What fractional inspired oxygen (FIO2) setting on the ventilator should be used when the patient has a PaO2 of 53 mm Hg while receiving 50% oxygen and the target PaO2 is 90 mm Hg?

• A. 0.80
• B. 0.85
• C. 0.50
• D. 0.95

Answer: (B)

The key idea is estimating how much FiO2 is needed to raise PaO2 when oxygenation is inadequate. If PaO2 rises roughly in proportion to FiO2, you can scale the current PaO2 by the same factor as the FiO2 change.

Here, PaO2 is 53 mm Hg on FiO2 0.50, and the target PaO2 is 90 mm Hg. Using proportional reasoning:

FiO2 needed ≈ (target PaO2 / current PaO2) × current FiO2

FiO2 needed ≈ (90 / 53) × 0.50 ≈ 0.85

So about 0.85 is the best estimate to reach the target PaO2. A FiO2 of 0.80 would likely give around 85 mm Hg, and 0.95 could push PaO2 well above 90, with more risk of oxygen toxicity. 0.50 would do nothing to improve the current PaO2.

In practice, after adjusting FiO2, recheck ABG or SpO2 and consider other measures (like PEEP) if needed, to optimize oxygenation while avoiding unnecessary high FiO2 for extended periods.