Structural and Functional Changes Occuring During Growth of the Respiratory System Can Be Quantified and Classified

This paper describes the nonlinear effects in the respiratory signals captured by means of the forced oscillation technique (FOT) non-invasive lung function tests. The measurements are performed using a prototype device developed such that it overcomes the limitations present in commercial FOT devices and allows the generation of multisine signals below 4 Hz. The principle of sending detection lines in the frequency domain for characterizing odd and even nonlinear contributions from a nonlinear system are introduced briefly to the reader. Two detection methods are presented: a robust method based on multiple measurements and a fast method based on a single measurement. The ingenious combination of the device and the method allow to detect the nonlinear contributions in the respiratory signals: pressure and flow. The intrinsicly pesent nonlinear effects are quantified by means of a novel index and analyzed in two groups of healthy volunteers, aged 14 years and aged 17 years, respectively. The results we obtain suggest that the proposed device, method and index are a successful combination of lung function testing, signal processing and classification items.