Adapted from

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PaO2/FiO2 ratio is the ratio of arterial oxygen partial pressure to fractional inspired oxygen
aka the Carrico index and the PF ratio.

To be honest, I feel we do not use this enough when at the end of the bed space.How many times have you heard the phrase, ‘they have ARDS’, but backed by no ratio evidence.

So, we need to use this more. Simple bedside calculators are available and you can find a link to one in the menu on this site.

Normal level should be >500


  • Quick and simple to do by the bed.
  • Shows you whether there is a large A-a gradient present: PaO2 should = FiO2 x 500 (e.g. 0.21 x 500 = 105 mmHg) [Then divide by 7.5 to convert to kPa (15.3).
  • Used in the SMART-COP risk score for intensive respiratory or vasopressor support in community-acquired pneumonia (PF ratio <333 mmHg if age <50y or PF ratio <250mmHg if age >50y)
  • Used as part of the Berlin definition of ARDS (PF ratio <300mmHg), and correlates with mortality

Screen Shot 2016-10-14 at 09.43.51.pngDISADVANTAGES

  • Dependent on barometric pressure – so must be adjusted if used in divers / climbers etc
    • Normal lungs (with a normal A-a gradient) will have lower PF ratios at high altitude and higher PF ratios at supra-atmospheric pressures
  • Can not distinguish hypoxaemia due to alveolar hypoventilation (high PACO2) from other causes such as V/Q mismatch (whereas the A-a gradient can)
  • Markedly dependent on FiO2, in part due to the shape of the Hb-O2 dissociation curve
  • Highly dependent on CaO2-CvO2 which tends to fluctuate markedly in sepsis
  • PF ratio should only be used as a rule of thumb for detecting an A-a gradient when:
    • the PaCO2 is normal
    • shunt is not suspected

This post has prompted us to emphasise the fact that we should probably be looking at this more in our patients and also, it has prompted us to pop a calculator onto the site for you all!