Definitions

# Alveolar gas equation

The alveolar pO2 is not routinely measured but is calculated from blood gas measurements by the Alveolar gas equation:

$p_AO_2 = p_IO_2 - p_ACO_2/R + F ,$

where:

• R is the Respiratory quotient (normally about 0.8)
• pAO2 is the Alveolar pO2
• pIO2 is the Inspired pO2, equal to about 150 mm Hg (0.21 x 713 mmHg) at sea level. The given pressure at sea level is due to atmospheric pressure (760 mmHg) minus the partial pressure of water vapor (47 mmHg), as alveolar gas is completely saturated with water. The mole fraction of oxygen is about 0.21 in dry atmospheric gas.
• pACO2 is the Alveolar pCO2 (assumed to be equal to the measured arterial pCO2)
• F is a correction factor (usually less than 2 mmHg)