(12) United States Patent
`US 6,343,223 B1
`(10) Patent No.:
`Jan. 29, 2002
`(45) Date of Patent:
`Chin et al.
`
`US006343223B1
`
`(54)
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`(75)
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`(73)
`
`OXIMETER SENSOR WITH OFFSET
`EMITTERS AND DETECTOR AND HEATING
`DEVICE
`
`Inventors: Rodney Chin, Oakland; Steven Hobbs,
`Pasadena, both of CA (US)
`
`Assignee: Mallinckrodt Inc., Hazelwood, MO
`(US)
`
`(*)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`US.C. 154(b) by 0 days.
`
`(21)
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`(22)
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`(63)
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`(61)
`(52)
`(58)
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`(56)
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`Appl. No.: 09/483,098
`
`Filed:
`
`Jan. 14, 2000
`
`Related U.S. Application Data
`
`Continuation-in-part of application No. 09/447,449,filed on
`Nov. 22, 1999, and a continuation-in-part of application No.
`08/903,120,filed on Jul. 30, 1997.
`
`Unt, C07 oieeeeccceecscesseseeseeseeseereeseeseeseesees A61B 5/00
`
`... 600/323; 600/334
`Field of Search ..........00..00000. 600/310, 322-324,
`600/334-344
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,805,623 A *
`4,807,631 A *
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`4,928,692 A *
`5,007,423 A *
`5,028,787 A *
`5,048,524 A *
`5,119,815 A *
`
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`4/1991 Branstetteret al.
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`6/1992 Chance
`
`5/1993 Tripp, Jr.
`5,213,099 A *
`6/1993 Jacotetal.
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`5,246,002 A *
`5,259,381 A * 11/1993 Cheungetal.
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`4/1994 Dahlin et al.
`5,313,940 A *
`5/1994 Fuseetal.
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`9/1994 Hollub
`5,351,685 A * 10/1994 Potratz
`5,355,882 A * 10/1994 Ukawaet al.
`5,368,224 A * 11/1994 Richardsonetal.
`5,372,134 A * 12/1994 Richardson
`5,373,850 A * 12/1994 Kohnoetal.
`5,379,238 A *
`1/1995 Stark
`5,408,998 A *
`4/1995 Mersch
`5,413,101 A *
`5/1995 Sugiura
`5,490,523 A *
`2/1996 Isaacsonetal.
`5,503,148 A *
`4/1996 Pologeetal.
`5,551,422 A *
`9/1996 Simonsenetal.
`5,551,423 A *
`9/1996 Sugiura
`5,596,986 A *
`1/1997 Goldfarb
`5,770,454 A *
`6/1998 Essenpreis etal.
`5,800,349 A *
`9/1998 Isaacsonetal.
`5,817,008 A * 10/1998 Rafertet al.
`
`* cited by examiner
`
`Primary Examiner—Linda C. M. Dvorak
`Assistant Examiner—David M. Ruddy
`(74) Attorney, Agent, or Firm—Townsend & Townsend &
`Crew LLP
`
`(57)
`
`ABSTRACT
`
`A method and apparatus for improving blood perfusion by
`both heating a patient’s skin and providing emitters and a
`detector which are offset from each other. Since the emitters
`and detector are not directly opposite each other,the light is
`forced to pass through more blood perfused tissue (with
`blood perfusion enhanced by heating) to pass from the
`emitters to the detector. This causes the light emitted by the
`emitters to pass through more blood-perfusedtissue to reach
`the detector than it would on the direct path through the
`appendage if the emitters and detector were opposite each
`other.
`
`26 Claims, 5 Drawing Sheets
`
` 1
`
`APPLE 1006
`
`APPLE 1006
`
`1
`
`

`

`U.S. Patent
`
`Jan. 29, 2002
`
`Sheet 1 of 5
`
`US 6,343,223 B1
`
`INPUT
`AMPLIFIER
`
`Pate
`oe Le
`SENSOR
`50
`
`16
`PHOTO-
`DETECTOR| |THERMISTOR
`
`
`
`
`
`
`|
`
`| |
`
`| | |
`
`14
`
`SENSOR
`
`LEDS||memory|
`
`62
`
`THERMISTOR
`CONTROL
`
`FIG. 1.
`
`2
`
`

`

`U.S. Patent
`
`Jan. 29, 2002
`
`Sheet 2 of 5
`
`US 6,343,223 B1
`
`EASUREMENT
`
`VOLTAGE
`
`MONITOReee J
`
`CONNECTOR
`
`SENSOR
`
`FIG. 2.
`
`THERM/ISTOR
`
`3
`
`

`

`U.S. Patent
`
`Jan. 29, 2002
`
`Sheet 3 of 5
`
`US 6,343,223 B1
`
`PRIOR ART
`118 F./G. 5B. SENSOR
`
`
`
`e-emitters
`d=-detector
`
`Telee feeorer
`
`
`
`
`
`
`4
`
`

`

`U.S. Patent
`
`Jan. 29, 2002
`
`Sheet 4 of 5
`
`US 6,343,223 B1
`
`62
`
`164
`
`92
`
`186
`
`(84
`
`60
`
`FIG. 6A.
`
`FIG. 6B.
`
`-
`FIG. &.
`
`182. 160
`
`
`5
`
`

`

`U.S. Patent
`
`Jan. 29, 2002
`
`Sheet 5 of 5
`
`US 6,343,223 B1
`
`
`
`FIG.
`
`II.
`
`6
`
`

`

`US 6,343,223 B1
`
`1
`OXIMETER SENSOR WITH OFFSET
`EMITTERS AND DETECTOR AND HEATING
`DEVICE
`
`RELATED APPLICATIONS
`
`This application is a continuation-in-part of applications
`Ser. No. 08/903,120, filed Jul. 30, 1997, entitled “Oximetry
`Sensor with Offset Emitters and Detector” and Ser. No.
`09/447,449, filed Nov. 22, 1999, entitled “Single Device for
`Both Heating and Temperature Measurementin an Oximeter
`Sensor.”
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates to oximeter sensors, and in
`particular oximeter sensors with a heating element
`to
`improve perfusion.
`Pulse oximetry is typically used to measure various blood
`characteristics including, but not
`limited to,
`the blood-
`oxygen saturation of hemoglobin in arterial blood, and the
`rate of blood pulsations corresponding to a heart rate of a
`patient. Measurement of these characteristics has been
`accomplished by use of a non-invasive sensor which passes
`light through a portion of the patient’s tissue where blood
`perfuses the tissue, and photoelectrically senses the absorp-
`tion of light in such tissue. The amountof light absorbed is
`then used to calculate the amount of blood constituent being
`measured.
`
`The light passed throughthetissue is selected to be of one
`or more wavelengths that are absorbed by the blood in an
`amountrepresentative of the amount of the blood constituent
`present in the blood. The amountof transmitted or reflected
`light passed through the tissue will vary in accordance with
`the changing amountof blood constituent in the tissue and
`the related light absorption. For measuring blood oxygen
`level, such sensors have been provided with light sources
`and photodetectors that are adapted to operate at two dif-
`ferent wavelengths, in accordance with known techniques
`for measuring blood oxygen saturation.
`Heaters have been used in sensors to improve the
`perfusion, or amountof blood, adjacent the sensor. This will
`thus improve the measurementsince the light will encounter
`a larger volumeof blood, giving a better signal-to-noise ratio
`for the oximeter reading.
`USS. Pat. No. 4,926,867 showsa piece of metal used as a
`heater in an oximeter sensor. A separate thermistor is used to
`measure the amount of heat so that
`the heater can be
`controlled to avoid burning the patient.
`USS. Pat. Nos. 5,299,570 and 4,890,619 both show ultra-
`sonic elements being used for perfusion enhancement.
`Because the normal human body core temperature is
`approximately 37° C., and burningoftissue could take place
`for temperatures above approximately 42-43° C., a tight
`range of control of the heating element is required. Another
`challenge is the heat gradient and delay time between the
`heating element and the temperature measuring element.
`Pulse oximeter sensors are often attachedto a digit, or ear.
`These sites on a patient provide an adequate level of blood
`perfusion for measuring the oxygenation of the blood hemo-
`globin. In addition, the distance across these appendagesis
`sufficiently short to allow the detection of transmitted red or
`infrared light.
`One type of sensor is a clothespin-type clip which
`attaches across the earlobe, with the emitter and detector
`opposite each other. Such conventional sensors sometime
`suffer from poorsensitivity and poorcalibration or accuracy.
`
`15
`
`20
`
`25
`
`30
`
`35
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`40
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`45
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`50
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`55
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`60
`
`2
`This type of sensor often applies pressure which exsan-
`guinates the tissue and alters the blood present leading to
`accuracy errors.
`One type of oximeter sensor will add a diffusing optic to
`diffuse the light emitted from the light-emitting diodes
`(LEDs) to cause it to pass through more tissue, and thus
`more blood. An example of a pulse oximeter sensor using
`such a diffusing element is shown in U.S. Pat. No. 4,407,
`290.
`
`Onetechnique for limiting the exsanguination effect is to
`separate the light emitters and detector from the portion of
`the sensor which holdsit to the appendage and applies the
`pressure. Examples of sensors where the light emitters and
`detector avoid the point of pressure are set forth in U.S. Pat.
`Nos. 5,413,101 and 5,551,422.
`Another type of clip-on sensor is marketed by Nonin
`Medical, Inc. for attaching to an ear. Instead of using a
`transmission sensor where light
`is transmitted from an
`emitter on one side of the ear through the ear to a detector
`on the other side, a reflectance sensor is used with both the
`emitter and detector on the same side of the ear. The Nonin
`
`medical sensor has spacing between the emitter and the
`detector of approximately 4 mm, which is similar to the
`thickness of a typical earlobe. On the opposite side of the ear
`a reflective surface is provided to reflect the light from the
`emitter back to the detector.
`
`The typical distance of a standard, bandaid-type reflec-
`tance sensor which can attach to the forehead or other part
`of the body is 6-10 mm. Traditionally, a spacing of this
`magnitude was felt
`to be appropriate to ensure that a
`measurable amountof light could be detected with sufficient
`pulsatile signal components.
`SUMMARYOF THE INVENTION
`
`The present invention provides a method and apparatus
`for improving blood perfusion by both heating a patient’s
`skin and providing emitters and a detector which are offset
`from each other. Since the emitters and detector are not
`
`is forced to pass
`the light
`directly opposite each other,
`through more blood perfused tissue (with blood perfusion
`enhanced by heating) to pass from the emitters to the
`detector. This causes the light emitted by the emitters to pass
`through more blood-perfused tissue to reach the detector
`than it would on the direct path through the appendageif the
`emitters and detector were opposite each other.
`In one embodiment, the heater is a thermistor. The ther-
`mistor generates controlled heat, and is not just used for
`sensing the temperature. In an oximetry sensor, the ther-
`mistor is located in the vicinity of the light emitter and
`photodetector to warm the optically-probed tissue region. As
`heat is dissipated, temperature changes are sensed asresis-
`tance changes according to Ohm’s law. Active thermal
`regulation by varying the amountof thermistor current and
`powercan safeguard against burning the tissue while maxi-
`mizing perfusion. The combination of heating and offset
`increase the amountof bloodthatthe light from the emitters
`passes through.
`It has been shownrecently that general warming of the
`tissue region increases the amount of blood perfused in the
`tissue. This increased perfusion substantially strengthens the
`pulse oximetry signal. Benefits include quick signal
`acquisition,
`increased accuracy, and greater tolerance to
`motion artifact.
`
`In one embodiment, the thermistor is a positive tempera-
`ture coefficient (PTC)
`thermistor rather than the more
`common, negative temperature coefficient
`(NTC)
`ther-
`7
`
`7
`
`

`

`US 6,343,223 B1
`
`3
`mistor. The PTC providesa highly desirable safety feature as
`poor connections yield a perceived, higher-than-normal
`resistance indication. As a result,
`the actual
`thermistor
`temperature is regulated at
`a lower-than-expected
`temperature, avoiding the chance of burns.
`Another advantage of the same thermistor being used for
`both generating heat and temperature measurementis that
`there is no thermal gradient between the heating element and
`the sensing element as in the prior art. This allows for a
`faster response time, whichis critical for maintaining a tight
`temperature range.
`The thermistor’s resistance can be conventionally deter-
`mined either by a two-wire or a four-wire method. The
`four-wire method is typically used when the connections
`used in the two-wire method would have resistances that
`
`could significantly affect the measurement. In the four-wire
`method, one pair of wires is used to inject a Known current
`through the thermistor, while the other pair is used to sense
`the voltage across the thermistor. This enables a highly
`accurate determination of the thermistor’s temperature.
`In an alternate embodiment, a simple bridge circuit with
`a setpoint resistor may be used to automatically bias the
`thermistor at a particular resistance/temperature. Once the
`thermistor’s desired operating resistance is known from the
`factory, the appropriate value of the setpoint resistor can be
`employed in the circuit. This simple circuit could be inte-
`grated into the sensoritself or in the remote monitor.
`In one offset, the sensor includes at least one reflecting
`surface for redirecting light back to the blood-perfused
`tissue in the region of the offset between the emitters and
`detector. Preferably, the offset distance is at least greater
`than, and more preferably at least twice as great as,
`the
`direct, shortest path through the appendage.
`In an alternate embodiment, a reflectance-type sensoris
`used, with a reflective surface on the opposite side of the
`appendage. Unlike the prior art, however,
`the distance
`between the emitter and detector is greater than, and pref-
`erably twice as great as, the shortest, direct distance through
`the appendage.
`For a further understanding of the nature and advantages
`of the invention, reference should be madeto the following
`description taken in conjunction with the accompanying
`drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a diagram of a pulse oximetry system including
`the present invention.
`FIG. 2 is a diagram illustrating four-wire measurementin
`one embodiment of the invention.
`
`FIG. 3 is a diagram of an embodimentusing a large area
`thermistor and a reflective type oximeter sensor.
`FIG. 4 is a circuit diagram of an embodimentof a bridge
`circuit for regulating the thermistor temperature.
`FIG. 5A is a diagram of a prior art emitter and detector
`configuration.
`FIGS. 5B-5G are diagrams of different embodiments of
`the configuration of the emitter and detector accordingto the
`invention.
`FIGS. 6A and 6B are end and side viewsof an ear sensor
`
`according to the invention.
`FIGS. 7A and 7Bare side and top viewsofa nostril sensor
`according to the invention.
`FIGS. 8 and 9 are diagrams of alternate embodiments
`illustrating curves in the sensor.
`
`10
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`15
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`20
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`25
`
`30
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`35
`
`40
`
`45
`
`50
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`55
`
`60
`
`65
`
`4
`FIG. 10 is a diagram of a sensor with foam for distributing
`applied pressure.
`FIG. 11 is a diagram of an adhesive reflectance sensor
`with a thermistor.
`
`DESCRIPTION OF THE SPECIFIC
`EMBODIMENTS
`
`FIG. 1 is a block diagram of one preferred embodiment of
`the invention. FIG. 1 showsa pulse oximeter 17 (or sensor
`reader) which is connected to a non-invasive sensor 15
`attached to patient tissue 18. Light from sensor LEDs 14
`passes into the patient tissue 18, and after being transmitted
`through or reflected from tissue 18, the light is received by
`photosensor 16. Either two or three LEDs or other light
`sources can be used depending upon the embodimentof the
`present invention. The LEDs and photosensorare offset as
`described in more detail below with respect to FIGS. 5-10.
`Photosensor 16 converts the received energy into an elec-
`trical signal, which is then fed to input amplifier 20.
`Light sources other than LEDs can be used. For example,
`lasers could be used, or a white light source could be used
`with appropriate wavelength filters either at the transmitting
`or receiving ends. The light could be delivered to the patient
`site with fiber optics, with the light source in the sensor or
`remotely located.
`Time Processing Unit (TPU) 48 sends control signals to
`the LED drive 32, to alternately activate the LEDs,typically
`in alteration. Again, depending on the embodiment, the drive
`may control two or any additional desired number of LEDs.
`The signal received from input amplifier 20 is passed
`through three different channels as shown in this embodi-
`mentfor three different wavelengths. Alternately, two chan-
`nels for two wavelengths could be used, or N channels for
`N wavelengths. Each channel includes an analog switch 40,
`a low passfilter 42, and an analog to digital (A/D) converter
`38. Control lines 69 from TPU 48 select the appropriate
`channel at
`the time the corresponding LED 14 is being
`driven, in synchronization. A queued serial module (QSM)
`46 receives the digital data from each of the channels via
`data lines 79. CPU 50 transfers the data from QSM 46 into
`RAM 52 as QSM 46 periodically fills up.
`In one
`embodiment, QSM 46, TPU 48, CPU 50 and RAM 352 are
`part of one integrated circuit, such as a microcontroller.
`A thermistor 60 is shown mounted in sensor 15. Ther-
`mistor 60 could be mounted adjacent the photodetector or
`the LEDs,or nearby. A thermistor control circuit 62 provides
`the power and currentto the thermistor to deliver the desired
`heat, while measuring the resulting resistance, and thus the
`temperature. The thermistor can either be a positive tem-
`perature coefficient (PTC) or a negative temperature coef-
`ficient (NTC) thermistor.
`The thermistor is used in a dual capacity to dissipate
`thermal heat energy and self-monitor its temperature for the
`safe operation in a “warmed” oximeter sensor.
`A positive temperature coefficient (PTC) thermistor is
`more desirable than a negative temperature coefficient
`(NTC) thermistor for oximetry/medical applications. For a
`given voltage source applied to the thermistor, the power
`dissipation decreases with increasing temperature due to the
`increased resistance at higher temperatures. Additionally, if
`there exists connection resistances within the sensor cable
`and/or connections,the increased series resistance would be
`perceived by the oximeter as a falsely higher temperature.
`This is desirable as the oximeter would regulate the sensor
`at a lower (safe) temperature and avoid the possibility for
`patient burns. Since PTC thermistors generally have thermal
`8
`
`8
`
`

`

`US 6,343,223 B1
`
`5
`coefficients that are smaller than for NTC, special PTC
`thermistors may be used. The nonlinear behavior of the
`switching or nonlinear PTC thermistors is desirable. These
`are available from Advanced Thermalproducts, St. Mary’s,
`PA and other sources. The material
`is processed so the
`switching temperature is between 40-50° C., generally.
`In one embodiment, it is desirable to have a PTC tran-
`sistor with a phase transition, where the resistance suddenly
`increases, in the region between 40-50° C. This can be
`controlled in a numberof different ways, such as by appro-
`priate doping of the thermistor material.
`In practice, the PTC thermistor is regulated at 39-41° C.
`This is just slightly above normal (37° C.) core body
`temperature but below the burn threshold of 42—43° C. It has
`been shown recently that general warming of the tissue
`region probed by the oximetry sensor increases localized
`perfusion and increasesthe strength of the pulsatile oximetry
`signal. The benefit of this includes an increase in the
`acquisition and accuracy of the oximetry measurement and
`an increase in the tolerance to motion artifact.
`
`An advantage of the same thermistor being used for both
`generating heat and for measuring it
`is that there is no
`thermal gradient between the heating element and the sens-
`ing element as in the prior art. This allows for a faster
`response time, whichiscritical in maintaining a temperature
`within a tight range, as required.
`FIG. 2 illustrates a four-wire measurement system for a
`thermistor of the present invention. FIG. 2 shows a monitor
`64 with a currentdrive circuit 66 and a voltage measurement
`circuit 68. Each are separately connected by two wires to a
`connector 70 close to sensor 15. From connector 70, the four
`wires are converted into two wires for connecting to the
`actual sensor. Alternately, the four wires can extend all the
`way to thermistor 60.
`Current drive circuit 66 is programmable to provide the
`appropriate amount of current to achieve the desired power
`dissipation and temperature through thermistor 60. Voltage
`measurementcircuit 68 simultaneously measuresthe result-
`ing voltage, which will allow the determination of the
`resistance from the knowndrive current. By using four wires
`to a position close to the sensor, the resistance effects of the
`wiring and any connections are also taken into account.
`The other connections in FIG. 2 are not shownin order not
`
`to obscure the connections of the thermistor. Memory chip
`12 in one embodimentis used to store thermal coefficients
`of the thermistor or other thermal parameters of the sensor.
`These parameters can then be read by the oximeter monitor
`64 and used by its CPU 50 to determine an appropriate drive
`current for the thermistor. The temperature control is done in
`part by the hardware andin part by software in the CPU. The
`amount of powerdissipated in the thermistor is controlled by
`the resistance measurement, which corresponds to a tem-
`perature measurement.
`The sensor could be any type of sensor, such as a durable
`sensor or a disposable sensor. It could attach to any body
`part, such as the earlobe, finger, etc. The sensor could be a
`reflectance or a transmittance sensor.
`
`10
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`20
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`25
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`40
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`50
`
`Since commercially available thermistors often vary sig-
`nificantly in their actual resistance value, the thermistors can
`either be trimmedat the factory, or a precision resistor could
`be placed in series or in parallel to adjust the resistance to the
`desired value.
`
`60
`
`In one embodiment, shown in FIG. 3, the sensor 72 uses
`a single thermistor element 74 with a reflectance geometry.
`The thermistor is opposite to the reflectance sensor emitter
`76 and detector 78. this allows a large warming surface to
`contact the tissue 80 for the ear sensor.
`
`6
`The thermistor need not directly contact the skin because
`the thermal loading could be asymmetrically strong to cause
`a lengthwise thermal gradient and an error in the tempera-
`ture measurement. The thermistor is in close contact for
`maximum heat transfer but is somewhat embedded inside
`
`the sensor housing. A thin layer between the thermistor and
`contact surface may act as a buffer to allow a uniform,
`heat-spreading action.
`FIG. 4 is a circuit diagram of an alternate embodiment
`which allows a thermistor to be set to a desired temperature
`without intervention by a microprocessor.
`A floating resistive bridge circuit 80 can be biased at high
`or low current. Alternately, this current bias can be made
`continuously adjustable. The nulling of the bridge signifies
`when the setpoint temperature has been met. A setpoint
`resistor 82 is adjusted for the proper setpoint temperature
`(resistance) of the thermistor 84. When the thermistor’s
`resistance (temperature) is too high, a comparator circuit 86
`is switched to cause the bridge to be biased in the low current
`mode to minimize the current through the thermistor (by
`turning off transistor Q1, forcing the current throughresistor
`RQ). Conversely, when the thermistor’s resistance
`(temperature) is too low, the comparatorcircuit is switched
`to cause the bridge to be biased in the high current mode
`supplying more current and thus more powerto the ther-
`mistor (turning on transistor Q1, bypassing resistor RQ).
`There must be some voltage (current) supplied to the bridge
`to allow for sensing of the thermistor’s resistance for the null
`measurementof the bridge circuit.
`Obviously, a more elaborate thermal regulation circuit
`could be built. However, it has been found that this circuit
`works very well with no significant temperature overshoot/
`undershoot. This is due to the intrinsic self-measurement
`
`nature of the system with no thermal delay time between the
`warming element and the temperature sensor. Typical maxi-
`mum power dissipation for effective application of a
`warmedearlobe sensoris less than 0.5 watts per side. With
`properheat spreading,the thermistoris efficient at delivering
`the thermal energy without incurring a large thermal gradi-
`ent from the thermistor to the tissue. This would give the
`best tissue temperature and the best performance.
`Because of the simplicity of the circuit with few
`components,it is possible to integrate the whole circuit in
`the oximetry sensor assembly. The circuit consists of only a
`few components as shown. The benefit of this would be the
`requirement of only a single power supply connection and
`utilizing an existing ground connection. An adapter cable
`could be used with older instruments to supply the additional
`supply lead.
`FIG. 5Aillustrates a prior art configuration in which an
`emitter 112 is opposite a detector 114 across an earlobe,
`nostril, digit, or other appendage 116. The present invention
`provides an offset emitter and sensor to improve upon this
`arrangement, providing morearea for the light to penetrate
`between emitter and detector. In addition, the figures below
`show thermistor 60, which provides a heating function to
`further enhance blood perfusion. Alternately, a simple heater
`can be substituted for thermistor 60. Also, although clip type
`sensors are shown below, an adhesive sensor could be used,
`with reflectance oximetry. Alternately, adhesives could be
`used to attach a transmissive sensor on an appendage, with
`adhesives on one or both sides of the appendage.
`FIG. 5B illustrates an offset configuration in which an
`emitter 118 is offset from a detector 120 as can been seen,
`providing a longer light transmission path 122. Emitter 118
`is typically a pair of emitters, an infrared range emitter and
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`a red range emitter, which are mounted in a portion 124 of
`a sensor probe. Detector 120 is a photodetector which is
`mounted in a portion 126 of a sensor probe.
`FIG. 5C illustrates an alternate embodiment in which
`emitter 128 is spaced from a detector 130 by an offset
`distance which is more than twice the width of appendage
`132. As can be seen, this provides a much longer transmis-
`sion path 138.
`FIG. 5D illustrates an embodiment similar to FIG. 5C,
`wherea pair of reflectors 133 and 134 have been added. As
`can be seen, the reflectors 133 and 134 cause the light path
`136 in FIG. 5D to be longerthan the light path 138 in FIG.
`5C. This is due to light which goes across the entire
`appendagebeing reflected back in, and then back in from the
`other surface, bouncing back and forth between the reflec-
`tors until it reaches the detector from the emitter. In FIG. 1C,
`by contrast, the light which reaches the detector from the
`emitter is substantially the light which moves in a path
`through the body of the appendage, since light which would
`hit the edges would typically be absorbed,rather than being
`reflected.
`
`The reflective surface 133 may be, for instance, a white
`surface which will reflect both red and infrared light. This
`will enhance the path length of both red and infrared light.
`Alternatively, the reflective surface 133 may be “colored” to
`reflect red light more than infrared light (or vice versa) to
`compensate for skin pigmentation effects.
`FIGS. 5E and 5F show alternate embodiments in which
`the emitter and detector are on the sameside of the append-
`age in a reflectance configuration. As shown in FIG. 1E,an
`emitter 140 and a detector 142 are in a portion 144 of a
`sensor attached to an appendage 146, such as an earlobe. The
`sensor, which may be a clip-on type sensor, has a second
`portion 148 opposite portion 144. Portion 148 includes a
`reflective surface 150. As can be seen, the light path 152 will
`thus be reflected back from surface 150, providing more
`light
`to detector 142 than would be found in a typical
`reflectance configuration. (Please note that the light path
`shown in these figures is merely illustrative). The use of
`reflector 150 allows not only more light to be directed back
`into the tissue to arrive at detector 142, but allows a larger
`space between emitter 140 and detector 142. As in FIGS. 5C
`and 5D, the distance L between the emitter and detector in
`FIG. 5E is preferably greater than the width t of the
`appendage, and preferably a value of L whichis at least
`twice t.
`FIG. 5F showsan alternate embodimentto that of FIG. 5E
`in which a secondreflector 154 is added between the emitter
`140 and detector 142 in portion 144 of the sensor probe. This
`prevents the light from being absorbed in the body of the
`sensor 144 between emitter 140 and detector 142 on the
`
`sameside. A reflector on one side will improve performance
`over a sensor without such a reflector, while a reflector on
`both sides would typically give even more enhanced per-
`formance. However, even a single reflector provides a
`significant improvement in the amountof light reaching the
`detector.
`Also shown in FIG. 5F is a shunt barrier 156. Shunt
`
`barrier 156 prevents light from shunting directly between
`emitter 140 and detector 142 through sensor body 144
`without passing through appendage 146. Examples of shunt
`barriers are set forth in commonly-owned copending appli-
`cation entitled SHUNT BARRIER IN PULSE OXIMETER
`
`SENSOR, application Ser. No. 08/611,151, filed Mar. 5,
`1996.
`FIG. 5G shows an alternate embodiment in which two
`emitters, 161 and 163, have a different offset distance from
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`the detector. This can be used to partially compensate for a
`difference in absorption of red and infrared.
`FIG. 6A showsan end view of one embodimentof an ear
`
`invention. Other
`clip sensor according to the present
`embodiments are possible, but this embodiment shows a
`simple, inexpensive, disposable-type sensor. A bent piece of
`metal 160 holds pads 162 and 164, which contain the light
`emitters and detector, respectively. Bent metal 160 is springy
`to provide pressure applying the pads 162, 164 against the
`earlobe. The pads (162 and 164) are rigid since the earlobe
`conformseasily. Preferably, slowly deformable spring mate-
`rial
`is used, which is an assembly which provides the
`gripping action but has a damping component which pre-
`vents quick movements. (e.g., metal sheet as the spring with
`a rubber coating of laminate).
`In the side view of FIG. 6B, pad 162 is shown, along with
`the position of an emitter 166. Shown in phantom is the
`position on the other pad where detector 168 would be
`located.
`
`FIGS. 7A and 7B show a similar configuration for a
`nostril sensor, which is basically more slender and narrow.
`As shown in FIG. 7B, a bent metal 170 provides the
`springiness for pads 172 and 174. Pad 172 includes an
`emitter 176, while pad 174 includes a detector 178. Also
`shown is an optional optical diffuser 180 for diffusing the
`light from emitter 176, which causes a further spreading or
`mixing of light and may enhance the amount of tissue
`penetrated in some instances.
`FIG. 7A showsa side view with the relative position of
`emitter 176 and detector 178 shown in phantom.
`FIG. 8 illustrates an exaggerated view of the construction
`of one embodimentof the sensor of FIGS. 6A, 6B, 7A and
`7B. In the view of FIG. 8, an emitter 180 and detector 182
`are shown.
`
`Emitter 180 is mounted on the edge of a curved portion
`184 of one end of the sensor, while detector 182 is mounted
`near the end of a curved portion 186 on the other side of the
`sensor. The curvature in FIG. 8 would range from zero (no
`curvature) to less than 15% depth ofoffset distance orto less
`than 30% depth of offset distance. These curved portions
`ensure that less pressure will be applied to the appendage
`in-between the emitter and detector. Instead, more pressure
`is applied, for instance, to points 188 and 190, which are
`outside of the region in-between the emitter and detector.
`Thus, this configuration reduces the exsanguination of the
`tissue in-between the emitter and detector.It is desirable that
`
`somepressure is applied throughout to reduce the amount of
`venous pooling in the tissue.
`Preferably,
`the spring force of the metal clip in the
`embodiments of FIGS. 6-10 has sufficient pressure so that
`it exceeds the typical venous pressure of a patient, but does
`not exceed the diastolic arterial pressure. The signal received
`by the detector will include both a DC component and an AC
`component. The AC and DC components are monitored to
`determine variations in the oxygen saturation. By having a
`pressure greater than the venous pressure, contributions to
`the AC waveform from the venous blood are limited, thus
`enhancing the sensitivity to variations in the arterial blood
`pressure. Since the pressure of the clip is less than that of the
`arterial pressure, it does not inhibit the arterial AC signal
`significantly.
`The pressure applied to the spring is such that the pressure
`exerted on the tissue is equal to the force applied by the
`spring divided by the contact area to the tissue. Since the
`system is in steady state, the compressed tissue will be at a
`minimum pressure exerted by the contact surfaces.
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`modulation of the DC signal at 100% SpO2). This increased
`Typical venous pressure, diastolic and systolic arterial
`ACcardiac signal level is believed to be due to the longer
`pressures are <10-35 mmHg, 80 mmHg, and 120 mmHg,
`absorption path length. The increased AC cardiac signal
`respectively. Functionally, these vary due to the location of
`amplitude allows it
`to be more easily processed by the
`the vascular bed and the patient’s condition. Low arterial
`oximeter electronics and software. In addition, the increased
`diastolic blood pressure (~30 mmHg) may occur in sick
`AC cardiac modulation level limits the sensor’s susceptibil-
`pa