A MODEL OF TRANSIENT OSCILLATORY PRESSURE-FLOW RELATIONSHIPS OF CANINE AIRWAYS

In a previous paper (27) we developed a lumped parameter model of cani ne pulmonary airway mechanics featuring airway wall elasticity, gas in ertance, and laminar and turbulent gas flow. The model accurately acco unted for the steady-state pressure-flow data we obtained during sinus oidal cycling of the lung following a period of apnea. In the present paper, we extend the model to account for the transient decrease in th e amplitude of the trans-airway pressure swings that we observed immed iately following the apnea, which we have shown to be due to a vagally mediated bronchodilatation reflex. The extended model accounts for th is transient in terms of a sudden change in airway smooth muscle tone acting on the viscoelastic properties of the airway wall and tissues m echanically coupled to it. Consequently, this model is able to tempora rily store a volume of gas in the conducting airway tree as its volume changes cyclically with that of the whole lung. This means that the f low entering the airway tree from the trachea at any instant (V) is no t precisely equal to that entering the alveoli (V-alv) even when the g as is considered incompressible We found that assuming V to be equal t o V-alv can lead to errors in estimating respiratory tissue impedance of as much as 10%. However, tissue hysteresivity remained almost unaff ected, suggesting that the hysteretic properties of respiratory system tissues and airway wall are well matched.