Optimization of flow setting during high-flow nasal cannula (HFNC) with a new spirometry system

High-flow nasal cannula (HFNC) is frequently used to treat respiratory distress in infants and children because of its beneficial effects on alveolar ventilation and respiratory mechanics. Setting an adequate flow rate that meets a patient's peak inspiratory flow (PIF) is thus crucially important to achieve such effects. HFNC flow rate is typically set at 1 L/min/kg +1 as suggested by the manufacturer and increased to 2 L/min/kg according to the degree of respiratory distress. However, whether this empirical flow setting actually meets a patient's PIF has not yet been investigated. In this study, we implemented our previously described respiratory mechanics monitoring system (MAES) with a new spirometry function (NSS) that allows for a simultaneous visualization of the flow tracings of HFNC and the patient's spontaneous breathing. We tested the ability of NSS-MAES to determine the adequacy of empirically set flow rates of 1 L/min/kg +1 or 2 L/min/kg on 9 infants with respiratory distress receiving HFNC. HFNC flow rate was considered adequate if its tracing was just above the patient's respiratory flow. In patients in whom 1 L/kg/min +1 was inadequate, we used NSS-MAES to identify the adequate flow by raising the HFNC flow until it reached the patient's PIF (HFNC_NSS-MAES). We also investigated which flow rate was associated with the maximal decrease of respiratory effort, namely, Pressure Time Product (PTP) and Work of Breathing (WOB). We found that 1 L/min/kg +1, but not 2 L/min/kg was often unable to meet the patient's PIF. In these cases HFNC_NSS-MAES values were around 1.6 L/min/kg. Conversely, HFNC at 2 L/min/kg always exceeded the patient's PIF. All breathing effort indexes tested improved after HFNC treatment with the maximal unloading seen at 2 L/min/kg for PTP and at HFNC_NSS-MAES.