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M F Pimentel
Unit-4
Blood Pressure Measurement
August 26, 2019Blood Pressure - Biomedical Signal Processing
Page 1
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 2
Outline
Short history
Definition of Blood Pressure
What is hypertension?
Various methods used for measurements of blood
pressure
How often should we measure blood pressure?
Sampling and Aliasing
New resources – An Oxford University Project
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 3
The first to measure BP
1733 - Reverend Stephen Hales first to measure BP
Measured the height of a column of blood after cannulating the carotid artery in a horse with a brass pipe.
I’m that
good!!!
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 4
The first measurement of BP
The brass pipe was attached to a 12 inch glass tube
Tube was connected to the pipe via a trachea of a goose
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 5
A second try…
100 years later: a mercury manometer connected to the artery was used to measure the BP
However:
Both of these methods are invasive
… And maybe… a
bit uncomfortable
for the patient!
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 6
Eureka!
In late 1890s, an Italian physician – Scipione Riva Rocciinvented the Sphygmomanometer
Shygmo (from the Greek) => pulse
Manometer => pressure meter
- Consisted of a mercury column Manometer, a rubber sleeve whichis filled with air, and a squeeze bagto inflate the rubber sleeve I had an
idea!
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 7
Nowadays BP monitors
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 8
What is Blood Pressure?
Blood Pressure is a measurement of the force against the walls of the arteries as the heart pumps blood throughout the body
Blood pressure is measured in mmHg (millimeters of mercury)
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 9
What is Blood Pressure?
Blood Pressure is a measurement of the force against the walls of the arteries as the heart pumps blood throughout the body
Blood pressure is measured in mmHg (millimeters of mercury)
1,000 Pa is about 7 mmHg!
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 10
What is Blood Pressure?
The blood pressure reading is taken in 2 numbers:systolic and diastolic.
Systolic
Diastolic
Measure of
pressure as the
heart is beating
Measure of
pressure while
the heart is at rest
between beating
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 11
Hypertension
Blood pressure changes all the time:
It decreases when we sleep, or when we are at rest
It increases when we are active, excited, practicing sports, stressed and nervous
Changing blood pressure is a vital part of a healthy cardiovascular system
Hypertension, or high blood pressure, happens when the blood pressure stays too high over an extended period of time:
Can cause the heart to have to work too hard and the force of the blood flow can damage your arteries, heart, kidneys, brain and eyes.
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 12
Hypertension
Know your numbers:
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 13
Hypertension
Hypertension depends on several factors:
HYPERTENSION
StressDiabetesTobacco
AgeGenderEthnicity
ObesityActivity
Diet
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 14
How to measure?
Invasive blood pressure (IBP):
It measures the BP internally by using a sensitive IV catheter inserted into an superficial artery. This is the procedure taken where continuous BP monitoring is required.
Non-invasive blood pressure (NIBP):
Auscultation
Oscillometry
Doppler (determines the flow distal to the artery)
Arterial tonometry, Arterial volume clamp are other types of NIBP measure
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 15
How to measure?
Non-invasive blood pressure
Auscultation
Oscillometry
Mercury sphygmomanometer+ stethoscope
Mechanical manometer + stethoscope
M F PimentelAugust 26, 2019
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How to measure?
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 17
The auscultation method
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 18
The auscultation method
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 19
The auscultation method
Systolic BP
Diastolic BP
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 20
The oscillometric method
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 21
The oscillometric method
It is based on the change of the magnitude of oscillation
MAP – Mean Arterial Pressure
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 22
The oscillometric method
It is based on the change of the magnitude of oscillation
MAP – Mean Arterial Pressure
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 22
How often should we measure?
Lets first consider an example…
MATLAB Demo
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 22
How often should we measure?
Why is this important for blood pressure monitoring?
How much does your blood pressure vary in one day?
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 22
How often should we measure?
Why is this important for blood pressure monitoring?
How much does your blood pressure vary in one day?
Kotsis et al. Hypertension, 2005
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 22
How often should we measure?
Treat
Do not
treat
8am
DBP
4am2pm 4am
Treatment
threshold
SBP
time
24hr mean
BP
10pm
M F Pimentel
Under-sampling
August 26, 2019Blood Pressure - Biomedical Signal Processing
Page 22
How often should we measure?
Treat
Do not
treat
8am4am2pm
10pm 4amtime
++
+
Treatment
threshold
24hr mean
BP
M F Pimentel
Aliasing effect
August 26, 2019Blood Pressure - Biomedical Signal Processing
Page 22
How often should we measure?
Treat
Do not
treat
8am4am2pm
10pm 4am
BP
time
++
+
Treatment
threshold
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 22
Under-sampling and aliasing
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 22
Sampling and Aliasing
Under-sampling looses (important) wave details!
It also may change the wave form/shape due to Aliasing
Sampling is a discrete observation or measurement, while Aliasing is an illusion, appearance of something that is not, due to shortcomings of sampling
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 22
New resources
http://www.worldlifeexpectancy.com/cause-of-death/hypertension/by-country
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 22
Low-cost BP monitor
http://code.google.com/p/ewh-bp-project/
M F PimentelAugust 26, 2019
Blood Pressure - Biomedical Signal ProcessingPage 22
Low-cost BP monitor
http://code.google.com/p/ewh-bp-project/
LECTURE NOTES
BLOOD FLOW METERS
1.Electromagnetic Blood Flow Meters
• Measures instantaneous pulsatile flow of blood
• Works based on the principle of electromagnetic induction
• The voltage induced in a conductor moving in amagnetic field is proportional to the velocity of theconductor
• The conductive blood is the moving conductor
Principle of Electromagnetic Blood Flow Meters
Principle of Electromagnetic Blood flow Measurement
BME 312-BMI II-L3-ALİ IŞIN 2015
Principle of Electromagnetic Blood Flow Meters
• A permanent magnet or electromagnetpositioned around the blood vessel generatesa magnetic field perpendicular to thedirection of the flow of the blood.
• Voltage induced in the moving blood columnis measured with stationary electrodeslocated on opposite sides of the blood vesseland perpendicular to the direction of themagnetic field.
Principle of Electromagnetic Blood Flow Meters
• The Induced emf
• Where• B = magnetic flux density, T• L = length between electrodes, m• u = instantaneous velocity of blood, m/s
Principle of Electromagnetic Blood Flow Meters
• This method requires that the blood vessel beexposed so that the flow head or themeasuring probe can be put across it.
Design of Flow Transducers
• The electromagnetic flow-transducer is a tube of non-magnetic material to ensure that the magnetic flux doesnot bypass the flowing liquid and go into the walls of thetube.
• The tube is made of a conducting material and generallyhas an insulating lining to prevent short circuiting ofinduced emf.
• The induced emf is picked up by point electrodes madefrom stainless steel or platinum.
Design of Flow Transducers• The flow head contains a slot through which the
intact blood vessel can be inserted to make a snugfit.
• Several probes of different sizes must thereforeaccompany the flowmeter to match the full rangeof sizes of the blood vessels which have variousdiameters.
• Flow heads having as small as 1mm are available.
BME 312-BMI II-L3-ALİ IŞIN 2015
Types of Electromagnetic Blood Flow Meters
• DC Flow meters
• Use DC Magnetic field.• Cause electrode polarization and amplifier drift.
• AC Flow meters• Electromagnets are driven by alternating
currents.• The transducer acts like a Transformer and
induces error voltages that often exceed thesignal levels by several orders of magnitude.
BME 312-BMI II-L3-ALİ IŞIN 2015
Electromagnetic AC flow meters• Error recovery is achieved by using several different
waveforms for magnet current
• Sine, Square, Trapezoidal.
• Suitable balancing circuits are used to balance outthe error voltage.
BME 312-BMI II-L3-ALİ IŞIN 2015
Sine wave Flowmeters• The transformer induced voltage is 90˚ out of
phase and is eliminated by
• Injecting a voltage of equal strength andopposite phase into the signal.
• Using a gated amplifier.• Permit the amplification of the signal
only during the flow induced voltages aremaximum and the transformer inducedvoltages are minimum.BME 312-BMI II-L3-ALİ IŞIN 2015
Square wave Flowmeters• The transformer induced voltage is only a
spike.
• Separation is easier as the amplifier can begated only for a very short period.
• Blanking is required only when the current inthe magnet is reversing its direction and theamplifier works during the flat portion of thesquare wave.
Magnetic Flowmeter Block Diagram
Magnetic Flowmeter Block Diagram
• The oscillator, which drives the magnetprovides a control signal for the gate, operatesat a frequency of between 60 and 400 Hz.
• The frequency response is high enough toallow the recording of the flow pulses.
• The mean or average flow can be derived byuse of a low-pass filter.
BME 312-BMI II-L3-ALİ IŞIN 2015
2. Ultrasonic Blood Flow Meters
• A beam of ultrasonic energy is used tomeasure the velocity of flowing blood.
• Two types:
• Transit time flow meters
• Doppler type.
BME 312-BMI II-L3-ALİ IŞIN 2015
Transit-Time Ultrasonic Flow Meters
Ultrasonic Transducer
Ultrasonic Transducer
Transit-Time Ultrasonic Flow Meters
• Where• t - transit time• D - Distance between the transducers• c - Sound velocity• u - blood flow velocity
Transit-Time Ultrasonic Flow Meters
• The pulsed beam is directed through a bloodvessel at a shallow angle and its transit time ismeasured.
• The transit time is shortened when the bloodflows in the same direction as the transmittedenergy
• The transit time is lengthened otherwise.
Doppler Type Ultrasonic Flow Meters
Doppler type Ultrasonic Flow Meters
• Based on the Doppler principle
• A transducer sends an ultrasonic beam with afrequency F into the flowing blood.
• A small part of the transmitted energy isscattered back and is received by a secondtransducer arranged opposite the first one.
• The reflected signal has a different frequencyF + FD or F – FD due to Doppler effect.
Doppler Frequency equation
•Where• fd = Doppler frequency shift• f0 = source frequency• u = target velocity• c = velocity of sound
Doppler type Ultrasonic Flow Meters…
• The Doppler component FD is directlyproportional to the velocity of the flowingblood.
• A fraction of the transmitted ultrasonic energyreaches the second transducer directly withthe frequency being unchanged.
Doppler Type Ultrasonic Flow Meters
Doppler type Ultrasonic Flow Meters…
• After amplification of the composite signal,the Doppler frequency can be obtained at theoutput of the detector as the differencebetween the direct and the scattered signalcomponents.
• For normal blood velocities, the Dopplersignal is typically in the low audio frequencyrange.