oy 4,015,595
`United States Patent,
`[45]
`Apr. 5, 1977
`Benjamin, Jr.
`
`
`,
`
`3,698,382 10/1972 Howell ..........cee 128/2.05 V X
`3,787,119
`1/1974 Rybak wo. cceeneeteeeee 356/39 X
`——3'910,701_
`10/1975 Henderson et al. 356/39
`
`Kleinerman...........
`... 356/39 X
`3,916,205 10/1975
` SHUCK .....eessssseccssseecceseensenes 356/39
`3,923,397 12/1975
`.
`.
`Primary Examiner—Kyle L. Howell
`Attorney, Agent, or Firm—Smith, Harding, Earley &
`_—Follmer
`
`[54] PHOTOPLETHYSMOGRAPHS
`.
`-
`[76]
`Inventor:
`J. Malvern Benjamin,Jr., c/o Bionic
`Instruments, Inc., 221 Rock Hill
`Road, Bala Cynwyd, Pa. 19004
`Sept. 15, 1975
`[22] Filed:
`[21] Appl. No.: 613,301
`[52] US. Ch. wceccecseseeeee 128/2.05 V; 128/2.05 P
`FSV] Unt, Ch? ee eceseseseneeeensenees A61B 5/02
`ABSTRACT
`|
`[57]
`[58] Field of Search ..........0...... 128/2.05 V, 2.05 P,
`128/2.05 T, 2 L, 2 A; 356/39 A photoplethysmograph probeincludingalight source,
`
`References Cited
`[56]
`a photo-sensitive cell and a light controlfilm positioned
`in front of the light source and photo-sensitive cell for
`UNITED STATES PATENTS
`collimating the light emittedfrom the light source and
`Smith woe 128/2.05 P
`9/1963
`reflected back to the photo-sensitive cell.
`1/1965
`Richter ........
`vere 128/2.05 P X
`
`12/1967
`Sherman ...........cccccsecssceees 128/2 L
`Shaw oes eeeeseseceseesesceseeseeers 128/2 L
`2/1972
`
`3,103,214
`3,167,658
`3,359,975
`3,638,640
`
`7 Claims, 3 Drawing Figures
`
`APPLE 1007
`
` 1
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`1
`
`APPLE 1007
`
`

`

`4,015,595
`
`U.S. Patent
`
`FIG. 2.
`
`April 5, 1977
`
`
`
`
`
`2
`
`

`

`PHOTOPLETHYSMOGRAPHS
`
`BACKGROUNDOF THE INVENTION
`
`A photoplethysmographis a device usedfor the mea-
`surementofa peripheral pulsatile blood flow. It senses
`blood flow by meansof a-probe placed on the surface
`of the skin of any part of the body. The probe contains
`a tiny light source and a specially selected photo-sensi-
`tive cell that responds to light absorbed by the arterial
`blood in the peripheral vascular bed over which the
`sensor is placed. Since the cell responds to the light
`absorbed bythe blood in its view, the amountofpulsat-
`ing light it registers is proportional to the amount of
`pulsating arterial blood in its field.
`In practice, however, the amplitude of electrical sig-
`nal produced by the photo-sensitive cell not only varies
`with the amountof pulsation of blood, but also varies
`with the pressure of application of the probeto the skin
`surface. As the pressure is gradually increased, at first
`the amplitude of the signal gradually rises. It reaches a
`maximum, and then, with further increases in pressure,
`once again diminishes until it finally drops off to zero.
`The drop-off of signal beyond the maximum point as
`the pressure increasesis easily explained in terms of the
`high pressure’s squeezing the bload vessels closed and
`thusactually cutting off the blood flow. The increase in
`amplitudeatfirst with increasing pressure is caused by
`changes in the amount of steady state static light re-
`turned to the photocell. It is the average amount of
`light returned to the photocell that establishes the aver-
`age resistanceofthe cell (the operating point) and thus
`the average gain or sensitivity of the cell as regards the
`pulsatile signal which is being measured.
`Oneof the problems involved in the photoplethysmo-
`graphic pickupof the bloodflow pulseis that variations
`in the amountofscattered light reaching the photocell
`cause variations in the operating pointof the photocell.
`This adversely affects the accuracy of the measure-
`ment.
`Another problem associated with the photoplethys-
`mographic pickupof the blood flow pulse is artifactual
`noise produced by motion of the photo-sensitive cell
`and light source with respect to the pickupsite. If the
`site is illuminated by broad spectrum “‘white”’ light,all
`ofthis light will be reflected from the structures within
`the tissue and as these structures move with respect to
`the pickup, noise will be generated which adversely
`affects the accuracy of the measurement.
`SUMMARYOF THE INVENTION
`
`It is the general object of this invention to provide
`improvements in photoplethysmographs and, more
`particularly, to improve the accuracy of the photople-
`thysmographic pickup of the blood flow pulse.
`In accordance with one feature of the invention |
`meansare provided for holding the operating point of
`the photocell constant by overcoming the adverse af-
`fects of the scattered light reaching the photocell. To
`this end, the amount of scattered light reaching the
`photocell can be made closer to constant by placingin
`front of the photocell and light source a small piece of
`light controlfilm. This film has the effect of collimating
`the light thereby to make the sensor more nearly de-
`pendent only upon the light beam directly reflected
`from the pulsating bloodfield.
`In accordance with another feature of the invention,
`the artifactual noise produced by motion of the photo-
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`cell and light source with respect to the pickupsite is
`reducedbythe provision of the photoplethysmographic
`probein which thelight is emitted in a narrow band of
`wave lengths centering in the green. By this construc-
`tion only green light will be absorbed by the oxygen-
`ated arterial blood andit is this arterial blood pulsing
`through the vessels which produces thesignals that are
`desired. Specifically, oxygenated hemoglobin reflects
`light maximally at 640 nanometers (red) and therefore
`absorbs maximally at the complimentary color, green.
`Thus, if light in and around the. green wave lengths is
`transmitted into the structure, the motion artifact will
`be minimized.
`
`BRIEF DESCRIPTION OF THE DRAWING
`
`FIG.1 is a sectional view of a photoplethysmographic
`probe in accordance with the invention;
`FIG. 2 is a plan view of the probe shown in FIG. 1;
`and
`FIG. 3 is a schematic illustration of the electrical
`circuitry of a photoplethysmograph incorporating the
`probe shown in FIGS. 1 and 2.
`DETAILED DESCRIPTION OF THE PREFERRED
`EMBODIMENTSOF THE INVENTION
`
`The photoplethysmographic probe in accordance
`with the invention is indicated generally at 10 and com-
`prises a casing 12 having a generally rectangular bot-
`tom side 14 provided with a window 16. A light source
`18 is mounted within the casing 12 to overlie the win-
`dow 16 so thatit directs light through the window 16
`toward a field to be measured. A photo-sensitive cell 20
`is mounted within the casing 12 adjacent the light
`source 18 so that it respondsto light reflected from the
`field to be measured and passing through the window
`16. Thecell 20 is preferably a cadmium selenide photo-
`cell. In the form of the invention shown in FIGS. 1 and
`2, the light source 18 is a miniature incandescent lamp.
`A flexible multi-conductor cable 24 extends from the
`casing 12 and contains various conductorsfor the elec-
`trical circuitry to be described hereafter.
`In accordancewith a feature ofthe invention,a light
`contro! film 22 is mounted within the casing 12 to
`-extend across the window 16. Accordingly, the light
`emitted from the light source 18 passes through the
`light control film 22 to the field to be measured and
`light is reflected from this field back through thelight
`control film 22 to the photo-sensitive cell 20. The light
`control film 22 is made of a 0.030 inch thick clear
`cellulose acetate butyrate film. Light control film of
`this type is known in the art and is commercially avail-
`able, such as from the EdmundScientific Company.
`The light control film 22 has the effect of collimating
`the light passing therethrough to thereby make the
`photo-sensitive cell 20 more nearly dependent only
`upon the light beam directly reflected from the field
`being measured. Thus, the amount of scattered light
`reaching the photo-sensitive cell is made closer to con-
`stant. This serves to hold the operating point of the
`photo-sensitive cell more constant whichiimproves the
`accuracy of the measurement.
`In accordance with another feature of the invention,
`the light source 18 is constructed to emit light in a
`narrow band of green wave lengths. This may be
`achieved by the use of an appropriate light emitting
`diode or a miniature incandescent lamp with an appro-
`priate filter. By this construction, the artifactual noise
`produced by motion of the photo-sensitive cell 20 and
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`a casing having a windowtherein,
`light source 18 with respect to the pickupsite is re-
`a light source and a photo-sensitive cell mounted in
`duced. This design avoids the problems resulting from
`said casing to overlie said window,
`illuminating the pickup site by “white” light in which
`said light source being arranged to direct
`case all of this light will be reflected from the structures
`through said window,
`within the tissue causing noise generation as these
`said photo-sensitive cell being arranged adjacent said
`Structures move with respect to the pickup. At the ~
`light source to respondtolight passing through said
`sametime, only the sourcelight will be reflected by the
`windowasit is reflected fromafield in line with
`oxygenated arterial blood and it is this arterial blood
`said light source, and
`pulsing through the vessels which produces the signal
`a light control film mounted to extend across said
`that is desired in a photoplethysmographic pickup.
`windowforcollimating the light emitted from said
`In FIG.3, there is provided a schematicillustration of
`light source to direct the same toward said field and
`the electrical circuitry of a photoplethysmograph in-
`for collimating the light reflected to said photo-sen-
`corporating a probe in accordance with the invention.
`sitive cell and passing through said window.
`There is shown a probe 10 provided with a light source
`2. A probe according to claim 1 wherein said light
`18 and a photo-sensitive cell 20, The light source 18
`control film is made of a clear cellulose acetate buty-
`may be an incandescent lampor it may be designed to
`rate film.
`emit light
`in a narrow band of green wave lengths.
`3. A probe according to claim 2 wherein said light
`There is also provided a suitable power source 30, an
`source comprises an incandescent lamp.
`amplifier 32 and a chart recorder 34. The electrical
`4. A probe according to claim 1 wherein said light
`output from the photo-sensitive cell 20 is delivered to
`source comprises an incandescent lamp.
`the amplifier 32 which delivers an amplified signal to
`5. A probe according to claim 1 wherein said light
`operate the chart recorder 34.
`source is constructed to emit light in a narrow band of
`In the use of the probe in accordance with the inven-
`green wavelengths.
`tion for the measurementof peripheral pulsatile blood
`6. A probe according to claim 2 wherein said light
`flow, the sensing probe 10 is applied to the skin of the
`source is constructed to emit light in a narrow band of
`patient abovethefield to be measured. Thelight source
`green wavelengths.
`18 emits light and the photo-sensitive cell 20 responds
`7. A probe for use with a photoplethysmograph for
`to the light absorbed bythearterial blood in the periph-
`the measurementofperipheral pulsatile blood flow by
`eral vascular bed over which the probe 10 is placed.
`application to the surface of the patient’s skin compris-
`Since the photo-sensitive cell 20 responds linearly to
`ing:
`the light reflected from the blood in its view,
`the
`a casing having a window therein,
`amountof pulsating light it registers is directly propor-
`a light source and a photo-sensitive cell mounted in
`tional to the amountof pulsating arterial blood. The
`said casing to overlie said window,
`photo-sensitive cell 20 delivers its output to the ampli-
`said light source being arranged to direct
`fier 32 which amplifies the signal and transmitsit to the
`through said windowto a field of blood,
`chart recorder 34 which records the variations in the
`said photo-sensitive cell being atranged adjacentsaid
`pulse amplitudes toprovide a written record which can
`light source to respond to light reflected from a
`be analyzed to determine the blood flow through the
`field of blood in line with said light source, and
`said light source being constructed to emit light in a
`field being measured.
`I claim:
`narrow band of green wave lengthsso that substan-
`tially all of said light is absorbed: by the blood so as
`1. A probe for use with a photoplethysmograph for
`to minimize the motion artifact.
`the measurement of peripheral pulsatile blood flow
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