Pulse wave velocity ssid02688 03

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Clinical Innovation:

Pulse Wave Velocity

Aixplorer®

What are the known cardiovascular risks?

The most common causes and risk factors

Reliable predictors

The arterial pressures

The arterial wall stiffness

Arterial pulse wave

Arterial stiffness E and c velocity of the arterial pulse wave

Ultrafast imaging and the wall motion

The wall motion and the velocity of the arterial pulse wave

User interface: PWV mode through images

Main stiffness evaluation by competition

The cardiovascular risks

Measuring the Pulse Wave Velocity with Aixplorer®

Conclusion

The Cardiovascular risks

The cardiovascular risks

Cardiovascular diseases (CVD) are the major cause of deaths worldwide:

• 17.3 million people died from CVDs in 2008 (30% of all deaths) • Over 80% of CVD deaths in low- and middle-income countries. • By 2030, 23.6 million people will die from CVDs. • WHO South-East Asia Region, CVD cause 3.6 million deaths/year (1/4 all deaths). • Europe: 49% of all deaths; estimated cost €169 billion/year

• They involve the heart and the bloods vessels

• They can affect the brain, the heart, kidneys, the abdomen...

The most common causes and risk factors

Amongst several risk factors: ageing, smoking, cholesterol, blood pressure,

diet

Their consequences: Atheroma, distensibility of the vessels, blood flow

decrease, aneurysm, high arterial pressure

Reliable predictors

I. The arterial pressures

1. Definitions

Systolic peak pressure: P1 (comes earlier with ageing)

Semi-delayed systolic peak: SP

Difference between the 2 pressures: DP

Diastolic pressure: DP

Mean pressure: MP= DP + 1/3 PP= Q. R

with Q the blood flow and R the resistivity of the arterial

system

PP

Pulsed pressure: PP = SP –DP

It seems to be the best preditor of the state of the big arterial vessels and of the left ventricle.

PP depends on the speed of the LVE, on the peripheral resistances, and on the mainly on the stiffness of the aortic tree.

120 mmHg

60 mmHg

2. Measurements in different places along the vasculature

• Aorta: central pressure, the most representative of the

cardiovascular risk, but invasive method, very rarely used

• Carotid artery: results are supposed to be the closest to aortic

measurements

• Brachial artery: the most commonly used, because easy, non-

invasive, long history (Chinese practice)

• Femoral artery

Raw measurements performed in different places cannot be compared. They need to be transformed if one wants to compare them.

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Reliable predictors

II. The arterial wall stiffness

Arterial wall

stiffness

Blood Pressure

• Blood pressure (and PP) affects the wall stiffness during the

cardiac cycle: ongoing phenomenon

(non-linearity of vessel wall)

• But the global vessel wall stiffness affects the way the blood

pressure changes (Pulsed Pressure) during the cardiac cycle (the

stiffer the vessel, the higher the PP)

• Knowing the PP (brachial measurement) is not enough.

• Assessing the stiffness of the vessel wall during the cardiac

cycle may bring additional information to the clinician, as

another predictor of cardiovascular impairment.

Reliable predictors

II. The arterial wall stiffness

• Vessel wall stiffness seems to increase due to several factors:

• Natural ageing of the wall

• Atheromateous disease

• Hemodynamic changes

• Increased vessel wall stiffness also becomes a cardiovascular risk:

• Hypertrophy of the LV

• Vessel wall structural lesions

• Expected benefits

• Knowing the Pulsed Pressure and the vessel Wall Stiffness may have

a high potential in enabling the clinician to distinguish between causes

and consequences of a given cardiovascular impaired status.

• It may also help in assessing the efficacy of cardiovascular drug

treatments.

Reliable predictors

II. The arterial wall stiffness

• At each contraction of the heart, the left

ventricle sends 40% of the volume of blood

ejected towards periphery.

This is called the systole.

• Because of the contracting heart, the blood

flow running in the arteries takes the form

of a pulsed flow.

• It is described as a pulse wave,

propagating from hearth towards periphery

with a velocity depending on physical

properties of the vessels, and

especially on their stiffness.

NB: At every arterial bifurcation, reflected

pulse wave propagates backwards and mixes

with the original, incident pulse wave.

From the heart

towards periphery

The arterial pulse wave

During the systole:

increase of the intravascular pressure,

expansion of the wall of the artery,

energy storage.

During the diastole:

basal shape recovered, delivery of the

stored energy, decrease of the

intravascular pressure.

The stiffness of the arterial wall regulates

the blood flow, i.e. the pulse wave.

The stiffer the arterial wall, the faster the

pulse wave.

The Pulse Wave Velocity is a relevant indication of arterial stiffness.

Arterial stiffness and velocity of the arterial pulse wave

Measuring the Pulse Wave Velocity

with Aixplorer®

• Thanks to the UltraFast™ Imaging technology, available on Aixplorer®, the vessel walls can be imaged in B-mode at 2,000 Hz (i.e. 2,000 flat acquisitions per second).

• Aixplorer® uses Tissue Doppler Imaging algorithms on the B Mode acquisitions to calculate the speed at which the diameter of the vessel is enlarging or reducing.

• With TDI, both the mean velocity of deformation and its direction, towards or away from the probe, can be retrieved.

Electrocardiogram of the sane volunteer

UltraFast™ Imaging and the wall motion (1/2)

x

x(mm)

color code: v in mm/s

time (s)

Direction of blood flow

A given color shows the group of points with a given velocity reported to time.

Ultrafast imaging and the wall motion (2/2)

x(mm)

c = Dx /Dt

PWV ~ 5 m/s (ES) and 6 m/s (LS)in the carotid

2 points of the wall have the same velocity with a time shift Dt

This time shift is the time needed by the arterial pulse wave to travel from the first point to the second one.

The velocity of the pulse wave is C

x x

.

Dx

Dt

t

A

A’

v

v

The wall motion and the velocity of the arterial pulse wave

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t

V (cm/s) C (m/s)

x

x(mm)

color code: v in mm/s

time (s)

Direction of blood flow

ES

Ultrafast imaging and the wall motion (2/2)

x(mm)

We measure the PWV at 2 different times of the cardiac cycle: • When the vessel diameter is enlarging the fastest: fastest increase

of blood pressure => Beginning of the systole • When the vessel diameter is decreasing the fastest: fastest

decrease of the blood pressure => End of the systole

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PWV through images

Acquisition

Application, preset, probe

Select Vascular / Carotid with SL15-4 or SL10-2

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• The walls have to be as parallel as

possible to the probe (the intima-media

must be very clearly seen)

• Scanning plane: make sure to scan

following a diameter of the artery

• Avoid the bifurcation

✖

On the Touchscreen, press PWV

(or "S" shortcut if configured)

Acquisition time: 2s

No movement is required during the acquisition

Select the B-Mode

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2

After the acquisition

New page of the Touchscreen

After the acquisition

The B-Mode image on the monitor

Segmentation

At the bottom of the monitor

If the segmentation is not adequate, change the position and/or the size of the box.

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Values of the velocity

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Press Select on the control panel.

The map of the wall velocities over time is displayed as well as the values

velocity at the beginning of the systole and at the end of the systole.

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Other techniques to evaluate PWV

Competition

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Measure of the Pulse Pressure

• The PP is well correlated with the stiffness.

• Method : aplanation tonometry.

• At the wrist, the radial artery is slightly aplaned with a

micromanometer-typed probe (pencil probe).

• The measured and registered pressure is equal to the

transmural pressure.

• The brachial pressure at the left arm is registered for the

calibration of the tonometer and a transfer function gives the

aortic PP

SphygmoCor Requires calibration with a sphygmometer

Gives a measurement of the PP, and of the PWV

Simple and robust technique

Dedicated apparatus and learning curve needed

High quality of the transducer is needed

Difficult with high BMI

Issue with estimation of D

Not a local technique

Measurement of the PWV

CompliorSP

• Measures the transit time of the

pulse wave between to distant

pressure sensors and the distance

between the 2 selected points

• PWV = D/T and PW≈ (E)1/2

• Gives an estimation of the aortic

stiffness

Technique called "Echotracking", based on the Doppler effect.

Measurements needed:

• The diameter of the vessel: D

• The variations of D

• The thickness of the artery

• The brachial pressure

Calculates the PWV based on a relationship from Moëns et Korteweg (1878):

c= (E h0 /r D0)1/2

Conditions of validity of the relationship are not well satisfied.

Local measure of the PWV

Local measurement Non direct Hypotheses

Esaote

SSI PWV Echotracking Complior

Direct measure

Local

Easy to perform

Aortic PWV

Clinical value New tool Publications Gold standard

- We have a new tool that combines most advantages of other techniques

- One problem: lack of some clinical feedback (new tool)

- Solutions:

On carotid: correlation study with echotracking and ShygmoCor (ongoing)

Comparison with our new feature

Conclusion

• Quick acquisition that can be performed in less

than 1 min at the end of a conventional

vascular Doppler exam

• Direct measurement (time of flight)

• Easy to perform (with a good experience in carotid scanning)

• Has the potential to become a new tool to be used by radiologists

New tool for the clinicians to measure PWV