PiO2 calculation: FiO2 = 0.25 at sea level. PiO2 is approximately:

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PiO2 calculation: FiO2 = 0.25 at sea level. PiO2 is approximately:

The key idea is how to get the inspired oxygen pressure by adjusting for humidity and the air’s oxygen fraction. PiO2 is FiO2 times the amount of pressure available after removing the water vapor in humidified air: PiO2 = FiO2 × (Pb − PH2O). At sea level, barometric pressure Pb is about 760 mmHg and the water vapor pressure PH2O at body temperature is about 47 mmHg. So with FiO2 = 0.25: PiO2 ≈ 0.25 × (760 − 47) = 0.25 × 713 ≈ 178 mmHg. Thus PiO2 is approximately 178 mmHg.

The key idea is how to get the inspired oxygen pressure by adjusting for humidity and the air’s oxygen fraction. PiO2 is FiO2 times the amount of pressure available after removing the water vapor in humidified air: PiO2 = FiO2 × (Pb − PH2O).

At sea level, barometric pressure Pb is about 760 mmHg and the water vapor pressure PH2O at body temperature is about 47 mmHg. So with FiO2 = 0.25:

PiO2 ≈ 0.25 × (760 − 47) = 0.25 × 713 ≈ 178 mmHg.

Thus PiO2 is approximately 178 mmHg.

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