Umted States Patent [191
`
`nu Patent Number:
`
`4,955,379
`
`
`Hall
`1
`[45] Date of Patent:
`Sep. 11, 1990
`
`[54] MOTION ARTEFACI‘ REJECTION SYSTEM
`F0“ Pm-SE °"IMEm‘5
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3/19'3'S Bond et al.
`4.109.643
`9/1979 Nielsen
`4,157,331
`4/1931 Lewyn
`4,260,951
`4.353.152 ID/1982 0’Connoretal.
`P37 I-EPPET
`.,
`98'.-'Pa.ssafaro
`4.723.554 2/1983 Oman
`
`
`
`128/666
`356/41
`. 123/590
`.. 128/666
`-
`.
`
`128/633
`
`Primary Exam:'ner—Kyle L. Howell
`Ashton: Examir:er—John C. Hanley
`Attarrsey, Agent, or F:rm—Howard F. Mandelbaum
`[57]
`ABSTRACT
`
`A pulse oximeter apparatus characterized in that it com-
`prises 3 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
`
`[75]
`
`.
`I'“'°“‘°"
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`3 °m
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`[73] Assignee: National Research Development
`cm-pofi]_d_E
`d
`H ml’
`on on
`ugh“
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`[2]] App], No,; 229,692
`
`[22] Filed:
`
`Aug. 3’ 1988
`
`F°”i3“ APl’“"3fi°‘“ P1‘i°1'i'V D3”
`[301
`Aug. 14, 1937 [on] United Kingdom
`
`3119333
`
`[51] ht CL:
`[52] U.S. Cl.
`
`[58] Field of Search
`
`A513 5/on
`128/633; 128/664;
`128/666; 128/687
`128/633, 634, 664, 665,
`123/666, 63?; 356/39, 41
`
`
`
` 11! up
`lune ilur
`SEILI
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`IIII
`main;
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`
`0001
`
`Apple I
`APL10
`
`U.S. Patent No. 89239
`
`Apple Inc.
`APL1048
`U.S. Patent No. 8,923,941
`
`0001
`
`

`
`U.S. Patent
`
`Sep. 11,1990
`
`Sheet 1 014
`
`4,955,379
`
`Pulse rate
`
`I.
`
`1
`
`B
`
`Frequency {Hz}
`
`iflflflnrn channel]
`
`1
`
`Pulserate
`
`1.7
`
`I]
`
`Frequency I Hz}
`
`[13U!}nm channel}
`
`Fig.2
`
`0002
`
`0002
`
`

`
`U.S. Patent
`
`Sep. 11,1991»
`
`' si11e1 2 .114
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`4,955,379
`
`Pulse rate
`
`15
`
`5
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`2.5 U
`
`I]
`
`Frequency {Hz}
`
`(13Uflnm channel}
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`3
`
`time
`
`after
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`iiflering
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`beiore
`
`filtering
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`0003
`
`0003
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`

`
`U.S. Patent
`
`Sep. 11, 1990
`
`Sheet 3 of 4
`
`4,955,379
`
`129 tap
`bandpassfflter
`-BJ5-Lflz
`
`0004
`
`0004
`
`

`
`U.S. Patent
`
`Sep.ll, 1990
`
`Sheet 4 of4
`
`4,955,379
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`0005
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`

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`1
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`4,955,379
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`2
`skin, their distance from it may vary slightly when the
`patient moves. By simple l/dl 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
`change if 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
`fimction of intensity versus distance is more compli-
`cated than ld/1. 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 may be pulled laterally along the tissue
`under measurement, effectively changing the measure-
`ment site. This typically occurs when the cable connect-
`ing the sensor to the instnnnent 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 movement artefact.
`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. That is,
`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 of the pulse rate. Accompanying FIGS.
`2 and 3 illustrate the frequency versus power spectra 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
`artefaot signals from wanted plethysmograph signals.
`SUMMARY OF THE INVENTION
`
`5
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`10
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`15
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`20
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`30
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`MOTION ARTEFACT REJECTION SYSTEM FOR
`PUISEO
`
`BACKGROUND OF THE INVENTION
`
`This invention relates to a motion artefact rejection
`system for pulse oximeters; more particularly, it relates
`to a system for filtering 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. 0xim-
`eters have difficulty in distinguishing the pulsating sig-
`nals due to arterial blood flow from the pulsating 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 and the 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 LEDs of different wavelength (commonly
`660 nm and 940 nm), and a photodetector, which are
`applied directly to a patient. The sensor is connected to
`the main instrument by 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 to isolate
`the pulsatile components of these incoming light signals.
`The nonvarying ("DC signals”) at each wavelength are
`either maintained equal by the LED power adjusting
`system, whereby the effects thereof cancel, or they may
`themselves be isolated and measured. The time-varying
`signals (“AC signals”) 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 DC signals are
`then taken into a microprocessor, which analyses the
`AC signals 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:
`
`ACHDCI
`AC2/DC:
`
`and reading the result from an experimentally-deter
`mined reference table. The results may be displayed on
`LEDs or LCDS. There is 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 sensor is 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 AC signal is obtained. The emergent light
`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 and intensity 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
`photodetector is not fixed directly in contact with the
`
`55
`
`65
`
`In a first embodiment, the present invention relates 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 invtion.
`FIG. 2 is a graphical view of the plethysmographic
`signals with random motion artefacts found in the envi-
`ronmt of the preferred embodiment of the invention.
`FIG. 3 is a graphical view of the plethysmographic
`signals with periodic motion artefacts found in the envi-
`ronmt of the preferred embodiment of the invention.
`
`0006
`
`0006
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`

`
`3
`FIG. 4 is a graphical view of plethylsmographic sig-
`nals demonstrating the effectiveness of the preferred
`embodiment of the invention.
`FIG. 5 is a schtic block diagram view of the
`preferred embodiment of 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 AC signals
`whether motion artefact is present. If artefact is not
`judged present, the bandpass filter is timed 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 artefact is 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 output of this
`frequency detector passes through a. low-pass loop fil-
`ter, whose output in turn directly turns the bandpass
`filter. The system thus formed is a frequency-locked
`loop or tracking filter.
`Thus, when motion artefact is present, the bandpass
`filters can stay tuned to the pulse rate,
`tracking its
`change. The filters 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 exemplifrcation. 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.
`AC signals are first passed through a high grade 5.5 Hz
`Iowpass filter, 129 tap FIR filter, which is a necessary
`anti-aliasing filter at the lowest bandpass filter sampling
`rates. The lowpam filter sampling rate is fixed at 100 Hz.
`The bandpass filter has fixed coefiicients, 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 of this filter is the result of a
`number of conflicting requirements which are outlined
`beknm
`(i) optimal ariefact 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.
`One suitable filter is a 129 tap FER of sampling rate
`15-80 Hz, with -3 dB points $1696 of centre fre-
`quency and stopband rejection of -40 dB at :!:50% of
`centre frequency.
`I claim:
`
`20
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`SD
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`55
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`65
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`4,955,379 '
`
`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 timing 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. Apparatirs according to 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 having a 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 by a 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 artefaet signals on said measurement of
`blood oxygen saturation comprising
`a first tunable bandpass filter disposed in said fir-st
`channel and having an input to which said fir-st
`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 output of said at least.
`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.
`
`0007
`
`0007
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`

`
`
`
`UNITED STATES PATENT OFFICE
`
`CERTIFICATE OF CORRECTION
`
`PATENT NO.
`DATED
`
`; 4,955,379
`.- Sep. 11, 1990
`
`It~tVE.\tTOR{S)
`
`: Peter R. Hall
`
`-
`
`i
`
`I’
`
`1
`
`I
`
`It is certified that enor appears in the above—:dentified patent and that said Letters Patent
`is hereby corrected as shown below;
`'
`At Column 1,
`llne 1]., change
`
`2:1
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`2
`to --1/d --.
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`n
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`161/
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`At column 4,
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`line 39, after "digital" insert --band--.
`
`Signed and Sealed this
`
`Eleventh Day of February, 1992
`
`Arrest:
`
`HARRY F. MANBECK. JR.
`
`'-
`
`‘
`
`_____
`
`_______________________________._.____________,_________________________ ___._
`
`Commissioner of Parents and Trademarks
`
`Arresting Oflicer
`
`0008
`
`0008