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`U8005575284A
`
`United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,575,284
`
`
`Athan et at.
`[45} Date of Patent:
`Nov. 19, 1996
`
`[54} PORTABLE PULSE OXEMETER
`
`OTHER PUBLICATIONS
`
`[751
`
`Inventors: Stephan R Aw“, Tampa; John E
`smart, Oldfifm.1 both of Fla‘
`
`[73] Assignce: University of South Florida, Tampa,
`1:];
`
`.
`[211 Am NO“ 221353
`[22]
`Filed:
`Apr. 1, 1994
`{51]
`Int. Cl.“
`A6113 5:02
`....................
`[52] U.S. Cl.
`1281633; 356141
`
`[581 Field of Search --------------- --
`1281633, 903;
`
`356141; 2
`A. 114 L; 507/50
`
`[59]
`
`1231903
`
`
`
`.. 1281633
`
`_
`References C1199
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`4111974 Tchang -
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`$1974 Scheidt
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`41407290 10,1933 “HEEL
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`511984 Heinemaim .
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`9.11989 Stone et al..
`.
`4,883,353
`1111989 Hausman et a}.
`4,911,157
`311990 Comm a 3L ,
`4,934,372
`611990 Ccrertmart et a1.
`.
`5,078,136
`111992 Stone ct nl.
`,
`5.111.817
`et a].
`............................. 1231633
`1113351
`511992 R011” -
`g3” “1 81'
`""""""
`131,1993 P0150:
`5‘190‘038
`'
`'
`’
`FOREIGN PATENT DOCUMENTS
`1377605
`21'1983
`11.55.11,
`9207505
`511992 WTPO .......
`
`“Optimizition of Portable Pulse Oximetry Through Fourier
`Analysts,
`John
`Seharf at 3.1.. 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. 24, 1993.
`“Pulse Oximetry Through Spectral Analysis." John E.
`Scharf et at, 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, hold A 1‘. 2—4. 1993.
`'
`“Dim, Digital Capm of Pulse Oximm Wavefom!”
`John E 5,;th et a1“ 1933 TREEI rum 1993‘ ml 230_232,
`first available on Apr. 2, 1993. at the IEEE, 1211-. Southern
`Biomedical Conference at Tulane University, New Orleans.
`LA, held Apr. 2—4, 1993.
`Light—To—Frequency Converter—TSLZZ'D, Texas Instru-
`11361118 1.116., D3619, Aug. 1990, REV. Jun. 1991.
`Programmable Ltghl—To—Frequency Convener-—-TSL230,
`Texas Instruments Inc, SOESDUTA, Dee. 1992, Rev. Dec.
`1991
`'
`‘
`'
`SMPfS—ftl B'Efim’d Magma“
`'
`'
`13“
`‘1“— “ “5 ‘mvem‘s’ .c“ ‘pp‘ ‘
`Burr—Brown ACFZlO] Advertisement and Product Data
`Sheet (PBS-1079, Mar. 1991]-
`“Integrator [C Convene Pieoamperes to Volts." Frank Good-
`enough, Electronic Design, .lun. 13, 1991, pp. 132~134.
`
`Primary Examiner—Angela D. Sykes
`Assistant Examiner—Erie F. Winakur
`Attorney. Agent, or Finn—Calfee, Halter & Griswold
`-
`
`[57]
`
`ABSTRACT
`
`A diagnostic instrument for determining a cardiovascular
`system parameter. In one embodiment, the instrument takes
`the form of a portable pulse oximeter comprising a light to
`frequency converter
`as a sensor. Also pmvidad is 3
`light to frequency converter comprising a photoresistor and
`capacitor
`in circuit communication with an inverting
`Schmitt trigger and configured such that the inverter gener-
`ates a periodic electrical signal corresponding to the amount
`of electromagnetic radiation illuminating the photoresistor.
`
`22 Claims, 6 Drawing Sheets
`
`
`'
`
`1,281,633
`
`
`
`2501214L
`1281903
`
`DISPLAY
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`64
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`COUNTER
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`NP St'STEH
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`62
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`LED
`Drivers
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`0001
`
`Apple Inc.
`APLl O3 8
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`US. Patent No. 8,923,941
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`FITBIT, Ex. 1038
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`Apple Inc.
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`US. Patent
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`Nov. 19, 1996
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`U.S. Patent
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`Nbv.19,1996
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`Sheet 2 of 6
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`5,575,284
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`US. Patent
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`Nov. 19, 1996
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`US. Patent
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`Nov. 19, 1995
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`Nov. 19,1996
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`US. Patent
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`Nov. 19, 1996
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` I00
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`InEIIQIize
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`SyStEm
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`three
`Collect
`quarters of
`data
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`102
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`FFT to determine:
`Red AC. Red DC,
`IR AC. and IR DC
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`112
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`110
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`Display SpOE
`or
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`discrete LEDS
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`FIG. 5
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`1
`PORTABLE PULSE OXIMETER
`
`FIELD OF THE lNVENTION
`
`The present invention relates generally to medical diag-
`nostic instruments and, more specifically, to a portable pulse
`oxitncter with a remote light-to-frequency converter as a
`sensor and a telemetry system to telemeter the calculated
`sanitation value to a remote display.
`
`BACKGROUND OF THE INVENTION
`
`The degree of oxygen saturation of hemoglobin, SpOz, in
`arterial blood is often a vital index of the condition of a
`patient. As blood is pulsed through the lungs by the heart
`action, a certain pcrcentage of the deoxyhcmoglobin, RHb.
`picks up oxygen so as to become oxyhemoglobin, HbOz.
`From the lungs, the blood passes through the arterial system
`until it reaches the capillaries at which point a portion of the
`l-l'bC‘l2 gives up its oxygen to support the life processes in
`adjacent cells.
`By medical definition, the oxygen saturation level is the
`percentage of HbCl2 over the total hemoglobin; therefore.
`SpObeOJfiRHbl-Hboz). The saturation value is a very
`important physiological value. A healthy, conscious person
`will have an oxygen saturation of approximately 96 to 98%.
`A person can lose consciousness or sufl'er permanent brain
`damage if that person’s oxygen saturation value falls to very
`low levels for extended periods of time. Because of the
`importance of the oxygen saturation value, “Pulse oximetry
`has been rcconunended as a standard of care for every
`general anesthetic." Kevin K. Tremper & Steven J. Barker,
`Pulse Grimerry, Anesthesiology, January 1989. at 98.
`An oxirneter determines the saturation value by analyzing
`the change in color ofthe blood. When radiant energy passes
`through a liquid, certain wavelengths may be selectively
`absorbed by particles which are dissolved therein. For a
`given path length that thc light traverses through the liquid,
`Beer's law (the Beer—Lambert or Bonguer~Becr relation)
`indicates that
`the relative reduction in radiation power
`(PlPo) at a given wavelength is an inverse logaritlnnic
`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 norlinvasivcly measuring oxygen
`saturation in arterial blood utilize the relative difference
`between the electromagnetic radiation absorption coefficient
`of deoxyhcmoglobin, RHb, and that of oxyhcmoglobiu,
`HbOz. The electromagnetic radiation absmption coefficients
`of RHb and Hsz are characteristically tied to the wave‘
`length of the electromagnetic radiation traveling through
`lhern.
`
`It is well known that deoxyhcmoglobin molecules absorb
`more red light than oxyhemoglobin molecules. and that
`absorption of infrared electromagnetic radiation is not
`afi’ected by the presence of oxygen in the hemoglobin
`molecules. Thus, both Ill-[b and HblE)2 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 coclficient for RHb is
`quite different from the light absorption coefficient of HbO2
`
`5 ,575 ,284
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`
`because HbO2 absorbs significantly more light in the visible
`spectrum than RHb.
`in practice of the pulse oximctty technique, the oxygen
`saturation of hemoglobin in intravascular blood is deter-
`mined by (l) alternatively illuminating a volume of intra-
`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 electro-
`magnetic radiation intensity transmitted through or reflected
`back by the intravascular blood for each of the wavelengths,
`and (3) calculating oxygen saturaan values for 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,
`Whereas apparatus is available for making accurate mea-
`surements on a sample of blood in a cuvettc, it is not always
`possible or desirable to withdraw blood {rum a patient, and
`it obviously impracticable to do so when continuous moni-
`toring is required, such as while the patient is in surgery.
`Therefore, much effort has been expanded in devising an
`instrument for making the measurement by noninvasive
`11185118.
`
`The pulse oximeters used today are desk-top models or
`handheld models that are interfaced to the patient through
`the use of a multi-vvire bundle. Despite their size and level
`of technology. these units are still bound by several limita-
`tions.
`A critical limitation is that of measurement accuracy. In
`pulse oximeu'y, 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. ("Anifact” 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.
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`Another important limitation is patient confinement to the
`pulse oximeler, 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 oximctry. 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 inaccmacies.
`The third problem relates to restricted patient mobility and
`probe placement due to the wire bundle that physically
`couples the patient to the oxirnetcr unit and the exclusive use
`of transmittanceatypc probes.
`Due to the non-linear nature of human physiology, engir
`nears were forced to employ techniques for calculating the
`final saturation value based not on an analytic solution, but
`rather, on a calibration curve or loolcup table derived from
`empirical data. This is data that has been collected over
`hundreds or possibly thousands of patients and stored as a
`look-up table in the system memory. This technique leads to
`obvious inaccuracies in the final saturation value since the
`SpO2 value in the look-up table is only as accurate as the
`calibration curve programmed into the system memory,
`which in turn 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 difi'crcnces over the skin surface of the
`samc patient.
`Signal artifact has three major sources: (1) ambient light
`(which causes an ACIDC masking signal), [2) low perfusion
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`(in which the intensity of the desired ACIDC signal is very
`low thereby allowing other artifact sources to mask the
`desired signal more easily). and (3) patient or sensor motion
`(which generates a large ACJDC artifact masking the 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 rcflcctance- and uansmittance—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 mold-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 die cars, fingers, or toes and,
`thus. require
`physical access to these areas exclusively.
`Oximeters are large because of the circuitry heretofore
`believed necessary to capture the signals and because such
`higher-powered circuitry shortens battery life. Typical digi-
`tal oximeters use a silicon photodiode, a current-to—voltage
`converter (a tt'ansimpedance amplifier), a preamplifier. filter
`stage, a sample and hold, and an analog-todigital (AID)
`converter to capture the oximetry signal. These components
`make the creation of truly portable oxirneters diflieult
`because of the large footprint and high power requirements
`of each device. The AID converter, in particular, is typically
`large and power-hungry.
`SUMMARY OF THE INVENTION
`
`According to the present invention, an oximeter is pro-
`vided with a light-to—fiequency converter as a sensor and a
`telemetry system to telemeter the calculated saturation value
`to a remote station. The light-to-frequency converter elimi-
`nates the need for a separate photodiode, a currentrto-
`voltage converter, a preamplifier, a filter, a sample and hold,
`and an analog-to—digital (AID) converter found in typical
`digital oximcters, thereby significantly reducing the circuit
`footprint and power consumption. In short,
`the light—to-
`frequency converter can be directly connected to an input or"
`a. microcontroller or other CPU. The use of telemetry allows
`accurate hemoglobin saturation level determination to be
`made without the patient being tethered by a wire bundle to
`a remote display. Powerful portable systems can be realized
`using very large-scale integrated circuit (VLSI) multichip
`module (MGM) technology.
`An oxirneter made under the present invention is 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 conununicates 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
`techniques allow highly secure and noise-immune telemetry
`of saturation values in noisy clinical and healthcare envi-
`moments.
`
`The oximeter of the present invention uses a pair of light
`emitting diodes, a lighten—frequency converter. a high-speed
`counter, a computer system, and an display or other output.
`According to the present invention, two light emitting
`diodes (LEDs), a red LED and an infrared LED, alternae
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`tively illuminate an intravascular blood sample with two
`wavelengths of electromagnetic 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-frcqucncy converter (LFC) col-
`lects oximetry data from the intravascular blood sample
`illuminated by the two LEDs. The LFC produces a periodic
`electrical signal
`in the form of a pulse train having a
`frequency, the logarithm of which is in linear relationship to
`the logarithm of the 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, :1 CPU with an internal counter can be
`used, thereby eliminating the need for an external counter
`and further reducing the system sine.
`Once inside the CPU, the time-domain data is converted
`into the frequency domain by, for example. performing the
`well—known Fast Fourier Transform (FFI') on the time—
`domain data. The frequency domain data is then processai
`to determine the samration 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 count of prior art systems.
`These and other advantages of 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 incorporated in
`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 example the principles of
`this invention.
`
`FIG. I is an electrical schematic representation of a
`generic prior an pulse oximeter;
`FIG. 2A is an electrical schematic representation of one
`embodiment of a pulse oximeter of the present invention;
`FIG. ZB 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 lightetoefrequency converter
`in the oxirneter of the present invention;
`FIG. BB is an elecu-ical schematic representation of the
`implementation of the TSL2301ight-to—frequeney converter
`in the oximctcr of the present invention;
`FIG. 4A is an electrical schematic representaan of an
`implementation of a light-to-frequency converter of the
`present invention;
`FIG. 43 is another embodiment of the LFC shown in FIG.
`4A;
`FIG. 4C is yet another embodiment of the LFC shown in
`FIG. 4A; and
`FIG. 4D is still another embodiment of the LFC shown in
`FIG. 4A; and
`
`FIG. 5 is a flow chart showing the major process steps
`taken by the computer system in calculating the saturation
`value.
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`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 oximc-
`ter of the present invention. Reference is bad, therefore. to
`FIG. 1., which shows a generic prior art pulse oximeter 10.
`A typical prior art oximeter 10 has a photodiodc 12 for
`detecting an optical signal 14 reflected from or transmitted
`through a volume of intravaseular 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. Thc 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 H1 noise
`generated by fluorescent lighting. The filtered 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 coellicient from a look—up table
`stored in memory. The final saturation value is displayed on
`a display 26.
`Thus.
`the generic prior art pulse oximcter 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.
`
`Under the 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 embodiment of a pulse oximeter 51] of
`the present invention. The oximcter 50 of the present inven-
`tion comprises a light-to-frequency converter (LFC) 52 for
`detecting an optical signal 54 from a volume of intravenoular
`volume of blood 56 illuminated by one or more light
`emitting diodes {LEDsJ 58, 60. The 1.15135 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 intravaseular blood
`for each of the wavelengths. [n the preferred embodiment,
`the reflected optical signal 54 is analyzed to determine the
`saturation value. The LFC 52 produces a periodic electrical
`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
`becomes an input to a highwspeed 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 vaiuc. In one embodi—
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`merit, the data is converted into the frequency domain by, for
`example, performing the well-known Fast Fourier Trans-
`form (FFTJ on the data. It is also believed that other common
`techniques of convening time—domain data to the frequency
`domain will sufiicc: e.g., discrete cosine transform, wavelet
`transform, discrete Henley transform, and Gabor transform.
`The frequency domain data is then analyzed to determine the
`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 value is displayed on
`a display 68.
`the
`lo addition to performing saturations calculations.
`computer system 64 controls LED drivers 66, which control
`the LEDs 58, 60.
`FIG. 213 shows another embodiment of the pulse oximeter
`of the present invention. The embodiment of FIG. 2B differs
`from the embodiment in FIG. 2A in two respects. First. the
`computer system 64 and counter 62 are implemented by a
`rrtierocontroller 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. The receiver 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 components are
`connected in electrical circuit communication as shown in
`FIGS. 2A and 23. One suitable LFC 52 is the TSLZZU,
`manufactured and sold by Texas Instruments, PO. Box
`655303, Dallas, Tex. 75265. FIG. 3A is an electrical sche-
`matic representation showing the use of the TSLZZD inrthe
`ofimeter of the present invention. The capacitor 70 and
`resistor 72 are in circuit communication and have the values
`as shown in that figure. Another suitable LFC 52 is the
`TSL230, shown in FIG. SE,
`is manufactured by Texas
`Instruments. Unlike the TSLZZD, the TSL230 requires no
`external capacitor and provides microprocessor compatible
`control lines; therefore, the TSL23D is a one—chip sensor.
`Yet another suitable LFC 52 is a novel LFC circuit, 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 conununication 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 photorcsistor 73, a
`capacitor 74, and an inverter '15 are placed in circait com-
`munication and have the values shown in that figure. The
`photorcsistor 73 is placed across the input node 76 and the
`output node 77 of the 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 photorcsistor 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 known in the art, the photoresistor
`73 has a variable resistance that depends on the amount of
`electromagnetic radiation 73 being emitted onto the photo-
`resistor. The photoresisror 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
`output of the inverter 75 is a periodic signal, the period of
`which depends on the amount of electromagnetic radiation
`being emitted onto the photoresistor 73.
`
`0010
`
`FITBIT, EX. 1038
`
`0010
`
`FITBIT, Ex. 1038
`
`

`

`5,575 ,284
`
`7
`As shown in FIG. 4B, a resiStor 79 with a substantially
`fixed resistance can be placed in 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 photorcsistor 73 and placed across the input
`76 and the output 77 of the inverter 75. As shown in that
`figure. the MDAC 80 is 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 more or least resistance in series with the photore—
`sistor 73. One suitable MDAC is the AD7524 available from
`Analog Devices, which is essentially a computer controlled
`R2R network. which is known in the art.
`As shown in FIG. 40. a bank of capacitors with varying
`capacitance values can be connected in the circuit of FIG.
`4A. The capacitors are interfaced to the circuit via a com—
`puter controlled bank of analog switches, as shown in that
`figure. The microeontroller 84 can control the parameters of
`the circuit, and therefore the sensitivity (i.e., shifting the
`frequency associated with a given amount of illumination to
`accommodate a broader range of light frequencies), by
`selectively connecting one or more of the capacitors to line
`76.
`
`While the LFC of FIGS. 4A~tlD is believed to be par-
`ticularly uscfiil in connection with the portable pulse oxime-
`ter of the present invention, it is also believed to have utility
`beyond that of oximctry or other cardiovascular measure-
`ment.
`
`Referring back to FIGS. 2A and 2B, the Red LED 58 is
`a red LED, entitling light having a wavelength of approxi-
`mately 660 nm. (line suitable LED is the P417-ND, which is
`available from by Digiltey, 701 Brooks Avenue South, Thief
`River Falls, Minn. 56701. It is believed that an LED emitting
`any wavelength of light 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
`known in the art).
`
`The [R LED 60 is an infrared LED. emitting electromag—
`netic radiation having a wavelength of approximately 940
`rim. One suitable LED is the FSFIQT-ND. which is also
`available from Digikey. It is believed that to be suitable. the
`IR LED 60 most emit electromagnetic radiation at a wave-
`length such that the absorption of the emitted electromag-
`netic radiation by the blood 56 is unafi'ected 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, c.g., Texas Instruments, HO. Box
`655303, Dallas, 'Dex. 75265, as is well known in the art.
`Interfacing the counter 62 to the computer system 64 may
`be done in several ways. The counter 62 and computer
`system 64 may be configured to either (1) count the pulses
`generated by the LFC 52 during a given time period or (2)
`count the number of pulses of 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 cart 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
`
`ll}
`
`15
`
`25
`
`3D
`
`35
`
`40
`
`45
`
`50
`
`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 difietence 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 counter periodically
`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
`done either in real time or after collection of desired data)
`and capable of interfacing with a counter 62, a display 63,
`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
`known in the art. To be suitable, the computer system must
`be capable of being a signal analyzer. That is, the computer
`system 64 must have the computational capacity to deter-
`mine the saturation value from the periodic pulses.
`One suitable computer system 64 is any of several micro—
`controllers 84, which are known in the an; The GSHCIG
`microcontroller manufactured by Motorola, Inc, Austin Tex.
`78735, is one example. The 68HC16 is suitable for systems
`requiring low-level digital signal processing and has on-
`board erasablet'prog-rammable 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 output from the
`LFC 52 may be directly connected to the counter input of the
`68HC]6,' thereby allowing the elimination of another dis—
`crete device (the separate counter 62). Another suitable
`microcontroller 84 is the SUCXSIFA, which is manufactured
`by Intel Corp, Santa Clara, Calif. 95051.
`If more processing power than either the 68HC16 or the
`BDCXSIFA can provide is required to determine the satura-
`tion value, a digital signal processor or floating pomt copro-
`cessor (not shown} can be added to the computer system 64.
`One suitable digital signal processor is the TMSSZOCXO
`digital signal processor, manufactured by Texas Instruments.
`This device can calculate highly accurate oxygen saniration
`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-driving circuits 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.
`'
`Some prior an. LED drivers have a normalizing function
`that increases or decreases the intensity of electromagnetic
`radiation generated by the LEDs in the system. It is desirable
`to be able u