PaO₂/FiO₂ ratio (ARDS severity, Berlin)
Oxygenation index with Berlin ARDS severity grading; PaO₂ in kPa or mmHg
- ESICM 2023 ARDS guideline (Berlin definition)
Sources last checked: 2026-10-13
PaO₂/FiO₂ ratio (ARDS severity, Berlin)
Oxygenation index with Berlin ARDS severity grading; PaO₂ in kPa or mmHg
The ratio alone does not diagnose ARDS. Berlin requires PEEP/CPAP ≥ 5 cmH₂O at the time of measurement; the 2024 global definition also accepts high-flow nasal oxygen ≥ 30 L/min.
Interpretation
| ≤ 100 mmHg — severe ARDS range | Berlin: severe ARDS if the other criteria are met (PEEP/CPAP ≥ 5 cmH₂O, onset within 1 week, bilateral opacities, not fully explained by cardiac failure or fluid overload). |
|---|---|
| > 100–200 mmHg — moderate ARDS range | Berlin: moderate ARDS if the other criteria are met (PEEP ≥ 5 cmH₂O). |
| > 200–300 mmHg — mild ARDS range | Berlin: mild ARDS if the other criteria are met (PEEP or CPAP ≥ 5 cmH₂O). |
| > 300 mmHg | Does not meet the Berlin oxygenation criterion for ARDS. |
What is the PaO₂/FiO₂ ratio (ARDS severity, Berlin)?
The PaO₂/FiO₂ ratio (in German practice the Horovitz quotient) expresses arterial oxygenation relative to the inspired oxygen fraction. The Berlin definition of ARDS, developed through an ESICM-initiated consensus in 2012, uses it to grade ARDS as mild, moderate or severe at cut-offs of 300, 200 and 100 mmHg, measured with PEEP or CPAP of at least 5 cmH₂O.
When to use it
- Grading severity in a patient who otherwise meets the ARDS definition
- Daily monitoring of oxygenation on invasive or non-invasive ventilation
- Checking eligibility for interventions whose evidence is stratified by P/F (e.g. prone positioning in moderate–severe ARDS)
When not to use it
- As a stand-alone diagnosis of ARDS — timing, imaging and exclusion of cardiac failure are also required
- On low-flow oxygen devices where the true FiO₂ is unknown
- Comparing values obtained at very different PEEP levels without noting the PEEP
How it is calculated
P/F = PaO₂ (mmHg) ÷ FiO₂ (fraction) kPa → mmHg: × 7.50 Berlin cut-offs: 300 / 200 / 100 mmHg = 40 / 26.7 / 13.3 kPa
Clinical pearls and limitations
- The ratio alone does not diagnose ARDS. Berlin requires PEEP/CPAP ≥ 5 cmH₂O at the time of measurement; the 2024 global definition also accepts high-flow nasal oxygen ≥ 30 L/min.
- Enter FiO₂ as a percentage; 21 % is room air.
- In kPa the Berlin cut-offs are about 40, 26.7 and 13.3 kPa.
- The 2024 global ARDS definition keeps the same P/F cut-offs but also accepts high-flow nasal oxygen ≥ 30 L/min and SpO₂/FiO₂ when blood gases are unavailable.
- P/F depends on PEEP and FiO₂ itself, so trends on stable settings are more informative than single values.
Frequently asked questions
What P/F ratio defines severe ARDS?
Under the Berlin definition, P/F ≤ 100 mmHg (about 13.3 kPa) with PEEP ≥ 5 cmH₂O, provided the other ARDS criteria are met.
How do I calculate P/F from kPa?
Multiply PaO₂ in kPa by 7.5 to get mmHg, then divide by FiO₂ as a fraction. For example 9 kPa at FiO₂ 0.6 gives 67.5 / 0.6 = 112.5 mmHg.
Is the Horovitz quotient the same as the P/F ratio?
Yes. German-speaking ICUs often call PaO₂/FiO₂ the Horovitz quotient or oxygenation index.
Does the P/F ratio apply on high-flow nasal oxygen?
Not under Berlin, which requires PEEP/CPAP. The 2024 global definition allows ARDS to be diagnosed on high-flow oxygen at ≥ 30 L/min.
Evidence and guidelines
- Primary source: ARDS Definition Task Force, Ranieri VM, Rubenfeld GD, 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;49(7):727-759.
- Matthay MA, Arabi Y, Arroliga AC, et al. A new global definition of acute respiratory distress syndrome. Am J Respir Crit Care Med 2024;209(1):37-47 (same P/F cut-offs; adds HFNO ≥ 30 L/min and SpO₂/FiO₂).
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