A-a Gradient Calculator
Enter 21 to 100 (%) or 0.21 to 1.0 (fraction). Room air is 21.
What is A-a Gradient Calculator
The alveolar-arterial (A-a) oxygen gradient is the difference between the calculated alveolar oxygen tension and the measured arterial PaO2. It tells you whether hypoxaemia comes from pure hypoventilation, which leaves the gradient normal, or from a gas-exchange problem like shunt, V/Q mismatch or diffusion impairment, which widens it. The alveolar value comes from the alveolar gas equation using FiO2 and PaCO2.
How to use
- Enter the FiO2 as a percentage or a fraction.
- Enter the PaO2 and PaCO2 from the arterial blood gas in mmHg.
- Optionally add the age to see the expected upper-normal gradient.
- Read the gradient and whether it is elevated for the age.
When to use it
Working up hypoxaemia and need to know if the lungs are the problem? Enter room air (21%), a PaO2 of 90 and a PaCO2 of 40 and you get a gradient of about 10, within normal. If the PaO2 drops to 60 while the PaCO2 stays at 40, the gradient widens and points to shunt or V/Q mismatch rather than simple hypoventilation. Handy at the bedside or when reading a gas alongside the clinical picture.
Frequently asked questions
What is a normal A-a gradient?
It rises with age. A common estimate for the upper limit of normal on room air is age divided by 4, plus 4. So a 40-year-old sits around 14 and an 80-year-old around 24. Enter the age to see this expected value.
Which equation does this use?
The alveolar gas equation: PAO2 = FiO2 times (atmospheric pressure minus water vapour pressure) minus PaCO2 divided by the respiratory quotient. This tool assumes 760 mmHg at sea level, 47 mmHg water vapour and a respiratory quotient of 0.8.
What does a widened A-a gradient mean?
A gradient above the expected value points to a gas-exchange problem: shunt, ventilation-perfusion mismatch, or a diffusion barrier. A normal gradient with hypoxaemia suggests hypoventilation or a low inspired oxygen, not a lung parenchymal issue.
Can I share this calculator with the values pre-filled?
Yes. The URL updates as you type the gas values. Copy it from the address bar or use the Copy link button, and anyone who opens it sees the same A-a gradient ready to go.
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