WO2004028361A1

WO2004028361A1 - Wearable device for transcutaneous blood flow monitoring

Info

Publication number: WO2004028361A1
Application number: PCT/IB2003/003598
Authority: WO
Inventors: Ralf Schmidt; Harald Reiter; Gereon Vogtmeier; Josef Lauter; Olaf Such; Christian Reichinger
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-09-27
Filing date: 2003-08-08
Publication date: 2004-04-08
Also published as: AU2003255907A1

Abstract

The invention relates to a monitoring device (1a) arranged to perform a transcutaneous blood flow monitoring. The monitoring device (1a) comprises means (4,6) to generate a magnetic field B through a cross-section of a vessel 7, where a net blood flow V exists. The monitoring device (1a) further comprises sensing means, for example a set of electrodes (8a,8b) arranged to measure a voltage U generated by electrically charged blood particles intercepting the magnetic flux (B). The means (4,6) for generating the magnetic field and the sensing means (8a,8b) are spatially arranged on a ring conceived to be arranged relative to the vessel such that magnetic poles (P1,P2) of said means for generating the magnetic field are substantially diametrically arranged with respect to said cross-section. The monitoring device 1a according to the invention uses a ring-shaped carrier (2) to house the means for generating the magnetic field and the sensing means, which is better adapted to the physics by surrounding the flowing medium. The crosswise opposing magnet poles and sensing means ensure the maximum signal amplitude. The monitoring system according to the invention can be arranged to measure the net flow through an extremity, for example the wrist, which is advantageous for purposes of sole monitoring of an existence of the net blood flow.

Description

Wearable device for transcutaneous blood flow monitoring

The invention relates to a monitoring device for non-invasively monitoring of a blood flow in a vessel, said vessel having a cross-section, said device comprising means for generating a magnetic field in a volume comprising the cross-section of the vessel, the direction of the field lines being substantially perpendicular to a direction of the blood flow, the device further comprising sensing means arranged to pick-up a signal corresponding to said blood flow.

The invention further relates to a cardiac monitoring system.

A monitoring device as set forth in the opening paragraph is known from US

5,935,077. The known monitoring device belongs to the class of transcutaneous blood flow monitoring devices intended to measure a blood flow in a vessel when put into contact with a person's skin. The known device comprises means for generating an external varying magnetic field in order to produce a field component in the blood vessel. The orientation of the thus produced magnetic field is such that a varying magnetic filed has a component parallel to the skin and transverses the blood vessel in the depth substantially at a right angle to the direction of the blood flow. The known monitoring device comprises a sense electrode conceived to be positioned on the skin adjacent to the blood vessel. When blood particles transverse the magnetic field an electrical signal is produced according to the principle of electromagnetic induction. This electrical signal is measured by the sense electrode in order to determine the actual value of the blood flow.

A disadvantage of the known monitoring device is due to the usage of the oscillating magnetic field. The major disadvantage of oscillating magnetic fields is the necessity to use an electromagnet. The field strengths which can be achieved with conventional electromagnets suitable for such purposes is very small compared to the state of the art permanent magnets. For generating comparable field strengths a bulky, power consuming electromagnet design is necessary. Thus a limited field strength is the major reason for the poor signal to noise ratio characteristic of the known monitoring devices utilizing oscillating fields. Moreover, the oscillating magnetic field is a source of noise by itself. Electrostatic coupling and electromagnetic coupling between exciting and detecting circuits and phase changes are problems which are inherent to the design of such systems.

It is an object of the invention to produce a monitoring system having an improved signal to noise ratio.

The monitoring device according to the invention is characterized in that the means for generating the magnetic field and the sensing means are spatially arranged on a ring conceived to be arranged relative to the vessel such that magnetic poles of said means for generating the magnetic field are substantially diametrically arranged with respect to said cross-section, the sensing means being situated on the ring substantially half-way between the magnetic poles. In contrast to the known magnet, where the poles of the magnetic system are positioned on the skin adjacent to each other thus producing a field component propagating parallel to the skin, the monitoring device according to the' invention uses a ring- shaped solution which is better adapted to the physics by surrounding the flowing medium. The opposing magnet poles ensure the maximum signal amplitude, while the corresponding crosswise placing of the electrodes ensures the optimum signal pick-up. The monitoring system according to the invention can be arranged to measure the net flow through an extremity, for example the wrist, which is advantageous for purposes of sole monitoring of an existence of the net blood flow.

An embodiment of the monitoring device according to the invention is characterized in that the magnetic poles of the means for generating the magnetic field are arranged on a ring-shaped carrier. It is advantageous to realize the monitoring system on a watch-like carrier, which can be easily worn by the person. It is also possible to mount the parts of the monitoring device on a bracelet, elastic band or another suitably adjusted carrier to realize a good contact between at least the electrodes of the monitoring system and the skin.

A further embodiment of the monitoring device according to the invention is characterized in that the means for generating the magnetic field comprise a permanent magnet arranged substantially on a central portion of a substantially half-circularly shaped piece of a ferromagnetic material. The advantages of permanent magnets are that they provide stronger fields without consuming power and can be implemented in small size. Thus only permanent magnets can provide a convenient low-power consuming solution suitable for long-term wearable monitoring. The signal to noise ratio benefits from use of permanent magnets as they produce stronger fields and do not have a coupling between exciting and detecting circuits. Suitable permanent magnet materials for this application are those with high energy product and high coercive force like SmCo, NdFeB and the like. With those permanent magnet materials a field strength up to 1 T is achievable. A still further embodiment of the monitoring device according to the invention is characterized in that the means for generating the magnetic field comprise two permanent magnets. An advantage of this set-up is a further reduction of the weight of the whole monitoring device. This is particularly advantageous when such monitoring system is intended to be worn permanently. The magnets are to be placed at opposite sides of the wrist enabling a light weight design since no ferromagnetic material is required.

A still further embodiment of the monitoring device according to the invention is characterized in that the means for generating the magnetic field comprise two electromagnets. An advantage of placing two electromagnets on a monitoring device suitable to be put on a wrist is that the magnetic field can be modulated within the wrist. This feature is useful for reducing a noise content of the signal. By orienting the electromagnets diametrically with respect to the vessel a better signal can be produced as the blood particles which are translated through the magnetic flux by the blood flow experience substantially the same magnetic flux. Therefore, substantially all levels in the blood flow produce substantially the same induced electric voltage. This feature makes the system more stable and reliable with respect to possible geometrical misalignments.

A still further embodiment of the monitoring device is characterized in that the device further comprises a magnetic field sensor (45) arranged in a direct vicinity of one of the magnetic poles. It is advantageous to use the Hall sensor to measure the strength of the magnetic field, which may vary due to changes of the geometry of the set-up caused by motion. The measurement of an actual magnetic flux can be performed by a Hall sensor positioned close to or above the magnetic poles. For purposes of qualitative analysis, the measured fluctuations in the actual value of the magnetic field can be corrected for using the signal from the Hall sensor and can be used in the system to calculate the actual value of the blood flow. The design of such a correction loop is straightforward and will not be explained in detail here.

These and other aspects of the invention will be further elaborated with reference to the figs. Fig. la is a schematic view of an embodiment of the monitoring device according to the invention where a single permanent magnet is used to create a magnetic field propagating through the vessel.

Fig. lb is a schematic view of an embodiment of the monitoring device according to the invention where two permanent magnets are used to create a magnetic field propagating through the vessel.

Fig. 2 is a block diagram of a signal processing chain. Fig. 3 is a schematic view of an embodiment of the monitoring device according to the invention where an electromagnet is used to create a magnetic field propagating through the vessel.

Fig. 4 is a block diagram of the signal processing chain for the monitoring device utilizing the electromagnet.

Fig. 5 is a schematic view of an embodiment of the monitoring device according to the invention where a Hall sensor is used to measure a strength of the magnetic field propagating through the vessel.

Fig. 6 is a block diagram of the signal processing chain for the monitoring device utilizing the Hall sensor.

Fig. 7 is a block diagram representing the units of a cardiac monitoring system based on the monitoring device according to the invention.

Fig. la shows a schematic view of an embodiment of the monitoring device 1 according to the invention where a single permanent magnet 6 is used to create a magnetic field propagating through the vessel. The vessel 7 is for our purposes schematically represented by the direction of the blood flow V. As is explained earlier, the monitoring device according to the invention belongs to the class of transcutaneous monitoring devices, where the blood flow is monitored by means of electromagnetic interaction between the electrically charged blood particles with an externally induced magnetic field. It is known that the blood is a conductive fluid containing positively and negatively charged particles. When such a particle passes through the magnetic field B, oriented perpendicular to the direction of the blood flow V, a voltage U is induced in a direction oriented orthogonally to V and B. Therefore, the geometry of the monitoring device 1 is particularly suited to measure such voltage by surrounding the blood vessel with diametrically positioned magnetic poles PI, P2 and a set of electrodes 8a, 8b. This geometry is conveniently realized by using a ring 2 on which a permanent magnet 6, positioned in a center of a horse-shoe shaped ferromagnetic material 4, and the electrodes 8a, 8b are positioned. This set-up enables creating magnetic fields with magnetic strength of about 1 T. Due to such magnetic field strength a sensitive monitoring device is implemented which is suitable to monitor weak flows or flows in vessels with small diameter. The signal to noise ratio of such a monitoring device is significantly improved with respect to systems using electromagnets positioned adjacent each other. By using a ferromagnetic material the required magnetic material can be minimized, thus reducing the system costs. The ferromagnetic material guides the magnetic flux forming the horseshoe magnet with the poles PI, P2. An example of an implementation of the ring 2 is a wristband, for example a watch-band, or any other suitable band designed to circumvent an extremity or a neck for monitoring purposes. The monitoring device according to the invention further comprises electronics 10 designed to feed the electrodes as well as to perform a measurement of the induced voltage U and to perform the analysis of the measured data. The electronics 10 may also be housed in the ring 2. The details of the electronics 10 will be discussed in detail with reference to Fig. 2.

Fig. lb presents a schematic view of an embodiment of the monitoring device lb according to the invention where two permanent magnets are used to create a magnetic field propagating through the vessel. The vessel 7 is for our purposes schematically represented by the direction of the blood flow V. The principle of operation of the device lb is similar to the principle of operation of the device 1 presented in Fig. la. The device lb uses two permanent magnets 3 and 5, accordingly, to create the magnetic field B through the vessel. The magnetic field B is oriented perpendicular to the direction of the blood flow V. The advantage of using two pieces of permanent magnet is that no ferromagnetic material is required and therefore a lighter design is achieved, compared to the design of the monitoring device of Fig. la. Also with the device lb a high magnetic field strength of about 1 T is achieved resulting in an improved signal to noise ratio of the monitoring device. The electronics 10 of the monitoring device lb will be explained with reference to Fig. 2. Fig. 2 shows a block diagram of a signal processing chain of the electronics

10, shown in Figs, la and lb. According to principles of electromagnetic induction the electrodes 8 a and 8b measure an induced voltage U in a closed frame. The signal from the electrodes 8a, 8b is passed through a preamplifier 11 followed by a high-pass filter 13 to filter out base-line variations. The signal is further amplified with an amplifier 15 which has a galvanic isolation from the measurement side followed by a low-pass filter 17 to remove high frequency disturbances. In order to make the signal suitable for processing by means of a digital processor, the signal is converted by an analog to digital converter 18. Finally the digitized signal is made available to a processing unit 19 which performs a suitable data analysis. For example, the processing unit can be suited to analyze the blood flow frequency, or alternatively it can be limited to monitor the presence of the blood flow. The former feature is valuable for, for example monitoring of a sport activity and the latter feature is valuable for implementations aimed at cardiac patients, or persons with a risk of a sudden heart failure. Thus, such monitoring device can be a part of a cardiac monitoring system and can alarm other persons or remote medical personnel upon an event of a cardiac arrest. The data processing of the signal from the electrodes 8a and 8b can be performed locally by the processing unit 19, or alternatively, the unit 19 can be suited to perform a preprocessing and to transmit the data to a distant terminal by means of a RF-transmitter [not shown]. It must be understood, that the technical features of the processing unit and a suitable unit to perform the RF-transmission lies within the technical scope of the person skilled in the art and will not be detailed any further.

Fig. 3 shows a schematic view of an embodiment of the monitoring device according to the invention where an electromagnet is used to create a magnetic field propagating through the vessel. The monitoring device 20 comprises a wristband 21 where electrodes 25 a and 25b are mounted. The wristband 21 comprises further an electromagnet with magnetic poles 23a, 23b enabled by coils 22. The operational principle of the device 20 is similar to the devices shown in Figs, la and lb. The electromagnet is fed by a power supply 24. The power supply 24 together with the electrodes 25 a and 25b are fed by the electronics 30 housed on the wristband 21 as well. The power supply 24 can comprise means for modulation of the electromagnetic field, alternatively it can comprise a DC source with a separate modulator. The possibility to modulate the magnetic field leads to a further noise reduction in the signal. However, the achievable field strength assuming a battery-powered device is smaller than that possible with permanent magnets. The electronics 30 of the set-up shown in Fig. 3 differs from the set-up shown in Fig. 2 in that it needs a signal generation means which drives the current of the field coils 22. In addition, a synchronization signal is fed into the signal processing block to enable a synchronous detection.

Fig. 4 shows a block diagram of the signal processing chain for the monitoring device utilizing the electromagnet. The electronics 30 comprises a field coil 23 that is fed by a signal which is amplified by an amplifier 32. The amplifier 32 is arranged with a galvanic isolation from the coil 23. According to principles of electromagnetic induction the electrodes 25 a and 25b measure an inducted voltage U in a closed frame. The signal from the electrodes 25a, 25b is passed through a preamplifier 31 followed by a high-pass filter 33 to filter out base-line variations. The signal is further amplified with an amplifier 35 which has a galvanic isolation from the measurement side, which is followed by a low-pass filter 37 to remove high frequency disturbances. In order to make to signal suitable for processing by means of a digital processor, it is converted by an analog to digital converter 38. Finally, the digitized signal is made available to a processing unit 39 which performs a suitable data analysis. The processing unit 39 enables a driving unit 34 by means of a synchronization signal. This signal is suited to synchronize the power supply to the coil 23 and the detection circuit comprising electrodes 25a and 25b. For the sake of simplicity the block-scheme 30 does not illustrate power supply means which can comprise a simple battery or a rechargeable accumulator battery. The technical principles to energize the electronic scheme 30 lie within the scope of knowledge of the person skilled in the art and therefore will not be discussed in great detail.

Fig. 5 shows a schematic view of an embodiment of the momtoring device according to the invention where a Hall sensor is used to measure a strength of the magnetic field propagating through the vessel. The monitoring device 40 can be implemented according to the rationale set out referring to Figs, la, lb and 3. To enable the monitoring of the blood flow, the monitoring device comprises means 41, 41a to generate a magnetic flux B through the vessel comprising flowing blood V. In an example shown in Fig. 5 a combination of a permanent magnet 21 and a horse-shoe shaped ferromagnetic material 41a is shown. As can be easily understood other technical implementations, for example those set out in Figs, lb and 3 can be used for the purpose of generating a magnetic field. In order to measure the induced voltage U resulting from the blood flow V intercepting the magnetic flux B, a set of electrodes 43 a, 43b is used. The electrodes are fed and read-out by the electronics 50 which is mounted on the carrier 42. According to the invention the carrier is to be positioned spatially circumventing the cross-section of a vessel to be monitored. Preferably, the carrier is a wristband-like construction and is worn on the wrist. However, due to movements of the person wearing the monitoring device the device can be misaligned with respect to the cross- section of the vessel resulting in another absolute magnitude of the measured signal. In order to compensate for the artifacts in the signal a Hall-sensor 45 is used. The purpose of the Hall sensor 45 is to measure the strength of the magnetic field which may vary due to motion induced changes in the geometry of the set-up. This measurement can be performed by the Hall-sensor placed in the vicinity of the magnetic poles, for example as shown in Fig. 5. The principles of the operation of the Hall-sensor lies within the scope of knowledge of the person skilled in the art and therefore will not be explained here for the sake of conciseness. The signal from the Hall-sensor is assessed separately by the electronics 50, which is schematically shown in Fig. 6. It is also possible to use other sensors to measure the local magnetic field, for example magnetoresistive sensors.

Fig. 6 shows a block diagram of the signal processing chain for the monitoring device utilizing the Hall sensor in case electromagnets are used to generate the magnetic field. The electronics 50 comprises a field coil 53 which is fed by a signal that is amplified by an amplifier 54. The amplifier 54 is arranged with a galvanic isolation from the coil 54.

According to principles of electromagnetic induction the electrodes 51a and 51b measure an inducted voltage U in a closed frame. The signal from the electrodes 51a, 51b is passed through a preamplifier 52 followed by a high-pass filter 56 to filter out base-line variations. The signal is further amplified with the amplifier 57 which has a galvanic isolation from the measurement side, which is followed by a low-pass filter 58 to remove high frequency disturbances. In order to make to signal suitable for processing by means of a digital processor, it is converted by an analog to digital converter 59. Finally the digitized signal is made available to a processing unit 60 which performs a suitable data analysis. The processing unit 60 enables a driving unit 55 by means of a synchronization signal. This signal is suited to synchronize the power supply to the coil 53 and the detection circuit comprising electrodes 51a and 51b. The processing unit 60 is further arranged to analyze the signal from the Hall-sensor 45, which is preamplified by the preamplifier 62, and an amplifier 67 and put through a high-pass filter 66 and a low-pass filter 68. Finally the thus preprocessed signal is digitized by the Analog to Digital converter 69 and is fed into the processing unit 60. For the sake of simplicity the block diagram 50 does not illustrate power supply means which can comprise a simple battery or a rechargeable accumulator battery. This set-up of the electronics chain is particularly suited for cases when the magnetic field is modulated to gain a better signal to noise ratio or when the blood flow through the vessel is quantified based on the applied magnetic field. For the purposes of the solely monitoring the presence of the net blood flow, the Hall sensor can be omitted and it suffices to use the set-ups schematically presented in the embodiments of Figs. 2 and 4.

Fig. 7 shows schematically an embodiment of the components of the user-side 70 of the cardiac monitoring system according to the invention. The user-side 70 comprises monitoring means 72, comprising the monitoring device according to the invention arranged to monitor the blood flow of the user. The monitoring means 72 thus comprise a set of electrodes 74 arranged on the body of the user to pick-up a signal characteristic to the blood flow, which is a voltage induced in the electric circuit comprising the electrodes 74. Additionally, the monitoring means 72 can comprise a sensor 74' arranged to monitor a signal not directly related with the blood flow, for example a movement detector, a respiration rate monitor, body temperature monitor and so on. In a preferred embodiment the monitoring means 72 are arranged to perform a continuous monitoring of the blood flow of the user and are further arranged to provide a corresponding signal to the front-end electronics 76 of the user-side 70 of the cardiac monitoring system. The monitoring means 72 and the front-end electronics 76 are worn on the body of the user, preferably at the wrist area. The front-end electronics 76 is arranged to analyze the signal from the sensors 74, 74' in order to quantify the value of the blood flow in case the monitoring device is arranged to monitor the blood flow in healthy individuals. Alternatively, the front-end electronics can be arranged to produce an alarm in case no net flow is detected. This feature is particularly suited to monitor patients at risk of a heart failure. The user-side 70 of the system operates as follows: the monitoring means 72 acquire the raw data which are delivered to the front-end electronics 76. The front-end electronics 76 comprises means for receiving the signals from the monitoring means 72, performs suited signal processing by means of the circuits shown in Figs. 2, 4 or 6. The processing unit 78 of the front-end electronics 76 sends out a value of the blood flow or in simpler arrangements it sends out a trigger signal T to the alarm generating means 80 when a value of the blood flow reaches a critical value. It is possible to arrange the system so that more than one critical value is assigned with corresponding different alarms. In the most simple arrangement the system assigns one critical value of the net blood flow, which is set to zero. The alarm generating means 80 can be arranged to produce a local alarm by means of an acoustic and/or visual signal 82 or they can be arranged to additionally forward the alarm to a remote location 84 where a medical personnel is located. Due to this feature the medical assistance can be delivered promptly to a person suffering from a cardiac failure.

Claims

CLAMS:

1. A monitoring device for non-invasively monitoring of a blood flow in a vessel, said vessel having a cross-section, said device comprising means for generating a magnetic field in a volume comprising the cross-section of the vessel, the direction of the field lines being substantially perpendicular to a direction of the blood flow, the device further comprising sensing means arranged to pick-up a signal corresponding to said blood flow, characterized in that the means for generating the magnetic field and the sensing means are spatially arranged on a ring conceived to be arranged relative to the vessel such that magnetic poles of said means for generating the magnetic field are substantially diametrically arranged with respect to said cross-section, the sensing being situated on the ring substantially half- way between the magnetic poles .

2. A monitoring device according to claim 1 , characterized in that the magnetic poles of the means for generating the magnetic field are arranged on a ring-shaped carrier.

3. A monitoring device according to claim 1 or 2, characterized in that the means for generating the magnetic field comprise a permanent magnet arranged substantially on a central portion of a substantially half-circularly shaped piece of a ferromagnetic material.

4. A monitoring device according to claim 1 or 2, characterized in that the means for generating the magnetic field comprise two permanent magnets.

5. A monitoring device according to claim 1 or 2, characterized in that the means for generating the magnetic field comprise two electromagnets.

6. A monitoring device according to any one of the preceding claims, characterized in that the device further comprises a magnetic field sensor arranged in a direct vicinity of one of the magnetic poles.

7. A cardiac monitoring system comprising a monitoring device according to any one of the preceding claims.