Mechanical Ventilation- Phase Variables
Above is the flow curve generated by the ventilator when delivering a breath in assist control ventilation.
This has a number of phases which can be described, and will help us understand some of the principles of ventilation.
1. Start of inspiration
2. Inspiratory phase
3. End inspiration
What is it that dictates when the ventilator moves between these phases?
This process will depend upon three different types of phase variables:
Trigger- What is it that causes the start of inspiration?
Limit- limited during inspiration, but does not cycle the breath. So it does not cause the breath to end.
Cycle- what causes the breath to go from end of inspiration to the start of expiration. Or what cycles inspiration? For example is there a certain pressure that needs to be reached or have we set a particular volume that needs to be delivered?
To demonstrate this principle lets talk about the differences between controlled breaths and assisted breaths in volume controlled ventilation.
So the trigger variable in controlled breaths, i.e. whats starts inspiration, is time. Every time 6 seconds passes by, if the rate is 10 breaths per minute, another breath is triggered.
The limit variable would be, for example, the flow rate we set. So if we set a flow rate of 60 l/min then this is what will be reached during inspiration and it will go no higher.
What cycles the breath, i.e. what moves us from inspiration to expiration, is volume. When the set volume is reached, for example 500 mls we change to expiration. So the cycle variable is volume
The trigger variable with the assisted breath will not be time but would be initiated by the patient. The ventilator will be set to register if the patient triggers a certain amount of flow e.g. 3 l/min. The ventilator can also be set to detect a certain pressure change generated by the pressure. So the trigger variable with an assisted breath can be either flow or pressure.
The limit variable will still be flow and the cycle variable will still be the volume.
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