So that was the hemoglobin. Now
let's look at the hematocrit.
Remember, the hematocrit is a
percentage of the overall solution.
So we're looking at the packed cell volume.
Look at that first test tube.
You've got a purple top
because that's the one that
we put blood in for a CBC.
You draw the blood, fill the tube,
and we used to have to put
it in a centrifuge. Historically,
it was this machine that would whip
these tubes all the way around.
It would help it separate
to show us the 55%, 1%, and 45%.
Now we can just calculate that.
So we're looking at hematocrit
as the packed cell volume, PCV.
It's measured as a volume percentage.
Remember, hematocrit is the
volume of the red blood cells,
the percentage of that volume,
and we get that from the CBC test.
But here's something that
starts to get really cool.
Now that you've done the work
of laying all that groundwork
of understanding what red
blood cells are, what they do,
looking at what hemoglobin
and hematocrit is,
now let's talk about hematocrit and
hydration status. Because remember,
the percentage of red blood cells is
compared to the total volume of the blood.
Now, can you recall, do we have a higher
percentage of plasma or red blood cells?
Well, look at your picture for a clue. Right.
Normally, you have a higher
percentage of plasma.
Now, when red blood cells and
hemoglobin are normal --
like your patient isn't going
through any chronic issues,
the red blood cells and
hemoglobin are normal --
it's usually a matter of 3 times the
hemoglobin will give you the hematocrit.
So, remember, you have to have a normal
red blood cell and a normal hemoglobin.
3 times a hemoglobin will
equal your hematocrit.
So that's just a quick number game
that you can play the next time you look
at a patient's values that
are within normal status.
Now here's what happens with hematocrit
when you have a + or - in the plasma volume.
Here's what the impact
that you're going to see.
If the patient has lost fluid or lost plasma
volume from the vascular space,
it will raise their percentage
of red blood cells.
Now, you don't have to do
tricky math to do that,
but if you look at that test tube and
if the amount of plasma or fluid volume
decreases in the blood,
that way, the blood is going to
now take up a higher percentage
because there's less plasma.
Now, reasons you would have that
would be like intravascular dehydration.
That means that the amount of
volume is in my intravascular space --
my veins and my arteries -- is less fluid.
Not less red blood cells, but less fluid.
So how that can happen? I can
have intravascular dehydration,
I might have a severe burn.
In severe burns, those capillaries are
damaged. Remember, the capillaries are what
help us keep fluids in because we
have that oncotic pressure from protein.
But if I'm severely burned, those
capillaries are damaged and fluid
is going to leak out of my intravascular
space into my tissues.
So I'm going to have an elevated hematocrit,
not technically because I have
more red blood cells --
remember, it's a overall percentage.
So if I have this reason for having less
fluid volume in my intravascular space,
then my percentage of blood is
now going to be higher,
because the ratio is different.
I have less fluid volume, so it looks like
I have a higher percentage of blood.
Now, what do you think happens if you become
over hydrated, if you have too much
fluid in your intravascular space?
Do you predict that the hematocrit
would go up or down?
Remember, hematocrit is the
total percentage, right,
of the red blood cells compared
to the whole volume.
If you have overhydration,
we're going to have extra fluid
in the intravascular space.
So what's your guess?
Right. You're going to have
a decrease in hematocrit.
So this plays a role when you're
looking at your patients, right?
If they're dehydrated, their hematocrit
is going to look like it's really low.
So when you rehydrate them -- boom --
it's going to drop, and you're going to think
like, "Oh my goodness. Are they bleeding out?"
No. When you recognize that
a patient is dehydrated,
and that was the rationale for
the hematocrit being low,
you give them IV fluids to
replace that volume,
you would expect the hematocrit to drop.
Just to go over that one more time.
The percentage of red blood cells as
compared to the total volume of blood --
So, if I have more or less in the plasma
volume, it will impact your hematocrit.
It doesn't mean your patient
is bleeding. It just means
that a + or - in the plasma volume
will impact your hematocrit.
Now let's get real fancy. Let's
look at a trauma patient.
So when they have equal loss
of red blood cells and plasma,
that means they've had a significant
So, it's not a matter of dehydration
where they lost more fluid volume
and didn't lose red blood cells.
We're talking about, you've cut a major artery
and blood is just leaking, or you
have a big internal bleed.
So immediately after that hemorrhage,
there's no significant change
in the hematocrit
because you've lost equal parts of
the fluid and the red blood cells.
But hours later,
your body's going to rush where you're like,
"Oh, my goodness. We do not have
enough volume in the intravascular space."
So what it does is it will
shift interstitial fluid
into the vascular space to try to
compensate for that fluid loss.
Okay, now what's going to happen?
What's going to happen to that hematocrit
if we shift extra fluid into
the intravascular space
because the body registers that
you've hemorrhaged and bled out
equal parts, plasma and red blood cells.
Now it's shifting interstitial fluid space
in, what's going to happen to the hematocrit?
Yeah, that's why you're going to
see a decrease in the hematocrit.
The bone marrow takes 10 days
to mature a red blood cells.
That's why people who've lost a lot of blood
have to receive packed red blood
cells or they're not going to make it.
So, immediately after hemorrhage, no
significant changes in the hematocrit
because you've lost equal
amounts of blood and plasma.
Hours later, when you have that fluid shift
to compensate for the fluid loss
into the intravascular space,
the hematocrit will then
show you a decrease.
And remember, the body can't
respond to this quickly.
It's going to take 10 days for
extra red blood cells.
So whether it's an acute loss, like
we just talked about with hemorrhage,
or a chronic blood loss, it matters.
So, star this note
and let's walk through why it does.
An acute loss -- the body doesn't
have time to respond --
and a chronic loss -- it's a more gentle
change and the body has time to adapt.
So if I have an acute massive hemorrhage,
I'm going to have an increased
risk for hypovolemic shock.
If I have somehow severed an artery
and blood is leaking out everywhere,
I'm at a risk to have low volume
in my intravascular space,
which means I'm at risk to go into shock.
Now shock means I don't have enough oxygen
getting to my tissues to
meet its metabolic needs.
I don't have enough volume
in my intravascular space.
That's the reason why I don't
have enough oxygen.
People who have chronically low hemoglobin,
like dialysis patients who
have renal failure,
they'll tolerate a much lower
hematocrit than you or I would
without going into shock.
Because the body came to
that state gradually,
it had time to try and compensate.
Remember, renal failure patients
are the ones that have to get
that erythropoietin, like my dad,
because it stimulated his bone
marrow to make red blood cells.
Now, here's another thing I
want you just to file away.
Each unit of packed red blood cells
should raise the hematocrit
approximately by 3%.
So by looking at what your
patient's somatic rate is,
you'll know when you hang a unit of blood,
we would expect it to raise about 3%,
if they're not still chronically bleeding.
Okay, so let's hit the critical values.
Hematocrit of <15% can
result in cardiac failure.
A hematocrit of >60%
can result in spontaneous blood clotting.
Think of the blood as being too thick
and where it's at risk for extra clotting.
But there's a special note about pregnancy.
They have a decreased hematocrit,
especially in the last trimester because
that plasma volume increases.
Now, see if you can remember why
does a plasma volume increase
cause a lower hematocrit?
Right. Because hematocrit is a percentage,
and if you increase the plasma volume,
then ratio wise, you're going to
have a lower hematocrit.
In children, it just really varies with age.
I know. That was a lot of help, but
just keep that concept in mind.
For kids, it will vary with age.