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.