The Effects of Expiratory Flow Limitation and Different Inspiratory and Expiratory Airway Resistances on Dynamic Hyperinflation of the Lungs: A Bench Study

Dynamic hyperinflation occurs when mean alveolar pressure exceeds the mean airway pressure measured in the airway opening. The dynamic hyperinflation develops as a consequence of insufficiently long expiration terminated by the subsequent inspiration. Two main principles are supposed to create the dynamic hyperinflation during high frequency oscillatory ventilation (HFOV): expiratory flow limitation and different inspiratory and expiratory airway resistances (Re > Ri). The aim of the study is to design and test a physical model of the respiratory system comprising both the mechanisms causing dynamic hyperinflation and to characterize their effect upon dynamic hyperinflation or hypoinflation development during HFOV. The models were created using a rigid volume and passive pneumatic components mimicking the required characteristics of the airway resistance. The models were connected to a HFOV ventilator and different ventilator settings were applied in order to investigate the parameters having the most significant effect on dynamic hyperinflation development. The main result of the study is that the expiratory flow limitation corresponds better with the observed and published properties of the respiratory system during HFOV. The magnitude of dynamic hyperinflation depends on ventilator setting. The acquired results are supported by recently published studies using mathematical-physical modeling and correspond with results of the published clinical trials.