What it is
The Oxygenation Index (OI) quantifies how much ventilator support is being spent to achieve a given arterial oxygen level. Because it incorporates mean airway pressure, it captures the cost of oxygenation — unlike a bare PaO₂/FiO₂ ratio. Under PALICC-2, OI (or its non-invasive cousin OSI, the Oxygen Saturation Index) is the primary metric for defining and stratifying pediatric ARDS (PARDS) in children on invasive mechanical ventilation, with PaO₂ used preferentially when an arterial line is available.
The method
OI = (FiO₂ × mean airway pressure) / PaO₂
FiO₂ as a percentage, mean airway pressure in cmH₂O, PaO₂ in mmHg. Higher OI = worse disease. The runner accepts PaO₂ in kPa and converts internally (×7.501).
PALICC-2 severity bands (invasive ventilation)
- OI 4 to <8 — mild PARDS
- OI 8 to <16 — moderate PARDS
- OI ≥16 — severe PARDS
PALICC-2 advises optimising oxygenation to a target SpO₂ of 88–97% before computing the index, and stratifying severity at least 4 hours after the initial diagnosis rather than on the first gas.
Why children use OI, not the adult Berlin P/F
The adult Berlin definition grades ARDS on PaO₂/FiO₂ at PEEP ≥5. PALICC-2 deliberately moved pediatric stratification to OI/OSI because they reflect the applied airway pressure and perform better across the wide range of pediatric ventilation. Reserve the P/F ratio for the non-invasive/contextual situation; for an intubated child with an arterial line, OI is the metric of record.
Worked example
FiO₂ 60%, mean airway pressure 14 cmH₂O, PaO₂ 70 mmHg → OI = (60 × 14) / 70 = 12.0 → moderate PARDS. If, four hours later, the same child needs FiO₂ 80% and MAP 18 to hold a PaO₂ of 65, the OI climbs to (80 × 18) / 65 = 22.2 — now severe, and a clear signal that lung-protective strategy and adjuncts deserve escalation. Combine the absolute value with its trend and the broader clinical picture before acting.
Pitfalls and caveats
- MAP stays in cmH₂O — do not divide it by 1.36. A common spreadsheet error inflates or deflates OI; this runner keeps the units correct.
- Time it right: stratify ≥4 h post-diagnosis, after oxygenation has been optimised, not on a transient desaturation.
- OSI substitutes when no arterial line exists — use SpO₂-based indices only while SpO₂ ≤97% (above that the value saturates and underestimates severity).
- OI guides severity, not a specific therapy; couple it with the full ventilation and hemodynamic picture.
Run it: Oxygenation Index
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.