Welcome to another MedCram lecture.
We just talked about peak pressures and plateau pressures and we wanted to get into that a
little bit more and talk about what the difference is between those two.
Going back to our drawing, I can explain those two types of pressures here using our picture.
When air is going actually into the lung, the biggest resistance to that air going in
or the biggest pressure that's being delivered is a result of the airways.
When air is actually moving, when it's dynamic, when it's actually going in, the thing that's
generating the pressure that you'll see up here if you had a pressure gauge is actually
the airways.
Now, once the air is in the lungs and it's actually in the air sacks and there's no more
air movement and that's the key, once the air has stopped, once the lungs have inflated,
once there is no more movement in the airways, the thing that determines the pressure in
the air circuit is no longer the resistance of the airways, it is now the actual compliance
of the lung because the pressure that is being transmitted back into the main airway is what
is the compliance throughout the entire lung.
That's important to know.
There's two things that you'll see on pressure, there's a peak pressure.
The peak pressure is when there is air flow.
Then there's something called the plateau pressure.
That is when the air stops.
The air flow, the peak pressure has to do with the airways.
The plateau pressure, when the air stops, has to do with lung compliance.
That is extremely important because here's the point, if there is a problem with the
airways, you're going to see the peak pressure's going up and we'll explain what that is.
If you see a problem with lung compliance itself, you're going to see a problem with
the plateau pressures.
Let's show you what the definition is for those.
Let's look at pressure and let's look at flow.
First of all on pressure, if I am ventilating a patient and I inflate their lungs, the pressure's
going to go up and when I release it the pressure's going to go down.
How does that look in terms of flow?
Well, when you're talking about flow, obviously when pressure is going up flow is going up.
Then there's a certain amount of flow and then when the pressure starts to come down,
flow goes in the opposite direction until it finally comes back to baseline.
That's what it looks like.
Now, let's say I do a specific maneuver.
Let's say I go ahead and I have the lungs inflated but then I hold the lungs at inflation.
There's going to reach an equilibrium when the air has gone in, that there will be a
holding or a plateau of pressure and then a release.
What that means is air is going in, so there's flow.
When I go ahead and I hold that breath ... This is a breath hold.
When I hold that breath, air flow is going to go to zero until I release that breath
and then it's going to go down and come back up.
This right here is inhalation and this right here is exhalation.
This is not a normal breath.
This is where I actually do a breath hold.
The purpose of this is to show you the difference here.
This area right here is the peak pressure.
Whereas this area here is the plateau pressure.
Peak pressure is at the beginning.
This has to do with ... Let's do it in the right color.
This peak pressure right here has to do with airways.
This plateau pressure here has to do with lung compliance.
Based on those readings, let's give an example where we would have a high peak pressure.
This peak pressure ... Specifically what we're looking at is the difference right there.
Let's go ahead and reconstruct what that would look like.
This is an example of a high peak pressure.
If the difference between the peak pressure and the plateau pressure is greater than five
millimeters of mercury, that is an elevated peak pressure.
Let's take a look at the other extreme.
Here, clearly the problem is not with the peak pressure.
Here the problem is with a high plateau pressure.
Again, we have a plateau pressure elevation and we have a peak pressure elevation.
This is something that you can actually do at the bedside with a ventilator, if the pressure
alarm is going off, you're going to want to ask your respiratory therapist or if you are
a respiratory therapist, you're going to want to know is it a problem with plateau pressure
or peak pressure.
Let's talk about why that's important.
A high peak pressure remember it has to do with airways.
What are some things that could happen on a ventilator that would give you a high peak
pressure?
One of them is bronchospasm.
If a patient all of the sudden becomes bronchospastic, their peak airway pressure is going to go
up.
What happens if there are secretions?
Specifically dried secretions that are plugging up the endotracheal tube.
That could do it as well.
A mucus plug, kind of the same lines or the tip of the endotracheal tube is occluded.
These are all things that could cause peak airway pressure alarms in a patient with AC
mode ventilation.
What about plateau pressure?
Remember this has to do with the lung compliance.
What are some things that would drop lung compliance?
Pneumothorax could drop lung compliance.
Pulmonary edema, that could do it pretty quickly.
ARDS, pneumonia.
ARDS and pneumonia are things that happen more chronically.
You could have flash pulmonary edema.
Obviously pneumothorax would happen rather quickly as well.
Knowing the difference between plateau pressure and peak pressure could actually be very diagnostically
helpful in a patient who is acutely decompensating so you should know it.
With that knowledge, let's go back to our four inputs in terms of AC mode ventilation.
We said earlier that there were a few things that we could set things up with.
For instance, we could set the rate, the backup rate that is, the title volume, the FIO2,
and the peep.
We said that these things here, the rate and the title volume would effect carbon dioxide
and these things here would effect oxygen and that's correct.
Let's further qualify these things.
The rate, obviously if you increase the rate you're going to get increased ventilation.
Remember when you're increasing ventilation, every time a patient breathes there's a little
bit of dead space so you're increasing the amount of dead space.
Probably a more efficient way of increasing ventilation would just be to increase the
title volume because remember, with every breath there's about a hundred and fifty cc's
of dead space.
When you increase the title volume, you're actually getting 100% ventilation for every
amount of increase in title volume that you're getting.
When you increase the respiratory rate, you're increasing the number of times you breathe.
With that is included a dead space of about 150 cc's.
This is air that goes into the trachea and the bronchi where there is not gas exchange.
The bottom line here is that you could increase ventilation by either increasing the rates
or increasing the title volume.
A 5% increase in title volume is probably more efficient than a 5% increase in the rate.
The one thing that I would say is that if you're talking about a patient however in
COPD, remember they have a hard time getting air out.
It's possible that the air might not be able to get out before the next breath.
This is particularly important and if you have too high of a respiratory rate or too
much of a title volume.
If you give a breath before the first breath is able to be exhaled, you could get something
called breath stacking in COPD and actually paradoxically increasing either the rate or
the title volume and a patient with COPD could actually decrease your minute ventilation
and actually increase the CO2.
Just be careful of that.
The other thing that I would say about title volume is that be careful of it being too
high because when the title volume goes in and out, remember your alveoli are going in
and out.
As a result of that you could be getting collapsing and opening of your alveoli back and forth.
That type of movement can cause inflammation.
That's something that you don't want to have in things like ARDS.
In ARDS we typically like to have low title volumes.
We can talk about that more later.
Now, what about oxygen?
FIO2 is pretty self-explanatory.
Obviously the higher the FIO2, the higher the PAO2 is going to be.
We'd like to have it, if you can, less than 50% because FIO2 is greater than 50% can cause
bronchitis in the ICU.
The peep remember is the pressure left in the circuit, in the lungs, in the alveoli
at the end of exhalation.
The more pressure there is, the more likely those alveoli are going to be open and therefore
oxygen exchange is going to go up.
Peep opens up more alveoli.
Here's the other problem with peep.
Remember that if you increase the pressure in the lungs at the end of exhalation you're
also increasing the intrathoracic pressure.
That could have good effects and that could have bad effects but remember what major organ
sits inside the thorax?
Your heart.
If you're putting pressure on your lungs to keep the alveoli open, you're also putting
pressure on your heart.
Now if you put pressure on your heart, that's going to increase the pressure in the right
atrium.
Remember in the venous system blood flows back to the heart in a process known as venous
return and it's a down slope hill.
In other words, blood is going to go from an area of high pressure to an area of low
pressure.
The more there is a difference between these two pressures, the faster blood is going to
come back to the heart.
If we are increasing the peep intrathoracically, that's going to increase the pressure in the
right atrium and this low pressure system is going to be not as low.
If it's not going to be as low, that means the gradient between these is not going to
be as much.
Therefore we're going to have less venous return.
Less venous return translates into less cardiac output.
Less cardiac output means lower blood pressure.
Here's the trade off, increasing the peep may improve your oxygenation but it can lower
blood pressure.
That could be a problem.
Sometimes having less venous return is not a problem.
In fact, it may be beneficial.
If for instance someone is in congestive heart failure, the pressure behind this heart can
... Or the amount of blood behind this heart can be alleviated by increasing the pressure
in the right atrium.
In patients who have congestive heart failure and they get intubated, increasing the PEEP
can actually improve lung aeration.
You could also get the same effect by the way with just using Bi-PAP or CPAP on patients
with congestive heart failure.
That's why it works so well and increasing the end expiratory pressure is very beneficial.
Since you've got the basics down for AC ventilation, let's go ahead and work on a sample problem
and we'll do that in the next lecture.