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Description  |
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BACKGROUND OF THE INVENTION
This invention relates in general to blood pressure measuring device and in
particular to such devices which are automatic and do not require a
stethoscope for identifying systolic and diastolic conditions and which
may also provide mean pressure and heart rate in a direct reading manner.
Blood pressure measurement as a medical procedure is not new nor are the
basic concepts or equipment which are employed new. Conventional
measurement procedures require that a compression cuff be snugly wrapped
around the patient's right arm approximately one inch above the
antecubital fossa location. The compression cuff includes an inflatable
bladder at one end and this bladder must be applied directly over the
artery with the remainder of the compression cuff being used as a wrap to
secure the bladder in place. The compression cuff (actually the inflatable
bladder) is inflated rapidly to a pressure above the assumed or previously
determined systolic pressure. By means of a manually released valve, the
cuff is deflated at a rate of two to three millimeters (mm) of mercury
(Hg) per heartbeat. With the bell of a stethoscope placed snugly over the
artery (thereby producing an airtight seal) the physician may either watch
an aneroid dial or column of Hg of a suitable sphygmomanometer and note
the pressure at which characteristic changes in the Korotkoff sounds
occur. From these sounds, created by pressure controlled compression
waves, the systolic and diastolic blood pressures can be determined.
This conventional method has various shortcomings in terms of convenience,
versatility and accuracy. It is necessary for the physician or medical
personnel taking the blood pressure to accurately position the inflatable
bladder over the artery, to make an air tight seal with the bell of a
stethoscope over the artery, to manually control the pressure release rate
and to attempt to read the corresponding pressure when particular
characteristic sounds are detected through the stethoscope.
In an attempt to improve blood pressure measuring techniques, certain
modifications have been made to conventional blood pressure measuring
equipment. Examples of some of these modifications are shown by the
following patents:
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Pat. No. Patentee Issue Date
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3,651,798 Egli et al. 3/28/72
3,993,047 Peek 11/23/76
4,005,701 Aisenberg et al.
2/01/77
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Egli et al. discloses an electronic blood pressure indicator for use with
conventional blood pressure measuring equipment. This device includes a
microphone for translation of arterial blood flow sounds into electrical
signals. The electronics are somewhat complicated due to the need to
filter out undesired and extraneous frequencies, and to distinguish the
characteristic changes in the Korotkoff sounds.
Peek discloses a type of electronic blood monitoring device which is used
to monitor blood circulation conditions. This device includes an infared
radiation source and a corresponding sensor which are placed in contact
with the skin of the subject. This device is not related to conventional
sphygmomanometers nor is it usable for reading blood pressure.
The patent to Aisenberg et al. discloses a noise-rejecting electronic
sphygmomanometer which utilizes two microphones positioned adjacent the
inflatable bladder in a conventional compression cuff. The first
microphone is used to pick up Korotkoff sounds as the cuff is deflated and
blood flow in the patient's arm resumes. The second microphone is used to
pick up background noises so that electronic circuit discrimination and
signal comparisons can be used to provide an accurate sound indication
which is unaffected by such background noises.
With each of the aforementioned devices there are numerous shortcomings
which exist. To begin with, all presently available blood pressure
measuring devices require the accurate placement of the bladder directly
over the artery (this is true even with the use of microphones in lieu of
a stethoscope). Secondly, known electronic blood pressure measuring
devices rely entirely on pickup of the characteristic changes in Korotkoff
(pressure wave) sounds and thus employ conventional microphones. The need
to filter and isolate the correct sounds results in relatively complex and
thus costly circuitry. Furthermore, the number of circuit components
required is large and this has an effect on labor costs and system
reliability. A third shortcoming is that each device is designed for only
taking the blood pressure of humans and for using, for example, the left
arm of the patient for the measurement. It is a recognized rule that in
order to obtain an accurate reading, the width of the inflatable bladder
should be approximately 20% greater than the diameter of the limb on which
it is used. Therefore, it would be an advantage if blood pressure devices
could be provided with detachable and interchangeable cuffs so that the
blood pressure of different sizes of patients, and especially infants,
could be accurately measured by using a single measurement unit, such as a
sphygmomanometer, but with different, interchangeable, compression cuffs.
Another shortcoming with conventional devices is that a portion of the
medical personnel's attention is taken up with the activity of manually
releasing the pressure from the bladder and attempting to do so at a
regular rate, such as 2-3 mm of Hg per second. Automatic air pressure
release means would permit the medical personnel to devote their full
attention to hearing and noting the corresponding pressure at systolic and
diastolic conditions and the monitoring of a dial to make such
determinations.
A further shortcoming of such present-day devices involves the fact that
these conventional devices are only usable on human beings and are not
acceptable for animals due to the differences in anatomy between such
animals and human beings. For example, in dogs, the arteries within the
animal's limbs are somewhat recessed and a conventional cuff and bladder
applies pressure to only one side of the limb and will act to push the
artery back and away from the surface of the limb where the stethoscope
bell is normally placed. The surface of the artery will thus not be in
close proximity to the bladder portion of the cuff as is required for
accurate readings. A further difficulty with animals, such as dogs, is
that the animal's artery in an appendage, such as a hind leg, may be
somewhat randomly positioned and thus may be difficult to locate and the
requisite precise placement of the stethoscope bell over the artery may
not be possible. In addition, when the size of the animal's appendage is
small, there may not be sufficient surface skin area to permit the bell of
the stethoscope to make a seal over the artery and due to the fact that
the edges of the stethoscope overhang the sides of the appendage, the
accuracy of any readings taken will be subject to noticeable error.
Therefore, arterial catheterization--a method which is both lengthy and
complicated--must be used on animals in order to accurately measure their
blood pressure.
Although only two classes of subjects have heretofore been mentioned, adult
human beings and animals, a third class exists. This class includes
infants whose physical size requires a specially sized compression cuff
and whose artery size and position make conventional devices unreliable.
Infants' arteries are so small, such as the brachial artery which is often
used, that they are difficult to find and when a stethoscope must be used
(as with conventional blood pressure measuring devices) positioning
variations can cause a 10 to 20 percent difference in the readings. Also,
as previously mentioned, there may not be adequate skin area over the
artery for the bell of the stethoscope to make an air-tight seal.
Consequently when an infant's condition is being monitored, there is no
way to tell if blood pressure variations are due to the measuring
equipment and technique or due to a physical change in the infant. This
problem can be eliminated by providing measurement means which does not
require a stethoscope or the precise positioning of pick-up microphones.
It would also be an improvement to such devices if other useful data could
be obtained at the same time blood pressure readings are taken, and
displayed in a conveniently readable manner, without having to subject the
patient to further medical procedures or testing.
SUMMARY OF THE INVENTION
A blood pressure measuring device according to one embodiment of the
present invention comprises a sphygmomanometer, a compression cuff having
an inflatable bladder therein, a flexible tube, means for inflating the
inflatable bladder and an air leak valve having a continuously open air
escape aperture therein sized for a predetermined leak rate. The flexible
tube is connected between the inflatable bladder and the sphygmomanometer
and the air leak valve is coupled to and coaxial with this tube.
A diagnostic device for determining characteristics of a blood circulation
system according to another embodiment of the present invention comprises
means for applying artery occluding pressure to an appendage artery, means
for gradually releasing the occluding pressure, means for sensing the
pressure of blood pulsating in the artery, means for indicating pressure
changes occurring in the artery and means for determining and displaying
systolic blood pressure, diastolic blood pressure, heart rate and mean
blood pressure.
One object of the present invention is to provide an improved blood
pressure measuring device which is equally well-suited for use on adults,
infants and animals.
Another object of the present invention is to provide a blood pressure
measuring device which is virtually fully automatic with the only manual
activities being the wrapping of the compression cuff and the inflating of
the bladder.
Yet another object of the present invention is to provide an electronic
indicator circuit which utilizes pressure changes in lieu of sounds in
order to detect systolic and diastolic conditions.
Related objects and advantages of the present invention will be apparent
from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a blood pressure measuring device according
to a typical embodiment of the present invention.
FIG. 2 is a perspective view of an air leak valve comprising a portion of
the FIG. 1 device.
FIG. 3 is a perspective view of a piezoelectric crystal apparatus
comprising a portion of the FIG. 1 device.
FIG. 4 is a schematic diagram of an indicator circuit comprising a portion
of the FIG. 1 device.
FIG. 5 is a graph of pressure change relative to time.
FIG. 6 is a graph of pressure relative to time.
FIG. 7 is a diagrammatic view of a blood diagnostic device according to an
alternate embodiment of the present invention.
FIG. 8 is a perspective view of a digital display console comprising a
portion of the FIG. 7 device.
FIG. 9 is a schematic diagram of circuit subsections comprising a portion
of the FIG. 7 device.
FIG. 10 is a block diagram of the circuitry of the FIG. 8 console.
FIGS. 11 through 11e are detailed schematics corresponding to portions of
the FIG. 10 block diagram.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
For the purposes of promoting an understanding of the principles of the
invention, reference will now be made to the embodiments illustrated in
the drawings and specific language will be used to describe the same. It
will nevertheless be understood that no limitation of the scope of the
invention is thereby intended, such alterations and further modifications
in the illustrated device, and such further applications of the principles
of the invention as illustrated therein being contemplated as would
normally occur to one skilled in the art to which the invention relates.
Referring to FIG. 1, there is illustrated a blood pressure measuring device
20 according to the present invention. Device 20 includes compression cuff
21 with inflatable bladder 22 therein, first flexible tube 23, control
unit 24, dial indicator 25, junction 26, second flexible tube 27 and bulb
28. The general appearance of device 20 is that of a conventional
sphygmomanometer device for measuring a person's blood pressure. However,
there are important distinctions which will be discussed hereinafter. The
particular manometer shown is that of an aneroid manometer which consists
of a metal bellows, the inside of which is connected to the compression
cuff. Variations of pressure within the system cause the bellows to expand
and collapse and this movement rotates a gear that turns needle 29 to make
the indicated dial reading. Device 20 is equally suited for use with a
mercury-gravity manometer in lieu of an aneroid manometer and although
such a device is not shown, it is to be understood that the principles and
features discussed herein are equally applicable. Control unit 24 houses a
control circuit, described hereinafter, which is energized by means of
ON-OFF switch 24a.
Although the exterior appearance of compression cuff 21 is much like that
of a conventional compression cuff as is well known to the blood pressure
measuring art, the internal design of compression cuff 21 is unique.
Flexible tube 23 connects to and opens into inflatable bladder 22 and this
bladder extends longitudinally for substantially the entire length of
compression cuff 21. This elongated condition of inflatable bladder 22,
which is significantly different than conventional bladders, permits the
entire bladder 22 to be wrapped around the portion of anatomy which is
being used for the blood pressure measurement. Conventional inflatable
bladders are constructed of an elastomeric compound which provides
sufficient expansion to accomodate the air pumped into the bladder. Due to
the increased size of bladder 22, the degree of elasticity may be less and
vinyl material is suitable. The result is that pressure is applied
radially inward to that portion of anatomy from all sides in a uniform
circumferential manner. Although the flow of blood will be restricted by
this type of design in much the same manner as a conventional compression
cuff restricts the flow of blood, the unique distinction is that when
using the described device on animals such as, for example, dogs, the
artery involved will not be pushed back and away from the bladder into a
recessed location. Thus an artery of such an animal which may normally be
in a recess or depression can be used for accurate blood pressure
measurement due to the fact that this type of compression cuff will
virtually hold the artery in its normal position and the bladder will be
placed in close proximity to this artery as the bladder is inflated.
Bulb 28, which acts as a hand pump, is used as the means to inflate bladder
22. As the attending medical personnel squeezes on bulb 28, air will be
introduced into bladder 22 and at the same time the corresponding pressure
will be indicated by means of needle 29 on dial indicator 25. Junction 26
serves as a connector between flexible tube 27, flexible tube 23 (23a) and
tubes 23b and 23c which enter control unit 24 and connect to the aneroid
manometer. With conventional blood pressure measuring devices, there would
be a small manually operated valve adjacent bulb 28 within tube 27 which
would allow air to be released from bladder 22 thereby lowering the system
pressure. In lieu of this manually operated conventional device, there is
included with the present system an air leak valve 30. Once bulb 28 has
been used to rapidly inflate bladder 22 to a pressure level somewhat above
the presumed systolic condition, valve 30 which has a continuously open
aperture 34, releases the air from bladder 22 at a predetermined rate.
Aperture 34 is intermediate the ends of valve 30 and the air release flow
rate is dependent on the pressure within bladder 22, this rate gradually
decreasing over the time interval between systolic and diastolic
conditions. A manual valve 28a for rapid deflation is also provided.
Although mention has been made of the fact that device 20 is usable on
animals for measuring their blood pressure, due to the design of
compression cuff 21 and inflatable bladder 22, it is to be noted that
device 20 is equally applicable to adults, children and infants. It is a
recognized rule that in order to obtain an accurate reading, the width of
the inflatable bladder 22 should be approximately 20% greater than the
diameter of the limb around which the compression cuff 21 is wrapped.
Therefore, a single size of compression cuff 21 and inflatable bladder 22
would not be usable for all classes and sizes of patients. In order that
the control unit 24 and the corresponding sphygmomanometer which is used
with control unit 24 do not have to be unnecessarily duplicated, device 20
is provided with means by which compression cuff 21, inflatable bladder 22
and flexible tube 23 can be removed and a differently sized compression
cuff attached. This detachable compression cuff feature is provided by
means of air leak valve 30 which is tapered on each end and extends
outwardly in order to accommodate main tube 23 on one end and a smaller
branch tube 23a on the opposite end. Referring to FIG. 2, it is shown that
tubes 23 and 23a are merely pressed over the ends 35 and 36 respectively
of air leak valve 30. Suitable surface deforming may be provided on ends
35 and 36 such as knurling, ridges or lips in order to securely hold tubes
23 and 23a in position while still permitting the quick and easy
disconnect of these tubes. The region surrounding aperture 34 may be
recessed in order to keep foreign matter from clogging aperture 34 and for
preventing aperture 34 from resting against a surface which might restrict
the air leak flow. As will be described hereinafter, it is also possible
to replace air leak valve 30 with a new valve having a differently sized
aperture 34 in order to vary the air leak rate. Thus air leak valve 30 may
either be detached with compression cuff 21 and tube 23 by disconnecting
at end 36, or air leak valve 30 may remain with the basic unit and
compression cuff 21 and tube 23 by disconnecting at end 35.
The conventional method of taking a blood pressure measurement is to apply
a compression cuff around the limb of the patient, restrict the blood
flow, gradually release the pressure and as the pressure decreases and
blood flow resumes listen by way of a stethoscope to Korotkoff sounds
which, to the trained ear, are indicative of systolic and diastolic
conditions. Even newer electronic blood pressure measuring devices still
rely on using these sounds in order to determine systolic and diastolic
conditions. The present device employs a piezoelectric crystal 40 within
control unit 24 as part of the control and display circuitry. Leading from
junction 26 within control unit 24 are two additional sections of tubes,
23b which leads to the sphygmomanometer, and 23c which leads to
piezoelectric crystal apparatus 41. Apparatus 41 includes crystal 40 which
is sealed around its periphery and mounted atop housing 42 which has a
hollow cavity 43 therein. Tube 23c opens into cavity 43 and as pulsed
variations in pressure are delivered through tubes 23, 23a and 23c,
piezoelectric crystal 40 flexes and undergoes stress variations which are
converted into electrical signals and delivered to the control and
indicating circuitry by means of wires 44 and 45 which are bonded to the
surface of piezoelectric crystal 40.
Referring to FIG. 4, the signals out from piezoelectric crystal 40 are
introduced into a high pass RC filter comprising capacitor 51 and resistor
52 and from there into the positive input of one amplifier 53 of a dual
operational amplifier, such as, for example, an LM 747 offered by Motorola
Semiconductor Products, Tempe, Arizona, and which is used in a
noninverting mode. Variable resistor 54 and feedback resistor 55 provide
gain adjustment for this portion of the indicating control circuit and a
minimum gain of 16 is selected. The output of amplifier 53 is AC coupled
to the comparator-timing portion of circuit 50 by means of capacitor 56.
The selected value of capacitor 56 is large enough so that amplified heart
sounds are allowed to pass unattenuated. The comparator-timing portion of
circuit 50 includes resistors 57, 58, 59 and 60, capacitor 61, diodes 62
and 63 and the remaining operational amplifier on the LM 747 integrated
circuit chip, amplifier 64. A negative reference voltage is established by
a series diode configuration including diodes 65 and 66, resistor 67 and a
power supply potential of negative 8.4 volts. The pulse width is a
function of resistors 57 and 58, capacitor 61, reference voltage and
negative and positive saturation voltages of amplifier 64. The pressure
fluctuations received by piezoelectric crystal 40 and which are converted
and amplified by amplifier 53 will cause the comparator-timing portion of
circuit 50 to illuminate light-emitting diodes 68 and 69 if the amplified
signal out of amplifier 53 is greater than the selected reference voltage.
The duration of illumination, approximately 0.25 seconds, is controlled by
the selection of components and this duration is selected so that the
illumination of the light-emitting diodes is easily visualized and yet
does not conflict with rapid heartbeats which one might expect to occur
over a variety of patients and circumstances.
Light-emitting diodes (LED's) 68 and 69 require approximately 0.015 amps in
order to illuminate and the output portion of circuit 50 includes an
output booster consisting of resistors 70 and 71 and transistor 72. A
suitable component for transistor 72 is a 2N5172 offered by Motorola
Semiconductor Products. The value of resistors 70 and 71 are analytically
determined in order to deliver the necessary light-emitting diode current
as well as to saturate transistor 72. Two mercury cell 8.4 volt batteries
are used for the negative and positive power supply potentials. Although
it is clear how the circuit just described can produce a current signal in
response to a pressure change, the important feature of blood pressure
measuring device 20 is how various pressure levels are detected so that
the blood pressure at diastolic and systolic conditions can be identified.
Referring to FIGS. 5 and 6, there are illustrated two graphs which have the
same horizontal (X) time axis and represent the interval during which a
blood pressure measurement is taken. FIG. 5 has as its vertical (Y) axis
scale, change in pressure as seen by piezoelectric crystal 40. An upper
level indicated by line 76 must be exceeded in order to exceed the
threshold or turn-on level for LED's 68 and 69. As the air within inflated
bladder 22 escapes through opening 34 in air leak valve 30, the change in
pressure will be relatively constant even though the actual pressure level
is decreasing. This nearly constant pressure change indicated by line 77
is below the threshold level and the LED's remain off.
Referring now to the graph of FIG. 6, the vertical (Y) axis is actual
pressure within tube 23 leading from bladder 22 to control unit 24. The
decreasing bladder pressure is indicated by line 78 and the heartbeat of
the subject is represented by repeating wave signal 79. When the pressure
in bladder 22 exceeds the maximum pulsing pressure due to the subject's
heartbeat, the blood flow will be temporarily occluded. As the pressure in
bladder 22 is released through air leak valve 30, there will come a point
in time (T.sub.1) when the maximum heartbeat pressure first exceeds the
occluding pressure of the bladder and blood begins to flow past the
compression cuff. The flow of blood causes an increase in the
circumference of the appendage at the location of the compression cuff and
this size increase causes a pressure pulse in tube 23 which appears as
spike 80 in FIG. 5. This spike corresponds to systolic condition and since
spike 80 exceeds the turn-on level of line 76, LED's 68 and 69 will
momentarily flash on. This initial flashing on signifies when dial
indicator 25 should be read in order to obtain the systolic blood pressure
measurement.
With subsequent heartbeats, additional spikes will occur causing LED's 68
and 69 to flash on and off. As the pressure in bladder 22 decreases even
further there will occur a point in time (T.sub.2) when the lowest
pressure level of the heartbeat will exceed the occluding pressure and all
the blood of each pulse will flow without causing any change in the
appendage circumference. Consequently, there will not be any pressure
spikes and the LED's will turn off and remain off until the measurement
procedure is repeated.
Although device 20 has been discussed and described solely as a blood
pressure measuring device, the indicating and control circuit 50 and other
features of this device will permit basic modifications to be incorporated
such as counting the rythymic beats with respect to a time interval and
thereby determine the patient's heartbeat rate. In addition, it would be
possible to convert dial indicator 25 to a digital readout means whereby
the blood pressure at systolic and diastolic conditions could be
numerically derived and presented as well as the mean pressure. As
described in detail hereinafter, each of these various readings; heartbeat
rate, systolic pressure, diastolic pressure and mean pressure can be
individually and simultaneously displayed.
As has been previously described control unit 24 works in combination with
dial indicator 25 to both indicate the pressure level within tube 23 and
to flash OFF and ON light-emitting diodes 68 and 69 when the systolic
pressure is present. Similarly, diastolic pressure is identified by taking
the dial reading when the diodes turn off and remain off. In certain
situations it may be desirable to obtain further information about a
patient such as, for example, heart rate and mean blood pressure.
Conventional manometers are not suitable for obtaining this type of blood
circulation system data because these conventional manometers are designed
solely for a single dial reading at any given instant of time, and such
devices neither retain the data nor provide circuitry for obtaining other
data.
FIG. 7 illustrates an alternative blood pressure measuring device 89 which
includes additional circuitry and additional capabilities over what has
been previously described for device 20. Blood pressure measuring device
89 includes some of the same basic components used with device 20, such
as, compression cuff 21, inflatable bladder 22, first flexible tube 23,
second flexible tube 27, bulb 28, tubes 23a, 23b and 23c, air leak valve
30 and piezoelectric crystal apparatus 41. Additionally device 89 includes
a pressure transducer device 90, solenoid air valve 91 and digital display
console 92. Digital display console 92 houses the pressure transducer
device 90, the piezoelectric crystal apparatus 41 and suitable analog and
microprocessor circuitry for the operation of device 89 as will be further
described hereinafter.
Console 92 is illustrated in FIG. 8 and is diagrammatically represented by
the broken lines in FIG. 7. Solenoid air valve 91 is located in line with
flexible tube 27 and is powered by a D.C. to D.C. converter 93, also part
of console 92, such that when energized, valve 91 connects the compression
cuff 21 to the inflation bulb 28 by providing an open path through tube
27. Converter 93 is powered by a five volt battery 94 (see FIG. 10) and
converter 93 provides outputs of plus and minus 15 volts. With initial
turn on of device 89, solenoid air valve 91 is open to atmospheric
pressure so that the pressure sensing components of device 89 may be
automatically zeroed to the atmospheric pressure before blood pressure
readings are taken. Once the diastolic blood pressure has been computed
and displayed, the solenoid will hold that pressure for a predetermined
interval of time and will then open in order to dump the pressure in the
cuff and return the components to the atmospheric pressure level.
Digital display console 92 includes four display window groups 96, 96a, 96b
and 96c each having three, seven-segment digital displays, a tube inlet
port 97, a start switch 98, a redisplay button 99, ON-OFF switch 100 and a
recharging input 101. Window group 96 is used to display the systolic
blood pressure of the patient which is accurately computed by the
monitoring, determining and display circuitry within console 92, and the
direct reading capability of the digital displays eliminate the need to
read a mechanically driven dial. Similarly, window group 96a displays the
diastolic blood pressure, window group 96b displays the mean pressure of
the patient and window 96c group displays the patient's heart rate. The
mean pressure is derived from the systolic and diastolic blood pressure
readings in accordance with the following expression:
P.sub.M =P.sub.D +(P.sub.s -P.sub.D)/(3)
where P.sub.M is the mean pressure, P.sub.D is the diastolic pressure and
P.sub.S is the systolic pressure. Each of these readings is in terms of
millimeters of mercury. Heart rate is derived by counting received heart
beat pulses for 15 seconds and then multiplying the total number by 4 in
order to express the heart rate in pulses per minute.
With the availability of blood circulation system characteristic data such
as systolic pressure, diastolic pressure, mean pressure and heart rate,
the programming options for the disclosed circuitry can be utilized to
generate and compute yet further data such as the pressure pulse product.
Digital display console 92 includes three main circuit sections (see FIG.
10), including analog section 104, microprocessor section 105 and digital
display section 106, and each of these sections will be described
hereinafter. Analog section 104 (see FIG. 9) includes three circuit
subsections 104a, 104b and 104c, two of which, 104a and 104b, derive their
output signals from pressure transducer device 90 inputs and the third of
the three (104c) generates a Korotkoff pulse from piezoelectric crystal
apparatus 41. Pressure transducer device 90 may be, for example, a model
LX1602G offered by National Semiconductor Corporation of Santa Clara,
California. The output of pressure transducer device 90 is coupled to a
unity gain amplifier 107 and the output of amplifier 107 is applied to a
unity gain inverting amplifier 108 which inverts the pressure voltage so
that the output voltage will increase in proportion to increasing
pressure. The output voltage scale factor is approximately 12.9 mV per 1
millimeter of mercury pressure. The D.C. pressure voltage output of
amplifier 107 is also applied to circuit subsection 104b and is input to
an A.C. coupled amplifier 109. An A.C. signal is superimposed on this D.C.
pressure voltage input to amplifier 109 and amplifier 109 has a gain of
approximately 50. The output of amplifier 109 is filtered by a
second-order low pass filter 110 which passes only frequencies below
approximately 20 HZ. The resultant output of filter 110 is amplified again
by amplifier 111, which has a gain of approximately 20, and the output of
amplifier 111 is compared by comparator 112 with a preset threshold
voltage. When the input signal from amplifier 111 is larger than the
preset threshold level, a logic level at the output of comparator 112
indicates the presence of a pressure wave which is the result of artery
expansion as the patient's heart beats.
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