🫁 a/A Oxygen Ratio
UseWhen to use: tracking oxygenation efficiency across changing FiO₂ (with A–a gradient).
StrataLower a/A suggests V/Q mismatch, shunt, or diffusion limitation cues.
LimitsWhen not to use: wrong FiO₂. · Pitfall: % vs fraction. · Not a PE rule-out.
SourceStandard a/A ratio from alveolar gas equation
This-shift next steps
After scoring: open the scene pack, peer scores, and the matching pathway. Local reference — no HIS write.
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Methodology · worked example, guidelines & full notes
PaO₂/PAO₂.
When to use
When to use: tracking oxygenation efficiency across changing FiO₂ (with A–a gradient).
How it works
a/A = PaO₂ / PAO₂; PAO₂ = FiO₂×(PB−47) − PaCO₂/R.
Key points
- PaO₂/PAO₂.
- Also shows A–a.
- Next: P/F / imaging.
References
- Standard a/A ratio from alveolar gas equation
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 | 80 mmHg |
|---|---|
| PaCO₂ | 40 mmHg |
| FiO₂ | 0.21 fraction (0–1) or % |
| Barometric pressure | 760 mmHg |
| R (RQ) | 0.8 |
→a/A ratio0.80
- PAO₂：99.7 mmHg
- A–a gradient：19.7 mmHg
- Cue：>=0.75 — nearer normal a/A teaching band (FiO₂-dependent)
Verification & sources · governance
FAQ
Frequently asked questions
- Does a/A Oxygen Ratio 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 Ratio calculated in practice? Can you show a worked example?
- Inputs: PaO₂ 80 mmHg, PaCO₂ 40 mmHg, FiO₂ 0.21 fraction (0–1) or %, Barometric pressure 760 mmHg, R (RQ) 0.8 → Result: a/A ratio 0.80（PAO₂: 99.7 mmHg, A–a gradient: 19.7 mmHg, Cue: >=0.75 — nearer normal a/A teaching band (FiO₂-dependent)）