PaO₂/FiO₂ Ratio Calculator

About The PaO2/FiO2 Ratio

The PaO2/FiO2 ratio, also known as the P/F ratio, is a simple calculation used in medicine to assess the efficiency of oxygen transfer from the lungs to the bloodstream. It's particularly helpful in evaluating the severity of various respiratory conditions, like ARDS.

Here's how to calculate the PaO2/FiO2 ratio:

1. Gather the necessary values:

  • PaO2: This is the partial pressure of oxygen in arterial blood, measured in millimeters of mercury (mmHg). It usually requires an arterial blood gas (ABG) test.
  • FiO2: This is the fraction of inspired oxygen, which represents the percentage of oxygen in the air a patient is breathing. It can be obtained from the oxygen delivery device settings.

2. Perform the calculation:

Divide the PaO2 value by the FiO2 value. The formula is: 

                  P/F ratio = PaO2 / FiO2

3. Interpret the result:

A normal P/F ratio in adults breathing room air (FiO2 = 0.21) is generally considered to be above 300 mmHg. Lower values may indicate impaired oxygenation due to various lung issues. However, it's important to remember that the interpretation of the P/F ratio can vary depending on several factors like:

  • Underlying medical condition
  • Patient's age and overall health
  • Use of additional respiratory support like mechanical ventilation

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