WO2015165785A1

WO2015165785A1 - Low power device for monitoring a vital sign

Info

Publication number: WO2015165785A1
Application number: PCT/EP2015/058699
Authority: WO
Inventors: Ronaldus Maria Aarts; Cristian Nicolae Presura
Original assignee: Koninklijke Philips N.V.
Priority date: 2014-04-28
Filing date: 2015-04-22
Publication date: 2015-11-05

Abstract

The present invention relates to a device for monitoring a vital sign of a subject (14) when attached to the subject's body. The device (12, 12', 12", 12" ') comprises an optical sensor arrangement (18) including a light source (20) for emitting light into tissue of the subject, a light sensor (22) for receiving at least part of the emitted light after an interaction of the emitted light with the tissue and an evaluation unit (24) for deriving an optical monitoring signal from the received light being indicative of the vital sign of the subject (14). The device (12, 12', 12", 12' ") further comprises an auxiliary sensor arrangement (26) different from the optical sensor arrangement (18) for obtaining an auxiliary monitoring signal being indicative of the vital sign of the subject (14). Still further, the device (12, 12', 12", 12" ') comprises a fusion unit (28) for determining an output signal based on the optical monitoring signal and the auxiliary monitoring signal, said output signal being representative of the vital sign of the subject (14); and a control unit (30) for controlling the optical sensor arrangement (18), the auxiliary sensor arrangement (26) and the fusion unit (28). The optical monitoring signal and the auxiliary monitoring signal are indicative of the same vital sign of the subject. The present invention further relates to a corresponding method as well as to a monitoring apparatus (10) including a device as described above.

Description

Low power device for monitoring a vital

FIELD OF THE INVENTION

The present invention relates to a device and method for monitoring a vital sign of a subject as well as to a monitoring apparatus comprising such a device. BACKGROUND OF THE INVENTION

Vital signs of a person, for example the heart rate (HR), the respiration rate (RR) or the blood oxygen saturation, serve as indicators of the current state of a person and as powerful predictors of serious medical events. For this reason, vital signs are extensively monitored in inpatient and outpatient care settings, at home or in further health, leisure and fitness settings.

A possible approach for continuously monitoring vital signs is the integration of a vital sign sensor into a handheld, wearable or other device for determining a vital sign signal when in contact with a user's body. Particularly, wrist-worn or arm band worn device can be attached to the body of a user to provide a comfortable monitoring. Application areas of such a device can particularly include personal fitness applications, in which, e.g., the heart rate or heart rate variability may be monitored. The measured heart rate or other vital sign is then provided to the user via a user interface to allow him adapting his training intensity and preventing ineffective or dangerous training schedules.

Different measurement principles are used in this context. One way of measuring vital signs is plethysmography. Plethysmography generally refers to the measurement of volume changes of an organ or a body part and in particular to the detection of volume changes due to a cardio-vascular pulse wave traveling through the body of a subject with every heart beat. Photoplethysmography (PPG) is an optical measurement technique. A time- variant change of light reflectance or transmission of an area or volume of interest is evaluated. PPG is based on the principle that blood absorbs light more than surrounding tissue, so variations in blood volume with every heart beat affect transmission or reflectance correspondingly. Besides information about the heart rate, a PPG waveform can comprise information attributable to further physiological phenomena such as the respiration. By evaluating the transmissivity and/or reflectivity at different wavelengths (for example red and infrared), the blood oxygen saturation can be determined. The MIO APLHA (e.g. disclosed at http://www.mioglobal.com) represents an implementation of a wristwatch-like heart rate monitoring system that is based on an optical measurement principle.

In US 2012/0283525 Al a biological inforamtion processing device is presented. The device includes a measuring unit that measures the pulse rate of a test subject; a detector that detects a body motion of the test subject; a computation unit that computes an intensity of exercise performed by the test subject using a result of detection by the detector; a first estimation unit that estimates an estimated pulse rate using the exercise intensity computed by the computation unit; and a controller that controls so that the pulse rate from a result of the measurement is displayed in an instance in which measurement by the measuring unit is possible, and the estimated pulse rate estimated by the first estimation unit is displayed in an instance in which measurement by the measuring unit is not possible.

In DE 10 2006 024 459 Al a sensor for measuring a vital parameter of a living organism is disclosed. The device includes a light source and a photo receiver and an acceleration sensor.

In AT 010 035 U1 a system for the mobile assessment and processimng of vital sign values is presented. The system includes at least two optical pulse detection units.

In US 20012/0150047 Al a pulse wave sensor is disclosed that includes a measurement unit to measure the pulse wave, a power source unit to supply power to the measurement unit, a cable to connect between the measurement unit and the power source unit electrically, and a armlet type housing to contain the measurement unit, the power source unit, and the cable.

The known devices have, however, some shortcomings particularly with respect to energy consumption or robustness to movement artifacts. Consequently, there is a need for further monitoring devices and in particular mobile monitoring devices.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved device for monitoring a vital sign of a subject when attached to the subject's body. It is further an object of the present invention to provide an improved method for monitoring a vital sign of the subject and an improved monitoring apparatus.

In a first aspect of the present invention there is presented a device for monitoring a vital sign of a subject when attached to the subject's body. The device comprises: an optical sensor arrangement including a light source for emitting light into tissue of the subject, a light sensor for receiving at least part of the emitted light after an interaction of the emitted light with the tissue and an evaluation unit for deriving an optical monitoring signal from the received light being indicative of the vital sign of the subject; an auxiliary sensor arrangement different from the optical sensor arrangement for obtaining an auxiliary monitoring signal being indicative of the vital sign of the subject; a fusion unit for determining an output signal based on the optical monitoring signal and the auxiliary monitoring signal, said output signal being representative of the vital sign of the subject; and a control unit for controlling the optical sensor arrangement, the auxiliary sensor arrangement and the fusion unit, wherein the optical monitoring signal and the auxiliary monitoring signal are indicative of the same vital sign of the subject.

In another aspect of the present invention there is presented a method for monitoring a vital sign of a subject. The method comprises the steps of: emitting light into tissue of the subject, receiving at least part of the emitted light after an interaction of the emitted light with the tissue and deriving an optical monitoring signal from the received light being indicative of the vital sign of the subject; obtaining an auxiliary monitoring signal being indicative of the vital sign of the subject; determining an output signal based on the optical monitoring signal and the auxiliary monitoring signal, said output signal being representative of the vital sign of the subject; and controlling an optical sensor arrangement, an auxiliary sensor arrangement and a fusion unit, wherein the optical monitoring signal and the auxiliary monitoring signal are indicative of the same vital sign of the subject.

In yet another aspect of the present invention there is presented a monitoring apparatus comprising a device as described above and a support band for supporting the device at a limb of the subject, in particular an arm of the subject.

In yet further aspects of the present invention, there are provided a computer program which comprises program code means for causing a computer to perform the steps of the method disclosed herein when said computer program is carried out on a computer as well as a non-transitory computer-readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method disclosed herein to be performed.

Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed method, computer program and medium have similar and/or identical preferred embodiments as the claimed system and as defined in the dependent claims. According to the present invention two different sensor arrangements are included in a single device to monitor a vital sign of the subject when the device is attached to the subject's body. The two sensor arrangements are different, i.e. rely on different measurement principles. Both sensor arrangements provide an output that is indicative of the same vital sign of the subject. Particularly, the present invention aims at monitoring the heart rate or parameters derived therefrom, such as the heart rate variability, of the subject.

However, also other vital signs may be monitored. The readings provided by the two different sensor arrangements are fused in a fusion unit and an output signal is determined based on the two readings of the same vital sign provided by the two sensor arrangements. Thus, the fusion unit provides a signal that is representative of the vital sign of the subject. Furthermore, the device includes a control unit, by means of which the sensor arrangements and the fusion unit are controlled.

The basic concept underlying the present invention is to exploit the advantages of different sensor arrangements by combining them (or more precisely their readings). For instance, a disadvantage of the optical sensor arrangement may be that it consumes a comparably high amount of energy due to the requirement of having an LED switched on during the whole measurement procedure. Another sensor arrangement, i.e. the auxiliary sensor arrangement, may consume less energy. On the downside, the auxiliary sensor arrangement may, however, have the disadvantage that movements of the body of the monitored subject may decrease the quality and/or validity of the monitoring signal. Other possible advantages/disadvantages may exist with regard to the accuracy of different sensor principles at varying environmental conditions (e.g. temperature, humidity, light, etc.). One sensor arrangement may be more suitable under given conditions whereas the other may be more suitable under other conditions. Furthermore, different sensor arrangements may be better or worse adapted to properties of the subject to be monitored (skin color, age, transpiration, etc.).

The present invention provides an approach to exploit the advantages of both sensor arrangements and avoid their respective disadvantages. The relative advantages of the optical sensor arrangement versus an auxiliary sensor arrangement are exploited and vice versa. To this end, the present invention comprises a fusion unit along with a control unit to allow an intelligent combination (fusion) of the signals provided by the two sensor arrangements as well as an intelligent control approach for controlling the sensor

arrangements and the fusion unit. The present invention allows combining two sensing approaches via an intelligent fusion system. Thereby, it may, e.g., become possible that the monitoring becomes more robust, particularly with respect to motion of the user. Also, it may become possible that the monitoring device consumes a lower amount of energy in comparison to previous systems. Further advantages may arise with respect to providing a more reliable signal under varying environmental conditions or for different subjects.

In a preferred embodiment, the auxiliary sensor arrangement is represented by a capacitive sensor arrangement including two electrodes forming a capacitor for measuring a capacitance when the two electrodes are in contact with a skin portion of the subject and an evaluation unit for deriving the auxiliary monitoring signal from the measured capacitance.

In a first embodiment the control unit is configured to switch off the optical sensor arrangement; and control the fusion unit to determine the output signal based on the auxiliary monitoring signal when the optical sensor arrangement is switched off.

One preferred control approach consists in that only the auxiliary monitoring signal forms the basis for the output signal when the optical sensor arrangement is switched off. When the optical sensor arrangement is switched off the control unit controls the fusion unit to (solely) base the output signal on the signal provided by the auxiliary sensor arrangement. Thus, when the optical sensor arrangement is switched off, its monitoring signal is not used. Thereby, it becomes possible to save energy. Whenever possible, the optical sensor arrangement is switched off and it is relied on the auxiliary sensor arrangement to provide an appropriate output signal. The control unit performs the necessary control operations for appropriately controlling the sensor arrangements as well as the fusion unit. If, e.g., it is assumed that the optical sensor arrangement consumes more energy than the auxiliary sensor arrangement this control approach makes it possible that less energy is consumed than if the optical sensor arrangement was operated continuously. In contrast to continuously operating the auxiliary sensor arrangement, however, it becomes possible that the more reliable measurement of the optical sensor arrangement is used as a basis for the output signal in certain cases.

In another preferred embodiment the control unit is further configured to switch off the auxiliary sensor arrangement when the optical sensor arrangement is switched on and vice versa and to control the fusion unit to determine the output signal based on the optical monitoring signal when the optical sensor arrangement is switched on. According to this embodiment only one of the two sensor arrangements is switched on during operation of the device. When one sensor arrangement is switched off the other one is switched on and vice versa. Thus, it becomes possible to save even more energy.

In yet another embodiment the device comprises a reliability unit for determining a reliability parameter being indicative of the validity of the auxiliary monitoring signal with respect to the vital sign of the subject. Such a reliability parameter may be used as an input to the control unit in order to determine an appropriate control approach. Thereby, a reliability parameter may be indicated on a relative or absolute scale and may be compared to previously determined values. It may also be possible that the reliability parameter is a binary value indicating whether the signal that is currently provided by the auxiliary sensor arrangement is considered to be valid or not. The advantage of this reliability parameter is that it becomes possible to control the optical sensor arrangement, the auxiliary sensor arrangement and the fusion unit based on the reliability of the current output of the auxiliary sensor arrangement. The reliability unit may also determine a reliability parameter that includes information on the reliability of the optical sensor arrangement. If it is assumed that the reliability of the auxiliary sensor arrangement is unchanged, a change in the reliability parameters means that the optical sensor arrangement is now considered to be less reliable. Reliability and the reliability parameter herein particularly refer to the validity with respect to the monitored vital sign of the subject, i.e. how accurate the current reading is.

In a preferred embodiment the reliability unit is configured to determine the reliability parameter based on at least one of:

a sample of the optical monitoring signal;

a sample of the auxiliary monitoring signal;

an optical sensor parameter being indicative of an output quality of the optical sensor arrangement; and

an auxiliary sensor parameter being indicative of an output quality of the auxiliary sensor arrangement.

Herein, a sample of the signal may particularly refer to a recording of the signal for a short time period, such as a few seconds. A sensor parameter may particularly refer to a quality parameter being derived from a property of a sensor or sensor arrangement, e.g. determined during a calibration procedure and referenced in a datasheet or the like for the sensor. Such a sensor parameter may describe a property of a sensor arrangement.

According to this embodiment it may, e.g., be possible that a sample of the auxiliary monitoring signal is used to determine the reliability parameter of the auxiliary monitoring signal. For instance, it can be evaluated whether the sample fulfills certain predefined criteria with respect to predetermined parameters such as frequency, intensity, variability, variance, etc. It may also be possible that the validity of the auxiliary monitoring signal is assessed based on a sample of the optical monitoring signal. For instance, the optical monitoring signal may be used as a reference to which the auxiliary monitoring signal is compared. If the differences between the two signals are above a certain predefined threshold (or fulfill another predefined or continuously updated criterion), the reliability parameter may be determined appropriately. Further, it may also be possible that the signal of the auxiliary sensor arrangement is used for determining a validity of the optical sensor arrangement. It may, e.g., be possible that an optical sensor parameter or an auxiliary sensor parameter is predetermined during a calibration procedure or the like or that it is determined during the use of the device, i.e. continuously updated.

In a preferred embodiment the control unit is configured to switch on the optical sensor arrangement when the reliability parameter is below a reliability threshold. In this case, the optical sensor arrangement is only switched on when the auxiliary monitoring signal is determined to not being valid (i.e. that the reliability parameter indicates that the monitoring signal has only a low validity with respect to the monitored vital sign). This means that if the signal of the auxiliary sensor arrangement is considered to not carry sufficient information, the optical sensor arrangement is switched on. Thereby, it becomes possible to operate the optical sensor arrangement only when required. This allows the device according to the present invention to consume less energy than if the optical sensor arrangement was operated continuously. This also allows, however, the device according to the present invention to provide a more reliable output signal than if only the auxiliary sensor arrangement was used continuously, given that the auxiliary sensor arrangement is more prone to errors than the optical sensor arrangement.

In another embodiment the fusion unit is configured to determine the output signal further based on the reliability parameter.

It may also be possible that the fusion unit relies on the reliability parameter when determining the output signal. For instance, it may be possible that the two signals provided by the two sensor arrangements are combined based on an operation in which also the reliability parameter is considered. In case of a binary reliability parameter, a

combination may, e.g., correspond to a selection.

Preferably, the control unit is configured to control the fusion unit to determine the output signal based on the auxiliary monitoring signal when the reliability parameter is above a reliability threshold. As soon as a certain predefined or continuously updated reliability threshold is acceded the optical monitoring signal is not used any more and the fusion unit determines the output signal based on the auxiliary monitoring signal alone. A reliability parameter that is determined based on a threshold thus substantially corresponds to a binary reliability parameter.

Further preferably, the fusion unit is configured to calculate a weighting parameter representing a relative weight of the auxiliary monitoring signal relative to the optical monitoring signal; and use the weighting parameter to determine an output signal based on a weighted average of the auxiliary monitoring signal and the optical monitoring signal. In this embodiment, the signals provided by the optical sensor arrangement and the auxiliary sensor arrangement are fused by means of a weighted average. Thereby, it becomes possible to provide an output signal that has a higher validity with respect to the vital sign of the subject than each of the optical monitoring signal and auxiliary monitoring signal alone. Consequently, the quality of the output signal, in particular with the validity of the determined output signal with respect to the monitored vital sign of the subject, is higher in comparison to monitoring devices that include only one sensor arrangement or multiple sensor arrangements but do not fuse the signals of the different sensor arrangements. The weighting parameter can thus be calculated based on the reliability parameter. For instance, it may be possible that the auxiliary monitoring signal is attributed a higher relative weight when the reliability parameter is high. Equivalently, the optical monitoring signal may be attributed a higher weight when the reliability parameter indicating the validity of the auxiliary monitoring signal with respect to the vital sign of the subject is lower. Herein, higher or lower may refer to (predetermined) thresholds or function values of a function determined during a calibration procedure. If the reliability parameter is determined on a scale of 0 to 1 it may directly be used as a weighting parameter. Also transformations or inverse functions or a binary value may be applied. By making use of a weighting parameter to calculate a weighted average of the two monitoring signals it becomes possible to obtain an output signal with a higher validity than each of the two monitoring signals alone. In this case, both the optical monitoring signal and the auxiliary monitoring signal have to be indicated on the same scale as the output signal. In case of a heart rate, e.g., this scale may be beats per minute. In case of heart rate variability, this scale may be a parameter derived from a histogram or the like such as a variance.

In a preferred embodiment the auxiliary sensor arrangement corresponds to one of a capacitive sensor arrangement including two electrodes forming a capacitor for measuring a capacitance when the two electrodes are in contact with a skin portion of the subject and an evaluation unit for deriving the auxiliary monitoring signal from the measured capacitance; an expansion sensor arrangement including a stress measurement sensor attached to an elastic band for measuring a stress of the elastic band when the elastic band is arranged around a body part of the subject and an evaluation unit for deriving the auxiliary monitoring signal from the measured stress; and a bio-impedance sensor arrangement including two electrodes, a current source connected to the two electrodes for applying a current to the two electrodes, an impedance sensor for measuring the impedance when the two electrodes are in contact with a skin portion of the subject and an evaluation unit for deriving the auxiliary monitoring signal from the measured impedance.

The different sensor arrangements may all be used to determine a monitoring signal being indicative of a vital sign of a subject, in particular the heart rate of a subject. It may also be possible that measurements provided by the auxiliary sensor arrangement are used to merely determine whether the signal provided by the optical sensor arrangement is valid or not. For instance, it may be possible that the capacitive sensor arrangement can be used to determine whether or not the device is in contact with a skin portion of the subject. If it is not in contact with a skin portion of the subject the optical sensor arrangement may not provide a meaningful signal.

In a preferred embodiment the monitored vital sign corresponds to the heart rate or the heart rate variability of the subject. As described above, the application of the present invention is particularly advantageous when the heart rate or heart rate variability of the subject is monitored. This is of particular interest in fitness applications. Thereby, the device according to the present invention will usually include a feedback unit, such as a display or the like for providing the measured heart rate or heart rate variability to the subject. Then, the subject may adapt his current workout program or fitness program to his heart rate (or other vital sign).

In yet another embodiment the device further comprises a movement sensor for providing a movement signal being indicative of a movement of the subject, wherein the reliability unit is configured to determine the reliability parameter based on the movement signal. A movement sensor, such as an acceleration sensor or the like may be used for determining whether the subject moves or not. Usually, if the subject moves, the

measurement provided by the auxiliary sensor arrangement will be of a lower validity than the measurement provided by the optical sensor arrangement. This is particularly important if the auxiliary sensor arrangement corresponds to a capacitive sensor arrangement. According to this embodiment, the reliability parameter is thus also based on a measurement of a motion sensor. For instance, it may be possible that the auxiliary monitoring signal is considered to not be valid when the subject moves. The advantage of this embodiment is that it is not required to perform any further signal processing but that the determination of the reliability parameter can be based on an additional sensor. Thereby, artifact effects can be reduced and the reliability can be further improved.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment s) described hereinafter. In the following drawings

Fig. 1 shows an illustration of a monitoring apparatus according to an aspect of the present invention;

Fig. 2 shows a schematic illustration of a device for monitoring a vital sign of a subject according to an aspect of the present invention;

Fig. 3 shows an illustration of an embodiment of a device for monitoring a vital sign in a backside view;

Fig. 4 shows another illustration of an embodiment of a device for monitoring a vital sign;

Fig. 5 shows yet another illustration of an embodiment of a device for monitoring a vital sign according to the present invention; and

Fig. 6 schematically illustrates a method for monitoring a vital sign according to an aspect of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 illustrates a monitoring apparatus 10 according to a first aspect of the present invention. The apparatus 10 includes a device 12 for monitoring a vital sign of a subject 14 when attached to the subject's body. In the illustrated example the device 12 is attached to the subject's 14 body by means of a support band 16. As illustrated, a preferred embodiment of a monitoring apparatus 10 according to the present invention corresponds to a wristwatch-like apparatus that can be worn by the human being at his wrist. Other embodiments of the present invention may, however, also include devices to be attached to the leg of a human being by means of a strap or band or the like. Further, a device according to the present invention may be represented by a device to be attached to the breast of a human being by means of a breast belt. Still further, embodiments of the present invention may take the form of hand-held devices that are not attached to the subject's body but that are adapted to be manually applied to a body part. The concept disclosed herein may also be applied in other forms.

The determined output signal is provided to the user by means of a user interface. In particular this user interface may include a digital display. Further, the apparatus may include control elements such as buttons or a touch screen for allowing the user to change parameters, switch between different display options, chose settings etc.

The present invention provides an approach for monitoring a vital sign of a subject when attached to the subject's body. In the following, the concept of the present invention is outlined with particular regard to monitoring the heart rate of the subject. The present invention is, however, not limited to be used for monitoring the heart rate but can also be used for monitoring other vital sings such as the breathing rate, the blood oxygen saturation, etc.

Particularly in fitness and leisure applications, heart rate sensors are widely used. In addition to breast belt devices, it has been demonstrated that it is also possible to monitor the heart rate of a user by means of a wristwatch-like device, i.e. a system in form of a wristwatch to be worn at a wrist of a user, which includes a heart rate sensor. For this, basically two different solutions have been demonstrated. On the one hand it is possible to monitor the heart rate of the subject by means of an optical sensor, i.e. a PPG sensor, which basically relies on the principle that light interacts with pulsating blood in tissue of the subject. These interactions can be evaluated and the heart rate can be derived therefrom. Such a PPG sensor is usually substantially in direct contact with a skin portion of the subject. On the other hand, it has been demonstrated that pulsating blood also causes a change in capacitance that can be measured by electrodes attached to the subject's skin. Both approaches, however, have disadvantages. The optical monitoring approach requires a LED to be operated most of the time and thus has relative high power consumption. The capacitive method can be implemented to only consume very little energy but is very susceptible to motion artifacts (i.e. disturbances caused by a moving user). In particular, in fitness applications where a user wears a monitoring device during his workout, this may result in a high number of errors (i.e. inaccurate readings).

The main idea of the present invention is to intelligently combine two sensing modalities and thereby increase the robustness of the monitoring system and reduce its power consumption. Although it may look straightforward to combine different types of sensors, the present invention makes it possible to fuse the signals captured by two different sensing modalities for the same vital sign in order to increase robustness and/or reduce power consumption.

Fig. 2 schematically illustrates a device 12 for monitoring a vital sign of a subject 14 according to the present invention. As used herein, the distinction between device for monitoring 12 and monitoring apparatus 10 aims at clarifying that the present invention may also take other forms than devices that are to be attached to the subject's 14 body by means of a support band.

As illustrated in Fig. 2, the device includes an optical sensor arrangement 18. This optical sensor arrangement 18 comprises a light source 20 that emits light into tissue of the subject 14. Usually, the light will be emitted into a skin portion of the subject 14, e.g., the skin portion under the wristwatch-like apparatus. This emitted light interacts with the tissue of the subject 14 and in particular with the pulsating blood in this tissue. Part of the emitted light may be transmitted through the tissue and part of it may be reflected. After this interaction, at least part of the emitted light is received by means of a light sensor 22 in the optical sensor arrangement 18. Based upon the received light, an optical monitoring signal is derived in an evaluation unit 24. This optical monitoring signal is indicative of the vital sign of the subject 14. The optical monitoring signal may particularly correspond to the heart rate of the subject 14 itself or to a parameter derived therefrom, such as the heart rate variability. The optical monitoring signal may, however, also correspond to the PPG waveform that allows deriving other vital signs therefrom or to another vital sign.

Usually, the optical sensor arrangement 18 includes a LED. This LED is usually operated in pulsed mode, i.e. emits light in short pulses. This may also be referred to as burst mode. The LED may be operated periodically, e.g. 0.5 ms on per 8 ms period time. Continuously operating the optical sensor arrangement particularly refers to operating the LED in such a pulsed mode. Normally, it is not required that the LED is powered on all the time. Power considerations require the LED to be in its on state as little as possible. It may, however, also be possible that the LED is operated in a continuous mode, i.e. always on.

The device 12 further comprises an auxiliary sensor arrangement 26 that is different from the optical sensor arrangement 18. This difference may particularly be with respect to a different measurement principle (i.e. physical measurement principle) that is applied. It may, however, also be possible that the auxiliary sensor arrangement is also an optical sensor arrangement that is different from with respect to the wavelength of the emitted light, the accuracy or another parameter. In a preferred embodiment the auxiliary sensor arrangement 26 may be represented by a capacitive sensor arrangement that includes two electrodes that form a capacitor for measuring a capacitance when the two electrodes are in contact with a skin portion of the subject 14 along with an evaluation unit for deriving an auxiliary monitoring signal that is representative of the vital sign of the subject 14 from the measured capacitance. Other possible embodiments of the present invention include embodiments wherein the auxiliary sensor arrangement 26 is represented by an expansion sensor arrangement or by a bioimpedance sensor arrangement or by another type of sensor arrangement. The auxiliary sensor arrangement 26 generates an auxiliary monitoring signal that is indicative of the same vital sign as the optical monitoring signal generated by the optical sensor arrangement 18.

Usually, the auxiliary monitoring signal and the optical monitoring signal will be indicated on the same scale. For the example of the heart rate of the subject both the auxiliary monitoring signal and the optical monitoring signal may be represented by a figure indicating a number of heart beats per minute. This number is usually calculated from the received light or from the physical phenomenon measured in the auxiliary sensor

arrangement 26 in the evaluation unit 24 of the optical sensor arrangement or in an evaluation unit in the auxiliary sensor arrangement 26.

The two monitoring signals form the basis for determining an output signal in a fusion unit 28 that is also comprised in the device 12. In this fusion unit 28 the two monitoring signals are combined in order to derive therefrom an output signal that usually has a higher validity with respect to the monitored vital sign of the subject than each of the two monitoring signals provided by the sensor arrangements alone.

Still further, the device 12 includes a control unit 30. This control unit 30 is also connected to the sensor arrangements and the fusion unit 28 and allows applying a control to the sensor arrangements and/or the fusion unit 28. Controlling as used herein may particularly refer to adjusting settings of the sensor arrangements and/or the fusion unit 28 in order to configure the sensor arrangements and/or the fusion 28 unit to perform their functionalities according to preset parameters or current requirements. In particular, the control unit 30 may control the two sensor arrangements in that it switches them on or off depending on the respective monitoring signal that is currently delivered by one of the two sensor arrangements or by each of them. For instance, if there are no movement artifacts in the monitoring signal provided by the auxiliary sensor arrangement 26, the LED and possibly other parts of the circuitry of the optical sensor arrangement 18 can be switched off to save power. Herein, switching off not only refers to actually powering off the whole sensor arrangement but may also refer to powering off parts therefrom, i.e. putting the sensor arrangement into a sleep mode or power saving mode or low power mode. When the optical sensor arrangement 18 is switched off the output signal is determined in the fusion unit 28 based on the monitoring signal provided by the auxiliary sensor arrangement 26 alone. As soon as, however, the auxiliary sensor arrangement 26 produces an unreliable signal, the control unit 30 controls the fusion unit 28 to determine the output signal based on the optical monitoring signal provided by the optical sensor arrangement 18.

In a device 12 according to the present invention it may be possible that the auxiliary sensor arrangement 26 is used to check whether the optical sensor arrangement 18 makes contact with the skin of the subject 14 or not. If the optical sensor arrangement 18 is not in contact with the skin the optical monitoring signal may not be reliable. In case of a wristwatch-like apparatus, the user may be advised to tighten the band of the watch.

In yet other embodiments the device 12 may be used to combine the monitoring signals provided by the optical sensor arrangement 18 and the auxiliary sensor arrangement 26, e.g. for obtaining medical information. Usually, an optical sensor arrangement is particularly sensitive to blood volume changes in the surface layers of the skin of the subject 14. In contrast thereto, the auxiliary sensor arrangement 26 may be particularly sensitive to blood volume changes in deeper layers of the tissue (e.g. when the electrodes of a capacitive sensor arrangement representing the auxiliary sensor arrangement are placed on opposite sides of a limb of the subject). Then, the ratio of the monitoring signals provided by the two sensor arrangements may be indicative of the structure and the response of the blood vessels. In this case, the control unit 30 may be configured to control the fusion unit 28 to calculate an output signal that basically corresponds to an average of the monitoring signals provided by the two sensor arrangements. For instance, a weighted average of the two signals may be determined. For this, a weighting factor may be derived based on a reliability of at least one of the two sensor arrangements. This reliability (reliability parameter) may, e.g. be predetermined based on a property of one of the sensor arrangements or may also be continuously updated.

In order to determine such a reliability parameter, an embodiment of the present invention may include a reliability unit 32 for determining a reliability parameter being indicative of the validity of the auxiliary monitoring signal with respect to the vital sign of the subject 14. As illustrated in Fig. 2 by the dashed line, this reliability unit 32 is optional and not necessarily included in all embodiments of the present invention. Such a reliability unit 32 may particularly be configured to determine the reliability parameter based on a sample of one or more of the monitoring signals provided by on of the sensor arrangements and/or based on a sensor parameter that is inherent to one of the sensor arrangements. Such a sensor parameter may be predetermined, e.g. during a calibration procedure wherein the robustness with respect to motion artifacts is empirically assessed, or may be continuously updated, e.g. in the form of a moving average or the like. It may also be possible that combinations of parameters, e.g. thresholds, and/or samples of the monitoring signals are used. Particularly, a sensor parameter may carry information on a quality of the monitoring signal provided by a sensor arrangement. A quality measure may, e.g., be an empirically determined accuracy, robustness to environmental influences etc. A sensor parameter may also be a threshold or constant value.

The reliability unit 32 determines a reliability parameter that is used by the fusion unit 28 for determining the output signal. Furthermore, the reliability parameter may also be used by the control unit 30 for adapting the applied control. For instance, possible control approaches may include switching one of the sensor arrangements on or off when the reliability parameter is above or below a certain predefined threshold (i.e. a sensor parameter). When the reliability unit 32 provides a reliability parameter that indicates that the monitoring signal provided by the auxiliary sensor arrangement 26 is sufficiently valid, the control unit 30 may switch off the optical sensor arrangement 18 in order to save energy. Particularly, it is possible that the optical monitoring signal is used whenever the auxiliary monitoring signal is determined to be disturbed by movement artifacts for instance by means of a signal analysis of the auxiliary monitoring signal with respect to frequency, frequency variation or amplitude and adequate thresholds.

Alternatively, it may be possible that the weighting factor for fusing the monitoring signals provided by the two sensor arrangements is determined based on the reliability parameter. For instance, if the monitoring signal of the auxiliary sensor arrangement has a low reliability (e.g. lower than a predetermined threshold), the weighting of the auxiliary monitoring signal in a weighted average calculation of the output signal in the fusion unit 28 may be reduced versus the weighting of the optical monitoring signal.

In yet another embodiment there may further be included a movement sensor 34 for providing a movement signal being indicative of a movement of the subject 14. Such a movement sensor 34 may particularly be represented by an acceleration sensor. The movement signal provided by the movement sensor 34 may be used in the reliability unit 32 in order to determine the reliability parameter based thereupon. For instance, it may be possible that the reliability parameter is calculated as to indicate that the validity of the auxiliary monitoring signal is low with respect to the heart rate of the subject 14 when the movement signal indicates that the subject moves. This may, e.g., be the case if the auxiliary sensor arrangement 26 is represented by a capacitive sensor arrangement that is prone to movement artifacts due to the electrodes being displaced versus the skin of the subject 14 when the subject 14 moves.

Fig. 3 illustrates a perspective illustration of an embodiment of a device 12' according to the present invention in backside view. Therein, the device 12' is again included in a wristwatch-like monitoring apparatus. Fig. 3 illustrates a backside view of the apparatus, i.e. the side of the apparatus that will be in direct contact with the skin of a subject having the apparatus attached to his wrist.

In the illustrated embodiment of the device 12' the auxiliary sensor arrangement 26 is represented by a capacitive sensor arrangement. This capacitive sensor arrangement particularly includes two electrodes 36, which, when being in contact with the skin of the subject, can measure a change in capacitance that is caused by pulsating blood. The electrodes 36 are usually connected by wires integrated in the support band 16 to an evaluation unit of the auxiliary sensor arrangement that is integrated in the housing 38 of the watch-like apparatus.

The light source 20 of the optical sensor arrangement is represented by two LEDs 40 facing the skin of the subject when the wristwatch-like apparatus is worn by the subject. These LEDs 40 may emit light of the same or of different wavelengths. For instance, green and red light may be emitted or red and infrared light may be emitted. After interacting with the skin of the subject the light is captured by a light sensor. This light sensor may particularly be represented by a photodiode 42 that is also integrated in the housing 38 of the apparatus and that is arranged in between the two LEDs and also in contact with the skin of the subject when the subject has the wristwatch-like apparatus attached to his wrist. The photodiode 42 is susceptible to light that is reflected at the skin of the subject. The evaluation unit of the optical sensor arrangement is usually also integrated in the housing 38. Fig. 3 also illustrates two buttons 43 that are part of a user interface allowing the user to control some of the functionalities of the monitoring apparatus 12' .

In Fig. 4 another possible embodiment of the device 12" is illustrated, in which the auxiliary sensor arrangement is represented by a capacitive sensor arrangement. In comparison to the embodiment illustrated in Fig. 3, the electrodes 36 of the capacitive sensor arrangement are, however, also integrated in the housing 38 and not integrated in the support band 16. Depending on the desired application area differently positioned electrodes may advantageous in comparison to others.

Other embodiments of the present invention that also rely on a capacitive sensor arrangement representing the auxiliary sensor arrangement exist. For instance, it may be possible that textile electrodes are integrated in the support band. It may also be possible that the electrodes are applied to the skin of the subject at other positions, e.g. by means of a cantilever structure attached to the housing of the device or the like.

Fig. 5 illustrates yet another embodiment of a device 12" ' according to the present invention. As illustrated in Fig. 5, the auxiliary sensor arrangement is represented by an expansion sensor arrangement including a stress measurement sensor 44. This stress measurement sensor may particularly include one or two strain gauges 44 to be integrated in an elastic band 46 representing the support band for supporting the device 12' " at a limb of the subject. When the blood pulsates in the limb of the subject the elastic band 46 is stretched. This stretch can be measured by means of a strain gauge 44 as illustrated in Fig. 5. Based on the signal derived therefrom, the auxiliary monitoring signal being indicative of the heart rate of the subject may be obtained.

Still further embodiments of the present invention include embodiments in which the auxiliary sensor arrangement is represented by a bio-impedance sensor

arrangement (not illustrated). This bio-impedance sensor arrangement may, comparable to the illustrations in Figs. 3 and 4, also include two electrodes. These two electrodes, however, are connected to a current source, which applies a current to them. Thus, the skin or the tissue of the subject is subject to an electrical current flow. Then, the impedance is measured by means of an impedance sensor. This measured impedance is influenced by the pulsating blood in the tissue of the subject. Consequently, the auxiliary monitoring signal being indicative of the vital sign of the subject can be obtained from evaluating the measured impedance.

In yet further embodiments of the present invention it may be possible that one or more of the evaluation unit of the optical sensor arrangement, an evaluation unit of the auxiliary sensor arrangement, other parts of the sensor arrangements, the control unit and the reliability unit are partly or entirely implemented in hard- or in software. It may be possible that some or all of the functions provided by the units or other components are carried out by one or more processors (e.g. an IC, ASIC, FPGA, etc.).

Fig. 6 illustrates a method for monitoring a vital sign of the subject. The method comprises the steps of emitting (step S10) light into tissue of a subject, receiving (step SI 2) at least part of the emitted light after it has interacted with tissue of the subject and deriving (step SI 4) therefrom an optical monitoring signal that is indicative of the vital sign of the subject. The method further comprises a step of obtaining (step SI 6) an auxiliary monitoring signal. This auxiliary monitoring signal is representative of the same vital sign of the subject as the optical monitoring signal. Both the auxiliary monitoring signal as well as the optical monitoring signal form the basis for a step of determining (step S 18) an output signal. This output signal is consequently also representative of the same vital sign of the subject. Still further, the method comprises a step of controlling an optical sensor

arrangement which is used for obtaining the optical monitoring signal, an auxiliary sensor arrangement, which is used for obtaining the auxiliary monitoring signal and a fusion unit which is used for determining the output signal. Such a method may particularly be executed by a processor included in a mobile device such as a wristwatch-like device.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

Herein, the terms "device for monitoring" and "monitoring device" are used synonymously. Attached to the subject's body may particularly refer to attached to a limb, such as an arm or a leg of a person. A monitoring device may also be integrated into a breast belt. A device (device for monitoring) as used herein means that the device physically forms a single unit. Usually, all parts of the device are integrated in a single housing. This housing may particularly be hold at the subject's body by means of a strap or band or the like. The different parts of the device according to the present invention are physically connected. It may, however, be possible that some parts are attached to the housing in the form of a cantilever structure a wire connection or the like in some embodiments. Embodiments of the present invention may, e.g., also include parts of the optical or auxiliary sensor arrangement not being integrated in the same single housing but being connected thereto via a wired connection. For instance, an electrode of a capacitive sensor arrangement representing the auxiliary sensor arrangement may be integrated in a support band supporting the housing, in which the units, the optical sensor arrangement and the other parts of the auxiliary sensor arrangement are integrated, in order to provide a larger distance between two electrodes forming a capacitor for carrying out a capacitive measurement of the heart rate of the subject. A subject as used herein may particularly refer to a human being. The optical sensor arrangement as used herein particularly corresponds to a PPG sensor arrangement. The light source particularly corresponds to an LED. This LED emits light into tissue of the subject, i.e. into a skin portion of the subject that is substantially in contact with the device. This light interacts with this skin portion. This means that part of the light may transmitted and part of the light may be reflected. The light sensor particularly corresponds to a photodiode. Based on the light that this photodiode detects the vital sign of the subject is derived.

The auxiliary sensor arrangement as used herein may particularly include a capacitive, optical and/or piezoresistive sensor that allows measuring the same vital sign as the optical sensor arrangement by means of a different measurement principle. The auxiliary sensor arrangement is based on a measurement principle different from the measurement principle of the optical sensor arrangement. A different measurements principle may particularly refer to a different physical measurement principle. The auxiliary sensor arrangement has different properties with respect to energy consumption, accuracy, physical size and robustness versus motion of the subject etc. It may also be possible that the auxiliary sensor arrangement is substantially based on the same measurement principle but exhibits different properties due to another data processing, other components, another calibration etc.

A sensor arrangement is used herein particularly refers to the sensor, i.e. the physical sensing component itself, as well as the necessary signal processing equipment for deriving a signal being indicative of the vital sign of the subject. A sensor arrangement may particularly include an evaluation unit that allows evaluating the analog or digital signal provided by the physical sensing unit and deriving therefrom a meaningful signal. Different filters or signal shaping components may be included. The optical monitoring signal provided by the optical sensor arrangement and the auxiliary monitoring signal provided by the auxiliary sensor arrangement to the fusion unit are usually indicated on the same scale. For instance, a monitoring signal may correspond to a heart rate signal indicated on a beats per minute (bpm) scale.

Herein, switching off a sensor arrangement may particularly refer to putting the sensor arrangement into a state in which it does not consume energy or consumes less energy. Consequently, switching off as used herein may also refer to putting the sensor arrangement into a low power state or sleep state.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

A computer program may be stored/distributed on a suitable non-transitory medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. Device for monitoring a vital sign of a subject (14) when attached to the subject's body, said device (12, 12', 12", 12"') comprising:

an optical sensor arrangement (18) including a light source (20) for emitting light into tissue of the subject, a light sensor (22) for receiving at least part of the emitted light after an interaction of the emitted light with the tissue and an evaluation unit (24) for deriving an optical monitoring signal from the received light being indicative of the vital sign of the subject (14);

an auxiliary sensor arrangement (26) different from the optical sensor arrangement (18) for obtaining an auxiliary monitoring signal being indicative of the vital sign of the subject (14);

a fusion unit (28) for determining an output signal based on the optical monitoring signal and the auxiliary monitoring signal, said output signal being representative of the vital sign of the subject (14); and

a control unit (30) for controlling the optical sensor arrangement (18), the auxiliary sensor arrangement (26) and the fusion unit (28);

wherein the optical monitoring signal and the auxiliary monitoring signal are indicative of the same vital sign of the subject.

2. Device (12, 12', 12", 12" ') as claimed in claim 1 , wherein the control unit (30) is configured to

switch off the optical sensor arrangement (18); and

control the fusion unit (28) to determine the output signal based on the auxiliary monitoring signal when the optical sensor arrangement (18) is switched off.

3. Device (12, 12', 12", 12" ') as claimed in claim 2, wherein the control unit

(30) is further configured to

switch off the auxiliary sensor arrangement (26) when the optical sensor arrangement (18) is switched on and vice versa; and control the fusion unit (28) to determine the output signal based on the optical monitoring signal when the optical sensor arrangement (18) is switched on.

4. Device (12, 12', 12", 12" ') as claimed in claim 1 , further comprising a reliability unit (32) for determining a reliability parameter being indicative of the validity of the auxiliary monitoring signal with respect to the vital sign of the subject (14).

5. Device (12, 12', 12", 12" ') as claimed in claim 4, wherein the reliability unit (32) is configured to determine the reliability parameter based on at least one of:

a sample of the optical monitoring signal;

a sample of the auxiliary monitoring signal;

an optical sensor parameter being indicative of an output quality of the optical sensor arrangement (18); and

an auxiliary sensor parameter being indicative of an output quality of the auxiliary sensor arrangement (26).

6. Device (12, 12', 12", 12" ') as claimed in claim 4, wherein the control unit (30) is configured to switch on the optical sensor arrangement (18) when the reliability parameter is below a reliability threshold.

7. Device (12, 12', 12", 12" ') as claimed in claim 4, wherein the fusion unit (28) is configured to determine the output signal further based on the reliability parameter.

8. Device (12, 12', 12", 12" ') as claimed in claim 7, wherein the control unit (30) is configure to control the fusion unit (28) to determine the output signal based on the auxiliary monitoring signal when the reliability parameter is above a reliability threshold.

9. Device (12, 12', 12", 12" ') as claimed in claim 1 , wherein the fusion unit (28) is configured to

calculate a weighting parameter representing a relative weight of the auxiliary monitoring signal relative to the optical monitoring signal; and

use the weighting parameter to determine an output signal based on a weighted average of the auxiliary monitoring signal and the optical monitoring signal.

10. Device (12, 12', 12", 12" ') as claimed in claim 1 , wherein the auxiliary sensor arrangement (26) corresponds to one of a:

a capacitive sensor arrangement including two electrodes (36) forming a capacitor for measuring a capacitance when the two electrodes (36) are in contact with a skin portion of the subject (14) and an evaluation unit for deriving the auxiliary monitoring signal from the measured capacitance;

an expansion sensor arrangement including a stress measurement sensor (44) attached to an elastic band (46) for measuring a stress of the elastic band (46) when the elastic band (46) is arranged around a body part of the subject (14) and an evaluation unit for deriving the auxiliary monitoring signal from the measured stress; and

a bio-impedance sensor arrangement including two electrodes, a current source connected to the two electrodes for applying a current to the two electrodes, an impedance sensor for measuring the impedance when the two electrodes are in contact with a skin portion of the subject (14) and an evaluation unit for deriving the auxiliary monitoring signal from the measured impedance.

1 1. Device (12, 12', 12", 12" ') as claimed in claim 1 , wherein the monitored vital sign corresponds to the heart rate or the heart rate variability of the subject (14).

12. Device (12, 12', 12", 12" ') as claimed in claim 4, further comprising a movement sensor (34) for providing a movement signal being indicative of a movement of the subject,

wherein the reliability unit (32) is configured to determine the reliability parameter based on the movement signal.

13. Monitoring apparatus (10), comprising

a device (12, 12', 12", 12" ') as claimed in claim 1 ;

a support band (16) for supporting the device (12, 12', 12", 12"') at a limb of the subject (14), in particular an arm of the subject (14).

14. Method for monitoring a vital sign of a subject (14) comprising the steps of:

emitting (S10) light into tissue of the subject, receiving (SI 2) at least part of the emitted light after an interaction of the emitted light with the tissue and deriving (SI 4) an optical monitoring signal from the received light being indicative of the vital sign of the subject (14);

obtaining (SI 6) an auxiliary monitoring signal being indicative of the vital sign of the subject (14);

determining (SI 8) an output signal based on the optical monitoring signal and the auxiliary monitoring signal, said output signal being representative of the vital sign of the subject (14); and

controlling (S20) an optical sensor arrangement (18), an auxiliary sensor arrangement (26) and a fusion unit (28),

15. Computer program comprising program code means for causing a computer to carry out the steps of the method as claimed in claim 14 when said computer program is carried out on the computer.