Alveolar Gas Equation Calculator — Free Online Calculator
Input
Alveolar PO₂ (PAO₂)
—
About This Calculator
The alveolar gas equation calculates the partial pressure of oxygen in the alveoli (PAO₂). It is fundamental to understanding gas exchange and is used to calculate the A-a gradient. PAO₂ depends on the fraction of inspired oxygen (FiO₂), atmospheric pressure, water vapor pressure, PaCO₂, and the respiratory quotient (RQ).
Formula
Interpretation
| PAO₂ (mmHg) | Interpretation |
|---|---|
| ≥ 80 | Normal |
| 60 – 79 | Mildly reduced |
| < 60 | Significantly reduced |
References
- West JB. Respiratory Physiology: The Essentials. 10th ed. Wolters Kluwer; 2015.
- Curran-Everett D. A classic learning opportunity from Fenn, Rahn, and Otis (1946): the alveolar gas equation. Adv Physiol Educ. 2006;30(2):58-62.
Frequently Asked Questions
What does the alveolar gas equation calculate?
The alveolar gas equation calculates the partial pressure of oxygen in the alveoli (PAO₂): PAO₂ = FiO₂ × (Patm − PH₂O) − (PaCO₂ ÷ RQ). At sea level on room air: PAO₂ ≈ 0.21 × (760 − 47) − (PaCO₂ ÷ 0.8).
What is the respiratory quotient (RQ)?
The respiratory quotient is the ratio of CO₂ produced to O₂ consumed. A standard value of 0.8 is used for a mixed diet. RQ varies with substrate metabolism: carbohydrates = 1.0, proteins = 0.8, fats = 0.7. It affects the calculated PAO₂.
How does altitude affect the alveolar gas equation?
At higher altitudes, atmospheric pressure (Patm) decreases, which directly reduces PAO₂. For example, at 5,000 feet (Patm ≈ 632 mmHg), the PAO₂ on room air drops to approximately 80 mmHg, compared to ~100 mmHg at sea level.
Related Calculators
⚠ Medical Disclaimer
This tool is for educational and informational purposes only. It is not intended to replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional for clinical decisions.