United States Patent £191
`Hall
`
`[11] Patent Number:
`[45} Date of Patent:
`
`4,955,379
`Sep. 11, 1990
`
`[54] MOTION ARTEFACI REJECI'ION SYSTEM
`FOR PULSE OXIMETERS
`
`[75)
`
`Inventor: Peter R. Hall, Dyfed. United
`Kingdom
`
`[73] Assignee: National Research DeTelopment
`Corporation, London, England
`
`[21] Appl. No.: .229,692
`
`[22] Filed:
`
`Aug. 8,1988
`
`Foreign Application Priority Data
`[30]
`Aug. 14, 1987 [GB) United IGngdom ................. 8719333
`
`Int. Q,$ ................................................ A61B 5/00
`[51]
`(52) u.s. a. .................................... 128/ 633; 128/664;
`128/666; 128/687
`(58] Field of Search ............... 128/633, 634, 664, 665,
`128/666, 687; 356/39, 41
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`4,109,643 8/1978 Bond et al ........................... 128/666
`4,167,331 9/1979 Nielsen .................................. 356/41
`4,260,9Sl 4/1981 Lewyn .•.........•.................... 128/690
`4,3S3,1S2 10/1982 O'Connor et al ................... 128/666
`4,641,658 2/1987 Lepper ................................ 128/633
`4,6SI,741 3/1987 Passafaro .........•.................. 128/633
`4,723,554 2/1988 Oman .................................. 128/633
`Primary Examiner-Kyle L. Howell
`Assistant Examiner-John C. Hanley
`Attorney, Agent, or Finn-Howard F. Mandelbaum
`ABSTRAcr
`[57]
`A pulse oximeter apparatus characterized in that it com(cid:173)
`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(cid:173)
`gen saturation.
`
`6 Claims, 4 Drawing Sheets
`
`'"''
`
`•ito>~ il
`
`ttl AC
`si;•llool
`
`lr AC
`sl;oal .. l
`
`Apple Inc.
`APL1036
`U.S. Patent No. 9,289,135
`
`001
`
`

`

`U.S. Patent
`
`Sep. 11, 1990
`
`Sheet 1 of 4
`
`4,955,379
`
`Pulse rate
`
`(Hz l
`Frequency
`(BOOnm channel)
`
`0
`
`Fig.1
`
`Pulse rate
`
`1.7
`
`0
`
`Frequency (Hz)
`( 1300 n m channel)
`
`Fig.2
`
`002
`
`

`

`U.S. Patent
`
`Sep. 11, 1990
`
`Sheet 2 of 4
`
`4,955,379
`
`Pulse rate
`
`.
`
`7.5
`
`5
`Frequency (Hz)
`( 1300 nm channel)
`
`2.5
`
`1.7
`
`0
`
`Fig. 3
`
`after
`filtering
`
`before
`filtering
`
`r - - - - - -- --
`
`-
`
`- - - time
`
`Fig.4
`
`003
`
`

`

`U.S. Patent
`
`Sep. 11, 1990
`
`red AC
`signal in
`
`129 tap
`lowpass fi~er
`5.5 Hz
`
`ir AC
`signal in
`
`129 tap
`lowpass filter
`5.5 Hz
`
`4,955,379
`
`red AC
`signal out
`
`Sheet 3 of 4
`
`r
`
`129 tap
`band pass filter
`0.75-,Hz
`I
`
`T
`
`129 tap
`bandpass filter
`0.75-,Hz
`I
`
`zero
`crossing
`counter
`
`ir AC
`si gnat out
`""
`
`loop filter
`(lowpass)
`
`calculate
`sample
`rate
`
`.
`Frg. 5
`
`004
`
`

`

`~
`~
`
`\C
`.._.J;;..
`
`~
`
`~ g,
`[
`
`i
`...
`,.....
`~ ,.....
`
`rJJ
`
`~ ......
`~ ......
`~ rn
`
`8 bit
`
`X 2
`t1mers
`116 bil
`
`oximeter
`to main
`communications
`
`8 bit
`
`65C22
`I. A.
`
`V.
`
`I
`
`I
`
`I
`
`I
`
`EPROM
`X 16
`32k
`
`RAM
`x16
`BK
`
`from main oximeter
`signal
`ir front end
`
`signal
`red front end
`
`Fig.6
`
`~
`
`DC
`ir
`
`AC
`ir
`
`I
`
`System Buses
`
`I
`
`I
`
`I
`
`I
`
`1
`
`AC
`red
`
`DC
`red
`
`I dual 12 bil I l 1
`
`O.
`
`I
`{AGC)
`A.C.
`
`CPU
`68000
`
`lOMHz
`
`AOC
`12 bit
`
`hold
`
`1 sample
`
`---1 MPX
`
`~way
`
`~
`
`,
`
`-
`
`,------
`
`005
`
`

`

`1
`
`4,955,379
`
`MOTION ARTEFACT REJECTION SYSTEM FOR
`PULSE OXIMETERS
`
`2
`skin, their distance from it may vary slightly when the
`patient moves. By simple 1/d2 function through air,
`measured light levels may change disastrously in real-
`life situations.
`S Additionally, even if the optical components are ide-
`BACKGROUND OF THE INVENTION
`ally fixed to the skin, the path length between them may
`This invention relates to a motion artefact rejection
`change if the tissue is slightly deformed by the move-
`system for pulse oximeters; more particularly, it relates
`ment. Again, light level changes by this mechanism may
`to a system for filtering out signals due to patient move-
`seriously interfere with measurements. In this case, the
`ment, i.e. motion artefact signals, from wanted signals.
`The operation of pulse oximeters which measure 10 function of intensity versus distance is more compli·
`cated than Id!2, since, as tissue is deformed, its optical
`arterial blood oxygen saturation and pulse rate is preju-
`charactenstics change. This is because of the mobility
`diced when the patient performs any movement. Oxim-
`eters have difficulty in distinguishing the pulsating sig-
`of the blood, the major absorbing species at the wave·
`nals due to arterial blood flow from the pulsating signals
`lengths in use; for instance, as the fmgertip is com-
`due to patient movement. Since the results are calcu- IS pressed, the path length between the optical compo-
`nents will reduce, but, additionally, venous and capil·
`lated from this pulsatile signal and the size thereof, it is
`highly desirable to be able to distinguish signals from
`lary blood is squeezed out of the light path.
`these two sources. The present invention, which en-
`Furthermore, during severe motion, one or both opti-
`cal transducers may be pulled laterally along the tissue
`compasses an apparatus and the use thereof, reduces the
`severity of this problem and offers significant advan- 20 under measurement, effectively changing the measure·
`tages to a clinician.
`ment site. This typically occurs when the cable connect-
`In general terms, a pulse oximeter apparatus will
`ing the sensor to the instrument is pulled and may cause
`typically comprise the following units: a sensor, con-
`major optical disturbance.
`taining two LEDs of different wavelength (commonly
`Since the AC signal is typically only 2-5% of the
`660 nm and 940 nm), and a photodetector, which are 2S amplitude of the DC signal, it is this which is propor(cid:173)
`tionally most seriously affected by movement artefact.
`applied directly to a patient. The sensor is connected to
`the main instrument by a cable. The instrument contains
`.
`.
`.
`Considering this, it is a reasonable approximation to
`a system to adJUSt LED power, hence controllmg hght
`·
`al
`'thm t
`th AC · al
`d t
`1 mt
`0
`~PP Y a e~g gon
`intensity, and a system to analyse the incoming light
`.
`e
`Stgn s an o
`from the photodetector. Means are provided to isolate 30 Ignore e.n:ors m ~he DC stgnals.
`.
`Surpnsmgly_. 1t has now ~en discovered that the
`the pulsatile components of these incoming light signals.
`wanted AC Signals, othe~tse known as plethysmo-
`The nonvarying ("DC signals") at each wavelength are
`either maintained equal by the LED power adjusting
`graph waveforms, have typtcal frequency versus power
`spectra as illus~ted in ac~ompan~g FIG. L That is,
`system, whereby the effects thereof cancel, or they may
`themselves be isolated and measured. The time-varying 3S about 90% of thelt' energy IS con~ed a~ the ~damen-
`tal f~equency (the pul:U: rate) w1th relattvely little. har-
`signals ("AC signals") then pass through an AGC (auto•
`matic gain control) system to ensure that they supply an
`moruc energy.. Additionally,
`the un:-vanted . stgnal
`adequate signal to an analogue-to-digital converter,
`caused by monon artefact frequently lies o~tstde the
`where they are digitised. The AC and DC signals are
`frequen~y band of the pulse rate. AccompanytDg FIGS.
`then taken into a microprocessor, which analyses the 40 2. and 3 ~ustrat~ the fre~uency versus power spectra ~f
`AC signals for amplitude and frequency (corresponding
`SJgnals wtth which motton artefacts, random and pen-
`to pulse rate). Oxygen saturation is calculated by the
`odic, respectively, are interfering. It follows from these
`microprocessor by inserting the amplitudes of the vari-
`realisations that a bandpass filter may be adapted selec-
`tively to exclude motion artefact from wanted signals.
`ous signals into the following formula:
`45 Accompanying FIG. 4 illustrates the effectiveness of
`the present system in the removal of unwanted motion
`artefact signals from wanted plethysmograph signals.
`
`and reading the result from an experimentally-deter(cid:173)
`mined reference table. The results may be displayed on SO
`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 ss
`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
`LEOs needs to pass through a well-perfused region to
`ensure a good AC signal is obtained. The emergent light 60
`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 th.e LEOs or
`photodetector ~ not fixed directly in contact with the
`
`SUMMARY OF THE INVENTION
`In a first embodiment, the present invention relates to
`a pulse oximeter apparatus characterised in that it com(cid:173)
`prises a bandpass filter adapted selectively to exclude
`motion artefact from wanted signal.
`In order to achieve this, the ftlter must initially be
`tuned to the pulse rate. M~reover, 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·
`65 ronment 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-
`ronment of the preferred embodiment of the invention.
`
`006
`
`

`

`4,955,379
`
`3
`FIG. 4 is a graphical view of plethylsmographic sig(cid:173)
`nals demonstrating the effectiveness of the preferred
`embodiment of the invention.
`FIG. 5 is a schematic block diagram view of the
`preferred embodiment of the invention.
`FIG. 6 is a schematic block diagram view of appara(cid:173)
`tus comprising an environment for the preferred em(cid:173)
`bodiment of the invention.
`
`4
`1. In a pulse oximeter for making a measurement of
`blood oxygen saturation which produces pulsatile sig(cid:173)
`nals in response to a patient's pulsating arterial blood
`flow in a first variable range of frequencies and motion
`s artefact signals at frequencies outside of said fli'St vari(cid:173)
`able range of frequencies, apparatus for minimizing the
`effect of said motion artefact signals on said measure(cid:173)
`ment of blood oxygen saturation comprising
`a tunable bandpass fllter having an input to which
`said pulsatile signals and said motion artefact sig(cid:173)
`nals are applied;
`a frequency determining means connected to the
`output of said tunable bandpass ftlter for determin(cid:173)
`ing the frequency of the pulsatile signals at the
`output of said tunable bandpass fllter;
`and a tuning means operatively connected to said
`frequency determining means and said tunable
`bandpass ftlter for tuning said tunable bandpass
`ftlter in response to said determined frequency to
`align the pass band of said band pass falter 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 ftlter.
`3. Apparatus according to claim 1 further comprising
`a loop filter connected between said frequency deter(cid:173)
`mining means and said tuning means.
`4. Apparatus according to claim 1 wherein said band
`pass filter is a digital pass falter 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(cid:173)
`quency determining means comprises a zero crossing
`counter.
`6. In a pulse oximeter for making a measurement of
`blood oxygen saturation having a rust 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 fli'St vari(cid:173)
`able range of frequencies are produced iin said fli'St and
`second channels, apparatus for m.inimizing the effect of
`said motion artefact signals on said measurement of
`blood oxygen saturation comprising
`a first tunable bandpass filter disposed in said fmt
`channel and having an input to which said first
`pulsatile signals and said first channel motion
`artefact signals are applied;
`a second tunable bandpass ftlter disposed in said sec(cid:173)
`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 ftrst and second tun(cid:173)
`able bandpass falters 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(cid:173)
`able bandpass falters to align the pass bands of the
`band pass filters with the determined frequency.
`• • • • •
`
`10
`
`DESCRIPTION OF TilE PREFERRED
`EMBODIMENT OF TilE INVENTION
`As mentioned above, a motion artefact detector sys(cid:173)
`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 IS
`judged present, the bandpass fJ.lter 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 ftlter such that there is a large
`overload margin, for example x16, above the incoming 20
`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(cid:173)
`ters is substantially sinusoidal and so a simple frequency 25
`detector, for example a zero crOMing counter, is suitable
`to determine its output frequency. The output of this
`frequency detector passes through a low-pass loop fil(cid:173)
`ter, whose output in turn directly turns the bandpass
`filter. The system thus formed is a frequency-locked 30
`loop or tracking fLlter.
`Thus, when motion artef~t is present, the bandpass
`filters can stay tuned to the pulse rate, tracking its
`change. The fLlters selectively exclude motion artefact
`during operation and the amplitude of the AC signals 35
`emergent from the fllters may be used by the oximeter
`as normal. The errors in oxygen saturation measure(cid:173)
`ments, as well as pulse rate, caused by patient move(cid:173)
`ment are thus advantageously reduced.
`For purposes of exemplification, the present system 40
`has been incorporated into a Novametrix oximeter
`model 500 as an additional68000-10 slave processor. A
`hardware block diagram is illustrated in accompanying
`FIG. 6.
`Regarding digital signal processing algorithms, the 45
`present system is illustrated in ~companying FIG. 5.
`AC signals are fii'St passed through a high grade 5.5 Hz
`lowpass ftlter, 129 tap FIR fLlter, which is a necessary
`anti-aliasing filter at the lowest bandpass fllter sampling
`rates. The lowpus falter sampling rate is fixed at 100Hz. 50
`The bandpass falter has fiXed coefficients, and is tuned
`by varying its sample rate as illustrated in accompany(cid:173)
`ing FIG. 5. Finite impulse response (FIR) fllters have
`been used for their predictable frequency versus delay
`characteristics. The design of this filter is the result of a ss
`number of conflicting requirements which are outlined
`below:
`(i) optimal artefact filtering demands a narrow pass(cid:173)
`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 FIR of sampling rate
`15-80 Hz, with -3 dB points ± 16% of centre fre- 6S
`quency and stopband rejection of -40 dB at ±50% of
`centre frequency.
`I claim:
`
`60
`
`007
`
`

`

`UNITED STATES PATENT OFFICE
`CERTIFICATE OF CORRECTION
`: 4 , 955,379
`PATENT NO.
`Sep . ll, 1990
`LJA TED
`INVENTO R(S) : Peter R. Hall
`
`It is certified that error appears in the above-I dentified patent and that sa1d Letters Patent
`is hereby corrected as shown below:
`line 11 , change " ld/2
`" to -- l/d 2-- .
`At
`column 1,
`
`At column 4 , line 30 , afte r "digital " insert --band--.
`
`Signed and Sealed this
`
`Eleventh Day of February, 1992
`
`Attest:
`
`HARRY F. MANBECK. JR.
`
`Attesting Officer
`I
`Commissioner of Paumts and TrrJdemar/cs
`~---------------------------------------------------------------~
`
`008
`
`