Alveolar Arterial Oxygen Gradient

Alveolar Arterial Oxygen Gradient (A-a gradient) is a measure of the difference between the Alveolar Concentration of Oxygen (PAO2) and the Arterial Concentration of Oxygen (PaO2). The A–a gradient is useful in determining the source of hypoxia.

Alveolar Arterial Oxygen Gradient

How to calculate Alveolar Arterial Oxygen Gradient

A-a Oxygen Gradient = PAO2 – PaO2
PAO2 = ( FiO2 * (760 – 47)) – (PaCO2 / RQ)
PaO2 = Arterial PO2 (measured in arterial blood)
A-a Oxygen Gradient (expanded form) = (FiO2 * (Patm – PH20) ) – (PaCO2/RQ) – PaO2

Testogen Hormone

What is the normal A-a Gradient

A normal A–a gradient for a young adult is between 5–10 mmHg. A–a gradient increases with age. For every decade a person has lived their expected A–a gradient typically increases by 1 mmHg. Furthermore a higher A-a gradients is associated with oxygen transfer / gas exchange problems.


Hypoxemia is a below normal level of oxygen in your blood. Moreover hypoxemia is a sign of a variety of problems related to breathing or circulation, and may result in various symptoms, such as shortness of breath.

  • Hypoventilation
  • Right-to-left shunt
  • V/Q mismatch
  • Diffusion impairment
  • Low inspired O2

A-a Gradient Calculator

Input Atmospheric Pressure (Patm):
Input Vapor Pressure (PH2O):
Input FiO2:
Input PaCO2:
Input RQ:
Input PaO2: