Mechanical ventilation explained with illustrations by Dr. Roger Seheult. Get CME / CE for this video series here: …
welcome to another MedCram lecture
we're going to talk about mechanical
ventilation and this is meant to be a
introduction to mechanical ventilation
so if you've never done mechanical
ventilation before we're going to
introduce you to the basics so you can
go in and actually feel competent about
managing a patient on the ventilator
this is often a daunting task because
typically these patients are critical
but actually the basics are fairly
graspable and this is a series that's
going to actually go through a number of
different lectures and we're gonna start
with the basic starting right now the
first thing you've got to know is you've
got another definition of some of these
things okay so you've got the patient
then you've got this thing coming out of
their mouth that's the endotracheal tube
we're gonna show this a little bit more
later and then you have it hooked up to
a big machine with a bunch of knobs on
it and dials and output this is what we
know as the ET tube
that's the endotracheal tube and then
finally you've got the actual ventilator
that's important to know because
sometimes people are intubated that
means we put a tube down into their
mouth because they need airway
protection in other words because they
can't protect their airway they can't
protect liquids and solids from going
down their airway work that stuff
shouldn't go and because of this it's
not too comfortable we've got to sedate
them and when we sedate them we've got
to put them on a ventilator so that
might be one reason why we would have to
do this the other reason is because they
can protect their airway okay but they
just can't breathe on their own they're
struggling to breathe and so we help
them out with the mechanical portion of
breathing and that's where the
ventilator comes in and the way we
deliver that is through the endotracheal
tube so it's kind of important to know
what an endotracheal tube looks like and
the basic is pretty much the same all
the way around it's this long tube kind
of looks like this and that's the part
that connects to the ventilator this is
the part that goes inside the patient
and actually you'll see that there is a
balloon on the end of that endotracheal
the thing that allows you to blow it up
it's a little thing that goes up the
pilot part comes out and it's like a
little pilot balloon that you can kind
of feel what the pressure is and then
there's a little port where you can
inject air into it so when this goes
down and you intubate somebody goes into
their mouth past their vocal cords
specifically and down into the trachea
so the vocal cords usually end up about
right here and so this is going down
into somebody's trachea okay and then
usually it branches off you've got the
left and the right mainstem bronchus
so here you have the endotracheal tube
going down now this balloon gets
inflated here so that stuff that might
make it down here doesn't go past and go
into the lung this is called airway
protection and we blow up the balloon
here after we intubate them to make sure
that that happens now in some versions
of this they'll have like a little
device right here that also comes out
the purpose of that is to suck
secretions that might come up and go out
and that's subglottic suctioning that's
kind of an option but this is the basic
anatomy of an endotracheal tube now of
course we just talked about the
ventilator that's got a bunch of buttons
and whistles and things and we're gonna
talk about them a little bit okay so
going back to our patient again we've
got our endotracheal tube we've got our
ventilator so what's the purpose of this
ventilator the purpose of the ventilator
is to maintain homeostasis between the
dew gas concentrations that we're
talking about here which is carbon
dioxide and oxygen okay
so oxygen is being put into the patient
and carbon dioxide is coming out and for
the most part we want to keep those
close to normal there's some exceptions
to that here's the point though there's
many different ways to put air into
somebody okay we can say we're gonna put
air into somebody based on volume so
we're going to put a certain X amount of
volume into somebody and then let it
come back out that's one way of doing it
another way of ventilate
somebody is saying we're going to
inflate them to a certain pressure so
we're gonna have this ventilator put a
certain amount of pressure into the
patient and then when that pressure is
released then it's gonna come back out
so we can do that now we can do it at a
certain rate we can do this fast and we
can do it slow okay so in other words
how many breaths per minute we could
also adjust the flow rate so in other
words yes we can port a certain volume
in but we can give that volume slowly or
we can give that volume very quickly the
other thing that we can do is we can
decide how much pressure to leave in
there at the end of when we put the air
in and then we can decide how much
pressure to leave in there after we're
done putting the air in finally we can
decide how much oxygen we want to put in
there we can put a lot or we can put a
little now just to further complicate
this just to kind of see where we're
going with this we can have the
ventilator be in charge of when the
patient gets a breath or we can have the
patient be in charge of when they want
to get a breath so think about all of
these different variabilities and now
you can quickly see how there are so
many different ways that you can
ventilate somebody and each one of these
ways is a different mode of ventilation
you may have heard of these before like
AC or simv
or pressure support or CPAP these are
all different modes and we're going to
go through some of these modes and show
you how this is working
okay so here's our system over here
we've got the ventilator here we've got
the tubing that goes to the endotracheal
tube down into the lungs and we've got
our balloon here filled with air to make
sure nothing else gets down there and
we're ventilating our right lung and our
left lung so let's talk about the first
mode of ventilation and this will become
important later
the first mode that I want to talk about
is AC okay the other way we call it as
assist control the other name for it
also is continuous mandatory ventilation
or CMV now this is the most common mode
of ventilation that you'll see
especially on a medicine floor or
medicine units the key here is that the
patient triggers the vent how does that
happen well the patient takes a breath
in and therefore there's a negative
pressure here which causes a negative
pressure to be sensed here at the
ventilator the other way you could sense
it is by flow if there is a flow that
actually goes through here by the
negative pressure now as soon as the
ventilator picks up on that negative
pressure it's going to deliver a
specific volume and so there's an actual
dial on here where you can actually turn
the knob to a specific volume or you can
enter it in and that volume can be
anywhere from 500 CC's all the way up to
600 cc's usually the ideal way of
ventilating somebody would be around
eight milliliters per kilogram ideal
body weight anyway whatever that volume
is it's good to liver that specific
volume in AC mode ventilation now the
patient can trigger it you could also
set up a backup mode or a rate what does
that mean if I set the rate to for
instance 12 because there are 12
five-second intervals in one minute that
means every five seconds the ventilator
will give a breath to the patient of
a specific volume only if the patient
does not take a breath if the patient is
breathing above 12 then the ventilator
will only give breaths when the patient
triggers it by trying to take a breath
in in other words if you set the mode to
AC set in a volume and set a rate of 12
the patient can never breathe less than
12 times per minute now there's
something that you should understand
about this which is very important you
may recall from chemistry an equation
that says P V equals n R T now in this
system temperature is constant R of
course is always a constant n is a
constant and so the thing that you must
realize is that pressure and volume are
inversely proportional to themselves in
other words as the volume of a gas goes
up the pressure goes down if you have
the same amount of gas however the other
way of looking at this is compliance
which I'll abbreviate as a/c compliance
is equal to the change in volume over
the change in pressure which means to
say that if the pressure changes a
little bit and the volume changes a lot
then you have a very compliant lung if
you don't have a very compliant lung
it's gonna take a lot of pressure to
make just a small amount of change
here's the point the point is is that
these set of lungs have a specific
compliance and if you are delivering a
specific volume into these lungs you are
going to get a specific pressure after
you deliver that volume that pressure
can change depending on the compliance
the point of this is is that you need to
have a readout that tells you what the
pressure is in that lung so you can know
what the compliance is so in other words
in this mode of ventilation you set the
tidal volume and the ventilator will
tell you what the pressure is so you're
setting the tidal volume you're setting
how much volume of gas is going to go
into the lung and based on the
compliance of the lung it will tell you
what the pressure is
if the compliance of the lung goes down
then typically you'll have higher
if the compliance of the lung is very
high in other words a very compliant
lung then your pressures are going to
tend to be on the lower side now let's
make this converse to pressure control
in pressure control what we're doing is
we're setting a pressure in other words
we're going to decide how much pressure
we're going to ventilate this patient
with and so as you can imagine if we're
setting a pressure there is a specific
compliance to this lung depending on
what state it is in and if we set a
certain pressure if the compliance of
this lung is very low then you can
imagine we're gonna have lower volumes
however if the compliance of this lung
is very high then we're gonna have
higher volumes because you remember
compliance is equal to the change in
volume over the change in pressure so in
pressure control you can also have the
patient or time
triggering a set change in pressure and
depending on the compliance the volume
can change and so the key here is that
you need to have alarm setup and you
need to know and understand what those
alarms mean what could happen here let's
for instance say in a pressure control
situation where you're giving a specific
pressure if the compliance of these
lungs somehow drop precipitously all of
a sudden because of some pathology which
we'll get into you will notice that the
volumes will drop you would want to know
that and so you could set a alarm on the
lower side of the volume so that if the
volumes did go down an alarm would go
off saying that you're not ventilating
conversely if you were back in our
previous mode which was assist control
and you're setting a certain volume if
the compliance of the lung dropped in
that situation then as you would realize
the pressure would start to go up
because you're trying to put a set
amount of volume in
to a low compliant lung and when that
happens the pressure goes up the
pressure would then trigger an alarm so
the point here is in pressure control
you're setting a pressure and your
output to read is your volume in AC it's
the flip of this so when we come back
we'll talk about the pressure volume
relationship and a few more modes of
mechanical ventilation

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