PaO2/FiO2 Ratio (P/F Ratio)


PaO2/FiO2 ratio is the ratio of arterial oxygen partial pressure (PaO2 in mmHg) to fractional inspired oxygen (FiO2 expressed as a fraction, not a percentage)

  • also known as the Horowitz index, the Carrico index, and (most conveniently) the P/F ratio
  • at sea level, the normal PaO2/FiO2 ratio is ~ 400-500 mmHg (~55-65 kPa)
  • MD Calc is an example of an online P/F ratio calculator – however it is quite easy to do “in your head”

P/F ratio is a widely used clinical indicator of hypoxaemia, though its diagnostic utility is controversial.


Alternative indices of oxygenation include:

  • Oxygen saturations in arterial blood (SpO2 and SaO2)
  • S/F ratio (SpO2 to FiO2 ratio)
  • PaO2 (arterial oxygen tension)
  • A-a gradient (difference between alveolar oxygen tension (PAO2) and PaO2)
  • Oxygenation index (OI) (the reciprocal of P/F times mean airway pressure (MAP): OI = (FiO2×MAP)/PaO2)
  • P/FP Ratio (PaO2/(FiO2 X PEEP)
  • a/A ratio (ratio of PaO2 and PAO2)
  • Respiratory index (RI) (RI = pO2(A-a)/pO2(a), ie. the A-a gradient divided by the PaO2; normal RI is <0.4)
  • Shunt fraction


  • Quick and simple (probably the main reason for it’s widespread use)
  • can be used as a rough guide to whether there is a significant A-a gradient present:
    • PaO2 should = FiO2 x 500 (e.g. 0.21 x 500 = 105 mmHg)
    • see caveats below
  • More practical than the a/A ratio, as measurement of alveolar oxygen tension (PAO2) is not required
  • Used in severity scoring systems
    • e.g. SMART-COP risk score for intensive respiratory or vasopressor support in community-acquired pneumonia (P/F ratio <333 mmHg if age <50y or PF ratio <250mmHg if age >50y)
    • e.g. part of the Berlin definition of Acute Respiratory Distress Syndrome (ARDS) (P/F ratio <300mmHg), and correlates with mortality (see below)
 ARDS Severity  PaO2/FiO2
Mild200 – 30027%
Moderate100 – 20032%
Severe< 10045%
Note that the ratios for the Berlin definitions was with a PEEP setting of 5 cmH20 or more


  • A better P/F ratio may not be associated with better outcomes
    • e.g. In the ARDSNet ARMA study the high tidal volume strategy had better P/F ratios, but worse outcomes
  • P/F ratio is dependent on barometric pressure (it is a “tension-based index”)
    • normal lungs (with a normal A-a gradient) will have lower PF ratios at high altitude and higher PF ratios at supra-atmospheric pressures
  • P/F ratio alone cannot distinguish hypoxaemia due to alveolar hypoventilation (high PACO2) from other causes such as V/Q mismatch and shunt
    • whereas A-a gradient based indices (e.g. a/A ratio and RI) can exclude alveolar hypoventilation
    • as shunt increases, the PaO2 tends to become less and less sensitive to the PAO2 and to the FIO2, and more dependent on the mixed venous O2 content and saturation
  • markedly dependent on FiO2
    • May be unreliable unless FiO2 > 0.5 and PaO2 < 100 mmHg
    • variation occurs with both right-to-left shunt (e.g. ARDS) and with widespread V/Q scatter (e.g. COPD)
    • varies with degree of shunt present – increasing the FIO2 causes the PaO2/FIO2 ratio to rise if intrapulmonary shunt is small, but to drop if the shunt is large 
    • as a result, P/F ratio will vary according to the chosen SpO2 (and hence PaO2) target, as the required FiO2 will vary
  • does not account for mean airway pressure or PEEP
    • The Oxygenation Index (OI) may be a more accurate measure of oxygenation dysfunction in ventilated patients as it accounts for mean airway pressure
    • P/FP Ratio adjusts the P/F ratio for the set PEEP
  • requires and arterial blood gas measurement
    • S/F ratio tends to correlates with P/F ratio and is non-invasive
  • highly dependent on CaO2-CvO2 (oxygen extraction)
    • arterial blood may appear well oxygenated despite lung dysfunction if mixed venous oxygen tension is high due to poor oxygen extraction by tissues, e.g. sepsis
    • P/F ratio may appear worse due to high oxygen extraction ratio (e.g. cardiogenic shock)
  • does not indicate oxygen content of the blood (dependent on haemoglobin) or oxygen delivery to tissues ( dependent on cardiac output and oxygen content)


P/F ratio should only be used as a rule of thumb for detecting an A-a gradient when:

  • the PaCO2 is normal, and
  • shunt is not suspected

The FiO2 used should always be specified.

A quick comparison of the patient’s PaO2 to the product of “500 x FiO2” is a magic trick for estimating a-A gradient that should be in the arsenal of every intensivist!


Journal articles

  • Broccard AF. Making sense of the pressure of arterial oxygen to fractional inspired oxygen concentration ratio in patients with acute respiratory distress syndrome. OA Critical Care 2013 Jun 01;1(1):9. [article]
  • The ARDS Definition Task Force*. Acute Respiratory Distress Syndrome: The Berlin Definition. JAMA. 2012;307(23):2526–2533. doi:10.1001/jama.2012.5669
  • Cane RD, et al. “Unreliability of oxygen tension-based indices in reflecting intrapulmonary shunting in critically ill patients.” Critical care medicine 16.12 (1988): 1243-1245.
  • Hahn CEW, Editorial I: KISS and indices of pulmonary oxygen transfer, BJA: British Journal of Anaesthesia, Volume 86, Issue 4, 1 April 2001, Pages 465–466, https://doi.org/10.1093/bja/86.4.465
  • Horovitz JH, Carrico CJ, Shires GT. Pulmonary response to major injury. Arch Surg. 1974;108(3):349–355. doi:10.1001/archsurg.1974.01350270079014
  • Karbing DS, Kjaergaard S, Smith BW, Espersen K, Allerød C, Andreassen S, Rees SE. Variation in the PaO2/FiO2 ratio with FiO2: mathematical and experimental description, and clinical relevance. Crit Care. 2007;11(6):R118.[pubmed]
  • Kathirgamanathan A, Mccahon RA, Hardman JG. Indices of pulmonary oxygenation in pathological lung states: an investigation using high-fidelity, computational modelling. Br J Anaesth. 2009;103(2):291-7. [article]
  • Nirmalan M, et al. Effect of changes in arterial‐mixed venous oxygen contentdifference (C (a–v̄) O2) on indices of pulmonary oxygen transfer in a model ARDS lung†,††. British journal of anaesthesia 86.4 (2001): 477-485. https://doi.org/10.1093/bja/86.4.477
  • Rice TW, Wheeler AP, Bernard GR, et al. Comparison of the SpO2/FIO2 ratio and the PaO2/FIO2 ratio in patients with acute lung injury or ARDS. Chest. 2007;132(2):410–417. doi:10.1378/chest.07-0617 [pubmed]
  • Wandrup JH. Quantifying pulmonary oxygen transfer deficits in critically ill patients. Acta Anaesthesiologica Scandinavica 39.s107 (1995): 37-44. https://doi.org/10.1111/j.1399-6576.1995.tb04328.x
  • Whiteley JP, Gavaghan DJ, Hahn CE. Variation of venous admixture, SF6 shunt, PaO2, and the PaO2/FIO2 ratio with FIO2. Br J Anaesth. 2002 Jun;88(6):771-8. PMID: 12173192.
  • Zetterström H. Assessment of the efficiency of pulmonary oxygenation. The choice of oxygenation index. Acta anaesthesiologica scandinavica 32.7 (1988): 579-584. https://doi.org/10.1111/j.1399-6576.1988.tb02789.x

FOAM and web resources

CCC 700 6

Critical Care


Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of three amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC


  1. You note that, “at sea level, the normal PaO2/FiO2 ratio is > 500 mmHg”.

    Not to quibble since I may not be understanding, but isn’t the normal PaO2/FiO2 ratio typically less than 500 at sea level?

    Assuming a PaO2 of 100 and FiO2 of 0.21, the ratio would be about 476, i.e. less than 500.


  2. What criteria are you using with reference to ARDS severity? I’m pretty sure that a baseline measurement or application of PEEP (>/= 5 cmH20) is required in for the Berlin criteria. The AECC criteria is no longer used to stratify severity of ARDS.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.