United States Patent
`
`[191
`
`[11] Patent Number:
`
`5,024,226
`
`Tan
`[45] Date of Patent:
`Jun. 18, 1991
`
`[54] EPIDURAL OXYGEN SENSOR
`
`[75]
`
`Inventor:
`
`Josef K. S. Tan, Tampa, Fla.
`
`[73] Assignee:
`
`Critikon, Inc., Tampa, Fla.
`
`[21] App]. No.: 394,997
`
`Aug. 17,1989
`[22] ‘Filed:
`[51]
`Int. 01.5 ................................................ A61B5/00
`[52] U.S. c1. ...........'.......................... 128/633; 128/666
`[58] Field of Search ............... 128/632, 633,634, 665,
`128/666; 356/40
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`
`4,621,643 11/1986 New Jr. et a].
`............... 128/666
`.
`4,623,789 11/1986 Ikeda et al. .......... 604/175
`
`..
`..... 128/666
`4,714,080 12/1987 Edgar Jr. et a1.
`
`..... 128/664
`4,784,150 11/1988 Voorhies et al.
`
`5/1989 Tun et a1. ............. 128/665
`4,825,872
`
`9/1984 Rich et a1. ............ 128/665
`4,865,038
`
`
`....... 356/41
`4,867,557
`9/1989 Takatani et a].
`.................... 128/633
`4,928,691
`5/1990 Nicolson et a1.
`4,938,218
`7/1990 Goodman et al.
`.................. 128/633
`
`FOREIGN PATENT DOCUMENTS
`
`............ 128/633
`0094749 11/ 1983 European Pat. Off.
`............ 128/633
`0135840 4/1985 European Pat. Off.
`3152963 10/1983 Fed. Rep. of Germany ...... 128/633
`
`OTHER PUBLICATIONS
`
`“Cerebral Oxidative Metabolism and Blood Flow Dur-
`ing Acute Hypoglycemia and Recovery in Unaesthe-
`tized Rate“, Joum. of Neurochem., 1982 p. 397.
`“Regional Acetylcholine Metabolism in Brain During
`Acute Hypoglycemia and Recovery”, Journ. Neuro- .
`chem, 1985 at p. 94 et seq.
`Yee, Sinclair et al., “A Proposed Minature Red/Infra—
`
`. .”, IEEE Trans, Biomed. Eng. (USA),
`red Oximeter .
`vol. BME—24 NO. ‘2 (Mar. 1977).
`
`Primary Examiner—Lee S. Cohen
`Assistant Examiner—John D. Zele
`Attorney. Agent, or Firm—Paul A. Coletti
`
`[5 7]
`
`ABSTRACT
`
`A sensor for measuring the oxygen availability of blood
`flow within the skull is described. In a first embodiment
`the sensor comprises a photodetector and a pair of light
`emitting diodes surface mounted near the end of a
`length of flexible printed wiring. The sensor is sealed by
`a coating of rubber or polymeric material which has an
`optical window over the photodetector and light emit-
`ting diodes. The sensor is inserted through a burr hole
`drilled in the skull and slides between the skull and the
`dura of the drain. The light emitting diodes are pulsed
`to illuminate blood flow in the brain beneath the dura
`with light, and light reflected by the blood is received
`by the photodetector and converted to electrical sig-
`nals. The signals are processed by a pulse oximeter to
`provide an indication of blood availability. In a second
`embodiment the photodetector and light emitting di-
`odes are mounted at the end of a core of compressible
`foam extending from the end of a hollow bone screw.
`As the bone screw is screwed into a burr hole in the
`
`skull the photodetector and light emitting diodes will
`contact the dura and the foam will compress to maintain
`optical contact between the electrical components and
`the dura. Light from the diodes is reflected by blood in
`the dura and brain, received by the photodetector, and
`the resultant electrical siganls are processed by the
`pulse oximeter.
`
`12 Claims, 3 Drawing Sheets
`
`
`
`MASIMO 2011
`MASIMO 2011
`Apple v. Masimo
`Apple V. Masimo
`IPR2020-01526
`IPR2020-01526
`
`

`

`US. Patent
`
`June 18, 1991
`
`V Sheet 1 of 3
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`5,024,226
`
`
`
`

`

`US. Patent
`
`June 18, 1991
`
`Sheet 2 of 3
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`5,024,226
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`
`
`FIG-4a
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`FIG-4b
`
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`
`

`

`US. Patent
`
`June 18, 1991
`
`Sheet 3 of 3
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`5,024,226
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`FIG-7
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`FIG-80
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`FIG-8b -
`
`
`
`

`

`1
`
`EPIDURAL OXYGEN SENSOR
`
`5,024,226
`
`This invention relates to sensors for determining the
`oxygen availability of tissues within the skull and, in
`particular, to such sensor which are placed epidurally
`through the skull to measure oxygen availability.
`During neurological and neurologically related surgi-
`cal procedures it is oftentimes desirable to continuously
`monitor the oxygenation of blood which is supplied to
`the brain. Frequently access is gained to the brain
`through a borehole in the skull, and a sensor which
`optically measures oxygenation can then be inserted
`through such a borehole. An optical sensor should then
`exhibit numerous design and performance criteria in
`order to operate satisfactorily in this environment. The
`sensor must be capable of insertion through the bore-
`hole so as to contact tissue where oxygen availability is
`to be measured The sensor must be soft so that it does
`not damage neurological tissue, yet be sufficiently rigid
`in certain dimensions so that it can be maneuvered from
`outside the skull. It also must be sized to fit inside the
`borehole and in the location where measurements are to
`be taken. Furthermore, the sensor must be designed so
`as to eliminate detection of ambient light which will
`interfere with detection of the desired optical signals
`The sensor must also prevent the detection of directly
`transmitted light from the light source of the sensor.
`In accordance with the principles of the present in-
`vention, an optical sensor is provided for epidural mea-
`surement of blood oxygenation In a first embodiment
`the sensor comprises a pair of light emitting diodes
`(LED’s) which emit light at two predetermined wave-
`lengths. The sensor also includes a photodetector for
`receiving light emitted by the LED’s which has been
`reflected from adjacent blood perfused tissue. The
`LED’s and the photodetector are mounted on flexible
`printed wiring which transmits signals to the LED’s
`and from the photodiode. The components are encapsu-
`lated in a soft polymer which is biocompatible. The
`resultant sensor is thus capable of operation in an epidu-
`ral environment, and is further capable of being maneu-
`vered into the desired position for epidural measure—
`ments.
`
`In a second embodiment the LED’s and photodetec-
`tor are located in a hollow bone screw, with the compo-
`nents opposing the tissue from which measurements are
`to be taken. The components are backed by a resilient
`member such as a spring or soft polymeric foam which
`will compress under gentle pressure within the bone
`screw to cause the components to contact the dura and
`maintain optical contact with the dura as it moves with
`the patient’s respiration.
`In the drawings:
`FIG. 1 illustrates a cross-sectional view of the use of
`an epidural oxygenation sensor constructed in accor-
`dance with the present invention;
`FIG. 2 is a side cross-sectional view of an epidural
`oxygenation sensor constructed in accordance with the
`principles of the present invention;
`FIG. 20 is a perspective view of an epidural oxygena-
`tion sensor constructed in accordance with the princi-
`ples of the present invention;
`FIGS. 30-36 are cross-sectional views of different
`embodiments of epidural oxygenation sensors of the
`present invention;
`
`2
`FIGS. 4a-4c, 50—5c and 6a—6c are plan views of dif-
`ferent placements of LED’s and photodiodes of epidu-
`ral oxygenation sensors of the present invention;
`FIG. 7 is an electrical schematic of the components of
`the epidural oxygenation sensor of FIG. 2; and
`FIGS. 80—80 are cross-sectional,
`top, and bottom
`views of an epidural oxygenation sensor mounted in a
`hollow bone screw.
`
`10
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`Referring first to FIG. 1, a skull is shown in which a
`burr hole 12 has been drilled. Underlying the skull is the
`dura 16 which encases the brain, and beneath the dura is
`the cerebrum 14. An epidural oxygenation sensor 20 is
`inserted through the burr hole 12 for measurement of
`the oxygenation of blood flowing in the brain. The
`sensor 20 is inserted through the burr hole and slides
`between the skull 10 and the dura 16, where it
`is
`shielded from ambient light entering the burr hole. At
`the distal end of the sensor 20 is a photodetector 24 and
`LED’s 22 which face the dura through optical windows
`in the sensor. The photodetector and LED’s are
`mounted on flexible printed wiring which is connected
`to a sensor cable 26. The sensor cable is connected to a
`pulse oximeter
`(not shown), which provides drive
`pulses for the LED’s, receives electrical signals from
`the photodetector, and processes the received electrical
`signals to produce an indication of the oxygen availabil-
`ity of blood in the brain. The sensor is operated in a
`reflective mode, whereby light of different wavelengths
`emitted by the LED’s is reflected by the blood in the
`brain and the reflected light is received by the photode-
`tector.
`
`As shown in FIG. 2, the sensor 20 comprises a photo—
`detector 24 and an adjacent pair of LED’s 220 and 22b
`which are surface mounted to leads of flexible printed
`wiring 28 such as 0.001 inch Kapton TM based printed
`wiring. The use of surface mounted components and the
`printed wiring provide a thin sensor which minimizes
`cerebral compression. Separating the LED’s and the
`photodetector is a light barrier 25 which prevents the
`direct transmission of light from the LED’s to the pho-
`todetector. The light barrier may be provided by an
`opaque epoxy material, but in a preferred embodiment
`the light barrier is formed of a thin sheet of copper foil.
`The copper foil not only effectively blocks light from
`the LED’s, but is also connected to a grounded lead of
`the flexible printed wiring. The copper foil thus shields
`the photodetector from radio frequency interference
`such as that emanated during pulsing of the LED’s.
`The foregoing components are encapsulated by a soft
`coating 30 of silicone rubber or polyurethane material.
`The soft coating smoothly rounds the corners and edges
`of the sensor which prevents injury to the dura by the
`sensor. The coating also seals the components from
`moisture and other environmental factors. The coating
`30 is optically transmissive to light at the wavelengths
`of the LED’s where it overlies the lower surfaces of the
`phOtodetector and the LED’s from which light is trans-
`mitted and received by these components.
`FIG. 2a is a perspective view of the sensor 20 of FIG.
`2, referenced to x, y, and z axes. As mentioned above,
`the coating 30 provides the sensor with a 'smooth,
`gently rounded profile such as the rounded distal end
`27. The sensor is relatively stiff along the portion of the
`printed wiring where the components are mounted to
`maintain their relative alignment. In the x dimension the
`sensor is fairly stiff so that it may be inserted and guided
`beneath the skull and in contact with the dura. In the z
`dimension the sensor is stiff to provide maneuverability
`
`

`

`3
`during placement of the sensor. In the y dimension the
`sensor proximal the components is flexible to curve
`through the burr hole and under the skull, which may
`have a thickness of 2 to 20 mm depending upon the
`patient.
`.
`In order to be capable of sliding between the skull
`and the dura the sensor should be thin in the y dimen-
`sion so as not to injure the patient. Preferably the sensor
`thickness in this dimension should be not greater than 4
`mm, ‘and most preferably not greater than 2.5 mm. The
`sensor should also be not less than about one millimeter
`in thickness to maintain continuous contact with the
`dura. This will reduce the occurrence of motion arti-
`facts, as the dura can move as much as £ mm or more
`away from the skull during hyperventilation of the
`patient, for instance.
`In the embodiment of FIG. 2 the photodetector 24 is
`located toward the distal end 27 of the sensor with
`respect to the LED’s 22. This distal placement of the
`photodetector keeps the photodetector well removed
`from the burr hole and ambient light passing through
`the burr hole. FIGS. 3a—3c show other component
`orientations which may be employed in different sensor
`embodiments. In FIG. 3a the LED’s 22' are canted
`toward the dura where the dura overlies the photode-
`tector 24, which improves the efficiency of light reflec—
`tance. The canted LED’s are supported by a filler of the
`coating material 30. In FIG. 3b two pairs of LED’s 22"
`are located on either side of the phototdetector 24 to
`illuminate the dura from both sides of the photodetec-
`tor. In FIG. 3c the pair of LED’s 22 is centrally located
`between a pair of photodetectors 24’, the latter being
`canted toward the area of the dura illuminated above
`the LED’s.
`It is desirable for the optical windows of the compo-
`nents which face the dura to be as large as possible so as
`to maximize optical transmission efficiency. Opposing
`this desire is the constraint that the sensor must be sized
`to fit through the burr hole in the skull. To determine
`the largest components which may fit through a given
`burr hole diameter, rectangular configurations of com-
`ponents may be calculated which are capable of fitting
`through the burr hole. FIGS. 4a—6c show component
`configurations which can fit through a 14 mm diameter
`burr hole. FIGS. 4a—4c show plan views of component
`layouts for the single photodetector and pair of LED’s
`employed in the sensors of FIGS. 2 and 3a. In FIG. 4a
`the LEDfs 22’ and the photodetector are arranged in a
`layout which measures 8.2 mm by 6.3 mm. In FIG. 4b
`the rectangular layout measures 10.5 mm by 5.3 mm,
`and in FIG. 4c the rectangular layout measures 9.3 mm
`by 5.7 mm. In each layout the coating material 30 is
`shown in the area outside the boundaries of the electri-
`cal components.
`FIGS. 5a-Sc show component layouts for a 14 mm
`diameter burr hole using two pairs of LED’s 22" and
`one photodetector 24. In FIG. 50 the rectangular layout
`measures 10.8 mm by 7.0 mm; in FIG. 5b the layout
`measures 13.0 mm by 5.5 mm; and in FIG. 5c the layout
`measures 8.2 mm by 7.3 mm. In a similar manner, FIGS.
`6a—6c show component layouts using two photodetec-
`tors 24' and one or two pairs of LED’s 22 or 22”. In
`FIG. 6a the rectangular layout measures 8.1 mm by 6.8
`mm; in FIG. 6b the layout measures 11.5 mm by 6.8 mm;
`and in FIG. 6c the layout measures 8.6 mm by 7.5 mm.
`In FIGS. 4a—6c each LED pair had an area of 3.0 mm
`by 4.2 mm. The photodetectors in FIGS. 60 and 6c had
`an area of 2.25 mm by 6.25 mm. In the remaining lay-
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`5,024,226
`
`4
`cuts the photodetectors each had an area of 4.0 mm by
`6.25 mm.
`FIG. 7 is an electrical schematic of the sensor 20 of
`FIG. 2. The two LED’s 22a and 22b emit light at a red
`wavelength and are connected in parallel. These two
`LED’s are paralleled by LED‘s 23a and 23b, which
`emit light at an infrared wavelength. The 'LED’s are
`connected in series with respective resistors 25, 25' of
`values chosen in correspondence with the drive current
`to be supplied to the LED’s. The LED’s are also cou-
`pled to a biasing resistor 27. The resistors may be
`mounted in line with the flexible printed wiring, such as
`the points 320—32d at which the wiring joins the cable to
`the pulse oximeter monitor. Points 32c, 32d are con-
`nected to leads from photodetector 24. Alternatively
`the resistors can be functionally incorporated into the
`printed wiring by proper selection of materials and'
`dimensions.
`FIGS. 8a—8c illustrate a further embodiment of an
`epidural sensor in which the sensor components are
`located in a hollow bone screw 40. The screw 40 is
`threaded as indicated at 44 to screw into the skull, and
`the head of the screw has a slot 42 to turn the screw
`with an adjustment instrument as more clearly shown in
`the top plan view of FIG. 8b. A photodetector 24 and a
`pair of LED’s 22 are located at the bottom of a core of
`soft, compressible foam material 52 in the center of the
`screw, as shown in the bottom plan view of FIG. Be. A
`resilient member 50 is located above the compressible
`foam 52 in the center of the screw. The resilient member
`may comprise a metallic spring or a core of silicone
`rubber or polyurethane. The electrical leads 26’ from
`the LED's and photodetector pass through the foam
`material 52 and the resilient member 50 and exit through
`the top of the hollow screw as shown in FIG. Be.
`In use of the sensor embodiment of FIGS. 8a—8c, a
`hole is drilled in the skull into which the bone screw 40
`is screwed. As the bone screw is screwed into the skull,
`the oximeter monitor is continuously monitored for the
`onset of oxygen availability readings. When the bottom
`of the screw with the sensor components contacts the
`dura, oxygen readings will commence, and will initially
`occur erratically. As the bone screw is slowly turned
`the sensor components will make better contact with
`the dura and the signal quality will
`improve The
`contact between the sensor components and the dura is
`induced in a gentle manner by the compressible foam
`52, which will readily compress as the components
`make contact with the dura to prevent damage to the
`dura. The resilient member acts to maintain the com-
`pression of the foam 52. When consistent readings occur
`no further turning of the screw is necessary, as the
`sensor components are in good surface contact with the
`dura and will gently ride on the dura due to the com—
`pressibility of the foam 52.
`A preferred technique for using the embodiment of
`FIGS. 80—8c is to drill a hole in the skull and screw the
`bone screw into the hole before locating the sensor in
`the screw. After the bone screw is in place the sensor
`with its foam backing material 52 slides into the hollow
`bone screw, followed by insertion of the resilient mem-
`ber 50. The foam is gently compressed by the resilient
`member to cause the sensor to make good contact with
`the dura, a condition which is detected by monitoring
`the signal quality of the monitor. This technique obvi-
`ates problems of entangling the sensor wires as the bone
`screw is screwed into the skull with the sensor already
`positioned within the bone screw. Only after the bone
`
`

`

`5,024,226
`
`5
`screw is emplaced is the sensor with its backing and
`filling materials inserted into the screw, which can then
`be done without twisting the sensor components and
`the wiring to the sensor.
`This hollow bone screw embodiment is desirable for
`
`5
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`10
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`6
`5. The sensor of claim 1, wherein said flexible wiring
`comprises flexible printed wiring.
`6. The sensor of claim 1, wherein said coating is sili-
`cone rubber.
`
`7. The sensor of claim 1, wherein said coating is poly-
`urethane.
`8. The sensor of claim 1, wherein the optical window
`of said light emitting diodes is canted toward the area
`above said photodetector.
`9. The sensor of claim 1, further comprising a second
`pair of light emitting diodes located adjacent said pho-
`todetector and on the opposite side of said photodetec-
`tor as said first-named pair of light emitting diodes.
`10. The sensor of claim 1, further comprising a sec-
`ond photodetector located adjacent said light emitting
`diodes on the opposite side of said light emitting diodes
`as said first-named photodetector.
`11. The sensor of claim 10, wherein said photodetec-
`tors are canted toward the area above said light emitting
`diodes.
`12. A method of epidurally sensing oxygen availabil-
`ity comprising the steps of:
`drilling a burr hole in a skull;
`inserting a length of flexible wiring connected to a
`photodetector and a pair of light emitting diodes
`mounted in the proximity of the distal end of said
`wiring, said wiring, said photodetector, and said
`light emitting diodes encapsulated in a coating
`through said burr hole and between the skull and
`the dura, said encapsulated photodetector and said
`light emitting diodes having a width less than about
`20 mm to fit through the diameter of said burr hole
`and a thickness less than about 4 mm to slide be-
`tween the skull and dura, with the optical windows
`of said photodetector and said light emitting diodes
`opposing the dura;
`energizing said light emitting diodes, and receiving
`electrical signals from said photodetector resulting
`from the reception of reflected light emanating
`from said diodes, by way of said flexible wiring;
`and
`
`processing said electrical signals to produce an indi-
`cation of blood oxygen availability.
`II
`t
`t
`t
`it
`
`its ability to completely block ambient light from the
`sensor components, and by plugging the burr hole with
`the bone screw infection of the dura is retarded. The
`sensor can be safely left in place in the burr hole for
`extended periods of time.
`What is claimed is:
`1. A sensor for measuring cerebral oxygen availabil-
`ity through a burr hole in the skull by optical reflec-
`tance comprising:
`a length of flexible wiring having (a) a distal end and
`(b) a proximal end which is to be connected to an
`oximeter;
`a photodetector electrically connected to said flexible
`wiring in the proximity of said distal end;
`a pair of light emitting diodes connected to said flexi-
`' ble wiring adjacent to said photodetector; and
`a coating encapsulating said photodetector, said light
`emitting diodes, and said flexible wiring in the
`proximity of said photodetector and said light emit-
`ting diodes, said coating including optical windows
`where said coating overlies the optical windows of
`said photodetector and said light emitting diodes
`which is transmissive to light at the wavelengths of 30
`said light emitting diodes, said encapsulated photo-
`detector and said light emitting diodes having a
`width less than about 20 mm to fit through the
`diameter of said burr hole and a thickness less than
`about 4 mm to slide between the skull and dura.
`
`20
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`25
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`35
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`2. The sensor of claim 1, further including a light
`barrier positioned between said photodetector and said
`light emitting diodes which shields said photodetector
`from the direct reception of light from said light emit-
`ting diodes.
`_3. The sensor of claim 2, wherein said light barrier
`comprises opaque epoxy.
`4. The sensor of claim 2, wherein said light barrier
`comprises metal foil.
`
`45
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