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`USlll167ll275232
`
`(12) Unlted States Patent
`(10) Patent 1%.:
`US 6,702,752 B2
`
`Dekker
`(45) Date of Patent:
`Mar. 9, 2004
`
`(54) MONITORING RESPIRA’I‘ION BASED ON
`PLETHYSMOGRAPHIC HEART RATE
`SIGNAL
`
`(75)
`
`Inventor: Andreas Lubbertus Aloysius Johannes
`Dekker' Maaslrichl (NL)
`
`(13) Assignee: Datex-Ohmeda. tnc., Madim, WI
`(US)
`
`(fl Nmm:
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`thmmdeMmmmuwmdmb
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) ApPl- NO-i 1010311719
`(22)
`Fixed:
`Feb. 22, 20112
`
`(55)
`
`Prior I’uhllcatltm Data
`US 21113111153054 Al Aug. 28, 2mm
`
`(51)
`(52)
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`Int' Cl",
`11.3. (:1.
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`(58) Field of Search
`(1(1).t48l,
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`476, 479, 480, 473, 475, 529
`
`(50)
`
`References Cited
`US. PATEN'I' DOCUMENTS
`
`3112:9222 2
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`
`EEEIEM H a"
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`
`Q
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`.
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`1111994 Richardson el 11].
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`ywfitm¥un
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`(‘iark el al.
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`[H1990 Diah et al.
`2.199!) Dial) Cl
`‘41.
`411990 801221me
`9’199" CW“ °‘ a"
`
`(Li-<1 continued on ncxl page.)
`OTHER PUBLICAJ‘IONS
`S ectral Anal sis: Review "Heart Rate Variabilit “. Lukas
`Sfiicker, hemufdynamicsncdavisedu, Unknown Pihlicalion
`Dale.
`
`Primary Examiner-41“ F. [Iindcnburg
`Assistant Emminer-Navin Natnilhithaclha
`(74) Attorney, Agent. or Firm—Marsh Fischmann 81
`Breyfogle LL1’
`_
`(:17)
`
`ABSTRACT
`
`A plelh signal is analymd to identify a heart rate variahilily
`parameter associated with respiration rate.
`In one
`embodiment, an aseocietect process involves obtaining a
`pholoplelhysmograple s1gnal. procen‘smg the plcth Signal to
`obtain heart rate samples, monitoring the heart rate sample
`to identify a heart rate variability associated with tespiralion,
`and determining a respiration rate based on the heart rate
`variability. The photnplclhysmograph 1‘: signal may be based
`on one or more channel signals of a conventional pulse
`oximelcr. The invention Ihus allows for noninvasive moni-
`luring of respiration rate and expands the functionality of
`"130 oximclcm
`p
`"
`
`38 Claims, 8 Drawing Sheets
`
`Apple Inc.
`APL1012
`
`US. Patent No.
`
`8,929,965
`
`FITBIT
`
`, Ex. 1012
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`
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`OXYGENATIO
`CALCULATION
`
`
`0001
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`Apple Inc.
`APL1012
`U.S. Patent No. 8,929,965
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`0001
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`US 6,702,752 B2
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`Page 2
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`311999 Mon:
`911999 Deming el 0L 1.
`1011999 nghanawi .... .1
`110999 Brockway el 31.
`1111999 Kikuchi
`1211999 9113:1000 cl :0
`1:20:00 Alhan c1111.
`212000 Inukaiet a].
`22000 Sodickson cl al.
`5.12000 Andaman" el a|_ __
`5,2000 Diab (3| a|_
`012000 Amanc e1 al
`812000 Daniels el al.
`10120011 .Tzly
`1212000 Elcnning ct a].
`1112002 Turcoll
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`5,934,277 A
`5,954,644 A
`5971930 A
`5,980,463 A
`5,993,893 A
`5,997,482 A
`5,011,915 A
`0027.45 A
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`6,067,462 A
`6,081.?42 A 2
`15,099,481 A
`0,129,075 A
`6,155,992 A
`6,400,233 01
`
`* cited by examiner
`
`us, pATENT DOCUMENTS
`
`6001822
`
`6001323
`6001323
`6001310
`
`9f1996 Richardson et al-
`1111906 Athan et al-
`411997 Ammo elal-
`511998 Amano CI al-
`011998 Pologe cl all.
`71'1998 Inukai et 211-
`110998 Scharf
`12,0998 Jarman
`121'1998 Baken Jr. al :11.
`“1999 Nilmn
`211900 (indik
`2/1999 Feel, 111
`311999 Richardson el al-
`50099 Irwis ct all
`711999 Diah
`811999 Arakaki 91 a1.
`
`
`
`$55,832 A
`5,575,284 A
`5,923,933 A
`51755229 A
`5360,12? A
`5.3761071 A
`53301137 A
`5,842,979 A
`5,853.364 A
`5,362,805 A
`1805,16? A
`5,895,759 A
`5,885,213 A
`5,902,235 A
`5,919,134 A
`5,931,779 A
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`0002
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`US. Patent
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`Mar. 9, 2004
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`Sheet 1 of 8
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`US 6,702,752 B2
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`100
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`______ __/fi~z‘°‘____#____
`10
`fl
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`r
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`118
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`riudfi
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`102
`
`
`
`OXYGENATION
`CALCULAT10N
`
`
`
`
`
`SIGNAL
`PROCESSING
`11g.
`
`
`
`SOURCE
`
`DRIVES
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`m
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`0003
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`US. Patent
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`Mar. 9, 2004
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`Sheet 2 of 8
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`US 6,702,752 B2
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`DETECTOR OUTPUT V. TIME
`
`r—Tp—al
`
`DETECTOROUTPUT
`
`FIG.2
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`US. Patent
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`Mar. 9, 2004
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`Sheet 3 of 8
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`US 6,702,752 B2
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`LOW FREQUENCY
`PLETH VARIAB!LlTY POWER SPECTRUM
`
`Pla.u.]
`
`FIG.3
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`US. Patent
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`Mar. 9,2004
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`Sheet 4 of 8
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`US 6,702,752 B2
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`HEART RATE VARIABILITY
`
`
`
`TB
`
`EXPIRATION
`
`iNSPIRATION
`
`
`HEART
`
`RATEFILTEROUTPUT(HEART
`
`RATE)
`
`FIG.4
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`0006
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`US. Patent
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`Mar. 9, 2004
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`Sheet 5 of 8
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`US 6,702,752 B2
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`PLETH POWER SPECTRUM
`
`
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`0007
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`US. Patent
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`Mar. 9, 2004
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`Sheet 6 of 8
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`US 6,702,752 B2
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`RESPIRATORY POWER SPECTRUM
`
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`US. Patent
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`Mar. 9, 2004
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`Sheet 7 of 8
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`US 6,702,752 B2
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`#00
`
`OBTAIN DETECTOR OUTPUT
`(PLETH SIGNAL)
`
`FILTER PLETH TO OBTAIN
`HEART RATE
`
`RESPIRATION RATE
`
`MONITOR HEART RATE OVER TIME
`TO OBTAIN HEART RATE SIGNAL
`@
`
`FILTER HEART RATE SIGNAL TO
`OBTAIN FUNDAMENTAL FREQUENCY
`@
`
`OUTPUT FUNDAMENTAL FREQUENCY
`OF HEART RATE SIGNAL AS
`
`FIG.7
`
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`US. Patent
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`US 6,702,752 B2
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`1
`MONITORING RESPIRATIDN BASED ON
`PIJ‘JTHYSMUGRAPHIC HEART RATE
`SIGNAL
`
`FIELD OF THE INVENTION
`
`The present invention relates, in general, to the noninvl -
`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
`
`it]
`
`Photoplcthysmography relates 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 andror
`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 because it is noninvasive, typi-
`cally yields substantially instantaneous and accurate results,
`and utilizes minimal medical resources, thereby proving to
`be cost elfcctivc.
`
`30
`
`A common type of photoplethysmographic instrument is
`the pulse oximeter. Pulse oximetors determine an oxygen
`saturation level of a patient’s blood, or related analyte _
`values, basad on transmissionlabsomtion characteristics of
`Light
`transmitted through or reflected from the patient’s
`tissue. In particular, pulse oximetcrs generally include a
`probe for attaching to a patient’s appendage such as a linger,
`earlobe or nasal septum. The probe is used to transmit pulsed
`optical signals of at 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 diiferent 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-
`cessing unit. Accordingly, one or more analog to digital
`(A/D) eonveners 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
`
`60
`
`Such digital information defines input photoplethysmo-
`graphic signals or "pleths." These pleths generally contain
`twu components. The first component of interest is a low
`frequency or substantially invariant component in relation to
`the time increments considered for blood oxygen saturation
`calculations, sometimes termed the “ DC component," which
`
`65
`
`0011
`
`2
`generally corresponds to the attenuation related to the non-
`pulsatile volume of the 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
`quency to that of the heartbeat. In contrast, the DC compo-
`nent is a more steady baseline component, since the 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 inter
`mation for generating each of these outputs is generally
`obtained from the AC component of the pleth. In this regard,
`some pulse oximeters attempt 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, eg, 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—
`tors 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 oxirneters to monitor other parameters
`including respiration rate, it is apparent that such proposed
`uses have not gained general commercial acceptance.
`SUMMARY 0t“ 'l‘I—IE INVEN’I'ION
`
`The present invention is directed to monitoring patient
`respiration based on a pleth signal. The invention thus
`provides important diagnostic or monitoring information
`noninvasivcly. Moreover, various aspects of the invention
`can be implemented using one or more channels andfor other
`components of a conventional pulse oximcter, 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. II is known that heart rate varies with the respiration
`cycle, an cfl‘ect called Respiratory Sinus Arrhythmia. The
`present invention provides a robust process for monitoring
`this etfect and determining respiration rate based on pleth
`signals. A novel processor and pulse oximcter 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 he provided using components of
`a conventional pulse oximcter. Pulse oximetors 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, either
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`US 6,702,752 B2
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`it]
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`3
`pleth can be used to yield the desired heart rate inforrnalion.
`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-tonpeak 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 filled to the pleth.
`Allemalively, other points on the waveform, such as nomi-
`nal zero (or average pleth value) crossings may he moni-
`tored. Such 'Iiero cromsings 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—
`lined.
`Additionally. the heart rate calculations may be performed _
`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 monitored to 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 9—5 Hz to accommodate the
`higher respiration rates of newborns.
`One or more fillers 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 puls: rate. Forexample, such a filter
`may function as a narrow band pass filter having a band pass
`that is centered on the fundamental frequency of the pleth.
`The transfer function of the 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 time series of pulse rate values, whether obtained as
`an output of an adaptive filter system or otherwise, may be
`filtered using a static band pass filter 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 noninvasiver monitoring patient
`respiration based on pleth signals.
`The present invention is based in part on a recognition that
`the pleth signal includes a variety of information in addition
`to the pulsatile waveform that
`is generally the focus of
`plethysmographic processing. In particular, it has been ree-
`ognjzed that the pleth signal includes at least three additional
`or related components: 1) a component related to 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-
`
`4
`lure control. Regarding the second of these. the origin and
`nature of the Mayer wave is not fully settled. For present
`purposes, the Mayer wave relates to a low frequency varia-
`tion in blood pressure, heart rate, andtor vase 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 of a patient’s stress
`levels and to identify patient
`respiratory distress.
`It
`is
`expected that both the Mayer and respiration waves influi
`encc heart rate (and related parameters such as variations in
`blood pressure and blood volume) by direct influence on the
`vaso motor center. 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 asses
`cialed with these waves. particularly in view of the 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
`lower end of preferably at least about 0.15 Hz, for example,
`U.]5—U.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 I’ll or, especially,
`about 0.3 Hz. In the case of tracking respiration rate using
`an adaptive filter 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 0.3 112. 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 waveform] ofa 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 time 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 filler 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.
`
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`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 number of algorithms for performing such calcula—
`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.
`
`FIG. 2 illustrates an exemplary waveform of a pleth as
`such information may be obtained by the processor of a
`pulse oximeter.
`In particular, such information may be
`obtained as a digital signal output by the M) converter, i.e.,
`a time series of values related to the detector output. Such
`values are shown graphically in FIG. 2. As noted above. the
`plclh corresponding to either of the oximetry channels, or a
`combination of the channels. may be used in accordance
`with the present invention. It is desirable to obtain a strong
`pleth signal so that
`the waveform and pulse rate can be
`accurately identified. Accordingly, for normally oxygenated
`patients,
`the infrared channel pleth may be utiliried. For
`poorly oxygenated patients, the red plcth may be preferred.
`In this regard, a cut off oxygenation level such as 85% may
`be used in determining whether to use the infrared or red
`pleth. Alternatively, the two pleth signals maybe mathemati—
`cally blended, depending on the current oxygenation level to
`obtain an optimized pleth for subsequent analysis in accor-
`dance with thc present invention. Appropriate techniques for
`obtaining an optimized pleth signal are disclosed in US.
`patent application Ser. No. U9r975,289, which is disclosed
`herein by reference.
`As shown in FIG. 2, the pleth signal includes a pulsatilc
`component having a period designated T,,. This period
`corresponds to the patient’s hean rate. The bean rate can be
`determined by monitoring this pleth in a variety of ways
`such as identifying a change in sign of a differential value of
`the waveform (e.g., to perform a peak-to—peak period mea-
`surement or pcak-to-trough ‘1’; period measurement), track-
`ing crossings of an average value indicated by A, or, as will
`be discussed in more detail below, by using a filter to track
`the fundamental frequency of the pleth.
`In accordance with the present invention, the patient’s
`respiration is monitored by tracking low frequency heart rate
`changes. FIG. 3 shows an exemplary pleth power spectrum.
`The spectrum is characteriaed by three discrete peaks. These
`include a peak typically around 0.3 Hit-0.5 Hz, a peak
`typically around 0.1 Hz and a peak below 0.05 Hz. The peak
`below 0.05 Hz is generally linked with vaso motor control
`and temperature control. The peak at around 0.] Hz is
`generally associated with the Mayer wave. As noted above,
`this phenomenon is not Well understood but has been
`correlated to hypertension, sudden cardiac death, ventricular
`tachycardia, coronary anery disease, myocardial infarction,
`heart failure. diabetes, and autonomic neuropatlty and has
`been seen to change after heart transplantation. The remain-
`ing peak, at about 0.3—0.5 Hz is believed to be correlated
`with respiration and is of panicular interest for purposes of
`the present invention. It will be appreciated that this peak
`may be as high as l 112 or greater for newborns.
`FIG. 4 graphically illustrates the respiratory Sinus
`Arrhythmia phenomenon associated with the above noted
`respiration wave. in particular, FIG. 4 is a graph plotting the
`output of a heart
`rate filter, as will be discussed below,
`against time. As shown, the result is a periodic waveform
`having a period designated TB. This generally corresponds to
`a reduction in hean rate during the expiration portion of the
`respiratory cycle and an increase in heart rate during the
`inspiration portion of the cycle. The period of this waveform
`generally corresponds to the respiration rate and is tracked
`using a pulse oximeter in accordance with the present
`invention.
`
`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
`Mayor 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—
`tion utilizing components of a conventional pulse oximeter.
`The invention has particular advantages in this regard as
`such an implementation enhances the functionality of con-
`ventional pulse oximeters and provides important physi~
`ological parameter information in a cost effective manner.
`However, it will be appreciated that various aspects of the
`invention are not limited to such a pulse oximeter or other
`multi-channel signal implementation and the invention may
`be embodied in a dedicated single or multi-channel photop~
`lethysmography instrument. Accordingly, the following dis-
`cussion should be understood as exemplifying the invention
`and not by way of limitation.
`Referring to FIG, 1, a schematic diagram of a pulse
`oximeter 100 in accordance with the present invention is
`shown. The oximeter [00 generally includes an instrument
`housing 102 and a probe 104 for attachment to a finger 101
`or other appendage of a patient under analysis.
`in the _
`illustrated embodiment, the probe 104 includes two or more
`sources 106 and a detector 110. It will be appreciated that
`either or both of these components may allematively be
`located in the housing .102 and may be optically connected
`to the probe 104 by fiber optics or the like. Additionally, the
`sources 106 andfor detector 110 may be located in the cable
`or other coupling operativcly between the probe 104 and the
`housing 102. The sources 106 are driven by source drives
`108. The drives 108 serve to modulate the signals 103 in any
`of various ways. In this regard, the signals 103 transmitted
`by the sources 106 may be time division multiplexed,
`frequency division multiplexed, code division multiplexed,
`or the like. Such multiplexing facilitates separation of the
`signals from each of the channels during hardware or
`software based signal processing. The souroes 106 provide
`two or more channels of signals 103. Each channel has a
`unique spectral content, e.g., wavelength or wavelength
`band. In the illustrator] embodiment. two sources 106 are
`shown; one of the sources may have a red-centered wave-
`length and the other may have an infrared-centered wave-
`length.
`The signals 103 may be transmitted through or reflected
`by the patient's tissue. In either case, the signals are modu-
`lated by the patient’s tissue to provide information regarding
`blood oxygen saturation in a manner that is well known. The
`transmitted signals 103 are received by the detector 110
`which,
`in the illustrated embodiment, provides an analog
`current output signal 105 representative of the detected
`signals 103. This detector signal 105 is then processed by
`signal processing module 112. The processing module 112
`may include a number of components that may be embodied
`in software, firmware and/or hardware. These components
`may include components for amplifying the signal 105 and
`converting the signal from a current signal
`to a voltage
`signal, filtering the signal to remove certain components of
`noise and otherwise conditioning the signal. In the illus-
`trated embodiment, the signal processing module 112 also
`includes an analog to digital converter for convening the
`signal into a digital signal and a demultiplexer component
`for providing two separate output signals 118 or pleths that
`generally correspond to the two separate channel signals
`103. These pleths 118 are then used by oxygenation calcu-
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`US 6,702,752 B2
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`From the foregoing discussion, it will be appreciated that
`respiration rate can be monitored by: l) 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 andtor static band pass filters as discussed
`below.
`FIG. 5 illustrates a pleth power spectrum. Such a power
`spectmm may be obtained by configuring the oximeter
`processor to mathematically obtain a Fourier transform of
`the time domain pieth signal. As shown, the pleth power
`spectrum has a fundamental frequency at tD corresponding to
`the patient's heart rate. Additional peaks of the illustrated
`power spectrum relate to harmonics thereof. The present
`invention utilizes an adaptive fitter adapted to function as a
`band pass filter having a narrow band pass encompassing the
`fundamental frequency. The transfer function of this filter is
`generally indicated by function 500. A variety of different
`types of filters may be used in this regard. Generally, such
`fillers 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. such a filter can provide as an output a time series _
`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 of the heart rate lilter can be
`processed to provide a respiratory power spectrum as shown
`in FIG. 6. For example,
`the oximeter processor can he
`configured to perform a Fourier transform on the time series
`of pulse rate values output by the head rate filter. The
`resulting respiratory power spectrum includes a frequency
`peak correlated to the respiration rate designated as to. The
`additional peaks shown in the power spectrum of FIG. 6
`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 of this
`filter
`is
`a periodically updated respiration rate value.
`Alternatively, a static band pass filter 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 (1—05
`It: or, to accommodate neonatal applications, U—l.5 111.
`FIG. 7 is. a flow chart illustrating a process for determin-
`ing respiration rate based on pleth signals in accordance with
`the pre5ent invention. The process 700 is initiated by obtain-
`ing a detector output or pleth signal‘ In the context of a pulse
`oximeter, this may involve receiving the digital output from
`an AID 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 of the channelsor an optimized pleth based
`on both of the channel comprtnean. 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 oximcter programmed to provide such
`information.
`The corresponding components of a pulse oximeter pro-
`cessing unit are illustrated in FIG, ti. The illustrative unit 800
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`includes an IUD converter 802. The NT} converter receives
`an analog signal representative of the optical signal received
`by the pulse oximetcr 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 sig