A 25-year-old unconscious patient has PaO2 190 mmHg with an SpO2 of 90%. The most likely explanation is the presence of which factor?

A 25-year-old unconscious patient has PaO2 190 mmHg with an SpO2 of 90%. The most likely explanation is the presence of which factor?

• A. Methemoglobinemia
• B. Sickle cell hemoglobin
• C. Carboxyhemoglobinemia
• D. Hemoglobin shifted to the right

Answer: (C)

PaO2 reflects the amount of dissolved oxygen in the blood, not how much oxygen the hemoglobin is carrying. When PaO2 is high but the saturation measured by pulse oximetry is reduced, it points to a dyshemoglobinemia where hemoglobin’s ability to bind or release oxygen is impaired. Carbon monoxide binding to hemoglobin to form carboxyhemoglobin fits this pattern: CO binds tightly to hemoglobin, drastically reducing the oxygen-carrying capacity and shifting the dissociation curve to the left, so oxygen is not readily released to tissues. Oxygen tension in the血—reflected by PaO2—can be normal or even high, especially with supplemental oxygen, yet tissue oxygen delivery remains inadequate, which helps explain unconsciousness. Pulse oximetry, which estimates saturation without distinguishing different forms of hemoglobin, can give an apparently misleading reading in CO poisoning, often around a normal or near-normal value, even though tissues are hypoxic. Co-oximetry would reveal elevated carboxyhemoglobin levels, confirming the diagnosis. Treat with 100% oxygen and consider hyperbaric oxygen in severe cases to rapidly displace CO from hemoglobin and restore oxygen delivery.

PaO2 reflects the amount of dissolved oxygen in the blood, not how much oxygen the hemoglobin is carrying. When PaO2 is high but the saturation measured by pulse oximetry is reduced, it points to a dyshemoglobinemia where hemoglobin’s ability to bind or release oxygen is impaired. Carbon monoxide binding to hemoglobin to form carboxyhemoglobin fits this pattern: CO binds tightly to hemoglobin, drastically reducing the oxygen-carrying capacity and shifting the dissociation curve to the left, so oxygen is not readily released to tissues. Oxygen tension in the血—reflected by PaO2—can be normal or even high, especially with supplemental oxygen, yet tissue oxygen delivery remains inadequate, which helps explain unconsciousness. Pulse oximetry, which estimates saturation without distinguishing different forms of hemoglobin, can give an apparently misleading reading in CO poisoning, often around a normal or near-normal value, even though tissues are hypoxic. Co-oximetry would reveal elevated carboxyhemoglobin levels, confirming the diagnosis. Treat with 100% oxygen and consider hyperbaric oxygen in severe cases to rapidly displace CO from hemoglobin and restore oxygen delivery.