AACN Charles Towne Chapter

How to Calculate the PaO2 / FIO2 Ratio

A PaO2/FIO2 ratio is an index to characterize the acute respiratory distress syndrome (ARDS), which
involves severe hypoxemia (insufficient oxygen content in blood).

PaO2 is the partial pressure of oxygen in arterial blood. It's usually measured in millimeters of mercury
(mmHg or Torr) by the test called arterial blood gas (ABG) analysis. PaO2 of 75 to 100 mmHg is
considered normal.

FIO2 is the fraction of inspired oxygen or, simply percentage of oxygen, in a gas mixture. For example,
the atmospheric air has FIO2 of 21 percent.

Instructions

1. Obtain a PaO2 value in mmHg
     Example PaO2 = 92 mmHg


2. Convert the FIO2 (%) value into decimal form
     FIO2 = FIO2 (%) / 100%.

     Example:
     FIO2=32% / 100%=0.32


3. Calculate the PaO2/FIO2 ratio taking numbers from Step 1 and 2.

     Example:
     PaO2 / FIO2 = 92 mmHg / 0.32 = 287.5 mmHg.
     If PaO2 / FIO2 < 200 mmHg   =  Adult Respiratory Distress Syndrome (ARDS)
     If PaO2 / FIO2 < 300 mmHg = Acute Lung Injury (ALI)