## 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).

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

The partial pressure gradient of O_{2} between the alveolus and the pulmonary capillaries is one of the key factors that determine the rate of O_{2} diffusion across the alveolar–capillary barrier (see Chapter 10). Unfortunately, it is not possible to directly measure the *P*_{A}O_{2}. Instead, *P*_{A}O_{2} can be estimated using the alveolar gas equation (AGE).

### What is the AGE?

The AGE^{1} allows *P*_{A}O_{2} to be estimated from variables that are easily measured.

where *F*_{i}O_{2} 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*_{a}CO_{2} is measured in kilopascals. *R* is the respiratory quotient, usually taken as 0.8.

The AGE tells us that *P*_{A}O_{2} is essentially dependent on three variables:

**The inspired fraction of O**_{2}. According to the AGE, increasing*F*_{i}O_{2}will result in a greater*P*_{A}O_{2}, thus increasing the pressure gradient across the alveolar–capillary barrier; the rate of O_{2}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*_{A}O_{2}and thus a reduced rate of O_{2}diffusion across the alveolar–capillary barrier.

**Alveolar ventilation**. As*P*_{a}CO_{2}is inversely proportional to*V̇*_{A}(see Chapter 11):

– An increase in

*V̇*_{A}(hyperventilation) results in a decrease in*P*_{a}CO_{2}. According to the AGE,*P*_{A}O_{2}will increase, thus increasing the rate of O_{2}diffusion across the alveolar–capillary barrier.

– A decrease in

*V̇*_{A}(hypoventilation) results in an increase in*P*_{a}CO_{2}. According to the AGE,*P*_{A}O_{2}will decrease, thus reducing the rate of O_{2}diffusion across the alveolar–capillary barrier.

### What is the respiratory quotient? Why does it differ for different dietary substrates?

The respiratory quotient *R* is the ratio of CO_{2} production and O_{2} consumption: