[54]
`
`{75]
`
`[73]
`(21)
`
`[22]
`
`[63]
`
`[51]
`[52]
`
`[58]
`
`[56]
`
`United States Patent
`Pologe
`
`19)
`
`COQACATAA
`US005297548A
`5,297,548
`[11] Patent Number:
`Mar,29, 1994
`
`[45] Date of Patent:
`
`ARTERIAL BLOOD MONITORING PROBE
`
`5,137,023
`
`8/1992 Mendelsonetal. ...........0.. 128/633
`
`Inventor:
`
`Jonas A. Pologe, Boulder, Colo.
`
`Assignee:
`
`OhmedaInc., Murray Hill, N.J.
`
`Appl. No.:
`Filed:
`
`45,962
`
`Apr. 12, 1993
`
`Related U.S. Application Data
`Continuation-in-part of Ser. No. 832,551, Feb. 7, 1992.
`Tht 00S cccscesencceessenssreeeneerenes
`. A61B 5/00
`WSs Oly csasasnisausuiaaviiin-.128/633; 128/665;
`356/41
`Field of Search...........000
`. 128/633-634,
`128/664-667; 356/39-41
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,223,680 9/1980 Jobsis ......ccssssseseeseenesneeeeees 128/633
`4,824,242 4/1989 Frick et al.
`...
`. 128/666 X
`
`4,867,165 9/1989 Noller et al. occ 128/666 X
`5,078,136
`1/1992 Stone et al. occ 128/666 X
`
`5,127,406
`7/1992 Yarmaguchi
`......seseressenenees 128/633
`
`FOREIGN PATENT DOCUMENTS
`
`0303502 2/1989 European Pat. Off.
`9004353 5/1990 PCT Int'l Appl.
`.
`911136
`8/1991 PCT Int'l Appl.
`.
`
`.
`
`Primary Examiner—Angela D. Sykes
`Attorney, Agent, or Firm—Roger M. Rathbun; Larry R.
`Cassett; James M. Graziano
`
`ABSTRACT
`[57]
`This arterial blood monitoring system takes advantage
`ofthe basic statistical property that arterial blood con-
`tains a plurality of dominant absorbers, whose measured
`light absorption spectra appear as a constant Over a
`short interval of time. By measuring the transmitted
`light as it varies with arterial pulsation at selected wave-
`lengths oflight, over a commonlightpath,the relative
`amount of these dominant absorbers in the arterial
`blood can noninvasively be determined. To ensure the
`common light path, a sandwich construction light de-
`tector is used.
`
`10Claims, 4 Drawing Sheets
`
`yoo
`
` PROCESSOR
`
`SYNCHRONOUS
`,
`woe
`MPLI-|DEMODULA-
`TION
`VOLTAGE
`FIER
`105 CONVERTER]_
`
`
`DISPLAY
`DRIVER
`
`DISPLAY MODULE
`
`PLETHYSMOGRAPHIC
`WAVEFORM
`
`0001
`
`Apple Inc.
`APL1020
`U.S. Patent No. 8,652,040
`
`0001
`
`Apple Inc.
`APL1020
`U.S. Patent No. 8,652,040
`
`

`

`U.S. Patent
`
`Mar. 29, 1994
`
`Sheet 1 of 4
`
`5,297,548
`
`Lv 100
`
`EMITTER DRIVER
`CIRCUIT
`
`VOLTAGE
`CONVERTER
`
`SYNCHRONOUS
`DEMODULA-
`
`oow
`
`)
`YW)
`uJ
`oO
`
`WAVEFORM
`
`ocO
`
`o.
`
`DISPLAY MODULE
`
`PLETHYSMOGRAPHIC
`
`DISPLAY
`DRIVER
`
`0002
`
`0002
`
`

`

`U.S. Patent
`
`©
`
`Mar. 29, 1994
`
`Sheet 2 of 4
`
`5,297,548
`
`
`
`
`
`
`ARTERIAL PULSATION
`ABSORPTION
`
`ARTERIAL BLOOD
`ABSORPTION
`
`VENOUS BLOOD
`ABSORPTION
`
`ABSORPTION
`
`
`OTHER TISSUE
`ABSORPTION
`
`0003
`
`0003
`
`

`

`U.S. Patent
`
`Mar, 29, 1994
`
`Sheet 3 of 4
`
`5,297,548
`
`RECEIVE DATA FROM PROBE AND
`
`410
`
`TRANSMIT DATA TO DISPLAY
`DRIVER TO PRODUCE
`
`STORE IN MEMORY
`WAVEFORMDISPLAY
`
`
`402 COMPUTE DIFFERENTIAL CHANGEIN
`
`ABSORPTION AT FIRST WAVELENGTH
`
`403
`
`COMPUTE DIFFERENTIAL CHANGEIN
`
`ABSORPTION AT SECOND WAVELENGTH
`
`
`
`COMPUTE RATIO OF COMPUTEDDIFFERENTIAL
`
`CHANGEIN ABSORPION FOR FIRST WAVELENGTH
`TO COMPUTED DIFFERENTIAL CHANGEIN
`
`ABSORPTION FOR SECOND WAVELENGTH
`
`
`
`
`DRIVER TO PRODUCE NUMERIC OUTPUT
`
`TRANSMIT COMPUTEDtHb TO DISPLAY
`
`FIGURE
`
`4
`
`0004
`
`0004
`
`

`

`U.S. Patent
`
`Mar. 29, 1994
`
`Sheet 4 of 4
`
`5,297,548
`
`501
`
`510
`
`~~ RECEIVE DATA FROM PROBE AND
`— STORE IN MEMORY
`TRANSMIT DATA TO DISPLAY
`DRIVER TO PRODUCE
`WAVEFORM DISPLAY
`
`
`
`502 ~\\__[@OMPUTE DIFFERENTIAL CHANGE IN
`
`ABSORPTION AT FIRST WAVELENGTH
`
`503
`"\__ [COMPUTE DIFFERENTIAL CHANGE IN
`ABSORPTION AT SECOND WAVELENGTH
`
`504
`:
`~\_|coMPUTE DIFFERENTIAL CHANGE IN
`
`ABSORPTION AT THIRD WAVELENGTH
`505 =
`
`
`
`
`
`COMPUTE tHb FROM THE
`COMPUTE 8a02 FROM THE
`MEASURED DIFFERENTIAL
`
`MEASURED DIFFERENTIAL
`CHANGE IN ABSORPTION
`CHANGE IN ABSORPTION
`
`AT ALL WAVELENGTHS
`AT ALL WAVELENGTHS
`
`
`
`
`
`
`
`
`
`
`COMPUTE O2 ct FROM tHb AND 8a02
`
`508 ———___
`
`TRANSMIT tHb, 8a02 AND O2 ct DATA
`TO DISPLAY DRIVER TO PRODUCE
`NUMERIC OUTPUT
`
`FIGURE
`5
`
`0005
`
`0005
`
`

`

`1
`
`5,297,548
`
`ARTERIAL BLOOD MONITORING PROBE
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`This application is a continuation-in-part of U.S. pa-
`tent application Ser. No. 07/832,551, titled “Improved
`Arterial Blood Monitoring System”, filed Feb. 7, 1992,
`pending
`
`FIELD OF THE INVENTION
`
`This invention relates to non-invasive photoplethys-
`mographic measurementof blood analytes and, in par-
`ticular, to a probe for use in an arterial blood monitoring
`system to more accurately measure the changein inten-
`sity of the light transmitted throughthearterial blood of
`a patient.
`
`.
`
`10
`
`15
`
`PROBLEM
`
`20
`
`It is a problem in the field medical monitoring equip-
`ment to accurately measure various parameters of arte-
`rial blood in a noninvasive manner. For example, the
`oxygen saturation (Sg O2) of the hemoglobin in arterial
`blood is determined by the relative proportions of oxy-
`genated hemoglobin and reduced hemoglobin in the
`arterial blood. A pulse oximeter system noninvasively
`determines the oxygen saturation of the hemoglobin by
`measuring the difference in the light absorption ofthese
`two forms of hemoglobin. Reduced hemoglobin absorbs
`morelight in the red band (600-800 nm) than does oxy-
`hemoglobin while oxyhemoglobin absorbs morelight in
`the near infrared band (800-1000 nm) than does reduced
`hemoglobin.
`The pulse oximeterincludesa probethat is placed in
`contact with the skin, either on a flat surface in the case
`of reflectance probes or across some appendage in the
`case of a transmission probe. The probe contains two
`light emitting diodes, each of which emits a beam of
`light at a specific wavelength, one in the red band and
`one in the infrared band. The magnitude of red and
`infrared light transmitted through the intervening ap-
`pendage contains a non-pulsatile component which is
`influenced by the absorbency oftissue, venous blood,
`capillary blood, non-pulsatile arterial blood, and the
`intensity of the light source. The pulsatile component of
`the received signals is an indication of the expansion of
`the arteriolar bed in the appendage with arterial blood.
`Theeffects of different tissue thicknesses and skin pig-
`mentation in the appendage can be removed from the
`received signals by normalizing the change in intensity
`of the received signal by the absolute intensity of the
`received signal. Taking the ratio of the mathematically
`processed and normalized red and infrared signals re-
`sults in a number which is theoretically a function of
`only the concentration of oxyhemoglobin and reduced
`hemoglobin in the arterial blood. This assumes that
`oxyhemoglobin and reduced hemoglobin are the only
`substantial absorbers in the arterial blood.
`The amplitude of the pulsatile componentis a very
`small percentageof the total signal amplitude and de-
`pends on the blood volume change per pulse and the
`oxygen saturation (S,O2) of the arterial blood. The
`received red and infrared signals have an exponential
`relationship to the path length ofthe arterial blood. The
`photoplethysmographic measurementofthese analytes
`is predicated on the assumption that the light beams
`from the two light sources follow identical paths
`through the intervening appendageto thelight detec-
`
`2
`tor. The greater the departure of the light beams from a
`commonlight path, the more significant the opportu-
`nity for the introduction of errors into the resultant
`measurements. This is especially true if multiple inde-
`pendent discrete light sources and multiple discrete
`light detectors are used in the probe, resulting in sepa-
`rate light transmission paths through the intervening
`appendage. The use of multiple light detectors, each
`sensitive to different wavelength regions, becomes a
`necessity if the wavelengths of light selected are far
`apart in wavelength, since there does notexist a single
`light detector device that can detect a wide bandwidth
`oflight with significant speed, sensitivity and an accept-
`ably flat response. Therefore, existing probe designs can
`introduce errors into the measurements by their inabil-
`ity to transmit a plurality of light beams substantially
`along a commonlightpath throughthearteriolar bed of
`the appendage being monitored.
`SOLUTION
`
`The above described problemsare solved and a tech-
`nical advance achieved in the field by the probe for an
`arterial blood monitoring system that creates a single
`light path through an appendage to noninvasively mea-
`sure and calculate characteristics of arterial blood. This
`arterial blood monitoring system probe takes advantage
`of the basic statistical property that arterial blood con-
`tains a plurality of dominant absorbers, whose measured
`light absorption spectra appear as a constant over a
`short interval of time. The arterial blood characteristics
`to be measured are empirically related to the changes in
`the measured light transmission through the plurality of
`dominant absorbers as a function of the changes in arte-
`rial blood volume at the probesite. By measuring the
`transmitted light as it varies with arterial pulsation at a
`plurality of selected wavelengthsoflight, over a single
`commonlight path, the relative amount of these domi-
`nant absorbers in the arterial blood can noninvasively
`be determined.
`Byselecting one wavelengthoflight around 1270 nm,
`where water has a measurable extinction and second
`and third wavelengths at about 660 nm and 940 nm, a
`direct relationship between the transmitted intensities at
`these three wavelengths and the arterial hemoglobin
`concentration exists and can be calculated. The accu-
`rate detection of these three wavelengths of light is
`accomplished by the use of two different light detec-
`tors. To avoid the problem of different light paths
`through the intervening appendage, a sandwichorlay-
`ered detector design is used in the probe. The light
`detector consists of a multiple layer element that con-
`tains a germanium photodiode placed under, and coinci-
`dent with, a silicon photodiode. For the wavelengths of
`light shorter than approximately 1000 nm,thesilicon
`photodiode receives the incident light and produces a
`signal indicative of the intensity of the received light.
`Abovethis wavelength, the silicon photodiode becomes
`transparent and the germanium photodiode picks up the
`incident
`light. Thus,
`the light from the three light
`sources is transmitted through thetissue along substan-
`tially identical light paths to be detected by the coinci-
`dent light detectors at exactly the same “exit area”,
`regardless of wavelength. By constraining the detected
`: light to traverse one path through thetissue, regardless
`of wavelength, this apparatus avoids the inaccuracies
`caused by sampling different cross-sectionsoftissue, as
`
`35
`
`40
`
`45
`
`55
`
`60
`
`65
`
`0006
`
`0006
`
`

`

`5,297,548
`
`4
`The probe 101 consists of an exterior housing 104 that
`applies the active elements of the probe 101to thetissue
`undertest, such as a finger 105, containing an arterial
`blood flow that is to be monitored. Included within
`housing 104 is a plurality (at least two) of light emitting
`devices 111, 112 and at least one corresponding light
`detector 113.
`Emitter driver circuit 131 produces the analog drive
`signals to activate light emitting devices 111, 112 in
`probe 101. These analog drive signals are carried over
`cable 103 to probe 101. To measure the concentration of
`total hemoglobin (tHb), oxygen saturation (S,O02), or
`other blood analytes, in arterial blood, the concentra-
`tion of several dominant absorbers contained in the
`arterial blood must be measured. In particular, for the
`measurement of total hemoglobin (tHb), concentration
`of the water and hemoglobin components ofthe arterial
`blood must be measured. The light emitting devices 111,
`112 each produce an output light beam of predeter-
`mined wavelength which is directed at the finger 105
`enclosed by housing 104.
`In this embodiment,
`light
`emitting device 111 is selected to produce a beam of
`light at approximately 810 nm, which wavelength is
`substantially isobestic to the oxygenated and deoxygen-
`ated componentsof the hemoglobinin thearterial blood
`(that is, the extinction coefficients of the oxygenated
`and deoxygenated hemoglobin are substantially identi-
`cal). Light emitting device 112is selected to produce a
`beam oflight at approximately 1270 nm. Theselection
`of these two wavelengthsis such that water is transpar-
`entat thefirst wavelength oflight (810 nm) but detected
`at the second (longer) wavelengthoflight (1270 nm). In
`addition, these wavelengthsare such that the extinction
`coefficients of the two components (water and hemo-
`globin) differ at the first wavelength oflight. Further,at
`both wavelengths the two species of hemoglobin are
`substantially isobestic in extinction but not transparent.
`Thelight detector 113 monitors thelevel of light that
`is transmitted throughorreflected from finger 105. The
`analog data signals produced by light detector 113 in
`response to the received beams of light are received
`from probe 101 over conductors 103 and filtered by
`_ analog hardware 132-134in probe interface circuit 102.
`The input analog data from probe 101 may be decom-
`posed into its non-pulsatile and pulsatile sub-elementsin
`probeinterface circuit 102 in order to provide accurate,
`high resolution, measurements of these components.
`Thepulsatile componenttypically represents anywhere
`from 0.05% to 20% of the total input signal and the
`decomposition of the input signal
`into pulsatile and
`non-pulsatile components permits accurate analog to
`digital conversion of even the smallest of these pulsatile
`components.
`In order to distinguish between the light beams pro-
`duced by first 111 and second 112 light emitting de-
`vices, these light emitting devices 111, 112 are modu-
`lated in a manner to allow the outputof the light detec-
`tor 113 to be synchronously demodulated. Ambient
`light, being unmodulated, is easily eliminated by the
`demodulator process.
`
`15
`
`20
`
`25
`
`_
`
`30
`
`35
`
`40
`
`45
`
`50
`
`3
`with two or three discrete light detectors mounted side
`byside.
`BRIEF DESCRIPTION OF THE DRAWING
`
`FIG.1 illustrates in block diagram form the overall
`architectureof the arterial blood monitoring system and
`the probe of the present invention;
`FIG.2 illustrates in graphical form the various com-
`ponents of the input signal from the probe;
`FIG.3 illustrates a cross-section view of the light
`detector used in the probe of the present invention;
`FIG. 4 illustrates in flow diagram form the opera-
`tional steps taken by a two wavelength arterial blood
`monitoring system to measure selected components in
`arterial blood; and
`FIG.5 illustrates in flow diagram form the opera-.
`tional steps taken by a three wavelength arterial blood
`monitoring system to measure selected components in
`arterial blood.
`
`DETAILED DESCRIPTION
`
`An arterial blood monitoring system takes advantage
`of the basic statistical property that arterial blood con-
`tains a plurality of dominant absorbers, whose measured
`light absorption spectra appear as a constant over a
`short interval of time. The arterial blood characteristics
`to be measured are empirically related to the changes in
`the measured light transmission through the plurality of
`dominant absorbers as a function of the changes in the
`arterial blood volumeat the probe site. Therefore, by
`measuring the transmitted light as it varies with arterial
`pulsation, at selected wavelengths, the relative amount
`of these dominant absorbers in the arterial blood can
`noninvasively be determined. A single probe can be
`used to generate the plurality of wavelengths of hight,
`therefore simplifying the arterial blood monitoring sys-
`tem.
`
`Definition of Terms
`
`lo=The intensity of the beam oflight at a given wave-
`length incident on the tissue-under-test, where the
`wavelength is denoted by the subscript.
`l=Theinstantaneous value ofthe intensity of the light
`received by the detector. Thelight is at a given wave-
`length, which wavelengthis indicated by a subscript.
`€=The
`extinction coefficient of light by a given sub-
`stance (indicated by a superscript) at a given wave-
`length (indicated by a subscript).
`C=The concentration of a given substance (indicated
`by a superscript).
`L=Thepathlength of a given substance (indicated by a
`superscript).
`tHb=Total hemoglobin measured in arterial blood.
`Usually expressed in terms of grams per deciliter.
`O=Used as a superscript to represent oxyhemoglobin.
`R=Usedas a superscript to represent reduced hemo-
`globin.
`‘W=Used as a superscript to represent water.
`t= Used as a superscript to represent the combination of
`oxyhemoglobin and reduced hemoglobin.
`
`System Architecture
`FIG. 1 illustrates in block diagram form the overall
`architectureofthe arterial blood monitoring system 100
`and the probe 101 of the present invention. Thearterial
`blood monitoring system 100 consists of a probe 101
`connected to probe interface circuit 102 by means of a
`set of electrical conductors 103 and connector 1032.
`
`Signal Components
`FIG. 2 illustrates in graphical form (not to scale) the
`various componentsofthe total absorption produced by
`finger 105. The light detector output signal, high where
`absorption is low and visa versa, consists of a large
`magnitude non-pulsatile component and a small magni-
`tude pulsatile component. The non-pulsatile component
`
`0007
`
`0007
`
`

`

`5,297,548
`
`5
`represents light remaining after absorption due to a
`combination of venous blood, cutaneoustissue, bone,
`and constant arterial blood while the small pulsatile
`component is caused by the light absorption due to
`pulsatile arterial blood flow that is to be measured.
`Following synchronous demodulation, the data signals
`producedbylight detector 113 and transmitted to probe
`interface circuit 102 consist of a series of data points that
`are digitized and stored in memory 106. Since the first
`111 and second 112 light emitting devices are sampled
`simultaneously and inrapid succession, these digitized
`data points consist of a plurality of sets of measure-
`ments, with one set corresponding to samples of the
`light beam intensity at a first wavelength, the other set
`corresponding to samples of the light beam intensity at
`a second wavelength, and, in some schemes, a third set -
`correspondingto the intensity of the ambient light.
`Ideally, in pulse oximeter systems red and infrared
`wavelengths of light are used and the ratio of the nor-
`malized derivative (or logarithm) ofthe red intensity to
`the normalized derivative (or logarithm) of the infrared
`intensity is a constant. This constantis indicative of the
`partial oxygenation (S,O2) of the hemoglobin in the
`arterial blood flow.It is obvious that this ratio changes
`as $,02 changes but, for a short interval with rapid
`enough sampling rate, the ratio remains constant.
`Probe Interface Circuit
`
`20
`
`25
`
`6
`scaling amplifiers 135 such that they can be converted,
`with optimal resolution,
`to a digital equivalent. All
`channels output by scaling amplifiers 135 are then si-
`multaneously sampled by the sample/hold circuitry
`136a, 1366, .
`.
`. 136. The sampleddata is passed a chan-
`nel at a time via multiplexer 137 to the analogto digital
`converter 138. From therethe data, nowin digital form,
`is sent on to data processing circuit 107 whereit is
`stored in memory 106 for processing. The digital data
`represents the substantially simultaneously sampled
`amplitudes of the receivedlight intensities from each of
`the wavelengths used at a sampling frequency of typi-
`cally 30 Hzor greater. These data values are referred to
`as I, I, .
`.
`. Iv, where the subscript indicates the given
`wavelength. I, then indicates the received light inten-
`sity at any given wavelength.
`
`Data Processing Circuit
`In a two wavelength system, data processing circuit
`107 computes a ratio from the digital amplitude data
`measured at each wavelength oflight. In particular, this
`process used by data processingcircuit 107is illustrated
`in flow diagram form in FIG. 4. At step 401, data pro-
`cessing circuit 107 receives a set of digital input data
`indicative of the measured intensity of light at both
`wavelengths, as received by light detector 113. Data
`processing circuit 107 at step 410 transmits the received
`set of data to display driver 109 for display in graphical
`form on display 114. The displayed waveform repre-
`sents the pulsatile componentofthe arterial blood. Data
`processing circuit 107 also stores the received set of
`data in memory 106 and uses this set of data and thelast
`most recently received set of data to compute at steps
`402 and 403 the differential change in absorption of the
`arterial blood in finger 105 at the first and second se-
`lected wavelengthsoflight, respectively. The differen-
`tial change in absorption at wavelength n is computed
`by data processing circuit 107 as:
`
`dA, =
`
`dl,
`In
`
`1
`q)
`
`‘Because dI, is a mathematical construct, it is approxi-
`mated in arterial blood monitoring system 100 by Al,
`whereAI, is the difference between two consecutively
`received I, values. Only AI values that are caused by a
`small but non zero changein path length throughfinger
`105 are used and therefore AI, can also be a longer
`interval of time if necessary to obtain a sufficient change
`in received intensity of the beam of light. The I, value
`used in equation | is the average ofthe two successively
`received I, values used to compute AI,.
`In a two wavelength system,a final ratio is then cal-
`culated by data processing circuit 107 at step 404 as:
`
`dAy
`R= Gay
`
`(2)
`
`wherethe data values used to compute dA, are from the
`samepoints in time as the data values used to compute
`dA2.
`This ratio is then used in a calibration equation by
`data processing circuit 107 at step 405 to relate the R
`value to a specific blood analyte value. For example,
`when measuring total hemoglobin,the calibration equa-
`tion is approximated by a second order polynomial of
`the form:
`
`The actual analog data received by the probe inter-
`face circuit 102 can include a fairly significant noise
`component which is caused by a number ofsources
`including motion of finger 105, the introduction of am-
`bient light into housing 104, and various sources of
`electrical noise. These noise components skew the val-
`uesof either or both of the magnitudes measured in each
`set of data points destroying the correct relationship
`between the red and infrared signals. Existing pulse
`oximeter circuits make use of various filtering tech-
`niques to minimize the impact of noise on the S,O2
`value measured by the system. This filtering circuitry
`and software/algorithms are analogous to that used in
`the arterial blood monitoring system 100 and are there-
`fore not described in detail herein.
`Probe interface circuit 102 includes emitter driver
`circuit 131 that is capable of driving light emitting de-
`vices 111, 112 such that the light beams producedtra-
`verse finger 105 and sufficientlight intensity is incident
`onlight detector 113 to produce data indicative of the
`light absorption of the dominant absorbersin arterial
`blood. The data produced bylight detector 113 (voltage
`equivalent of the received light
`intensities) at each
`wavelength is kept distinct and can be processed inde-
`pendently. This can be done by any of the many
`schemespresently in use for pulse oximetry, such as
`time division multiplexing, or frequency division multi-
`plexing.
`Thelight received from finger 105 is converted to an
`equivalent current signal by the photodiodes of light
`detector 113, and then converted to a voltage signal by
`the current to voltage converter 132. The data is then
`amplified by amplifier 133, and demultiplexed via syn-
`chronous demodulation circuit 134. The demultiplexed
`data comprises analog voltage signals applied to leads
`CHAN 1, CHAN 2... CHAN n representative of the
`intensity of the received light at each of the wave-
`lengthsoflight produced by light emitting devices 111,
`112, respectively. The voltage signals on leads CHAN
`1, CHAN2 are then scaled (further amplification) by
`
`35
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`
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`
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`
`0008
`
`0008
`
`

`

`tHb=AR? +BR+C
`
`@)
`
`Where A, B, and C are constants that depend on the
`specific wavelengths oflight used.
`The tHb valueis then output by data processing cir-
`cuit 107 at step 406 to display driver 109 (and/or hard-
`copy) to display in human-readable form on display 115
`a numeric value of the concentration of total hemoglo-
`bin in the arterial blood of finger 105. Processing then
`returns to step 401.
`
`5,297,548
`
`8
`blood monitoring system 100 to read the differential
`changein absorption (dA values) as accurately as possi-
`ble. This leaves only the valuesof the differential path
`lengths dL as unknowns. With two equations, the two
`dL values can be uniquely and simply solved for.
`Writing the proportion of hemoglobinin thearterial
`blood as:
`
`0
`
`dL!
`Proportion Hb = GL+ aL”
`
`(8)
`
`Theory
`This device is based on the theory that:
`
`dA,S=esCdL*
`
`15
`
`(4)
`
`A differential change in absorption at a given wave-
`length n to a given substance (dA,‘), is equal to the
`extinction of that substance(e,*) times the concentration
`(C*) of that substance times the differential change in
`pathlength of that substance (dL*).
`Further the differential change in absorption can be
`defined as:
`
`dAn =
`
`dlp
`Tn
`
`25
`
`6)
`
`Note that no measurement ofthe incident light inten-
`sity, Io, is required to measurethedifferential change in
`absorption dA. However, samples of I, must be taken
`sufficiently close in time so that Al, represents a good
`mathematical approximation of dI,.
`To determine the relative proportions of two domi-
`nant absorbers, in this case water and hemoglobin, one
`chooses two wavelengths of light at which the two
`absorbers have extinctions, such that the following set
`of simultaneous equations has a unique solution forall
`possible concentrations and pathlengths of the two ab-
`sorbers.
`
`While this proportion is not directly the tHb,it is di-
`rectly related to it. And while this relationship could be
`theoretically derived, an empirical relationship (as de-
`fined in equation 3) is measured instead. This is neces-
`sary due to several ways in whichthe true optical sys-
`tem ofliving tissues and realistic optical elements devi-
`ate from the exact theoretical mode] developed here.
`Equation 3 is therefore referred to as the calibration
`equation andits coefficients A, B, and C, are experimen-
`tally derived via clinical testing. The coefficients are
`then installed in the arterial blood monitoring system
`software. It should be noted that these coefficients dif-
`fer for different wavelength emitters.
`The wavelengthsoflight produced by light emitting
`devices 111, 112 are also selected so as to optimize the
`performance of the entire electro optical system: low
`enoughlight absorption so that sufficient optical signal
`is received by light detector 113 and high enoughlight
`absorption so thatthere is an appreciable changein light
`absorption over the range of physiological changes in
`pathlength caused by the pulsation of arterial blood.
`Typical wavelengths of light selected for a realization
`of this system are 810 nm and 1270 nm, however many
`wavelength combinations meeting the above criteria
`can be used.
`
`Probe
`
`dAy=ey'dL'+e"dL”
`
`dAp=e7'dL'+e2"dL"
`
`Probe 101 contains a minimumoftwolights emitting ©
`devices 111, 112, each of which produces a beam of
`light centered abouta selected wavelength (810 nm and
`1270 nm, respectively). Probe 101 also contains light
`In this system of equations it is assumed that the only
`detector 113 capable of receiving the emitted wave-
`components which change in pathlength are those of
`lengthsoflight. In the present implementation, the light
`the arterial blood. Furtherit is assumed that the primary
`detector 113 consists of a multiple layer element, shown
`absorbers are those of water and hemoglobin where the
`in additionaldetail in FIG. 3, that contains a germanium
`hemoglobin species in the blood are essentially only
`50
`photodiode 113 placed underasilicon photodiode
`those of oxyhemoglobin and reduced hemoglobin.
`113a. For the wavelengths of light shorter than approxi-
`Choosing a wavelength of light
`that represents an
`isobestic point for the two species of hemoglobin, such
`mately 1000 nm, the silicon photodiode 113a receives
`as 804 manometers, minimizes the effects of changes in
`the incident light. Above this wavelength, the silicon
`oxygen saturation on the total hemoglobin readings.
`photodiode 113a becomes transparent and the germa-
`Notice that in equations 6 and 7 above, the concentra-
`nium photodiode 1136 picks upthe incidentlight. Probe
`tion term expressed in equation 4 has been eliminated.
`101 includes a cable 103 and connector 103a for trans-
`By viewing the optical system as compartmentalized,
`mitting and receiving signals between probe 101 and
`that is, looking at the tissue under test as one in which
`probe interface circuit 102. Probe 101is positioned on
`the light first passes through 100% skin tissue, followed
`the tissue either in the transmission mode: light emitting
`by 100% venousblood, followed by 100% arterial he-
`devices 111, 112 on one side and light detector 113 on
`moglobin, followed by 100% water, and so on, the
`the other side offinger 105, earlobe, toe or other appro-
`concentration terms expressed by equation 4 are actu-
`priate site through which light can be received by the
`ally constants. Thus, beginning with equation 6, the
`light detector 113 at acceptable signal levels; or in the
`extinction coefficients are meant to represent the combi-
`reflectance mode: in which the light emitting device
`nationofthe actual extinction coefficient and the actual
`111, 112 and light detector 113 are placed on the same
`concentration for 100% of any given absorber.
`side of the tissue under test, such as the forehead or
`In the system of equations (Equations6, 7) the extinc-
`forearm.
`tion € values are constantsandit is the job ofthe arterial
`
`(6)
`
`(7)
`
`45
`
`60
`
`65
`
`0009
`
`0009
`
`

`

`9
`
`5,297,548
`
`10
`(O2Hb), reduced hemoglobin (RHb), and water (H20).
`
`Note that this equation showsonly that the total hemo-
`globin is proportional
`to this ratio of path length
`changes, not equal to it. This is due,at least in part, to
`the fact the tHb is measured in terms of grams/deciliter
`of whole blood and this equation is a ratio of path
`lengths. Thereis a one to one correspondence between
`this ratio of path lengths and the tHb which is deter-
`mined, and curve fit, experimentally. The empirical
`curve fit also compensates for the differences between
`the theoretical models and the actual optical systems.
`
`dL?
`S02 = 70 + gEe
`
`qa)
`
`25
`
`For a three wavelength system, with the subscripts1,2,
`and 3 indicating the specific wavelengths used we can
`write following system of equations
`
`dAy =, 0dL+ €)RaLR 4 e,¥aL¥
`
`dAp=€)°%dLo+ep° dL +€)" dL”
`
`dA3=€3°dL9+ 38dLR 4 3%aL
`
`35
`
`In matrix notation:
`
`.
`
`
`
`6? e® 6
`
`(12)
`
`(13)
`
`(14)
`
`(15)
`
`Combination tHb Monitor and Pulse Oximeter
`The methodologies for pulse oximetry are well
`known. The method of obtaining tHb noninvasively
`10)
`R
`oO
`5
`and in real time has been disclosed above. Thearterial
`1Hb «——@L-+dl"ag
`aL? + diR + + dL
`blood monitoring system of the present invention can
`combine the two technologies to create a device for
`measurement of both parameters. tHb is an interfering
`substance in the measurement of S,O2 by the present
`technologies. By “interfering substance”it is meant that
`variations in tHb cause variations in the SO as ready
`by a pulse oximeter. These variations in SgO2 are corre-
`lated to, but not corrected for, the tHb level. A device
`capable of measuring tHb can therefor provide a means
`for eliminating the error it causes in determining S,Ob.
`The same holdstrue in terms of SO? being an interfer- _
`ing substance in the measurement of tHb. Thesolution
`to this problem lies in a combination device capable of
`reading both parameters. Such a device can be simply
`obtained by using two wavelengthsto derive the 8,02,
`and two more as described above for obtaining tHb.
`Theresulting values for SgO2 and Thb can then be used
`to correct the readings of the other. A more sophisti-
`cated system uses a three wavelength system, where the
`practical realization of this system utilizes the standard
`oximetry wavelengths of 660 nm and 940 nm produced
`by twolight emitting devices 111a, 1110, along with a
`wavelength of 1270 nm produced by a light emitting
`device 112. (Once again, any three wavelengths that
`meet the criteria stated above for a standalone tHb
`system can be used.) In addition, the two segmentlight
`detector 113 is activated in a mannerto reflect the use of
`three wavelengths of light. Silicon photodiode 113a
`detects both of the light beams (660 nm, 940 nm) pro-
`duced bylight emitter devices 111a, 111 andits output
`is demultiplexed to separate the two measured values of
`light intensity. Germanium photodiode 113d of light
`detector 113 measures the intensity of the third beam
`af a” @
`dL?
`dA,
`light at 1270 nm.
`the process used by data processing
`In particular,
`
`dAz aER|)9 eR e¥|=]
`circuit 107is illustrated in flow diagram form in FIG.5.
`dA3
`aL¥
`¥
`Atstep 501, data processing circuit 107 receives a set of
`digital input data i