Transcript BME 311: BIOMEDICAL INSTRUMENTATION I Lecturer: Ali Işın

FACULTY OF ENGINEERING
DEPARTMENT OF BIOMEDICAL ENGINEERING
BME 311: BIOMEDICAL INSTRUMENTATION I
Lecturer: Ali Işın
Lecture Note 3: Blood Pressure Measurement
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Blood Pressure
• An Individuals blood pressure is a standard clinical
measurement
• Is considered a good indicator of the status of the
cardiovascular system.
• Blood pressure values in the various chambers of the heart
and in the peripheral vascular system help the physician
determine the functional integrity of the cardio vascular
system.
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Blood Pressure inside the heart chambers
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Major Arteries & Veins of the Body
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Cardiovascular system-typical values
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Blood Pressure Measurement
• Direct (invasive)
1. Extravascular Method
• The vascular pressure is coupled to an external sensor
element via a liquid filled catheter.
[Catheter – is a long tube introduced into the heart or a major vessel by way of
a superficial vein or artery.]
2. Intravascular
• A sensor is placed into the tip of a catheter that is
placed in the vascular system.
• Indirect (non invasive)
• Sphygmomanometer
Consists of an inflatable pressure cuff and a manometer to
measure the pressure in the cuff.
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Direct Measurement
(Extravascular)
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Disposable blood-pressure sensor with integral flush device
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Direct Measurement
Extra Vascular
• The extra vascular sensor system is made up of a
catheter.
• The catheter is connected to a three way stopcock
and then to a pressure sensor
• It is filled with a saline-heparin solution.
• It must be flushed with solution every few minutes
to prevent blood clotting at the tip.
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Direct Measurement
Extra Vascular contd…
• Physician inserts the catheter
• Either by means of a surgical cut-down, which exposes the artery
or vein.
• or by means of percutaneous insertion which involves the use of a
special needle or guide-wire technique.
• Blood pressure is transmitted via the catheter column to
the sensor and finally to the diaphragm which is deflected.
• The displacement of the diaphragm is sensed electronically.
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Direct Measurement
Extra Vascular contd…
• Disadvantages
• The frequency response of the catheter-sensor system
is limited by the hydraulic properties of the system.
• Creates time delay in detection of pressures when a
pressure pulse is transmitted.
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Direct Measurement
Intravascular
• The sensor is placed at the tip of the catheter.
• Enables the physician to obtain a high frequency response
in detection of pressures at the tip of the catheter.
• Types of sensors
1. Strain-gage systems
• bonded onto a flexible diaphragm at the catheter tip.
2. Fibre-optic device
• Measures the displacement of the diaphragm optically by
varying reflection of light from the back of the deflecting
diaphragm.
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Bonded Strain Gage pressure transducer
•
Consists of strain-sensitive gages which are firmly
bonded with an adhesive to the membrane or
diaphragm whose movement is to be recorded.
•
Made by taking a length of a very thin wire or foil
which is formed into a grid pattern and bonded
to a backing material.
•
Is then attached to the diaphragm.
•
Deflection of the diaphragm causes
corresponding strain in the wire gage.
•
Causes a corresponding change in the resistance
which is proportional to the pressure.
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Fiber optic type pressure transducer
• Measures the displacement of the diaphragm
optically by the varying reflection of light from the
back of the deflecting diaphragm.
• Inherently safer electrically
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Blood Pressure Waveforms
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Harmonic Analysis of Blood Pressure
Waveforms
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Harmonic Analysis of Blood Pressure
Waveforms
• Using Fourier Analysis techniques
quantification of pressure and flow.
in
the
• Blood pressure pulse can be divided into its
fundamental component (of the same frequency as
the blood pressure wave) and its significant
harmonics.
• Analysis of the frequency components of the pulse
yield more information on arterial properties.
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Electrical Model of Catheter-Sensor system
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Testing technique for measuring the transient response of
the catheter-sensor system
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Transient response of the catheter-sensor system
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Pressure-waveform distortion
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Distortion during the recording of arterial pressure
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Venous pressure
• For determining the function of
• Capillary bed
• Right side of the heart
• The central venous pressure is measured in the central vein or in
the right atrium.
• It fluctuates above and below atmospheric pressure as the
subject breathes.
• The reference level for venous pressure is at the right atrium.
• Central venous pressure is an important indicator of myocardial
performance
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Central venous pressure
• Monitored for assessing proper therapy for
• heart dysfunction
• Shock
• Hypovolemic
circulating blood)
(Of or relating to a decrease in the volume of
or hypervolemic States
• Circulatory failure
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Central venous pressure
• Continuous dynamic measurements is
made by connecting a high sensitive
pressure sensor to the venous catheter.
• Normal venous pressure values range
widely from 0 to 1.2 kPa with a mean
pressure of 0.5 kPa.
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Heart Sounds
• Heart sounds are vibrations or sounds due to the
acceleration or deceleration of blood.
• Murmurs are vibrations or sounds due to blood
turbulence.
• The technique of listening to sounds produced by
the organs and vessels of the body is called
auscultation.
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Heart Sounds …
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Heart Sounds …
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Heart Sounds …
•
With each heartbeat, the normal heart produces two distinct sounds that are
audible in the stethoscope – often described as “lub-dub”
•
The “lub” is caused by the closure of atrioventricular valves and is called the first
heart sound
• occurs approximately at the time of QRS complex of the ECG and just before
ventricular systole.
•
The “dub” part of the heart sounds is called the second heart sound and is caused
by the closing of the semilunar valves
• Which closes at the end of the systole, just before the atrioventricular valve
opens.
•
•
Occurs at the time of the end of the T wave of the ECG
The third heart sound attributed to the sudden termination of the rapid filling
phase of the ventricles from the atria and the associated vibration of the
ventricular muscle walls., which are relaxed.
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Auscultation Techniques
• There are optimal recording sites for the various heart sounds.
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Auscultation Techniques…
• Heart sounds and murmurs have extremely small
amplitudes with frequencies from 0.1 to 2000 Hz.
• Thus the recording device must be carefully
selected for wide band frequency response
characteristics.
• Specially designed acoustically quiet environment is
needed for noise free recording of heart sounds.
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Stethoscope
• Mechanical stethoscopes amplifies sound because of Standing wave
phenomenon that occur at quarter-wavelengths of the sound.
• Uneven frequency response with many resonance peaks.
• Firm application of the chest piece makes the diaphragm taut
with pressure thereby causing an attenuation of low frequencies.
• Loose-fitting earpiece cause leakage which reduces the coupling
between the chest wall and the ear.
• Electronics stethoscopes has selectable frequency response
characteristics ranging from “ideal” flat-response to selectable bandpass response.
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Stethoscope Frequency response
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Phonocardiogram
• A Phonocardiogram is a recording of the heart sounds and
murmurs.
• Eliminates subjective interpretation of the heart sounds
• Enables evaluation of the heart sounds and murmurs with
respect to the electric and mechanical events in the cardiac
cycle.
• Evaluation of the result is based on the basis of changes in
the wave shape and various timing parameters.
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Phonocardiogram
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Frequency spectrum of a typical
Phonocardiogram
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Cardiac Catheterization
• The process of introducing a catheter into the heart for
diagnosis.
• Used to asses hemodynamic (circulation of the blood and
the forces involved) function and cardiovascular structure.
• Performed during most of the heart surgeries.
• Performed in specialized laboratories outfitted with x-ray
equipment for visualizing heart structures and the position
of various pressure catheters.
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Cardiac Catheterization …
• A radiopaque die is injected into the ventricles
or aorta through the catheter for assessing
the ventricular or aortic function
• Pressures in all four chambers of the heart
and in the great vessels can be measured by
positioning the catheters in such a way to
recognize the characteristics pressure
waveforms.
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Cardiac Catheterization …
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Angiography
• Roentgenographic examination of blood vessels
after injection of a radiopaque contrast medium;
• Angiographic visualization is an essential tool used
to evaluate cardiac structure.
• Specially designed catheters and power injectors
are used in order that a bolus of contrast material
can be delivered rapidly into the appropriate vessel
or heart chamber.
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Cardiac Catheterization & Angiogram
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Angiogram
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Angioplasty
• Surgical procedure to repair a
damaged blood vessel or unblock a
coronary artery
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Indirect Blood Pressure Measurement - Sphygmomanometer
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Indirect Blood Pressure Measurement - Sphygmomanometer
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Indirect Blood Pressure Measurement - Sphygmomanometer
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Indirect Blood Pressure Measurement - Sphygmomanometer
• The pressure cuff on the upper arm is first
inflated to a pressure well above the systolic
pressure.
• At this point no sound can be heard through
the stethoscope, which is placed over the
brachial artery, for that artery has been
collapsed by the pressure of the cuff.
• The pressure in the cuff is then gradually
reduced.
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Sphygmomanometery…
• When the systolic peaks are higher than the
occlusive pressure, the blood spurts under
that cuff and causes a palpable pulse in the
wrist (Riva-Rocci Method)
• Audible sounds (Korotkoff (named after Dr.
Nikolai Korotkoff) sounds) generated by the
flow of blood and vibrations of the vessel
under the cuff are heard through the
stethoscope.
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Sphygmomanometery …
• The pressure of the cuff that is indicated on the
manometer when the first Korotkoff sound is
heard is recorded as the systolic blood pressure.
• As the pressure in the cuff is continues to drop,
the Korotkoff sounds continue until the cuff
pressure is no longer sufficient to occlude the
vessel during any part of the cycle. Below this
pressure the Korotkoff sounds disappear,
marking the value of the diastolic pressure.
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Sphygmomanometery …
• Auscultatory (based on the Korotkoff sounds)
technique is simpler and requires a minimum of
equipment.
• Cannot be used in noisy environments.
• Palpation (based on pulse on the blood vessel)
technique doesn’t require a noise free
environment.
• Normal respiration and vasomotor waves
modulate the normal blood-pressure levels.
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Automated Indirect Blood Pressure measurement
techniques
• Involves an automatic sphygmomanometer
that inflates and deflates an occlusive cuff at a
predetermined rate.
• A sensitive detector is used to measure the
distal pulse or cuff pressure.
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Automated Auscultatory device
• Microphone replaces the stethoscope for
sensing the Korotkoff sounds.
• The process begins with a rapid (20-30mm
Hg/s) inflation of the occlusive cuff to a preset
pressure about 30mm Hg higher than the
suspected systolic pressure.
• The flow of blood beneath the cuff is stopped
by the collapse of the vessel.
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Automated Auscultatory device
• Cuff pressure is then reduced slowly (2-3 mm
Hg/s).
• The first Korotkoff sound is detected by the
microphone, at which time the level of the
cuff pressure is stored.
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Automated Auscultatory device
• The muffling and silent period of the Korotkoff
sound is detected, and the value of the
diastolic pressure is also stored.
• After a few minutes the instrument displays
the systolic and diastolic pressure and recycles
the operation
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Ultrasonic Based Blood Pressure Measurement
• Employs a transcutaneous Doppler sensor
that detects the motion of the blood-vessel
walls in the various states of occlusion.
• The Doppler ultrasonic transducer is focused
on the vessel wall and the blood.
• The reflected signal (shifted in frequency) is
detected by the receiving crystal and
decoded.
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Ultrasonic Based Blood Pressure Measurement…
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Ultrasonic Based Blood Pressure Measurement…
• The difference in frequency, in the range of 40
to 500 Hz, between the transmitted and
received signals is proportional to the velocity
of the wall motion and the blood velocity.
• As the cuff pressure is increased above
diastolic but below systolic, the vessels opens
and closes with each heartbeat, because the
pressure in the artery oscillates above and
below the applied external pressure in the
cuff.
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Ultrasonic Based Blood Pressure Measurement…
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Ultrasonic Based Blood Pressure Measurement…
• As the applied pressure is further increased, the time
between the opening and closing decreases until they
coincide. The reading at this point is the systolic
pressure.
• When the pressure is cuff is reduced, the time between
the opening and closing increases until the closing
signal from one pulse coincides with opening signal
from the next. The reading at this point is the diastolic
pressure.
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Ultrasonic Based Blood Pressure Measurement…
• Advantages:
• Doesn’t require a noise free environment.
• Disadvantage:
• Movement of the subject‘s body cause
changes in ultrasonic path between the
sensor and the blood vessel.
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