US 6,912,413 B2
`(10) Patent No:
`a2) United States Patent
`Rantalaet al.
`(45) Date of Patent:
`Jun. 28, 2005
`
`
`US006912413B2
`
`3/2002 Bernsteinetal. ........... 600/323
`2/2004 AI-Ali oe... ec eeeeeeeeeeeee 600/323
`
`
`.- 600/323
`3/2004 Mortz .......
`
`...eessseseeeees 600/473
`5/2004 Turcott
`1/2004 Huanget al... 600/300
`
`(54) PULSE OXIMETER
`
`(75)
`
`Inventors: Bérje Rantala, Helsinki (FI); Aki
`Backman, Helsinki (FI)
`,
`(73) Assignee: GE Healthcare Finland OY (FI)
`(*) Notice:
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`(21) Appl. No.: 10/661,012
`(22)
`Filed:
`Sep. 12, 2003
`(65)
`Prior Publication Data
`US 2004/0054269 A1 Mar. 18, 2004
`
`(60)
`
`6,356,774 B1 *
`6,697,658 B2
`6,714,803 B1 *
`6,731,967 B1 *
`2004/0002637 Al *
`* cited by examiner
`.
`.
`.
`.
`Prmary Examiner Eric F. Winakur
`Assistant Examiner—Matthew Kremer
`(74) Attorney, Agent, or Firm—Marsh Fischmann &
`Breyfogle LLP
`ABSTRACT
`67)
`The invention relates to pulsed oximeters used to measure
`blood oxygenation. The current
`trend towards mobile
`oximeters has brought the problem of how to minimize
`power consumption without compromising on the perfor-
`mance of the device. To tackle this problem, the present
`Related U.S. Application Data
`invention provides a methodfor controlling optical power in
`Provisional application No. 60/410,526, filed on Sep. 13,
`a pulse oximeter. The signal-to-noise ratio of the received
`2002.
`(51) Unt, C1 iceeccccccceccccceceseeessecsssessseeesees A61B 5/00—_baseband signal is monitored, and the duty cycle of the
`(52)
`UWS. Ch.
`ceececceccccstecssesssteseesssteneeen 600/322; 600/330
`_—-Ariving pulsesis controlled in dependence on the monitored
`(58) Field of Search oo... cece 600/322-324,
`signal-to-noise ratio, preferablysothat the optical Power 1S
`600/330
`minimized within the confines of a predetermined lower
`threshold set for the signal-to-noise ratio. In this way the
`optical power is made dependenton the perfusion level of
`the subject, whereby the power can be controlled to a level
`which does not exceed that needed for the subject.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`4,859,057 A *
`8/1989 Taylor et al. 0... 600/323
`5,348,004 A *
`9/1994 Hollub oe 600/323
`
`32 Claims, 3 Drawing Sheets
`
`
`
`SAMPLING CONTROL
`
`CONTROL OF PULSE WIDTH AND/OR REPETITION RATE
`
`
`
`
`R
`
`15
`
`
`
` 16
`
`
`
`
`A
`AMPLIFIER ADJUSTMENT, BANDWIDTH CONTROL
`
`IR
`SAMPLING
`
`1
`
`APPLE 1022
`
`APPLE 1022
`
`1
`
`

`

`U.S. Patent
`
`Jun. 28, 2005
`
`Sheet 1 of 3
`
`US 6,912,413 B2
`
`SAMPLING CONTROL
`
`CONTROL OF PULSE WIDTH AND/OR REPETITION RATE
`
`
`a AMPLIFIER ADJUSTMENT, BANDWIDTH CONTROL
`
`
`
`EXAMINE
`FIG. 2
`SIGNAL-TO-NOISE RATIO
`
`OF DEMODULATED
`BASEBAND SIGNAL
`
`OPTICAL POWER
`
`
`
`
`
`DECREASE
`OPTICAL POWER
`
`MAINTAIN
`
`2
`
`

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`U.S. Patent
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`Jun. 28, 2005
`
`Sheet 2 of 3
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`US 6,912,413 B2
`
`ms
`
`~~~a—>200 }15
`
`—__} Monutation BY HEART RATE
`
`PULSE AMPLITUDE
`
`RED
`
`JRED
`
`RED
`
` IRED
`
`mee
`
`SUM OF PEAKS AT1, 3, AND 5 KHz
`2 NORMALIZED BY DIVIDING BY OC LEVEL
`
`SUMOF NOISE 170 5 kHz
`
`} 1
`
`Hz
`
`FIG. 3d
`
`3
`
`

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`U.S. Patent
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`Jun. 28, 2005
`
`Sheet 3 of 3
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`US 6,912,413 B2
`
`10 ms
`
`ei 20 15
`
`PULSE AMPLITUDE
`
`| MODULATION BY HEART RATE
`
`
`
`
`
`RED—JRED (RED IRED
`
`
`
`—_———— —
`
`DC LEVEL
`
`PREFILTER
`
`Koff
`
`At
`
`SUM OF ALL HARMONICS
`AND NOISE
`
`
`
`FIG. 4d
`
`4
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`US 6,912,413 B2
`
`1
`PULSE OXIMETER
`
`RELATED APPLICATION
`
`This application claims priority under 35 U.S.C. § 119 to
`prior U.S. Provisional Patent Application No. 60/410,526,
`filed Sep. 13, 2002, entitled “PULSE OXIMETER’”,
`the
`entire contents of which are incorporated herein as if set
`forth herein in full.
`
`FIELD OF THE INVENTION
`
`The invention relates generally to devices used for non-
`invasively determining the amount of at
`least one light
`absorbing substancein a subject. These devices are typically
`pulse oximeters used to measure the blood oxygenation of a
`patient. More specifically, the invention relates to the opti-
`mization of power consumption in such a device.
`BACKGROUND OF THE INVENTION
`
`Pulse oximetry is at present the standard of care for the
`continuous monitoring of arterial oxygen saturation (SpO,).
`Pulse oximeters provide instantaneous in-vivo measure-
`ments of arterial oxygenation, and thereby provide early
`warning of arterial hypoxemia, for example.
`A pulse oximeter comprises a computerized measuring
`unit and a probe attached to the patient, typically to his or her
`finger or ear lobe. The probe includes a light source for
`sending an optical signal through the tissue and a photo
`detector for receiving the signal after transmission through
`the tissue. On the basis of the transmitted and received
`
`signals, light absorption by the tissue can be determined.
`During each cardiac cycle, light absorption by the tissue
`varies cyclically. During the diastolic phase, absorption is
`caused by venous blood,
`tissue, bone, and pigments,
`whereas during the systolic phase, there is an increase in
`absorption, which is caused by the influx of arterial blood
`into the tissue. Pulse oximeters focus the measurement on
`
`this arterial blood portion by determining the difference
`between the peak absorption during the systolic phase and
`the constant absorption during the diastolic phase. Pulse
`oximetry is thus based on the assumption that the pulsatile
`component of the absorption is due to arterial blood only.
`Light transmission through an ideal absorbing sample is
`determined by the known Lambert-Beer equation as follows:
`DC
`Tour=Tin€
`>
`
`@)
`
`whereI, is the light intensity entering the sample, I,,,, is
`the light intensity received from the sample, D is the path
`length through the sample, e€
`is the extinction coefficient of
`the analyte in the sample at a specific wavelength, and C is
`the concentration of the analyte. When I,,, D, and €
`are
`out
`known and I,,,,
`is measured, the concentration C can be
`calculated.
`In pulse oximetry, in order to distinguish between the two
`species of hemoglobin, oxyhemoglobin (HbO,), and deoxy-
`hemoglobin (RHb), absorption must be measured at two
`different wavelengths, i.e. the probe includes two different
`light emitting diodes (LEDs). The wavelength values widely
`used are 660 nm (red) and 940 nm (infrared), since the said
`two species of hemoglobin have substantially different
`absorption values at these wavelengths. Each LED is illu-
`minated in turn at a frequency which is typically several
`hundred Hz.
`
`The accuracy of a pulse oximeter is affected by several
`factors. This is discussed briefly in the following.
`Firstly, the dyshemoglobins which do not participate in
`oxygen transport, i.e. methemoglobin (MetHb) and carboxy-
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`hemogiobin (CoHb), absorb light at the wavelengths used in
`the measurement. Pulse oximeters are set up to measure
`oxygen saturation on the assumption that the patient’s blood
`composition is the same as that of a healthy, non-smoking
`individual. Therefore, if these species of hemoglobin are
`present in higher concentrations than normal, a pulse oxime-
`ter may display erroneous data.
`Secondly, intravenous dyes used for diagnostic purposes
`may cause considerable deviation in pulse oximeter read-
`ings. However, the effect of these dyes is short-lived since
`the liver purifies blood efficiently.
`Thirdly, coatings such as nail polish may in practice
`impair the accuracy of a pulse oximeter, even though the
`absorption caused by them is constant, not pulsatile, and
`thus in theory it should not have any effect on the accuracy.
`Fourthly, the optical signal may be degraded by both noise
`and motion artifacts. One source of noise is the ambient light
`received by the photodetector. Many solutions have been
`devised with the aim of minimizing or eliminating the effect
`of the movementof the patient on the signal, and the ability
`of a pulse oximeter to function correctly in the presence of
`patient motion depends on the design of the pulse oximeter.
`One way of canceling out the motion artifact is to use an
`extra wavelength for this purpose.
`One of the current trends in pulse oximetry is the aim
`towards lower power consumption, which is essential for
`battery-operated oximeters, for example. These oximeters
`are typically mobile and must therefore be used in various
`locations where both the characteristics of the patient and
`the surrounding measurement environment may vary. A
`problem related to these various measurement conditions is
`the optimization of power consumption without compromis-
`ing the performance of the device, i.e. how to guarantee
`reliable measurementresults even in difficult measurement
`conditions andstill keep the battery life as long as possible.
`The current straightforward solution for obtaining reliable
`measurementresults under tough measurementconditions is
`to increase the driving power of the LEDs. This approachis
`based on the transmittance of the tissue: if the level of the
`signal transmitted through the tissue is not enough to guar-
`antee reliable results, the level of the transmitted signal (i.e.
`the amplitude of the pulse train) is increased until the level
`of the signal receivedis sufficient. This is naturally contrary
`to the need to save power.
`It is an objective of the invention to bring about a solution
`by meansof whichit is possible to dynamically optimize the
`power consumption in a pulse oximeter, especially in a
`portable battery-operated pulse oximeter, and to maintain
`good performance even in tough measurement conditions,
`where the transmittance and/or the perfusion level, as indi-
`cated by the normalized pulsatile component, are low.
`SUMMARYOF THE INVENTION
`
`These and other objectives of the invention are accom-
`plished in accordance with the principles of the present
`invention by providing a power-saving scheme which allows
`the pulse oximeter to use no more power than that which is
`neededto drive the emitters while maintaining good perfor-
`mance of the oximeter. In this scheme, the signal-to-noise
`requirements are compromised in favor of power
`consumption, as long as this does not compromise measure-
`mentreliability.
`According to the invention, the patient-specific effect of
`the tissue on the measurement result is taken into account,
`whereby the optical power, i.e. the power supplied to acti-
`vate the emitters, can be controlled to a level which is no
`more than what is needed for each measurement. The idea
`
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`US 6,912,413 B2
`
`3
`behind the invention is that the measurementresults of the
`pulse oximeter depend on the perfusion level and on the
`transmittance of the tissue underillumination. Therefore, the
`optical power in the pulse oximeter of the invention can be
`controlled at each measurement occasion to a level which is
`
`in question, by
`the patient
`the minimum sufficient for
`monitoring the demodulated baseband signal indicative of
`the perfusion level of the patient and controlling the duty
`cycle of the driving pulses so that the optical power is
`minimized given a predetermined lowerthreshold set for the
`signal-to-noise ratio.
`The invention provides other ways to reach the level
`which is enough at each time, said ways being applicable
`alone or in combination. These ways can also be combined
`with the normal increase of the amplitude of the driving
`pulses.
`Under favorable conditions the requirements for the
`signal-to-noise ratio of the pulse oximeter are eased in favor
`of power consumption, and the optical power is dropped to
`the minimum level sufficient for measurement. Poweris then
`
`increased only when the minimum signal-to-noise ratio
`ensuring a reliable measurementis not otherwise reached.
`Thus, one aspect of the invention is providing a method
`for controlling optical power in a monitoring device
`intended for determining the amount of at least one light
`absorbing substance in a subject,
`the monitoring device
`comprising
`emitters for emitting radiation at a minimum of two
`wavelengths,
`driving meansfor activating said emitters, and
`a detector for receiving said radiation at said wavelengths
`and for producing an electrical signal in responseto the
`radiation,
`the method comprising the steps of
`supplying driving pulses from said driving meansto the
`emitters, the pulses having predetermined characteris-
`tics determining the optical power of the device,
`demodulating the electrical signal originating from said
`detector, whereby a basebandsignal is obtained,
`monitoring a signal-to-noise ratio of the basebandsignal,
`and
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`4
`responsive to the monitoring
`power control means,
`means, for controlling the duty cycle of the driving
`pulses.
`The power control means are preferably adapted to per-
`form the controlling of the duty cycle so that the optical
`poweris minimized within the confines of a predetermined
`lower threshold set for the signal-to-noise ratio.
`The power control scheme of the present invention pro-
`vides good performance even when the tissue is thick
`(requiring high drive current) and the perfusion (pulsatility)
`is low. In this case the front stage of the pulse oximeter tends
`to saturate, whereby a conventional pulse oximeter can no
`longer operate in a reliable way.
`In contrast,
`the pulse
`oximeter of the invention maystill obtain reliable readings
`by widening the pulses or increasing the pulse repetition
`rate, thereby increasing the signal-to-noise ratio.
`Other features and advantages of the invention will
`become apparent by reference to the following detailed
`description and accompanying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`In the following, the invention and its preferred embodi-
`ments are described more closely with reference to the
`examples shownin FIG. 1 to 4d in the appended drawings,
`wherein:
`
`FIG. 1 illustrates a typical embodimentof a pulse oxime-
`ter according to the present invention,
`FIG. 2 is a flow diagram illustrating one embodiment of
`the power control scheme of the present invention,
`FIG. 3a illustrates the timing sequence of the detector
`signal when a high duty cycle pulse sequence is used,
`FIGS. 3b and 3c illustrate the frequency spectrum of a
`detector signal according to FIG. 3a,
`FIG. 3d illustrates the spectrum of the baseband signal
`obtained from the signal of FIG. 3b after demodulation,
`FIG. 4a illustrates the timing sequence of the detector
`signal when a low duty cycle pulse sequence is used,
`FIGS. 4b and 4c illustrate the frequency spectrum of a
`detector signal according to FIG. 4a, and
`FIG. 4d illustrates the spectrum of the baseband signal
`obtained from the signal of FIG. 4b after demodulation.
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`controlling the duty cycle of the driving pulses in depen-
`dence on the monitored signal-to-noiseratio.
`The duty cycle is preferably controlled so that the optical
`FIG. 1 is a block diagram of one embodimentof a pulse
`poweris minimized within the confines of a predetermined
`oximeter according to the present invention. This embodi-
`lower threshold set for the signal-to-noise ratio.
`ment is based onatraditional pulse oximeter where syn-
`Another aspect of the invention is that of providing an
`50
`chronousdetection is used. At least two different LEDs, A
`apparatus for non-invasively determining the amountofat
`and B, are driven by a LED drive 10. Each LED operates at
`least one light absorbing substance in a subject, the appa-
`a respective wavelength, and the light emitted by the LEDs
`ratus comprising
`passes into patient tissue, such as a finger 11. The light
`emitters for emitting radiation at a minimum of two
`propagated through or reflected from the tissue is received
`different wavelengths,
`by a probe 12 including a photodetector. The photodetector
`driving means for activating said emitters, adapted to
`converts the optical signal received into an electrical signal
`supply driving pulses to the emitters, the pulses having
`and supplies it to an amplifier stage 13, which includes a
`predetermined characteristics determining currentopti-
`controllable preamplifier 13a and a variable low-passfilter
`cal powerof the device,
`13b. After the amplifier stage, an analog switch 14, con-
`a detector for receiving said radiation at said wavelengths
`trolled by the control unit 18, ensures that the signal is
`and producing an electrical signal in response to the
`zeroed between consecutive pulses, thereby removing back-
`radiation,
`ground light. The reception branch is then divided into two
`a demodulator unit for demodulating the electrical signal
`branches: the IR branch for the infrared signal and the R
`originating from said detector, whereby a baseband
`branch for the red signal. Each branch is preceded by an
`signal is obtained from the demodulator unit,
`analog switch (not shownin the figure), which is controlled
`monitoring means for monitoring a signal-to-noise ratio
`by the control unit 18 so that the pulses are connectedto their
`respective branch (the R pulses to the R branch and the IR
`of the baseband signal, and
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`US 6,912,413 B2
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`5
`pulsesto the IR branch). In each branch a sampling unit (15,
`16) then takes samples of the pulses received by the branch.
`The control unit controls the R sampling unit so that it
`samples the R pulses and the IR sampling unit so that it
`samples the IR pulses. The sampling units typically include
`a sampling switch and a capacitor charged to the pulse
`voltage prevailing at the sampling moment. The sampled
`signals are then supplied to an A/D converter 17, which
`converts them into digitized format for the control unit 18.
`The synchronous detection performed in the sampling units
`15 and 16 is also termed “demodulation” in this context,
`since it is the operation which extracts the original modu-
`lating signal from the detector signal.
`In order to introduce the power-controlling schemeof the
`invention into a pulse oximeter of the type shownin FIG. 1,
`the pulse oximeter structure is modified so that the control
`unit 18 monitors the baseband signal-to-noiseratio (i.e. the
`signal-to-noise ratio of the demodulated baseband signal)
`and selects the optical power in dependence on the moni-
`tored ratio. The power consumption is minimized
`dynamically, so that when the monitored signal-to-noise
`ratio is low, the control unit starts to compromise on power
`consumption in favor of performance,
`thereby ensuring
`reliable measurementresults. As discussed below, minimiz-
`ing power consumption involves changing at
`least one
`parameter of the duty cycle of the pulse train driving the
`LEDs so that
`the optical power changes in the desired
`direction. The parameters include the pulse width and the
`pulse repetition rate. When the control unit decreases the
`pulse width, it simultaneously widens the bandwidth of the
`low-passfilter 135 to allow the pulse to reach essentially its
`full height. Whenthe control unit increases the pulse width,
`it simultaneously narrows the bandwidth of the low-pass
`filter to decrease the amountof input noise. In addition to the
`pulse width and/or the repetition rate, the pulse amplitude
`can also be controlled.
`
`FIG. 2 illustrates one embodiment of the power control
`scheme. It is assumed here that the power control schemeis
`implemented in the pulse oximeter of FIG. 1. As discussed
`above, the control unit first defines the signal-to-noise ratio
`of the demodulated baseband signal (step 21) and compares
`the ratio to a first threshold, which defines the lowerlimit of
`an acceptable signal-to-noise ratio (step 22). If the current
`ratio is below the first ratio, the control unit increases the
`optical power by changing the duty cycle of the pulse train
`(step 23), and the process returns to step 21 to define the
`signal-to-noise ratio associated with the new characteristics.
`If it is detected at step 22 that the signal-to-noise ratio is
`abovethefirst threshold, it is examined at step 24 whether
`the signal-to-noise ratio is also above the second threshold,
`whichis slightly higher than the first threshold. If this is not
`the case, but
`the ratio is between the first and second
`thresholds, the current characteristics of the pulse train are
`maintained, i.e. the optical poweris maintainedatits current
`value (step 25). If it is detected at step 24 that the signal-
`to-noise ratio is above a second threshold, the duty cycle of
`the pulsetrain is changed at step 26 so that the optical power
`is decreased. The process then returns to step 21 to define the
`signal-to-noise ratio associated with the new duty cycle of
`the pulse train.
`The optical power can be increased in several waysat step
`23. The first method is to increase the pulse width, while
`simultaneously decreasing the bandwidth of the low-pass
`filter 135, which thereby decreases the amount of input
`noise. The second methodis to increase the pulse repetition
`rate in order to decrease noise aliasing, i.e. to decrease the
`numberof harmonics being down-converted by the synchro-
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`the
`nous demodulation. In addition to these operations,
`current or voltage of the pulses driving the LEDs can be
`increased.
`
`Accordingly, the optical power can be decreased in sev-
`eral ways at step 26, whenever it
`is detected that
`the
`signal-to-noise requirements can easily be met. Thefirst
`method is to narrow the pulses, simultaneously increasing
`the bandwidth of the low-passfilter 13b, thereby allowing
`the pulses to reach approximately their full height. The
`second methodis to use a lower pulse repetition rate, which
`allows more aliasing of interference/noise in the demodu-
`lation phase due to a lower sampling rate and thus degrades
`the signal-to-noise ratio on the baseband. The above opera-
`tions can be used alone or in combination to decrease the
`optical power. In addition to these operations, the current or
`voltage of the pulses driving the LEDs can be decreased. It
`is to be understood that steps 23 and 26 include the control
`of the bandwidth associated with the control of the pulse
`width.
`
`FIG. 3a to 3d illustrate noise aliasing in a conventional
`high duty cycle oximeter which uses LED pulses having a
`duty cycle greater than 10%. The power control scheme of
`the present invention uses a high duty cycle pulse train only
`when the desired signal-to-noise ratio cannot otherwise be
`reached,1.e. the situation of FIG. 3a to 3d is entered at step
`23 in FIG. 2. FIG. 4a to 4d correspond to FIG. 3a to 3d,
`respectively, except that in FIG. 4a to 4d the pulse oximeter
`is a narrow pulse oximeter where the LEDsare activated as
`briefly as possible in order to save power. This powersaving
`mode is entered whenever conditions permit easing the
`signal-to-noise requirements in favor of power consumption.
`As to the example of FIG. 2, the power saving mode is
`entered in step 26, and the mode is maintained in step 25.
`It is assumedhere that (1) in the high duty cycle mode the
`pulse width equals 200 us and the pulse repetition rate f, is
`equal to 1 kHz,i.e. the time period between two consecutive
`pulses is 1 ms, and (2) in the power saving modethe pulse
`width is equal to 20 us and the pulse repetition rate f, equals
`100 Hz. FIGS. 3a and 4a show the timing sequencesof the
`detector signal in the respective modes, whereas FIGS. 3b
`and 4b illustrate the frequency spectrum of the detector
`signal in the respective modes. FIGS. 3a@ and 4a also show
`the amplitude modulation appearing in the pulse train at the
`heart rate of the patient. FIGS. 3c and 4c show in moredetail
`the part of the spectrum denoted by circles Ain FIGS. 3b and
`4b, respectively.
`the
`As can be seen from FIGS. 3b, 3c, 4b, and 4c,
`spectrum comprises a main peak at
`the pulse repetition
`frequency and harmonic peaks at the odd harmonic frequen-
`cies of the repetition rate. Side peaks SP caused by the
`above-mentioned amplitude modulation appear around the
`main and harmonic peaks. The frequency deviation between
`a side peak and the associated main or harmonic peak
`corresponds to the heart rate, which is in this context
`assumed to be 1 Hz.
`
`FIGS. 3d and 44dillustrate the frequency spectrum of the
`baseband signal in the above-mentioned two modes,i.e. the
`frequency spectrum of the signal after synchronous detec-
`tion. The aliased peaks contribute to the amplitude Al of the
`signal at the heart rate, whereas the surrounding noise level
`A2 is determined by the noise aliased on the whole band
`(FIG. 3d). The amplitude of the baseband signal (A1)
`indicates the perfusion level of the patient, but the quantity
`to be controlled is the basebandsignal-to-noise ratio, which
`is directly dependent on the signal amplitude, i.e. on the
`perfusion level.
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`US 6,912,413 B2
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`7
`As to the power saving modeof FIG. 4a to 4d, narrowing
`the pulses and lowering their repetition rate has two conse-
`quences: the narrow pulses require the preamplifier to have
`a wide bandwidth, and the harmonic content of the detector
`signal is high (cf. FIG. 4b). When demodulating the narrow
`pulses, all harmonic components of the sampler are folded
`into the baseband. Therefore the noise level (A2) on the
`baseband (FIG. 4d) is higher than in the high duty cycle
`(FIG. 3d). In a sense the pulse harmonics belong to the
`“payload signal”, since they contribute to the amplitude Al
`of the signal at the heart rate, whereas the noise coming from
`the detector, the preamplifier, or other sources do not.
`The dashed lines in FIGS. 3b and 4b6illustrate the pass-
`band of the low-pass filter 13b contained in the amplifier
`stage, the passband being controlled by the control unit 18
`in the above-described manner in association with the con-
`trol of the pulse width. The actual width of the passband
`depends on many factors. However, the passband width is
`always kept at a value which allows the reception of a
`sufficient amount of pulse energy. As can be seen in FIG. 3c
`and 4c, the wider the pulses are the steeper the decline in
`harmonic amplitude.
`It was assumed above that the pulse oximeter is a con-
`ventional pulse oximeter based on synchronousdetection in
`the sampling units. However, the power control scheme of
`the invention can also be used with other types of pulse
`oximeters, for example, in a known oximeter based on fast
`A/D conversion.
`Although the invention was described above with refer-
`ence to the examples shown in the appended drawings,it is
`obviousthat the inventionis not limited to these, but may be
`modified by those skilled in the art without departing from
`the scope and spirit of the invention. For example, the pulse
`oximeter can be provided with more than two wavelengths
`and with auxiliary means for eliminating external
`interference, such as motionartifact. The numberofdistinct
`powerlevels depends on the implementation and can vary to
`a great extent. For example, the number of possible pulse
`width values depends on the resolution of the pulse width
`modulator used. Furthermore, the method can also be used
`in devices other than pulse oximeters, devices measuring
`other substances in a similar manner, i.e. non-invasively by
`radiating the patient. An example of such measurementis
`non-invasive optical monitoring of glucose orbilirubin, or
`simply an optical pulse rate monitor.
`Whatis claimedis:
`1. Amethod for controlling optical power in a monitoring
`device intended for determining the amountof at least one
`light absorbing substance in a subject, the monitoring device
`comprising
`emitters for emitting radiation at a minimum of two
`wavelengths
`driving meansfor activating said emitters, and
`a detector for receiving said radiation at said wavelengths
`and for producing an electrical signal in responseto the
`radiation,
`the method comprising the steps of
`supplying driving pulses from said driving meansto the
`emitters, the pulses having predetermined characteris-
`tics determining the optical power of the device,
`demodulating the electrical signal originating from said
`detector whereby a baseband signal is obtained;
`transforming the baseband signal into a frequency spec-
`trum to identify an amplitude and a noise level of the
`baseband signal, whereby a signal-to-noise ratio of the
`amplitude to the noise for
`the baseband signal
`is
`obtained;
`
`5
`
`10
`
`20
`
`25
`
`30
`
`35
`
`40
`
`50
`
`60
`
`65
`
`8
`monitoring the signal-to-noise ratio of the baseband
`signal, and
`controlling the duty cycle of the driving pulses in depen-
`dence on the monitored signal-to-noiseratio.
`2. A method according to claim 1, wherein said control-
`ling step includes controlling the duty cycle of the driving
`pulses so that the signal to noise ratio is maintained within
`the confines of a predetermined range for the signal-to-noise
`ratio.
`3. A method according to claim 2, wherein said control-
`ling step further includes comparing the monitored signal-
`to-noise ratio to said predetermined range, said predeter-
`mined range being defined by a predetermined lower
`threshold and a predetermined higher threshold.
`4. A method according to claim 3, further including the
`step of connecting the electrical signal originating from said
`detector through a preamplifier and a low-pass-filter prior to
`said demodulating step.
`5. A method according to claim 4, wherein said control-
`ling step includes
`least one operation in response to said
`performing at
`signal-to-noise ratio reaching said lower threshold, the
`said at least one operation being selected from a group
`of operations including (1) the increase of the width of
`said pulses and (2) the increase ofpulse repetition rate,
`and
`
`decreasing the bandwidth of said low-passfilter when the
`width of said pulses is increased.
`6. Amethod according to claim 5, wherein the controlling
`step further includes the step of increasing the amplitude of
`said driving pulses.
`7. Amethod according to claim 4, wherein said control-
`ling step includes
`selecting at least one operation in responseto said signal-
`to-noise ratio reaching said higher threshold, the said at
`least one operation being selected from a group of
`operations including (1) the decrease of the width of
`said pulses and (2) the decrease of pulse repetition rate,
`and
`
`increasing the bandwidth of said low-passfilter when the
`width of said pulses is decreased.
`8. A method according to claim 7, wherein the controlling
`step further includes the step of decreasing the amplitude of
`said driving pulses.
`9. A method according to claim 1, wherein said demodu-
`lating step includes sampling of the electrical signal by a
`synchronousdetector, taking one sample per each pulse of
`the electrical signal.
`10. Amethod according to claim 1, wherein the amount of
`at least one light absorbing substance is determined in the
`blood of a subject.
`11. A method according to claim 1, wherein the monitor-
`ing device is a pulse oximeter.
`12. An apparatus for non-invasively determining the
`amountofat least one light absorbing substance in a subject,
`the apparatus comprising
`emitters for emitting radiation at a minimum of two
`different wavelengths,
`driving means for activating said emitters, adapted to
`supply driving pulses to the emitters, the pulses having
`predetermined characteristics determining currentopti-
`cal powerof the device,
`a detector for receiving said radiation at said wavelengths
`and producing an electrical signal in response to the
`radiation,
`a demodulator unit for demodulating the electrical signal
`originating from the detector, whereby a baseband
`signal is obtained from the demodulator unit,
`
`8
`
`

`

`US 6,912,413 B2
`
`9
`
`monitoring meansfor:
`transfoming the basebandsignal into a frequency spec-
`trum;
`generating a signal-to-noise ratio of the transformed
`baseband signal; and
`monitoring the signal-to-noise ratio of the baseband
`signal, and
`power control means, responsive to the monitoring
`means, for controlling the duty cycle of the driving
`pulses.
`13. An apparatus according to claim 12, wherein the
`powercontrol meansare adapted to control the duty cycle so
`that the signal-to-noise ratio is maintained within a prede-
`termined range between a first
`threshold and a second
`threshold.
`
`14. An apparatus according to claim 13, further compris-
`ing a low-passfilter for filtering said electrical signal prior
`to said demodulating, the control means comprising at least
`onesetoffirst and second means, whereinthe first means are
`adapted to change the width of said pulses and of the
`passband of the low-passfilter, and the second meansare
`adapted to increase pulse repetition rate.
`15. An apparatus according to claim 14, wherein the
`control means further comprise means for changing the
`amplitude of said pulses.
`16. An apparatus according to claim 13, wherein said
`apparatus is a pulse oximeter.
`17. A method for controlling optical power in a monitor-
`ing device intended for determining the amountofat least
`one light absorbing substance in a subject, the monitoring
`device comprising
`emitters for emitting radiation at a minimum of two
`wavelengths,
`driving meansfor activating said emitters, and
`a detector for receiving said radiation at said wavelengths
`and for producing an electrical signal in responseto the
`radiation