What it is
The Oxygen Saturation Index (OSI) is the non-invasive twin of the Oxygenation Index (OI): it grades the severity of pediatric ARDS (PARDS) using pulse-oximeter saturation in place of an arterial PaO₂. Like OI, it folds in mean airway pressure, so it reflects the cost of oxygenation rather than oxygenation alone — but it needs no arterial line, which makes it the practical severity metric in many intubated children.
The method
OSI = (FiO₂ × mean airway pressure) / SpO₂
In the runner, FiO₂ is entered as a percentage (e.g. 60), mean airway pressure in cmH₂O, and SpO₂ as a percentage. Higher OSI = worse oxygenation.
PALICC severity bands (invasive ventilation)
- OSI < 5 — below the PARDS threshold
- 5 to < 7.5 — mild PARDS
- 7.5 to < 12.3 — moderate PARDS
- ≥ 12.3 — severe PARDS
These mirror the OI bands and apply to children on invasive mechanical ventilation.
When to use it
Use OSI to define and stratify PARDS in a ventilated child when an arterial gas is unavailable or you want continuous, non-invasive trending. PALICC-2 accepts OI or OSI as the primary severity metric on invasive ventilation, with PaO₂-based OI preferred whenever an arterial line exists. It is also handy for serial reassessment without repeated arterial sticks.
Worked example
FiO₂ 60%, mean airway pressure 14 cmH₂O, SpO₂ 92%:
- OSI = (60 × 14) / 92 = 840 / 92 = 9.1
That falls in the 7.5–12.3 band → moderate PARDS.
Pitfalls and caveats
- Valid only with SpO₂ ≤97%. Above that the SpO₂–PaO₂ relationship flattens onto the top of the dissociation curve, so OSI loses discrimination — titrate FiO₂ down to bring SpO₂ into the usable range before reading it.
- Enter FiO₂ as a percentage in this tool (60, not 0.6).
- It requires mean airway pressure, so it is meaningful on invasive ventilation, not bare oxygen by mask.
- Confirm against the OI when an arterial gas becomes available, particularly at decision points (proning, paralysis, ECMO referral).
- Poor pulse-oximetry tracing, dyshaemoglobinaemia, or marked perfusion abnormality degrade the SpO₂ and therefore the index.
Run it: Oxygen Saturation Index (OSI)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.