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🫁 A–a Oxygen Gradient Calculator

UseDistinguish causes of hypoxaemia (shunt/V-Q mismatch vs hypoventilation).

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SourceAlveolar gas equation (standard physiology references)

Runs locally in your browser (no PHI uploaded). For licensed clinicians — not a substitute for clinical judgement.
Stored on this device for revisit reminders

This-shift next steps

After scoring: open the scene pack, peer scores, and the matching pathway. Local reference — no HIS write.

Scene next step
📋electrolyteOpen scene pack →
Related scores
Matching pathway
🧭hyperkOpen pathway →
Score + version; reference only — not a substitute for care · no HIS write

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Methodology · worked example, guidelines & full notes
Clinical takeaway

Widened gradient: V/Q mismatch, shunt or diffusion defect.

When to use

Distinguish causes of hypoxaemia (shunt/V-Q mismatch vs hypoventilation).

How it works

PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 (sea level); A–a = PAO₂ − PaO₂. Expected ≈ age/4 + 4.

Key points

  • Widened gradient: V/Q mismatch, shunt or diffusion defect.
  • Normal gradient with hypoxaemia: hypoventilation or low inspired O₂.
  • Expected value rises with age and FiO₂.

References

Worked calculation

The values below come from this tool's own example placeholders and are computed server-side with the formula shown on this page, so the arithmetic can be checked quickly. It demonstrates how to substitute values only — it is not clinical advice and not a real case.

PaO₂ (arterial)90 mmHg
PaCO₂40 mmHg
FiO₂0.21 fraction
Age (for expected)50 years

→A–a gradient10mmHg

  • Alveolar PAO₂：100 mmHg
  • Formula：PAO₂ = FiO₂·(760−47) − PaCO₂/0.8 (sea level)
  • Expected (age)：≈ 17 mmHg (age/4 + 4)
Verification & sources · governance
FAQ

Frequently asked questions

What are the limits and cautions when using A–a Oxygen Gradient Calculator?
For licensed clinicians and clinical researchers. Interpret results with history, investigations and local protocols; not a diagnosis or prescription, and not a substitute for multidisciplinary decision-making or local guidelines.
Does A–a Oxygen Gradient Calculator upload patient data? Where is it calculated?
Calculation runs locally in the browser; inputs and results are not uploaded. For licensed clinicians only; interpret with history, investigations and local protocols — not a diagnosis or prescription.
How is A–a Oxygen Gradient Calculator calculated in practice? Can you show a worked example?
Inputs: PaO₂ (arterial) 90 mmHg, PaCO₂ 40 mmHg, FiO₂ 0.21 fraction, Age (for expected) 50 years → Result: A–a gradient 10 mmHg（Alveolar PAO₂: 100 mmHg, Formula: PAO₂ = FiO₂·(760−47) − PaCO₂/0.8 (sea level), Expected (age): ≈ 17 mmHg (age/4 + 4)）
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