blood pressure and flow measurements

8/2/2019 Blood Pressure and Flow Measurements

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Mohamad Nasser-EddinPh.D

Blood Pressure and FlowMeasurements


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Contents

Invasive

Non-Invasive

Blood Pressure Blood Flow

Palpatory Method(Riva-Rocci Method)

Auscultatory Method

Ultrasonic Method

Oscillometric Method

Tonometry

ExtravascularSensor

Intravascular Sensor

Ultrasound Doppler

Laser Doppler Flowmetry

Dye Dilution Method

Thermal Dilution Method

Strain Gage Plethysmography

Electric-Impedance Plethysmogr.

Photoelectric Plethysmography

Radioisotopes

Thermal Convection Probes

General on

System Parameters


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Blood Pressure (1)One of the oldest physiological measurements

Originates from the heart

Value depends on 3 factors:

cardiac output

diameter of arteries

the quantity of blood

Commonly refers to arterial blood pressure

Values should be lower than 120 / 80 mmHg(systolic pressure (SP) / diastolic pressure (DP))

peripheral

resistance

Observation of blood pressure allows dynamic tracking of pathology and

physiology affecting to the cardiovascular system, which has profound

effects to all other organs of the body


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Blood Pressure (2)

Pulse pressure (PP) = SP-DP

MP = DP+PP/3

Mean pressure (MP)

average pressure during one cardiac cycle

driving force of the peripheral perfusion.

an estimate can be done by using an empirical formula:

SP and DP may vary significantly throughout the arterial system but

MP is quite uniform (in normal situations)

High value increases the risk of heart attack and strokes

Low value increases the risk of lower oxygen perfusion e.g. in brains

However, the normal values differ from person to another


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Blood Pressure (3)


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Indirect Methodsin

Blood Pressure Measurements


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General Facts

Indirect measurement = non-invasive measurement

Brachial artery is the most common measurement site

Close to heart

Convenient measurement

Other sites are e.g.:

forearm / radial artery

wrist (tends to give much higher SP)

The most common indirect methods are auscultation

and oscillometry


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General Facts (cont.)

An occlusive cuff is placed on arm and inflated to P > SP.

Then the cuff is deflated gradually and the measurement of

blood flow is done

cuff

The occlusive cuff should be of a correct size in order totransmit the pressure to the artery evenly and thus to obtain

accurate results

A short cuff requires special attention in placement. Longer

cuff reduces this problem.

The cuff should be placed at the heart level in order to

minimize the hydrostatic effects


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Palpatory Method (Riva-RocciMethod)

When the cuff is deflated, there is a palpable

pulse in the wrist. P = BP

Several measurements should be done as the

respiration and vasomotor waves modulate theblood pressure levels

+) The blood pressure can be measured in noisy environment too

ADVANTAGES

-) Only the systolic pressure can be measured (not DP)

DISADVANTAGES

-) The technique does not give accurate results for infants and

hypotensive patients

cuff

+) Technique does not require much equipment
http://images.google.fi/imgres?imgurl=http://image3.greetingcards.com/dgc/i/c/isp_SaddieFace.jpg&imgrefurl=http://www.greetingcards.com/d/greeting_cards/card_306_805.html&h=500&w=500&sz=27&tbnid=lkSeb59upRUxUM:&tbnh=127&tbnw=127&hl=fi&start=5&prev=/images%3Fq%3Dsad%2Bface%26svnum%3D10%26hl%3Dfi%26lr%3Dhttp://images.google.fi/imgres?imgurl=http://www.e-partner.com/images/03-2005-PU-Picture/MB003-C%252070mm%2520Happy%2520Face.jpg&imgrefurl=http://www.e-partner.com/MB003-C-squueze%2520-toy.html&h=230&w=230&sz=8&tbnid=XQq8SLNZOj1rJM:&tbnh=103&tbnw=103&hl=fi&start=11&prev=/images%3Fq%3Dhappy%2Bface%26svnum%3D10%26hl%3Dfi%26lr%3D


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Auscultatory Method

The Korotkoff sounds are

ausculted with a stethoscope ormicrophone (automatic measurement)

Pulse waves that propagate

through the brachial artery,

generate Korotkoff sounds.

There are 5 distinct phases in the

Korotkoff sounds, which define SP

and DP

Also with this method, several measurements

should be done.

The frequency range is 20-300 Hz

and the accuracy is +/- 2mmHg (SP)

and +/- 4mmHg (DP)


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Auscultatory Method (cont.)

-) The observations differ from observer to another

-) A mechanical error might be introduced into the system e.g. mercuryleakage, air leakage, obstruction in the cuff etc.

-) The observations do not always correspond with intra-arterial

pressure

-) Auscultatory tecnique cannot be used in noisy environment

+) Auscultatory technique is simple and does not require much

equipment

ADVANTAGES

DISADVANTAGES

-) The technique does not give accurate results for infants and

hypotensive patients
http://images.google.fi/imgres?imgurl=http://www.e-partner.com/images/03-2005-PU-Picture/MB003-C%252070mm%2520Happy%2520Face.jpg&imgrefurl=http://www.e-partner.com/MB003-C-squueze%2520-toy.html&h=230&w=230&sz=8&tbnid=XQq8SLNZOj1rJM:&tbnh=103&tbnw=103&hl=fi&start=11&prev=/images%3Fq%3Dhappy%2Bface%26svnum%3D10%26hl%3Dfi%26lr%3Dhttp://images.google.fi/imgres?imgurl=http://image3.greetingcards.com/dgc/i/c/isp_SaddieFace.jpg&imgrefurl=http://www.greetingcards.com/d/greeting_cards/card_306_805.html&h=500&w=500&sz=27&tbnid=lkSeb59upRUxUM:&tbnh=127&tbnw=127&hl=fi&start=5&prev=/images%3Fq%3Dsad%2Bface%26svnum%3D10%26hl%3Dfi%26lr%3D


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Ultrasonic MethodA transcutaneous (through the skin)

Doppler sensor is applied here.

The motion of blood-vessel walls

in various states of occlusion is

measured.

The frequency difference between

transmitted (8 MHz) and received

signal is 40-500 Hz and it is

proportional to velocities of the wall motion and the blood.

The vessel opens and closes with

each heartbeat when

DP < P < SPcuff


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Ultrasonic Method (cont.)

+) Can be also used in noisy environment

ADVANTAGES & DISADVANTAGES

+) Can be used with infants and hypotensive individuals

-) Subjects movements change the path from sensor to vessel

As the cuff pressure is increased, the time between opening and closing

decreases until they coincide Systolic pressure

Again as the cuff pressure is decreased, the time between opening andclosing increases until they coincide Diastolic pressure
http://images.google.fi/imgres?imgurl=http://www.e-partner.com/images/03-2005-PU-Picture/MB003-C%252070mm%2520Happy%2520Face.jpg&imgrefurl=http://www.e-partner.com/MB003-C-squueze%2520-toy.html&h=230&w=230&sz=8&tbnid=XQq8SLNZOj1rJM:&tbnh=103&tbnw=103&hl=fi&start=11&prev=/images%3Fq%3Dhappy%2Bface%26svnum%3D10%26hl%3Dfi%26lr%3D


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Oscillometric Method

http://colin-europe.com/docpdfdemos/oscillo0104.wmv

The intra-arterial pulsation is

transmitted via cuff to transducer

(e.g. piezo-electric)

The arterial pressure oscillations(which can be detected throughout the

measurement i.e. when P > SP and

P < DP) are superimposed on the

cuff pressure

SP and DP are estimated from the amplitudes of the oscillation by using a

(proprietary) empirical algorithm.

cuff

cuff

The cuff pressure is deflated

either linearly or stepwise
http://colin-europe.com/docpdfdemos/oscillo0104.wmvhttp://colin-europe.com/docpdfdemos/oscillo0104.wmvhttp://colin-europe.com/docpdfdemos/oscillo0104.wmv


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Oscillometric Method (cont.)

DISADVANTAGE

-) Many devices use fixed

algorithms leading to

large variance in blood

pressures

+) In the recent years,

oscillometric methods have

become popular for their

simplicity of use andreliability.

ADVANTAGES

+) MP can be measured

reliably even in the case of

hypotension
http://www.biomedical-engineering-online.com/content/4/1/49/figure/F1?highres=y


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Tonometry

A sensor array is used here, because at

least one of the pressure sensors must lay

directly above the artery

Linear array of pressure sensors is pressed

against a superficial artery, which is

supported from below by a bone (radial artery).

The pressure is increased continuously andthe measurements are made when the

artery is half collapsed

When the blood vessel is partly collapsed,

the surrounding pressure equals the artery

pressure.

The hold-down pressure varies between

individuals and therefore a calibration must

be done


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Tonometry (cont.)

ADVANTAGES

+) Can be used fornon-invasive, non-painful, continuous measurement

DISADVANTAGES

-) Relatively high cost

-) The wrist movement and tendons result

in measurement inaccuracies


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Direct Methodsin

Blood Pressure Measurements


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General FactsDirect measurement = Invasive measurement

Used only when essential to determine the blood pressure continuously

and accurately in dynamic circumstances

A vessel is punctured and a catheter(a flexible tube) is guided in

The most common sites are brachial and radial arteries

but also other sites can be used e.g. femoral artery

A division is made into extravascular and

intravascular sensor systems

This method is precise but it

is also a complex procedureinvolving many risks.


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Extravascular Sensor

The sensor is located behind the

catheter and the vascular pressure

is transmitted via this liquid-filled

catheter.

The normal measuring system

The actual pressure sensor can be

e.g. strain gage

variable inductance

variable capacitance

optoelectronicpiezoelectric, etc


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Extravascular Sensor (cont.)The hydraylic link is the major source of errors. The systems naturalfrequency may be damped and degraded due (e.g.):

too narrow catheter

too long tubing

various narrow connections

air bubbles in the catheter

The catheter-sensor system must be

flushed with saline-heparine solution

every few minutes in order to prevent

blood from clotting at the tip.

.

p

R L

C

lichidcateter

senzor

diafragma

V

lungimeincrementala

c s

d

R c R cc

c C C C

Lc Lc

c c

R L

C

s

s

= Vp

(a)

(b)


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Extravascular Sensor (cont.)

Normally the interesting frequency range is 0 100 Hz.

If only MP is measured the bandwidth is 20 Hz (harmonics > 10 are ignored)


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Intravascular SensorThe sensor is located in the tip of the catheter. This way the hydraulicconnection is replaced with an electrical or optical connection

+) The frequency response is not

limited by the hydraulic properties of

the system. No time delay.

-) Breaks easily

-) More expensive

+) Electrical safety and isolation when

using fiber optics

The dispacement of the diaphragm is

measured


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Disposable SensorsDisposable sensors decrease the risk of patient cross-contamination andreduce the amount of handling by hospital personnel

Cheaper and more reliable than reusable pressure sensors

General on System ParametersEven minute air bubbles in catheter have a dramatic effect on frequency

response

The natural frequency and the length of the catheterhave a following relationship:

The catheter diameter has a linear relationship

to natural frequency

Lfn

1=

Stiffer catheters have a higher frequency response

Teflon

Polyethylene

Silicon rubber

BETTER

WORSE


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Indirect Methodsin

Blood Flow Measurements


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Blood Flow

O and other nutrition concentration in the cells are

one of the primary measurements.2

Blood flow helps to understand basic physiological

processes and e.g. the dissolution of a medicine into

the body.

Usually the blood flow measurements are more

invasive than blood pressure measurements / ECG

It also helps to understand many pathological conditions,

since many diseases alter the blood flow. Also the blood

clots in the arterial system can be detected.


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Blood Flow (2)

Normal blood flow velocity 0,5 m/s 1 m/s (Systolic, large vessel)


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Doppler Measurements (1)Ultrasound Doppler

The blood cells in the fluid scatter

the Doppler signal diffusively.

In the recent years ultrasoundcontrast agents have been used in

order to increase the echoes.

c

vff cd 2=

f = 2 10 MHzc

c = 1500 - 1600 m/s (1540 m/s)

f = 1,3 13 kHzd

The ultrasound beam is focused bya suitable transducer geometry and

a lens


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The flow velocity is obtained from

the spectral estimation of the

received Doppler signal

In order to know where along the beam

the blood flow data is colledted, a pulsed

Doppler must be used


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The ultrasound Doppler device can be eithera continuous wave ora pulsed

Doppler

CW DOPPLER PULSED DOPPLER

Range ambiguity

Low flow cannot be

detected

No minimum range

Simpler hardware

Minimum range

Accuracy

No minimum flow

(Maximum flow) x (range)

= limited


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GENERAL PARAMETERS

the power decays exponentially because of the heating of the tissue.

The absorption coefficient ~ proportional to frequency

the far field operation should be avoided due to beam divergence.

1

1Ddnf = D = Transducer diameter (e.g. 1 5 mm)

the backscattered power is proportional to f4

the resolution and SNR are related to the pulse duration. Improving either

one of the parameters always affects inversely to the other


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Doppler Measurements (5)Laser Doppler Flowmetry

The principle of measurement is the

same as with ultrasound Doppler

The laser parameter may have e.g.

the following properties:

5 mW

He-Ne-laser

632,8 nm wavelength

The moving red blood cells causeDoppler frequency 30 12 000 Hz.

The method is used for capillary

(microvascular) blood flow

measurements


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Direct Methodsin



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Indicator Dilution Methods (1)Dye Dilution Method

A bolus of indicator, a colored dye (indocyanine green), is rapidly injected in

to the vessel. The concentration is measured in the downstream

The blood is drawn through a colorimetric cuvette

and the concentration is measured using the

principle of absorption photometry

( )[ ] dttC

mF

t

=

1

0

amount ofdye

1% peak CAvg.

flow


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( ) dttTc

QF

t

bbb =

1

1

Indicator Dilution Methods (2)Thermal Dilution Method

A bolus of chilled saline solution is

injected into the blood circulation system

(right atrium). This causes decrease in the

pulmonary artery temperature.

A standard technique for measuring cardiac output in critically ill patients

An artery puncture is not needed in this technique

heat contentof injectate

Several measurements can be done in relatively short time

density of blood

(e.g. 1060 kg/m3)

specific heat of blood

(e.g. 3640 J/(kg*K)


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Plethysmographyin



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Plethysmography (1)Strain Gage Method

Plethysmographymeans the methods for recording volume changes of an

organ or a body part (e.g. a leg)

Strain gage is made of silicone rubber

tubes, which are filled with conductive

liquid (e.g. mercury) whose impedancechanges with volume.

Venous occlusion cuff is inflated to 40

50 mmHg. In this way there will be

the arterial inflow into the limb but novenous outflow.

If only a segment of limb is measured, there is a need for arterial occlusion

cuff also.


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Plethysmography (2)Chamber Method

Chamber plethysmograph is the only accurate non-invasive way to

measure changes in the blood volume

As the volume of the leg increases, the leg

squeezes some kind of bladder and decreases

its volume

Volume transducer can be e.g. water filled tube

(level) or air (pressure)

The speed of the return of the

venous blood is measured


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Plethysmography (3)Electric-Impedance Method

Different tissues in a body have a different resistivity. Blood is one of the best

conductors in a body ( = 1,5 m)

A constant current is applied via

skin electrodes

The change in the impedance is

measured

The accuracy is often poor or unknown

I = 0,5 4 mA rms (SNR)

f = 50 100 kHz

(Zskin-electrode+shock)

ZZ

LVol =

2

0

2


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Plethysmography (4)Photoelectric Method

A beam of IR-light is directed to the

part of the tissue which is to be

measured for blood flow (e.g. a finger

or ear lobe)

The light that is transmitted / reflected iscollected with a photodetector

The blood flow modulates the

attenuated / reflected light which is

recorded.

Poor measure for changes in volume

Very sensitive to motion artefacts

Method is simple

Heart rate is clearly seen


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Other Methodsin



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Radioisotopes

A rapidly diffusing, inert radioisotope of lipid-soluble gas ( Xe or Kr)is injected into the tissue or passively diffused

133 85

The elimination of the radioisotope from microcirculatory bed is related to

the blood flow:( )ktCtC = exp)( 0

2/1/2ln tk=


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Thermal Convection Probe

The rate of heat removal from the tissue under probe is measured

The concentric rings are

isolated thermally & electrically

from each other

The central disk is heated

1 2 C over the temperature

of tissue

A temperature difference of

2- 3 C is established betweenthe disks

o

o

The method is not very common due extreme nonlinear properties and difficulties

in practical use (e.g. variable thermal characteristics of skin)

This is one of the earliest techniques for blood flow measurements


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Summary (1)

BLOOD PRESSURE

Describes the physiology and pathology of cardiocvascular system

Normal values are 120 / 80 mmHg

High values may lead to heart attack and strokes

Low values may lead to low oxygen perfusion

All direct methods require skin punctuation and a use of

catheter. Methods are used only when continuous and

accurate measurements are needed.

Almost all indirect methods rely on an occlusive cuff

which is placed on the bracial artery. The actual

measurement is done when the cuff is deflated


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Summary (2)

BLOOD FLOW

Used for understanding physiological processes (e.g. medicine

dissolution). Also used for locating clots in arteries

Usually more invasive methods are used than with blood

pressure measurements

Normal velocity is 0,5 - 1 m/s

Indirect measurements are done by using ultrasound or

plethysmographic method

Direct measurements are done by dilution methods (dye / thermal)

blood pressure and flow measurements

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