A P/F ratio of which value indicates severe hypoxemia?

• A. less than 100
• B. less than 150
• C. less than 200
• D. less than 250

Answer: (B)

The P/F ratio shows how effectively oxygen moves from the lungs into the bloodstream relative to how much oxygen is being delivered. It’s calculated by dividing the arterial oxygen tension (PaO2) by the fraction of inspired oxygen (FiO2). When the lungs aren’t exchanging gases well, PaO2 drops even if FiO2 is high, so the ratio falls. A P/F ratio below 150 is used in many guidelines and tests as a marker of severe hypoxemia, indicating a substantial impairment in oxygen transfer.

For example, if PaO2 is about 70 mmHg with FiO2 0.60, the ratio is roughly 70/0.60 ≈ 117, which is severe. By contrast, a PaO2 of 90 mmHg on FiO2 0.40 gives a ratio of about 225, indicating less severe hypoxemia.