Alveolar–Arterial Gradient Calculator


For the calculation of the expected PAO2 only:

Alveolar PO2 : mmHg

Gradient A-a : mmHg

Expected (max) gradient: mmHg

PAO2 (min) expected: mmHg

Alveolar–Arterial Gradient

The alveolar-arterial oxygen gradient (A-aO2, or A-a gradient), is the difference between the partial pressure of alveolar oxygen and that of arterial oxygen. It is used in the diagnosis of the source of hypoxemia.

Its estimation makes it possible to assess the integrity of the alveolar capillaries. For example, at high altitude arterial oxygen PaO2 is low, but only because alveolar oxygen (PAO2) is also low. However, in conditions of ventilation-perfusion ratio abnormalities, such as pulmonary embolism or right-to-left shunts, oxygen is poorly transferred from the alveoli to the blood, resulting in an increase in the alveolar-arterial gradient.

Alveolar–Arterial Gradient Calculator

The alveolar-arterial oxygen gradient, or alveolar-arterial oxygen difference, labeled as P(A-a)O2 or AaDO2, is the difference between the calculated PAO2 from the alveolar gas equation and the measured PaO2. PAO2 represents the partial pressure of oxygen in the alveoli and PaO2 represents the arterial partial pressure of oxygen.

The alveolar-arterial gradient in a young and healthy subject is less than 10 mmHg (varying from 5 to 20 mmHg). It increases with age.