High-flow nasal cannula (HFNC) is a relatively new method for delivering heated and humidified oxygen gas through a nasal cannula at a high flow rate of up to 60 L/min. HFNC can provide up to 100 % oxygen and generate positive end-expiratory pressure, resulting in the elimination of some dead airway space and improvements in oxygenation [1]. Humidified oxygen also offers comfort and enhancement of mucociliary clearance [2]. Accordingly, previous studies have shown the relative effectiveness of HFNC to conventional oxygen therapy in reducing the rate of endotracheal intubation in adult patients with acute respiratory failure admitted to the intensive care unit or post-operation populations. Recent systematic reviews also reported that HFNC is more reliable than conventional oxygen therapy in reducing the rate of endotracheal intubation or reintubation in these populations [3], [4], [5], [6].

In patients presenting to emergency departments (EDs) for acute respiratory failure, the effect of HFNC in preventing endotracheal intubation and mortality is still controversial. Several studies reported that HFNC can improve the work of breathing, but did not show significant benefits in reducing the rates of admission, intubation, and mortality [7], [8], [9]. Nevertheless, the use of HFNC is increasing, which may entail the risk of delayed intubation if HFNC fails in a patient [10]. Therefore, a strong predictor for identifying patients at high risk of HFNC failure will be helpful.

Previous studies reported that the associated factors for HFNC failure such as heart rate, respiratory rate, bicarbonate levels, organ dysfunction, and ROX index [11], [12], [13].

The ROX index is a recently proposed objective bedside tool that is calculated using oxygen saturation (SpO2), fraction of inspired oxygen (FiO2), and respiratory rate, and was shown to be predictive of the success of HFNC [11]. However, the ROX index has not been validated well in the ED setting. Considering the heterogeneity and acuteness of patients presenting at ED, parameters for predicting HFNC failure would be valuable for appropriate monitoring in the ED environment.

The aim of this study was to identify the factors, including ROX index, associated with the failure of HFNC in adult patients with acute respiratory failure patients undergoing HFNC treatment at an ED.