In carbon monoxide poisoning, pulse oximetry readings can be misleading; which statement is true?

In carbon monoxide poisoning, pulse oximetry readings can be misleading; which statement is true?

• A. Cyanosis is typically present
• B. PaO2 is low despite a normal O2 saturation
• C. Pulse oximetry shows normal saturation despite CO poisoning
• D. The hemoglobin affinity for oxygen is unchanged

Answer: (C)

In carbon monoxide poisoning the problem isn’t how much oxygen is dissolved in the blood, but how much oxygen the hemoglobin can actually carry and release to tissues. CO binds to hemoglobin with very high affinity, forming carboxyhemoglobin, which both reduces the total oxygen-carrying capacity and shifts the oxyhemoglobin dissociation curve to the left. That leftward shift means hemoglobin holds onto oxygen more tightly and delivers less to the tissues, contributing to tissue hypoxia even when the arterial oxygen tension appears normal. PaO2 can stay normal because it reflects dissolved oxygen in plasma, not how much is bound to hemoglobin. Pulse oximetry, on the other hand, estimates arterial oxygen saturation by light absorption and assumes only oxyhemoglobin and deoxyhemoglobin. It cannot distinguish carboxyhemoglobin from oxyhemoglobin, so it can report a normal or falsely high saturation despite CO poisoning. That’s why pulse oximetry readings can be misleading in this scenario. Cyanosis isn’t a reliable early indicator in CO poisoning, and the hemoglobin affinity for oxygen is actually increased (not unchanged) due to CO binding, contributing to reduced oxygen delivery to tissues.

In carbon monoxide poisoning the problem isn’t how much oxygen is dissolved in the blood, but how much oxygen the hemoglobin can actually carry and release to tissues. CO binds to hemoglobin with very high affinity, forming carboxyhemoglobin, which both reduces the total oxygen-carrying capacity and shifts the oxyhemoglobin dissociation curve to the left. That leftward shift means hemoglobin holds onto oxygen more tightly and delivers less to the tissues, contributing to tissue hypoxia even when the arterial oxygen tension appears normal.

PaO2 can stay normal because it reflects dissolved oxygen in plasma, not how much is bound to hemoglobin. Pulse oximetry, on the other hand, estimates arterial oxygen saturation by light absorption and assumes only oxyhemoglobin and deoxyhemoglobin. It cannot distinguish carboxyhemoglobin from oxyhemoglobin, so it can report a normal or falsely high saturation despite CO poisoning. That’s why pulse oximetry readings can be misleading in this scenario.

Cyanosis isn’t a reliable early indicator in CO poisoning, and the hemoglobin affinity for oxygen is actually increased (not unchanged) due to CO binding, contributing to reduced oxygen delivery to tissues.