The A-a gradient is a measure of the drop in partial pressure of oxygen between the alveolus and arterial blood. It can be estimated from the arterial blood gases. A PaO2 of 14 kPA (107 mmHg) could be considered normal - until one realises that the patient is breathing 60% oxygen. The predicted PaO2 should be approximately 8 kPA (75 mmHg) below the FiO2. There is a significant problem with gas exchange in this example. This problem can be measured by the A-a gradient. In some situations, the A-a gradient can be a more sensitive measure than PaO2 alone in indicating a problem with gas exchange.

Alveolar oxygen can be estimated using the alveolar gas equation. Oxygen leaves the alveolus in exchange for carbon dioxide. The amount of CO2 entering the alveolus is known as this is almost the same as arterial CO2. Slightly less CO2 is excreted than O2 absorbed so a correction factor (0 8) is added.

The alveolar gas equation is:

where PB equals the ambient barometric pressure which equals 101 kPA at sea level and PAH2O equals the alveolar partial pressure of water which equals 6 kPA. This is often simplified to:

Once PAO2 has been estimated, the A-a gradient (PAO2 -PaO2) can be calculated. Any significant difference is due to abnormal gas exchange. Normal people have a small difference because the bronchial veins of the lung and thebesian veins of the heart carry unsaturated blood to the left ventricle, bypassing the alveoli. A normal A-a gradient is up to 2 kPA (15 mmHg) or 4 kPA (30 mmHg) in smokers and the elderly.

A person breathing air with a PaO2 of 12 0 kPA and a PaCO2 of 5 0 kPA has an A-a gradient as follows:

When calculating the A-a gradient on air, 0 21 x 95 is often shortened to 20:

The calculation of the A-a gradient illustrates the importance of always documenting the inspired oxygen concentration. Relative hypoxaemia cannot be detected without this.