Alveolar gas equation
The alveolar gas equation is the method for calculating partial pressure of alveolar oxygen. The equation is used in assessing if the lungs are properly transferring oxygen into the blood. The alveolar air equation is not widely used in clinical medicine, probably because of the complicated appearance of its classic forms.
The partial pressure of oxygen in the pulmonary alveoli is required to calculate both the alveolar-arterial gradient of oxygen and the amount of right-to-left cardiac shunt, which are both clinically useful quantities. However, it is not practical to take a sample of gas from the alveoli in order to directly measure the partial pressure of oxygen. The alveolar gas equation allows the calculation of the alveolar partial pressure of oxygen from data that is practically measurable. It was first characterized in 1946.
Assumptions
The equation relies on the following assumptions:- Inspired gas contains no carbon dioxide
- Nitrogen in the inspired gas are in equilibrium with their dissolved states in the blood
- Inspired and alveolar gases obey the ideal gas law
- Carbon dioxide in the alveolar gas is in equilibrium with the arterial blood i.e. that the alveolar and arterial partial pressures are equal
- The alveolar gas is saturated with water
Equation
where:
| Quantity | Description | Sample value |
| The alveolar partial pressure of oxygen | 107 mmHg | |
| The fraction of inspired gas that is oxygen. | 0.21 | |
| PATM | The prevailing atmospheric pressure | 760 mmHg |
| The saturated vapour pressure of water at body temperature and the prevailing atmospheric pressure | 47 mmHg | |
| The arterial partial pressure of carbon dioxide | 40 mmHg | |
| RER | The respiratory exchange ratio | 0.8 |
Sample Values given for air at sea level at 37°C.
Doubling will double.
Other possible equations exist to calculate the alveolar air.