Nonlinear phenomena in respiratory mechanical measurements.

The impact of nonlinearities on the assessment of respiratory mechanics was examined using a block-structured model (BSM) featuring both flow (V) and tidal volume (VT) nonlinearities. The model consists of an airway compartment (AC) in series with a tissue compartment (TC). The AC is a series connection of a resistance, an inertance, and a nonlinear V-dependent element. The TC is composed of a linear Kelvin body in cascade with a nonlinear polynomial system. Analytic results show that 1) the lack of VT dependence using sine waves does not mean that the system is linear and 2) the hysteresivity (J. J. Fredberg and D. Stamenović. J. Appl. Physiol. 67:2408-2419, 1989) of a wide class of such tissue models is independent of VT, offering a simple mechanism for the coupling between resistance (R) and elastance (E). Furthermore, R from the step response of the TC was approximately 30% smaller than from sinusoids. Below 1 Hz, R and E of the BSM showed a negative VT dependence. Above 1 Hz, R increased with frequency and VT because of V dependence. Analysis of the response of the BSM to composite signals revealed how linear airway resistance can be overestimated because of V dependence and how tissue properties can be under-estimated because of harmonic distortion and cross talk induced by VT-associated nonlinearities.