Mechanical Ventilation Explained Clearly by | 2 of 5

Welcome to another MedCram lecture, we’re
going to continue in our series here, we’re going to talk about additional modes of ventilation,
specifically now, CPAP or PEEP. CPAP is typically the term that is used if
this is non-invasive ventilation. In other words, instead of putting a tube
down into someone’s throat, we instead did a mask to fit over their face. Like a BiPAP mask or a CPAP mask, in fact,
it’s what it’s called. That would be more CPAP. If you have a ventilator on invasively then
it’s known as PEEP. What is this, what is this mode of ventilation? This mode of ventilation is simply, you’re
not dialing in a vent setting. You’re not dialing in a specific volume, in
fact, what you are dialing in however, is a pressure. But this pressure is continuous. In other words, it doesn’t matter whether
the patient is breathing in or whether the patient is breathing out. They’re always getting the same amount of
pressure. You can set CPAP anywhere from 5 centimeters
of water pressure all the way up to 20. PEEP is usually measured in millimeters of
mercury and that can be anywhere from 5 to 20 as well. These pressures that we see here are pressures
that are there on inspiration and specifically on exhilaration and that’s where this PEEP
comes from. PEEP stands for positive end-expiratory pressure. That positive end-expiratory pressure is there
even at the end of exhilaration so that when you breath out, instead of having 0 pressure
in your lungs, there’s actually still pressure in there. Let’s say it’s set to 5, there’s still pressure
of 5 in your airways. Now, what’s the purpose of this? Basically you’re breathing in and out with
the same pressure, this could have a lot of good impact in the non-invasive mode if you’ve
got obstructive sleep apnea because that pressure in your air way is going to keep your tongue
forward, so it’ll keep your air way open so you don’t have obstructive sleep apnea. In the invasive mode of ventilation here,
remember these air ways come down to small little tiny airways where they have little
grape light clusters. That PEEP or that positive end-expiratory
pressure is really good at keeping those alveoli open and what we call recruited. If you like to keep those alveoli open, you
can increase the PEEP on the ventilator, and usually is it’s own switch here, or basically,
it’s own dial where you can increase the positive and expiratory pressure. The next mode of ventilation is called pressure
support or just PS. This is like PEEP and CPAP, except it only
occurs on inhalation. This is like AC except that instead of delivering
a specific volume, this patient is going to get a certain amount of pressure. It delivers a specific pressure support for
each breath. The patient initiates all the breaths. Patients initiates each breath. This is a little different from pressure control,
where in pressure control you can actually set the rate. Higher pressure supports give bigger breaths. The bigger the pressure, the bigger the breath
is going to be. This is a very popular weaning mode of ventilation
as well as CPAP from what we talked about. We’ve already talked about 4 different modes. We’ve talked about the AC where the patients
triggers the ventilator and the patient gets a specific volume of breath, we’ve talked
about pressure control where the patient or the ventilator can both cause the ventilator
to give a specific pressure and then come back down to specific pressure and we’ve talked
about CPAP where the patient is on a continuous pressure regardless of whether or not they’re
taking a breath in or out. Then we just talked about pressure support
ventilation where on each breath that the patient triggers the ventilator on, they get
a specific amount of pressure. That could be anywhere from 5 to 15. We’re going to go into these modes a little
bit more carefully and kind of dissect them out a little bit more for your benefits. There’s 4 things that you should know about
writing the orders for vent. The first thing you need to know is you need
to write a mode. In this case we’re going to do AC. Remember that’s assist control continuous
mandatory ventilation. This means that whenever the patient triggers
the ventilator, he’s going to get a certain amount of volume. But you’re going to set up a backup rate. The other thing that you put with this is
a backup rate. If we put 16, that means, the patient is going
to receive at least 16 breaths per minute if he doesn’t breath, the ventilator’s going
to give that to him anyway. The next thing you’re going to put int, since
this is AC, you need to put in a tidal volume. Let’s just say it’s going to be 550 milliliters. The next thing you put in is the FIO2. What is that? The FIO2 is the fractional inspiration of
oxygen. Let’s just say we’re going to set it to 50%,
that means 50% of the volume the patient’s breathing is going to be oxygen. Finally, the last thing that we set is the
PEEP, let’s say it’s 5. These are the 4 things that generally are
set in AC mode ventilation which is the most common mode of ventilation that you’ll see
in a medical intensive care unit. What we’re saying here is we’re using assist
control, that means that the patient get’s a certain volume. We’re writing in what that volume is, we are
putting in how much FIO2 the patient’s going to be receiving, how much oxygen and then
how much pressure is going to be left in the circuit at the end of exhilaration. Remember this pressure is there to recruit
alveoli. What you’ll notice here is that the first
2 parameters are going to affect carbon dioxide. How fast you’re breathing and how much breath
you’re taking with your breath is going to affect your minute ventilation. The last 2 is going to affect your oxygenation. Obviously, the amount of oxygen you put in
is going to affect how much oxygen you read on your saturation. As it turns out, the higher the PEEP, the
more alveoli you can recruit and therefore the better the oxygenation is going to be. We can manipulate these values to get the
effect that we want with the blood gas. By the way, if there’s any questions about
blood gas, please refer to our acid base lectures on interpretation of acid base and blood gases. Let’s take a look and see what a pressure
volume flow graph would look for AC mode of ventilation. What we have here recall is AC 16, tidal volume
550, FIO2 of 50% and a PEEP of 5. The first thing you want to notice, let’s
look at the pressure diagram. Remember there’s a PEEP of 5 and so there
will always be a certain amount of pressure in the circuit until the patient takes a breath. When the patient takes a breath in, that pressure
is going to go down to a negative pressure. At that point, it’s going to trigger the ventilator
that it’s time to give a certain amount of breath. The volume that gets delivered is going to
go up to a certain preset volume tidal volume. As that volume start to enter into the lung,
the pressure in the lung is going to go up until it reaches the same point at a maximum. Flow into that lung is going to start right
at that time and it’s going to immediately go up and it’s going to be a constant flow
that you can preset until it reaches that point. This here, you can actually set, by the way,
it’s not here it the 4 different settings but you can actually set how fast that flow
is going to go in. That’s important when talking about ventilating
patients in COPD. When you have that preset title volume that
you’ve set into your ventilator, this is when your ventilator is going to stop giving flow
and what you’ll see is, flow will not only stop, but flow will start to come out of the
patient and start to go back to normal. When that happens, the pressure in the lung
will start to fall back down, but it will not go to 0 because remember we have a PEEP
of 5, that means there’s always a pressure of 5 left in there. Of course, when that occurs, volume will come
back out of the lung again and come back to 0. What we have now is the status quo where we
have a pressure of 5 left in the lung, flow is back to 0 and volume is back to 0, and
the same thing will happen again. If the patient decides to have another breath,
the pressure will go back to 0 and go beyond it which will trigger the ventilator to do
the same thing again. You’ll see that the flow rate will go up and
then back down again. Here, right when the ventilator is triggered,
you will see volume go back into the lungs and then come out again. Here, of course, as soon as the trigger is
set, you will see flow go up at a certain constant rate until the target volume is reached
then flow will come back out again, then it will go to 0. If the patient decides not to get a breath,
we’re talking about about a patient, let’s say who is overly sedated, but you’ve set
a backup rate. Because you set a backup rate, the ventilator
is not going to allow the patient to go long without a breath. That will look a little bit different. Because the patient will not have triggered
a breath, you will just see, instead of a negative deflection, you will just see it
go up. At that time when it decides to go up, everything
else on the ventilator will look the same. Flow up, flow across, flow down. Notice that in a patient triggered breath,
you will see a negative deflection in the pressure circuit, but in a ventilator given
breath, it will be missing. This is a good time to, again, talk about
compliance of the system. Notice that when we’re talking about the AC
mode of ventilation, there is a preset tidal volume that we are entering into in this ventilator. If for some reason the compliance of the system
goes down, the ventilator is still going to give the same volume, but what you would see,
you would see a higher pressure. That higher pressure is a result of decreased
compliance in the lung. Of course, the thing to know there is you
can actually set a pressure alarm here so that if the compliance of the lung does go
down, and what I mean when the compliance of the lung goes down is let’s say if he comes
with pulmonary edema or there’s pneumothorax or there’s something that prevents the lung
from expanding as easily as it would’ve normally been. If this pressure exceeds the set pressure
alarm, there will be a bell that goes off and the respiratory therapist or the nurse
will be drawn to the bedside because there’s a problem. Remember in AC mode, you dial in the volume,
the pressure is variable depending on the compliance. But, as most things in medicine, it’s not
always as simple as you may think. There’s actually 2 types of pressures that
you’ve got to be concerned about. 1 is a peak pressure and the other is a plateau
pressure and we’re going to talk about that in the next lecture.

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