A-a Oxygen Gradient calculator
Result
9.7mmHg
Normal A-a gradient
Expected for age 40: ~11 mmHg.
How to use it
- 1
Record FiO₂. 21% on room air; calculate or estimate on supplemental O₂.
- 2
From ABG: PaCO₂ and PaO₂. Ensure sample is arterial.
- 3
Interpret. Normal A-a with hypoxia → hypoventilation. Elevated A-a → V/Q mismatch, shunt, or diffusion limitation.
When to use it
- Hypoxia workup
- Differentiates hypoventilation (normal A-a, e.g., opioid) from parenchymal (elevated A-a, e.g., PE/pneumonia).
- PE assessment
- PE classically gives elevated A-a with low PaCO₂ (hyperventilation).
- ARDS
- PaO₂/FiO₂ <300 + elevated A-a in bilateral infiltrates = Berlin criteria.
A reference for clinicians and students, not medical advice. A score supports a decision; it does not make one. Check every result against the patient in front of you and your local protocol.
At a glance
- Specialty
- Pulmonology
- Inputs
- 4
- Type
- Formula
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