Unit-4 Blood Pressure Measurement - rmd.ac.in Materials/5/BBI/unit4.pdf · Unit-4 Blood Pressure Measurement August 26, 2019 Blood Pressure - Biomedical Signal Processing Page 1

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



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!!!

http://www.nsta.org/Energy/find/luminaries/hales2.html

http://www.nsta.org/Energy/find/luminaries/hales2.html



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



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!



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!



Nowadays BP monitors



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)




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!




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



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.



Hypertension

Know your numbers:



Hypertension

Hypertension depends on several factors:

HYPERTENSION

StressDiabetesTobacco

AgeGenderEthnicity

ObesityActivity

Diet



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



How to measure?

Non-invasive blood pressure

Auscultation

Oscillometry

Mercury sphygmomanometer+ stethoscope

Mechanical manometer + stethoscope



How to measure?



The auscultation method







Systolic BP

Diastolic BP



The oscillometric method




It is based on the change of the magnitude of oscillation

MAP – Mean Arterial Pressure




It is based on the change of the magnitude of oscillation

MAP – Mean Arterial Pressure



How often should we measure?

Lets first consider an example…

MATLAB Demo




Why is this important for blood pressure monitoring?

How much does your blood pressure vary in one day?




Why is this important for blood pressure monitoring?

How much does your blood pressure vary in one day?

Kotsis et al. Hypertension, 2005




Treat

Do not

treat

8am

DBP

4am2pm 4am

Treatment

threshold

SBP

time

24hr mean

BP

10pm

M F Pimentel

Under-sampling


Page 22


Treat

Do not

treat

8am4am2pm

10pm 4amtime

++

+

Treatment

threshold

24hr mean

BP

M F Pimentel

Aliasing effect


Page 22


Treat

Do not

treat

8am4am2pm

10pm 4am

BP

time

++

+

Treatment

threshold



Under-sampling and aliasing



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



New resources

http://www.worldlifeexpectancy.com/cause-of-death/hypertension/by-country



Low-cost BP monitor

http://code.google.com/p/ewh-bp-project/



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


• 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.


• The Induced emf

• Where• B = magnetic flux density, T• L = length between electrodes, m• u = instantaneous velocity of blood, m/s


• 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.


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.


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.


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.


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.


Transit-Time Ultrasonic Flow Meters

Ultrasonic Transducer

Ultrasonic Transducer


• Where• t - transit time• D - Distance between the transducers• c - Sound velocity• u - blood flow velocity


• 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.

Documents

Unit-4 Blood Pressure Measurement - rmd.ac.in Materials/5/BBI/unit4.pdf · Unit-4 Blood Pressure Measurement August 26, 2019 Blood Pressure - Biomedical Signal Processing Page 1