Detecting and analyzing non-linear effects in respiratory impedance measurements

This article describes the nonlinear effects in the respiratory impedance and in the related measurement instrumentation during the forced oscillation technique (FOT) measurements. First, the principle of FOT-measurements and nonlinear variance analysis is explained. Two methods are considered to detect nonlinear effects: a robust method and a fast method. These methods are employed to compare the nonlinear distortions in a prototype device and a commercial device, respectively. The identification signal for the respiratory impedance is optimized to reduce the nonlinear distortions. Finally, the nonlinear effects are measured in the respiratory impedance. A set of lung function tests are performed in several groups of patients with various lung conditions (healthy, asthma, cystic fibrosis, smoking). Based on the measured data from the patients, the corresponding nonlinear distortions can be quantified and evaluated for classification purposes.