Ventilator Modes Explained! PEEP, CPAP, Pressure vs. Volume


welcome to another MedCram lecture
we’re gonna continue in our series here we’re gonna talk about additional modes
of ventilation specifically now CPAP or peep okay
CPAP is typically the term that is used if this is non invasive ventilation in
other words if 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 is
what it’s called so that would be more CPAP if you have a ventilator on and are
invasive Li then it’s kind of noticed peep so 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 and so you can set
CPAP anywhere from five centimeters of water pressure all the way up to twenty
peep is usually measured in millimeters of mercury and that can be anywhere from
five to 20 as well but these pressures that we see here are pressures that are
there on inspiration and specifically on exhalation and that’s where this peep
comes from peep stands for positive and expert ory pressure that positive end
expiratory pressure is there even at the end of exhalation so that when you
breathe out instead of having zero pressure in your
lungs there’s actually still pressure in there so let’s say if it’s set to five
there’s still pressure of five in your Airways now what’s the purpose of this
so basically your 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 airway is going to keep your
tongue forward to keep your airway open so that you don’t have obstructive sleep
apnoea in the invasive mode of ventilation here
remember these Airways come down to small little tiny Airways where they
have little grape light clusters that P par that positive end expiratory
pressure is really good at keeping those alveoli open and what we call recruited
so if you’d like to keep those alveoli open you can increase the peep on the
ventilator and usually it’s its own switch hair or basically its own dial
where you could increase the positive end expiratory pressure the next mode of
ventilation is called pressure support or just PS now this is kind of like peep
and CPAP except it only occurs on inhalation so this is kind of like AC
except that instead of delivering a specific volume this patient is going to
get a certain amount of pressure so it delivers a specific pressure support for
each breath now the patient initiates all the breaths so patient initiates
each breath this is a little different from pressure control where in pressure
control you can actually set the rate so higher pressure supports give bigger
breaths the bigger the pressure the bigger the breath is going to be this is
a very popular weeding mode of ventilation as well as CPAP from what we
talked about so we’ve already talked about four different modes we’ve talked
about AC where the patient’s 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 and 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
five to fifteen okay now we’re gonna go into these modes a little
bit more carefully and kind of dissect them out a little bit more for your
benefit now there’s four things that you should know about writing the orders for
vent okay the first thing you need to know is you need to write a mode in this
case we’re gonna do AC remember that’s assist control
continuous mandatory ventilation this means that whenever the patient triggers
the ventilator he’s gonna get a certain amount of volume but you’re gonna set up
a backup rate okay so that’s the other thing that you put with this is the
backup rate so if we put 16 that means the patient is gonna receive at least 16
breaths per minute if he doesn’t breathe the ventilator is gonna give that to him
anyway the next thing you’re gonna put in since this is AC is you need to put
in a tidal volume and let’s just say it’s gonna be five hundred and fifty
milliliters the next thing you put in is the fio2 so what is that the fio2 is the
fractional inspiration of oxygen and let’s just say we’re gonna set it to
about fifty percent that means fifty percent of the volume of the patient’s
breathing is going to be oxygen and then finally the last thing that we set is
the peep let’s say it’s five so these are the four 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 so what we’re saying here is we’re
using assist control that means that the patient gets 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 exhalation number this pressure is there
to recruit alveoli and so what you’ll notice here is that the first two
parameters are going to affect carbon dioxide how fast you’re breathing and
how much breath you’re taking one each breath is go to effect your minute
ventilation and the last two 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 and it turns out the higher the peep the more alveoli you can
recruit and therefore the better the oxygenation is going to be and of course
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 gasses please refer to our
acid-base lectures on interpretation of acid-base and blood gases so let’s take
a look and see what a pressure volume flow graph would look for AC mode of
ventilation okay so what we have here recall is AC 16 tidal volume v 50 fio2
of 50% and a peep of five so the first thing you want to notice let’s look at
the pressure diagram remember there’s a peep of five and so there will always be
a certain amount of pressure in the circuit until the patient takes a breath
now when the patient takes a breath in that pressure is going to go down to a
negative pressure okay and at that point it’s going to trigger the ventilator
that it’s time to give a certain amount of breath and so the volume that gets
delivered is going to go up to a certain preset tidal volume okay and of course
as that volume starts to enter into the lung the pressure in the lung is going
to go up until it reach reaches the same point at a maximum now of course 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 so this here you can actually set by the way it’s not
here in the four different setting but you can actually set how fast that
flow is going to go in and that’s important when we talk about ventilating
patients in COPD now when you have that preset tidal volume that you’ve set into
your ventilator this is when the 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 and of course when that
happens the pressure in the lung will start to fall back down but it will not
go to zero because remember we have a peep of five that means there’s always a
pressure of five left in there and of course when that occurs volume will come
back out of the lung again and come back to zero and so what we have now is the
status quo where we have a pressure of five left and lung flow is back to zero
and volume is back to zero and the same thing will happen again if the patient
decides to have another breath the pressure will go back to zero and go
beyond it which will trigger the ventilator to do the same thing again
and so you’ll see that the flow rate will go up and then back down again and
here right when the ventilator is triggered you will see volume go back
into the lungs and then come back 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 and it will go back
to zero now if the patient decides not to get a breath okay so we’re talking
about a patient let’s say who is overly sedated but you’ve set a backup rate and
it because you’ve set a backup rate the ventilator is not going to allow the
patient to go long without a breath and 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 and at that time
that it decides to go up everything else on the ventilator will look the same flow up
flow across go 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 now 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 we’re still the
ventilator is still going to give the same volume but what you would see is
you would see a higher pressure and that higher pressure is a result of decreased
compliance in the lung and 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 it becomes
with pulmonary edema or there’s a pneumothorax or there’s something that
prevents the lung from expanding as easily as it would have 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 so 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 two types
of pressures that you’ve got to be concerned about one is a peak pressure
and the other is a plateau pressure and we’re going to talk about that in the
next lecture you