Which factor could NOT explain a PaO2 of 48 mm Hg in a patient breathing a mixture of nitrous oxide and oxygen?

Which factor could NOT explain a PaO2 of 48 mm Hg in a patient breathing a mixture of nitrous oxide and oxygen?

• A. Hypoxic gas mixture
• B. Eisenmenger syndrome
• C. Profound anemia
• D. Hypercarbia

Answer: (C)

Oxygen tension in arterial blood (PaO2) reflects the amount of oxygen dissolved in the blood and how well it is transferred from the alveoli. It depends on the inhaled oxygen fraction (FiO2) and the effectiveness of gas exchange. A PaO2 of 48 mm Hg while breathing a nitrous oxide–oxygen mix can be explained by a hypoxic gas mixture (low FiO2), a right-to-left shunt such as Eisenmenger syndrome, or hypoventilation with hypercarbia that lowers alveolar oxygen tension (PAO2), all of which reduce the amount of O2 diffusing into blood. Profound anemia, however, lowers the blood’s oxygen-carrying capacity (CaO2) because there is less hemoglobin available to carry oxygen, but it does not directly decrease PaO2. So severe anemia would not explain a low PaO2; it would cause reduced oxygen content rather than reduced oxygen tension.

Oxygen tension in arterial blood (PaO2) reflects the amount of oxygen dissolved in the blood and how well it is transferred from the alveoli. It depends on the inhaled oxygen fraction (FiO2) and the effectiveness of gas exchange. A PaO2 of 48 mm Hg while breathing a nitrous oxide–oxygen mix can be explained by a hypoxic gas mixture (low FiO2), a right-to-left shunt such as Eisenmenger syndrome, or hypoventilation with hypercarbia that lowers alveolar oxygen tension (PAO2), all of which reduce the amount of O2 diffusing into blood. Profound anemia, however, lowers the blood’s oxygen-carrying capacity (CaO2) because there is less hemoglobin available to carry oxygen, but it does not directly decrease PaO2. So severe anemia would not explain a low PaO2; it would cause reduced oxygen content rather than reduced oxygen tension.