# Alveolar air equation

The alveolar air equation calculates the partial pressure of oxygen within the alveoli (PAO2). The alveolar air equation is also referred to as the alveolar gas equation.

## Why is calculating the PAO2 useful?

Calculating the PAO2 is necessary to determine the A-a gradient.

## Why is calculating the PAO2 so difficult?

This calculation is difficult because oxygen is continuously entering and leaving the alveoli. It’s pretty easy to calculate the pressure of oxygen entering the alveoli. However, determining the pressure of oxygen leaving the alveoli is more complicated. For this second part we must use the respiratory quotient.

## How is the alveolar air equation calculated?

Practice problem: What is the PAO2 for an afebrile patient breathing room air at sea level and a PaCO2 of 40 mm Hg (Hint: the Ph2O for a patient with a normal temperature is 47 mm Hg.)

PAO2 = [(Patm-Ph2O)fio2] – [PaCO2/0.8]

PAO2 = [(760-47).21] – [40/0.8]

PAO2 = [(713).21] – 

PAO2 = 150 – 50

PAO2 = 100 mm Hg