Oxygenation Index and OSI Calculator
Oxygenation Index and OSI Calculator
The oxygenation index and the oxygen saturation index use MEAN AIRWAY PRESSURE, not PEEP, which is what separates them from the P/F ratio. Both are the oxygenation criterion of the paediatric ARDS definition.
Oxygenation index and OSI
MAP-based, PALICC strataOxygenation index; mean airway pressure 15 cmH₂O, FiO₂ 60%, PaO₂ 70 mmHg
Formula
OSI = (FiO₂ × mean airway pressure × 100) ÷ SpO₂ (%)
with FiO₂ as a fraction; entering FiO₂ as a percentage does the same arithmetic
- mean airway pressure
- the average airway pressure over the whole respiratory cycle, in cmH₂O. It is NOT PEEP and not the peak. It rises with PEEP, with inspiratory pressure and with inspiratory time, which is why these indices capture the cost of the oxygenation as well as the oxygenation
- why not just use P/F
- a P/F ratio of 150 on 5 cmH₂O of mean airway pressure and the same ratio on 25 are not the same patient. The oxygenation index puts the support in the numerator, so it rises when either the oxygenation worsens or the pressure sustaining it increases
- OSI, and its ceiling
- substituting SpO₂ for PaO₂ makes the index non-invasive and bounds it: SpO₂ cannot exceed 100, so the denominator barely moves while PaO₂ can range over hundreds. PALICC-2 therefore requires SpO₂ to be 97% or below when the OSI is used
- which definition, and when to measure
- PALICC 2015 published three strata for each index; PALICC-2 in 2023 published two, with the severe boundary at an OI of 16 or above or an OSI of 12 or above. The OI boundary is the same in both, the OSI boundary moved from 12.3 to 12. Both define the stratum on a measurement made at least 4 hours after the initial PARDS diagnosis
Worked example
Oxygenation index; mean airway pressure 15 cmH₂O, FiO₂ 60%, PaO₂ 70 mmHg
OI = (0.60 × 15 × 100) ÷ 70 = 900 ÷ 70 = 12.86
12.86 sits in the 8-to-16 band: moderate PARDS by PALICC 2015, and mild-to-moderate by PALICC-2, which uses one band below 16
The OSI on the same support with an SpO₂ of 92% is 900 ÷ 92 = 9.78, the moderate OSI band of 7.5 to 12.3 — the two indices agree on the stratum here, and they need not
For comparison the P/F ratio is 70 ÷ 0.6 = 117 mmHg, moderate ARDS by the adult Berlin definition, and it says nothing about the 15 cmH₂O of mean airway pressure used to achieve it
Halve the mean airway pressure to 7.5 for the same gas exchange and the OI halves to 6.43 — the index is linear in the support
Raise the PaO₂ to 140 mmHg at unchanged support and the OI halves to 6.43 as well, which is the point: the same movement can come from either term
PALICC oxygenation strata, 2015 and 2023
| Severity | PALICC 2015 OI | PALICC 2015 OSI | PALICC-2 2023 |
|---|---|---|---|
| Mild | 4 to under 8 | 5 to under 7.5 | Merged: mild or moderate is OI under 16, or OSI under 12 |
| Moderate | 8 to under 16 | 7.5 to under 12.3 | As above |
| Severe | 16 or above | 12.3 or above | OI 16 or above, or OSI 12 or above |
Which oxygenation criterion applies to which support
| Support | Criterion | Source |
|---|---|---|
| Invasive mechanical ventilation | Oxygenation index, or OSI where no arterial sample is available | PALICC 2015 and PALICC-2 2023 |
| Full-face non-invasive ventilation, CPAP or BiPAP with PEEP of at least 5 cmH₂O | PaO₂/FiO₂ of 300 or less, or SpO₂/FiO₂ of 250 or less | PALICC-2 2023 |
| Adults | PaO₂/FiO₂ bands of the Berlin definition, with PEEP of at least 5 cmH₂O | ARDS Definition Task Force, JAMA 2012 |
Why the paediatric definition counts the pressure
The adult Berlin definition grades oxygenation with the PaO₂/FiO₂ ratio and a single requirement that PEEP be at least 5 cmH₂O. That works where ventilator support is broadly similar between patients. In children it is not: a neonate and an adolescent, a child on 6 cmH₂O of mean airway pressure and one on 25, are not comparable on a P/F ratio alone, because the ratio says nothing about what it cost to achieve. So the paediatric definition puts mean airway pressure into the numerator. The oxygenation index is FiO₂ times mean airway pressure times 100, divided by PaO₂, and it rises both when oxygenation worsens and when the pressure sustaining it increases.
The oxygen saturation index is the same expression with pulse oximetry in place of the arterial tension, for the many children with no arterial line. The substitution has a built-in ceiling: saturation cannot pass 100, so the denominator barely moves while PaO₂ can range over hundreds. PALICC-2 therefore states that SpO₂ must be 97% or below when the OSI is used, and that PaO₂ is preferred wherever a sample can be obtained. That is not a formality — above 97% the oxyhaemoglobin dissociation curve is flat, and an OSI computed there will look better than the child is.
Which definition a number is read against matters, because there are two. The 2015 consensus conference published three strata for each index: for the oxygenation index, 4 to 8 mild, 8 to 16 moderate, 16 or above severe; for the OSI, 5 to 7.5, 7.5 to 12.3 and 12.3 or above. The 2023 second conference merged mild and moderate into one band and restated the severe boundaries as an oxygenation index of 16 or above or an OSI of 12 or above. The oxygenation-index boundary is unchanged; the OSI boundary moved slightly, so a value of 12.1 is moderate under one definition and severe under the other. Both conferences grade severity on a measurement taken at least four hours after the initial diagnosis, because the first hours reflect the support being set up rather than the disease.
This supports a clinician’s judgement rather than replacing it.
Frequently asked questions
How is the oxygenation index calculated?
FiO₂ as a fraction multiplied by mean airway pressure in cmH₂O multiplied by 100, divided by PaO₂ in mmHg. A mean airway pressure of 15, an FiO₂ of 0.6 and a PaO₂ of 70 mmHg give 900 ÷ 70 = 12.86. Entering the FiO₂ as a percentage and omitting the 100 is the same arithmetic.
What is the difference between the oxygenation index and the P/F ratio?
The oxygenation index includes mean airway pressure. A P/F ratio of 150 achieved on 5 cmH₂O of mean airway pressure and the same ratio on 25 cmH₂O describe very different patients, and only the oxygenation index distinguishes them. That is why the paediatric ARDS definition uses it while the adult Berlin definition uses P/F with a PEEP floor.
What are the PALICC severity cut-offs?
By the 2015 conference, on invasive ventilation: oxygenation index 4 to under 8 mild, 8 to under 16 moderate, 16 or above severe; OSI 5 to under 7.5, 7.5 to under 12.3, and 12.3 or above. PALICC-2 in 2023 uses two bands, severe at an oxygenation index of 16 or an OSI of 12.
Does the oxygenation index apply to adults?
It is the oxygenation criterion of the paediatric ARDS definition and its strata were defined in ventilated children. Adult ARDS severity is graded by the PaO₂/FiO₂ bands of the Berlin definition: the index can be computed in an adult, but the PALICC strata are not an adult classification.
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References
- Emeriaud G, López-Fernández YM, Iyer NP, et al. Executive summary of the Second International Guidelines for the Diagnosis and Management of Pediatric Acute Respiratory Distress Syndrome (PALICC-2). Pediatr Crit Care Med. 2023;24(2):143–65.
- Pediatric Acute Lung Injury Consensus Conference Group. Pediatric acute respiratory distress syndrome: consensus recommendations from the Pediatric Acute Lung Injury Consensus Conference. Pediatr Crit Care Med. 2015;16(5).
- Medicina Intensiva. Síndrome de distrés respiratorio agudo pediátrico: definición PALICC — a reproduction of the 2015 PALICC Table 1 with the OI and OSI equations and strata. medintensiva.org
- ARDS Definition Task Force. Acute respiratory distress syndrome: the Berlin definition. JAMA. 2012;307(23):2526–33.
- Rauf A, Sachdev A, Venkataraman ST, Dinand V. Dynamic airway driving pressure and outcomes in children with acute hypoxemic respiratory failure. Respir Care. 2021;66(3):403–9.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/