ABSTRACT

This is measured directly. The PaO2 is always less than the PAO2 because of shunt. The relationship between the PAO2 and the fractional inspired oxygen concentration (FIO2), is described by the alveolar gas equation:

When breathing air at an atmospheric pressure of 100kPa (760mmHg), the partial pressure of inspired oxygen is 20.9kPa (158mmHg). Having become fully saturated with water in the upper respiratory tract the partial pressure of oxygen falls to 19.5kPa (148mmHg). At the alveolus, oxygen is taken up and replaced by CO2, which reduces the PaO2 to 14kPa (106mmHg). Because of shunt, the arterial oxygen pressure is always slightly lower than that in the alveolus. Shunt increases with age. When breathing air, the normal PaO2 is 12.5kPa (92mmHg) at the age of 20 years and 10.8kPa (82mmHg) at 65 years.