A-a Gradients

When a patient is breathing room air (21% oxygen) it is easy to assess the PaO2 value of an Arterial Blood Gas (ABG) for hypoxia. We know that any value less than 80 torr is hypoxia and tells you that oxygen transfer is impaired. But how do you tell if the lungs and circulation are working properly when the patient is on supplemental oxygen? The A-a Gradient calculation can help.

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

Input FiO2(decimal)
Input PaCO2 (torr)
Input PaO 2 (torr)

Alveolar-arterial oxygen gradient =

A normal A-a gradient is less than 10 torr. High gradients result from impaired diffusion or by ventilation-perfusion inequality.