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