[19]
`United States Patent
`5,490,505
`[11] Patent Number:
`[45] Date of Patent: Feb. 13, 1996
`
`
`
`Diab et al.
`
`llllllIllllllllllllllllllllIllllllllllllllllllllllllllllll||l||||||||||l|||
`USOOS490505A
`
`[54] SIGNAL PROCESSING APPARATUS
`
`[75]
`
`Inventors: Mohamed K. Diab; Esmaiel
`Kiani-Azarbayjany, both of Laguna
`Niguel; Walter M. Weber, Dana Point,
`all of Calif.
`
`[73] Assignee: Masirno Corporation, Mission Viejo,
`Calif.
`
`[21] Appl. No.: 132,812
`
`[22]
`
`Filed:
`
`Oct. 6, 1993
`
`Brown, David P., “Evaluation of Pulse Oximeters Using
`Theoretical Models and Experimental Studies”, Master’s
`thesis, University of Washington, Nov. 25, 1987, pp. 1—142.
`Cohe, Amon, “Volume 1: Time and Frequency Domains
`Analysis”, Biomedical Signal Processing, CRC Press, Inc.,
`Boca Raton, Florida, pp. 152—159.
`Severinghaus, John W., “Pulse Oximetry Uses and Limita-
`tions”, pp. 1—4, ASA Convention, New Orleans, 1989.
`
`(List continued on next page.)
`
`Primary Examiner—Angela D. Sykes
`Attorney, Agent, or Firm—Knobbe, Martens, Olson & Bear
`
`Related US. Application Data
`
`[57]
`
`ABSTRACT
`
`[63] Continuation-impart of Ser. No. 249,690, May 26, 1994,
`which is a continuation of Ser. No. 666,060, Mar. 7, 1991,
`abandoned.
`
`Int. Cl.6 ........................................................ A613 5/00
`[51]
`[52] US. Cl.
`............................................... 128/633; 356/41
`[58] Field of Search ..................................... 128/633—634,
`128/664—667, 672, 687—688, 716; 356/39—41
`
`[56]
`
`‘ References Cited
`
`U.S. PATENT DOCUMENTS
`
`3/1972 Lavallee.
`3,647,299
`3,704,706 12/1972 Herczfeld et a1.
`4,063,551
`12/1977 Sweeny .
`4,086,915
`5/1978 Kofsky et a1.
`
`.
`
`.
`
`(List continued on next page.)
`OTHER PUBLICATIONS
`
`Harris, Fred et al., “Digital Signal Processing with Efficient
`Polyphase Recursive All—Pass Filters”, International Con—
`ference on Signal Processing, Florence, Italy, Sep. 4P6,
`1991, 6 pages.
`Rabiner, L. et al., Theory and Application of Digital Signal
`Processing, Prentice Hall, Englewood Cliifs, New Jersey
`1975, p. 260.
`Tremper, Kevin et al., “Pulse Oximetry: Techincal Aspects
`of Machine Design”, Advances in Oxygen Monitoring,
`Little, Brown and Comp., Boston, Mass., 1987, pp.
`137—153.
`
`A signal processor which acquires a first signal, including a
`first primary signal portion and a first secondary signal
`portion, and a second signal, including a second primary
`signal portion and a second secondary signal portion,
`wherein the first and second primary signal portions are
`correlated. The signals may be acquired by propagating
`energy through a medium and measuring an attenuated
`signal after transmission or reflection. Alternatively,
`the
`signals may be acquired by measuring energy generated by
`the medium, A processor of the present invention generates
`a primary or secondary reference signal which is a combi-
`nation, respectively, of only the primary or secondary signal
`portions. The secondary reference signal is then used to
`remove the secondary portion of each of the first and second
`measured signals via a correlation canceler, such as an
`adaptive noise canceler, preferably of the joint process
`estimator type. The primary reference signal
`is used to
`remove the primary portion of each of the first and second
`measured signals via a correlation canceler. The processor of
`the present invention may be employed in conjunction with
`a correlation canceler in physiological monitors wherein the
`known properties of energy attenuation through a medium
`are used to determine physiological characteristics of the
`medium. Many physiological conditions, such as the pulse,
`or blood pressure of a patient or the concentration of a
`constituent in a medium, can be determined from the pri-
`mary or secondary portions of the signal after other signal
`portion is removed.
`
`24 Claims, 20 Drawing Sheets
`
`
`
`
`
`1
`
`'
`
`APPLE 1014
`
`1
`
`APPLE 1014
`
`

`

`5,490,505
`Page 2
`
`
`U.S. PATENT DOCUMENTS
`6/1978
`10/1983
`8/1985
`3/1987
`9/1988
`1/1989
`1/1989
`4/1989
`7/1989
`8/1989
`9/1989
`9/1989
`9/1989
`9/1989
`11/1989
`1/1990
`3/1990
`3/1990
`5/1990
`5/1990
`8/1990
`9/1990
`9/1990
`
`4,095,117
`4,407,290
`4,537,200
`4,649,505
`4,773,422
`4,799,493
`4,800,495
`4,824,242
`4,848,901
`4,860,759
`4,863,265
`4,867,571
`4,869,253
`4,869,254
`4,883,353
`4,892,101
`4,907,594
`4,911,167
`4,927,264
`4,928,692
`4,948,248
`4,955,379
`4,956,867
`
`.
`
`Nagy .
`Wilbcr .
`Widrow .
`Zinser, Jr. et al.
`Isaacson et 2.1.
`.
`DuFault .
`Smith .
`Frick et a].
`Hood, Jr.
`.
`.
`Kahn et al.
`Flower et al.
`Frick et a1.
`.
`Craig, Jr. et a1.
`Stone et a1.
`.
`Hausman .
`Cheung et al. .
`Muz .
`Corenman et al.
`Shiga et al.
`.
`Goodman et a1.
`Lehman .
`Hall .
`Zurek et al. .
`
`.
`
`......................... 128/633 X
`
`.
`
`,
`.................
`
`.............
`
`5,057,695
`5,273,036
`
`10/1991 Hiro et a1.
`12/1993 Kronberg et al.
`
`
`
`128/633 X
`....... 128/666 X
`
`OTHER PUBLICATIONS
`
`Mook, G. A., et al. “Spectrophotometric determination of
`oxygen saturation of blood independent of the presence of
`indocyanine green”, Cardiovascular Research, v01. 13, pp.
`233—237, 1979.
`Neuman, Michael R., “Pulse Oximetry: Physical Principles,
`Technical Realization and Present Limitations”, Continuous
`Transcutaneous Monitoring, Plenum Press, New York,
`1987, pp. 135—144.
`Mook, G. A., et al., “Wavelength dependency of the spec-
`trophotometric determination of blood oxygen saturation",
`Clinical Chemistry Acta, vol. 26, pp. 1707173, 1969.
`Klimasuaskas, Casey, “Neural Nets and Noise Filtering", Dr.
`Doob’s Journal, Jan. 1989, p. 32.
`Haykin, Simon, Adaptive Filter Theory, Prentice Hall,
`Englewood ClilTs, NJ, 1991.
`Widrow, Bernard, Adaptive Signal Processing, Prentice
`Hall, Englewood Clzfiir, NJ, 1985.
`
`128/633
`
`128/633
`
`2
`
`

`

`US. Patent
`
`Feb. 13,1996
`
`Sheet 1 of 20
`
`5,490,505
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`

`US. Patent
`
`Feb. 13, 1996
`
`Sheet 2 of 20
`
`5,490,505
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`US. Patent
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`Feb. 13, 1996
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`5,490,505
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`US. Patent
`
`Feb. 13, 1996
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`5,490,505
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`US. Patent
`
`Feb. 13, 1996
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`5,490,505
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`

`US. Patent
`
`Feb. 13, 1996
`
`Sheet 6 of 20
`
`5,490,505
`
`FREQUENCY(f)
`
`TRANSFER FUNCTION
`
`FIG.5c
`
`8
`
`

`

`US. Patent
`
`Feb. 13, 1996
`
`Sheet 7 of 20
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`5,490,505
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`US. Patent
`
`Feb. 13, 1996
`
`Sheet 8 of 20
`
`5,490,505
`
`FIG.7b
`
`COEFFICIENTS
`
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`
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`ENERGY OUTPUT
`
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`

`

`US. Patent
`
`Feb. 13, 1996
`
`Sheet 9 of 20
`
`5,490,505
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`

`US. Patent
`
`Feb. 13, 1996
`
`Sheet 10 of 20
`
`5,490,505
`
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`

`US. Patent
`
`Feb.13,1996
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`
`5,490,505
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`

`US. Patent
`
`Feb. 13, 1996
`
`Sheet 12 of 20
`
`5,490,505
`
`LEAST SQURES LATTICE
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`US. Patent
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`Feb. 13, 1996
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`5,490,505
`
`1
`SIGNAL PROCESSING APPARATUS
`
`This application is a continuation-in—part of US. Ser. No.
`08/249,690, filed May 26, 1994, now allowed, which is a
`continuation of application Ser. No. 07/666,060, filed Mar.
`7, 1991, now abandoned.
`
`FIELD OF THE INVENTION
`
`The present invention relates to the field of signal pro-
`cessing. More specifically, the present invention relates to
`the processing of measured signals, containing a primary
`and a secondary signal, for the removal or derivation of
`either the primary or secondary signal when little is known
`about either of these components. The present invention also
`relates to the use of a novel processor which in conjunction
`with a correlation canceler, such as an adaptive noise can-
`celer, produces primary and/or secondary signals. The
`present
`invention is especially useful for physiological
`monitoring systems including blood oxygen saturation.
`
`BACKGROUND OF THE INVENTION
`
`Signal processors are typically employed to remove or
`derive either the primary or secondary signal portion from a
`composite measured signal including a primary signal por—
`tion and a secondary signal portion. If the secondary signal
`portion occupies a ditferent frequency spectrum than the
`primary signal portion,
`then conventional filtering tech-
`niques such as low pass, band pass, and high pass filtering
`could be used to remove or derive either the primary or the
`secondary signal portion from the total signal. Fixed single
`or multiple notch filters could also be employed if the
`primary and/or secondary signal portion(s) exit at a fixed
`frequency(s).
`It is often the case that an overlap in frequency spectrum
`between the primary and secondary signal portions exists.
`Complicating matters further, the statistical properties of one
`or both of the primary and secondary signal portions change
`with time. In such cases, conventional filtering techniques
`are totally ineffective in extracting either the primary or
`secondary signal. If, however, a description of either the
`primary or secondary signal portion can be made available
`correlation canceling, such as adaptive noise canceling, can
`be employed to remove either the primary or secondary
`signal portion of the signal leaving the other portion avail—
`able for measurement.
`
`Correlation cancelers, such as adaptive noise canoelers,
`dynamically change their transfer function to adapt to and
`remove either the primary or secondary signal portions of a
`composite signal. Correlation cancelers require either a
`secondary reference or a primary reference which is corre-
`lated to either the secondary signal or the primary signal
`portions only. The reference signals are not necessarily a
`representation of the primary or secondary signal portions,
`but have a frequency spectrum which is similar to that of the
`primary or secondary signal portions. In many cases,
`it
`requires considerable ingenuity to determine a reference
`signal since nothing is usually known a priori about the
`secondary and/or primary signal portions.
`One area where composite measured signals comprising a
`primary signal portion and a secondary signal portion about
`which no information can easily be determined is physi-
`ological monitoring. Physiological monitoring apparatuses
`generally measure signals derived from a physiological
`system, such as the human body. Measurements which are
`typically taken with physiological monitoring systems
`
`2
`include electrocardiographs, blood pressure, blood gas satu—
`ration (such as oxygen saturation), capnographs, heart rate,
`respiration rate, and depth of anesthesia, for example. Other
`types of measurements include those which measure the
`pressure and quantity of a substance within the body such as
`breathalyzer testing, drug testing, cholesterol testing, glu—
`cose testing, arterial carbon dioxide testing, protein testing,
`and carbon monoxide testing, for example. Complications
`arising in these measurements are often due to motion of the
`patient, both external and internal (muscle movement, for
`example), during the measurement process.
`Knowledge of physiological systems, such as the amount
`of oxygen in a patient’s blood, can be critical, for example
`during surgery. These data Can be determined by a lengthy
`invasive procedure of extracting and testing matter, such as
`blood, from a patient, or by more expedient, non-invasive
`measures. Many types of non—invasive measurements can be
`made by using the known properties of energy attenuation as
`a selected form of energy passes through a medium.
`Energy is caused to be incident on a medium either
`derived from or contained within a patient and the amplitude
`of transmitted or reflected energy is then measured. The
`amount of attenuation of the incident energy caused by the
`medium is strongly dependent on the thickness and compo-
`sition of die medium through which the energy must pass as
`well as the specific form of energy selected. Information
`about a physiological system can be derived from data taken
`from the attenuated signal of the incident energy transmitted
`through the medium if either the primary or secondary signal
`of the composite measurement signal can be removed.
`However, non—invasive measurements often do not afford
`the opportunity to selectively observe the interference caus—
`ing either the primary or secondary signal portions, making
`it difficult to extract either one of them from the composite
`signal.
`The primary and/or secondary signal portions often origi-
`nate from both AC and/or DC sources. The DC portions are
`caused by transmission of the energy through differing
`media which are of relatively constant thickness within the
`body, such as bone, tissue, skin, blood, etc. These portions
`are easy to remove from a composite signal. The AC
`components are caused by physiological pulsations or when
`differing media being measured are perturbed and thus,
`change in thickness while the measurement is being made.
`Since most materials in and derived from the body are easily
`compressed,
`the thickness of such matter changes if the
`patient moves during a non-invasive physiological measure-
`ment. Patient movement, muscular movement and vessel
`movement, can cause the properties of energy attenuation to
`vary erratically. Traditional signal filtering techniques are
`frequently totally ineffective and grossly deficient in remov—
`ing these motion induced effects from a signal. The erratic
`or unpredictable nature of motion induced signal compo-
`nents is the major obstacle in removing or deriving them.
`Thus, presently available physiological monitors generally
`become totally inoperative during time periods when the
`measurement site is perturbed.
`A blood gas monitor is one example of a physiological
`monitoring system which is based upon the measurement of
`energy attenuated by biological tissues or substances. Blood
`gas monitors transmit light into the tissue and measure the
`attenuation of the light as a function of time. The output
`signal of a blood gas monitor which is sensitive to the
`arterial blood flow contains a component which is a wave-
`form representative of the patient’s arterial pulse. This type
`of signal, which contains a component related to the
`patient’s pulse, is called a plethysmographic wave, and is
`
`10
`
`15
`
`20
`
`25
`
`30
`
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`shown in FIG. 1 as curve 3. Plcthysmographic waveforms
`are used in blood pressure or blood gas saturation measure-
`ments, for example. As the heart beats, the amount of blood
`in the arteries increases and decreases, causing increases and
`decreases in energy attenuation, illustrated by the cyclic
`wave 5 in FIG. 1.
`
`Typically, a digit such as a finger, an ear lobe, or other
`portion of the body where blood flows close to the skin, is
`employed as the medium through which light energy is
`transmitted for blood gas attenuation measurements. The
`finger comprises skin, fat, bone, muscle, etc., shown sche-
`matically in FIG. 2, each of which attenuates energy incident
`on the finger in a generally predictable and constant manner.
`However, when fleshy portions of the finger are compressed
`erratically, for example by motion of the finger, energy
`attenuation becomes erratic.
`
`An example of a more realistic measured waveform S is
`shown in FIG. 3,
`illustrating the effect of motion. The
`primary plethysmographic waveform portion of the signal s
`is the waveform representative of the pulse, corresponding
`to the sawtooth-like pattern wave in FIG. 1. The large,
`secondary motion-induced excursions in signal amplitude
`hide the primary plethysmographic signal s. It is easy to see
`how even small variations in amplitude make it difficult to
`distinguish the primary signal s in the presence of a sec-
`ondary signal component It.
`A specific example of a blood gas monitoring apparatus is
`a pulse oximeter which measures the arterial saturation of
`oxygen in the blood. The pumping of the heart forces freshly
`oxygenated blood into the arteries causing greater energy
`attenuation. The arterial saturation of oxygenated blood may
`be determined from the depth of the valleys relative to the
`peaks of two plethysmographic waveforms measured at
`separate wavelengths. Patient movement introduces signal
`portions mostly due to venous blood, or motion artifacts, to
`the plethysmographic waveform illustrated in FIG. 3. It is
`these motion artifacts which must be removed from the
`measured signal for the oximeter to continue the measure-
`ment of arterial blood oxygen saturation, even during peri—
`ods when the patient moves. It is also these motion artifacts
`which must be derived from the measured signal for the
`oximeter to obtain an estimate of venous blood oxygen
`saturation. Once the signal components due to either arterial
`blood or venous blood is known, its corresponding oxygen
`saturation may be determined.
`
`SUMMARY OF THE INVENTION
`
`This invention is an improvement of U.S. patent applica-
`tion Ser. No. 07/666,060 filed Mar. 7, 1991 and entitled
`Signal Processing Apparatus and Method, which earlier
`application has been assigned to the assignee of the instant
`application. The invention is a signal processor which
`acquires a first signal and a second signal that is correlated
`to the first signal. The first signal comprises a first primary
`signal portion and a first secondary signal portion. The
`second signal comprises a second primary signal portion and
`a second secondary signal portion. The signals may be
`acquired by propagating energy through a medium and
`measuring an attenuated signal after transmission or reflec-
`tion. Alternatively, the signals may be acquired by measur-
`ing energy generated by the medium.
`The first and second measured signals are processed to
`generate a secondary reference which does not contain the
`primary signal portions from either of the first or second
`measured signals. The remaining secondary signal portions
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`from the first and second measured signals are combined to
`form the secondary reference. This secondary reference is
`correlated to the secondary signal portion of each of the first
`and second measured signals.
`The secondary reference is then used to remove the
`secondary portion of each of the first and second measured
`signals via a correlation canceler, such as an adaptive noise
`canceler. The correlation canceler is a device which takes a
`first and second input and removes from the first input all
`signal components which are correlated to the second input.
`Any unit which performs or nearly performs this function is
`herein considered to be a correlation canceler. An adaptive
`correlation canceler can be described by analogy to a
`dynamic multiple notch filter which dynamically changes its
`transfer function in response to a reference signal and the
`measured signals to remove frequencies from the measured
`signals that are also present in the reference signal. Thus, a
`typical adaptive correlation canceler receives the signal from
`which it is desired to remove a component and a reference
`signal. The output of the correlation canceler is a good
`approximation to the desired signal with the undesired
`component removed.
`Alternatively, the first and second measured signals may
`be processed to generate a primary reference which does not
`contain the secondary signal portions from either of the first
`or second measured signals. The remaining primary signal
`portions from the first and second measured signals are
`combined to form the primary reference. The primary ref-
`erence may then be used to remove the primary portion of
`each of the first and second measured signals via a corre-
`lation canceler. The output of the correlation canceler is a
`good approximation to the secondary signal with the pri—
`mary signal removed and may be used for subsequent
`processing in the same instrument or an auxiliary instru-
`ment. In this capacity, the approximation to the secondary
`signal may be used as a reference signal for input to a second
`correlation canceler together with either the first or second
`measured signals for computation of, respectively, either the
`first or second primary signal portions.
`Physiological monitors can often advantageously employ
`signal processors of the present invention. Often in physi-
`ological measurements a first signal comprising a first
`primary portion and a first secondary portion and a second
`signal comprising a second primary portion and a second
`secondary portion are acquired. The signals may be acquired
`by propagating energy through a patient’s body (or a mate-
`rial which is derived from the body, such as breath, blood,
`or tissue, for example) or inside a vessel and measuring an
`attenuated signal after transmission or reflection. Alterna-
`tively,
`the signal may be acquired by measuring energy
`generated by a patient’s body, such as in electrocardio—
`graphy. The signals are processed via the signal processor of
`the present invention to acquire either a secondary reference
`or a primary reference which is input
`to a correlation
`canceler, such as an adaptive noise canceler.
`One physiological monitoring apparatus which can
`advantageously incorporate the features of the present
`invention is a monitoring system which determines a signal
`which is representative of the arterial pulse, called a plethys-
`mographic wave. This signal can be used in blood pressure
`calculations, blood gas saturation measurements, etc. A
`specific example of such a use is in pulse oximetry which
`determines the saturation of oxygen in the blood. In this
`configuration, we define the primary portion of the signal to
`be the arterial blood contribution to attenuation of energy as
`it passes through a portion of the body where blood flows
`- close to the skin. The pumping of the heart causes blood flow
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`5
`to increase and decrease in the arteries in a periodic fashion,
`causing periodic attenuation wherein the periodic waveform
`is the plethysmographic waveform representative of the
`arterial pulse. We define the secondary portion of the signal
`to be that which is usually considered to be noise. This
`portion of the signal is related to the venous blood contri-
`bution to attenuation of energy as it passes through the body.
`Patient movement causes this component to flow in an
`unpredictable manner, causing unpredictable attenuation
`and corrupting the otherwise periodic plethysmographic
`waveform. Respiration also causes secondary or noise com-
`ponent to vary, although typically at a much lower frequency
`than the patients pulse rate.
`A physiological monitor particularly adapted to pulse
`oximetry oxygen saturation measurement comprises two
`light emitting diodes (LED’s) which emit light at different
`wavelengths to produce first and second signals. A detector
`registers the attenuation of the two different energy signals
`after each passes through an absorptive. media, for example
`a digit such as a finger, or an earlobe. The attenuated signals
`generally comprise both primary and secondary signal por-
`tions. A static filtering system, such as a bandpass filter,
`removes a portion of the secondary signal which is outside
`of a known bandwidth of interest,
`leaving an erratic or
`random secondary signal portion, often caused by motion
`and often difficult to remove, along with the primary signal
`portion.
`Next, a processor of the present invention removes the
`primary signal portions from the measured signals yielding
`a secondary reference which is a combination of the remain—
`ing secondary signal portions. The secondary reference is
`correlated to both of the secondary signal portions. The
`secondary reference and at least one of the measured signals
`are input to a correlation canceler, such as an adaptive noise
`canceler, which removes the random or erratic portion of the
`secondary signal. This yields a good approximation to the
`primary plethysmographic signal as measured at one of the
`measured signal wavelengths. As is known in the art,
`quantitative measurements of the amount of oxygenated
`arterial blood in the body can be determined from the
`plethysmographic signal in a variety of ways.
`The processor of the present invention may also remove
`the secondary signal portions from the measured signals
`yielding a primary reference which is a combination of the
`remaining primary signal portions. The primary reference is
`correlated to both of the primary signal portions. The
`primary reference and at least one of the measured signals
`are input to a correlation canceler which removes the pri—
`mary portions of the measured signals. This yields a good
`approximation to the secondary signal at one of the mea-
`sured signal wavelengths. This signal may be useful for
`removing secondary signals from an auxiligry instrument as
`well as determining venous blood oxygen saturation.
`One aspect of the present invention is a signal processor
`comprising a detector for receiving a first signal which
`travels along a first propagation path and a second signal
`which travels along a second propagation path wherein a
`portion of the first and second propagation paths are located
`in a propagation medium. The first signal has a first primary
`signal portion and a first secondary signal portion and the
`second signal has a second primary signal portion and a
`second secondary signal portion. The first and second sec-
`ondary signal portions are a result of a change of the
`propagation medium. This aspect of the invention addition—
`ally comprises a reference processor having an input for
`receiving the first and second signals. The processor is
`adapted to combine the first and second signals to generate
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`a secondary reference having a significant component which
`is a function of the first and said second secondary signal
`portions. The processor may also be adapted to combine the
`first and second signals to generate a primary reference
`having a significant component which is a function of the
`first and second primary signal portions
`The above described aspect of the present invention may
`further comprise a signal processor for receiving the sec-
`ondary reference signal and the first signal and for deriving
`therefrom an output signal having a significant component
`which is a function of the first primary signal portion of the
`first signal. Alternatively, the above described aspect of the
`present invention may further comprise a signal processor
`for receiving the secondary reference signal and the second
`signal and for deriving therefrom an output signal having a
`significant component which is a function of the second
`primary signal portion of the second signal. Alternatively,
`the above described aspect of the present invention may
`further comprise a signal processor for receiving the primary
`reference and the first signal and for deriving therefrom an
`output signal having a significant component which is a
`function of the first secondary signal portion of the signal of
`the first signal. Alternatively, the above described aspect of
`the present invention may further comprise a signal proces-
`sor for receiving the primary reference and the second signal
`and for deriving therefrom an output signal having a sig-
`nificant component which is a function of the second sec-
`ondary signal portion of the second signal. The signal
`processor may comprise a correlation canceler, such as an
`adaptive noise canceler. The adaptive noise canceler may
`comprise a joint process estimator having a least-squares—
`lattice predictor and a regression filter.
`The detector in the aspect of the signal processor of the
`present invention described above may further comprise a
`sensor for sensing a physiological function. The sensor may
`comprise a light or other electromagnetic sensitive device.
`Additionally, the present invention may further comprise a
`pulse oximeter for measuring oxygen saturation in a living
`organism. The present invention may further comprise an
`electrocardiograph.
`Another aspect of the present invention is a physiological
`monitoring apparatus comprising a detector for receiving a
`first physiological measurement signal which travels along a
`first propagation path and a second physiological measure-
`ment signal which travels along a second propagation path.
`A portion of the first and second propagation paths being
`located in the same propagation medium. The first signal has
`a first primary signal portion and a first secondary signal
`portion and the second signal has a second primary signal
`portion and a second secondary signal portion. The physi-
`ological monitoring apparatus further comprises a reference
`processor having an input for receiving the first and second
`signals. The processor is adapted to combine the first and
`second signals to generate a secondary reference signal
`having a significant component which is a function of the
`first and the second secondary signal portions. Alternatively,
`the processor may be adapted to combine the first and
`second signals to generate a primary reference havinga
`component which is a function of the first and second
`primary signal portions.
`The physiological monitoring apparatus may further com-
`prise a signal processor for receiving the secondary refer-
`ence and the first signal and for deriving therefrom an output
`signal having a significant component which is a function of
`the first primary signal portion of the first signal. Altema—
`tively, the physiological monitoring apparatus may further
`comprise a signal processor for receiving the secondary
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`reference and the second signal and for deriving therefrom
`an output signal having a significant component which is a
`function of the second primary signal portion of the second
`signal. Alternatively, the physiological monitoring apparatus
`may further comprise a signal processor for receiving the
`primary reference and the first signal and deriving therefrom
`an output signal having a significant component which is a
`function of the first secondary signal portion of the first
`signal. Alternatively, the physiological monitoring apparatus
`may further comprise a signal processor for receiving the
`primary reference and the second signal and deriving there—
`from an output si