Alveolar-Arterial gradient calculation (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 hypoxima.

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.

A-a gradient Calculation (expanded form)

Input Atmospheric Pressure (Patm):
Input Vapor Pressure (PH2O):
Input FiO2: %
Input PaCO2:
Input RQ:
Input PaO2:
A-a gradient: mmHg

What is the normal A-a Gradient?

Normal A-a Gradient Estimate = (Age/4) + 4

Causes of Hypoxemia

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

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