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How to calculate the pao2/fio2 ratio

Updated March 23, 2017

A PaO2/FIO2 ratio is an index to characterise the acute respiratory distress syndrome (ARDS), which involves severe hypoxaemia (insufficient oxygen content in blood). PaO2 is the partial pressure of oxygen in arterial blood. It's usually measured in millimetres of mercury (mm Hg 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 per cent. If a patient needs mechanical ventilation, FIO2 is usually in the 30-to-40-percent range.

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  1. Obtain a PaO2 value in mm Hg from records or elsewhere. Example PaO2=92 mm Hg. Note: determination of PaO2 requires a sample of the arterial blood and special laboratory instruments such as blood gas analyzers or spectrofluorometers (see References).

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

  3. FIO2=FIO2(%)/100%.

  4. Example: FIO2=32%/100%=0.32

  5. Calculate the PaO2/FIO2 ratio taking numbers from Step 1 and 2 as an example.

  6. PaO2/FIO2=92 mm Hg/0.32=287.5 mm Hg.

  7. Compare the PaO2/FIO2 ratio to the hyperaemia criterion that is if PaO2/FIO2 < 200 mmHg.

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