Alveolar - arterial Oxygen gradient

A–a gradient measures the difference in alveolar oxygen tension and arterial oxygen tension 

A-a gradient = PAO2 - PaO2

Where 

PAO2 =(FiO2× [Patm - PH2O]) - (PaCO2/R)

FiO2 : Fraction of inspired oxygen

Patm : Atmospheric pressure (760 mmHg at sea level)

PH2O : partial pressure of water (47 mmHg at 37 degrees C)

PaCO2 :Arterial carbon dioxide partial pressure

R : respiratory quotient (which, in a normal resting steady state, is usually 0.8, depending on the patient’s nutritional intake, but may approach 1.0 with critical illness and high sympathetic tone)

A “normal” A–a difference on room air is often considered to be 10 mmHg.

Reference: Fishman 6th ed,page no. 2510