Circuit analysis justifies a reduced Mead´s model of the human respiratory impedance for impulse oscillometry data

Recent attempts at estimating the parameters for respiratory impedance models from data obtained by Impulse Oscillometry (IOS) have come across difficulties when using the well-established Mead´s model of human respiratory impedance. Unconstrained optimization of this model often yields values of chest wall compliance (C_W) and lung compliance (C_l) too large to be physiologically feasible. We hypothesize that IOS volume displacements are inconsequential to the lung tissue and chest wall due to the small contributions of these displacements relative to lung capacity. In order to explore the validity of this hypothesis we performed a detailed analysis of Mead´s impedance model. The IOS input flow signal was approximated by using a combination of typical waveforms, this signal was then used to excite Mead´s electrical circuit model of the respiratory impedance with physiologically realistic parameter values estimated using data obtained from one normal adult, ten adult patients with Cystic Fibrosis, ten patients with Asthma and ten normal children, with focus on normal adult data. Pressure waveforms, energy and integrated pressure values were then obtained and compared at different points of interest in the model. This investigation suggests that the pressures “felt” by the lung tissue and chest wall are too small to have a noticeable effect on them therefore making those particular circuit elements unnecessary when the respiratory system is subject to small displacement volumes such as those used in Impulse Oscillometry. Furthermore, we believe that the very large parameter values often obtained with unconstrained optimization of Mead´s model are evidence that C_l and C_w could be “shorted-out” when modeling IOS data.