Alveolar Oxygen Pressure (PAO₂) Calculator on Mechanical Ventilation (50% O₂)
ICU patient receiving 50% inspired oxygen via mechanical ventilation with normal CO₂ levels.
Estimates alveolar oxygen pressure using the alveolar gas equation. Enter your FiO₂ (Fraction of Inspired Oxygen), Atmospheric Pressure (Patm), Water Vapor Pressure (PH₂O), Arterial CO₂ Pressure (PaCO₂), Respiratory Quotient (R) to get an instant alveolar oxygen pressure (pao₂). Formula: (fio2 * (patm - ph2o)) - (paco2 / r).
Alveolar Oxygen Pressure (PAO₂)
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How It Works
How It Works
This calculator estimates the amount of oxygen available in the alveoli (air sacs of the lungs) using the alveolar gas equation. It combines the oxygen you breathe in with the effects of pressure and carbon dioxide levels in your blood.
First, it adjusts the total atmospheric pressure by subtracting water vapor pressure. Then it multiplies that result by the fraction of oxygen you are breathing (FiO₂). Finally, it subtracts the effect of carbon dioxide (PaCO₂ divided by the respiratory quotient) to give the final alveolar oxygen pressure (PAO₂).
- Subtract water vapor pressure (PH₂O) from atmospheric pressure (Patm)
- Multiply the result by FiO₂ to find available inspired oxygen
- Divide PaCO₂ by the respiratory quotient (R)
- Subtract that value to get PAO₂
Understanding the Results
The result shows the estimated oxygen pressure inside the alveoli, measured in mmHg. This value reflects how much oxygen is available to move from the lungs into the bloodstream.
Higher values generally mean more oxygen is available for gas exchange. Lower values may suggest reduced oxygen availability due to altitude, breathing problems, or abnormal carbon dioxide levels.
- Normal sea-level PAO₂ is usually around 75–100 mmHg on room air
- Higher FiO₂ increases PAO₂
- Higher PaCO₂ lowers PAO₂
- Changes in atmospheric pressure (such as altitude) affect the result
Frequently Asked Questions
What does the Alveolar Oxygen Pressure (PAO₂) calculator measure?
This calculator estimates the partial pressure of oxygen in the alveoli (PAO₂) using the alveolar gas equation. PAO₂ reflects the amount of oxygen available for gas exchange in the lungs. It is an important value for assessing oxygenation status and identifying potential respiratory issues.
When should I use the PAO₂ calculator?
You should use this calculator when evaluating a patient’s oxygenation status, especially in clinical or educational settings. It is commonly used in respiratory physiology, critical care, and anesthesia to assess gas exchange efficiency. It can also help calculate the alveolar-arterial (A–a) gradient when compared to measured arterial oxygen pressure (PaO₂).
What is the typical normal value for PAO₂?
At sea level, breathing room air (FiO₂ = 0.21), with normal values for atmospheric pressure (760 mmHg), water vapor pressure (47 mmHg), PaCO₂ (40 mmHg), and R (0.8), PAO₂ is typically around 100 mmHg. Normal values may vary with altitude, oxygen supplementation, or changes in ventilation. Always interpret results in the clinical context.
Why do I need to enter water vapor pressure (PH₂O)?
Water vapor pressure accounts for the humidification of inspired air in the respiratory tract. At normal body temperature (37°C), PH₂O is approximately 47 mmHg. Including this value ensures the calculation accurately reflects the effective oxygen pressure reaching the alveoli.
What is the respiratory quotient (R), and why is it important?
The respiratory quotient (R) represents the ratio of carbon dioxide produced to oxygen consumed, typically around 0.8 for a mixed diet. It adjusts the impact of carbon dioxide on alveolar oxygen levels in the equation. Using an accurate R value improves the precision of the PAO₂ estimate.
How does increasing FiO₂ affect PAO₂?
Increasing the fraction of inspired oxygen (FiO₂) raises the calculated PAO₂ because more oxygen is delivered to the alveoli. For example, administering supplemental oxygen (FiO₂ greater than 0.21) will significantly increase alveolar oxygen pressure. This is commonly used in clinical settings to improve oxygenation in patients with respiratory compromise.
Disclaimer
This health calculator is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Disclaimer.