IJS007415298B2
`
`(12) United States Patent
`US 7,415,298 B2
`Casciani et a].
`(45) Date of Patent:
`*Aug. 19, 2008
`
`(10) Patent No.:
`
`(54)
`
`(75)
`
`PULSE OXIMETER AND SENSOR
`OPTIMIZED FOR LOW SATURATION
`
`Inventors: James R. Casciani, Cupertino, CA (US);
`Paul D. Mannheimer, Belmont, CA
`(US); Steve L. Nierlich, Oakland, CA
`(US); Stephen J. Ruskewicz,
`Kensington, CA (US)
`
`(73)
`
`Assignee:
`
`Nellcor Puritan Bennett Inc.,
`Pleasanton, CA (US)
`
`(*)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(58) Field of Classification Search ................. 600/310,
`600/322, 323, 330, 331, 336, 338
`See application file for complete search history.
`References Cited
`
`(56)
`
`U.S. PATENT DOCUMENTS
`
`3,638,640 A *
`
`2/1972 Shaw ......................... 600/323
`
`(Continued)
`FOREIGN PATENT DOCUMENTS
`
`EP
`
`0522 674 Al
`
`1/1993
`
`(Continued)
`OTHER PUBLICATIONS
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`This patent is subject to a terminal dis-
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`
`Reynolds et a1., “Diffuse reflectance from a finite blood medium:
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`v01. 15, N0. 9, Sep. 1976.
`
`(21)
`
`Appl. No.: 11/710,084
`
`(22)
`
`Filed:
`
`Feb. 23, 2007
`
`(65)
`
`(60)
`
`Prior Publication Data
`
`US 2007/0156039 A1
`
`Jul. 5, 2007
`
`Related U.S. Application Data
`
`Division of application No. 10/698,962, filed on Oct.
`30, 2003, which is a continuation of application No.
`09/882,371, filed on Jun. 14, 2001,now Pat. No. 6,662,
`033, which is a continuation ofapplication No. 09/033,
`413, filed on Jan. 6, 1998, now Pat. No. 6,272,363,
`which is a continuation of application No. 08/413,578,
`filed on Mar. 30, 1995, now Pat. No. 5,782,237, which
`is a continuation-in-part of application No. 08/221,
`911, filed on Apr. 1, 1994, now Pat. No. 5,421,329.
`
`Int. Cl.
`
`(51)
`
`(2006.01)
`A6IB 5/1464
`(2006.01)
`A6IB 5/1455
`U.S. Cl.
`....................................... 600/338; 600/323
`
`(52)
`
`(Continued)
`
`Primary ExamineriEric F Winakur
`
`(57)
`
`ABSTRACT
`
`A pulse oximeter sensor with a light source optimized for low
`oxygen saturation ranges and for maximizing the immunity to
`perturbation induced artifact. Preferably, a red and an infrared
`light source are used, with the red light source having a mean
`wavelength between 700-790 nm. The infrared light source
`can have a mean wavelength as in prior art devices used on
`patients with high saturation. The sensor ofthe present inven-
`tion is further optimized by arranging the spacing between the
`light emitter and light detectors to minimize the sensitivity to
`perturbation induced artifact. The present invention opti-
`mizes the chosen wavelengths to achieve a closer matching of
`the absorption and scattering coefficient products for the red
`and IR light sources. This optimization gives robust readings
`in the presence ofperturbation artifacts including force varia-
`tions, tissue variations and variations in the oxygen saturation
`itself.
`
`16 Claims, 14 Drawing Sheets
`
`E.
`
`“5,.“ IO
`85;
`‘2
`(291—
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`{:9—
`§§5
`
`
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`€395
`2
`;
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`
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`600
`
`650
`
`700
`
`850
`800
`750
`WAVELENGTH (nm)
`
`900
`
`950
`
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`40mm
`0% SAT
`moo
`
`APPLE 1019
`
`APPLE 1019
`
`1
`
`

`

`US 7,415,298 B2
`
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`3,847,483 A
`4,114,604 A
`4,223,680 A
`4,281,645 A
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`4,623,248 A
`4,700,708 A
`4,714,341 A
`4,859,057 A
`4,908,762 A
`4,938,218 A
`4,975,581 A
`5,058,588 A
`5,109,849 A
`5,188,108 A
`5,247,932 A
`5,253,646 A
`5,299,570 A
`5,353,791 A
`5,355,880 A
`5,385,143 A
`5,402,778 A
`5,413,100 A
`5,419,321 A
`5,421,329 A
`5,431,159 A *
`5,494,032 A
`5,497,769 A
`5,242,545 A
`5,575,285 A
`5,772,589 A
`5,782,237 A *
`5,782,756 A
`5,782,757 A
`5,823,950 A
`5,902,235 A *
`6,011,986 A
`6,256,523 B1
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`6,334,065 B1
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`6,584,336 B1
`6,606,511 B1
`6,662,033 B2
`6,678,543 B2
`6,684,090 B2
`6,714,804 B2
`6,770,028 B1
`6,792,300 B1
`6,813,511 B2
`
`11/1974 Shaw et a1.
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`10/1983 Wilber
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`
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`
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`
`2
`
`

`

`US 7,415,298 B2
`
`Page 3
`
`Flewelling, “Noninvasive Optical Monitoring,” The Biomedical
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`
`* cited by examiner
`
`3
`
`

`

`US. Patent
`
`Aug. 19, 2008
`
`Sheet 1 of 14
`
`US 7,415,298 B2
`
`I00
`
`prOZi '0
`
`BHb;
`
`‘RED"
`
`"INFRARED"
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`

`

`US. Patent
`
`Aug. 19, 2008
`
`Sheet 2 of 14
`
`US 7,415,298 B2
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`

`

`US. Patent
`
`Aug. 19, 2008
`
`Sheet 3 of 14
`
`US 7,415,298 B2
`
`
`
`FIG. 5.
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`
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`

`

`U.S. Patent
`
`Aug. 19, 2008
`
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`US 7,415,298 B2
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`U.S. Patent
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`Aug. 19, 2008
`
`Sheet 5 of 14
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`US 7,415,298 B2
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`Aug. 19, 2008
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`US. Patent
`
`Aug. 19, 2008
`
`Sheet 7 0f 14
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`US 7,415,298 B2
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`

`US. Patent
`
`Aug. 19, 2008
`
`Sheet 8 of 14
`
`US 7,415,298 B2
`
`
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`U.S. Patent
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`Aug. 19, 2008
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`Sheet 9 of 14
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`US 7,415,298 B2
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`US. Patent
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`Aug. 19, 2008
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`Sheet 10 of 14
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`US 7,415,298 B2
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`US. Patent
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`Sheet 11 of 14
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`US 7,415,298 B2
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`U.S. Patent
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`Aug. 19, 2008
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`Sheet 13 of 14
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`US 7,415,298 B2
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`1
`PULSE OXIMETER AND SENSOR
`OPTIMIZED FOR LOW SATURATION
`
`CROSS-REFERENCES TO RELATED
`APPLICATIONS
`
`This application is a divisional ofUS. application Ser. No.
`10/698,962, filed Oct. 30, 2003, which is a continuation of
`US. application Ser. No. 09/882,371, filed Jun. 14, 2001 , now
`US. Pat. No. 6,662,033, which is a continuation of US.
`application Ser. No. 09/003,413, filed Jan. 6, 1998, now US.
`Pat. No. 6,272,363, which is a continuation ofU.S. applica-
`tion Ser. No. 08/413,578, filed Mar. 30, 1995, now US. Pat.
`No. 5,782,237, which is a continuation-in-part ofU.S. appli-
`cation Ser. No. 08/221,911, filedApr. 1, 1994, now US. Pat.
`No. 5,421,329, the disclosures of which are incorporated
`herein by reference.
`
`STATEMENT AS TO RIGHTS TO INVENTIONS
`MADE UNDER FEDERALLY SPONSORED
`RESEARCH OR DEVELOPMENT
`
`NOT APPLICABLE
`
`REFERENCE TO A “SEQUENCE LISTING,” A
`TABLE, OR A COMPUTER PROGRAM LISTING
`APPENDIX SUBMITTED ON A COMPACT DISK
`
`NOT APPLICABLE
`
`BACKGROUND OF THE INVENTION
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`Pulse oximetry is used to continuously monitor the arterial
`blood oxygen saturation of adults, pediatrics and neonates in
`the operating room, recovery room, intensive care units, and
`increasingly on the general floor. A need exists for pulse
`oximetry in the delivery room for monitoring the oxygen
`status of a fetus during labor and delivery, and for monitoring
`the oxygen status of cardiac patients.
`Pulse oximetry has traditionally been used on patient popu-
`lations where arterial blood oxygen saturation is typically
`greater than 90%, i.e., more than 90% ofthe functional hemo-
`globin in the arterial blood is oxyhemoglobin and less than
`10% is reduced hemoglobin. Oxygen saturation in this patient
`population rarely drops below 70%. When it does drop to
`such a low value, an unhealthy clinical condition is indicated,
`and intervention is generally called for. In this situation, a
`high degree of accuracy in the estimate of saturation is not
`clinically relevant, as much as is the trend over time.
`Conventional two wavelength pulse oximeters emit light
`from two Light. Emitting Diodes (LEDs) into a pulsatile
`tissue bed and collect the transmitted light with a photodiode
`positioned on an opposite surface (transmission pulse oxim-
`etry), or an adjacent surface (reflectance pulse oximetry). The
`LEDs and photodetector are housed in a reusable or dispos-
`able sensor which connects to the pulse oximeter electronics
`and display unit. The “pulse” in pulse oximetry comes from
`the time varying amount of arterial blood in the tissue during
`the cardiac cycle, and the processed signals from the photo-
`detector create the familiar plethysmographic waveform due
`to the cycling light attenuation. For estimating oxygen satu-
`ration, at least one ofthe two LEDs ’ primary wavelength must
`be chosen at some point in the electromagnetic spectrum
`where the absorption of oxyhemoglobin (HbOZ) differs from
`the absorption of reduced hemoglobin (Hb). The second of 65
`the two LEDs’ wavelength must be at a different point in the
`spectrum where, additionally,
`the absorption differences
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`between Hb and HbO2 are different from those at the first
`wavelength. Commercial pulse oximeters utilize one wave-
`length in the near red part of the visible spectrum near 660
`nanometers (nm), and one in the near infrared part of the
`spectrum in the range of 880-940 nm (See FIG. 1). As used
`herein, “red” wavelengths or “red” spectrum will refer to the
`600-800 nm portion of the electromagnetic spectrum; “near
`red”, the 600-700 nm portion; “far red”, the 700-800 nm
`portion; and “infrared” or “near infrared”, the 800-1000 nm
`portion.
`Photocurrents generated within the photodetector are
`detected and processed for measuring the modulation ratio of
`the red to infrared signals. This modulation ratio has been
`observed to correlate well to arterial oxygen saturation as
`shown in FIG. 2. Pulse oximeters and pulse oximetry sensors
`are empirically calibrated by measuring the modulation ratio
`over a range of in vivo measured arterial oxygen saturations
`(SaOZ) on a set ofpatients, healthy volunteers or animals. The
`observed correlation is used in an inverse manner to estimate
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`saturation (SpOZ) based on the real-time measured value of
`modulation ratios. (As used herein, SaO2 refers to the in vivo
`measured functional saturation, while SpO2 is the estimated
`functional saturation using pulse oximetry.)
`The choice of emitter wavelengths used in conventional
`pulse oximeters is based on several factors including, but not
`limited to, optimum signal transmission through blood per-
`fused tissues, sensitivity to changes in arterial blood oxygen
`saturation, and the intensity and availability of commercial
`LEDs at the desired wavelengths. Traditionally, one of the
`two wavelengths is chosen from a region of the absorption
`spectra (FIG. 1) where the extinction coefficient of HbO2 is
`markedly different from Hb. The region near 660 nm is where
`the ratio of light absorption due to reduced hemoglobin to that
`ofoxygenated hemoglobin is greatest. High intensity LEDs in
`the 660 nm region are also readily available. The IR wave-
`length is typically chosen near 805 nm (the isosbestic point)
`for numerical convenience, or in the 880-940 nm spectrum
`where additional sensitivity can be obtained because of the
`inverse absorption relationship of Hb and HbOZ. Unfortu-
`nately, pulse oximeters which use LED wavelengths paired
`from the 660 nm band and 900 nm bands all show diminished
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`accuracy at low oxygen saturations.
`
`BRIEF SUMMARY OF THE INVENTION
`
`According to the invention, more accurate estimates of low
`arterial oxygen saturation using pulse oximetry are achieved
`by optimizing a wavelength spectrum of first and second light
`sources so that the saturation estimates at low saturation
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`values are improved while the saturation estimates at high
`saturation values are minimally adversely affected as com-
`pared to using conventional first and second wavelength spec-
`trums. It has been discovered that calculations at low satura-
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`tion can be significantly improved if the anticipated or
`predicted rates of absorption and scattering of the first wave-
`length spectrum is brought closer to, optimally equal to, the
`anticipated or predicted rates of absorption and scattering of
`the second wavelength spectrum than otherwise exists when
`conventional wavelength spectrum pairs are chosen, such as
`when conventionally using a first wavelength centered near
`660 nm and a second wavelength centered anywhere in the
`range of 880 nm-940 nm.
`The present invention solves a long felt need for a pulse
`oximeter sensor and system which provides more accurate
`estimates of arterial oxygen saturation at low oxygen satura-
`tions, i.e. saturations equal to or less than 80%, 75%, 70%,
`65%, or 60%, than has heretofore existed in the prior art. The
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`US 7,415,298 B2
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`3
`sensor and system is particularly useful for estimating arterial
`saturation of a living fetus during labor where the saturation
`range of principal
`importance and interest
`is generally
`between 15% and 65%, and is particularly useful for estimat-
`ing arterial saturation of living cardiac patients who experi-
`ence significant shunting ofvenous blood into their arteries in
`their hearts and hence whose saturation range of principle
`importance and interest is roughly between 50% and 80%. By
`contrast, a typical healthy human has a saturation greater than
`90%. The invention has utility whenever the saturation range
`of interest of a living subject, either human or animal, is low.
`In addition to providing better estimates of arterial oxygen
`saturation at low saturations, the sensor, monitor, and system
`of the invention further provide better and more accurate
`oxygen saturation estimates when perturbation induced arti-
`facts exist and are associated with the subject being moni-
`tored.
`
`When the rates of absorption and scattering by the tissue
`being probed by the first and second wavelength spectrums
`are brought closer together for the saturation values of par-
`ticular interest, improved correspondence and matching of
`the tissue actually being probed by the first and second wave-
`lengths is achieved, thus drastically reducing errors intro-
`duced due to perturbation induced artifacts. For example,
`when light of one wavelength is absorbed at a rate signifi-
`cantly higher than that ofthe other wavelength, the light ofthe
`other wavelength penetrates significantly further into the tis-
`sue. When the tissue being probed is particularly in-homog-
`enous, this difference in penetrations can have a significant
`adverse impact on the accuracy of the arterial oxygen satura-
`tion estimate.
`Perturbation induced artifacts include, but are not limited
`to, any artifact that has a measurable impact on the relative
`optical properties of the medium being probed. Perturbation
`induced artifacts include but are not limited to the following:
`(1) variations in the tissue composition being probed by the
`sensor from subject to subject,
`i.e., variations in the
`relative amounts of fat, bone, brain, skin, muscle, arter-
`ies, veins, etc.;
`(2) variations in the hemoglobin concentration in the tissue
`being probed, for example caused by vasal dilations or
`vasal constrictions, and any other physical cause which
`affects blood perfusion in the tissue being probed; and
`(3) variations in the amount of force applied between the
`sensor and the tissue being probed, thus affecting the
`amount of blood present in the nearby tissue.
`In one embodiment, the present invention provides a fetal
`pulse oximeter sensor with a light source optimized for the
`fetal oxygen saturation range and for maximizing the immu-
`nity to perturbation induced artifact. Preferably, a far red and
`an infrared light source are used, with the far red light source
`having a mean wavelength between 700-790 nm. The infrared
`light source can have a mean wavelength as in prior art
`devices used on patients with high saturation, i.e., between
`800- 1000 nm. As used herein, “high saturation” shall mean an
`arterial oxygen saturation greater than 70%, preferably
`greater than 75%, alternatively greater than 80%, optionally
`greater than 90%.
`The fetal sensor of the present invention is further opti-
`mized by arranging the spacing between the location the
`emitted light enters the tissue and the location the detected
`light exits the tissue to minimize the sensitivity to perturba-
`tion induced artifact.
`
`According to a preferred embodiment, electrooptic trans-
`ducers (e.g., LEDs and photodetectors) are located adjacent
`to the tissue where the light enters and exits the tissue.
`According to an alternate embodiment, the optoelectric trans-
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`ducers are located remote from the tissue, for example in the
`oximeter monitor, and optical fibers interconnect the trans-
`ducers and the tissue with the tissue being illuminated from an
`end of a fiber, and light scattered by the tissue being collected
`by an end of a fiber. Multiple fibers or fiber bundles are
`preferred.
`The present invention recognizes that the typical oxygen
`saturation value for a fetus is in the range of 5-65%, com-
`monly 15-65%, compared to the 90% and above for a typical
`patient with normal (high) saturation. In addition, a fetal
`sensor is subject to increased perturbation induced artifact.
`Another unique factor in fetal oximetry is that the sensor is
`typically inserted through the vagina and the precise location
`where it lands is not known in advance.
`
`The present invention recognizes all of these features
`unique to fetal oximetry or oximetry for low saturation
`patients and provides a sensor which optimizes the immunity
`to perturbation induced artifacts. This optimization is done
`with a trade-off on the sensitivity to changes in saturation
`value. This trade-offresults in a more reliable calculation that
`
`is not obvious to those who practice the prior art methods
`which attempt to maximize the sensitivity to changes in the
`saturation value. The improvement
`in performance that
`results from these optimizations are applicable to both reflec-
`tance and transmission pulse oximetry. An example of a fetal
`transmission pulse oximetry configuration usable with the
`present invention is described in US. patent application Ser.
`No. 07/752, 1 68, assigned to the assignee ofthe present inven-
`tion, the disclosure of which is incorporated herein by refer-
`ence. An example of a non-fetal transmission pulse oximetry
`configuration usable with the present invention is described in
`US. Pat. No. 4,830,014, assigned to the assignee of the
`present invention, the disclosure of which is incorporated
`herein by reference.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a chart of the absorption characteristics of oxy-
`hemoglobin (HbOZ) and reduced hemoglobin (Hb) versus
`wavelength showing prior art near red and infrared LED
`wavelengths;
`FIG. 2 is a graph ofred/IR modulation ratio versus oxygen
`saturation;
`FIG. 3 is a diagram illustrating light penetration through
`different layers of tissue at different distances;
`FIG. 4A is a chart of the variation in extinction and scat-
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`tering coefficients over a range of wavelengths for different
`saturation values;
`FIG. 4B is a table of the values of FIG. 4A;
`FIG. 5 is a diagram illustrating the placing of a sensor on a
`fetus;
`FIG. 6 is a graph illustrating the spectrum of an LED
`according to the present invention;
`FIGS. 7-18 are graphs showing experimental modeling of
`the modulation ratio and saturation error as a function of
`
`saturation for different red and infrared wavelength combi-
`nations;
`FIGS. 19-23 are charts illustrating saturation and the error
`due to applied force for different combinations of emitter
`wavelength and emitter-detector spacing from experiments
`done on sheep;
`FIGS. 24 and 25 are diagrams illustrating the construction
`of a sensor according to the present invention;
`FIGS. 26A-B are diagrams of a single package, dual emit-
`ter package used in the present invention; and
`FIG. 27 is a block diagram ofa pulse oximeter according to
`the present invention.
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`US 7,415,298 B2
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`5
`DETAILED DESCRIPTION OF THE INVENTION
`
`An understanding of the design of the fetal sensor accord-
`ing to the present invention requires an understanding of the
`environment in which the sensor will operate. FIG. 3 illus-
`trates the layers of tissue in a typical fetus location where a
`sensor may be applied. Typically, there would be a first layer
`of skin 12, perhaps followed by a layer of fat 14, a layer of
`muscle 16, and a layer ofbone 18. This is a simplified view for
`illustration purposes only. The contours and layers can vary at
`different locations. For instance, bone would be closer to the
`surface on the forehead, as opposed to closer muscle on the
`neck. Such variations in sites can produce the first type of
`perturbation artifact mentioned in the summaryiartifact due
`to variations in tissue composition.
`The general paths of light from an emitter 20 to a photo-
`detector 22 are illustrated by arrows 24 and 26. Arrow 24
`shows light which passes almost directly from emitter 20 to
`detector 22, basically shunted from one to the other, passing
`through very little blood perfused tissue. Arrow 26, on the
`other hand, illustrates the deeper penetration of another path
`of the light. The depth of penetration is affected by the wave-
`length ofthe light and the saturation. At low saturation, infra-
`red light penetrates deeper than near red, for instance. The
`deeper penetration can result in an undesirable variation
`between the infrared and red signals, since the IR signal will
`pass through more different layers.
`Also illustrated in FIG. 3 is the effect ofusing an emitter 28
`which is spaced on the tissue at a greater distance from a
`detector 30 than the first pair 20, 22 described. As can be seen,
`this greater separation results in the penetration of a larger
`amount of tissue, as indicated by arrows 32 and 34. Thus, the
`greater spacing increases the depth of penetration, although it
`will reduce the intensity of the signal received at the detector
`due to more attenuation from more ofthe light being absorbed
`in the tissue and the greater light propagation distances
`involved.
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`The second type ofperturbation mentioned in the summary
`is variations in the concentration of blood in the tissue from
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`patient to patient or over time. A lower concentration results
`in less absorption,
`increasing the penetration depth. The
`inventors estimate that the mean penetration depth ofphotons
`in a medium is related to the product of the absorption and
`scattering coefficients, and this estimate is consistent with the
`findings of Weiss et al., “Statistics of Penetration Depth of
`Photons Re-emitted from Irradiated Tissue”, Journal ofMod—
`ern Optics, 1989, Vol. 36, No. 3, 349-359, 354, the disclosure
`of which is incorporated herein by reference.
`Absorption of light in tissue in the visible and near infrared
`region of the electromagnetic spectrum is dominated by the
`absorption characteristics of hemoglobin. Absorption coeffi-
`cients of hemoglobin can be found in the literature, for
`example Zijlstra et al., “Absorption spectra of human fetal
`and adult oxyhemoglobin, de-oxyhemoglobin, carboxyhe-
`moglobin and methemoglobin”, Clinical Chemistry, 37/9,
`1633-1638, 1991 (incorporated herein by reference). Mea-
`sured scattering coefficients of tissue are influenced by the
`methodology of measurement and the model used to fit the
`data, although there is general agreement in the relative sen-
`sitivity to wavelength regardless ofmethod. Tissue scattering
`coefficients used by the inventors are based on diffusion
`theory, and are taken from Schmitt, “Simple photon diffusion
`analysis of the effects of multiple scattering on pulse oxim-
`etry”, IEEE Transactions on Biomedical Engineering, Vol.
`38, No. 12, December 1991, the disclosure ofwhich is incor-
`porated herein by reference.
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`FIG. 4A is a graph showing the product of the absorption
`and scattering coefficients for 0%, 40%, 85% and 100% satu-
`rations for wavelengths between 600 nm and 1,000 nm. For
`85-100% tissue oxygen saturation, good balance or correla-
`tion exists between the product of the absorption and scatter-
`ing coefficients of convention