- [Voiceover] To understand the pathophysiology
of shock, we first need to remind ourselves
what shock is.
Shock at its basic level means
cells are not getting oxygen they need.
Essentially the O2 delivery, the amount of oxygen
that the body can deliver to its cells
becomes less than the amount of oxygen
that is required by the body.
Shock is essentially a failure to deliver oxygen
to the different tissues and organs and cells of the body.
Why do cells need oxygen?
Basically cells, in this little box here I' going
to designate as a cell, and cells need oxygen
to be able to create energy.
This is titled "aerobic metabilism."
Creating energy metabolism with the assistance
of oxygen, aerobic.
And this is really the reason we breathe.
We go through respiration because we need
the oxygen to help create energy.
However, the cells of the body can also
create energy through anaerobic metabolism.
The cells of the body can survive without oxygen
just for a little while.
However, there's a problem with going through
anaerobic metabolism versus aerobic metabolism.
With oxygen, you're able to create a lot more energy
to be able to satisfy the needs of each cell in the body.
Yet if the body is forced to undergo
anaerobic metabolism, not enough energy
can be created to meet the requirement to sustain life.
So that's one problem with undergoing
anaerobic metabolism and why we need oxygen.
But another issue with anaerobic metabolism is
a by-product of trying to create this energy
is this substance called "lactic acid."
I'll come back to this point a little bit later
to show what the issue is with creating lactic acid,
but for now it's important to remember
that anaerobic metabolism creates this by-product.
lactic acid.
Okay, so we know now that aerobic metabolism
is necessary to create the amount of energy
we need to sustain cellular function to sustain life.
We know that the body has to deliver oxygen.
And to understand how the body fails to do this in shock,
let's go ahead and take a look at a blood vessel.
I'm gonna go ahead and draw a blood vessel right here.
Note that this blood vessel is going to be
delivering oxygen through the blood
to distribute oxygen to the tissues.
So let's make oxygen this little light blue color.
This is oxygen.
As you know, oxygen is carried in red blood cells
in hemoglobin, so this is how oxygen
is delivered to the tissues, through hemoglobin,
through red blood cells.
Let's go ahead and draw some different cells.
These little boxes, of course,
are representing our cells.
Just a smaller version than this big one up here.
Now remember, in shock the issue is tissue perfusion.
Tissues and cells are not getting enough oxygen,
they're not getting enough blood that they need
for oxygenation, and without this oxygen
they can't create the energy necessary to sustain life.
In shock, these cells are desperate for oxygen.
There are two scenarios that we can see in shock.
In the first scenario, there is increased
extraction of oxygen.
This is because there's an increased demand
for the cells to have oxygen.
There might be an increased requirement.
These cells have so little oxygen in them.
They're oxygen-starved.
It's not really an active process,
but because there's so little oxygen up here
and more oxygen in the blood,
oxygen readily diffuses into the cells.
These cells are oxygen-starved.
They pull out more oxygen, therefore that would mean
that there's lower oxygen return to the heart.
Less oxygen is getting back to the heart.
Two types of shock that are an example
of this increased extraction are cardiogenic shock
and hypovolemic shock.
If you think about it, hypovolemic and cardiogenic shock,
blood is not getting pushed forward fast enough.
It's not getting delivered properly,
so the cells are using up their oxygen
quicker than it's being delivered.
Lower delivery, despite the required oxygen
remaining pretty much the same.
The cardiovascular system is just not able
to deliver that oxygen to these cells.
Second, let's take another look.
Let's look at these cells.
These cells are still desperate for oxygen
and in shock that's really the definition.
They require more than is able to be delivered.
But what if there's something that actually impedes
oxygen from being able to be delivered?
If oxygen isn't getting to the cells,
this is decreased extraction.
This is what happens in the type of shock
known as "distributive shock."
Oxygen can't be distributed to the cells.
For example, in septic shock when there's
a lot of inflammation and swelling in the space
in between the cells, the interstitial space,
the oxygen has a tougher time diffusing
through this space, so oxygen can't get through
all this thick fluid.
This extra fluid creates a diffusion barrier
so that oxygen can't be distributed.
So again, we have increased demand of oxygen
due to poor oxygen delivery.
Again, I'll go ahead and write these down.
This first scenario occurs with cardiogenic
and hypovolemic shock.
And the second scenario occurs
with distributive types of shock.
Septic shock, anaphylactic shock,
something that creates a barrier that
prevents oxygen from getting to the cells.
Now again, if oxygen isn't delivered to the cells,
it's going to stay in the blood,
so more oxygen will return to the heart.
That makes sense, if there's less extraction,
oxygen just kind of remains in the blood,
which shows up as increased oxygen return to the heart.
I go over this point in detail because if we can
figure out how much oxygen is extracted
from the tissues, we can have a better idea
of what type of shock it may be:
cardiogenic or hypovolemic versus
a distributive type of shock.
So how do we measure that?
That's where something called mixed venous oxygen,
or mixed venous oxygen saturation,
which is abbreviated SMVO2, saturation of
mixed venous oxygen, comes into play.
This term is actually interchangeable
with the other term, which is known as
central venous oxygen saturation,
abbreviated SCVO2, central venous oxygen saturation.
Now what are these two terms,
what do they mean?
It's very hard for us to measure how much
oxygen is getting pulled from the tissues
at each individual tissue.
So what we do is, we look at how much oxygen
is returning to the heart.
We take a look at the heart, and you see
that blood returns to the right side of the heart
through the superior vena cava, so up here,
and the inferior vena cava.
This is how blood returns from the top
of the body, the arms and the head,
versus the bottom of the body,
the legs, the abdomen, so on and so forth.
When blood from both the superior and the inferior
vena cava meet in the right atrium,
the oxygen of these two major veins,
these central veins, mix.
This is important because the oxygen extraction
may differ between the upper part of the body
and the lower part of the body.
So when they mix, we get an average of the total
oxygen coming from the upper and the lower
parts of the body.
Now we see if the mixed venous oxygen saturation,
also known as the central venous oxygen saturation,
is lower, that means less oxygen is returning
to the heart, and so therefore we can say
that oxygen is being extracted more than normal.
The same goes for if more oxygen is returning
to the heart, so if there's a higher
mixed venous oxygen saturation than normal,
then we can conclude that there's decreased
oxygen extraction from the tissues.
So again, we look at the mixed venous oxygen
to try to identify what type of shock
the patient may be experiencing.
From what we've covered so far,
what other tests do you think we could do
to understand if the patient is experiencing shock?
Well, remember this lactic acid.
In anaerobic metabolism when there is low oxygen
within the cells, energy is created through this process,
and as a by-product lactic acid was created.
Patients who have shock may experience
lactic acidosis where they have a
largely increased amount of lactic acid.
Initially this can be overcome
and doesn't cause damage to the body.
However, over time because of this increased
lactic acid, I'll say "LA," the body will have
an overall decreased pH, which means
more acidic composition and in the presence of acid,
if the body becomes too acidic,
different proteins and structures that are
normally intact in the cells start to
degredate and denature, which leads to
a cascade of events that can eventually mean
cellular death.
So though initially this process is reversible,
if shock continues long enough, cells may begin
to die, they're starved from oxygen
and starved from energy.
You can see differentiating these types of shocks
and understanding the basics of shock
can help health practitioners prevent
the potentially devastating problems
that can arise in a patient who has shock.