A-a Gradient
Compute the alveolar-arterial oxygen gradient using the standard alveolar gas equation: PAO2 = FiO2 * (760 - 47) - (PaCO2 / 0.8), then A-a = PAO2 - PaO2.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- FiO2
- 0.21
- PaCO2
- 40 mmHg
- PaO2
- 90 mmHg
Result: A-a gradient ~9.7 mmHg
The tool opens with these values already filled in. Replace them with your own.
What you enter
- FiO2 (0-1)
- PaCO2
- PaO2
What this is
Sophie Well returns the gradient in mmHg along with the age-adjusted upper limit (roughly age/4 + 4) so a normal-vs-elevated read is one glance away.
When to use it
Use this when a patient is hypoxemic and you need to differentiate the five physiologic causes: a normal A-a gradient points to hypoventilation or low inspired oxygen; a widened gradient points to V/Q mismatch, shunt, or diffusion limitation. The standard formula assumes sea-level barometric pressure and water-vapor partial pressure; at altitude or on supplemental O2, interpret the absolute value with care.
How this is calculated
West JB. Respiratory Physiology: The Essentials. PAO2 = (FiO2 * (760-47)) - (PaCO2 / 0.8).
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.