e.g. 0.6 = 60% oxygen
Enter FiO2 (0.21-1.0).
From arterial blood gas
Enter PaO2 (20-600 mmHg).
From ventilator display (optional for P/F only)
Pulse oximetry (leave blank if using PaO2 only)
Berlin criteria requires PEEP ≥ 5
P/F Ratio (PaO2/FiO2)
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⚠️ Medical Disclaimer: ARDS classification and clinical decisions require full physician assessment. This calculator is for educational reference only. OI and P/F ratio are one component of ARDS diagnosis, which also requires imaging, etiology, and clinical context.
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Sources & Methodology
OI and P/F ratio per Berlin ARDS Definition (JAMA 2012) and PALICC-2 Pediatric ARDS Guidelines (2023).
The ARDS Definition Task Force — JAMA (2012)
Berlin Definition of ARDS: P/F ratio classification with PEEP criteria. jamanetwork.com
PALICC-2 — Pediatric Acute Lung Injury Consensus Conference (2023)
Pediatric ARDS definition using OI and OSI as primary severity measures. PCCM Journal
Khemani RG et al. — Lancet Respiratory Medicine (2012)
Validation of Oxygen Saturation Index (OSI) as a non-invasive surrogate for OI in pediatric ARDS.
P/F ratio = PaO2 / FiO2
OI = (FiO2 x Pmean x 100) / PaO2
OSI = (FiO2 x Pmean x 100) / SpO2
Berlin ARDS: P/F 200-300 = Mild, 100-200 = Moderate, <100 = Severe (on PEEP ≥5 cmH2O)
OI = (FiO2 x Pmean x 100) / PaO2
OSI = (FiO2 x Pmean x 100) / SpO2
Berlin ARDS: P/F 200-300 = Mild, 100-200 = Moderate, <100 = Severe (on PEEP ≥5 cmH2O)
OI = (FiO2 x Pmean x 100) / PaO2
Example: FiO2 0.6, Pmean 16 cmH2O, PaO2 80 mmHg.
P/F = 80 / 0.6 = 133 mmHg (Moderate ARDS)
OI = (0.6 x 16 x 100) / 80 = 960 / 80 = 12 (Moderate ARDS)
P/F = 80 / 0.6 = 133 mmHg (Moderate ARDS)
OI = (0.6 x 16 x 100) / 80 = 960 / 80 = 12 (Moderate ARDS)
Last reviewed: April 2026
ARDS Classification and Oxygenation Indices
Acute Respiratory Distress Syndrome (ARDS) is a life-threatening form of respiratory failure characterized by bilateral lung injury, hypoxemia, and reduced lung compliance. Oxygenation indices quantify the severity of gas exchange impairment relative to the ventilator support required to achieve it.
The P/F ratio (PaO2/FiO2) is the simplest oxygenation metric and forms the basis of the Berlin Definition. The Oxygenation Index (OI) is more comprehensive — it accounts for mean airway pressure, reflecting the ventilator burden required to maintain oxygenation. Two patients with the same P/F ratio can have very different OIs if one requires much higher airway pressures.
Berlin ARDS Criteria (Adult)
| Category | P/F Ratio (mmHg) | OI | Mortality Risk |
|---|---|---|---|
| No ARDS | Above 300 | Below 4 | Low |
| Mild ARDS | 200-300 | 4-8 | ~27% |
| Moderate ARDS | 100-200 | 8-16 | ~32% |
| Severe ARDS | Below 100 | Above 16 | ~45% |
Pediatric ARDS (PALICC-2) — OI-Based Classification
| Severity | OI | OSI | ECMO Consideration |
|---|---|---|---|
| Mild PARDS | 4-8 | 5-7.5 | Not typically |
| Moderate PARDS | 8-16 | 7.5-12.3 | Consider if refractory |
| Severe PARDS | Above 16 | Above 12.3 | Consider at OI ≥ 25 |
⚠️ ECMO Consideration: In pediatric ARDS, an OI above 25 on two measurements 4 hours apart is a traditional trigger for ECMO referral discussion. Some centers use OI above 40 as a stronger threshold. Adult ECMO criteria differ and consider refractory hypoxemia, acidosis, and clinical trajectory. Always involve ECMO specialists early in severe ARDS.
Frequently Asked Questions
OI = (FiO2 x Mean Airway Pressure x 100) / PaO2. It measures respiratory failure severity while accounting for ventilator support. Higher OI = worse failure. Normal OI below 4.
Normal OI is below 4. OI 4-8: mild respiratory failure. OI 8-16: moderate ARDS. OI above 16: severe ARDS. OI above 25 in pediatrics: consider ECMO referral.
P/F ratio = PaO2 / FiO2. Normal above 400 mmHg. Berlin ARDS: 200-300 = mild, 100-200 = moderate, below 100 = severe. All require PEEP at least 5 cmH2O.
Berlin 2012: P/F 200-300 (mild), 100-200 (moderate), below 100 (severe) — all on PEEP ≥5 cmH2O. Plus: bilateral opacities on imaging, non-cardiac etiology, acute onset within 1 week.
OSI = (FiO2 x Pmean x 100) / SpO2. A non-invasive surrogate for OI using pulse oximetry SpO2 instead of arterial PaO2. Valid when SpO2 is 88-97%.
FiO2 = Fraction of Inspired Oxygen (0.21-1.0). Room air = 0.21. Ventilated patients: read directly from ventilator. Non-invasive oxygen: estimated from device flow rate table.
The average pressure applied throughout the respiratory cycle on a mechanical ventilator, displayed on modern ventilators. Influenced by PEEP, inspiratory pressure, I:E ratio, and respiratory rate.
OI is preferred when comparing patients on different ventilator settings. Two patients with P/F 150 can have very different OIs if one requires much higher airway pressure. OI corrects for ventilator support level.
Traditional ECMO referral: OI above 25 on two measurements 4 hours apart. Some centers use OI above 40. Adult ECMO criteria differ — involve ECMO specialists early in severe ARDS.
PaO2: partial pressure of oxygen in arterial blood (blood gas, mmHg). SpO2: oxygen saturation by pulse oximetry (%). PaO2 is more precise; SpO2 is non-invasive. OSI uses SpO2 as a surrogate when PaO2 is unavailable.
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