Alveolar Gas Equation

\[P_{AO_2}=F_{IO_2}\left(P_{atm}-P_{H_2O}\right)-\frac{P_{aCO_2}}{RQ}\]

Variables

PAO2alveolar oxygen partial pressure (mmHg)

FiO2fraction of inspired oxygen

Patmatmospheric pressure (mmHg)

PH2Owater vapor pressure (mmHg)

PaCO2arterial carbon dioxide partial pressure (mmHg)

RQrespiratory quotient

Description

What is this formula?

The Alveolar Gas Equation estimates the partial pressure of oxygen in the alveoli. It is one of the most important equations in respiratory physiology because it links inspired oxygen, atmospheric pressure, carbon dioxide levels, and metabolic gas exchange.

When to use it

Use this equation when evaluating oxygenation status, calculating the alveolar-arterial oxygen gradient, interpreting arterial blood gases, assessing respiratory failure, or studying pulmonary physiology.

Example

Data:

FiO2 = 0.21

Patm = 760 mmHg

PH2O = 47 mmHg

PaCO2 = 40 mmHg

RQ = 0.8

Formula:

PAO2 = FiO2(Patm − PH2O) − PaCO2/RQ

Substitution:

PAO2 = 0.21(760 − 47) − 40/0.8

PAO2 = 149.7 − 50

PAO2 = 99.7 mmHg

Result:

PAO2 ≈ 100 mmHg

Applications

- Arterial blood gas interpretation

- A–a oxygen gradient calculation

- Critical care medicine

- Pulmonology

- Mechanical ventilation assessment

- High-altitude physiology