To the editor,

We have read with much enthusiasm the article of Hancı et al.¹ who have analyzed respiratory rate oxygenation (ROX) index dynamics in patients receiving high-flow nasal oxygen (HFNO) for the treatment of acute respiratory failure in the background of severe acute respiratory syndrome-coronavirus-2 infection. While congratulating the authors for their commendable analysis, we would like to raise certain points for clarification and better understanding of the reported observations.

First, although the success of HFNO therapy can be predicted using the ROX index, it does not take into account the changes in the flow rate that can influence the success of HFNO therapy. Higher flow rates can generate a continuous pressure effect on the airway and favor the lavage of dead space, increased end-expiratory volume, decreased respiratory rate, and diminished work of breathing. They designed their study to switch over to non-invasive ventilation (NIV) or invasive mechanical ventilation (IMV) as rescue therapy in the case of deterioration of patients when patients were treated with HFNO with a flow rate of < 50 l/min  Increasing the flow rate would have achieved better outcomes.^3,4 A study reported a positive linear relationship between the amount of delivered flow and the generated mean airway pressure.⁵ For every 10 l/min increase in gas flow, the generated mean airway pressure can increase by approximately 1.16 cm H₂O.

Second, the analysis of HFNO failure and subsequent need for NIV/IMV could have been better validated by the incorporation of simultaneous lung ultrasound findings. Being noninvasive and performed at the bedside, lung ultrasound findings of B-line patterns and the lack of aeration in the presence of dyspnea and hypoxemia can be a very useful tool in predicting worsening of clinical condition and possible requirement of intubation.⁶

Third, the proportion of lung involvement can be assessed by computed tomography (CT), which can help in deciding whether the patient would benefit from HFNO/NIV or early intubation. Had the authors included the chest CT severity score as one of the parameters for evaluating ROX dynamics to determine HFNO success, the outcomes could have been better and more firmly established.⁷

Fourth, it is unclear from the reporting whether immunosuppressed states such as the use of systemic steroids have been taken into account for computing the Charlson comorbidity index. It would be interesting to comment whether the use of systemic steroids in this group of patients influenced the ROX index values in any way.

Finally, after addressing these points, this study can be a template for further research to consolidate the evidence about the role of the ROX index as a predictive tool in the early phase of management of coronavirus disease-19 pneumonia.

Authorship Contributions: Analysis or Interpretation- A.G., D.B., M.M., A.E.R.; Literature Search- ; Writing-  A.G., D.B., M.M., A.E.R.

Conflict of Interest: No conflict of interest was declared by the authors.

REFERENCES

1. Hancı P, Uysal A, Yüksel B, İnal V. Rox Index Dynamics According to High Flow Nasal Cannula Success in Intensive Care Unit Patients with COVID-19-Related Acute Respiratory Failure. Balkan Med J. 2023;40:111-116.
2. Roca O, Caralt B, Messika J, et al. An Index Combining Respiratory Rate and Oxygenation to Predict Outcome of Nasal High-Flow Therapy. Am J Respir Crit Care Med. 2019;199:1368-1376.
3. Mauri T, Alban L, Turrini C, et al. Optimum support by high-flow nasal cannula in acute hypoxemic respiratory failure: effects of increasing flow rates. Intensive Care Med. 2017;43:1453-1463.
4. Gallardo A, Gigliotti C, Saavedra S, et al. Strategies for monitoring and predicting failure to high-flow nasal cannula therapy in the ED. Am J Emerg Med. 2022;57:183-184.
5. Parke RL, Eccleston ML, McGuinness SP. The effects of flow on airway pressure during nasal high-flow oxygen therapy. Respir Care. 2011;56:1151-1155. 
6. Peixoto AO, Costa RM, Uzun R, et al. Applicability of lung ultrasound in COVID-19 diagnosis and evaluation of the disease progression: A systematic review. Pulmonology. 2021;27:529-562.
7. Goury A, Moussanang JA, Bard M, et al. Predictive factors associated with high-flow nasal cannula success for COVID-19-related acute hypoxemic respiratory failure. Health Sci Rep. 2021;4:e287.