In order to insert and withdraw the tidal volume of perfluorochemical liquid (PFC), the total liquid ventilation (TLV) can be efficiently performed with a pump system. The main originality of the developed prototype is that the expiratory pump is independent of the inspiratory one. Consequently, they are controlled independently. The main advantages are to allow the adjustment of the functional residual capacity (FRC) during ventilations and the optimization of the ventilation cycle. However, two main points must be considered in volume-controlled TLV: avoiding airway closure (chocked flow) during expiration and controlling the FRC volume in the lungs. For this purpose, a controller is implemented to manage the pumping system. The main function of the controller is to control with high precision the volume of liquid to insert and withdraw in the lungs. Due to uncertainties, the pump system generates small volume errors at each inspiratory and expiratory cycle. Over time, the addition of small volume errors may cause an FRC drift. To avoid this problem, the controller monitors the movement of the pumps in order to compute small corrections of the desired inspiratory and expiratory volumes. Consequently, cycle after cycle, the controller learns the command which ensures a constant FRC. In order to withdraw the tidal volume, the controller generates an exponential expiratory profile defined by a time constant. The implementation of this profile is motivated by the need to prevent the risk of airway closure. Moreover, the exponential expiratory profile reproduces the effect of gravity by which the tidal volume of PFC is withdrawn from the lungs. However, even if an exponential is used, airway closures can still appear (if the FRC is too low or the flow velocity is too high). The risk of airway closure during expiration is detected when a negative pressure limit is reached. In this case, the controller stops the expiratory pump, but adjusts the next inspiratory volume in order to ensure a constant FRC in the lungs. Experimental results in vivo and in vitro with newborn lambs confirmed the benefit of using the controller with the exponential expiratory profile.
The team is supported in part by the Foundation for Research into Children's Diseases and by the Fonds Québécois pour la Recherche sur la Nature et les Technologies.