Alveolar air equation

Alveolar air equation is a mathematical formula used in pulmonology and anesthesiology to calculate the partial pressure of oxygen (PaO2) in the alveoli of the lungs. The equation is essential in understanding the gas exchange process in the lungs and is often used in the diagnosis and treatment of respiratory diseases.

Overview

The alveolar air equation is derived from the ideal gas law and the principles of gas exchange in the lungs. It is used to estimate the alveolar oxygen tension, which is the partial pressure of oxygen in the alveoli. This value is crucial in assessing the efficiency of oxygen transfer from the lungs to the bloodstream.

Equation

The alveolar air equation is expressed as follows:

PAO2 = FiO2 (Patm - PH2O) - PaCO2/R

Where:

• PAO2 is the partial pressure of oxygen in the alveoli
• FiO2 is the fraction of inspired oxygen
• Patm is the atmospheric pressure
• PH2O is the partial pressure of water vapor
• PaCO2 is the partial pressure of carbon dioxide in arterial blood
• R is the respiratory quotient

Clinical significance

The alveolar air equation is used in clinical practice to assess the efficiency of gas exchange in the lungs. It can help diagnose conditions such as hypoxemia, pulmonary edema, and chronic obstructive pulmonary disease (COPD). The equation is also used to guide the administration of supplemental oxygen in patients with respiratory failure.

See also

• Gas exchange
• Respiratory quotient
• Pulmonary function testing