Chapter 18 – Alveolar Gas Equation

Abstract

The partial pressure gradient of O2 between the alveolus and the pulmonary capillaries is one of the key factors that determine the rate of O2 diffusion across the alveolar–capillary barrier (see Chapter 10). Unfortunately, it is not possible to directly measure the PAO2. Instead, PAO2 can be estimated using the alveolar gas equation (AGE).

Chapter 18 Alveolar Gas Equation

Can you measure the partial pressure of oxygen in the alveolus?

The partial pressure gradient of O₂ between the alveolus and the pulmonary capillaries is one of the key factors that determine the rate of O₂ diffusion across the alveolar–capillary barrier (see Chapter 10). Unfortunately, it is not possible to directly measure the P_AO₂. Instead, P_AO₂ can be estimated using the alveolar gas equation (AGE).

What is the AGE?

The AGE¹ allows P_AO₂ to be estimated from variables that are easily measured.

Key equation: simplified AGE

PAO2=FiO2PB−PSVPwater−PaCO2R

where F_iO₂ for dry air is 20.93%; P_B at sea level is 101.325 kPa. Inspired air becomes fully saturated with water vapour by the time it reaches the carina. P_{SVP water} is the saturated vapour pressure of water, which at body temperature is 6.3 kPa. P_aCO₂ is measured in kilopascals. R is the respiratory quotient, usually taken as 0.8.

The AGE tells us that P_AO₂ is essentially dependent on three variables:

The inspired fraction of O₂. According to the AGE, increasing F_iO₂ will result in a greater P_AO₂, thus increasing the pressure gradient across the alveolar–capillary barrier; the rate of O₂ diffusion will increase (Fick’s law – see Chapter 10).
Barometric pressure. P_B decreases exponentially with ascent to altitude (see Chapter 87). According to the AGE, as the elevation above sea level increases, the fall in P_B results in a lower P_AO₂ and thus a reduced rate of O₂ diffusion across the alveolar–capillary barrier.
Alveolar ventilation. As P_aCO₂ is inversely proportional to V̇_A (see Chapter 11):
1. – An increase in V̇_A (hyperventilation) results in a decrease in P_aCO₂. According to the AGE, P_AO₂ will increase, thus increasing the rate of O₂ diffusion across the alveolar–capillary barrier.
2. – A decrease in V̇_A (hypoventilation) results in an increase in P_aCO₂. According to the AGE, P_AO₂ will decrease, thus reducing the rate of O₂ diffusion across the alveolar–capillary barrier.