A patient receiving 60% oxygen from an air entrainment mask has PaO2 45 mm Hg. The patient is being intubated and the ventilator is being set up. What FIO2 should be used to achieve PaO2 60 mm Hg?

• A. 0.80
• B. 0.60
• C. 0.85
• D. 1.00

Answer: (A)

When you want to raise PaO2, you adjust the FiO2 on the ventilator. Here, the patient is at FiO2 of 0.60 with a PaO2 of 45 mmHg, which is below the target. Increasing FiO2 raises the alveolar oxygen tension, which tends to lift PaO2, provided there isn’t a severe shunt or other gas-exchange problem.

Going from 0.60 to 0.80 is a moderate increase that is typically enough to push PaO2 up toward 60 mmHg in many patients. A FiO2 of 0.60 would keep PaO2 around 45, so it’s not sufficient. A FiO2 of 0.85 or 1.00 would likely overshoot or create unnecessary risk of oxygen toxicity; 0.80 is the smallest step among the options that reasonably achieves the target PaO2 in this scenario.

In practice, aim for PaO2 about 60 mmHg (or SpO2 around 92–96%), adjusting FiO2 to the lowest value that maintains that target to minimize oxygen-related toxicity.