Usefulness of respiratory impedance by measuring at functional residual capacity position

Background: The Forced Oscillation Technique (FOT) can detect impairments of lung function by measuring the impedance of respiratory system during normal tidal breathing. Objective: We assessed the clinical meaning of the respiratory impedance components measured at functional residual capacity (FRC) position. Methods: We recruited 20 healthy male volunteers (age 23-57 years). We measured respiratory impedance by using a commercially available FOT device (MostGraph-01, Ver.1.70R) during normal tidal breathing. We analyzed respiratory resistance (Rrs) and reactance (Xrs) from 5 Hz to 20 Hz of oscillatory frequency at the level of FRC position defined as 0 of flow rate from expiratory phase to inspiratory phase. We also measured respiratory impedance during breath holding at the level of FRC position. We underwent standard pulmonary function tests. A P -value of <0.05 was considered significant. Results: We found negative relationships between height and Rrs at 5 Hz (R5), resonant frequency (Fres), and a positive relationship between height and Xrs at 5 Hz (X5). R5 adjusted by height was inversely correlated with FEV1(r=-0.57). X5 adjusted by height was positively correlated with FEV1 (r=0.62) and PEFR (r=0.55). Fres adjusted by height was inversely correlated with FEV1(r=-0.58). There were no relationships between the respiratory impedance components during breath holding and measurements of pulmonary function tests. Discussion: Our findings demonstrated that the respiratory impedance components at FRC, which was eliminated the effect of the elastic forces between lung and chest wall, had the correlation with FEV1, which would be represented dynamic phase of airway flow limitation.