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