Hemodynamics Flashcards
What is hemodynamics?
The study of blood flow in the circulatory system referring to power and blood
What is the circulatory system?
A closed circuit comprised of the heart, arteries, capillaries, and veins.
What is the venous heart?
The heart muscles contracting
How does the blood get back to the heart?
Muscles contract and act as a pump
What makes up blood?
1) Plasma
2) Erythrocytes
3) Leukocytes
4) Platelets
What makes up plasma?
Water and protein
What are erythrocytes?
Red blood cells
What are leukocytes?
White blood cells
What is the purpose of platelets?
Blood clotting
What is hematocrit?
The portion of blood that is made up of cells
How much of the blood volume is hematocrit?
40%
What is the definition of density in regards to blood?
Mass per unit volume OR the resistance of an object to accelerate
What is the density of blood?
1.05g/m
What is the name for “resistance to flow”?
Viscosity
What is the viscosity of blood?
0.035 poise
What is does 1 poise equal?
1 g/cm-s
Where is the greatest amount of blood in the circulatory system? How much is it?
The veins
65%
Describe the volume and velocity of blood in the capillaries?
Low volume, low velocity
What vessel can dilate more, a vein or an artery?
Veins can dilate 3X more than an artery
Describe the area of capillaries in comparison to the other vessels.
The capillaries have an area of 600-100x that of the other vessels.
Can you see blood flow in capillaries on doppler?
No, the velocity in capillaries is too low. Doppler only picks up med-large size vessels.
What is required for any fluid to flow?
A pressure difference
What creates a pressure difference in the circulatory system?
1) The heart
2) Gravity
How does pressure effect flow rate?
An increase in pressure results in an increase in flow rate.
If there is no pressure difference what happens to flow?
No flow
What creates flow in the heart?
A pressure difference between the left and right ventricles.
Little pressure = ?
Little flow
Like pressing on a pump
What law describes blood flowing in a vessel is synonymous with current flowing through a wire?
Ohm’s law
What is Ohm’s law?
Current = Voltage/Resistance
Regarding Ohm’s law, what is the “voltage” in the body and what is the “current”?
Voltage = Pressure gradient
Current = Volume flow rate
What is pressure gradient defined as?
The pressure difference divided by the distance between two pressure locations
What is the pressure gradient formula?
P Gradient = (P1 - P2)/L
What is Volume flow rate defined as?
The volume of blood passing a point per unit time
What is the abbreviation for Volume flow rate?
Q
What is the total adult blood flow rate?
5000ml/min
What is the cardiac output?
The total adult blood flow rate (5000ml/min)
What is the formula for Volume flow rate?
Q = Pressure change/Resistance
If the pressure gradient increases, what happens to the volume flow rate?
It increases
More pressure = more flow
What happens to the volume flow rate if the resistance to flow increases?
It decreases
More resistance = less flow
What factors effect resistance, and therefore flow?
1) Length
2) Viscosity
3) Radius
What is the formula for resistance?
R = (8 x length x viscosity) / (π x radius^4)
Why does length effect flow?
The longer the pathway the more energy is lost and therefore the greater the resistance.
More resistance = less flow
How does viscosity effect flow?
The greater the viscosity, the greater the resistance = less flow
How does radius effect flow?
The smaller the radius, the greater the resistance = less flow
Of all the factors, which one has the greatest effect on flow, why?
Radius.
Because length and viscosity don’t change much in the human body. Also if we look at the formula, radius is to the power of 4, this means it is 16x greater when it changes.
What is the law of resistance called?
Poiseuille’s Law for Resistance
What is the condition for when we can use Poiseuille’s law for resistance to calculate the volume flow rate?
For a long straight tube only
What is the formula for volume flow rate using poiseuille’s law?
Q = (Change in pressure x π x diameter^4) / (128 x length x viscosity)
How does an increase in pressure effect the flow rate?
Increase pressure = increase flow rate
How does diameter effect the flow rate?
Increase diameter = increase flow rate
How does the length of the tube affect flow rate?
Decrease in length = increase in flow rate
How does the viscosity of the fluid effect flow rate?
Decrease in viscosity = increase in volume flow rate
What factors determine the flow patterns of blood as it flows?
1) Diameter
2) Tortuosity
3) Bifurcations
4) Anastomoses
What is plug flow?
When almost all RBC’s are travelling at the same velocity
Where is plug flow seen?
At the opening of large vessels (like the aorta)
What is laminar parabolic flow?
The RBC’s move in concentric layers with the ones in the centre moving faster than the ones near the walls.
What is the most common flow pattern in normal arteries?
Laminar parabolic flow
What can happen to the RBC’s towards the edges of the vessel walls in laminar parabolic flow?
The velocity of the RBC’s might be so slow that they will not be picked up
How is Laminar parabolic flow on a spectral tracing represented?
1) A clear window
2) A thin envelope
If we dont scan exactly the middle of a vessel what can happen?
We won’t see a clean window on a spectral tracing
What is jet flow?
The velocities within a narrow opening are much faster than the rest (slow velocity before stenosis, fast velocity after).
When is jet flow seen?
When there is a significant reduction in the diameter of a vessel, pressure builds before the narrowing (slow velocity) and jets after the narrowing (fast velocity).
What is disturbed flow?
When not all layers of blood move in a laminar fashion (not quite laminar, not quite turbulent).
When does disturbed flow occur?
It can occur naturally when:
- A vessel tapers
- A vessel cuves
- Bifurcations
What is an example of disturbed flow?
Occurs at the carotid bulb
What is turbulent flow?
Flow velocities are greatly varied and flow directions are chaotic.
Is turbulence normal?
No, it is unusual except in and near the heart.
What type of flow is seen just past a stenosis?
Turbulent
What is reynolds number (RN)?
It can predict the onset of turbulence in a vessel
What is the formula for reynolds number?
RN = (Avg. Flow Speed x Diameter x Density) / Viscosity
Above what number does the RN have to be in order for turbulence to occur?
2000
Why does blood velocity increase in a stenosis when Poiseuille proved otherwise?
The rule of continuity (fluid is neither created or destroyed as it flows).
What is the rule of continuity? (IMPORTANT)
In the presence of a stenosis the volume of flow must remain constant proximal to, at and distal to the stenosis
How is the rule of continuity maintained in a stenosis?
The velocity of blood increases in a stenosis to keep the volume flow constant.
If the diameter of the stenosis is 1/2 the normal vessel then what happens to the velocity?
It doubles through the stenosis to maintain the volume flow rate.
When the velocity is doubled in a stenosis, how has the diameter changed?
50% of the diameter has been lost
What does Bernoulli’s Principle establish?
A relationship between kinetic energy, gravitational potential energy, and pressure in a frictionless fluid system.
What is the relationship established in Bernoulli’s principle?
That not only is volume flow rate maintained in the body but the energy within a fluid system is kept in balance as well.
What is the kinetic energy?
The movement of the RBC’s
What is the potential energy?
The energy stored in the elasticity of the vessel walls
Is there more potential or kinetic energy in vessels?
More potential
What type of energy is Total fluid energy (E) comprised of?
Potential and kinetic energy
What two things make up the potential energy?
The intravascular pressure plus the gravitational potential energy.
How is pressure produced?
The contraction of the heart, the hydrostatic pressure and the static filling pressure.
Describe the pressure at a stenosis?
It drops
How does pressure effect velocity at a stenosis?
The pressure drops which causes the fluid to accelerate through the stenosis and decelerate past the stenosis.
Describe the changes in energy through a stenosis?
Potential energy (pressure) is converted to kinetic energy (flow) on entry and back to pressure (potential energy) on exit.
How can we calculate the drop in pressure in a stenosis?
Using the velocity of blood flow in the jet
What is the formula to calculate the drop in pressure in a stenosis?
Change in pressure = 4(V2)^2
V2 = Velocity in stenosis
Why is the velocity before the stenosis not found in the formula to calculate the drop in pressure in a stenosis?
Because the velocity before a stenosis is small compared to the jet and can be ignored.
When in UT do we use the formula for calculating the pressure drop in a stenosis?
In echo for stenotic heart valves
What happens after a stenosis?
A drop in the acceleration time and peak systolic velocity of arterial flow.
What is Tardus Parvus?
When a stenosis is present it detracts from the pressure gradient that was created by the heart and there is a drop in acceleration time and peak systolic velocity of arterial flow after the stenosis. (It is “late”)
What is the windkessel effect?
The continued forward flow in the aorta during diastole as a result of the elasticity of the vessel walls contracting back to their natural diameter (can only flow fowards).
What prevents the flow from reversing back to the heart in the windkessel effect?
The aortic valve has closed
What is the Shanekessel effect?
Students continue learning in a forward motion due to the greatness of the teacher.
How does flow reversal work?
Pressure waves along the walls of the vessels travel faster than the blood flowing within, when a wave reaches the end of the arterial system a reflected wave causes the vessel wall to expand and contract against the flow of blood, the blood is then reversed back towards the heart because of the increased pressure down stream and the heart in diastole.
Does blood travel faster or slower away from the heart? Why?
The further away, the slower it travels.
Capillaries are stiffer than the compressible vessels near the heart therefore blood slows down away from the heart until the reflected wave in the wall of the vessel speeds it up.
What type of flow creates different flow profiles?
Flow reversal creates (Low/Med/High) flow profiles.
