Pulmonary Diffusion Explained Clearly by MedCram.com


welcome to another MedCram lecture we
could talk about hypoxemia again and this time we’re gonna talk about
diffusion again hypoxemia literally meaning low oxygen in the blood let’s
talk about diffusion as a possibility for a mechanism for hypoxemia to talk
about that though we’ve got to sort of break down the the alveolus and i’m
drawing that here and then we’ll switch to a blue color this is to demonstrate
the pulmonary artery which as you know is deoxygenated then
we have the capillary and then we’ll switch to red to show that in fact the
blood is now oxygenated and then this green is going to be oxygen because you
know that oxygen tanks are always green right okay so we’ve got oxygen coming
down into this area and then there’s something that we should talk about
called the capital a lowercase a gradient okay so you guys know how a
lung works john west is quoted as saying the lungs are very very easy to
understand air goes in and out and blood goes round and round
well let’s sort of break that down a little bit to understand it a little bit
better oxygen is going to come in into the alveolus and it’s going to diffuse
past this one cell layer thick epithelium which is the epithelium of
the lung and then it’s going to go through this one cell layer thick
endothelium which is could then go into the pulmonary artery pulmonary capillary
and then pulmonary vein which is going to become oxygenated the problem here is
that sometimes you can have a blockage in this area and that can cause a
diffusion abnormality where you’d see this is particularly in patients with
infiltrated disease the lung like pulmonary fibrosis
especially if they exercise let me give you an example if you’ve got a patient
with fibrosis and they’re at rest so that means there’s stuff in in the
middle here in this interstitial and it’s preventing oxygen from diffusing in
these red blood cells normally to get through take about 0.75 seconds to
transmit through this pulmonary capillary and to pop out the other side
with oxygen now as they’re going that slowly they’re able to equilibrate with
the oxygen here in the alveolus that’s no problem
that means the oxygen coming out the other end is you know 95 percent
saturated however when they start to exercise cardiac output goes up and even
though there are parallel systems that the blood can go through to pick it up
and the velocity of these red blood cells increase and so therefore the 0.75
seconds goes down and so it’s going through its equilibrating and it pops
out when they’re exercising before there’s full equilibration so now maybe
instead of 95 percent saturated it’s only 85% saturated let’s just say and so
what you get there is hypoxemia because the red blood cells sped up but this
happened primarily because there was some sort of a deposition interstitial
deposition in here that prevented this oxygen from diffusing into the pulmonary
capillary and there you’ll see an increased a a gradient capital a
standing for alveolus lowercase a standing for artery so that increased a
a gradient you’ll see and we’ll show you how to calculate that in a bit
here’s a equation for the AAA gradient and this kind of has some of the items
in it that we saw before this part up here refers to the capital a that’s the
amount of oxygen in the alveolus this here is the smaller case a this is the
amount of oxygen that is dissolved in the blood this is the pao2 that you pick
up on a blood gas so let’s plug in some numbers for somebody whose normal fio2
in this case would be point two one P atmosphere let’s say this is at sea
level that would be 760 millimeters the P of water is about 47 and the pco2
would be forty divided by 0.8 okay so let’s go through and oh let’s say po2
what’s a normal po2 around 86 so 760 minus 47 is 713 this times point to one
is going to give us 150 150 minus 50 is a hundred a hundred minus 86 is equal to
about 14 so the AAA gradient is about 14 which is
pretty normal in terms of trying to figure out or guesstimate what a normal
a a gradient would be it should be around 20 15 20 or so another way of
trying to predict it some people say is if you use this type of an equation
where you take the age of the patient and divide it by 4 and then you just add
4 so a 40-year old gentleman 40 divided by 4 is 10 plus 4 is 14 so that’s how
you calculate a an a a gradient now know if for some reason we ran into a
situation where there was a diffusion problem what would we see we would have
to see that the fio2 would go up because we’d have to give them more oxygen to
keep the oxygen the way it was correct this would not change this would not
change and this would either stay the same or go down we’d have to give a lot
of this to keep this the same you can see that in situations where we can’t
get oxygen through we’ve got to go up on the capital a to get the same or only a
small small increase in the little a and so what I’m saying here is that a large
AAA gradient means there’s some barrier or some problem in oxygenating the
tissue or getting oxygen into the pulmonary capillary okay then so let’s
review if we look at diffusion couple key points that you need to know first
of all it does respond to a hundred percent oxygen again if we look at our
alveolus and we’ve got our pulmonary artery and our pulmonary vein if there’s
a barrier hair just giving more oxygen will help push it through and get the o2
in the pulmonary vein to come up so it does respond to 100 percent oxygen it
has an increased AAA gradient where do we see this fibrosis any kind of
situation where there is a collagen deposition here in the interstitial of
the lung like in pulmonary fibrosis especially if they’re exercising and why
is that we’re going to emphasize that with exercising because the red blood
cells are going through here at a faster velocity and
therefore it accentuates this difficulty in diffusion but you’re gonna have to
have a lot of thickening for that to happen okay so that takes care of
diffusion we’re going to move on to other ones in the next lecture