Patent Publication Number: US-2020281498-A1

Title: Bio-signal detection

Description:
TECHNOLOGICAL FIEL 
     Embodiments of the present invention relate to bio-signal detection. In particular, they relate to improving bio-signal detection. 
     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. 
     Detected bio-signals can suffer from noise arising from many different sources. 
     It is desirable to remove noise from bio-signals. 
     BRIEF SUMMARY 
     According to various, but not necessarily all, embodiments of the invention there is provided an apparatus comprising: 
     a first displacement current sensor comprising a first sensing electrode and a first guard electrode, wherein the first displacement current sensor is configured to measure a first sensed signal dependent upon electrical activity of a subject&#39;s heart; 
     a second displacement current sensor comprising a second sensing electrode and a second guard electrode, wherein the second displacement current sensor is configured to measure a second sensed signal dependent upon electrical activity of a subject&#39;s heart; and 
     circuitry configured to process at least the first sensed signal to compensate for artefacts arising from motion of the subject. 
     The circuitry may comprise: at least one processor; and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor  90 , cause the apparatus at least to perform: processing the sensed signal to compensate for artefacts arising from motion of the subject. 
     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 compensate for artefacts arising from motion of the subject; 
         FIGS. 2 and 3  illustrates examples of an apparatus for processing one or more sensed electrical signals to compensate for artefacts arising from motion of the subject; 
         FIG. 4  illustrates an example of a configuration of a guard electrode and an ECG electrode; 
         FIG. 5  illustrates an example of a demodulation circuit; 
         FIGS. 6A, 6B, 6C  illustrate examples of motion-associated sensors which may, optionally, be fixed to articles; 
         FIGS. 7A to 7D  illustrate further examples of circuitry configured to process the one or more sensed electrical signals to compensate for artefacts arising from motion of the subject; 
         FIG. 8  illustrates an example of a controller configured to process the one or more sensed electrical signals to compensate for artefacts arising from motion of the subject; and 
         FIG. 9  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  dependent upon electrical activity of a subject&#39;s heart; to compensate for artefacts arising from motion of the subject. In this way, in some examples, the apparatus  10  obtains an electrocardiogram signal  41  from the one or more sensed electrical signals  21 . An 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 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  to assess heart 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 and 3 . 
     The displacement current sensor  20  is configured to measure one or more sensed electrical signals  21  dependent upon electrical activity of a heart of a subject  30 . 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 a value dependent upon D and its variation over time, dD/dt. 
     The displacement current sensor  20  comprises at least one electrode  22  in proximity to the skin  32  of the subject  30  but electrically insulated therefrom. 
     In some examples the displacement current sensor  20  comprises electrical insulation  24  for insulating the at least one electrode  22  from the subject&#39;s skin  32 . 
     In some examples, material between the electrode  22  and the subject&#39;s skin  30  additionally or alternatively provides electrical insulation of the electrode  22  from the subject&#39;s skin  32 . 
     The electrical insulation 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 compensate for artefacts arising from motion of the subject. The compensation for motion artefacts removes noise from the sensed electrical signal  21  producing a version of the electrical signal  21  that is less affected by noise. 
     The circuity  40  receives a motion associated signal  50  that is associated with motion of the electrode  22  of the displacement current sensor  20  relative to the user  30 . 
     For example, the motion associated signal  50  may be a motion dependent signal that is dependent upon or responsive to motion of the electrode  22  of the displacement current sensor  20  relative to the user  30 . Additionally or alternatively, the motion associated signal  50  may be a force dependent signal that is dependent upon or responsive to force that causes motion of the electrode  22  of the displacement current sensor  20  relative to the user  30 . 
     In some examples, the apparatus  10  comprises one or more motion-associated sensors  52  for producing one or more motion associated signals  50 . 
     For example, a motion-associated sensor  52  may be a motion sensor, a distance sensor; a force sensor, a pressure sensor, a deformation sensor and/or a body wearable motion sensor. 
     The motion-associated sensor  52  may, for example, be a capacitive sensor, a radio frequency sensor, an ultrasound sensor, an optical sensor; and electromagnetic film sensor, a piezoelectric sensor, a strain gauge sensor, an accelerometer, a gyroscope, and/or a magnetometer. 
     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 a variable capacitance caused by motion of the electrode  22  of the displacement current sensor  20  relative to the user  30 . The circuitry  40  may be configured to measure a variable reactance caused by motion of the electrode  22  of the displacement current sensor  20  relative to the user  30  by measuring, in the Imaginary domain, modulation of a reference signal by the variable reactance. In the examples illustrated in  FIGS. 2 and 3 , the reference signal is an external signal  45  applied to the subject  30 . 
     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 a noise-reduced electrocardiogram signal  41 . The circuitry  40  measures the artefacts caused by motion 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 process the sensed signal  21  to compensate for the artefacts arising from motion of the subject. 
     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, and measure a time-variable signal V out , for example at an output of a second operational amplifier. 
     In these examples, the first electrode  26  is in proximity to the skin  32  of the subject  30  but electrically insulated therefrom. In some examples, electrical insulation  24  insulates the electrode  26  from the subject&#39;s skin  32 . In some examples material between the electrode  26  and the subject&#39;s skin  30  additionally or alternatively provides electrical insulation of the 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. For example, the electrical reference signal  45  may have a significant component over 100 kHz, for example in the range 100-500 kHz, or in some embodiments may lie entirely within the range over 100 kHz or 100-500 kHz. The electrical reference signal  45  may, for example, be a pure tone (single frequency). 
     In the example of  FIG. 3 , a first displacement current sensor  20   1  comprises a first ECG electrode  60   1  for measuring a first ECG signal  41   1 , and a first electrode  26  for injection of a current (reference signal  45 ), and a second displacement current sensor  20   2  comprises a second ECG electrode  60   2  for measuring a second ECG signal  41   2 , and a second electrode  22  for measuring motion artefacts. The first displacement current sensor  20   1  and the second displacement current sensor  20   2  are physically distinct and separate. 
     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  FIG. 4 , the first electrode  26  and the second electrode  22  are guard electrodes for the respective first and second ECG electrodes  60   1 ,  60   2 . 
     Consequently, the first displacement current sensor  20   1  comprises a sensing electrode  60   1  and also at least one guard portion, the guard electrode  26 . Consequently, the second displacement current sensor  20   2  comprises a sensing electrode  60   2  and also at least one guard portion, the guard electrode  22 . 
     The ECG electrode is centrally located. In these examples it is circular but this is not necessarily essential. The electrode  26 / 22  is separated from the ECG electrode  60   1 / 60   2 , in this example, low relative permittivity gaps are used for separation. 
     The electrode  26 / 22  is a circle circumscribing but separated from the ECG electrode  60   1 / 60   2 . 
     Returning to  FIG. 3 , the ECG signal  41   1  received at the ECG electrode  60   1  is applied, via op-amp  90 , as a virtual earth at a +ve terminal of a first op-amp  70 . A voltage divider may be used in some examples. 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 . 
     The ECG signal  41   2  received at the ECG electrode  602  is applied, via op-amp  92 , as a virtual earth at a +ve terminal of a second op-amp  80 . A voltage divider may be used in some examples. For example, impedances Z A ′ and Z B ′ may be connected in series between the output of the op-amp  92  and ground, and an intermediate node between impedances Z A ′ and Z B ′ may be connected to the +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 fist guard electrode  26 . The first op-amp  70  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 variable voltage source  76  produces voltage V in (t). The impedance  74  has a value Z 1 . The first ECG signal  41   1  received at the first ECG electrode  60   1  is applied, after amplification by op-amp  90 , as a virtual earth at the +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 second ECG signal  41   2  received at the second ECG electrode  60   2  is applied, after amplification by op-amp  92 , as a virtual earth at the +ve terminal of the second op-amp  80 . The second op-amp  80  generates at its output a voltage V out  which is the motion artefact signal  43 . In some examples, 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 between the electrodes  22 ,  26  and 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 relative motion between the electrodes  22 ,  26  and the subject&#39;s body. The output of the op-amp  80  is therefore a variable impedance signal  43  caused by relative motion. 
     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  70  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  80  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 inter-electrode 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 part of V out  that arises as a consequence of variation in V in . 
     This may, for example, be achieved by removing 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 the electrodes  22 / 26  and the skin surface will be primarily reactive (capacitive). 
     Changes in the imaginary (reactive) part of the variable impedance signal  43  may therefore be attributed to variation in capacitance arising from the relative motion of the electrodes  22 , 26  and the body. These represent motion artefacts in the sensed signal  21 . 
       FIG. 5  illustrates an example of a demodulation circuit. The demodulation circuit 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 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 apparatus  10  therefore comprises: a first displacement current sensor  20   1  comprising a first sensing electrode  60   1  and a first guard electrode  26  and a second displacement current sensor  20   2  comprising a second sensing electrode  60   2  and a second guard electrode  22 . The first displacement current sensor  20   1  is configured to measure a first sensed signal  21   1  dependent upon electrical activity of a subject&#39;s heart. The second displacement current sensor  20   2  is configured to measure a second sensed signal  21   2  dependent upon electrical activity of a subject&#39;s heart. 
     The first displacement current sensor  20   1  comprises electrical insulation for insulating the first sensing electrode  60   1  from the subject&#39;s skin. The second displacement current sensor  20   2  comprises electrical insulation for insulating the second sensing electrode  60   2  from the subject&#39;s skin. 
     The first displacement current sensor  20   1  and the second displacement current sensor  20   2  are separated in space by at least several centimeters (more than 2 or 3 cm). The first displacement current sensor  20   1  is movable relative to the subject and is not affixed to the subject. The second displacement current sensor  20   2  is movable relative to the subject and the first displacement current sensor  20   1 , and is not affixed to the subject. The first displacement current sensor  20   1  and the second displacement current sensor  20   2  can be embedded in a bed or other furniture. 
     The apparatus  10  comprises circuitry configured to process the first sensed signal  21   1  to compensate for artefacts arising from motion of the subject and to process the second sensed signal  21   2  to compensate for artefacts arising from motion of the subject. 
     The circuitry is configured to apply a reference signal  45  to one of the first guard electrode  26  and the second guard electrode  22  and to sense at the other of the first guard electrode  26  and the second guard electrode  22  an additional signal dependent upon motion of the subject and to use the additional signal to estimate artefacts arising from motion of the subject and compensate the first sensed signal and/or the second sensed signal. In the illustrated example, the circuitry is configured to apply a reference signal  45  to the first guard electrode  26  and to sense at the second guard electrode  22  an additional signal dependent upon motion of the subject. The reference signal  45  is a first time-variable voltage and the additional signal is a second time-variable voltage dependent upon the first time-variable voltage and motion of the subject. In this example, the reference signal  45  is continuously applied and the additional signal is sensed continuously and contemporaneously. 
     The first sensed signal and the second sensed signal are used, as virtual earth, for applying the reference signal  45  and for sensing the additional signal. The circuitry is configured to apply the first time-variable voltage V 1  to the first guard electrode  26  via an operational amplifier  70 , measure a second time-variable voltage V out  at an output of another operational amplifier  80 . An input to the operational amplifier  80  is connected to the second guard electrode  22 . An impedance Z 3  is connected between the input and the output of the operational amplifier  80 . At least a part of the impedance between the first and second guard electrodes  26 ,  22  is measured using at least the first time-variable voltage and the second time-variable voltage to estimate motion of the subject. 
     The operational amplifier  80  is virtually earthed by an ECG signal received at the second sensing electrode  60   2  adjacent the second guard electrode  22  and guarded by the second guard electrode and the operational amplifier  70  is virtually earthed by an ECG signal received at the first sensing electrode  60   1  adjacent the first guard electrode  26  and guarded by the first guard electrode  26 . The apparatus  10  may be configured to use at least a part of the impedance between the first and second guard electrodes as an external reference signal for an adaptive filter for filtering the first sensed signal and/or the first sensed signal. 
     The circuitry is thus configured to process the first sensed signal to compensate for artefacts arising from motion of the subject, to provide an ECG signal. The circuitry is configured to process the second sensed signal to compensate for artefacts arising from motion of the subject, to provide an ECG signal. 
     It will be appreciated than in the apparatus  10 , the sensed signal  21  is used, as a virtual earth, to compensate for artefacts arising from motion of the subject. The sensed signal  21  is used, as virtual earth, for sensing variations, compared to an applied reference signal  45 , used to compensate for artefacts arising from motion of the subject. A first displacement current sensor  20   1  is configured to apply the reference signal  45 , using a first sensed signal as virtual earth. A second displacement current sensor  20   2  is configured to sense the additional signal using a second sensed signal as virtual earth. The first displacement current sensor  20   1  is configured to apply the reference signal  45  via first guard electrode  26  and sense the first sensed signal using an adjacent first sensing electrode  60   1 , guarded by the first guard electrode  26 . The second displacement current sensor  20   2  is configured to sense the additional signal via a second guard electrode  22  and sense the second sensed signal using an adjacent second sensing electrode  60   2 , guarded by the second guard electrode  22 . 
       FIGS. 6A, 6B, 6C  illustrate examples of motion-associated sensors  52  configured to provide one or more motion associated signals  50 . A motion-associated sensor  52  produces a motion associated signal  50  that is associated with motion of the electrode  22  of the displacement current sensor  20  relative to the user  30 . 
     For example, as illustrated in  FIG. 6A , the motion associated signal  50  may be a force dependent signal that is dependent upon or responsive to a force that causes motion of the electrode  22  of the displacement current sensor  20  relative to the user  30 . 
     The motion-associated sensor  52  may, for example, be a force sensor, a pressure sensor, and/or a deformation sensor. The motion-associated sensor  52  may, for example, be an electromagnetic film sensor, a piezoelectric sensor, a strain gauge sensor. 
     For example, as illustrated in  FIG. 6B , the motion associated signal  50  may be a motion dependent signal that is dependent upon or responsive to motion of the subject&#39;s body (e.g. relative to a fixed electrode  22  of the displacement current sensor  20 ). The motion-associated sensor  52  may, for example, be a body wearable motion sensor. The motion-associated sensor  52  may, for example, be an accelerometer, a gyroscope, and/or a magnetometer. 
     For example, as illustrated in  FIG. 6C , the motion associated signal  50  may be a motion dependent signal that is dependent upon or responsive to motion of the electrode  22  of the displacement current sensor  20  relative to the user  30 . The motion-associated sensor  52  may, for example, be a motion sensor or a distance sensor. The motion-associated sensor  52  may, for example, be a capacitive sensor, a radio frequency sensor, an ultrasound sensor, an optical sensor. 
     The circuitry  40  illustrated in  FIG. 3 , produces the variable impedance signal  43  as a motion associated signal  50 . The variable impedance signal  43  varies with the varying capacitance between ECG electrode  60   1  and the subject&#39;s body and ECG electrode  60   2  and the subject&#39;s body. 
     The motion-associated sensors  52  may be part of the apparatus  10  or separate to the apparatus  10 . 
     The motion-associated sensor  52  may perform one of more of the sensing functions described with reference to  FIGS. 6A to 6C . 
     In the example of  FIGS. 6A to 6C , the displacement current sensor  20  is movable relative to the subject and is not affixed to the subject. For example, the electrode  22  ( 26 ) of a displacement current sensor  20  is movable relative to the subject and is not affixed to the subject. 
     In these examples, but not necessarily all examples, the displacement current sensor  20  is fixed to an article  56  adjacent the subject  30 . The article  56  may, for example, be a bed or other furniture. 
       FIGS. 7A to 7D  illustrate further examples of circuitry  40  configured to process the one or more sensed electrical signals  21  to compensate for artefacts arising from motion of the subject. The compensation for motion artefacts removes noise from the sensed electrical signal  21  producing an electrical signal  21  less affected by noise. 
     In the example of  FIG. 7A , the circuitry  40  is configured to process the sensed signal  21  to compensate for artefacts arising from motion of the subject by passing the sensed signal  21  through a band pass filter  110  configured to filter the sensed signal  21  to compensate for artefacts arising from motion of the subject  30 . It may be desirable for the filter  110  to be adaptive and adapt in response to the motion associated signal  50 . In some but not necessarily all examples, the filter  110  may have a fixed bandwidth or an adaptive bandwidth in the range 1-40 kHz. 
     In the example of  FIG. 7B , the circuitry  40  is configured to process the sensed signal  21  to compensate for artefacts arising from motion of the subject by:
     i) determining a reliability of the sensed signal  21  over time based on the motion associated signal  50 . For example, if the motion associated signal  50  indicates motion for a first period of time, then the sensed signal  21  may be flagged as unreliable for that first period of time. If the motion associated signal  50  indicates no motion for a second period of time, then the sensed signal  21  may be flagged as reliable for that second period of time.   ii) processing the sensed signal  21  using weightings based on the determined reliability. For example, if the sensed signal  21  is flagged as reliable it may have a weighting of 1 (multiplication factor=1) and if the sensed signal  21  is flagged as unreliable it may have a weighting of 0 (multiplication factor=0). It is of course possible to have weightings between 0 and 1 depending upon the degree of reliability/unreliability.   

     In the example of  FIG. 7C , the circuitry  40  is configured to process the sensed signal  21  to compensate for artefacts arising from motion of the subject by:
     measuring artefacts arising from motion of the subject; and   in the frequency domain, removing the measured artefacts from the sensed signal.   

     The motion associated signal  50  may be used to represent artefacts arising from motion of the subject  30 . The motion associated signal  50  may be converted  112  to the frequency domain by, for example, using a transform function such as a Fourier Transform (FT). The sensed signal  21  may be converted  112  to the frequency domain by, for example, using a transform function such as a Fourier Transform (FT). 
     A processor  116  may be used to remove the measured artefacts from the sensed signal in the frequency domain. 
     For example, it may be determined that the spectral components  50 ′ of the motion associated signal  50  represent noise in the spectral components of the sensed signal  21 . The spectral components of the sensed signal that correspond to the spectral components of the motion associated signal  50  may be adapted to remove motion artefacts. In some examples they may be reduced. In some examples, they may be removed. 
     In the example of  FIG. 7D , the circuitry  40  is configured to process the sensed signal  21  to compensate for artefacts arising from motion of the subject by using an adaptive filter  118  for filtering the sensed signal  21 . The filtered signal or some other reference signal is used for updating the adaptive filter  118 . For example, at least a part of the impedance between the first and second electrodes  60   1 ,  60   2  may be used as an external reference signal  117  for the adaptive filter  118  for filtering the sensed signal  21 . In some but not necessarily all examples, the filter  118  may have an adaptive bandwidth in the range 1-40 kHz. 
     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). 
     As illustrated in  FIG. 8  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  FIGS. 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:   measuring a sensed signal, from a displacement current sensor  20 , dependent upon electrical activity of a subject&#39;s heart; and   processing the sensed signal  21  to compensate for artefacts arising from motion of the subject  30 .   

     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. 9  illustrates an example of a method  100  comprising:
     at block  102 , using a displacement current sensor  20  to measure a sensed signal  21  dependent upon electrical activity of a subject&#39;s heart;   at block  104 , processing the sensed signal  21  to compensate for artefacts arising from motion of the subject  30 .   

     The method, when applied to the apparatus illustrated in  FIG. 3 , comprises, at block  102 , using a first displacement current sensor, comprising a first sensing electrode and a first guard electrode, to measure a first sensed signal dependent upon electrical activity of a subject&#39;s heart and using a second displacement current sensor, comprising a second sensing electrode and a second guard electrode, to measure a second sensed signal dependent upon electrical activity of a subject&#39;s heart; and, at block  104 , processing the first sensed signal to compensate for artefacts arising from motion of the subject and/or processing the second sensed signal to compensate for artefacts arising from motion of the subject. 
     The blocks illustrated in the  FIGS. 9  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.