Role of the mechanical properties of tracheobronchial airways in determining the respiratory resistance time course

A physiologically based simulation model of breathing mechanics was conside red in an attempt to interpret and explain the time course of input respira tory resistance during the breathing cycle, observed in recent studies on v entilated patients. The model assumes a flow-dependent Rohrer resistance fo r the upper extrathoracic airways and volume-dependent resistance and elast ance for the intermediate airways. A volume-dependent resistance describes the dissipative pressure loss in the lower airways, and two constant elasta nces represent lung and chest wall elasticity. Simulated mouth flow and pre ssure signals obtained in a variety of well-controlled conditions were used to analyze total respiratory resistance and elastance estimated by an on-l ine algorithm based on a time-varying parameter model. These estimates were compared with those provided by classical estimation algorithms based on t ime-invariant models with two, three, and four parameters. The results show that the four-parameter model is difficult to identify, while the three-pa rameter one offers no substantial advantage for estimating input resistance with respect to the more simple two-parameter model. In contrast, the time -varying approach provides good on-line estimates of the simulated end-expi ration and end-inspiration resistances. These values provide further inform ation of potential clinical utility, with respect to time-invariant models. For example, the results show that the difference between the end-expirati on and end-inspiration resistance increases when obstructions shift from th e upper to the lower airways. The similarity of the results obtained with m easured and simulated data indicates that, in spite of its simplicity, the simulation model describes important physiological mechanisms underlying ch anges in respiratory input resistance, specifically the mechanical properti es of intermediate airways. (C) 2001 Biomedical Engineering Society.