Topic 2 - haemodynamics

Question 1

What are the three layers of vessel from inner to outer?

Answer 1

Tunica Intima
Tunica Media
Tunica Adventitia

Question 2

What is the function of the tunica intima?

Answer 2

• he inner most layer
• consists of endothelial cells and basement membrane
• variety of functions which include clot prevention and responding to local changes in blood flow by controlling artery diameter.

Question 3

What is the function of the tunica media?

Answer 3

• predominantly composed of smooth muscle
• arranged in circular and longitudinal orientation.
• principally concerned with controlling vessel diameter and contributing to the elasticity of the blood vessel.

Question 4

What is the function of the tunica adventitia?

Answer 4

• the outer most layer
• composed mostly of collagen and elastin
• gives support to the blood vessel and also contributes to the elasticity of the vessel.

Question 5

What is the difference in the tunica intima in veins and arteries?

Answer 5

• present in all vessels

* although it responds differently in arteries than veins in terms of the control it has over vessel tone (diameter).

Question 6

What is the difference in the tunica media in veins and arteries?

Answer 6

• changes significantly from relatively thick in the proximal arteries while thinning in the distal arteries and arterioles.
• In the capillaries, there is no media so that nutrients and waste chemicals can be transferred to and from the surrounding tissues.
• The venules also lack a tunica media but as the veins increase in size, the tunica media thickens.
• The tunica media however, remains significantly thinner in veins than in arteries.

Question 7

What is the difference in the tunica adventitia in veins and arteries?

Answer 7

• present in all of the vessels except the capillaries
• the thickness of the adventitia varies with more fibrous tissue present in the vessels closer to the heart.
• Again the arteries generally have a thicker layer of adventitia compared to the veins.

Question 8

How is blood flow velocity calculated in a tube?

Answer 8

• For a given diameter, the velocity of the blood flow is equal to the flow rate divided by the area.
• As the diameter of the vessel reduces, the velocity increases with a loss of lateral pressure on the vessel wall.
• Q = vXA
o where
o Q= ml/sec
o v= cm/s
o A= cm2

Question 9

How can you explain the movement of blood along a tube?

Answer 9

• this is achieved by creating a pressure difference between the two ends of the tube
• the relationship between flow and pressure is interchangeable.
• Blood which is moved or displaced in a vessel will cause a pressure
• similarly a pressure difference applied will displace blood and cause it to move.
• The Hagen-Poiseuillle equation (commonly called Poiseuilles equation) shows this concept and introduces the other factors which influence the relationship between pressure and flow

Question 10

What is Poiseuilles equation?

Answer 10

•	P1 – P2 = 8ηLQ/ π r4
o	Where
o	P1 – P2 : Tube Pressure difference
o	η: viscosity of the liquid
o	L: length of the tube
o	r: radius of the tube
o	Q: Flow in the tube

Question 11

When considering vessels in the body, what are the constants and variables of the equation?

Answer 11

• the viscosity of blood is relatively constant
• the length of an artery can be considered constant in most situations.
• This leaves the radius of the blood vessel to exert a significant influence on flow and pressure, particularly since radius is raised to the fourth power

Question 12

What does Poiseuilles equation tell us about vessel diameter?

Answer 12

• The application of Poiseuille’s equation highlights the dramatic effect that diameter has on pressure and flow.
• For a given artery with constant length, viscosity and pressure, doubling the radius of the artery will allow 16 times the blood flow

Question 13

Briefly comment on resistance in arteries

Answer 13

• R = P1-P2/Q = 8ηl/ πr4
• Arterial resistance (R) is used by the body to control the distribution of flow to different regions of the body by varying artery diameter in the different vascular beds.
• Changing diameter is the most powerful influence on resistance
• The total resistance seen across a series of changing blood vessel diameters (resistances) is the sum total of all the individual resistances (Eq. 2.4).
• If a number of vessels are parallel to one another then the inverse of the total resistance is equal to the sum of the inverse of each individual Resistance (1…n) (Eq. 2.5)
• As the arteries divide into smaller arteries and arterioles, the resistance of each individual vessel increases but the total effect of vastly larger numbers of arterioles and capillaries in parallel result in a far lower total resistance.

Question 14

What is the the Windkessel effect?

Answer 14

The changing pulsatile flow into continuous flow by the arteries.
• blood is pumped into the elastic arteries, which causes them to stretch and expand.
• The elastic artery stretches to a maximal diameter during systole
- some of the blood expelled by the heart is contained in the artery while the remainder is forced along the artery.
• As the artery relaxes and returns to it’s priginal diameter through diastole, the excess blood which was stored in the artery during systole is now moved forward
• This temporary storage and release of blood flow from within the artery reduces the pulsatility of the arterial flow throughout the large arteries

Question 16

What affect do arterioles have on resistance?

Answer 16

1. Arterioles dilate:
   • the amount of reflected wave reduces
   • the level of continuous flow increases in the artery.
   • this reduces the pulsatility of the wave and increases its continuous flow.
2. As arterioles constrict:
   • the amount of reflected wave increases
   • the level of continuous flow reduces
   • this increases pulsatility of the wave and reduces the continuous flow

Question 17

Comment on constant flow and pulsatile flow in arteries

Answer 17

arteries are a dynamic fluid system where both continuous flow and pulsatile flow co-exist.

Question 18

What is laminar flow?

Answer 18

• Laminar flow is the term used to describe smooth and stable blood flow. It is parabolic, with the fastest velocities in the centre of the vessel, steadily decreasing toward the wall of the vessel.

Question 19

What is plug flow?

Answer 19

• As blood leaves the heart, the velocities across the lumen of the artery are quite uniform since the arterial wall has yet to exert its slowing influence.
• In these proximal arteries, there is relatively little change in the velocity of the lamina over most of the arterial diameter and is termed Plug Flow
• As the blood moves further along the arteries, the effect of resistance further slows the outer lamina causing a parabolic velocity profile form across the artery which is accentuated as the blood moves distally (laminar flow)

Question 20

What is turbulent flow?

Answer 20

• occurs when blood is increased in speed until the cohesive nature of the lamina are disrupted by the increasing difference between the speed of adjacent lamina.
• Once the laminar flow is disrupted, the blood flows in random directions losing some of its energy (pressure) as heat and sound (bruit).
• Turbulent flow can be reflected in the spectral waveform by spectral broadening.

Question 21

How does helical flow form?

Answer 21

• When blood flows around a curved section of a vessel, such as the aortic arch, it will move helically.

Question 22

How does boundary layer separation occur?

Answer 22

• When blood fails to navigate a sharp bend, boundary layer separation occurs, it is common at the carotid artery bifurcation with swirling blood occurring in the carotid bulb

Question 23

Describe the formation of the waveform?

Answer 23

The form of the velocity(pressure) wave at a given point is created by the degree of continuous flow present in the artery together with the forward travelling pulse interacting with the reflected pulse.
The forward pulse will be added to the returning pulse (superposition) to create the final waveform shape at any given position.

Question 24

What is monophasic flow?

Answer 24

• shows continuous flow through diastole

* associated with a high level of continuous flow in the arteries and a low level of reflected wave

Question 25

What is biphasic flow?

Answer 25

• This waveform shows forward velocity in systole

* And brief reversal during the early part of diastole

Question 26

When does monophasic flow occur?

Answer 26

• may occur in normal arteries when the distal vessels are dilated such as in organs requiring constant flow (eg renal circulation) or in post exercise or reactive hyperaemia states.
• It may also occur distal to a stenosis

Question 27

When does bi phasic flow occur?

Answer 27

• associated with increasing constriction of the distal vessels which reduces the continuous flow and enhances the strength of the reflected wave.
• This is seen often in arteries where the distal circulation is in a moderate degree of constriction such as in leg arteries and arm arteries
• or if you are measuring close to a severely obstructed artery.

Question 28

What is triphasic flow?

Answer 28

• This waveform shape is created by an increasing distal resistance causing a stronger reflected wave and reducing the level of continuous flow.
• Although this is the same as the definition of biphasic flow, the triphasic waveform is also contributed to by the later arrival of the reflected pulse or pulses from multiple reflecting sites which contribute to the formation of the forward and reverse phases over a longer time than in the biphasic waveform.
• If the spectral trace is adjusted carefully, many arteries may show up to 4 or 5 phases during a cardiac cycle.

Question 29

What is the significance of a high resistance CCA, with an absence of plaque in the extracranial carotid system?

Answer 29

If the extracranial carotid system is free of plaque, what might cause this diminished diastolic velocity in either its source or the distal vascular bed?
Cardiac
- aortic incompetence. The incompetence will affect forward momentum of flow in the diastole, thus reducing diastolic velocity the waveform.
Distal vascular bed
• distal occlusion, tight stenosis or near occlusion in the ICA will certainly increase the resistance of the circulation dramatically and thus reduce diastolic velocity.
• Other pathologies that may also cause a similar effect include a cerebral space occupying lesion or vasculitis
While we may not be able to see the cardiac or distal changes, the observation of a high resistance waveform in the CCA is suggestive of either proximal or distal pathology.

Question 30

Describe the changes that will occur To the distal waveform when the patient is exercised

Answer 30

• The EDV will increase significantly
• the systolic velocity may be elevated, depending on the level of exercise undertaken.
• These are signs of a low impedance (resistance) in the distal circulation.
• This is a normal response of the arterioles as they dilate in response to the exercise.
Infection/inflammation of the distal tissues may also provoke vasodilation and cause the same changes in the waveform.

Question 31

Describe the changes that will occur to the radial artery after the hand has been in a warm bath

Answer 31

• The EDV will increase
• mild change in peak systolic velocity may also occur with the increased flow to the hand.
• The heating causes vaso dilation of the hand vessels, thus reducing the resistance to the blood flow and allowing greater flow to occur.
• This response may be enhanced by sympathetic nerve activation if other regions of the body are also heated.

Question 32

Describe the changes that will occur to the SMA after a sustained period of eating

Answer 32

• The EDV and systolic velocity will increase due to vasodilation and increased flow in response to the increased requirement for blood to the stomach and upper bowel as digestion occurs.

Question 33

List the essential features of plug flow pattern

Answer 33

• In plug flow nearly all the blood flow across the lumen is at the same velocity and it is only near the wall that the blood slows.
• This occurs when the speed of flow is much greater than the effect of friction in the blood lamina and you are looking at a vessel closer to the heart.
• The loss of energy due to friction between the lamina is relatively small and the flow has not had enough time to travel a great distance along the arterial system.
• The further away the blood flows, the greater is the cumulative effect of friction.

Question 34

How would plug flow be represented by a spectral wave from?

Answer 34

• Flow in the aorta is the closest we see to plug flow in the human body.
• The spectral trace is seen as a thin line with no spectral broadening of the wave form at all

Question 35

Discuss the profile of the common iliac artery and why it occurs

Answer 35

sharp systolic upstroke and multiple reverse and forward components during diastole.
The spectral envelope is clearly defined with minor broadening of the spectrum.

Question 36

What is a non pathological reason for spectral broadening?

Answer 36

• the blood vessels are so small, the spectral gate encompasses their entire width, thus all the velocities across the lumen of the artery are recorded, filling the spectral window
• sometimes physical movement of the artery during the cycle

Question 37

Where and why would you normally find bidirectional flow in the carotid artery?

Answer 37

In normal carotid artery bulbs, the dilated region caused flow lamina to reverse within the larger space.

Question 38

For a waveform taken proximal to a tight stenosis in the lower limb. Describe the characteristic features of the waveform and explain the reasons for its appearance.

Answer 38

• a pronounced flow reversal of greater velocity as the forward flow.
• This occurs because of a high distal resistance, usually in close proximity to the recording.
• A tight stenosis or occlusion may cause a strong reflected wave, which enhances the reverse flow component of the spectral waveform.
• Not all stenosis show this prestenotic waveform.

Question 39

What does pulsatility of the veins suggest?

Answer 39

• Like the arteries, the veins are also affected by the vascular bed to which they flow.
• Veins flow to the right side of the heart via the IVC.
• The waveform reflects an abnormality in the efficiency of the right side of the heart to transport blood to the pulmonary bed.
• ?regurgitation across the tricuspid valve.

Question 40

What happens to flow and energy at an arterial stenosis?

Answer 40

• Energy is dissipated in a stenosis through the production of turbulence and results in a loss of pressure
• this loss begins to become significant around a critical degree of stenosis
• however raised velocity and disturbed flow occur at milder degrees of stenosis.

Question 41

What are the spectral Doppler signs of a stenotic vessel?

Answer 41

• increased blood velocity within and immediately after the stenotic section of vessel
• spectral broadening in the section of vessel immediately after the stenosis.

Question 42

What causes spectral broadening after a stenosis?

Answer 42

• Once the fast-moving blood exits the stenotic section of the vessel it slows down causing eddy currents or, in more extreme cases, turbulence.
• The varying blood velocities present increase the range of Doppler shifts in the region of interest

Question 43

What is Bernoulli’s principle?

Answer 43

Bernoulli’s principle shows that energy is conserved between any two points in a tube assuming that no energy is lost due to friction against the artery wall. Therefore energy is conserved through a narrowing

Question 44

Relate Bernoulli’s principle to artery stenosis in an ideal scenario

Answer 44

• In an ideal stenosis the total energy is made up of the pressure (potential energy) and the kinetic energy (mass and velocity of blood flow).
• As the artery narrows, the pressure energy is converted into kinetic energy in the form of velocity (as mass does not change).
• The increase in velocity allows the same flow rate though the narrowing so that blood does not accumulate proximal to the stenosis.
• The conversion of potential to kinetic energy results in a loss of pressure within the stenosis
• but the pressure is totally returned to pressure energy as the artery diameter increases again and both pressure and velocity return to pre stenosis values.

Question 45

In simple terms write down Bernoulli’s principle equation?

Answer 45

Total Energy = Pressure energy + Kinetic energy = constant

Question 46

Relate Bernoulli’s principle to artery stenosis in an realistic scenario

Answer 46

• a stenosis will increase the velocity of the blood flow (reducing potential energy and increasing kinetic energy) to a high enough level to break the cohesion between the flow lamina and cause turbulent flow distal to the stenosis.
• The result of the turbulent flow is to permanently loose energy in the form of heat and sound (bruit).
• This loss of energy results in less energy being returned to potential energy and thus pressure is lost across the stenosis.
• The degree of narrowing and the shape of a stenosis will influence the degree of energy loss.

Question 47

Why is critical stenosis a difficult term?

Answer 47

• the definition of a critical stenosis is one which causes significant pressure and flow reduction in an artery.
• neither strictly defined in terms of the exact diameter reduction needed nor is it uniformly applicable in different arteries
Avoid the term critical stenosis

Question 48

What features of a stenosis can influence the production of turbulent flow?

Answer 48

o stenosis shape
o artery compliance
o length of stenosis etc.
o The distal arterial bed is also an important influence as a critical stenosis for the carotid arteries is different than that of the leg arteries or renal arteries.

Question 49

When is the term critical stenosis often used?

Answer 49

• When flow is reduced and appreciable pressure drops are seen after a stenosis it is often referred to as critical or hemodynamically significant.