1. What is the Alveolar Gas Equation?

The alveolar gas equation calculates the partial pressure of oxygen in the alveoli (PAO₂). It's fundamental for understanding pulmonary gas exchange and assessing respiratory function.

2. How Does the Calculator Work?

The calculator uses the alveolar gas equation:

Where:

• \( P_B \) — Barometric pressure (mmHg)
• \( P_{H_2O} \) — Water vapor pressure (47 mmHg at 37°C)
• \( FiO_2 \) — Fraction of inspired oxygen (0.21 for room air)
• \( PaCO_2 \) — Arterial carbon dioxide partial pressure (mmHg)
• \( R \) — Respiratory quotient (typically 0.8)

Explanation: The equation accounts for oxygen dilution by water vapor and the effect of CO₂ on alveolar oxygen concentration.

3. Importance of PAO₂ Calculation

Details: PAO₂ is essential for calculating the alveolar-arterial (A-a) gradient, which helps differentiate causes of hypoxemia and assess lung function.

4. Using the Calculator

Tips: Enter all required values. Defaults are provided for sea-level conditions (PB=760, PH₂O=47, FiO₂=0.21, PaCO₂=40, R=0.8). Adjust for altitude or different clinical scenarios.

5. Frequently Asked Questions (FAQ)

Q1: What is a normal PAO₂ value?
A: At sea level breathing room air, normal is about 100 mmHg. It decreases with altitude and increases with supplemental oxygen.

Q2: Why is water vapor pressure important?
A: Water vapor dilutes inspired gases in the alveoli. At body temperature (37°C), water vapor pressure is 47 mmHg.

Q3: What is the respiratory quotient (R)?
A: R is the ratio of CO₂ produced to O₂ consumed. It's typically 0.8 for normal diets but varies with metabolism.

Q4: How does altitude affect PAO₂?
A: Lower barometric pressure at altitude reduces PAO₂. For example, at 5000 ft, PB ≈ 632 mmHg.

Q5: What's the difference between PAO₂ and PaO₂?
A: PAO₂ is alveolar oxygen, while PaO₂ is arterial oxygen. The difference (A-a gradient) indicates gas exchange efficiency.