Ventilator Pearls Explained Clearly

welcome to another MedCram lecture we
can talk about ventilator pearls today if you haven’t done any work on the
ventilator look at our other lecture on ventilator explained clearly and there’s
a link there for that what today’s lecture is going to be on is just some
different vignettes about things that I use in general when I’m going to put
someone on a ventilator and I call this ventilator pearls so the first thing I
will talk about is just when you put someone on a ventilator in general when
you put someone on a ventilator you have to know what’s going to happen to their
pressure specifically their intrapleural pressure so if you look at the lung
there they sit in a cavity called the thoracic cage and there’s a pressure
associated with this space between the lungs and the chest wall so if we could
measure that and we would see it on a transducer we would notice something
that when someone is breathing spontaneously what’s going to happen is
their diaphragm is going to contract and instead of it being an ice dome it’s
going to contract down and it’s going to cause the intrapleural pressure to go
down okay that’s very important because when the intrapleural pressure goes down
the lungs are going to get bigger and therefore air is going to be sucked in
key word there is sucked in because there’s a negative pressure okay
that’s how you breathe in in a spontaneous fashion okay and then when
you release the air comes back up and back to zero again so that’s zero in the
situation where you have positive pressure ventilations so you’re putting
someone on a ventilator just the opposite happens you’re at a neutral
level here and then the ventilator goes ahead and pushes air not sucks but
pushes air into your lung and that’s going to make a positive pressure in
that plural space so you can see here that
when you put someone from spontaneous breathing on a ventilator they’re going
to go from negative pressure ventilation to positive pressure ventilation by the
way the same thing happens if they’re on BiPAP or CPAP it’s just that they’re not
intubated but when they’re intubated it’s even more so because you’ve
inflated the cuff and you’re basically pushing that air and you can measure the
pressures so this is positive pressure ventilation so you’ve just got to be
aware of that because there’s consequences to that and here’s one of
the biggest consequences to that let’s just draw this schematically here
imagine we’ve got the heart here and we know that the right atrium is where all
the venous return comes back to and here the pressure is fairly low let’s say
it’s zero to five millimeters of mercury okay
and the reason why blood flows back here is because blood in the venous system is
actually higher okay so it’s actually higher than it is in the right atrium
and as a result you know things go from a high pressure to a low pressure you’ve
got blood flowing back so this is venous return going back this way and that’s
the flow that it’s going at well here’s the problem when you put someone on
positive pressure ventilation this heart is also in that thoracic cage with the
lungs and so what happens is if you are putting someone from spontaneous
breathing to on a ventilator where you go from essentially negative pressure
ventilation to positive pressure ventilation the general average pressure
in the heart is going to go up because the heart is sitting in that very space
and so as a result of that your right atrial pressure also goes up so let’s
say it goes up to here now if that’s the case that means the slope has returned
is not going to be as steep as it was and so your venous return is actually
going to drop slightly okay now if that’s the case if your venous return
drop you know the heart can only pump what it’s given
right so cardiac output is also going to drop and that is going to drop blood
pressure incidentally the same thing happens when you’re talking about
positive end expiratory pressure or peep so peep is going to accentuate that so
where would this come in handy this is a patient who is septic let’s think about
this let’s think about a septic patient this is a patient specifically whose
venous return is going to be pretty low already and their septic they’ve got a
metabolic acidosis they’re looking kind of ill you may need to intubate them so
what’s the thing that you’re going to do you look at their blood pressure and
it’s maybe marginal you’ve got to know that as soon as you intubate this
patient this is going to go up their venous return is going to drop and their
blood pressure is going to drop so in these patients if I have time before I
have to intubate them if they don’t look like they have to be intubated right
away the first thing I do is make sure I’ve got good IV access or even put in a
central line and be ready to go of vasopressors you always got to think two
to three steps ahead in these patients so in septic patients I’m looking at
them they’re going downhill if they don’t have a central line they’re not
intubated I would rather put the central line in first and then intubate them
because if I intubate them and their pressure drops I’ve got access also if
I’ve got the central line in I can actually measure this pressure and I can
start working on getting it up so that when I do intubate I don’t get that drop
in pressure because these drops in pressure can cause problems with the
kidneys and etc so septic patient make sure I’ve got IV access make sure I’ve
got a central line in place and then go ahead and intubate okay so that’s just
the purl there when it comes to understanding heart lung interaction and
the fact that the heart is in the thoracic cage and it’s affected by
intrapleural pressures the next thing I want to talk about is with metabolic
acid we kind of alluded to this so probably
the same septic patient we’re talking about before but if you’ve got a
metabolic acidosis and I’ve seen this actually commonly okay you’ve got to be
careful with this so let’s pick a really bad blood gas let’s say their blood gas
is 7.20 2060 and something horrendous like I don’t know 15 I don’t even know
if this blood gas works or not but it’s it’s pretty low okay so what we know
here if you want to know more about this good our acid base lectures and there’s
a link for our acid base lecture you know that if the first if the first two
digits after the decimal point of the pH are equal to the co2 then what you have
here is a pure metabolic acidosis that’s compensated respiratory wise so
you can see here that normally our our p co2 level so by the way this is pH this
is P co2 this is po2 and this is the bike pump
you can hear that our p co2 which is normally 40
to 20 that’s because the lung is trying to compensate for this acidosis by
breathing off a Lewis acid in the form of carbon dioxide so let’s say this
patient’s crashing and you put the central line in and you’re giving IV
fluids and then you go and intubate them you may want to look at the respiratory
rates and the tidal volume before you intubated the patient because if the
vital signs on that patient are let’s say respiratory rate of 30 and tidal
volume let’s say it wasn’t that great let’s say it was 400 okay if you go
ahead and intubate that patient and sedate that patient and maybe even
paralyze that patient and you dial in your regular vent settings of a sea of
12 tidal volume of 450 and let’s say a peep of five and an fio2 of 100% these
are kind of generalized vent settings that people use when they innovate
people right look sure alveolar ventilation there okay
take a look at that that’s around an alpha by ventilation of 12 liters per
minute okay so that patients pushing it hard right
trying to try and get it done whereas down here
what’s this alveolar ventilation which is simply multiplying 12 times 0.45 zero
and we get five point four liters per minute so what you’ve done here is you
had a patient who was in metabolic acidosis and was trying to compensate
for that by breathing hard to get the pH up to seven point two zero because if
they weren’t breathing this hard the pH would be a lot lower and this is what
they’re doing this is what they’re doing twelve and then you sedate them paralyze
them and you put them on vent settings of this what’s going to happen they’re
not going to be blowing off as much carbon dioxide and this pH is going to
drop into the six point nine which is going to lead to cardiac arrest okay so
think about that in patient who is really severely metabolic is going
downhill they’re breathing hard and then you intubate them because if you do not
match these vent settings as appropriately as you possibly can
you’re going to get this drop in pH so I don’t set the respiratory rate to 30 not
unless reduce severe circumstances I may go up to 20 and I may increase the tidal
volume but the key there is to get a blood gas pretty quick follow up on your
pH and make sure you’re moving in the right direction if you’ve got a patient
without COPD you’re not going to really be too worried about the ventilator
rates okay but here’s the key the key point with this is you want to continue
the same amount of alveolar ventilation if possible before the ventilator as
after the ventilator okay so the last pearl I want to talk about is someone
that comes in with COPD let’s say it’s gold stage-four and by
that I mean the fev1 is less than 30 percent of predicted look at our COPD
lectures there’s a link or the pco2 and here’s the key the pco2 is greater than
45 if your pco2 on a patient with COPD is greater than 45 they’ve got chronic
respiratory failure okay pco2 should be less than 45 if it’s
greater than 45 then you’re dealing with chronic respiratory acidosis so let’s
imagine we’ve got somebody who is in that situation they’ve got chronic
respiratory acidosis and so a blood gas would look like that 7.3 for 60 55 and
let’s say a bicarb of 30 okay so there’s our chronic respiratory acidosis now
here’s the thing you know that they’ve got chronic respiratory acidosis because
their co2 is high I repeat normal pco2 should be about 40
yet in somebody who had a pco2 of 60 and it was acute their blood gas would look
more like this 7.2 660 55 and their bicarb would be more like you know 26 so
notice this is chronic this is acute and again if you need more review on this
you can go to the acid-base lectures that we had and there’s a link on that
so pco2 is high in both of these situations in this situation the patient
has not had enough time for their kidneys to hold on to that bicarbonate
and as a result the kidneys cannot compensate for the respiratory acidosis
therefore the pH drops more substantially in the chronic case the
pco2 went from 40 up to 60 over a long period of time because of the COPD for
instance and as a result the kidney has had the ability to retain more
bicarbonate in the body and as a result of that the pH is more close to no
normal which is 7.35 to 7.45 here’s the point with the ventilator if you have a
patient that comes in with chronic respiratory acidosis thereby carbs are
going to be high you can tell that as soon as they come in if I can just get a
chem seven and look at the bicarb and see that there are a chronic co2
retainer because they’re going to be in the mid 30s or 40s in those situations
it’s very important that when you put them on the ventilator you don’t blow
their co2 down to 40 what you’re trying to do is you’re trying to blow their co2
down so let’s say they’re normally at 60 but because they had a COPD exacerbation
on top of their chronic COPD it went up to 80 okay so they come in with 80 and
you put them on the ventilator and you’re trying to figure out how much do
I need to ventilate them don’t ventilate them down to 40 okay because if you
ventilate them down to 40 and they’re on the ventilator for any extended period
of time guess what’s going to happen to this
bicarb body’s going to say well we’re ventilating at 40 we can dump this
bicarb so it’s going to dump the bicarb and when it dumps the bicarb down to 24
and then you try to get these patients off the ventilator now they’ve got to
start retaining this bicarb again and they’re not going to be able to do it
quickly enough and they could go into respiratory failure so what do you do if
you have a patient who you know has got chronic COPD and the reason why you know
that is not because of the co2 that they come in with but it’s the bicarb that
they’ve adjusted to over the period before what you do is you ventilate them
to normalize the pH and you’ll see this over and over again in these patients
you’ve normalized the pH 735 to 7.45 but yet you’ll notice that the co2 s are
still elevated that’s okay that’s where they live and you can try to wean them
off the ventilator from there some helpful hints thanks for joining us you

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