Measuring the mechanical input impedance of the respiratory system with breath-driven flow oscillations

In recent years, the mechanical input impedance of the respiratory system (Z_rs) determined using the technique known as oscillometry has been gaining traction as a clinical diagnostic tool to complement conventional spirometry. Nevertheless, despite currently approved oscillometry devices being relatively compact and portable, they are still too heavy and bulky to be used in an ambulatory hands-free setting, mostly because of the mass of the motor and power supply. We therefore explored the possibility of using the subject's own respiratory musculature as the power source for creating flow oscillations at the mouth. We measured reference Z_rs in 8 normal volunteers by having them breathe tidally into a piston-driven oscillator powered by an external motor. We fit the measured Z_rs to the single-compartment model of the respiratory system characterized by the three parameters resistance (R_rs), elastance (E_rs), and inertance (I_rs). We then compared these parameter values to those obtained with two commercially available mucus-clearing devices that generate oscillations when expiratory flow drives a flapper valve. The estimates of R_rs agreed mostly within ±1 cmH₂O·s·L^-1, which is usefully accurate for most clinical needs. E_rs and I_rs agreed less well because the breath-driven oscillators provided data at essentially a single frequency close to the resonant frequency of the respiratory system. Nevertheless, we conclude that perturbing respiratory airflow and pressure with a breath-driven oscillator has the potential to provide measurements of Z_rs, possibly serving as the basis for a lightweight ambulatory oscillometry system.NEW & NOTEWORTHY The technique of oscillometry for measuring the mechanical input impedance of the respiratory system is gaining traction as a clinical diagnostic tool, but the portability of existing commercially available devices is limited by the size and weight of oscillator motors and power supplies. We show that impedance can be measured by oscillations in mouth pressure and flow generated by mucus-clearing devices that are powered by the subject's own respiratory flow.