PaO2/FiO2 Ratio (P/F Ratio)

OVERVIEW

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.

ALTERNATIVES TO P/F RATIO

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

ADVANTAGES OF P/F RATIO

• 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. APACHE IV, SOFA, SAPS-II and SAPS-III
  • 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	Mortality
Mild	200 – 300	27%
Moderate	100 – 200	32%
Severe	< 100	45%

DISADVANTAGES OF P/F RATIO

• 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)

RULE OF THUMB FOR USE OF P/F RATIO

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!

References and Links

LITFL

• CCC — Acute Respiratory Distress Syndrome (ARDS) Definitions
• CCC — Oxygen-Haemoglobin Dissociation Curve
• CCC — Oxygen Saturation Targets in Critical Care
• Part One – Shunt

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

• Deranged Physiology — Oxygen tension – based indices of oxygenation