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BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to apparatus and method for measuring arterial blood
pressure. More particularly, the blood pressure monitor of the present
invention allows noninvasive, instantaneous and continuous monitoring of
blood pressure and provides for producing a waveform which closely
represents the complete blood pressure waveform in a superficial artery.
Continuous, noninvasive blood pressure monitoring and recording are the
subject of extensive investigation. Both tonometric and sphygmomanometric
techniques are being considered. Where measurements are to be made while
the subject is engaged in normal activities and over long periods of time
tonometry is the preferred approach, because the pressure transducers used
in tonometry can be made small, light and easy to wear, whereas the
pressurized cuffs used in sphygmomanometry are relatively uncomfortable
and cuff pressure changes required to obtain significant parameters are
difficult to produce on a mobile subject. Further, the sphygmomanometer
only yields the systolic (highest pressure in the arteries during
contraction of the heart muscle) and diastolic (lowest pressure in the
arteries during relaxation of the heart muscle) pressures, whereas the
entire blood pressure waveform can be reproduced using the tonometric
approach.
2. Relation to Prior Art
The invention builds on and represents an improvement on the technique
described and claimed in U.S. Pat. No. 3,219,035, issued in the names of
Gerald C. Pressman and Peter M. Newgard and assigned to the assignee of
the present invention. The invention described in that patent relates to a
force-balance technique which provides a noninvasive, precalibrated blood
pressure measurement. The force-balance arrangement eliminates variations
in blood pressure measurements which are only a result of variations in
pressure between the transducer surface and the subject. A somewhat more
complex approach to solving the same problem is found in U.S. Pat. Nos.
3,880,145 to Edward F. Blick and 3,123,068 to Robert P. Bigliano. A major
difficulty encountered in the application of these devices has been
locating an initial position of the measuring transducer element relative
to the superficial artery and maintaining its position to obtain the
accuracy required.
The invention described and claimed in copending patent application Ser.
No. 848,753, filed even date herewith in the name of Peter M. Newgard and
assigned to the assignee of the present invention, represents an
improvement in the art in that it diminishes the positional accuracy
required of the transducer array by at least an order of magnitude. Hence,
initial placement of the device is trivial and the transducer design is
such that it easily tolerates the nominal variations in position that are
encountered during typical long term monitoring applications. Further, the
method of selecting the proper waveform from among those produced by the
array of individual transducers as set forth in that application is
responsible, at least in part, for both diminution of required positional
accuracy and tolerance for variations in position due to movements of the
subject during monitoring of the pressure.
The present invention constitutes an improvement over both the art and the
invention described and claimed in the Newgard application, supra, in that
the method and means of selecting the particular pressure sensitive
element of the array of transducers which most closely tracks the true
blood pressure waveform and gives the most accurate measure of blood
pressure are further refined.
SUMMARY OF THE INVENTION
The present invention, a noninvasive arterial blood pressure monitor,
incorporates a transducer array for generation of an electrical waveform
indicative of blood pressure in the artery and a novel method of
determining the waveform from among those generated by individual
transducers of the array which most closely matches the actual pressure in
the artery. Each individual force sensing element has at least one
dimension smaller than the lumen of the underlying artery wherein blood
pressure is to be monitored and the individual force sensing element is
selected to monitor blood pressure which is within one artery's diameter
of the pressure sensing element generating the maximum pulse amplitude and
which generates a waveform showing a local minimum of at least one of the
diastolic and systolic pressures.
An object of this invention is to provide an externally applied blood
pressure measuring transducer which is easy to position for accurate
continuous monitoring of arterial blood pressure even on a mobile subject.
Another object of the invention is to provide such a blood pressure
measuring transducer wherein an array of individual transducing arterial
riders is utilized and a means is provided to select and utilize the
arterial rider (pressure sensitive element) which most closely reproduces
the actual pressure waveform.
The novel features which are believed to be characteristic of the invention
are set forth with particularity in the appended claims. The invention
itself, however, both as to its organization and method of operation,
together with further objects and advantages thereof, may best be
understood by reference to the following description taken in connection
with the accompanying drawings, in which:
FIG. 1 shows the external appearance of a blood pressure transducer case,
typically positioned over an artery, for providing a continuous external
measurement of arterial blood pressure;
FIG. 2 is a schematic diagram illustrating the force balance between artery
and the multiple transducer elements (arterial riders), with the artery
wall properly depressed to give accurate blood pressure reading;
FIG. 3 is a perspective view of a sectioned transducer chip or substrate
showing the array of individual pressure sensitive transducer elements
(artieral riders) on the substrate and a preferred arrangement of the
individual pressure sensitive elements;
FIG. 4 is a plan view of a portion of the transducer array (silicon chip or
substrate) taken directly over one of the individual transducers 20 and
showing a transducer bridge and associated circuitry employed in
generating and measuring the blood pressure waveform;
FIG. 5 is a central, vertical section taken through the transducer case of
FIG. 1 showing the position of the transducer and its supporting
components;
FIG. 6 is a schematic circuit diagram showing circuit elements and their
connections which produce a waveform that tracks arterial blood pressure
and also selects the pressure sensitive element which produces the
waveform with maximum pulse amplitude, along with those pressure sensitive
elements immediately surrounding it;
FIG. 7 is a continuation of the schematic diagram of FIG. 6 showing circuit
elements which take the output signals from the circuit of FIG. 6, find
the waveform which contains a local minimum of diastolic or systolic
pressure among those waveforms generated by pressure sensitive elements
near the one generating the waveform of maximum amplitude and connect the
output of that one pressure sensitive element to a measuring and recording
device; and
FIG. 8 is a schematic circuit diagram showing a switch matrix which can be
substituted for the switches in the diagram of FIG. 6 (the switches 72
enclosed in the broken-line box 150) to ensure that the pressure sensitive
elements near the one which generates the waveform of maximum amplitude
are connected to the minimum finder circuit of FIG. 7, and hence ensure
selection of the pressure sensitive element which generates the waveform
most closely representing the complete blood pressure waveform in the
underlying artery.
DESCRIPTION OF A PREFERRED EMBODIMENT
A typical application of the transducer array for arterial tonometry is
illustrated in FIG. 1 wherein the transducer case 10, which has the
appearance of an ordinary wristwatch case, is held in place over the
radial artery in a human wrist 12 by an expansion band 14, also similar to
that of a wristwatch. Electrical wiring providing connections between the
transducer housing 10 and the circuitry (not shown in this figure) for
monitoring blood pressure is encompassed in an insulated cord 16 shown
emerging from the back of the transducer housing 10. Also enclosed in the
cord is a small tube that connects to a source of gas or air (not shown),
which source maintains a constant selected pressure in the case 10. The
particular location of the transducer case 10 on the body is not critical
as long as an artery is covered and contacted with sufficient force to
flatten the artery wall without occluding the artery. The artery must be
superficial in order to be available for contact, and the transducer must
be capable of sensing force applied to the artery. For example, any of the
pressure points, such as the temporal or dorsalis pedis artery, may be
contacted.
In order better to understand the basic principles of the invention,
consider first the diagrammatic mechanical model of FIG. 2, which is
representative of factors to be considered in the physical system. The
mechanical model will be recognized as that used in the IEEE article by G.
L. Pressman and P. M. Newgard entitled "A Transducer for the Continuous
External Measurement of Arterial Blood Pressure" (IEEE Trans. Bio-Med.
Elec., April 1963, pp. 73-81), adapted to the configuration used in the
present invention. For a more complete understanding of the model and its
role in the visualization of the elements of the blood pressure measuring
system reference should be made to that article, but a brief elementary
description is given here.
As shown in the figure, an array 18 of individual pressure sensitive
elements or transducers 20, which constitute the arterial riders, is
positioned so that one or more of the riders (as shown, four) are entirely
over an artery 23. It is emphasized here that the individual riders 20 are
small relative to the diameter of the artery 23, thus assuring that at
least one of the riders in its entirety is over the artery 23. Findings
are that accurate measurement of blood pressure is not only dependent upon
so positioning at least one arterial rider but also upon the skin surface
22 and the artery 23 being flattened below the transducers 20.
Consequently, the arterial riders 20 and side plates 21 are pressed
against the skin surface 22 with sufficient force to cause compression but
not occlusion of an underlying artery 23. The use of an array of arterial
riders 20, each of which is capable of performing the required
measurement, makes positioning of the case 10 trivial.
Pressing the transducers 20 and side plates 21 against the artery 23 with
sufficient loading force to flatten the wall, as illustrated, reduces the
effect of artery elasticity so that it is not a factor in the measurement
and does not appear as a factor in the mechanical model used. In practice,
the artery wall 23 responds elastically and acts as if it is resting on a
firm base (illustrated by ground symbol 25 under artery 23). Use of the
side plates 21 assures that the entire area covered by the transducers 20
will be flattened and, although there is a tangential force from arterial
wall tension, it does not affect the vertical force translated and
measured by the transducers 20.
As illustrated, the transducer case 10 and mounting strap 14 supply the
required compression force and hold the arterial riders 20 and side plates
21 in such a manner that arterial pressure changes are transferred to the
arterial riders 20 which overlie the artery 23. Diagrammatically this is
illustrated by showing the individual arterial riders 20 and side plates
21 backed by rider spring members 24, a spring backing plate 26 and a
backing spring 28 between the backing plate 26 and the mounting strap
system 30.
Ideally the coupling (backing spring 28) between the mounting strap system
30 and spring backing plate 26 should be infinitely stiff to restrain the
arterial riders 20 and side plates 21 rigidly with respect to the bone
structure and, hence, assure that they maintain a fixed position relative
to the artery 23. In practice, however, such a system is not practical,
and a pneumatic loading system is used which keeps constant the force
applied by the mounting strap system 30 to the arterial riders 20 and side
plates 21. In the mechanical model the spring constant (k, pressure per
unit of deflection) of the backing spring 28 representing such an
arrangement is nearly zero. A suitable pneumatic loading system is shown
and described in the Pressman-Newgard IEEE article and U.S. Pat. No.
3,219,035, both of which have been previously referenced. The system,
therefore, is not described in minute detail in this application.
In order to insure that the arterial riders 20 and side plates 21 flatten
the artery and provide a true force measurement (true blood pressure
measurement), they must be rigidly mounted. Hence the rider and side plate
springs 24 of the model ideally are infinitely rigid (spring constant
k=.infin.). It is found that as long as the system operates in such a
manner that it can be simulated by side plate and rider springs 24 having
a spring constant on the order of about ten times the value for the
artery-skin system, so that the deflection of riders 20 is on the order of
10 microinches, a true blood pressure measurement is obtained.
The actual physical structure of one practical array is shown in a
sectioned perspective view in FIG. 3. The array of individual transducers
20 (arterial riders) is formed in a thin (0.20 mm) rectangular (6.times.8
mm) monocrystalline silicon chip 32 which is made using modern but
conventional integrated circuit techniques. Each of the individual
transducers or diaphragms 20 in the illustration occupies a square area
(0.50 mm square) which is reduced by anisotropic etching to a thickness
about 15 microns (1 micron=1.times.10.sup.-6 m). One method which can be
used to form such a silicon chip 32 with the thin regions of predetermined
thickness in the chip 32 is described in U.S. Pat. No. 3,888,708, issued
June 10, 1975 to Kensall D. Wise et al. for "Method for Forming Regions of
Predetermined Thickness in Silicon". Contents of this patent are
incorporated herein by reference.
The array of transducers 20 in this case is made up of two side by side
sets 20a and 20b, each set arranged in a straight line parallel to the
other and each individual transducer 20 of one set offset lengthwise
(along its straight line), so that the individual transducers of one set
(say, 20a) are centered on a line midway in the space between the
individual transducers of the other set (20b). The central longitudinal
axis of each of the parallel sets 20a and 20b is intended to be positioned
essentially perpendicular to the artery 23 where pressure is monitored. In
this way the center-to-center spacing of the transducers is less than
could be achieved with a single line of transducers. In view of the fact
that the individual riders 20 are so small, a number of them will overlie
the artery 23 and receive pressure variations due to blood pressure
variations in the artery 23. With this arrangement, the transducer case 10
need only be placed generally over the artery to have an arterial rider 20
in optimum position for measurement. In order that the chip 32 present a
flat surface to the skin, the depressions formed by etching, that is, the
etched area under each individual transducers 20, is filled with a
silicone rubber filler 34.
Although the individual transducers 20 may utilize any of the well known
stress induced effects, such as piezoelectric, piezoresistive,
magnetorestrictive, etc., in the best embodiment of the invention now
known, pressure induced stresses (force) on the individual transducers 20
are sensed by piezoresistivity. An understanding of how the piezoresistive
effect is utilized may best be had by referring to FIG. 4, which shows a
plan view of a part of the silicon substrate or chip 32 that has one of
the individual transducers (diaphragms) 20 and the local associated
circuitry.
In this embodiment the pressure induced stresses transmitted to the
transducer or diaphragm 20 are sensed by a bridge circuit composed of four
individual piezoresistors 35 interconnected in conventional bridge circuit
form by conductors 37. The output of the bridge circuit appears on the
conductors 39, which are taken from the conductors 37 that interconnect
the individual resistors 35 of the bridge circuit. The piezoresistive
bridge circuit shown is formed on the surface of the silicon substrate 32,
which is opposite the surface that contacts the skin of the subject. The
resistors 35 and the conductors 37 and 39 of the bridge circuit and its
leads are formed by integrated circuit and photolithographic techniques
which are now conventional. In the embodiment shown they are formed and
interconnected in the manner described in detail in U.S. Pat. No.
3,918,019 and, therefore, the techniques are not described here,
particularly since they do not constitute a part of the present invention.
The arrangement of resistors 35 and conductors 37 along with the
connecting leads 39 thereto is essentially the same for each of the
individual transducers 20, and therefore the full circuit pattern on the
back of the substrate 32 is not shown here.
The complete silicon substrate 32 with all its interconnected bridge
circuits is mounted in what is known in the industry as a conventional
dual in line package. The dual in line package is not shown in detail;
however, the manner in which the silicon chip 32 and the dual in line
package are mounted in the watch-like transducer case 10 is shown in the
vertical cross section view of FIG. 5.
The transducer case 10 is generally a cylindrical, hollow container having
rigid back and side walls 40 and 42, respectively. The silicon transducer
chip 32 is mounted within the face 44 of the case (designated as the front
or operative face) in a cylindrical cup-like transducer housing 46. In the
practical arrangement shown, the operative face 44 includes a silicon
transducer chip 32 along with its included individual transducers are
arterial riders 20, the support structure within the transducer housing 46
which surrounds the silicon transducer chip 32 and the portion of the
transducer housing support structure on the same face. The pressure
sensitive transducers 20 at the operative face 44 of this figure
correspond directly to the arterial riders 20 in the mechanical model
illustrated in FIG. 2 and the remainder of the operative face 44 which is
contained within the housing 46 corresponds to the side plates 21 of FIG.
2.
The transducer housing 46 is fixed to the inside of the transducer case 10
by means of a cup-like silicone rubber bellows 48 which is sealed around
the lower outside lip of the cup-shaped transducer housing 46, extends up
inside the outer wall of the transducer case 10, and is hermetically
sealed to an annular ring 50, which in turn is fixed and sealed to the
inside back of the transducer case 10. The flexible silicone rubber
bellows 48 being sealed both to the transducer housing 46 and the inside
of the transducer case 10 allows air under pressure (air supply to
silicone rubber bellows not specifically illustrated) to be introduced
into the interior of the case 10 to allow the operative face 44 to be
pneumatically loaded, thereby keeping constant the force applied to the
arterial riders 20 and side plates 21, thus meeting the criteria
established for the model described in connection with the mechanical
model of FIG. 2. A mounting strap 52 is affixed to the transducer case 10.
The adjusted pneumatic pressure applied inside the silicone rubber bellows
48 supplies the compressional force required to provide the necessary
flattening of the artery wall 23.
In order to hold the transducer chip 32 in place and provide for electrical
connections to pressure sensitive elements or arterial riders 20, a
conventional dual in line package socket 54 is centrally located and fixed
to the inside back wall 56 of the cup-shaped aluminum transducer housing
46 with electrical conducting dual in line package socket terminals or
pins 58 extending through the back wall 56 of the housing 46 into the
pressurized cavity formed by the silicone rubber bellows 48. The dual in
line package socket pins 58 are connected by electrical wiring (not
shown), which wiring extends from the transducer case 10 through the
insulated cord 16 (FIG. 1) to analyzing circuitry (FIG. 6). The transducer
chip 32 is fixed to the conventional ceramic dual in line package 60 with
its electrical conducting pads making contact with the circuitry of the
package 60. Package circuitry is brought out through a series of dual in
line package pins 62 that are plugged into the dual in line package socket
54 and thus connected to the dual in line package socket terminal pins 58
and ultimately the analyzing circuitry. The connections in both the
ceramic dual in line package 60 and dual in line package socket 54 are
conventional, well known in the art and, therefore, not illustrated in
detail.
In order to be sure that the face of the silicon transducer chip 32 is
properly positioned relative to the rest of the operative face 44,
leveling screws 64 (a pair shown, but any required number may be used) are
provided and placed so that they extend through the back of the cup-shaped
transducer housing 46 and touch the outer edges of the back of the ceramic
dual in line package 60. In order to provide a good seal and prevent any
electrical leakage of contact between circuits from the transducers 20 to
the cup-shaped aluminum transducer housing 46, a silicone filler 66 is
provided inside the cup and around the dual in line package socket 54 and
package 60.
When the transducer case 10 is held in place on the wrist, generally over
the radial artery, as shown in FIG. 1, and the transducer housing 46 is
thus supported over the radial artery by the silicone rubber bellows 48,
air pressure inside the bellows 48 holds the operative face 44, including
the transducer chip 32 and its supporting structure, against the skin
surface 22 with sufficient force to achieve the desired degree of
flattening of the wall of the artery 23, and individual transducers 20 in
the array will each produce an output which is directly responsive to
pressure and variations in pressure on the individual transducers. Thus,
each of the individual transducers 20 produces an output indicative of the
entire pressure waveform exerted thereon.
In accordance with the invention illustrated, described and claimed in the
Newgard copending patent application Ser. No. 848,753 supra, the
individual transducer or force sensitive element 20 which generates the
waveform of maximum amplitude is selected to monitor blood pressure. Using
highly refined and sophisticated blood pressure measuring techniques, it
has been found that even though the Newgard mode of operation yields good
results, the pressure sensitive element 20 which produces the waveform of
maximum amplitude is not necessarily the one which is directly over the
center of the artery 23 and not necessarily the one which tracks the true
arterial pressure with greatest accuracy. Circuitry to carry out the
refined selection of the best individual pressure sensitive element 20 to
track the blood pressure is illustrated in FIGS. 6 and 7. The portion of
the circuit which finds the waveform of maximum amplitude is illustrated
in FIG. 6. This part of the circuit is essentially the same as that used
in the Newgard copending application Ser. No. 848,753 although, as is
described below, it is slightly modified and used in a slightly different
way here.
It will, of course, be appreciated that the circuitry shown, while
eminently practical, is not to be considered restrictive since the
pressure sensitive transducer selection can be carried out in many ways.
For example, from a description of the techniques described here, those
skilled in the art will be able to program a general purpose computer or
to provide a relatively simple, dedicated computer to perform the
techniques.
In the circuits of FIGS. 6 and 7 only a few (four) of the individual
pressure sensitive transducer elements 20 are shown and, as is
conventional, a series of dots is used to indicate that additional
elements, along with their circuitry, are used. The number of circuits
shown is reduced to simplify the drawings and the explanation. Since the
associated circuitry for each of the individual transducer elements 20 is
identical, only the circuitry for the first transducer element 20 (bottom
in FIG. 6) is shown and described in detail with respect to this figure;
the corresponding elements, if shown in other circuits, are given the same
reference numerals.
The output of each of the transducer elements 20 is connected directly to
an amplifier 68, for the purpose of raising the level of the signal
generated to a value more easily handled, and then by a main conductor 70
to one terminal 71 of a normally open switch 72. The opposite terminal 73
of the normally open switch is connected to one of the terminals 74 of a
local minimum finder circuit 76 illustrated in FIG. 7. Thus, when any one
of the main line switches 72 is closed, the output of the corresponding
pressure sensitive element is applied directly to the associated input
terminal 74 of the local minimum finder circuit 76 of FIG. 7.
Each of the switches 72 is provided with a closing coil 78 which closes the
corresponding switch when energized, and each of the coils 78 is connected
between ground potential and a corresponding one of a series of output
terminals 80 of a maximum pulse amplitude finder circuit 82, which is
illustrated within the confines of the block circumscribed by the broken
line so numbered. The input terminals 84 of the maximum pulse amplitude
finder circuit 82 are each connected to one of the main line circuit
conductors 70 so that the amplified output of each of the individual
pressure sensitive transducers 20 is applied to one channel of the maximum
pulse amplitude finder circuit 82.
Following the circuit generally from the input terminals 84 through the
peak pulse amplitude finder circuit 82, a high pass filter section 86 and
then a low pass filter section 88 are first encountered. The output of the
low pass filter section 88 is applied directly to a peak detector shown
generally in the block 90. The output of the low pass filter 88 is not the
pulse amplitude of the output from the pressure sensitive transducer 20
but is proportional to the pulse amplitude. Peak detector 90 receives the
pulse output of the low pass filter 88, operates on it, and generates an
output which is a direct function of the peak, or maximum value of the
applied waveform. Thus, a signal corresponding to the pulse amplitude of
the wave generated by each of the pressure sensitive transducers 20 is
applied to a corresponding one of the output terminals 92 of the
individual peak detector circuits.
The terminals 92 also constitute input terminals for a maximum finder
circuit 94, which is the final stage of the maximum pulse amplitude
circuit 82. In the copending Newgard application, supra, the function of
the maximum finder circuit is to find the one circuit input (at one of the
input terminals 92) which has the signal of greatest magnitude and produce
a relay enabling (actuating) voltage only at the one specific output
terminal 80 in the appropriate circuit. In this manner the particular
relay coil 78 is energized to close the switch 72 in the main circuit 70
of the specific pressure sensitive transducer element 20 that is producing
the output having the greatest pulse amplitude. Thus, the one pressure
sensitive transducer 20 which is producing an output waveform with the
maximum pulse amplitude is selected. However, in one embodiment of the
present invention the circuit not only selects the one pressure sensitive
transducer 20 producing the maximum pulse amplitude waveform but also some
transducers 20 which are surrounding it, preferably those within an area
which spans substantially the width of the artery 23 being measured.
A better understanding of the circuit operation may be had by considering a
functional description assuming that a given one of the individual
pressure sensitive transducer elements 20 produces the waveform having the
maximum pulse amplitude. Assume, for example, that the waveform of maximum
pulse amplitude is generated by the bottom transducer element 20 in the
figure. The waveform generated by each of the pressure sensitive
transducers 20 is amplified by its associated amplifier 68 and applied to
the associated, normally open switch 72 by means of its associated main
conductor 70. However, the waveform of maximum pulse amplitude is applied
from the bottom pressure sensitive transducer 20 to the bottom one of the
switches 72, and the remainder of the switches receive waveforms of lesser
pulse amplitude. Further, all of the waveforms generated by the pressure
sensitive transducer elements 20 are applied to the associated input
terminal 84 of the high pass filter circuit. Again, the lower one of the
terminals 84 receives the waveform having the maximum pulse amplitude. The
high and low pass filters 86 and 88, respectively, in each of the circuits
extract the Fourier components of the applied waveform, which components
are near the heart rate frequency. In practice, the high pass filter 86 is
not strictly necessary; however, it does remove any high frequency
components and reduces circuit noise problems. The filtered waveforms are
then applied to the connected peak detector circuit 90, which circuit
applies the amplitude of the filtered pulse to the respective output
terminals 92. Following the original peak waveform assumption, the lowest
output terminal 92 in the figure receives a higher voltage than any of the
other output terminals 92.
In the mode of operation for the Newgard invention, the maximum finder
circuit 94, which receives the peak detector outputs at the series of
terminals 92, is arranged so that only the circuit which receives the
maximum amplitude from peak detector 90 generates an output; therefore,
under the conditions assumed, the only one of the output terminals 80 of
the maximum finder circuit 94 which receives a voltage is the lower one in
the figure. As a consequence, none of the switch actuated coils 78 are
energized except the lower one in the matrix of coils which is connected
to the lower maximum pulse amplitude finder output circuit terminal 80. In
contradistinction for the mode of operation of the embodiment of this
invention (described here), the circuit elements and biases are chosen so
that the individual pressure sensitive transducer elements 20 surrounding
the one generating the waveform of maximum pulse amplitude within a span
covering the underlying artery 23 produce outputs which energize their
associated switch coils 78 and apply a waveform to the associated input
terminals 74 of the local minimum finder circuit 76 (FIG. 7).
The circuits utilized and illustrated are conventional and can be found in
electronic texts and handbooks. However, for the sake of clarity a circuit
suitable for performing each of the functions is illustrated and
described.
High pass circuit 86 incorporates a capacitor 96 in a series line followed
by a resistor 98 from line to ground. The resistor 98 and capacitor 96 are
selected, using well known design principles, so that the corner frequency
is approximately 0.1 Hz. The low pass filter 88, as is usual, is designed
with a resistor 100 in series on the high side of the line and a capacitor
102 shunted to ground. Again using conventional circuit design techniques,
the resistor 100 and capacitor 102 are selected to give a corner frequency
between 0.5 and 2.0 Hz.
The output of the filter section is applied to peak detector 90, which
incorporates first a conventional isolating voltage follower for accurate
voltage amplitude tracking and isolation. Next, a resistor 106 and
rectifying diode 108 are connected in series with the voltage follower
output to provide a pulsating output voltage that follows the output
waveform of the voltage follower 104. This circuit combination is followed
by a parallel shunt combination of capacitor 110 and resistor 112 (shunted
to ground) which provides a voltage proportional to the peak voltage of
the waveform applied to the peak detector 90.
The individual channels of the maximum finder circuit 94 are connected to
receive their input from terminals 92 which have the peak voltages applied
from peak detector 90. Each of the individual channels in the maximum
finder circuit is similar to the circuit of the individual channel in the
peak detector 90 and, in fact, forms a peak voltage detector. That is,
following any one of the channels from the terminal 92, the circuit
incorporates an isolating voltage follower circuit 114 followed by a
series diode 116, which in this case is a light emitting diode, a series
resistor 118 common to all of the channels, and a shunt capacitor 120 and
resistor 122 combination connected to ground. The combination of the shunt
resistor 122 and capacitor 120 operates in essentially the same fashion as
the corresponding combination (capacitor 110 and resistor 112) in the peak
detector circuit, with capacitor 120 being charged to a peak voltage
nearly equal to (but not equal to) the greatest voltage applied at the
input terminals 92. In this embodiment, the circuit components are so
chosen that the light emitting diodes 116 which will conduct over the back
bias of the capacitor 120/resistor 122 combination are the ones in the
channels which include pressure sensitive elements which are over the
artery 23 in question. Still assuming that the lower channel (connecting
to the bottom pressure sensitive transducer 20 in the figure) receives the
maximum applied voltage, the light emitting diode 116 in the bottommost
channel certainly conducts and emits light. A phototransistor 124 is
provided adjacent each light emitting diode to receive any light emitted
thereby, and each phototransistor has its collector connected to a
positive source of voltage (indicated by terminals +V) and its emitter
connected to the base of a conventional NPN transistor 126. Note that the
collectors of the transistors 126 are also connected to the +V terminal so
that they are forward biased, and the emitters of the transistors 126 are
connected to the output terminals 80 of the maximum finder circuit 94 and
the peak pulse amplitude finder circuit 82. The phototransistors 124 which
will be conductive are the ones activated by light from a light emitting
diode 116. Under the conditions assumed, the light emitting diode 116 of
the bottom channel emits light causing a voltage to be applied to its
associated maximum finder circuit terminal 80. In like manner the channel
associated with each pressure sensitive transducer 20 which is within the
designated span will have a voltage applied at its associated maximum
finder circuit terminal 80 and corresponding switches 72 will be closed to
apply the outputs generated by the associated pressure sensitive
transducers 20 to input terminals 74 of the local minimum finder circuit
76 (FIG. 7).
In order to obtain a better understanding of the invention and of the
operation of the local minimum finder circuit 76, make the assumption that
all of the input terminals 74 shown receive an input voltage. In fact,
only those input teminals in channels with pressure sensitive transducers
20 over the artery 23 being monitored would have an input and the
amplitude of each applied waveform would be different. The function of the
circuit then is to select from the transducer elements 20 which are close
to the one generating the waveform with maximum pulse amplitude that
individual transducer 20 corresponding to a minimum of diastolic pressure
and apply that waveform to a conventional recorder 130, so that the proper
blood pressure waveform with all of its included information may be of
record. Note here that the transducer corresponding to a minimum of
diastolic pressure is the transducer which measures the blood pressure
most accurately. Also, the same one corresponds to a minimum of systolic
pressure. Thus, either or both pressures can be used as a selecting
parameter, and if a corresponding point in time on all the waveforms
generated is used, the proper transducer 20 will still be selected.
Consider first just the circuit elements of the local minimum finder
circuit 76 which are between a pair of input terminals 74 (take the upper
pair) moving from the input terminals toward the output recorder 130 and
consider only the functions performed. Note that corresponding circuit
elements between each channel are given the same reference numerals to
simplify the description and drawings.
A conventional comparator 132 is connected between adjacent lines to
compare the voltage applied to each and generate an output equivalent to a
one (1) or a zero (0) in logic terms, depending upon which of its input
terminal receives the voltage of greater amplitude. The output of the
voltage comparator is applied to a conventional inverter circuit 134,
known as a NOT circuit, in its own circuit set and one terminal of a
conventional AND gate 136 in the next adjacent circuit set (below). The
NOT circuit 134 inverts a received logic one (1) to a zero (0) and vice
versa and applies it to a lower terminal of the AND gate 136 of the same
set. The upper terminal of each AND gate is connected to the converted
output of the comparator circuit from the next adjacent circuit set above.
The AND gate 136 produces an output only if both its terminals receive
ones (1). The AND gates 136 are each connected to supply a voltage across
a switch actuating coil 138. Thus, any AND gate which produces an output
closes a switch 140 on its associated terminals 141 and 142 in the main
line 70 of the next adjacent channel to complete a circuit from the
associated pressure sensitive element 20 to the recorder. Thus, the
recorder 130, having its terminals | | |
