A symmetrical computer model of nonlinear mechanical properties of the tracheobronchial airways

A nonlinear time-domain simulator of the tracheobronchial tree is proposed to model breathing mechanics during artificial ventilation. It Is based on Weibel's symmetrical geometric lung description which divides the airways into 24 generations, characterised by increasing numbers of branches. The tracheobronchial tree is represented as a nonlinear RLC ladder network, reflecting the viscoelastic and inertial properties of each generation. The simulator is also used to reproduce intermittent positive pressure ventilation with constant inspiratory flow, widely used in intensive care. The simulator, implemented in MATLAB-SIMULINK, enables modelling of different physiopathological and ventilatory conditions, calculation of flow and pressure time courses and analysis of the viscoelastic behaviour of the various airway generations. Results obtained with the simulator clarify time changes in generation resistances during the breathing cycle. In particular, at the beginning of expiration, upper airway resistance increases and then decreases under the influence of time-varying flow and airway volume. Since lower airway resistance only depends on airway volume, it increases rapidly in the first phase of expiration and then remains almost constant.