-
`United States Patent
`
`[19]
`
`(1500524399213.
`[11] Patent Number:
`
`5,243,992
`
`Eckerle et a1.
`
`[45} Date of Patent:
`
`Sep. 14, 1993
`
`||||1|||l|||l|||||||l||||||||||II||||l|||||l||||||||||||||||||||||||||||||Il
`
`[54] PULSE RATE SENSOR SYSTEM
`
`[75]
`
`[56]
`
`I
`luvs-1mm Joseph 5- Eckerle. Redwood City;
`Dale W. Ploeger, San Francrscc;
`Steven T. Holmes. Palo Alto; Thomas
`P. Low, Woudside; Rudull' Elbrecht,
`Los Altos; Philip H“ Jenck’ m,
`Memo Park; Rana“ E Peking!
`Menlo Park vicmr T. Newton, Jr"
`M l P k‘
`11
`f can.
`5“ 0 3’ 'a '3
`1 ’
`[73] Assignee: Colin Electronics Co" Ltd“ Aid“,
`Japan
`[21] Appl. No: 502,028
`.
`.
`,
`[22] Filed.
`M". 30 1990
`[51]
`Int. 01.5 .............................................. .. A6113 5/112
`
`------- 123/690; 123/672
`[52} Ursc CL
`[53] FlEld of Search ............. ..
`[231/587, 722,
`639,
`123/677' 631' 633' 690
`References Cited
`U s PATENT DOCUMENTS
`
`3399,53? 12/1976 Nodes ................................ .. 123/637
`128/105 '1'
`4.058.113 1111971 Stupay et al.
`4.086.916 5/1978 Freeman e1 al.
`128/690
`
`4.181,134
`11’1980 Mason e1 aL
`128/639
`5.!1950 Walton ................................ 123/690
`£1,202,350
`
`3fi Claims, 9 Drawing Sheets
`
`
`
`0001
`
`Apple Inc.
`APLl 047
`
`US. Patent No. 8,923,941
`
`FITBIT, Ex. 1047
`
`4,239,048 12/1980 Steuer
`4.301.723 12/1931 Wa1ton..
`4.353.312 111/1982 Ayer .... ..
`4.409.983 113/1983 Albert
`4,456,959
`6/1984 Hirano et a].
`4,646,149
`3/1987 Berger et a].
`11,667,680
`51’198'1 Ellts
`4,711,119 11/1937 Henna :.
`4,735,213 4/1988 Slums-ah
`6,799,491
`1f1959 Eckerlc
`4,302,433 2/1939 Eckerle
`4.836.213
`6f1989 Wenulet 2.1.
`Primary Examiner—Lee S. Cohen
`Assistant Examiner—Marianne H. Parker
`Attorney. Agent. or Firm—01111 dc Berridge
`[57]
`ABSTRACT
`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~
`sor is provided to detect heartbeat pressure waves pro-
`duced by a superficial artery. A microcomputer manip-
`ulates the unprocessed tonometer sensor element signals
`using multiple £8011!th to determine an accurate
`pulse rate‘
`
`
`
`128/666
`.. 123/68?
`128/640
`123/690
`364/417
`123/678
`IzB/6'12
`364/61?
`1232’631
`.. 125/672
`.. 123/512
`128/672
`
`.
`.
`
`Apple Inc.
`APL1047
`U.S. Patent No. 8,923,941
`
`0001
`
`FITBIT, Ex. 1047
`
`

`

`US. Patent
`
`Sep. 14, 1993
`
`Sheet 1 of 9
`
`5,243,992
`
`FIG.I
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`FITBIT, EX. 1047
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`0002
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`FITBIT, Ex. 1047
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`

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`US. Patent
`
`Sep. 14, 1993
`
`Sheet 2 of 9
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`5,243,992
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`
`
`FLEXIBLE
`EC. BOARD 10
`
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`_...AXIS #2 90
`GIMBAL 9
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`£003,511
`
`MOUNT TO SPRING 11
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`
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`
`0003
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`FITBIT, EX. 1047
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`US. Patent
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`Sep. 14, 1993
`
`Sheet 3 of 9
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`5,243,992
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`0004
`
`FITBIT, Ex. 1047
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`

`

`US. Patent
`
`Sep. 14, 1993
`
`Sheet 4 of 9
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`5,243,992
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`INITIALIZE
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`

`US. Patent
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`Sep. 14, 1993
`
`Sheet 5 of 9
`
`5,243,992
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`FITBIT, EX. 1047
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`0006
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`

`

`U.S. Patent
`
`Sep. 14, 1993
`
`Sheet 6 of 9
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`

`

`US. Patent
`
`Sep. 14, 1993
`
`Sheet 7 of 9
`
`5,243,992
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`1
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`
`0008
`
`FITBIT, Ex. 1047
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`

`

`US. Patent
`
`Sep. 14, 1993
`
`Sheet 8 of 9
`
`5,243,992
`
`
`
`
`
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`0009
`
`FITBIT, EX. 1047
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`U.S. Patent
`
`Sep. 14, 1993
`
`Sheet 9 of 9
`
`5,243,992
`
`.106
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`

`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
`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. creating additional de-
`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 rim and two falls in
`blood pressure during a single heartbeat. These can be
`mistakenly interpretul as two heartbeats. leading to a
`major inaccuracy in pulse rate measurement.
`The present invention overcomes the problems en-
`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 alge-
`rithms are used in signal processing and pulse rate cal-
`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-
`scribed in several publications including an article enti-
`tled "Tonornetry. Arterial." in Volume 4 of the Ency-
`clopedia of Medical Devices and Instruments. (J. G.
`Webster. Editor, John Wiley & Sons, £988). All of these
`references discuss arterial tonomctry 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-
`priate hold down force on the tonotneter 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.
`SUMMARY OF THE INVENTION
`
`5
`
`IO
`
`15
`
`25
`
`30
`
`35
`
`4o
`
`45
`
`1
`
`PULSE RATE SENSOR SYSTEM
`
`FIELD OF THE INVENTION
`
`The present invention relates to puISe 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
`with increased accuracy. More specifically. the present
`invention provides a method for increasing the accu-
`racy of a pulse rate sensing system by means of a novel
`pressure sensing array and multiple methods for identifi~
`cation and elimination of artifacts.
`Other methods and apparatus are known for measur-
`ing pulse rates and for rejecting pulse artifacts. For
`example. US. Pat. No. 4,409,983 shows a pulse measur-
`ing device which employs multiple transducers con-
`nected to averaging circuits and differential amplifiers.
`This invention helps separate signals conesponding to
`motion artifacts from the signal corresponding to a
`heartbeat pulse. Other apparatus and methods for re-
`moving motion artifacts are disclosed in US. Pat. N05.
`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
`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-
`ploying a single sensor element. (See U.S. Pat. Nos.
`4.202.350 and 4.239.048.) These systems have no physi-
`cal means for receiving both a pulse-plus—artifact signal
`and a separate artifact signal. Other means are required
`to compensate for. or eliminate. the error caused by
`artifacts such as motion artifacts. Signal processing
`techniques such as filtering and windowing are often
`used.
`Even those systems or methods which employ multi-
`ple sensor elements inaccurately measure pulse rate
`because only a single method is used for enhanced sig-
`nal processing. For example, different types of motion
`artifacts can occur simultaneOusly. and with other per-
`tubations. on the pulse sensor. It is also not unusual for
`signal errors to be interpreted as pulses or for actual
`pulses to be missed by the pulse sensor. Methods of
`pulse rate determination which do not compensate for
`these errors are inherently inaccurate under real-world
`conditions ere artifacts are present.
`For example. if a pulse rate system detects a “pulse”
`caused by noise. several adverse results may be seen.
`The pulse rate system could use the noise as the basis for
`windowing the signal. The pulse rate system could
`simply use this "pulse" as part of the overall pulse rate
`calculation. In addition. the pulse rate system could
`recognize the noise as noise and subtract out the noise,
`in some cases subtracting out a valid signal as well.
`Another source of inaccuracy that occurs using pulse
`measuring devices that measure pressure variations
`caused by a subject’s pulse (sec 11.8. Pat. No. 4.409.983
`for example) is inverted pulse waveforms. An inverted
`waveform can occur when the housing that holds the
`pressure sensitive element(s) is located on the artery.
`
`Accordingly, the present invention has been devel-
`oped to overcome the foregoing shortcomings of exist-
`ing pulse rate sensor systems.
`It is therefore an object of the present invention to
`_ prOvide a method and an apparatus for measuring pulse
`rates using arterial tonometry techniques including a
`50
`sensor array with multiple sensing elements disposed in
`an array. in order to provide increased accuracy in the
`determination of pulse rate.
`Another object of the present invention is to increase
`the accuracy of the displayed pulse rate by calculating
`the diSplayed pulse rate using only pulse rates deter-
`mined to be valid.
`A further object of the present invention is to deter-
`mine whether pulses detected are valid, based on the
`correlation between the present pulse and the previous
`pulse.
`A still further object of the present invention is to
`remove motion artifacts from the sensor element signals
`by subtracting a value from all these signals based on a
`spatially weighted average of these signals.
`An additional object of the present invention is to
`cancel out artifacts from a sensor element which exceed
`a level predetermined to be the maximum level of a
`valid blood pressure signal.
`
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`4
`FIGS. 8A and B are graphical representations of
`nonnal and inverted blood pressure waveforms. respec-
`tively.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`3
`Still another object of the present invention is to
`accurately process inverted waveforms caused by mis-
`alignment or shifting of the sensor elements relative to
`an underlying artery.
`These and other objects and advantages are achieved
`in accordance with the present invention by the steps
`of: sensing at least one blood pressure waveform signal
`at a predetermined sampling period using a tonometer
`sensor having a plurality of sensor elements disposed in
`an array; producing a plurality of sensor element sig-
`nals, at least one of the sensor element signals corre-
`sponding to the at least one blood pressure signal; cor-
`recting the sensor element signals using a correction
`factor based on one characteristic of the sensor element
`signals; calculating a plurality of slopes based on the
`corrected sensor element signals; selecting a corrected
`sensor element signal corresponding to one of the sen-
`sor elements, the selected sensor element signal having
`slopes greater than a predetermined slope threshold;
`determining a plurality of pulse rates based on the se-
`lected sensor element signal; computing a value corre-
`sponding to the autocorrclation of the corrected sensor
`element signal over a predetermined time period; and
`calculating a display pulse rate based on at least two of
`the pulse rates, each of the two pulse rates having the
`value within a predetermined range.
`These and other objects and advantages are achieved
`in accordance with the preferred embodiment of the
`present invention comprising: a tonometer sensor means
`having a plurality of pressure sensing elements disposed
`in an array, for sensing a blood pressure waveform on at
`least one of the pressure sensing elements and produc-
`ing a plurality of sensor element signals. at least one of
`the sensor element signals being indicative of the blood
`pressure acting on at least one of the pressure sensing
`elements; means for pivoting the tonometer sensor
`means about a pair of axes; means for pressing the to-
`nometer sensor means against a radial artery of a sub-
`ject; means for anchoring the pressing means on a dorsal
`side of the subject; central processing means for deter
`mining a pulse rate based on at least one of the sensor
`element signals received from the tonometer sensor
`means; means for displaying the pulse rate; and means
`for holding the anchoring means on the subject, the
`holding means at no time contacting the pressing means.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The preferred embodiments are described with refer-
`ence to the drawings in which:
`FIG. 1 is a general arrangement of a pulse rate sensor
`system illustrating a pulse rate sensor connected to a
`case assembly;
`FIG. 2 is a perspective view of the tonometer sensor
`supported in a gimbal assembly;
`FIGS. 3A. B and C show low, medium and high
`curvature spring profiles, respectively.
`FIG. 4 is a bloclt diagram of the pulse rate processing
`circuitry in accordance with the preferred embodiment
`of the present invention;
`FIGS. 5A, B, C and D are a flowchart describing a
`pulse rate measurement and compensation method in
`accordance with one embodiment of the present inven-
`tlon;
`FIG. 6 is a table illustrating the effect of differential
`enhancement on raw sensor element signal levels;
`FIG. 7 is a table illustrating the calculations of the
`correlation algorithm; and
`
`10
`
`15
`
`Referring to FIG. 1, a pulse rate monitor 1 in accor-
`dance with the preferred embodiment of the present
`invention is shown having a tonometer sensor 2 and a
`case 3 housing processing and display circuitry. The
`tonometer sensor 2 is mounted in a gimbal assemny 9
`whichin turnisconneetedtocaae3byaspring4.
`Spring 4 includes a sensor position adjustment 5 section
`at the connection point between gimbal assembly 9 and
`spring 4.
`Further details of this arrangement are shown in FIG.
`2. which uses the same component designations found in
`FIG. I, where possible. The tonometer sensor 2 is at-
`tached to a sensor adapter 12 which1 in turn is pinned to
`the gimbal assembly 9 at axis number 2. A spring mount-
`ing pad 1] is pinned along axis number I. of gimbal
`assembly 9. mounting pad 11 being the point at which
`Sensor positiOn adjustment 5 connects to gimbal assem-
`bly 9. Flexible printed circuit ll] connects the tonometer
`5 sensor 2 to circuitry (shown in FIG. 4) in case 3.
`Tonometer sensor 2 is an array of pressure or force
`sensitive elements fabricated into a single structure.
`Standard photolithographic manufacturing techniques
`can be used to construct the tonometer sensor. Experi-
`mental testing indicates that about 3 to 6 individual
`sensor elements are necessary for good accuracy in
`pulse rate measurement but even a single sensor element
`adapted for use with the method and apparatus of the
`present invention will produce more accurate pulse rate
`determinations.
`
`20
`
`30
`
`35
`
`Referring again to FIG. 1., the remainder of the pulse
`rate monitor 1 will be described in terms of wearing the
`pulse rate monitor 1. In the preferred embodiment, the
`pulse rate monitor is worn on the operator‘s wrist like a
`wrist watch. When the wearer dons the pulse rate moni-
`tor t, the tonometer sensor 2 is positioned above a radial
`artery and the case 3 is positioned on the opposite side
`of the wrist. The case 3, which anchors one end of
`spring 4, is held in place by strap 8 by cinching an flexi-
`ble portion 8a of a strap 8 and locking the strap in posi-
`tion by means of a buckle T. Strap 8 is prevented from
`directly contacting the tonometer sensor 2. the gimbal
`assembly 9 and the spring 4 by a protective band 6
`which is part of the strap 8. Protective band 6 has a box
`shaped cutout section which allows it to fit around the
`tonometer sensor 2,
`the gimbal assembly 9 and the
`spring 4. without contacting any of these elements.
`The tonometer sensor 2 is held against the artery with
`a thin cantilever spring 4 which is attached to the case
`3. The spring reaches around the wrist to position the
`tonometer sensor 2. A low profile gimba] assembly 9
`(See FIGS. 1 and z) connects the tonometer sensor 2 to
`the spring It and allows the tonometer sensor 2 to pivot
`so as to lie flat against the wrist while being worn. Gim-
`bal assembly 9 allows about 20 degrees of rotation about
`each of its two axes. The position on spring 4 of the
`tonometer sensor-gimbal assembly 2. 9 is adjustable by
`means of the sensor position adjustment 5 section of
`spring 4.
`Sensor hold down force must be controlled to pro-
`vide enough pressure to partially flatten the radial ar-
`tery but not enough pressure to cause discomfort to the
`wearer. The optimum hold down force is unique to
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`each individual but ranges from about 100 grams to
`about 500 grams. Some wearers may require a higher
`hold down force to obtain a reliable pulse signal from
`the tonorneter sensor 2. Other wearers may be sensitive
`to the hold down force of the tonometer sensor 2
`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
`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,
`provide adequate length and curvature adjustment to
`cover the general population. The Springs 4A, 4B and
`4C differ only in their radius of curvature.
`The discussion below of the computation of the pulse
`rate can hurt be understood by first understanding the
`important features of a normal blood pressure wave-
`form. Referring to FIG. 8A a normal blood pressure
`waveform 100 with an average blood prmsure 102 is
`shown. The point 104 on the waveform 100 where
`blood pressure is maximum is referred to as systole.
`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
`103, when the blood pressure is rising from diastole. A
`dichrotic notch 110 is present in the blood pressure
`wavefonn 100 of some subjects. Referring to FIG. SB.
`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. 5A, B,
`C and D.
`Referring first to FIG. 4. circuitry to process tonome-
`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-dig'ital digital converter (ADC) 66'. a cen-
`tral processing unit (CPU) 60; random access memory
`(RAM) 70; read only memory (ROM) 72; input/output
`unit (HQ) 74 and display 76. As shown in FIG. 4, the
`preamplifier 58. filters 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 filtering. The output of pre-
`amplifier 58 is the input to high pass filter 60 which
`removes the DC and very low frequency components
`of the signals. The output of the high pass filter 60 is the
`input to the low pass filter 62 which removes some of
`the high frequency components ofthe signal. (High pass
`filter 60 coupled with low pass filter 62 effectiver act as
`a band pass filter.) The preferred bandwidth of the
`effective band pass filer is about 0.1-30 Hz. After filter-
`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
`1/0 14. CPU 68 reads the digital data form [/0 '74 and
`stores the digital data in RAM 70.
`RAM 70 is segmented to form a plurality of data
`buffers for storage of digital data representing sensor
`
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`element signals, counts. flag settings and calculated
`values. A ROM 72 contains the operating program
`software for CPU 68. CPU 68, through 1/0 74. receives
`digital data from ADC 66 and outputs pulse rate calcu-
`lations to display '76.
`The operation of CPU 68 is best understood by refer-
`ring to the flow chart of FIGS. 5A. B, C and D.
`When the pulse rate sensor system is turned on or
`reset by the Wearer, the system goes through an initial-
`ization routine to clear stored data. At this point. a
`clock (not shown) starts and a oloelt signal is provided
`to the CPU 68 to set a predetermined sampling period.
`Referring to FIG. 5A, while performing segment 1 of
`the program, sub-step 1a checks to see if a clock ticlr has
`occurred. If a clock tick is not detected at sub-step 1n.
`the cycle repeats. If a clock tick is detected. the CPU 68
`executes sub-step lb and samples the outputs of all sen-
`sor elements of the tonometer sensor 2. In addition,
`system timers are updated. Control then passes to pro-
`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 es-
`ceeds the predetermined maximum value. i.e. the prede-
`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 342. 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-
`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-
`nals.
`The differential enhancement algorithm reduces mo-
`tion artifacts in the tonorneter sensor signals. This algo-
`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 hand, 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 forms a spatially
`weighted average signal. If each sensor element is af-
`fected in the same way by a motion, the spatially
`weighted average signal will be an accurate representa-
`tion of the motion artifact. This spatially weighted aver-
`age signal is then subtracted from each individual sensor
`element signal to form a differentially enhanced signal
`for each sensor element signal. (Le. 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-
`nals plus blood pressure signals. For example. FIG. 6
`shows the effect of the differential enhancement algo-
`rithm on the signals from a three element sensor array.
`Signals for only one clock tick are shown.
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`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-
`ple. differential enhancement of three raw values pro-
`duces a processed output for value 1 as follows:
`
`Processed Value I s: (2Han value I) +
`{—l/Slraw value 2) + (-— l/Jllraw value 3).
`
`Other extensions of this procedure are possible and
`different weighting factors than those used by the basic
`differential enhancement algorithm are possible without
`departing from the teachings of this disclosure. For
`example. it may be advantageous to assign large nega-
`tive weights to sensor element signals far from a se—
`lected sensor element and large positive weights to
`sensor element signals located near to a selected sensor
`element.
`Returning to FIG. 5A. after performing program
`segment 3. program control passes to program segment
`4 where the differentially enhanced signal data is pro-
`cessed by a correlation algorithm.
`The correlation algorithm computes a quantitative
`measure of the similarity between sensor waveforms
`over a predetermined time period of about two consec-
`utive heartbeats. In principle, the shape of a person's
`blood pressure waveform will be fairly constant from
`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 l. ICOR is then incre-
`mented by one for each subsequent clock tick (i.e. each
`time data is sampled from the sensor) When ICOR is set
`equal to I. 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
`time when the processor starts looking for a systole and
`the time when it actually finds a (presumed valid) sys-
`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
`ticks have occurred (after the start of looking for a
`systole) is stored in CORELAUCOR).
`For examlale. suppose a similar procedure had been
`mod for the previous heartbeat, and the values from the
`selected element had been stored in another array, C0-
`RELB. After the systole is found for the present heart-
`beat, the correlation coefficient, COR. may be calcu-
`lated. The correlation coefficient between the pressure
`wavefonns for the current and previous heartbeats is
`mathematically defined as:
`
`'CORE
`. CORE-isz
`CO“ = ’ (WEMUi‘EmEI—Aom—i
`
`(1)
`
`is over all values of elapsed
`,
`where the summation.
`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 to 1. 1f COR differs greatly from 1, the
`two waveforms are not similar and at least one of them
`is probably distorted by a movement artifact. The pro-
`gram accepts values of COR between about 0.6 and 2.0
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`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
`this disclosure.
`The traditional mathematical definition of the corre-
`lation coefficient applies only when the two waveforms
`being correlated (CORELA and CORELB of the
`above example) have exactly the same duration. In
`other words, Eq. (I) is mathematically rigorous only
`when both heartbeats‘ durations are equal. The present
`invention departs from this mathematical constraint and
`allows COR to be computed even if the durations of the
`two heartbeats are not exactly equal. Eq. (1) can be used
`to advantage in the case where the two waveforms do
`not have equal duration since Eq. (1) can be used very
`effectively to recognize movement artifacts even
`though it is not used in a mathematically rigorous way.
`In the preferred embodiment, the array. CORELB. of
`the above illustration is not used. Instead, a running
`summation of the numerator of Eq. (1), called NSUM.
`and of the denominator of Eq. (1), called DSUM, are
`updated after each sample of sensor data is obtained.
`Specifically. if BPGLEMENT) is the pressure mea-
`sured by a selected element at time ICOR, the following
`sequence is executed:
`
`IO DSUM = DSUM + (HHELEMENT) >( BH'ELEMENTll
`
`20 NSUM = MUM + EHELEMENT) x CORELAUCOR)
`30 CORELAUCDR) = BHELEMENT)
`
`The correlation coefficient is then simply calculated
`after systole is found as COR=NSUMIDSUM. This
`procedure works because in line 20 of the above code
`CORELAUCCIR) 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. (1)) is advantageous
`because it reduces computer speed and memory re-
`quiremcrtts. 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-
`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
`
`COR =NSUM/D50M: 3359/5325=0.723 I. Feed
`
`and this heartbeat would be accepted as valid by the
`acceptance criterion described above.
`_
`Finally. the correlation coefficient in other applica-
`tions is sometimes also defined as:
`
`COR =(COREL4 Ul'CORELWJIMMPANAMPE)
`
`where AMPA:sqrt(DSUM). AMPB=sqrt(DSUM'),
`and DSUM'
`is the value of DSUM for the previous
`heartbeat. This definition makes the correlation coeffi-
`cient independent of any overall gain