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Haemodynamics Book > Haemodynamics - Chapter 3 > Flashcards

Haemodynamics - Chapter 3 Flashcards

1
Q

Which cardiac phase is usually displayed in spectral waveforms?

A

Both although in healthy large arteries, there is nearly no diastolic flow in diastole
- diastolic is usually phases 2+3 of triphasic

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2
Q

What are the 2 fundamental properties of a fluid that are important when determining fluid motion?

A

Density and Viscosity

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3
Q

Why is density of a fluid (p) assumed as constant?

A

Fluid is generally assumed incompressible

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4
Q

What is viscosity?

A

‘Stickiness’ caused by the internal friction and shear (sliding of layers over each other)

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5
Q

What is a Newtonian fluid?

A

A fluid with constant viscosity for all velocity changes or shear rates

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6
Q

What is shear rate?

A

The velocity gradient caused by fluid layers sliding over each other

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7
Q

What does viscosity of fluid depend upon?

A

Temperature and haematocrit

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8
Q

Why is volume flow innacurate?

A

Vessel wall diameter changes at different points in the cardiac cycle

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9
Q

How does flow change in a tube with no branches?

A

Volume flow is constant throughout its length

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10
Q

How does flow change post-bifurcation?

A

The sum of the flow in the 2 vessels must equal the flow in the parent vessel

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11
Q

What is the continuity equation?

A

A2 = A1 v1/v2

A = area
v = velocity

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12
Q

What are the two energy components of blood flow?

A

Kinetic Energy and Potential Energy

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13
Q

Why do we use energy density rather than total energy for blood flow?

A

Blood is liquid and will change shape

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14
Q

What is potential energy equal to?

A

The pressure of the fluid on an object placed within it - blood pressure or fluid pressure

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15
Q

How does fluid incompressibility impact pressure changes?

A

A change in pressure at one point will be transmitted to all other points

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16
Q

What is pressure?

A

Force / area (N / m^2, Pascals)

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17
Q

How does mmHg compare to N?

A

1mmHg = 133.3 Pa

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18
Q

What are the 3 components to fluid pressure?

A
  1. Static filling pressure
  2. Hydrostatic pressure
  3. Dynamic pressure
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19
Q

What is static filling pressure?

A

The residual pressure that exists in a supine person in the absence of any blood flow - e.g. dead person
- results from fact circulation is closed system e.g. pressure in a bike tyre
- is typically 5 - 10 mmhg

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20
Q

What is hydrostatic pressure?

A

The fluid pressure due to the force of gravity acting between 2 points

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21
Q

Why does hydrostatic pressure have a negative sign?

A

An increase in height causes a decrease in hydrostatic pressure

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22
Q

What is the equation for hydrostatic pressure?

A

Hydrostatic pressure = pgh

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23
Q

Why is hydrostatic pressure not available to do work on blood flow?

A

Circulation is a closed system - increases in pressure at one point will cause decrease in pressure elsewhere

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24
Q

What is dynamic pressure?

A

The increase in pressure as a result of contraction of the ventricles
- it is the only component of the total fluid pressure that is available to do work on the blood

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25
Q

What is kinetic energy?

A

The energy associated with a moving mass

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26
Q

What is the equation for kinetic energy?

A

KE = 1/2 mv**2

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27
Q

What is p(rho)?

A

Density

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28
Q

How does KE relate to density?

A

KE = 1/2 pv**2

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29
Q

How much energy us stored as pressure?

A

120mmHg * 133.3 = 15,996Jm-3

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30
Q

How big is kinetic energy in normal arteries?

A

p = 1.06x10-3 kgm-3
v = 0.6m/s
KE = 190Jm-3

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31
Q

How does KE compare to the energy due to fluid pressure?

A

It is much smaller

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32
Q

Why does blood flow around the circulation?

A

Due to differences in total fluid energy - in particular blood flows down a pressure gradient

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33
Q

What is the Bernoulli Principle?

A

Just as mass of fluid does not change, the total energy of blood remains constant
- assuming no energy is lost to friction and there is steady flow

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34
Q

How does the Bernoulli principle relate to vessel diameter decreases?

A

Area decreases so velocity will increase, so for energy to be conserved, fluid pressure must decrease

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35
Q

Why does fluid pressure drop as blood moves around the circulation?

A

Energy losses due to acceleration and deceleration (inertial losses) and viscous losses (friction)

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36
Q

What is the main source of energy loss as fluid moves steadily along a smooth tube?

A

Frictional losses due to viscosity of the fluid

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37
Q

What equation relates viscosity and energy loss?

A

Poiseuille’s Law

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38
Q
A
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39
Q

What can Poiseuille’s Law be used to calculate?

A

The work that must be done to move fluid along the length of a tube

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40
Q

What is Poiseuille’s Law dependent on?

A
  1. Strongly dependent on radius (r**2)
  2. Linearly dependent on length of the tube
  3. Linearly dependent on the velocity of fluid
41
Q

What is approx. resistance of large arteries?

A

Low < 5 mmHg

42
Q

What is total resistance of vessels 1 & 2 in series?

A

RT = R1 + R2

43
Q

What is total resistance of vessels 1 & 2 in parallel?

A

1 / Rt = 1/R1 + 1/R2

44
Q

What is EDV very dependent on?

A

Peripheral Resistance

45
Q

Why does distal artery disease cause increases in EDV/

A

Distal disease can restrict the outflow - this causes the pressure gradient to remain positive throughout the whole cardiac cycle

46
Q

What are inertial forces?

A

The forces causing fluid to resist changes in direction or velocity
- The fluid tends to resist in proportion to its mass

47
Q

What is Newton’s second law?

A

F = ma

48
Q

What are inertial losses?

A

The energy losses when there is a change in direction or velocity of blood

49
Q

How does inertial energy loss compare with viscous energy loss?

A

Inertial energy loss is proportional to v**2 and viscous energy loss is just proportional to v
- hence inertial losses often exceed viscous

50
Q

What are streamlines?

A

Lines indicating the direction of flow of individual particles - tangents of flow

51
Q

When does plug flow occur?

A

In large arteries e.g. aorta

52
Q

What are the units for Reynolds number?

A

No units - they cancel

53
Q

What is Reynolds number dependent on?

A

Vessel diameter, density, velocity and viscosity

54
Q

What is the Reynolds equation?

A

Re = (Dpv) / u

55
Q

At what Reynolds numbers does the transition from laminar to turbulent flow occur?

A

2000 - 2500

56
Q

What is critical velocity?

A

The velocity above which turbulence is likely to occur

57
Q

How does critical velocity change with vessel diameter?

A

Smaller vessel diameter = bigger critical velocity
- unlikely to get turbulence in healthy, straight small vessel

58
Q

When can turbulence be seen in normal vessel?

A

When the vessel is acting as a collateral for other vessels e.g. the vertebral for an occluded ICA
- turbulence is due to high flow

59
Q

How does pulsatile flow impact turbulence?

A

Turbulence is more likely to occur with continuous flow when compared to pulsatile

60
Q

What is the boundary layer?

A

The layer of fluid adjacent to the vessel wall, and is influenced by the solid drag of this surface

61
Q

What is boundary layer separation?

A

When the boundary layer becomes detached
- can occur at low or high Reynolds numbers
- the boundary layer velocity may reverse
- can occur when flow decelerates when entering a wider vessel

62
Q

What is post-stenotic dilatation?

A

Turbulence causing vibrations on the vessel wall may increase its diameter

63
Q

How does flow through mild stenoses compare to severe?

A

Flow is still likely to be laminar in mild stenoses
However in severe stenoses, flow will become turbulent and losses will be almost entirely inertial

64
Q

Why is energy lost according to Bernoulli equation in stenoses?

A

Heat in turbulent flow

65
Q

What are entrance and exit losses?

A

Inertial losses due to the acceleration and deceleration of blood before and after stenoses

66
Q

What provides most of the energy in blood flow?

A

Fluid pressure

67
Q

What are the energy losses through a stenosis?

A

Entrance loss - inertial loss due to increase in speed
Viscous loss due to friction through the stenosis
Exit loss - inertial loss due to decrease in speed post-stenosis
- note exit loss is usually very large

68
Q

What accounts for most of the pressure drop through a stenosis?

A

Entrance and exit losses
- hence two localised stenoses are likely to cause greater energy drop than one long stenosis

69
Q

What percentage stenosis is needed to cause a significant pressure drop?

A

50% diameter reduction

70
Q

What is viscous diffusion?

A

The process of the boundary layer gradually growing away from the vessel wall

71
Q

What is the inlet length?

A

The distance along a narrowed tube that it takes for flow to become fully parabolic
- is also called entrance length

71
Q

What does inlet length depend on?

A

Vessel Diameter and Reynold’s number

71
Q

Why should you place Doppler sample volume at least 4 cm distal to any bifurcations?

A

This will avoid most distortions of the waveform caused by entrance effects

71
Q

What is a fluid jet?

A

The fast jet that occurs when blood goes from stenosis to a larger vessel
- the jet will show turbulence even at low Reynolds numbers (e.g. Re > 10)

72
Q

Why do eddies form?

A

Following stenosis, there is a free boundary layer between the fluid in the jet and the surrounding static fluid.
The shear flow along the boundary layer quickly becomes unstable and eddies form

72
Q

What is a boundary layer?

A

The layer of fluid adjacent to a vessel wall, where the vessel wall is asserting viscous forces on blood flow

72
Q

Why does the jet spread out in a cone shape post stenosis?

A

Small eddies along the boundary layer will transfer their energy to adjacent stationary fluid by viscous frictional forces

72
Q

What is entrainment?

A

The process of the jet spreading out like a cone shape post-stenosis

72
Q

What forces carry the energy in a jet forwards?

A

Inertial forces acting on the larger eddies

73
Q

What are the differences between small and large eddies?

A

Large eddies carry most of the energy of the jet, mainly through inertial forces
Small eddies form around the large eddies and mainly dissipate energy through viscous forces

73
Q

What forces need to be considered in a curved tube?

A

Centrifugal force which will push blood to the outside of the bend
- centrifugal forces are strongest on blood at highest velocity - is at the centre of the tube, centrifugal forces dominate pressure forces here

At the near side of the curve, viscous forces slow the fluid and pressure forces dominate.

This induces a circumferential motion along the edge of tube

73
Q

What happens to flow post-bifurcation?

A

Velocity increases - fast streamlines will be immediately adjacent to the vessel wall
- this causes formation of another boundary layer

74
Q

What is boundary layer separation?

A

Reverse flow close to the vessel wall that can cause eddies and vortices
- Occurs when the boundary layer has travelled far enough in an adverse pressure gradient

75
Q

In which branch of a bifurcation is flow separation most likely to occur in?

A

The smaller of the two branches

76
Q

What is a likely reason for why atheroma often builds up near to bifurcations?

A

Boundary layer and flow separation causes flow to be less mobile - causing regions of higher shear stress
- this can cause endothelial erosion
Hence, why plaque often builds up on the outer wall of the carotid bulb

77
Q

What is extramural pressure?

A

The pressure outside the vessel wall

78
Q

What is transmural pressure?

A

The difference between blood pressure and extramural pressure

79
Q

What does the absolute tension in vessel walls depend on?

A

Elasticity and any muscle tone within the wall

80
Q

What is Laplace’s law?

A

It states the relationship between vessel wall tension, transmural pressure and muscle tone

81
Q

What is the usual extramural pressure at rest?

A

Close to atmospheric pressure, so can be considered as zero

82
Q

What happens to extramural pressure as muscles contract?

A

This can increase extramural pressure and cause compression of the vessel wallsq

83
Q

How can respiration influence extramural pressure?

A

During inhalation it can decrease extramural pressure

84
Q

What 2 factors can cause the radius of a vessel to change?

A
  1. Change in transmural pressure
  2. If the stress in the vessel wall changes, through active contraction of smooth muscle or structural changes in elastic components of the wall
85
Q

Is a small or large vessel easier to increase radius?

A

Larger - think of blowing up balloon
- demonstrated by Laplace’s law

86
Q

What is strain?

A

The change in radius size divided by the original radius

87
Q

What is the effect of increasing smooth muscle tone in a vessel wall?

A

The wall becomes stiffer, so the stress for a given radius is increased

88
Q

What is circumferential stress?

A

The circumferential force per unit area exerted on the vessel wall

89
Q
A

Haemodynamics Book (5 decks)

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