Measuring Nonlinear Effects in Respiratory Mechanics: A Proof of Concept for Prototype Device and Method

The forced oscillation technique (FOT) is a lung function test used in clinical practice to evaluate the respiratory impedance. One of the main advantages of FOT over other lung function tests is that it does not require any special breathing maneuvers from the subject, making it one of the simplest tests to evaluate respiratory mechanics. This paper describes the nonlinear effects in the respiratory signals and related measurement instrumentation during the FOT tests. First, this paper discusses some improvements made on a prototype FOT device to allow the generation of multisines below 4 Hz. Second, two methods are evaluated to detect the nonlinear effects: a robust method and a fast method. These methods allow a comparison of the nonlinear distortions in a prototype FOT device and a commercial FOT device. The nonlinear effects are also quantified using a new index definition. FOT lung function tests are performed to obtain two distinct data sets: 1) one mixed group of patients diagnosed with asthma and cystic fibrosis and 2) one group of healthy volunteers. With the extracted nonlinear contributions, a significant difference has been observed between the two groups. This delivers the proof of concept that low-frequency measurements of the respiratory mechanics are useful to evaluate lung pathologies.