00:00
Another way to cause a hypoxemia
is via diffusional impairment
and that is seen upwards
on this particular diagram
by not letting O2 get from
the lungs into the blood.
00:14
And this means there’s inadequate
amount of gas exchange
that’s occurring across
that blood gas barrier.
00:23
So let’s go through this in a little bit more
detail because it’s an important process
and oftentimes pathologies result
because of diffusional impairments.
00:33
So normally, you have
diffusion that occurs
because you have to get the O2 from the
lung to the plasma, to the red blood cells.
00:42
Diffusion is occurring between
both of these places,
between the lung and the plasma and
the plasma and the red blood cell.
00:50
Then you need to of course
get that particular O2 bound
to hemoglobin on your red
blood cell or erythrocyte.
01:01
There is a way to calculate this and that
is taking into account these variables.
01:07
We need to know what the
partial pressure of O2 is
and we need to know how
far it needs to diffuse.
01:15
So this is oftentimes denoted
by this theoretical equation
in which the diffusion of a particular gas
is related to the surface area available
divided by the thickness or the distance
it needs to travel times a coefficient.
01:32
And this coefficient is specific
to each gas such as O2.
01:37
It is its solubility divided by the
square root of its molecular weight.
01:42
So if we’re dealing with any gas,
we can pretty much ignore
that factor because
it’ll be the same no
matter what particular –
If we’re talking about oxygen, it will
always be the same diffusion coefficient.
01:54
The other variable that comes very
important is this pressure differential.
01:59
You need to know
what P1 minus P2 is.
02:02
That would be the partial pressure
in the lung versus the plasma
and the partial pressure between
the plasma and the red blood cell.
02:13
To better understand
diffusional impairments,
we need to compare and contrast
a diffusion-limited gas versus
a perfusion-limited gas.
02:21
To do that, we’re going to use
these particular figures.
02:25
Along the X-axis is going
to be the capillary length
from the beginning of the capillary
to the end of the capillary.
02:33
Along the Y-axis is going to be the
partial pressure within the capillary.
02:40
The very top dashed line is the partial
pressure in the alveolar space.
02:47
So if we take a gas
like carbon monoxide,
you can see that it never quite gets
to the area of the alveolar space.
02:57
Let’s contrast that with nitrous
oxide, which is an anesthetic gas.
03:03
Here, if we look at the beginning of the
capillary to the end of the capillary,
nitrous oxide is rapidly
diffused across the membrane
so it matches the pulmonary
alveolar gas tension.
03:21
This allows for there to be quick diffusion
across the particular lung tissue.
03:28
So a perfusion-limited substance
has very fast diffusion
and a diffusion-limited substance
is very slow to diffuse
across the length of the
pulmonary capillary.
03:41
Now, let’s use our two gasses that we
deal with in physiology, O2 and CO2.
03:50
So here is the same type of
graph with the beginning
of the capillary through
the end of the capillary.
03:55
We have the partial pressure
of the gas, in this case, O2.
03:59
And then we have a dashed line
along the top that is the
partial pressure of O2
in the alveolar space.
04:08
Normally, you have a fairly
quick equilibration of the
gas in the pulmonary capillary for
what is in the alveolar capillary
meaning that it diffused
across quite well.
04:24
If we take a diffusional-limited person
such as someone that has
interstitial fibrosis,
you can see that they have
trouble getting their gas
from the lungs into the
pulmonary capillary.
04:37
They never reach alveolar
gas concentrations.
04:42
This also occurs if you lower
the PO2 in the alveolar gas.
04:48
Normally, you’ll take a little bit longer
for you to get to that equilibration phase
so you get all of the gas from the
alveolar space into the capillary.
04:58
And a fibrotic person has even lower
amounts of diffusion that occurs.
05:05
So if you’re not able to reach up to the
PAO2, a diffusion-limitation has occurred.
05:13
The next type of hypoxemia is something
called a right to left shunt.
05:17
And for this, it is blood
that’s travelling from the
right side of the heart to the left side of
the heart without undergoing oxygenation.
05:26
This can be seen in this diagram
where some of the venous
blood is bypassing the lungs
and going directly to the
arterial circulation.
05:36
And of course, if you don’t get oxygenated,
you’re going to be dumping deoxygenated
blood into the oxygenated blood.
05:43
And that’s going to
lower your PaO2.
05:49
Now, there is some natural
nonpathological right to left shunt.
05:56
And that occurred from
the physiological shunt.
05:59
Remember that was the draining of
the blood from the thebesian veins
as well as the bronchial circulation,
but that’s perfectly normal.
06:07
But again that should be less
than 15 millimeters of mercury.
06:12
Pathological right to left shunts occur
because the severity has increased.
06:19
But it’s going to be very dependent upon the
amount of cardiac output that is shunted.
06:24
So if you have a small hole
in the septum of your heart
that allowed some blood to pass from the
right ventricle to the left ventricle
without going to the lungs, that is a
great example of a right to left shunt.
06:40
Because some blood went from
the right side of the heart
to the left side of the heart
without going to the lungs.
06:47
That sometimes does happen
especially for infants
and they will require some surgery to
help fix that particular shunt problem.
06:59
The final type of hypoxemia
that we’re going to go through
is one called a ventilation
to perfusion inequality.
07:07
And some people will refer to this as
ventilation to perfusion mismatch.
07:12
Now, I’m going to go
through this in two forms.
07:15
One is in the upright lung
and one is in the pathology.
07:18
The upright lung allows us to have a very
good example of this process in physiology.
07:24
And then you can simply apply it
to a pathophysiological state.
07:35
So in our ventilation to
perfusion inequality diagram,
here I’ve simplified the lungs into
three different alveoli or air sacs.