PaO₂/FiO₂ Ratio (ARDS)
Calculates the PaO₂/FiO₂ ratio and grades hypoxemia with the Berlin definition severity bands.
When to use
Adults with hypoxemic respiratory failure and an arterial blood gas, to describe oxygenation severity and compare it with the Berlin definition. It does not replace assessment of the full criteria.
Pearls and pitfalls
- The definition requires PEEP of at least 5 cm H2O, which in the mild group may be delivered by CPAP, plus onset within one week of the insult or of new or worsening symptoms, bilateral opacities on imaging, and edema not fully explained by cardiac failure or fluid overload.
- The ratio alone does not diagnose ARDS: the severity class only applies after the timing, imaging, origin of edema and minimum PEEP criteria are confirmed.
- FiO₂ is entered as a decimal fraction, from 0.21 to 1.0. Entering 40 instead of 0.40 divides the result by one hundred.
- Above 1,000 m of altitude, the paper corrects the ratio for barometric pressure: (PaO₂/FiO₂) x (barometric pressure/760).
- In the derivation cohort, mortality increased from 27% in mild ARDS to 32% in moderate and 45% in severe ARDS.
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Formula
PaO₂ (mmHg) divided by FiO₂ as a decimal fraction.
Evidence
Consensus definition evaluated in 4,188 patients from four multicenter cohorts and in 269 patients from three single-center studies. Mortality was 27% in mild ARDS, 32% in moderate and 45% in severe, and the area under the curve for mortality was higher than with the previous definition.
References
- ARDS Definition Task Force, Ranieri VM, Rubenfeld GD, Thompson BT, et al. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526-2533. doi:10.1001/jama.2012.5669
Frequently asked questions
What is PaO₂/FiO₂ Ratio (ARDS)?
Calculates the PaO₂/FiO₂ ratio and grades hypoxemia with the Berlin definition severity bands. Adults with hypoxemic respiratory failure and an arterial blood gas, to describe oxygenation severity and compare it with the Berlin definition. It does not replace assessment of the full criteria.
How is the result calculated?
PaO₂ (mmHg) divided by FiO₂ as a decimal fraction.
How should the result be interpreted?
Consensus definition evaluated in 4,188 patients from four multicenter cohorts and in 269 patients from three single-center studies. Mortality was 27% in mild ARDS, 32% in moderate and 45% in severe, and the area under the curve for mortality was higher than with the previous definition.
What are the sources?
ARDS Definition Task Force, Ranieri VM, Rubenfeld GD, Thompson BT, et al. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526-2533.