains arsenic

Incorrect. Arsenic poisoning is not associated with house fire exposure. Arsenic can cause acute hemolytic anemia, and chronic exposure causes microcytic hypochromic anemia (as with other heavy metal toxicities). In this clinical scenario of a house fire victim, carbon monoxide poisoning is the appropriate diagnosis, not arsenic toxicity.

It has decreased erythropoietin

Incorrect. This option is incorrect for the acute house fire scenario. While chronic low-level carbon monoxide exposure can stimulate erythropoietin production (leading to secondary polycythemia as a compensatory response to chronic tissue hypoxia), this is not relevant to acute CO poisoning. In acute CO poisoning from a house fire, the primary pathophysiology involves CO binding to hemoglobin with increased oxygen affinity and impaired oxygen delivery to tissues.

Summary

Summary: Carbon monoxide poisoning occurs from inhalation of CO-rich smoke in house fires. CO binds to hemoglobin with high affinity, reducing oxygen-carrying capacity and causing a leftward shift of the oxygen-hemoglobin dissociation curve (increased oxygen affinity). This increased affinity impairs oxygen release to tissues, causing tissue hypoxia despite potentially normal oxygen saturation readings. HbF (fetal hemoglobin) is particularly sensitive to CO effects, making fetuses and infants more vulnerable. Patients may appear cherry red due to carboxyhemoglobin.
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