• Robert D Guglielmo, Justin C Hotz, Patrick A Ross, Timothy W Deakers, Jennifer E L Diep, NewthChristopher J LCJLDepartment of Anesthesiology and Critical Care Medicine, Children's Hospital Los Angeles, Los Angeles, CA; Department of Pediatrics, Keck School of Medicine, University of Southern California, Los Angeles, CA., and Robinder G Khemani.
• Division of Pediatric Critical Care, Department of Pediatrics, Loma Linda University Children's Hospital, Loma Linda, CA; Loma Linda University School of Medicine, Loma Linda, CA. Electronic address: rguglielmo@llu.edu.
• Chest. 2022 Oct 1; 162 (4): 861871861-871.

BackgroundHigh-flow nasal cannula (HFNC) therapy reduces the effort of breathing in patients with bronchiolitis, but the mechanisms are not understood. Theorized mechanisms include dead space washout and positive end-expiratory pressure (PEEP) application.Research QuestionWhat are the mechanisms of action of HFNC therapy in patients with bronchiolitis?Study Design And MethodsProspective, single-center study of children 3 years of age or younger with bronchiolitis from January 2020 through March 2021. Flow was titrated between 0.5 and 2 L/kg/min. Electrical impedance tomography measured end-expiratory lung impedance (EELZ) change as an end-expiratory lung volume (EELV) change surrogate and change in tidal impedance difference (ΔZ) as a tidal volume (VT) surrogate. A subset showed manometry measuring esophageal pressure change (ΔPes; transpulmonary pressure surrogate) and pressure rate product (PRP; effort of breathing metric). We hypothesized that EELV and VT would not change and that effort would reduce via respiratory rate (not ΔPes). Measurements were reported as the difference from 0.5 L/kg/min.ResultsWe studied 22 patients in total, 10 with esophageal manometry. Median EELZ increased by 0.36 arbitrary unit (AU), 2.42 AU, and 4.8 AU at 1 L/kg/min, 1.5 L/kg/min, and 2 L/kg/min (P = .01, 2 L/kg/min vs 0.5 L/kg/min), which corresponded to a median increase in EELV of 1.8 mL/kg between 0.5 and 2 L/kg/min. Seven patients showed an increase in EELZ of > 5 AU, 12 showed no change in EELZ (± 5 AU), and three showed a decrease in EELZ of > 5 AU. ΔZ (ie, VT) did not change from 0.5 L/kg/min to 2 L/kg/min (median change, 0.29 AU; P = .48). Median PRP decreased by 78 cm H2O/min from 0.5 L/kg/min to 2 L/kg/min (P = .02), with all patients demonstrating a reduction in PRP, with a nonsignificant change in ΔPes (P = .68).InterpretationIncreasing HFNC in children with bronchiolitis reduces the effort of breathing, but no consistent increase occurs in end-expiratory lung volume and no significant change occurs in VT or transpulmonary pressure. This suggests that PEEP application is not the primary mechanism of action of HFNC in children with bronchiolitis.Copyright © 2022 The Author(s). Published by Elsevier Inc. All rights reserved.

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