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Dissertations and Theses Dissertations and Theses

1982

Investigation of blood pressure measurement using Investigation of blood pressure measurement using

a hydraulic occlusive cuff a hydraulic occlusive cuff

Kusha R. Bhattarai Portland State University

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Recommended Citation Recommended Citation Bhattarai, Kusha R., "Investigation of blood pressure measurement using a hydraulic occlusive cuff" (1982). Dissertations and Theses. Paper 3140. https://doi.org/10.15760/etd.3131

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AN ABSTRACT OF THE THESIS OF Kusha R. Bhattarai for the Master of

Science in Applied Science presented May 18, 1982.

Title: Investigation on Blood Pressure Measurement Using a Hydraulic Occlusive Cuff

APPROVED BY MEMBERS OF THESIS COMMITTEE:

Pah I. Chen, Advisor

Loarn Robertson

This thesis presents an improved oscillotonometric system for

the measurement of human blood pressure. The study included:

l. The design of a hydraulic occlusive cuff,

2. The in~estigation of the wave forms taken from the blood

pressure measurements, and

3. The design of a mechanism for the simulation of human blood

pressure pulse.

In this study, an experimental system consisting of a rigid

shell occlusive cuff, a constant volume displacement pump, a

transducer, and a chart recorder was designed and used for data

collection.

Data from more than sixty human subjects were collected and

analyzed in this study. Criteria were established to determine the

diastolic and systolic pressures. The data obtained by the hydraulic

scheme were compared to that achieved from the conventional

auscultatory method with remarkable agreements.

The hydraulic occlusive cuff was further used in the mechanical

pressure pulse simulation system. This study was done with the

intention of studying the physical properties of blood vessel.

Since the pressure can be transmitted hydrostatically almost

without loss of amplitude and without change of frequency, it is

believed that this system would be a more accurate instrument to

measure human blood pressure. Furthermore, the possibility of

extrapolating more information of diagnostic significance becomes

available by studying the recorded wave forms.

INVESTIGATION OF BLOOD PRESSURE MEASUREMENT

USING A HYDRAULIC OCCLUSIVE CUFF

by KUSHA R. BHATTARAI

A thesis submitted in partial fulfillment of the requirement for the degree of

MASTER OF SCIENCE in

APPLIED SCIENCE

PORTLAND STATE UNIVERSITY 1982

TO THE OFFICE OF GRADUATE STUDIES AND RESEARCH:

The members of the Committee approve the thesis of Kusha R.

Bhattarai presented May 18, 1982.

APPROVED:

William Savery. De

Pah I. Chen

~ Department of Mechanical Engineering

I • .. Rauch, Dean of Graduate Studies and Research

.... ~

ACKNOWLEDGMENTS

I wish to express my deepest gratitude to Dr. Pah I. Chen, my

graduate study advisor, for his valuable advice, encouragement and

confidence in my ability to make this thesis a reality. My heartfelt

thanks to Or. Patrick J. Reynolds for his sincere cooperation and

constructive criticism throughout the project.

I would like to acknowledge Or. C. William Savery for his support

and critical evaluation. Committee member, Dr. Loarn Robertson's

genuine interest and suggestions are also appreciated.

The cooperation of Portland State University's machine shop

technician, Jon Griffin, for building the cuff and pressure pulse

mechanism is appreciated. Its successful operation during the test

period is praiseworthy.

My appreciation goes to David Partin for excellent photography and

to Donna Mikulic for typing the thesis.

It is needless to say that the continued advice and encouragements

of my friends was a source of encouragement.

Lastly, I acknowledge my parents for their perseverance, deep

understanding and the best support through my extended period of

study. The patience and mutual understanding of my wife and children

has been a never ending source of inspiration.

TABLE OF CONTENTS

PAGE

ACKNOWLEDGMENTS............................................... iii

LIST OF TABLES................................................ v

LIST OF FIGURES............................................... vi

CHAPTER

I INTRODUCTION........................................ 1

II HISTORICAL REVIEW................................... 3

III METHODS FOR MONITORING AND RECORDING BLOOD PRESSURE. 5

IV COMPONENTS AND SYSTEM DESIGN........................ 13

Design of the hydraulic occlusive cuff........... 13 Design of the mechanism to simulate pressure

pu l se. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 17

V EXPERIMENTAL SET UP AND TEST PROCEDURE.............. 26

Recording human blood pressure................... 26 Testing using mechanical pressure pulse pump..... 29 Equipment used................................... 32

VI EXPERIMENTAL RESULTS AND ANALYSIS................... 38

VII CONCLUSIONS AND RECOMMENDATIONS..................... 54

REFERENCES. • . . . . • • • • • • • • . • • • . . • • • . . . • • • . • . . . • . • • • • • • • . • • • • • • • • 56

TABLE

LIST OF TABLES

I Comparison of blood pressure records

measured by pneumatic and hydraulic

PAGE

scheme. . • • • • • • . • • • • • • • • • • • • • • • . . • • • • • . . • . • • • • . . . • 40

II Percent of independent hydraulic cuff vs.

auscultatory pressure measurements

falling within selected difference limit......... 42

LIST OF FIGURES

FIGURE PAGE

1 Hydraulic occlusive cuff............................... 15

2 Photograph of the hydraulic occlusive cuff............. 16

3 Detail of foreann with rubber plug attachments......... 18

4 Experimental set up to simulate blood pressure

wave farm •••. ~......................... . . . • . • . . . . . . . 19

5 Human blood pressure wave form......................... 20

6 Carn configuration...................................... 21

7 Pressure pulse mechanism and pump detail............... 23

8 Fulcrum detail......................................... 24

9 Recording system block diagram......................... 27

10 Experimental set up.................................... 28

11 Calibration chart...................................... 30

12 Schematic test set up for simulated arm................ 31

13 Syringe pump........................................... 33

14 Bridge conditioner circuit............................. 35

15 Bar chart to display the records shown in Table I...... 41

16 Typical one-cycle record generated during a

recording session................................... 44

17 Enlarged view of rising phase of Fig. 16 with

the diastolic and systolic pressures indicated

according to the criteria described in the text..... 45

18 Comparison of various types of pressure gradient

for human subjects •••••• ;........................... 47

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PAGE

19 Various types of oscillations........................... 48

20 Pulse contour obtained during the operation

of mechanical pressure pulse pump................... 50

21 Records obtained from simulated arm with

latex tube fully open and kinked position........... 52

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l I INTRODUCTION

A non-invasive technique, known as oscillometry, for measuring

the human arterial pressure pulse was introduced just around the turn

of the century (2,4). The method consisted of applying a pneumatic

occlusive cuff around a limb and observing the dynamic pressure varia-

tions within the cuff caused by the pulsating blood flow in arteries

passing through the region subject to occlusion. Criteria were establi­

shed to correlate oscillographic events with the cuff pressures that

matched diastolic and systolic blood pressure (i.e., the pressure

corresponding to the lowest and highest values, respectively, of the

pulsatile blood pressure). The oscillatory phenomenon was first detected

when the diastolic pressure is exceeded due to the increasing static

pressure in the cuff. The oscillation then ceases when the systolic

pressure is exceeded. This technique is distinctly different from that

presently used clinically. Although the clinical method (auscultatory)

also employs a pneumatic occlusive cuff, it uses as criteria for sys­

tolic and diastolic pressure levels the changes in the different phases

of sound pattern detected with a stethoscope while deflating the

pressure in the cuff.

The method of oscillography has essentially been abandoned as a

means of measuring blood pressure. The abandonment was forced by tech­

nical problems but the method merits further investigation, since as

yet unresolved controversy surrounds the auscultatory technique (7).

This project presents a variation on principle involved in

oscillometry which is believed to be conceptually and fundamentally

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advanced. A rigid volume container was used to convert the measurement

from a pneumatic (compressible) scheme to a hydraulic (incompressible)

scheme. Thus, a positive fluid displacement entering the cuff will

result in an increase in pressure inside the cuff. Since the system

would be theoretically incompressible, vascular pressure would be trans-

mitted directly (minus compliance effects) to the hydraulic cuff.

This enables us to plot accurately the volume displacement against

the arterial pressure. In addition, a great deal more data, possibly

of diagnostic significance, are available from a record of the pressure

pulsations in a hydraulic occlusive system than that provided by the

auscultatory pulse pressure measurement technique.

1 I HISTORICAL REVIEW

The concept of measuring blood pressure in living beings was first

recorded in 1733 when S. Hales measured direct blood pressure in an

unanesthetized horse (1). He noticed that there were small oscillations

synchronous with the heart beat while he was using a U-tube Hg-manometer

for the observation. The investigations were further noted by many

researchers, since then, to ascertain an accurate and reliable instrumen-

tations for measuring b1ood pressure on both men and animals by direct

and indirect techniques. Much of the research were focused on indirect

techniques because of the invasive risk in direct methods. However,

the direct readings give more accurate results.

On the pace of investigation, Von Basch was the first to intro­

duce in 1876 (1) a non-invasive technique for measuring blood pressure

by applying the water filled bag to the skin over a superficial artery.

Counter pressure was applied by pressing down on the water filled

bag. By palpating the distal artery, the force on the water bag is

increased until the radial pulse disappears. The force was lessened

until pulse reappears. The pressure indicator was noted when pulse

reappeared and then disappeared. The average of the two was taken as

systolic pressure. In the meantime, von Basch and Potain (1) carried

out the same procedure by using air-filled instruments. The method

served some clinical purposes. French physiologist, E. J. Marey, in

1876 (1), attempted to investigate hydraulic counter pressure to the fore­

arm. The bare arm was placed inside a closed cylinder fitted with a

window for viewing the arm. The pressure in the system was increased

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by raising a water reservoir. The pulsatile oscillations were observed

in a recording manometer as the reservoir was elevated. The applied

counter pressure for maximum oscillation was considered as diastolic

pressure and the point when oscillation ceased was taken as systolic

pressure. The use of water as an occluding medium caused a serious

problem in sealing. The leaking problem encountered by Marey were

later overcome by Hurthle, who in 1886 used a U-shaped bladder in a

glass cylinder to apply hydraulic counter pressure to vessels in an

extremity. Several other occluding devices such as Marey's single

digit occluding instrument, Mosso's four finger occluding instrument,

etc., were tried at that time. The oscillations were easily detectable

with these devices but none of the methods received clinical acceptances

and the use of oscillatory method was diminishing due to lacking of

sensitive pressure recording devices.

METHODS FOR MONITORING AND RECORDING BLOOD PRESSURE

The investigations of van Basch, Potain and Marey brought up two

important facts: (l) the application of counter-pressure could occlude

an artery with the indication of visible oscillations, and (2) with the

application of adequate pressure, the skin blanches and the color only

returns to normal when the pressure is reduced below the systolic

pressure level.

The above observations led to the method of arterial occlusion

for the measurement of blood pressure in the human subjects and experi-

mental animals. There are five basic methods which employ an externally

applied arterial-occluding device: (1) the palpatory method, (2) the

flush method, (3) the oscillometric method, (4) the auscultatory

method, and (5) the ultrasound method. A brief summary of the develop-

ment, complexity, validation and accuracy of each of these methods are

reviewed here.

PALPATORY METHOD

The palpatory method introduced by Noris (9) is applied to

measure blood pressure, routinely, when the other indirect methods fail.

In this technique, the stethoscope can be replaced by electronic, mech­

anical or manual tactile sensing of the arterial pulse. This technique

has been standardized by the American Heart Association in 1951.

In this method, the pressure in the cuff is raised, as used in

the auscultatory technique, to a level about 30 mm Hg above the point

where the radial pulse disappears. The pressure is then released grad-

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ually (2 to 3 mm Hg per beat) and the systolic pressure is read at the

time the distal pulse first reappears regularly. The distal pulse may

be manually palpated by placing the finger over the radial artery during

cuff deflation.

A reinvestigation of palpatory method carried out by Van Bergen

(14) in relation to direct method shows that the systolic (palpatory)

pressure was found to be below intra-arterial systolic pressure by about

30 mm Hg. Another study performed by Segall in 1940 (10) reported that

diastolic pressure can be determined with this palpatory technique using

the thumb and the results agreed within the accuracy of 2-10 mm Hg.

Rogge and Meyer re-examined the Segall's method using auscultatory

method and found the correlation coefficients of 0.91 for systolic pre­

ssure and 0.96 for diastolic pressures. Geddes (3) investigated that

the transition described by Segall is inconsistent because the transition

is so small and hence difficult to detect. However, this phenomenon

of palpatory method is still in existence and cannot be condemned

without further careful investigation.

FLUSH METHOD

The flush method was introduced by Gaertner in 1899 (3). He used

a small occlusive cuff placing over the finger where pressure is to be

measured. With no pressure applied in the cuff, the finger was pushed

into a cylinder with a rubber membrane stretched across the top. The

tip of the finger will be compressed because of the elasticity of the

rubber and the blood will be expelled, in the meantime out of the finger.

The cuff is then inflated to above suspected systolic pressure and the

finger is removed from the rubber. The cuff pressure is now slowly

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decreased until the blanched finger starts to return to the normal

color. At this point, the occluding pressure is re~orded as systolic

pressure.

The flush method did not receive the clinical acceptance because

of the discomfort (as pain) with the patient and difficulty of

detecting the color change needed for recording pressure. The major

disadvantage of this technique is that vase-constriction of the

digital arteries and, accordingly, the finger color are directly

affected by a variety of extraneous factors such as pain, temperature

and "emotion"(?). However, this method appears to have been most

effectively applied in clinical pediatric practices when other

indirect methods were not practical for new-born infants.

OSCILLOMETRIC METHOD

The oscillometric method was initially investigated by Howell

and Brush in 1901 (4) who observed the relation between the arterial

pressure and the amplitude of the oscillation as the counter pressure

was reduced in the cuff. Von Recklinghausen found that with

decreasing counter pressure, the amplitude of the oscillations were

increased just below systolic pressure; slightly below the point of

maximum oscilations the occluding pressure was equal to diastolic

pressure(l,3). Martin in 1903 showed that the point of maximum

oscillation was not coinciding with the diastolic pressure(l,3). In

1903, a detailed experimental study based on the same principle

performed by Erlanger (2) brought much to encourage the use of this

oscillometric method. All the studies performed above consisted of an

arm occluding cuff which is used to apply a counter pressure to the

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radial ~rtery. When the counter pressure in the cuff is reduced from

above systolic pressure to below diastolic pressure, the oscillations

in the cuff reflect amplitude changes which increase to a maximum and

then rapidly fall to a minimum during the dropping of the cuff

pressure below the diastolic level. A sensitive pressure gage or

calibrated recording chart-devices were used in conjunction with the

cuff for recording the oscillatory pulsations. Even though the

oscillations were clearly observed, the diastolic pressure indication

became conflicting observation and could not easily be identified.

The oscillometric method could not achieve acceptance as an

experimental or clinical tool without further verifications. The

measuring instruments and devices available during the early

investigations were also inadequate. In the meantime, the

investigation was hampered because of the advancement of the

auscultatory technique due to its portability and accuracy.

Recently, in connection with the research of Uzman and Wood

(13), Kraudsman (7) compiled NASA investigation on an advanced

technique based on using infrared photoelectric detector cells,

coupled with an illuminating infrared light source and a miniature

pressure capsule that is clamped to the pinna of the human ear. By

using this technique, a good correlation was found between direct and

indirect blood pressure. But it is yet to be recognized that this

method is another version of the oscillometric method. The pressure

measured is that in the ear vessels, not aortic. Nonetheless, because

the photoelectric oscillometric method is easy to apply and works

well, it will probably experience further growth.

8

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AUSCULTATORY METHOD

The auscultatory method of measurement was first introduced by

Korotkoff in 1905 (5) and became a universally adopted tool in

clincial practices for measuring human blood pressures. The methods

for inflating, deflating, pulse detection and reading of cuff pressure

is thoroughly verified by several investigators and has been

standardized by the American Heart Association since then.

In this auscultatory technique, an external force is applied to

produce deformation of the brachial artery. The largely laminar flow

in the major arteries can be changed to turbulent as a result of the

vascular deformation due to this applied force. In clinic

application, a pneumatic occlusive cuff is placed around the upper-arm

that can be inflated to a pressure above systolic pressure which

results in a flow stopage where no sound can be detected by a

stethoscope. As the cuff pressure is reduced gradually, the systolic

pressure is identified as the first sound is detected. Further

reducing the pressure, diastolic pressure is detected when the

Korotkoff sounds become dull and muffled. Thus, the noises due to

turbulent flow in the human arteries are considered as the Korotkoff

sounds for detecting the systolic and diastolic pressures.

Steele (11) verified the accuracy between auscultatory and

direct blood pressure measurements and found out that auscultatory

systolic was on average 10 nm Hg too low and diastolic pressure was on

average 8.8 nm Hg too high. In 1944 Kotte, Iglauer, and McGuier (6)

also investigated and verified that systolic and diastolic levels were

2 nm Hg below and 7 nm Hg above direct arterial pressures,

respectively.

9

The different phases of sound characteristics described by

Korotkoff requires a trained observer using the auscultatory technique

to achieve readings of systolic and diastolic levels. The pressure

recorded varies by only a few percent from those simultaneously

recorded by an invasive arterial catheter connected to a pressure

transducer. Such technique records only two data; the values of

systolic and diastolic pressures occurring during the brief period of

observation. However, such technique cannot be used by a person who

has a hearing problem. Also, the arm size of the subjects plays an

important role and requires correction factor to adjust the readings.

The coin operated blood pressure machines, currently available for

public use, have an automatic recording and display system. These

machines utilize the auscultatory technique with a pneumatic cuff and

the readouts display very large errors in blood pressures.

ULTRASOUND METHOD

The ultrasound method, described by Ware and Laenger, is an

advanced auscultatory technique (3) in which an ultrasonic transducer

and receiver are used to detect the pulsatile movement of the brachial

artery wall when the pressure is decreased in the pneumatic occluding

cuff. The changes in wall movement correspond to changes in occluding

pressure. At systolic occluding pressure, the vessel first begins to

move. Movements become larger as pressure drops, then gradually

diminish. At diastolic occluding pressure, movement ceases. A small

flat transducer assembly, mounted on the inside of the cuff, which

transmits and receives ultrasonic energy, is used instead of

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stethoscope or microphone. With this method, vessel wall position and

velocity of movement were obtained and it was found that velocity

(Doppler) signals that clearly identifies the closing (systolic) and

opening (diastolic) of the artery as the cuff pressure is gradually

reduced. This ultrasonic detector provides reliable monitoring of the

opening and closing vessel sounds during cuff deflation and

accordingly, valid detection of both systolic and diastolic

pressures. Several investigators validated the studies (7) by using

the Doppler ultrasound method and compared the results with direct

intra-art~rial cannulation and reported the average error in systolic

pressure +0.1 ±2.2 rrm Hg, and -0.3 ±2.1 rrm Hg for diastolic pressure.

These pressure levels can be significantly identified even in an

extremely noisy environment.

Reviewing the foregoing methods of blood pressure measurements,

they all provide an unambiguous indication of systolic pressure. But,

only the auscultatory method provides an indication of diastolic

pressure with acceptable accuracy. However, none has completely

overcome all of the basic deficiencies such as varying degree of

accuracy and reliability. The palpatory method served some clinical

purposes but the indication of pressure pulse was unstable and the

transition was very small. The flush method was applied in pediatric

practice but it was difficult to detect the appropriate color change

in the skin for reading the needed pressures. In the oscillometric

technique the instrumentation involved during the early research

period were not sufficient to indicate the pressure reading

sensitively. During the period of testing, the oscillations were

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clearly observed but due to the conflicting observation of detecting

diastolic pressure, this method could not obtain clinical acceptance

and it was abandoned in favor of auscultatory technique.

The auscultatory technique, which identifies both the systolic

and diastolic pressures, has dominated the daily clinical practices.

The correlation of the pressure readings has been found in good

agreement with the intra-arterial methods. However, the successful

recording of pulse pressure by this technique depends on the auditory

acuity of the observer, the lack of interfering acoustic noise of

certain frequencies during the observation period, and the adequacy of

the auditory pickup system with respect to frequency response

characteristics. Also, the cuff size used plays an important factor

when applied to children with small arms or adult subjects with larger

arms. With a standard cuff applied to subjects with arm circumference

ranging from about 15 to 50 cm, the correction factor extending from

as much as -25 nm Hg to +15 nm Hg are typically required for agreement

with true arterial pressure.

Thus among the two distinct methods of measuring human arterial

blood pressure, (1) direct or invasive, and (2) indirect or

non-invasive, the indirect method has become universally used in both

clinical and experimental environments. However, because of the

complexity of the instrumentation and methodology of indirect

techniques which leads to inaccuracies and certain unreliabilities,

more careful examination into this area is required.

12

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COMPONENTS AND SYSTEM DESIGN

The overall system design in this project is described in three

phases.

1. Design of a hydraulic occlusive cuff

2. Design of instrumentation system

3. Design of a mechanism to simulate human arterial pressure

pulse

DESIGN OF THE HYDRAULIC OCCLUSIVE CUFF

First of all, a specially designed hydraulic cuff is developed

and constructed. The unit consists of an enclosed cuff using water to

transmit pressure. The cuff is designed to allow the insertion of a

human forearm. Several alternate designs were investigated before the

final system was chosen. Major problems encountered during the

construction of the cuff were due to the unavailability of proper

bladder material as well as proper bonding glue to withstand the fluid

pressure applied.

The design of the hydraulic cuff system is developed by

considering the following important requirements:

1. The hydraulic system should be completely leak proof.

2. The shell of the cuff should be rigid.

3. It should be convenient to use by clinical personnel while

comfortable for patients.

4. The pressure transducer used should respond sensitively to

small pressure changes.

5. There is a need of an accurate recording system.

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Figure 1 shows the detail of the cross-sectional view of the

hydraulic occlusive cuff. The photograph of the cuff is shown in

Figure 2. A rigid transparent circular tube (3.5 11 i.d. and 5.011 long)

made of plexiglas is used as the outer shell of the pressure cuff.

The applied hydraulic pressure will be transmitted over the length of

2.7" only. The transparency facilitates the detection of air bubbles

in the system. The inner diameter of the tube is selected in such a

way that it fits to an average human forearm. Two holes opposite each

other are drilled on the cuff as shown, one for connecting the system

from the pump and the other to a pressure transducer. A domed shape

arrangement with a hole on the top of the tube is provided to remove

the air bubbles easily.

A thin membrane tube of highly flexible rubber material is

placed around the compression rings at both ends of the rigid cuff (as

shown). This helps to seal the torus of both ends. The compression

plugs are tightened against the compression rings with nuts and bolts

to seal .the rubber bladder tight. Thus the entire system can become

leak proof.

Since the human arm varries, special attachments were used to

prevent side thrust due to those rather tapered shaped forearms.

Rubber plugs of constant outside diameter which is equal to the inner

diameter of compression plug and rings are used for this purpose.

Different sizes of rubber plugs were prepared in order to fit various

sizes of forearms. A plug is placed at each end of the cuff to seal

the gaps between the forearm and the cuff. The rubber plugs were made

14

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from liquid rubber. Figure 3 shows the detail of the forearm and cuff

using the side thrust attachments (rubber plugs).

DESIGN OF A MECHANISM TO SIMULATE PRESSURE PULSE

Secondly, a mechanical working model for simulating human

arterial pressure pulse was designed and constructed. The mechanism

was designed according to the standard human arterial pulse contour.

Figure 4 shows the installation diagram for complete experimental

set up. Simulated arterial pressure pulse is created in the latex

tubing. Static pressure head is maintained by the water level in the

reservoir. Oscillations and pressures are caused by the movement of

the piston in the cylinder. The piston is actuated by the lever arm

which is driven about the fulcrum by the action of the cam. The

design of each individual mechanical component is presented as

follows:

l. CAM

With reference to the standard human pulse contour (12), an

enlarged cam profile is drawn and designed (8) for each rise

and return of 211 stroke. To simulate the best pressure wave

form as in Figure 5 a rise of 128°, return of 36°, constant

of 52°, and return of 144° is established. Figure 6 shows

the configuration of the cam. It was done by hand filing,

later polished with sand paper to achieve smooth surface

finish and close accuracy. The p·olishing work was needed to

exclude any vibration during the operation. This cam is

mounted on the shaft of a de motor.

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2. LEVER ARM

Figure 7 shows a lever ann of length 811 with a cam follower

attached at one end and a 1 i nkage joint at the other end.

This lever ann is supported by a fulcrum with frictionless

pivotal mechanism. A small roller bearing is mounted in the

follower to serve the function of a roller. During the

rotation of cam, the motion is transmitted to the linkage

via the lever ann resulting a reciprocating motion. The

piston rod and the linkage is connected by a pin. The

rotation of cam has direct control on the movement of the

piston to obtain desired pulse contour.

3. FULCRUM

Figure 8 is the detail plan and cross-sectional view of the

fulcrum. The lever ann can be held tightly by two screws to

protect it from ~liding during the operation. The stroke of

the piston can be readjusted by changing the arm length on

either side of the lever arm. Two side plates with shielded

bearings hold the rectangular hollow piece tightly with the

help of two bolts on each side (as shown). The shielded

bearing provides smooth operation. The overall fulcrum

assembly is mounted on a metal base plate for stability.

4. PISTON & CYLINDER

A steel hollow cylinder of 111 internal diameter with 1/16"

wall thickness was selected. The cylinder has a maximum

stroke of 3.5". The hollow plug attached to the top of the

cylinder helps to guide the piston rod. The plug also

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functions as a seal to prevent the water from leaking. A

T-joint is also attached at the bottom of the cylinder to

connect the pipe coming from the reservoir and flowing

towards the latex tube. A 1" diameter self lubricating

piston (Teflon material) inside the hollow cylinder

reciprocates according to the motion of the cam. A spring

is also used as a return mechanism for the piston.

5. RESERVOIR & PIPINGS

A reservoir of constant head (150 mm Hg) is installed. The

water inlet to the reservoir is connected to a regular water

tap. The outlet connects to a 2" internal diameter

galvanized pipe. It further connects to a T-joint attached

at the bottom of the cylinder. The other end of the T-joint

connects to a 211 pipe th at reduces to a 1/ 4" diameter

fitting. Lastly, the latex tube is connected to the fitting

as a simulated blood vessel. A manometer is also installed

at the end of the latex tube as a visual aid for identifying

the head loss and the pressure oscillation in the system.

25

I I '

I

EXPERIMENTAL SET UP AND TEST PROCEDURES

The experimental set up and test procedure for recording the

human blood pressure and the testing using the mechanical pressure

pulse pump are described as follows:

RECORDING HUMAN BLOOD PRESSURE

The experimental system for measuring human blood pressure

consists of a hydraulic occlusive cuff, a constant volume displacement

syringe pump, a pressure transducer, and a chart recorder. This

experimental system block diagram is shown in Fig. 9. A photograph of

the experimental setup is also shown in Figure 10. The "heart" of the

system is the occlusive cuff which is connected to the pressure

transducer and the syringe pump. The chart recorder provides a record

for s~bsequent derivation of pulse pressure. An air relief valve is

attached on the top of the cuff in order to remove the air bubbles

from the system.

The test procedures for a human subject are as follows:

1. Fill water (from a reservoir) in the syringes by reversing

the pump (thus providing adequate water for operation).

2. Remove any air bubbles contained in the system.

3. Carefully insert the subject's forearm into the cuff.

4. Insert a suitable side thrust attachment (rubber plug) at

both ends, if needed, to seal the gaps between the arm and

the cuff as shown in Fig. 3.

5. Introduce sufficient water to bring the elastic tube into

contact with the forearm.

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6. Close the valve through which the water is flowing from the

reservoir. At this time, the foreann experiences a pressure

equal to the head in ~he reservoir.

7. Inflate the bladder by actuating the pump while the subject

is at rest. Start the chart recorder simultaneously.

8. Watch carefully the hydraulic pressure to a point judged to

be well above systolic pressure and reverse the pump in

order to bring the pressure in the cuff back to ambient.

9. Perform a second subsequent trial immediately in an

identical fashion.

10. Turn off the pump and the chart recorder, then withdraw the

forearm.

11. Record the subject's blood pressure by the conventional

method (auscultatory) immediately after the measurement by

the hydraulic scheme.

During the period of pressurization, the pressure pulse is

picked up by a pressure transducer. The amplified voltage depicting

blood pressure wave forms are recorded on the strip chart recorder.

The voltage recorded on the chart is calibrated to read in mm of Hg as

shown in Fig. 11.

TESTINGS USING MECHANICAL PRESSURE PULSE PUMP

The experimental set-up for recording blood pressure is also

used for monitoring the simulated arterial pressure pulse. The set-up

system is shown in Fig. 4. A schematic diagram of the test set up is

shown in Fig. 12. An oscilloscope was used to check the frequency and

amplitude of the mechanical pulse during the experiment. Beef kidney

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is found to have sufficient elastic behavior which somewhat resembles

human ann muscles under the test arrangement. The blood vessel is a

latex tube implanted in the beef kidney (as arm muscle). There is a

1" diameter wooden rod inserted in the middle of the kidney to

simulate the arm bone.

A constant head reservoir is filled with water during the

operation of the mechanical pump. The water from the.reservoir

flowing through the latex tube simulates the blood flow.

The experiment is performed by operating two pumps, one to

provide arterial pressure waves and the other to inflate the hydraulic

cuff. To simulate the condition of arteriosclerosis (which causes

vessels to become restricted), the water flow in the latex tube was

restricted by clamping the tube upstream from the cuff. The results

are recorded in the chart. Comparisons have been made on restricted

vs. unrestricted latex tubes.

EQUIPMENT USED

The following equipment was used for the experiments involving

blood pressure measure.ment:

1. HARVARD SYRINGE PUMP

A constant volume displacement pump equipped with

multi-speed forward and reverse transmission arrangement (by

a gear reduction mechanism) is used for supplying water thus

pressure to inflate the rubber bladder. Figure 13 shows the

photograph of the pump.

32

Specifications: Constant volume displacement pump

Manufacturer: Harvard Apparatus Co.

Volume Displacement Rate: 0.43-2178 ml/hr

Syringe Size: 2/50 cc

The pump is modified to supply 4356 ml/hr of water during

the blood pressure measurements. Both plungers of the

syringes are arranged to !Tk)Ve together in the same direction

in the experiment in order to provide a sufficient amount of

water for compressing the forearm such that the artery can

be occluded.

The delivery end of the syringes is connected to the inlet

of the hydraulic cuff via a Y-junction tube. The

experiments were performed with a constant volume

displacement rate of 3480 ml/hr on human foreann and 1744

ml/hr on simulated (beef kidney) arm.

2. PRESSURE TRANSDUCER

A Statham Pressure Transducer which converts pressure input

signal to voltage output signal was used for measuring

static and dynamic pressures.

Specifications: Differential Pressure Transducer

Manufacturer: Gould Statham, CA

Model: PL 280 TC

Pressure Ranges: !0-50 psi differential

Excitation: 5 volt DC

The transduction of this device is through a resistive,

balanced and fully active strain gage bridge of resistance

34

350 ohms. The combined nonlinearity and hysteresis is less

than ±1% of full scale.

The outlet of the hydraulic cuff is connected to the inlet

pressure port of the transducer. A strip chart recorder

and/or oscilloscope is connected to the transducer via a

digital signal conditioner for recording the corresponding

responses.

3. DIGITAL PROCESS INDICATOR

A Digital Process Indicator is used to supply power to the

transducer.

Specifications: Digital Process Indicator

Manufacturer: VISIPAK Action Instrument

Model: VIP 504

Excitation: 5 volt to 15 volts ·

This indicator serves as a dual function direct process

indicator and a signal conditioner for driving recorder or

controls. Figure 14 consists of a 4-arm bridge conditioner

+

Figure 14. Bridge conditioner circuit

for direct display of strain gage process. Excitation is

field adjustable from 5-15 volts de. The display is

specified to read directly in % of voltage change.

35

4. STRIP CHART RECORDER

A SOLTEC Strip Chart Recorder is used for the entire

experiment. The pressure response in the transducer is

monitored in the recorder.

Specifications: SOLTEC Chart Recorder

Manufacturer: SOLTEC Corporation, CA

Chart Speed: 1.5 cm/hr to 120 cm/min

Scale: ±0.SmV to 200V

Response Time: 0.25 sec

For best experimental results, a 30 cm/min of chart speed

and 200mV scale were selected during the blood pressure

measurement in human subjects. Next, a 60 cm/min of chart

speed and 500 mV scale were used for testing on the

mechanical heart system. The input signal to the chart

recorder is directly connected from output of digital

display indicator.

5. OSCILLOSCOPE

A Tektronix Storage Oscilloscope is used to display the

pressure pulse when mechanical heart simulation system is in

operation.

Specifications: 5111 Storage Oscilloscope

Manufacturer: Tektronix, Inc., OR

Plug-in Unit: 5A22N Differential Amplifier

Type 3C66 Carrier Amplifier

A vertical scale of 50mV and a horizontal scale of 0.2

sec/div were used to receive the best pulse contour.

36

6. GEAR MOTOR

The Dayton Gear Motor is used to drive the mechanical heart

pump.

Specifications: Model 2Z802

Manufacturer: Dayton Electric Manuf. Co.,

IL

Full Load Torque: 45 in-lb

Powered By: 1/15 hp, 5000 rpm, full load

series wound motor

The speed of the motor is variable according to the load

applied. The unit is equipped with triple reduction gears

(ratio 100:1) wound motor. The cam is mounted on the shaft

of the motor. The speed of the motor is set approximately

at 105 rpm. At that speed the form of pressure pulse as

observed in the oscilloscope is quite similar to a normal

arterial pulse.

37

EXPERIMENTAL RESULTS AND ANALYSIS

According to the concept hypothesized previously on the

development of the hydraulic occlusive cuff blood pressure measurement

technique, roore than sixty data had been collected from different

subjects. Immediately after each measurement by the hydraulic scheme,

the subject's blood pressure was taken by the conventional pneumatic

(auscultatory) method. The data obtained by two methods were

compared.

The main differences between the two blood pressure measurement

schemes are as follows:

Hydraulic

1. Identification of diastolic

and systolic levels by pre­

ssure pattern on the compr­

ession curve.

2. Curve contains more than

just diastolic and systolic

points, e.g., display of

pulse contour as well as

pressure-volume displace­

ment recording for vascular

properties.

3. Imcompressible fluid (water)

is used as medium for trans­

mitting hydraulic pressure.

Pneumatic (auscultatory)

Identification on diastolic

and systolic levels by tur­

bulence sound on the decom­

pression side.

Only diastolic and systolic

points are obtainable.

Compressible fluid (air) is

used for transmitting

acoustic sound.

4. Readings sensitive to small Readings rather insensi-

movement in arm, hand, and tive to all these, but room

finger, even talking. noise may impair good read-

ing, acoustically.

5. More instrumentation is involved. Device is simple and portable.

6. Readings are taken on the

forearm.

Reading are taken on the upper

arm.

Regardless of these differences in concept and in measurement,

the results obtained by both methods are in remarkable agreement.

Table I shows the list of subjects whose systolic and diastolic

pressures are recorded by both hydraulic and pneumatic systems and its

percent of deviation between the two. A bar chart has been prepared

in Fig. 15 to display the results shown in Table I. Table II shows a

summary for number of cases, in percent, which agree within various

confidence limits measured by the individual methods. It can be

observed that, for a ±s% comparative differences, both systolic and

diastolic values are over 80% in agreement. There is no consistent

difference, however, existing between these readings. Most subjects

were young males, aged 20-30. Seven young females as well as a few

males up to ages 58 participated. Only two were hypertensive, and of

the normotensive subjects, none reported any known cardiovascular

abnormality.

The pressure records on the chart demonstrated a similar

pattern, regardless of slope and pulse frequency, for all the

subjects. The first phase of the record began with the increasing

static pressure up to a certain point. The second phase shows the

39

40

TABLE I COMPARISON OF BLOOD PRESSURE RECORDS MEASURED

BY PNEUMATIC ANO HYDRAULIC SCHEMES

Pneumatic Hydraulic Test No. nm Hg nm Hg % Deviation

Sys/Dias Sys/Dias Sys/Dias

1 114/82 115/84 -1.0/-2 .4 2 124/85 123/84 +0.8/+1.1 3 130/90 131/86 -0.7/+4.4 4 115/88 120/88 -4.3/ o.o 5 125/88 125/88 0.0/ o.o 6 108/76 107/70 +0.9/+7.8 7 116/76 117 /85 -0.9/-11.8 8 96/70 98/73 -2.0/-4.2 9 100/70 104/71 -4.0/-1.4

10 106/71 106/72 0.0/+2.7 11 118/78 115/80 +2.5/-2.5 12 114/88 117/85 -2.6/+3.4 13 104/75 106/74 -1.9/+1.3 14 125/82 119/80 +4.8/+2.4 15 145/92 133/88 +8.3/+4.3 16 160/100 160/98 0.0/+2.0 17 145/98 144/98 +0.7/ o.o 18 125/85 124/90 +0.8/-5.8 19 122/82 122/82 0.0/ 0.0 20 118/90 122/72 -3.4/+18.8 21 118/82 122/81 -3.4/+l.2 22 118/84 120/80 -1.7/+4.7 23 110/64 120/66 -9.0/-3.0 24 130/76 117/75 +10.0/+l .3 25 118/75 118/72 0.0/ 0.0 26 130/86 138/90 -6.2/-4.6 27 118/75 120/75 -1.7/ o.o 28 100/65 104/66 -4.0/-1.5 29 115/82 135/81 -17.4/+l.2 30 130/80 128/82 +1.5/-2.5 31 123/75 120/75 +2.4/ 0.0 32 95/62 95/61 0.0/+1.6 33 148/92 143/93 +3.4/-1.0 34 108/72 112/56 -3.7/+22.2 35 118/72 117 /74 +0.8/-2.7 36 110/75 120/88 -9.1/-17.3 37 110/74 111/76 -0.9/-2.7 38 115/74 115/76 0.0/-2.7 39 105/74 106/78 -0.9/-5.4 40 126/86 128/88 -1.6/-2 .2 41 115/72 112/74 +2.6/-2.7 42 115/82 115/80 0.0/+2.4 43 120/72 120/77 0.0/-6.9 44 125/84 123/85 +1.6/-1.2 45 128/78 115/75 +10.0/+3.8 46 120/76 109/75 +9.2/-1.3 47 120/68 125/72 -4.2/-5.8 48 122/72 120/69 +1.6/+1.4 49 122/74 122/74 0.0/ c.o 50 115/80 120/80 -4.3/ o.o 51 112/80 111/82 +0.9/ o.o 52 110/78 109/77 +0.9/+l.2 53 112/70 109/71 +2 .6/-1.4 54 145/102 149/101 -2.8/+1.0 55 106/68 122/67 -13.2/+l.4 56 135/72 131/72 +3.0/ o.o

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PERCENT OF INDEPENDENT HYDRAULIC CUFF VS. AUSCULTATORY PRESSURE MEASUREMENTS

FALLING WITHIN SELECTED DIFFERENCE LIMITS

Difference limit Cases falling within difference limit percent percent

Systolic Diastolic

1 32 18

2 53 46

3 61 70

4 75 77

5 82 84

6 82 89

7 84 91

8 84 93

9 86 93

10 94 93

42

oscillations due to the dynamic pressure caused by the pulsatile flow

in the blood vessel during collapsing. Finally, in the third phase,

the oscillations almost ceased after the blood vessel collapsed

completely. Due to the extreme sensitive experimental set up, low

amplitude oscillations still exist as heart pulses continue to be

transmitted to the upstream side of the cuff.

CRITERIA FOR PULSE PRESSURE

The records obtained during the rising pressure phase were more

useful than the records obtained during the pressure dropping phase on

the chart. In many cases, features of actual intra-arterial pressure

including the presence of a dicrotic notch were observed. Pressure

fell in a non-linear fashion on the decompression curve as the fluid

was withdrawn from the cuff, yielding oscillatory records whose shape

was difficult to analyze. A typical record is shown in Fig. 16.

Upon examination of preliminary results, some unique features of

the oscillatory records were found which correspond closely with

auscultatory diastolic/systolic pressure values. These are indicated

in the expanded view of Fig. 17. The diastolic value corresponds

closely to the low amplitude pulsations where a rather flat (zero

slope) inter-pulse profile first appears as pressure rises.

The pressure oscillations continue after the diastolic point

during the rising pressure period. The pulse amplitudes generally

rise to a peak, then diminish. Concurrently, the inter-pulse profile

goes through changes in slope. The pulse just prior to the point

where inter-pulse profiles seem to passively follow the rising cuff

43

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pressure corresponds quite closely to systolic pressure. From there

on, the oscillatory pulse profile reduces significantly as the vessel

becomes fully.collapse. From here and beyond, small amplitude

oscillations, due to the arterial pulse at the upstream side of the

occluded vessel, are observed. It is likely that the changes of pulse

profile are closely related to the sound producing events which are

detectable with a stethoscope.

The above description forms the criteria used in obtaining the

diastolic and systolic pressures for purpose of comparison.

No two records were exactly alike and we attribute the

difference to size of forearms, relative volume of muscle/vascular

tissue and possibly other factors. The slope of the risi~g pressure

curve, with constant pump displacement, varied considerably between

subjects. Figure 18 shows the sample of selected records for

comparing pressure gradient for different subjects (regardless of

oscillations). It can be observed that curve I has the largest

gradient, while curve VI has the smallest pressure gradient. About

80% of the records fell between the curve III and V in which 60%

(approx.) fell between III and IV. The curve pattern between

diastolic and systolic point is also different for different

subjects. Greater nonlinearity (as concave behavior) appears as curve

approaches near or beyond the systolic level.

In addition to the difference in slope of rising pressure,

amplitude and waveform of pulsatile pressure oscillations also varied

markedly. Among the various types of oscillations observed, Fig. 19

shows the typical form of oscillation. The pulse pressure waves, as

46

PRESSURE

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I ~

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~ ""'O m en

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t

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B17

49

those measured by direct intra-arterial pressures, can be observed in

each record. The dicrotic notch exists if the amplitudes of

oscillation are bigger. Curve I is an example which shows big

oscillation with dicrotic notch clearly visible. Curve II and III are

the most common types of oscillation observed while curve IV and V are

the examples of small oscillations. The latter are distinguished by

low and high pulse rate.

The readings are further recorded as the pressure was decreased

by reversing the speed of the constant volume displacement pump. This

process is similar to that used in the pneumatic occlusive technique.

However, no distinct criteria have been found which yield a good

correlation with the pneumatic auscultatory data. Data curves in this

portion are more nonlinear than those on the increasing side. Sharp

"saw-tooth" forms are superimposed on rather steep curve. The time

taken to return to zero pressure level is also shorter than on the

increasing side. It is also observed that between every two trials

the first record always took a little longer than the second reading.

This may be caused by the muscle relaxation properties.

The hydraulic occlusive cuff, designed for measuring blood

pressure, is further used in the mechanical pressure pulse simulation

system. Figure 20 shows a photograph of the pressure pulse recorded

when the mechanical pressure pulse pump is operating. The pulse is

similar to those measured by direct invasive method in which the

systolic and diastolic levels as well as dicrotic notch can be

observed clearly. To observe the puls_e contour, the inlet port of the

transducer is directly connected to a T-connection at the rubber

tubing through which the water is flowing. The changes in pressure

levels during the pump operation were received by transducer and

displayed on the storage oscilloscope.

The experimental setup as shown in Fig. 9 is utilized again to

demonstrate the use of mechanical heart beat pump. Beef kidney was

found to be the best material that can simulate the muscle properties

of the human arm. However, the kidney used was not living tissue

which also lost its elastic properties with time. The simulated arm

is inserted inside the hydraulic cuff and accordingly the pressure is

increased by operating the constant volume displacement pump. The

date is recorded during both pressure increasing and decreasing

periods.

Figure 21 is the complete record measured when the latex rubber

tube, which simulates blood vessel, is fully open and in kinked (low)

position. This is also similar to the pattern obtained in the

indirect blood pressure measurement in humans by using the hydraulic

occlusive cuff. Oscillation begins when the rubber tube starts

collapsing and finally ceased after the tube is completely collapsed.

The record as shown for the fully open and kinked positions did not

follow the same pattern as differences were found in various aspects.

The most significant difference is observed at point 37 of the chart

on each experimental operation.

The oscillation begins at point 37 in the fully open position

and again diminishes at point 40 of the recorded chart. Some ·small

oscillations still exist beyond point 40. This oscillation is growing

bigger slowly and approaches maximum amplitudes up to point 67.

51

2S

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PRESSURE

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i " !

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I ; I

: i I . , . I . ! .

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Similarly, it followed theLsame pattern in the kinked position

except there are fewer activities in terms of oscillations. At point

37 the pressure gradient decreases in contrast to the fully open

position. The pressure gradient picks up again and follows the same

as in fully open later. Oscillations also began after point 50 only.

However, fewer oscillations are observed in the kinked position as

compared to the bigger oscillations and more distinctive dicrotic

notches in the fully open position. During the period of pressure

drop, there are more irregularities in the kinked position, such as

sudden change in the uniform pressure gradient. The results are

caused by the restriction in the flow from which the pressure wave

does not travel freely, and, the effect of compliance (beef kidney)

and rubber tube properties.

53

CONCLUSIONS AND RECOMMENDATIONS

An oscillometric arterial pulse-pressure measurement system ~as

been developed and tested. The design and the construction of a rigid

occlusive cuff were presented. An instrumentation and a testing

procedure were described. Criteria were established as how to

determine the systolic and diastolic pressure levels.

A mechanical model for simulating human blood pressure pulse was

also studied. The records taken on the simulated arm by operating the

mechanical model resembled those measured from the human subjects.

Doe to the difference in muscle properties between the human arm and

simulated arm, these records showed different frequency and

amplitude. Based on the limited number of test runs, in connection

with the demonstration of arteriosclerosis properties, it was

speculated that the changes in the pulse profile were due to the

constriction of the latex rubber tubing used as blood vessel. A

detailed study would be required to verify this behavior by testing

directly on either human subjects or using experimental animals.

The results obtained from the oscillometric method using the

hydraulic occlusive cuff have shown close correlation with respect to

the current auscultatory technique. Additionally, it is believed that

the data obtained would be suitable to convert automatically to

digital display system. This will allow independence from human

observer error in determining the systolic/diastolic pressure values.

The improved oscillometric system may overcome the problems

associated with auscultatory blood pressure measurements. Electronic

pressure transducers as well as the monitoring equipment are available

now with high frequency response to accurately signal the pressure

oscillations. A further advantage of the hydraulic cuff-oscillometric

system is that much more data can be generated in a single pulse

pressure determination, potentially of diagnostic utility, than

results from auscultatory pressure measurements. In addition, it is

speculated that features of the pulsatile pressure records might be

detected that correspond to the various Korotkoff sound phases

identified by a stethoscope. It is believed that this system could be

useful not only for determining the usual pulse pressure reading

(systolic/diastolic), but it would also be useful in evaluating

vascular compliance, vascular volume, and changes in vascular

resistance.

Much further work remains to be done, specifically, including:

1) build a more compact and convenient unit; ideally the unit

should be housed in a compact enclosure with only a "power"

switch and an "operate" button on the front panel,

2) establish adequacy of pulse pressure criteria,

3) develop a satisfactory algorithmn for automatic pulse

pressure determination from an oscillometric record, and

4) establish potential diagnostic criteria that corresponds to

quantifiabale features of a standard oscillometric pressure

record.

55

REFERENCES

1. Bruner, John M. R., "Handbook of Blood Pressure Monitoring," PSA

Publishing Co., 1978.

2. Erlanger, J., "A New Instrument for Determining the Minimum and

Maximum Blood Pressures in Man," John Hopkins Hospital Report,

Vol. 12, 1903-1904, p. 53-110.

3. Geddes, L. A., "The Direct and Indirect Measurement of Blood

Pressure," Chicago: Year Book Medical Publisher, 1970.

4. Howell, W. H., & Brush, C. E., "A Critical note Upon Clinical

Methods of Measuring Blood Pressure, 11 Boston Medi ca 1 and

Surgical Journal, 1901, 145, p 146-151

5. Korotkoff, N. S., "On the Subject of Methods of Measuring Blood

Pressure," Bulletin of the Imperial Military Medical Academy,

St. Peterberg, Vol. 11, 1905, p. 365-367.

6. Kotte, J. H., Iglauer, A., & McGuire, J., "Measruements of

Arterial Blood Pressure in the Arm and Leg: Comparison of

Sphygmomanometric and Direct Intra-arterial Pressures, with

special attention to their Relationship in Aortic

Regurgitation," American Heart Journal, Vol. 28, 1944, p.

476-490.

7. Kraudsman, David T ., "Methods and Procedures for Monitoring and

Recording Blood Pressure," American Physiologist, March 1975,

p. 285-294.

8. Paul, Burton, "Kinematics and Dynamics of Planar Machinery,"

Prentice Hall, Inc., 1979.

9. Norris, G. W., "Blood Pressure and its Clinical Application,"

Philadelphia: Lea & Febiger, 1916.

10. Segall, H. N., "A Note on the Measurement of Diastolic and

Systolic Blood Pressure by the Palpation of Arterial Vibrations

(sounds) over the Brachial Artery, Canadian Medical Association

Journal, Vol. 42, 1940, p. 311-313.

11. Steele, J.M., "Comparison on Simultaneous Indirect (Auscultatory)

and Direct (Intra-arterial) Measurements of Arterial Pressure

in Man, 11 Journal of Mount Sinai Hospital, Vol. 8, 1941, p.

1042-1050.

12. Strong, Peter, "Biophysical Measurements," Tektronix, Inc., 1970.

13. Uzman, J. W., & Wood, E. H., "Oximeter Earpiece for Measurement of

Systemic Blood Content and Arteri a 1 Pressure in Human Ear,"

American Journal of Physiology, Vol. 163, 1950, p. 756-757.

14. Van Bergen, F. H., Weatherhead, D. S., Treolar, A. E., et. al.

"Comparison of Indirect and Direct Methods of Measuring

Arterial Blood Pressure," Circulation, Vol. 10, 1950, p.

481-92.

57