United States Patent 15
`Jaeb et al.
`
`[11] Patent Number:
`
`[45] Date of Patent:
`
`4,880,304
`Nov. 14, 1989
`
`[54] OPTICAL SENSOR FOR PULSE OXIMETER
`[75]
`Inventors:
`Jonathan P. Jaeb; Dennis W. Gilstad;
`Ronald L. Branstetter, all of San
`Antonio, Tex.
`[73] Assignee: Nippon Colin Co., Ltd., Komaki,
`Japan
`
`[21] Appl. No.: 33,406
`
`Apr. 1, 1987
`[22] Filed:
`[51]
`Int. Ch oe GOIN 33/49; A61B 5/00
`[52] ULS. Ch. oncecsesesesveoseseencssnseeseenes 356/41; 128/633
`
`[58] Field of Search ..............c000 356/39, 40, 41, 409,
`356/414, 446; 250/343; 128/633
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,910,701 10/1975 Henderson etal. ..........sc00 356/39
`+» 356/407 X
`3,922,090 11/1975 Fain...
`ease 356/41 X
`4,086,915
`5/1978 Kofsky et al.
`
`356/41 X
`4,167,331 9/1979 Nielsen.........
`soves 356/41
`4,447,150 5/1984 Heinemann ..
`
`356/71 X
`4,451,530 5/1984 Kaule et al.
`..
`
`- 356/446
`4,484,819 11/1984 Ulrich..........
`
`4,586,513
`5/1986 Hamagutri .....
`a 356/41 X
`4,624,572 11/1986 van den Bosch...........ue 356/446 X
`
`OTHER PUBLICATIONS
`
`Takatani et al. “A Noninvasive Tissue Reflectance Ox-
`imeter” Annals of Biomedical Engineering, vol. 8, No.
`I, pp. 1-15 (1980).
`
`Primary Examiner—Vincent P. McGraw
`- Attorney, Agent, or Firm—Matthews & Branscomb
`
`ABSTRACT
`[57]
`An improved optical sensor which has increased sensi-
`tivity and which is resistant to the effects of ambient
`light. In one embodiment of the invention, the sensor
`housing has a flat lower face with a central protrusion in
`whicha plurality of light emitting diodes and an optical
`sensor are mounted. When the sensor is placed on the
`patient’s tissue, the portion of the sensor face containing
`the LEDsand detector protrudes slightly into the tissue
`to provide improved optical coupling of the sensor to
`the skin. A light absorbing compliant material is at-
`tached to the perimeter of the sensor to reduce the
`effects of ambient light and to provide a cushion to
`minimize discomfort to the patient. In an alternate em-
`bodiment of the sensor, the LEDs and detector are
`mounted in a horizontal configuration substantially
`parallel to the surface ofthe tissue. The light produced
`bythe LEDsis projected into a central chamberof the
`housing where the respective beams are combined and
`directed toward the tissue. In this emodiment, the de-
`sired combining of the beams.can be achieved through
`the use of a set of mirrors or a prism. Various combina-
`tions of the improvements provided by each of the
`embodiments described above can be incorporated into
`either a transmission or backscatter optical sensor to
`provide a compact, sensitive optical sensor which is
`resistant to interference caused by ambient light.
`5 Claims, 5 Drawing Sheets
`
`
`
`APPLE 1017
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`APPLE 1017
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`US. Patent
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`Nov.14, 1989
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`Sheet 1 of 5
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`4,880,304
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`US. Patent
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`Nov. 14, 1989
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`Sheet 2 of 5
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`4,880,304
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`AMBIENT
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`US. Patent
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`Nov. 14, 1989
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`Sheet 3 of 5
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`4,880,304
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`4,880,304
`U.S. Patent—Nov. 14, 1989
`Sheet 4 of 5
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`US. Patent Nov.14,1989—Sheet5of5~~4,880,304
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`1
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`4,880,304
`
`OPTICAL SENSOR FOR PULSE OXIMETER
`
`FIELD OF THE INVENTION
`
`The present invention relates generally to optical
`sensors used in biomedical applications. Specifically,
`the present invention provides an improved optical
`sensor which can be used in conjunction with monitor-
`ing equipment used to estimate the degree of oxygen
`saturation in blood. The sensor of the present invention
`has increased sensitivity and is resistant to interference
`due to ambient light.
`,
`BACKGROUND
`
`40
`
`‘
`
`.
`2
`thus requiring higher power sources or very sophisti-
`cated amplifying circuitry to obtain a usable signal.
`A number of other problems have been encountered
`in prior art optical sensors used in noninvasive oximetry
`systems. For example,it is often difficult to obtain suffi-
`cient signal strength for transmitted or reflected light in
`the red. spectrum. Another common problem experi-
`enced with prior art optical sensors is interference
`caused by ambientlight entering at the perimeter of the |
`sensor housing. In addition to the problems discussed
`above, prior art sensors tend to have an inherent inaccu-
`facy associated with the spacing of the light sources in
`the sensor housing. Since the spacing of the sensors
`causes different portions of the underlying tissue to be
`illuminated with the light produced by the respective
`LED,the light detected by the optical detector neces-
`sarily represents the oxygen saturationat different loca-
`tions in the tissue.
`Various methods and apparati forutilizing the optical
`properties of blood to. measure blood oxygen saturation
`have been shown in the patent literature. Representa-
`tive devices for utilizing the transmission method of
`oximetry have been disclosed in U.S. Pat. Nos.
`4,586,513; 4,446,871; 4,407,290; 4,226,554; 4,167,331;
`and 3,998,550. In addition, reflectance oximetry devices
`and techniques are shown generally in U.S. Pat. Nos.
`4,447,150; 4,086,915; and 3,825,342.
`Numerous other works have disclosed theoretical
`approaches for analyzing the behavioroflight in blood
`and other materials. The following is a brieflist of some
`of the most relevant of these references: “New Contri-
`butions to the Optics of Intensely Light-Scattering Ma-
`terials, Part 1,” by Paul Kubelka, Journal of the Optical
`Society ofAmerica, Volume 38, No. 5, May 1948; “Opti-
`cal Transmission and Reflection by Blood,” by R.J.
`Zdrojkowski and N.R.Pisharoty, IEEE Transactions on
`Biomedical Engineering, Vol. BME-17, No. 2, April
`1970; and “Optical Diffusion in Blood,” by Curtis C.
`Johnson, LEEE Transactions on Biomedical Engineering,
`Vol. BME-17, No. 2, April 1970.
`The effectiveness of noninvasive oximetry systems
`such as those described above could be significantly
`enhanced by an improved optical sensor. Specifically,
`there is a need for an optical sensor having increased
`sensitivity and increased resistance to the effects of
`ambientlight.
`SUMMARYOF THE INVENTION
`
`In manyclinical situations, it is extremely important
`to be able to obtain continuous measurements of a pa-
`tient’s tissue oxygenation. One of the most common
`methods for measuring blood oxygen saturation re-
`quires removal and analysis of a sample of the patient’s
`blood. Analysis of an actual sample of blood isstill
`considered the most accurate method for obtaining a
`reading of absolute blood oxygen saturation. However,
`this method is undesirable in cases whereit is necessary
`to monitor blood oxygen saturation over long periods
`of time. While it is desirable to have an absolute mea- 25
`sure of blood oxygen saturation, it is often sufficient to ~
`measure relative changes in the saturation. For example,
`in the operating room, the physician is typically con-
`cerned only with significant changes in the patient’s
`blood oxygen saturation, and is less concerned with the
`measurement of absolute saturation. In this situation, a
`noninvasive oximeter which is capable of detecting
`significant changes in the blood oxygen content would
`be especially useful.
`Hemoglobin oxygen saturation (OS) of blood is de-
`fined’as the ratio of the oxyhemoglobin (HbO2) concen-
`tration to the total hemoglobin (Hb) concentration.It is
`well known that hemoglobin and oxyhemoglobin have
`different optical absorption spectra and that this differ-
`ence in absorption spectra can be used as a basis for an
`optical oximeter. Specifically, the difference between
`the absorption spectra for red and infrared light can be
`used to determine blood oxygen saturation. Most of the
`currently available oximeters using optical methods to
`determine blood oxygen saturation are based on trans-
`mission oximetry. These devices operate by transmit-
`ting light through an appendagesuch as a finger or an
`earlobe. By comparing the characteristics of the light
`transmitted into one side of the appendage with that
`detected on the opposite side, it is possible to compute
`oxygen concentrations. The main disadvantage of trans-
`mission oximetry is that it can only be used on portions
`of the body which are thin enough to allow passage of
`light.
`There has been considerable interest in recent years
`‘in the development of an oximeter which is capable of
`using reflected light to measure blood oxygen satura-
`tion. A reflectance oximeter would be especially useful
`for measuring blood oxygen saturation in portions of
`the patient’s body whichare not well suited to transmis-
`sion measurements. Experimental results suggest thatit
`is possible to obtain accurate indications of blood oxy-
`gen content through the use of reflectance techniques.
`One of the most common optical sensors for oxime-
`ters employs a plurality of optical fibers for directing
`light to and from the tissue. These sensors tend to be
`relatively expensive and they are bulky and fragile. In
`addition, optical fibers have a high transmission loss,
`
`50
`
`35
`
`The present invention overcomes the difficulties of
`the prior art by providing an improved optical sensor
`which has increased sensitivity and whichis resistant to
`the effects of ambient light. In one embodiment of the
`invention, the sensor housing has a flat lower face with
`a central protrusion in which a plurality of light emit-
`ting diodes and an optical sensor are mountedin a verti-
`cal configuration. When the sensor is placed on the
`patient’s tissue, the portion of the sensor face containing
`the LEDs and detector protrudesslightly into thetis-
`60
`sue. This feature provides .a more repeatable coupling
`_ effect between the face of the sensor and the tissue and
`increasesthe sensitivity of the sensor. A light absorbing
`compliant material is attached to the perimeter of the
`sensor to reduce the effects of ambient light and to
`provide a cushion to minimize discomfort to the patient.
`In an alternate embodimentof the sensor, the LEDs
`and detector are mounted in a horizontal configuration
`substantially parallel to the surface of the tissue. The
`
`65
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`4,880,304
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`3
`light produced by the LEDsis projected into a central
`chamberof the housing where the respective beamsare
`combined and directed toward the tissue. In this em-
`bodiment, the desired combining of the beams can be
`achieved through the use of a set of mirrors or a prism.
`Various combinations of the improvements provided
`by each of the embodiments described above can be
`incorporated into either a transmission or backscatter
`optical sensor to provide a compact, sensitive optical
`sensor whichis resistant to interference caused by ambi-
`ent light.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`20
`
`25
`
`30
`
`FIG.1a is a schematic block diagram ofa simplified
`embodiment of a noninvasive blood oxygen saturation
`monitoring system using a backscatter optical sensor.
`FIG.15 is a schematic block diagram of a simplified
`embodiment of a noninvasive blood oxygen saturation
`monitoring system using a transmissive optical sensor.
`FIG. 2a is a bottom view of a typical backscatter
`optical sensor showingdetails relating to the spacing of
`the LEDsin relation to the optical detector.
`FIG.20 is an elevational side view of the sensor of
`FIG. 2a showing the effects of ambient light entering
`from the perimeter of the sensor housing.
`FIG.3a is a bottom view of a preferred embodiment
`of the improved optical sensor for use with a backscat-
`ter pulse oximeter.
`FIG. 3b is an elevational side view of the optical
`sensor of FIG. 3a showing details relating to the light
`absorbing pad and the mounting of the LEDs.
`FIG. 4a is an elevational side view of an alternate
`embodiment of the sensor of the present invention hav-
`ing the LEDs mounted in a horizontal configuration
`and the optical detector mounted in a vertical configu-
`ration.
`FIG. 45 is an elevational side view of an alternate
`embodimentof the sensor of the present invention hav-
`ing the LEDs and the optical detector mounted in a
`horizontal configuration.
`FIG.52.is a top view of an alternate embodiment of
`the present invention showing details relating to the
`overlapping, staggered configuration for mounting the
`light sources in the housing.
`FIG. 55 is a cross-sectional side view taken along
`lines 56—5d of FIG. 5a, showing details relating to the
`half-silvered mirrors used to combine the beams emitted
`by the light sources.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`4
`requiring a highly sensitive optical sensor whichis resis-
`tant to the effects of ambient light.
`Referring to FIGS. 1a and 18, a sensor 10 is shown
`positioned over a portion of the patient’s tissue 14 such
`that light produced by twolight emitting diodes (LED)
`16 and 18 will be reflected by (or transmitted through)
`arterial blood in the tissue and detected by a photode-
`tector 20. In the preferred embodiment, the LED 16
`emits light having a wavelength of 600 nm (red) and the
`LED 18 emits light having 2 wavelength of 800 nm
`(infrared). However, the invention is not intended to be
`limited to any specific wavelength of light produced by
`the above-mentioned LEDs. Proper operation of the
`invention requires only that one sourceoflight having
`a wavelength for which the absorption coefficients of
`hemoglobin and oxyhemoglobin are approximately
`equal and that the second sourceoflight have a wave-
`length for which these absorption coefficients differ
`from one another. In an alternate embodiment of the
`invention, each of the LEDscould be replaced with an
`appropriate source of laser radiation providing mono-
`chromatic light at the desired wavelengths.
`The output of the photodetector 20 will be an electri-
`cal signal representing a combination of direct-current
`(DC) and alternating-current (AC) components of the
`light reflected by (or transmitted through) the arterial
`blood in the tissue 14. This output signal is processed by
`an appropriate filter 22 to produce signals correspond-
`ing to the AC voltage components of each of the wave-
`lengths of incident light. These AC voltage signals are.
`then processed by a voltage amplitude ratio circuit 24
`which provides a voltage amplitude ratio signal to the
`microprocessor 20. The microprocessor processes the
`voltage amplitude ratio of the AC voltage signals in
`accordance with a correlation algorithm to obtain an
`indication of blood oxygen saturation which is dis-
`played on the display 28.
`The functional features of the above-described sys-
`tem components can be accomplished thoroughthe use
`of electronic components and techniques which are
`well known in the art. For example, U.S. Pat. No.
`4,447,150, issued to Heinemann, which bythis reference
`is incorporated for all purposes, shows a system for
`illuminating a sample of blood with light at two wave-
`lengths and for detecting light signals reflected by the
`blood. In addition, a system for obtaining electrical
`representations of the AC componentsofthe reflected
`signals is shown in U.S. Pat. No. 4,586,513, issued to
`Hamaguri, which by this reference is incorporated for
`all purposes.
`FIG.2a is a bottom view of a typical optical sensor 10
`Referring to the drawings in more detail, and to
`used in backscatter oximetry systems. The sensor con-
`FIGS. 1¢ and 10 in particular, simplified schematic
`sists of a disc-shaped housing 12 havingaflat lower face
`block diagrams are shown of noninvasive measurement
`whichis placed on the surface of the tissue, as shown in
`systems for determining blood oxygen saturation. The
`FIG. 2b. The lower face of the housing 12 contains
`twosystems are essentially identical, except for the type
`LEDs16 and 18 which emit light in the red and infrared
`of optical sensor. In the system shown in FIG. 1a, a
`specta, respectively, and an optical sensor 20 for detect-
`reflectance optical sensor 10 is employed, while the
`ing light reflected by subsurface tissue
`system of FIG. 1b employs a transmission optical sensor
`The sensor design shownin FIGS.2a and 2b presents
`10’. In the following discussion these two types ofopti-
`a number of problems related to signal strength and
`cal sensors will be identified generally by the reference
`susceptibility to interference. The red light signal trans-
`number 10, although it is to be understood that the
`mitted through orreflected by thetissue 14 is typically
`improvements provided by the various embodiments of
`much weaker than the signal corresponding to light in
`the present invention can be incorporated into either
`the infrared spectrum. In systems employing one red
`backscatter or transmission type sensors. Indeed, the
`LED,therefore, it is necessary to intermittently over-
`-improvements provided by the optical sensor of the
`drive this LED beyondits maximum continuous power
`present invention are not limited to oximetry measure-
`rating. This tends to shorten thelife of the LED and can
`ments, but can be applied in virtually any application
`causeslight shifts in the wavelength of the emittedlight.
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`Another problem with the sensor design shown in
`FIGS.2a and 20 is interference caused by ambient light
`entering from the perimeter of the sensor housing.
`One embodiment of an optical sensor which over-
`comes the difficulties discussed above is shown in
`FIGS. 3a and 3b. The problem of low signal strength is
`solved by placing a total of four LEDsin a radial pat-
`tern around theoptical detector 20. In this embodiment,
`three red LEDs and oneinfrared LED are mounted in
`the lower face of the sensor. These LEDs are mounted
`in central protrusion 12’ in the lower face of the sensor
`housing 12. When the sensor housing is pressed against
`the surface of the tissue, as shown in FIG.3a, the por-
`tion of the sensor. face containing the LEDs and the
`optical detector protrudes into the tissue slightly,
`thereby increasing the signal strength of the detected
`signal. Interference due to ambient light entering from
`the edges of the sensor is reduced by a compliant, light
`absorbing pad 30 attached to the perimeter of the lower
`face of the sensor. This pad also provides a cushion to
`reduce discomfort to the patient. Interference due to
`’ ambientlight is further reduced by recessing the LEDs
`16, 18 and the detector 20 within the housing and by
`coating the respective LEDs with a polymersealant 32.
`The coating provides improved optical index matching
`and also provides an electrical and biological seal be-
`tween the sensor and thetissue.
`In addition to the two problems discussed above, the
`sensor shown in FIGS.2a and 2b tendsto be inherently
`inaccurate because of the spacing of the LEDs 16 and
`18 in relation to the detector 20. Ideally, the light pro-
`duced by each of the LEDs should illuminate the same
`portion ofthe tissue 14 underlying the sensor to ensure
`accurate calculation of the voltage amplitude ratio for
`the reflected (or transmitted light). A second embodi-
`mentof the present invention for solving this problem is
`shown in FIGS. 4a and 48. In this embodiment, the
`LEDs16 and 18 are mountedin a horizontal configura-
`tion with the light beams from the LEDsdirected into
`a central chamber 12a in the housing. The light beams
`are combined in the chamber 12¢ and are directed
`toward the tissue by an appropriate optical guide. For
`example, in FIG.4a, the light beams emitted by LEDs.
`16 and 18 are directed towardthe skin by reflectors 34a
`and 34). The reflectors are spaced such that light re-
`flected by the tissue 14 can pass therebetween to be
`detected by detector 20. In FIG. 44, the beams emitted
`by the LEDsare directed toward the tissue 14 by a
`prism 36. In this illustration, the detector 20 is shownin
`the opposite side of the tissue 14. This embodiment
`could easily be modified, however, to operate as a re-
`flectance sensor with the detector contained in the
`housing 12. In addition, either of the embodiments can
`be modified to incorporate the protruding lower face
`12 and the compliant light absorbing pad 30.
`Thehorizontal mounting configuration of the LEDs
`shown in FIGS. 4@ and 45 provides an extremely com-
`pact profile for the sensor. The dimensionsof the sensor
`can be further reduced by mounting the LEDsin the
`vertically overlapping staggered configuration shown
`in FIGS.5a and5b.In this embodiment, the beams from
`the various LEDsare directed toward a chamber 12a’
`wherethey are combined and directed towardthe tissue
`14 by mirror 38a and by half-silvered mirrors 385, and
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`38c, shownin FIG. 5b. Light emitted by upper LED 16'
`is reflected downward by the mirror 38¢ and passes
`throughhalf-silvered mirrors 384 and 38c. Similarly, the
`light emitted by LED 18 is reflected by half-silvered |
`mirror 386 and passes through half-silvered mirror 38c.
`Finally, the light emitted by LED 16is reflected down-
`ward by half-silvered mirror 38. The various beams
`emitted by the LEDsare thus combined to form a com-
`posite beam which illuminates the same portion of the
`underlying tissue 14. Light reflected by the tissue 14 is
`received in the chamber 125’ and is directed toward the
`optical detector 20 bythe reflector 34.
`While the optical sensor of the present invention has-
`been described in connection with the preferred em-
`bodiment, it is not intended to be limited to the specific
`form set forth herein, but on the contrary, it is intended
`to cover such alternatives, modifications and equiva-
`lents as may be reasonably included within thespirit and
`scope of the invention as defined by the appended -
`claims
`Weclaim:
`1. An optical sensor, comprising:
`a housing, said housing having a general flat lower
`surface with a central protrusion extendinga pre-
`determined distance from said lower surface, said
`protrusion adapted to provide an optical couple
`between said sensor and the surface of a patient’s
`tissue;
`a first sourceoflight at a first wavelength for which
`the absorption coefficients of hemoglobin and oxy-
`hemoglobin are approximately equal and a second
`source oflight at a second wavelength for which
`said absorption coefficients differ from one an-
`other,
`said first and second sources of light
`mounted in said central protrusion extending from
`said lower surface;
`light detecting means mounted in said protrusion of
`said housing to detect light at said first and second
`wavelengths after contact with arterial blood; and
`light absorbing means on said lower surface. of said
`housing surrounding said central protrusion for
`preventing the transmission of ambient light be-
`tween the lower surface of said housing andsaid. .
`tissue.
`2. The optical sensor according to claim 1, said first
`light source comprising at least one light emitting diode
`emitting light in the infrared spectrum, said secondlight
`source comprising at least two light emitting diodes
`emitting light in the red spectrum.
`3. The optical sensor according to claim 2, said first
`light emitting diode producing light at approximately
`660 nanometers, said second light emitting diode pro-
`ducing light at approximately 800 nanometers.
`4. The optical sensor according to claim 3, each said
`light emitting diode being recessed in the lower face of
`said protrusion, defining a plurality of generally cylin-
`drical chambers in the lowerface said protrusion, each
`of said chambers being filled with a polymersealant.
`5. The optical sensor according to claim 4, said means
`for absorbing light comprising a compliant, light ab-
`sorbing pad attached to the lower face of said housing
`along the perimeter thereof.
`~
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