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