A patient's baseline arterial blood gas reveals PaO2 78 mm Hg while receiving 35% supplemental oxygen. To achieve a target PaO2 of 95 mm Hg, what should the ventilator FIO2 be set at?

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

Answer: (C)

Raising FiO2 increases the amount of oxygen available for transfer into the blood, so PaO2 rises when you adjust the ventilator settings. Here, the patient is at 78 mm Hg on 35% oxygen and you want about 95 mm Hg. A modest uptick in FiO2 is typically enough to reach that target without jumping to very high oxygen levels. Setting FiO2 to around 0.43 (a small increase from 0.35) provides enough additional inspired oxygen to lift PaO2 into the 90s, matching the goal without overshooting or increasing the risk of oxygen toxicity. Lowering FiO2 would drop PaO2, while larger increases (0.50 or 0.60) would be more aggressive than necessary and could expose the patient to unnecessary higher oxygen concentrations.