`
`.
`Ulllted States Patent
`
`[19]
`
`l‘llllllllllllllll|I|||IllllIllllllllllllllllllllllllllllllllllllllllllll
`U5005297548A
`0
`er:
`t N mb
`
`‘
`
`11
`
`a en
`P t
`
`5,297,548
`
`Pologe
`
`[45] Date of Patent: Mar. 29, 1994
`
`[54] ARTERIAL BLOOD MONITORING p1:an
`
`5,137,023
`
`3/1992 Mendelson et a1.
`
`.............. ..
`
`l28/633
`
`[75]
`
`Inventor:
`
`Jones A. Foliage, Boulder. Colo.
`
`FOREIGN PATENT DOCUMENTS
`
`[73] Assignee: Ohmeda Ine., Murray Hill, NJ.
`
`[21] Appl. No: 45,962
`
`[22] Filed:
`
`Apr. 12, 1993
`
`Related (LS. Application Data
`Continuation-impart at Ser. No. 832,551, Feb. '1, 1992.
`
`[63}
`
`A613 5/00'
`Int. C1,:
`[51]
`[52] us. (:1. .................................. .. 128/633; 128/665;
`_
`356/41
`[53] Field ofSeueh
`123/633—634,
`"lu28/664—667; 355/3941
`
`[55]
`
`References Cit“
`U.S. PATENT DOCUMENTS
`4,223,680 9/1980 Jobsis .................................. 128/633
`4 324 242 4/1989 Flick at a]
`lzaffiéé‘x
`
`..
`4,367,165 9/1989 Noller et al.
`....'.'. 123/666 x
`5;0TB.136
`1/1992 Stone et a]. ........... .. 128/666 x
`
`5.127.406
`7/1992 Yamaguchi ....................... .. 123/633
`
`0303503 1/1939 Europa“ Pat 0“ -
`9004353 5/1990 PCT Int'I App].
`.
`911136
`8/1991 PCT lnt‘l Appl.
`.
`
`Primary Examiner—Angela D. Sykes
`Attorney, Agent, or Firm—Roger M. Rathbun; Larry R.
`Case“; Jamar M- Graziano
`[57]
`ABSTRACT
`
`This arterial blood monitoring system takcs 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. By measuring the transmitted
`light as it varies with arterial pulsation at selected wave-
`lengths of light, over a common light path, the relative
`amount of these dominant absorbers in the arterial
`'3in can nnnmvasmly be datemmed' T° ensure the
`common light path, a sandwich construction light de-
`‘3“01' is “50d-
`
`.
`
`Claims, 4 Drawing Sheets
`
`
`
`K400
`
`
`
`
`cunazm
`smcmonous
`
`TO
`PL" DEMODULA-
`vet-rte:
`'
`PIER
`non
`
`Iconvam'ea
`
`
`
`
`s—el
`
`ETECTOR "'3‘
`
`
`
`SCALI NG
`RHPLIFIERS
`
`
`I35
`
`
`
`[LITAPROCESSOR
`DISPLAY
`
`DIS PLAY MODULE
`
`PLETHYSMOGRAPH IC
`WAV E FOR M
`
`
`HEART RATE
`'1 BPMI
`
`
`
`"4
`
`“5|:
`
`IISC
`
`0001
`
`Apple
`APLl
`
`US. Patent No. 8,652
`
`
`
`0001
`
`Apple Inc.
`APL1020
`U.S. Patent No. 8,652,040
`
`

`

`
`
`US. Patent
`
`Mar. 29, 1994
`
`Sheet 1 0:4
`
`5,297,548
`
`IOI
`
`inO
`
`
`
`EM ITTER DRIVER
`CIRCUIT
`
`
`
`
`SYNCHRONOUS
`
`CURRENT
`DEMODULA—
`TO
`
`
`
`TION
`VOLTAGE
`
`
`
`CONVERTER
`105
`
`
`
`DATAPROCESSOR
`
`
`
`
`SCALING
`AMPLIFIERS
`
`
` DISPLAY MODULE
`
`PLETHYSMOGRAPH I C
`
`WAVEFORM 7.80 02
`
`
`
`
`
`HEART RATE
`
`(8PM)
`
`
`
`H4
`
`“Sb
`
`H5O
`
`0002
`
`
`
`0002
`
`

`

`
`ABSORPTION
`
`
`TIME
`
`ARTERIAL BLOOD
`ABSORPTION
`
`VENOUS BLOOD
`ABSORPTION
`
`OTHER TISSUE
`ABSORPTION
`
`US. Patent
`
`'
`
`Mar. 29, 1994
`
`Sheet 2 of4
`
`5,297,548
`
`ARTERIAL PULSATION
`ABSORPTION
`
`
`
`
`
`0003
`
`0003
`
`

`

`US. 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
`WAVEFORM DISPLAY
`
`
`402x COMPUTE DIFFERENTIAL CHANGE IN
`
`
`
`ABSORPTION AT FIRST WAVELENGTH
`
`403
`
`COMPUTE DIFFERENTIAL CHANGE IN
`
`ABSORPTION AT SECOND WAVELENGTH
`
`
`COMPUTE RATIO OF COMPUTED DIFFERENTIAL
`
`CHANGE IN ABSORPION FOR FIRST WAVELENGTH
`TO COMPUTED DIFFERENTIAL CHANGE IN
`ABSORPTION FOR SECOND WAVELENGTH
`
`
`
`
`
`
`
`
`
`DRIVER TO PRODUCE NUMERIC OUTPUT
`
`TRANSMIT COMPUTED tHb TO DISPLAY
`
`'FIGURE
`
`4
`
`0004
`
`0004
`
`

`

`US. Patent
`
`Mar. 29, 1994
`
`Sheet 4 of 4
`
`5,297,548
`
`501
`
`—\ menu mm non man: mm
`P STORE I)! most
`
`'
`
`no
`
`rmsnn mm re main!
`DRIVER TO PRODUCE
`IIYEIORH DISPLAY
`
`
`
`so:
`
`\ comm: DIFFERENTIAL came: 13!
`nanny-um: m nus-r “nuns-tn
`
`—\. COMPUTB DIFFERENTIAL came: :1!
`neonnxou n- anoorm annexe :-
`
`-
`—\A\ comma nrrrzumm came: IN
`ABSORPTION AT THIRD IIVELENGTK
`
`03
`
`504
`
`
`
`
`
`COMPUTE 8802 THOR TEE
`
`
`
`MEASURED nrrrxnzxwan
`
`CHANGE IN 1380RPTIOH
`AT ALL IIVBLINGTDB
`
`
`
`
`
`
`CORPUTB tflb THOR TEE
`IZLBURSD DIFFERENTIAL
`CHANGE IN IBBORPTION
`IT ALL IIYELEHGTEB
`
`
`
`
`
`
`COKPUTB 02 ct [ROE tnh 13D 3302
`
`533N831! tEb, 8&02 IND 02 at DITA
`TO DISPLAY DRIVER T0 PRODUCE
`NUMERIC OUTPUT
`
`
`FIGURE
`
`5
`
`0005
`
`0005
`
`

`

`
`
`1
`
`5,297,548
`
`ARTERIAL BLOOD MONITORING PROBE
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`_
`
`5
`
`This application is a continuation-in-part of US. pa— .
`tent application Ser. No. 07/832,551, titled “Improved
`Arterial Blood Monitoring System", filed Feb. 7, 1992,
`d.
`pen mg
`
`10
`
`FIELD OF THE INVENTION
`
`This invention relates to non-invasive photoplethys-
`mographic measurement of blood analytes and, in par-
`ticuIar, to a probe for use in an arterial blood monitoring
`system to more accurately measure the change in inten-
`sity of the light transmitted through the arterial blood of
`a patient.
`
`15
`
`PROBLEM
`
`2
`tor. The greater the departure of the light beams from a
`common light 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 not exist a single
`light detector device that can detect a wide bandwidth
`of light 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 common light path through the arteriolar bed of
`the appendage being monitored.
`SOLUTION
`
`caused by sampling different cross-sections of tissue, as
`
`The above described problems are 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 noninvasiver 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 probe site. By measuring the
`transmitted light as it varies with arterial pulsation at a
`plurality of selected wavelengths of light, over a single
`common light path, the relative amount of these domi-
`nant absorbers in the arterial blood can noninvasiver
`be determined.
`
`By selecting one wavelength of light around 1210 am,
`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 sandwich or lay-
`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, the silicon
`photodiode receives the incident light and produces a
`signal indicative of the intensity of the received light.
`Above this 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 the tissue 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 the tissue, regardless
`of wavelength, this apparatus avoids the inaccuracies
`
`It is a problem in the field medical monitoring equip- 2°
`ment to accurately measure various parameters of arte-
`rial blood in a noninvasive manner. For example, the
`oxygen saturation (Sc 0;} 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 of these
`two forms of hemoglobin. Reduced hemoglobin absorbs
`more light in the red band (600-800 nm) than does oxy-
`hemoglobin while oxyhemoglobin absorbs more light in
`the near infrared band (800-1011) nsn) than does reduced
`hemoglobin.
`The pulse oximeter includes a probe that 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 40
`infrared light transmitted through the intervening ap-
`pendage contains a non-pulsatile component which is
`influenced by the absorbency of tissue, venous blood,
`capillary blood, non-pulsatile arterial blood, and the
`intensity of the light source. The pulsatile component of 45
`the received signals is an indication of the expansion of
`the arteriolar bed in the appendage with arterial blood.
`The effects of different tissue thicknesses and skin pig-
`mentation in the appendage can be removed from the
`received signals by normalizing the change in intensity so
`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 55
`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 component is a very
`small percentage of the total signal amplitude and de-
`pends on the blood volume change per pulse and the
`oxygen saturation (8501) of the arterial blood. The
`received red and infrared signals have an exponential
`relationship to the path length of the arterial blood. The
`photoplethysmographic measurement of these analytes
`is predicated on the assumption that the light beams
`from the two light sources follow identical paths
`through the intervening appendage to the light detec—
`
`25
`
`30
`
`35
`
`65
`
`0006
`
`0006
`
`

`

`
`
`5,29?,548
`
`3
`with two or three discrete light detectors mounted side
`by side.
`-
`BRIEF DESCRIPTION OF THE DRAWING
`
`FIG. 1 illustrates in block diagram form the overall
`architecture of 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. 6 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 volume at 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
`noninvasiver be determined. A single probe can be
`used to generate the plurality of wavelengths of light.
`therefore simplifying the arterial blood monitoring sys-
`tem.
`
`Definition of Terms
`
`lo =The intensity of the beam of light at a given wave-
`length incident on the tissue-under—test, where the
`wavelength is denoted by the subscript.
`l=The instantaneous value of the intensity of the light
`received by the detector. The light is at a given wave-
`length, which wavelength is 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=The pathlength of a given substance (indicated by a
`superscript).
`tHb=Total hemoglobin measured in arterial blood.
`Usually expressed in terms of grams per deciliter.
`0=Used as a superscript to represent oxyhemoglobin.
`[t=Used as 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.
`
`4
`The probe 101 consists of an exterior housing 104 that
`applies the active elements of the probe 101 to the tissue
`under test. 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 (ti-lb). oxygen saturation (Scot). 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 {ti-lb), concentration
`of the water and hemoglobin components of the 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 310 nm. which wavelength is
`substantially isobestic to the oxygenated and deoxygen-
`ated components of the hemoglobin in the arterial blood
`(that is, the extinction coefficients of the oxygenated
`and deoxygenated hemoglobin are substantially identi-
`cal). Light emitting device 112 is selected to produce a
`beam of light at approximately 1270 run. The selection
`of these two wavelengths is such that water is transpar-
`ent at the first wavelength of light (810 run) but detected
`at the second (longer) wavelength of light (1270 rim). In
`addition, these wavelengths are such that the extinctiou
`coefficients of the two components (water and hemo-
`globin) differ at the first wavelength of light. Further, at
`both wavelengths the two species of hemoglobin are
`substantially isobestic in extinction but not transparent.
`The light detector 113 monitors the level of light that
`is transmitted through or reflected 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—134 in probe interface circuit 102.
`The input analog data from probe 101 may be decom-
`posed into its non-pulsatile and pulsatile sub-elements in
`probe interface circuit 102 in order to provide accurate,
`high resolution, measurements of these components.
`The pulsatile component typically 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 output of the light detec-
`tor 113 to be synchronously demodulated. Ambient
`light, being unmodulated, is easily eliminated by the
`demodulator process.
`
`5
`
`10
`
`15
`
`20
`
`25
`
`_
`
`30
`
`_3s
`
`45
`
`50
`
`55
`
`60
`
`tude pulsatile component. The non-pulsatile component
`
`Signal Components
`
`FIG. 2 illustrates in graphical f0rm (not to scale) the
`various components of the 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-
`
`System Architecture
`
`FIG. 1 illustrates in block diagram form the overall
`architecture of the arterial blood monitoring system 100
`and the probe 101 of the present invention. The arterial
`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 1039.
`
`65
`
`0007
`
`0007
`
`

`

`
`
`5,297,548
`
`5
`represents light remaining after absorption due to a
`combination of venous blood. cutaneous tissueI 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
`produced by light 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 in rapid 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 -
`corresponding to 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) of the red intensity to
`the normalized derivative (or logarithm) of the infrared
`intensity is a constant. This constant is indicative of the
`partial oxygenation (890;) of the hemoglobin in the
`arterial blood flow. It is obvious that this ratio changes
`as 5,102 changes but, for a short interval with rapid
`enough sampling rate, the ratio remains constant.
`Probe Interface Circuit
`
`lo
`
`15
`
`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
`1360. 136b, . . . 136:1. The sampled data is passed a chan-
`nel at a time via multiplexer 137 to the analog to digital
`converter 138. From there the data, now in digital form,
`is sent on to data processing circuit 107 where it is
`stored in memory 106 for processing. The digital data
`represents the substantially simultaneously sampled
`amplitudes of the received light intensities from each of
`the wavelengths used at a sampling frequency of typi-
`cally 30 Hz or greater. These data values are referred to
`as 11, 12,
`.
`.
`. IN, where the subscript indicates the given
`wavelength. 1,, then indicates the received light inten-
`sity at any given wavelength.
`
`Data Processing Circuit
`
`In a two wavelength system, data processing circuit
`10‘! computes a ratio from the digital amplitude data
`measured at each wavelength of light. In particular, this
`process used by data processing circuit 107 is 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 component of the arterial blood. Data
`processing circuit 107 also stores the received set of
`data in memory 106 and uses this set of data and the last
`most recently received set of data to compute at steps
`402 and 403 the differential change in absorption of the
`arterial blood in linger 105 at the first and second se-
`lected wavelengths of light. respectively. The differen—
`tial change in absorption at wavelength n is computed
`by data processing circuit 107 as:
`
`dAn_
`
`all,
`In
`
`1
`()
`
`‘Because dln is a mathematical construct, it is approxi-
`mated in arterial blood monitoring system 100 by :11",
`where A1,. is the difference between two consecutively
`received 1,, values. Only AI values that are caused by a
`small but non zero change in path length through finger
`105 are used and therefore All" 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 1 is the average of the two successively
`received 1,, 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:
`
`at
`R = —M2
`
`(2)
`
`where the data values used to compute mm are from the
`same points in time as the data values used to compute
`dAz.
`This ratio is then used in a calibration equation by
`data processing circuit 107 at step ‘05 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 of sources
`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-
`ues of 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 8,0:
`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 produced tra-
`verse fmger 105 and sufficient light intensity is incident
`on light detector 113 to produce data indicative of the
`light absorption of the dominant absorbers in arterial
`blood. The data produced by light 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
`schemes presently in use for pulse oximetry, such as
`time division multiplexing, or frequency division multi-
`pleaing.
`The light 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 couverter 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-
`lengths of light produced by light emitting devices 111,
`112, respectively. The voltage signals on leads CHAN
`1, CHAN 2 are then sealed (further amplification) by
`
`30
`
`35
`
`45
`
`55
`
`65
`
`0008
`
`0008
`
`

`

`
`
`5,297,548
`
`ins=nn1+an+c
`
`(3)
`
`Where A, B, and C are constants that depend on the
`specific wavelengths of light used.
`The tl-l'b value is then output by data processing cir-
`cuit 107 at step 406 to diSplay driver 109 (and/or hard-
`oopy) to display in human-readable form on diSplay 115
`a numeric value of the coucentration of total hemoglo-
`bin in the arterial blood of finger 105. Processing then
`returns to step 401.
`
`blood monitoring system 100 to read the differential
`change in absorption (dA values) as accurately as possi-
`ble. This leaves only the values of 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 hemoglobin in the arterial
`blood as:
`
`10
`
`61'
`Proportion as x a! + de
`
`(3)
`
`Theory
`This device is based on the theory that:
`
`dig-en’C‘dL‘
`
`15
`
`(4}
`
`A differential change in absorption at a given wave-
`length n to a given substance (dAn-l). is equal to the
`extinction of that substance (ens) 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:
`
`an a
`
`er,
`in
`
`(5}
`
`Note that no measurement of the incident light inten-
`sity, 10, is required to measure the differential change in
`absorption dA. However, samples of 1,. must be taken
`sufficiently close in time so that A1,, represents a good
`mathematical approximation of til”.
`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 for all
`possible concentrations and pathlengths of the two ab-
`sorbers.
`
`dA1=e1idU+ (1"de
`
`dA2=ez‘u'L’+eszL”
`
`(5)
`
`(7)
`
`In this system of equations it is assumed that the only
`components which change in pathlength are those of
`the arterial blood. Further it is assumed that the primary
`absorbers are those of water and hemoglobin where the
`hemoglobin species in the blood are essentially only
`those of oxyhemoglobin and reduced hemoglobin.
`Choosing a wavelength of light
`that represents an
`isobestic point for the two species of hemoglobin, such
`as 804 manometers, minimizes the effects of changes in
`oxygen saturation on the total hemoglobin readings.
`Notice that in equations 6 and 'i' above, the concentra-
`tion term expressed in equation 4 has been eliminated.
`By viewing the optical system as compartmentalized.
`that is, looking at the tissue under test as one in which
`the light first passes through 100% skin tissue, followed
`by 100% venous blood, followed by 100% arterial he-
`moglobin. followed by 100% water. and so on, the
`concentration terms expressed by equation 4 are actu-
`ally constants. Thus, beginning with equation 6, the
`extinction coefficients are meant to represent the combi-
`nation of the actual extinction coefficient and the actual
`concentration for 100% of any given absorber.
`In the system of equations (Equations 6, T) the extinc-
`tion e values are constants and it is the job of the arterial
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`55
`
`65
`
`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 which the true Optical sys-
`tem of living tissues and realistic optical elements devi-
`ate from the exact theoretical model developed here.
`Equation 3 is therefore referred to as the calibration
`equation and its coefficients A, B, and C. are experimen-
`tally derived via clinical testing. The coell‘icients are
`then installed in the arterial blood monitoring system
`software. It should be noted that these coefficients dif-
`fer for different wavelength emitters.
`The wavelengths of light produced by light emitting
`devices 111, 112 are also selected so as to optimize the
`performance of the entire electro optical system: low
`enough light absorption so that sufficient optical signal
`is received by light detector 113 and high enough light
`absorption so that there is an appreciable change in 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 rim. however many
`wavelength combinations meeting the above criteria
`can be used.
`
`Probe
`
`Probe 101 contains a minimum of two lights emitting '
`devices 111, 112, each of which produces a beam of
`light centered about a selected wavelength (810 nm and
`1270 nm, respectively). Probe 101 also contains light
`detector 113 capable of receiving the emitted wave-
`lengths of light. In the present implementation, the light
`detector 113 consists of a multiple layer element, shown
`in additional detail in FIG. 3, that contains a germanium
`photodiode 113b placed under a silicon photodiode
`1130. For the wavelengths of light shorter than approxi-
`mately 1000 nm, the silicon photodiode 113:: receives
`the incident light. Above this wavelength, the silicon
`photodiode 1130 becomes transparent and the germa-
`nium photodiode 1135 picks up the incident light. Probe
`101 includes a cable 103 and connector 103:: for trans-
`mitting and receiving signals between probe 101 and
`probe interface circuit 102. Probe 101 is positioned on
`the tissue either in the transmission mode: light emitting
`devices 111, 112 on one side and light detector 113 on
`the other side of finger 105, earlobe, toe or other appro-
`priate site through which light can be received by the
`light detector 113 at acceptable signal levels; or in the
`reflectance mode: in which the light emitting device
`111, 112 and light detector 113 are placed on the same
`side of the tissue under test. such as the forehead or
`forearm.
`
`0009
`
`
`
`0009
`
`

`

`
`
`9
`
`5,297,548
`
`10
`(021%). reduced hemoglobin (RI-lb), and water (H20).
`
`If?!)
`
`d1.“
`6L0
`‘ dL0+ rLR+ +dLW
`
`(ID)
`
`Note that this equation shows only that the total hemo-
`globin is preportional
`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. There is 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.
`
`at."
`5'02 = «v.0 + so“
`
`(11)
`
`For a three wavelength system, with the subscripts 1, 2,
`and 3 indicating the specific wavelengths used we can
`write following system of equations
`
`d41=qodLo+quLR+quLW
`
`u2=q°sro+quLR+anLW
`
`M3=e306Lo+e3RdLR+r3Wde
`
`35
`
`In matrix notation:
`
`dLO
`'14:
`0312 = at.“
`#3
`at)“
`
`V
`
`“0 £13 E1
`£20 £2“
`62W
`'w
`
`:30 e33 (3
`
`(12)
`
`(13)
`
`(14)
`
`(I5)
`
`Combination tHb Monitor and Pulse Oximeter
`
`The methodologies for pulse oximetry are well
`known. The method of obtaining tHb noninvasiver
`and in real time has been disclosed above. The arterial
`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 $002 by the present
`technologies. By “interfering substance” it is meant that
`variations in tI-lb cause variations in the 3,0; as ready
`by a pulse oximeter. These variations in 5,.01 are corre-
`lated to, but not corrected for, the till: level. A device
`capable of measuring tl-Ib can therefor provide a means
`for eliminating the error it causes in determining 5,702.
`The same holds true in terms of 8,302 being an interfer- ,
`ing substance in the measurement of tl-Ib. The solution
`to this problem lies in a combination device capable of
`reading both parameters. Such a device can be simply
`obtained by using two wavelengths to derive the 5,02.
`and two more as described above for obtaining tHb.
`The resulting vain for 5,0; 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 two light emitting devices 111a, 111b, along with a
`wavelength of 1270 um 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 segment light
`detector 113 is activ