High flow nasal cannula (HFNC) therapy reduces effort of breathing in bronchiolitis, but the mechanisms are not understood. Theorized mechanisms include dead space washout and positive end-expiratory pressure (PEEP) application.
What are the mechanisms of action of HFNC therapy in bronchiolitis?
Prospective, single-center study for children ≤3 years with bronchiolitis from January 2020-March 2021. Flow was titrated between 0.5-2L/kg/min. Electrical Impedance Tomography (EIT) measured end-expiratory lung impedance change (ΔEELZ) as an end-expiratory lung volume change (ΔEELV) surrogate and tidal impedance difference (ΔZ) as a tidal volume (V) surrogate. A subset had esophageal manometry measuring pressure change (ΔPes, transpulmonary pressure surrogate) and pressure rate product (PRP, effort of breathing metric). We hypothesized EELV and V would not change and that effort would reduce via respiratory rate (not ΔPes). Measurements were reported as the difference from 0.5L/kg/min.
We studied 22 total patients, 10 with esophageal manometry. Median EELZ increased by 0.36, 2.42, and 4.8AU at 1, 1.5, and 2L/kg/min (p=0.01, 2 vs 0.5L/kg/min), which corresponded to a median increase in EELV of 1.8mL/kg between 0.5-2L/kg/min. 7 patients had an increase in EELZ of greater-than 5AU, 12 had no change in EELZ (± 5AU), and 3 had a decrease in EELZ greater-than 5AU. Tidal impedance difference (ΔZ, i.e. tidal volume) did not change from 0.5-2L/kg/min (median change 0.29AU, p=0.48). Median PRP decreased by 78cmHO/min from 0.5 to 2L/kg/min (p=0.02), with all patients demonstrating a reduction in PRP, with a non-significant change in ΔPes (p=0.68).
Increasing HFNC in children with bronchiolitis reduces effort of breathing, but there is no consistent increase in end-expiratory lung volume and no significant change in tidal volume or transpulmonary pressure. This suggests that PEEP application is not the primary mechanism of action of HFNC in children with bronchiolitis.Abstract Word Count: 298.

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