Linear-lumped-parameter modeling of pulmonary impedance in monkeys

Pulmonary impedance, ZL, measured from 2 to 32 Hz in anesthetized, intubated and paralyzed bonnet monkeys (Macaca radiata) was fitted to a variety of linear-lumped parameter mechanical networks. Parameter values for each network were obtained by minimizing the average of the percent distance, Dr, between the computed network impedance and measured ZL at all frequencies. Measured resistance, RL, decreased from 2 to 8 Hz and increased from 8 to 32 Hz indicating that a single series resistance-inertance-compliance (RIC) network was not optimal (Dr approximately 19%). Networks consisting of two series RIC pathways in parallel resulted in a lower Dr (approximately 14%), but parameter values were difficult to interpret. Despite not modeling the decrease in RL with frequency below 8 Hz, an airway wall compliance, Caw, network in which the airways were separated into central and peripheral components resulted in an even lower Dr (approximately 11%). In addition, parameter values were easy to interpret, consistent among our "normal" monkeys and changed consistently and explainably with change in lung mechanics induced by decrease in lung volume. We conclude that networks containing both parallel pathways and Caw are necessary to model ZL over the entire frequency range (2-32 Hz), the effect of Caw is an important determinant of ZL above 8 Hz, and a six-parameter Caw network with the ratio of Caw to parenchymal compliance, Cp, fixed may prove useful in interpreting changes in ZL induced by alterations in lung mechanics in monkeys.