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8/2/2019 Blood Pressure and Flow Measurements
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Mohamad Nasser-EddinPh.D
Blood Pressure and FlowMeasurements
8/2/2019 Blood Pressure and Flow Measurements
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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%3D8/2/2019 Blood Pressure and Flow Measurements
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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%3D8/2/2019 Blood Pressure and Flow Measurements
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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%3D8/2/2019 Blood Pressure and Flow Measurements
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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.wmv8/2/2019 Blood Pressure and Flow Measurements
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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=y8/2/2019 Blood Pressure and Flow Measurements
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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
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%3D8/2/2019 Blood Pressure and Flow Measurements
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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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Doppler Measurements (2)Ultrasound Doppler
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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Doppler Measurements (3)Ultrasound Doppler
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
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%3D8/2/2019 Blood Pressure and Flow Measurements
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Doppler Measurements (4)Ultrasound Doppler
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
Blood Flow Measurements
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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)
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%3D8/2/2019 Blood Pressure and Flow Measurements
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Plethysmographyin
Blood Flow Measurements
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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
Blood Flow Measurements
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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)