Description and Validation of a Novel Score (Flow Index) as a Clinical Indicator of the Level of Respiratory Support to Children on High Flow Nasal Cannula

This study's objective was to describe and validate flow index (flow rate × FiO 2 /weight) as a method to report the degree of respiratory support by high flow nasal cannula (HFNC) in children. We conducted a retrospective chart review of children managed with HFNC from January 1, 2015 to December 31, 2019. Variables included in the flow index (weight, fraction of inspired oxygen [FiO 2 ], flow rate) and outcomes (hospital and intensive care unit [ICU] length of stay [LOS], escalation to the ICU) were extracted from medical records. Max flow index was defined by the earliest timestamp when patients FiO 2  × flow rate was maximum. Step-wise regression was used to determine the relationship between outcome (LOS and escalation to ICU) and flow index. Fifteen hundred thirty-seven patients met the study criteria. The median first and maximum flow indexes of the population were 24.1 and 38.1. Both first and maximum flow indexes showed a significant correlation with the LOS ( r  = 0.25 and 0.31, p  < 0.001). Correlation for the index was stronger than that of the variables used to calculate them and remained significant after controlling for age, race, sex, and diagnoses. Mild, moderate, and severe categories of first and max flow index were derived using quartiles, and they showed significant age and diagnosis independent association with LOS. Patients with first flow index >20 and maximum flow index >59.5 had increased odds ratio of escalation to ICU (odds ratio: 2.39 and 8.08). The first flow index had a negative association with rapid response activation. Flow index is a valid measure for assessing the degree of respiratory support for children on HFNC.