To investigate the application value of ROX index in evaluating the effect of high-flow nasal cannula oxygen therapy (HFNC) on patients diagnosed with respiratory failure, and to find a simpler and more effective method to observe the efficacy of HFNC.

A retrospective cohort study was conducted. Patients who were admitted to department of critical care medicine of the Tianjin Third Central Hospital from April 2020 to August 2022, diagnosed with type Ⅰ respiratory failure, and treated with HFNC after failure of conventional oxygen therapy were enrolled. Oxygenation index (PaO₂/FiO₂), fraction of inspired oxygen (FiO₂), gas flow rate at the initial time of admission, and pulse oxygen saturation (SpO₂), FiO₂ and respiratory rate (RR) at 2, 4, 6, 8, 10 and 12 hours of HFNC were collected, and ROX index was calculated. The patients with symptoms and PaO₂/FiO₂ improved after HFNC treatment and without higher respiratory support lately were defined as HFNC success, while other patients with symptoms worsening and needing follow-up non-invasive positive pressure ventilation (NIPPV) or invasive positive pressure ventilation (IPPV) were defined as HFNC failure. The tendency of changes in the ROX index at each time point was observed. Receiver operator characteristic curve (ROC curve) was plotted to obtain the optimum cut-off value of ROX index for predicting HFNC outcome and the optimal monitoring time point for HFNC.

A total of 142 patients were eventually enrolled, among whom 96 patients (67.61%) were in treated with HFNC successfully, while 46 patients (32.39%) were recorded as HFNC failure (39 patients and 7 patients received NIPPV or IPPV, respectively), with an overall intubation rate of 4.93% (7/142). Compared with the HFNC success group, the HFNC failure group had lower PaO₂/FiO₂ [mmHg (1 mmHg ≈ 0.133 kPa): 208.8±37.3 vs. 235.7±48.3, P < 0.01] and higher initial gas flow rate (L/min: 46.4±3.9 vs. 42.3±4.9, P < 0.01). However, there was no significant difference in gender, age, primary diagnosis, severity of disease, hemoglobin (Hb), C-reactive protein (CRP), and brain natriuretic peptide (BNP) between the two groups. In the HFNC failure group, there were 12 patients (26.09%) received progressive oxygen therapy within 12 hours of HFNC, of which 3 patients (6.52%) occurred within 6 hours, while the other 9 patients (19.57%) occurred after 6 hours. The initial ROX index was not statistically significant between the two groups. Both groups showed a continuous increasing ROX index with longer treatment duration of HFNC, and the ROX index at all of the time points of the HFNC failure group was significantly lower than that of the HFNC success group with statistically significant difference (2 hours: 9.39±2.85 vs. 10.91±3.51, 4 hours: 8.62±2.29 vs. 11.40±3.18, 6 hours: 7.62±1.65 vs. 11.85±3.45, 8 hours: 7.79±1.59 vs. 11.62±3.10, 10 hours: 7.97±1.62 vs. 12.44±2.75, 12 hours: 8.84±2.51 vs. 12.45±3.03, all P < 0.05). The ROC curve analysis showed that the areas under the ROC curve (AUC) of ROX index assessing the effect of HFNC at the time of treating 6, 8 and 10 hours were better than 2, 4 and 12 hours (0.890, 0.903, 0.930 vs. 0.585, 0.738 and 0.829), indicating that the ROX index could determine the efficacy at the early stage of HFNC (within 6 hours). When the optimum cut-off value of ROX index was 8.78, the sensitivity was 90.6%, and the specificity was 76.5%.

The ROX index at 6 hours of HFNC has a certain predictive value for the efficacy of HFNC with an optimum cut-off value of 8.78, which can provide clinical health care personnel a method for observing the efficacy of HFNC, and guide the correct selection of oxygen therapy modality at an early stage and timely adjustment of oxygen therapy strategy.

High-flow nasal cannula oxygen therapy; ROX index; Non-invasive positive pressure ventilation