A
`
`a2) United States Patent
`Dekker
`
`US006702752B2
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 6,702,752 B2
`Mar. 9, 2004
`
`(54) MONITORING RESPIRATION BASED ON
`PLETHYSMOGRAPHIC HEART RATE
`SIGNAL
`
`(75)
`
`Inventor: Andreas Lubbertus Aloysius Johannes
`Dekker, Maastricht (NL)
`(73) Assignee: Datex-Ohmeda,Ine., Madison, WI
`(US)
`
`(*) Notice:
`
`Subjecttoany disclaimer, theterm of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 10/081,719
`(22)
`Filed:
`Feb, 22, 2002
`(65)
`Prior Publication Data
`US 2003/0163054 Al Aug. 28, 2003
`Int. C1?eee
`(SI)
`(52) US.CD) aes
`
`- AGIB 5/02
`_. 600/484; 600/483; 600/500;
`600/479
`
`... 600/484, 483,
`(58) Field of Search .........0.......
`
`
`600/481, 500, 5 01, 502, 504, 507, 508,
`476, 479, 480, 473, 475, 529
`
`(50)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`oe . nee a? etal.
`Soreaso A
`4/1983 Tal
`4.404.974 A
`9/1983 Titus
`45 10.944 A
`4/1985 Porges
`4,765,340 A
`8/1988 Sakai et al.
`4,777,960 A
`10/1988 Berger etal.
`4,781,201 A
`11/1988 Wrightetal.
`4,813,427 A
`3/1989 Schlaefke et al.
`4,858,038 A
`8/1989) Ceti vee IST/LIS
`BRONTDD th
`S108) Kaho Stal.
`4,863,265 A
`9/1989 Floweret al.
`4,869,254 A
`9/1989 Stoneet al.
`4,884,578 A
`12/1989 Morgenstern
`4899760 A
`2/1990 Jaeb etal.
`
`6/1990 Cohen et al.
`4,930,517 A
`9/1990 Sharpeetal.
`4,958,638 A
`10/1990 DePaola et al.
`4,960,129 A
`11/1990 Korten etal.
`4,972,842 A
`7/1991 Sato etal.
`3,033,472 A
`1/1992 Stone et al.
`5,078,136 A
`5/1992 Clark et al.
`5,111,817 A
`:
`=
`:
`ak Gee eae
`5,368,224 A
`11/1994 Richardson et al.
`5,385,144 A
`1/1995 Yamanishi et al.
`5,396,893 A
`3/1995 Oberg et al.
`5,423,322 A
`6/1995 Clark et al.
`5,431,159 A
`7/1995 Baker et al.
`5,482,036 A
`1/1996 Diab et al.
`5,490,505 A
`2/1996 Diab et al.
`5,511,553 A
`4/1996 Segalowitz
`5,553,615 A
`9/1996 Carim et al.
`(List continued on next page.)
`OTHER PUBLICATIONS
`Spectral Analysis: Review “Heart Rate Variability”, Lukas
`Spicker, hemodynamicsuedavisedu, Unknown Publication
`Pate,
`
`Primary Examiner—Max ¥. Hindenburg
`Assistant Examiner—Navin Natnithithadha
`(74) Attorney, Agent, or Firm—Marsh Fischmann &
`Breyfogle LLP
`_
`(57)
`
`ABSTRACT
`
`Apleth signal is analyzed to identify a heart rate variability
`parameter associated with respiration rate.
`In one
`embodiment, an associated process involves obtaining a
`photoplethysmograpie signal, processing the pleth signal to
`obtain heart rate samples, monitoring the heart rate sample
`to identify a heart rate variability associated with respiration,
`and determining a respiration rate based on the heart rate
`variability. The photoplethysmographic signal may be based
`on one or more channel signals of a conventional pulse
`oximeter. The invention thus allows for noninvasive moni-
`toring of respiration rate and expands the functionality of
`filse cieimeters
`P
`>.
`
`38 Claims, 8 Drawing Sheets
`
`
`
`0001
`
`Apple Inc.
`APL1012
`8,929,965
`
`U.S. Patent No.
`
`FITBIT,
`
`Ex. 1012
`
`Apple Inc.
`APL1012
`U.S. Patent No. 8,929,965
`
`0001
`
`FITBIT, Ex. 1012
`
`

`

`US 6,702,752 B2
`
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`5,555,882 A
`5,575,284 A
`5,623,933 A
`5,755,229 A
`5,766,127 A
`5,776,071 A
`5,830,137 A
`5,842,979 A
`5,853,364 A
`5,862,805 A
`5,865,167 A
`5,865,756 A
`5,885,213 A
`5,902,235 A
`5,919,134 A
`5,931,779 A
`
`9/1996 Richardsonet al.
`11/1996 Athanet al.
`4/1997 Amanoet al.
`5/1998 Amanoetal.
`6/1998 Pologeetal.
`7/1998 Inukai et al.
`11/1998 Scharf
`12/1998 Jarman ......ccceeeees GOO/322
`12/1998 Baker, Jr. et al.
`1/1999 Nitzan
`2/1999 Godik
`2/1999 Peel, III
`
`3/1999 Richardson et al.
`+. 600/323
`5/1999 Lewis etal. ...
`
`7/1999 Diab...
`vee 600/323
`8/1999 Arakaki et al. vs... 600/310
`
`5,934,277 A
`5,954,644 A
`5,971,930 A
`5,980,463 A
`5,993,893 A
`5,997,482 A
`6,011,985 A
`6,027,455 A
`6,028,311 A
`6,064,910 A
`6,067,462 A
`6,081,742 A *
`6,099481 A
`6,129,675 A
`6,155,992 A
`6,480,733 Bl
`
`* cited by examiner
`
`8/1999 Mortz
`
`9/1999 Dettling et al. se... 600/322
`
`seve 600/483
`10/1999 Elghazzawi......
`11/1999 Brockwayet al.
`11/1999 Kikuchi vaca 427/8
`12/1999 Vaschillo et al
`. 600/484
`1/2000 Athan etal. .....
`2/2000 Inukai et al.
`....
`2/2000 Sodickson et al.
`5/2000 Andersson et al.
`5/2000 Diab et al.
`....
`6/2000 Amanoetal
`8/2000 Daniels etal.
`:
`10/2000 Tay .oeesseeeeetseeeeseeeees
`
`-
`12/2000 Henning et al.
`11/2002 Turcott wesc 600/516
`
`
`
`
`..
`
`0002
`
`FITBIT, Ex. 1012
`
`0002
`
`FITBIT, Ex. 1012
`
`

`

`U.S. Patent
`
`Mar.9, 2004
`
`Sheet 1 of 8
`
`US 6,702,752 B2
`
`
`
`
`SOURCE
`DRIVES
`108
`
`0003
`
`FITBIT, Ex. 1012
`
`0003
`
`FITBIT, Ex. 1012
`
`

`

`U.S. Patent
`
`Mar. 9, 2004
`
`Sheet 2 of 8
`
`US 6,702,752 B2
`
`DETECTOR OUTPUTV. TIME
`
`/+——tp——
`
`DETECTOROUTPUT
`
`FIG.2
`
`0004
`
`FITBIT, Ex. 1012
`
`0004
`
`FITBIT, Ex. 1012
`
`

`

`U.S. Patent
`
`Mar. 9, 2004
`
`Sheet 3 of 8
`
`US 6,702,752 B2
`
`LOW FREQUENCY
`PLETH VARIABILITY POWER SPECTRUM
`
`P[a.u.]
`
`FIG.3
`
`0005
`
`FITBIT, Ex. 1012
`
`0005
`
`FITBIT, Ex. 1012
`
`

`

`U.S. Patent
`
`Mar.9, 2004
`
`Sheet 4 of 8
`
`US 6,702,752 B2
`
`HEART RATEVARIABILITY
`
`1B
`EXPIRATION
`
`
`HEART
`
`RATEFILTEROUTPUT(HEART
`
`
`
`INSPIRATION
`
`RATE)
`
`FIG.4
`
`0006
`
`FITBIT, Ex. 1012
`
`0006
`
`FITBIT, Ex. 1012
`
`

`

`U.S. Patent
`
`Mar. 9, 2004
`
`Sheet 5 of 8
`
`US 6,702,752 B2
`
`PLETH POWER SPECTRUM
`
`0007
`
`FITBIT, Ex. 1012
`
`0007
`
`FITBIT, Ex. 1012
`
`

`

`U.S. Patent
`
`Mar. 9, 2004
`
`Sheet 6 of 8
`
`US 6,702,752 B2
`
`RESPIRATORY POWER SPECTRUM
`
`0008
`
`FITBIT, Ex. 1012
`
`0008
`
`FITBIT, Ex. 1012
`
`

`

`U.S. Patent
`
`Mar. 9, 2004
`
`Sheet 7 of 8
`
`US 6,702,752 B2
`
`START
`
`
`
`
`reas
`
`
`
`OBTAIN DETECTOR OUTPUT
`(PLETH SIGNAL)
`
`
`FILTER PLETH TO OBTAIN
`HEART RATE
`
`104
`
`
`
`MONITOR HEART RATE OVER TIME
`TO OBTAIN HEART RATE SIGNAL
`
`706
`
`
`
`FILTER HEART RATE SIGNAL TO
`
`OBTAIN FUNDAMENTAL FREQUENCY
`708
` OUTPUT FUNDAMENTAL FREQUENCY
`
`OF HEART RATE SIGNAL AS
`RESPIRATION RATE
`
`FIG.7
`
`0009
`
`FITBIT, Ex. 1012
`
`0009
`
`FITBIT, Ex. 1012
`
`

`

`U.S. Patent
`
`Mar.9, 2004
`
`Sheet 8 of 8
`
`US 6,702,752 B2
`
`
`
`g0gSLYNOILVuldS3yJIVYLNV3HJO
`
`SaiNaSSIL
`
`008
`
`aeOleaNa9|cogYOLVINGOW3CNOS
`
`YOL93S130dN¥TANNVHOWLSIG
`
`=£089TANNVHO208
`
`WHOSSAVMHLS1dGAZINILdO
`
`108
`
`0010
`
`
`
`NEIMIEyaL4
`
`
`
`AVIASIGFLVYNOLLVuldSauSVLYV3H
`
`Z18e198LLe908
`
`FITBIT, Ex. 1012
`
`0010
`
`FITBIT, Ex. 1012
`
`
`
`

`

`US 6,702,752 B2
`
`1
`MONITORING RESPIRATION BASED ON
`PLETHYSMOGRAPHIC HEARTRATE
`SIGNAL
`
`FIELD OF THE INVENTION
`
`The present invention relates, in general, to the noninva-
`sive monitoring of respiration rate based on optical (visible
`and/or non-visible spectrum) signals and, in particular, to
`monitoring respiration based on the processing of received
`optical signals to identify heart rate variability associated
`with respiration. The invention can be readily implemented
`in connection with pulse oximetry instruments so as to
`expand the utility of such instruments.
`BACKGROUND OF THE INVENTION
`
`Photoplethysmographyrelates to the use of optical signals
`transmitted through or reflected by a patient’s blood, e.g.,
`arterial blood or perfused tissue, for monitoring a physi-
`ological parameter of a patient. Such monitoring is possible
`because the optical signal is modulated by interaction with
`the patient’s blood. That is, interaction with the patient's
`blood generally involving a wavelength and/or time depen-
`dent attenuation due to absorption,
`reflection and/or
`diffusion, imparts characteristics to the transmitted signal
`that can be analyzed to yield information regarding the
`physiological parameter of interest. Such monitoring of
`patients is highly desirable becauseit is noninvasive, typi-
`cally yields substantially instantaneous and accurate results,
`and utilizes minimal medical resources, thereby proving to
`be cost effective.
`
`30
`
`2
`generally corresponds to the attenuation related to the non-
`pulsatile volumeofthe perfused tissue and other matter that
`affects the transmitted plethysmographic signal. The second
`component, sometimes termed the “AC component,” gen-
`erally corresponds to the change in attenuation due to the
`pulsation of the blood.
`In general,
`the AC component
`represents a varying waveform which corresponds in fre-
`quencyto that of the heartbeat. In contrast, the DC compo-
`nent is a more steady baseline component, sincethe effective
`volume of the tissue under investigation varies little or at a
`low frequency if the variations caused by the pulsation of the
`heart are excluded from consideration.
`
`Pulse oximeters typically provide as outputs blood oxy-
`gen saturation values and, sometimes, a heart rate and a
`graphical representation of a pulsatile waveform. The infor-
`mation for generating each of these outputs is generally
`obtained from the AC componentof the pleth. In this regard,
`some pulse oximeters allempt to filter the DC component
`from the pleth, e.g., in order to provide a better digitized AC
`component waveform. Other pulse oximeters may measure
`and use the DC component, e.g.,
`to normalize measured
`differential values obtained from the AC component or to
`provide measurements relevant
`to motion or other noise
`corrections. Generally, though, conventional pulse oxime-
`ters do not monitor variations in the DC component of a
`pleth or pleths to obtain physiological parameter information
`in addition to the outputs noted above. Although it has been
`proposed to use pulse oximeters to monitor other parameters
`including respiration rate, il is apparent that such proposed
`uses have not gained general commercial acceptance.
`SUMMARY OF THE INVENTION
`
`The present invention is directed to monitoring patient
`respiration based on a pleth signal. The invention thus
`provides important diagnostic or monitoring information
`noninvasively. Moreover, various aspects of the invention
`can be implemented using one or more channels and/or other
`components of a conventional pulse oximeter, thereby pro-
`viding additional functionality to instruments that are widely
`available and trusted, as well as providing access to impor-
`tant information for treatment of patients on a cost-effective
`basis.
`
`In accordance with one aspect of the present invention, a
`pleth signal is analyzed to identify a heart rate variability
`parameter associated with respiration rate. The associated
`process involves obtaining a pleth signal, processing the
`pleth signal to obtain heart rate samples, monitoring the
`heart rate samples to identify a heart rate variability, and
`determining a respiration rate based on the heart rate vari-
`ability. It is known that heart rate varies with the respiration
`cycle, an effect called Respiratory Sinus Arrhythmia. The
`present invention provides a robust process for monitoring
`this effect and determining respiration rate based on pleth
`signals. A novel processor and pulse oximeter incorporating
`such processing are also provided in accordance with the
`present invention.
`The step of obtaining a pleth signal generally involves
`receiving a digital signal representative of an optical signal
`modulated based on interaction with perfused tissue of a
`patient. Such a signal may be provided using components of
`a conventional pulse oximeter. Pulse oximeters typically
`transmit red and infrared signals, thereby yielding red and
`infrared pleths. Either or both of these pleths may be utilized
`in accordance with the present invention. In particular, each
`of these pleths generally has a fundamental frequency cor-
`responding to the patient’s heart rate. Accordingly, cither
`
`FITBIT, Ex. 1012
`
`Acommon type of photoplethysmographie instrument is
`the pulse oximeter. Pulse oximeters determine an oxygen
`saturation level of a patient’s blood, or related analyte
`values, based on transmission/absorption characteristics of
`light
`transmitted through or reflected from the patient's
`tissue. In particular, pulse oximeters generally include a
`probe for attaching toa patient’s appendage suchas a finger,
`earlobe or nasal septum. The probe is used to transmit pulsed
`optical signals ofat least two wavelengths, typically red and
`infrared, through the patient’s appendage. The transmitted
`signals are received by a detector that provides an analog
`electrical output signal representative of the received optical
`signals. By processing the electrical signal and analyzing
`signal values for each of the wavelengths at different por-
`tions of a patient’s pulse cycle, information can be obtained
`regarding blood oxygen saturation.
`The algorithms for determining blood oxygen saturation
`related values are normally implemented in a digital pro- s
`cessing unit. Accordingly, one or more analog to digital
`(A/D) converters are generally interposed between the
`detector and the digital processing unit. Depending on the
`specific system architecture employed, a single multi-
`channel digital signal may be received by the digital pro-
`cessing unit or separate digital signals for each channel may
`be received. In the former case, the digital processing, unit
`may be used to separate the received signal into separate
`channel components. Thus, in either case,
`the digital pro-
`cessing unit processes digital information representing each
`of the channels.
`
`40
`
`unPry
`
`60
`
`Such digital information defines input photoplethysmo-
`graphic signals or “pleths.” These pleths generally contain
`two components. The first component of interest is a low
`frequencyor substantially invariant component in relation to
`the time increments considered for blood oxygen saturation
`calculations, sometimes termed the “DC component,” which
`
`65
`
`0011
`
`0011
`
`FITBIT, Ex. 1012
`
`

`

`US 6,702,752 B2
`
`_ry
`
`30
`
`40
`
`3
`pleth can be used to yield the desired heart rate information.
`In general, for normally oxygenated patients, the infrared
`channel typically has the stronger pleth waveform and may
`be preferred for heart rate calculations. For poorly oxygen-
`ated patients, the red pleth may be preferred. In many cases,
`a combination of the two signals may provide a better
`waveform for heart rate analysis than either signal alone.
`The pleth may be processed to obtain heart rate samples
`in a variety of ways. As noted above, the pleth is generally
`a periodic signal having a fundamental frequency corre-
`sponding to the patient’s heart rate. Accordingly, heart rate
`may be determined by performing peak-to-peak measure-
`ments on the pleth to determine the pulse period and, hence,
`pulse frequency. For example, such maxima may be
`obtained by identifying a change in sign of differential
`values between successive samples or groups of samples
`along the pleth or of a
`function fitted to the pleth.
`Alternatively, other points on the waveform, such as nomi-
`nal zero (or average pleth value) crossings may be moni-
`tored. Such zero crossings would be expected to have a
`frequency of twice the heart rate. Such period measurements
`can be complicated due to the typically noisy waveform of
`the pleths. Accordingly, multiple waveforms may be uti-
`lized.
`3wr
`Additionally, the heart rate calculations may be performed 5
`in the frequency domain. In this regard, a processor may be
`configured to obtain a Fourier transform of the pleth. Once
`the Fourier transform is obtained,
`the pulse rate can be
`identified as the fundamental frequency of the pleth corre-
`sponding to the patient’s heart rate. In any case, once the
`heart rate is determined, it can be monitoredto identify low
`frequency variations associated with respiration.
`In
`particular, oscillatory variations having a
`frequency of
`between about 0.15 and 0.5 Hz and, especially, between
`about 0.2 and 0.4 Hz, are indicative of respiration rate. This
`range may be expanded to (-5 Hz to accommodate the
`higher respiration rates of newborns.
`One or more filters may be used in determining respira-
`tion rate information based on a pleth signal in accordance
`with the present invention. In this regard, an adaptive filter
`may be used to track the fundamental frequency of the pleth
`and, hence, the patient’s pulse rate. For example, sucha filter
`may function as a narrow band passfilter having a band pass
`that is centered on the fundamental frequency of the pleth.
`The transfer function ofthe filter may be varied, e.g., based
`on analysis of successive waveforms, to track the changing
`fundamental frequency. The filter or associated logic may
`thus be adapted to output a time series of pulse rate values.
`Such a timeseries of pulse rate values, whether obtained as
`an output of an adaptivefilter system or otherwise, may be s
`filtered using a static band passfilter having a pass band
`including the noted frequencies of interest, or using an
`adaptive filter that tracks a selected spectral peak of the time
`series to provide an output indicative of respiration rate.
`Such filtering provides a fast, robust and computationally
`efficient mechanism for noninvasively monitoring patient
`respiration based on pleth signals.
`The present invention is based in part on a recognition that
`the pleth signal includesa variety of information in addition
`to the pulsatile waveform that
`is generally the focus of
`plethysmographic processing. In particular, it has been rec-
`ognizedthat the pleth signal includesat least three additional
`or related components: 1) a component relatedto respiration
`or the “respiration wave”, 2) a low frequency component
`associated with the autonomic nervous system or vaso motor
`center, sometimes termed the “Mayer wave,” and 3) a very
`low frequency component which is associated with tempera-
`
`unPry
`
`60
`
`65
`
`4
`ture control. Regarding the second ofthese, the origin and
`nature of the Mayer wave is not fully settled. For present
`purposes, the Mayer waverelates to a low frequency varia-
`tion in blood pressure, heart rate, and/or vaso constriction.
`The first two components noted above have particular
`significance for diagnostic and patient monitoring purposes.
`In particular,
`the amplitude and frequency of the Mayer
`wave are seen to change in connection with hypertension,
`sudden cardiac death, ventricular
`tachycardia, coronary
`artery disease, myocardial infarction, heart failure, diabetes,
`and autonomic neuropathy and after heart transplantation.
`Respiration rate is monitored during a variety of medical
`procedures, for example, as an indication ofa patient’s stress
`levels and to identify patient
`respiratory distress.
`It
`is
`expected that both the Mayer and respiration waves influ-
`ence heart rate (and related parameters such as variations in
`blood pressure and blood volume) by direct influence on the
`vaso motorcenter. In the latter case, this is by a “spillover”
`from the breathing center to the vaso motor center, which
`increases heart rate during inspiration.
`A difficulty associated with obtaining physiological
`parameter information based on the Mayer wave and the
`respiration wave relates to distinguishing the effects asso-
`ciated with these waves,particularly in view ofthe fact that
`each of these waves can occur within overlapping frequency
`ranges. In accordance with the present invention, respiration
`information is obtained by monitoring heart rate variability
`within a specific frequency band as noted above.
`In
`particular, by monitoring in a frequency range having a
`lowerend ofpreferably at least about 0.15 Hz, for example,
`0.15—-0.5, interference due to Mayer wave effects can gen-
`erally be minimized. Still better results may be obtained by
`monitoring a range between about 0.2—0.4 Hz or, especially,
`about 0.3 Hz. In the case of tracking respiration rate using
`an adaptivefilter relative to a time series of pulse rate values
`or a corresponding frequency spectrum, the transfer function
`may be limited to track the respiration related peak only
`within these ranges using 0.3 Hz as an initial condition.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`For a more complete understanding of the present inven-
`tion and further advantages thereof, reference is now made
`to the following detailed description, taken in conjunction
`with the drawings, in which:
`FIG. 1 is a schematic diagram of a pulse oximeter in
`accordance with the present invention;
`FIG.2 illustrates the waveformofa pleth that may be used
`to obtain respiratory information in accordance with the
`present invention;
`FIG. 3 illustrates a pleth power spectrum showing the
`respiration related peak that is used in accordance with the
`present invention;
`FIG. 4 illustrates a heart rate Lime series generated using
`an appropriate filter in accordance with the present inven-
`tion;
`FIG. 5 is a pleth power spectrum illustrating a transfer
`function of a filter in accordance with the present invention;
`FIG, 6 is a respiratory power spectrum illustrating a
`transfer function of another filter in accordance with the
`present invention;
`FIG. 7 is a flow chart illustrating a process for using a
`pleth signal to monitor respiration in accordance with the
`present invention; and
`FIG. 8 illustrates a signal processing system in accor-
`dance with the present invention.
`
`0012
`
`FITBIT, Ex. 1012
`
`0012
`
`FITBIT, Ex. 1012
`
`

`

`5
`DETAILED DESCRIPTION
`
`US 6,702,752 B2
`
`6
`lation module 116 to compute a value related to blood
`oxygen saturation, e.g., a blood oxygen saturation percent-
`age. A numberof algorithms for performing such caleula-
`tions are known and such calculation techniques are dis-
`closed in U.S. Pat. Nos. 5,934,277 by Mortz and 5,842,979
`by Jarman, both of which are incorporated herein by refer-
`ence,
`
`The present invention relates to obtaining physiological
`parameter information for a patient based on an analysis of
`a pleth involving distinguishing an effect associated with a
`Mayer wave component from an effect associated with a
`respiration wave component. In the following discussion,
`the invention is described in the context of an implementa-
`FIG,2 illustrates an exemplary waveform of a pleth as
`lion utilizing components of a conventional pulse oximeter.
`such information may be obtained by the processor of a
`The invention has particular advantages in this regard as
`pulse oximeter.
`In particular, such information may be
`obtained asa digital signal output by the A/D converter,i.c.,
`such an implementation enhances the functionality of con-
`a time series of values related to the detector output. Such
`ventional pulse oximeters and provides important physi-
`values are shown graphically in FIG. 2. As noted above, the
`ological parameter information in a cost effective manner.
`pleth corresponding to either of the oximetry channels, or a
`However, it will be appreciated that various aspects of the
`combination of the channels, may be used in accordance
`invention are not limited to such a pulse oximeter or other
`with the present invention. It is desirable to obtain a strong,
`multi-channel signal implementation and the invention may
`pleth signal so that
`the waveform and pulse rate can be
`be embodied in a dedicated single or multi-channel photop-
`accurately identified. Accordingly, for normally oxygenated
`lethysmography instrument. Accordingly, the following dis-
`patients,
`the infrared channel pleth may be utilized. For
`cussion should be understood as exemplifying the invention
`poorly oxygenated patients, the red pleth may be preferred.
`and not by wayoflimitation.
`In this regard, a cut off oxygenation level such as 85% may
`Referring to FIG, 1, a schematic diagram of a pulse
`be used in determining whether to use the infrared or red
`oximeter 100 in accordance with the present invention is
`pleth. Alternatively, the two pleth signals may be mathemati-
`shown. The oximeter 100 generally includes an instrument
`cally blended, depending on the current oxygenation level to
`housing 102 and a probe 104 for attachmentto a finger 101
`obtain an optimized pleth for subsequent analysis in accor-
`or other appendage of a patient under analysis.
`In the 5
`dance with the present invention. Appropriate techniques for
`obtaining an optimized pleth signal are disclosed in U.S.
`illustrated embodiment, the probe 104 includes two or more
`patent application Ser. No. 09/975,289, which is disclosed
`sources 106 and a detector 110. It will be appreciated that
`herein by reference.
`either or both of these components may alternatively be
`As shownin FIG. 2, the pleth signal includes a pulsatile
`located in the housing 102 and may be optically connected
`to the probe 104 by fiber optics or the like. Additionally, the
`component having a period designated T,. This period
`corresponds to the patient’s heart rate. The heart rate can be
`sources L106 and/or detector L10 may be located in the cable
`determined by monitoring this pleth in a variety of ways
`or other coupling operatively between the probe 104 and the
`such as identifying a change in sign of a differential value of
`housing 102. The sources 106 are driven by source drives
`the waveform (e.g., to perform a peak-to-peak period mea-
`108. The drives 108 serve to modulate the signals 103 in any
`surement or peak-lo-trough 4 period measurement), track-
`ofvarious ways. In this regard, the signals 103 transmitted
`ing crossings of an average value indicated by A,or, as will
`by the sources 106 may be time division multiplexed,
`be discussed in more detail below, by usingafilter to track
`frequency division multiplexed, code division multiplexed,
`the fundamental frequency ofthe pleth.
`or the like. Such multiplexing facilitates separation of the
`In accordance with the present invention, the patient's
`signals from each of the channels during hardware or
`respiration is monitored by tracking low frequency heart rate
`software based signal processing. The sources 106 provide
`changes. FIG. 3 shows an exemplary pleth power spectrum.
`two or more channels of signals 103. Each channel has a
`The spectrum is characterized by three discrete peaks. These
`unique spectral content, ¢.g., wavelength or wavelength
`include a peak typically around 0.3 Hz-0.5 Hz, a peak
`band. In the illustrated embodiment, two sources 106 are
`typically around 0.1 Hz and a peak below 0.05 Hz, The peak
`shown; one of the sources may have a red-centered wave-
`below 0.05 Hz is generally linked with vaso motor control
`length and the other may have an infrared-centered wave-
`and temperature control. The peak at around 0.1 Hz is
`length.
`generally associated with the Mayer wave. As noted above,
`this phenomenon is not well understood but has been
`The signals 103 may be transmitted through or reflected
`correlated to hypertension, sudden cardiac death, ventricular
`by the patient's tissue. In either case, the signals are modu-
`tachycardia, coronary artery disease, myocardial infarction,
`lated by the patient’s tissue to provide information regarding
`heart failure, diabetes, and autonomic neuropathy and has
`blood oxygen saturation in a manner that is well known. The 5
`been seen to change after heart transplantation. The remain-
`transmitted signals 103 are received by the detector 110
`ing peak, at about 0.3-0.5 Hz is believed to be correlated
`which,
`in the illustrated embodiment, provides an analog
`with respiration and is of particular interest for purposes of
`current output signal LOS representative of the detected
`the present invention. It will be appreciated that this peak
`signals 103. This detector signal 105 is then processed by
`may be as high as 1 Hz or greater for newborns.
`signal processing module 112. The processing module 112
`may include a number of components that may be embodied
`FIG. 4 graphically illustrates the respiratory Sinus
`in software, firmware and/or hardware. These components
`Arrhythmia phenomenon associated with the above noted
`may include components for amplifying the signal 105 and
`respiration wave,In particular, FIG. 4 is a graph plotting the
`converting the signal from a current signal
`to a voltage
`output of a heart
`rate filter, as will be discussed below,
`signal, filtering the signal to remove certain components of
`against time, As shown, the result is a periodic waveform
`noise and otherwise conditioning the signal. In the illus-
`having a period designated T,,. This generally corresponds to
`trated embodiment, the signal processing module 112 also
`a reduction in heart rate during the expiration portion of the
`includes an analog to digital converter for converting the
`respiratory cycle and an increase in heart rate during the
`signal into a digital signal and a demultiplexer component
`inspiration portion of the cycle. The period of this waveform
`for providing two separate output signals 118 or pleths that
`generally corresponds to the respiration rate and is tracked
`generally correspond to the two separate channel signals
`using a pulse oximeter in accordance with the present
`invention.
`103. These pleths 118 are then used by oxygenation calcu-
`
`30
`
`35
`
`40
`
`45
`
`60
`
`65
`
`0013
`
`FITBIT, Ex. 1012
`
`0013
`
`FITBIT, Ex. 1012
`
`

`

`US 6,702,752 B2
`
`5
`
`30
`
`7
`From the foregoing discussion,it will be appreciated that
`respiration rate can be monitored by: 1) determining heart
`rate based on an analysis of the pleth signal, 2) monitoring
`this heart rate over time to obtain a time series heart rate
`values, and 3) analyzing the time series heart rate values to
`identify a respiration rate. These steps can be executed using
`adaptive filters and/or static band pass filters as discussed
`below.
`FIG. § illustrates a pleth power spectrum. Such a power
`spectrum may be obtained by configuring the oximeter
`processor to mathematically obtain a Fourier transform of
`the time domain pleth signal. As shown, the pleth power
`spectrum has a fundamental frequencyat t, corresponding to
`the patient’s heart rate. Additional peaks of the illustrated
`power spectrum relate to harmonics thereof. The present
`invention utilizes an adaptive filter adapted to function as a
`bandpass filter having a narrow band pass encompassing the
`fundamental frequency. The transfer function ofthisfilter is
`generally indicated by function 500. A variety ofdifferent
`types offilters may be used in this regard. Generally, such
`filters track the fundamental frequency of a signal based on
`certain programmed information regarding the nature of the
`signal as well as by monitoring successive signal wave-
`forms. Such filters are robust in operation and can provide
`a continually updated output, in this case, regarding pulse
`rate. Thus, sucha filter can provide as an output a time series 9
`of pulse rate values such as illustrated in FIG. 4.
`An additional digital filter can be used to track respiration
`rate. In particular, the output ofthe heartrate filter can be
`processed to provide a respiratory power spectrum as shown
`in FIG. 6, For example,
`the oximeter processor can be
`configured to perform a Fourier transform on the time series
`of pulse rate values output by the heart
`rate filter. The
`resulting respiratory power spectrum includes a frequency
`peak correlated to the respiration rate designatedas t,. The
`additional peaks shown in the power spectrum of FIG. 6 3
`relate to harmonics thereof or other heart rate variations. An
`adaptive filter having a transfer function, generally indicated
`by function 600, can be used to track the fundamental
`frequency. Such a filter may be similar to the heart rate filter
`as described above and is programmed to adaptively track
`the noted frequency of the respiratory power spectrum
`which corresponds to respiration rate. The output ofthis
`filter
`is
`a periodically updated respiration rate value.
`Alternatively, a static band passfilter may be used to isolate
`the peak related to respiration and, hence,
`identify the
`respiration rate. Such a filter may have a pass band of 0-0.5
`Hz or, to accommodate neonatal applications, O-1.5 Hz.
`FIG. 7 is a flow chart illustrating a process for determin-
`ing respiration rate based on pleth signals in accordance with
`the present invention. The process 700 is initiated by obtain-
`ing a detector outputor pleth signal. In the context of a pulse
`oximeter, this may involve receiving the digital output from
`an A/D converter that reflects the detector signal, demodu-
`lating this signal to obtain individual channel components
`and selecting a pleth for further processing. The selected
`pleth may be one ofthe channels or an optimized pleth based
`on both of the channel components. The pleth is then filtered
`(704) to obtain a time series of heart rate values. These
`values are monitored (706) over time to obtain a heart rate
`signal. The heart rate signal is then filtered (708) to identify
`a frequency peak correlated to respiration. The frequency of
`this peak is then output (710) as a respiration rate. This
`respiration rate may be displayed in the display area of a
`conventional pulse oximeter programmed to provide such
`information.
`The corresponding components of a pulse oximeter pro-
`cessing unit are illustrated in FIG. 8. The illustrative unit 800
`
`40
`
`45
`
`50
`
`60
`
`65
`
`8
`includes an A/D converter 802. The A/D converter receives
`an analog signal representative of the optical signal received
`by the pulse oximeter detector. This analog input signal is
`processed by the converter (802) to provide a digital detector
`signal 803. The digital detector signal 803 is then processed
`by demodulator 804 to provide two separate channel signals
`designated channel A (805) and channel B (807), that may
`correspond, for example, to the red and infrared channels of
`the pulse oximeter.