Calculates estimated alveolar oxygen tension from inspired oxygen, atmospheric pressure, PaCO₂ and respiratory quotient. This guide explains the method, appropriate use, interpretation and limitations.
Overview
Calculates estimated alveolar oxygen tension from inspired oxygen, atmospheric pressure, PaCO₂ and respiratory quotient.
Clinical Significance
The alveolar gas equation estimates PAO₂ and underpins assessment of the alveolar–arterial oxygen gradient. Assumptions become less reliable at unusual FiO₂ or respiratory quotient.
When to Use
- Use when the stated clinical calculation is relevant to the patient and decision being considered.
- Use contemporaneous measurements obtained with appropriate technique.
- Confirm that the equation and units match local practice before acting on the result.
How It Is Calculated
- PAO₂ = FiO₂ × (atmospheric pressure − 47) − PaCO₂ ÷ respiratory quotient.
Interpretation
- The alveolar gas equation estimates PAO₂ and underpins assessment of the alveolar–arterial oxygen gradient. Assumptions become less reliable at unusual FiO₂ or respiratory quotient.
- Interpret trends and the full clinical picture rather than relying on a single calculated value.
Worked Example
Enter a valid set of the required measurements. MedicalC applies: PAO₂ = FiO₂ × (atmospheric pressure − 47) − PaCO₂ ÷ respiratory quotient. The displayed result should then be checked against the clinical context and local guidance.
Patient Considerations
- Consider whether physiology, treatment or sampling conditions changed between measurements.
- Consider whether the patient differs from the population in which the equation is normally used.
Limitations
- The result is only as reliable as the entered measurements and assumptions behind the equation.
- The calculator does not replace clinical assessment, definitive measurement or specialist advice.
- Thresholds and interpretation may vary between populations, laboratories and clinical pathways.
Clinical Pearls
- Confirm units before calculation.
- Use values obtained at the same clinical time point.
- Document the component measurements with the calculated result.
Common Mistakes
- Mixing SI and conventional units.
- Using estimated or non-contemporaneous measurements without acknowledging uncertainty.
- Treating a calculated value as a diagnosis or treatment instruction.
Evidence Base
The implemented equation follows the established published or guideline-based relationship: PAO₂ = FiO₂ × (atmospheric pressure − 47) − PaCO₂ ÷ respiratory quotient.
Frequently Asked Questions
What does the Alveolar Gas Equation calculator do?
Calculates estimated alveolar oxygen tension from inspired oxygen, atmospheric pressure, PaCO₂ and respiratory quotient.
Can the result be used alone?
No. It should support, not replace, clinical assessment and current guidance.
Why might the result be unreliable?
Incorrect units, poor measurement quality, non-contemporaneous inputs or use outside the intended setting can produce misleading results.
Should the calculation be repeated?
Repeat it when relevant measurements or the patient’s clinical condition change.
References
- Standard alveolar gas equation. — Primary publication, guideline or established physiologic source supporting the equation.
Reviewed by: MedicalC Clinical Editorial Team
Last reviewed: July 2026