How to Calculate the PaO2/FIO2 Ratio

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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. If a patient needs mechanical ventilation, FIO2 is usually in the 30-to-40-percent range.

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

Convert the FIO2 (%) value into decimal form. FIO2=FIO2(%)/100%.
Example: FIO2=32%/100%=0.32

Calculate the PaO2/FIO2 ratio taking numbers from Step 1 and 2 as an example. PaO2/FIO2=92 mmHg/0.32=287.5 mmHg.

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

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  • The Ins and Outs of Respiration: Gas Exchange and the Alveolar Air Equation; S. Kraman, Clinical Pulmonary Medicine, Vol. 13, pp 296-300; Sep 2006
  • Relation between PaO2/FIO2 ratio and FIO2: a mathematical description; J. Aboab, B. Louis, B. Jonson and L. Brochard, Intensive Care Medicine, Vol. 32, pp.1494–1497; 2006
  • Determination of PO2 and its heterogeneity in single capillaries; L. Zheng, A.S. Golub and R.N.Pittman, Am J Physiol Heart Circ Physiol, Vol. 271, pp. H365-H372; 1996
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