Our next concept is mixed
venous oxygen saturation.
In my practice, it took me more than
two years to completely understand this.
So if you don't get it right
off the bat, it's okay,
just go back and keep going over
the slides until you finally get it.
Mixed venous oxygenation,
the normal is about 60%-80%.
What this means is, it's the percentage
of oxygen that is returning to the heart.
So the lungs put oxygen into the blood,
and they attach to the hemoglobin,
and then the heart ejects
it out to the body,
the body then uses it,
but it doesn't use all of it,
we only want it to
use about 20%-40%.
So that means that 60%-80%
is returning to the heart.
So the lungs doesn't have
to pump so much oxygen
back into the heart and back into the
blood, it only needs to do about 20%.
So again, think about
mixed venous oxygenation
is just the oxygen that's left
over after the body uses it.
Now we call this a mixed
venous oxygen saturation
because we're getting oxygen
returning from the brain,
the arms, the coronary
sinuses or the coronary veins,
that gut, the legs, everywhere,
it's gets all mixed up
into that pulmonary artery.
And then we draw an
oxygenation from it.
Now, if we have a central line,
and we draw an oxygenation from
that, that's called an ScvO2.
So you get a snapshot of what's happening
but you may not see everything in the body.
So a mixed venous is a little
bit better than an ScvO2.
What are the
determinants of an SvO2?
That's oxygenation, hemoglobin,
cardiac output and metabolic demand.
So we've discussed
the SvO2 determinants.
Now let's break
those down further
so we can understand them
a little bit more clearly.
We break the determinants
into two sections,
oxygen delivery and
We're going to focus on
oxygen delivery right now.
When we look at oxygen delivery,
I want you to think of three things:
lungs, blood and heart.
We're learning all of these
things so if we have a low SvO2,
we can try to analyze and find out
what's the cause of my low SvO2.
First off, we're going to look at
The whole goal of the lungs is to
get oxygen into the bloodstream.
So how do we know if we're getting
enough oxygen into the bloodstream?
One thing we can do is
look up at our pulse ox.
And see if we are
almost to 100%.
We like our oxygen saturation
to be between 92% and 100%.
But that doesn't give
us a clear understanding
of what the lungs
are completely doing.
It's just a snapshot.
So there's another test
that we can perform
in order to get a clearer
picture of the lungs
to make sure they're doing everything
that they're supposed to be doing.
That test is an arterial
blood gas analysis.
When you get an ABG,
there's two things I want you to look at
when you're trying to
determine why my SvO2 is low.
First off is PaO2.
And then we look at SaO2.
With PaO2, this is just how many oxygen
molecules are diffused in the bloodstream.
Think about a soda bottle or a coke
can, when you open that up,
What comes out?
Carbonation comes out,
that CO2 that's diffused in the soda.
It's the same thing
with oxygen and blood.
When we're looking at PaO2,
it's how many molecules of oxygen
are diffused in that bloodstream.
We'd like it to be
about 80 to 100,
anything greater than 100 and we're
starting to get into hyperoxia anemia.
So we want between 80 and 100.
If we're less than 80, then we're not
getting enough oxygen into the bloodstream.
we're going to look at SaO2.
This is how many of your hemoglobin
are fully saturated with oxygen.
This correlates with your pulse ox,
so we want it to be about 92%-100%.
If we're under 92%, then we don't have
enough oxygen that to test your hemoglobin.
So think about it.
If our SvO2 we want it
to be normals of 60%-80%.
And our pulse ox,
we want it to be 92%-100%.
If we start with a pulse
ox that's already low,
meaning let's say our pulse ox is
about 88% or our SaO2 is about 88%.
Then the body uses it,
it's going to be low when it gets
back to that right so the heart
and when we take a sample of it.
It may be, you know,
the body may be pulling 20% of that away.
So for starting at 88%,
we're going to finish at 68%.
If we're going to start at let's say 75%
oxygen saturation, which is a big deal.
We're going to go down to 55%.
So when you identified that your
oxygenation may be the issue,
what can we do about it?
Well, we can increase the amount of
oxygen that they're breathing in,
maybe by nasal cannula or increasing
the FiO2 if they're intubated.
Maybe we need to increase
the respiratory rate,
maybe we need to increase
the amount of ventilation,
or the PEEP,
the positive end-expiratory pressure.
All of these things can affect how many
oxygen molecules get into the bloodstream.
The second part we look at,
remember lungs, blood, heart,
the second thing we're
going to look at is blood.
So, if your lungs are getting
enough oxygen into the bloodstream,
the part of the blood that's
carries that away is the hemoglobin.
We need to make sure that
we have enough hemoglobin
to carry the oxygen
to the tissues.
So we may have an oxygen
saturation of 100%.
But if you've got a
hemoglobin that's really low,
the body's just going to extract all of
that and your SvO2 is going to be low.
So what causes
hemoglobin to be too low?
Things such as bleeding,
hemodilution and anemia.
So we want to look at also on your ABG,
you should have a hemoglobin level.
So let's make sure our hemoglobin,
we have enough oxygen carrying capacity,
meaning we have enough hemoglobin
to support the metabolic demands.
Now the third part we're going to
look at with oxygen delivery is heart.
blood and heart.
If our lungs are getting enough
oxygen into the bloodstream,
and our blood has enough hemoglobin
or oxygen carrying capacity
to take that oxygen to
where it needs to go,
we need to have a strong
enough left ventricle
to get that oxygen to perfuse
every part of the body.
We're going to look at cardiac output
and cardiac index to determine that.
If our cardiac output
or index is too low,
then we're not basically perfusing the
body and getting enough oxygen to it,
which means our SvO2
is going to be too low.
Now remember, the determinants of cardiac
output are heart rate and stroke volume.
Now, when we break
down stroke volume,
the determinants of that is preload,
afterload and contractility.
So we need to make sure
if our SvO2 is too low,
and we say, "Okay, I've got enough oxygen
of my ABG and I've got enough hemoglobin,
the last part of the oxygen delivery,
it could be my heart's not strong enough."
So we need to figure out
what's going on with the heart
that we're not getting
enough cardiac output
or in the flow of
that left ventricle.
We're going to dive into that a little
bit later so we can get an understanding.
So I'm not going to talk
all about it right now.
But if we determine that our oxygen, our
hemoglobin and our cardiac output is good,
meaning if we see that our
lungs, our blood and our heart
are doing what they're
supposed to do,
then that means our
oxygen delivery is great.
So the only thing left to determine why
our SvO2 is low is our metabolic demand,
meaning our body is
consuming too much oxygen.
We see this in shivering,
fever, anxiety, pain,
increased muscle activity,
and increased work of breathing.
All of these increase the amount of
oxygen that we're going to be consuming,
which means the amount
of oxygen returning
to the right side of the
heart is going to be low.
Now, in our patients that are
really, really sick
in their critical
many times there's multiple
things attributing to a low SvO2.
So what I'm saying is you
can't look at one thing and go,
"Okay, it has to be the lungs
or it has to be low hemoglobin."
You could have low
oxygenation and low hemoglobin
or you could have low oxygenation
and a borderline cardiac output
and an increased metabolic
demand that are all three
causing your SvO2 to be low.
So we have to look at all these
all the time when our SvO2 is low
to determine what are the
causes of our SvO2 being low.
our SvO2 normal is 60%-80%.
Now this can fluctuate when
we increase patient activity,
or we such as turning,
your SvO2 may drop
and that's okay.
It needs to return to
normal within a few seconds.
If it doesn't,
if it slowly recovers,
meaning let's say we were at 60%
when the patient was just supine,
then we turned them,
we bathe them and we brought them back
and now it's dropped to 40% and it's taking
minutes to get back up to a normal SvO2.
This is indication of
you've got a weak heart, or you've
got bad lungs or a combination of both
that are not able to allow that
patient to recover quickly.
Just keep that in mind.
Let's break these
down, low SvO2.
What can cause a low SvO2?
My hemoglobin is too low,
my SaO2 is too low,
or my cardiac output is not high
enough to meet tissue needs.
Remember, lungs, blood, heart.
My lungs aren't doing their
job so my SaO2 is too low.
I've got anemia or I've got bleeding or
hemodilution, so my hemoglobin is too low.
So I don't have enough
oxygen carrying capacity,
or my heart isn't strong
enough that left ventricle
is not doing what
it's supposed to.
And my cardiac output is too low
not meeting my tissue demands.
If those are okay, then remember
my oxygen consumption is too much.
I had a patient one time that
was intubated and he was sedated,
and everything was fine on
the oxygen delivery side.
But my SvO2 remained in the 40s.
Well, what we found out was
the patients were breathing
and he was a little bit agitated,
and his muscles were just pretty rigid,
causing his oxygen consumption
to be way too high.
So we ended up having to
paralyze this patient,
which greatly increased my SvO2.
So this patient could
recover more quickly.
Now let's look at the
causes of a high SvO2.
What would cause your mixed venous
oxygenation to be higher than 80%?
The first cause could be the
initial stages of sepsis.
Remember, we have a bacterial infection
that's causing mass vasodilation.
And to over compensate for that, our
left ventricle begins to contract harder,
so our cardiac
output goes way high.
Because of this,
our SvO2 will increase greater than 80%.
The second cause could
be an intracardiac shunt.
This is a hole in the septum that allows
oxygenated blood to not go out to the body,
but rather go back
over to the right side.
This is your atrial
your ventricular septal defects
or your patent foramen ovales.
So you got a hole and the
oxygenated blood goes back over
and then your pulmonary artery
catheter reads that oxygenated blood
so you'll have SvO2
that are 85%, 95%
and that's because you've
just got oxygenated blood
before it goes back
into the lungs.
The last cause of this,
though rare is cyanide poisoning,
which can cause
an elevated SvO2.