Factors affecting FiO 2 and PEEP during high‐flow nasal cannula oxygen therapy: A bench study

It is unknown if set‐flow, peak inspiratory flow (PIF), tidal volume and set fraction of inspired O 2 (FiO 2 ) affect actual‐FiO 2 and positive end‐expiratory pressure (PEEP) during high‐flow nasal cannula (HFNC) oxygen therapy. In addition, the extent of their influence is also unknown. Objectives To investigate the effects of set‐FiO 2 , set‐flow, PIF and tidal volume on the actual‐FiO 2 and PEEP during HFNC oxygen therapy. Methods A lung simulation model was used to study the factors that might affect FiO 2 and PEEP during HFNC therapy in vitro. These factors included set‐flow (20, 40, 60, 80 and 120 L/min), PIF (40, 80 and 120 L/min), tidal volume (400 and 600 mL), and set‐FiO 2 (30%, 50% and 70%). Actual‐FiO 2 and PEEP were recorded for 10 consecutive breaths. Actual‐FiO 2 and PEEP were compared under different conditions. Multivariate linear regressions were performed to investigate the effects of these factors on actual‐FiO 2 and PEEP. Results and conclusion Regression formula were: (a) actual‐FiO 2 as the dependent variable: actual FiO 2  = 2.71 + 0.78 × set‐FiO 2  + 0.17 × set‐flow − 0.09 × PIF ( F test, F  = 3866.62, P  < 0.001, R 2  = 0.94), (b) PEEP as the dependent variable: PEEP = 1.35 + 0.15 × set‐flow − 0.02 × PIF + 0.01 × tidal volume ( F test, F  = 4082.39, P  < 0.001, R 2  = 0.95). The following factors were found to affect actual‐FiO 2 (in descending order): set‐FiO 2 , set‐flow and PIF. Tidal volume had little effect on actual‐FiO 2 . Factors which affected PEEP were (in descending order): set‐flow, peak inspiratory flow and tidal volume.