welcome to another MedCram lecture we're gonna 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 and 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 gonna
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 diseases of 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 the middle here in this
interstitial and it's preventing oxygen from diffusing in these red blood cells
normally to get through take about zero point seven five 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 equilibrate 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
percent 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 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 2 1 p.m. is
fair let's say this is at sea level that would be 760 millimeters the P of water
is about 47 and the pco2 you would be 40 divided by 0.8 okay so
let's go through and oh let's say po2 what's the normal po2 around 86 so 760
minus 47 is 713 this times point to one is gonna give us 150 150 minus 50 is a
hundred a hundred minus 86 is equal to about 14 so the a a gradient is about
fourteen 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 twenty fifteen twenty
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 patients and divide it by
four and then you just add four so a 40-year old gentleman 40 divided by four
is ten plus four is fourteen so that's how you calculate a an a a gradient now
notice 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 a a 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% 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 component 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
could 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