Assessing peak inspiratory flow for initial HFNC flow setting: the end point or a first step towards a new approach?

We read with interest the report by Milesi et al1 showing a new physiologic approach for setting high flow nasal cannula (HFNC) therapy in infants with bronchiolitis, based on actual inspiratory flow demands, by measuring peak tidal inspiratory flow (PTIF) before HFNC therapy begins. In this study, 29% of PTIF values were >2 L/kg/min. The Authors thus cautiously propose a higher initial HFNC flow rate of 2.5 L/kg/min (or >3 L/kg/min in particular cases) than the commonly used 2 L/kg/min. We acknowledge the authors’ effort to identify a physiologic variable that may help to couple HFNC flow delivery and patients’ flow demands, the basic principle for HFNC efficacy. However, the study did not evaluate whether HFNC therapy itself influences patients’ PTIF and thus if the set HFNC flow becomes inadequate or exceedingly high after HFNC begins. We wish to raise some concerns that we see as crucial for their subsequent clinical implications. Although their findings question whether 2 L/kg/min is adequate in young infants with bronchiolitis, previously published clinical and physiologic studies suggest that this setting may be the correct choice in most infants. Previous clinical findings show that flow rates higher than 2 L/kg/min (i.e., 3 L/kg/min) bring about no further improvement.2 Weiler et al found a dose-dependent relationship between increasing HFNC flow rates and the reduced effort of breathing, optimal flow rates ranging between 1.5 and 2.0 L/kg/min .3 Thus, we wonder whether evaluating PTIF only once before HFNC therapy, and then using this PTIF value to set HFNC without further verification, is the right way to match patients and HFNC flow. An empirically consistent scenario is that PTIF will change once HFNC therapy begins. Indeed, breathing frequency slows during HFNC, and longer times flatten the flow tracings, thus reducing PTIF. Equally important, because HFNC therapy reduces esophageal pressure swings, PTIF values will likely diminish. Thus, after HFNC starts, reduced PTIF values might meet a 2 L/kg/min set flow rate or an even lower threshold. Although HFNC was developed as a procedure with no monitoring requirement, today’s expanded use requires extended physiologic studies to guide clinicians in setting HFNC flow. This study has uncovered the important issue of measuring PTIF to set the optimal initial HFNC flow rate and for that the authors should be complimented; however, we believe that further work is needed to verify what happens to PTIF during HFNC therapy.