well 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%
shot 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 in 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 too is directly proportional to the
saturation of oxygen so that's going to be very important is the oxygen
saturation that's the line 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 get Sri oxygenated 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 percent saturated blood here and we've got 70 percent 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%
is not going to have as much weight but because I've said it's a 50% shunt
meaning 50% of the flow is going to the lungs and 50% 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 twelve and a half
twelve and a half plus seventy 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 one 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 going to add a hundred percent oxygen now what's going
to happen when we add one hundred percent oxygen that one hundred 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 seventy percent saturated
instead of it going to ninety five percent 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 a hundred percent oxygen that's
essentially 760 millimeters of oxygen we're going to be way off the
scale over here but we're only going to 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
70 percent in this limb and instead of having an eighty two point five percent
saturation the new saturation is going to be only eighty five percent now this
is something that's different in response to high-altitude diffusion
hypoventilation what we'll talk about later is VQ mismatch and this is a very
important point is that one hundred 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 interventricular 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 going to
of 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 is 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
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