WO2014043739A1

WO2014043739A1 - A system for measuring physiological signals

Info

Publication number: WO2014043739A1
Application number: PCT/AU2013/001005
Authority: WO
Inventors: Gaetano Gargiulo
Original assignee: Heard Systems Pty Ltd
Priority date: 2012-09-19
Filing date: 2013-09-06
Publication date: 2014-03-27

Abstract

A system (10) for measuring physiological signals includes a sensing element (12) for sensing a biological signal generated in a subject's body, the biological signal including a noise component, having a profile of which is at least partially known, and an information carrying component. A filter module (26) separates the noise component and the information carrying component of the biological signal from each other, the filter module generating a filtered signal from the biological signal input into the filter module (26). An amplifier (24) has an inverting input and a non-inverting input, the filter module (26) being connected to one of the inputs of the amplifier (24) for receiving the filtered signal and the sensed biological signal being input to the other of the inputs so at least the noise component of the biological signal is attenuated relative to the information carrying component. The system (10) includes a reference electrode (16) relative to which the sensing element senses the biological signal.

Description

"A system for measuring physiological signals"

Cross-Reference to Related Applications

The present application claims priority from Australian Provisional Patent Application No 2012904093 filed on 19 September 2012, the contents of which are incorporated herein by reference.

Field

This disclosure relates, generally, to the measurement of physiological signals and, more particularly, to a system for, and a method of, measuring physiological signals.

Background

Current bio-potential measurement systems use a minimum number of three electrodes connected to the body of the subject. The bio-potential measurement is obtained by measuring the difference between the signals read at the recording electrodes relative to a reference electrode. Some techniques are available which make the use of the reference electrode redundant or suitable for loose connection relative to the body of the subject. However, the bio-potential recording is always obtained in a bipolar manner between two measurement electrodes by taking the difference between the two measurements. This includes systems making use of the so called active electrodes (with an amplifier embedded in the electrode) which have the sole role of lowering the contact impedance of the measurement electrodes. Summary

In a first aspect, there is provided a system for measuring physiological signals, the system including

a sensing element for sensing a biological signal generated in a subject's body, the biological signal including a noise component, having a profile of which is at least partially known, and an information carrying component;

a filter module for separating the noise component and the information carrying component of the biological signal from each other, the filter module generating a filtered signal from the biological signal input into the filter module;

an amplifier having an inverting input and a non-inverting input, the filter module being connected to one of the inputs of the amplifier for receiving the filtered signal and the sensed biological signal being input to the other of the inputs so at least the noise component of the biological signal is attenuated relative to the information carrying component;, and

a reference electrode relative to which the sensing element senses the biological signal.

The system may include a grounding circuit which is responsive to the sensed biological signal, the reference electrode being connected to the grounding circuit.

The system may include a buffer for buffering the sensed signal prior to feeding the sensed signal to the filter module and the amplifier.

At least one of the inputs of the amplifier may have an active guard associated with it. Both inputs of the amplifier may have active guards associated with them, the active guards being driven by the buffer. The buffer may be a part of the amplifier. The active guard may be coupled to the grounding circuitry.

The amplifier may be a high common-mode rejection ratio amplifier for improved noise attenuation.

The sensing element may be connected to the amplifier via a coupling impedance. The coupling impedance may be a calibrated coupling impedance. In the case of DC coupling, the coupling impedance may be a protective resistor. In the case of AC coupling, the impedance may be a capacitive impedance.

The filter module may be an active filter configured to filter noise of the known profile.

The reference electrode may be connected by a coupling impedance to a summing amplifier of the grounding circuitry.

The system may include a further filter module for filtering the information carrying component of the biological signal, the further filter module being complementary to the filter module.

In a second aspect, there is provided a method of measuring physiological signals, the method including

sensing a biological signal generated in a subject's body, the biological signal including a noise component, having a profile of which is at least partially known, and an information carrying component;

separating the noise component and the information carrying component of the biological signal from each other to form a filtered signal; and

feeding the filtered signal to one input of an amplifier and the sensed biological signal to another input of the amplifier so at least the noise component of the biological signal is attenuated relative to the information carrying component. The method may include also feeding the biological signal to grounding circuitry.

The method may include buffering the sensed biological signal prior to filtering the sensed signal and feeding the sensed signal to the amplifier.

The method may include connecting a sensing element, which senses the biological signal, to the amplifier via a coupling impedance.

The method may include shielding at least one of the inputs of the amplifier. The method may include shielding both inputs of the amplifier.

The method may include coupling the shielding to the grounding circuitry.

The method may include filtering the information carrying component of the biological signal as well prior to feeding the biological signal to the amplifier.

Brief Description of Drawings

Fig. 1 shows a schematic block diagram of an embodiment of a system for measuring physiological signals;

Fig. 2 shows a circuit diagram of the system;

Fig. 3 shows a graphic representation of a signal generated from an output of two such systems;

Figs. 4-9 show circuit diagrams of further embodiments of the system for measuring physiological signals; and

Fig. 10 shows a graphic representation of a 12 lead ECG measurement taken on a human subject.

Detailed Description of Exemplary Embodiment

In Fig. 1 of the drawings, reference numeral 10 generally designates an embodiment of a system for measuring physiological signals. The system 10 includes a sensing element in the form of a sensing electrode 12 which is placed on a subject's body as indicated schematically by reference numeral 14. The system 10 further includes a reference electrode 16 which is attached to the subject's body remote from the sensing electrode 12.

A biological signal sensed by the sensing electrode 14 is fed to a pre-amplifier stage 18 where the signal is processed before being output as an output signal on line 20. The biological signal sensed by the sensing electrode 12 contains an information carrying component and a noise component, the noise component having an at least partially known profile. The pre-amplifier stage 18 comprises a buffer 22, to which the biological signal is fed via a buffer 22, and a difference amplifier 24. A filter module 26 is connected to an output of the buffer 22 and is interposed between the buffer 22 and the amplifier 24. The system 10 includes an optional signal filter 28 which, if provided, is fed with the biological signal from the output of the buffer 22.

The sensing electrode 12 is connected to the pre-amplifier stage 18 via a coupling impedance 30. Similarly, the reference electrode 16 is connected to grounding circuitry 32 of the pre-amplifier stage 18 via a coupling impedance 34. In the case of DC coupling, the coupling impedances 30 and 34 are protection resistors. In the case of AC coupling, the coupling impedances 30 and 34 are capacitive.

The value of the protection resistor is calculated using the following formula:-

Λ = -^- χ 10- 200« where n is the number of leads connected and V_s is the voltage supply value.

The value of the coupling capacitor is selected according to the desired bandwidth and must not exceed 15 nF with a parallel parasitic resistor having a value exceeding 100 GQ.

The pre-amplifier stage 18 includes an active guard or shield 36 which shields a lead 38 connecting the sensing electrode 12 to the pre-amplifier stage 18 and also shields the inputs to the amplifier 24. The shield is shown, schematically, at 40 in Fig. 1 of the drawings and contains a facsimile of the biological signal.

Generally, in the measurement of physiological signals, the noise component of the signal has a reasonably well known profile. For example, for measuring external ECGs the information carrying component of the signal has a range from about 0.5 Hz to 150 Hz. For an internal ECG, the signal of interest, i.e. the information carrying component, of the signal has a frequency which is typically in the range of from about 0.05 Hz to 1000 Hz. For an EEG, the signal of interest has a frequency range of about 0.05 Hz to 45 Hz. For an E G, the useful signal has a range of from about 2 Hz to about 1000 Hz. Components of the signals above or below the specified frequency ranges set out above can be assumed to be noise. Hence, for example, in the case of an external ECG, any signal components below, for example, approximately 0.5 Hz, such as occurs as a result of respiration, is regarded as noise and is to be attenuated using the filter module 26. The sensing electrode 12 is a passive electrode having no internal amplification. The electrode 12 and lead 38 are, for example, as described in the applicant's International Patent Application No. PCT/AU2009/000873 with the active guard 36 of the pre-amplifier stage 18 providing the shielding 40 for the lead 38. If desired, the shielding 40 could also shield the electrode 12 as described in the above International Patent Application.

The filter module 26 is designed so that a transition bandwidth between the noise component and information carrying , component of the biological signal sensed by the sensing electrode 12 is as small as possible. It will be appreciated that the portion of the information carrying component of the biological signal in the transition bandwidth will contain some residual noise which is attenuated using the amplifier 24.

The amplifier 24 is implemented using an appropriate integrated circuit. An integrated circuit which the applicant has found to be suitable for the present purpose is the INAl 16 integrated circuit available from Texas Instruments. This integrated circuit provides an integrated buffer 22 and active guard 36 and has a high input impedance.

In the embodiment of the invention illustrated in Fig. 2 of the drawings, it is assumed that the noise component of the biological signal is required to filter out low frequency noise arising due to thermal emissions, respiration etc. and any signal having a frequency below about 0.5 Hz. Thus, the filter module 26 is implemented as a low pass filter. In the illustrated embodiment, the filter 26 is an active, non-inverting low pass filter having a low frequency cut off of approximately 0.16 Hz with an approximate 0.3Hz transition bandwidth. The low cut off frequency is set regulating the value of resistor 42 to 100 kQ and the value of a capacitor 44 to 10 μΡ. The filter gain is set to approximately 11 volts fixing the value of the resistor 46 at about 10 kQ. Generally, the noise cut off frequency can be calculated using the following formula: noise frequency—— -—

2iTL noise noise*

The noise filter module 26 is implemented using a suitable operational amplifier 48.1 of a dual operational amplifier having Serial No. OPA2244 available from Texas Instruments. A second part 48.2 of the operational amplifier forms a summing amplifier of the grounding circuitry 32.

Instead of the OPA2244 operational amplifier, other suitable operational amplifiers of similar specifications such as, for example, an OPA 2277 or an OPA 333 could be used if single supply or lower power consumption is required. As a result of the filtering by the filter module, an increase in noise amplitude occurs from the gain of the operational amplifier 48.1 of the noise filter module 26 and this is compensated for by a compensating circuit in the form of a resistor divider circuit 50 having resistors 52 and 54. The resistance of the resistor 52 is selected to be 9.1 kQ and the resistance of resistor 54 is the same as that of resistor 42, i.e 100 kQ. The lower value of resistor 52 is selected to obtain an attenuation factor of approximately 11. The compensation circuit can be replaced, if desired, by a single potentiometer.

As indicated above, the amplifier 24 is implemented using the integrated circuit INA116. In the preferred embodiment, the gain of the amplifier 24 is set to 6 volts by selecting the value of resistor 56 to be 10 kQ. As described above, the amplifier 24 also makes available a direct buffered output to implement the active guard 36 and the shielding 40 to the printed circuit board and the sensing electrode 12.

In most applications, the system will comprise a plurality of sensing electrodes 12 each with their own pre-amplifier stage 18. Absolute values of the signals from other electrodes 12 are summed using the summing amplifier 48.2 of the grounding circuit as indicated schematically at 57 in Figs. 1 and 2 of the drawings. The summing amplifier also drives a power supply ground terminal 58. The driving of the ground terminal 58 does not occur directly but, instead, via a low pass filter network 60 connected between the amplifier 48.2 and the ground terminal 58. This dampens any abrupt oscillations which may occur at the connection of one or more of the electrodes 12 to avoid saturation of limitation of the amplifier dynamics due to ground potential wandering. In case of a single electrode 12, the low pass filter network 60 is also used to create a virtual signal ground.

A signal detected by the sensing electrode 12 is fed via the shielded lead 38 to the buffer 22 of the pre-amplifier 18. The buffer then directs the signal to the noise filter module 26, to the active guard 36 which forms shielding for the printed circuit board containing the amplifier 24 and also the shielding 40 for the lead 38. In addition, the signal from the buffer 22 is fed to the optional signal filter 28. The signal filter 28 is designed either to enhance the information carrying component of the biological signal sensed by the electrode 12 or to compensate phase delays introduced from the noise filter module 26.

Further, the signal from the sensing electrode 12 is fed via the active guard 36 to the grounding circuitry 32. The grounding circuitry 32 is designed to increase the common-mode rejection ratio of the amplifier 24. In the selected embodiment, the noise filter module 26 is fed to the inverting input of the amplifier 24 and the unfiltered signal, or, if applicable, the signal filtered via the signal filter 28 is fed to the non-inverting input of the amplifier 24. In this way, the noise in the biological signal is attenuated by negative feedback in the amplifier 24. If noise attenuation via positive feedback is required, the inputs to the amplifier 24 are reversed so that the noise filter module 26 connects to the non-inverting input of the amplifier 24 with the signal, or the signal filtered by signal filter module 28, if used, being fed to the inverting input of the amplifier 24.

The grounding circuitry 32 is shared between more than one pre-amplifier stage 18 of the system 10 if more than one sensing electrode 12 is used to perform the measurements. Connection between any point on the subject's body and the preamplifier 18 is protected by the calibrated coupling impedance 30 and 34, as described above. This provides safety both for the subject and the circuitry of the pre-amplifier stage 18.

In use, for example, in the measurement of an external ECG, at least two sensing electrodes 12 are used and are placed at spaced locations, such as, for example, on the left arm and right arm of the subject's body, with the reference electrode 1,6 being placed at a further part on the subject's body spaced from the locations of the two sensing electrodes 12 of the system 10. The two pre-amplifier stages 18 share the same grounding circuitry 32.

Using the two electrodes 12, it is possible to record the potential at the location of each of the sensing electrodes 12 separately. Hence, two signals are generated as shown at 62 and 64 in Fig. 3 of the drawings. The signal 62 is measured by the electrode 12 on the left arm of the person's body and the signal 64 is measured by the electrode 12 on the right arm of the person's body.

The summing amplifier 48.2 is used to compute an output signal 66 which is subtraction of the signal 64 from the signal 62 or the summing of the absolute values of the signals 62 and 64.

Hence, signals of interest between active electrodes, in other words sensing electrodes 12, at different parts of a subject's body can be obtained rather than from a sensing electrode relative to a reference or grounding electrode. Hence, a more accurate mapping of a subject's organs, such as, for example, brain function or heart function can be obtained than has heretofore been possible. In addition, the number of electrodes required to obtain comprehensive data can also be reduced.

In Figs. 4-9 of the drawings, different embodiments of the system 10 are illustrated and, in particular, the configuration of the noise filter module 26. With reference to the previous drawings, like reference numerals refer to like parts, unless otherwise specified. Thus, in Fig. 4 of the drawings, the noise filter module 26 is implemented as a passive low pass filter. In Fig. 5 of the drawings, the noise filter module 26 is implemented as a passive, high pass filter. In Fig. 6 of the drawings, the noise filter module 26 is implemented as an active, high pass filter. In Fig. 7 of the drawings, the noise filter module 26 is implemented as a combined low pass filter and high pass filter, both being active filters.

In Fig. 8 of the drawings, the noise filter module 26 is implemented as a bandpass filter. This is also the case for the noise filter module 26 shown in Fig. 9 of the drawings. It will be appreciated that the noise filter modules shown in Figs. 8 and 9 of the drawings could be added to any of the other noise filter modules where it is desired to filter out narrow bandwidth noise such as may arise due to power line noise.

Fig. 10 shows traces of an ECG measured from a human subject using the 0.16 Hz low frequency noise definition. Dashed lined 68 represent traces measured by the sensing electrodes 12 of the system. The solid lines 78 represent computed ECG traces as computed by the system 10 adding together different signals sensed by sensing electrodes 12 of the system 10 after such signals have been processed to attenuate the noise components of the signals. The dotted line 72 is the computed Wilson Central Terminal Potential.

It is therefore an advantage of the above described embodiment that a unipolar- type measurement arrangement is able to be implemented as a result of this configuration of the system 10. Thus, by having an idea of the noise profile, steps can be taken to filter out noise and to enhance the information carrying component of the signal to enable physiological signals to be measured and displayed. This has particular application in uses such as EEGs, ECGs and EMGs.

Thus, the system 10 can be used to detect true signals at the specific sites of the electrodes and then via subtraction, allow the calculation of a normally seen ECG. Current 12 lead ECG systems have been referenced to the Wilson Central terminal since 1934, and are a combination of signals from multiple points. An array of single electrodes 12 of the system 10 across a subject's chest has the potential to detect more accurately a subject's heart condition due to the higher signal specificity from each of the electrodes 12.

In addition, the system 10 can be used for measuring true unipolar signals, i.e. signals that are specific with no far field component for investigating atrial fibrillation/CFE analysis, better sensing in the brain for placement of electrodes for deep brain stimulation and/or ablation. The system can also be used for obtaining unipolar signals for infarct analysis to determine whether a person has had a heart attack or whether or not there is some other cause for the chest pain. The use of the system will also allow better analysis for diagnosis. In addition, outputs from the system can be used with other signals for combination analysis, for example, with PBGs or for vital signs analysis.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. A system for measuring physiological signals, the system including

an amplifier having an inverting input and a non-inverting input, the filter module being connected to one of the inputs of the amplifier for receiving the filtered , signal and the sensed biological signal being input to the other of the inputs so at least the noise component of the biological signal is attenuated relative to the information carrying component; and

2. The system of claim 1 which includes a grounding circuit which is responsive to the sensed biological signal, the reference electrode being connected to the grounding circuit.

3. The system of claim 2 which includes a buffer for buffering the sensed signal prior to feeding the sensed signal to the filter module and the amplifier.

4. The system of claim 3 in which at least one of the inputs of the amplifier has an active guard associated with it.

5. The system of claim 4 in which both inputs of the amplifier have active guards associated with them, the active guards being driven by the buffer.

6. The system of claim 4 or claim 5 in which the active guard is coupled to the grounding circuitry.

7. The system of any one of the preceding claims in which the amplifier is a high common-mode rejection ratio amplifier for improved noise attenuation.

8. The system of any one of the preceding claims in which the sensing element is connected to the amplifier via a coupling impedance.

9. The system of any one of the preceding claims in which the filter module is an active filter configured to filter noise of the known profile.

10. The system of claim 2 in which the reference electrode is connected by a coupling impedance to a summing amplifier of the grounding circuitry.

11. The system of any one of the preceding claims which includes a further filter module for filtering the information carrying component of the biological signal, the further filter module being complementary to the filter module.

12. A method of measuring physiological signals, the method including

feeding the filtered signal to one input of an amplifier and the sensed biological signal to another input of the amplifier so at least the noise component of the biological signal is attenuated relative to the information carrying component.

13. The method of claim 12 which includes also feeding the biological signal to grounding circuitry.

14. The method of claim 12 or claim 13 which includes buffering the sensed biological signal prior to filtering the sensed signal and feeding the sensed signal to the amplifier.

15. The method of any one of claims 2 to 14 which includes connecting a sensing element, which senses the biological signal, to the amplifier via a coupling impedance.

16. The method of any one of claims 12 to 15 which includes shielding at least one of the inputs of the amplifier.

17. The method of claim 16 which includes shielding both inputs of the amplifier.

18. The method of claim 16 or claim 17, insofar as either claim is dependent on claim 13, which includes coupling the shielding to the grounding circuitry.

19. The method of any one of claims 12 to 18 which includes filtering the information carrying component of the biological signal as well prior to feeding the biological signal to the amplifier.