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`US0055'?5234A
`
`United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,575,284
`
`Athan et al.
`[453 Date of Patent:
`Nov. 19, 1996
`
`
`[S4} PORTABLE PULSE OXIMETER
`
`OTHER PUBLICATIONS
`
`[751
`
`Inventors. Stephan P. Athan, Tampa. John E_
`Scharf’ Oldsmarj both of 1:13‘
`
`[73] A5-sigma; University flf south F101-1,53‘ Tampa,
`1:]a_
`
`J 2
`
`21353
`A r. 1, 1994
`1]
`
`[591
`
`“0ptin1iztttion of Portable Pulse Oximetry Through Fourier
`Analysis,
`John
`Scharf et al., IEEE, Jun. 1993, pp.
`233-235, first available on Apr. 2, I993, at the IEEE, 12th
`Southern Biomedical Conference at Tulane University, New
`Orleans, LA. held Apr. 24, 1993.
`“Pulse Oximctry Through Spectral Analysis." John E.
`_
`‘
`Scharf er al., 1993 IEEE, Jun. 1993, pp. 227-229.
`firsi
`[211 App] N°
`available on Apr. 2. 1993, at
`the IEEE, 12th Southern
`[22]
`Filed:
`Biomedical Conference at Tulane University, New Orleans.
`LA h ld A
`2-4 1993
`'
`[51]
`Int. c1.°
`A6113 sm ..,1,,’,,.,,.,‘’, 01391;, C;,,,,,,.,.',,,= pm Oximw, ,.,,,.,.1,,,..,,,.»
`[52] U.S. Cl.
`. 123.1633; 355341
`John E‘ scharf at al“ 1933 IEEE‘ _fun_ 1991 ml 230_23»2’
`
`[58] Field of Search ................................... .. 128.3633. 903;
`1-1151 31131151311; an AP; 2, 1993, 31 11-11; 1551; 1211-, 591,11-1em
`3553413 2503214 Au 7-14 Li 507950
`Biomedical Conference at Tulane University, New Orleans.
`_
`LA, held Apr. 2-4, 1993.
`References C999
`Light—To—Frequency Converter—TSL220, Texas Instru-
`U_s_ PATENT DOCUMENTS
`l'lIIEl'lIS ].l'lC.,
`Aug. 1990, REV. Jun.
`Programmable Ltght—To—Frequency Converter-—-TSL230,
`Texas Instruments Inc., SOESDOTA, Dec. 1992, Rev. Dec.
`1991
`CMOS——8—Bit Buffered Multiplying oAc—Ai:rr524,
`Digita.1—to—Analog Converters, iicv. A, pp. 2-399, 402-403.
`Burr-Bro\vri ACF2l01 Advertisement and Product Data
`Sheet (PDS-1079, Mar. 1991).
`“Integrator [C Converts Picoamperes to Volts." Frank Good-
`enough, Electronic Design, Jun. 13, 1991, pp. 132~134.
`
`l28f633
`
`Primary Examiner——Angela D. Sykes
`Assistant Emmr‘rier—Eric F. Winakur
`Attorney, Agent, or Fi'rm—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
`frgquency cgnvgrtcf
`as a 531-1§{}r_ A]5g prgvidgd is 3
`li ht to frequency Convener comprising a photoresistor and
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`ates a periodic electrical signal corresponding to the amount
`of electromagnetic radiation illuminating the photoresistor.
`
`_
`22 Claims, 6 Drawing Sheets
`
`2sor2i4L
`128803
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`1377505
`9207505
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`
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`52
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`0001
`
`Apple I
`APL10
`
`U.S. Patent No. 89239
`
`3»397’--774
`3.315.533
`
`1281903
`
`'
`
`431974 TCh3-¥1E-
`511974 Scheitlt
`uh
`4557:] 05 H,1982 Lem:
`4!4m_29U 10,1933 wflber_
`41441151]
`511934 Heinemam 1
`4,498,020
`911985 Gloima el: al. .
`4,586,5t3
`5.0986 Hamaguri .
`4,694,333
`9.F198'I Haniaguri.
`4,300,495
`H1939 smith _
`4307.630
`31939 Malinouskas .
`4397-531
`211939 Hefsh El 31- -
`4.824.242
`411989 Frick ct al.
`4,869,254
`9t19s9 Stone et al..
`4,883,353
`1131989 Haustnan et al..
`4.911357
`3119911 comm-am 3131 _
`4,934,302
`6.-‘I990 Coreriman et al. .
`5.073.136
`1.61992 Stone ct al.
`.
`5,111,317
`H1992 Clark 61 31.
`........................... .. 1231633
`5’113v351
`$1992 R9‘-9" -
`5149503
`9l1992 Kohno eta!
`5:l6'I:230 1211992 Chance ......'..:........................... izsnsss
`5 190 038 M993 P0150“
`'
`’
`'
`FOREIGN PATENT DOCUMENTS
`
`Apple Inc.
`APL1038
`U.S. Patent No. 8,923,941
`
`0001
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`U.S. Patent
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`Nov. 19, 1995
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`Sheet 2 of 6
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`

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`US. Patent
`
`Nov. 19, 1995
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`Sheet 4 of 6
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`5,575,284
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`V00
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`70\
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`I2UpF
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`0005
`
`0005
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`

`
`U.S. Patent
`
`Nov. 19,1995
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`Sheet 5 of 6
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`5,575,284
`
`Anulo
`Switc
`Bank
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`84
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`76
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`0006
`
`0006
`
`

`
`U.S. Patent
`
`Nov. 19, 1996
`
`Sheet 6 of 6
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`5,575,284
`
`109
`
`Iniriuiize
`System
`
`102
`
`
`
`three
`Collect
`quarters of
`data
`
`
`
`
`IIO
`
`
`
`Display SpO2
`or
`illuminate
`discrete LEDS
`
`FIG- 5
`
`0007
`
`104
`
`Coiiect Fourth
`quarter of
`new data
`
`I12
`
`
`
`106
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`FFT to determine:
`Red AC. Red DC.
`IR AC. and IR DC
`
`Discard oldest
`quarter 0F data
`
`I08
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`Cufcuiate R and
`then SpO2
`
`
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`0007
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`2
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`1
`PORTABLE PULSE OXEMETER
`
`FIELD OF THE INVENTION
`
`The present invention relates generally to medical diag-
`nostic instrurnents and, more specifically, to a portable pulse
`oximeter with a remote light-to-frequency converter as a
`sensor and a telemetry system to telctneter the calculated
`saturation value to a remote display.
`
`BACKGROUND OF THE INVENTION
`
`The degree of oxygen saturation of hemoglobin, Sp02. 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 percentage of the deoxyhemoglobin. Rllb,
`picks up oxygen so as to become oxyhernoglobin, HbO2.
`From the lungs, the blood passes through the arterial system
`until it reaches the capillaries at which point a portion of the
`HbO1 gives up its oxygen to support the life processes in
`adjacent cells.
`By medical definition, the oxygen saturation level is the
`percentage of Hb0-J over the total hemoglobin; therefore,
`Sp02=l-lhO2I(RHb+l-IbO2). 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 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 of the oxygen saturation value, “Pulse oximetry
`has been recommended as a standard of care for every
`general anesthetic." Kevin K. Tremp-er 8: Steven J. Barker,
`Pulse Oximerry, Anesthesiology, January 1989. at 98.
`An oximeter determines the saturation value by analyzing
`the change in color of the 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 the light traverses through the liquid,
`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 logarithrnic
`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 coetficient
`of deoxyhemoglobin, RI-lb, and that of oxyhcmoglobin.
`Hb02. The electromagnetic radiation absorption coeflicients
`of RHb and HbO2 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 RHh and Hb02 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 coelficient for RI-lb is
`quite different from the light absorption coeflicient of Hboz
`
`because Hb02 absorbs significantly more light in the visible
`spectrum than R1-lb.
`in practice of the pulse oximetty technique, the oxygen
`saturation of hemoglobin in intravascular blood is deter-
`mined by (1) 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 saturation 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 irorn 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
`means.
`
`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.
`
`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.
`
`5
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`20
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`Another important limitation is patient confinement to the
`pulse oximcter, 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
`couples the patient to the oximeter unit and the exclusive use
`of transmittance-type probes.
`Due to the non-linear nature of human physiology, engi-
`neers were foroed 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 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
`Sp02 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 diiferences over the skin surface of the
`same patient.
`Signal artifact has three major sources: (1) ambient light
`(which causes an ACJ'DC masking signal), (2) low perfusion
`
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`(in which the intensity of the desired ACJDC 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 ACIDC 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 reflectance- and 1:ransmittance—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 of the circuitry heretofore
`believed necessary to capture the signals and because such
`higher-powered circuitry shortens battery life. Typical digi-
`tal oxirneters use a silicon photodiode, a current-to-voltage
`Convener (a transimpedance amplifier), a preamplifier, filter
`stage, a sample and hold, and an analog-to~digital (AID)
`converter to capture the oximetry signal. These components
`make the creation of truly portable oxirneters diflicult
`because of the large footprint and high power requirements
`of each device. The AID converter, in pa.rticular, is typically
`large and power-hungry.
`SUMMARY OF THE l.'NVEN'I‘lON
`
`According to the present invention, an oximeter is pro-
`vided with a light-to-frequency 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 current-to-
`voltage converter, a preamplifier, a filter, a sample and hold,
`and an analog-to-digital (AID) converter found in typical
`digital oximeters, thereby significantly reducing the circuit
`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 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 (MCM) 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 ca.regiver'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
`techniques allow highly secure and noise-immune telemetry
`of saturation values in noisy clinical and healtheare envi-
`ronrncnts.
`
`The oxirneter of the present invention uses a pair of light
`emitting diodes, a light-to-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, a.ltema—
`
`20
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`30
`
`45
`
`S5
`
`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 refiected and transmitted by the blood. A
`photodiode in the light-to-fncqueney 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, 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 for1'n 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 art pulse oximeter;
`FIG. 2A is air electrical schematic representation of one
`embodiment of a 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 elecu-ical schematic representation of the
`implementation of the TSL230 light-to-frequency converter
`in the oximeter of the present invention;
`FIG. 4A is an elecuical schematic representation of an
`implementation of a light-to-frequency converter of the
`present invention;
`FIG. 4B 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 sanitation
`value.
`-
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`0009
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`0009
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`5,575,284
`
`5
`DETAILED DESCRIPTION OF THE
`PREFERRED E\dBOD1'MENT
`
`Before describing the details of the present invention, a
`description of a generic prior an pulse oximeter may be
`helpful in understanding the advantages of the pulse oxinic-
`ter of the present invention. Reference is bad, therefore. to
`FIG. I, which shows a generic prior art pulse oximeter 10.
`A typical prior a.rt oximetcr 10 has a photodiodc 12 for
`detecting an optical signal 14 reflected from or transmitted
`through a volume of 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 reflectcd
`back from the inlravascular 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 (transirnped-
`ancc] configuration.
`The signal is then filtered with a filter stage 18 to remove
`unwanted frequency components, such as arty 60 H2 noise
`generated by fluorescent lighting. The filtered signal is then
`amplified with an amplifier 20 and the amplified signal is
`sampled and hold by a sample and hold 21 while the signal
`is digitized with a high—rcsolution (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 coefiicicnt for the
`oxygen saturation value from the digitized signal and deter-
`mines the final sanitation value by reading the saturation
`value for the calculated coefiicient from a look-up table
`stored in memory. The final 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.
`
`Under the present invention. the prior art oxinieter 10 is
`modified so that
`the photodiode 12, current to voltage
`converter 16, filter 18, amplifier 21}, 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 50 of
`the present invention. The oxirncter 50 of the present inven-
`tion comprises a light-to-frequency converter (LPG) 52 for
`detecting an optical signal 54 from a volume of intravascular
`volume of blood 56 illuminated by one or more light
`emitting diodes (I_.EDsJ 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 produces a periodic electrical
`signal
`in the forth 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 l1igh~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-
`
`6
`ment. the data is converted into the frequency domain by, for
`example, performing the well-known Fast Fourier Trans-
`form (FFT) on the data. It is also believed that other common
`techniques of converting time—doma.irt data to the frequency
`domain will suflice: e.g., discrete cosine transform, wavelet
`transform, discrete Hartley 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
`In addition to performing saturations calculations.
`computer system 64 controls LED drivers 66, which control
`the LEDS 58, 60.
`FIG. 2B shows another embodiment of the pulse oximcter
`of the present invention. The embodiment of FIG. 2B differs
`front the embodiment in FIG. 2A in two respects. First, the
`computer system 64 and counter 62 are implemented by a
`microcorrtroller 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 ir1
`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 TSL220 in_ the
`oxitneter 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. 3B,
`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 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 photorcsistor '73, a
`capacitor 74, and an inverter '75 are placed in circuit com-
`munication and have the values shown in that figure. The
`photoresistor 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 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 known in 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
`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.
`
`30
`
`35
`
`45
`
`50
`
`55
`
`0010
`
`0010
`
`

`
`5 ,5'?5 ,284
`
`'7
`
`As shown in FIG. 4B, a resistor 79 with a substantially
`fixed resistance can he 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 pholoresistor 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 less 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. 413. 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 eontrolled 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 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~4D is believed to he par-
`ticularly useful in connection with the portable pulse oxirne-
`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, emitting light having a wavelength of approxi-
`mately 660 nm. One suitable LED is the P41?-ND, which is
`available from by Digikey, 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 IR LED 60 is an infrared LED. emitting electromag»
`netic radiation having a wavelength of approximately 940
`nm. One suitable LED is the FSFIQT-ND, which is also
`available from Digikey. It is believed that to be suitable, the
`ER 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 unafiected 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.0. Box
`655303, Dallas, ‘Tex. 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 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
`
`30
`
`35
`
`4t}
`
`45
`
`50
`
`8
`and subtracted from the value stored at the previous edge.
`Either way, the result is a counter value corresponding to the
`time dificnznce 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 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
`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 68HCl6
`microcontroller manufactured by Motorola, Inc., Austin Tex.
`78735, is one example. The 68HCl6 is suitable for systems
`requiring low-level digital signal processing and has on-
`board erasablelprograrnmable ROM (EPROM) and RAM. It
`also has an on-board 16-bit high-speed counter 82 elin1inat—
`ing the need for an external counter 62. The output from the
`LFC 52 may be directly connected to the counter input of the
`68HC16,' 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 point copro-
`cessor {not shown) can be added to the computer system 64.
`One suitable digital signal processor is the TMS32{lCXO
`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-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 I54.
`'
`Some prior an LED drivers have a nonnalizing function
`that increases or decreases the intensity of electromagnetic
`radiation generated by the LEDs in the system. It is desirable
`to be able use a 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 and hair of an infa