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`US 20030065269A1
`
`(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2003/0065269 A1
`
`Vetter et al.
`(43) Pub. Date:
`Apr. 3, 2003
`
`(54) METHOD AND DEVICE FDR PULSE RATE
`DETECTION
`
`(52) use].
`
`
`
`riotous
`
`(3'5)
`
`Inventors; Rolf Vetter, Yverdon (Cl-I); Philippe
`Rene-veg. Lausanne ((TII); Roland
`Gentsch. Ilaulerive ((TII}; Jens Krauss.
`Neuchatel (Cit); Yves Depeursinge,
`Servion (CII)
`
`Correspondence Address:
`PARKHURST 8»: WENDEL, L.L.P.
`1421 PRINCE STREET
`SUITE 210
`ALEXANDRIA, VA 22314-2805 [US]
`
`Assignee: CSEM Centre Suisse d’l‘llectmnique et
`de Micmteehnique SA, Neuchatel (CH)
`
`Appl. No:
`
`10}255,068
`
`(22
`
`Filed:
`
`Sep. 26, 2002
`
`(30)
`
`Foreign Application Priority Data
`
`Sep. 28, 260]
`
`(LP) ....................................... {112036869
`
`l'ublieatlon Classification
`
`(51)
`
`Int. Cl.7 ....................................................... A6111 51'02
`
`(57)
`
`ABSTRACT
`
`There is described a device and a method for detecting the
`pulse rate. The measurng principle oonsists ol' emitting
`radiant energy at
`the surface of or through human body
`tissue (5) by means 01‘ a light-emitting source (10)I measur-
`ing the intensinr of the radiant energy after propagation
`through the human hotly tissue by means ofat leasl first and
`second light detectors (21, 22, 23, 24} located at a deter-
`mined distance from the light-emitting source and providing
`first and second input signals (y1(t), hm) representative of
`this propagation. Simultaneously, a motion detecting device
`(40), such as a three dimensional acceierometers, provides a
`motion reference signal (Mt). a,(l), alto) representative of
`motion of the detecting device on and with respect to the
`human body tissue (5). 111:.) input signals are then processed
`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 lirst and second
`enhanced signals. This processing basically comprises the
`elaboration ol‘ a model ol‘ the motion-related contributions
`based on the motion reference signal and the subtraction ol-
`this mode] from each of the input signals. Puise rate is then
`extracted from the enhanced signals using for instance a
`maximum likelihood histogram clustering technique.
`
`
`
`
`
`001
`
`Apple Inc.
`APLl 01 1
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`US. Patent No.
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`8,923,941
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`FITBIT, EX. 1011
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`Apple Inc.
`APL1011
`U.S. Patent No. 8,923,941
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`001
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`Patent Application Publication
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`Apr. 3, 2003 Sheet 1 0f 6
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`US 2003f0065269 A1
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`Fig.1
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`Patent Application Publication
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`Apr. 3, 2003 Sheet 2 of 6
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`US 2003(0065269 A1
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`Fig .3
`
`MOTION
`REFERENCE
`SIGNAL
`
`ax(t),ay(t), 326'.)
`
`
`
`
`
`
`
`
`
`MEASUREMENT NOISE
`AND NON-MODELLED
`CONTRIBUTIONS
`REMOVAL USING
`NOISE REDUCTION
`ALGORITHM
`
`
`
`
`
`
`110
`
`
`
`
`ENHANCED
`
`SIGNALS
`
`PULSE RATE
`EXTRACTION
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`
`
`SELECTION OF MOST
`RELIABLE CANDIDATE
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`120
`
`130
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`003
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`INPUT SIGNALS
`
`y1(t),y2(t)
`
`MOTION ARTEFACTS
`REMOVAL USING MON
`LINEAR MODEL-BASED
`NOISE CANCELLING
`
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`
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`100
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`Patent Application Publication
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`Apr. 3, 2003 Sheet 3 0f 6
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`US 2003f0065269 A1
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`Patent Application Publication
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`Apr. 3, 2003 Sheet 4 of 6
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`US 2003/0065269 A1
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`Patent Application Publication
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`Apr. 3, 2003 Sheet 5 of 6
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`US 2003/0065269 A1
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`Fig. 50
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`Patent Application Publication
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`Apr. 3, 2003 Sheet 6 0f 6
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`US 20032’0065269 A1
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`US 2003/0065269 A1
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`Apr. 3, 2003
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`METHOD AND DEVICE FOR PULSE RATE
`DIC'I‘ECTION
`
`[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-
`cally composed of a processing device and an external probe
`(e.g. electronic stethoscope, optical measure at ear lobe,
`chest belt for electrocardiogram—ECG-based measnrerneut,
`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 W0 00751680.
`[0003] Various pulse rate detection systems are known in
`the art. Pulse rate detection devices using pressure sensitive
`transducers such as piezoelectric elemean are for instance
`disclosed in documents U.S. Pat. Nos. 3,838,684, 4,195,642,
`4,331,154, 5,807,267 and W0 80700912.
`
`[0004] More recently, measuring techniques based on so-
`called photoplethysmography (or FPO) have been proposed.
`PPG is an electro-optic technique of measuring the cardio-
`vascular pulse wave found throughout the human body. This
`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-
`surement system classically consists of a source of radiant
`energy (usually an infra—red light source), at
`least one
`detector for detecting the intensity ofthe radiant energy after
`propagation through the human body tissue and a data
`processing means for extracting bodily parameters such as
`pulse rate or oxygen concentration in the blood. Infra—red
`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-
`sonable contrast. The principal advantage of PPG measure-
`ment resides in the fact that it is entirely non-invasive and
`can be applied to any blood bearing tissue, typically a finger,
`nail, ear lobe, nose and, in some instances, wrist.
`[0005] Since light is highly scattered in tissue, a detector
`positioned on the surface of the skin can measure reflections
`(or transmissions) from a range of depths and those reflec-
`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 the
`detector at
`the surface sinCe increasing (or decreasing)
`volume will cause more (or less) absorption. The effect will
`be averaged over many arteries and veins. In the absence of
`any blood volume changes, the signal level will be deter-
`mined by the tissue type, skin type, probe positioning, static
`blood volume content and of course the geometry and
`sensitivity of the sensor itself,
`[0006]
`PPG systems ditl'ercntiate between light absorption
`due to blood volume and that of other fluid and tissue
`constituents by observation that arterial blood flow pulsates
`while tissue absorption remains static As the illuminated
`blood flow pulsates.
`it alters the optical path length and
`therefore modulates the light absorption throughout
`the
`cardiac cycle. Non-pulsating fluids and tissues do not modu-
`late the light but have a fixed level of absorption (assuming
`there is no movement).
`[0007'] The result of this absorption is that any light
`reflected from (or transmitted through) the pulsating vascu-
`
`lar bed contains an AC component which is proportional to
`and synchronous with the patients plethysmographic signal.
`it
`is this modulated component which is known as the
`photoplethysmograpltic signal. This PPG signal is superimA
`posed onto a DC level which represents the difference
`between incident radiant energy and the constant absorption
`ol‘the tissue, blood and anything else in the optical path with
`constant absorption.
`[0008] PPS measurement can be achieved by measure-
`ment of the intensity of radiant energy transmitted through
`(transmission mode systems) or reflected by (reflection
`mode systems) body tissue. A reflection mode system typi-
`cally has much poorer signal to noise ratio, resulting from
`the fact that a smaller proportion of the light which is not
`absorbed will be reflected than transmitted. That
`is the
`reason why most of the prior art devices and systems use at
`detecting arrangement that is placed on the user's linger,
`nail, ear lobe, nose or part of the body through which light
`can easily be transmitted.
`[0009]
`PPG has widely been used for measuring arterial
`oxygen saturation known as pulse oximetry, The technique
`relies on the knowledge that haemoglobin and oxyhaemo-
`globin absorb light
`to varying degrees as a function of
`wavelength. In particular, the absorption characteristics 01'
`red and near infrared light are inverted [or the two species.
`It is thus possible to derive the proportion of oxyhaemoglo-
`bin and therefore the arterial oxygen saturation from a
`knowledge of the absorption characteristics of the arterial
`blood at these two wavelengths. Pl’G-based oximetry sens-
`ing devices employing sensors which are typically in 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, W0 99752420, WO 99162399 and
`WO 01725802. FPO-based oximetry and heart rate detecting
`devices intended to be worn on or around other pans of the
`human body such as the wrist or car, are also known, for
`instance from documents U.S. Pat. No. 5,807,267 and W0
`97fl4357.
`
`[0010] One of the main 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-
`pretation of PPG signals and degrade the accuracy and
`reliability of l’PG-based algorithms for the estimation of
`cardiovascular parameters.
`[0011]
`Processing of ambient light artefacts is not critical
`because the influence of ambient
`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", 'l‘rivedi N.
`et al., Journal of Clinical Anaesthesia, vol 9, pp. 179483,
`1997,
`for a description of these problems.
`In contrast,
`processing of motion artefaets 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
`mechanical forces that induces changes in the optical cou-
`pling and the optical properties of the tissue. Motion arte-
`l'acts are a particularly critical problem for the design of a
`wrist-located pulse detecting device.
`[0012] Several methods have been proposed to reduce
`motion artefacts in PPS signals. Feature-based algorithms
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`
`have been proposed to discard the corrupted segments from
`the signals for instance in document WO 94/22360 (corre-
`sponding to US. Pat. No. 5,368,036). This kind ofapproach
`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.
`
`In order to circumvent this drawback, model-based
`[0013]
`noise cancelling techniques have been applied more recently
`for the enhancement of optical signals. Examples are for
`instance described in documents US. Pat, No, 5,490,505,
`WO 94103102 and in articles “Simple photon dilIusion
`analysis of the clfects of multiple scattering on pulse oxim—
`etry", Schmitt 1., IEEE Transactions on Biomedical Engi-
`neering, vol. 38, pp.
`1194—2002, December 1991, and
`“Noise-resistant oxintetry using a synthetic reference sig-
`nal”, Coetzec F. M. et all, IEEE Transactions on Biomedical
`Engineering, vol. 47, pp. MIR-1026, August 2000. In such
`approaches a reference signal of motion is recorded and a
`parametric model
`is used subsequently to retrieve motion
`related influences
`in the optical signals. Nevertheless,
`motion references are classically obtained by piemo-sensors
`or optical measures and convey therefore only incomplete or
`local information of motion. This degrades the performance
`of model-based noise cancelling techniques since they
`require complete and low-noise motion reference signals.
`
`It is thus a principal object of the present invention
`[0014]
`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-
`tion is to provide a solution that allows for adcqu ate removal
`of ambient
`light and motion contributions in the optical
`signals.
`
`[0016] Another object of the invention is to provide a
`solution that
`is suitable for enabling measurement and
`detection to happen at the wrist level.
`
`[0017] Accordingly there is provided a portable pulse rate
`detecting device the features of which are recited in claim I.
`
`[0018] There is also provided a method for detecting pulse
`rate the features of which are recited in claim 17.
`
`[0019] Other advantageous embodimean of the invention
`are the object of the dependent claims.
`
`[0020] According to the present invention, an accurate
`motion detecting device is used to provide a reliable motion
`reference signa L This motion detecting device is preferably
`a fully integrated three dimensional accelerometer which
`exhibits a high accuracy and very low noise.
`
`In order to achieve efficient removal of motion
`[0021]
`related artefacts in the optical signals, nonlinear model-
`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.
`
`Furthermore, in order to grasp the spatial diversin
`[0022]
`ofthe Optical characteristiis of the tissue, at least IWo Optical
`sensors are uned. This two—channel arrangement, associated
`with an adequate noise reduction algorithm (preferably an
`algorithm based on so-called spat [rt-temporal Principal
`
`Component Analysis or PCA]. allows one to remove mean
`suremcnt noise and non-modelled stochastic signal contri-
`btltions as well as reduce artefacts related to finger movc~
`merits which are generally not recorded by the accelerometer
`and therefore not initially cancelled.
`[0023] Eventually,
`the heart
`rate is estimated from the
`enhanced signals using inter-beat extraction based on physi-
`ological properties of cardiac cells and maximum likelihood
`histogram clustering of the 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. This is mainly due to the inclusion
`of the parsimonious MDL-based model selection, which
`avoids art overfitting of the time series. Indeed,
`the full
`nonlinear model would retain pulse related components in
`the estimate of the motion artcfacts. Since these components
`are subtracted from the optical signals, the quality of the
`enhanced signal and consequently the 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 pulse
`estimation in adverse noisy environments.
`[0025] Other aspects,
`features and advantages of the
`present invention will be apparent upon reading the follow-
`ing detailed description of non—limiting examples and
`embodiments made with reference to the accompanying
`drawings, in which:
`[0026]
`FIt}. 1 is a schematic view of the bottom side
`(intended to come into contact with the body tissue) of a
`portable pulse rate detecting device according to the inven-
`tion which is adapted to be worn on the wrist and comprising
`a light source and two pairs oflighl detectors arranged at the
`bottom side;
`[0027] FIG. 2 is a schematic side view of the device of
`FIF. 1 further illustrating the arrangement of the acceler-
`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 pulse detection algorithm according to the present
`invention which is based on nonlinear model-based motion
`artefacl cancelling, coherence-based reduction of measure-
`ment noise and stochastic signal contributions, and a pulse
`detection using maximum likelihood histogram clustering;
`and
`
`[0030] FIGS. 5a to 5e are diagrams respectively illustrat-
`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 the two optical signals after moa-
`suremcnt noise removal (using I’CA) and (e) a correspond
`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 1,
`according to a preferred embodiment of the present inven-
`tron.
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`[0032] While 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 light
`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 linger, nail, ear lobe or any other suitable
`member or part of the human body. In addition, the same
`principles could be applied to a detecting device based on
`the measurement of light transmitted through the body tissue
`(such as those typically used in pulse oximetry) where the
`signal to noise ratio is higher. In 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 pat ient's wrist. Housing 2 comprises, located
`in a bottom side 2a of the device in Contact with the skin, a
`light source 10 for emitting radiant energy at the surface of
`(or through) the human body tissue, designated by reference
`numeral 5. Light source 10 is preferably an infrared light
`emitting device (LED).
`
`[0034] According to the preferred embodiment, housingZ
`further includes two pairs of light detectors 21, 22 and 23,
`24 [or detecting the intensity of the radiant energy after
`propagation through the human body tissue. Such light
`detectors may conveniently be photodiodes. Preferably, the
`pairs 21, 22 and 23, 24 of light detectors are respectively
`disposed along first and second axes, indicated by references
`A and B, which are substantially perpendicular and parallel
`to the longitudinal axis of the strap, respectively. More
`specifically,
`light source 10 is located in a substantially
`central part of bottom side 2a and light detectors 21 to 24 are
`disposed around and at a determined distance from light
`source 10. In this example, this distance is advantageously
`selected to be approximately equal to 10 mm.
`
`[0035] According to the invention, it will be appreciated
`that at
`least
`two light detectors are 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 [our light detectors. The number and spatial
`arrangement of these light detectors should however be
`selected in an adequate manner to provide sufficient spatial
`diversity for removing light-related artefacls 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 arrangement illustrated in FIGS. I and 2 has the
`advantage of allowing a good detection of such finger—
`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 2!: of housing 2. This
`motion detecting device 40 is preferably a three dimensional
`accelerometer,
`that
`is,
`in elfecl, three accelerometers dis—
`posed along three orthogonal measurement axes and pro-
`viding three dimensional acceleration data representative of
`the acceleration to which the device is subjected. This
`accelerometer is preferably and advantageously an acceler-
`ometer of the type manufactured by the company Colibrys
`S. A. under reference MS 6100. It will however be appre-
`ciated that other types ofacceleromcters 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 tissue.
`
`Processing of the signals can either be done by an
`[0037]
`external processing unit linked to the portable device (by
`means ofa direct or wireless connection) or preferably by an
`adequately programmed digital signal processor or DSP
`(indicated schematically by reference numeral 50 in FIG. 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 pulse rate in the form
`of an optical, audible signal, or other sensorial signal Such
`means could be a display, a buzzer. a vibrating device or any
`other suitable device adapted for transmitting information
`representative of the pulse rate measurement to the user.
`Additionally, the detecting device may also comprise alarm
`means for generating an alarm when the 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
`emitting an optical infrared (IR) signal at the surface of the
`human body tissue (or alternatively through the body tissue).
`This signal is then propagated through the tissue where it is
`submitted to modifications due to reflection, refraction,
`scattering and absorption. The resulting signal, after propa~
`gation through the tissue is grasped by the light detectors.
`Since variations of optical tissue characteristics are related
`to variations in the subcutaneous blood flow, the received
`signal can be used for 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 relleclion,
`refraction, scattering and absorption. Reflection and refrac-
`tion occur at
`the interfaces between the probe and the
`subject. Scattering is due to the microscopic variations of the
`dielectric properties of the tissue. ’l‘hesc variations are due to
`the cell membranes and the sub-cellular components (e.g.
`mitochondria and nuclei). For infra-red light, the absorption
`is mainly due to chromophores such as haemoglobin, myo-
`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-slate condition, the received IR
`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 well as reflectance loss.
`The time varying component reflects the modification ofthe
`etfeetive path length due to the expansion of the tissues
`subject to the varying blood pressure.
`
`For the near IR wavelength. the light propagation
`[0042]
`into the tissue is governed by scattering and absorption. The
`so-callcd Beer-Lambert equation is generally used to
`describe the phenomenon of light absorption in biological
`tissue:
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`
`u
`i'.,tr't = Mr) ~c1p{—Z ci‘Jc‘Jtrhijtrl]
`J2. I
`
`il l
`
`[0048] where 51(1), sgtt) are pulse pressure related signal
`contributions, nm1(l), nm2(t) are artefaets due to motion and
`gravity, n,(t), n20) include measurement noise and non—
`modelled stochastic signal contributions and NL is the num-
`ber of observed samples.
`
`[0043] where 130) and 1,,(t) are the input and output light
`intensity, 3» is the wavelength of light and 00), dJ-(t) and exj
`represent, respectively, the concentrations, the spanning path
`length and the absorption coefficient of the different com~
`ponents. For further information about
`this subject, refer-
`ence can be made to the articles “Noise-resistant oximetry
`using a synthetic reference signal”, Coetzee F. M. et al.,
`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., [BEE
`Journal of Quantum Electronic, vol. 26, pp. 2166-2185,
`1990.
`
`[0044] As 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-
`ally accepted that motion artel’acts are mainly due to modi-
`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,
`in presence of
`motion artefacts, the received intensity can be rewritten in
`function of the major contributions
`(3)
`[sill-it(U‘Trint-p'Tpqu-(I’i'lyzmyi” 'l'mmianl
`[0045] where yn-sm is the static attenuation due to the
`tissue, T “1560) is due to pulsatile absorption of the blood,
`Y ‘mmttl’ is due to change of position and ymoliwtt) is due to
`dynamic changes of the tissue induced by the movement of
`the am (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
`trauma] changes with time and it is then straightforward to
`remove the other contributions using a high-pass filtering.
`When 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 he removed from the signals in order to
`allovtr an accurate estimation of the heart rate. The contri-
`butions ol' 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 frequency band as the heart rate. Therefore
`techniques have to be developed in order to remove the
`motion artet'acls to obtain an accurate estimation of the heart
`rate.
`
`It has been shown above that IR-signals recorded at
`[0047]
`the wrist are mainly affected by perturbations, such as tissue
`modifications, motion and gravity related artefacts. The
`main issue resides in the estimation of the mean heart rate
`from short time recordings of Ill—signals (e.g. 10 seconds).
`It is assumed that the tissue properties do not vary over the
`observed duration and for a dual channel approach,
`the
`log-corrected observer] lR-signals [y,(l), y2(t)) given by
`expression (2) can be written as
`.V.(0=~‘>':(fl+"mitfl+v‘ni0
`(=0, .
`.
`. ,N,—r
`)’:(-'l=‘5:(ll'+"m:{n+-‘*:(U
`
`(3)
`
`In order to obtain a robust pulse detection in a large
`[0049]
`variety of experimental conditions, 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.
`
`the observed optical signals
`In a first step 100,
`[0050]
`yJ(t), 312(1) are enhanced using nonlinear, model—based noise
`cancelling techniques (see for instance “Adaptive Filter
`Theory", [Iaykin 3., Prentice Hall, 1991). For this to be
`achieved, according to the present invention, an accurate
`motion reference signal (ie. acceleration signals ax“), 11,0)
`and a,,(t)) is provided by the accelerometer. 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-
`niques are already known as such by those skilled in the art.
`Reference can for instance he made to "Nonlinear Biomedi-
`cal Signal Processing“ Celka R et al., vol. 2. IEEE Press,
`2000, and to the PhD thesis of M. R. Vetter (co-inventor)
`entitled “Extraction of eficient and characteristic features of
`multidimensional time series”, El’liL Lausanne (Switzer-
`land) 1999, which are both incorporated herein by reference.
`
`[0051] The use of the parsimonious MDI.. selection crite-
`rion avoids an overfitting of the time series and ensures in
`this way that no pulse pressure related signal contributions
`are cancelled.
`
`In a second step 110, measurement 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 invenA
`tion, by a noise reduction algorithm based on spatio-tcm-
`poral Principal Component Analysis (PCA). For further
`information about
`this PCA algorithm, reference will be
`made to the article “Blind source separation in highly noisy
`environments", Vetter R. et al., in First International Work-
`shop on Independent Component Analysis and Signal Sepa-
`ration" (ICA’99), Aussois (France), pp. 491-490, 1999,
`which is also incorporated herein by reference. This step is
`not as such compulsory since a pulse rate measurement
`could be derived from the input signals af1er removal of the
`motion-related contributions.
`
`In addition to the removal of measurement noise
`[0053]
`and non-modelled signal contributions, spatio-temporal
`FCA allows one to reduce artefacts related to finger move-
`ments, which are generally not cancelled in step 100. Indeed,
`finger movements do not necessarily imply a global dis—
`placement of the forearm and are therefore not grasped by
`the accelerometer.
`l-‘inger movements. ot‘ten imply tiny,
`reciprocal tendon related displacement of the forearm tissue,
`which yields reciprocal artefact contributions in the two
`channels. Due to the reciprocity of these signal contribu-
`tions, they can elficiently be cancelled by a spatio-temporal
`FCA.
`
`011
`
`FITBIT, EX. 1011
`
`011
`
`FITBIT, Ex. 1011
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`

`

`US 2003/0065269 A1
`
`Apr. 3, 2003
`
`In a third step 120, the pulse rate is extracted from
`[0054]
`the enhanced IR-signals. This extraction essentially consists
`of an inter—beat interval extraction achieved through a clas~
`sical maximum detection procedure, preferably with inhibi—
`tion of peak detection during the refractory period of cardiac
`cells. In addition, a maximum likelihood histogram cluster—
`ing of the resulting inter-beat
`intervals is performed (ci'.
`"Vector Quantization and Signal Compression“, Gersho A.
`et al., Kluwer Academic Publishers. [992].
`
`[0055] Eventually, in a fourth step 130, the most reliable
`candidate can be selected. A robust and reliable estimate of
`the pulse rate can be obtained through a nonlinear mapping
`of the two candidate values in function of their reliability
`measures. This nonlinear mapping is advantageously
`achieved by Multiple Layer l’erceptron (MLP), which has
`been trained on data of various experimental setups as
`described in "Neural Networks", Haykin 8., Macmillan
`College Publishing Company Inc, [994.
`
`the preferred
`[0056] A more detailed description of
`embodiment of the present invention will now be described
`in reference to the diagrams of FIG. 4 and FIGS. So to Sc.
`FIG. 4 shows a diagram illustrating the preferred algorithm
`according to the invention where block 200 refers to the
`nonlinear modelling based on the motion reference signal
`(3,,(1), a,_(t), 55(1)), block 210 refers to the measurement noise
`and nonsmodelletl contributions cancellation using PCA,
`block 220 refers to the inter-beat interval extraction on the
`two enhanced signals. block 225 refers to the maximum
`likelihood histogram clustering, block 226 refers to the
`detection of the non-stationary signal segments, and block
`230 refers to the final selection of the most reliable candidate
`using a nonlinear mapping technique.
`
`[0057] One of the key element in the pro