okay welcome to another MedCram lecture we're gonna talk about a case
today so imagine that you go down to the emergency room you get called and
there's a 25 year old Caucasian female who is about four months postpartum and
she's having a hard time breathing so she's got shortness of breath it's been
going on for about a week and it's progressive and she has a history of
asthma but she has been remiss in using her inhaler and on physical examination
you noticed that she's got bilateral expert ory wheezing and her respiratory
rate is in the 20s and she certainly is looking anxious and in fact she's got a
history of anxiety disorder as well her vital signs she's afebrile her blood
pressure is 150 over 70 pulse is already in the 120s and that's because she's
already received an albuterol and atrovent nebulizer
she's saturating in the low 90s about 92% and as we said her respiratory rate
is in the 20s you do a blood gas and her pH looks something like seven point
three five pco2 is in the high 40s po2 is in the 80s and bicarb is 24 so you
give her a nebulizer treatment again and you start steroids so steroids
nebulizers give her a little bit of oxygen but she doesn't get better she
continues to have issues so you start her on some BiPAP because she looks like
she's using some accessory muscle use and you start her on regular setting of
about ten over five when you increased that to 12 over 6 and you draw another
blood gas and instead of 7.35 after about two or three hours of battling
with her and everybody else in the emergency room you now get a pH of seven
point two zero sixty four seventy five and 24 and that's on BiPAP she is awake
alert oriented but she's a little bit more lethargic than she was initially
okay let's stop there and talk a little bit about what's going on with her and
the diagnosis so most likely this lady has an asthma exacerbation and remember
with asthma you've got a bronchus and you've got smooth muscle that's going
all throughout this and because of inflammation that is occurring in this
area so inflammation causes contraction of this down to a small hole and as a
result of that the air has a hard time getting out and that's the key word
there is out so air can come in but air has a very hard time coming out and you
have a problem therefore on exhalation if you were to look at a flow volume
loop you would see a normal flow volume loop looks like this you have a lot of
air coming out initially and then you get to the small Airways and then you
take a deep breath in so that's what a flow volume look will look like with
flow here on the y-axis and volume on the x-axis in small Airways disease
either in COPD or when you have an asthma flare you are going to look very
similar on inhalation it's not gonna affect about of air that can go into the
lung but what you will see on exhalation is no issues with the large Airways but
you'll notice that the small Airways are definitely decreased and so you can see
her flow rates on exhalation are incredibly reduced and again that's
primarily because these small muscles here in the Airways in the small Airways
particularly are contracted now there's two receptors that you've
got to be aware of on here there is a muscarinic receptor and there is a beta
receptor the beta receptor actually causes muscular relaxation and so you
want to activate this receptor with a beta agonist
however the muscarinic receptors you want to block
with a muscarinic antagonist and so those are just about all of the
medications that end in I um so tiotropium epitope reham you metla diem
all of those anticholinergic medications whereas these beta agonists are the ones
that end in oh L that would be like albuterol salmeterol for model so the I
um s again are the muscarinic antagonists there is one that does not
end with I um and that is glyco pi relate so that's the one exception to
this but the I UMS are the muscarinic antagonist the OLS are the beta agonists
and what that's going to do is it's going to open up those Airways which is
what you need to get air out and so that's the real problem here air cannot
get out and of course it goes without saying that the air must come out during
the exhalation phase and so you want the exhalation phase of breathing to be as
long as possible well that's hard to do when the patient's breathing fast
because that gives a short amount of time for both inhalation and exhalation
so that is the issue with asthma and that's the issue with this patient
that's coming to the hospital okay so let's continue as it goes our patient
continues to get worse and now she's becoming more confused and so the
decision is made to intubate but before the patient gets that intubation there's
another Blood gas that's gotten on this patient and it clearly shows a worsening
acute respiratory acidosis this time 7.10
80 70 and 24 remember now that the bicarb has not changed the po2 is a
little bit less the pco2 has greatly increased and that's cause the pH to
drop pretty precipitously and so the patient's intubated with an endotracheal
tube and put on the ventilator and the initial settings for the patient are AC
of 20 a tidal volume of 500 a peep of 5 and an fio2 of 100% so this means that
the patient's got to get a breath at least 20 times per minute more if the
patient demands that the tidal volume of each breath is good
500 MLS or half a liter the amount of pressure left in the circuit is going to
be 5 at the end of exhalation and the patient's going to be breathing in a
hundred percent oxygen so after a while of the patient being on the ventilator
and x-ray is taken it shows that the patient has clear lung fields there's a
heart but that the Hemi diaphragms are very flattened and the rest of the lungs
are consistent with hyper expansion the patient's blood pressure starts to drop
slightly that could be because of the fact that patient was sedated and had
and intubated and sedated for that or it could be because of hyperinflation and
so what we are concerned about is the patient is having something called
dynamic hyperinflation and let me explain to you what this is what happens
here is that the air goes into the lungs as the air goes into the lungs it
inflates the lungs in this case we're giving 500 MLS of air but because the
airways are so obstructed that it takes such a long time for the air to come out
that let's say only 450 MLS of air come out before the next breath which is
either determined by a time constant or when the patient takes another breath
another 500 goes in when only 450 came out and so what happens is with that
next breath you're gonna get a little bit more hyper-inflated and with the
next breath a little bit more hyper-inflated and so that's called
dynamic because this happens over a period of time hyperinflation so what
happens there is that the intrapleural pressure start to increase and that can
prevent venous return that can also increase your plateau pressures it can
also cause issues with peak pressures and so the compliance of the lung
becomes less and less and less and with less venous return you're going to get
less cardiac output and a decrease in blood pressure and so really the crux
here is that when patients have severe asthma exacerbations and they're put on
the ventilator and they start to dynamically hyper inflate the real key
here is that there is not enough time for exhalation
exclamation point triple underline that could be potentially a serious phase
situation where the patient could die because there's not enough time for
exhalation if you ever see a situation like this where the patient is
dynamically hyper inflated and they can't trigger the ventilator because
there's so much positive pressure inside their lungs and their blood pressure is
going down and their heart rates going up
you should detach them from the ventilator and allow a appropriate
amount of time for all of the air to come out of their lungs so that blood
can come back to their heart and they can get a blood pressure short of that
how would you prevent a patient from getting in this situation well join us
for our next video where we talk about ventilator management issues and things
that you can do to prevent specifically patients with obstructive lung disease
asthma or COPD to ventilate without running into these situations but again
the failsafe ways to disconnect the patient immediately and let that lung
deflate if you're having any questions about what we're doing here in terms of
PHP co2 oxygen bicarb please visit us at Metron comm for a
ventilator course that will answer almost all of your questions that you
will have thanks for joining us
you