NONLINEAR PHENOMENA IN RESPIRATORY MECHANICAL MEASUREMENTS

The impact of nonlinearities on the assessment of respiratory mechanic s was examined using a block-structured model (BSM) featuring both flo w (V) and tidal volume (VT) nonlinearities. The model consists of an a irway compartment (AC) in series with a tissue compartment (TC). The A C is a series connection of a resistance, an inertance, and a nonlinea r V-dependent element. The TC is composed of a linear Kelvin body in c ascade with a nonlinear polynomial system. Analytic results show that 1) the lack of VT dependence using sine waves does not mean that the s ystem is linear and 2) the hysteresivity (J. J. Fredberg and D. Stamen ovic. J. Appl. Physiol. 67: 2408-2419, 1989) of a wide class of such t issue models is independent of VT, offering a simple mechanism for the coupling between resistance (R) and elastance (E). Furthermore, R fro m 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 depende nce. Above 1 Hz, R increased with frequency and VT because of V depend ence. Analysis of the response of the BSM to composite signals reveale d how linear airway resistance can be overestimated because of V depen dence and how tissue properties can be underestimated because of harmo nic distortion and cross talk induced by VT-associated nonlinearities.