Welcome another MedCram lecture we're
talking about hypoxemia and specifically
the mechanisms that cause 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 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
here 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 determinants about
where this blood goes and so what
actually happens
is that you have more profusion to the
lower areas of the lung than you do the
higher areas of the lung and areas in
the upper part have lower profusion 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 units 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 in 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 where is 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 is 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 would 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 hypoxia and then we call
that shunt just to kind of give you an
idea
what the extremes are okay but we
digress a little bit let's get back to
what a v/q ratio is so if we have an
area of high VQ 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 an 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 gonna 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
profusion that's why I've drawn this
limb
Guinea because there's not gonna be a
lot of blood going through this area
okay whereas by definition areas with
low vq are gonna 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 gonna
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 of 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 gonna be 89%
it's gonna 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 a hundred
percent oxygen okay that 100 percent
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 in 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 we're
a hundred 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 VQ mismatch 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 instance pneumonia pulmonary
embolism what about a COPD all of these
are reasons for the patient to have EQ
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 gonna 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 alright
number one it responds to a hundred
percent o2 remember the only one that
did not respond to a hundred percent 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