Factors affecting FiO2 and PEEP during high‐flow nasal cannula oxygen therapy: A bench study

INTRODUCTION: It is unknown if set-flow, peak inspiratory flow (PIF), tidal volume and set fraction of inspired O2 (FiO2 ) affect actual-FiO2 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-FiO2 , set-flow, PIF and tidal volume on the actual-FiO2 and PEEP during HFNC oxygen therapy. METHODS: A lung simulation model was used to study the factors that might affect FiO2 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-FiO2 (30%, 50% and 70%). Actual-FiO2 and PEEP were recorded for 10 consecutive breaths. Actual-FiO2 and PEEP were compared under different conditions. Multivariate linear regressions were performed to investigate the effects of these factors on actual-FiO2 and PEEP. RESULTS AND CONCLUSION: Regression formula were: (a) actual-FiO2 as the dependent variable: actual FiO2 = 2.71 + 0.78 x set-FiO2 + 0.17 x set-flow - 0.09 x PIF (F test, F = 3866.62, P < 0.001, R(2) = 0.94), (b) PEEP as the dependent variable: PEEP = 1.35 + 0.15 x set-flow - 0.02 x PIF + 0.01 x tidal volume (F test, F = 4082.39, P < 0.001, R(2) = 0.95). The following factors were found to affect actual-FiO2 (in descending order): set-FiO2 , set-flow and PIF. Tidal volume had little effect on actual-FiO2 . Factors which affected PEEP were (in descending order): set-flow, peak inspiratory flow and tidal volume.