This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
Watching high-flow oxygen
High-flow nasal cannula (HFNC) delivers warmed, humidified oxygen at up to 60 liters per minute through the nose. It helps many patients with acute respiratory failure avoid a breathing tube. But when it fails, waiting too long before intubation is linked with worse outcomes. The ROX index, published by Oriol Roca and colleagues in 2016 and validated in 2019, gives a simple way to track whether the treatment is working.
The formula
ROX = (SpO₂ ÷ FiO₂) ÷ respiratory rate
- SpO₂ is the oxygen saturation from the pulse oximeter, in percent.
- FiO₂ is the fraction of inspired oxygen set on the device, as a fraction (50% = 0.50).
- Respiratory rate is in breaths per minute.
A higher number is better: good oxygen levels on little oxygen with a calm breathing rate.
Worked example
After 6 hours on high-flow oxygen, a patient has an SpO₂ of 94% on an FiO₂ of 50% and breathes 24 times a minute:
- SpO₂/FiO₂ = 94 ÷ 0.50 = 188.
- ROX = 188 ÷ 24 = 7.83, above 4.88: lower risk of needing intubation.
- To stay at 4.88 or more with this saturation and oxygen, the breathing rate would have to stay at or below about 39 a minute.
A second patient, after 2 hours, has an SpO₂ of 88% on 90% oxygen and breathes 36 times a minute: 97.8 ÷ 36 = 2.72, below the 2-hour cutoff of 2.85.
The thresholds
| Time on HFNC | Lower risk | High risk of failure |
|---|---|---|
| 2 hours | 4.88 or more | below 2.85 |
| 6 hours | 4.88 or more | below 3.47 |
| 12 hours | 4.88 or more | below 3.85 |
Between the two thresholds, the usual advice is to repeat the measurement within an hour or two. The trend matters as much as a single value: a ROX that keeps falling is a warning sign even above the cutoff.
Limits
- The index was developed in adults with pneumonia; it has been studied in COVID-19 and other conditions, with similar but not identical cutoffs.
- Pulse oximetry becomes less accurate at low saturations and in people with darker skin, and an SpO₂ of 98 to 100% hides how much reserve remains.
- It does not measure the work of breathing, the blood gases or mental status, all of which guide the decision.
The decision to intubate always rests with the critical care team, who weigh the ROX index with everything else they see at the bedside.