Variable expiration control for an intensive care ventilator
Abstract: Critical care patients are often connected to ventilators, to support or replace their breathing. The ventilators deliver a mixture of gas to the patient by applying a specific volume or pressure, and then the patient exhales passively. This thesis is based of the hypothesis that a slower reduction of the expiration pressure could benefit intensive care patients connected to a ventilator. To enable research within the area, a device which can control the expiration is needed. In this thesis project, an expiration valve was controlled to create different pressure patterns during expiration. To facilitate the research and the usage of the expiration control, an application software was created with the purpose to simulate relevant pressure, flow and volume curves. The prototype is an expiration cassette created for the ventilator Servo-i by Maquet Getinge Group. To enable flexibility, the prototype is external and no information is transmitted from or to the ventilator. The prototype has its own flow and pressure sensors. The different pressure patterns which the prototype uses are designed as a linear decrease and as if a constant resistance was added to the system. The user can also create their own pressure pattern, by deciding 20 pressure points in the duration of two seconds. The simulation application was designed with the ability to simulate the same pressure patterns available with the prototype. By using a lung model, it is possible to simulate the ideal pressure, flow and volume in the lungs which can be expected from the chosen expiration control. During the implementation, two different types of lung models were evaluated in order to determine the specificity required. The prototype was tested with settings which were chosen to challenge the performance of the control. Some problematic areas were detected, such as high pressures or large volumes. However, the prototype was judged to perform well enough to be used in animal trials. The lung model used for the simulation application was a simple model of the lung, consisting of a resistor and a capacitor in series. The simulations were compared with the real system with the purpose to get an indication on the difference between theory and reality. The application presents the expected behavior when using the expiration control. However, it should be kept in mind by the user that the application represents a theoretical model.
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