4,955,379
`[11] Patent Number:
`United States Patent 11)
`Hall
`[45] Date of Patent:
`Sep. 11, 1990
`
`
`[54] MOTION ARTEFACT REJECTION SYSTEM
`FOR EULESS OXIMETERS
`
`_[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`[75]
`
`.
`Inventor:
`
`Heicas R. Hall, Dyfed, United
`gdom
`
`[73] Assignee: National Research Development
`Corporation, London, England
`
`[21] Appl. No.: 229,692
`
`[22] Filed:
`
`Aug. 8, 1988
`
`Foreign Application Priority Data
`[30]
`Aug. 14, 1987 [GB] United Kingdom................. 8719333
`
`Unt, CUS ceccccsscsesecsseessstssssseterrseseeseneesss AGIB 5/00
`(51)
`sesssssnsssse 128/633; 128/664;
`[52] US. Cd. cesccscessececceenee
`128/666; 128/687
`[58] Field of Search ............. 128/633, 634, 664, 665,
`128/666, 687; 356/39, 41
`
`8/1978 Bond et al. svseeseneeneraene 128/666
`4,109,643
`9/1979 Nielsen .....
`veeee 356/41
`4,167,331
`
`4/1981 Lewyn ....
`». 128/690
`4,260,951
`4,353,152 10/1982 O’Connoret al.
`". 128/666
`
`4,641,658 2/1987 Lepper........
`... 128/633
`
`". 128/633
`3/1987 Passafaro .
`4,651,741
`
`"128/633
`4,723,554 2/1988 Oman ...
`
`Primary Examiner—Kyle L. Howell
`Assistant Examiner—John C. Hanley
`Attorney, Agent, or Firm—Howard F. Mandelbaum
`
`ABSTRACT
`[57]
`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
`
`
`
`
`
`
`
`
`
`
`129 tap
`bandpass filter
`
`OF5-bHr
`
`red AC
`signal out
`
`
`
`129 tap
`\ewpase fer
`55 le
`
`
`ir AC
`‘signalin
`
`0001
`
`Apple Inc.
`APL1048
`U.S. Patent No. 8,923,941
`FITBIT, Ex. 1048
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`Apple Inc.
`APL1048
`U.S. Patent No. 8,923,941
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`0001
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`FITBIT, Ex. 1048
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`

`

`
`
`US. Patent—Sep. 11, 1990 Sheet 1 of 4 4,955,379
`
`
`
`Pulse rate
`
`Frequency (Hz)
`
`(800nm channel)
`
`Fig.1
`
`Pulse rate
`
`7
`
`0
`
`Frequency (Hz)
`(1300 nm channel}
`
`Fig.2
`
`0002
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`

`US. Patent
`
`sep. 11,1999
`
` ‘Sheet20f4
`
`4,955,379
`
`Pulse rate
`
`75
`
`5
`
`yA 0
`
`Frequency (Hz)
`
`(1300nm channel)
`
`Fig. 3
`
`
`
`after
`
`filtering
`
`before
`
`filtering
`
`time
`
`Fig.4
`
`0003
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`FITBIT, Ex. 1048
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`0003
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`US. Patent
`
`Sheet 3 of 4
`Sep. 11, 1990
`
`
`
`
`red AC
`red AC
`
`
`
`129 tap
`129 tap
`
`
`
`signal in
`signal out
`lowpass filter
`bandpassfilter
`
`
`
`5.5 Hz
`0.75-Hz
`
`
`
`
`4,955,379
`
`zero
`crossing
`counter
`
`ir AC
`signal in
`
`
`
` ir AC
`
`
`129 tap
`123 tap
`signal out
`
`
`
`bandpassfilter
`lowpassfilter
`0.75-4Hz
`5.5 Hz
`
`
`
`loop filter
`{lowpass}
`
` calculate
`
`
`sample
`rate
`
`
`Fig. 5
`
`0004
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`

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`U.S. Patent
`
`Sep. 11, 1990
`
`Sheet 40f4
`
`4,955,379
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`

`

`1
`
`4,955,379
`
`MOTION ARTEFACT REJECTION SYSTEM FOR
`PULSE OXIMETERS
`
`BACKGROUND OF THE INVENTION
`
`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 when the patient performs any movement. Oxim-
`eters havedifficulty in distinguishing the pulsating sig-
`nals dueto arterial blood flow from the pulsating signals
`due to patient movement. Since the results are calcu-
`lated from this pulsatile signal and thesize thereof, it is
`highly desirable to be able to distinguish signals from
`these two sources. The present invention, which en-
`compasses an apparatus andtheuse 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 incoming light
`from the photodetector. Means are provided toisolate
`the pulsatile components of these incominglightsignals.
`The nonvarying (“DC signals’) 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 of the vari-
`ous signals into the following formula:
`
`AC/DC;
`AC/DC
`
`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 message is given.
`In use, the sensoris closely applied to a well-perfused
`region of a 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 for the light is increased andintensity falls. Arte-
`rial blood is examined exclusively since it alone is the
`cause of the AC signals.
`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
`
`40
`
`45
`
`55
`
`65
`
`2
`skin, their distance from it may vary slightly when the
`patient moves. By simple 1/d? function throughair,
`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 out of the light path.
`Furthermore, during severe motion, one or both opti-
`cal transducers maybe pulled laterally along the tissue
`under measurement, effectively changing the measure-
`mentsite. This typically occurs when the cable connect-
`ing the sensor to the instrument is 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 DCsignals.
`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% of their 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 band ofthe pulse rate. Accompanying FIGS.
`2 and 3 illustrate the frequency versus power spectra of
`signals with which motion artefacts, random and peri-
`odie, 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-
`bodiment of the 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 foundin the envi-
`ronmentof the preferred embodiment of the invention.
`
`0006
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`FITBIT, Ex. 1048
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`4,955,379
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`FIG.4 is a graphical view of plethylsmographic sig-
`nals demonstrating the effectiveness of the preferred
`embodimentof the invention.
`FIG. 5 is a schematic block diagram view of the
`preferred embodimentof the invention.
`FIG.6 is a schematic block diagram view of appara-
`tus comprising an environment for the preferred em-
`bodiment of the invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT OF THE INVENTION
`
`As mentioned above, a motion artefact detector sys-
`tem decides by examination of the variability of the
`amplitude and frequency of the incoming ACsignals
`whether motion artefact is present. If artefact is not
`judged present, the bandpass filter is tuned to the pulse
`rate as determined by the normal oximeter algorithms.
`Additionaly, the AGC system adjusts the input signal
`levels to the bandpass filter such that there is a large
`overload margin, for example x16, above the incoming
`wanted AC signals. When artefactis present, the AGC
`system is frozen, fixing the gain level, and the bandpass
`filter is configured in a feedback loop as illustrated in
`accompanying FIG. 5. The output of the bandpass fil-
`ters is substantially sinusoidal and so a simple frequency
`detector, for example a zero crossing counter,is suitable
`to determine its output frequency. The outputofthis
`frequency detector passes through a low-pass loopfil-
`ter, whose output in turn directly turns the bandpass
`filter. The system thus formed is a frequency-locked
`loop or trackingfilter.
`Thus, when motion artefact is present, the bandpass
`filters can stay tuned to the pulse rate,
`tracking its
`change. Thefilters selectively exclude motion artefact
`during operation and the amplitude of the AC signals
`emergent from the filters may be used by the oximeter
`as normal. The errors in oxygen saturation measure-
`ments, as well as pulse rate, caused by patient move-
`ment are thus advantageously reduced.
`For purposes of exemplification, the present system
`has been incorporated into a Novametrix oximeter
`model 500 as an additional 68000-10 slave processor. A
`hardware block diagram is illustrated in accompanying
`FIG.6.
`Regarding digital signal processing algorithms, the
`present system is illustrated in accompanying FIG.5.
`ACsignals are first passed through a high grade 5.5 Hz
`lowpass filter, 129 tap FIR filter, which is a necessary
`anti-aliasingfilter at the lowest bandpass filter sampling
`rates, The lowpass filter samplingrate is fixed at 100 Hz.
`The bandpass filter has fixed coefficients, and is tuned
`by varying its sample rate as illustrated in accompany-
`ing FIG. 5. Finite impulse response (FIR)filters have
`been used for their predictable frequency versus delay
`characteristics. The design ofthis filter is the result of a
`number of conflicting requirements which are outlined
`below:
`(i) optimal artefact filtering demands a narrow pass-
`band and high stop-band rejection,
`implying long
`tap-length filters;
`(ii) adequate tracking of changes in pulse rate demands
`a wide pass-band and fast servo loop performance,
`implying short tap-length filters.
`Onesuitablefilter is a 129 tap FIR of sampling rate
`15-80 Hz, with —3 dB points +16% of centre fre-
`quencyand stopband rejection of —40 dB at +50% of
`centre frequency.
`I claim:
`
`25
`
`40
`
`45
`
`50
`
`65
`
`4
`1. In a pulse oximeter for making a measurement of
`blood oxygen saturation which produces pulsatile sig-
`nals in response to a patient’s pulsating arterial blood
`flow in a first variable range of frequencies and motion
`artefact signals at frequencies outside of said first vari-
`able range of frequencies, apparatus for minimizing the
`effect of said motion artefact signals on said measure-
`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
`output of said tunable bandpass filter for determin-
`ing the frequency of the pulsatile signals at the
`output of said tunable bandpass filter;
`and a tuning means operatively connected to said
`frequency determining means and said tunable
`bandpass filter for tuning said tunable bandpass
`filter in response to said determined frequency to
`align the pass band of said band pass filter with the
`determined frequency of said pulsatile signals
`whereby motion artefact signals are attenuated.
`2. Apparatus according to claim 1 further comprising
`a first low pass filter connected to the input of said
`tunable bandpass filter.
`3. Apparatus accordingto claim 1 further comprising
`a loop filter connected between said frequency deter-
`mining means and said tuning means.
`4. Apparatus according to claim 1 wherein said band
`pass filter is a digital pass filter tunable by changing its
`sampling rate, and said tuning means comprises means
`for changing said sampling rate in accordance with the
`output of said frequency determining means.
`5. Apparatus according to claim 6 wherein said fre-
`quency determining means comprises a zero crossing
`counter.
`6. In a pulse oximeter for making a measurement of
`blood oxygen saturation havinga first channel wherein
`there are produced first pulsatile signals in response to
`red light absorbed by a patient’s pulsating arterial blood
`flow and a second channel wherein there are produced
`second pulsatile signals in response to infrared light
`absorbed bya patient’s pulsating arterial blood flow in a
`first variable range of frequencies, and in which motion
`artefact signals at frequencies outside of said first vari-
`able range of frequencies are produced in said first and
`second channels, apparatus for minimizing the effect of
`said motion artefact signals on said measurement of
`blood oxygen saturation comprising
`a first tunable bandpass filter disposed in said first
`channel and having an input to which said first
`pulsatile signals and said first channel motion
`artefact signals are applied;
`a second tunable bandpass filter disposed in said sec-
`ond channel and having an input to which said
`second pulsatile signals and said second channel
`motion artefact signals are applied;
`a frequency determining means connected to the
`output of at least one of said first and second tun-
`able bandpass filters for determining the frequency
`of the pulsatile signals at the outputofsaid atleast.
`one tunable bandpass filter;
`and a tuning means operatively connected to said
`frequency determining means and said first and
`second tunable bandpass filters for tuning said tun-
`able bandpass filters to align the pass bands of the
`*
`*
`*
`*
`*
`band pass filters with the determined frequency.
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`UNITED STATES PATENT OFFICE
`CERTIFICATE OF CORRECTION
`
`PATENT NO.
`
`: 4,955,379
`
`DATED
`INVENTOR(S)
`
`: Sep. li, 1990
`: 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:
`At
`column 1,
`line 11, change "iqs2m to cet
`
`At column 4,
`
`line 39, after "digital" insert --band--.
`
`
`
`Signed and Sealed this
`
`Eleventh Day of February, 1992
`
`Altest:
`
`HARRY F. MANBECK, JR.
`
`
`
`:
`
`Artesting Officer
`
`Commissioner of Patents and Trademarks —_
`
`
`
`0008
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`FITBIT, Ex. 1048
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`0008
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