Master Alveolar Gas Equation with Picmonic for Medicine

This is the partial pressure of oxygen (PO2) in the pulmonary alveoli, typically measured in millimeter of mercury (mmHg). This value is important for the calculation of other clinically relevant values, such as the A-a gradient.

Oxygen exerts a partial pressure, which is determined by the environmental pressure. At sea level, the atmospheric pressure is 760 mmHg, and oxygen makes up 21 percent (20.094 percent to be exact) of inspired air. Since alveolar gas is assumed to be saturated with water vapor, the vapor pressure of water (~ 47 mmHg) must be subtracted from atmospheric pressure (760 - 47 = 713). So oxygen exerts a partial pressure of 713 x 0.21 = 150 mmHg.

PIO2 in normal atmospheric settings is approximated as 150 mmHg.

The arterial carbon dioxide partial pressure (PaCO2) is an indicator of carbon dioxide in the blood. A high PaCO2, or hypercapnia, is indicative of underventilation, or more rarely, hypermetabolic activity. A low PaCO2, or hypocapnia, is indicative of hyperventilation. This value can be obtained clinically via an arterial blood gas measurement (ABG).

The respiratory quotient is the amount of CO2 produced, divided by the amount of O2 consumed. This varies from organism to organism, but can be approximated at 0.8 for humans.

Respiratory quotient is approximately 0.8 in humans.

This is the equation used to calculate the alveolar partial pressure of oxygen. This equation can normally be approximated as PAO2 = 150 - (PaCO2/0.8).