A-a Gradient Calculator
Estimate alveolar oxygen on room air and calculate the alveolar-arterial oxygen gradient. The A-a gradient helps separate hypoventilation from oxygen transfer problems like V/Q mismatch, diffusion limitation, and shunt.
Calculate A-a Gradient
This beginner calculator uses the simplified room-air, sea-level estimate: PAO₂ ≈ 150 − (PaCO₂ ÷ 0.8).
A-a Gradient Formula
Simplified room-air PAO₂ estimate:
PAO₂ ≈ 150 − (PaCO₂ ÷ 0.8)
Example: PaCO₂ 40, PaO₂ 75
PAO₂ ≈ 150 − 50 = 100
A-a Gradient = 100 − 75 = 25 mmHg
Capital A Minus Little a
The A-a gradient compares oxygen expected in the alveoli to oxygen measured in arterial blood.
Capital A = alveolar oxygen. Lowercase a = arterial oxygen. The gradient is the gap between what should be available and what made it into arterial blood.
Age-Based Expected A-a Gradient
This is a rough clinical estimate. A widened gradient suggests an oxygen transfer problem rather than pure hypoventilation.
How to Interpret A-a Gradient
| Pattern | Meaning | Common Clinical Direction |
|---|---|---|
| Normal A-a gradient with hypoxemia | Alveolar-to-arterial transfer is relatively preserved | Think hypoventilation or low inspired oxygen. |
| Elevated A-a gradient with hypoxemia | Oxygen is available in alveoli but not reaching arterial blood well | Think V/Q mismatch, diffusion limitation, or shunt. |
| Very wide A-a gradient | Significant oxygen transfer problem | Consider severe V/Q mismatch, ARDS, shunt, pneumonia, edema, or PE. |
The A-a Gradient Helps Explain Why PaO₂ Is Low
When a patient is hypoxemic, the A-a gradient helps you ask whether the problem is mostly reduced alveolar ventilation or impaired transfer from alveoli to blood.
Think hypoventilation or low inspired oxygen.
Think V/Q mismatch, diffusion limitation, or shunt.
V/Q mismatch often improves; true shunt responds poorly.
Use with ABGs, imaging, FiO₂, P/F ratio, and patient presentation.
Avoid These Errors
Capital A is alveolar. Lowercase a is arterial.
This simplified formula assumes room air at sea level.
The expected normal gradient increases with age.
The A-a gradient guides reasoning but does not replace clinical context.
Connect A-a Gradient to Oxygenation
Review P/F ratio, causes of hypoxemia, and ABG interpretation to understand oxygenation problems from multiple angles.