In assist control mode of ventilation, the ventilator settings are include
besides fio2, the target that could be the volume or the pressure. In our
example here it's a volume that you are targeting to deliver 500 ml of air into
the lung, and this will be delivered at a flow rate of 30 liters per minute
and the set rate on the machine is here at 12 breaths per minute
indicating that every five seconds there will be a breath, so the trigger here is
time which is set at five seconds. Now the inspiratory time is set at 1.5 seconds
indicating that the whole inspiratory time will be
1.5 seconds, and since we are able to deliver 0.5 liter at a
flow rate of 30 liters per minute in one second only, that means that we are left
with 0.5 second in inspiration where the volume is already into the lung and the
flow reaches zero here, there is no need for more flow to deliver any further
volume, in order to keep that volume inside the long you need to overcome the
lung elastance, and that's why we need a pressure here, which is a plateau
pressure there is no more need of pressure to overcome the airway resistance
that's why the peak pressure pressure goes down to the plateau pressure
but this breath is a mandatory breath, and this breath is mandatory.. you can see the
patient is not initiating any efforts and this patient is breathing now with a
controlled mode of ventilation and the control here is a volume
Now once this patient starts to breathe, at the time every inspiratory effort
will be assisted with the same control that is tidal volume. So you can see
here the patient now is breathing at a rate close to 18 per minute, so almost
every three seconds there is a breath, and you can notice the
scalloping of the inspiratory pressure indicating that the patient is
initiating inspiratory breath. Now every time the patient is triggering the
ventilator, the ventilator will deliver the control which is a tidal volume
In order to trigger the ventilator the patient has to reach a flow trigger
level, which is set here at 4 liters per minute, so the patient has to initiate a
flow of 4 liters per minute before the machine is triggered and deliver and
delivering the tidal volume. Now if the patient does not breathe for five
seconds, then the machine will be triggered with a mandatory breath, and
freeze it so you can see it.. and you can see the difference between mandatory
breath and assisted breath. Now this breath is an assisted breath and this breath
is an assisted, it occurred at almost three seconds. But the patient here for
five seconds, one two three four and five did not breathe so the trigger which has
set that every five seconds now is activating another breath that is
mandatory and you can see that we lost the scalloping here that was caused by
the patient's effort, and this is a mandatory breath, so this is assisted breath
and this is mandatory breath, and this mode is called assist control
mode of ventilation or CMV.
Thank you very much