A–a Oxygen Gradient
The alveolar–arterial oxygen gradient separates hypoxemia caused by hypoventilation or low inspired oxygen from hypoxemia caused by V/Q mismatch, diffusion impairment, or shunt. This calculator applies the alveolar gas equation and reports the A–a gradient and a/A ratio with interpretation.
760 at sea level; lower at altitude
Usually 0.8
Alveolar gas equation. PH₂O = 47 mmHg at body temperature; R ≈ 0.8.
Use with caution in neonates — combine with oxygenation index and echocardiography for shunt physiology.
Frequently asked questions
What is a normal A–a gradient in a newborn?
It rises with FiO₂ and is not firmly established in neonates. A widened gradient suggests V/Q mismatch, shunt, or diffusion impairment rather than hypoventilation.
Why use the a/A ratio?
It is less dependent on FiO₂ than the A–a gradient; a value below about 0.75 suggests impaired gas exchange.
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