0AAA
`
`United States Patent 19
`
`5,575,284
`Nov. 19, 1996
`Date of Patent:
`Athanet al.
`[45]
`
`(11]
`
`Patent Number:
`
`[54] PORTABLE PULSE OXIMETER
`
`{75]
`
`Inventors: Stephan P. Athan, Tampa; John E.
`Scharf, Oldsmar, both of Fla.
`
`[73] Assignee: University of South Florida, Tampa,
`Fla.
`
`[21]
`
`Appl. No.: 221,958
`
`[22]
`
`Filed:
`
`Apr. 1, 1994
`
`(51)
`[52]
`[58]
`
`[56]
`
`Tint. C19eeceeeeetttecceeeeeeecssssssssseeseeseeeseneeeseees ALB 5/02
`
`TNE, cecsprrveprscroryrcestsrsyens
`. 128/633, 356/41
`Field. of Seareha:
`ccssccisccsscisscssnssscoseasicceeses 128/633, 903;
`356/41; 250/214 A, 214 L; 607/60
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`ssesseeveene 128/903
`
`OTHER PUBLICATIONS
`
`“Optimization of Portable Pulse Oximetry Through Fourier
`Analysis,” John E. Scharf et al., IEEE, Jun. 1993, pp.
`233-235, first available on Apr. 2, 1993, at the IEEE, 12th
`Southern Biomedical Conference at Tulane University, New
`Orleans, LA, held Apr. 2-4, 1993.
`“Pulse Oximetry Through Spectral Analysis,” John E.
`Scharf et al., 1993 IEEE, Jun. 1993, pp. 227-229,
`first
`available on Apr. 2, 1993, at
`the IEEE, 12th Southern
`Biomedical Conference at Tulane University, New Orleans,
`LA,held Apr. 2-4, 1993.
`:
`“Direct Digital Capture of Pulse Oximetry Waveforms,”
`John E. Scharf et al., 1933 IEEE, Jun. 1993, pp. 230-232,
`first available on Apr. 2, 1993, at the IEEE, 12th Southern
`Biomedical Conference at Tulane University, New Orleans,
`LA,held Apr. 2-4, 1993.
`Light-To-Frequency Converter—TSL220, Texas Instru-
`ments Inc., D3619, Aug. 1990, Rev. Jun. 1991.
`Programmable Light-To—Frequency Converter—TSL230,
`Texas Instruments Inc., SOESOO7A, Dec. 1992, Rev. Dec.
`1993.
`CMOS—8-Bit Buffered Multiplying DAC—AD7524,
`Digital-to—Analog Converters, Rev. A, pp. 2-399, 402-403.
`Burr-Brown ACF2101 Advertisement and Product Data
`Sheet (PDS-1079, Mar. 1991).
`“Integrator [C Converts Picoamperesto Volts,” Frank Good-
`enough, Electronic Design, Jun. 13, 1991, pp. 132-134,
`
`3,802,774
`4/1974 Tchang .
`6/1974 Scheidt «2.2.2...
`3,815,583
`9/1979 Nielsen .
`4,167,331
`4,266,554
`5/1981 Hamaguri .
`11/1982 Loretz .
`4,357,105
`10/1983 Wilber .
`4,407,290
`5/1984 Heinemann .
`4,447,150
`2/1985 Gloima et al. .
`4,498,020
`4,586,513
`5/1986 Hamaguri -
`4,694,833
`9/1987 Hamaguri .
`1/1989 Smith .
`4,800,495
`2/1989 Malinouskas .
`4,807,630
`2/1989 Hersh et al. .
`4,807,631
`Af1989 Frick et al.
`.....ccsssenssssesseereeeere
`4,824,242
`9/1989 Stone et al. .
`4,869,254
`11/1989 Hausman et al. .
`4,883,353
`3/1990 Corenman et al. .
`A diagnostic instrument for determining a cardiovascular
`4,911,167
`6/1990 Corenman etal. .
`system parameter. In one embodiment, the instrumenttakes
`4,934,372
`1/1992 Stoneetal. .
`5,078,136
`the form of a portable pulse oximeter comprising a light to
`SINGGD.|Charicet al) siscccittcassinsestersieiee 128/633
`5,111,817
`frequency converter (LFC) as a sensor. Also providedis a
`§/1992 Rother.
`5,113,861
`light to frequency converter comprising a photoresistor and
`9/1992 Kohnoetal. .
`5,149,503
`capacitor
`in circuit communication with an inverting
`S167290|Y2s199 Chance cccsain 128/633
`Schmitt trigger and configured such that the inverter gener-
`5,190,038
`3/1993 Polson .
`ates a periodicelectrical signal corresponding to the amount
`of electromagnetic radiation illuminating the photoresistor.
`
`Primary Examiner—Angela D. Sykes
`Assistant Examiner—Eric F. Winakur
`Attorney, Agent, or Firm—Calfee, Halter & Griswold
`
`128/633
`
`(57)
`
`ABSTRACT
`
`FOREIGN PATENT DOCUMENTS
`
`1377605
`9207505
`
`ccsesssesetssessessneeneee 250/214 L
`—-2/19BB USSR.
`5/1992 WIPO oeeesceeeteee
`veveee 128/903
`
`22 Claims, 6 Drawing Sheets
`
`COUNTER
`
`ui P-SYSTEM
`
`
`DISPLAY
`[ke
`
`
`LEO
`Orivers
`
`0001
`
`Apple Inc.
`APL1038
`U.S. Patent No. 8,923,941
`
`Apple Inc.
`APL1038
`U.S. Patent No. 8,923,941
`
`0001
`
`

`

`U.S. Patent
`
`Nov. 19, 1996
`
`Sheet 1 of 6
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`5,575,284
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`WILSAS-d7Ja
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`H/SYastIdWyYsTd
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`A=
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`(LYVYOIed)f814
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`0002
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`0002
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`

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`U.S. Patent
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`Noy. 19, 1996
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`Sheet 2 of 6
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`5,575,284
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`asasPay
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`WILSAS-d7|
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`037
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`SUGA!Ug
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`ogYara
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`99
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`0003
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`0003
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`

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`U.S. Patent
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`Nov. 19, 1996
`
`Sheet 3 of 6
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`5,575,284
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`68
`
`
`DISPLAY
`
`
`
`Microcontroller
`
`
`FIG.2B
`
`LEDDrivers
`
`0004
`
`0004
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`

`

`U.S. Patent
`
`Nov.19, 1996
`
`Sheet4 of 6
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`5,575,284
`
`70
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`Yoo
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`120pF
`
`
`0005
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`0005
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`

`

`Nov. 19, 1996
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`Sheet 5 of 6
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`U.S. Patent
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`5,575,284
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`FIG. 4C
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`76
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`Analo
`Switc
`Bank
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`0006
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`0006
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`

`

`U.S. Patent
`
`Nov. 19, 1996
`
`Sheet 6 of 6
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`5,575,284
`
` 100
`Initialize
`
`System
`
`102
`
`
`
`
`three
`Collect
`quarters of
`
`data
` 104 Collect fourth
`
`quarter of
`new data
`
` FFT to determine: 112
`
`Red AC. Red OC.
`IR AC. and IR OC
`
`
`
`quarter of data
`
` Calculate R and
`
`Discard oldest
`
`106
`
`108
`
`then Sp02
`discrete LEDs
`
`110
`
`
`
`
`Display Sp02
`
`or
`illuminate
`
`FIG. 3
`
`0007
`
`0007
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`

`

`5,575,284
`
`1
`PORTABLE PULSE OXIMETER
`
`FIELD OF THE INVENTION
`
`The present invention relates generally to medical diag-
`nostic instruments and, more specifically, to a portable pulse
`oximeter with a remote light-to-frequency converter as a
`sensor and a telemetry system to telemeter the calculated
`saturation value to a remote display.
`
`10
`
`BACKGROUND OF THE INVENTION
`
`2
`because HbO,absorbssignificantly more light in the visible
`spectrum than RHb.
`In practice of the pulse oximetry technique, the oxygen
`saturation of hemoglobin in intravascular blood is deter-
`mined by (1) alternatively illuminating a volumeofintra-
`vascular blood with electromagnetic radiation of two or
`more selected wavelengths, e.g., a red wavelength and an
`infrared wavelength, (2) detecting the time-varying eleciro-
`magnetic radiation intensity transmitted through orreflected
`back by the intravascular blood for each of the wavelengths,
`and (3) calculating oxygen saturation valuesfor the patient's
`blood by applying the Lambert-Beer’s transmittance law to
`the detected transmitted or reflected electromagnetic radia-
`tion intensities at the selected wavelengths.
`The degree of oxygen saturation of hemoglobin, SpO,,in
`Whereas apparatusis available for making accurate mea-
`arterial blood is often a vital index of the condition of a
`surements on a sample of blood in a cuvette,it is not always
`patient. As blood is pulsed through the lungs by the heart
`possible or desirable to withdraw blood fromapatient, and
`action, a certain percentage of the deoxyhemoglobin, RHb,
`it obviously impracticable to do so when continuous moni-
`picks up oxygen so as to become oxyhemoglobin, HbO).
`toring is required, such as while the patient is in surgery.
`From the lungs, the blood passes through the arterial system
`Therefore, much effort has been expanded in devising an
`until it reaches the capillaries at which point a portion ofthe
`instrument for making the measurement by noninvasive
`means.
`HbO, gives up its oxygen to support the life processes in
`adjacentcells.
`By medical definition, the oxygen saturation level is the
`percentage of HbO, over the total hemoglobin; therefore,
`SpO,=HbO./(RHb+HbO,). The saturation value is a very
`important physiological value. A healthy, conscious person
`will have an oxygensaturation of approximately 96 to 98%.
`A person can lose consciousness or suffer permanent brain
`damage if that person’s oxygen saturation value falls to very
`low levels for extended periods of time. Because of the
`importance ofthe oxygen saturation value, “Pulse oximetry
`has been recommended as a standard of care for every
`general anesthetic.” Kevin K. Tremper & Steven J. Barker,
`Pulse Oximetry, Anesthesiology, January 1989, at 98.
`An oximeter determines the saturation value by analyzing
`the change in color of the blood. Whenradiant energy passes
`through a liquid, certain wavelengths may be selectively
`absorbed by particles which are dissolved therein. For a
`given path length that the light traverses throughtheliquid,
`Beer’s law (the Beer-Lambert or Bouguer-Beer relation)
`indicates that
`the relative reduction in radiation power
`(P/Po) at a given wavelength is an inverse logarithmic
`function of the concentration of the solute in the liquid that
`absorbs that wavelength.
`For a solution of oxygenated human hemoglobin, the
`absorption maximum is at a wavelength of about 640
`nanometers
`(red),
`therefore,
`instruments
`that measure
`absorption at this wavelength are capable of delivering
`clinically useful information as to oxyhemoglobin levels.
`In general, methods for noninvasively measuring oxygen
`saturation in arterial blood utilize the relative difference
`between the electromagnetic radiation absorption coefficient
`of deoxyhemoglobin, RHb, and that of oxyhemoglobin,
`HbO,,. The electromagnetic radiation absorption coefiicients
`of RHb and HbO,are characteristically tied to the wave-
`length of the electromagnetic radiation traveling through
`them.
`It is well known that deoxyhemoglobin molecules absorb
`more red light than oxyhemoglobin molecules, and that
`absorption of infrared electromagnetic radiation is not
`affected by the presence of oxygen in the hemoglobin
`molecules. Thus, both RHb and HbO,absorb electromag-
`netic radiation having a wavelength in the infrared (IR)
`region to approximately the same degree; however, in the
`visible region, the light absorption coefficient for RHb is
`quite different from the light absorption coefficient of HbO,
`
`A critical limitation is that of measurement accuracy. In
`pulse oximetry, signal artifact from patient-probe motion,
`ambient light, and low perfusion (low blood circulation
`through the extremities) is one of the primary causes of
`inaccurate saturation readings. (“Artifact” is any component
`of a signal that is extraneous to variable represented by the
`signal.) Inaccuracies are also caused from physiologic non-
`linearities and the heuristic methods used to arrive at the
`final saturation values.
`Another importantlimitation is patient confinementto the
`pulse oximeter, due to the wired probe connecting the patient
`to the unit. This limits patient mobility in every application
`_of its use, including the emergency room, operating room,
`intensive care unit, and patient ward.
`Thus,
`three problems plague pulse oximetry. The first
`problem relates to signal artifact management and inaccu-
`racies of the saturation values due to the nonlinear nature of
`the sample tissue bed. The second problem relates to noise
`from signal artifact which introduces further inaccuracies.
`The third problem relates to restricted patient mobility and
`probe placement due to the wire bundle that physically
`couplesthe patient to the oximeterunit andthe exclusiveuse
`of transmittance-type probes.
`Dueto the non-linear nature of human physiology, engi-
`neers were forced to employ techniques for calculating the
`final saturation value based not on an analytic solution, but
`rather, on a calibration curve or look-up table derived from
`empirical data. This is data that has been collected over
`hundreds orpossibly thousandsof patients and stored as a
`look-up table in the system memory. This techniqueleadsto
`obvious inaccuracies in the final saturation value since the
`SpO, value in the look-up table is only as accurate as the
`calibration curve programmed into the system memory,
`which in tur is only as accurate as the in vitro laboratory
`oximeter used to generate it. These inaccuracies are com-
`pounded by differences in skin characteristics between
`patients, as well as differences over the skin surface of the
`same patient.
`Signal artifact has three major sources: (1) ambientlight
`(which causes an AC/DC maskingsignal), (2) low perfusion
`
`The pulse oximeters used today are desk-top models or
`handheld models that are interfaced to the patient through
`the use of a multi-wire bundle. Despite their size and level
`of technology,these units are still bound by several limita-
`tions.
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`5,575,284
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`4
`tively illuminate an intravascular blood sample with two
`wavelengths ofelectromagnetic radiation. The electromag-
`netic radiation interacts with the blood and a residual optical
`signal
`is both reflected and transmitted by the blood. A
`photodiode in the light-to-frequency converter (LFC) col-
`lects oximetry data from the intravascular blood sample
`illuminated by the two LEDs. The LFC producesa periodic
`electrical signal
`in the form of a pulse train having a
`frequency,the logarithm of whichis in linear relationship to
`the logarithm ofthe intensity of the optical signal received
`by the LFC. The data becomes an input to a high-speed
`digital counter, which converts the pulsatile signal into a
`form suitable to be entered into a central processing unit
`(CPU)of a computer system.
`In the alternative, a CPU with an internal counter can be
`used, thereby eliminating the need for an external counter
`and further reducing the system size.
`Once inside the CPU, the time-domain data is converted
`into the frequency domain by, for example, performing the
`well-known Fast Fourier Transform (FFT) on the time-
`domain data. The frequency domain data is then processed
`to determine the saturation value.
`
`It is therefore an advantage of the present invention to
`provide a portable, low-power oximeter.
`It is a further object of this invention to provide an
`improved sensor in the form of a light-to-frequency con-
`verter to reduce the parts countof prior art systems.
`These and other advantagesof the present invention shall
`become more apparent from a detailed description of the
`invention.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`In the accompanying drawings, which are incorporatedin
`and constitute a part of this specification, embodiments of
`the invention are illustrated, which, together with a general
`description of the invention given above, and the detailed
`description given below serve to examplethe principles of
`this invention.
`
`15
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`20
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`25
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`
`3
`(in whichthe intensity of the desired AC/DC signal is very
`low thereby allowing other artifact sources to mask the
`desired signal moreeasily), and (3) patient or sensor motion
`(which generates a large AC/DCartifact maskingthe desired
`signal), When the oximetry signal is amplified, the noise
`components are amplified along with the desired signal. This
`noise acts to corrupt the primary signal, during both pre-
`processing as well as post-processing, thereby reducing the
`accuracy of the pulse oximeter reading. Signal artifact is
`prevalent with both refiectance- and transmittance-type
`probes.
`Restricted patient mobility is due to the hard wired
`interface that
`links the patient probe to the large, bulky
`oximeter unit. This link is a multi-wire bundle that is used
`to provide an electrical path for the LED drivers and the
`photodiode located at the end of the wire bundle in the
`probe. Probes employing transmittance-type method are
`restricted to the ears, fingers, or toes and,
`thus, require
`physical access to these areas exclusively.
`Oximeters are large because ofthecircuitry heretofore
`believed necessary to capture the signals and because such
`higher-poweredcircuitry shortensbattery life. Typical digi-
`tal oximeters usea silicon photodiode, a current-to-voltage
`converter(a transimpedance amplifier), a preamplifier,filter
`stage, a sample and hold, and an analog-to-digital (A/D)
`converter to capture the oximetry signal. These components
`make the creation of truly portable oximeters difficult
`because ofthe large footprint and high power requirements
`of each device. The A/D converter, in particular, is typically
`large and power-hungry.
`SUMMARYOF THE INVENTION
`
`According to the present invention, an oximeter is pro-
`vided with a light-to-frequency converter as a sensor and a
`telemetry system to telemeterthe calculated saturation value
`to a remotestation. The light-to-frequency converter elimi-
`nates the need for a separate photodiode, a current-to-
`voltage converter, a preamplifier, a filter, a sample and hold,
`and an analog-to-digital (A/D) converter found in typical
`digital oximeters, thereby significantly reducing thecircuit
`footprint and power consumption. In short, the light-to-
`frequency converter can be directly connected to an input of
`a microcontroller or other CPU.The useoftelemetry allows
`accurate hemoglobin saturation level determination to be
`made withoutthe patient being tethered by a wire bundleto
`a remote display. Powerful portable systems can be realized
`using very large-scale integrated circuit (VLSI) multichip
`module (MCM)technology.
`An oximeter made under the presentinventionis a truly
`portable unit, capable of capturing and processing oximetry
`data in a very small package and transmitting calculated
`saturation values to a remote receiver. The type of receiver
`that is particularly useful in the context of the present
`invention is a caregiver’s wrist receiver or other type of
`receiver that communicates to a primary caregiver. In addi-
`tion, this invention can communicate with other types of
`receivers, such as a nurses’ station receiver or some other
`personal data receiver. Spread spectrum communication
`techniquesallow highly secure and noise-immunetelemetry
`of saturation values in noisy clinical and healthcare envi-
`ronments,
`
`FIG. 1 is an electrical schematic representation of a
`generic priorart pulse oximeter;
`FIG. 2Ais an electrical schematic representation of one
`embodimentofa pulse oximeter of the present invention;
`FIG. 2B is an electrical schematic representation of
`another embodiment of a pulse oximeter of the present
`invention;
`FIG. 3A is an electrical schematic representation of the
`implementation of the TSL220 light-to-frequency converter
`in the oximeter of the present invention;
`FIG, 3B is an electrical schematic representation of the
`implementation of the TSL230 light-to-frequency converter
`in the oximeter of the present invention;
`FIG, 4A is an electrical schematic representation of an
`implementation of a light-to-frequency converter of the
`present invention;
`FIG.4Bis another embodimentof the LFC shownin FIG.
`4A;
`FIG, 4C is yet another embodiment of the LFC shown in
`FIG. 4A; and
`The oximeter of the present invention usesapair oflight
`FIG. 4Disstill another embodimentof the LFC shown in
`emitting diodes,a light-to-frequency converter, a high-speed
`FIG. 4A; and
`counter, a computer system, and an display or other output.
`FIG, 5 is a flow chart showing the major process steps
`According to the present invention, two light emitting
`taken by the computer system in calculating the saturation
`value.
`diodes (LEDs), a red LED and an infrared LED, alterna-
`
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`5,575,284
`
`5
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`Before describing the details of the present invention, a
`description of a generic prior art pulse oximeter may be
`helpful in understanding the advantages of the pulse oxime-
`ter of the present invention. Reference is had, therefore, to
`FIG. 1, which shows a generic prior art pulse oximeter 10.
`A typical prior art oximeter 10 has a photodiode 12 for
`detecting an optical signal 14 reflected from or transmitted
`through a volumeof intravascular blood (not shown) illu-
`minated by one or more light emitting diodes (LEDs, not
`shown). The LEDs emit electromagnetic radiation at a
`constant intensity; however, an optical signal 14 with a
`time-varying intensity is transmitted through or reflected
`back from the intravascular blood for each of the wave-
`lengths. The photodiode 12 generates a low-level current
`proportional to the intensity of the electromagnetic radiation
`received by the photodiode 12. The current is converted to
`a voltage by a current to voltage converter 16, which may be
`an operational amplifier in a current to voltage (transimped-
`ance) configuration.
`The signal is then filtered with a filter stage 18 to remove
`unwanted frequency components, such as any 60 Hz noise
`generated by fluorescentlighting. Thefiltered signal is then
`amplified with an amplifier 20 and the amplified signal is
`sampled and held by a sample and hold 21 while the signal
`is digitized with a high-resolution (12-bit or higher) analog
`to digital converter (ADC) 22.
`The digitized signal
`is then latched by the CPU (not
`shown) of the computer system 24 from the ADC 22. The
`computer system 24 then calculates a coefficient for the
`oxygen saturation value from the digitized signal and deter-
`mines the final saturation value by reading the saturation
`value for the calculated coefficient from a look-up table
`stored in memory. Thefinal saturation value is displayed on
`a display 26.
`Thus,
`the generic prior art pulse oximeter 10 requires
`numerous devices to determine the oxygen saturation value
`from the optical signal. Moreover, these devices, particularly
`the ADC 22, require a relatively large amount of space and
`electrical power, thereby rendering a portable unit imprac-
`tical.
`
`Underthe present invention, the prior art oximeter 10 is
`modified so that
`the photodiode 12, current to voltage
`converter 16,filter 18, amplifier 20, sample and hold 21, and
`analog-to-voltage converter 22 are replaced with a light-to-
`frequency converter and a high speed counter.
`FIG. 2A shows one embodimentofa pulse oximeter 50 of
`the presentinvention. The oximeter 50 of the present inven-
`tion comprisesa light-to-frequency converter (LFC) 52 for
`detecting an optical signal 54 from a volumeof intravascular
`volume of blood 56 illuminated by one or more light
`emitting diodes (LEDs) 58, 60. The LEDs 58, 60 emit
`electromagnetic radiation at a constant intensity; however,
`an optical signal 54 with a time-varying intensity is trans-
`mitted through or reflected back by the intravascular blood
`for each of the wavelengths. In the preferred embodiment,
`the reflected optical signal 54 is analyzed to determine the
`saturation value. The LFC 52 producesa periodicelectrical
`signal
`in the form of a pulse train having a frequency
`corresponding to the intensity of the broadband optical
`signal received by the LFC 52. The periodic data then
`becomesan input to a high-speed digital counter 62, which
`converts the periodic signal into a form suitable to be entered
`into a computer system 64.
`Once inside the computer system 64, the LFC signal is
`analyzed to determine the saturation value. In one embodi-
`
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`ment, the data is converted into the frequency domain by, for
`example, performing the well-known Fast Fourier Trans-
`form (FFT) onthe data.It is also believed that other common
`techniquesof converting time-domain data to the frequency
`domain will suffice: e.g., discrete cosine transform, wavelet
`transform,discrete Hartley transform, and Gabortransform.
`The frequency domain data is then analyzed to determinethe
`saturation value by code executing on the computer system
`64,as will be more fully explained in the text accompanying
`FIG. 4. Once calculated, the saturation valueis displayed on
`a display 68.
`the
`In addition to performing saturations calculations,
`computer system 64 controls LED drivers 66, which control
`the LEDs 58, 60.
`FIG. 2B shows another embodimentof the pulse oximeter
`of the present invention. The embodimentof FIG. 2B differs
`from the embodiment in FIG. 2A in tworespects. First, the
`computer system 64 and counter 62 are implemented by a
`microcontroller 84 having an internal high-speed counter 82
`associated therewith. Second, the microcontroller 84 and the
`display 68 are placed in circuit communication using a
`transmitter 86 and receiver 88. The transmitter 86 transmits
`a signal 90 through an antenna 92. Thereceiver 88 receives
`the signal 90 through a second antenna 94 and passes the
`information to the display circuit 68. The LFC 52,
`the
`counter 62, the computer system 64, the display 68, the LED
`drivers 66, the LEDs 58, 60, and the other componentsare
`connectedin electrical circuit communication as shownin
`FIGS. 2A and 2B. One suitable LFC 52 is the TSL220,
`manufactured and sold by Texas Instruments, P.O. Box
`655303, Dallas, Tex. 75265. FIG. 3A is an electrical sche-
`matic representation showing the use of the TSL220inthe
`oximeter of the present invention. The capacitor 70 and
`resistor 72 are in circuit communication and havethe values
`as shown in that figure. Another suitable LFC 52 is the
`TSL230, shown in FIG. 3B,
`is manufactured by Texas
`Instruments. Unlike the TSL220, the TSL230 requires no
`external capacitor and provides microprocessor compatible
`controllines; therefore, the TSL230 is a one-chip sensor.
`Yet another suitable LFC 52 is a novel LFCcircuit, which
`was invented by Stephan Peter Athan,one of the coinventors
`of this invention, and is shown in FIG.4A. In that circuit, a
`photoresistor 73 having a variable resistance is placed in
`circuit communication with a pulse generating circuit that is
`configured to generate a periodic electrical signal corre-
`sponding to the value of the variable resistance of the
`photoresistor. In one embodiment, a photoresistor 73, a
`capacitor 74, and an inverter 75 are placed in circuit com-
`munication and have the values shownin that figure. The
`photoresistor 73 is placed across the input node 76 and the
`output node 77 ofthe inverter 73. The capacitor 74 is placed
`between the input node 76 and ground. The inverter 75 is
`ideally an inverting Schmitt trigger with hysteresis at its
`input; however, other inverters are also believed to be
`suitable.
`The photoresistor 73 can be a standard cadmium sulfide or
`cadmium selenide photoresistor, which are both widely
`available from many sources. Other types of photoresistors
`are also available. As is knownin the art, the photoresistor
`73 has a variable resistance that depends on the amount of
`electromagnetic radiation 78 being emitted onto the photo-
`resistor. The photoresistor 73, capacitor 74, and inverter 75
`are configured such that the period of time in which the
`capacitor 74 charges and discharges corresponds to the value
`of the variable resistance of the photoresistor 73. Thus, the
`outputof the inverter 75 is a periodic signal, the period of
`which depends on the amountof electromagnetic radiation
`being emitted onto the photoresistor 73.
`
`0010
`
`0010
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`

`

`5,575,284
`
`7
`As shownin FIG.4B, a resistor 79 with a substantially
`fixed resistance can be placedin series with the photoresistor
`78 and placed across the input 76 and output 77 of the
`inverter 75. In addition, as shown in FIG. 4C, a multiplying
`digital to analog converter (MDAC) 80 can be placed in
`series with the photoresistor 73 and placed across the input
`76 and the output 77 of the inverter 75. As shownin that
`figure, the MDAC80is interfaced to the microcontroller 84,
`which can then control the parameters, and therefore the
`sensitivity (i.e., shifting the frequency associated with a
`given amount of illumination to accommodate a broader
`range of light frequencies), of the circuit by selectively
`asserting moreorless resistance in series with the photore-
`sistor 73, One suitable MDACis the AD7524available from
`Analog Devices, whichis essentially a computer controlled
`R2R network, which is known inthe art.
`As shownin FIG. 4D, a bank of capacitors with varying
`capacitance values can be connectedin the circuit of FIG.
`4A. The capacitors are interfacedto the circuit via a com-
`puter controlled bank of analog switches, as shown in that
`figure. The microcontroller 84 can control the parameters of
`the circuit, and therefore the sensitivity (i.e., shifting the
`frequency associated with a given amountofillumination to
`accommodate a broader range of light frequencies), by
`selectively connecting one or moreofthe capacitorsto line
`76.
`
`While the LFC of FIGS. 4A-4D is believed to be par-
`ticularly useful in connection with the portable pulse oxime-
`ter of the presentinvention,it is also believed to haveutility
`beyond that of oximetry or other cardiovascular measure-
`ment.
`
`Referring back to FIGS. 2A and 2B, the Red LED 58 is
`a red LED, emitting light having a wavelength of approxi-
`mately 660 nm. Onesuitable LEDis the P417-ND, whichis
`available from by Digikey, 701 Brooks Avenue South, Thief
`RiverFalls, Minn. 56701. It is believed that an LED emitting
`any wavelength oflight in the visible spectrum is suitable;
`however, because a solution of human hemoglobin has an
`absorption maximum at a wavelength of about 640 nanom-
`eters (red), the closer to that wavelength, the more accurate
`the results (otherwise, calibration curves are required,as is
`knownin theart).
`The IR LED 60is an infrared LED, emitting electromag-
`netic radiation having a wavelength of approximately 940
`nm. One suitable LED is the FS5FIQT-ND, which is also
`available from Digikey.It is believed that to be suitable, the
`IR LED 60 must emit electromagnetic radiation at a wave-
`length such that the absorption of the emitted electromag-
`netic radiation by the blood 56 is unaffected by the presence
`or absence of oxygen bound to the hemoglobin molecules.
`The counter 62 may be any high speed counter capable of
`being interfaced to a computer system. One suitable counter
`is the 4020 CMOS counter, which is manufactured by
`numerous manufacturers, e.g., Texas Instruments, P.O. Box
`655303, Dallas, Tex. 75265, as is well knownin theart.
`Interfacing the counter 62 to the computer system 64 may
`be done in several ways. The counter 62 and computer
`system 64 may be configuredto either (1) count the pulses
`generated by the LFC 52 during a given time period or(2)
`count the numberofpulsesof a free-running clock (corre-
`sponding to the amount of time) between the individual
`pulses of the LFC 52. Either method will provide satisfac-
`tory data. The latter method can be implemented in several
`ways. For example,the counter can be reset at each period
`of the LFC signal. In the alternative, at each edge of LFC
`pulse train, the value in the counter can be saved to a register
`
`25
`
`30
`
`35
`
`40
`
`55
`
`65
`
`8
`and subtracted from the value stored at the previous edge.
`Either way,the result is a counter value corresponding to the
`time difference between the two pulse edges. Many con-
`figurations are possible. The counter 62 can either count
`pulses or elapsed time between edges and the computer
`system 64 either reads the value in the counterperiodically
`by polling the counter, or the computer system 64 reads the
`value whenever the counter 62 generates an interrupt. Again,
`many configurations are possible.
`The computer system 64 can be any computer system
`capable of performing oximetry calculations to the desired
`accuracy in the desired period of time (calculations may be
`doneeither in real time or after collection of desired data)
`and capable of interfacing with a counter 62,a display 68,
`and LED drivers 66. The computer system 64 may include
`a CPU,random access memory (RAM), read-only memory
`(ROM), and associated control circuitry, such as decoders
`and multi-phase clocks, in circuit communication, as is well
`knownin the art. To be suitable, the computer system must
`be capable of being a signal analyzer. Thatis, the computer
`system 64 must have the computational capacity to deter-
`mine thesaturation value from the periodic pulses.
`Onesuitable computer system 64 is any of several micro-
`controllers 84, which are known in the art. The 68HC16
`microcontroller manufactured by Motorola, Inc., Austin Tex.
`78735,is one example. The 68HC16is suitable for systems
`requiring low-level digital signal processing and has on-
`board erasable/programmable ROM (EPROM)and RAM. It
`also has an on-board 16-bit high-speed counter 82 eliminat-
`ing the need for an external counter 62. The outputfrom the
`LFC 52 may bedirectly connected to the counter inputofthe
`68HC16,thereby allowing the elimination of another dis-
`crete device (the separate counter 62). Another suitable
`microcontroller 84 is the 80CX51FA, which is manufactured
`by Intel Corp., Santa Clara, Calif. 95051.
`If more processing power than either the 68HC16 or the
`80CX51FA can provide is required to determine the satura-
`tion value, a digital signal processororfloating point copro-
`cessor (not shown) can be added to the computer system 64.
`One suitable digital signal processor is the TMS320CX0
`digital signal processor, manufactured by Texas Instruments.
`This device can calculate highly accurate oxygen saturation
`values in a period of time on the order of microseconds.
`The LED drivers 66 may be any driver capable of
`providing a signal capable of causing one or more LEDs to
`illuminate. Numerous LED-drivingcircuits are well known
`in the art. The drivers 66 must allow the LEDs 58, 60 to be
`alternatively illuminated under control of the computer
`system 64.
`:
`Someprior art LED drivers have a normalizing function
`that increases or decreases the intensity of electromagnetic
`radiation generated by the LEDsin the system.It is desirable
`to be able usea single oximeter configuration to measure the
`oxygen saturation of an infant and later to use the same
`oximeter configuration to measure’ oxygen saturation levels
`of an adult. Since the nature of skin andhair of an infant are
`different from that of an adult, it is generally accepted that
`an LEDintensity calibrated to measure the oxygen satura-
`tion level of an adult will be too bright