`Eckerle et al.
`
`lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
`5,243,992
`Sep. 14, 1993
`
`US005243992A
`[11) Patent Number:
`[45] Date of Patent:
`
`[54] PULSE RATE SENSOR SYSTEM
`
`(75)
`
`Inventors: J oseph S. Eckerle, Redwood City;
`Dale W, Ploeger, San Francisco;
`Steven T. H olmes, Palo Alto; Thomas
`P. Low, Woodside; Rudolf Elbrec:bt,
`Los A ltos; Philip R. J euck, ill,
`Menlo Park; Ronald E. Pelrine,
`Menlo Park; Victor T. Newton, Jr.,
`Menlo Park, all of Calif.
`
`[73] Assignee: Colin Electronics Co., Ltd., Aichi,
`Japan
`[21] Appl. No.: 502,028
`[22] Filed:
`Mar. 30, 1990
`Int. CI,S ................................................ A61B 5/02
`(51)
`[52] u.s. Cl. '"'""""""""""""""""' l l8/690; 128/672
`[58] Field of Search ............... 128/687, 722, 699, 689,
`128/677, 681, 683, 690
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`3,999,537 12/1976 Noiles .................................. 128/687
`4,058,118 11/1977 Stupay et al. .................. 128/2.05 T
`4,086,916 5/1978 Freeman et al. .................... 128/690
`4,181,134 1/1980 Mason et al. ........................ 128/689
`4,202,350 5/1980 Walton ................................ 128/690
`
`4,239,048 12/1980 Steuer ................................. 128/666
`4,307,728 12/1981 Walton ................................ 128/687
`4,353,372 10/1982 Ayer .................................... 128/640
`4,409,983 10/1983 Alben ................................. 128/690
`4,456,959 6/1984 Hirano et al. ....................... 364/417
`4,646,749 3/1987 Berger et al ........................ 128/678
`4,667,680 5/1987 Ellis ..................................... 128/672
`4,712,179 12/1987 Heimer ................................ 364/417
`4,735,213 4/1988 Shirasaki ............................. 128/681
`4,799,491 1/1989 Eckerle ............................... 128/672
`4,802,488 2/1989 Eckerle ............................... 128/~72
`4,836,213 6/1989 Wenzel et al ....................... 128/672
`
`Primary Examiner-Lee S. Cohen
`Assistant Examiner- Marianne H . Parker
`Attorney, Agent, or Firm--Oliff & Berridge
`
`[57]
`ABSTRACf
`A pulse rate sensor system is packaged in a wristwatch
`sized assembly and is worn by the user to provide an
`accurate determination of pulse rate. A tonometer sen(cid:173)
`sor is provided to detect heartbeat pressure waves pro(cid:173)
`duced by a superficial artery. A microcomputer manip(cid:173)
`ulates the unprocessed tonometer sensor element signals
`using multiple algorithms to determine an accurate
`pulse rate.
`
`36 Claims, 9 Drawing Sheets
`
`8
`
`Sa
`
`Apple Inc.
`APL1034
`U.S. Patent No. 9,289,135
`
`001
`
`
`
`U.S. Patent
`U.S. Patent
`
`Sep. 14, 1993
`Sep. 14, 1993
`
`Sheet 1 of 9
`Sheet 1 of 9
`
`5,243,992
`5,243,992
`
`-"
`
`FIG.|
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`U.S. Patent
`
`Sep. 14, 1993
`
`Sheet 2 of 9
`
`5,243,992
`
`SENSM
`ADAPTER 12 ,,-
`AXIS #1
`9b
`
`MOUNT TO SPRING 11
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`U.S. Patent
`U.S. Patent
`
`Sep. 14, 1993
`Sep. 14, 1993
`
`Sheet 4 of 9
`Sheet 4 of 9
`
`5,243,992
`5,243,992
`
`INITIALIZE
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`
`U.S. Patent
`
`Sep. 14, 1993
`
`Sheet 5 of 9
`
`5,243,992
`
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`
`SEGMENT
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`
`
`U.S. Patent
`
`Sep. 14, 1993
`
`Sheet 6 of 9
`
`5,243,992
`
`r----------~
`SEGMENT 1
`1
`2
`# 7
`I
`I
`~TERMINE ELEMENT
`Wl'Tli MAXIMUM
`(PULSE AMPLITOOE)
`
`:
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`
`DETERMINE ELEME'NT
`WI'Tli MAXIMUM
`POSITtVE Pll.SE
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`WITH MOST MAXIMJMS
`IN LAST 5 TRIGGERS
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`
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`
`
`U.S. Patent
`
`Sep. 14, 1993
`
`Sheet 7 of 9
`
`5,243,992
`
`SEGMENT
`#f4
`
`C
`
`I
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`
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`to
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`
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`
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`10
`
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`2
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`
`SIGNAL
`RAW
`
`ELEMENT#
`
`009
`
`
`
`U.S. Patent
`
`Sep. 14, 1993
`
`Sheet 9 of 9
`
`5,243,992
`
`104
`
`too'
`
`I
`
`" 104
`
`102
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`----.L-
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`
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`FIG. 88
`
`010
`
`
`
`1
`
`5,243,992
`
`2
`but the pressure sensitive element{s) itself is located off
`the artery. In this case the subject's pulse can push up on
`the housing and lessen the pressure on the pressure
`sensitive element. The result is that a pulse waveform is
`5 still received, but it is inverted and shows a negative
`relative pressure. Pulse measuring devices which rely
`on pressure measurements but can correctly interpret
`only positive pressure waveforms must be placed and
`held accurately on the artery, c reating additional de-
`10 mands on attachment of the device and/or lowering
`comfort to the user.
`Additionally, pronounced dichrotic notches can be
`found in the pulse of many people. When dichrotic
`notches are present there are two rises and two fall~ in
`15 blood pressure during a single heartbeat. These can be
`mistakenly interpreted as two h.eartbeats, leading to a
`major inaccuracy in pulse rate measurement.
`The present invention overcomes the problems en(cid:173)
`countered with other pulse rate sensors by applying the
`principles of arterial tonometry for signal acquisition
`for a pulse rate sensor. In the invention, multiple algo-
`rithms are used in signal processing and pulse rate cal(cid:173)
`culation to compensate for multiple signal errors which
`could occur during pulse rate measurement.
`The principles of arterial tonometry are described in
`several U.S. Patents including: U.S. Pat. Nos. 3,219,035;
`4,799,491 and 4,802,488. These principles are also de(cid:173)
`scribed in several publications including an article enti(cid:173)
`tled "Tonometry, Arterial," in Volume 4 of the Ency(cid:173)
`clopedia of Medical Devices and Instruments. (J. G.
`Webster, Editor, John Wiley & Sons, 1988). All of these
`references discuss arterial tonometry as used for the
`measurement of blood pressure.
`For blood pressure measurement, it is desirable to
`flatten a section of the arterial wall as described in these
`references. flattening is produced by exerting an appro(cid:173)
`priate hold down force on the tonometer sensor. For
`pulse sensing, significant flattening of the arterial wall is
`not necessary and a lower hold down force can be used.
`This results in greater comfort for the wearer.
`
`PULSE RATE SENSOR SYSTEM
`
`FIELD OF THE INVENTION
`The present invention relates to pulse monitors with
`visual readouts of pulse rate and more particularly to a
`tonometer sensor pulse rate monitor which employs
`multiple noise and motion artifact rejection methods to
`determine an accurate pulse rate.
`
`BACKGROUND OF THE INVENTION
`The present invention relates generally to a method
`and apparatus for measuring and displaying pulse rate
`wit.h increased accuracy. More specifically, the present
`invention provides a method for increasing the accu(cid:173)
`racy of a pulse rate sensing system by means of a novel
`pressure sensing array and multiple methods for identifi(cid:173)
`cation and elimination of artifacts.
`Other methods and apparatus are known for measur(cid:173)
`ing pulse rates and for rejecting pulse artifacts. For 20
`example, U.S. Pat. No. 4,409,983 shows a pulse measur(cid:173)
`ing device which employs multiple transducers con(cid:173)
`nected to averaging circuits and differential amplifiers.
`This invention helps separate signals corresponding to
`motion artifacts from the signal corresponding to a 25
`heartbeat pulse. Other apparatus and methods for re(cid:173)
`moving motion artifacts are disclosed in U.S. Pat. Nos.
`4,307,728, 4,202,350, 4,667,680, 4,239,048, 4,181,134 and
`4,456,959. Methods used to reduce signal errors include
`the use of windowing and averaging techniques and 30
`auto correlation algorithms.
`The pulse rate sensor systems described above are
`subject to several sources of inaccuracies. First, it is
`difficult to reject motion and noise artifacts in many of
`these systems. This is especially true for systems em- 35
`ploying a single sensor element. (See U.S. Pat. Nos.
`4,202,350 and 4,239,048.) These systems have no physi(cid:173)
`cal means for receiving both a pulse-plus-artifact signal
`and a separate anifact signal. Other means are required
`to compensate for, or eliminate, the error caused by 40
`artifacts such as motion artifacts. Signal processing
`techniques such as filtering and windowing are often
`SUMMARY OF THE INVENTION
`used.
`Accordingly, the present invention has been devel-
`Even those systems or methods which employ multi-
`ple sensor elements inaccurately measure pulse rate 45 oped to overcome the foregoing shortc·omings of exist-
`ing pulse rate sensor systems.
`because only a single method is used for enhanced sig-
`It is therefore an object of the present invention to
`nal processing. For example, different types of motion
`artifacts can occur simultaneously, and with other per-
`provide a method and an apparatus for measuring pulse
`tubations, on the pulse sensor. It is also not unusual for
`rates using arterial tonometry techniques including a
`signal errors to be interpreted as pulses or for actual SO sensor array with multiple sensing elements disposed in
`an array, in order to provide increased accuracy in the
`pulses to be missed by the pulse sensor. Methods of
`pulse rate determination which do not compensate for
`determination of pulse rate.
`Another object of the present invention is to increase
`these errors are inherently inaccurate under real-world
`the accuracy of the displayed pulse rate by calculating
`conditions ere artifacts are present.
`For example, if a pulse rate system detects a "pulse" 55 the displayed pulse rate using only pulse rates deter-
`caused by noise, several adverse results may be seen.
`mined to be valid.
`The pulse rate system could use the noise as the basis for
`A further object of the present invention is to deter-
`windowing the signal. The pulse rate system could
`mine whether pulses detected are valid, based on the
`simply use this "pulse" as part of the overall pulse rate
`correlation between the present pulse and the previous
`calculation. In addition, the pulse rate system could 60 pulse.
`recognize the noise as noise and subtract out the noise,
`A still further object of the present invention is to
`in some cases subtracting out a valid signal as well.
`remove motion artifacts from the sensor element signals
`Another source of inaccuracy that occurs using pulse
`by subtracting a value from all these signals based on a
`measuring devices that measure pressure variations
`spatially weighted average of these signals.
`caused by a subject's pulse (see U.S. Pat. No. 4,409,983 65 An additional object of the present invention is to
`cancel out artifacts from a sensor element which exceed
`for example) is inverted pulse waveforms. An inverted
`a level predetermined to be the maximum level of a
`waveform can occur when the housing that holds the
`pressure sensitive element(s) is located on the artery,
`valid blood pressure signal.
`
`011
`
`
`
`5,243,992
`
`4
`FIGS. 8A and B are graphical representations of
`normal and inverted blood pressure waveforms, respec(cid:173)
`tively.
`
`3
`Still another object of the present invention is to
`accurately process inverted waveforms caused by mis(cid:173)
`alignment or shifting of the sensor elements relative to
`an underlying artery.
`DESCRIPTION OF THE PREFERRED
`These and other objects and advantages are achieved S
`EMBODIMENTS
`in accordance with the present invention by the steps
`Referring to FIG. 1, a pulse rate monitor 1 in accor-
`of: sensing at least one blood pressure waveform signal
`dance with the preferred embodiment of the present
`at a predetermined sampling period using a tonometer
`invention is shown having a tonometer sensor l and a
`tensor having a plurality of sensor elements disposed in
`an array; producing a plurality of sensor element sig- 10 case 3 housing processing and display circuitry. The
`tonometer sensor l is mounted in a gimbal assembly 9
`nals, at least one of the sensor element signals corre-
`which in turn is connected to case 3 by a spring 4.
`sponding to the at least one blood pressure signal; cor-
`Spring 4 includes a sensor position adjustment 5 section
`recting the sensor element signals using a correction
`factor based on o ne characteristic of the sensor element
`at the connection point between gimbal assembly 9 and
`signals; calculating a plurality of slopes based on the IS spring 4.
`corrected sensor element signals; selecting a corrected
`Further details of this arrangement are shown in FIG.
`l , which .uses the sat?e component designations fou~d in
`sensor element signal corresponding to one of the sen-
`FIG. 1, where possible. The to~om~ter se~r. lJs at-
`1
`ts th
`lected sensor element signal having
`ta.ched to a sensor adapter 1l which, m tum JS p1nned to
`sor e emen • e se
`.
`slopes greater than a predetermmed slope threshold; 20 h
`. bal
`·
`·
`bl 9 t
`be 2 A
`assem y a IXIS num
`f u1
`· ·
`·
`t e gun
`r
`.
`spnng mount-
`b
`d
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`· bal
`.
`determmmg a pluraltty o p se rates ase on t e se-
`pad 11 ·
`· ed al
`·
`
`
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`·
`·
`al
`mg
`JS p1nn
`o gtm
`lected. sensor element stgnal;. computmg a v ue corre-
`assembly 9, mounting pad 11 being the point at which
`spondmg ~o the autocorrelation °.f the ~orrecte~ ~nsor
`sensor position adjustment 5 connects to gimbal assem-
`element. stgnal . over a predeterrnmed ttme penod, and
`bly 9. Flexible printed circuit 10 connects the tonometer
`calculating a dtsplay pulse rate based on at least .two of 2S sensor 2 to circuitry (shown in FIG. 4) in case 3.
`the pul~ r~tes, each of t~e two pulse rates havmg the
`Tonometer sensor 2 is an array of pressure or force
`value Wtthm a predet~rmmed range.
`.
`sensitive elements fabricated into a single structure.
`. These and othe! ObJects and advantages .are achteved
`Standard photolithographic manufacturing techniques
`m acco~danc~ With the. ~referred embodiment of the
`can be used to construct the tonometer sensor. Experi-
`present mventt~n compnsmg: a to?ometer senso~ means 30 mental testing indicates that about 3 to 6 individual
`having a plural tty of pressure semmg elements dtsposed
`sensor elements are necessary for good accuracy in
`in an array, for sensing a bl~ pressure waveform on at
`pulse rate measurement but even a single sensor element
`~east one o~ the pressure serJSIJlg ~lements and produc-
`adapted for use with the method and apparatus of the
`mg a plurahty of sensor element stgnals, at least one of
`present invention will produce more a ccurate pulse rate
`the sensor element signals being indicative of the blood 35 determinations.
`pressure acting on at l~t ~ne of the pressure sensing
`Referring again to FIG. 1., the remainder of the pulse
`elements; means . for ptvottng the tonome.ter sensor
`rate monitor 1 will be described in terms of wearing the
`means about a parr of axe~; means ~or pressmg the to-
`pulse rate monitor 1. In the preferred embodiment, the
`nometer sensor mean~ agamst a r~dtal artery of a sub-
`pulse rate monitor is worn on the operator's wrist like a
`ject; means for anchonng the pressmg means on a dorsal 40 wrist watch. When the wearer dons the pulse rate moni-
`tor 1, the tonometer sensor 2 is positioned above a radial
`side of the subject; central processing means for deter-
`mining a pulse rate based on at least one of the sensor
`artery and the case 3 is positioned on the opposite side
`element signals received from the tonometer sensor
`of the wrist. The case 3, which anchors one end of
`spring 4, is held in place by strap 8 by cinching an flexi-
`means; means for displaying the pulse rate; and means
`for holding the anchoring means on the subject, the 45 ble portion 8a of a strap 8 and locking the strap in posi(cid:173)
`tion by means of a buckle 7. Strap 8 is prevented from
`holding means at no time contacting the pressing means.
`directly contacting the tonometer sensor 2, the gimbal
`BRIEF DESCRIPTION OF THE DRAWINGS
`assembly 9 and the spring 4 by a protective band 6
`which is part of the strap 8. Protective band 6 has a box
`The preferred embodiments are described with refer-
`so shaped cutout section which allows it to fit around the
`ence to the drawings in which:
`tonometer sensor 2, the gimbal assembly 9 and the
`FIG. 1 is a general arrangement of a pulse rate sensor
`spring 4, without contacting any of these elements.
`system illustrating a pulse rate sensor connected to a
`The tonometer sensor 2 is held against the artery with
`case assembly;
`a thin cantilever spring 4 which is attached to the case
`FIG. 2 is a perspective view of the tonometer sensor
`supported i.n a gimbal assembly;
`S5 3. The spring reaches around the wrist to position the
`FIGS. 3A, B and C show low, medium and high
`tonometer sensor l . A low profile gimbal assembly 9
`(See FIGS. 1 and 2) connects the tonometer sensor 2 to
`c urvature spring profiles, respectively.
`FIG. 4 is a block diagram of the pulse rate processing
`the spring 4 and allows the tonometer sensor 2 to pivot
`circuitry in accordance with the preferred embodiment
`so as to lie flat agairJSt the wrist while being worn. Giro-
`of the present invention;
`60 bal assembly 9 allowsabout 20 degrees of rotation about
`each of its two axes. The position on spring 4 of the
`FIGS. SA, B, C and D are a flowchart describing a
`pulse rate measurement and compensation method in
`tonometer sensor-gimbal assembly 2, 9 is adjustable by
`accordance with one embodiment of the present inven-
`means of the sensor position adjustment 5 section of
`tion;
`spring 4.
`FIG. 6 is a table illustrating the effect of differential 6S
`Sensor hold down force must be controlled to pro-
`enhancement on raw sensor element signal levels;
`vide enough pressure to partially flatten the radial ar-
`FIG. 7 is a table illustrating the calculations of the
`tery but not enough pressure to cause discomfort to the
`correlation algorithm; and
`wearer. The optimum hold down force is unique to
`
`012
`
`
`
`5,243,992
`
`35
`
`s
`each individual but ranges from about 100 grams to
`about SOO grams. Some wearers may require a higher
`bold down force to obtain a reliable pulse signal from
`the tonometer sensor 2. Other wearers may be sensitive
`to the bold down force of the tonometer sensor 2 s
`against their wrists and desire the lowest possible hold
`down force.
`Ideally, since the hold down force of the tonometer
`sensor 2 is controlled only by the deflection of the
`spring 4 and since the size and shape of the wrist can 10
`vary greatly between individuals, each spring 4 would
`have to be custom fit for each wearer. In general, three
`configurations for spring 4, shown in FIGS. 3A, B and
`C. will suffice to cover the majority of the population.
`Springs 4A, B and C, shown in FIGS. 3A, B and C, IS
`provide adequate length and curvature adjustment to
`cover the general population. The springs 4A, 48 and
`4C differ only in their radius of curvature.
`The discussion below of the computation of the pulse
`rate can best be understood by first understanding the 20
`important features of a normal blood pressure wave(cid:173)
`form. Referring to FIG. 8A a normal blood pressure
`waveform 100 with an average blood pressure 102 is
`shown. The point 104 on the waveform 100 where
`blood pressure is maximum is referred to as systole, 25
`while the point 106 where blood pressure is a minimum
`is referred to as diastole. It is known from scientific
`studies that the maximum absolute value of the slope of
`wave form 100 occurs just prior to systole, in region
`108, when the blood pressure is rising from diastole. A 30
`dichrotic notch 110 is present in the blood pressure
`waveform 100 of some subjects. Referring to FIG. 88,
`an inverted waveform 100' is shown having systole
`(104), diastole (106), the region (108) and dichrotic
`notch (110).
`Computation of pulse rate is performed by electronic
`circuitry shown in FIG. 4 and located in case 3, in
`accordance with the flow chart shown in FIGS. SA, B,
`C and D.
`Referring fU"St to FIG. 4, circuitry to process tonome- 40
`ter signals and compute pulse rate in accordance with
`the present invention comprises: a preamplifier 58; a
`high pass filter 60; a low pass filter 62; an amplifier 64;
`an analog-to-digital digital converter (ADC) 66; a cen(cid:173)
`tral processing unit (CPU) 68; random access memory 45
`(RAM) 70; read only memory (ROM) 72; input/output
`unit (I/0) 74 and display 76. As shown in FIG. 4, the
`preamplifier 58, ftlters 60 and 62, amplifier 64 and ADC
`66 each include inputs corresponding to the individual
`sensor elements.
`During operation, the output of all sensor elements of
`tonometer sensor 2 are routed to preamplifier 58 which
`amplifies the signals before ftltering. The output of pre(cid:173)
`amplifier 58 is the input to high pass ftlter 60 which
`removes the DC and very low frequency components 55
`of the signals. The output of the high pass ftlter 60 is the
`input to the low pass ftlter 62 which removes some of
`the high frequency components of the signal. (High pass
`filter 60 coupled with low pass filter 62 effectively act as
`a band pass filter.) The preferred bandwidth of the 60
`effective band pass filer is about 0.1-30 Hz. After filter(cid:173)
`ing, the signals are amplified by amplifier 64 to a level
`compatible with the ADC 66. ADC 66 multiplexes the
`signals and convertS them to digital data which is sent to
`110 74. CPU 68 reads the digital data form I/ 0 74 and 65
`stores the digital data in RAM 70.
`RAM 70 is segmented to form a plurality of data
`buffe.rs for storage of digital data representing sensor
`
`SO
`
`6
`element signals, counts, flag settings and calculated
`values. A ROM 72 contains the operating program
`software for CPU 68. CPU 68, through I/ 0 74, receives
`digital data from ADC 66 and outputs pulse rate calcu(cid:173)
`lations to display 76.
`The operation of CPU 68 is best understood by refer(cid:173)
`ring to the flow chart of FIGS. SA, B, C and D .
`When the pulse rate sensor system is turned on or
`reset by the wearer, the system aoes through an initial(cid:173)
`ization routine to clear stored data. At this point, a
`clock (not shown) starts and a clock signal is provided
`to the CPU 68 to set a predetermined sampling period.
`Referring to FIG. SA, while performing segment 1 of
`the program, sub-step 1a checks to see if a clock tiel- bas
`occurred. If a clock tick is not detected at sub-step l a,
`the cycle repeats .. If a clock tick is detected, the CPU 68
`executes sub-step 1b and samples the outputs of all sen(cid:173)
`sor elements of the tonometer sensor 2. In addition,
`system timers are updated. Control then passes to pro(cid:173)
`gram segment 2.
`During program segment 2, each data element for
`each sensor element is checked to see if it exceeds a
`predetermined maximum value. If an actual value ex(cid:173)
`ceeds the predetermined maximum value, i.e. the prede(cid:173)
`termined maximum amplitude, the data from that sensor
`element is set to zero. The program also sets a flag so
`that all data from that sensor element is set to zero for
`the next five seconds. This eliminates unusually large
`signals which are usually noise. Program control then
`passes to program segment 3.
`Differential enhancement of the signal occurs during
`execution of program segment 3. During sub-step 3a, all
`signals from sensor elements not previously set to zero
`are averaged to produce a spatially weighted average
`signal. In the preferred embodiment, the weighted fac(cid:173)
`tor is about 1.0, but can be adjusted as described below.
`At sub-step 3b, the spatially weighted average signal is
`subtracted from each of the actual sensor element sig(cid:173)
`nals.
`The differential enhancement algorithm reduces mo(cid:173)
`tion artifacts in the tonometer sensor signals. This algo(cid:173)
`rithm can only be used when multiple sensor elements
`are employed simultaneously. Since all of the sensor
`element signals are often affected equally by a motion
`artifact, the differential enhancement algorithm aids in
`distinguishing between artifacts and blood pressure
`signals. For example, motion artifacts such as footsteps
`usually affect all sensor elements in the same way.
`Blood pressure signals, on the other band, affect only
`sensor elements which are directly over or very near to
`the artery.
`Differential enhancement adds all of the signals from
`all of the sensor elements together and form.s a spatially
`weighted average signal. If each sensor element is af(cid:173)
`fected in the same way by a motion, the spatially
`weighted average signal will be an accurate representa(cid:173)
`tion of the motion artifact. This spatially weighted aver(cid:173)
`age signal is then subtracted from each individual sensor
`element signal to form a differentially enhanced signal
`for each sensor element sjgnal. (I.e. an approximation of
`the motion artifact is subtracted from the raw sensor
`element signals to produce corrected sensor element
`signals.) The raw sensor element signals are expected to
`be either motion artifact signals or motion artifact sig(cid:173)
`nals plus blood pressure signals. For example, FIG. 6
`shows the effect of the differential enhancement algo(cid:173)
`rithm on the signals from a three element sensor array.
`Signals for only one clock tick are shown.
`
`013
`
`
`
`5,243,992
`
`7
`Differential enhancement is part of a larger class of
`algorithms where the processed output for each value is
`a weighted sum of the unprocessed values. For exam(cid:173)
`ple, differential enhancement of three raw values pro(cid:173)
`duces a processed output for value 1 as follows:
`
`10 DSUM .. DSUM + (B~LEM E.)'o.'T) X B.m;LEMENT))
`3
`O 20 NSUM • NSUM + B~LEMENT) X CORELA(ICOR)
`
`30 CORELA(ICOR) "' B~LEMENT)
`
`The correlation coefficient is then simply calculated
`after systole is found as COR= NSUM/DSUM. This
`procedure works because in line ZO of the above code
`CORELA(ICOR) still contains the pressure data from
`the previous heartbeat. Line 30 updates CORELA for
`the calculation on the next (future) heartbeat. Use of
`this running summation (in place of the two arrays,
`CORELA and CORELB, of Eq. (I)) is advantageous
`because it reduces computer speed and memory re(cid:173)
`quirements. However, Eq. (1) defines the correlation
`coefficient so it may be calculated by other procedures
`without departing from the teachings ·of this disclosure.
`The table shown in FIG. 7 gives an example of what
`the relevant variables hold for a few samples of hypo(cid:173)
`thetical data. If systole occurred at ICOR = 3 (in reality
`the waveform normally extends for many more samples
`before a systole is found) the correlation coefficient
`would be calculated as
`
`8
`as valid pulses, but rejects or ignores pulses that have
`correlation coefficients outside this predetermined
`range. Of course, different acceptance ranges for COR
`may be used without departing from the teachings of
`5 this disclosure.
`The traditional mathematical definition of the corre-
`Proc:aaed Value I '"' (2/3)(raw value I) +
`lation coefficient applies only when the two waveforms
`( - 1/l)(raw value 2) + (- 1/3)(raw value 3).
`being correlated (CORELA and CORELB of the
`above example) have euctly the same duration. In
`.
`.
`.
`. Other ext~SI?ns of this procedure are posstble a~d 10 other words, Eq. (I) is mathematically rigorous only
`f
`ual Th
`dtffe.rent we1ghtmg factors than those used by the baste
`whe both heartbeats' d
`thiu~a tont.hs are t~cal ·
`• e ~resendt
`nt'o d. parts f
`differential enhancement algorithm are possible without
`.
`departing from the teachings of this disclosure. For m;en 1 n e
`s ma e~ 1
`rom
`co~tramt an
`al ows COR to be computed even tf the durations of the
`example, it may be advantageous to assign large nega-
`rive weights to sensor element signals far from a se- 15 two heartbea~ are not exactly equal. Eq. (1) can be ~d
`to advantage m the ~ ~here the two waveforms do
`Jected sensor element and large positive weights to
`not have equal duratton smce Eq. (1) can be used very
`aensor element signals located near to a selected sensor
`element.
`effectively to recognize movement artifacts even
`though it is not used in a mathematically rigorous way.
`Returning to FIG. SA, after performing program
`In the pr~ferred ~m~iment, the ar:ray, CORELB, of
`segment 3, program control passes to program segment 20
`4 where the differentially enhanced signal data is pro-
`the above illustration IS not used. Instead, a running
`cessed by a corre.lation algorithm.
`summation of the numerator of Eq. (I), called NSUM,
`and of the denominator of Eq. (I), called DSUM, are
`The correlation algorithm computes a quantitative
`measure of the similarity between sensor waveforms
`updated after each sample of sensor data is obtained.
`o~er a predetermined t~me. period of about two consec- 25 Specifically, if BP(ELEMEN!) is the pressure mea-
`sured by a selected element at ume ICOR, the following
`uuve heartbeats. In pnnctple, the shape of a person's
`blood pressure waveform will be fairly constant from
`sequence is executed:
`one heartbeat to the next. The correlation algorithm
`compares the blood pressure waveform for the current
`heartbeat with a previously recorded waveform for the
`preceding heartbeat.
`The processor executes the correlation algorithm as
`follows. A variable, ICOR, is used as a counter to keep
`track of the number of clock ''ticks" (i.e. the elapsed
`time) that the processor has spent looking for a systole.
`At the start, ICOR is set equal to I. ICOR is then incre- 35
`mented by one for each subsequent clock tick (i.e. each
`time data is sampled from the sensor) When ICOR is set
`equal to 1, the variables DSUM and NSUM, described
`below, are both set equal to zero. The data from the
`selected element of the tonometer sensor between the 40
`time when the processor starts looking for a systole and
`the time when it actually finds a (presumed valid) sys(cid:173)
`tole is stored in an array, called CORELA. ICOR is
`used as a pointer for the array CORELA, i.e. the value
`of the data from the selected element after ICOR clock 45
`ticks have occurred (after the start of looking for a
`systole) is stored in CORELA(ICOR).
`For example, suppose a similar procedure had been
`used for the previous heartbeat, and the values from the
`selected element had been stored in another array, CO- SO
`RELB. After the systole is found for the present heart(cid:173)
`beat, the correlation coefficient, COR, may be calcu(cid:173)
`lated. The correlation coefficient between the pressure
`waveforms for the current and previous heartbeats is
`mathematically defined as:
`
`. CCOREUWCORELB@
`COR "'1 (CORELA(J)"CORELA(J))
`
`(I)
`
`55
`
`and this heartbeat would be accepted as valid by the
`acceptance criterion described above.
`.
`Finally, the correlation coefficient in other applica(cid:173)
`tions is sometimes also defmed as:
`
`where the summation, , is over all values of elapsed 60
`time, j, from the start of looking for the systole. If the
`current heartbeat is exactly identical to the previous
`heartbeat, the array, CORELB, will be equal to the
`array, CORELA, and the correlation coefficient, COR,
`will be equal
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