Success or Failure of High-Flow Nasal Oxygen Therapy: The ROX Index Is Good, but a Modified ROX Index May Be Better

To the Editor:

Predicting the failure of oxygen therapy or noninvasive ventilation has remained an important area of study, and late intubation has been shown to be associated with poor clinical outcome (1). High-flow nasal oxygen (HFNO) therapy is gaining popularity, and overenthusiastic use leading to delayed intubation cannot be denied (2). In this situation, an objective method to identify patients who are likely to fail to respond to HFNO is very much needed. Thus, we read with interest the article by Roca and colleagues (3). Their article evaluates the capability of the ROX index to predict failure of HFNO therapy. First, we congratulate the authors for their contribution and effort, which is definitely going to impact clinical practice. There is no doubt that the authors have done commendable work; still, we believe that there is scope for further thinking.

Roca and colleagues have calculated the ROX index using the respiratory rate and oxygen saturation as measured by pulse oximetry (Sp_O2)/Fi_O2. Although the Sp_O2/Fi_O2 ratio compares well with the Pa_O2/Fi_O2 ratio when a patient is receiving low concentrations of supplemental oxygen, whether the relationship fares well with an Fi_O2 of 1 is not well established. Even the relationship of Sp_O2/Fi_O2 with Pa_O2/Fi_O2 is not so linear (4). Similarly, the fall of Sp_O2 and Pa_O2 is also not linear (5). In their study, Roca and colleagues have used HFNO therapy with up to 60 L/min and Fi_O2 of 1. Considering the facts mentioned above, an expectation of better results and correlation using a modified ROX index calculated from respiratory rate and Pa_O2/Fi_O2 cannot be ruled out. Moreover, during noninvasive/assisted breathing, especially HFNO therapy, oxygenation will depend on the respiratory pattern of the patient as well. Therefore, Pa_O2/Fi_O2 data, which can provide data from blood levels, probably would have given more predictability or accuracy. If the authors have correlated their data with Pa_O2/Fi_O2 and prediction of failure, this information will be more contributory in further validation.

Oxygen-carrying capacity correlates with Sa_O2 and Pa_O2. Sa_O2 can fall drastically from the Sp_O2 below 90%, as evident from the oxyhemoglobin association–dissociation curve. Moreover, Hb of the patient is a major determinant of oxygen-carrying capacity and oxygen delivery. Therefore, we believe that the ROX criteria need to be assessed using Pa_O2/Fi_O2 as well and for different Hb levels. Use of ROX criteria with Sp_O2/Fi_O2 as described by Roca and colleagues and of modified ROX criteria using Pa_O2/Fi_O2 in patients with different severity of respiratory failure will further help researchers in the future.