`
`(19) World Intellectual Property Organization
`International Bureau
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`(51)
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`(21)
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`(25)
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`(26)
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`(30)
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`(71)
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`(43) International Publication Date
`1 November 2007 (01.11.2007)
`
`International Patent Classilicatjon:
`A6IB 5/I-155 (2006.01)
`A6135/08 (2006.01)
`A613 5102-! (2006.0l)
`
`(10) International Publication Number
`
`W0 2007/122375 A2
`
`(74) Agent: CHARIG, Raymond; liric Potter (Ilarkson l.l.l’,
`Park View House. 58 The Ropcwalk, Nottingham NGI
`SDD (GB).
`
`International Application Number:
`PC'l”tGB20lT’(0O I 355
`
`International Filing Date:
`
`1
`
`l April 2007 (I 1.04.2007}
`
`Filing Language:
`
`Publication Language:
`
`Priority Data:
`t]6Di'2i'(l.6
`
`English
`
`English
`
`I
`
`1 April 2(l0(i (I l.[)4.2tl(l6)
`
`GB
`
`Applicant (for all de.t't‘gttateu' Strttes except US}: THE
`UNIVERSITY OF NOTTINGHAM IGB.-’GB|: Univer-
`sity Park, Nottingham NG7 2RD (GB).
`
`Inventors; and
`lnventorsmpplicants (for US only): CROWE, John
`[GBIGB]; School of Electrical and Electronic Engineer-
`ing, The University of Nottingham, University Park,
`Nottingham NG7 2RD (GB). GRUBB. Mark IGBIGBI;
`School of lilectrical and iilectronic Engineering, The
`University of Nottingham, University Park, Nottingham
`NG7 2RD (GB). HAYES-GILL, Barrie [GBIGB]; School
`of Electrical and Electronic Engineering. The University
`of Nottingham, University Park, Nottingham NU?‘ 2RD
`(GB 1. MILES, Nicolas {GI-3t’GB]‘. School of lilectricai and
`Electronic Engineering, The University of Nottingham,
`University Park, Nottingham NG7 2RD (GB).
`
`Designated States (t.ntte.r5 otherwise trtritcated. for every
`kind of rtatiortal protection available}: Ali. AG. AL. AM.
`AT, A1J_.A'/C, BA, BB. BG, BH. BR, BW, BY, BZ, CA, CH.
`(IN, (10, CR, CU, CZ, DIE, DK, DM, I)’/1, l':‘(I, lili, 15G, liS,
`Fl, GB, GD, GE, GH, GM, GT, HN, HR, IIU. ID, IL, IN,
`IS, JP, KE, KG, KM, KN, KP, KR,
`LA, LC, LK, LR,
`LS. l.'l'. LU, LY. MA. MD. MG. MK. MN. MW. MX. MY.
`MZ, NA, NG, N1, NO, NZ, OM, PG, PH, PL, PT, Rt), RS,
`RU, SC, SD, SE, SG, SK, Sl., SM, SV, SY, TJ, TM, TN,
`TR. T1", TZ. UA, UG, US. UZ, VC. VN, ZA, ZM, ZW.
`
`Designated States (.t.tm"e.s‘.t' nthen-t=.t'se indir:£tted. for every
`kind of regiottrtl pt‘0te{.‘t.t£Jtt available): ARIPO (BW, GI-l,
`GM, KE, LS, MW, MZ, NA, SD, SL, SZ, TZ, UG, ZM,
`KW). Eurasian (AM. AZ. BY. KG. K7,. MI). RU. TJ. TM).
`Iiuropean (AT, HIS, BG, CH, CY, CZ, DE, IJK, lili, IES. l-‘I,
`FR, GB, GR, IIIJ, I13, IS, IT, LT, LU, LV, MC, MT, NL, PL,
`PT, RO, SE, SI, SK, TR), OAPI (BF, BJ, CF, CG, CI, CM,
`GA, GN, GQ, GW. ML, MR, NE. SN, TD, TGI.
`
`Published:
`
`without ftttematiotrat _rer.tr¢.‘It report am)‘ to be repttb(i.rtted'
`trprm receipt of that report
`
`For two—t'etter r:rm'e.s' and other abbretrirtttrjns. refer to the "Guid-
`cmce Note: on CcJu'e.s‘ ort.u‘Abbrevtatiorts " appettrtng at the begin-
`ning ofeaclt regular issue ofthe PCT Gazette.
`
`(54) Title: PH()’l‘()l’l.tL'I‘t-[YSMOGRAPHY
`
`100
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`«-4
`<2‘
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`tn
`r-
`to
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`NN!
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`(57) Abstract: A photoplethysmograph device includes a light source for illuminating a target object. A modulator drives the light
`—
`sotuce suclt that the output intensity varies as a function of a modulation signal at a modulation frequency. A detector receives light
`"“"--.
`from the target object and generates an electrical output as a function of the intensity of received light. A demodulator with a local
`r-
`; oscillator receives the detector output and produces a demodulated output representative of the modulation signal. The demodulator
`= is insensitive to any phase difference between the modulation signal and the oscillator of the demodulator. lirom the demodulated
`(‘I output, a signal indicative ofblood volume as a function of time and for blood composition is generated. A number ofdemodulators
`may be provided to derive signals from multiple light sources of different wavelengths, or from an array of detectors. The plethys-
`mograph may operate in a transmission mode or a reflectance mode. When in a reflectance mode, the device may use the green part
`of the optical spectrum and may use polarising filters.
`
`U.S. Patent No. 8 923 9
`
`Apple Inc.
`APL1049
`U.S. Patent No. 8,923,941
`
`0001
`
`
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`W0 2007!] 22375
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`PCTIG B2007l00 I355
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`PHOTOPLETHYSMOGRAPHY
`
`The present invention relates photopletliysmography and in particular to a method and
`
`apparatus for measuring pulse rate, breathing rate and blood constituents in the human
`
`or animal body.
`
`The word plethysmography is a combination of the Greek words Plethysmos,
`
`meaning increase, and graph, meaning write. A plethystnograph is an instrument,
`method or apparatus used to measure the variations in blood volume in the body.
`Photoplethysmography iheremafier also referred to as 'PPG') refers to the use of light
`
`to measure these changes in volume, and therefore a photoplethysmograph is an
`
`instrument, method or apparatus that uses light to perform these measurements.
`
`Although the human or animal body is generally assumed to be opaque to light, most
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`soft
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`tissue will
`
`transmit and reflect both visible and near-infrared radiation.
`
`Therefore, if light is projected onto an area of skin and the emergent light is detected
`
`after its interaction with the slcin, blood and other tissue, time varying changes of light
`
`intensity having a relation with blood volume, known as the plethyslllogranl, can be
`
`observed. This time varying light intensity signal will depend on a number of factors
`
`including the optical properties of the tissues and blood at the measurement site, and
`
`the wavelength of the light source. The signal results because blood absorbs light and
`
`the amount of light absorbed, and hence the intensity of remaining light detected,
`
`varies in relation with the volume of blood illuminated.
`
`Variation i.n the
`
`plethysrnogram is caused by the variation in blood volume flowing in the tissue.
`
`This technique was introduced in 1937 by Hertzman. He was the first to use the term
`
`photoplethysrnography and suggested that the resultant plethysmograrn represented
`
`volumetric changes of blood in the skin's vessels.
`
`The plethysrnogram is usually described with respect to its AC and DC components.
`
`The absorption of light by non-pulsatile blood, bone and tissue is assumed to be
`
`constant and gives rise to the DC component. The DC component represents the
`
`volume of non-pulsatile blood below the sensor, plus light reflected and scattered off
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`the skin, bone and other tissues. The AC component is caused by the time varying
`
`absorption of light caused by temporal changes in blood volume below the sensor.
`
`Changes in the blood volume can be caused by cardiovascular regulation, blood
`
`pressure regulation, thermoregulation and respiration. Thus the plethysmograin can
`
`be analysed to determine information on such parameters as pulse rate, breathing rate,
`
`blood pressure, perfusion, cardiac stroke volume and respiratory tidal volume. These
`
`can be observed as periodic and non-periodic changes in the amplitude of AC and DC
`
`components in the plethysmogram. This has been described in more detail in Kamal
`
`er.‘ al:
`
`‘Stein Photoplerhysmography — a review’, Computer Methods and Programs in
`
`Biornedicine, 28 (1989) 257-269). The plethysrnograrn can also be analysed to
`
`detennine blood constituents.
`
`One such technique is pulse oxirnetty, which
`
`determines the relative amount of oxygen in the blood. Other blood constituents can
`
`also be measured by using photoplethysmography.
`
`There are two modes of photoplethysrnography, the transmission mode and the
`
`reflection mode. In transmission mode the light source is on one side of the tissue and
`
`the photodetector is placed on the other side, opposite the light source. The use of
`
`transmission mode is limited to areas Where the tissue is thin enough to allow light to
`
`propagate, for example the fingers, toes and earlobes of a human subject.
`
`In reflection mode the light source and photodetector are place side-by-side. Light
`
`entering the tissue is reflected and a proportion of this is detected at the photodetector.
`
`This source—detector configuration is more versatile and allows measurements to be
`
`performed on almost any area of tissue. However, the use of reflectance mode is
`
`much harder to design than transmission because the signal level is significantly lower
`
`at the most effective wavelengths. Thus, considerable attention must be given to
`
`maximising signal-to-noise ratio. As a result, the most common PPG sensors use
`
`transmission mode and hence are restricted to positions where light can pass through
`
`tissue.
`
`As a photodetector is used to measure light from the source, the photoptethysrnograph
`
`can also respond to interfering signals from other sources of light,
`
`for‘ example
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`fluorescent lighting and computer monitors. The sensor must also respond to changes
`
`in the light propagating through tissue, i.e. the plethysmogram. These physiological
`
`changes contain frequency components between DC and 25 Hz. However, it is
`
`desirable for the sensor not to respond to ambient light noise. Accordingly, the
`
`photoplethysmograph should reject ambient
`
`light noise while detecting the
`
`plethysmogram in the bandwidth of interest.
`
`A second source of interference is other electrical apparatus. Other electrical devices
`
`can generate radio frequency signals that a photoplethysmograph can detect.
`
`It is
`
`desirable to minimise the sensitivity of the system to interfering sources of this nature.
`
`A third source of
`
`interference
`
`is
`
`the
`
`electrical noise generated by the
`
`photoplethysmograph itself. Such noise can be generated by electrorlic components,
`
`and can include thermal noise, flicker noise, shot noise, as well as noise spikes, for
`
`example, harmonics generated by missing codes in an analogue-to—d.igitaI converter.
`
`It is also desirable to minimise the sensitivity of the system to interference from these
`SOUYCBS.
`
`A known technique for reducing the noise generated by these three sources of
`
`interference is to drive the sensor's light source with a carrier modulated at a
`
`frequency that is not present, or dominant,
`
`in the ambient
`
`light, electrical radio
`
`frequency signals, or photoplethysrnograph system noise. This can be done by
`
`modulating the sensor’s light source with a square wave, by pulsing it on and off. The
`
`detected signals are then band pass filtered to attenuate interference outside the
`
`frequency range of
`
`interest.
`
`Subsequent demodulation will
`
`recover
`
`the
`
`plethysmograni.
`
`In general, any periodic signal such as a sine wave may be used to
`
`modulate the light source.
`
`Though modulated light photoplethysmography exists in the prior art, there are still
`
`critical limitations in how it has been applied, especially in terms of suitable signal
`
`conditioning circuits for attenuating or removing noise, and demodulation. For
`
`example, EP033535?, BP0314324, WO0144780 and WO9846125 disclose modulated
`
`light photoplethysmography. However,
`
`they use a demodulation method. and
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`apparatus that
`
`requires
`
`the modulating and demodulating carrier phase to be
`
`synchronised. Error in the synchronisation timing will add noise to the demodulated
`
`signal (timing jitter or phase noise). The prior art also fails to make full use of band
`
`pass filter characteristics to remove ambient interfering light, by still relying on a
`
`separate channel to measure ambient light, and later subtracting it from the signal,
`
`which adds
`
`fiirther complexity and is arguably less efficient at attenuating
`
`interference. These limitations reduce immunity to broadband and narrowband noise
`
`from sources such as fluorescent lighting, computer monitors, sunlight, incandescent
`
`light, electrical RF interference, thermal noise, flicker noise, and shot noise.
`
`A further limitation in the prior art is the choice of wavelength for reflectance mode
`
`sensors. Both reflection mode and transmission mode sensors use light sources in the
`
`red andfor infrared part of the spectrum, wavelengths between 600nm and 1000nm
`
`being typical. However, red l infrared reflectance sensors do not function well
`
`because light at red and infrared wavelengths is poorly absorbed by blood. This
`
`results in low modulation of the rcflectecl signal and therefore a small AC component.
`
`Therefore red I infiared reflectance probes give poor results when compared to
`
`transmittance probes. It has been shown in Weija Cui er al: “In Vivo Reflectance of
`
`Blood and Tissue as a Function of Light Wavelength”,
`
`IEEE Transactions on
`
`Biomedical Engineering, Volume 37, No 6, June 1996), that a larger plethysmogram
`
`AC component amplitude can be recorded if a reflectance mode sensor uses light of
`
`wavelengths between 500nm and 600nm (green light).
`
`A continuous non-modulated green light photoplethysmograph was described in W0
`
`9822018A1. However, the objective of this invention was reflectance pulse oximetry,
`
`and the patent does not explain the steps necessary to produce a reliable
`
`photoplethysmograph suitable for measuring the plethysmogram AC and DC
`
`component. Such a green light sensor would be necessary to reliably detect the AC
`
`component, for example heart rate, but moreover the breathing signal, which is
`
`extremely small and was not detected by this system.
`
`In Benten er al: “Integrated synchronous receiver ohannelfiar optical z'nstrnmentarion
`
`applications” Proceedings of SPIE — The International Society for Optical
`
`4
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`Engineering, Volume
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`3100,
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`75-88,
`
`1997),
`
`a modulated
`
`light
`
`reflectance
`
`photoplethysmograph is described that uses a switching multiplier to systematically
`
`changelthe gain of the signal path between +1 and -1. This is the equivalent of
`
`mixing the modulated signal with a square wave. to recover the plethysmogram.
`
`However, similar to the other prior art described previously, this method needs the
`
`modulating carrier and dernodulating local oscillator signals to be in-phase.
`
`It is an object ofthe present invention to provide an improved plethysmograph.
`
`According to one aspect,
`
`the present
`
`invention provides a photoplethysrnograph
`
`device comprising:
`
`a light source for illuminating a target object;
`
`a modulator for driving the light source such that the output intensity varies as
`
`a function of a modulation signal at a modulation frequency;
`
`a detector for receiving light from the target object and generating an electrical
`
`output as a function of the intensity of received light;
`
`a demodulator for receiving the detector output, having a local oscillator and
`
`producing a demodulated output representative of the modulation signal and any
`
`sidebands thereof, in which the demodulator is insensitive to any phase difference
`
`between the modulation signal and the oscillator of the demodulator; and
`
`means for generating, from the demodulated output, a signal indicative of
`
`blood volume as a function of time and I or blood composition.
`
`According to another aspect, the present invention provides a method of generating a
`
`plethysmograrn, comprising the steps of:
`
`illuminating a target object with a light source;
`
`driving the light source with a modulator such that the output intensity varies
`
`as a function of a modulation signal at a modulation frequency;
`
`receiving light from the target object with a detector and generating an
`
`electrical output as a function of the intensity of received light;
`
`receiving the detector output in a demodulator having a local oscillator and
`
`producing a demodulated output representative of the modulation signal and any
`
`0006
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`sidehands thereof, in which the demodulator is insensitive to any phase difference
`
`between the modulation signal and the oscillator of the demodulator; and
`
`generating, from the demodulated output, a signal indicative of blood volume
`
`as a fiinction of time and I or blood composition.
`
`According to another aspect, the present invention provides a photoplethysmogtuph
`
`device comprising:
`
`one or more light sources each for illuminating a portion of a target object;
`
`one or more modulators for driving the light sources such that the output
`
`intensity of each light source varies as a function of a modulation signal at a
`
`modulation frequency;
`one or more detectors for receiving light from the target object and generating
`
`one or more electrical outputs as a function of the intensity of received light;
`
`a plurality of dernodulators each for receiving one or more of the electrical
`
`outputs and producing a demodulated output representative of the modulation signal
`
`of one of the modulated light sources and any sidebands thereof, to thereby produce a
`
`plurality of demodulated outputs corresponding to the plurality of light sources andfor
`
`plurality of detectors; and
`
`means for generating, from the demodulated outputs, plethysinograrn signals
`
`indicative of blood volume as a function of time and I or blood composition for each
`
`of the demodulator outputs.
`
`According to another aspect, the present invention provides a method of generating a
`
`plethysmogram, comprising the steps of:
`
`illuminating a portion of a target object with one or more light sources;
`
`driving the light sources with one or more modulators such that the output
`
`intensity of each light source varies as a function of a modulation signal at a
`
`modulation frequency;
`
`receiving light from the target object with one or more detectors and
`
`generating one or more electrical outputs as a filnction of the intensity of received
`
`light;
`
`receiving one or more of the electrical outputs with a plurality of
`
`dernodulators, each producing a demodulated output representative of the modulation
`
`6
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`signal of one of the modulated light sources and any sidebands thereof, to thereby
`
`produce a plurality of demodulated outputs corresponding to the plurality of light
`
`sources; and
`
`generating, from the demodulated outputs, plethysmogram signals indicative
`
`of blood volume as a function of time and / or blood composition for each of the
`
`demodulator outputs of the pixel array.
`
`According to another aspect, the present invention provides a photoplethysmograph
`
`device for non-contact use, comprising:
`
`a light source for illuminating a target object via a first polarising filter;
`
`a modulator for driving the light source such that the output intensity varies as
`
`a function of a modulation signal at a modulation frequency;
`
`a detector for receiving light from the target object via a second polarising
`
`filter having a different polarisation state than the first polarising filter, the detector
`
`adapted to generate an electrical output as a function of the intensity ofreceived light;
`
`a demodulator for receiving the detector output and producing a demodulated
`
`output representative of the modulation signal and any sidebands thereof; and
`
`means for generating, from the demodulated output, a signal iridicative of
`
`blood volume as a fimction of time and I or blood composition.
`
`According to another aspect, the present invention provides a method of generating a
`
`photoplethysmogram, comprising the steps of:
`
`illuminating a target object with a light source via a first polarising filter;
`
`driving the light source with a modulator such that the output intensity varies
`
`as a fimction of a modulation signal at arnodulation frequency;
`
`receiving light from the target object with a detector via a second polarising
`
`filter having a difierent polarisation state than the first polarising filter, the detector
`
`generating an electrical output as a function of the intensity of received light;
`
`receiving the detector output with a demodulator and producing a demodulated
`
`output representative of the modulation signal and any sidebands thereof; and
`
`generating, from the demodulated output, a signal indicative of blood volume
`
`as a function of time and I or blood composition.
`
`0008
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`According to another aspect, the present invention provides a photoplethysmograph
`
`device for non-contact use, comprising:
`
`a light source for illuminating a target object with optical
`
`radiation of
`
`wavelength less than 600 nm;
`
`a modulator for driving the light source such that the output intensity varies as
`
`a function of a modulation signal at a modulation frequency;
`
`a detector for receiving light from the target object and adapted to generate an
`
`electrical output as a function of the intensity of received light, the light source and
`
`detector being disposed laterally adjacent to one another on a substrate such that the
`
`active surfaces thereof can be directed towards substantially the same point on a
`
`surface of the target body;
`
`a demodulator for receiving the detector output and producing a demodulated
`
`output representative of the modulatzion signal and any sidebands thereof; and
`
`means for generating, from the demodulated output, a signal indicative of
`
`blood volume as a function of time and I or blood composition.
`
`According to another aspect, the present invention provides a method of generating a
`
`photopletliysmograrn, comprising the steps of:
`
`illuminating a target object with optical radiation of wavelength less than 600
`
`nm from a light source;
`
`driving the light source with a modulator such that the output intensity varies
`
`as a function of a modulation signal at a modulation firequency;
`
`receiving light from the target object with a detector to generate an electrical
`
`output as a function of the intensity of received light, the light source and detector
`
`being disposed laterally adjacent to one another on a substrate such that the active
`
`surfaces thereof can be directed towards substantially the same point on a surface of
`
`the target body;
`
`receiving the detector output with a demodulator and producing a demodulated
`
`output representative ofthe modulation signal and any sidebands thereof; and
`
`generating, from the demodulated output, a signal indicative of blood volume
`
`as a functzion of time and I’ or blood composition.
`
`0009
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`The invention provides a modulated light photoplethysmograph device.
`
`In selected
`
`embodiments, it combines the features of modulated light, band pass filtering, and IQ
`
`demodulation to give a plethysmogram of perfuse tissue. When used in reflectance
`
`mode, light in the blue andfor green portion of the optical spectrum is used which
`
`gives a larger pulsatile signal and improved signal to noise ratio.
`
`Selected embodiments of the invention provide improved reliability through the
`
`reduction of noise when the photoplethysmograph device is used in transmission
`
`mode.
`
`In addition, the choice of light in the blue 2' green portion of the optical
`
`spectrum (i.e. wavelengths of between 400 nm and 600nm) gives improved reliability
`
`through the reduction of noise and the increase in AC component signal amplitude,
`
`when the photoplethysrnograph device is used in reflection mode.
`
`Selected embodiments can be applied to different photoplethysroography techniques
`
`including
`
`single wavelength
`
`photoplethysmography, multiple wavelength
`
`photoplethysmography,
`
`pixel
`
`array
`
`photoplethysmography,
`
`and
`
`non-contact
`
`photoplethysmography.
`
`Embodiments of the present invention will now be described by way of example and
`
`with reference to the accompanying drawings in which:
`
`Figure
`
`1
`
`is
`
`a
`
`functional block diagram of
`
`a
`
`single wavelengtlri
`
`photoplethysrnograph device;
`
`Figure 2 is a functional block diagram of a demodulator suitable for use in the
`
`photoplethysmograph device of figure 1;
`
`Figure 3
`
`is
`
`a functional block diagram of a multiple wavelength
`
`photoplethysmograph device;
`
`Figure 4 is a schematic plan view of a pixel array photoplethysmograph
`
`device;
`
`Figure 5a is a schematic side view of a non-contact photoplethysmograph
`
`device with polarising filters;
`
`Figure 5b is a plan view of a polarising filter for use with the reflectance rnode
`
`photoplethysmo graph device of figure 7;
`
`0010
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`Figure
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`6
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`is
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`a
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`functional
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`block diagram of
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`a
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`single wavelength
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`pllotoplethysmograph device;
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`Figure Tr‘ is a schematic plan view, side view and end view of a reflectance
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`mode photopletbysrnograph device;
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`Figure 8 is a circuit diagram of a transirnpedance amplifier suitable for use in
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`the photoplethysmograph devices described herein;
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`Figure 9 is a circuit diagram of a band pass filter circuit suitable for use in the
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`photopletbysrnograph devices described herein;
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`Figure 10 is a circuit diagram of a light source driver circuit suitable for use in
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`the photoplethysmograph devices described herein;
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`Figure 11 is a process flow diagram illustrating a demodulation algorithm
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`suitable for use in the photopletliysrnograph devices described herein;
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`Figure 12 is a functional block diagram of a light source brightness control
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`loop suitable for use in the photoplethysrnograph devices described herein;
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`Figure 13a is a photoplethysrnogram showing a combined AC and DC output
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`of a photoplethysmograph device;
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`Figure 13b is a photoplethysmogram showing the magnified AC component
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`from figure 13a;
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`Figure 14a is a photoplethysmogram showing combined pulsatile and
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`breathing signal;
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`Figure 14b is a photoplethysrnogram showing the breathing signal of figure
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`14-a only;
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`Figure 15a is a photoplethysrnograrn showing a breathing signal only;
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`Figure 15b is a corresponding breathing signal as measured by an oral
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`thermistor;
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`Figure 16 is a photoplethysrnogram recorded using a green light source of
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`wavelength 510 um;
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`Figure 1? is a photoplethysmogram recorded using a red light source of
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`wavelength 644 nm;
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`Figure 18 is a fimctional block diagram of an alternative demodulator suitable
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`for use in the photoplethysrnograph device of figure 1;
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`Figure 19 is a functional block diagram of an alternative demodu1ator- suitable
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`for use in the photoplethysniograph device of figure 1;
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`Figure 20 is a functional block diagram of an alternative demodulator suitable
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`for use in the photoplethysrnograph device of figure 1.
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`Single wavefengrh photopfethysntograph device
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`With reference to figure 1, a photoplethysmograph device 100 comprises a driver
`circuit 101 which is coupled to energise a light source 102 with modulated drive
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`signal such that the output intensity of the light source varies as a function of a
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`modulation signal having a specific modulation frequency (fm) and modulation
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`amplitude (Ml(t)). The waveform driving the light source is therefore a modulating
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`carrier characterised by its frequency and amplitude.
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`The light source 102 is configured to illuminate a target object 103 such as an area of
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`tissue of the human or animal body. The light source 102 preferably comprises one or
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`more light emitting devices each of a given wavelength or range of wavelengths.
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`A photodetector 104 is configured to receive light from the target object 103 after its
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`interaction therewith. Depending on the relative positioning of the light source 102,
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`the target object 103 and the photodetector 104, thisreceived light may be one or
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`more of light that has been transmitted through the target object, light that has been
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`reflected from the surface of the target object, and light that has been scattered by and
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`K or reflected from structures or fluids within the target object. The photodetector will
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`generate an electrical current that is a function of, e.g. proportional to, the amount of
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`light incident to its active area.
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`A detector 105 may be provided to convert
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`the electrical current
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`fiom the
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`photodetector 104 to a voltage that is proportional to the current. The detector 105
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`may incorporate an amplifier (not shown). The gain of that amplifier can be rolled off
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`at a frequency greater than the modulation frequency. The detector 105 and amplifier
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`can, with careful design, minimise the noise at the input to a band pass filter 106
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`coupled thereto.
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`In a general sense, the photodetector 104 and detector 105 functions
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`may be provided by any detector capable of receiving light from the target object and
`generating an electrical output that is a_ function of the intensity ofthe received light.
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`The band pass filter 106 may be provided for attenuating signals outside a bandwidth
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`of interest. The filter bandwidth is preferably centred on the modulation frequency fm
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`and is sufficiently wide to pass the modulating carrier and sidebands caused by
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`plethysmograrn amplitude modulation, but narrow enough to attenuate frequency
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`components of interference and noise. To reduce noise, the handwidth of the band
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`pass filter 106 should be as narrow as possible.
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`It need only be wide enough to pass
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`the upper and lower sidehands of the plethysmogram, typically but not limited to 50
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`Hz. The band pass filter 106 may incorporate an amplifier (not shown) to provide
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`additional gain. The band pass filter 106 and amplifier are preferably designed to
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`minimise noise at the input of the following stage, namely a demodulator 107. It will
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`be appreciated that the provision of a band pass filter 106 is not always necessary but,
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`if employed, an increase in signal-to-noise ratio (SNR) may result.
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`A preferred arrangement of demodulator 107 is shown in more detail in figure 2. The
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`demodulator 107 is adapted to demodulate the output of the band pass filter 106 and
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`hence recover a plethysmogram from the detected light received from the target
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`object. The preferred demodulator 10? uses a method that is insensitive to the phase
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`difference between the modulation carrier and a demodulation carrier. In other words,
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`the demodulator is insensitive to any phase difference between the modulation signal
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`and an oscillator in the demodulator, as will he explained later.
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`Thus,
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`it
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`is
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`unnecessary to maintain a predetermined phase relationship between the modulation
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`and demodulation process.
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`The demodulator 10'?‘ may comprise a multiplexer 210 for splitting the modulated
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`signal M1(t) into two channels. A first channel processes a first modulated input
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`signal M1(t)a and a second channel processes a second modulated input signal
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`M1(t)h. The first modulated input signal Ml(t)a is provided as input to a first
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`multiplier 201 together with an output of a first demodulator local oscillator (LO)
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`signal 204, D1(t).
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`"The frequency of the local oscillator signal 204 is preferably
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`substantially equal to the frequency of the modulation signal and therefore equal to
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`the modulating carrier
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`frequency of input
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`signal M1(t).
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`The result of the
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`multiplication of Ml(t)a with the first LO signal 204 is anl (‘in phase’) signal. In the
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`second channel, the second modulated input signal is multiplied, using a multiplier
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`12
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`206, with a second demodulator local oscillator (LO) signal that also has a fiequency
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`preferably substantially equal to the fiequency of the modulation signal. However,
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`the second demodulator LO signal is phase shifted by phase shifter 205 with respect
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`to the first demodulator LO signal.
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`The phase difference between the first
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`demodulator L0 and second demodulator L0 is preferably 90 degrees. The result of
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`the multiplication of Ml(t)b with the second demodulator LO signal
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`is the Q
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`(‘quadrature phase‘) signal.
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`It will be understood that the local oscillator, although
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`shown as a producing a sine wave output, could produce other waveforms of the
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`required frequency.
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`The separate I and Q signals are preferably separately low pass filtered in filter
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`elements 202 and 207 respectively to remove unwanted harmonics and products of the
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`multiplication process. Optionally,
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`the resulting signals may be decimated in
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`decimators 203 and 208 respectively to reduce the sample rate. The results of this are
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`the I’ and Q’ signals.
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`The I’ and Q’ signals can be demultiplexed back into one signal at mixer 209 to
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`provide the demodulated plethysmogram Sl(t). The demultixplexing process can
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`include an algorithm or circuit that determines the square root of the sum of the
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`squares of the I’ and Q‘ signals.
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`The demodulator arrangement of figure 2 can be modified while still providing a
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`demodulator that is insensitive to any phase difference between the modulation signal
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`and the oscillator in the demodulator. Figures 18 to 20 show alternative arrangements
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`each providing two channels in which, in the first channel the detector output is mixed
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`with a local