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Vascular Ultrasound. Fluid Hemodynamics. Fluid Hemodynamics. Blood flow influenced by Cardiac function Elasticity of the vessel Tone of vascular smooth muscle Dimension, pattern and interconnection of branching vessels. Fluid Hemodynamics.

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vascular ultrasound

Vascular Ultrasound

Fluid Hemodynamics

fluid hemodynamics
Fluid Hemodynamics

Blood flow influenced by

  • Cardiac function
  • Elasticity of the vessel
  • Tone of vascular smooth muscle
  • Dimension, pattern and interconnection of branching vessels.
fluid hemodynamics3
Fluid Hemodynamics
  • In order for blood to flow between two there has to be a difference in the energy level between these two points.
  • This difference in energy level is translated in a difference of pressure.
circulatory system
Circulatory System

Arterial Reservoir:

- High energy

- High pressure

Venous Pool

- Low energy

- Low pressure

energy level
Energy Level

The large volume of blood entering the

arterial reservoir is responsible for the

high energy level.

the heart as a pump
The Heart as a Pump

The function of the heart and blood

vessels is normally regulated to maintain

volume and pressure in the arteries within

the limits required for smooth function.

the heart as a pump7
The Heart as a Pump

To achieve this there must be a balance

between the amount of blood that enters

and the amount of blood that leaves the

arterial reservoir

arterial reservoir
Arterial Reservoir
  • Amount of blood entering – depends on the CO
  • Amount of blood leaving – depends on the AP and the TPR
total peripheral resistance
Total Peripheral Resistance
  • Controlled by the amount of vasoconstriction in the microcirculation.
determinant of blood flow
Determinant of Blood Flow

Flow to all the body tissues is adjusted according to

the tissues’ particular need at that instance.

  • High need – Vasodilatation
  • Low need - Vasoconstriction
energy in flowing blood
Energy in Flowing Blood

Main form is Potential Energy

  • Created by the pumping action of the heart.
  • Pressure distending the vessel.
kinetic energy
Kinetic Energy
  • Small compared with PE
  • Proportional to its density and square of its velocity.
kinetic energy13
Kinetic Energy


  • High flow states (exercise)
  • Stenotic lesion
energy pressure differences
Energy/Pressure Differences

Hydrostatic Pressure

  • Pressure that depends on the weight of column of blood resting on the blood at that level.
  • Increases in the more dependent part of the body.
hydrostatic pressure
Hydrostatic Pressure

As the hydrostatic pressure increases:

  • The transmural pressure increases
  • The vessel become more distended.
gravitational potential energy
Gravitational Potential Energy
  • Gravitational PE is reduced in the dependent part of the body by the same amount of the increase, resulting from the hydrostatic pressure.
fluid hemodynamics17
Fluid Hemodynamics
  • Therefore differences in the level of the body parts usually do not lead to changes in the driving pressure along the vascular tree unless the column of blood is interrupted.
laminar flow
Laminar Flow

Flow in which blood moves in concentric


parabolic velocity profile
Parabolic Velocity Profile
  • Laminar flow
  • Highest velocity is in the middle of the vessel
  • Gradual decrease in velocity as you move towards the walls of the vessel.
parabolic velocity profile20
Parabolic Velocity Profile
  • The rate of change of velocity is great near the vessel wall and decreases towards the center
  • Mean velocity is half the maximum velocity.
frictional losses
Frictional Losses

Loss of energy during blood flow is:

  • Due to friction
  • Is determined by the vessel dimension.
poiseuille s law and equation
Poiseuille’s Law and Equation

Describes flow in a cylindrical tube model. The

mean linear velocity of laminar flow is

  • Proportional to the pressure difference between the end of the tube.
poiseuille s law and equation23
Poiseuille’s Law and Equation
  • Proportional to the radius squared.
  • Inversely proportional to the length of the tube.
  • Inversely proportional to the viscosity of the fluid
poiseuille s law and equation24
Poiseuille’s Law and Equation

V α(P1- P2) x r2

L h

V = Mean linear velocity

Q = V x CSA = V x pr2

poiseuille s law and equation25
Poiseuille’s Law and Equation

Q a (P1- P2) x r2 x pr2 = p (P1- P2) x r4

L h 8Lh

8Lh = P1- P2

pr4 Q

poiseuille s law and equation26
Poiseuille’s Law and Equation

R = P1- P2


Q = P1- P2



Vessel Interconnection

Resistance to flow is influenced by the presence of

numerous interconnected vessels.


Vessels in series:

RT = R1 + R2 + R 3 + …………….. Rn


Vessels in parallel:

1 = 1 + 1 + 1 + ………. 1

RT R1 R2 R3 Rn


Thus the contribution of any single vessel to the total

resistance of a vascular bed, or the effect of a change in

the dimension of a vessel, depends on the presence and

relative size of the other vessels linked in series or


non laminar flow
Non Laminar Flow

Occurs due to

  • Changes in flow velocity during the cardiac cycle.
  • Alteration in lines of flow whenever vessel changes dimensions.
  • Distortion of line of flow at curves, bifurcation and branches

The factors that affect the development of

turbulence are expressed by the dimensionless

Reynolds number (Re):

Re = vq2r


h = viscosity

q = density

r = radius of the tube

v = velocity

  • In the tube model turbulence occurs at Re > 2000
  • In the circulatory system, turbulence occur at Re < 2000 due to:
  • Body movement
  • Pulsatile nature of the blood
  • Changes in vessel dimension
  • Roughness of the endothelial surface.
pressure wave
Pressure Wave
  • Ventricular contraction leads to

- stroke volume.

  • Stroke volume leads to

- pressure wave.

pressure wave35
Pressure Wave
  • The pressure waves changes as it traverses the arterial system.
  • There are changes in its:
  • Shape
  • Speed
  • Amplitude
ventricular contraction
Ventricular Contraction
  • 2 Phases:
  • Rapid phase
  • Late Phase
ventricular contraction37
Ventricular Contraction

Rapid Phase:

  • Outflow through the peripheral resistant vessel is less than volume ejected by the heart.
  • Increased volume in the arterial end.
  • Increased pressure to systolic peak.
ventricular contraction38
Ventricular Contraction

Late Phase

  • Decrease in cardiac ejection
  • Outflow through the peripheral resistant vessel is greater than volume ejected by the heart.
  • Decrease in pressure.
ventricular contraction39
Ventricular Contraction

Cardiac contraction leads to:

  • Forward flow
  • Distension of the arteries
ventricular contraction40
Ventricular Contraction
  • The distention of the arteries serve as a reservoir for storing blood volume and energy.
  • This is responsible for the continuous flow to tissue during diastole.
pressure wave41
Pressure Wave

The shape and amplitude are affected by

  • Stroke volume
  • Time course of ventricular ejection
  • Peripheral resistance
  • Stiffness of the arterial wall
pressure wave42
Pressure Wave

An increase in any of these factors

will lead to an increase in the pulse

amplitude and the systolic pressure

propagation speed
Propagation Speed

Depends on

  • Stiffness of the arterial wall
  • Ratio of the wall thickness to diameter.
propagation speed44
Propagation Speed
  • In circulation, the arteries become progressively stiffer from aorta toward the periphery.
  • Therefore the speed of propagation increases as it moves peripherally.
pressure wave46
Pressure Wave
  • Changes in pressure as wave travels from aorta to small limb arteries.
  • Slight decrease in MAP
  • Minor changes in DAP
  • The amplitude and SAP increases (Systolic amplification).
pressure wave47
Pressure Wave

Systolic Amplification is due to

  • Increase stiffness
  • Presence of reflected waves:

- changes in vessel diameter

- dividing branches

pulsatility changes
Pulsatility Changes

Pulsatility changes

Small and Medium arteries

  • Vasoconstriction leads to increased pulsatilty.
  • Vasodilatation leads to decreased pulsatility.
pulsatility changes49
Pulsatility Changes

Minute arteries, arterioles and capillaries

  • Vasoconstriction leads to decreased pulsatilty.
  • Vasodilatation leads to increased pulsatility.
flow reversal
Flow Reversal

Reversal of flow due to

  • Pressure gradient
  • Arterial branches supplying both high and low resistance vascular area.
pressure gradient
Pressure Gradient

Pressure gradient between two arteries

changes due to difference in the

  • Shape
  • Magnitude
  • Time of arrival

of the pressure wave.

critical stenosis
Critical Stenosis
  • Stenosis that causes a reduction in flow and pressure.
critical stenosis55
Critical Stenosis

Whether a hemodynamic abnormality result

from a stenosis and how severe it may be

depends on:

  • The length and diameter of the narrowed segment.
  • The roughness of the endothelial surface
  • The degree of irregularity of the narrowing and its shape
critical stenosis56
Critical Stenosis
  • The ratio of the CSA of the narrowed segment to the normal vessel.
  • The rate of flow.
  • The arterio-venous pressure gradient.
  • The peripheral resistance beyond the stenosis
pressure change
Pressure Change


  • A decrease in systolic BP is a sensitive index of reduction in both the mean pressure and the amplitude of the pressure wave distal to a minor stenosis.
pressure change58
Pressure Change


  • The diastolic pressure does not fall until the stenosis is quite severe.
fluid hemodynamics59
Fluid Hemodynamics

Other changes distal to the stenosis include

  • Damping of the waveform
  • Increased time to peak
  • Greater width of the wave at half-amplitude.
changes in blood flow
Changes in Blood Flow

Blood flow can be normal in the presence

of severe stenosis due to:

  • Collateral circulation
  • Compensatory decrease in PR.
changes in blood flow63
Changes in Blood Flow

Resting blood flow decreases only

  • Acute occlusion
  • Extensive chronic arterial obstruction with 2 or more lesions in series.
changes in blood flow65
Changes in Blood Flow

The inability for an increase in blood flow during exercise is due to the fact that the sum of the resistances of the obstructions (stenosis, collateral resistance, or both) and the peripheral resistance prevent a normal increase in flow.

velocity changes
Velocity Changes

Proximal to the stenosis

  • Increased pulsatility.
  • Decreased diastolic flow
velocity changes67
Velocity Changes

At to the stenosis

  • Increased systolic velocity
  • Increased diastolic velocity
  • Spectral broadening
  • Decrease velocity.
velocity changes68
Velocity Changes

Distal to the stenosis

  • Turbulence
  • Damped waveform
  • Disappearance of the flow reversal
  • Decrease pulsatility
disappearance of flow reversal
Disappearance of Flow Reversal
  • Maintenance of high level of forward flow throughout the cardiac cycle because of the pressure gradient across the stenosis
  • Resistance to reverse flow created by the stenotic lesion.
disappearance of flow reversal70
Disappearance of Flow Reversal
  • A decrease in PR as a result of relative ischemia.
  • Damping of the pressure wave by the lesion resulting in attenuated pressure pulses which are less subject to reflections and amplifications.