Carbon monoxide poisoning causes a leftward shift of the oxyhemoglobin dissociation curve. Which option correctly describes this effect?

Carbon monoxide poisoning causes a leftward shift of the oxyhemoglobin dissociation curve. Which option correctly describes this effect?

• A. Rightward shift
• B. Leftward shift
• C. No shift
• D. Upward shift

Answer: (B)

The main idea here is that carbon monoxide binds to hemoglobin with very high affinity, forming carboxyhemoglobin, which makes the remaining hemoglobin sites hold onto oxygen more tightly. This makes the oxyhemoglobin dissociation curve shift to the left. A leftward shift means increased affinity for oxygen and reduced release of O2 to tissues, so even with adequate arterial oxygen content, tissues can become hypoxic. The shift is reflected by a decreased P50, indicating you need a lower PO2 to saturate hemoglobin. So the effect described is a leftward shift. Rightward shift would imply easier oxygen unloading (decreased affinity), which CO poisoning does not cause; upward shift isn’t the standard way this curve is described.

The main idea here is that carbon monoxide binds to hemoglobin with very high affinity, forming carboxyhemoglobin, which makes the remaining hemoglobin sites hold onto oxygen more tightly. This makes the oxyhemoglobin dissociation curve shift to the left. A leftward shift means increased affinity for oxygen and reduced release of O2 to tissues, so even with adequate arterial oxygen content, tissues can become hypoxic. The shift is reflected by a decreased P50, indicating you need a lower PO2 to saturate hemoglobin. So the effect described is a leftward shift. Rightward shift would imply easier oxygen unloading (decreased affinity), which CO poisoning does not cause; upward shift isn’t the standard way this curve is described.