WO2013132378A1

WO2013132378A1 - System and method for measuring a physiological rhythm of a subject

Info

Publication number: WO2013132378A1
Application number: PCT/IB2013/051403
Authority: WO
Inventors: Klaus Steffen LEONHARDT; Tobias Wartzek; Christoph BRÜSER; Anna KEREKES
Original assignee: Koninklijke Philips N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2012-03-06
Filing date: 2013-02-21
Publication date: 2013-09-12

Abstract

A system for measuring a physiological rhythm of a subject comprises a light source arranged to emit light, a photodetector arranged to receive the emitted light, a layer of compressible material directly contacting the light source, and a processing device connected to the photodetector and arranged to detect changes in the emitted light received by the photodetector. In a preferred embodiment, the system further comprises multiple light sources each directly contacting the layer of compressible material, wherein each light source is arranged to emit light in turn, so that no more than one light source is emitting light at any one time.

Description

SYSTEM AND METHOD FOR MEASURING A PHYSIOLOGICAL RHYTHM OF A SUBJECT

TECHNICAL FIELD OF THE INVENTION

This invention relates to a system for measuring a physiological rhythm of a subject. In one embodiment, the invention provides methods and apparatus for unobtrusive measurement of vital signs and activity of a subject by means of bed-mounted, optical sensors.

BACKGROUND TO THE INVENTION

Cardiovascular diseases in general, and heart failure in particular, are among the commonest reasons for hospitalization in industrialized countries. In order to deal with the growing number of patients, technical solutions which enable a personalized monitoring and treatment, preferably at home, are desirable. In recent years, the bed has emerged as a promising place for long-term monitoring of cardiopulmonary activity at home, as virtually everyone spends a significant portion of their day in bed. Furthermore, instrumented beds could be applied in the general wards of hospitals where fully automatic, unobtrusive monitoring systems could help to reduce the workload of the staff and increase the safety of the patients.

One promising approach for measuring cardiopulmonary activity is the integration of highly sensitive mechanical sensors into the bed- frame or mattress of the subject's bed that record the vibrations of the body caused by the mechanical activity of the heart as well as the respiratory movements of the thorax. This basic principle, known under the term "ballistocardiography" (BCG), has first been reported in the late 19th century. However, through improvements in sensor technologies and digital signal processing techniques, the field has gained renewed interest in recent years. Using a variety of different sensors such as strain gauges, PVDF and EMFi sensors, accelerometers, hydraulic and pneumatic sensors as well as optical devices, BCG systems have been integrated into objects of daily life, such as beds, chairs and even weighing scales. These systems share the common advantage that they are unobtrusive and that they do not require direct skin contact, such as, for example, a conventional ECG. Hence they are very well suited for long-term monitoring. The paper entitled "A System For Monitoring Cardiac Vibration, Respiration, And Body Movement In Bed Using An Infrared" by Maki et al, 32^nd Annual International Conference of the IEEE Engineering in Medicine and Biology Society, discloses a noninvasive system for monitoring cardiac vibrations, respiration and body movement of in-bed hospitalized patients and elderly people who need constant care. The physiological parameters are recorded by an infrared emitting diode and a photo transistor, which are attached between spring coils in a bed mattress. The infrared emitting diode diffuses infrared light into the mattress. The diffusion of this energy is changed by mattress shape variations and spring coil vibrations, which modulate the intensity of the received infrared signal. The intensity is also modulated by physiological parameters such as heart pulse, respiration and body movement. The physiological parameters are detected from the received infrared intensity signal by low, high and band pass filters. This sensor uses infrared light reflection from a hollow cavity and has to be embedded into a hollow space in the mattress itself in order to be functional.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to improve upon the known art.

According to a first aspect of the present invention, there is provided a system for measuring a physiological rhythm of a subject comprising a light source arranged to emit light, a photodetector arranged to receive the emitted light, a layer of compressible material directly contacting the light source, and a processing device connected to the photodetector and arranged to detect changes in the emitted light received by the photodetector.

According to a second aspect of the present invention, there is provided a method of operating a system for measuring a physiological rhythm of a subject, the system comprising a light source, a photodetector, a layer of compressible material directly contacting the light source and a processing device connected to the photodetector, the method comprising the steps of emitting light from the light source, receiving the emitted light at the photodetector, and detecting changes in the emitted light at the processing device.

Owing to the invention, it is possible to provide a system for the unobtrusive measurement of vital signs by means of one or more bed-mounted, optical sensors. The sensors are placed underneath a bed's mattress and emit light into the mattress. A

photodetector continuously records the intensity of the light that is scattered back through the mattress over time. Any type of movement, such as respiratory movement, cardiac vibrations, as well as any other body movements, of the subject lying on the mattress causes slight deformations of the mattress. Through this change in geometry, the optical properties of the mattress change which in turn causes a change in the intensity of light which is reflected or scattered back to the photodetector. By recording the light intensity over time, a curve containing respiratory, cardiac, and other activity can be obtained.

The system therefore provides a method and apparatus for the unobtrusive measurement of vital signs by means of one or more bed-mounted, optical sensors. In one embodiment, the system enables the recording of mechanical vibrations and movements caused by respiratory and cardiac activity such as a ballistocardiogram of a subject lying in bed. The sensor also works on full foam mattresses and does not have to be integrated into the mattress itself.

Preferably, the system further comprises a mounting strip arranged to mount both the light source and the photodetector, wherein the mounting strip can be flexible and/or adhesive. The use of a mounting strip, which can be rigid or flexible, provides a simple arrangement to mount the light source and the photodetector and allows a user to easily locate the transmitting and receiving parts of the system in the desired position. If the mounting strip is flexible then it supports a wider variety of positions than a rigid mounting strip. For example chairs, which do not always have fiat cushions, can support the use of the system. If the mounting strip is adhesive, then it can be easily fixed in position and the chance of movement of the light source and the photodetector is greatly reduced.

Advantageously, the system further comprises multiple light sources each directly contacting the layer of compressible material, wherein each light source is arranged to emit light in turn (i.e. time division multiplexed), so that no more than one light source is emitting light at any one time. If multiple light sources are used in the system, then several advantages are gained. Firstly, the system is more robust in relation to the failure of a single component such as a light source no longer working. Secondly light is transmitted into the layer of compressible material from a variety of different locations, which increases the likelihood that the physiological rhythm being monitored will be picked up by the photodetector. Furthermore, the redundancy offered by the use of multiple sensors can be exploited by advanced signal processing algorithms in order to improve the reliability and accuracy of the physiological monitoring. When the system is operating by cycling through the light sources in turn, any potential crosstalk is eliminated.

Ideally, in the system, each light source is arranged to emit infrared light and each light source is arranged to emit light at a maximum intensity at a specific wavelength and each photodetector is arranged to have a maximal sensitivity at the same specific wavelength. The system will give the best results if the light source(s) and the photodetector(s) are effectively tuned so that they emit and receive the same wavelength of light. This will give the greatest sensitivity to the movements of the subject that is pressing on the layer of compressible material and will be the most efficient arrangement in terms of power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic diagram of a subject being monitored while in bed,

Fig. 2 is a schematic diagram of a sensor system,

Fig. 3 is a pair of graphs showing ECG and optical sensor outputs,

Figs. 4 and 5 are schematic diagrams of further embodiments of the sensor system,

Fig. 6 is a top plan view of another embodiment of the sensor system, Figs. 7a and 7b are block and circuit diagrams of part of the sensor system, Fig. 8 is a schematic diagram of a filter stage of the sensor system,

Fig. 9 is a schematic diagram of two different locations for the sensor system, Fig. 10 is a top plan view of three different arrays of sensor systems underneath respective mattresses, and

Fig. 11 is a set of graphs of the output of different photodetectors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 illustrates a bed 10 with a mattress 12 on which a subject 14 being monitored will sleep at night. A sensor system 16 is placed between the bed 10 and mattress 12 and will generate a signal that represents a physiological rhythm of the subject. This rhythm could be the subject's heartbeat or the user's breath frequency, for example. The sensor system 16 is connected to a local processing unit 18 which can perform analysis of the received signal in order to perform constant monitoring of the health of the subject 14. The sensor system 16 can be connected to remote systems that provide further processing and or monitoring functions.

The purpose of the system shown in Figure 1 is to provide continual monitoring of the health of the subject 14, in such a way that the subject 14 is not required to wear any sensors on their body nor has their natural sleep disturbed in any way. Constant monitoring is provided by the sensor system 16, which may be monitoring multiple different physiological parameters of the subject 14. A continuous signal is provided from the sensor system 16 that is processed by the device 18 for monitoring the health of the subject 14. These systems allow subjects to be monitored in their own home, without the need for expensive hospital monitoring of the subject 14.

Figure 2 shows a schematic diagram of one embodiment of the sensor system 16. The sensor system 16 consists of a thin mounting strip 22 with two light sources 22 and a photodetector 24 that can be placed underneath the mattress 12 of the bed 10. The sensor system 16 contains light sources 22 which emit light that then permeates the mattress 12 above the sensor system 16. In a preferred embodiment of the invention, light emitting diodes (LEDs) are used to produce light at one particular wavelength (for instance infra-red light between 850 nm and 950 nm). In an embodiment the sensor system 16 is integrated in a bed frame that supports a subject 14 lying in the bed. The sensor system may for example be integrated in a bed slat of the bed frame that supports the mattress.

Furthermore the sensor system 16 consists of a photodetector 24 which measures the intensity of the light that was reflected or scattered within the structure of the mattress 12. In a preferred embodiment, the photodetector 24 consists of a photodiode which has its peak sensitivity matched to the output of the LEDs 22 and is equipped with a daylight filter. Alternative embodiments of the system can be used with multiple light sources 22 with different wavelengths and time-division multiplexing to simultaneously perform

measurements using different wavelengths.

Any type of movement such as respiratory movement, cardiac vibrations, as well as any other body movements, of the subject 14 lying on the mattress 12 causes slight deformations of the mattress 12. Through this change in geometry, the optical properties of the mattress 12 change which in turn causes a change in the intensity of light which is reflected or scattered back to the photodetector 24. By recording the light intensity over time, a curve containing respiratory, cardiac, and other activity can be obtained.

Figure 3 shows an example of the cardiac component of a signal recorded by the photodetector 24 (upper graph) as well as a simultaneously recorded reference ECG signal (lower graph). It can be seen from these graphs that the sensor system 16 can be used to monitor a subject's heart rate, for example. The signal produced from the photodetector 24 is of sufficient quality that monitoring of the subject 14 can be carried out using the processing device 18. Other implementations of the system are possible that do not place the sensor system 16 under the bed's normal mattress 12. Instead, the sensor system 16 is mounted underneath a special overlay mattress which is placed on top of the regular mattress 12. The placement of the sensor system 16 under other compressible objects such as seat cushions etc. is also possible. Further embodiments of the system place the sensor system 16 beneath a spacing textile which has optimized optical properties to further enhance the signal quality.

Instead of using a single photodetector 24, embodiments of the sensor system 16 are possible that use an array of multiple photodetectors 24 which are located at different positions under the mattress 12. Multiple photodetectors 24 improve the overall signal quality of the system by exploiting redundancies in the signal. The use a photodetector array can also ensure that the subject 14 is always situated right on top of at least one photodetector 24. Systems using multiple photodetectors 24 can apply time-division or frequency-division multiplexing to eliminate cross-talk between different photodetectors 24 of the array.

The system has several advantages over known systems. For example, the sensor system 16 contains no moving parts and its performance does not degrade over time (as opposed to, for instance, EMFi or PVDF foil-based sensor). The sensor system 16 can be miniaturized to have an area of only a few square cm. The sensor system 16 can be easily retro-fitted into nearly any type of bed. The sensor system 16 is very cheap to produce and consists only of few components.

The principle of operation allows only local movements to be measured which is advantageous for a multi-sensor system to ensure little cross-talk between various channels and the sensor system 16 is less sensitive to external vibrations (i.e. a person walking next to the bed) than comparable state-of-the-art sensors.

Figure 4 shows an embodiment of a sensor array system. This Figure provides a system overview using analogue filtering stages. Each photodetector 24 is connected in series to a respective analogue filter 26, and the analogue filters are connected to an analogue to digital converter 28, which connects to the processing device 18. This Figure shows the measurement system comprised of a plurality of photodetectors 24 that are each connected to an analogue filtering stage 26. The filtered signals are then fed to the analogue to digital converter 28 which transmits the digitized signals to the signal processing and display device 18 for further analysis, storage, and/or transmission.

Figure 5 shows a further embodiment of the sensor system 16, using a direct digital sampling technique. In this implementation, each sensing element 24 is directly connected to a high-resolution (24-bit) analogue to digital converter 30 that is located directly on the sensing element. This technique minimizes the influence of external noise sources which can affect the signal quality of an analogue system. The necessary signal conditioning (especially the removal of the significant signal offset) is then performed using high-order digital filters 32.

As an additional, optional improvement, the digital system allows the use of a time-multiplexing method. This means that the light sources 22 are not constantly emitting light, but instead only one light source 22 is ever emitting light and measuring at any given time. The light sources 22 are consecutively switched on and off in rapid succession in order to eliminate any crosstalk, i.e. stray light coming from neighboring light sources 22, between the different light sources 22. The time-multiplex controller 34 is controlling the light sources 22 to operate them alternately. Multiple sensors 24 are shown in this Figure, but there could be a single sensor 24 with multiple light sources 22.

Figure 6 is a top plan view of a further embodiment of the sensor system 16. This Figure shows a top plan view of one embodiment of the sensor system 16 consisting of three IR light emitting diodes 22 and a photodetector 24 which is a photodiode. The light sources 22 are arranged in a triangle formation around the central photodetector 24. The light sources 22 can be controlled either so that all are illuminated at the same time or so that each light source 22 is arranged to emit light in turn, so that no more than one light source 22 is emitting light at any one time.

In another embodiment the sensor system comprises one central light emitting diode 16 surrounded in a triangle formation by three photodetectors. Multiple photodetectors 24 improve the overall signal quality of the system by exploiting redundancies in the signal. Time-division or frequency-division multiplexing may be used to eliminate cross-talk between the photodetectors 24.

Figures 7a and 7b show block and circuit diagrams of the receiving part of the sensor system 16. The photodiode 24 connects in series to a transimpedance amplifier 36. Light hitting the photodiode 's sensitive surface generates a small electrical current related to the intensity of the light hitting the diode. The transimpedance amplifier amplifies this current and converts it into a voltage signal. This voltage signal can then either be digitized directly or after further analogue filtering, as described above. Figure 7b exemplarily shows a possible embodiment of the transimpedance amplifier. Those skilled in the art will appreciate that the external resistor R and capacitor C can be selected in order to optimize the amplification and frequency characteristics of the amplifier. Figure 8 shows more detail of an analogue/digital filtering stage in the sensor system 16. This Figure shows the details of the analogue (as shown in Figure 4) or digital (Figure 5) filter stages. Depending on which of these systems is being used, the filters are realized as analogue filters or as digital filters. The high-pass filter 38 removes the signal baseline and has a cut-off frequency below the typical respiratory frequency. The output signal is passed to an amplifier 40. After amplification, the signal is low-pass filtered at filter 42 in order to remove unwanted high-frequency noise. An additional notch filter 44 removes 50/60Hz power line noise.

Figure 9 illustrates two possible positions for the sensor system 16, either (a) underneath an additional mattress overlay 46 or (b) beneath the regular mattress 12. In both embodiments, two sets of sensing systems 16 are used, but only one is necessary to achieve the required result of accurately monitoring the subject's physiological rhythm. The light sources 22 that are within each sensor system 16 emit their light into the layer of

compressible material above, which is directly contacting the light source 22. The subject's heart rate can be monitored by the detection in the very slight changes in the mattress 12 or mattress overlay 46 caused by the subject's movement.

In addition to the two sensor arrangements shown in Figure 9, further sensor arrangements are possible in order to monitor the subject's physiological rhythms. The sensor systems 16 as shown in the Figure 9 embodiment (a) can, for instance, also be mounted with their optical side facing down towards the regular mattress 12. Thus, the light sources 22 emit their light into the compressible mattress 12 below instead of the mattress overlay 46 above. As with the other arrangements, slight changes in the geometry of the regular mattress 12 caused by the subject's movements and cardiopulmonary activity can be detected by the sensor systems 16.

It is also possible to place a layer of compressible material similar to the mattress overlay 46 beneath the regular mattress 12, instead of on top of it, and mount the optical sensor systems 16 to the bed frame 10 in such a way that the light source 22 of the sensor system 16 emits light into the compressible material 46 above. If an additional mattress is used underneath the conventional mattress 12, then the sensor systems 16 can be built into that mattress so that the light source(s) 22 and photodetector(s) 24 are integral in the additional mattress. The sensor systems 16 can face either up or down, depending upon which side of the additional mattress is facing upwards. The additional mattress, when located below the regular mattress 12, can be so arranged that each light source 22 and each photodetector 24 are directed downwards. All of these sensor arrangements rely on the fact that the deformations caused by the subject's movements propagate through the multiple layers of compressible material 12 and/or 46.

Figure 10 illustrates top plan views of three examples of possible sensor arrangements for a four sensor system comprising sensor systems SI - S4. Each sensor system 16 is places roughly in the region of a subject's thorax, in order to best pick up the tiny movements that can be used to determine the subject's heart rate, for example. Each mattress 12 has an array of four sensor systems 16 underneath that will detect changes in the deformable mattresses 12. Each sensor system 16 may be comprised of a single light source 22 and a single photodetector 24 mounted on a mounting strip 20 or comprises multiple light sources 22 and/or multiple photodetectors 24. When an array of sensor systems 16 are used in this manner, all of the sensor systems 16 for an individual mattress 12 (and hence subject 14) will connect to a single local processing device 18.

Figure 11 shows sample signals as generated by three sensor systems 16, measured in arbitrary units (a.u.) wherein sensor system 1 had been positioned in an area beneath the thorax, sensor system 2 in an area beneath the head and sensor system 3 in an area beneath the abdomen. This Figure 11 shows a segment of signals recorded by an array of three sensor systems comprising three photodetectors 24. Depending on the positioning of the sensor systems 16, the signal shape detected changes. Each of the three signals shows a clear high frequency oscillation which is caused by the subject's heart. Sensors system 1 and 3 also show the lower frequency respiratory movement. This is barely visible in Sensor system 2. The processing device 18 can combine the three signals or can process them separately to provide an output that is useful in monitoring the wellbeing of the subject. In an embodiment the system for measuring a physiological rhythm of a subject comprises two or more sensor systems, a bed and a mattress. Each light source included in a sensor system directly contacts a layer of compressible material. One sensor system is located in an area that is beneath the abdomen of a subject lying in the bed and one sensor system is beneath the thorax of said subject.

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.

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. 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 processor 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 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. A system for measuring a physiological rhythm of a subject comprising:

a light source arranged to emit light,

a photodetector arranged to receive the emitted light,

a layer of compressible material directly contacting the light source, and a processing device connected to the photodetector and arranged to detect changes in the emitted light received by the photodetector.

2. A system according to claim 1, and further comprising a mounting strip arranged to mount both the light source and the photodetector.

3. A system according to claim 2, wherein the mounting strip is flexible and/or adhesive.

4. A system according to claim 1, 2 or 3, and further comprising multiple photodetectors.

5. A system according to any preceding claim, and further comprising multiple light sources each directly contacting the layer of compressible material.

6. A system according to claim 5, wherein each light source is arranged to emit light in turn, so that no more than one light source is emitting light at any one time.

7. A system according to any preceding claim, wherein the or each light source is arranged to emit infrared light.

8. A system according to any preceding claim, wherein the or each light source is arranged to emit light at a maximum intensity at a specific wavelength and the or each photodetector is arranged to have a maximal sensitivity at the same specific wavelength.

9. A system according to any preceding claim, and further comprising a bed and a mattress, wherein the layer of compressible material directly contacting the or each light source comprises an additional mattress for use above or below the bed mattress.

10. A system according to claim 9, wherein the additional mattress is in direct contact with the bed mattress.

11. A system according to claim 9 or 10, wherein the or each light source and the or each photodetector are integral within the additional mattress.

12. A system according to claim 11, wherein the additional mattress is located below the bed mattress and wherein the or each light source and the or each photodetector are directed downwards.

13. A method of operating a system for measuring a physiological rhythm of a subject, the system comprising a light source, a photodetector, a layer of compressible material directly contacting the light source and a processing device connected to the photodetector, the method comprising the steps of:

emitting light from the light source,

receiving the emitted light at the photodetector, and

detecting changes in the emitted light at the processing device.

14. A method according to claim 13, wherein the system further comprises multiple light sources each directly contacting the layer of compressible material and the method further comprises controlling each light source to emit light in turn, so that no more than one light source is emitting light at any one time.

15. A method according to claim 13 or 14, wherein the or each light source is arranged to emit light at a maximum intensity at a specific wavelength and the or each photodetector is arranged to have a maximal sensitivity at the same specific wavelength.