US009241676B2
`
`a2z United States Patent (10) Patent No.: US 9,241,676 B2
`Lisogurski et al. 45) Date of Patent: *Jan. 26, 2016
`(54) METHODS AND SYSTEMS FOR POWER 5,485,847 A 1/1996 Baker, Jr.
`OPTIMIZATION IN A MEDICAL DEVICE 5,515,847 A 5/1996 Braigetal.
`5,560,355 A 10/1996 Merchant et al.
`(75) Inventors: Daniel Lisogurski, Boulder, CO (US); g:g;g:égg fi li;}ggg figljan etal
`Clark R. Baker, Jr., Newman, CA (US) 5746,697 A 5/1998 Swedlow et al.
`5,766,127 A 6/1998 Pologe et al.
`(73) Assignee: Covidien LP, Mansfield, MA (US) 5,846,190 A 12/1998 Woehrle
`5,924,979 A 7/1999 Swedlow et al.
`(*) Notice: Subject to any disclaimer, the term of this 6,005,658 A 12/1999 Kaluza et al.
`patent is extended or adjusted under 35 (Continued)
`U.S.C. 154(b) by 128 days.
`. . . . . FOREIGN PATENT DOCUMENTS
`This patent is subject to a terminal dis-
`claimer. WO WO 2006/080856 8/2006
`WO WO 2006/083180 8/2006
`(21) Appl. No.: 13/484,770 OTHER PUBLICATIONS
`(22) Filed: May 31, 2012 Takada, H. et al., “Acceleration Plethysmography to Evaluate Aging
`(65) Prior Publication Data Effect in Cardiovascular System,” Medical Progress Through Tech-
`nology, vol. 21, pp. 205-210, 1997.
`US 2013/0324856 Al Dec. 5, 2013 (Continued)
`(51) Imt.ClL
`A61B 5/00 (2006.01) Primary Examiner — Mark Remaly
`A61B 5/021 (2006.01) (74) Attorney, Agent, or Firm — Shvarts & Leiz LLP
`A61B 5/024 (2006.01)
`AG61B 5/1455 (2006.01) 57 ABSTRACT
`(52) US.CL A physiological monitoring system may use photonic signals
`CPC ..o A61B 5/7285 (2013.01); A61B 5/021 to determine physiological parameters. The system may vary
`(2013.01); A61B 5/02416 (2013.01); A61B parameters of a light drive signal used to generate the photo-
`5/02433 (2013.01); A61B 5/14551 (2013.01) nic signal from a light source such that power consumption is
`(58) Field of Classification Search reduced or optimized. Parameters may include light intensity,
`CPC e A61B 5/021; A61B 5/02416; A61B firing rate, duty cycle, other suitable parameters, or any com-
`5/14551; A61B 5/7285 bination thereof. In some embodiments, the system may use
`See application file for complete search history. information from a first light source to generate a light drive
`. signal for a second light source. In some embodiments, the
`(56) References Cited system may vary parameters in a way substantially synchro-
`
`U.S. PATENT DOCUMENTS
`
`nous with physiological pulses, for example, cardiac pulses.
`In some embodiments, the system may vary parameters in
`response to an external trigger.
`
`5,343,818 A 9/1994 McCarthy et al.
`5,349,952 A 9/1994 McCarthy et al.
`5,351,685 A 10/1994 Potratz 32 Claims, 30 Drawing Sheets
`100
`/ 08 )
`Light Drive Circuitry
`Control
`Circuitry User Interface
`110 180
`o2 User Input
`“\ Light Source 182
`130
`Front End Processing Clrcuitry 150 ’Tplay‘
`Analog-to- 184
`Co‘r\\:iatli?:\er c:nisi::e . Back End P g
`152 154 Clrcultry 170 +—— ‘ Speaker
`§s 186
`Detector 4
`140 Bermultiol Digital L 172
`156 o
`\ / Memory Communication
`174 — interface
`Decimator/ Dark 190
`Interpolator Subtractor
`160 162
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`(56) References Cited 7,623,990 B2 11/2009 Anderson et al.
`2005/0084202 Al 4/2005 Smith et al.
`U.S. PATENT DOCUMENTS 2005/0109339 Al 5/2005 Stahmann et al.
`
`2006/0264720 Al 11/2006 Chew et al.
`6,217,523 Bl 4/2001 Amano et al. 2007/0038049 Al 2/2007 Huang
`6,226,539 Bl 5/2001 Potratz 2007/0149871 Al 6/2007 Sarussi et al.
`6.697.655 B2 2/2004 Sueppel et al. 2007/0208240 Al 9/2007 Nordstrom et al.
`6.697.658 B2 2/2004 Al-Ali 2011/0213397 Al 9/2011 Mathonnet
`6,731,967 Bl 5/2004 Turcott 2011/0237911 Al 9/2011 Lamego et al.
`6.863.652 B2 3/2005 Huang et al. 2011/0245636 Al 10/2011 Li et al.
`6,912,413 B2 6/2005 Rantala et al.
`7,003,339 B2 2/2006 Diab et al. OTHER PUBLICATIONS
`;’}ég’g;g E% 1?;388? I(\:I}(;%Vsti)frll' et al. International Search Report and Written Opinion of the International
`7:295:866 B2 11/2007 Al-Ali Searching Authority for application No. PCT/US 2013/043338,
`7,382,247 B2 6/2008 Welch et al. mailed on Oct. 2, 2013.
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`102
`
`N\ | Light Source
`130
`
`Light Drive Circuitry
`
`120
`
`Control
`Circuitry
`110
`
`104
`
`~
`
`Front End Processing Circuitry 150
`
`| Detector
`R 140
`
`N
`
`Analog Ana‘iolg—to—
`o Digital
`Conditioner
`152 Converter
`T 154
`Demultiplexer Dlg.'Fai
`156 Conditioner
`T 158
`Decimator/ Dark
`interpolator Subtractor
`160 162
`
`Back End Processing
`
`Circuitry 170
`
`Processor
`172
`
`User Interface
`180
`
`User Input
`182
`
`Display
`184
`
`Speaker
`186
`
`Memory
`174
`
`Communication
`Interface
`190
`
`FIG. 1
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`U.S. Patent Jan. 26, 2016 Sheet 2 of 30 US 9,241,676 B2
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`Light Drive Signal
`
`Detector Current
`
`/216 /218
`
`1 eyele |
`202
`/ 220 220
`204
`Time
`FIG. 2A
`214
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`FIG. 2B
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`U.S. Patent Jan. 26, 2016 Sheet 4 of 30 US 9,241,676 B2
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`U.S. Patent Jan. 26, 2016 Sheet 5 of 30 US 9,241,676 B2
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`00
`
`Generating a first light drive signal
`corrasponding to a first photonic
`signal
`402
`
`l
`
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`corresponding to the first photonic
`signal
`404
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`;
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`to determine when to activate a
`second light source
`406
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`signal corresponding to a second
`photonic signal
`408
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`;
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`Determining a physiological
`parameter
`410
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`FiG. 4
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`Sheet 6 of 30
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`U.S. Patent Jan. 26, 2016 Sheet 8 of 30 US 9,241,676 B2
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`U.S. Patent Jan. 26, 2016 Sheet 11 of 30 US 9,241,676 B2
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`50
`
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`part correlated to physiclogical
`pulses
`802
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`light drive signal
`504
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`parameter
`206
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`FIG. 9
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`U.S. Patent Jan. 26, 2016 Sheet 19 of 30 US 9,241,676 B2
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`1760
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`first rate
`1702
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`second rate
`1704
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`signals at a constant rate
`1706
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`Perform a physiological
`measuyrement in a first mode
`1802
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`Detect a physiological condition
`1904
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`Parform a physiological
`measurement in a second mode
`1806
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`2202
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`the first signal
`2204
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`1
`METHODS AND SYSTEMS FOR POWER
`OPTIMIZATION IN A MEDICAL DEVICE
`
`The present disclosure relates to power optimization, and
`more particularly relates to conserving and optimizing power
`in a photoplethysmography system or other medical device.
`
`SUMMARY
`
`Systems and methods are provided for optimizing power
`consumption in an optical physiological monitoring system.
`The system may vary light drive signal parameters to reduce
`power consumption or vary power use. The system may vary
`parameters in a technique correlated to cardiac pulse cycles.
`In some embodiments, reducing power consumption may
`allow for increased battery life in portable systems or
`increased portability. In some embodiments, varying light
`output during a cardiac cycle may reduce heating effects of
`the emitters. Parameters that may be varied include light
`intensity, firing rate, duty cycle, other suitable parameters, or
`any combination thereof. The generated signals may be used
`to determined physiological parameters such as blood oxygen
`saturation, hemoglobin, blood pressure, pulse rate, other suit-
`able parameters, or any combination thereof.
`
`In some embodiments, the system may use information
`from a first light source to control a second light source. The
`system may generate a first light drive signal for activating a
`first light source to emit a first photonic signal. The first light
`source and second light source may each include one or more
`emitters. The system may receive a light signal attenuated by
`the subject, wherein the light signal comprises a component
`corresponding to the first photonic signal. The system may
`analyze the component of the light signal to determine when
`to activate a second light source. The system may generate a
`second light drive signal, based on the analysis of the first
`component, for activating the second light source to emit one
`or more second photonic signals. The system may determine
`one or more physiological parameters based on the light
`signals.
`
`In some embodiments, the system may vary a light drive
`signal in a way substantially synchronous with physiological
`pulses, for example, cardiac pulses. The system may generate
`a light drive signal for activating a light source to emit a
`photonic signal, wherein at least one parameter of the light
`drive signal is configured to vary substantially synchronously
`with physiological pulses of the subject. The system may
`receive a light signal attenuated by the subject, wherein the
`signal comprises a component corresponding to the emitted
`photonic signal. The system may determine physiological
`parameters based on the signal. In some embodiments, the
`system may vary light levels with other periodic (or mostly
`periodic) physiological changes. For example, venous return
`changes with intrathoracic pressure during a respiration cycle
`can affect the baseline level of the photoplethysmography
`waveform. The system may vary the emitter output such that
`similar signal quality is available at the detector over time
`varying volumes of venous blood present in the path of light.
`
`In some embodiments, the system may vary a light drive
`signal based on a received external trigger. The system may
`receive an external trigger based on a signal other than a light
`signal received by the physiological monitor. The trigger may
`include a signal received from an ECG sensor, an ECG sensor
`configured to detect an R-wave, a blood pressure sensor, a
`respiration rate sensor, any other suitable sensor, or any com-
`bination thereof. The system may, in response to the external
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`trigger, vary the light intensity, duty cycle, light source firing
`rate, any other suitable parameter, or any combination
`thereof.
`
`BRIEF DESCRIPTION OF THE FIGURES
`
`The above and other features of the present disclosure, its
`nature and various advantages will be more apparent upon
`consideration of the following detailed description, taken in
`conjunction with the accompanying drawings in which:
`
`FIG. 1 is a block diagram of an illustrative physiological
`monitoring system in accordance with some embodiments of
`the present disclosure;
`
`FIG. 2A shows an illustrative plot of a light drive signal in
`accordance with some embodiments of the present disclo-
`sure;
`
`FIG. 2B shows an illustrative plot of a detector signal that
`may be generated by a sensor in accordance with some
`embodiments of the present disclosure;
`
`FIG. 2C shows illustrative timing diagrams of a drive cycle
`modulation and cardiac cycle modulation in accordance with
`some embodiments of the present disclosure;
`
`FIG. 3 is a perspective view of an embodiment of a physi-
`ological monitoring system in accordance with some embodi-
`ments of the present disclosure;
`
`FIG. 4 is a flow diagram showing illustrative steps for
`determining a physiological parameter in accordance with
`some embodiments of the present disclosure;
`
`FIG. 5 shows an illustrative timing diagram of a physi-
`ological monitoring system in accordance with some embodi-
`ments of the present disclosure;
`
`FIG. 6 shows another illustrative timing diagram of a
`physiological monitoring system in accordance with some
`embodiments of the present disclosure;
`
`FIG. 7 shows another illustrative timing diagram of a
`physiological monitoring system in accordance with some
`embodiments of the present disclosure;
`
`FIG. 8A shows another illustrative timing diagram of a
`physiological monitoring system in accordance with some
`embodiments of the present disclosure;
`
`FIG. 8B shows another illustrative timing diagram of a
`physiological monitoring system in accordance with some
`embodiments of the present disclosure;
`
`FIG. 9 is a flow diagram showing illustrative steps for
`determining a physiological parameter in accordance with
`some embodiments of the present disclosure;
`
`FIG. 10 shows another illustrative timing diagram of a
`physiological monitoring system in accordance with some
`embodiments of the present disclosure;
`
`FIG. 11 shows another illustrative timing diagram of a
`physiological monitoring system in accordance with some
`embodiments of the present disclosure;
`
`FIG. 12 shows another illustrative timing diagram of a
`physiological monitoring system in accordance with some
`embodiments of the present disclosure;
`
`FIG. 13 shows another illustrative timing diagram of a
`physiological monitoring system in accordance with some
`embodiments of the present disclosure;
`
`FIG. 14 shows another illustrative timing diagram of a
`physiological monitoring system in accordance with some
`embodiments of the present disclosure;
`
`FIG. 15 shows another illustrative timing diagram of a
`physiological monitoring system in accordance with some
`embodiments of the present disclosure;
`
`FIG. 16 shows another illustrative timing diagram of a
`physiological monitoring system in accordance with some
`embodiments of the present disclosure;
`
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`FIG. 17 is a flow diagram showing illustrative steps for
`decimating and interpolating a signal in accordance with
`some embodiments of the present disclosure;
`
`FIG. 18 shows an illustrative timing diagram of a physi-
`ological monitoring system including sampling rate variation
`in accordance with some embodiments of the present disclo-
`sure;
`
`FIG. 19 is a flow chart showing steps to adjust a cardiac
`cycle modulation based on a physiological condition in accor-
`dance with some embodiments of the present disclosure;
`
`FIG. 20 is an illustrative timing diagram of a system oper-
`ating in a first and second mode following detection of a
`physiological condition in accordance with some embodi-
`ments of the present disclosure;
`
`FIG. 21 is another illustrative timing diagram of a system
`operating in a first and second mode following detection of a
`physiological condition in accordance with some embodi-
`ments of the present disclosure;
`
`FIG. 22 is a flow diagram showing illustrative steps for
`identifying features in a signal in accordance with some
`embodiments of the present disclosure;
`
`FIG. 23 is an illustrative plot of a waveform showing iden-
`tification of fiducials in accordance with some embodiments
`of the present disclosure;
`
`FIG. 24 is another illustrative plot of a waveform showing
`identification of fiducials in accordance with some embodi-
`ments of the present disclosure;
`
`FIG. 25 is another illustrative plot of a waveform showing
`identification of fiducials in accordance with some embodi-
`ments of the present disclosure;
`
`FIG. 26 is an illustrative plot of waveforms showing pulse
`identification in accordance with some embodiments of the
`present disclosure;
`
`FIG. 27 is an illustrative plot of waveforms showing
`dicrotic notch identification in accordance with some
`embodiments of the present disclosure; and
`
`FIG. 28 is an illustrative plot of waveforms showing PPG
`signals in accordance with some embodiments of the present
`disclosure.
`
`DETAILED DESCRIPTION OF THE FIGURES
`
`The present disclosure is directed towards power optimi-
`zation in a medical device. A physiological monitoring sys-
`tem may monitor one or more physiological parameters of a
`patient, typically using one or more physiological sensors.
`The system may include, for example, a light source and a
`photosensitive detector. Providing a light drive signal to the
`light source may account for a significant portion of the
`system’s total power consumption. Thus, it may be desirable
`to reduce the power consumption of the light source, while
`still enabling high quality physiological parameters to be
`determined. The system may reduce the power consumption
`by modulating parameters associated with the light drive
`signal in techniques correlated to the cardiac cycle or other
`cyclical physiological activity. For example, the system may
`decrease brightness during a particular portion of the cardiac
`cycle. It may also be desirable to reduce the power consump-
`tion by the light drive signal to reduce heating effects caused
`by an emitter.
`
`An oximeter is a medical device that may determine the
`oxygen saturation of an analyzed tissue. One common type of
`oximeter is a pulse oximeter, which may non-invasively mea-
`sure the oxygen saturation of a patient’s blood (as opposed to
`measuring oxygen saturation directly by analyzing a blood
`sample taken from the patient). Pulse oximeters may be
`included in patient monitoring systems that measure and dis-
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`play various blood flow characteristics including, but not
`limited to, the oxygen saturation of hemoglobin in arterial
`blood. Such patient monitoring systems may also measure
`and display additional physiological parameters, such as a
`patient’s pulse rate and blood pressure.
`
`An oximeter may include a light sensor that is placed at a
`site on a patient, typically a fingertip, toe, forehead or earlobe,
`or in the case of a neonate, across a foot. The oximeter may
`use a light source to pass light through blood perfused tissue
`and photoelectrically sense the absorption of the light in the
`tissue. In addition, locations which are not typically under-
`stood to be optimal for pulse oximetry serve as suitable sensor
`locations for the blood pressure monitoring processes
`described herein, including any location on the body that has
`a strong pulsatile arterial flow. For example, additional suit-
`able sensor locations include, without limitation, the neck to
`monitor carotid artery pulsatile flow, the wrist to monitor
`radial artery pulsatile flow, the inside of a patient’s thigh to
`monitor femoral artery pulsatile flow, the ankle to monitor
`tibial artery pulsatile flow, and around or in front of the ear.
`Suitable sensors for these locations may include sensors for
`sensing absorbed light based on detecting reflected light. In
`all suitable locations, for example, the oximeter may measure
`the intensity of light that is received at the light sensor as a
`function of time. The oximeter may also include sensors at
`multiple locations. A signal representing light intensity ver-
`sus time or a mathematical manipulation of'this signal (e.g., a
`scaled version thereof; a logarithm taken thereof, a scaled
`version of a logarithm taken thereof; a derivative taken
`thereof, a difference taken thereof, etc.) may be referred to as
`the photoplethysmograph (PPG) signal. In addition, the term
`“PPG signal,” as used herein, may also refer to an absorption
`signal (i.e., representing the amount of light absorbed by the
`tissue), a transmission signal (i.e., representing the amount of
`light received from the tissue), any suitable mathematical
`manipulation thereof; or any combination thereof. The light
`intensity or the amount of light absorbed may then be used to
`calculate any of a number of physiological parameters,
`including an amount of a blood constituent (e.g., oxyhemo-
`globin) being measured as well as a pulse rate and when each
`individual pulse occurs.
`
`In some applications, the photonic signal interacting with
`the tissue is selected to be of one or more wavelengths that are
`attenuated by the blood in an amount representative of the
`blood constituent concentration. Red and infrared (IR) wave-
`lengths may be used because it has been observed that highly
`oxygenated blood will absorb relatively less red light and
`more IR light than blood with a lower oxygen saturation. By
`comparing the intensities of two wavelengths at different
`points in the pulse cycle, it is possible to estimate the blood
`oxygen saturation of hemoglobin in arterial blood.
`
`The system may process data to determine physiological
`parameters using techniques well known in the art. For
`example, the system may determine blood oxygen saturation
`using two wavelengths of light and a ratio-of-ratios calcula-
`tion. The system also may identify pulses and determine pulse
`amplitude, respiration, blood pressure, other suitable param-
`eters, or any combination thereof, using any suitable calcula-
`tion techniques. In some embodiments, the system may use
`information from external sources (e.g., tabulated data, sec-
`ondary sensor devices) to determine physiological param-
`eters.
`
`In some embodiments, it may be desirable to implement
`techniques to optimize power consumption in an oximeter or
`other system. For example, in a battery powered system,
`reducing the power requirements may allow for smaller
`devices, longer life, or both. In some embodiments, powering
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`the light source may include a large amount of the power load
`a device may experience. In some embodiments, variation of
`parameters in the light drive signal may enable a particular
`amount of power to be used more efficiently. For example, the
`brightness of a light source may be decreased during a less
`important period and increased during a more important
`period. In some embodiments, parameter variation may
`reduce the impact of heating effects caused by a light source
`on a subject. Techniques to vary the amount of time a light
`source is turned on, to vary the brightness of the light source,
`other techniques, or any combination thereof, may be
`employed to modify power consumption.
`
`In some embodiments, the brightness of one of more light
`sources may be modulated in a technique that is related to the
`cardiac cycle. The cardiac cycle is the substantially periodic
`repetition of events that occur, for example, during heart-
`beats. The cardiac cycle may include a systole period and
`diastole period. The cardiac cycle may include pressure
`changes in the ventricles, pressure changes in the atria, vol-
`ume changes in the ventricles, volume changes in the atria,
`opening and closing of heart valves, heart sounds, and other
`cyclic events. In some embodiments, the heart may enter a
`non-periodic state, for example, in certain types of arrhythmia
`and fibrillation.
`
`Asused herein, “cardiac cycle modulation” will refer to the
`modulation techniques generally correlated to the cardiac
`cycle. It will be understood that cardiac cycle modulation
`may include modulation aligned with pulses of the heart,
`pulses of a particular muscle group, other suitable pulses, any
`other suitable physiological cyclical function, or any combi-
`nation thereof. In some embodiments, the system may use a
`cardiac cycle modulation with a period on the order of the
`cardiac cycle period. For example, the cardiac cycle modula-
`tion may repeat every cardiac cycle. In some embodiments,
`the system may use a cardiac cycle modulation with a period
`on the order of some multiple of the cardiac cycle period. For
`example, the cardiac cycle modulation may repeat every three
`cardiac cycles. In some embodiments, the cardiac cycle
`modulation may relate to both a cardiac cycle and a respira-
`tory cycle. The cardiac cycle and the respiratory cycle may
`have a time varying phase relationship. It will be understood
`that cardiac cycle modulation techniques, while generally
`related to the cardiac cycle, may not necessarily be precisely
`correlated to the cardiac cycle and may be related to prede-
`termined parameters, other physiological parameters, other
`physiological cycles, external triggers (e.g., respiration), user
`input, other suitable techniques, or any combination thereof.
`
`As used herein, “drive cycle modulation” (described
`below) will refer to a relatively higher frequency modulation
`technique that the system may use to generate one or more
`wavelengths of intensity signals. Cardiac cycle modulation
`may have a period of, for example, around 1 second, while
`drive cycle modulation may have a period around, for
`example, 1.6 milliseconds.
`
`In some embodiments, conventional servo algorithms may
`beused in addition to any combination of cardiac cycle modu-
`lation and drive cycle modulation. Conventional servo algo-
`rithms may adjust the light drive signals due to, for example,
`ambient light changes, emitter and detector spacing changes,
`sensor positioning, other suitable parameters, or any combi-
`nation thereof. Generally, conventional servo algorithms vary
`parameters at a slower rate than cardiac cycle modulation. For
`example, a conventional servo algorithm may adjust drive
`signal brightness due to ambient light every several seconds.
`The system may use conventional servo algorithms in part to
`keep received signal levels within the range of an analog to
`digital converter’s dynamic range. For example, a signal with
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`amplitudes that are large may saturate an analog to digital
`convertor. In response to a signal with high amplitudes, the
`system may reduce emitter brightness. In a further example,
`the quality of a low amplitude signal may be degraded by
`quantization noise by an analog to digital converter. In
`response, the system may increase the emitter brightness.
`
`In some embodiments, a technique to remove ambient and
`background signals may be used in addition to or in place of
`a power saving light modulation scheme. In a drive cycle
`modulation technique, the system may cycle light output at a
`rate significantly greater than the cardiac cycle. For example,
`a drive cycle modulation cycle may include the system turn-
`ing on a first light source, followed by a “dark™ period, fol-
`lowed by a second light source, followed by a “dark™ period.
`The system may measure the ambient light detected by the
`detector during the “dark” period and then subtract this ambi-
`ent contribution from the signals received during the first and
`second “on” periods. In some embodiments, drive cycle
`modulation may be implemented using time division multi-
`plexing as described above, code division multiplexing, car-
`rier frequency multiplexing, phase division multiplexing,
`feedback circuitry, DC restoration circuitry, any other suit-
`able technique, or any combination thereof. For example, the
`system may use frequency division multiplexing in a drive
`cycle modulation technique. The cardiac cycle modulation
`may represent a lower frequency envelope function on the
`higher frequency drive cycle. For example, cardiac cycle
`modulation may be an envelope on the order of 1 Hz super-
`imposed on a 1 kHz sine wave drive cycle modulation.
`
`In some embodiments, the system may use various cardiac
`cycle modulation schemes to adjust the brightness of a light
`source controlled by the light drive signal used in determining
`physiological parameters. The system may modulate the
`brightness of the light source using a periodic waveform, for
`example, a sinusoidal or triangle wave. The period of the
`waveform may be substantially related to the cardiac pulse
`rate, for example, in a one-to-one relationship, a two-to-one
`relationship, any other suitable relationship, or any suitable
`combination thereof. The system may align the peak of