Changes in respiratory mechanics with increasing degrees of airway obstruction in COPD: Detection by forced oscillation technique

Summary

The Forced Oscillation Technique (FOT) is a method for non-invasively assessing respiratory mechanics during spontaneous breathing, demanding little cooperation. The aim of this study was to test the ability of FOT to describe the changes in respiratory mechanics in progressive COPD. The study was performed in a control group formed by 21 healthy subjects and 79 outpatients with COPD, which were classified by spirometry, according to the degree of airway obstruction, in mild, moderate and severe groups. Resistive impedance data were submitted to linear regression analysis over the 4–16 Hz frequency range, which yielded the total respiratory system resistance extrapolated at 0 Hz (R₀), the respiratory system conductance (G_rs), mean respiratory resistance (R_m), and the resistance/frequency slope (S). Reactance data were interpreted using the mean values (X_m) over the 4–32 Hz frequency range, the dynamic compliance (C_rs,dyn), the dynamic elastance (E_rs,dyn), and the resonant frequency (f_r) data.

Considering the control and mild groups, the increase of airway obstruction resulted in a significant increase of R₀ ($P<0.008$), R_m ($P<0.001$), and a significant reduction in G_rs ($P<0.002$). Reactive parameters, C_{rs, dyn} and E_rs,dyn also presented significant modifications. The subsequent increase (mild to moderate) showed a significant raise of R₀ ($P<0.007$), S ($P<0.001$), and a reduction in G_rs ($P<0.015$), while significant increases in X_rs ($P<0.001$), and E_rs,dyn ($P<0.02$), and also a reduction in C_{rs, dyn} ($P<0.02$) were also observed. In contrast to earlier stages, in the late stage of the airway obstruction increase (moderate to severe obstruction), resistive parameters did not present statistically significant modifications, while significant modifications were observed in X_rs ($P<0.02$), C_{rs, dyn} ($P<0.003$) and E_rs,dyn ($P<0.003$).

The results of this study demonstrated that the FOT is useful for detecting the respiratory mechanics modifications in COPD patients. The initial phases of airway obstruction in COPD can be described mainly by resistive parameters, while in more advanced phases, reactive parameters seem to be more useful. Since the FOT has the advantage of being a simple method, such a technique may give a significant clinical contribution, representing an alternative and/or complement to the evaluation of respiratory mechanics by means of forced expiration.