(12) United States Patent
`(10) Patent N0.:
`US 6,178,343 B1
`
`Bindszus et al.
`(45) Date of Patent:
`Jan. 23, 2001
`
`US006178343B1
`
`(54) PULSE RATE AND HEART RATE
`COINCIDENCE DETECTION FOR PULSE
`OXIMETRY
`
`............................. 600/323
`1/1996 Baker, Jr.
`5,485,847 *
`5,971,930 * 10/1999 Elghazzawi .......................... 600/483
`
`(75)
`
`Inventors: Andreas Bindszus, Boeblingen;
`Andreas Boos, Bondorf, both of (DE)
`
`(73) Assignee: Hewlett Packard Company, Palo Alto,
`CA (US)
`
`( * ) Notice:
`
`Under 35 U.S.C. 154(b), the term of this
`patent shall be extended for 0 days.
`
`(21) Appl. No.: 09/315,698
`
`(22)
`
`Filed:
`
`May 20, 1999
`
`(30)
`
`Foreign Application Priority Data
`
`Jun. 5, 1998
`
`(EP)
`
`................................................. 98110266
`
`Int. Cl.7 ........................................................ A61B 5/00
`(51)
`(52) US. Cl.
`........................... 600/323; 600/324; 600/483
`(58) Field of Search ..................................... 600/323, 324,
`600/333, 336, 479, 483
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,858,574
`4,928,692
`5,123,420
`5,285,784 *
`
`1/1975 Page.
`5/1990 Goodman et a1.
`6/1992 Paret.
`2/1994 Seeker .................................. 600/331
`
`.
`
`* cited by examiner
`
`Primary Examiner—John P. Lacyk
`
`(57)
`
`ABSTRACT
`
`A coincidence recognition unit receives a first Signal indica-
`tive of a pulse rate derived from pulse oximetry and a second
`Signal indicative of a heart rate. Acoincidence detection unit
`generates a third Signal
`indicative of the coincidence
`between the first Signal and the second Signal. The pulse rate
`of a patient (detected by pulse oximetry) can thus be
`compared With the heart rate of the patient (e.g. from EKG
`or ultrasound). An indicator is preferably generated when the
`pulse rate and the heart rate do not match e.g. Within a
`pre-given limit. Awarning Signal might further be generated
`indicating that the oxygen saturation value as measured by
`the pulse oximetry is not sufficiently accurate and/or invalid.
`A pulse oximetry unit according to the invention comprises
`a pulse oximeter for generating the first Signal, a heart rate
`determination unit for generating the second Signal, and the
`coincidence recognition unit receiving the first and second
`Signals. The coincidence recognition unit provides the third
`Signal indicative of the coincidence between the first and the
`second Signal
`to the pulse oximeter for validating the
`accuracy of measured oxygen saturation values of any kind
`of patient such as adults, pediatrics, or neonates.
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`10 Claims, 1 Drawing Sheet
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`US. Patent
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`Jan. 23,2001
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`US 6,178,343 B1
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`US 6,178,343 B1
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`1
`PULSE RATE AND HEART RATE
`COINCIDENCE DETECTION FOR PULSE
`OXIMETRY
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates to the measuring of blood
`oxygen saturation, and in particular to validating the accu-
`racy of measured oxygen saturation values.
`Pulse oximetry is commonly used for measuring and
`displaying various arterial blood characteristics including
`blood oxygen saturation of hemoglobin in arterial blood, the
`pulse rate as the rate of blood pulsation in the arteries
`corresponding to the heart rate of the patient, or a perfusion
`indicator. Pulse oximetry represents a well-established tech-
`nique in the art and needs only to be briefly discussed herein.
`Pulse oximeters generally determine the arterial oxygen
`saturation of hemoglobin (also called Sp02 or Sa02
`measurement) by way of a non-invasive technique using two
`different monochromatic light sources typically formed by
`light emitting diodes (LEDs). An example for a pulse
`oximeter is the Hewlett Packard Component Monitoring
`System with the Pulse Oximeter Module, the ‘HP M1020A’.
`As known in the art of pulse oximetry, the light of both
`light sources is attenuated by static and dynamic absorbers
`on its path through the patient’s body to a light detector. The
`arterial blood whose quantity varies with the time synchro-
`nously with the patient’s heartbeat represents the only
`dynamic absorber during the pulse period. All other
`absorbers, such as skin, tissue or bone, are not time-variant.
`Thus, pulse oximeters make use of the pulsatile component
`of arterial blood generated by the heartbeat at only two
`spectral lines.
`The light detector receives the modulated light intensities
`of each wavelength. The signals are usually amplified, low
`pass filtered, converted from analog to digital and further
`processed. Apulse finding algorithm analyses the received
`signals, which are so-called spectrophotometric signals, for
`identifying the pulses and for determining the pulse. After
`identifying the pulse period, the diastolic and systolic values
`of the spectrophotometric signals are determined and the
`so-called relative absorption ratios are derived therefrom.
`Subsequently, in a saturation calculation algorithm the arte-
`rial oxygen saturation is computed from the relative absorp-
`tion ratio using calibration data and so-called extinction
`coefficients from the absorption spectrum of hemoglobin
`and oxyhemoglobin at
`the appropriate wavelengths. The
`mathematical background therefor, which makes use of
`Lambert-Beer’s law, has been described in sufficient detail
`in a multiplicity of former publications such as EP-A-262
`778.
`
`In parallel to the calculation of the oxygen saturation, the
`period between pulses is converted into the beat-to-beat
`pulse rate (rate=1/period). The beat-to-beat pulse rates are
`then averaged over a certain intervals or number of beats to
`generate a more or less stable value of the pulse rate. Typical
`averaging is done over 4,8 or 18 beats, or over 5 to 20
`seconds.
`
`Since the early 1980s, when pulse oximetry was
`introduced,
`this non-invasive method of monitoring the
`arterial oxygen saturation level
`in a patient’s blood has
`become a standard method in the clinical environment
`
`because of its simple application and the high value of the
`information applicable to nurses and doctors. It has become
`as common in patient monitoring to measure the oxygen
`level in the blood as to monitor heart activity with the ECG.
`In some application areas,
`like anesthesia in a surgical
`procedure, it is mandatory for doctors to measure this vital
`parameter.
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`More background information about pulse oximetry is
`given e.g. by S. Kastle et al., “A New Family of Sensors for
`Pulse Oximetry”, Hewlett-Packard Journal, February 1997,
`pages 39—53.
`US. Pat. No. 4,928,692 (Goodman) discloses a method
`for synchronizing the sampling of a signal that is then to be
`processed by a pulse oximeter. The described technique acts
`as a filter that gates the input signal for further processing in
`the pulse oximeter. The method is based on the real time
`ECG-Signal (and only on this one) with its known QRS
`shape characteristic as the gating trigger.
`Pulse oximetry, however, relies on the fact that the arterial
`blood is the only pulsating component that causes a pulsatile
`change of the light absorption used to determine the oxygen
`saturation. When the source of the pulsatile component is
`not the patient’s arterial blood flow, the oxygen saturation
`measurement, however, might derive inaccurate values. In
`case of standard pulse oximetry (e.g. adult, pediatric,
`neonatal) motion artifacts can cause other non-arterial pul-
`sating components. In case of e. g. fetal pulse oximetry using
`reflectance sensors, the sensor can accidentally pick up the
`mother’s pulsating blood instead of the fetal pulsating blood
`and lead to a wrong value of the oxygen saturation. In
`general, oxygen saturation values derived by pulse oximetry
`might not be sufficient accurate due to a strong impact of
`pulsatile sources other than the patient’s arterial blood flow.
`It is therefore an object of the present invention to provide
`an improved pulse oximetry.
`SUMMARY OF THE INVENTION
`
`The object is solved by the independent claims. Preferred
`embodiments are given by the dependent claims.
`The invention makes use of the fact that the pulse rate
`determined by the pulse oximetry has to be correlated—for
`physical reasons—to the patient’s heart rate. The patients
`heart rate can be measured directly by applying electrodes to
`the skin of the patient and measure the electrical activity of
`the contracting heart muscle (e.g. electrocardiography—
`EKG). Further more, the heart rate can also be measured
`indirectly by listening to (e.g. acoustically monitoring) the
`heart beat or by measuring the Doppler-shift of an ultra-
`sound wave reflected by the moving parts of the heart.
`It is to be understood that the term ‘pulse rate’, as used
`herein, shall refer to a pulsating value determined by pulse
`oximetry, whereas the term ‘heart rate’, as used herein, shall
`refer to a pulsating value determined by any kind of direct
`(e.g. EKG) or indirect (e.g. ultrasound) heart monitoring
`other than pulse oximetry.
`According to the invention, a coincidence recognition unit
`receives a first signal indicative of a pulse rate derived from
`pulse oximetry and a second signal indicative of a heart rate.
`A coincidence detection unit generates a third signal indica-
`tive of the coincidence between the first signal and the
`second signal. The pulse rate of a patient (detected by pulse
`oximetry) can thus be compared with the heart rate of the
`patient (e.g. from EKG or ultrasound). An indicator is
`preferably generated when the pulse rate and the heart rate
`do not match e.g. within a pre-given limit. Awarning signal
`might further be generated indicating that the oxygen satu-
`ration value as measured by the pulse oximetry is not
`sufficiently accurate and/or invalid.
`A pulse oximetry unit according to the invention com-
`prises a pulse oximeter for generating the first signal, a heart
`rate determination unit for generating the second signal, and
`the coincidence recognition unit receiving the first and
`second signals. The coincidence recognition unit provides
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`US 6,178,343 B1
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`3
`the third signal indicative of the coincidence between the
`first and the second signal to the pulse oximeter for validat-
`ing the accuracy of measured oxygen saturation values. The
`invention thus allows validating the accuracy of measured
`oxygen saturation values of any kind of patient such as
`adults, pediatrics, or neonates.
`In fetal pulse oximetry, the invention allows validating
`that the measured oxygen saturation comes from the fetus
`and not from the mother. In that context, the invention might
`be applied in combination with the so-called cross-channel
`verification method as disclosed in US. Pat. No. 5,123,420
`by the same applicant. The cross-channel verification allows
`discriminating heart rates of the mother and up to two
`fetuses within a multi-channel
`fetal monitor
`(twin
`monitoring). The fetal monitor is capable of recording the
`heart rate trace (e.g. the beat-to-beat heart rate trace) of a
`fetus and a second heart rate trace of the mother or of a
`second fetus. Coincidence between the heart rate traces is
`
`detected by means of a direct or indirect comparison of the
`two traces and comparison of the difference with a pre-
`defined or adaptive limit. A warning signal is generated if
`coincidence is detected or, in other words, when the heart
`rate that should come from either the mother, the first or the
`second fetus is equal or similar. In this case the user will be
`warned that the monitor might not monitor three individual
`objects, but instead may duplicate one object on (an) other
`channel(s).
`The comparison between the pulse rate and the heart rate
`can be accomplished by any comparison method and/or
`apparatuses as known in the art, and preferably, by the
`comparison method or means as disclosed in US. Pat. No.
`5,123,420. Accordingly, any criteria as known in the art can
`be applied for determining the correlation between the pulse
`rate and the heart rate, or in other words, whether the pulse
`rate and the heart rate match. Suitable measures, such as a
`warning signal, might be initiated when a mismatch between
`the pulse rate and the heart rate is detected.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Other objects and many of the attendant advantages of the
`present invention will be readily appreciated and become
`better understood by reference to the following detailed
`description when considering in connection with the accom-
`panied drawings in which:
`FIG. 1 shows a coincidence recognition unit according to
`the invention,
`FIG. 2 shows an improved pulse oximetry unit according
`to the invention, and
`FIG. 3 shows a preferred embodiment of the coincidence
`detection unit.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`FIG. 1 shows in a principal block diagram a coincidence
`recognition unit 10 according to the invention an apparatus
`for providing a coincidence detection between a heart rate
`and a pulse rate. The coincidence recognition unit 10 com-
`prises a coincidence detection unit 20 receiving a first signal
`PR indicative of the pulse rate derived from pulse oximetry
`and a second signal HR indicative of the heart rate measured
`either directly (e.g. by ECG) or
`indirectly (e.g. by
`ultrasound). The coincidence detection unit 20 generates
`therefrom a third signal 30 indicative of the coincidence of
`the first signal PR and the second signal HR. The third signal
`30 is coupled to an optional coincidence evaluation unit 40
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`for generating a fourth signal 50, which can be a warning
`signal, when the third signal 30 exceeds a predefined relation
`to a given limit.
`According to the invention, the coincidence recognition
`unit 10 determines whether the pulse rate and the heart rate
`coincide, or in other words, whether the pulse rate and the
`heart rate probably originate from the same patient. For that
`purpose, the coincidence detection unit 20 preferably deter-
`mines the correlation between the pulse rate and the heart
`rate and the coincidence evaluation unit 40 generates the
`fourth signal 50 when the correlation between the pulse rate
`and the heart rate exceeds a given limit.
`FIG. 2 shows an improved pulse oximetry unit according
`to the invention. Apulse oximeter 60 provides the first signal
`PR indicative of the pulse rate to the coincidence recognition
`unit 10, which further receives the second signal HR indica-
`tive of the heart rate from a heart rate determination unit 70.
`
`The heart rate determination unit 70 can be e.g. an ECG or
`an ultrasound-providing unit. The coincidence recognition
`unit 10 provides the third signal 30 and/or the fourth signal
`50 to the pulse oximeter 60. The coincidence recognition
`unit 10 validates the accuracy of measured oxygen satura-
`tion values by determining the coincidence between the
`pulse rate and the heart rate. A mismatch between the pulse
`rate and the heart rate is signaled by the third signal 30
`and/or the fourth signal 50 indicating that
`the oxygen
`saturation value as measured by the pulse oximetry is not
`sufficiently accurate and/or invalid.
`FIG. 3 shows a preferred embodiment of the coincidence
`detection unit 20. The heart rate signal HR (e.g. derived from
`ultrasound or ECG, such as a direct scalp ECG in fetal
`monitoring) is fed to a first (e.g. positive) input terminal of
`a summing circuit 100 and the pulse rate PR (derived from
`pulse oximetry) is fed to a second (e.g. negative) terminal
`thereof. The summing circuit 100 generates a difference
`signal on an output terminal thereof indicative of the dif-
`ference between the heart rate HR and the pulse rate PR. The
`difference signal is fed to an absolute value generator 110
`that produces the absolute value of the provided difference
`signal.
`A threshold comparator 120 compares the absolute value
`of the difference signal, derived from the absolute value
`generator 110, with a predetermined limit and outputs e.g. a
`positive constant if the absolute value is below the limit.
`Correspondingly, a negative constant is output if the abso-
`lute value is above the limit. It is to be understood that the
`
`threshold limit can also be an adaptive limit and that the
`output signal of the threshold comparator 120 can be a
`variable value instead of a constant value. This allows giving
`the output signal of the threshold comparator 120 a weight-
`ing dependent on the absolute value of the heart rate HR and
`the difference value between the heart rate HR and the pulse
`rate PR.
`
`The output of the threshold comparator 120 is summed up
`by an integrator circuit 130. The integrator circuit 130
`provides a built-in limitation function that avoids an over-
`flow if the input signal is positive for a long period of time
`or an underfiow if it is negative.
`A second threshold comparator 140 compares the output
`signal from the integrator circuit 130 with one or more
`thresholds and outputs a first (e.g. positive) signal if the
`integration output is above a predetermined or adaptive
`limit. Correspondingly, a second (e.g. negative) signal is
`output if the integration output is below a predetermined or
`adaptive limit. Preferably, the second threshold comparator
`140 has a built-in hysteresis to avoid a fast changing output
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`US 6,178,343 B1
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`5
`if the output of the integrator circuit 130 is permanently
`changing around the threshold values of the second com-
`parator 140.
`The second threshold comparator 140 preferably provides
`a positive output when the heart rate HR and the pulse rate
`PR are within a certain range. A negative output is provided
`when the heart rate HR and the pulse rate PR are not within
`the given range.
`When the output of the second threshold comparator 140
`is negative, the heart rate HR, derived e.g. from ultrasound
`or ECG, and the pulse rate PR, derived from pulse oximetry,
`are different, thus indicating that the sources of the heart rate
`HR and the pulse rate PR are probably not the same. This
`can be the case, e.g. in fetal monitoring, when the pulse rate
`PR is derived from maternal pulses instead of from fetal
`pulses, or if a fetal arrhythmia is present.
`When the output of the second threshold comparator 140
`is negative, thus indicating a mismatch between the pulse
`rate PR and the heart rate HR, a warning signal is preferably
`generated indicating to a user to check the measurement
`conditions and to reposition or to replace a pulse oximetry
`sensor, if required. Typically the values of the heart rate HR
`derived by ultrasound or by direct scalp ECG measurements
`are more accurate and reliable than e.g. optically derived
`pulse rates PR from pulse oximetry. In addition, during fetal
`and/or maternal movements, the SpO2 pulse signals can be
`influenced by those movements, so that wrong pulse rates
`PR could by delivered. In these cases, the warning given to
`the user might help to make a better interpretation of the
`measurement result, thus improving patient management.
`The coincidence evaluation unit 40 can be embodied by
`any hardware or software means known in the art.
`In a preferred embodiment,
`the coincidence detection
`according to the invention is implemented using digital
`components and an integrated microprocessor. However,
`analog or partly analog implementations are possible
`accordingly.
`Further embodiments for the coincidence detection
`
`according to the invention, and in particular for the coinci-
`dence detection unit 20, can be derived from US. Pat. No.
`5,123,420 by replacing HR1 by the pulse rate PR and HR2
`by the heart rate HR, or vice versa. In particular, the various
`coincidence detection methods and apparatuses described in
`FIGS. 2—4, 6—7, 8—9, and 10 plus corresponding parts of the
`description of US. Pat. No. 5,123,420 are applicable for the
`purpose of the present invention and shall be incorporated
`herein by reference.
`What is claimed is:
`
`1. A signal matching recognition unit receiving a first
`signal indicative of a pulse rate derived from pulse oximetry
`and a second signal, indicative of a heart rate, that is derived
`from a technique other than pulse oximetry,
`the signal
`matching recognition unit comprising:
`a signal matching detection unit for generating a third
`signal
`indicative of a signal match between the first signal and
`the second signal.
`2. The signal matching recognition unit of claim 1, further
`comprising:
`a signal matching evaluation unit receiving the third
`signal for generating therefrom a fourth signal when the
`third signal exceeds a predefined relation to a given
`limit.
`
`3. The signal matching recognition unit of claim 1,
`wherein the signal matching detection unit comprises means
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`for determining a signal match between the first signal and
`the second signal.
`4. The signal matching recognition unit of claim 1,
`wherein the signal matching detection unit comprises:
`a summing circuit, receiving the first signal and the
`second signal, for generating a difference signal indica-
`tive of the difference between the first signal (PR) and
`the second signal (HR),
`a first comparator for comparing the difference signal with
`a predetermined limit and for providing an output
`corresponding the difference signal,
`an integrator circuit for summing up the output received
`from the first comparator, and
`a second comparator for comparing a signal provided by
`the integrator circuit with one or more threshold values
`and for providing the third signal.
`5. The signal matching recognition unit of claim 4,
`wherein the signal matching detection unit further comprises
`an absolute value generator, receiving the difference signal,
`for generating the absolute value of the difference signal,
`whereby the first comparator receives and compares the
`absolute value of the difference signal with a predetermined
`limit and provides an output corresponding the received
`absolute value of the difference signal.
`6. A pulse oximetry unit comprising:
`a pulse oximeter generating a first signal indicative of a
`pulse rate,
`a heart rate determination unit generating a second signal,
`indicative of a heart rate, that is derived from a tech-
`nique other than pulse oximetry, and
`a signal matching recognition unit wherein said signal
`matching recognition unit receives the first signal from
`the pulse oximeter and the second signal from the heart
`rate determination unit and provides a third signal
`indicative of the signal matching between the first
`signal and the second signal to the pulse oximeter for
`validating the accuracy of measured oxygen saturation
`values.
`
`7. The pulse oximetry unit of claim 6, wherein the signal
`matching recognition unit provides a warning signal to the
`pulse oximeter, indicating that the oxygen saturation value is
`not sufficiently accurate and/or invalid, when the signal
`matching recognition unit recognizes a mismatch between
`the pulse rate and the heart rate.
`8. A method for validating the accuracy of oxygen satu-
`ration values measured by a pulse oximeter, comprising the
`steps of:
`indicative of a pulse rate
`(a) receiving a first signal
`derived from pulse oximetry and a second signal,
`indicative of a heart rate, that is derived from a tech-
`nique other than pulse oximetry, and
`(b) generating a third signal
`indicative of the signal
`matching between the first signal and the second signal.
`9. The method of claim 8, further comprising the step of:
`(c) generating a fourth signal from the third signal when
`the third signal exceeds a predefined relation to a given
`limit.
`
`10. The method of claim 8, further comprising the step of:
`(d) providing a warning signal to the pulse oximeter,
`indicating that the oxygen saturation value as measured
`by the pulse oximeter is not sufficiently accurate and/or
`invalid, when a mismatch between the pulse rate and
`the heart rate is recognized.
`*
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