The P/F ratio is calculated as:

The P/F ratio is calculated as:

• A. PaO2 / FiO2
• B. FiO2 / PaO2
• C. PaO2 / SaO2
• D. PaO2 / CaO2

Answer: (A)

The P/F ratio measures how effectively the lungs transfer oxygen into the blood by comparing the arterial oxygen tension to the amount of oxygen the patient is receiving. It is calculated by dividing PaO2 (arterial oxygen tension, in mmHg) by FiO2 (fraction of inspired oxygen, written as a decimal). For example, on room air FiO2 is 0.21; if PaO2 is 100 mmHg, the P/F ratio is about 476, which is typical of good oxygenation. If PaO2 is 80 mmHg on 40% oxygen (FiO2 = 0.40), the P/F ratio is 200, indicating impaired oxygenation. The reason this is the right formula is that it directly relates the oxygen available in the blood to the oxygen being delivered, independent of how much oxygen the patient is given. Using the reciprocal would not reflect oxygen transfer, and using SaO2 or CaO2 introduces other variables (saturation in a nonlinear range or total oxygen content that includes hemoglobin), which do not measure how well the lungs are oxygenating blood.

The P/F ratio measures how effectively the lungs transfer oxygen into the blood by comparing the arterial oxygen tension to the amount of oxygen the patient is receiving. It is calculated by dividing PaO2 (arterial oxygen tension, in mmHg) by FiO2 (fraction of inspired oxygen, written as a decimal). For example, on room air FiO2 is 0.21; if PaO2 is 100 mmHg, the P/F ratio is about 476, which is typical of good oxygenation. If PaO2 is 80 mmHg on 40% oxygen (FiO2 = 0.40), the P/F ratio is 200, indicating impaired oxygenation. The reason this is the right formula is that it directly relates the oxygen available in the blood to the oxygen being delivered, independent of how much oxygen the patient is given. Using the reciprocal would not reflect oxygen transfer, and using SaO2 or CaO2 introduces other variables (saturation in a nonlinear range or total oxygen content that includes hemoglobin), which do not measure how well the lungs are oxygenating blood.