ROX (Respiratory rate-OXygenation) index to predict early response to high-flow nasal cannula therapy in infants with viral bronchiolitis

Introduction: High-flow nasal cannula (HFNC) is commonly used as first step respiratory support in infants with moderate-to-severe acute viral bronchiolitis (AVB). This device, however, fails to effectively manage respiratory distress in about a third of patients, and data are limited on determinants of patient response. The respiratory rate-oxygenation (ROX) index is a relevant tool to predict the risk for HFNC failure in adult patients with lower respiratory tract infections. The primary objective of this study was to assess the relationship between ROX indexes collected before and 1 h after HFNC initiation, and HFNC failure occurring in the following 48 h in infants with AVB.Method: This is an ancillary study to the multicenter randomized controlled trial TRAMONTANE 2, that included 286 infants of less than 6 months with moderate-to-severe AVB. Collection of physiological variables at baseline (H0), and 1 h after HFNC (H1), included heart rate (HR), respiratory rate (RR), fraction of inspired oxygen (FiO(2)), respiratory distress score (modified Wood's Clinical Asthma Score [mWCAS]), and pain and discomfort scale (EDIN). ROX and ROX-HR were calculated as (SpO(2)/FiO(2))/RR and 100xROX/HR, respectively. Predefined HFNC failure criteria included increase in respiratory distress score or RR, increase in discomfort, and severe apnea episodes. The accuracies of ROX, ROX-HR indexes and clinical variable to predict HFNC failure were assessed using receiver operating curve analysis. We analyzed predictive factors of HFNC failure using multivariate logistic regressions.Result: HFNC failure occurred in 111 of 286 (39%) infants, and for 56 (50% of the failure) of them within the first 6 h. The area under the curve of ROX indexes at H0 and H1 were, respectively, 0.56 (95% confidence interval [CI] 0.48-0.63, p = 0.14), 0.56 (95% CI 0.49-0.64, p = 0.09). ROX-HR performances were better but remained poorly discriminant. HFNC failure was associated with higher mWCAS score at H1 (p < 0.01) and lower decrease in EDIN scale during the first hour of HFNC delivery (p = 0.02). In the multivariate analyses, age and mWCAS score were were found to be independent factors associated with HFNC failure at H0. At H1, weight and mWCAS were associated factors.Conclusion: In this study, neither ROX index, nor physiological variables usually collected in infants with AVB had early discriminatory capacity to predict HFNC failure.