Oxygenation & Gas Exchange Tool
A–a Gradient Calculator
Quickly calculate the alveolar–arterial oxygen gradient to evaluate hypoxemia causes including V/Q mismatch, shunt, and diffusion impairment. This page also includes interpretation help, clinical scenarios, and supporting oxygenation resources so you can use the A–a gradient in a more complete hypoxemia workup.
Mini interpretation table
Within expected range
A normal age-adjusted A–a gradient makes hypoventilation alone more plausible when oxygen is low.
Mild to moderate elevation
Think about V/Q mismatch, early pneumonia, COPD, asthma, or evolving gas exchange impairment.
Marked elevation
Larger gradients raise concern for shunt physiology, severe V/Q mismatch, or diffusion problems.
How to calculate the A–a gradient
- Step 1Enter the measured PaO₂ and current FiO₂.
- Step 2Add PaCO₂, atmospheric pressure, water vapor pressure, and respiratory quotient to complete the alveolar gas equation.
- Step 3Enter age so the page can compare your result with an expected normal A–a gradient.
- Step 4Review whether the gradient is within the expected range or elevated, then use the interpretation table and related tools to frame the hypoxemia differential.
An elevated A–a gradient indicates a problem with oxygen transfer from the alveoli to the bloodstream. It helps distinguish the cause of hypoxemia, whether it is due to ventilation-perfusion mismatch, diffusion defect, or shunting. A normal age-adjusted gradient makes hypoventilation alone more plausible.
Pair this tool with the ABG Analyzer for full blood gas interpretation, review the A–a Gradient guide, compare oxygenation severity with the P/F Ratio article, or jump into the P/F Ratio & Oxygenation Index tool.
Clinical scenarios where the A–a gradient is useful
Hypoxemia with suspected V/Q mismatch
Helpful in COPD, asthma, pneumonia, and pulmonary embolism when you are trying to decide whether oxygen transfer is impaired beyond simple hypoventilation.
Shunt physiology
A markedly elevated A–a gradient can support concern for significant shunt, especially when paired with severe hypoxemia and poor oxygenation response.
Diffusion impairment
Interstitial lung disease and other diffusion problems can widen the gradient, especially when interpreted alongside the clinical picture.
Hypoventilation vs oxygen transfer problem
This is one of the most useful bedside distinctions: is the low oxygen mostly from low ventilation, or is the lung failing to transfer oxygen effectively?
Supporting guides & oxygenation tools
Use these pages together to build a more complete hypoxemia workup, from blood gas interpretation to oxygenation severity and differential thinking.