United States Patent [19J
`Athan et al.
`
`111111111111111111111111111111111111111111111111111111111111111111111111111
`US005575284A
`[111 Patent Number:
`[451 Date of Patent:
`
`5,575,284
`Nov. 19, 1996
`
`[54) PORTABLE P ULSE OXIMETER
`
`OTHER PUBLICATIONS
`
`[75)
`
`Inventors: Stephan P. Athan, Tampa; John E.
`Scha r f, Oldsmar, both of Fla.
`
`[73) Assignee: University of South Florida, Tampa,
`A a.
`
`[21) Appl. No.: 221,958
`
`(22) Filed:
`
`Apr. 1, 1994
`
`Int. Cl.6
`•••••••••••••••••••••••••••••••••••••••••.•••••.•••.•••• A61B 5/02
`[51]
`[52] U.S. Cl . ............................................... 128/633; 356/41
`[58) Field of Search ..................................... 128/633, 903;
`356/41; 2501214 A, 214 L; 607/60
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
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`211989 Malinouskas .
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`211989 Hersh et al ..
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`9/1989 Stone et al . .
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`4,883,353 11/1989 Hausman et al . .
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`3/1990 Corenman et al ..
`4,934,372 6/1990 Corenman et al ..
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`J/1992 Stone et al ..
`5/1992 Clark Cl al .............................. 1281633
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`511992 Rother .
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`5,149,503
`9/1992 Kohno et al ..
`5,167,230 1211992 Chance .................................... 128/633
`3/1993 Polson .
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`
`FOREIGN PATENT DOCUMENTS
`
`1377605
`9207505
`
`211988 U.S.S.R . ....... : .................... 2501214 L
`5/1992 WIPO .................................... 128/903
`
`"Optimization of Portable Pulse Oximetry Through Fourier
`Analysis," John E. Scharf ct al., IEEE, Jun. 1993, pp.
`233- 235, first available on Apr. 2, 1993, at the fEEE, 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 Thlane University, New Orleans,
`LA, held Apr. 2-4, 1993.
`.
`"Direct Digital Capture of Pulse Ox.imctry 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 Convertcr-TSL220, Texas Instru(cid:173)
`ments Inc., D3619, Aug. 1990, Rev. Jun. 1991.
`Programmable Light-To-Frequency Converter-TSL230,
`Texas Instruments Inc., SOES007A, 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 IC Converts Picoamperes to Volts," Frank Good·
`enough, Electronic Design, Jun. 13, 1991, pp. 132-134.
`
`Primary Examiner- Angela D. Sykes
`Assistant Examiner-Eric F. Winak.ur
`Attorney, Agent, or Firnz-Calfee, Halter & Griswold
`
`[57)
`
`ABSTRACT
`
`A diagnostic instrument for determi ning a cardiovascular
`system parameter. In one embodiment, the instrument takes
`the form of a portable pulse ox.imeter comprising a light to
`frequency converter (LFC) as a sensor. Also provided is a
`light to frequency converter comprising a photoresistor and
`capacitor in circuit communication with an
`inverting
`Schmitt trigger and configured such that the inverter gener(cid:173)
`ates a periodic electrical signal corresponding to the amount
`of electromagnetic radiation illuminating the photoresistor.
`
`22 Claims, 6 Drawing Sh eets
`
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`Apple Inc.
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`U.S. Patent No. 8,942,776
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`U.S. Patent
`U.S. Patent
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`Nov. 19, 1996
`Noy. 19, 1996
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`Sheet 1 of 6
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`Nov. 19, 1996
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`Nov. 19, 1996
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`U.S. Patent
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`Nov. 19, 1996
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`Sheet 4 of 6
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`5,575,284
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`U.S. Patent
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`Nov. 19, 1996
`
`Sheet 5 of 6
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`5,575,284
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`

`U.S. Patent
`
`Nov. 19, 1996
`
`Sheet 6 of 6
`
`5,575,284
`
`100
`
`In it i a I i ze
`System
`
`102
`
`104
`
`Collect three
`quarters of
`data
`
`Collect fourth
`quar_ter of
`new data
`
`106
`
`FFT to determine:
`Red AC. Red DC.
`IR AC. and IR DC
`
`108
`
`Calculate R and
`then Sp02
`
`110
`
`Display Sp02
`or illuminate
`discrete L[Os
`
`FIG. 5
`
`112
`
`Discard oldest
`quarter of data
`
`007
`
`

`

`1
`PORTABLE PULSE OXIMETER
`
`FIELD OF THE INVENTION
`
`The present invention relates generally to medical diag(cid:173)
`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.
`
`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
`patienL As blood is pulsed through the lungs by the heart
`action, a certain percentage of the deoxyhemoglobin, RHb,
`picks up oxygen so as to become oxyhemoglobin, Hb02 •
`From the lungs, the blood passes through the arterial system
`until it reaches the capillaries at which point a portion of the
`Hb02 gives up its oxygen to support the life processes in
`adjacent cells.
`By medical definition, the oxygen saturation level is the
`percentage of Hb02 over the total hemoglobin; therefore,
`Sp02=Hb0zi(RHb+Hb02) . 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. 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. 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 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
`satur!ltion in arterial blood utilize the relative difference
`between the electromagnetic radiation absorption coefficient
`of deoxyhemoglobin, RHb, and that of oxyhemoglobin,
`Hb02. The electromagnetic radiation absorption coefficients
`of RHb and Hb02 are characteristically tied to the wave(cid:173)
`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 Hb02 absorb electromag(cid:173)
`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 Hb02
`
`35
`
`40
`
`45
`
`5,575,284
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`30
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`2
`because Hb02 absorbs significantly 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-
`s mined by (l) alternatively illuminating a volume of intra(cid:173)
`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(cid:173)
`magnetic radiation intensity transmitted through or reflected
`10 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(cid:173)
`tion intensities at the selected wavelengths.
`15 Whereas apparatus is available for making accurate mea-
`surements on a sample of blood in a cuvette, it is not always
`possible or desirable to withdraw blood from a patient, and
`it obviously impracticable to do so when continuous moni(cid:173)
`toring is required, such as while the patient is in surgery.
`Therefore, much effort has been expanded in devising an
`20 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
`25 of technology, these units are still bound by several limita(cid:173)
`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(cid:173)
`Jinearitics and the heuristic methods used to arrive at the
`final saturation values.
`Another important limitation is patient confinement to 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(cid:173)
`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
`50 of transmittance-type probes.
`Due to the non-linear nature of human physiology, engi(cid:173)
`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
`ss empirical data. This is data that bas 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
`60 calibration curve programmed into the system memory,
`which in tum is only as accurate as the in vitro laboratory
`oximeter used to generate iL These inaccuracies are com(cid:173)
`pounded by differences in skin characteristics between
`patients, as well as differences over the skin surface of the
`65 same patient.
`Signal artifact has three major sources: (1) ambient light
`(which causes an AC/DC masking signal), (2) low perfusion
`
`008
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`

`

`5,575,284
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`3
`(in which the intensity of the desired AC/DC 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 largeAC/DC artifact masking the desired
`signal). When the oximetry signal is amplified, the noise
`components arc amplified along with the desired signal. This
`noise acts to corrupt the primary signal, during both pre(cid:173)
`processing as well as post-processing, thereby reducing the
`accuracy of the pulse oximeter reading. Signal artifact is
`prevalent with both reflectance- 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 15
`photodiode located at the end of the wire bundle in the
`probe. Probes employing transmittance-type method arc
`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 oximeters use a silicon photodiode, a current-to-voltage
`convener (a transirnpedance amplifier), a preamplifier, filter
`stage, a sample and hold, and an analog-to-digital (AID)
`convener to capture the oximetry signal. These components
`make the creation of truly portable oximeters difficult
`because of the large footprint and high power requirements
`of each device. The AID convener, in particular, is typically
`large and power-hungry.
`SUMMARY OF THE INVENTION
`
`4
`tively illuminate an intravascular blood sample with two
`wavelengths of electromagnetic radiation. The electromag(cid:173)
`netic radiation interacts with the blood and a residual optical
`signal is both reflected and transmitted by the blood. A
`5 photodiode in the light-to-frequency converter (LFC) col(cid:173)
`lects oximetry data from the intravascular blood sample
`iJluminated 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 nee(! 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
`20 well-known Fast Fourier Transform (FFT) on the time(cid:173)
`domain data. The frequency domain data is then processed
`to determine the saturation value.
`It is therefore an advantage of the present invention to
`25 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(cid:173)
`verter to r~uce the parts count of prior art systems.
`These and other advantages of the present invention shall
`30 become more apparent from a detailed description of the
`invention.
`
`According to the present inventipn, an oximeter is pro(cid:173)
`vided with a light-to-frequency converter as a sensor and a
`telemetry system to telemeter the calculated saturation value 35
`to a remote station. The light-to-frequency convener elimi(cid:173)
`nates the need for a separate photodiode, a current-to(cid:173)
`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 40
`footprint and power consumption. In short, the light-to(cid:173)
`frequency convener 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 45
`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 present invention is a truly 50
`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(cid:173)
`tion, this invention can conununicate 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 60
`of saturation values in noisy clinical and healthcare envi(cid:173)
`ronments.
`The oximeter 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. 65
`According to the present invention, two light emitting
`diodes (LEOs), a red LED and an infrared LED, altema-
`
`55
`
`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. 1 is an electrical schematic representation of a
`generic prior art pulse oximeter;
`FIG. 2A is an 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 convener
`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. 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 saruration
`value.
`
`009
`
`

`

`5
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`5,575,284
`
`6
`ment, the data is converted into the frequency domain by, for
`example, performing the well-known Fast Fourier Trans(cid:173)
`form (FFT) on the data. It is also believed that other common
`techniques of converting time-domain data to the frequency
`5 domain will suffice; 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
`10 a display 68.
`In addition to performing saturations calculations, the
`computer system 64 controls LED drivers 66, which control
`the LEDs 58, 60.
`FIG. 2B shows another embodiment of the pulse oximeter
`of the present invention. The embodiment of FIG. 28 diliers
`from the embodiment in FIG. 2A in two respects. 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
`20 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
`25 counter 62, the computer system 64, the display 68, the LED
`drivers 66, the LEOs 58, 60, and the other components are
`connected in electrical circuit communication as shown in
`.FIGS. 2A and 2B. One suitable LFC 52 is the TSL220,
`manufactured and sold by Texas Instruments, P.O. Box
`30 655303, Dallas, Tex. 75265. FIG. 3A is an electrical sche(cid:173)
`matic representation showing the use of the TSL220 in. the
`oximeter 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 TSL220, the TSL230 requires no
`external capacitor and provides microprocessor compatible
`control lines; therefore, the TSL230 is a one-chip sensor.
`Yet another suitable LFC 52 is a novel LFC circuit, which
`40 was invented by Stephan Peter Athan, one of the coinvcntors
`of this invention, and is shown in FIG. 4A. In that circuit, a
`pbotoresistor 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-
`45 sponding to the value of the variable resistance of the
`pbotoresistor. In one embodiment, a photorcsistor 73, a
`capacitor 74, and an inverter 75 are placed in circuit com(cid:173)
`munication and have the values shown in that figure. The
`photoresistor 73 is placed across the input node 76 and the
`50 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 photoresistor73 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
`e lectromagnetic radiation 78 being emitted onto the photo(cid:173)
`resistor. The photoresistor 73, capacitor 74, and inverter 75
`are configured such that the period of time in which the
`capacitor74 charges and discharges corresponds to the value
`of the variable resistance of the photoresistor 73. Thus, the
`65 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.
`
`15
`
`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(cid:173)
`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 volume of intravascular blood (not shown) illu(cid:173)
`minated by one or more light emitting diodes (LEOs, 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(cid:173)
`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(cid:173)
`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 fluorescent lighting. The filtered signal is then
`amplified with an awplifier 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(cid:173)
`mines the final saturation value by reading the saturation
`value for the calculated coefficient 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(cid:173)
`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 wilh a light-to(cid:173)
`frequency converter and a high speed counter.
`FIG. 2A shows one embodiment of a pulse oximeter 50 of
`the present invention. The oximeter 50 of the present inven(cid:173)
`tion comprises a light-to-frequency converter (LFC) 52 for
`detecting an optical signal 54 from a volume of intravascular
`volume of blood 56 illuminated by one or more light
`emitting diodes (LEOs) 58, 60. The LEOs 58, 60 emit
`electromagnetic radiation at a constant intensity; however,
`an optical signal 54 with a time-varying intensity is trans- 55
`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 form of a pulse train having a frequency 60
`corresponding to the intensity of the broadband optical
`signal received by the LFC 52. The periodic data then
`becomes an 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 cmbodi-
`
`35
`
`010
`
`

`

`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 photoresistor 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 rnicrocontroller 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 les~ resistance in series with the photore(cid:173)
`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. 4D, 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(cid:173)
`puter controlled bank of analog switches, as shown in that 20
`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 25
`76.
`While the LFC of FIGS. 4A-4D is believed to be par(cid:173)
`ticularly useful in connection with the portable pulse oxime-
`ter of the present invention, it is also believed to have utility
`beyond that of oximetry or other cardiovascular measure- 30
`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 P417-ND, which is
`available from by Digikey, 701 Brooks Avenue South, Thief 35
`River Falls, Minn. 5670I.lt 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(cid:173)
`eters (red), the closer to that wavelength, the more accurate 40
`the results (otherwise, calibration curves are required, as is
`known in the art).
`The IR LED 60 is an infrared LED, emitting electromag(cid:173)
`netic radiation having a wavelength of approximately 940 45
`nm. One suitable LED is the F5FIQT-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(cid:173)
`length such that the absorption of the emitted electromag(cid:173)
`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 55
`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) 60
`count the number of pulses of a free-running clock (corre(cid:173)
`sponding to the amount of time) between the individual
`pulses of the LFC 52. Either method will provide satisfac(cid:173)
`tory data. The latter method can be implemented in several
`ways. For example, the counter can be reset at each period 65
`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
`
`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(cid:173)
`figurations are possible. The counter 62 can either count
`5 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
`15 a CPU, random access memory (RAM), read-only memory
`(ROM), and associated control circuitry, such as decoders
`and multi-phase clocks,