The system so far accomplishes about as much as if you had an automated ambu-bag running. It could provide repeated breaths of a controllable volume, and a fairly consistent rate but there is no pressure monitoring or fail-safe should something go wrong. What we want to have is to be able to determine if an overpressure situation arises and so be able to stop the ventilator to prevent lung injury. We should also have some sort of backup failsafe that does not depend on the PLC or exhale valve functioning properly, in case the source of the problem is in fact one of these components. We should also have some sort of visual indication of the pressure at any time in order to aid the clinician that is monitoring the device.
The design for this is very basic. It is a manometer with a float, magnet and sensors. The floating magnet and sensors provide a signal to the PLC if the positive over pressure or lower pressure limits are exceeded. The lower pressure sensor can also be used to operate the ventilator in a demand mode or to implement PEEP. A third or fourth sensor could also be added later to provide redundant alarm points for tubing disconnect or blockage.
In the following diagrams you can see the function of the manometer.
- In Figure 1, the manometer is at its neutral state and both sides are at the same level.
- In Figure 2, the manometer is under negative pressure and the lower sensor is activated.
- In Figure 3, the manometer is under a positive pressure and the maximum sensor is activated.
- In Figure 4, the manometer has gone past the positive control point and it is acting as a safety overpressure. The air will leak past the bottom of the U portion of the curve and bubble up through the top.
Figure 2 Manometer Minimum Pressure
Figure 4 Manometer Over Pressure