Alveolar Arterial Oxygen Gradient (A-a Gradient)

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.

How to calculate Alveolar Arterial Oxygen Gradient

A-a Oxygen Gradient = P_AO₂ – P_aO2
P_AO₂ = ( F_iO₂ * (760 – 47)) – (P_aCO₂ / RQ)
P_aO2 = Arterial PO₂ (measured in arterial blood)
A-a Oxygen Gradient (expanded form) = (FiO2 * (Patm – PH20) ) – (PaCO2/RQ) – PaO2

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

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