Shunting Explained Clearly (Pulmonary Shunt) – Remastered
25
August

By Adem Lewis / in , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , /


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


One thought on “Shunting Explained Clearly (Pulmonary Shunt) – Remastered

  1. I’m an anesthesiology resident and sometimes I just need a quick refresher on concepts and these videos are perfect. Thank you!

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