BME 311: BIOMEDICAL INSTRUMENTATION ILecturer: Ali Işın

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BME 311: BIOMEDICAL INSTRUMENTATION I Lecturer: Ali Işın

Lecture Note 3: Blood Pressure Measurement

FACULTY OF ENGINEERING

DEPARTMENT OF BIOMEDICAL ENGINEERING

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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.

2 BME 311 LECTURE NOTE 3 - ALİ IŞIN, 2014

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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

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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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• 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.

Direct Measurement

Extra Vascular contd…

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• 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.

Direct Measurement

Extra Vascular contd…

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• 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.

Direct Measurement

Intravascular

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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 in the quantification of pressure and flow.

• 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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• 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

Venous pressure

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• Monitored for assessing proper therapy for

• heart dysfunction

• Shock

• Hypovolemic (Of or relating to a decrease in the volume of circulating blood) or hypervolemic States

• Circulatory failure

Central venous pressure

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• 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.

Central venous pressure

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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 band- pass 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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• 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

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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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• 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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• 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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• 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

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• 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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• 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.