US005396893A
`115
`United States Patent
`5,396,893
`[11] Patent Number:
`Mar, 14, 1995
`[45] Date of Patent:
`Oberget al.
`
`[54] METHOD AND APPARATUS FOR
`ANALYZING HEART AND RESPIRATORY
`FREQUENCIES
`PHOTOPLETHYSMOGRAPHICALLY
`
`[76]
`
`Inventors: Ake P. Oberg, Ugglebovagen 79,
`S-590 60 Ljungsbro; Lars-Goran
`Lindberg, Knektgatan 18, S-582 65
`Linképing, both of Sweden
`
`[21] Appl. No.:
`
`920,274
`
`[22]
`
`PCTFiled:
`
`Feb. 14, 1991
`
`[86] PCT No.:
`
`PCT/SE91/00106
`
`§ 371 Date:
`
`Aug, 3, 1992
`
`§ 102(e) Date:
`
`Aug. 3, 1992
`
`[87] PCT Pub. No.: W0O91/11956
`
`PCT Pub. Date: Aug. 22, 1991
`
`Foreign Application Priority Data
`[30]
`Feb. 16, 1990 [SE]
`Sweden .......eccssssccssessnesenee 9000564
`
`Tint, C16 oeeeecece eeeessetteseeeeeses A61B 5/0205
`SL]
`[52] U.S. C1, oeeeeecetesseesseeeeseees 128/671; 128/687;
`128/666
`[58] Field of Search ...............c00 128/633, 664-667,
`128/670-671, 668, 687-690
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`.
`
`1/1980 Carlson et al.
`4,183,360
`4,379,460 4/1983 Judell ........
`4,781,201 11/1988 Wrightet al......
`4,788,982 12/1988 Gedeon et al.
`.
`4,934,372 6/1990 Corenman etal. ............. 128/666 X
`5,078,136
`1/1992 Stone et ab...ce eeee 128/666 X
`
`.. 128/671
`« 128/671
`
`FOREIGN PATENT DOCUMENTS
`
`0109826 5/1984 European Pat. Off.
`319158 6/1989 European Pat. Off.
`0341327 11/1989 European Pat. Off.
`0349755
`1/1990 European Pat. Off.
`0335357 9/1990 European Pat. Off.
`3811689 6/1989 Germany .
`03211
`8/1985 WIPO .
`
`.
`.
`.
`.
`.
`
`Primary Examiner—Angela D. Sykes
`Attorney, Agent, or Firm—Sughrue, Mion, Zinn,
`Macpeak & Seas
`ABSTRACT
`[57]
`A pulse-frequency analyzing monitor which is provided
`with an optical sensor and whichis intended for measur-
`ing photoplethysmographically the blood circulation of
`a subject, such as a body part of a human being or an
`animal, includes a light source, preferably a light-emit-
`ting diode for infrared light, fibre optics, a detector unit
`for detecting the alternating voltage, or AC-component
`of a generated PPG-signal for the purpose of determin-
`ing the heart frequency of the subject, an electronic
`amplifying unit and a presentation unit, for instance an
`oscilloscope of a display unit. The apparatus also in-
`cludes meansfor separating from the detected PPG-sig-
`nal a signal component which indicates the respiration
`frequency of the subject. The PPG-signal is coupled to
`a filter, preferably a digital filter, which enables limit
`frequencies and the degree of amplification to be set.
`Thedigital filter may be a componentof a microproces-
`sor which is able to eliminate disturbances, for instance
`disturbances emanating from 50 Hzstray light, in addi-
`tion to separating signals concerning the heart and res-
`piration frequency of the subject. The invention also
`relates to a method of carrying out such photople-
`thysomographic measuring processes.
`
`11 Claims, 4 Drawing Sheets
`
`PULSE
`PART
`
`
`
`
` MEASURING
`PROBE
`
`
` HP-FILTER
`0, 1 Hz
`
` 20 Hz
`
`
`
`
`
` LP-FILTER
`
`LP-FILTER
`10 Hz
`
`OUTPUT
`SIGNAL
`
`OUTPUT
`SIGNAL
`
`0001
`
`Apple Inc.
`APL1062
`U.S. Patent No. 8,923,941
`
`Apple Inc.
`APL1062
`U.S. Patent No. 8,923,941
`
`0001
`
`

`

`U.S. Patent
`
`Mar. 14, 1995
`
`Sheet 1 of 4
`
`5,396,893
`
` LP-FILTER
`
`200 Hz
`
`MEASURING
`PROBE
`
`
`
`LP-FILTER
`20 Hz
`
`LP-FILTER
`10 Hz
`
`OUTPUT
`SIGNAL
`
`OUTPUT
`SIGNAL
`
`FIG. 6
`
`
`
`0002
`
`0002
`
`

`

`U.S. Patent
`
`Mar. 14, 1995
`
`Sheet 2 of 4
`
`5,396,893
`
`FIG. 2
`
`MG
`
`“WG.
`iSTaPZ
`KEE[iff
`SCZ
`
`0003
`
`
`
`

`

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`ZHOl
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`
`U.S. Patent
`
`Mar. 14, 1995
`
`Sheet 3 of 4
`
`5,396,893
`
`
`LAdLAO’<rnonota<rbuZbuGastion|cauOLofibeahizey1889!
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`19So
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`0004
`
`0004
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`
`

`

`U.S. Patent
`
`Mar, 14, 1995
`
`Sheet 4 of 4
`
`5,396,893
`
`
`
`0005
`
`0005
`
`

`

`1
`
`5,396,893
`
`METHOD AND APPARATUS FOR ANALYZING
`HEART AND RESPIRATORY FREQUENCIES
`PHOTOPLETHYSMOGRAPHICALLY
`
`TECHNICAL FIELD
`
`The present invention relates to monitoring apparatus
`provided with an optical sensor and functioning to ana-
`lyze pulse frequencies by measuring the blood circula-
`tion of a measuring object, such as a part of the human
`body or an animal body using photoplethysmography
`(PPG-measuring), said apparatus being of the kind set
`forth in the preamble of claim 1. The invention also
`relates to a method of taking such measurements.
`Photoplethysmography (hereinafter abbreviated to
`PPG)has been knownto the art for more than 50 years
`and is applied technically for measuring peripheral
`blood circulation. The method is primarily used for
`measuring heart frequencies and blood circulation when
`performing surgery. The method has many advantages,
`since, among other things, it is non-invasive and does
`not subject the patient to any appreciable trauma. Fur-
`thermore, the method requires no highly expensive or
`complicated equipment in order to be put into effect.
`When light from a suitable light source impinges on
`the skin, the light is dampened or attenuated according
`to the nature of the tissue on whichthe light impinges.
`This light attenuation is assumed to be constant. The
`light also passes through a numberof blood vessels and
`is also attenuated by the blood present. The light is
`attenuated as a result of a number of complex processes,
`such as absorption, reflection and different forms of
`scattering. The PPG-technique is based on the assump-
`tion that the more blood that is present in the volume
`investigated, the more the light is attenuated. This re-
`sults in two signal components of interest, namely a
`DC-component which correspondsto the total amount
`of blood in the investigated volume and an AC-compo-
`nent which signifies pulsation of the blood flow.
`In order to study the aforedescribed phenomenon,it
`is necessary to use a light source,a light detector, ampli-
`fying electronics and a display unit, for instance an
`oscilloscope or a printer.
`The light source and the detector may be placed on a
`respective side of the object on which blood circulation
`is to be measured, and the detector consequently mea-
`sures the light transmitted. This technique is, at times,
`referred to as transmission-photoplethysmography and
`can only be applied to a few skin surfaces, such as fin-
`gers, ear lobes andtoes.
`A more general method is to place both the light
`source and the detector in one and the same probe and
`measurethe light reflected. This technique is known as
`reflection * and is the dominating technique. It has long
`been understood that the pulsating component or AC-
`componentof the PPG-signal is contingent on changes
`in blood volume during each heart beat. The greater the
`volume of blood, the less light will impinge on the de-
`tector.
`
`It is obvious, however, that this is not the whole
`truth. Tests have been carried out in which blood has
`been allowed to pulsate in rigid glass tubes, where
`changesin volume are impossible and wheresolely the
`flow rate pulsates. A pulsating PPG-signal is also ob-
`tained in this case, which can be explained by the detec-
`tion of changes in orientation of the erythrocytes,
`which varies during each heart beat.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`65
`
`0006
`
`2
`In summary, there are at least two reasons for the
`AC-component, namely a change in blood volume and
`the orientation of the erythrocytes.
`A typical PPG-signal has, in the time plane, the form
`of a blood-pressure curve having the same periodicity
`as the heart beats. The signal also includes a numberof
`low frequencies.
`Thepresent invention is based on the realization that
`the lower frequencies occur as a result of changes in
`blood flow caused by the sympathetic nerve system and
`by respiration,
`this realization being based on the
`known fact that the intrathoracic pressure is lowered
`when breathing-in, or inspiring. This subpressure is
`utilized to “suck” the venous blood into the atrium and
`ventricle.
`The invention departs from this starting point and
`assumes that this subpressure causes variations in blood
`flow in venousplexus andalso that it should be possible
`to detect this variation with the aid of the PPG-tech-
`nique, particularly by using a technique which enables
`measurements to be made in the venous plexus.
`Nerve signals in the sympathetic nerve system also
`influence the blood flow. The smooth muscle around
`the vessels pulsates at a frequency which lies close to
`the respiration frequency. This pulsation is normally
`referred to as Traube-Hering’s waves, after the scien-
`tists’ Traube-Hering. Waves ofa still lower frequency
`are also found, these waves normally being referred to
`as Mayer’s waves.
`The blood also pulsates through the so-called arterio-
`venous anastomises, so as to control body temperature.
`This normally occurs at a frequency of about 0.3 Hz and
`is designated Burton waves.
`DISCLOSURE OF THE INVENTION
`
`The present invention is based on the aforesaid real-
`ization and on measurements, or assays, carried out with
`the aid of apparatus that has been constructed in accor-
`dance with the theory on which the present invention is
`based.
`The inventive monitoring apparatus intended for
`measuring blood circulation is of the kind set forth in
`the preamble of claim 1 and has the characterizing fea-
`tures set forth in the characterizing clause ofsaid claim.
`The exhaustive experimentation, which forms the
`basis of the invention and which is briefly described
`herebelow, has shown that respiration is the totally
`dominating low-frequency component of the PPG-sig-
`nal.
`It was found from the series of experiments per-
`formed that the extracted signal can be encountered
`substantially irrespective of where in the body the
`probe is placed. This leads to the conclusion that the
`extracted PPG-signal constitutes a measurement of var-
`iations in blood pressure caused by respiration, and
`therewith also variations in the flow of blood through
`the object under examination.
`It is expected that the inventive monitoring apparatus
`will find universal use within human care establish-
`ments.In the case of patients in intensive care wards or
`under anaesthetic or under postoperative conditions, it
`is important to monitor heart and respiration frequen-
`cies. When these two physiological variables are
`known, the doctor or nursing syster will have a good
`pictureofthe patient’s general condition. Monitoring of
`these variables under anaesthetic conditions can facili-
`tate the assessment of the depth of anaesthesia.
`
`0006
`
`

`

`5,396,893
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`4
`adapt to the prevailing heart and/or respiration fre-
`quency, so as to optimize the filter properties.
`Furthermore, the monitoring apparatus will prefera-
`bly be provided with means for DC-compensating the
`PPG-signal, so as to balance-out the low frequency
`components of said signal automatically, without expe-
`riencing harmful energy losses.
`In the case of one embodimentofthe invention which
`is particularly beneficial in practical application, the
`monitoring apparatus includes a probe which is in-
`tended to be placed on a suitable part of the body, for
`example a finger, and which includes meansfor deliver-
`ing light to said body part and meansfor capturing light
`which passes through said body part or which is re-
`flected therein, for the purpose of passing this light to
`the detector unit. This monitoring apparatus is charac-
`terized by optical fibres connected to the probe and
`functioning to conductlight from a light source to the
`skin and from the skin to the detector unit respectively.
`By conducting, in said apparatus, the light through
`optical fibres to and from the skin, there is obtained a
`system which is highly resistant to electromagnetic
`disturbances or interference, a feature which is ex-
`tremely important within the sphere of medical treat-
`ment. This particular feature enables the heart fre-
`quencyand respiration frequency to be recorded during
`surgery in which diathermyis applied. An apparatus of
`this kind which is insensitive to disturbances during
`surgery represents a very important step forwardsin the
`art.
`The inventive monitoring apparatus preferably in-
`cludes a microprocessor which is programmed to calcu-
`late the Fourier transform, and/or to separate signals
`concerning the heart and respiration frequencies of the
`object by digital filtration, and/or to eliminate disturb-
`ances, emanating, for example, from stray light of fre-
`quency 50 Hz.
`A further possibility afforded by the use of micro-
`processor technology is that of combining the measur-
`ing process with SaO2-measuring with pulsoxymetry.
`The present invention also relates to a method of
`carrying out photoplethysmographic measuring pro-
`cesses, this method being characterized mainly by the
`characteristic features set forth in claim 9.
`
`3
`Aboveall, the invention avoids those serious disad-
`vantages that are associated with respiratory frequency
`monitoring methods and apparatus hitherto used, all of
`which are normally unreliable, besides being both stren-
`uous and complex.
`It is generally more important to monitor respiration
`parametersin the case of infant care than in the case of
`adults under intensive care. In this regard, the inventive
`monitoring apparatusis superior to the technique which
`has been used most frequently hitherto, namely the use
`of impedance plethysmography with the aid of ECG-
`electrodes placed on the surface of the skin, among
`other things because such electrodes (normally three)
`take-up a relatively large area of the thorax. When
`requiring to make X-ray examinations,it is necessary to
`removethe electrodes, since they are not transparent to
`X-rays.
`Furthermore, light disturbances from peripheral elec-
`trical apparatus are induced in ECG-cables with associ-
`ated input amplifiers. The signal cables are coupled both
`inductively and capacitively. When carying out sur-
`gery, it is impossible to carry out ECG-recordings and
`to measure heart frequencies over prolonged periods of
`time, due to the surgical application of diathermy.
`ECG-electrodes and the paste used together there-
`with cause irritation of the skin, particularly when mon-
`itoring is effected over a prolonged period and particu-
`larly in the case of infants whose skin is very tender and
`sensitive. The electrodes and associated leads or cables
`also limit the ability of the child to move.
`The present invention provides important direct ad-
`vantages in relation to the aforesaid, and also affords
`indirect advantages with respect to methods of measur-
`ing respiration frequency.
`For example, the inventive monitoring apparatus can
`be applied and handled with ease; it avoids the aforesaid
`problems associated with prolonged use of skin elec-
`trodes; it is free from disturbances during surgery in
`which diathermyis applied; and affords a wide degree
`of freedom with regard to positioning of the sensor. For
`example, a sensor-provided probe can be placed on a
`finger or on a toe, at a distance from the thorax region
`where another investigation is being made. Further-
`more, the sensor element can be made very small, such
`as not to interfere with X-ray examinations to any ap-
`preciable extent.
`An additional, very important advantage afforded by
`the inventive monitoring apparatus is that it can be
`With the intention of confirming the aforedescribed
`integrated with a number of different medical instru-
`theory scientifically, namely the theory that it is also
`ments of the kind where heart frequency andrespiration
`possible to separate from a PPG-signal whose dominat-
`frequency are important parameters, for instance pul-
`ing component forms a measurement of the heart fre-
`soxymetry and defibrillators.
`quency of the object being examined, a signal compo-
`In order to enable measurements to be taken directly
`nent which discloses the respiration frequency of said
`on patients, it is necessary to equip the monitoring appa-
`object, a simple photoplethysmograph was constructed.
`35
`ratus with one or morefilters. This will result, however,
`Fourdifferent measuring probes were mounted on the
`in difficulty in selecting limit frequencies, since the
`photoplethysmograph,all of which probes usedalight-
`signal can exhibit pronounced variations.
`emitting diode as a light source. One probeutilized the
`Accordingly, one preferred embodiment of the in-
`wavelength 875 nm, two utilized the wavelength 940
`vention is characterized in that the apparatus includes
`nm and one utilized the wavelength 950 nm. All probes
`one or morefilters, preferably digital filters, which have
`measured reflected light. The photoplethysmograph
`means for setting limit frequencies and the degree of
`operated either within the frequency range of 0.2-10 Hz
`amplification. Filters of this kind are suitably incorpo-
`or the frequency range 0.2-20 Hz.
`rated in the apparatus, which will also preferably in-
`The photoplethysmograph was used to measure the
`clude meansfor electronically detecting the frequency
`blood circulation of dogs, cats, adult males aged 35
`content of the signal for selection and setting of limit
`years and infants in incubators. In order to show both
`frequencies.
`the respiration frequency and heart frequency in the
`According to another embodiment of the monitoring
`photoplethysmograph signal, the heart frequency and
`apparatus,the filters are adaptive and are constructed to
`respiration frequency of the adults and the infants were
`
`DISCLOSURE OF THE EXPERIMENTS
`PERFORMED
`
`60
`
`65
`
`0007
`
`0007
`
`

`

`5,396,893
`
`6
`beneath 20 Hz. The pulse frequency selected was 1 kHz,
`whichfulfils the sampling theorem more than well. The
`diode illuminating time was 40 us, which constitutes a
`fraction of the period time 1 ms. Since the illuminating
`time is so short in relation to the dark time,it is possible
`to use a very high diode current without destroying the
`light-emitting diode.
`For the purpose of obtaining a continuous measure-
`ment response, the measuring values are maintained
`constant between each new measuring process, with the
`aid of a sample-and-hold circuit. In order to guarantee
`that a measurement value is obtained when the diode
`emits light at full intensity, the sample-and-hold circuit
`is closed (and therewith holds the measurement value)
`before extinguishing the light-emitting diode.
`Thesignal from the sample-and-hold circuit is equal-
`ized by passing said signal through a low-passfilter.
`Theprinciple construction of the measuring electron-
`ics will be seen from FIG.1. References are made to the
`circuit diagram shown in FIG.4.
`
`15
`
`20
`
`5
`recorded separately with the aid of other methods. In
`the case of the animals used in this experiment, solely
`the respiration frequency was recorded separately.
`These measurements were used as reference signals in
`the measuring-data analysis. All measurements were
`recorded on a measurement tape-recorder.
`The measurement data was analyzed partly in the
`time plane, where the two components in the photople-
`thysmographsignal werefiltered out, and partly in the
`frequency plane, wherein the power spectrum was cal-
`culated?. A cross-correlation function for the photople-
`thysmograph signal and the reference signals was also
`calculated. The following conclusions can be drawn
`from these analyses:
`The apparatus functions well on adults. The heart
`frequency and respiration frequency can be separated
`by means offilter techniques. The heart frequency is the
`dominant signal component. The two components are
`clearly evident in the power spectrum and the cross-
`correlation function shows correlation with the refer-
`encesignals.
`In the case of infants in respirators, respiration is the
`totally dominant component.It is slightly moredifficult
`to filter-out the two components in the case of infants
`than in the case of adults. Although the two frequencies
`are evident in the power spectrum, respiration domi-
`nates the spectrum totally. The cross-correlation func-
`tion showscorrelation with the reference signals.
`The respiration frequency is the dominant signal
`component in the case of animals. Although a high
`frequency component can be filtered-out, it cannot be
`guaranteed that this componentis the heart frequency.
`Wavelength
`Probe
`Light Power
`The powerspectrum has a broad band with many peaks
`0.5 mW
`875 nm
`Philips CQY 58
`A
`or spikes whose origin cannot readily be established.
`
`B,C—_Telefunken TSUS 5400 950 nm 15 mw
`
`35
`The cross-correlation function was constructed solely
`(corresponds to
`for respiration, where correlation can be shown.
`Philips CQY 99)
`Hewlett-Packard HEMT
`BRIEF DESCRIPTION OF THE DRAWINGS
`1001
`
`The Measuring Probe
`Four different measuring probes, referenced A-D,
`were constructed during the experimental period. All of
`these measuring probes included a detector in the form
`of a light-emitting diode model S-4C from United De-
`tector Technology.
`Three different
`namely:
`
`light-emitting diodes were used,
`
`25
`
`30
`
`D
`
`940 nm
`
`2.55 mW
`
`A more detailed account of the experiments carried
`out is given below, with reference to the accompany
`drawings, in which
`FIG. 1 is a block schematic which illustrates the
`principle construction of the measuring electronics
`used;
`FIGS. 2 and 3 are schematic views of measuring
`probes used in the experimentseries;
`FIGS. 4 and 5 are circuit diagrams; and
`FIG.6 illustrates the principle of transmission mea-
`suring with the aid of fibre optics and with the aid of a
`probefitted to one finger of the object.
`DETAILED DESCRIPTION OF THE
`DRAWINGS
`
`Infrared light-emitting diodes were used as the light
`source. Light-emitting diodes are small, mechanically
`insensitive and have a light intensity which is propor-
`tional to the diode current. The AC-component of the
`PPG-signal is weak and must be greatly amplified. The
`light intensity should be high, in order to obtain a high
`signal level. This is achieved by pulsating the light-emit-
`ting diode with a high current. This enables a much
`higher diode current to be used than in the case of con-
`tinuous light. The use of pulsated light results in time-
`discrete measurement of a time-continuoussignal.
`According to the sampling theorem, it is necessary
`for the pulse frequency to be twice as large as the fre-
`quency content of the signal, in order to recreate the
`continuous signal. This frequency content should be
`
`45
`
`50
`
`35
`
`60
`
`65
`
`It should be observedin this respect that the light was
`comparitively broad-bandlight and that the wavelength
`values refer to maximum intensity. The power value
`denotes the powerradiated totally in a hemisphere and
`is estimated from existing data sheets in the case of
`HEMT1001. It should also be mentioned that the light-
`emitting diodes spread light differently.
`FIGS.2 and 2a illustrate a measuring probe compris-
`ing an acrylic tube 4 in which a light-emitting diode 1
`and a detectorin the form ofa light-emitting diode 2 are
`embodied or cast with epoxy resin 5. The tube 4 has a
`diameter of 8 mm and is surrounded by a tube 3 of
`diameter 12 mm.
`The probeillustrated in FIG.3 differs from the probe
`illustrated in FIG. 2, in that the detector 2 is angled in
`relation to the detector surface.
`The following light-emitting diodes were used with
`the different probes:
`Probe A uses light-emitting diode CQy 58
`Probes B and C uses light-emitting diode HEMT
`1001
`Probe D useslight-emitting diode TSUS 5400
`Probes A and D were constructed in the same wayas
`the probe B shown in FIG.2.
`The cables used between probes and electronic de-
`vices were very thin and flexible, screened four-con-
`ductor cables sold by Telko underthe trade name Pick-
`uptrad PU 402.
`
`0008
`
`0008
`
`

`

`7
`FIGS.4 and 5 are circuit diagrams of embodiments of
`the pulse electronics circuity employed in the present
`invention. Exemplary values for the components used
`in these embodiments are set forth in Tables 1 and 2 as
`follows:
`
`TABLE1
`
`5,396,893
`
`8
`
`Pulse Electronics
`
`component
`Ici
`IC2
`1c3
`1C4
`ICs
`1c6
`Ic7 .
`1C8
`1c9
`IC10
`cll
`
`Tl
`
`PHO
`
`LED
`
`value or type
`401i
`4050
`4538
`TLO71
`NES37
`TLO72
`TLO71
`TLO72
`TLO72
`TLO71
`AD580
`
`BD667A
`
`UDT S4C
`
`probe A:CQY 58
`probe B:HEMT 1001
`probe C:HEMT 1001
`probe D:CQY 99
`
`value or type
`component
`10M
`RI
`15k
`R2
`3.9k
`R3
`3.3k
`R4
`22k
`RS
`180
`R6
`0-100k
`R7
`ik
`R8
`27k
`R9
`2.2k
`R10
`39k
`Rll
`39k
`R12
`2.2k
`R13
`39k
`R14
`39k
`Ri5
`1002
`R16
`1.8M.
`R17
`3.9M
`R18
`15M
`R19
`480k
`R20
`150k
`R21
`4.7k
`R22
`0-100k
`R23
`75k
`R24
`820
`R25
`2.7k
`R26
`480k
`R27
`150k
`R28
`4.7k
`R29
`0-50k
`R30
`92k
`R31
`33k
`R32
`680,
`R33
`2.2k
`R34
`Ik
`R35
`0-500k
`R36
`1k
`R37
`10k
`R38
`8.2k
`R39
`8.2k
`R40
`180
`R41
`560k
`R42
`560k
`R43
`560k
`R44
`1M
`R45
`1M
`R46
`1M
`R47
`0-100k
`R48
`
`R49 L5k
`
`ci
`C2
`C3
`4
`C5
`C6
`C7
`cs
`c9
`C10
`Cll
`C12
`C13
`Cl4
`Cis
`Ci6
`Ci7
`C18
`c19
`C20
`C21
`C22
`C23
`C24
`C25
`
`22 nF
`10 nF
`10 nF
`0-40 pF
`33 nF
`Sn
`0.22 pF
`6.8 nF
`10 pF
`0.22 pF
`0.22 pF
`47 nF
`47 nF
`47 nF
`22 nF
`22 nF
`22 nF
`22 oF
`i nF
`0.1 pF
`22 nF
`1laF
`0.1 pF
`15 nF
`1.0 pF
`
`The digital IC-circuits are supplied with a voltage
`+Vcc of +5 V whereas the OP-amplifiers are supplied
`with + Vcc and a voltage — Vcc of —5 V. Also, each of
`the digital circuits are provided with a switch-offcapac-
`itor of 0.1 uF between °Vcc and ground. The OP ampli-
`fiers have similar switch-off capacitors between +-Vcc
`and ground as well as between — Vccand ground.
`TABLE 2
`
`component
`Trafo
`
`component
`
`value or type
`
`value or type
`Tradania
`type TD3701
`50 mA
`$2
`50 mA
`$1
`PBDF104
`Ic2
`PBDF104
`ICI
`0.1 uF
`C2
`1000 pF
`cl
`0.33 pF
`C4
`1000 pF
`C3
`0.1 pF
`C6
`1000 pF
`cS
`0.33 pF
`cs
`1000 uF
`C7
`79L05
`REG2
`7805
`REG1
`
`
`
`REG3 79L05 78L05 REG4
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`The pulse electronics used had two functions, firstly
`to drive the light-emitting diodes and secondly to gen-
`erate and deliver control signals to the sample-and-hold
`circuit. The fundamental component ofthe pulse elec-
`tronics is a bistable flip-flop which generates a square
`wave having a frequency of 1 kHz. This flip-flop is
`constructed around two Nand-gates (IC 1A andB in the
`circuit diagram of FIG. 4), and a buffercircuit (IC 2).
`The clock frequency is proportional to the product R2
`and C1. The capacitor C24 is required, to lead away
`disturbing high frequencies.
`The control pulses are generated by a monostable
`flip-flop (C3), which is triggered on positive flanks by
`flanks of the square wave.
`Those times at which the monostable flip-flop is
`“high” is determined by the product R3 and C2 for the
`light-emitting diode, and by the product R4 and C3 for
`the sample-and-hold circuit.
`Current
`is supplied to the light-emitting diode
`through a transistor stage built around a Darlington
`transistor (T1 in FIG. 4). A Darlington transistor is
`actually two transistors connected in series and has the
`positive property of having a very high current-ampli-
`fying factor.
`The voltage drop across collector and emitter
`reached about 1.4 V.
`The light-emitting diode control pulse was connected
`to the base of the transistor via a resistor (R5), which
`was dimensioned so that the transistor would bottom at
`high pulse values and throttle at low pulse values.
`The resistor R41 was coupled in parallel with the
`collector resistor R6 by means of a switch on the front
`panel of the apparatus used, such as to obtain a high
`collector current and a high light intensity. The collec-
`tor current was 130 mA in position “low” and 180 mA
`in position “high”.
`
`The Current-Voltage Converter
`The light-emitting diode in the measuring probe was
`biased electrically with +2.5 V in the reverse direction.
`This voltage was produced by IC11, whichis a preci-
`sion regulator and which held the voltage stable. A
`linear detector response was guaranteed in this way.
`The light-emitting diode now delivered a reverse cur-
`rent which was proportional to the detected light inten-
`sity. This current was converted to a voltage, by a cur-
`rent-voltage converter constructed around an opera-
`tional amplifier (IC4).
`It shall be noted that the current-voltage converter
`was an inverting circuit.
`
`35
`
`65
`
`The Sample-and-Hold Circuit (C5)
`The circuit functioned to hold the time-discrete mea-
`surement values constant between each new measuring
`process. Sample-and-hold circuits, however, are en-
`cumbered with the disadvantage that disturbances in
`the form of spikes from the control logic leak through
`to the measurement value. This is particularly pro-
`nounced in respect of the weak AC-component. In
`order to reduce this disturbance, the amplitude of the
`control signal was scaled down to about 2 V over the
`resistors R39 and R40. An external holding capacitor
`C15 on 1.0 82 F was connected to the circuit. This
`capacitor also assisted in damping the disturbance
`spikes.
`
`0009
`
`0009
`
`

`

`9
`
`Low-Pass Filter 200 Hz
`
`5,396,893
`
`10
`transformer which had two secondary windings, each
`producing a secondary voltage of 12 V. This alternating
`voltage wasrectified to +5 V with the aid ofrectifying
`bridges, smoothing and disturbance-eliminating capaci-
`tors and integrated regulators. Particular mention can
`be madeto the fact that the light-emitting diode of the
`probe was powered by a current of between 100 and
`200 mA. Consequently, there was used a more powerful
`regulator capable of delivering more current to the
`digital +5 V-side (cf the circuit diagram shownin FIG.
`5).
`
`The Light Intensity of the Probes
`The following measuring process was carried out in
`order to obtain an estimation of the mutual intensity
`relationship between the various probes.
`The probes were connected and a photometer was
`held directly against the probes. The luminousintensity
`of the probes was observed from the photometer. It
`shall be noted that this is an integrated measurement
`value and not the intensity when the diodesare illumi-
`nated. The measurement values are shown in the fol-
`lowing table.
`
`Probe
`A
`B
`Cc
`D
`
`“Low”
`25.1 pW
`15.2 pW
`18.4 pW
`32.1 pW
`
`Intensity
`
`“High”
`-
`19.0 pW
`22.7 pW
`38.7 pW
`
`It was found that probe D could not be used to take
`measurements in practice, since the system became
`self-oscillating. This problem was not rectified. Probes
`B and C were found to provide a better result than
`probe A, probably because the light emitted had a
`longer wavelength (875 nm for probe A, 950 nm for
`probes B and C and 940 nm for probe D).
`Measurements and Results
`
`The auto-measuring process was effected by record-
`ing the PPG-signal on tape. The respiration frequency
`was also taped at the same time. When measuring blood
`circulation on human beings, the heart frequency was
`also measured, but with other methods. These signals
`were used as reference signals for the two components
`of the PPG-signal.
`Measurements were taken on three different groups
`of objects, namely animals, infants in incubators, and
`adult males aged 35-years.
`A narrow selection of the measurements taken are
`presented below. This selection is neither a random
`selection or a particularly representative selection.It is
`rather an example of those measurements which were
`considered to be of interest in evaluating the technique.
`An attempt to form a conclusion from these measuring
`processes is made below.
`
`Measuring Equipment
`In addition to the aforedescribed PPG-equipment,
`the respiration frequency of animals and adult human
`beings was measured with the aid of Strain Gauge
`equipment, which had the form of a strain sensor con-
`sisting of a thin rubber hose filled with mercury and
`connected to a measuring bridge. The heart frequency
`of the adult human beings was measured with the aid of
`laser-doppler equipment. When laser-doppler equip-
`
`The purpose ofthe first low-pass filter was to elimi-
`nate the disturbance spikes deriving from the sample-
`and-hold circuit. The filter was an active Tjebychev
`filter of the fourth order, followed by a passive RC-link.
`Thefilter had been designed to permit 0.5 dB ripple in
`the pass band andto have a cut-off frequency of 200 Hz.
`The filter was built-up around two cascade-coupled
`operational amplifiers (IC 6A and B) and the RC-link
`R16 and C9.
`
`The High-Pass Filter 0.1 Hz
`The high-pass filter eliminated the DC-component,
`thereby enabling amplification of the AC-component.
`Thefilter was an active Tjebychevfilter of the second
`order, constructed around an operational amplifier
`(IC7). Thefilter was designed to permit 0.5 dB ripple in
`the pass band andto havea cut-off frequency of 0.1 Hz.
`
`Amplification 1-500 Times
`The amplifier was a non-inverting amplifier con-
`structed around a offset-compensated operational am-
`plifier (C10). The amplification was varied with the aid
`of a potentiometer positioned on the front panel.
`Notch Filter 50 and 100 Hz
`
`The weak AC-component was greatly disturbed by
`the net frequency 50 Hz, and also by disturbances from
`lamps and fluorescent tubes at 100 Hz. These disturb-
`ances were eliminated in two cascade-coupled notch
`filters. The notch filters were constructed around two
`operational amplifiers (IC 8A for 50 Hz and IC 8B for
`100 Hz). Thefilters could be adjusted in the frequency
`direction?, with the aid of potentiometers R29 for the 50
`Hz-filter and R30 for the 100 Hz-filter, s