Critical Care
P/F Ratio (PaO₂/FiO₂)
Oxygenation; ARDS severity (Berlin).
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Background
The P/F ratio is the partial pressure of arterial oxygen (PaO₂) divided by the fraction of inspired oxygen (FiO₂), and it is the core measure of oxygenation efficiency used to define and grade acute respiratory distress syndrome (ARDS). It normalises the arterial oxygen level for the amount of supplemental oxygen being given, so a low ratio indicates that high oxygen delivery is achieving poor arterial oxygenation. Under the Berlin definition, ARDS grading assumes the patient is receiving at least 5 cmH₂O of PEEP or CPAP. The ratio is expressed in mmHg.
Interpreting the result
Using the Berlin thresholds (on PEEP/CPAP ≥5 cmH₂O): a ratio of 200–300 indicates mild ARDS, 100–199 moderate ARDS and below 100 severe ARDS, while values of 300 or above lie in the normal or non-ARDS range. Lower ratios correlate with worse mortality and identify patients who may benefit from interventions such as prone positioning or lung-protective strategies. The ratio is sensitive to PEEP and FiO₂, so it should always be interpreted with the ventilator settings in mind.
Worked example
A ventilated patient on FiO₂ 0.8 has a PaO₂ of 80 mmHg: P/F = 80/0.8 = 100. At a PEEP of at least 5 cmH₂O this meets the threshold for moderate ARDS (just above the severe cut-off of 100).
Critical actions
Enter PaO₂ in mmHg (multiply kPa by 7.5) and FiO₂ as a fraction. Berlin ARDS grading assumes PEEP/CPAP ≥5 cmH₂O.
Pearls / pitfalls
- PaO₂ must be in mmHg — multiply a kPa value by 7.5 — and FiO₂ entered as a fraction.
- The ratio depends on PEEP and FiO₂: the same patient can shift ARDS category as ventilator settings change, so record the conditions of measurement.
- At high altitude or low barometric pressure the ratio falls for a given lung injury, which the unadjusted value does not account for.
- Berlin grading requires PEEP/CPAP ≥5 cmH₂O; a low ratio off PEEP does not by itself confirm ARDS.
Evidence & validation
The PaO₂/FiO₂ ratio underpins the Berlin definition of ARDS produced by the ARDS Definition Task Force, which replaced the earlier American–European Consensus Conference criteria, and remains the standard oxygenation measure for ARDS classification and trial enrolment.
Frequently asked questions
How do I convert PaO₂ from kPa to mmHg?
Multiply the kPa value by 7.5. For example, a PaO₂ of 10 kPa is 75 mmHg. The P/F ratio is conventionally expressed using mmHg.
What P/F ratio defines ARDS?
On at least 5 cmH₂O of PEEP or CPAP, a ratio of 200–300 is mild, 100–199 moderate and below 100 severe ARDS. A ratio of 300 or more is the normal or non-ARDS range.
Why does PEEP matter for the P/F ratio?
PEEP recruits collapsed lung and improves oxygenation, raising the ratio. The Berlin definition therefore specifies a minimum PEEP/CPAP of 5 cmH₂O so that grading is comparable between patients.
Can the P/F ratio change without the lungs changing?
Yes. Adjusting FiO₂ or PEEP alters the ratio even when lung injury is unchanged, so the value must be read together with the ventilator settings at the time of the blood gas.
Does a low P/F ratio mandate any specific treatment?
It identifies severity rather than prescribing therapy. A low ratio supports lung-protective ventilation and may prompt prone positioning, but management is individualised within ARDS care pathways.
References
- ARDS Definition Task Force; Ranieri VM, Rubenfeld GD, Thompson BT, et al. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526–2533.
- Grasselli G, Calfee CS, Camporota L, et al. ESICM guidelines on acute respiratory distress syndrome: definition, phenotyping and respiratory support strategies. Intensive Care Med. 2023.
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