Patent Publication Number: US-2021177334-A1

Title: Bio-signal detection

Description:
TECHNOLOGICAL FIELD 
     Embodiments of the present invention relate to bio-signal detection. In particular, they relate to detecting bio-signals simultaneously. 
     BACKGROUND 
     Bio-signals are signals that provide information about the functioning of a subject&#39;s body. There are a very large number of bio-signals. 
     Bio-signals that relate to the heart and circulation include, for example, systolic blood pressure, diastolic blood pressure, heart rate, electrocardiogram, pulse wave velocity, phonocardiogram, ballistocardiogram, echocardiogram etc. 
     It is desirable to obtain multiple different bio-signal measurements simultaneously. 
     BRIEF SUMMARY 
     According to various, but not necessarily all, embodiments of the invention there is provided an apparatus comprising: 
     a displacement current sensor configured to measure for a subject one or more sensed electrical signals; and 
     circuitry configured to process the one or more sensed electrical signals to obtain an electrocardiogram signal and a variable impedance signal caused by an arterial pulse wave. 
     According to various, but not necessarily all, embodiments of the invention there is provided examples as claimed in the appended claims. 
    
    
     
       BRIEF DESCRIPTION 
       For a better understanding of various examples that are useful for understanding the detailed description, reference will now be made by way of example only to the accompanying drawings in which 
         FIG. 1  illustrates an example of an apparatus for processing one or more sensed electrical signals to obtain an electrocardiogram signal and a variable impedance signal caused by an arterial pulse wave; 
         FIGS. 2 and 3  illustrates examples of an apparatus for processing one or more sensed electrical signals to obtain an electrocardiogram signal and a variable impedance signal caused by an arterial pulse wave, where the variable impedance signal is measured as a modulation of an applied electrical reference signal; 
         FIGS. 4A and 4B  illustrate example of configurations of guard electrodes for an ECG electrode; 
         FIG. 5  illustrates an example of a demodulation circuit; 
         FIG. 6  illustrates an example of an apparatus for processing one or more sensed electrical signals to obtain an electrocardiogram signal and a variable impedance signal caused by an arterial pulse wave, where the variable impedance signal is measured as a modulation of an internal naturally-generated electrical; 
         FIG. 7  illustrates an example of a displacement current sensor comprising an ECG electrode with guard electrodes, suitable for use in the apparatus illustrated in  FIG. 6 ; 
         FIG. 8  illustrates how a displacement current sensor may be placed on a body of a subject; 
         FIGS. 9A, 9B, 9C  illustrate that the apparatus may be configured as part of an article worn by the subject; 
         FIG. 10  illustrates an example of circuitry; 
         FIG. 11  illustrates an example of a method. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example of an apparatus  10  for processing one or more sensed electrical signals  21  to obtain an electrocardiogram signal  41  and a variable impedance signal  43  caused by an arterial pulse wave. 
     The electrocardiogram signal  41  is a signal that depends upon the electrical polarization and depolarization of the heart muscles. It is indicative of heart function. 
     The variable impedance signal  43  is a signal that depends upon both the generation of an arterial pulse wave and the transport of the arterial pulse wave, which is a pressure wave, through the arterial system. It is indicative of heart output and arterial function. 
     The apparatus  10  may therefore be, or be a part of, a circulation monitoring system or a health monitoring system that uses the electrocardiogram signal  41  and the variable impedance signal  43  to assess heart function/output and arterial function. This may find application for patient monitoring, for personal health monitoring, for fitness assessment, for exercise effectiveness monitoring etc. 
     The apparatus  10  comprises at least a displacement current sensor  20  and circuitry  40  operatively connected to the displacement current sensor  20 . The connection may, for example, be a direct galvanic connection via a lead, as illustrated in  FIGS. 2, 3, 6 . 
     The displacement current sensor  20  is configured to measure for a subject  30  one or more sensed electrical signals  21 . The sensor  20  detects the one or more sensed electrical signals  21 . The sensor  20  may or may not further process the detected electrical signal to produce the one or more sensed electrical signals  21 . Measurement does not therefore imply that the one or more sensed electrical signals is quantised, although it may be. 
     The total current density (defined by curl H) has a galvanic component J and a displacement component dD/dt, where D=εE. The displacement current sensor  20  measures D and its variation over time, dD/dt. 
     The displacement current sensor  20  comprises at least one electrode  22  adjacent the skin  32  of the subject  30  and comprises electrical insulation  24  for insulating the at least one electrode  22  from the subject&#39;s skin  32 . This prevents the displacement current sensor  20  from receiving the galvanic component J of the total current density. 
     The circuitry  40  is configured to process the one or more sensed electrical signals  21  to obtain an electrocardiogram signal  41  and a variable impedance signal  43  caused by an arterial pulse wave. 
     The circuitry  40  may be any suitable circuitry. It may be an arrangement of discrete components, and/or may comprise programmable gate arrays, and/or may comprise programmed processors, for example. 
     In some but not necessarily all examples, the circuitry  40  is configured to measure the variable impedance signal  43  caused by an arterial pulse wave as a modulation of a reference signal. In the examples illustrated in  FIGS. 2 and 3 , the reference signal is an external signal  45  applied to the subject  30 . In the example illustrated in  FIG. 6 , the reference signal is an internal naturally-generated signal, generated by the subject&#39;s heartbeat. 
     In the examples of  FIGS. 2 and 3 , the circuitry  40  is configured to process the one or more sensed electrical signals  21  to obtain an electrocardiogram signal  41  and a variable impedance signal  43  caused by an arterial pulse wave. The circuitry  40  measures the variable impedance signal  43  caused by an arterial pulse wave as a modulation of the electrical reference signal  45  provided to the subject  30  via the first electrode  26  by the circuitry  40 . 
     The circuitry  40  is configured to apply a time variable voltage V 1 , as the reference signal  45 , to a first electrode  26 , for example via a first operational amplifier  70 , and measure a time variable signal  43  (voltage V out ), for example at an output of a second operational amplifier  80 . 
     In these examples, the additional first electrode  26  is adjacent the subject&#39;s skin  32  and comprises electrical insulation  24  for insulating the first electrode  26  from the subject&#39;s skin  32 . 
     The electrical reference signal  45  has one or more high frequency components, for example, greater than 1 kHz. In some examples, the electrical reference signal  45  may have a significant component greater than 100 kHz, for example, in the range 100-500 kHz, or in some embodiments may lie entirely within the range 100-500 kHz. 
     The electrical reference signal  45  may, for example, be a pure tone (single frequency). 
     The displacement current sensor  20  is located at a location defined by a desired ECG vector. The first electrode  26  may be adjacent the electrode  22  in the displacement current sensor  20 . 
     In the example of  FIG. 3 , the displacement current sensor  20  comprises an ECG electrode  60  for measuring an ECG signal  41 , a first electrode  26  for injection of a current (reference signal  45 ), and a distinct second electrode  22  for providing the variable impedance signal  43  caused by an arterial pulse wave. 
     The circuitry  40  is configured to apply a time variable voltage V 1  to a first electrode  26  via a first operational amplifier  70 , and provide a time variable signal  43  (voltage V out ) at an output of a second operational amplifier  80 . 
     As illustrated in more detail in  FIGS. 4A and 4B , the first electrode  26  and the second electrode  22  are guard electrodes for the ECG electrode  60 . The ECG electrode is centrally located. In these examples it is circular but this is not necessarily essential. The first electrode  26  and the second electrode  22  are separated from the ECG electrode  60  and from each other, in these example, low relative permittivity gaps are used for separation. 
     In the example of  FIG. 4A , the first electrode  26  and the second electrode  22  are arranged to both lie on a common shape that encloses the ECG electrode  60 , in this case a circle circumscribing but separated from the ECG electrode  60 . In this example, the displacement current sensor  20  has 180° rotational symmetry. In this example, the first electrode  26  and the second electrode  22  have the same contact area. 
     In the example of  FIG. 4B , the first electrode  26  and the second electrode  22  are arranged to both lie on differently sized shapes that enclose the ECG electrode  60 , in this case circles of different radius circumscribing but separated from the ECG electrode  60  and each other. In this example, the displacement current sensor  20  has 360° rotational symmetry. In some examples, the first electrode  26  and the second electrode  22  have the same contact area. 
     Returning to  FIG. 3 , the ECG signal  41  received at the ECG electrode  60  is applied, via op-amp  90 , as a virtual earth at a +ve terminal of a first op-amp  70  and is applied as a virtual earth at a +ve terminal of the second op-amp  80 . A voltage divider may be used in some examples (not illustrated). For example, impedances Z A  and Z B  may be connected in series between the output of the op-amp  90  and ground, and an intermediate node between impedances Z A  and Z B  may be connected to +ve terminal of the first op-amp  70  and +ve terminal of the second op-amp  80 . 
     The first op-amp  70  generates at its output a current and a voltage V 1  at the first guard electrode  26 . The first op-amp is arranged for closed loop negative feedback via an impedance  72  connected between its output and its −ve terminal. The impedance  72  has a value Z 2 . The first op-amp  70  is arranged to receive an input at its −ve terminal, via an impedance  74 , from a variable voltage source  76 . The impedance  74  has a value Z 1 . The ECG signal  41  received at ECG electrode  60  is applied, after amplification by op-amp  90 , as a virtual earth at a +ve terminal of the first op-amp  70 . 
     The second op-amp  80  receives at a −ve terminal a voltage from the second guard electrode  22 . The second op-amp  80  is arranged for closed loop negative feedback via an impedance  82  connected between its output and its −ve terminal. The impedance  82  has a value Z 3 . The ECG signal  41  received at ECG electrode  60  is applied, after amplification by op-amp  90 , as a virtual earth at a +ve terminal of the second op-amp  80 . The second op-amp  80  generates at its output a voltage V out  which is the variable impedance signal  43 . In some examples, but not this example, an impedance Z 4  may be connected between the second guard electrode  22  and the −ve terminal of the second op-amp  80 . 
     The current at the −ve terminal of the second op-amp  80  depends on the voltage (V 1 ) at the first electrode  26  (relative to virtual earth) and an unknown impedance Z associated with the current path through the subject&#39;s body. The impedance Z is comprised of a steady state value and a variable value that may be assumed to arise substantially from the arterial pulse wave. The output of the op-amp  80  is therefore a variable impedance signal  43  caused by an arterial pulse wave. 
     The current at the −ve terminal of the first op-amp  70  is V in /Z 1 , where V in  is the variable voltage (relative to virtual earth) applied to a −ve input of the first op-amp  70  via impedance Z 1 . The first op-amp is arranged for closed loop, negative feedback. The output of the first op-amp  70  is therefore V 1 =V in *(1+Z 2 /Z 1 ). The current at the −ve terminal of the second op-amp  70  is V 1 /Z where V 1  is the variable voltage (relative to virtual earth) applied to a −ve input of the second op-amp  80  via the body impedance Z. The second op-amp  80  is arranged for closed loop, negative feedback. The output of the second op-amp  80  is therefore V out =Z 3 *V 1 /Z=V in *(Z 3 /Z)*(1+Z 2 /Z 1 ). 
     It is possible to separate the part of V out  that arises as a consequence of variation in Z from the the part of V out  that arises as a consequence of variation in V in . 
     This may, for example, be achieved by subtracting V in  from V out  in the frequency domain using a demodulator. 
     If Z 1 , Z 2 , Z 3  are resistors, then at the higher frequencies of V in  the steady state impedance of the body may be considered to be primarily resistive. The changing impedance arising from the distance between an arterial wall and the skin surface decreasing with a passing arterial pulse wave will be primarily reactive (increasing capacitance). The changing impedance arising from the increasing blood volume in an artery with a passing arterial pulse wave will be primarily resistive (decreasing resistance). 
     Changes in the imaginary (reactive) part of the variable impedance signal  43  may therefore be attributed to variation in capacitance arising from the arterial pulse wave. 
     Changes in the real (resistive) part of the variable impedance signal  43  may therefore be attributed to variation in resistance arising from the arterial pulse wave. 
     The parameter indicative of the elasticity of the artery may therefore be determined from the imaginary (reactive) part of the variable impedance signal  43  and the real (resistive) part of the variable impedance signal  43 . 
       FIG. 5  illustrates an example of a demodulation circuit  90 . The demodulation circuit  90  processes the input time-variable voltage V in  and the output time-variable voltage V out  to obtain Imaginary and Real components of the variable impedance signal  43 . 
     The demodulation circuit  90  may be configured to calculate how the complex transfer function between the input time-variable voltage V in  and the output time-variable voltage V out  varies over time. The complex transfer function is related to the variable impedance Z via constant impedances and may be used as the variable impedance signal  43  or may be further processed to obtain the variable impedance signal  43 . 
     In the examples of  FIG. 6 , the circuitry  40  is configured to process the one or more sensed electrical signals  21  to obtain an electrocardiogram signal  41  and a variable impedance signal  43  caused by an arterial pulse wave. The circuitry  40  measures the variable impedance signal  43  caused by an arterial pulse wave as a modulation of the electrical reference signal (the ECG signal  41 ) which is an internal naturally-generated signal, generated by the subject&#39;s heartbeat. 
     The displacement current sensor  20  comprises a displacement current electrode  22  having a capacitance that varies with varying arterial blood volume under the displacement current sensor  20  when in situ. The displacement current electrode  22  may be a modified ECG electrode  60 . 
     The displacement current electrode  22  operates as one side of a capacitor, the other side being provided by the body of the subject. The displacement current is dependent upon relative permittivity between the displacement current electrode  22  and the skin  32  and the physical separation (distance d) between the displacement current electrode  22  and the skin  32 . 
     The displacement current sensor  20  is adapted to allow an arterial pulse wave to modulate the relative permittivity between the displacement current electrode  22  and the skin  32  and/or modulate the physical separation (distance d) between the displacement current electrode  22  and the skin  32 . 
     A material that polarizes with applied force, a piezoelectric material, may, for example, be used as a dielectric  24  between the displacement current electrode  22  and subject&#39;s skin  32  to vary the relative permittivity between the displacement current electrode  22  and the skin  32  in response to an arterial pulse wave. 
     A material that resiliently deforms with applied force, an elastic material, may, for example, be used as a dielectric  24  between the displacement current electrode  22  and subject&#39;s skin  32  to vary physical separation (distance d) between the displacement current electrode  22  and the skin  32  in response to an arterial pulse wave. 
     The circuitry  40  is, in this example, configured to process the one or more sensed electrical signals  21  to obtain an electrocardiogram signal  41  and a variable impedance signal  43  caused by an arterial pulse wave by measuring the variable impedance signal  43  caused by an arterial pulse wave and removing the variable impedance signal  43 , in the frequency domain or the time domain, from the one or more sensed electrical signals  21  to obtain an electrocardiogram signal  41 . 
     The variable impedance signal  43  caused by an arterial pulse wave may be time separated from the electrocardiogram signal  41  because, for example, the transport time for an arterial pulse wave is greater than the transport time for the ECG signal. 
     Also, if one samples over a period of time that captures a single PQRST profile of an ECG signal and a single arterial pulse wave, then higher frequency components are associated with the QRS morphology of the ECH signal. 
     Other techniques such as, for example, adaptive filtering may be used to remove the variable impedance signal  43  from the measured signal to obtain a ‘clean’ ECG signal. The variable impedance signal  43  may be used as an external reference signal for an adaptive filter for filtering the sensed signal  21 . 
     As illustrated in  FIG. 7 , the displacement current sensor  20  the ECG electrode  60 , may use guard electrodes G, G′. The guard electrodes may, for example, be connected as the first and second guard electrodes  22 ,  26  of  FIG. 3 , with or without the application of the input time-variable voltage  76 . 
       FIG. 8  illustrates that the displacement current sensor  20  may be placed on a body of a subject on the skin  32  of the subject  30 , at a location that overlies an artery of the subject  30 . In some examples, the displacement current sensor  20  is placed over the brachial artery or a sub-branch of the brachial artery such as the radial artery. 
     In some examples, multiple displacement current sensors  20  are used to provide differential measurements across the subject&#39;s body. For example, displacement current sensor  20  may be placed on a left side or left limb of the body (e.g. left wrist) and a different displacement current sensor  20  may be placed on a right side or right limb of the body (e.g. right wrist). Measurements may be made for the same heartbeat at both sensors  20  and compared. 
       FIGS. 9A, 9B, 9C  illustrate that the apparatus  10  may be configured as part of an article  80  worn by the subject  30 . In  FIG. 9A , the article  80  is a spectacle frame. In  FIG. 9B , the article  80  is a wrist band. In  FIG. 9C , the article  80  is clothing, a shoe. 
     As illustrated in  FIG. 10  the controller  96  may be implemented using instructions that enable hardware functionality, for example, by using executable instructions of a computer program  94  in a general-purpose or special-purpose processor  90  that may be stored on a computer readable storage medium (disk, memory etc) to be executed by such a processor  90 . 
     The processor  90  is configured to read from and write to the memory  92 . The processor  90  may also comprise an output interface via which data and/or commands are output by the processor  90  and an input interface via which data and/or commands are input to the processor  90 . 
     The memory  92  stores a computer program  94  comprising computer program instructions (computer program code) that controls the operation of the apparatus  10  when loaded into the processor  90 . The computer program instructions, of the computer program  94 , provide the logic and routines that enables the apparatus to perform the methods illustrated in  FIG. 11 . The processor  90  by reading the memory  92  is able to load and execute the computer program  94 . 
     The apparatus  10  therefore comprises: 
     at least one processor  90 ; and 
     at least one memory  92  including computer program code 
     the at least one memory  92  and the computer program code configured to, with the at least one processor  90 , cause the apparatus  10  at least to perform: 
     using a displacement current sensor configured to sense for a subject one or more electrical signals; and 
     processing the one or more sensed electrical signals to obtain an electrocardiogram signal and a variable impedance signal caused by an arterial pulse wave. 
     The computer program  94  may arrive at the apparatus  10  via any suitable delivery mechanism. The delivery mechanism may be, for example, a non-transitory computer-readable storage medium, a computer program product, a memory device, a record medium such as a compact disc read-only memory (CD-ROM) or digital versatile disc (DVD), an article of manufacture that tangibly embodies the computer program  94 . The delivery mechanism may be a signal configured to reliably transfer the computer program  94 . The apparatus  10  may propagate or transmit the computer program  94  as a computer data signal. 
     Although the memory  92  is illustrated as a single component/circuitry it may be implemented as one or more separate components/circuitry some or all of which may be integrated/removable and/or may provide permanent/semi-permanent/dynamic/cached storage. 
     Although the processor  90  is illustrated as a single component/circuitry it may be implemented as one or more separate components/circuitry some or all of which may be integrated/removable. The processor  90  may be a single core or multi-core processor. 
     References to ‘computer-readable storage medium’, ‘computer program product’, ‘tangibly embodied computer program’ etc. or a ‘controller’, ‘computer’, ‘processor’ etc. should be understood to encompass not only computers having different architectures such as single/multi-processor architectures and sequential (Von Neumann)/parallel architectures but also specialized circuits such as field-programmable gate arrays (FPGA), application specific circuits (ASIC), signal processing devices and other processing circuitry. References to computer program, instructions, code etc. should be understood to encompass software for a programmable processor or firmware such as, for example, the programmable content of a hardware device whether instructions for a processor, or configuration settings for a fixed-function device, gate array or programmable logic device etc. 
     As used in this application, the term ‘circuitry’ refers to all of the following: 
     (a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry) and 
     (b) to combinations of circuits and software (and/or firmware), such as (as applicable): (i) to a combination of processor(s) or (ii) to portions of processor(s)/software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions and 
     (c) to circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present. 
     This definition of ‘circuitry’ applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term “circuitry” would also cover an implementation of merely a processor (or multiple processors) or portion of a processor and its (or their) accompanying software and/or firmware. The term “circuitry” would also cover, for example and if applicable to the particular claim element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in a server, a cellular network device, or other network device. 
       FIG. 11  illustrates an example of a method  100  comprising: 
     at block  102 , using a displacement current sensor  20  configured to sense for a subject  30  one or more electrical signals  21 ; and 
     at block  104 , processing the one or more sensed electrical signals  21  to obtain an electrocardiogram signal  41  and a variable impedance signal  43  caused by an arterial pulse wave. 
     Implementation of the circuitry  40  may be as a controller  96 , for example. The controller  96  may be implemented in hardware alone, have certain aspects in software including firmware alone or can be a combination of hardware and software (including firmware). 
     The blocks illustrated in  FIG. 11  may represent steps in a method and/or sections of code in the computer program  94 . The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some blocks to be omitted. 
     Where a structural feature has been described, it may be replaced by means for performing one or more of the functions of the structural feature whether that function or those functions are explicitly or implicitly described. 
     As used here ‘module’ refers to a unit or apparatus that excludes certain parts/components that would be added by an end manufacturer or a user. The apparatus  10  may be a module. 
     The term ‘comprise’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use ‘comprise’ with an exclusive meaning then it will be made clear in the context by referring to “comprising only one.” or by using “consisting”. 
     In this brief description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term ‘example’ or ‘for example’ or ‘may’ in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus ‘example’, ‘for example’ or ‘may’ refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all of the instances in the class. It is therefore implicitly disclosed that a feature described with reference to one example but not with reference to another example, can where possible be used in that other example but does not necessarily have to be used in that other example. 
     Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. 
     Features described in the preceding description may be used in combinations other than the combinations explicitly described. 
     Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. 
     Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not. 
     Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.