Ventilation Perfusion (VQ) Mismatch Explained Clearly

welcome to another MedCram lecture
we’re talking about hypoxemia and specifically the mechanisms that caused
it and today we’re going to talk about VQ mismatch let’s talk a little bit
about what VQ mismatch means V stands for ventilation this is basically air
going in and out of the lung and Q stands for perfusion this is basically
blood circulating to that area of the lung and it kind of goes along with what
they say about the lungs and learning about the lungs is basically air goes in
and out and blood goes round and round and ventilation and perfusion occur
simultaneously in the lung and how they occur and in what ratio can actually
have a lot to do with the different pathologies and specifically hypoxemia
let’s give it a little bit of background though on ventilation and perfusion the
first thing to remember when we talk about ventilation perfusion is remember
the heart okay here’s my little diagram of the heart and there’s the right side
of the heart and the left side of the heart and we have the tricuspid and
mitral valves you remember that the right ventricle pumps blood out to both
the right and the left lungs through the pulmonary artery and then of course the
left side pumps blood out through the aorta first of all you know the left
side when it pumps blood to the aorta the pressures that we usually see there
are anywhere from 80 to 120 millimeters of mercury not so on the right side the
right side is very is used to pumping very low pressures in fact the typical
what we call pulmonary artery systolic pressure ranges around 30 at most in
terms of normal 20 to 30 is the general range why that is important is we have
to realize the lungs are sitting here on both sides of the heart and this blood
gets pumped to them because it’s it’s done at such a low pressure around 30
millimeters of mercury gravity actually has a very large
determinant about where this blood goes and so what actually happens is that you
have more profusion to the lower areas of the lung then you do the higher areas
of the lung and areas in the upper part have lower perfusion now there is also a
very similar distribution of ventilation there’s more ventilation in the lower
areas but that that differential in ventilation between the upper and the
lower is much less variable than it is in perfusion so there’s a big difference
and as a result of all of this what we see is when we look at a ventilation –
perfusion ratio and what I mean by a ventilation – perfusion ratio is when we
look at the alveolus and the capillary that goes along with it we look at how
much ventilation is occurring in the alveolus and how much perfusion is
occurring in that unit so in this unit here specifically what is the perfusion
to ventilation ratio now why that becomes important we’ll get into but
suffice to say at this point is that areas in the top part of the lung
generally have high VQ ratio and and the reason why that is the case is because
there is more ventilation the upper portions of the lung and there is less
perfusion okay so perfusion is lower in the upper portions of the lung that
makes the VQ ratio higher whereas the lower portion of the lungs the VQ ratios
are generally lower and why is that because there’s more perfusion so you
could probably see that if you have a lot of ventilation
what’s the extreme if you had a lot of ventilation but no perfusion that would
be kind of like your trachea okay your trachea has got a lot of ventilation but
there is no perfusion to that area in terms of capillaries and so we call that
dead space so that will be an extreme that would be an infinitely high VQ
ratio whereas the alternative is with a low VQ where
basically ventilation is zero well we just talked about that type of a of a
mode or mechanism of hypoxemia and then we call that shunt just to kind of give
you an idea about what the extremes are okay but we digress a little bit let’s
get back to what a V q ratio is okay so if we have an area of high V Q again
here is our alveolus and here’s our capillary pulmonary artery pulmonary
vein what this means is that there’s quite a lot of ventilation occurring but
there’s not a lot of perfusion this type of situation is going to cause for
generally speaking a high oxygen situation now that is opposed to an area
of low VQ in the area of low VQ we have here’s our alveolus again we do have
some ventilation but the majority of this here what we have is we’ve got a
lot of perfusion as a result of that this ventilation is not able to saturate
this area of perfusion and so we typically have low oxygenation okay
notice however the ventilation is occurring in both these situations there
is no shunt going on anywhere and there’s no dead space the situation that
we talked about here in this case is something called VQ mismatch and that’s
a situation where there are certain lung units that are high VQ and certain lung
units that are low that is what we call VQ mismatch and that’s what gets us into
trouble in terms of hypoxia and now let me demonstrate what it is that happens
in that situation okay so I’m going to kind of draw a diagram that we had when
we talked about shunt but it’s not shunt these are two options that the blood can
go through and these represent two different areas of the lung the top part
is going to represent an area where we have high vq and the lower area is going
to represent where we have low vq now this is important areas of high vq by
definition are going to have low perfusion that’s why I’ve drawn this
limb skinny because there’s not going to be a lot of blood going through this
area okay whereas by definition areas with low vq are going to have quite a
bit of perfusion going through this area okay
so let’s again look at our numbers your typical saturation of oxygen in the
venous blood is 70% now what’s going to happen as this blood splits notice it’s
not 50/50 like it wasn’t shot there is definitely a smaller area of blood going
to the areas of high V Q versus air is going to the low V Q but anyhow the
areas of high V Q will go and they will be fairly well oxygenated so let’s say
that after it goes through these alveolar units that the saturation here
is 98% and why is that that’s because we have little perfusion but very good
ventilation so we’re going to oxygenate these lung units very well let’s go to
the area of low VQ mismatch I’m sorry the low V Q I should say the mismatch is
the fact that these two are different and in that situation the saturation is
going to be it’ll still be better than 70% but it won’t be much better let’s
say it’ll be 80% saturation and why is that
it’s because we have a lot of perfusion going to this areas of lung units and we
have low low ventilation and so we cannot oxygenate this very well so now
when these two limbs come back together the question is what’s the new
saturation going to be and the thing you’ve got to realize here is that
there’s many more red blood cells that are 80% saturated then there are red
blood cells that are 98% saturated and as a result the arithmetic mean of these
two is not sufficient to tell the answer because it’s always going to be weighted
more toward the lower saturation area because that has the more perfusion so
that’s always the way it is by definition is that it’s not going to be
the mean of these two what would be the average of 98 and 80 well it would be
around 89% it’s not going to be 89% okay it’s not going to be 89% it’s going to
be closer to 80% and so a more realistic number there would be 85% okay and
that’s the mechanism that gives us hypoxemia so let’s think about this
let’s go ahead and give 100% oxygen okay that 100% oxygen is going to go here and
that hundred percent oxygen unlike shunt is also going to go here and so in fact
what happens is the hundred percent oxygen is able to overcome this this
inequality and VQ mismatch and as a result the eighty-five percent will
correct to 95 percent plus depending on the amount of oxygen that you use and so
as opposed to shunt where 100 percent oxygen doesn’t really make a difference
with oxygenation with VQ mismatch in this case on a percent oxygen does
actually make a difference now let me go ahead and show you an example of the
Hume is match and for that I’ll show you the prototypical now VQ mismatch is the
most common if they ever ask you that on a test it is the most common cause of
high pox amia and the reason for that is because
of the causes are so common for this for in
since pneumonia pulmonary embolism what about a COPD all of these are reasons
for the patient to have you mismatch to the point where is if they ever ask you
this on a question and you don’t know the answer to it and you see VQ mismatch
then I would select that one let’s go ahead though and focus on one of these
to sort of illustrate that let’s focus on pulmonary embolism okay so you’ve got
your lungs let’s go ahead and show these schematically and of course you’ve got
your pulmonary artery coming out and going to the different lungs and it
splits off and splits off again this is your heart here so when you get a clot that forms the
clot goes gets pumped in and it goes over to a specific area and lodges what
happens at that point is that there is no more perfusion to this segment of the
lung and as a result of that if there is no perfusion you’re going to have an
area of high V Q because there’s very low perfusion now the blood that would
normally go to this area what happens to it it’s got to go everywhere else so
perfusion increases there it increases here
it increases here it increases here it increases everywhere so in other words
everywhere else you get a area of low V Q low V Q in comparison to this area
here as a result of this the whole situation is is that you get low oxygen
in the blood okay so let’s summarize the situation with VQ mismatch all right
number one it responds to a hundred percent o2 remember the only one that
did not respond to 100% o2 was shunting number two you have an increased a a
gradient number three the causes so here they are
COPD you can even get fibrosis causing this asthma pulmonary embolism pulmonary
hypertension pulmonary hypertension and even pneumonia and as a result of all of
these causes it is the most common form okay well thank you for joining us you