PiO2 at sea level with FiO2 of 0.40 is approximately how many mmHg?

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Multiple Choice

PiO2 at sea level with FiO2 of 0.40 is approximately how many mmHg?

PiO2 represents the amount of oxygen tension in the inspired air. It’s found by multiplying the fraction of inspired oxygen (FiO2) by the pressure difference between atmospheric pressure and the water vapor pressure in the airways: PiO2 = FiO2 × (Pb − PH2O). At sea level, Pb is about 760 mmHg and PH2O (water vapor pressure at body temperature) is about 47 mmHg, so (Pb − PH2O) ≈ 713 mmHg. With FiO2 = 0.40, PiO2 ≈ 0.40 × 713 ≈ 285 mmHg. So the inspired oxygen tension is roughly 285 mmHg. This value is a starting point used in the alveolar gas equation to estimate PAO2. The other numbers would result from different FiO2 values, but with 0.40 FiO2 you land around 285 mmHg.

PiO2 represents the amount of oxygen tension in the inspired air. It’s found by multiplying the fraction of inspired oxygen (FiO2) by the pressure difference between atmospheric pressure and the water vapor pressure in the airways: PiO2 = FiO2 × (Pb − PH2O). At sea level, Pb is about 760 mmHg and PH2O (water vapor pressure at body temperature) is about 47 mmHg, so (Pb − PH2O) ≈ 713 mmHg. With FiO2 = 0.40, PiO2 ≈ 0.40 × 713 ≈ 285 mmHg. So the inspired oxygen tension is roughly 285 mmHg. This value is a starting point used in the alveolar gas equation to estimate PAO2. The other numbers would result from different FiO2 values, but with 0.40 FiO2 you land around 285 mmHg.

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