A patient’s baseline ABG shows PaO2 of 67 mm Hg while receiving 40% FiO2. To achieve a target PaO2 of 80 mm Hg, what FiO2 should be set?

• A. 0.50
• B. 0.30
• C. 0.60
• D. 0.47

Answer: (D)

Understanding how FiO2 affects PaO2 helps you estimate how much to raise FiO2 to reach a target oxygen level. In the moderate range, PaO2 roughly scales with FiO2, so you can use the PaO2/FiO2 ratio as a rough guide.

Here, baseline PaO2 is 67 mmHg on FiO2 0.40, giving a PaO2/FiO2 ratio of about 67 / 0.40 = 167.5. To reach a target PaO2 of 80 mmHg with the same relationship, the required FiO2 is 80 / 167.5 ≈ 0.478, i.e., about 0.48. The closest option is 0.47.

This is a quick estimate; after adjusting FiO2, confirm with another ABG since factors like shunt, diffusion, and V/Q mismatch can affect the actual response.