ROX’ing with HFNC

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Sabrina Kroft
Sabrina Kroft
Registered Respiratory Therapist for 18 years working in all areas of healthcare. Professional interest is the nuances of mechanical ventilation and just learning new/old technology.

ROX Index 

How the utilization of non-invasive monitoring allows for up to date data points without increasing stress to the patient.

The use of high flow nasal cannula (HFNC) has increased as a treatment for Acute Hypoxic Respiratory Failure patients. Knowing if these patients are going to fly or fail on HFNC is crucial, as we do not want to delay intubation.  ROX index is a tool for this scenario. 

To use the ROX index we need to first discuss the SpO2/FiO2 (S/F) ratio. This ratio can be used as an alternative to the PaO2/FiO2 ratio and reduce the need for serial arterial blood gases (ABGs).  The PaO2/FiO2 (P/F) is widely known for its classification of severity of ARDS.  S/F has shown to correlate with P/F ratio and be a reliable estimation of the P/F ratio. S/F ratios of 235 corresponds with a P/F ratio of 200 and a S/F ratio of 315 corresponds with a P/F ratio of 300.  Multiple studies show the S/F ratio to be a dependable means of hypoxemia screening for patients.

What is the ROX index? 

 The ROX index (Respiratory rate OXygenation) is defined as the ratio of pulse oximetry oxygenation saturation/FiO2 to respiratory rate. To calculate the ROX Index you will first need to figure out the S/F ratio. The SpO2 will be obtained and divided by the FiO2 (in decimal form) the patient is currently receiving. Next, an accurate respiratory rate will be counted, the ROX Index depends on the accuracy of this count. Once the two numbers are gathered, the S/F ratio will be divided by the respiratory rate.  The end result will be the ROX index.

This tool can be used as a predictor in the success of HFNC on Acute Hypoxic Respiratory Failure patients. Calculations should at least be made at the 2, 6, and 12 hour mark. Studies showed that patients with a ROX of 4.88 or higher throughout treatment with HFNC, were less likely to be intubated. The probability of HFNC failure increased when the ROX at the 6 hour mark decreased. Patients that showed a minimal increase in ROX from hours 2, 6, and 12 were also more likely to fail HFNC. Patients with minimal increase or slight decrease in ROX will need closer monitoring to identify their potential need for intubation. 

Adding the ROX index to assessments in HFNC patients will allow for an early prediction of failure. The use of ROX can also decrease the amount of ABGs performed on Acute Hypoxic Respiratory Failure patients. 


Non-invasive tools that practitioners can calculate quickly and accurately by visual assessments have never been more necessary in triage management. Practitioners can be directed to who is priority and provide timely interventions.

Editors Commentary

Avoiding Hyperoxia is the key to an accurate S/F ratio. Hyperoxia will cause this ratio to be falsely low. Give the patient as much oxygen as they need, but no more!

-Steve Haywood MD

The Debrief

  • S/F ratio can be utilized  as an alternative to the P/F ratio
  • The ROX index allows practitioners to quickly assess patients for possible HFNC failures
  • The ROX index is just one of many assessment tools needed for proper care of Acute Hypoxic Respiratory Failure patients


  1.  Rice TW, Wheeler AP, Bernard GR, Hayden DL, Schoenfeld DA, Ware LB; National Institutes of Health, National Heart, Lung, and Blood Institute ARDS Network. Comparison of the SpO2/FIO2 ratio and the PaO2/FIO2 ratio in patients with acute lung injury or ARDS. Chest. 2007 Aug;132(2):410-7. doi: 10.1378/chest.07-0617. Epub 2007 Jun 15. PMID: 17573487.
  2. Catoire, P., Tellier, E., de la Rivière, C., Beauvieux, M. C., Valdenaire, G., Galinski, M., Revel, P., Combes, X., & Gil-Jardiné, C. (2021). Assessment of the SpO2/FiO2 ratio as a tool for hypoxemia screening in the emergency department. The American journal of emergency medicine, 44, 116–120.
  3. Ricard, J. D., Roca, O., Lemiale, V., Corley, A., Braunlich, J., Jones, P., Kang, B. J., Lellouche, F., Nava, S., Rittayamai, N., Spoletini, G., Jaber, S., & Hernandez, G. (2020). Use of nasal high flow oxygen during acute respiratory failure. Intensive care medicine, 46(12), 2238–2247.
  4. Gianstefani, A., Farina, G., Salvatore, V., Alvau, F., Artesiani, M. L., Bonfatti, S., Campinoti, F., Caramella, I., Ciordinik, M., Lorusso, A., Nanni, S., Nizza, D., Nava, S., & Giostra, F. (2021). Role of ROX index in the first assessment of COVID-19 patients in the emergency department. Internal and emergency medicine, 1–7. Advance online publication.


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