Hemodynamics for the Sonographer - SD

Introduction to Hemodynamics

Hi, my name is Tracy Fox.
I'm an instructor from Thomas Jefferson University.
Today we're going to have a review on hemodynamics.
It's important to understand hemodynamics when interpreting
vascular sonography, so sit back and enjoy.
Thank you.

Principles of Flow

Let's begin our discussion on hemodynamics.
For the stenographer. Why is there a flow?
Why do we have flow?
We have flow because there's a pressure difference,
there needs to be high pressure at one end,
lower pressure at the other
fluid will flow from a high energy state
to low energy state.
And in the cardiovascular system that energy is pressure.
Flow travels from the left heart to the right heart,
from the region of highest pressure to the region
of lowest pressure.
The heart creates the potential energy
of the system in the form of the blood pressure.
The flowing blood is the kinetic energy.
The total fluid energy, which is a constant,
is the potential energy plus the
kinetic energy in the system.
The total fluid energy in the system will never change.
Even as the potential energy is converted into kinetic
energy, that total fluid energy never changes
as the blood travels.
Some of the kinetic energy is continuously lost in the form
of heat and this is due to friction.
The red blood cells rubbing against the vessel walls.
However, again, the net energy remains unchanged.
So a red blood cell, which starts out with a certain amount
of potential energy as it flows,
that energy is converted into kinetic energy
and there will also be some heat loss that occurs
as a result of friction.
However, that kinetic energy plus whatever heat
energy was lost,
will equal the potential energy at the beginning
of the system flow represented
by the letter Q is the volume of blood flow per unit time.
Again, in order further to be flowed,
there must be a pressure difference,
higher pressure at one end, lower pressure at the other.
The pressure gradient represented by delta P
or P one minus P two is the difference
between two pressures at different points in the vessel.
The higher the pressure gradient,
meaning the higher the difference between pressures,
the greater the amount of flow.
If there is no difference between the two pressures,
there is no flow.
Let's look at this vessel
where P one equals a hundred millimeters of mercury
and P two equals a hundred millimeters of mercury.
There won't be flow at this vessel
because there's no pressure difference.
However, let's look at this vessel
with P one a hundred millimeters of mercury
and P 2 99 millimeters of mercury.
Will there be flow? Yes, there will be.
It won't be a lot of flow
because again, the amount of flow is directly proportional
to the pressure difference.

Bernoulli's Law

Bern's law says that
there you can have the same amount of total energy
even if it's in different forms.
There has to be an equilibrium.
Energy cannot be created or destroyed.
So in other words, you always end up with the same amount
of total energy, the beginning versus the end of a system,
even if it's in different forms.
So for example, as velocity increases, pressure decreases,
the total amount of energy will remain change.
But in order to conserve energy, something has
to decrease when something else increases.
And in the case of velocity
and pressure, they're inversely related to each other.
Let's look at this equation. Delta P equals four V squared.
This is the formula used to calculate the pressure drop
pressure drop across a stenotic heart valve
and this is the modified rnli equation.
As we increase the velocity such as tho a stenotic jet,
the pressure difference increases,
meaning there is a pressure drop.
Looking at it this way, notice how the total energy
of the system never changes.
As the velocity goes up, the pressure decreases.
As the pressure increases, the velocity decreases,
the total energy remains unchanged.
Try this, take two pieces of paper, hold them
between your fingers and now blow between them.
Logic would dictate that when you blow between them,
the pieces of paper will blow apart.
But what Bernardi's law tells us is that as the velocity
of air increases between those two pieces
of paper from you blowing between them,
there will be a pressure drop.
And actually what happens is the pieces
of paper are drawn together.

Continuity Equation

The continuity equation Q equals VA says
that if we increase the velocity
or we increase the area, we increase flow.
This is best demonstrated with a garden hose.
When you turn on a garden hose
and put your thumb over the front of the hose, notice
how the water shoots out much faster.
This is because as we decrease the area,
we must increase the velocity in order to maintain flow.
Well, why is flow maintained? Flow comes from the spigot.
If you don't touch the spigot
and decrease the amount of flow,
but you decrease the area, the velocity must increase.
Let's look at this example here where we have flow moving
through a system, the turquoise
line represents the flow rate.
Now I'm gonna change the area in a moment
and when I do, I want you to watch what happens to velocity,
the blue line and pressure the red line.
So watch as I decrease the area, watch that red
and blue line and see what happens.
Notice as I decrease the area, the velocity increases
and the pressure decreases.
Notice what happens to flow. Now move it again.
Notice that flow never changes. Flow is a constant.
Your heart will not change how it beats
because there's a stenosis flow is a constant.
So in order for these red blood cells to get
through this narrowed area, they must
increase in velocity in order
to maintain the continuity to flow.
That decrease in area causes an increase in velocity.
If the velocity increases, the pressure must decrease.

Types of Flow

There are three types of flow commonly found in the body
and these are plug flow, laminar flow and disturbed
or turbulent flow plug flow occurs in the aortas
that exits the heart and at the entrance of vessels
with plug flow, the blood has not had time
to experience much frictional losses yet a result
as a result of red blood cells rubbing
against the vessel wall.
And in fact the flow is traveling at almost a uniform
velocity as this blood moves through the vessel.
However, it does start experiencing friction as a result
of the heat loss from the rubbing
of the red blood cells on the enter,
along the vessel walls.
And notice what happens. The flow profile changes shape
to a more laminar flow and we call that laminar flow.
Laminar flow is the most common type
of flow found in the body
and it's again represented by this parabolic shape.
The boundary layer is this very thin layer
of red blood cells that remain stationary along the wall
because they're experiencing the most amount of friction.
As we move further towards the center of the vessel,
there are less and less frictional losses.
So the fastest flow will be found in the center
of the vessel and the slowest flow found
towards the vessel walls.
This velocity profile again shows you the fastest flow will
be in the center, progressively getting slower
and slower as we move towards the vessel walls.
This color Doppler image also demonstrates this.
As you can see, the brightest colors,
the fastest flow are in the center of the vessel
and the darkest colors representing the slowest flow
or towards the vessel walls.
Turbulent flow is often called chaotic flow
because flow is no longer traveling predominantly down
the long axis of the vessel.
And in fact, the red blood cells are just tumbling
around in all different directions.
In the presence of turbulent flow,
turbulent flow which doesn't always indicate pathology
can occur just in the presence of large vessels
or high velocities distal
to an obstructive lesion or stricture.
And within a kink
or sharp turn within a vessel when there is obstruction,
obstruction is commonly caused from plaque.
And again, remember that flow doesn't change even
if there's an obstruction.
However, if flow does change,
we call this lesion hemodynamically significant.
We're able to quantitate the amount of turbulence
that's seen and that is done by an equation known
as a formula known as Reynolds number.
And this formula, when it's calculated out,
if we get a number greater than 2000, that usually tells us
that turbulence is present.

Poiseuille's Law

Pier's law. Pier's law says that the amount
of flow is dependent on the pressure difference, the radius
of the vessel, the length of the vessel in the viscosity
of the blood and of these
radius is the most significant factor.
This is ER's law which says that flow is equal
to the pressure difference times pi times the radius
to the fourth power divided by eight times the length
of the vessel times the viscosity of the blood.
And I think you can see why we say radius is the most
significant factor and we'll go into that shortly.
Now there's a catch of course Pier's law assumes that it,
it's a rigid pipe with no elasticity.
It's a straight pipe and there are no curves or bends.
And that the fluid has a constant viscosity
and constant flow rate, meaning it's not pulsatile.
Now of course this does not
accurately describe the cardiovascular system,
however the general principles still apply.
So let's look at the individual components.
Desi's law length, the length of the vessel.
Why is the length of vessel here?
This says that if you increase the length,
you decrease flow.
Well I would think that if you increase the length
of the vessel, you increase the amount of flow, right?
Well it doesn't work that way
because the longer the vessel,
the more time these red blood cells can spend rubbing along the side
of the vessel wall.
That's more friction.
And again, friction causes energy loss in the form of heat.
So the longer the vessel,
the greater the amount of frictional losses.
The shorter the vessel,
the more flow you're gonna have at the end of the system
because there's gonna be less energy losses.
Viscosity blood is thicker than water.
It's not just a saying.
If you increase the viscosity,
you're gonna decrease the amount of flow.
Flow is generally considered to be a constant,
but there are certain medical conditions
that can alter the viscosity such as poly,
polycythemia and sickle cell anemia.
The radius most important factor
because the radius, the volume
of flow is proportional to the fourth power of the radius.
Meaning even a small change in radius results in a
significant change in flow.
Decreasing the radius
by half will cause a 16 fold decrease in flow.
So do we have to memorize PO's law?
No, we have to think about it. Just think about it.
Anything that's gonna increase friction, decreases flow.
Longer vessels, real thin
or narrow vessels, thick blood, these are things
that will increase friction, increase resistance, and
therefore decrease flow.
The short form of ER's law can be written as Q is equal
to the pressure difference divided by resistance.
And where does the long form of ER's law come from?
When we break resistance down into its individual
components, why is there a pressure drop
in the legs in the presence of significant disease?
Well, what happens when there's a significant obstruction?
The body part distal to the obstruction needs more oxygen,
therefore the arterials which have a muscular medial wall
are able to dilate causing an increase in radius.
This increase in radius causes a pressure drop
and that's why there's a pressure drop distal
to a significant stenosis.
Most arterial resistance occurs at the level
of arterials in the peripheral extremities.
For example, the arterials are normally constricted.
Normally we have high resistance bed,
the arterials are constricted.
However, when they sense in need such
as increased demand from exercise
or if there's an obstruction, they're able
to selectively dilate or constrict
because of their muscular wall.

Vessels in Series and Parallel

Vessels that are in series meaning end
to end will have more resistance than
vessels that are in parallel.
What does that mean? Well these are in series vessels.
In series vessels, you calculate their total resistance
by adding the resistance of each segment.
So R one plus R two plus R three.
In parallel vessels that are in parallel such
as you see here, you calculate the total resistance
by adding their reciprocal of the resistance.
One over R one plus one over R two.
Well let me explain that a different way.
Let's say a bunch of cars are trying to leave an island
and this island has one bridge.
You can imagine that this would cause increased resistance.
This is a high resistance state.
You have a lot of cars all trying
to leave off of one bridge.
If we had bridges in parallel, multiple bridges,
multiple pathways, this would decrease the resistance.
We would be able to move a lot more cars per unit time
because this would cause a decrease in the resistance
and we'd be able to move more
of these cars out over the same amount of time.

Inertia and Stenosis Effects

Let's talk about inertia.
Newton said that an object at rest stays at rest
and an object in motion stays in motion
unless acted on by an outside force.
The dividing of vessels causes inertial loss.
Every time the blood cells have
to make a change in direction, we lose some energy.
Eventually this slows the blood down the region
of the capillaries where we want the blood to be.
Its slowest and we'll talk about that in a little bit.
When an astronaut is in space, if he had a ball
and he were to just let go of it
or throw it, it would keep moving until it hit a planet
or fell into the sun or something like that.
Because an object at motion stays in motion
unless acted on by an outside force, there's no air,
there's no resistance up in space on the earth.
However, we have air resistance, we have friction
as a result of the ball rubbing on the ground.
We have the force of gravity applying a downward force and
therefore inertia acts upon the ball
and it does not travel forever.
What happens with a stenosis?
Remember that when we decrease the area,
we must increase the velocity in order to maintain flow.
And oli told us that if the velocity increases,
there has to be a pressure drop.
Flow does not change with a non hemodynamically
significant stenosis.
So at the region of a narrowing in the vessel
or a stenosis, there will be an increase in velocity and
because the velocity increases, the pressure must decrease.
Therefore there is a pressure drop within a stenosis.
Because of these increased velocities distal
to the stenosis, we commonly have turbulence
because now the diameter of the vessel increases to
where it was, to the pres stenotic state.
So let's take a look at what happens pre stenosis,
we have our typical laminar flow as we expect at the region
of the stenosis, the velocity markedly increases
and in this case is about five meters per second,
which is pretty high distal to the stenosis.
When the radius goes back to its pres stenotic state,
there is turbulence as all of these red blood cells now have
to occupy this increased area.
And we do get turbulence as a result.
We might even have some downstream turbulence,
a little ways from the stenosis as well.
But remember this is the region of highest
pressure right here and the pressure will
continually decrease.
Remember this is the area of lowest
pressure right here because this is the
area of highest velocity.
As we leave the stenosis
and go back to our normal flow state,
the pressure will continually increase
as the velocity decreases.
It's important to remember that a 75% reduction in area
corresponds to a 50% reduction in diameter
and that two or more steno lesions
that occur in series have a more significant
or negative impact downstream than one single lesion.
In other words, as blood is traveling through a region
where there are two stenosis, one right
after the other, it takes an energy, hit the flow,
takes an energy hit from that first stenosis,
and while it's still weak from that first stenosis,
it runs into another stenosis,
it causes a very significant energy drop
versus having one single lesion.

Waveforms and Resistivity

Let's talk about the ity of these vessels.
Phasic and biphasic flow, also known as mono,
multiphasic flow indicate high resistance vessels.
Distally. Monophasic flow like you see here
would indicate a more low resistance downstream bed
in the lower extremity arteries of a resting patient.
Monophasic flow is always abnormal.
This should never be the wave form
of a femoral artery like it is in this case.
This tells me that the distal bed downstream is low
resistance, which would be abnormal in the resting patient.
It's telling me that there are persistently dilated
downstream vessels as a result of disease.
Ity always relates to the bed being fed
and not the vessel itself.
For example, this doesn't tell me
that the problem is in the femoral artery.
This tells me that the arterials downstream are dilated
as a result of problems somewhere.
The problem may be in the common femoral artery,
for example, when the waveform looks like this,
what this tells us is that the downstream arterial,
when the waveform looks like this, this tells us
that the downstream arterial is constricted.
This is a high resistance, arterial high resistance bed.
Therefore we have high resistance, monophasic flow,
more high resistance flow, more proximally.
When the waveform looks like this, for example,
a high resistance waveform, it tells us
that there are constricted arterials downstream,
whereas this low resistance waveform tells us
that there are dilated arterials downstream.
So why do we see monophasic change
to flow in a normally high resistance vessel?
Well, what happens is when there's an obstruction
there ends up being hypoxia downstream.
We don't have enough oxygen getting through to the body part
as a result of this obstruction.
Therefore, these arterials dilate
to get more flow to the body part.
This increased dilatation causes a low resistance bed and
therefore monophasic flow.
Tardis parvis is a delay in the upstroke
of the arterial waveform.
And this tells us that there is disease
proximal to the point of sampling.
So when you see this waveform, this delayed upstroke,
you know that there is disease more proximal
to where you're sampling.
Let me describe Tardis parvis, as two runners
that are gonna be in a race.
And this runner, the shoot,
the starting pistol and off he goes.
Notice he's immediately able
to start running at its top speed.
But if we tied one of the runner's legs down
and tied it to a stake,
this representing the proximal obstruction, he's not able
to get going like the other guy was.
And in fact, it's gonna take him longer to get
to this top speed.
Then the other runner, here's
what it looks like on a spectral wave form.
So this is a normal vessel with a nice strong sharp upstroke
versus this tardis parvis
or delayed upstroke as a result of proximal obstruction.

Collaterals and Path of Least Resistance

What happens with collaterals?
Well, blood flow increases in branches in order
to compensate for disease in the main vessel.
This occurs over time with chronic disease.
In fact, if a patient's collateral network is extensive
enough, then net flow
to the distal body part may even be normal.
Again, collaterals form over time with chronic disease,
if a patient throws an embolus, there just isn't enough time
for these collaterals to be formed
and you will get a hypoxia in that body part.
So it doesn't occur in acute disease
processes, it occurs over time.
Blood will always take the path of least resistance,
the lowest pressure.
So in a normal artery, this is the path of least resistance.
This the collateral vessels are very, very small.
They are much higher resistance than the normal pathway
in the presence of an obstruction.
However, this is now the highest resistance pathway.
This is the highest pressure.
Blood will always seek the lowest resistance pathway
and in this case it will be the collaterals which will
increase in size as a result of increased flow.
Remember we said that inertia causes a decrease in the
velocity because of all the changing in direction
as a result of all the branching.
And this is to get the blood vessels.
This is to get the blood cells to the point
where they're moving very, very slowly.
And in fact, one of the reasons why vessels get smaller
and smaller is to increase the amount of resistance
to these red blood cells and at the capillaries.
In fact, they are just about the size of a red blood cell to
maximize the amount of resistance in order.
Well, in other words, to slow this flow down,
we don't want red blood cells flying through a capillary
because it won't allow the time to exchange nutrients
and waste, which is the whole point of the capillaries.
Imagine getting a cafeteria tray
and running down that cafeteria line.
How much food will you be able to put on your tray
versus if you very slowly walk down that line
and put food on your tray?
Same thing with the capillaries.
They need to travel very slowly through
the capillaries in order to exchange nutrients
and waste veins
that are farthest from the heart have the least ity
peripheral veins.
We don't expect them to be pulsatile.
In fact, we expect them to have a ity, meaning
that they change with respiration.

Venous System

Veins can transmit nearby arterial pulsations,
which is important to keep in mind,
but typically in the peripheral extremities,
we expect the veins to be phasic change with respiration.
If venous flow is phasic, meaning continuous
and there is no change as a result of respiration,
that tells us that there may be some obstruction between
where you're sampling and the heart.
In other words, an extrinsic mass on the IBC for example,
or a clot inside one of the veins.
The venous system acts as a reservoir
for the cardiovascular system, the ability of the veins
to expand and we know how flexible the veins are allows
for increased capacitance.
In other words, the veins can store the blood
until it's needed at rest.
About 65% of your blood volume is in your venous system.
We know that arterial blood travels distally
because there is a pump the heart making it go there.
So how does blood get back to the heart?
Well, there's a couple different ways.
One is that there actually is a pressure gradient
in the venous system.
There is a pressure gradient.
It's not as significant a pressure gradient
as we find in the arterial system,
but it's there in the legs.
The pressure is approximately 15 millimeters of mercury
compared to the right heart, which is somewhere between zero
and eight millimeters of mercury.
So again, not a significant pressure gradient,
but a pressure gradient nonetheless.
However, when somebody stands up,
how do we get the blood back to the heart?
How do we fight gravity?
Well, there are three different ways.
The first are the intrathoracic pressure changes.
Every time you inhale
or exhale, the pressure inside your chest changes.
During inspiration, there is a pressure drop
inside the chest which causes increased venous return.
So every time somebody inhales, there is a pressure drop
inside the chest and venous flow is
sucked up into the chest.
Another way we get blood back to the heart is as a result
of venous valves.
If you're standing, gravity wants
to pull the blood back down to your feet.
But these valves when they work,
keep the blood traveling in a forward direction
towards the heart.
It's like jacking up a car. The car keeps going up and up.
It doesn't fall back down to the ground
unless your jack doesn't work.
Finally, while there is no heart per se in the legs,
we do have a pump and that is the calf muscle pump.
The calf muscles act as a pump.
Every time you move your legs, the calf muscles contract
and propel the blood in a forward
direction towards the heart.
And again, as long as the valves work,
the blood will only move in a forward direction.

Hydrostatic Pressure

Hydrostatic pressure is the amount
of force measured in a vein
and it is the effect of gravity on the blood.
So again, when you stand,
gravity is affecting the blood trying to pull it downward.
The weight of the blood itself affects
all the blood below it.
Picture your venous system as being a column of blood.
The heart is the reference point at the level of the heart.
The hydrostatic pressure is zero.
When you're standing, anything superior
to the heart is a negative pressure.
Anything inferior to the heart is a positive pressure.
So when you're standing, the farther you get from the heart,
the closer you get towards the ground,
the higher the hydrostatic pressure above the heart.
The hydrostatic pressure is a negative,
and at the heart, again, it's zero.
So again, and somebody who's standing picture this column
of blood as having weight to it,
the highest weight is gonna be found near the ankles
because it's supporting the weight
of all the blood superior to it.
So who has it best? Well, the blood closest to the heart.
Who has it worse? The blood at the ankles.
It's supporting the weight of all the blood above it.
Why do we see ulcers in the gator zone?
Venous ulcers are always seen in the region
of the ankles and the shins.
How come? Well,
because that's where the highest hydrostatic pressure is.
So again, in the person who's standing,
the pressure will be highest
as you get closer to the ground.
The heart is the reference point, that's zero.
And anything above the heart will have a negative
hydrostatic pressure.
You can demonstrate this
by putting your arm down by your side.
When you put your arm down by your side,
you'll see your dorsal hand veins increase in size
as a result of the increased hydrostatic pressure.
Put your arm up over your head
and you'll see the veins collapse as a result
of negative hydrostatic pressure.
When the patient is supine
and all body parts are equal to the heart,
the hydrostatic pressure in these parts is zero.

Transmural Pressure

And finally we'll discuss transmural pressure,
which is the difference between the intravascular pressure,
the pressure inside the vein versus the pressure
of the surrounding tissue.
When a patient is supine, the pressure
outside the vein is higher than the
pressure inside the vein.
And we say that has a low transmural pressure.
This is why the veins are elliptical in shape.
When a patient is supine low transmural pressure,
the vein is not pushing on the wall.
When you stand up, the transmural pressure increases.
There is now increased pressure.
And then if say a patient does a al Salva
and they really increase that venous pressure,
we can dilate the veins even further.

Conclusion

Thank you very much for spending the time
to watch this lecture.