Alveolar-arterial Gradient

Calculate the Alveolar - arterial gradient given FiO2, PaCO2 and PaO2.

High gradients result from impaired diffusion or, more commonly, by ventilation-perfusion inequality of the "shunting" variety. A normal A-a gradient is less than 10 torr. The age (years) / 4 + 4 is another conservative estimate of a normal gradient.

The calculations above assume 100% humidity at sea level and a respiratory quotient of 0.8, using the alveolar gas equation to determine PAO2:

P_AO₂ = ( F_iO₂ * (760 - 47)) - (P_aCO₂ / 0.8)

A-a gradient = P_AO₂ - P_aO₂

See the Alveolar Gas Equation calculator that allows manipulation of all the variables.

Created: January 7, 2000
Revised: December 10, 2001