Which FiO2 corresponds to a target PaO2 of 95 mm Hg when starting from PaO2 78 mm Hg on 35% oxygen?

• A. 0.25
• B. 0.43
• C. 0.50
• D. 0.60

Answer: (B)

Raising the amount of oxygen the patient breathes increases the amount of oxygen that dissolves in the blood, so PaO2 and FiO2 are roughly related by a proportional change when oxygenation is not severely impaired. To estimate the needed FiO2, scale by the ratio of the target PaO2 to the current PaO2.

Current PaO2 is 78 mm Hg on an FiO2 of 0.35. Target PaO2 is 95 mm Hg. The required FiO2 ≈ 0.35 × (95 / 78) ≈ 0.35 × 1.217 ≈ 0.43.

So about 0.43 is the best fit to reach a PaO2 of 95 mm Hg.

Notes: if you don’t achieve the target even with higher FiO2, investigate other factors like shunt, V/Q mismatch, or ventilation settings. The other options (0.25, 0.50, 0.60) would either under- or overestimate the needed oxygenation in this simple proportional estimate.