The patient triggers the ventilator by taking a breath in which creates a negative pressure. The ventilator detects this negative pressure change. It will then deliver a specific volume to the patient which can be controlled by the user.
Ideally a patient should be ventilated with 6mls/kg. Mechanical ventilation with lower tidal volumes and positive end-expiratory pressure prevents pulmonary inflammation in patients without preexisting lung injury. Anesthesiology 2008.
A rate can also be set up as back up. So if the rate is also set to 10 per minute. for example, then the ventilator will deliver a breath to the patient every 6 seconds if the patient does not take a breath. So the patient can never breathe less than 10 times per minute.
Here we have set the volume delivered with each breath at 5oo mls; this is the tidal volume. We have also set the respiratory rate to 10 breaths per minute. This means that each breath will take 6 seconds from start on inspration to end of expiration. This is the total cycle time.
So with assist control ventilation the ventilator will deliver this set breath every 6 seconds- where you can see the ticks on the picture above and each breath will be 500 mls.
These breaths are known as the controlled breaths. There is no input from the patient with this breath (the patient could have been paralysed for example).
What triggers the breath to start in this example is time.
After the 6 seconds have passed the machine is triggered to deliver another breath. This then is time triggered ventilation.
The patient my take a breath in this mode of ventilation and this would then be a patient triggered breath. This will still be delivered at a set volume, in this case 500mls.
So rather than a controlled breath, this is an assisted breath.
So in assist control ventilation you can have a controlled breath, which is time triggered or an assisted breath which is patient triggered.
Assist control/volume control ventilation is determined by the volume set rather than the pressure set.
With this mode of ventilation the compliance of the patients lungs will become important.
Compliance of the lung is the relation between pressure change and volume change. Does it take a large rise in pressure to get a certain volume in or a small rise in pressure? In this mode the ventilator will deliver a set volume regardless of the pressure generated to do so.
With the less compliant lung it will take a larger rise in pressure to get the set volume into the lung. This rise in pressure with each breath can cause damage to the lung over time, so is not always the ideal situation.
In this mode we set a certain pressure for the ventilator to achieve with each breath rather than a certain volume.
So if the lung is very compliant, that is it only takes a small change in pressure to increase the volume in the lung, then the pressures set to achieve good tidal volumes will only have to be relatively low.
However, if we have what are sometimes referred to as stiff lungs i.e. the compliance is poor, then the pressures will have to be set to a higher level to achieve the same volume targets.
Setting the alarms on the ventilator becomes very important here. If the compliance of the lung changes over time you need to be aware that the ventilator is may not be delivering adequate tidal volumes or may be delivering too much.
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