ROX index performance to predict high-flow nasal oxygen outcome in Covid-19 related hypoxemic acute respiratory failure.

Background: Given the pathophysiology of hypoxemia in patients with Covid-19 acute respiratory failure (ARF), it seemed necessary to evaluate whether ROX index (ratio SpO/FiO to respiratory rate) could accurately predict intubation or death in these patients initially treated by high-flow nasal oxygenation (HFNO). We aimed, therefore, to assess the accuracy of ROX index to discriminate between HFNO failure (sensitivity) and HFNO success (specificity).

Methods: We designed a multicentre retrospective cohort study including consecutive patients with Covid-19 ARF. In addition to its accuracy, we assessed the usefulness of ROX index to predict HFNO failure (intubation or death) via logistic regression.

Results: Among 218 ARF patients screened, 99 were first treated with HFNO, including 49 HFNO failures (46 intubations, 3 deaths before intubation). At HFNO initiation (H0), ROX index sensitivity was 63% (95%CI 48-77%) and specificity 76% (95%CI 62-87%) using Youden's index. With 4.88 as ROX index cut-off at H12, sensitivity was 29% (95%CI 14-48%) and specificity 90% (95%CI 78-97%). Youden's index yielded 8.73 as ROX index cut-off at H12, with 87% sensitivity (95%CI 70-96%) and 45% specificity (95%CI 31-60%). ROX index at H0 was associated with HFNO failure (p = 0.0005) in univariate analysis. Multivariate analysis showed that SAPS II (p = 0.0003) and radiographic extension of pulmonary injuries (p = 0.0263), rather than ROX index, were predictive of HFNO failure.

Conclusions: ROX index cut-off values seem population-specific and the ROX index appears to have a technically acceptable but clinically low capability to discriminate between HFNO failures and successes in Covid-19 ARF patients. In addition, SAPS II and pulmonary injuries at ICU admission appear more useful than ROX index to predict the risk of intubation.