Detection of parasitic signals during pulsoxymetric measurement

In the proposed method for detecting spurious signals caused by movements of the patient or his environment in the pulse oximetric measurement of the arterial oxygen saturation, the intensity of the light emerging from the test specimen is measured by two photodiodes (7, 8). The signals (S.sub.1, S.sub.2) generated at these photodiodes (7, 8) are normalized to equal levels of their DC components. Finally, to detect spurious signals caused by movements, the differences (.DELTA.S.sub.n) of these normalized signals (S.sub.1n, S.sub.2n) are formed.

FIELD OF THE INVENTION
 The invention relates to a method for detecting spurious signals caused by
 movements of a patient or his environment in the pulse oximetric
 measurement of the arterial oxygen saturation, as well as to a pulse
 oximeter for carrying out the method.
 BACKGROUND OF THE INVENTION
 It is known to measure the oxygen saturation of the haemoglobin in the
 arterial blood (arterial oxygen saturation) by means of a non-invasive
 method which is designated as pulse oximetry. This method is used for
 monitoring patients, e.g. during anaesthesia and intensive care. The
 principle of the measurement is based on the difference between the
 optical absorbtivities of haemoglobin in its form saturated with oxygen
 and its reduced form. In the case of red light, the absorption coefficient
 of blood is greatly dependent upon the oxygen content and, in the case of
 light in the near infrared range, almost independent thereof. By measuring
 the ratio of the intensities of the absorbed light of both wavelengths, it
 is possible to determine the arterial oxygen saturation.
 In pulse oximetry, as a rule the light sources employed are two closely
 mutually adjacent light emitting diodes (LED) having wavelengths of
 approximately 660 nm (red) and approximately 890 nm (infrared). The light
 emitted by the two LEDs is passed into a body part (e.g. the pad of the
 finger) which is well supplied with blood, and is there scattered and
 partially absorbed. The light emerging is measured by a photodiode which,
 as a rule, is disposed to be situated opposite the LEDs. The LEDs and the
 photodiode are usually integrated in an assembly which is designated as a
 pulse oximetric sensor. The separate measurement of the red and infrared
 light using only one photodiode is made possible by the use of alternating
 light pulses of the two wavelengths, which are separately metrologically
 picked up and evaluated.
 The light of both wavelengths which is measured by the photodiode consists
 of a steady and a time dependent component. The steady component is
 essentially determined by absorption by bones, tissue, skin and
 non-pulsating blood. The time dependent component is caused by changes in
 absorption in the specimen under test, which, in the ideal case, are
 caused only by the arterial blood flowing in in pulsed fashion. To
 determine the arterial oxygen saturation (SaO.sub.2), the steady
 components (DC.sub.R, DC.sub.IR) and the time dependent components
 (AC.sub.R, AC.sub.IR) of the measured red (R) and infrared (IR) light
 intensities are utilized. Usually, the arterial oxygen saturation is
 determined using the relation:
 ##EQU1##
 where f represents an empirically determined function.
 A problem which has not yet been satisfactorily solved in pulse oximetric
 measurement resides in that disturbances to the measurement signals which
 are caused by movements of the patient or his environment cannot be
 eliminated entirely. Such disturbances are critical particularly in
 circumstances in which they occur periodically, since in this case, they
 may lead under specified conditions to false measurement results. Since
 the frequency distribution of movement artifacts may overlap that of the
 physiological signal, conventional band pass filters or selective filters
 are not suitable for reliably separating movement artifacts from the
 physiological signal. Even adaptive filter techniques, such as for example
 the method of adaptive spurious frequency suppression, cannot be directly
 applied to pulse oximetry, since these presuppose that either the spurious
 frequencies or the physiological signals exhibit predictable frequency
 characteristics. This prerequisite is not satisfied either in respect of
 movement artifacts or in respect of the pulse frequency. In particular in
 the case of patients having cardiovascular disorders, the latter may
 exhibit a high variability.
 In principle, it has to be stated that, as a consequence of the nature of
 the problem, limits are set to any solution which is based solely on an
 improvement of the signal processing. These limits are caused by the fact
 that disturbances due to movement artifacts cannot be entirely eliminated,
 since they are not always detected as such. Primarily, it is accordingly
 necessary to seek a solution via the route of a differentiated signal
 extraction which permits a separation of the spurious signals from the
 physiological signals. Various solution routes have already been proposed
 in this sense.
 WO-A-94/03102 contains a description of an optical monitoring device which
 comprises:
 a) a sensor having a transmitter part which has three light emitting diodes
 which emit light of differing wavelength, and having a receiver part to
 measure the light intensity, which receiver part has three photodetectors,
 and
 b) a control and evaluating part.
 To suppress the spurious signals caused by movements of a patient or his
 environment, it is proposed to normalize the signals generated at the
 photodetectors to equal levels of their DC components. Proceeding on the
 basis of the assumption that the amplitudes of the spurious components of
 these normalized signals are equally large, the differences of the
 normalized signals are then formed to eliminate the spurious components.
 However, in practice the amplitudes of the spurious components of the
 normalized signals are different, as is also mentioned on page 6, lines 4
 to 6 of this publication. Accordingly, a complete suppression of spurious
 signals does not take place.
 U.S. Pat. No. 4,802,486 contains a description, for example, of a method
 which is based on using specified measurement signals, derived from the
 ECG of the patient, to identify the arterial pulsations. Signals which are
 not identified as such (i.e. spurious signals) can thus be suppressed.
 This method, which is designated as ECG-synchronized pulse oximetry, has
 the disadvantage that spurious signals which occur simultaneously with a
 pulse signal are not picked up. Moreover, the simultaneous measurement of
 the ECG is presumed. However, this is not always available.
 In U.S. Pat. No. 5,226,417, it is proposed to incorporate into the pulse
 oximetric sensor a measured value pickup which detects movements at the
 location of the pulse oximetric test point. Piezoelectric films,
 acceleration transducers and wire strain gauges are mentioned as examples
 of such measured value pickups. However, such a solution demands a
 considerable expenditure in the manufacture of the sensor; this leads to a
 substantial increase in the cost of the product, which is often designed
 as a disposable article. Moreover, by reason of the extremely stringent
 requirements imposed on the sensitivity of the movement detection, a
 considerable expenditure in respect of signal processing is necessary.
 A similar idea is described in U.S. Pat. No. 5,025,791. In that document,
 it is proposed to incorporate into the pulse oximetric sensor a movement
 detector which is specifically designed for this purpose and which is
 based on an electromechanical or a magnetic measurement method or a
 combination of both of these methods. The objections which have already
 been set forth hereinabove are factors against such a concept.
 Another solution is proposed in WO-A-91/18550. That document contains a
 description of an arrangement which is provided for the measurement of the
 pulse frequency, the concept of which could however also be transferred to
 pulse oximetry. In a sensor, which is applied to the forehead of a person,
 there are incorporated a LED emitting in the infrared and a LED emitting
 in the yellow frequency range as well as two photodiodes. The light
 emitted into the tissue of the forehead is back-scattered there and
 measured by the two photodiodes. The signal generated by the infrared
 light contains components which are caused both by the pulsating arterial
 blood and also by movements. In contrast, the signal generated by the
 yellow light is to a large extent independent of blood pulsations and
 contains only the components caused by movements. This may be explained in
 that infrared light is able to penetrate deeply into the forehead tissue
 which is well supplied with blood, while yellow light has a substantially
 smaller depth of penetration and therefore picks up only processes in the
 vicinity of the surface of the skin of the forehead, i.e. in a region
 whose blood supply is weak. The two signals can now be analyzed by means
 of known processes, and the components of the infrared signal which are
 caused by movements can be removed. In the case of a transfer of this
 concept to pulse oximetry, it has to be borne in mind that, for reasons
 which are predominantly practical, but also physiological, the forehead
 lacks suitability as a test point. The test point which is most frequently
 used for pulse oximetric measurements is the finger. Measurements are
 often made also on the ear lobe and on the toe. It is common to these
 three test points that even the tissue parts which are close to the
 surface have a good supply of blood. There, a separation of the signal
 components caused by pulsations and by movements is accordingly not
 readily possible by the use of light of differing depths of penetration.
 OBJECTS OF THE INVENTION
 The object of the invention is to provide an improved method for
 differentiated signal extraction for pulse oximetric measurements, which
 method permits a detection of the spurious signals caused by movements,
 and which does not exhibit the disadvantages and limitations of the known
 methods.
 This object is achieved by the method according to the invention for
 detecting spurious signals, and the pulse oximeter according to the
 invention for carrying out this method. Preferred variant embodiments are
 also disclosed.
 SUMMARY OF THE INVENTION
 According to the invention, the pulse oximetric sensor which is used is a
 device having a transmitter part which has two light emitting diodes which
 emit light of differing wavelengths, and having a receiver part to measure
 light intensity, which receiver part comprises at least two photodiodes.
 The two or at least two of the photodiodes are, as in the case of the
 previously known pulse oximetric sensors, disposed opposite the red and
 infrared LEDs, but offset to different sides with respect to the
 longitudinal central line of that bearing surface of the receiver part
 which faces the transmitter part.
 An embodiment of such a sensor has the form of a clamp, which is suitable
 to be secured to a finger. In one of the clamp limbs there are
 incorporated the red and infrared LEDs, the light of which is emitted into
 the pad of the finger. Opposite them, in the other limb of the clamp,
 there are two mutually adjacent photodiodes. The light emitted by the LEDs
 is scattered in multiple fashion in the pad of the finger, in part
 absorbed and in part scattered out. A part of the light scattered out
 passes to the photodiodes. As initially described, the light received by
 the photodiodes consists of a steady and a time dependent component which
 is caused by blood pulsations. In the case of movements of the patient or
 his environment which are transferred to the test point, this causes the
 addition of a further time dependent component. Accordingly, the signals
 S.sub.1 and S.sub.2 measured at the two photodiodes 1 and 2 (light
 intensities) are made up as follows:
EQU S.sub.1 =DC.sub.1 +AC.sub.P1 +AC.sub.B1 (2)
EQU S.sub.2 =DC.sub.2 +AC.sub.P2 +AC.sub.B2
 In these expressions, DC.sub.1 and DC.sub.2 are the steady signal
 components, i.e. those light intensities which would be measured in the
 complete absence of blood pulsations and movement artifacts. AC.sub.P1 and
 AC.sub.P2 respectively are the signal components caused by blood
 pulsations, and AC.sub.B1 and AC.sub.B2 respectively are the signal
 components caused by movements.
 In the absence of movement artifacts, the signals S.sub.1 and S.sub.2
 measured at the two photodiodes are very similar. However, in the case of
 movements very marked differences between them appear. In order to pick up
 these differences, S.sub.1 and S.sub.2 are in the first instance
 normalized to an approximately equal amplitude; expediently, this takes
 place by means of division by the respective DC values:
 ##EQU2##
 The thus obtained normalized signals S.sub.1n and S.sub.2n are subsequently
 subtracted from one another, and the result is the difference signal
 .DELTA.S.sub.n :
 ##EQU3##
 In these expressions, .DELTA.S.sub.nP are the components of the difference
 signal caused by pulsations, and .DELTA.S.sub.nB those caused by
 movements. Measurements which are explicitly described later show that in
 the absence of movements (AC.sub.B1 =AC.sub.B2 =0) the difference signal
 .DELTA.S.sub.n determined using red light and also that determined using
 infrared light is almost equal to zero. This means that the changes in the
 test specimen which are caused by blood pulsations, at the two adjacent
 photodiodes, generate almost identical signals. This may be explained in
 that the red and infrared light is abundantly scattered in the test
 specimen and, accordingly, the tissue of the pad of the finger is
 uniformly illuminated. Accordingly, those changes to the optical
 properties of the test specimen which are caused by blood pulsations act
 symmetrically on both test points.
 In contrast to this, those changes to the optical properties of the test
 specimen which are caused by movements have different effects. In this
 case, it is possible to find marked fluctuations of the difference signal
 .DELTA.S.sub.n which proceed in a manner temporally synchronous with the
 movements. Thus, the difference signal .DELTA.S.sub.n proves to be an
 extremely sensitive indicator for the detection of movement artifacts of
 various types, i.e. irrespective of whether these are caused by the
 patient himself, by external effects on the sensor and the cable, or by
 other extraneous influences. In particular, it is possible to detect
 periodically occurring movement artifacts, even when these occur with a
 frequency which is in the vicinity of the pulse frequency or is even
 identical therewith. An explanation for this unexpected result must be
 sought in that movements (in contrast to blood pulsations) result in a
 non-uniformly distributed change to the optical properties of the test
 specimen. In the case of movements, the tissue of the pad of the finger is
 asymmetrically displaced in relation to the positions of the LEDs and the
 photodiodes; this has the effect of an unequal change to the signals
 measured at the two adjacent photodiodes.
 The advantages attained with the method according to the invention reside
 in that for the first time a method, which can be realized in technically
 simple fashion and which is economical, sensitive and reliable, for
 detecting movement artifacts during pulse oximetric measurements is
 available. The design of a pulse oximetric sensor having two or more
 photodiodes differs only in insubstantial fashion from the previously
 customary designs, since the same optical components are used. It is not
 necessary to use a separate measured value pickup for the detection of
 movements, as is proposed in the above cited U.S. Pat. No. 5,226,417 and
 U.S. Pat. No. 5,025,791. The signal processing is extremely simple and can
 build on the methods currently used in pulse oximetry. The method can be
 used at each of the test points which are customary for pulse oximetry.
 Furthermore, there are no restrictive conditions with respect to the
 simultaneous availability of a second measurement parameter as in the case
 of ECG-synchronized pulse oximetry (see U.S. Pat. No. 4,802,486). A
 further advantage of the method resides in that, with the amplitude of the
 difference signal .DELTA.S.sub.n, a measure of the magnitude of the
 spurious signal is available and, accordingly, the disturbance can be
 quantified in a simple manner.
 The detection of movement artifacts with the aid of the method according to
 the invention makes it possible to take various suitable measures. Thus,
 with the aid of known methods of analogue or digital electronics, which
 will not be discussed in greater detail here, the spurious signals
 determined from the difference signal .DELTA.S.sub.n can be analyzed,
 edited and separated from the measured signal. The adjusted physiological
 signal obtained thereby permits a more precise and more reliable
 determination of the arterial oxygen saturation. Furthermore, in the case
 of the occurrence of movement artifacts, an alarm message can be
 triggered, which causes the user of the pulse oximeter to eliminate the
 causes of the disturbances. It is also possible to compare the amplitude
 of the spurious signal with that of the physiological signal and, with
 effect from a specified ratio of these two values, to trigger an alarm
 message or no longer to display the measurement result. This may become
 necessary, for example, in circumstances in which during weak
 physiological signals, disturbances occur which are so great that the
 requirements imposed on the accuracy of measurement appertaining to the
 arterial oxygen saturation can no longer be satisfied.

DETAILED DESCRIPTION OF THE DRAWINGS
 The sensor 1 shown in FIG. 1A has the form of a clamp which is secured to a
 finger 2. In the bearing surface 3 of the upper limb of the clamp there
 are a red LED 4 and an infrared LED 5, the light of which is emitted into
 the pad of the finger. The bearing surface 6 of the opposite limb of the
 clamp contains two photodiodes 7 and 8 to measure the transmitted light.
 The four components 4, 5, 7 and 8 are connected via a cable 9 to the
 control and evaluating part of the pulse oximeter. FIGS. 1B and 1C show in
 each instance a view of the bearing surfaces 3 and 6, in order to
 illustrate the positions of the LEDs and of the photodiodes. The LEDs 4
 and 5 are incorporated at the smallest possible spacing (approximately 1
 to 2 mm) along the central line of the bearing surface 3, which
 approximately corresponds to the central line of the finger. In contrast,
 the photodiodes 7 and 8 are disposed to be obliquely offset in relation to
 the central line of the bearing surface 6. Depending upon their size and
 the size of the sensor, the spacing between the photodiodes may be
 approximately 1 to 10 mm. The aim of this geometric arrangement of the
 photodiodes is the following: as has been described hereinabove, the idea
 according to the invention resides in detecting those changes to the
 optical properties of the test specimen which are caused by movements on
 the basis of their unequal effect on two adjacent test points. Depending
 upon the nature of the movement, such changes may act more in the
 direction of the central line of the finger or perpendicular thereto. With
 the aid of the arrangement of the photodiodes which is offset diagonally
 to the central line of the finger, it is thus possible to pick up changes
 in both directions.
 It is understood that instead of the clamp-type finger sensor described
 here, it is also possible to use other types of sensor which are customary
 in pulse oximetry, e.g. sensors with flexible bearing surfaces.
 Furthermore, besides the finger, all other body parts which are
 customarily used for pulse oximetric measurements (e.g. ears, toes) may be
 used. Furthermore, the concept of the invention is not necessarily
 restricted to the use of only two photodiodes and to the geometric
 arrangement which is described here. It is evident that the sensitivity of
 the detection of movement artifacts can be enhanced by the use of three or
 more photodiodes; in this case, depending upon the type of sensor, the
 test point and the number of photodiodes used, the geometric arrangement
 thereof must be designed in optimal fashion in the individual case.
 FIG. 2 shows a simplified functional diagram of those circuit parts which
 are necessary for the determination of the difference signal
 .DELTA.S.sub.n. For the sake of simplicity, all circuit elements which are
 not directly important to an understanding of the method according to the
 invention have not been separately shown, but are combined in diagrammatic
 fashion in the electronic unit 10. The red and infrared LEDs 4 and 5 of
 the sensor 1 are driven via the electronic unit 10 and the multiplexer 11
 in such a way that alternating light pulses of both wavelengths are
 generated. As has been described hereinabove (see Equation 2), the signals
 S.sub.1 and S.sub.2 obtained at the photodiodes 7 and 8 of the sensor 1
 consist in each instance of a DC component (DC.sub.1, DC.sub.2), a first
 AC component, which is caused by blood pulsations (AC.sub.P1, AC.sub.P2),
 and a second AC component, which is caused by movements (AC.sub.B1,
 AC.sub.B2). Each one of these signals is present both for the red and also
 for the infrared lights; in this case, however, in the description which
 follows it is immaterial which one of the two optical wavelengths is used
 for the determination of .DELTA.S.sub.n. S.sub.1 and S.sub.2 are in the
 first instance logarithmized by means of the amplifiers 12 and 13.
 Subsequently, the DC components of the logarithmized signals are removed
 by means of the high pass filters 14 and 15. In mathematical terms, the
 high pass filtering of the logarithmized signal S.sub.1 may be described
 as follows:
 ##EQU4##
 S.sub.1n is defined by Equation 3. Since, as a rule, the AC components of
 the signal are substantially smaller than the DC component, i.e., since
 ##EQU5##
 the following relation is approximately applicable:
 ##EQU6##
 where k=log e=0.434.
 In the case of S.sub.2n, the following applies in corresponding fashion:
 ##EQU7##
 The following difference is formed by the amplifier 16:
EQU logS.sub.1n -logS.sub.2n =k.DELTA.S.sub.n (9)
 The difference signal .DELTA.S.sub.n obtained in this way (for definition,
 see Equation 4) is now available for further processing and evaluation by
 the electronic unit 10. The methods used in this case will not be
 discussed in greater detail here, since they do not form part of the
 subject of the invention.
 It is understood that, instead of the circuit concept described here, it is
 also possible to use other circuits provided that they are suitable for
 the determination of the normalized difference signal .DELTA.S.sub.n in
 accordance with the above definition or a quantity equivalent thereto.
 Thus, by way of example, it is also possible to use circuits in which,
 instead of the logarithmization route chosen here, the AC components of
 the signals S.sub.1 and S.sub.2 are directly divided by their DC
 components, and subsequently the difference of the two quotients thus
 obtained is formed. It is also possible in the first instance to amplify
 the signals S.sub.1 and S.sub.2 to equal levels of their DC components and
 subsequently to form the difference of the signals normalized in this way.
 FIG. 3 shows, by way of example, a measurement result which was obtained in
 the absence and presence of movement artifacts. The upper two curves 17
 and 18 show the values, picked off downstream of the high pass filters 14
 and 15, of log S.sub.1n =k(S.sub.1n- 1) and log S.sub.2n =k(S.sub.2n -1).
 The bottom curve 19 shows the signal .DELTA.S.sub.n obtained downstream of
 the difference amplifier 16. At the instant A, an individual brief
 disturbance occurs by reason of a movement of the sensor. During the time
 intervals B and C periodic disturbances take place, which were caused in
 each instance by rapid (B) and slow (C) rhythmic movements of the test
 point (finger) . It can be recognized that in the absence of movements log
 S.sub.1n and log S.sub.2n proceed in very similar fashion and
 .DELTA.S.sub.n exhibits only slight, pulse-synchronous fluctuations. As
 has been mentioned hereinabove, this is to be ascribed to the fact that
 those signal components of S.sub.1n and S.sub.2n which are caused by blood
 pulsations have almost equal amplitudes. In the case of movements, in
 contrast, it is possible to detect great fluctuations of .DELTA.S.sub.n
 which take place in a manner temporally synchronous with the movement
 sequences. In this case, it should be emphasized that the spurious signals
 occurring during the time B have approximately the same frequency as the
 pulse rate and, in this special case, generate for log S.sub.1n and log
 S.sub.2n curve forms which cannot be distinguished from purely
 physiological signals. Accordingly, spurious signals of this extreme type
 cannot be detected using conventional methods of signal analysis. It is a
 particular advantage of the invention reliably to detect even disturbances
 of this type.