US005924979A
`5,924,979
`(114) Patent Number:
`United States Patent 55
`Swedlowet al.
`[45] Date of Patent:
`*Jul. 20, 1999
`
`
`[54] MEDICAL DIAGNOSTIC APPARATUS WITH
`SLEEP MODE
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`[75]
`
`Inventors: David B. Swedlow, Danville; Michael
`J. Bernstein, San Ramon; Charles E.
`Porges, Orinda; James E. Luecke,
`Dublin; Michael W. Nootbaar, Benicia,
`all of Calif.
`
`[*] Notice:
`
`.
`.
`.
`.
`.
`This patent is subject to a terminal dis-
`claimer.
`
`[21] Appl. No.: 08/919,540
`.
`Filed:
`
`Aug. 28, 1997
`
`[22]
`
`8/1993 Gallant et al. oes 600/513
`5,238,001
`
`1/1994 Hankinson et al.
`wo... 600/537
`5,277,196
`5/1998 Swedlow et al. oe
`eeeeee 600/323
`5,746,697
`5/1998 Flach et al. beesee cee cesses eee seseeeees 600/509
`5,748,103
`Primary Examiner—Cary O’ Connor
`Assistant Examiner—Eric F. Winakur
`[73] Assignee: Nellcor Puritan Bennett Incorporated,|Attorney, Agent, or Firm—Townsend and Townsend and
`Pleasanton, Calif.
`Crew LLP
`[57]
`ABSTRACT
`A method and apparatus for conserving power in a medical
`diagnostic apparatus by using a sleep mode during a moni-
`toring state. The sleep mode allowsnot only the processorto
`be put to sleep, but other detection circuitry as well. This is
`accomplished by not relying on detecting events to awaken
`the sleeping circuitry, but rather establishing the stability of
`a physiological parameter before going to sleep. The inven-
`tion monitors a physiological parameter of the patient and
`enters a sleep mode only after it has been stable for a
`Related U.S. Application Data
`predetermined period of time. The apparatus is periodically
`oo,
`i
`awakened from the sleep mode to take additional measure-
`coonTuationinpailofapplication No. 08/599,255, Feb. 9,
`[63]
`
`
`
`2 INO9LODE”EE ments and to ascertain that the stability of the physiological
`[51]
`Tint, Cd ccc ccccssseeecesssseessssseneeesssseees A61B 5/00
`parameter has not changed. In one embodiment, the sleep
`[52] U.S. Ch. caececcssecsseeesneen 600/300; 600/310; 600/323
`Period is chosen to be consistent with the period in which an
`[58] Fleld of SearCh sense 6901300, 310,
`armconditionwouldneedtobegenerated if a patent’
`
`600/322, 323, 324, 326, 333, 336, 483;
`607/16
`
`17 Claims, 4 Drawing Sheets
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`U.S. Patent
`
`Jul. 20, 1999
`
`Sheet 1 of 4
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`5,924,979
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`

`

`U.S. Patent
`
`Jul. 20, 1999
`
`Sheet 2 of 4
`
`5,924,979
`
`fA
`
`Automatic Sleep
`Started
`
`Cc
`
`Sleep for
`1 second
`
`Rate values
`
`Sensor
`Attached
`
`fH
`
`Read SAT & rate for
`next
`two complete pulses
`
`"two pulses
`unchanging from
`baseline
`values test"
`passes
`
`FIG. 2
`
`? A
`
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` Save baseline SAT and
`
`Sleep for 20 seconds
`
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`?
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`
`Collect 20 secs
`worth of SAT & rate
`
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`SAT & rate
`values stable
`test”
`passes
`
`
`
`
`
`
`Sleep for 20 seconds
`
`
`3
`
`

`

`U.S. Patent
`
`Jul. 20, 1999
`
`Sheet 3 of 4
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`5,924,979
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`
`
`FIG. 3
`
`4
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`

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`U.S. Patent
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`Jul. 20, 1999
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`Sheet 4 of 4
`
` MODULE
`
`aEs4Ed=BiSiS3
`INPUT MODULE
`
`5,924,979
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`150
`
`FIG.
`
`4,
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`5
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`

`

`5,924,979
`
`1
`MEDICAL DIAGNOSTIC APPARATUS WITH
`SLEEP MODE
`
`This application is a continuation-in-part of application
`Ser. No. 08/599,255, filed Feb. 9, 1996, now U.S. Pat. No.
`5,746,697.
`
`BACKGROUND OF THE INVENTION
`
`invention relates to medical diagnostic
`The present
`devices, and particularly to battery-powered pulse oxime-
`ters.
`
`10
`
`2
`Sleep-mode techniques have been used in other
`technologies, notably for lap-top computers which run on
`batteries. In a typical sleep-mode, power to certain compo-
`nents of the computer is turned off, or they are slowed down
`by reducing the clock speed, to reduce power consumption.
`Typically, these take advantage of the fact that a computer
`user is not always using the computer. Thus, for instance,
`circuitry in the computer may detect
`the absence of a
`keystroke for a certain amount of time, and enter a sleep
`mode in response. Enough circuitry is left on to detect an
`interrupt due to a keystroke, and the rest of the circuitry is
`awakened on such an occurrence. Certain microprocessors
`have a sleep or standby mode built in, with some micropro-
`cessors being able to be shut down completely, while others
`accept a vastly reduced clock speed. The microprocessor
`will automatically save the state of its registers and take any
`other action necessary to be able to resume from whereit left
`off in its program.
`Certain aspects of a computer system may be required to
`have power supplied to them constantly. For instance,
`DRAM memoryis required to be periodically refreshed in
`order to save the memory contents. Other types of memory
`which are non-volatile, and do not require refreshing, are
`available. However, non-volatile memory is typically much
`more expensive, and thus there is a cost/power savings
`trade-off.
`
`The *972 discusses a sleep mode in which the micropro-
`cessor is put to sleep, but not the other circuitry for detecting
`physiological events. The microprocessor can then be awak-
`ened either by a timer or upon the detection of a psycho-
`logical event. When the microprocessor awakens,
`it can
`examine any events which may bestored or latched that
`occurred while the microprocessor wasasleep.
`SUMMARYOF THE INVENTION
`
`The present invention provides a method and apparatus
`for conserving power in a medical diagnostic apparatus by
`6
`
`Diagnostic monitors are used to monitor various physi-
`ological parameters of a patient. In particular, such monitors
`are used for heart rate, respiration rate, blood pressure,
`temperature, and arterial oxygen saturation. Pulse oximeters,
`for example, illustrate the different aspects of such monitors,
`and are used as an example herein.
`Pulse Oximeters are commonly used to monitor the level
`of oxygen in the blood of a patient. This is particularly
`critical during an operation, or during post-operation recov-
`ery. In addition, pre-delivery monitoring of the oxygen in a
`fetus provides important information. A typical oximeter
`directs light to the skin of a patient, with either transmitted
`or reflected scattered light being measured by a light detec-
`tor. The amountof light detected will be diminished by the
`amount of light absorbed by the oxygen in the patient’s
`Sleep mode techniques have also been used in other
`blood. By using appropriate wavelengths of light emitters,
`technologies. For example, U.S. Pat. No. 4,716,463 dis-
`and their knownabsorption characteristics along with appro-
`cusses the use of batteries to keep a television powered
`priate mathematical algorithms, the oxygen saturation of a
`during a powerfailure. Sleep mode is entered automatically
`patient can be monitored.
`upon detection of a power failure. U.S. Pat. No. 5,142,684
`Because of the need to quickly react to a change in a
`discusses the use of a sleep mode in a portable bar code
`patient’s condition, it is oftentimes important for a pulse
`reader. U.S. Pat. No. 4,924,496 discusses the use of a sleep
`oximeter to be in a continuous monitoring mode, with alarm
`mode in a telephone with incoming telephone call number
`limits set to generate an alarm in case the patient exceeds
`display capability.
`certain parameters. It is also desirable to be able to provide
`A numberof patents discuss various uses of a sleep mode
`a portable pulse oximeter sothat it can be moved from room
`in an implantable device such as a pacemaker. Examples
`to room without requiring it to be plugged in to a power
`include U.S. Pat. No. 4,554,920, U.S. Pat. No. 4,561,442,
`outlet. In such a portable pulse oximeter, power consump-
`and U.S. Pat. No. 4,856,524. Clearly,
`in an implantable
`40
`tion is of concern, andit is desirable to minimize the power
`device which is required to run onabattery, extending
`consumption. Such a portable oximeter may not only con-
`battery life is important so that another operation is not
`sume powerin performing the measurements, but might also
`necessary to remove and replace the implanted device.
`transmit signals to a remote host computer for monitoring.
`Pacemakers can be put into a sleep mode for a variety of
`In a portable oximeter, such transmissions may be done
`conditions. In particular, these patents discuss putting the
`using wireless methods.
`pacemaker into a sleep mode during the refractory period,
`the Nellcor N-20, a
`In one existing pulse oximeter,
`whichis a period between heart beats when the heart muscle
`snapshot mode is provided. In this mode, the oximeter can
`is non-responsive to electrical stimuli. U.S. Pat. No. 4,404,
`be turned on for a short period long enough to acquire a
`972 discusses not only implantable pacemakers, but also
`signal and provide a pulse oximeter reading, and then
`implantable devices for controlling bladder function, pro-
`automatically shuts down. This method is useful primarily
`ducing muscle contractions effective to combatscoliosis, to
`for taking a reading of a healthy or stable patient, and is not
`assist in countering pain-producing nerve impulses, and to
`useful for a patient which requires continuing monitoring
`control the infusion of varioussolutions into the body. These
`due to the patient’s condition. When the snapshot mode
`devices are all therapeutic, delivering material or energy to
`button is pushed, five pulses are qualified and an oxygen
`the body at controlled times. They do not collect data for
`saturation and pulse rate are displayed. No more measure-
`diagnostic purposes, where the condition of the patient is
`unknown.
`ments are made unless the snapshot button is pressed again.
`If the snapshot button is not pressed for 30 seconds, the N-20
`automatically turns itself off. The N-20 also has an extended
`mode in whichit continuously takes data and calculates and
`displays oxygen saturation and pulse rate.
`The inventors of the present invention recognized that
`sleep-mode techniques used in other technologies could be
`imported into medical diagnostic devices, such as pulse
`oximetry, under the appropriate conditions. In particular, a
`sleep mode could be entered under carefully prescribed
`conditions for short periods while a patient’s physiological
`state is stable.
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`5,924,979
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`3
`using a sleep mode during a monitoring state. The sleep
`mode allows not only the processor to be put to sleep, but
`other detection circuitry as well. This is accomplished by not
`relying on detecting events to awaken the sleeping circuitry,
`but ratherestablishing the stability of a physiological param-
`eter before going to sleep. The invention monitors a physi-
`ological parameter of the patient and enters a sleep mode
`only after it has been stable for a predetermined period of
`time. The apparatus is periodically awakened from the sleep
`modeto take additional measurements and to ascertain that
`
`the stability of the physiological parameter has not changed.
`In one embodiment, the sleep period is chosen to be con-
`sistent with the period in which an alarm condition would
`need to be generated if a patient’s condition started to
`quickly change.
`In one embodiment, the processing portion of the appa-
`ratus goes into a sleep modeafter it has finished processing
`available data. Data will then continue to accumulate while
`
`the processor is asleep. The processor will be awakened
`whensufficient data has been accumulated, or the processor
`is otherwise programmed to awaken.
`The stable period during which sleep mode can be used
`may vary depending on the type of diagnostic apparatus and
`the characteristic being measured. Physical characteristics
`can include the five vital signs: heart rate, respiration rate,
`blood pressure, temperature and arterial oxygen saturation.
`Blood constituents typically include oxygen, carbon
`dioxide, blood glucose, hemoglobin concentration and blood
`analytes, such as sodium, potassium, chlorine, bicarbonate
`and blood urea nitrogen. Stability is defined as a condition
`where one or more predetermined variables change within a
`predetermined window of values defined by a predetermined
`rule set, the rule set allowing for the change to be measured
`in relative or absolute values, and the rule set allowing for
`the passage of time to constitute an integral part of the rules.
`In one preferred embodiment, the apparatus is a pulse
`oximeter, and both the blood oxygen saturation and heart
`rate are monitored. These may be considered stable if the
`heart rate does not vary by more than 3—-20% and the blood
`oxygen saturation does not vary by more than 2-10 satura-
`tion percentage points for a predeterminedstable time period
`of 5-50 seconds (other preferred percentages and time
`periods are set forth in the description of the preferred
`embodiments). Upon attaining such stability, sleep mode can
`be entered. In a preferred embodiment, the sleep period is
`between 20 and 60 seconds, after which the pulse oximeter
`is awakened for a sufficient amount of time to make new
`
`measurements of blood oxygen saturation and heart rate.
`Since the patient is presumed to still be stable, the pulse
`oximeter does not need to use an initialization routine to
`acquire pulses which would typically require at least five or
`more pulses. Rather, data for 1-2 pulses (alternately, 1-15
`pulses) is acquired and compared to the previously acquired
`data before re-entering the sleep mode (assumingstability is
`re-confirmed). If the patient has not remained stable, the
`sleep mode is discontinued and continuous monitoring is
`done until the patient 1s again determinedto bestable.
`In a preferred embodiment, a majority of the pulse oxime-
`ter electronic circuitry is turned off, excepting in particular
`a memory storing the data acquired while the patient was
`stable. Elements which are turned off in a sleep mode
`include the CPU,
`the light emitting diodes and driver
`circuitry, as well as the analog-to-digital converter con-
`nected to the detector. Preferably, the circuitry is turned off
`for a period of 20 seconds, and then awakened to acquire
`data associated with two pulse maximums. Alternately, the
`pulse oximeter could be put in a sleep mode in between
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`pulses, such as preferably during the diastolic decay portion
`of a heart pulse (after the maximum and before the
`minimum).
`An additional requirement for entering sleep mode in one
`embodimentis that the patient be stable at a “high” satura-
`tion value. High is preferably defined as being a predeter-
`mined amounthigher than the alarm limit of the oximeter for
`low saturation or heartrate.
`
`For a fuller understanding of the nature and advantages of
`the invention, reference should be made to the ensuing
`detailed description taken in conjunction with the accom-
`panying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram of a pulse oximeter with sleep
`mode according to the present invention.
`FIG. 2 is a flow chart of the automatic sleep mode
`according to the present invention.
`FIG. 3 is a diagram of a heart pulse illustrating periods
`during which sleep mode can be entered.
`FIG. 4 is a block diagram of a pulse oximeter with a
`separate buffer for storing data while the processoris asleep.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`invention applies to a diagnostic device
`The present
`which enters a sleep mode while a patient is stable. By way
`of example, a pulse oximeter is described, although the
`invention could apply to any diagnostic device. FIG. 1 is a
`block diagram of one embodiment of a pulse oximeter
`accordingto the present invention,illustrating an example of
`the type of components which could be put into a sleep mode
`to conserve power. The pulse oximeter includes a CPU 12
`which may be connected bya serial I/O port 14 to a remote
`host computer. Port 14 can either be a hardwired connection
`or a wireless connection. CPU 12 is connected to its own
`
`memory 14. The CPU is also connected to electronic cir-
`cuitry in the form of an Application Specific Integrated
`Circuit (ASIC) 18 which includes the drive and detection
`circuitry for a pair of Light Emitting Diodes (LEDs) 20
`which provide light to the patient’s skin.
`As shown, ASIC 18 includes a numberof registers 22 and
`a state machine 24. State machine 24 off-loads some of the
`routine functions from CPU 12, such as the alternate switch-
`ing and controlling of powerlevels for the drive circuitry 26
`connected to the two LEDs20. In addition, the state machine
`24 adjusts the gain of a programmable gain circuit 28 which
`adjusts the signal detected in a light detector 21 associated
`with LEDs 20 and provided through a preamplifier 30.
`Typically, this gain is adjusted to take maximum advantage
`of the range of the analog-to-digital converter to provide
`more sensitivity for smaller signals, and more range for
`larger signals.
`Completing the circuitry is a demodulator/demultiplexer
`32 which provides the detected signal through two different
`filters 34 and 36 and a switch 38 to an analog-to-digital
`converter (ADC) 40.
`During a sleep mode,all the elements shown in FIG. 1
`except for memory 16 are put into a sleep mode. “Sleep
`mode” may mean the removalof power,or the use of a lower
`powerstate, such as a slower clock frequency for CPU 12.
`The oximeteralso has a display 13 and a display driver 15.
`This will typically include a numeric display of the heart rate
`and the oxygen saturation. In addition, there may be a bar
`graph whichrises and falls with the heart rate, or a waveform
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`5,924,979
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`5
`display. In a sleep mode, someorall of the display may be
`put into a sleep mode. Preferably, the numerical displays
`continue to receive power and are active in a sleep mode,
`while any bar/blip display and waveform display is disabled.
`The CPU can thus provide the display driver 15 with the
`latest data before the CPU goesto sleep, with display driver
`15 staying awake and maintaining the last data throughout
`the sleep mode period. Power is saved by limiting the
`amount of display illuminated. Alternate methods are
`possible, such as dimming the numerical display during
`sleep mode or having a slow, blinking display to save
`additional power. Preferably, the numeric display is main-
`tained constantly to make the sleep mode somewhattrans-
`parent to the user.
`Whenput into the sleep mode, the pulse oximeter can be
`awakenedby a signal from the remote host on I/O line 14
`(which can be a wire or wireless telemetry connection).
`Alternately, a stand-alone pulse oximeter may have its own
`optional timer 46 whichis set by the CPU before going into
`sleep mode. The timer will then generate an interrupt to the
`CPU upon expiration of the sleep mode time to awaken the
`CPU.
`
`FIG. 2 is a flow chart illustrating the automatic sleep
`mode control flow of the present invention. It should be
`noted that in addition to the automatic sleep mode, a manual
`mode can be provided wherein the host can provide a signal
`to instruct the pulse oximeter to go into a sleep mode, and
`remain there until awakened by the host. In such a mode, the
`host would provide the necessary timing.
`As shown in FIG. 2, in a first step A, automatic sleep
`modeisstarted. After the pulse oximeter is turned on,itfirst
`determines whethera sensoris attached to the oximeter(step
`B). If no sensor is attached, the pulse oximeteris put into a
`sleep mode for one second, and then is reawakenedto test
`again whether a sensor is attached. Since typically only
`around 200 microseconds are required to determine if a
`sensoris attached, this provides a significant power savings
`even though the sleep modeis for only one secondat a time.
`In this mode, the pulse oximeter is thus asleep for around
`80% of the time.
`
`Once a sensor has been attached, the oximeter searches
`for a detectable pulse pattern to lock onto or “acquire.” In
`this type of pulse oximeter, the cardiac pulse is first detected
`so that the oximeter readings can be made at
`the same
`position in subsequent pulses. This is done because the
`volume of blood changes depending uponthe portion of the
`pulse, thus changing the oximeter reading due to morelight
`being absorbed with the presence of more oxygenated blood
`in one portion of the pulse compared to another. Once a
`pulse has been acquired, there is no longer any pulse search,
`and the oximeter then proceeds to collect blood oxygen
`saturation data and heart rate data (step E). The data is
`continuously tested to see whether it passes the stability test
`compared to previous samples during a 20-second period
`(step F). If at any time during the 20 second period the
`stability test fails, the test is reset.
`A 20-second stability period is chosen for the preferred
`embodiment, although other periods of time may be chosen,
`preferably 5, 7, 10, 15, 20, 25, 30, 40 or 50 seconds, and
`most preferably at least 15—20 seconds. The stability criteria
`in the preferred embodimentis that the heart rate not vary
`more than 5%, although other variation limits may be used,
`preferably no more than 20%, more preferably 15%, more
`preferably 10%, optionally more than 5% or 3%. The
`clinical empirical tests known to the inventors show that a
`+/-5 beats per minute (bpm) variation of heart rate can be
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`expected from a normal patient during the course of oxim-
`etry monitoring. In additionto the heartrate, in the preferred
`embodiment, the oxygen saturation value may not vary more
`than two saturation points out of a maximum scale of 100 in
`order to be stable. Alternately, this stability limit is prefer-
`ably chosen to be less than 10, 8, 6, 5, 4, 3 or 2 saturation
`percentage points. The inventors have determined by
`empirical test that a +/-2% variation of oxygen saturation
`(SAT) can be expected from a normal patient during the
`course of oximetry monitoring. Alternately, the saturation
`variation can be a percentage of the optimum saturation for
`the patient orlast saturation value. This would be significant,
`for example, where a normal adult with a saturation in the
`high nineties is compared to a fetus with a saturation
`typically in the seventies or lower. An absolute limit of 1%
`of a 100-point range producesdifferent percentage values of
`a normalsaturation in a healthy adult versus a fetus, and
`accordingly, different limits may optionally be imposed.
`If the patient is determined to be stable, sleep mode is
`entered (step G). Sleep modeis entered byfirst saving the
`baseline oxygen saturation value (SAT) and heart rate value
`in memory 16. The pulse oximeter then goesto sleep for a
`period of 20 seconds in the preferred embodiment. In the
`study of apnea, the normally accepted times when a patient
`is to be checked are 15, 20 or 30 seconds. Preferably, the
`sleep period is no more than 60, 50, 40, 35, 30, 25, 20 or 15
`seconds. Optionally, the same period of time used to deter-
`mine stability (preferably 20 seconds) is also used for the
`sleep period between checksofthe patient (also 20 seconds).
`Upon awakening from the sleep mode, the saturation and
`heart rate value are read for the next two complete pulses
`(step H). These are compared to the baseline values stored
`in memory from the stability period (step I). If the values
`collected during the two pulses are within the baseline value
`limits, indicating the patient is still stable, sleep mode is
`reentered for another 20 seconds (step J). Otherwise, the
`continuous monitoring operation is reinstated, and the sleep
`mode cannot be entered again until at least 20 seconds of
`stable data have again been collected.
`The two pulses read when the pulse oximeter awakes
`from the sleep modeare preferably not used to adjust the
`baseline value, though optionally they could be so used.
`Keepingthe baseline fixed prevents the baseline from slowly
`changing without having to be stable for a 20-second period.
`The numberof pulses read upon awakening need not be two,
`but is preferably 1, 2, 3, 5, 8, 10 or 15 pulses. In one
`embodiment, if the oximeter has been in the sleep mode for
`a series of 20 second periods, more time maybe spentin the
`awakestate to re-establish the average, stable values, while
`still realizing a significant power savings. This can be done
`without a significant overall effect on power consumption,
`since significant power savings have already been achieved.
`For example, in one embodiment, if the oximeter has been
`in a continuous series of sleep modes for more than 1-2
`minutes,
`the oximeter could remain awake for 5 pulses,
`rather than 2, to establish and calculate a new average heart
`rate and oxygen saturation. The awake periods chosen can
`alternatively be time based, as opposed to pulse based as
`previously described, in which case preferable awake time
`periods could include any one of 1, 2, 3, 5, 8, 10, and 15
`seconds, with 2, 3, and 5 seconds being preferred embodi-
`ments. Some pulse oximeters use saturation calculation
`algorithms which are not event based (e.g., pulse based) but
`rather use data for saturation is calculation without regard to
`where the data is located relative to the cardiac pulse.
`The limits for heart rate and blood oxygen saturation to
`determine stability could be varied depending upona variety
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`5,924,979
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`7
`of factors. For example, the type of patient could be used to
`vary the standard, especially for blood oxygen saturation
`between a fetus and an adult. Alternately, depending upon
`the method used to acquire the data, a different limit could
`be used depending upon the amountof averaging usedin the
`evaluation method. The host computer dynamically adjusts
`these limits in one embodiment.
`
`In one embodiment, the pulse oximeter is not allowed to
`go into sleep mode unless the patient is stable at a “high”
`saturation value. “High” could be defined as a predeter-
`mined number,or in relation to a low alarm limit, if one is
`used. For example, high could be greater than 90 or 95
`saturation points for an adult, and greater than 50, 55, 60, or
`65 for a fetus. Alternately, high could be 5, 10 or 15
`saturation points above a low alarm limit, which could be,
`Le., 70, 75, 80, 85, 90 or 95 for an adult, or 10, 15, 20, 25,
`30, 35, 40, 45, 50, 55, 60 or 65 for a fetus. Similarly, sleep
`mode could berestricted if the heart rate is outside a
`predetermined range, or if any other monitored physiologi-
`cal parameter is outside a predetermined range. The host can
`adjust the high saturation and heartrate test settings, depend-
`ing on the type of patient being monitored, the condition of
`the patient, whetherthe patient is awakeorasleep, or for any
`other reason.
`
`In addition, other aspects of the pulse oximeter operation
`may be modified during a sleep mode. In particular, a pulse
`oximeter includes alarm limits, such as an alarm which may
`be generated if no pulse is detected for a predetermined
`period of time (such as 10 seconds). It may be desirable to
`impose a shorter limit upon awakening from a sleep mode
`since the condition may have been continuing undetected
`sometime prior to the awakening. In one embodiment, the
`“no pulse” alarm will be generated if no pulseis detected for
`5 seconds after awakening, as opposed to the normal 10
`seconds.
`
`A description of different states an oximeter could be in,
`including noise, motion and alarm states, is set forth in U.S.
`Pat. No. 5,368,026 (the “’026 patent”), the disclosure of
`which is hereby incorporated herein by reference.
`Optionally, an oximeter may be required to be in a normal
`state, as set forth in the °026 patent, for a period of 20
`seconds before sleep mode is entered.
`In another
`embodiment, sleep mode is allowed in the presence of
`motion, but upon awakening from the sleep mode, it must be
`additionally determined that motionis absent long enough to
`confirm stable readings before the oximeter is put back to
`sleep.
`A manual or remotely controlled sleep mode is also
`provided, where the host computer controls when the oxime-
`ter is put
`to sleep and when it
`is awakened. The host
`computer may receive inputs from other monitors, such as
`an EKG or a CO2 monitor, and could use heart
`rate
`information derived from these, for instance, to determine if
`a patient is stable. The CO2 monitor could also provide an
`indication of the amount of oxygen the patient is receiving.
`The host could also be operated in response to a human
`operator viewing the patient through a remote TV monitor,
`or the human operator could simply decide the patient
`doesn’t need to be monitored while awake and/or during the
`daytime, or only periodically during the night, or for other
`reasons.
`In one embodiment,
`the host can periodically
`determine stability separately from the pulse oximeter moni-
`tor with data from other sources, eliminating the need for the
`pulse oximeter to establish or reconfirm stability itself.
`Preferably, the pulse oximeter responds differently to a
`command from the host computer to go into sleep mode,
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`30
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`depending on the mode the pulse oximeteris in. If the pulse
`oximeter does not have a sensorattached, it will wakeitself
`from the sleep mode every second to check for a sensor
`attached, and will continue sending no sensor attached
`messages to the host. If the oximeter is in the middle of
`performing a noise measurement whenit receives the sleep
`command,it will preferably complete that measurement and
`defer entering the sleep mode for up to 2 seconds. If the
`oximeter is in the process of adjusting LED brightness or
`amplifier gains, sleep mode maybe deferred for up to one
`second.
`
`Depending on the state the oximeter was in when it
`received a sleep mode command from the host,
`it will
`respond differently when it is awakened by the host. If was
`doing a pulse search, it will continue the pulse search upon
`being awakened. If it was reporting valid SAT andheart rate
`information, it will wait for the next good pulse. If a good
`pulse is detected in 10 seconds,it will continue with normal
`pulse oximetry measurements. If no good pulse is detected
`in 10 seconds, a pulse time out occurs, with an alarm being
`generated, unless a probationary state has been entered for
`motion as set forth in the 026 Patent.
`
`Upon reawakening the pulse oximeter from a manual
`sleep mode, it can go into an automatic sleep mode opera-
`tion. Preferably, the oximeter must always cycle through a
`normal monitoring mode (20 seconds) before entering an
`automatic sleep mode after a manual sleep mode.
`FIG. 3 is a diagram of the optical pulse which may be
`received by the pulse oximeter. This waveform 50 includes
`a series of peaks 52, 54, and 56, along with intervening
`minimums 58, 60, and 62. Typically,
`the pulse oximeter
`measurements are made on the rising edge of the pulse
`oximeter waveform, such as between minimum 58 and
`maximum 54 or between minimum 60 and maximum 56.
`Accordingly,in an alternate embodiment, the pulse oximeter
`could be put
`to sleep during a diastolic decline after a
`maximum and before the next minimum. For example, the
`sleep mode could be entered in the period between dotted
`lines 64 and 66 for each pulse. Preferably, a margin of at
`least 5% of the length ofthe trailing edge of the pulse is used
`after the maximum and before the expected minimum to
`allow for variations in the occurrence of the minimum and
`
`to avoid false peak triggering on the maximum. Since the
`trailing edge of the pulse is typically 75% ofthe total pulse
`time,
`this sleep mode can produce a significant power
`savings.
`To achieve additional power savings, the two techniques
`can be combined. That is, during normal operation, when it
`is being determinedif a patient is stable, the pulse oximeter
`could still sleep between points 64 and 66 for each pulse.
`Since only the rising edge of the pulse wave form is needed,
`this would not degrade the determination of blood oxygen-
`ation and heart rate so long as the pulse does not vary so
`much from pulse to pulse that the sleep window ends up
`extending beyond the falling edge. This can be avoided by
`providing sufficient margin for the next anticipated mini-
`mum to reawaken from sleep mode. Upon entering a normal
`sleep mode, the operation of the pulse oximeter to acquire
`data during two pulses could also be put to sleep during the
`falling edges of those two pulses to further conserve power.
`In a preferred embodiment, a calibration indicator 111 is
`included in sensor 110. The calibration indicator can be an
`
`impedance, such as a resistor, corresponding to an actua