US 7,415,298 B2
`(10) Patent No.:
`a2) United States Patent
`Cascianiet al.
`(45) Date of Patent:
`*Aug. 19, 2008
`
`
`US007415298B2
`
`(54) PULSE OXIMETER AND SENSOR
`OPTIMIZED FOR LOW SATURATION
`
`(58) Field of Classification Search ................. 600/310,
`600/322, 323, 330, 331, 336, 338
`See application file for complete search history.
`Inventors: James R. Casciani, Cupertino, CA (US);
`(75)
`Ref
`Cited
`56
`Paul D. Mannheimer, Belmont, CA ererences©Ne(56)
`
`
`(US); Steve L. Nierlich, Oakland, CA
`U.S. PATENT DOCUMENTS
`(US); Stephen J. Ruskewicz,
`3,638,640 A *
`2/1972 Shaw.
`cecccccsssssssseeseeeee 600/323
`Kensington, CA (US)
`.
`(Continued)
`FOREIGN PATENT DOCUMENTS
`0522 674 Al
`1/1993
`
`(73) Assignee: Nelleor Puritan Bennett Inc.,
`Pleasanton, CA (US)
`
`EP
`
`*)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`ject
`Y
`:
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`.
`.
`.
`.
`.
`This patent is subject to a terminal dis-
`claimer.
`
`(21) Appl. No.: 11/710,084
`
`(22)
`
`Filed:
`
`Feb. 23, 2007
`
`(65)
`
`Prior Publication Data
`
`US 2007/0156039 Al
`
`Jul. 5, 2007
`on
`Related U.S. Application Data
`(60) 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, whichis a continuation ofapplication No. 09/033,
`413, filed on Jan. 6, 1998, now Pat. No. 6,272,363,
`whichis a continuation ofapplication 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.
`
`(51)
`
`Int. Cl.
`(2006.01)
`AGIB 5/1464
`(2006.01)
`AGIB 5/1455
`(52) US. Ch ccc ceceecsecnecreeneeneeeeee 600/338; 600/323
`
`:
`(Continued)
`
`OTHER PUBLICATIONS
`Reynolds et al., “Diffuse reflectance from a finite blood medium:
`applications to the modelingoffiber optic catheters,’ Applied Optics,
`vol. 15, No. 9, Sep. 1976.
`
`(Continued)
`
`Primary Examiner—Eric 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 inducedartifact. Preferably, a red and an infrared
`light source are used, with the red light source having a mean
`wavelength between 700-790 nm. Theinfrared light source
`can have a mean wavelength as in prior art devices used on
`patients with high saturation. The sensorofthe present inven-
`tion is further optimized by arranging the spacing between the
`light emitter and light detectors to minimizethesensitivity to
`perturbation induced artifact. The present invention opti-
`mizes the chosen wavelengthsto achieve a closer matching of
`the absorption andscattering coefficient products for the red
`and IR light sources. This optimization gives robust readings
`in the presenceofperturbation artifacts including force varia-
`tions, tissue variations andvariationsin the oxygen saturation
`itself.
`
`16 Claims, 14 Drawing Sheets
`
`
`
`= E
`
`u (0
`Sa
`2
`QF
`ee is
`re
`a)
`Se
`=
`=
`oS
`
`4
`600
`
`100%SAT
`B5%SAT
`40% SAT
`0% SAT
`1000
`
`650
`
`700
`
`850
`800
`750
`WAVELENGTH (nm)
`
`900
`
`950
`
`1
`
`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
`4,407,290 A
`4,446,871 A
`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
`§,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 Bl
`6,272,363 B1*
`6,285,896 Bl
`6,298,253 Bl
`6,334,065 Bl
`6,397,091 B2
`6,584,336 Bl
`6,606,511 Bl
`6,662,033 B2
`6,678,543 B2
`6,684,090 B2
`6,714,804 B2
`6,770,028 Bl
`6,792,300 Bl
`6,813,511 B2
`
`11/1974 Shawetal.
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`what’s critical?”,
`in Oxygen Transport to Tissues XIV, ed. R.
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`genation,” Clinical Pediatrics, May 1992, pp. 258-273.
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`neonatal skin,” Applied Optics, vol. 34, No. 31, Nov. 1, 1995.
`
`2
`
`

`

`US 7,415,298 B2
`Page 3
`
`Flewelling, “Noninvasive Optical Monitoring,” The Biomedical
`engineering Handbook, CRCPress 1995.
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`comparedin piglets,” Am. J. Gynecol., 172 (1 (part 2)):38X6 (1995).
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`fetal pulse oximetry sensors,” European Journal of obstetrics &
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`S9-S19.
`
`Kaestle et al., “A New Family of Sensors for Pulse Oximetry,”
`Hewlett Packard Journal, pp. 1-17 (Feb. 1997).
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`Oximetry,” IEEE Transaction on Biomedical Engineering, vol. 44,
`No. 3, Mar. 1997.
`
`* cited by examiner
`
`3
`
`

`

`U.S. Patent
`
`Aug.19, 2008
`
`Sheet 1 of 14
`
`US 7,415,298 B2
`
`
`
`
`
` MOLAREXTINCTIONCOEFFICIENT
`
`1 500 550 600 650 700 750 800 650 900 950 1000
`4;
`
`|.
`
`OXY - HEMOGLOBIN
`
`REDUCED
`HEMOGLOBIN
`
`SKIN [2
`
`FIC. I
`
`4
`
`RED/IR
`MODULATION 2
`RATIO
`
`0
`
`0
`
`80
`60
`40
`20
`ARTERIAL OXYGEN SATURATION (%)
`FIG. 2.
`
`100
`
`2 pa
`
`2
`
`30 i
`
`
`
`BONE 18
`
`4
`
`

`

`U.S. Patent
`
`Aug.19, 2008
`
`Sheet 2 of 14
`
`US 7,415,298 B2
`
`COEFFICIENTPRODUCTOFTISSUE
`EXTINCTION-SCATTERING
`
`00 650
`
`700
`
`850
`800
`10
`WAVELENGTH (nm)
`
`900
`
`950
`
`1000
`
`FIG. 4A.
`
`COEFFICIENT PRODUCT
`
`
`
`
`
`
`EXTINCTION-SCATTERIN 860m|78am
`(L/mmole-cm2)|ig1>
`[sd
`
`.64
`1.63
`
`
`|Ae
`u's B40%
` |59
`u's" Bb aCe
`
`
`||2
`
`
`
`FIG. 4B.
`
`5
`
`

`

`U.S. Patent
`
`Aug.19, 2008
`
`Sheet 3 of 14
`
`US 7,415,298 B2
`
`
`
`FIG. 5.
`
`
`
`735 am
`
`FIG. 6.
`
`6
`
`

`

`U.S. Patent
`
`Aug. 19,2008
`
`Sheet 4 of 14
`
`US 7,415,298 B2
`
`4
`
`0
`
`acs
`So
`=
`
`s 20
`
`=
`=>
`=
`B00
`
`1 oH 10
`—perTuRBED
`°*2
`‘*
`- - BASIS
`
`FIG. 7A.
`
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`°° 2040
`—perturnen 2 (%)
` -BASIS
`
`FIG.
`
`7B.
`
`= 20
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`= Be
`
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`
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`BASIS
`FIG. 8A.
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`i
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`
`60
`5902 (%)
`
`60
`
`100
`
`FIG. 8B.
`
`= 20
`
`S 0
`=
`—
`H-20
`
`60
`0
`2
`“o
`— perTurgep 2002 (%)
`= BASIS
`
`700
`
`FIG. YA.
`
`FIG.
`
`JB.
`
`7
`
`

`

`U.S. Patent
`
`Aug.19, 2008
`
`Sheet 5 of 14
`
`US 7,415,298 B2
`
`MODULATIONRATIO
`
`noSo
`
`
`
`SATURATIONERROR
`
`a aa
`—PERTURBED 5°02 ()
`—PERTURBED 5902 (%)
`--BASIS
`= -BASIS
`
`FIG. 10A.
`
`FIG. 1OB
`
`
`
`SATURATIONERROR
`
`ERRORSsoSs
`SATURATION
`
`MODULATIONRATIO
`
`0
`0
`2
`4
`6
`—perruraen °02
`- ~BASIS
`
`FIG. IA.
`
`8
`
`0
`
`MODULATIONRATIOrR=
`
`20 40608100
`—perrurgen 5002 %!
`~ “BASIS
`
`FIG. 12A.
`
`Ss
`
`i00
`
`0
`2
`40
`S
`—perTursen 5°02 (%)
`= “BASIS
`
`FIG. IB
`
`80
`
`100
`
`o
`2
`40
`60
`—pertureen 5002 “%)
`ae
`
`FIG. 2B
`
`MODULATIONRATIOie
`
`
`
`SATURATIONERRORsi)8iSo
`
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`
`
`
`
`
`
`o
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`—perrurpep 2 (!
`~ ~BASIS
`~ ~BASIS
`
`
`
`
`
`8
`
`

`

`U.S. Patent
`
`Aug.19, 2008
`
`Sheet 6 of 14
`
`US 7,415,298 B2
`
`4
`
`s
`
`ee 20
`wo
`
`= |ee Boy
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`
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`
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`
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`
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`
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`
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`
`FIG. 148.
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`%
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`--BASIS
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`
`I6A.
`
`FIG. 168.
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`—PERTURBED 9902 (%)
`“BASIS
`FIG. 7A.
`
`80
`
`100
`
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`
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`—PERTURBED 5902 (%)
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`/7B.
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`
`FIG.
`
`[5A.
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`—perturpep 02 (%)
`--BASIS
`
`FIG. 158
`
`80
`
`100
`
`9
`
`

`

`U.S. Patent
`
`Aug.19, 2008
`
`Sheet 7 of 14
`
`US 7,415,298 B2
`
`MODULATIONRATIORM&
` ATURATIONERRORLo
`
`9
`
`8
`
`10
`
`
`
`0
`2
`—perursen
`—~ BASIS
`
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`6
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`60
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`~~ BASIS
`
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`
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`
`FIG.
`
`I8A.
`
`FIG. 8B.
`
`
`
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`
`[LILLE
`Sp,L222
`
`124
`
`FIG 25.
`
`122
`
`10
`
`

`

`U.S. Patent
`
`Aug.19, 2008
`
`Sheet 8 of 14
`
`US 7,415,298 B2
`
`
`
`
`
`PULSEOXIMETRY(%Sp02)
`
`o NECK
`
`* HEAD
`
`0
`
`0
`
`2
`
`30
`
`©6©6580066000670l068Oll(90
`©4
`CO-OXIMETRY (% $002)
`
`100
`
`Fig. 19.
`
`11
`
`

`

`U.S. Patent
`
`Aug. 19, 2008
`
`Sheet 9 of 14
`
`US 7,415,298 B2
`
`Si)>SSooS=SSsSoS4ouoeoS
`
`
`
`(2095%)AYLIWIXO3S7Nd
`
`ptt}EttQe
`
`
`Pi}Ppaptt|ING
`Pt}ANEE
`
`70=80s 90—s«*100
`
`30
`20
`10
`40
`90
`
`60
`CO-OXIMETRY (%Sa02)
`FIG.
`20.
`
`12
`
`12
`
`
`
`
`
`

`

`U.S. Patent
`
`Aug. 19, 2008
`
`Sheet 10 of 14
`
`US 7,415,298 B2
`
`FORCE
`CHANGEINPULSEOXIMETERSATURATIONREADINGDUETOAPPLIED
`
`
`
`
`e HEAD
`
`o NECK
`
`CO-OXIMETER SATURATION
`
`FIG. 21.
`
`13
`
`

`

`U.S. Patent
`
`Aug. 19, 2008
`
`Sheet 11 of 14
`
`US 7,415,298 B2
`
`
`
`
`
`PULSEOXIMETRY(%Sp02) S
`
` 0
`
`0
`
`80
`
`90
`
`100
`
`0
`
`20
`
`30
`
`60
`50
`40
`CO-OXIMETRY (% S002)
`
`FIG. 22.
`
`14
`
`

`

`U.S. Patent
`
`Aug. 19, 2008
`
`Sheet 12 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 13 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 14 of 14
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`US 7,415,298 B2
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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 1s a divisional ofU.S. application Ser. No.
`10/698,962, filed Oct. 30, 2003, which is a continuation of
`USS. application Ser. No. 09/882,371, filed Jun. 14, 2001, now
`USS. Pat. No. 6,662,033, which is a continuation of U.S.
`application Ser. No. 09/003,413, filed Jan. 6, 1998, now U.S.
`Pat. No. 6,272,363, which is a continuation of U.S. applica-
`tion Ser. No. 08/413,578, filed Mar. 30, 1995, now U.S. Pat.
`No. 5,782,237, which is a continuation-in-part of U.S. appli-
`cation Ser. No. 08/221,911, filed Apr. 1, 1994, now U.S. 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
`
`REFERENCETO A “SEQUENCELISTING,” A
`TABLE, OR A COMPUTER PROGRAMLISTING
`APPENDIX SUBMITTED ON A COMPACTDISK
`
`NOT APPLICABLE
`
`BACKGROUND OF THE INVENTION
`
`Pulse oximetry is used to continuously monitorthe arterial
`blood oxygensaturation 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 hastraditionally 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. Oxygensaturation 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 muchasis the trend over time.
`Conventional two wavelength pulse oximeters emit light
`from two Light. Emitting Diodes (LEDs) into a pulsatile
`tissue bed andcollect 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 connectsto the pulse oximeter electronics
`and display unit. The “pulse” in pulse oximetry comes from
`the time varying amountofarterial blood in thetissue 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
`wherethe absorption of oxyhemoglobin (HbO,)differs from
`the absorption of reduced hemoglobin (Hb). The second of
`the two LEDs’ wavelength must beat a different point in the
`spectrum where, additionally,
`the absorption differences
`
`2
`between Hb and HbO, 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 modulationratio 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 modulationratio
`over a range of in vivo measuredarterial oxygen saturations
`(SaO,) ona set ofpatients, healthy volunteers or animals. The
`observed correlation is used in an inverse mannerto estimate
`
`saturation (SpO,) based on the real-time measured value of
`modulationratios. (As used herein, SaO, refers to the in vivo
`measured functional saturation, while SpO, 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 changesin arterial blood oxygen
`saturation, and the intensity and availability of commercial
`LEDsat 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 HbO,is
`markedly different from Hb. The region near 660 nm is where
`the ratio of light absorption due to reduced hemoglobinto that
`ofoxygenated hemoglobinis 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 HbO,. Unfortu-
`nately, pulse oximeters which use LED wavelengths paired
`from the 660 nm band and 900 nm bandsall show diminished
`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 offirst and second light
`sources so that the saturation estimates at low saturation
`
`values are improved while the saturation estimates at high
`saturation values are minimally adversely affected as com-
`pared to using conventionalfirst and second wavelength spec-
`trums. It has been discovered that calculations at low satura-
`tion can be significantly improved if the anticipated or
`predicted rates of absorption and scattering ofthefirst wave-
`length spectrum is brought closer to, optimally equalto, the
`anticipated or predicted rates of absorption andscattering 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 ofarterial 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 wherethe saturation
`range of principal
`importance and interest
`is generally
`between 15% and 65%,andis particularly useful for estimat-
`ing arterial saturation of living cardiac patients who experi-
`ence significant shunting ofvenousbloodintotheirarteries in
`their hearts and hence whosesaturation range of principle
`importance andinterest is roughly between 50% and 80%. By
`contrast, a typical healthy human hasa saturation greater than
`90%. The invention has utility wheneverthe saturation range
`of interest of a living subject, either humanor animal,is low.
`In addition to providing better estimates ofarterial oxygen
`saturation at low saturations, the sensor, monitor, and system
`of the invention further provide better and more accurate
`oxygen saturation estimates when perturbation inducedarti-
`facts exist and are associated with the subject being moni-
`tored.
`
`4
`ducers are located remote from thetissue, for example in the
`oximeter monitor, and optical fibers interconnect the trans-
`ducers andthetissue withthe tissue being illuminated from an
`end ofa fiber, andlight 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 abovefor a typical
`patient with normal (high) saturation. In addition, a fetal
`sensor is subject to increased perturbation inducedartifact.
`Another unique factor in fetal oximetry is that the sensor is
`typically inserted through the vagina andthe precise location
`whereit lands is not knownin 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 morereliable 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 optimizationsare applicable to both reflec-
`tance and transmission pulse oximetry. An example ofa fetal
`transmission pulse oximetry configuration usable with the
`present invention is described in U.S. patent application Ser.
`No. 07/752,168, assigned to the assignee ofthe present inven-
`tion, the disclosure of which is incorporated herein byrefer-
`ence. An example of a non-fetal transmission pulse oximetry
`configuration usable with the present invention is described in
`USS. Pat. No. 4,830,014, assigned to the assignee of the
`present invention, the disclosure of which is incorporated
`herein by reference.
`
`Whenthe rates of absorption and scattering by the tissue
`being probedbythe 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 probedbythefirst 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 furtherinto thetis-
`sue. Whenthetissue being probedis particularly in-homog-
`enous, this difference in penetrations can have a significant
`adverse impact on the accuracyofthearterial oxygen satura-
`tion estimate.
`Perturbation inducedartifacts include, but are not limited
`to, any artifact that has a measurable impact on the relative
`optical properties of the medium being probed. Perturbation
`inducedartifacts include but are notlimited to the following:
`(1) variationsin the tissue composition being probed by the
`sensor from subject to subject,
`i.e., variations in the
`FIG.1 is a chart of the absorption characteristics of oxy-
`relative amounts offat, bone, brain, skin, muscle, arter-
`hemoglobin (HbO.) and reduced hemoglobin (Hb) versus
`ies, veins, etc.;
`wavelength showing prior art near red and infrared LED
`(2) variations in the hemoglobin concentrationin the tissue
`wavelengths;
`being probed, for example caused by vasal dilations or
`FIG.2 is a graph ofred/IR modulation ratio versus oxygen
`vasal constrictions, and any other physical cause which
`saturation;
`affects blood perfusion in the tissue being probed; and
`FIG. 3 is a diagram illustrating light penetration through
`(3) variations in the amountof force applied between the
`different layers of tissue at different distances;
`sensor and the tissue being probed, thus affecting the
`FIG. 4A is a chart of the variation in extinction and scat-
`amountof blood present in the nearbytissue.
`tering coefficients over a range of wavelengths for different
`In one embodiment, the present invention providesa fetal
`saturation values;
`pulse oximeter sensor with a light source optimized for the
`FIG.4Bis a table of the values of FIG. 4A;
`fetal oxygen saturation range and for maximizing the immu-
`FIG. 5 is a diagram illustrating the placing of a sensor on a
`nity to perturbation inducedartifact. Preferably, a far red and
`fetus;
`an infrared light source are used, with the far red light source
`FIG. 6 is a graph illustrating the spectrum of an LED
`having a mean wavelength between 700-790 nm.The infrared
`according to the present invention;
`light source can have a mean wavelength as in prior art
`FIGS. 7-18 are graphs showing experimental modeling of
`devices used on patients with high saturation, i.e., between
`the modulation ratio and saturation error as a function of
`800-1000 nm.As used herein,“high saturation” shall mean an
`saturation for different red and infrared wavelength combi-
`arterial oxygen saturation greater than 70%, preferably
`nations;
`greater than 75%, alternatively greater than 80%, optionally
`FIGS. 19-23 are charts illustrating saturation and the error
`greater than 90%.
`due to applied force for different combinations of emitter
`The fetal sensor of the present invention is further opti-
`wavelength and emitter-detector spacing from experiments
`mized by arranging the spacing between the location the
`done on sheep;
`emitted light enters the tissue and the location the detected
`FIGS. 24 and 25 are diagramsillustrating the construction
`light exits the tissue to minimize the sensitivity to perturba-
`tion inducedartifact.
`of a sensor according to the present invention;
`FIGS. 26A-Bare diagrams ofasingle package, dual emit-
`According to a preferred embodiment, electrooptic trans-
`65
`ter package usedin the present invention; and
`ducers (e.g., LEDs and photodetectors) are located adjacent
`to the tissue where the light enters and exits the tissue.
`FIG. 27 isa block diagram ofa pulse oximeter according to
`Accordingto an alternate embodiment, the optoelectric trans-
`the present invention.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
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`5
`DETAILED DESCRIPTION OF THE INVENTION
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`US 7,415,298 B2
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`6
`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 balanceor correla-
`tion exists between the productof the absorption and scatter-
`ing coefficients of conventionally chosen wavelength pairs
`(1.e., 660 nm and 892 nm), as illustrated by points A and B on
`curve 101.
`
`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, anda layer ofbone 18. This is a simplified view for
`illustration purposes only. The contours andlayers can vary at
`different locations. For instance, bone would becloserto the
`surface on the forehead, as opposed to closer muscle on the
`neck. Such variations in sites can producethefirst type of
`perturbation artifact mentioned in the summary—artifact 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
`showslight which passes almost directly from emitter 20 to
`detector 22, basically shunted from oneto 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 penetrationis 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.
`Alsoillustrated in FIG.3 is the effect ofusing an emitter 28
`which is spaced on the tissue at a greater distance from a
`detector 30 thanthefirst pair 20, 22 described. As can be seen,
`this greater separation results in the penetration of a larger
`amountoftissue, as indicated by arrows 32 and 34. Thus, the
`greater spacing increasesthe depth of penetration,althoughit
`will reduce the intensity of the signal receivedat the detector
`due to more attenuation from moreofthe light being absorbed
`in the tissue and the greater light propagation distances
`involved.
`
`For low tissue oxygen saturation, points C and D on curve
`102 graphically indicate that there is a very significant mis-
`match between the product of the absorption and scattering
`coefficients of the 660 nm near red and 892 nm infrared light,
`with the near red light being more strongly absorbed and
`scattered. This very significant absorption and scattering mis-
`matchresults in very differenttissue being probed by the near
`red and infrared light which significantly degrades the accu-
`racy ofthe arterial oxygen saturation calculation. In addition,
`whena large range of low arterial oxygen saturations need to
`be accurately calculated, as when monitoring a fetus during
`labor where the range of arterial oxygen saturations can
`extend between 15% and 65%,it is evident from FIG. 4A that
`not only does a significant mismatch between the rates of
`absorption and scattering of the near red and infrared light
`exist, but that the amount of mismatch will vary significantly
`as arterial oxygen saturation varies, thus causing a differential
`inaccuracy of oxygen saturation estimates which varies with
`the arterial saturation.
`Onthe other hand, points D and E on curve 102 in FIG. 4A
`illustrate advantages of a preferred embodimentofthe inven-
`tion of choosing first and second wavelengths, i.e., 732 nm
`and 892 nm, which have absorption and scattering character-
`istics which are more closely balanced as compared to the
`priorart pairing of 660 nm and 892 nm for 40% tissue oxygen
`saturation. As can be appreciated, since the 732 nm extinction
`and scattering coefficients more nearly match the 892 nm
`extinction and scattering coefficients, improved overlap of
`the tissue being probed by the two wavelengthsoflight result.
`In addition, 732 nm results ina smaller variation ofthe extinc-
`The secondtype ofperturbation mentioned in the summary
`tion andscattering coefficients as a function of oxygen satu-
`is variations in the concentration of bloodin