In carbon monoxide poisoning, tissue hypoxia occurs primarily because carboxyhemoglobin reduces oxygen delivery by causing which of the following?

In carbon monoxide poisoning, tissue hypoxia occurs primarily because carboxyhemoglobin reduces oxygen delivery by causing which of the following?

• A. Increased O2 unloading
• B. Leftward shift of oxyhemoglobin dissociation curve
• C. Decreased Hb affinity for O2
• D. Increased PaO2

Answer: (B)

In carbon monoxide poisoning, the crucial factor for tissue oxygen delivery is how readily hemoglobin releases oxygen to the tissues, not just how much oxygen is carried. CO binds to hemoglobin with high affinity to form carboxyhemoglobin, which lowers the overall oxygen-carrying capacity. But it also causes a leftward shift of the oxyhemoglobin dissociation curve, so the remaining hemoglobin holds onto oxygen more tightly and is less willing to release it to tissues. This impaired unloading leads to tissue hypoxia even when arterial oxygen tension might be normal. The other possibilities would either make unloading easier, increase dissolved oxygen, or both, which would not explain the hypoxia seen in CO poisoning.

In carbon monoxide poisoning, the crucial factor for tissue oxygen delivery is how readily hemoglobin releases oxygen to the tissues, not just how much oxygen is carried. CO binds to hemoglobin with high affinity to form carboxyhemoglobin, which lowers the overall oxygen-carrying capacity. But it also causes a leftward shift of the oxyhemoglobin dissociation curve, so the remaining hemoglobin holds onto oxygen more tightly and is less willing to release it to tissues. This impaired unloading leads to tissue hypoxia even when arterial oxygen tension might be normal. The other possibilities would either make unloading easier, increase dissolved oxygen, or both, which would not explain the hypoxia seen in CO poisoning.