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`US 20030065269Al
`
`(19) United States
`(12) Patent Application Publication
`Vetter et al.
`
`(10) Pub. No.: US 2003/0065269 Al
`Apr. 3, 2003
`(43) Pub. Date:
`
`(54) METHOD AND DEV ICE FOR PULSE RATE
`DETECTlON
`
`(75)
`
`inventors: Rolf Vetter, Yverdon (CH); Philippe
`Renevey, Lausa nne (CJ I); Roland
`Gentsch, llauterive (CH); .Tens Krauss,
`Neuchatel (CH); Yves Depeursinge,
`Servion (CH)
`
`Correspoodeoce Address:
`PARKHURST & WENDEL, L.L.P.
`1421 PRINCE STREET
`SUITE 210
`ALEXANDRIA, VA 22314-2805 (US)
`
`(73) Assignee: CSEM Centre Suisse d 'Electronique et
`de Microtechnique SA, Neucbatel (CH)
`
`(21) Appl. No.:
`
`10/255,068
`
`(22) Filed:
`
`Sep. 26, 2-002
`
`(30)
`
`Foreign Application Priority Data
`
`Sep. 28, 2001
`
`(EP) ........................................ 01203686.9
`
`Publication C lassification
`
`(51)
`
`Int. C l.7
`
`............................. . .... . ... ......... . ....... A61B 5/02
`
`(52) U-S. C l.
`
`··························- ································· 600/503
`
`(57)
`
`ABSTRACT
`
`There is described a device and a met bod [or de tecting the
`pulse rate. The measuring principle consists of emitting
`radiant energy at tbe surface of or through humao body
`tissue (5) by meaos of a light-emitting source (10), measur(cid:173)
`ing the intensity of the radiant energy after propagation
`th rough the human body tissue by means of at least first and
`second light detectors (21, 22, 23, 24) located at a deter(cid:173)
`mined distance (rom the light-emitling source and providing
`first and second input signals (y 1(t), Yz(t)) representative of
`this propagation. Simultaneously, a motion detecting device
`(40), such as a three dimensional accelerometers, provides a
`motion reference signal (a,.(t), ay(t), a,.(t)) representative of
`motion of the detecting device on and with respect to the
`human body tissue (5). The input signals are then proces.sed
`in order to remove motion-related contributions due to
`motion of the detecting device (1) on and with respect to the
`human body tissue (5) and to produce first and second
`enhanced signals. Th.is processing basically comprises the
`elaboration of a model of the motion- related contributions
`based on the motion reference signal a·nd the subtraction o(
`tb.is model from each of the input signals. Pulse rate is tben
`extracted from the enhanced signals using for instance a
`maximum likelihood histogram clustering technique.
`
`B
`(_
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`22
`
`001
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`Apple Inc.
`APL1032
`U.S. Patent No. 8,942,776
`
`

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`Patent Application Publication Apr. 3, 2003 Sheet 1 of 6
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`US 2003/0065269 Al
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`1
`
`B
`( _
`
`·-·: 8
`
`. - - - -
`
`- - - I - -
`
`24
`
`Fig.1
`
`3
`
`Fig.2
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`I
`
`10
`
`22
`
`10
`
`40
`
`IR
`
`WRIST
`
`002
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`

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`Patent Application l)ublication Apr. 3, 2003 Sheet 2 of 6
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`US 2003/0065269 Al
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`Fig.3
`
`INPUT SIGNALS
`y1(t),y2(t)
`
`MOTION
`REFERENCE
`SIGNAL
`ax(t),ay(t),az(t)
`
`MOTION ARTEFACTS
`REMOVAL USING NON- I - -100
`LINEAR MODEL-BASED
`NOISE CANCELLING
`
`MEASUREMENT NOISE
`AND NON-MODELLED
`CONTRIBUTIONS
`REMOVAL USING
`NOISE REDUCTION
`ALGORITHM
`
`r---__ 110
`
`ENHANCED
`SIGNALS
`
`~
`
`PULSE RATE
`EXTRACTION
`
`r----120
`
`SELECTION OF MOST I - -130
`RELIABLE CANDIDATE
`
`PULSE
`
`003
`
`

`

`;so.. -
`~ 0
`c
`
`~
`0\
`N
`(JJ
`0\
`0
`
`N
`\J).
`
`0\
`0 .....,
`~
`
`t'> ....
`\J). =(cid:173)
`~ s
`... ~
`:-s
`"0
`;J;..
`
`t'>
`
`0' c::
`~
`
`n = .... -· 0 =
`£ .... a· =
`
`'";I = ~ = ....
`
`c::
`:g
`;J;..
`
`t Pulse
`
`"i-230
`
`mapping
`Nonlinear ~ 1
`
`I
`
`130
`
`--,
`
`225
`
`!
`
`~ ___ I ___ -I-_-_I
`I
`I
`l
`I
`I
`l
`I
`I
`l
`l
`l
`l
`l
`I
`
`I
`l
`l
`I
`l
`I
`I
`I
`
`extraction
`interval
`
`J_-----,
`
`120
`
`~-----__ _ H Inter-beat
`
`,
`
`Spatio-temporal
`
`PCA
`
`~
`
`~
`
`_i_
`
`\
`210
`
`I
`110
`
`I
`
`226
`
`Non-stationarity
`
`detector
`
`200
`
`----
`I
`I
`I
`I
`l
`I
`I
`I
`I
`I
`I
`I
`I
`
`modelling
`Nonlinear
`
`I
`I
`I
`I
`I
`
`I i +
`_______ , __ ,
`
`+qr
`
`I
`
`~ 2:
`
`I
`100
`
`Fig. 4
`
`Signals
`Reference az(t)
`Motion ay(t)
`ax(t)
`
`y2(t)
`
`y1 (t)
`
`Signals
`
`IR
`
`004
`
`

`

`Patent Application Publication Apr. 3, 2003 Sheet 4 of 6
`
`US 2003/0065269 At
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`Fig. 5a
`
`Fig. 5b
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`005
`
`

`

`Patent Application Publication Apr. 3, 2003 Sheet 5 of 6
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`US 2003/0065269 Al
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`Fig. 5c
`
`006
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`

`

`Patent Application Publication
`
`Apr. 3, 2003 Sheet 6 of 6
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`US 2003/0065269 Al
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`Fig. 5d
`
`3
`
`2
`
`1
`
`0
`
`-1
`
`-2
`
`-3
`
`-40
`
`Fig. 5e
`ECG
`
`1
`
`2
`Time [s]
`
`3
`
`4
`
`007
`
`

`

`US 2003/0065269 Al
`
`Apr. 3, 2003
`
`1
`
`METHOD AND DEVICE FOR PULSE RATE
`DETECTION
`[0001] This invention is in the field of signal processing
`and is more specifically directed to pulse rate detection.
`[0002] Portable heart rate monitoring devices are classi(cid:173)
`cal ly composed of a processing device and an external probe
`(e.g. electronic stethoscope, optical measure at ear lobe,
`chest belt for electrocardiogram-ECG-based measurement,
`etc.). The use of an external probe is often considered as a
`reduction of the user's comfort. ECG-based pulse rate
`detecting devices using external electrode probes are for
`instance disclosed in documents U.S. Pat. Nos. 4,108,166,
`6,018,677, 6,149,602 and WO 00/51680.
`[0003] Va rious pulse rate detection systems are known in
`the art. Pulse ra te detection devices using pressure sensitive
`transducers such as piezoelectric elements are for instance
`disclosed in documents U.S. Pat. Nos. 3,838,684, 4,195,642,
`4,331,154, 5,807,267 and WO 80/00912.
`[0004] More recently, measuring techniques based on so(cid:173)
`called photoplethysmography (o r PPG) have been proposed.
`PPG is an electro-optic technique of measuring the cardio(cid:173)
`vascular pulse wave found throughout the human body. Tbis
`pulse wave is caused by the periodic pulsations of arterial
`blood volume and is measured by the changing optical
`absorption of radiant energy which this induces. The mea(cid:173)
`surement system classically consists of a source of radiant
`energy (usually an infra-red light source), at least one
`detector for detecting the intensity of the radiant energy after
`propagat..ioo through the human body tissue and a data
`processing means for extracting bodily parameters such as
`pulse rate or oxygen concentration in the blood. Jn(Ta-rcd
`light is predominantly used since it is relatively well
`absorbed in blood and weakly absorbed in body tissue.
`Blood volume changes are therefore observed with a rea(cid:173)
`sonable contrast. TI1e princ ipal adva ntage of PPG measure(cid:173)
`ment resides in the fact that it is entirely non-invasive and
`can be applied to any blood bearing tissue, typically a finger,
`nai l, ear lobe, nose and, in some instances, wrist.
`[0005] S ince light is highly scattered in tissue, a detector
`positioned on the surface of the skin can measure reflections
`(or transmissions) fTom a range of deptlls and those reflec(cid:173)
`tions (or transmissions) are variously absorbed depending on
`whether the light encounters weakly or highly absorbing
`tissue. Any change in blood volume will be registered by tbe
`detector at the surface since increasing (or decreasing)
`volume will cause more (or less) absorption. T he effect will
`be averaged over many arteries and veins. To the absence of
`any blood volume changes, the signal level will be deter(cid:173)
`mined by the tissue type, ski.n type, probe positioning, static
`blood volume content and of course the geometry and
`sensitivity of the sensor itself.
`[0006] PPG systems differentiate between light absorption
`due to blood volume and that of other fluid and tis.5ue
`constituents by observation that arterial blood flow pu lsates
`wh ile tissue absorption remains s tatic. As the il luminated
`blood flow pulsates, it alters the optical path length and
`therefore modulates the light absorptioo throughout the
`cardiac cycle. Non-pulsating fluids and tiss11es do not modu(cid:173)
`late tbe tight but have a fixed level of absorption (assuming
`there is no movemen t).
`[0007] The result of this absorpt..ion is that any light
`reflected from (or transmiued through) the pulsating vascu-
`
`lar bed contains an AC component which is proportional to
`and synchronous with the patients plethysmograpbic signal.
`It is this modulated component which is known as the
`photoplethysmographic signal. This PPG signal is superim(cid:173)
`posed onto a DC level which represents the difference
`between incicleot radiant energy and tbe constant absorption
`of the tissue, blood and anything else in the optical path with
`constant absorption.
`[0008] PPG measurement can be achieved by measure(cid:173)
`ment of the intensity of rad iant energy transmillcd through
`(transmission mode systems) or reflected by (reflection
`mode systems) body tissue. A reflection mode system typi(cid:173)
`cally bas much poorer signal to noise ratio, resulting from
`the fact that a smaller proportion of tbe light which is no t
`absorbed will be reflected than transmiued. That is the
`reason why most of the prior an devices and systems use a
`detecting arrangement that is p laced o n the user's fi nger,
`nai l, car lobe, nose or part of the body through which light
`can easily be transmitted.
`[0009] PPG bas widely been used for measuring arterial
`oxygen saturation known as pulse oximetry. lbe technique
`relies on the know ledge that haemoglobin and oxyhaemo(cid:173)
`globin absorb light to varying degrees as a function of
`wavelength. In particular, the absorption characteristics o(
`red and near infrared light are inverted for the two species.
`It is thus possible to derive the proportion of oxybaemoglo(cid:173)
`bin and therefore the arterial oxygen saturation from a
`knowledge of the absorption characteristics of the arterial
`blood at these two wavelengths. PPG-based oximetry sens(cid:173)
`ing devices employing sensors which are typically io contact
`with the user's finger or nail are for instance disclosed in
`documents U.S. Pat. No. 5,237,994, 5,645,060, 5,662,106,
`5,934,277, 6,018,673, WO 99/52420, WO 99/62399 and
`WO 01/25802. PPG-based oximetry and heart rate detect..i ng
`devices intended to be worn on or around other parts of the
`human body such as the wrist or ear, are also known, for
`instance from documen ts U.S. Pat. No. 5,807,267 and WO
`97/ 14357.
`[0010] One of the maio problems of PPG measurement is
`corruption of the useful signal by ambient light and other
`electromagnetic radiations (so-called light artefacts) and by
`voluntary or involuntary subject movement (so-called
`motion artefacts). These artefacts lead to erroneous inter(cid:173)
`pretation of PPG signals and degrade the accuracy and
`reliability of PPG-based algorithms for the estimation of
`cardiovascular parameters.
`[OOll] Processing of ambient Light artefacts is not critical
`because the influence of ambien t light can be measured
`using multiplexing techniques and the PPG signal can be
`restored using subtractive-type techniques. Reference can
`here be made to the article "Effect of motion, ambient light,
`and hypoperfusion on pulse oximeter function", Trivedi N.
`et al., Journal of Clinical Anaesthesia, vol 9, Pi>· 179-183,
`1997, for a description of these problems. In contrast,
`processing of motion artefacts is a tough task since its
`contribution often exceed that of the useful pulse-related
`signal by an order of magnitude. It is essentially caused by
`mccbanica l forces that induces changes in the optical cou(cid:173)
`pling and the optical properties of the tissue. Motion arte(cid:173)
`facts are a particularly critica l problem for Lbe design of a
`wrist-located pulse detecting device.
`[0012] Several methods have been proposed to reduce
`motion artefacts in PPG signals. Feature-based algorithms
`
`008
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`

`US 2003/0065269 Al
`
`Apr. 3, 2003
`
`2
`
`have been proposed to discard the corn1ptcd segments from
`the s ignals for instance in document WO 94/22360 (corre(cid:173)
`sponding to U.S. Pat. No. 5,368,026). This kind of approach
`allows one to reduce the occurrence of false alarms in
`clinical environments, but it often degrades the signals with
`small motion artefacts contributions. This could lead to
`erroneous estimation of cardiovascular parameters.
`[0013]
`In order to circumvent this drawback, model-based
`noise canceJljng techniques have been applied more recently
`for the enhancement of optical signals. Examples are for
`instance described in documents U.S. Pat. No. 5,490,505,
`WO 94/ 03102 and in articles '·Simple photon difl.'usion
`analysis of the effects of multiple scattering on pulse oxim(cid:173)
`etry", Schmitt J., IEEE Transactions on Biomedical Engi(cid:173)
`neering, vol. 38, pp. 1194-2002, December L99l, and
`"Noise-resistant oximetry using a synthetic reference sig(cid:173)
`nal", Coetzee F. M. et al., IEEE Transactions on Biomedical
`Engineering, vol. 47, pp. 1018-1026, August 2000. In such
`approaches a reference signal of motion is recorded and a
`parametric model is used subsequently to retrieve motion
`the optical signal<>. Nevertheless,
`related i.n£Juences in
`motion references are classically obtained by piezo-sensors
`or optical measures and convey therefore only incomplete or
`local information of motion. 1l1is degrades ihe performance
`of model-based noise cancelling techniques since they
`require complete and low-noise motion reference signaLs.
`[0014]
`It is thus a principa l object of tbe present invention
`to provide a device and method for accurately monitoring
`and detecting heart rate based on photoplethysmography,
`even under intense physical activity.
`
`[0015] More particularly, an object of the present inven(cid:173)
`tion is to provide a solution that allows for adequate removal
`of ambient light and mo tion contributions in the optical
`signals.
`
`[001 6] Ano ther object o( the invention is to provide a
`solution that is suitable for enabling measurement and
`detection to happen at tbe wrist level.
`[0017] Accordingly there is provided a portable pulse rate
`detecting device the features of which are recited in claim 1.
`(0018] There is also provided a method for detecting pulse
`rate the features of wbicb are recited in claim 17.
`
`[0019] Other advantageous embodiments of tbe invention
`are the object of the dependent claims.
`[0020] Accordi ng to the present invention, an accurate
`motion detecting device is used to provide a reliable motion
`reference signal. This mo tion detecting device is preferably
`a fully integrated three dimensional accelerometer which
`exhibits a high accuracy and very low noise.
`
`[0021]
`In order to achieve efficient removal of mo tion
`related artefacts in the optical signals, nonlinear model(cid:173)
`based techniques are applied. This nonlinear modelling
`preferably consists in a polynomial expansion model using
`a moving average and an associated model selection based
`on the Minimum Description Length (MDL) criterion.
`
`[0022] Furthermore, in order to grasp the spatial diversity
`of the optical characteristics of the tissue, at least two optical
`sensors are used. This two-channel arrangement, associated
`with an adequa te noise reduction algorithm (preferably an
`algorithm based on so-called spatio-tcmporal Principal
`
`Component Analysis or PCA), allows one to remove mea(cid:173)
`surement noise and non-modelled stochastic signal contri(cid:173)
`butions as well as reduce artefacts related to finger move(cid:173)
`ments which arc generally not recorded by the accelerometer
`and therefore not initially cancelled.
`[0023] Eventually, the heart rate is estimated from tbe
`enhanced signals using inter-beat extraction based on physi(cid:173)
`ological properties of cardiac cells and maximum likelihood
`bistogram clustering of tbc resulting time series.
`[0024] An assessment of the performance of the proposed
`solution according to the invention has shown its high
`robustness and accuracy. It has to be pointed out that the
`application of nonlinear
`instead of linear modelling
`decreases the standard deviation of the detected heart rate of
`about one to two percent. 1l1is is mainly due to the inclusion
`of the parsimonious MDL-based model selection, which
`avoids an overfilling of the time series. Indeed, the full
`nonlinear model would retain pulse related components in
`the estimate of the motion artefacts. Since these components
`are subtracted from the optical signals, the quality of the
`enhanced signal and consequently tbe reliability of the
`estimated pulse are reduced. In contrast, MDL selects only
`movement related parameters in the model, which yields
`higher enhancement performance and a more accurate puJse
`estimation in adverse noisy environments.
`[0025] Other aspects, features and advantages of the
`present invention will be apparent upon reading tbe follow(cid:173)
`ing detailed description of non-limiting examples and
`embodiments made with reference to the accompanying
`drawings, in wbich:
`[0026] FIG. 1 is a schematic view of tbe bottom side
`(intended to come into comact with the body tissue) of a
`portable pulse rate detecting device according to tbe inven(cid:173)
`tion which is adapted to be worn on the wrist and comprising
`a light source and two pairs of light detectors arranged at tbe
`bouom side;
`[0027] FIG. 2 is a schematic side vjew of the device o(
`FIG. 1 further illustrating the arraogemcot of tbe acceler(cid:173)
`ometer;
`[0028] FIG. 3 is a flow chart of the preferred method for
`pulse rate detection according to the invention;
`[0029] FIG. 4 is a block diagram illustrating a dual
`channel pttlse detection algorithm according to the present
`invention which is based on nonlinear model-based motion
`artefact cancelling, coherence-based reduction of measure(cid:173)
`ment noise and stochastic signal contributions, and a pulse
`detection using maximum likelihood histogram clustering;
`and
`[0030] FIGS. Sa to Se are diagrams respectively illustrat(cid:173)
`ing the evolution, as a function of time, (a) of optical signals
`provided by two light detectors, (b) of acceleration signals
`detected by the accelerometer along three measurement
`axes, (c) of the two optical signals after removal of the
`motion artefacts, (d) of tbe two optical signals after mea(cid:173)
`surement noise removal (using PCA) and (e) a correspond(cid:173)
`ing ECG electrocardiogram.
`[0031] FIGS. 1 and 2 schematically show a top view of
`the bottom side and a side view of a wrist-located pulse rate
`detecting device, indicated globally by reference numeral !,
`according to a preferred embodiment of !he present inven(cid:173)
`tioo.
`
`009
`
`

`

`US 2003/0065269 Al
`
`Apr. 3, 2003
`
`3
`
`[0032) White the invention will be described hereinbelow
`with respect to a portable device which is adapted to be worn
`on the wrist and which is based on the measurement of tight
`reflected in the body tissue, it will be appreciated that the
`detecting device according to the present invention could be
`designed to be worn on other parts of the human body such
`as a patient's finger, nail, ear lobe or any other suitable
`member or part of the bumao body. In addition, the same
`principles could be applied to a detecting device based on
`the measurement of light transmitted through the body tis.~ue
`(such as those typica ll y used in pulse oximetry) where the
`signal to noise ratio is higher. ln addition, these principles
`could be applied for pulse oximetry on the red and IR
`signals.
`
`[0033) As shown in FIGS. 1 and 2, detecting device 1
`comprises a housing 2 and a strap 3 for attaching detecting
`device 1 on the t>a tient's wrist. Housing 2 comprises, located
`in a bottom side 2a of the device in contact wi th the skin, a
`tight source 10 for emitting radiant energy at the surface of
`(or through) the httman body tissue, designated by reference
`numeral 5. Light source 10 is preferably an infrared light
`emilling device (LEO).
`
`[0034) Accorcling to the preferred embodiment, housing 2
`further includes two pairs of light detectors 21, 22 and 23,
`24 fo r detecting the intensity of the radia nt energy after
`propagation through the bumao body tissue. Sucb light
`detectors may conveniently be pbotodiodes. Preferably, the
`pairs 21, 22 and 23, 24 of tight detectors arc respectively
`disposed along first and second axes, incHcated by references
`A and B, which are substantially perpendicular and parallel
`to the longitudinal axis of !be strap, respectively. More
`specifically, light source 10 is located in a substantially
`central part of bottom side 2a and light detectors 21to 24 are
`disposed around and at a determined distance from light
`source 10. Ln this example, this distance is advantageously
`selected to be approximately equal to 10 rom.
`
`[0035) According to the invention, it wiU be appreciated
`that at least two light detectors arc required for a proper
`detection of the heart rate. The detecting device of FIGS. 1
`and 2 could thus be designed to have only one pair, three or
`even more than four light detectors. The number and spatial
`arrangemen t of these light detectors should however be
`selected in an adequate manner to provide sutlicieot spatial
`d iversity for removing light-related artefacts and, as this will
`be seen hereinafter, to remove other contributions which
`cannot be detected by the accelerometer, such as reciprocal
`contributions due to finger movements. In that regard, the
`two-axes arrangemen t illustrated in FIGS. 1 a nd 2 bas the
`advantage of aLlowing a good detection of such finger(cid:173)
`related reciprocal contributions.
`
`[0036) Referring again to FIGS. 1 and 2, housing 2
`further comprises a motion detecting device 40 which is for
`example disposed in an upper part 2b of housi ng 2. This
`motion detecting device 40 is preferably a three dimensional
`accelerometer, that is, in effect, three accelerometers dis(cid:173)
`posed along three orthogonal measurement axes aocl pro(cid:173)
`viding three dimensional acceleration data representa tive of
`the acceleration to which the device is subjected. This
`accelerometer is preferably and advantageously an acceler(cid:173)
`ometer of the type manufactured by the company Colibrys
`S. A. under reference MS 6100. It will however be appre(cid:173)
`ciated that other types of accelerometers or motion detecting
`
`devices could be used provided they deLiver a reliable
`measure of motion of the pulse rate detecting device on and
`with respect to the human body tis.~ue .
`
`[0037) Processing of the signals can either be done by an
`external processing unit linked to tbe portable device (by
`means of a direct or wireless connection) or preferably by an
`adequately programmed digital signal processor or DSP
`(indicated schematicaLly by reference numeral 50 in FlG. 2)
`housed within the device.
`
`[0038) Optionally, the portable pulse rate detecting device
`according to the invention may further comprise means for
`outputting an indication of the detected pu lse rate in the form
`of an optical, audible signal, or other sensorial signal. Such
`means could be a display, a bu7..zer, a vibrating device or any
`other suitable device adapted for transmining information
`representative of the pulse rate measurement to the user.
`Additionally, the detecting device may also comprise alarm
`means for generating an alarm when tbe detected pulse rate
`reaches a determined threshold, which could be either a low
`or high threshold or both.
`
`[0039) The basic principle of the invention resides in
`emilling an optical infrared (IR) signal at the surface of the
`buman body tissue (or alternatively through tbc body tissue).
`This signal is then propagated through the ti'>.~ue where it is
`subm itted to modifications due to reflection, refraction,
`scattering and absorption. lbe resulting signal, after propa(cid:173)
`gation through the tissue is grasped by tbe light detectors.
`Since variations of optical tissue characteristics are related
`to variations in the subcutaneous blood flow, the received
`signal can be used fo r the estimation of the heart rate.
`
`[0040) When light is transmitted through biological tissue,
`several mechanisms are involved in the interaction between
`the Light and the tissue. These interactions are reflection,
`refraction, scattering and absorption. Reflection and refrac(cid:173)
`tion occur at the interfaces between the probe and the
`subject. Scaucring is due to them icrosc<Jpic varia tions of the
`dielectric properties of the tissue. These variations are due to
`the cell membranes and the sub-cellular components (e.g.
`mitochondria and mtclei). For infra-red Light, the absorption
`is mainly due to chromophores such as haemoglobin, myo(cid:173)
`globin, cytochrome, melanin, lipid, bilirubin, and water. The
`relative importance depends on the wavelength considered
`and their distribution in the tissue.
`
`[0041) Under ideal steady-state condition, the received LR
`light signal contains both a constant (DC) and a time varying
`(AC) component. The constant component is generally
`ascribed to baseline absorption of blood and soft tissue, non
`expansive tissue such as bone, as welJ as reflectance loss.
`The time varying component reflects the modificatjon of the
`effective path length due to the expa.nsion of the tissues
`subject to the varying blood pressure.
`
`[0042) For the near lR wavelength, the light propagation
`into the tissue is gove rned by scattering and absorption. The
`so-called Beer-Lambert equation is generally used
`to
`describe the phenomenon of light absorption in biological
`tis.~ue:
`
`010
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`(I)
`
`[0048) where s1(l), s2(t) are pulse pressure related signal
`contributions, D011(t), nua(t) are artefacts due to motion and
`gravity, n1(t), o2(t) include measurement ooi<;e and non(cid:173)
`modelled stochastic signal contributions and N, is the num(cid:173)
`ber of observed samples.
`
`[0043) where li(t) and l0 (t) are the input and output light
`intensity, A. is the wavelength of ljgbt and e.(t), di(t) and €>..j
`represent, respectively, the concentrations, d1e spanning path
`length and the absorption coefficient of the different com(cid:173)
`ponents. For further information about this subject, refer(cid:173)
`ence can be made to the articles " Noise-resistant oximetry
`using a synthetic reference signal", Coetzee F. M. et a!.,
`IEEE Transactions on Biomedical Engineering, vol. 47, pp.
`1018-1026, August 2000, and "A review of the optical
`properties of biological tissues", Cheong W -F. et al., IEEE
`Journal of Quantum Electronic, vol. 26, pp. 2166-2185,
`1990.
`
`[0044) A<; briefly mentioned in the preamble part of the
`description, voluntary or involuntary movements corrupt the
`PPG signal and create motion-related artefacts. It is gener(cid:173)
`ally accepted that motion artefacts are mainly due to modi(cid:173)
`fication of the optical properties of the tissue (modification
`of blood pressure, modification of the optical path, etc.).
`These modifications affect the corresponding components of
`the Beer-Lambert equation. Therefore, io presence of
`motion artefacts, the received intensity can be rcwrillen in
`function of the major contributions
`
`(2)
`
`/ 0 (1)•i;(l)·y,,,,..,·y,.....(t)·ygo>vioy(l)·y.., 0 ,;00(1)
`[0045) where Ytissue is the static attenuation due to the
`tissue, y ulse(t) is due to pulsatile absorption of the blood,
`Y_grnvity(tJ is due to change of position a ad Ymotiou(l) is due to
`dynamic changes of the tissue induced by the movement of
`the arm (assuming the device is worn on the wrist). It is
`obvious that the different contributions become additive if
`one takes the logarithm of expression (2) above.
`
`[0046) When the subject is static, only the contributions of
`YJ>ul••(t) changes with time aod it is thea straightforward to
`remove the other contributions using a high-pass filtering.
`Wben the subject is moving, however, the contribution of
`the gravity and the modification of the interface between the
`detecting device and the body tissue are varying with time
`and they have to be removed from the signals in order to
`allow an accurate estimation of the heart rate. The contri(cid:173)
`butions of the gravity are at low frequency and can be
`removed quite easily by an adaptation of the gain. The
`contributions of the motion is difficult to remove, especially
`if it is in the same frequeocy band as the heart rate. Therefore
`techniques have 10 be developed in order to remove the
`motion artefacts to obtain an accurate estimation of the bean
`rate.
`
`It bas been shown above that IR-signals recorded at
`[0047)
`the wrist are mainly affecled by perturbations, such as tissue
`modifications, motion and gravity related artefacts. The
`main issue resides in tbe estimation of the mean heart rate
`from short lime recordings of IR-signals (e.g. 10 seconds).
`It is assumed that the tL'>Sue properties do not vary over the
`observed duration and for a dual channel approach, tbe
`Jog-corrected observed IR-sigoals (y1(t), yit)) given by
`expression (2) can be written as
`
`y 1(t)=s1(t}+nm 1(t)+111(t)
`
`Y2(t)=s0(t}+nm2(t)+112(t)
`
`(3)
`
`In order to obtain a robusl pulse detection in a large
`[0049)
`variety of experimental condilions, namely non-stationary
`environments, the proposed method according to the present
`invention works on a frame-to-frame basis with a frame
`duration of e.g. 3 seconds and it consists of mainly a three
`step algorithm as shown in FIG. 3.
`
`In a first step 100, the observed optical signals
`[0050)
`y1(t), yz(t) are enhanced using nonlinear, model-based noise
`cancelling 1ecbniques (see for instance " Adaptive Filter
`Theory", Haykin S., Prentice Hall, 1991). For this to be
`achieved, according to the present invention, an accurate
`motion reference signal (i.e. acceleration signals a,lt), ay(t)
`and a,(t)) is provided by the acceleronoeler. The non-linear
`modelling essentially consists in a polynomial expansion
`model and an associated model selection based on the
`Minimum Description Length (MDL) criterion. Such tech(cid:173)
`niques are already known. as such by those skilled in the art.
`Reference can for instance be made to '·Nonlinear Biomedi(cid:173)
`cal Signal Processing" Celka P. et al., vol. 2, IEEE Press,
`2000, and to the PhD thesis of M. R. Vetter (co-inventor)
`entitled "Extraclion of efficienl and characteristic features of
`multidimensional time series", EPFL Lausanne (Switzer(cid:173)
`land) 1999, which arc both incorporated herein by reference.
`
`[0051) The use of the parsimonious MDL selection crite(cid:173)
`rion avoids an overfilling of the time series and ensures in
`this way that no pulse pressure re lated signal contributions
`are cancelled.
`
`In a second step UO, measuremeot noise and
`[0052)
`non-modelled stochastic signal contributions in the two
`recorded channels are preferably removed. This is achieved,
`according to the preferred embodiment of the present ioven(cid:173)
`lion, by a noise reduction algorithm based on spatio-tem(cid:173)
`poral Principal Component Analysis (PCA). For further
`information aboul this PCA algorithm, reference will be
`made to the article ··Blind source separation in highly noisy
`environments", Yeller R. et aL, in First International Work(cid:173)
`shop on Independent Component Analysis and Signal Sepa(cid:173)
`ration" (ICA'99), Aussois (France), pp. 491-496, 1999,
`which is also incorporated bereio by reference. This step is
`not as such compulsory since a pulse rate measurement
`could be derived from the input signals after removal of tbe
`motion-relaled contributions.
`
`In addition to the removal of measurement noise
`[0053)
`and non-modelled signal contribulions, spatio-lemporal
`PCA allows one to reduce artefacts related to finger move(cid:173)
`meats, which are gene raJ ly not cancelled in step 100. Indeed,
`linger movements do oot necessarily imply a global dis(cid:173)
`placement of the forearm and arc therefore not grasped by
`the accelerometer. Finger movements, often imply tiny,
`reciprocal tendon related displacement of the forearm !issue,
`which yields reciprocal artefact contributions in tbe 1wo
`channels. Due to the reciprocity of these signal contribu(cid:173)
`tions, they can efficiently be cancelled by a spatio-temporal
`PCA
`
`011
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`In a third step UO, the pulse rate is extracted from
`[0054)
`the enhanced TR-siguals. This extraction essentially consists
`of an inter-beat interval extraction achieved through a clas(cid:173)
`sical maximum detection procedure, preferably with inhibi(cid:173)
`tion of peak detection during the refractory period of cardiac
`cells. In addition, a maximum likelihood histogram cluster(cid:173)
`ing of the restdting inter-beat intervals is performed (cf.
`" Vector Quantization and Signal Compression", Gersbo A.
`et al.,