Mechanical Ventilation- Pressure Support Ventilation Graphs
This is the information from a pressure supported breath. We have the pressure generated at the top, the flow generated in the middle and the subsequent volume at the bottom.
There are a number of important points to note. If we look at the pressure graph, we can see a slight dip below the base line before each breath, where each red arrow is pointing. This is the negative pressure created by the patient as they try to initiate a breath.
Therefore the breath will always be patient triggered in pressure support ventilation.
Each breath, after initiation, will then be pressure limited as you can see by the plateau on each breath. We set that pressure when we decide how much pressure support the patient needs.
If we look at the flow curves in the middle section we can see the difference between pressure controlled and pressure support ventilation. In pressure support ventilation the breath is cycled by flow.
So as the patient takes a breath in they generate an inspiratory flow which will peak at point 1 on this diagram and will then decline down the point two, which is when the ventilator will then cycle to the next breath. In pressure controlled ventilation, where we set a rate, the breath is time cycled.
The consequence of this on the volume curve is that we don't see an inspiratory pause. As soon as the ventilator detects a reduction in the flow rate it triggers to expiration. The breath is not held for a period of time as it would be in pressure controlled ventilation.
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- Lung Compliance
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