What it is
The PaO₂/FiO₂ ratio (P/F ratio) is the classic oxygenation index for acute respiratory distress. It is the backbone of the adult Berlin definition of ARDS, dividing arterial oxygen tension by the inspired oxygen fraction to give a single number that falls as the lung fails.
The method
P/F = PaO₂ (mmHg) / FiO₂ (as a decimal)
So a PaO₂ of 90 mmHg on 50% oxygen gives 90 / 0.50 = 180. Lower = worse.
Berlin severity bands (at PEEP ≥5 cmH₂O)
- >300 — no ARDS / normal
- 200–300 — mild ARDS
- 100–200 — moderate ARDS
- ≤100 — severe ARDS
The PEEP ≥5 requirement matters: the same gas at low PEEP overstates severity.
When to use it — and the pediatric caveat
The P/F ratio is the right tool for adult ARDS grading and remains useful as a quick, widely understood oxygenation index at the bedside. In children, PALICC-2 prefers the Oxygenation Index (OI) or Oxygen Saturation Index (OSI) as the primary severity metric whenever an arterial line and ventilator settings are available, because those indices incorporate mean airway pressure and track pediatric disease better across the range of support children receive. Use the P/F ratio in the pediatric context only as a supportive number — for example when mean airway pressure is unavailable, or to communicate severity in a shared adult/pediatric vocabulary — and reach for OI/OSI on an intubated child with an arterial line. When neither PaO₂ nor an arterial line is available, the SpO₂/FiO₂ (S/F) ratio is the non-invasive surrogate, with the same caveat that it saturates once SpO₂ exceeds ~97%.
Worked example
PaO₂ 80 mmHg on FiO₂ 60% → 80 / 0.60 = 133 → moderate ARDS by Berlin. In a ventilated child, recompute the OI before assigning a PARDS severity grade.
Pitfalls and caveats
- FiO₂ is a fraction, not a percent, in the formula — the runner takes the percentage and divides correctly; doing it by hand, use 0.60 not 60.
- Read it at PEEP ≥5 — a low-PEEP gas exaggerates severity.
- It ignores airway pressure, so two patients with the same P/F can be on very different support; that is precisely why pediatrics moved to OI.
- A single gas is a snapshot; stratify on a stable, optimised measurement, not a transient dip.
Run it: PaO₂/FiO₂ Ratio (P/F Ratio)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.