The effect of dynamic hypoinflation during high frequency oscillatory ventilation in an in vitro model of the respiratory system

Dynamic hyperinflation during high frequency oscillatory ventilation (HFOV) is defined as a situation when the mean alveolar pressure (mPalv) is higher than the mean airway pressure (mPaw). Lower inspiratory to expiratory time ratio (I:E) is recommended to prevent dynamic hyperinflation. Some studies document a possibility of the opposite effect - dynamic hypoinflation (mPalv<;mPaw). The aim of this study is to investigate whether dynamic hypoinflation may occur during HFOV when a linear or parabolic resistance simulating the airway resistance is present in the model of the respiratory system (RS) and whether the magnitude of the dynamic hypoinflation can be predicted based on the ventilatory parameters and setting of the HFOV ventilator. Models of the RS with various linear and parabolic resistors were ventilated with SensorMedics 3100B ventilator with various preset parameters. The preset I:E of 1:2 and the presence of a parabolic resistance in the circuit increase the magnitude of the generated dynamic hypoinflation. The study also documents that the developed pressure gradient cannot be predicted easily as its magnitude is strongly dependent not only on the preset ventilatory parameters and mechanical properties of the RS, but also on the mechanical characteristics of the HFOV ventilator which are very complex to be described.