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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:
PAO2 = ( FiO2 * (760 - 47)) - (PaCO2 / 0.8)
A-a gradient = PAO2 - PaO2
See the Alveolar Gas Equation calculator that allows manipulation of all the variables.
Revised: December 10, 2001