Rox = (SpO2/FR)/FiO2
During high-flow nasal cannula (HFNC) therapy in patients with acute hypoxemic respiratory failure, it can be desirable not to delay intubation and have an adverse event.

The ROX index, defined as the ratio of oxygen saturation as measured by pulse oximetry/FIO2 to respiratory rate, has been assessed as a predictor of the need to intubate in patients received HFNC oxygen therapy.

Prediction accuracy of the ROX index increased over time with AUC of 0.679 at 2 h, 0.703 at 6 hours and 0.759 at 12 hours.

ROX ≥4.88 at 2, 6 and 12 hours after HFNC initiation was associated with a lower risk for intubation.

Predictors of HFNC failure include:

ROX <2.85 at 2 hours
ROX <3.47 at 6 hours
ROX <3.85 at 12 hours

References

Roca O et al.
An Index Combining Respiratory Rate and Oxygenation to Predict Outcome of Nasal High-Flow Therapy.
Am J Respir Crit Care Med. 2019 Jun 1;199(11):1368-1376. doi: 10.1164/rccm.201803-0589OC.