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