Tracking of respiratory mechanics at multiple oscillation frequencies

Intra-breath oscillometry (IBOsc) is an emerging approach to characterize dynamic changes in respiratory mechanical impedance (Zrs). IBOsc utilizes a small-amplitude sinusoidal signal superimposed on quiet breathing to track Zrs with sufficient temporal resolution to find specific time points, such as end-expiration (eE) and end-inspiration (eI). IBOsc has demonstrated superiority to conventional multifrequency oscillometry in detecting abnormal respiratory function and predicting future impairment in several clinical settings. The aim of the present study was to construct intra-breath Zrs spectra from multifrequency recordings to demonstrate how the Zrs spectrum and its measures change during breathing.

Conventional oscillometric recordings from groups of healthy subjects and patients with interstitial lung disease, asthma and chronic obstructive pulmonary disease (N=40 each group) were analyzed. Zrs was computed at each component of the multifrequency (5-37-Hz) signal to establish the Zrs spectra at eE and eI. This multi-frequency tracking method was validated on simulated Zrs data generated by a non-linear model of respiratory mechanics. The 2-way median test and Wilcoxon signed rank test were used to compare Zrs values and derived measures between groups and respiratory phases, respectively.

Large intra-breath changes in Zrs were found in all subject groups. Most pairwise comparisons of Zrs measures (such as resistance, resonance frequency, reactance area and effective compliance) revealed significant (P<0.05) or highly significant (P<0.001) differences between groups at eE, which became more uniform at eI. Similarly, the changes between eE and eI were significant in most Zrs measures and subject groups, indicating the tidal improvement of lung mechanics in the obstructive patients.

Our results demonstrate that re-processing of archived datasets is feasible and can provide useful additional data to further characterize respiratory mechanical phenotypes. In particular, the estimation of Zrs spectra at zero respiratory flow minimizes the contribution of upper airway nonlinearities and thus improves the assessment of intrapulmonary dynamics. However, as this study points out, most current multifrequency signals are suboptimal for exploiting the potential of IBOsc due to low signal-to-noise ratio and interaction between adjacent frequency components.