RATIONALE:It is difficult to evaluate the physiological effect of HFNC. In the current study, we used the forced oscillation technique (FOT) to evaluate the Methods: The study were evaluated by the esophageal balloon method, the electrical impedance method, and the FOT and on a flow rate applied at 30 and 50 L/min through HFNC. RESULTS:During HFNC, there was no significant difference in resistance at 5 Hz and 20 Hz . respiratory system reactance at 5Hz(X5) significantly decreased (OFF,30,50L/min:0.06±0.15,0.02±0.19, ?0.12±0.17 cmH2O/L/sec), and both the resonance frequency (Fres) and the low-frequency reactance area (ALX) significantly increased. (Fres?Off,30,50 L/min:4.80±0.81, 5.03±1.37, 6.42±0.67 cmH2O/L/sec) (ALX?Off, 30, 50 L/min:0.15±0.18, 0.30±0.4,?0.61±0.50cmH2O/L/sec)There was a flow rate-dependent end-expiratory esophageal pressure (EEEP) increase, and the positive end-expiratory pressure (PEEP) effect of HFNC was confirmed. There was a correlation between the difference in X5 and the difference in EEEP during HFNC 30  and 50L/min, with correlation coefficients of 0.534 and 0.404 (p = 0.112, 0.281). The amount of change in EEEP and the fluctuation in X5 were positively correlated. CONCLUSION: The results of the FOT studies showed that HFNC did not cause any change in expiratory airway resistance, but it did cause flow rate-dependent increases in the elastic resistance and inertial resistance of the lungs. The amounts of change in EEEP and X5 were determined by the esophageal balloon method and shown to have a significant positive correlation.PEEP effect of HFNC was thus confirmed by FOT.