Alveolar-arterial Partial Pressure Oxygen Gradient (A-a O2 gradient)

Alveolar-arterial gradient (A-a gradient) is a measurement of the difference between the alveolar concentration of oxygen and the arterial concentration of oxygen.

It can be used in diagnosing the source of hypoxemia.

For example, in conditions of high altitude or hypoventilation in which the lung parenchyma is normal, the A-a gradient should be within normal limits.

In contrast, in persons with diffusion defects, ventilation-perfusion mismatch, or right-to-left shunting, oxygen is not effectively transferred from the alveoli to the blood and this results in an elevated A-a gradient.

A-a gradient = PAO2 - PaO2

* PAO2 = alveolar PO2 (calculated from the alveolar gas equation)
* PaO2 = arterial PO2 (measured in arterial gas)

PAO2:
(PB - PH2O)*FiO2 - (PaCO2/RQ)

where
PB = Barometric pressure
PH2O = Water vapor pressure, 47 mmHg
FiO2 = Fractional inspired O2 concentration, in room air, 21% or 0.21

PaCO2 can be derived from ABG. In this case, for normal subject, assume 40 mmHg

RQ is the respiratory quotient, or gas exchange ratio; normally 0.8

PAO2 - PaO2
= [(760 - 47)*FiO2 - PaCO2/0.8] - PaO2
= [150 - PaCO2/0.8] - PaO2

* Note:
At sea level, in room air, the humidified air is usually about 150 mmHg

Therefore, in this case
PAO2 = 150 - 40/0.8 = 100 mmHg

PaO2
PaO2 can be calculated from the ABG.
There will be a decline in PaO2 as someone increases in age.
This decrease in PaO2 with increasing age is most likely due to increase in V/Q mismatch

The relationship/formula to estimate PaO2 with regards to age is stated by Sorbini et al even since 1968.

PaO2 = 109 - 0.43 *age (in years)

(Sorbini, L. A., V. Grass, E. Solinas, and G. Muiesan. 1968. Arterial oxygen tension in relation to age in healthy subjects. Respiration 25: 3-13)

In other words, Sorbini et al., found a decline with age of 0.43 mm Hg/yr.

Normal A-a gradient
A normal A-a gradient is less than 10 mmHg, but can range from 5-20 mmHg.

Normally, the A-a gradient increases with age.

1. A normal A-a gradient should be less than the (patient's age divided by 4) + 4.
For example:
Age: 36 years old
Normal A-a gradient should be: 36/4 + 4 = 13

Note:
A-a gradient is always positive because without this gradient, oxygen would not move out of the lungs, across the Alveolar-capillary membrane, and into the blood. (Remember gas always flows from an area of high pressure to an area of low pressure).

A-a gradient can also be widened under normal conditions of age, obesity, fasting, supine position, and heavy exercise (great for MCQ!).

Causes of hypoxemia WITH increased A-a gradient:
1. Diffusion defect
2. V/Q mismatch
3. Shunting

Causes of hypoxemia WITHOUT increased A-a gradient:
1. Alveolar hypoventilation
2. Low FiO2 (FiO2 <21%)

More on A-a gradient. Click here.