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`US 20090105556Al
`
`(19) United States
`(12) Patent Application Publication
`Fricke et aJ.
`
`{10) P ub. No.: US 2009/0105556 Al
`Apr. 23, 2009
`(43) Pub. Date:
`
`(54) MEASUREMENT OF PHYSIOLOGlCAL
`SIGNALS
`
`(75)
`
`Inventors:
`
`.John Rober t F ricke, Lexington.
`MA (US): Matthew C or b in
`W iggins, Concord, MA (US)
`
`Correspondence Address:
`OCCHlUTl ROHLICEK & TSAO, LLP
`10 FAWCETT STREET
`CAMBRIDGE, MA 02138 (US)
`
`(73) Assignee:
`
`T iax LLC. Cambridge, MA (US)
`
`(21) Appl. No.:
`
`12/240,651
`
`(22) Filed:
`
`Sep.29,2008
`
`Related U.S. Application Data
`
`(60) Provisional application No. 60/995,723, filed on Sep.
`28, 2007.
`
`Publication C lassification
`
`(51)
`
`Int. CJ.
`A61B 5/00
`(2006.01)
`A6IB 5/1455
`(2006.01)
`(52) U.S. C l . ........... ... ........................... 600/301; 600/310
`(57)
`ABSTRACT
`
`A system inc.ludes an optical sensor and a signal processing
`module. The optical sensor is configured to be positioned on
`an area of skin of a patient. The optical sensor includes a light
`source for illuminating a capillary bed in the area of skin and
`a photodetcctor. n1e photodetector is configured to receive an
`opticaJ signal from the capillary bed resulting from the illu(cid:173)
`mination and to convert the optical signal into an electrical
`signal, the optical signal characterizing a fluctuation in a level
`of blood in the capillary bed. The signal processing module is
`configured to process the e lectric signal using a nonstationary
`frequency estimation method to obtain a processed signal
`related to at least one of a heart rate and a respiration rate of
`the patient. Another aspect relates to obtaining a quantity
`related to the blood pressure of the patient in addition to or
`instead of o btaining a processed signal related to at least one
`of the heart rate and the respiration rate of the patient.
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`U.S. Patent No. 8,942,776
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`Patent Application Publication Apr. 23, 2009 Sheet 10 of 20
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`US 2009/0105556 A1
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`

`Patent Application Publication Apr. 23, 2009 Sheet 12 of 20
`
`US 2009/0105556 A1
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`

`

`US 2009/0105556 Al
`
`Apr. 23, 2009
`
`1
`
`MEASUREMENT OF PHYSIOLOGICAL
`SIGNALS
`
`CROSS-REFERENCE TO RELATED
`APPL!CATlONS
`
`10001] Tl1ls application claims priority to U.S. provisional
`application No. 60/995,723, filed Sep. 28, 2007, entitled
`"Method and Devices for Measurement of Multi-modal
`Physiological Signals," wluch is incorporated herein by ref(cid:173)
`erence.
`
`STATEMENT REGARDING FEDER.t\LLY
`SPONSORED RESEARCH
`
`10002] The subject matter described in trus application was
`partially fu nded by the Government of the United States
`under Contract No. W91 ZLK-04-P-0239 awarded by the
`U.S. Department of the Army. The government has certaln
`rights in the invention.
`
`FIELD OF THE INVENTION
`I 0003] The invention relates to measurement of physiologi(cid:173)
`cal signals.
`
`BACKGROUND
`
`1 0004) Physiologjcal signals are important for morutoring a
`subject's physical and cognitive state. Often, heart rate
`parameters are measured directly via electrocardiogram
`(ECG) measurements of a heart beat. Respiration rate data
`can be obtained from a respiration chest strap. Physiologica l
`signals can also be extrac ted from infrared (IR) photoplethys(cid:173)
`mographs (PPG). The signals of interest include heart rate,
`respiration rate, continuous blood pressure, and intrathoracic
`pressure. With respect to blood pressure, there is technology
`related to collecting data at two locations on the body and
`using pulse transit time and other parameters as the basis of
`the pressure estimate.
`
`SUMMARY
`
`10005]
`In a general aspect. a system includes an optical
`sensor and a signal processing module. The optical sensor is
`configured to be positioned on an area of skin of a patient. The
`optical sensor includes a light source for ilJumiuating a cap(cid:173)
`illary bed in the area of skin and a photodetector. The photo(cid:173)
`detector is configured to receive an optical signal from the
`capillary bed resulting from the illumination and to convert
`the optical signal into an electrical signal, the optical signal
`characterizing a fluctuation in a level of blood in the capillary
`bed. The signal processing module is configured to process
`the electric signalusinga nonstationary frequency estimation
`method to obtain a processed signal related to at least one of
`a heart rate and a respiration rate of the patient.
`10006] Embodiments may include one or more of the fol (cid:173)
`lowing. The system includes an output for providing infor(cid:173)
`mation detennined from the processed signal. l b e nonsta(cid:173)
`tionary frequency estimation method includes a Hilbert
`transform method or an instantaneous frequency estimation
`method. The processed signal includes at least one of instan(cid:173)
`taneous heart rate. inter-beat interval, heart rate variability,
`wgh-low heart rate ratios, respiration rate, inter-breath inter(cid:173)
`val, and respiration rate variability.1ne fluctuation in tlte level
`of blood in tlte capillaty bed relates to a change in a t least one
`
`of volume and pressure of the tl1oracic cavity or to a change in
`at least one of volume and pressure of an organ in the thoracic
`cavity.
`[0007] The system includes an auxiliary sensor configured
`to detect an ambient signal. The auxiliary sensor includes at
`least one o f accelerometer, a pressure sensor. an optical detec(cid:173)
`tor, a temperan1re sensor, and a piezoelectric device. The
`signal processing module is configured to remove an effect of
`tbe ambient signal from tl1e e lectrical signal. 11Je optical
`signal is a reflectance or a transmittance oft he capillary bed.
`[0008]
`ln another general aspect. a method includes illtuni(cid:173)
`nating a capillary bed in an area of skli1 of a patlent, receiving
`an optical signal from the capillary bed resulting from the
`illumlnation, converting the optical signal into an electrical
`signal, and processing the electrical signal using a nonstation(cid:173)
`ary frequency estimation method to obtain a processed signal
`related to at least one of a heart rate and a respiration rate of
`the patient.11te optical signal c haracterizes a nuctltation in a
`level of blood in the capillary bed.
`(0009] Embodiments may include one or more of the fol(cid:173)
`lowing. The method includes outputting information deter(cid:173)
`mined from the processed signaL Processing the electrical
`signal using the nonstationary frequency estimation method
`includes perfonning a Hilbert transfonu or proc(.'Ssing the
`electrical signal using an instantaneous frequency estimation
`method. Processing the electrical signal using the instanta(cid:173)
`neous frequency method includes band pass filtering the elec(cid:173)
`trical signal, determining an instantaneous frequency of the
`electrical signal, and using the instantaneous freq11ency to
`obtain the processed signaL
`[0010] 1l1e method further includes processing the electri(cid:173)
`cal s ignal using a model to obtain a blood pressure signal
`related to a blood pressure of the patient. The optlcal signal
`characterizes a capillary refill time in the capillary bed. Pro(cid:173)
`cessing the electrical signal includes processing the electrical
`signal in real time.
`[0011]
`ln another aspect, a method for moru toring blood
`pressure includes illuminating a capillary bed in an area of
`skin of a patient, receiving au optical signal from the capillary
`bed resulting from the illumination, converting the optical
`signal into an electrical signal. and processing the electrical
`signal using a model characterizing a relationslup of the fluc(cid:173)
`tuation in the level of blood and the blood pressure of the
`patient to obtain a quantity related to the blood pressure of the
`patient. ·n1e optical signal characterizes a fluctuation in a
`level of blood in the capillary bed o f the patient
`[0012] Embodiments may include one or more of the Jot(cid:173)
`lowing. The method includes outputting informatlon deter(cid:173)
`milled based on the quantity related to the blood pressure of
`the patient. The optical signal characterizes a capillary refill
`time. The method further includes engaging a device to
`restrict circulation in the capillary bed of the patient and
`disengaging the device prior to receiving the optical signal
`from the capillary bed. 111e disengaging of the device occurs
`gradually. The device is an active clan1ping device.
`10013] H1e quantity related to the blood pressure of the
`patient is a quantity related to the continuous blood pressure
`of the patient. Applying the model includes applying a model
`including circuit elements or properties of the capillary bed.
`The method further includes calibrating the model on the
`basis of a blood pressure of the patient determined by using a
`blood pressure cuff.
`
`022
`
`

`

`US 2009/0105556 Al
`
`Apr. 23, 2009
`
`2
`
`10014] Embodiments may include one or more of the fol(cid:173)
`lowing advantages.
`I 00151 A system or method as described above can be used
`for both military and civilian applications. Combat casualty
`care requires close monitoring of vital signs from the moment
`that a medic first attends to a wounded soldier in the battle(cid:173)
`field and thence through the many transfer stages to the point
`of f11ll hospital care. generally removed from the combat
`scene. Simi lar needs are evident in the civilian community
`where critical care is administered by first responders at the
`scene of accidents, by emergency room sta·tT, and by imensive
`care unit stafT. lt is often desirable to obtain maximum infor(cid:173)
`mation using as little equipment as possible. The system and
`method described herein support tbis need. They reduce the
`burden of equipment logistics. the burden of extra wires and
`sensors on and around d1e patient, and the complexity and
`cost of using multiple devices.
`10016] For both military and civilian applications, a dispos(cid:173)
`able, wearable device in keeping with the system and metl1od
`described herein can be adapted to stay with a patient a nd to
`report vital signs throughout the care and transport processes.
`Further, the system ca11 be configured to provide medical
`personnel with real-time visibility of vital signs as well as
`recording of this information for playback by attending medi(cid:173)
`cal staff at a later time. 111e disposability of the device allows
`it to be fabricated with low cost parts and eliminates the need
`for sanitization and asset tracking logistics in large scale
`clinical or military uses.
`10017] Such system and methods additionally support
`applications in fitness monitoring. where their ease of use and
`robustness make them a compelling alternative to chest strap
`monitors for the monitoring of cardiac and respiratory param(cid:173)
`eters during exercise. An ear-wom device can also integrate a
`speaker llllit for mobile electronic devices such as mobile
`phones or music players.
`10018] An advantage of applying a nonstationary fre(cid:173)
`quency estimation method (e.g., analysis involving monitor(cid:173)
`ing the frequency changes of tbe signal over time, such as
`monitoring changes in the instantaneous principal frequency
`over time) is that it is possible to avoid a tradeoff inherent in
`many stationary estimation methods between frequency reso(cid:173)
`lution and duration of data signals being analyzed. For
`example, if the signal is ass umed to be stationary withhl each
`of a series of data windows, the frequency resolution is gen(cid:173)
`erally inversely proportional to the duration of the window.
`As the window duration increases, tl1e assumption of a sta(cid:173)
`tionary signal is increasingly violated and/or nonstationary
`events (e.g .. transients) are more difficult to detect. At least
`some nonstationary frequency analysis methods, which may
`be based, without limitation, on a Hilbert transform approach,
`tracking of a oonstationary model, nonstatiooary principal
`frequency analysis. or other time-frequency methods. miti(cid:173)
`gate the effects of such a time-frequency tradeoff. Further(cid:173)
`more, useofsucb nonstationary techniques, as opposed to use
`of time domain peak picking and/or threshold based tech(cid:173)
`niques, can provide robustness of algorithm against artifacts,
`and provide sensitivity to periodicity without being burdened
`by a window tl1at can reduce U1e time resolution.
`I 00191 Other feat11res and adva ntages are apparenr from the
`following description and from the appended claims.
`
`BRI EF DESCRIPTION OF DRAWINGS
`
`10020) FIG. l is a schematic diagram of a photopletl1ysmo(cid:173)
`graph (PPG) sensor system.
`
`[0021] FIG. 2 is a graph of a PPG detector signal taken over
`a 25 second period by ~Ul earlobe PPG device.
`10022] FIG. 3 is a flow diagram of signal processing of a
`detector sig nal from a PPG device to obtain heart rate and
`respiration rate parameters.
`[0023) FIG. 4 is a graph of a result ofba11d-pass filtering the
`data shown in FIG. 2 between 0.5 Hz and 5.5 l-Iz to extract a
`cardiac signal.
`[0024) FIG. 5 is a graph of a result of band-pass filtering the
`data shown in FIG. 2 between 0.17 Hz and 0.5 1-lz to extract a
`respiration signal.
`10025] FIG. 6 is a graph of an inter-beat interval obtained by
`applying an instantaneous frequency method to the cardiac
`signal shown in FIG. 4.
`[0026] FIG. 7 is a graph of a spectral analysis of the inter(cid:173)
`beat interval data shown in FIG. 6.
`10027] FIG. 8 is a graph of the respiration rate obtained by
`applying a n instantaneous frequency method to the respira(cid:173)
`tion signal shown in FIG. 5.
`[0028] FIG. 9 is a diagram ofPPG measurements related to
`physiological states used to determine intrathoracic pressure.
`[0029] FIG. 10 is a graph of the output of a matched filter(cid:173)
`ingprocess using the PPGdetectorsignal shown in FIG. 2 and
`a pulse pilot signal.
`[0030] FIG. 11 is a block diagram of a least mean squares
`(LMS) adaptive filter.
`[0031] FIG. 12 is a schematic diagram of an active clamp(cid:173)
`ing mechanism used to stimulate capillary refill.
`[0032] FIG. 13 is a diagram of a system model relating a
`PPG signal to blood pressure.
`[0033] FIG. 14 is a graph of trends in various physiological
`parameters before and during a stress event.
`10034] FIG. 15 is a block diagram of a portable electronics
`unit.
`[0035] FlG. 16 is a flow diagram of methods to estimate a
`heart rate and a respiration rate.
`[0036] FlG. 17 is a flow diagram of a processing delay in
`the estimation of a heart rate.
`[0037) FJG. 18 is a flow diagram of a processing delay in a
`first method for the estimation of a respiration rate.
`[0038) FlG. 19 is a flow diagram of a processing delay in a
`second method for the estimation of a respiration ra te.
`10039] FIG. 20 is a flow diagram of a processing delay in a
`third method for the estimation of a respiration rate.
`
`DE1AILED DESCRIPTION
`
`10040] Referring to FIG. 1, examples of an infrared pho(cid:173)
`toplethysmograph (PPG) device 100 are used to obtain physi(cid:173)
`ological signals related to o ne or more of heart rate, respira(cid:173)
`tion rate, blood pressure, and intrad1oracic pressure. Such
`signals may be releva11t for monitoring a person's s tate,
`including one or more of the person's physical state, long(cid:173)
`term health, psychological state. and/or cognitive state. More
`generally, the physiological signals may provide information
`about the activity of the person's sympathetic and parasym(cid:173)
`pathetic nervous system. The PPG device 100 illustrated in
`FIG. 1 is attached to an earlobe 102 of a person, for example.
`using a clamping or adhesive approach. However. in other
`embodiments, PPG device l 00 is used on other areas of the
`skin of a person, including but not limited to a portion of a
`forehead, a neck, an arm, a forearm, a finger, a leg, a back. an
`abdomen, or a stomach. In general. a requirement for the
`positioning of PPG device 100 is that the PPG sensor be
`located such tl1at it can obtain a measurement via tl1e skin that
`
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`US 2009/0 I 05556 A 1
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`Apr. 23, 2009
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`is related to blood flow or pressure. for example to measure a
`level of blood in a capillary bed 104. for example. a blood
`volume. a rate of blood flow, or a rate of change of blood
`volume. Note also that the approach is not limited to use of a
`single PPG device on an individual. ln some embodimems.
`multiple PPG devices are used, for exan1ple, on the torso
`ancVor at different extremities. and signals obtained at the
`different PPG devices are processed independently or in com(cid:173)
`bination to determine underlying characteristics of the indi(cid:173)
`vidual's state.
`10041]
`In some embodiments, such as thatshowninFlG.l,
`an infrared light source 106 illuminates the earlobe 102. The
`blood leve l in capillary bed 104 aflects the amount of light
`108 that is baekscaucred or reflected by earlobe 102. Light
`108 backscaucrcd by earlobe 102 is received by an optical
`transducer such as a photodetector 110 and converted into a
`detector signal J 12. Since the blood flow iu capillary bed 104
`is controlled by the hean beat oft he person and tlms the blood
`level in the capillary b(.'(! varies with time, the backscauered
`light 108 and hence the detector signal 112 are also tinle(cid:173)
`varying. In another embodiment. the PPG sensor operates in
`transmission mode and the light transmincd through the cap(cid:173)
`illary bed is received by tbe photodetector.
`100421 The detector signal 112 is sentto a signal processing
`unit 114 '' bich processes the detector signaL which contains
`information about the person's pulse, to extract desired physi(cid:173)
`ological data. in various embodiments including one or more
`ofin~tantanoou~ hean mte, inter-beat interval, heart rate vari(cid:173)
`ability, high-low heart rate ratio. respiration rate. inter-breath
`interval, respiration rate variability, blood pressure. and
`intrathoracic pressure. A single PPG device 100, referred to
`below as an Integrated Multi-Modal Physiological Sensor
`(IMMPS). is capable of produci ng multiple (or all) of such
`ty pes of physiological data.
`10043]
`In some embodiments, the PPGdevice 100 provides
`real-time visibility of rhysiological parameters and vital
`signs, which can be transmitted to other equipment for real(cid:173)
`time processing or for playback or off-line processing at a
`later time. In some embodiments, the PPG device includes
`user output devices, such as a set of light emitting diodes
`(Lims) (e.g., a red LED 11.6, a yellow LED 118. and a green
`LED 120) or an audio device for producing alert sounds.
`which provide on-device stants on PPG device 100. As an
`example for usc of such output devices, wheu a selected
`pliysiological parameter is in a normal range. green LED 120
`is turned on; when the physiological parameter is in a slightly
`abnormal range. yellow LED 118 is turned on: when the
`physiological parameter is in a dangerous range. red LED 116
`is turned on. In some embodiments, the audio output device is
`used to provide other audio output. such as the output for an
`electronic device such as a mobile phone or a music player. ln
`some embodiments, a wireless link 122 to an external moni(cid:173)
`toring system 124, such as a bedside system or a wearable
`system, provides sensor data to the external system enabling
`a numeric readout 126 of various physiological parameters.
`In some embodiments. the PPG device, or at least some
`wearable ponion of the