Alveolar Gas Equation Calculator
Calculate the partial pressure of alveolar oxygen (PAO2) using the alveolar gas equation. PAO2 = FiO2 x (Patm - PH2O) - PaCO2/RQ, where standard values are Patm = 760 mmHg, PH2O = 47 mmHg, and RQ = 0.8. Essential for computing the A-a gradient.
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Formula
Determines the theoretical maximum oxygen pressure in the lungs based on altitude (Patm) and carbon dioxide levels.
Worked Example
The Alveolar Gas Equation
- Formula: PAO2 = FiO2 x (Patm - PH2O) - (PaCO2 / RQ)
- Standard sea-level constants: Patm = 760 mmHg, PH2O = 47 mmHg, RQ = 0.8
- On room air at sea level with PaCO2 of 40: PAO2 = 0.21 x 713 - 50 = approximately 100 mmHg
- The equation is the first step in calculating the A-a gradient for hypoxemia workup
- At altitude, reduced Patm significantly lowers PAO2, explaining altitude-related hypoxia
PAO2 is a calculated value, not a measured one. It requires an accurate PaCO2 from an arterial blood gas and knowledge of the patient's FiO2 and altitude.
You can also calculate changes using our A-a Gradient Calculator or Oxygen Requirement Calculator.
Frequently Asked Questions
What is PAO2?
PAO2 (capital A) is the partial pressure of oxygen inside the alveoli. On room air at sea level, normal PAO2 is approximately 100 mmHg.
What is the Respiratory Quotient (R)?
It is the ratio of CO2 produced to O2 consumed. A standard value is 0.8, reflecting a normal mixed diet. Pure carbohydrate metabolism gives RQ of 1.0, while pure fat gives 0.7.
How does altitude change this?
At higher altitudes, barometric pressure (Patm) is lower, which lowers the PAO2. For example, at Denver (5,280 feet), Patm is about 630 mmHg, giving a PAO2 of approximately 73 mmHg on room air.
Why is water vapor pressure subtracted?
Inspired air is humidified to 100% in the upper airways, so 47 mmHg of the total pressure is occupied by water vapor at body temperature (37C). This leaves less pressure available for oxygen and nitrogen.
How is PAO2 used clinically?
PAO2 is primarily used to calculate the A-a gradient (PAO2 - PaO2). This helps determine whether hypoxemia is caused by lung parenchymal disease or by extrapulmonary factors like hypoventilation.
What is a normal PAO2 on room air at sea level?
Using standard values (FiO2 0.21, Patm 760, PH2O 47, PaCO2 40, RQ 0.8), PAO2 = 0.21 x 713 - 50 = approximately 100 mmHg. Actual values vary with PaCO2 and altitude.
Medical Disclaimer
This calculator is provided for informational and educational purposes only. It is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making any decisions based on these results. Do not disregard or delay seeking medical advice because of information obtained from this tool.
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