4,955,379
`[11] Patent Number:
`United States Patent 115)
`Hall
`[45] Date of Patent:
`Sep. 11, 1990
`
`
`[54] MOTION ARTEFACT REJECTION SYSTEM
`FOR PULSE OXIMETERS
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`(75]
`
`.
`‘
`Inventor: eae Hall, Dyfed, United
`
`som
`[73] Assignee: National Research Development
`Corporation,
`London, E
`d
`ration,
`London,
`agian
`[21] Appl. No.: 229,692
`[22] Filed:
`Aug. 8, 1988
`
`Foreign Application Priority Data
`[30]
`Aug. 14, 1987 [GB] United Kingdom .............. 8719333
`
`Int, CLS veeccsssssesecssssssssssueesssssnessseneeeeee AGIB 5/00
`[51]
`[52] US. CL. ooeeeesestesesteseessesseeeseeee 128/633; 128/664;
`128/666; 128/687
`[58] Field of Search ............... 128/633, 634, 664, 665,
`128/666, 687; 356/39, 41
`
`4,109,643
`4,167,331
`
`8/1978 Bond et all. csscsssssseeessee 128/666
`swe 356/41
`9/1979 Nielsen .....
`
`
`4/1981 Lewyn ...
`4,260,951
`. 128/690
`4,353,152 10/1982 O’Connoret al.
`.. 128/666
`. ieee
`peonee os Lepper sss.
`
`;
`651,
`987 Passafaro
`4,723,554 2/1988 Oman crssssssccesessssseqeeeseeeeeeee 128/633
`Primary Examiner—Kyle L. Howell
`Assistant Examiner—John C. Hanley
`Attorney, Agent, or Firm—Howard F. Mandelbaum
`[57]
`ABSTRACT
`A pulse oximeter apparatus characterized in that it com-
`prises a bandpass filter adapted selectively to exclude
`motion artefact from wanted signal is disclosed.
`Also disclosed is the use of such an apparatus for the
`determination of pulse rate and/or arterial blood oxy-
`gen saturation.
`
`6 Claims, 4 Drawing Sheets
`
`
`
`3
`red AC
`
`
`123 tap
`
`129tap
`signal in
`bandpass filter
`lowpace {Her
`O79-bHz
`55 He
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`ir AC
`signalin
`
`red AC
`
`crossing
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`signal oul Tero
`counter
`
`
`0001
`
`Apple Inc.
`APL1048
`U.S. Patent No. 8,923,941
`
`Apple Inc.
`APL1048
`U.S. Patent No. 8,923,941
`
`0001
`
`

`

`
`
`US. Patent—Sep. 11, 1990 Sheet 1 of 4 4,955,379
`
`
`
`Pulse rate
`
`Frequency (Hz)
`
`(800nm channel)
`
`Fig.1
`
`Pulse rate
`
`17
`
`0
`
`Frequency (Hz)
`(1300 nm channel)
`
`Fig.2
`
`0002
`
`0002
`
`

`

`
`
`US. Patent ‘Sheet20f4=—--4,955,379sep. 11,1999
`
`
`
`
`
`after
`
`filtering
`
`before
`
`filtering
`
`Pulse rate
`
`75
`
`5
`
`25.
`
`47,
`
`0
`
`Frequency (Hz)
`
`(1300nm channel)
`
`Fi g. 3
`
`time
`
`Fig.4
`
`0003
`
`0003
`
`

`

`US. Patent
`
`Sheet 3 of 4
`Sep. 11, 1990
`
`
`4,955,379
`
`red AC
`red AC
`
`
`
`129 tap
`129 tap
`
`
`
`
`signal in
`signal out
`lowpass filter
`bandpassfilter
`
`
`
`5.5 Hz
`-0.75-LHz
`
`
`
`zero
`crossing
`counter
`
`
` ir AC
`ir AC
`
`129 tap
`129 tap
`signal out
`signalin
`
`
`
`
`lowpassfilter
`bandpassfilter
`
`
`5.5 Hz
`0.75-4Hz
`
`
`
`
`loop filter
`{lowpass}
`
`
`
` calculate
`sample
`
`rate
`
`
`Fig.5
`
`0004
`
`0004
`
`

`

`US. Patent
`
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`4,955,379
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`Sheet 40f4
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`Sep. 11, 1990
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`0005
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`
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`

`

`1
`
`4,955,379
`
`2
`skin, their distance from it may vary slightly when the
`patient moves. By simple 1/d? function through air,
`measured light levels may change disastrously in real-
`life situations.
`Additionally, even if the optical components are ide-
`ally fixed to the skin, the path length between them may
`changeif the tissue is slightly deformed by the move-
`ment. Again, light level changes by this mechanism may
`seriously interfere with measurements. In this case, the
`function of intensity versus distance is more compli-
`cated than 1d/2, since, as tissue is deformed, its optical
`characteristics change. This is because of the mobility
`of the blood, the major absorbing species at the wave-
`lengths in use; for instance, as the fingertip is com-
`pressed, the path length between the optical compo-
`nents will reduce, but, additionally, venous and capil-
`lary blood is squeezed outof the light path.
`Furthermore, during severe motion, one or both opti-
`cal transducers may be pulledlaterally along thetissue
`under measurement, effectively changing the measure-
`mentsite. This typically occurs when the cable connect-
`ing the sensorto the instrumentis pulled and may cause
`major optical disturbance.
`Since the AC signal is typically only 2-5% of the
`amplitude of the DC signal, it is this which is propor-
`tionally most seriously affected by movementartefact.
`Considering this, it is a reasonable approximation to
`apply a filtering algorithm to the AC signals and to
`ignore errors in the DC signals.
`Surprisingly,
`it has now been discovered that the
`wanted AC signals, otherwise known as plethysmo-
`graph waveforms, have typical frequency versus power
`spectra as illustrated in accompanying FIG.1. Thatis,
`about 90% oftheir energy is contained at the fundamen-
`tal frequency (the pulse rate) with relatively little har-
`monic energy. Additionally,
`the unwanted signal
`caused by motion artefact frequently lies outside the
`frequency bandof the pulse rate. Accompanying FIGS.
`2 and 3 illustrate the frequency versus powerspectra of
`signals with which motion artefacts, random and peri-
`odic, respectively, are interfering. It follows from these
`realisations that a bandpass filter may be adapted selec-
`tively to exclude motion artefact from wanted signals.
`Accompanying FIG.4 illustrates the effectiveness of
`the present system in the removal of unwanted motion
`artefact signals from wanted plethysmographsignals.
`SUMMARY OF THE INVENTION
`
`In a first embodiment, the present inventionrelates to
`a pulse oximeter apparatus characterised in that it com-
`prises a bandpass filter adapted selectively to exclude
`motion artefact from wanted signal.
`In order to achieve this, the filter must initially be
`tuned to the pulse rate. Moreover, as the pulse rate
`changes, the filter is so-adapted that its pass-band will
`follow the frequency change.
`DESCRIPTION OF THE DRAWINGS
`
`FIG.1 is a graphical view of the plethysmographic
`signals found in the environment of the preferred em-
`bodimentofthe invention.
`FIG.2 is a graphical view of the plethysmographic
`signals with random motion artefacts found in the envi-
`ronment of the preferred embodimentof the invention.
`FIG.3 is a graphical view of the plethysmographic
`signals with periodic motion artefacts found in the envi-
`ronment of the preferred embodimentof the invention.
`
`MOTION ARTEFACT REJECTION SYSTEM FOR
`PULSE OXIMETERS
`
`BACKGROUND OF THE INVENTION
`
`5
`
`This invention relates to a motion artefact rejection
`system for pulse oximeters; more particularly, it relates
`to a system forfiltering out signals due to patient move-
`ment, i.e. motion artefact signals, from wanted signals.
`The operation of pulse oximeters which measure
`arterial blood oxygen saturation and pulse rate is preju-
`diced whenthe patient performs any movement. Oxim-
`eters havedifficulty in distinguishing the pulsating sig-
`nals dueto arterial blood flow from thepulsating signals
`due to patient movement. Since the results are calcu-
`lated from this pulsatile signal and the size thereof, it is
`highly desirable to be able to distinguish signals from
`these two sources. The present invention, which en-
`compasses an apparatus andthe use thereof, reduces the
`severity of this problem and offers significant advan-
`tages to a clinician.
`In general terms, a pulse oximeter apparatus will
`typically comprise the following units: a sensor, con-
`taining two LEDsofdifferent wavelength (commonly
`660 nm and 940 nm), and a photodetector, which are
`applied directly to a patient. The sensor is connected to
`the main instrumentby a cable. The instrument contains
`a system to adjust LED power, hence controlling light
`intensity, and a system to analyse the incominglight
`from the photodetector. Means are provided to isolate
`the pulsatile components ofthese incominglightsignals.
`The nonvarying (“DCsignals”) at each wavelength are
`either maintained equal by the LED poweradjusting
`system, wherebythe effects thereof cancel, or they may
`themselves be isolated and measured. The time-varying
`signals (“ACsignals”) then pass through an AGC(auto-
`matic gain control) system to ensure that they supply an
`adequate signal
`to an analogue-to-digital converter,
`where they are digitised. The AC and DCsignals are
`then taken into a microprocessor, which analyses the
`ACsignals for amplitude and frequency (corresponding
`to pulse rate). Oxygen saturation is calculated by the
`microprocessor by inserting the amplitudes ofthe vari-
`ous signals into the following formula:
`
`AC/DC;
`AC2/DC2
`
`and reading the result from an experimentally-deter-
`mined reference table. The results may be displayed on
`LEDs or LCDs. Thereis additionally provided a sys-
`tem to judge whether motion artefact is present by
`examination of variability of AC signal frequency. If
`motion is judged to be present, displayed values are
`frozen and, if this state of affairs continues for any
`length of time, a warning messageis given.
`In use, the sensoris closely applied to a well-perfused
`region ofa patient, such as a fingertip. Light from the
`LEDs needs to pass through a well-perfused region to
`ensure a good ACsignal is obtained. The emergentlight
`pulsates in intensity due to arterial pulsation. Since dur-
`ing systole the internal vessels are dilated, the total path
`length forthe light is increased and intensity falls. Arte-
`rial blood is examined exclusively since it alone is the
`cause of the ACsignals.
`Patient movement interferes with the operation of
`pulse oximeters in several ways.If either the LEDs or
`photodetectoris not fixed directly in contact with the
`
`25
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`40
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`45
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`LA0
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`55
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`60
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`65
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`0006
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`0006
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`

`4,955,379
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`10
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`20
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`25
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`30
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`35
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`DESCRIPTION OF THE PREFERRED
`EMBODIMENT OF THE INVENTION
`
`4
`1. In a pulse oximeter for making a measurement of
`FIG.4 is a graphical view of plethylsmographic sig-
`blood oxygen saturation which produces pulsatile sig-
`nals demonstrating the effectiveness of the preferred
`nals in response to a patient’s pulsating arterial blood
`embodimentofthe invention.
`flow inafirst variable range of frequencies and motion
`FIG. 5 is a schematic block diagram view of the
`artefact signals at frequencies outside of said first vari-
`preferred embodimentof the invention.
`able range of frequencies, apparatus for minimizing the
`FIG.6 is a schematic block diagram view of appara-
`effect of said motion artefact signals on said measure-
`tus comprising an environment for the preferred em-
`bodimentof the invention.
`ment of blood oxygen saturation comprising
`a tunable bandpass filter having an input to which
`said pulsatile signals and said motion artefact sig-
`nals are applied;
`a frequency determining means connected to the
`As mentioned above, a motion artefact detector sys-
`output ofsaid tunable bandpass filter for determin-
`tem decides by examination of the variability of the
`ing the frequency of the pulsatile signals at the
`amplitude and frequency of the incoming AC signals
`output of said tunable bandpass filter;
`whether motion artefact is present. If artefact is not
`and a tuning means operatively connected to said
`judged present, the bandpass filter is tuned to the pulse
`frequency determining means and said tunable
`rate as determined by the normal oximeter algorithms.
`bandpass filter for tuning said tunable bandpass
`Additionaly, the AGC system adjusts the input signal
`filter in response to said determined frequency to
`levels to the bandpass filter such that there is a large
`align the pass band of said band pass filter with the
`overload margin, for example x16, above the incoming
`determined frequency of said pulsatile signals
`wanted AC signals. When artefact is present, the AGC
`whereby motion artefact signals are attenuated.
`system is frozen, fixing the gain level, and the bandpass
`2. Apparatus according to claim 1 further comprising
`filter is configured in a feedback loopas illustrated in
`a first low pass filter connected to the input of said
`accompanying FIG. 5. The output of the bandpass fil-
`tunable bandpass filter.
`ters is substantially sinusoidal and so a simple frequency
`3. Apparatus according to claim 1 further comprising
`detector, for example a zero crossing counter,is suitable
`a loop filter connected between said frequency deter-
`to determine its output frequency. The output of this
`mining means and said tuning means.
`frequency detector passes through a low-pass loopfil-
`4. Apparatus according to claim 1 wherein said band
`ter, whose output in turn directly turns the bandpass
`pass filter is a digital pass filter tunable by changing its
`filter. The system thus formed is a frequency-locked
`sampling rate, and said tuning means comprises means
`loop ortrackingfilter.
`for changing said sampling rate in accordance with the
`Thus, when motion artefact is present, the bandpass
`output of said frequency determining means.
`filters can stay tuned to the pulse rate,
`tracking its
`5. Apparatus according to claim 6 wherein said fre-
`change. Thefilters selectively exclude motion artefact
`quency determining means comprises a zero crossing
`during operation and the amplitude of the AC signals
`counter.
`emergent from thefilters may be used by the oximeter
`6. In a pulse oximeter for making a measurement of
`as normal, The errors in oxygen saturation measure-
`blood oxygen saturation havingafirst channel wherein
`ments, as well as pulse rate, caused by patient move-
`there are produced first pulsatile signals in response to
`mentare thus advantageously reduced.
`red light absorbed bya patient’s pulsating arterial blood
`For purposes of exemplification, the present system
`flow and a second channel wherein there are produced
`has been incorporated into a Novametrix oximeter
`second pulsatile signals in response to infrared light
`model 500 as an additional 68000-10 slave processor. A
`absorbed bya patient’s pulsating arterial blood flow ina
`hardware block diagram is illustrated in accompanying
`FIG.6.
`first variable range of frequencies, and in which motion
`artefact signals at frequencies outside of said first vari-
`Regarding digital signal processing algorithms, the
`able range of frequencies are produced in said first and
`present system is illustrated in accompanying FIG.5.
`second channels, apparatus for minimizing the effect of
`ACsignals are first passed through a high grade 5.5 Hz
`said motion artefact signals on said measurement of
`lowpass filter, 129 tap FIR filter, which is a necessary
`blood oxygen saturation comprising
`anti-aliasing filter at the lowest bandpass filter sampling
`a first tunable bandpass filter disposed in said first
`rates. The lowpass filter sampling rate is fixed at 100 Hz.
`channel and having an input to which said first
`The bandpass filter has fixed coefficients, and is tuned
`pulsatile signals and said first channel motion
`by varying its sample rate as illustrated in accompany-
`artefact signals are applied;
`ing FIG.5. Finite impulse response (FIR) filters have
`a second tunable bandpass filter disposed in said sec-
`been used for their predictable frequency versus delay
`ond channel and having an input to which said
`characteristics. The design ofthis filter is the result of a
`second pulsatile signals and said second channel
`number of conflicting requirements which are outlined
`below:
`motion artefact signals are applied;
`a frequency determining means connected to the
`(i) optimal artefact filtering demands a narrow pass-
`output of at least one ofsaid first and second tun-
`band and high stop-band rejection,
`implying long
`able bandpass filters for determining the frequency
`tap-length filters;
`of the pulsatile signals at the output ofsaid at least.
`(ii) adequate tracking of changes in pulse rate demands
`one tunable bandpass filter;
`a wide pass-band and fast servo loop performance,
`and a tuning means operatively connected to said
`implying short tap-length filters.
`frequency determining means and said first and
`Onesuitablefilter is a 129 tap FIR of sampling rate
`second tunable bandpass filters for tuning said tun-
`15-80 Hz, with —3 dB points +16% of centre fre-
`able bandpass filters to align the pass bands of the
`quency and stopband rejection of —40 dB at +50% of
`band pass filters with the determined frequency.
`-
`=
`=
`=
`centre frequency.
`I claim:
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`40
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`55
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`60
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`65
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`0007
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`0007
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`

`

`-—_SSTWTfHSWTS
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`
`
`UNITED STATES PATENT OFFICE
`CERTIFICATE OF CORRECTION
`: 4,955,379
`: Sep. li, 1990
`
`.
`
`PATENT NO.
`DATED
`
`INVENTOR(S)
`
`: Peter R. Hall
`
`it is certified that error appears in the above—identified patent and that said Letters Patent
`is hereby corrected as shown below:
`2
`2
`At
`column 1,
`line 11, change "1d/“" to --1/d“--.
`
`At column 4,
`
`line 30, after "digital" insert --band--.
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`1
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`!
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`Signed and Sealed this
`
`Eleventh Day of February, 1992
`
`Attest:
`
`HARRY F. MANBECK, JR.
`
`Commissioner of Patents and Trademarks
`Attesting Officer
`——— a npepee Set
`
`
`
`0008
`
`0008
`
`