Medical AIAlveolar-arterial (A-a) oxygen gradient
The A-a gradient is the difference between the calculated alveolar oxygen tension and the measured arterial oxygen tension. A gradient that is normal for age suggests hypoventilation or a low inspired oxygen tension (such as altitude), whereas a widened gradient points to V/Q mismatch, right-to-left shunt, or impaired diffusion. The expected upper limit rises with age, roughly (age / 4) + 4 mmHg on room air at sea level.
How to measure each input
- FiO2
- Enter the inspired oxygen fraction as a decimal: 0.21 on room air, higher on supplemental oxygen. The gradient is most interpretable on room air; on high FiO2 it is harder to read.
- PaO2 and PaCO2
- Take both directly from a contemporaneous arterial blood gas, ideally drawn while the patient is on the FiO2 you entered.
- Atmospheric pressure
- Use 760 mmHg at sea level. At altitude the barometric pressure is lower, which lowers the alveolar PO2 and must be entered for the gradient to be valid.
- Age-adjusted normal
- The expected upper limit of a normal gradient rises with age, approximated as (age / 4) + 4 mmHg. Compare the calculated gradient against this rather than a single fixed cutoff.
Interpretation
| Band | Meaning |
|---|---|
| Gradient < 15 mmHg, generally normal | A gradient under about 15 mmHg is normal for most younger adults on room air. If the patient is hypoxaemic despite a normal gradient, suspect hypoventilation (raised PaCO2) or a low inspired oxygen tension such as altitude. Compare against the age-adjusted expected value shown beside the result. |
| Gradient 15 to 30 mmHg, interpret with age | This range may be normal in older patients (the expected limit is roughly age / 4 + 4) or mildly elevated in younger ones. Compare directly with the age-adjusted expected value before calling it abnormal. |
| Gradient > 30 mmHg, widened | A clearly widened gradient indicates a problem with gas exchange: V/Q mismatch (for example pulmonary embolism, pneumonia, COPD), right-to-left shunt, or a diffusion limitation. Pursue the cause; this does not implicate hypoventilation or altitude alone. |
Pitfalls, exclusions and caveats
- The equation assumes a respiratory quotient of 0.8 and water vapour pressure of 47 mmHg at body temperature; these are standard approximations, not measured values.
- On high supplemental oxygen the gradient widens for reasons unrelated to disease and becomes hard to interpret; it is most useful on room air.
- Failing to enter a reduced atmospheric pressure at altitude overestimates the alveolar PO2 and therefore the gradient.
- A normal gradient does not exclude lung disease: it simply means alveolar gas exchange is not the dominant cause of the measured hypoxaemia (consider hypoventilation or low inspired oxygen).
- The age-adjusted normal ((age / 4) + 4) is a conservative estimate from healthy populations on room air at sea level; other references use slightly different forms (such as (age + 10) / 4), so treat the cutoff as approximate.
PAO2 = FiO2 x (Patm - 47) - PaCO2 / 0.8, then A-a gradient = PAO2 - PaO2. (47 mmHg is the water vapour pressure at body temperature; 0.8 is the assumed respiratory quotient.) Estimated normal upper limit = (Age / 4) + 4 mmHg.Frequently asked
What is a normal A-a gradient?
It rises with age. A common estimate of the upper limit is (age / 4) + 4 mmHg on room air at sea level, so roughly 9 mmHg at age 20 and around 24 mmHg at age 80. Compare the calculated gradient against this age-adjusted value rather than a single fixed number.
What does a widened A-a gradient mean?
A gradient above the age-adjusted normal indicates a problem with gas exchange in the lungs: ventilation-perfusion mismatch (for example pulmonary embolism, pneumonia, COPD), a right-to-left shunt, or impaired diffusion. The wider the gradient, the more the hypoxaemia is attributable to lung pathology rather than to hypoventilation or altitude.
Why can a patient be hypoxaemic with a normal A-a gradient?
A normal gradient with hypoxaemia points to causes outside the alveolar membrane: hypoventilation (which raises PaCO2 and lowers alveolar oxygen across the board) or a low inspired oxygen tension such as at high altitude. In both cases the alveolar and arterial oxygen fall together, so the difference between them stays normal.
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