Using the formula for desired FiO2, if PaO2 desired is 100 mmHg, FiO2 known is 0.40, and PaO2 known is 80 mmHg, what is the required FiO2?

• A. 0.40
• B. 0.60
• C. 0.50
• D. 0.70

Answer: (C)

The idea is to adjust FiO2 in proportion to the change in PaO2, assuming a roughly linear relationship within the practical range. You scale the known FiO2 by the ratio of the desired PaO2 to the current PaO2.

FiO2 desired = FiO2 known × (PaO2 desired / PaO2 known)

= 0.40 × (100 / 80)

= 0.40 × 1.25

= 0.50

So the required FiO2 is 0.50.