00:01
So now to apply the ventilation
to perfusion inequality
from that theory example
that we just went through
to something that is practical,
this is the upright lung.
00:11
To do that, you’ll have to
remember about blood flow
in the different zones
of the upright lung.
00:16
Remember in the apex of the lung,
there’s going to be a low blood flow.
00:22
In the base of the lungs, there’s
going to be high blood flow.
00:25
This will be important as we start trying
to match ventilation to perfusion.
00:31
Now, pulmonary blood flow can also be
looked at on its effects of gravity.
00:35
The lung in the upright condition can
be broken into three different zones.
00:41
Zone 1, zone 2 and zone 3.
00:44
Zone 1 is denoted by
this type of a situation
in which there’s very little with any
blood flow travelling through it.
00:52
And that is because the
arterial side of the capillary
to venous side of the capillary is being
impinged on by the alveolar pressure.
01:02
So in this case, P, capital A,
for alveolar is larger than
P small A for the arterial
side of the capillary
to P, small V which is the
venous side of the capillary.
01:18
A condition where there is higher
blood flow occurs in this condition
in which P, small A, is greater than P,
capital A, which is greater than P, small V.
01:29
How this condition works is on the
arterial side of the capillary,
there is enough pressure
in it that it will
be able to push pass
the alveolar pressure.
01:41
And therefore, there
will be some flow.
01:46
Where there is a lot of flow or the
highest flow in the lungs is in zone 3
where P, small A, is larger than P small
V, which is larger than P, capital A.
01:57
And this condition, the
arterial side of the capillary
has enough pressure in it that
there is no effect of the alveoli.
02:07
And therefore, flow goes
through continuously
and that has a highest
amount of blood flow.
02:12
When dealing with ventilation
to perfusion inequalities,
we also need to now take into
account the ventilation component.
02:19
Now ventilation will be very dependent
upon what part of the lung you’re in
and this is based upon
the gravity effect.
02:26
Just like this particular coil,
it’s stretched out at the
top of the lung or the apex
and it’s not stretched out as much on the
bottom part of the lung or the base.
02:39
The lung works in the same parameter in
which the top parts of the lung are pulled
and the bottom part is compressed.
02:47
And what this allows for is there to
be different interpleural pressures
between each of these particular
components of the lung.
02:57
At the base of the lung,
there is compression which
increases pleural pressure
and that is different from the apex
of the lung in which it is stretched
and therefore pleural pressure is larger.
03:11
This allows for a decrease in
compliance in the apex of the lung
and an increase in the compliance
at the base of the lung.
03:21
What this is going to mean is
as flow goes into the lung,
it will preferentially go to the area that
has the higher compliance or the base
rather than going to the
top portions of the lung
because of a decrease
in compliance.
03:38
So now let’s take with
some practical examples.
03:42
So here, we have the
apex of the lung
so this will be a
PAO2 of 125 up here.
03:55
Now at the apex of lung, there is a
very large decrease in blood flow
that we just went through because
of this effect of gravity.
04:05
There’s also a decrease in ventilation
because of the lower compliance.
04:11
When we look at the ventilation
to perfusion ratio,
this increases the ventilation
to perfusion ratio.
04:18
Why?
Because the denominator in
this particular component
went down to a greater
degree than the numerator.
04:24
The result or the rationale that happens
because of this increase in
ventilation to perfusion ratio,
there’s an increase in PO2, a decrease
in PCO2, and pH rises slightly.
04:38
In the base of the lung,
so this goes down here to
the P, capital A, of 90.
04:43
There’s an increase in blood flow,
an increase in ventilation,
but the overall effect
is a decrease in the ventilation
to perfusion ratio.
04:52
Again, because blood flow
increases to a greater degree
than the amount of
ventilation increase.
04:59
What happens to the
portions of blood that is
leaving the bottom portion
of the lung or the base,
PO2 is lower,
PCO2 is higher and pH is also lower.
05:13
Therefore, a person that
is in the upright position
always has a ventilation
to perfusion inequality.
05:22
Because there is a lower
ventilation to perfusion
ratio in the base of your
lungs compared to the apex.
05:28
All due to gravity, its effect
on blood flow and compliance.
05:35
We can graph this in terms
of a curve that we look at
the PO2 on the X-axis and
on the Y-axis, PCO2.
05:47
So if we have a place in which there’s
a high ventilation to perfusion ratio,
it’s at the edge of the ventilation
to perfusion ratio graph
and it can be seen in zone 1.
06:00
Both of those have a high
ventilation to perfusion ratio.
06:04
A normal ventilation to perfusion ratio will
then continually move towards the left.
06:13
And if you have a low ventilation to
perfusion ratio such as in zone 3,
it continues to move towards the
left on this graph and finally here.
06:24
So we can quantify what part
of a ventilation to perfusion
ratio we have based upon
the PO2s and PCO2s.
06:36
Okay. We’ve covered the four
different areas of the hypoxemias.
06:40
Now, let’s cover hypercapneas.
06:43
Hypercapneas are a lot simpler but
what it has to do with in the
red portion of this particular
graph is a high CO2 component.
06:53
So that PaCO2 is higher than normal.
06:58
This could occur because of a
decrease in alveolar ventilation,
a very severe ventilation
to perfusion inequality
and finally, I could occur if there’s
an increase in CO2 production
without an appropriate
ventilation compensation.
07:17
So let’s go through these
couple hypercapneas.
07:22
Hypercapnea is all related
to one particular formula
which is known as the alveolar
ventilation equation.
07:29
Luckily, unlike the alveolar gas equation,
it is not something that we
calculate all that often
but I’ll show it to you here
because of the relationship
that you can see as this occurs.
07:42
And that is there’s an
inverse relationship
between alveolar ventilation
and carbon dioxide.
07:49
If alveolar ventilation goes
up, carbon dioxide goes down.
07:53
And the opposite occurs as
carbon dioxide levels go up,
alveolar ventilation goes down.
08:00
These always are
inverse of each other.
08:03
So if you measure a CO2
level that is too high,
you know that alveolar
ventilation rate is low.
08:10
If you measure a CO2
rate that is too low,
you’ll know that alveolar
ventilation rate is high.
08:17
These will always be
opposite of each other.
08:20
So we utilize this equation
simply because of its relationship
between these two variables.