Calculator Form
Example Data Table
| Case | FiO2 (%) | PaCO2 | Patm | PH2O | RQ | PaO2 | Estimated PAO2 | A-a Gradient |
|---|---|---|---|---|---|---|---|---|
| Sea level room air | 21 | 40 | 760 | 47 | 0.80 | 95 | 99.73 | 4.73 |
| Supplemental oxygen | 40 | 35 | 760 | 47 | 0.80 | 150 | 241.45 | 91.45 |
| Lower pressure setting | 21 | 30 | 700 | 47 | 0.80 | 70 | 99.63 | 29.63 |
Formula Used
Alveolar gas equation: PAO2 = [(Patm - PH2O) × FiO2] - (PaCO2 / RQ)
Inspired oxygen pressure: PIO2 = (Patm - PH2O) × FiO2
Alveolar to arterial gradient: A-a Gradient = PAO2 - PaO2
Expected A-a gradient by age: (Age + 10) / 4
FiO2 must be entered as a percentage in the form. The calculator converts it to a decimal before computing PIO2 and PAO2. Patm reflects barometric pressure. PH2O is the humidified airway water vapor pressure. PaCO2 represents arterial carbon dioxide. RQ adjusts for metabolism. When PaO2 is entered, the page also shows the A-a gradient for quick oxygenation review.
How to Use This Calculator
- Enter FiO2 as a percent. Room air is usually 21.
- Enter PaCO2 from the blood gas result.
- Use atmospheric pressure for the setting being reviewed.
- Keep PH2O at 47 mmHg unless you need a different assumption.
- Use 0.8 for RQ when no better estimate is available.
- Enter measured PaO2 if you want the A-a gradient.
- Enter age if you want the age-based gradient estimate.
- Press calculate to show the result above the form.
- Use the CSV or PDF buttons to save the current result.
About This Alveolar Gas Equation Calculator
The alveolar gas equation calculator estimates alveolar oxygen pressure, also called PAO2. It helps connect inspired oxygen, airway humidity, carbon dioxide, and barometric pressure. Many clinicians compare this estimate with an arterial blood gas. Students use it to understand pulmonary gas exchange. Respiratory teams use it to organize oxygenation review during bedside assessment.
Why PAO2 Matters
PAO2 is not measured directly in routine practice. It is estimated from known physiologic inputs. That estimate gives context to a measured PaO2. The difference between them is the A-a gradient. This value is useful when hypoxemia needs structured review. It can support thinking about hypoventilation, low inspired oxygen, shunt, diffusion limitation, or ventilation perfusion mismatch.
Why Each Input Changes the Answer
FiO2 raises the oxygen made available to the alveoli. Atmospheric pressure changes with altitude and environment. Water vapor pressure reduces dry gas pressure once air is humidified in the airways. PaCO2 reflects carbon dioxide balance and alveolar ventilation. Respiratory quotient adjusts the relationship between oxygen and carbon dioxide exchange. Small changes in these inputs can move the final estimate in a meaningful way.
How the A-a Gradient Helps
When you add measured PaO2, the calculator shows the alveolar to arterial gradient. This gradient helps separate causes of low oxygen. A normal or near expected gradient may fit low inspired oxygen or hypoventilation. A higher gradient may suggest gas exchange problems inside the lungs. The age-based comparison gives an extra reference point. It is helpful during chart review, teaching, and quick case discussion.
When to Use This Tool
Use this alveolar gas equation calculator when you need fast respiratory math. It is useful for bedside review, classroom learning, pulmonary case notes, and blood gas interpretation. It can also help compare sea level assumptions with lower barometric pressure settings. This tool supports structured thinking. It does not replace direct examination, laboratory quality control, imaging, pulse oximetry, or clinical judgment.
FAQs
1. What does this calculator estimate?
It estimates alveolar oxygen pressure, called PAO2. If you enter PaO2, it also calculates the A-a gradient. If you enter age, it adds an age-based A-a estimate.
2. What FiO2 should I use for room air?
Use 21 for room air. The calculator converts that percentage to 0.21 before running the equation.
3. Why is atmospheric pressure included?
Atmospheric pressure changes available oxygen pressure. This matters at altitude, in different environments, and whenever you want a more realistic estimate.
4. Why is water vapor pressure subtracted?
Inspired air becomes humidified in the airways. That humidity reduces dry gas pressure, so it must be accounted for in the alveolar oxygen estimate.
5. What is the A-a gradient?
It is the difference between estimated alveolar oxygen and measured arterial oxygen. It helps frame oxygen transfer efficiency across the lungs.
6. What RQ value should I enter?
A value of 0.8 is commonly used when no better estimate exists. It is a practical default for many teaching and clinical review situations.
7. Can this calculator diagnose disease?
No. It is a support tool for calculations and review. Final interpretation still depends on the full clinical picture and other test results.
8. What does a negative A-a gradient mean here?
It usually suggests the entered values should be rechecked. Sampling issues, timing differences, or inconsistent assumptions can produce that result.