US 20110237911A1
`a9y United States
`
`a2y Patent Application Publication o) Pub. No.: US 2011/0237911 Al
`
`Lamego et al. 43) Pub. Date: Sep. 29, 2011
`(54) MULTIPLE-WAVELENGTH (60) Provisional application No. 61/318,735, filed on Mar.
`PHYSIOLOGICAL MONITOR 29, 2010, provisional application No. 60/586,069,
`filed on Jul. 7, 2004.
`(75) Inventors: Marcelo M. Lamego, Coto De
`Caza, CA (US); Mohamed Diab, Publication Classification
`Weber, Lagua ills, CA (U (51 1nt.CL
`Ammar AL-Ali, Tustin, CA (US); AGIB 51455 (2006.01)
`Massi Joe E. Kiani, Laguna (52) US.ClL oo 600/323
`Niguel, CA (US)
`(57) ABSTRACT
`
`(73) Assignee: MASIMO LABORATORIES,
`
`- A physiological monitor for determining blood oxygen satu-
`INC., Irvine, CA (US)
`
`ration of a medical patient includes a sensor, a signal proces-
`sor and a display. The sensor includes at least three light
`emitting diodes. Each light emitting diode is adapted to emit
`light of a different wavelength. The sensor also includes a
`detector, where the detector is adapted to receive light from
`the three light emitting diodes after being attenuated by tis-
`sue. The detector generates an output signal based at least in
`(63) Continuation-in-part of application No. 12/045,309, part upon the received light. The signal processor determines
`filed on Mar. 10, 2008, which is a continuation of blood oxygen saturation based at least upon the output signal,
`application No. 11/139,291, filed on May 27, 2005, and the display provides an indication of the blood oxygen
`now Pat. No. 7,343,186. saturation.
`
`(21) Appl. No.: 13/073,778
`(22) Filed: Mar. 28, 2011
`
`Related U.S. Application Data
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`MULTIPLE-WAVELENGTH
`PHYSIOLOGICAL MONITOR
`
`RELATED APPLICATIONS
`
`[0001] This application claims priority from U.S. Provi-
`sional No. 61/318,735, filed Mar. 29, 2010 and is a continu-
`ation-in-part of U.S. application Ser. No. 12/045,309, filed
`Mar. 10,2008, which is a continuation of U.S. application Ser.
`No. 11/139,291, filed May 27, 2005, now U.S. Pat. No. 7,343,
`186, which claims priority from U.S. Provisional No. 60/586,
`069, filed Jul. 7, 2004. All of the foregoing are expressly
`incorporated by reference herein.
`
`BACKGROUND
`
`[0002] 1. Field
`
`[0003] The present invention relates to the field of signal
`processing. More specifically, the present invention relates to
`the processing of measured signals which contain a primary
`signal portion and a secondary signal portion for the removal
`or derivation of either signal portion. The present invention is
`especially useful for physiological monitoring systems,
`including blood oxygen saturation measurement systems and
`oximeters.
`
`[0004] 2. Description of the Related Art
`
`[0005] Blood oxygen saturation measurement systems,
`oximeters, and physiological monitors of the prior art gener-
`ally utilize two different wavelengths of light to determine a
`patient’s blood oxygen saturation level. In general, such sys-
`tems provide two wavelengths of light to a target location on
`apatient’s body. The systems then measure at least one signal
`indicative of the transmission or reflection of the two light
`wavelengths with respect to the tissue at the target location.
`[0006] One such physiological monitor is taught by Diab et
`al. in U.S. Pat. No. 5,632,272, incorporated by reference
`herein in its entirety. One embodiment of Diab’s physiologi-
`cal monitor provides light having a red wavelength and light
`having an infrared wavelength to one side of a patient’s finger.
`A detector on the opposite side of the patient’s finger mea-
`sures the red and infrared wavelength light transmitted
`through the patient’s finger and generates a measurement
`signal. A processor analyzes the measurement signal to deter-
`mine red and infrared component signals. Possible saturation
`values are input to a saturation equation module which pro-
`vides reference coefficients. The red or infrared component
`signal is processed with the reference coefficients to yield
`reference signal vectors.
`
`[0007] The reference signal vectors and the red or infrared
`component signal are processed by a correlation canceller to
`generate output vectors. The output vectors are input into a
`master power curve module, which provides a blood oxygen
`saturation value for each possible saturation value input to the
`saturation equation module. The patient’s blood oxygen satu-
`ration is determined based upon the power curve module
`output.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0008] FIG.1illustrates an arterial blood oxygen saturation
`curve in accordance with the prior art.
`
`[0009] FIG. 2 illustrates an arterial blood oxygen saturation
`curve of a multi-wavelength physiological monitor in accor-
`dance with one embodiment of the present invention.
`
`[0010] FIG. 3 is an example of a physiological monitor in
`accordance with one embodiment of the present invention.
`
`Sep. 29, 2011
`
`[0011] FIG. 3A illustrates one embodiment of the low noise
`emitter current driver of the physiological monitor of FIG. 3.
`[0012] FIG. 4 illustrates one embodiment of the front end
`analog signal conditioning circuitry and the analog to digital
`conversion circuitry of the physiological monitor of FIG. 3.
`[0013] FIG. 5 illustrates one embodiment of the digital
`signal processing circuitry of FIG. 3.
`
`[0014] FIG. 6 illustrates one embodiment of additional
`operations performed by the digital signal processing cir-
`cuitry of FIG. 3.
`
`[0015] FIG. 7 illustrates one embodiment of the demodu-
`lation module of FIG. 6.
`
`[0016] FIG. 8 illustrates one embodiment of the sub-sam-
`pling module of FIG. 6.
`
`[0017] FIG. 9 illustrates one embodiment of the statistics
`module of FIG. 6.
`
`[0018] FIG. 10 illustrates a block diagram of the operations
`of one embodiment of the saturation transform module of
`FIG. 6.
`
`[0019] FIG. 10A illustrates an adaptive noise filter that may
`be used as the multi-variate process estimator of FIG. 10.
`[0020] FIG. 11 illustrates a saturation transform curve in
`accordance with one embodiment of the present invention.
`[0021] FIG. 12 illustrates a block diagram of the operation
`of one embodiment of the saturation calculation module of
`FIG. 6.
`
`DETAILED DESCRIPTION
`
`[0022] Spectroscopy is a common technique for measuring
`the concentration of organic and some inorganic constituents
`of a solution. The theoretical basis of this technique is the
`Beer-Lambert law, which states that the concentration ¢, of an
`absorbent in solution can be determined by the intensity of
`light transmitted through the solution, knowing the path-
`length d,, the intensity of the incident light I, ,, and the
`extinction coeflicient €, , at a particular wavelength A. In
`generalized form, the Beer-Lambert law is expressed as:
`
`I = foaeH02 )
`
`n @
`
`Hox = Z &yt Ci
`
`i=1
`
`[0023] where p,, is the bulk absorption coefficient and
`represents the probability of absorption per unit length. The
`minimum number of discrete wavelengths that may be
`required to solve Equations 1 and 2 is at least the number of
`significant absorbers that are present in the solution. Least
`squares or other estimation techniques can be used to
`approximate a solution to these equations for underdeter-
`mined or overdetermined systems. The system of equations is
`underdetermined if there are fewer equations or wavelengths
`than unknowns or significant absorbers (e.g., blood constitu-
`ents). Conversely, the system is overdetermined if there are
`more equations than unknowns.
`
`[0024] A practical application of this technique is pulse
`oximetry, which utilizes a noninvasive sensor to measure
`blood oxygen saturation (SpO,) and pulse rate. A multi-
`wavelength physiological monitor in accordance with one
`embodiment determines blood oxygen saturation by propa-
`gating multi-wavelength energy through a medium, such as a
`portion of a patient’s body where blood flows close to the
`
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`body surface. For example, in one embodiment, energy is
`propagated through an ear lobe, a digit (such as a finger or
`toe), a forehead, or a scalp (such as a fetus’s scalp). An
`attenuated signal is measured after energy propagation
`through, or reflection from the medium. The physiological
`monitor determines the saturation of oxygenated blood in the
`patient based at least in part upon the measured signal.
`[0025] It is well known by those of skill in the art that
`freshly oxygenated blood is pumped at high pressure from the
`heart into the arteries for use by the body. The volume of
`blood in the arteries varies with the heartbeat. This variation
`gives rise to a variation in energy absorption at the heartbeat
`rate, or the pulse.
`
`[0026] Oxygen depleted, or deoxygenated, blood is
`returned to the heart through the veins with unused oxygen-
`ated blood. Unlike the arteries, the volume of blood in the
`veins varies with the rate of breathing, which is typically
`much slower than the heartbeat. Since the blood pressure in
`the veins is typically much lower than that of the arteries, the
`volume of blood in the veins varies in response to motion,
`such as a patient raising or lowering her arm. Changes in
`blood volume within the veins cause changes in vein thick-
`nesses. Therefore, when there is no motion induced variation
`in the thickness of the veins, venous blood causes a low
`frequency variation in energy absorption, which is related to
`the rate of breathing. However, when erratic, motion-induced
`variations in the thickness of the veins occur, the low fre-
`quency variation in absorption is coupled with an erratic
`variation in energy absorption due to the erratic motion.
`[0027] In one embodiment, absorption measurements are
`based upon the transmission of energy through a medium. In
`one embodiment, multiple light emitting diodes (LEDs) are
`positioned on one side of a portion of the body where blood
`flows close to the body’s surface, such as a finger, and a
`photodetector is positioned on the opposite side of the sur-
`face. In another embodiment one or more such LEDs emit
`light of different wavelengths. In one embodiment, one LED
`emits a visible wavelength, such as red, and the other LED
`emits an infrared wavelength. However, one skilled in the art
`will realize that other wavelength combinations could be
`used.
`
`[0028] The finger comprises skin, tissue, muscle, both arte-
`rial blood and venous blood, fat, etc., each of which absorbs
`light energy differently due to different absorption coeffi-
`cients, different concentrations, different thicknesses, and
`changing optical pathlengths. When the patient is not moving,
`absorption is substantially constant except for variations due
`to the flow of blood through the skin, tissue, muscle, etc. A
`constant attenuation can be determined and subtracted from
`the measured signal via traditional filtering techniques. How-
`ever, when the patient moves, perturbations such as changing
`optical pathlengths occur. Such perturbations may be due to
`movement of background fluids, (such as venous blood,
`which has a different saturation than arterial blood). There-
`fore, the measured signal becomes erratic. Erratic, motion-
`induced noise typically cannot be predetermined and sub-
`tracted from the measured signal via traditional filtering
`techniques. Thus, determining the oxygen saturation of arte-
`rial blood and venous in erratic, motion-induced noise envi-
`ronments, blood becomes more difficult.
`
`[0029] In one embodiment, a physiological monitor mea-
`sures light transmission through a patient’s finger to deter-
`mine arterial blood oxygen saturation. In some cases, how-
`ever, the measured light signal contains noise, or other
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`Sep. 29, 2011
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`secondary signal, due to an event, such as patient movement
`during signal measurement. In such case, the signal measured
`by the physiological monitor includes a primary portion,
`related to the blood oxygen saturation of the patient, and a
`secondary portion, related to the noisy, erratic, motion-in-
`duced secondary signal. The physiological monitor processes
`the measured signal to determine the patient’s blood oxygen
`saturation based upon the signal’s primary portion.
`
`[0030] In one embodiment, the physiological monitor uti-
`lizes a processor to determine a secondary reference signal
`n'(t) or N, The secondary reference signal n'(t) is used to
`determine the primary portion of the measured signal. In one
`embodiment, the secondary reference signal n'(t) is input to a
`multi-variate process estimator, which removes the erratic,
`motion-induced secondary signal portions from the measured
`signal. In another embodiment, the processor determines a
`primary signal reference signal s'(t) which is used for display
`purposes or for input to a multi-variate process estimator to
`derive information about patient movement and venous blood
`oxygen saturation.
`
`[0031] FIG. 1illustrates an arterial blood oxygen saturation
`curve 100 representative of the sensitivity of blood oxygen
`saturation systems of the prior art. Such systems utilize two
`different wavelengths of light (such as red and infrared wave-
`length light) to determine blood oxygen saturation. The wave-
`lengths used may be, for example, about 660 nm and about
`905 nm. At 660 nm, deoxyhemoglobin has a higher absorp-
`tion than oxyhemoglobin, while the reverse is the case at 905
`nm. This difference in absorption between oxyhemoglobin
`and deoxyhemoglobin allows for the calculation of blood
`oxygen saturation levels.
`
`[0032] InFIG. 1, arterial blood oxygen saturation (Sa0,) is
`represented on the y-axis of the curve 100. The x-axis repre-
`sents ratios of a red wavelength light transmission signal and
`an infrared wavelength light transmission signal. An ideal
`saturation curve 102 would be highly accurate at all values.
`However, due to the limitations of using only two wave-
`lengths of light, such systems typically operate between a
`lower range curve 104 and an upper range curve 106. Such
`blood oxygen saturation systems typically exhibit highly
`accurate, low tolerance 107 measurements at high saturation
`values 108, but at lower saturation values 110, that accuracy
`decreases, and increased tolerance 111 results.
`
`[0033] The arterial blood oxygen saturation curve 120 of a
`multi-wavelength physiological monitor in accordance with
`one embodiment of the present invention is shown in FIG. 2.
`The multi-wavelength system utilizes at least three different
`wavelengths of light (A, A,, . . . A,) to determine blood
`oxygen saturation. Arterial blood oxygen saturation (S,0,) is
`represented on the y-axis of the curve 120. The x-axis repre-
`sents ratios of composite signals, each comprising signals
`based upon the light transmission of the various light wave-
`lengths (A, A, . . . A,,). Each ratio r can be expressed by:
`
`n
`Z @;NPpus,i
`=
`
`re
`
`BiNPrus,i
`
`s T
`
`where n is the number of wavelengths of light utilized by the
`multi-wavelength physiological monitor, NPy, ; is the nor-
`malized plethysmographic waveform of the ith wavelength
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`Petitioner WHOOP, Inc. Ex1062
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`light source, and ¢, and p, are vector coefficients of known
`constants that are determined based upon fitting and/or cali-
`bration using experimental data and/or model(s).
`
`[0034] The curve 120 contains lower and upper limit range
`curves 122, 124. However, the lower and upper limit range
`curves 122, 124 of the multi-wavelength physiological moni-
`tor are more linear than the lower and upper range curves 104,
`106 of the dual-wavelength blood oximeter described above,
`as illustrated in FIG. 1. In addition, the lower and upper limit
`range curves 122, 124 of the multi-wavelength physiological
`monitor exhibit higher accuracy and lower tolerance 126 than
`the dual-wavelength blood oximeter of FIG. 1, particularly at
`lower arterial blood oxygen saturation levels. In certain cases,
`such as during the monitoring of neonates, or persons with
`low blood-oxygen saturation, it is desirable for the physi-
`ological monitor to exhibit increased accuracy at lower satu-
`ration levels. For example, when providing oxygen to a neo-
`nate, it is often critical that neither too much oxygen nor too
`little oxygen is provided. A multi-wavelength physiological
`monitor provides improved accuracy as well as an improved
`signal-to-noise ratio at lower perfusion levels.
`
`[0035] A multi-wavelength physiological monitor provides
`additional advantages over a two-wavelength device, as well.
`For example, utilizing multiple wavelengths provide a multi-
`dimensional calibration curve, which can be used to provide
`multiple degrees of freedom to compensate for variation in
`other physiologically-related parameters. As discussed
`above, when only two wavelengths are used but there are
`more than two significant absorbers in the patient’s tissue, the
`system may be underdetermined. Therefore, adding wave-
`lengths sensitive to additional significant absorbers can help
`the system compensate for the additional significant absorb-
`ers and enable more accurate calculation of the concentration
`of each absorber and the blood oxygen saturation. For
`example, an eight wavelength physiological monitor utilizes
`an eight-dimensional calibration curve, which provides eight
`degrees of freedom to compensate for various physiologi-
`cally-related parameters. Such parameters can include, for
`example, noise, motion, various hemodynamic parameters,
`and/or blood constituent concentrations and/or levels. On the
`other hand, the system may be overdetermined if, for
`example, more wavelengths are used than there are significant
`absorbers and/or wavelengths are used for constituents not
`present in the particular patient’s blood.
`
`[0036] Furthermore, traditional physiological monitors
`that utilize two light sources to derive a patient’s plethysmo-
`graphic signal generally require one light source’s wave-
`length to fall in the red spectrum and the other light source’s
`wavelength to fall in the infrared spectrum. However, a multi-
`wavelength physiological monitor advantageously provides
`the ability to utilize all infrared wavelength light sources
`and/or other non-red infrared light sources to derive an accu-
`rate plethysmographic waveform. One advantage of utilizing
`non-red wavelength light sources (for example, all infrared
`wavelength light sources) is that the multi-wavelength physi-
`ological monitor can be further configured to determine sev-
`eral blood constituent concentrations based upon signals
`measured from just the non-red light sources. For example,
`such a multi-wavelength physiological monitor can deter-
`mine levels and/or concentrations of: methemoglobin
`(MetHb), carboxyhemoglobin (COHb), low hemoglobin lev-
`els, high hemoglobin levels, bilirubin, methylene blue,
`deoxyhemoglobin, and lipids.
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`Sep. 29, 2011
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`[0037] The ability to measure a physiological parameter
`with a multi-dimensional system allows the system to com-
`pensate and account for the presence of other conditions that
`may affect the particular physiological parameter being mea-
`sured. For example, many drugs cause the level of MetHb
`within a patient’s blood to increase. MetHb also absorbs light
`at the two wavelengths used in a typical two-wavelength
`physiological monitor, which may cause inaccurate S,0,
`readings. Increased levels of MetHb generally cause blood
`oxygen concentration level (e.g., S,0,) readings to decrease.
`A two-wavelength physiological monitor would provide
`reduced readings of blood oxygen concentration, but would
`not be able to identify the cause for the decreased reading, or
`to compensate for such readings. On the other hand, a multi-
`wavelength physiological monitor, such as any of the multi-
`wavelength physiological monitors described below, could
`not only identify the cause of decreased S,0, readings, but
`also (or alternatively) compensate for such cause when cal-
`culating the patient’s S,0, level and/or signal. For example,
`to compensate for MetHb, one or more wavelengths sensitive
`to MetHb can be used. The wavelength(s) used can be, for
`example, about 760 nm and/or about 805 nm or the like. This
`allows for one or more additional equation (s) and degree(s)
`of freedom so that solving the system may account for the
`effect of the level of MetHb on blood oxygen saturation
`readings.
`
`[0038] Similarly, the system can compensate for other
`blood constituents with the addition of wavelengths sensitive
`to the other blood constituents of interest. For example, car-
`boxyhemoglobin absorbs about the same amount of 660 nm
`light as oxyhemoglobin. Therefore, a typical two-wavelength
`physiological monitor can mistake carboxyhemoglobin for
`oxyhemoglobin, which can result in normal blood oxygen
`saturation level readings even though the actual level is abnor-
`mal. For example, for every about 1% of carboxyhemoglobin
`circulating in the blood, the monitor may over read by about
`1%. Selecting one or more additional wavelengths sensitive
`to carboxyhemoglobin, for example, about 610, 630, and/or
`640 or the like, can allow the system to compensate for this
`effect. The system can compensate for other blood constitu-
`ents, such as deoxyhemoglobin or lipids, with the addition of
`wavelengths sensitive to those constituents. More wave-
`lengths can allow the system to compensate for more vari-
`ables.
`
`[0039] The various blood constituents mentioned above
`may not be orthogonal; rather they are often highly correlated
`with one another. Therefore, even using multiple wavelengths
`may not fully eliminate the effect of such constituents on the
`blood oxygen saturation level calculations, but it can reduce
`or minimize the effect and produce more accurate blood
`oxygen saturation level readings. In one embodiment, using
`eight wavelengths (including, for example, any of the wave-
`lengths mentioned herein) can provide relatively more accu-
`rate readings of blood oxygen saturation levels relative to
`other numbers of wavelengths. However, the number of
`wavelengths that is most appropriate for a particular patient
`and that will provide the most accurate readings can vary
`based on such factors as the actual concentrations of blood
`constituents in the patient’s blood, as well as the patient’s
`condition, gender, age, or the like. In addition to improving
`the accuracy of blood oxygen saturation level readings, using
`multiple wavelengths can also provide relatively more accu-
`rate measurements of other parameters, including, for
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`Petitioner WHOOP, Inc. Ex1062
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`Page 17 of 27
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`example, the concentrations of any of the blood constituents
`or other parameters mentioned herein.
`
`[0040] In one embodiment, the multi-wavelength physi-
`ological monitor compensates for conditions, such as motion
`and/or blood constituents that would cause a standard two-
`wavelength system to over- or under-react to the presence of
`such condition. Such compensation provides a more accurate,
`stable reading or a patient’s physiological condition, includ-
`ing, but not limited to, their blood oxygen concentration
`and/or plethysmographic signal.
`
`[0041] A schematic of one embodiment of a multi-wave-
`length physiological monitor for pulse oximetry is shown in
`FIGS. 3-5. FIG. 3 depicts a general hardware block diagram
`of a multi-wavelength pulse oximeter 299. A sensor 300 has
`n LEDs 302, which in one embodiment are at least three light
`emitters. The n LEDs 302 emit light of different wavelengths
`(M5 Ay, - .o ). In one embodiment, the n LEDs 302 include
`four, six, eight or sixteen LEDs of different wavelengths. In
`one embodiment, the n LEDs 302 are placed adjacent a finger
`310. A photodetector 320 receives light from the n LEDs 302
`after it has been attenuated by passing through the finger. The
`photodetector 320 produces at least one electrical signal cor-
`responding to the received, attenuated light. In one embodi-
`ment, the photodetector 320 is located opposite the n LEDs
`302 on the opposite side of the finger 310. The photodetector
`320 is connected to front end analog signal conditioning
`circuitry 330.
`
`[0042] The front end analog signal conditioning circuitry
`330 has outputs that are coupled to an analog to digital con-
`version circuit 332. The analog to digital conversion circuit
`332 has outputs that are coupled to a digital signal processing
`system 334. The digital signal processing system 334 pro-
`vides desired parameters as outputs for a display 336. Outputs
`for the display 336 include, for example, blood oxygen satu-
`ration, heart rate, and a clean plethysmographic waveform.
`[0043] The signal processing system also provides an emit-
`ter current control output 337 to a digital-to-analog converter
`circuit 338. The digital-to-analog converter circuit 338 pro-
`vides control information to emitter current drivers 340. The
`emitter drivers 340 are coupled to the n light emitters 302. The
`digital signal processing system 334 also provides a gain
`control output 342 for front end analog signal conditioning
`circuitry 330.
`
`[0044] FIG. 3A illustrates one embodiment of the drivers
`340 and the digital to analog conversion circuit 338. As
`depicted in FIG. 3 A, the digital-to-analog conversion circuit
`338 includes first and second input latches 321, 322, a syn-
`chronizing latch 323, a voltage reference 324, and a digital to
`analog conversion circuit 325. The emitter current drivers 340
`include first and second switch banks 326, 327, and n voltage
`to current converters 328. LED emitters 302 of FIG. 3 are
`coupled to the output of the emitter current drives 340.
`[0045] The driver depicted in FIG. 3A is advantageous in
`that the present inventors recognized that much of the noise in
`the oximeter 299 of FIG. 3 is caused by the LED emitters 302.
`Therefore, the emitter driver circuit of FIG. 3A is designed