welcome to another MedCram lecture we're talking about hypoxemia and
today's topic is shunting we've talked briefly about high-altitude diffusion
and hypoventilation the fourth mechanism that we want to talk about is the
shunting mechanism and this is a little bit different than the previous two and
I'd like to sort of diagram what exactly is a shunt okay so if we look at the
pulmonary vascular system we start out with the pulmonary artery and it's in
blue because it's deoxygenated and of course it ends up in the pulmonary vein
which is oxygenated and the way to look at this is two different systems so I've
drawn this so that it's pretty easy to tell that 50% is going in this direction
and 50% is going in this direction this is what we call a 50% shunt so that a
50% of the blood goes to in this case the lungs and 50% is going to a shunt or
basically it's not going anywhere it's not going anywhere where there isn't
it's not going anywhere where there's an alveoli or the ability for oxygen to get
to it okay so these are the lungs oxygen can come here oxygen can't come here
because it's being shunted away from the lungs and so even though I've drawn this
with kind of a blue and red you must understand that there is no o2 coming
here whatsoever and this is exactly what a shunt is is a 50% shot as I mentioned
but it doesn't have to be 50 you can have less you can have more I'm just
making it 50% for illustrative purposes now before we get started with that kind
of a discussion I want to sort of give you a little bit of a background about
the red blood cells as you know red blood cells carry oxygen
and they look like this kind of from the side because there is no nucleus well
inside each red blood cell is the human globin molecule which is actually made
up of for binding parts we'll talk a little bit more about this later and
this is where oxygen binds to it and you get something called the hemoglobin
oxygen dissociation curve which basically looks like this okay and
basically po2 is on the bottom and saturation is on the side and what it
says is as po2 goes up so does the saturation but there comes a point where
increasing po2 doesn't increase the saturation that much more the important
thing to take from this is that this goes to the body after the left
ventricle pumps it and the amount of oxygen that is delivered to the body so
the do two is directly proportional to the saturation of oxygen so that's going
to be very important is the oxygen saturation
that's the lion's share of oxygen that is delivered to the peripheral tissues
and so what I want to do here to avoid confusion is talk a little bit about the
saturation of the blood in the pulmonary artery so pulmonary artery blood is
basically blood that is coming back to the heart
it is not oxygenated it is blood that has already been spent oxygenation wise
in the tissues and the saturation of blood in that area is about 70 percent
so if you were to get a pulse oximeter or some sort of way of measuring the
saturation of oxygen in the venous blood that gets eventually pumped to the
pulmonary artery the answer is 70 percent and in normal situations this
blood goes to the lungs and gets reoxygenate it and it comes back to the
pulmonary vein to the left atrium left ventricle gets pumped out to the body
but in a shunt situation like we have drawn here where in this case 50 percent
of the blood is diverted away from it let's go through and figure out the
arithmetic about what happens so we've got 70 percents at
created blood here and we've got 70% oxygen saturated blood here well because
there's lungs here that area becomes oxygenated and let's just say that it
becomes oxygenated up to the point that it is now 95 percent saturated with
oxygen well in the shunt side of this that 70 percent blood 70 percent
saturated blood doesn't get to see any alveoli and so it remains at 70 percent
saturated now because this is 50 percent of the blood and this is 50 percent of
the blood when these two combine in the pulmonary vein you simply what you have
here is the arithmetic mean of these two saturation obviously now if it's a
smaller shunt fraction this 70 percent is not going to have as much weight but
because I've said it's a 50 percent shot meaning 50 percent of the flow is going
to the lungs and 50 percent of the flow is not going to the lungs and in fact
never sees the lungs the arithmetic mean of 70 and 95 is the difference between
these two is 25 half of 25 is 12 and a half
12 and a half plus 70 is eighty two point five percent saturation that is
definitely a situation where you have pulmonary hypoxemia
okay so hypoxemia is caused by this shunt fraction so as a result of that
even though you've got good oxygen here in the alveoli you see and that's
capital a you see that your lowercase a is low that means you're going to have
an increased a a gradient what about if we give a hundred percent oxygen and and
this is probably the most important point to understand and this whole
lecture is what happens next when we add a hundred percent oxygen so we're going
to add we're gonna add a hundred percent oxygen now what's going to happen when
we add 100 percent oxygen that 100 percent oxygen is going to affect this
side of the equation okay but it's not going to affect this side of
the situation because this shunt by definition is not seeing any oxygen in
the lungs now when we give 100% oxygen to this side what's going to happen over
here the blood which is 70% saturated instead of it going to 95%
saturated remember here's our human globin binding curve it's going to go up
like this and whereas at this po2 the oxygen saturation was 95 now if we give
100% oxygen that's essentially 760 millimeters of oxygen we're gonna be way
off the scale over here but we're only gonna be up to a hundred percent so
instead of 95 let's say that the new saturation for just the blood coming off
this limb is now a hundred percent saturation but because this area here
never got to see the hundred percent oxygen because by definition it is a
shunt we still have seventy percent in this limb and instead of having an
eighty two point five percent saturation the new saturation is going to be only
85 percent now this is something that's different
in response to high-altitude diffusion hypoventilation of what we'll talk about
later is VQ mismatch and this is a very important point is that 100 percent
oxygen treatment in these type of hypoxemia x' do not really improve your
hypoxemia and that's a clue that the mechanism of action here is is a shunt
okay so let's review shunting the first thing that we said and this is probably
the most important is that it does not respond to 100 percent oxygen that's the
first time we've come across one of those it does have an increased AAA
gradient now what are some examples well if you look at the heart and here is my
schematic drawing of a heart we've got the immature
ventricular septum we've got the tricuspid valve and the mitral valve any
type of a situation where blood on the right side of the heart goes over to the
left side so like an ASD or like a VSD or even like a patent ductus arteriosus
if it's going from the left to the right that could do it as well
in this situation you're gonna have right to left shunting now these are
congenital heart anomalies but I can think of another reason why you could
have right to left shunting and that would be in something called a r d s
what is a RDS well in a RDS remember your alveolus looks like this okay and
you've got your capillary coming in like this and it gets oxygenated and it comes
out like this but in a RDS a RDS stands for acute respiratory distress syndrome
this is a situation where a lot of basically Highland containing high
protein fluid leaks out into the alveolus if you would like you can
imagine that there's just basically fluid in the alveolus so in fact what
happens when oxygen tries to get down there there is no oxygen that can come
through and as a result of that this deoxygenated blood goes right through
without seeing any oxygen whatsoever so that's another way you could have
shunting also pulmonary edema could also do this so that does it for shunting
thanks for joining us you