Concentric electrode construction for an electrocardiogram transmitter

Abstract

An electrode assembly for generating electrocardiogram bioelectric potential signals suitable for wave-form analysis is disclosed. The electrode assembly comprises two separate transmitter electrodes adapted to be placed on the skin of a subject. Each electrode includes a central conducting surface and an outer conducting surface disposed in surrounding relationship about the periphery of the central conducting surface, the outer conducting surface being spaced from the central conducting surface by an intermediate insulating barrier region. The outer conducting surfaces of each electrode are connected in common and to a common ground of a differential amplifier. The central conducting surface of each of the electrodes is connected to a different respective input of the differential amplifier whereby a signal representative of the bioelectric potential is provided at the differential amplifier output.

Description

BACKGROUND OF THE INVENTION

This invention generally relates to electrocardiogram techniques, and particularly concerns a novel transmitting electrode assembly therefor by which bioelectric potential signals are generated of a quality suitable for wave-form analysis.

With particular reference initially being directed to FIG. 1 of the appended drawings, a conventional prior art technique and apparatus is shown by which bioelectric potential signals are typically generated in the art, such signals being utilized to drive a chart recorder of an electrocardiogram monitoring machine. Since a complete electrocardiogram diagnosis of heart disorders requires an analysis of the bioelectric potential waveform, three separate surface electrodes are normally provided, such as electrodes 10, 12, and 14, each of these electrodes being disposed on the skin of a patient. Electrode 14 is conventionally coupled to the common ground 16 of a differential amplifier 18, whereas electrodes 10 and 12 are conventionally coupled to different respective inputs 20 and 22 of differential amplifier 18. Three separate signals are therefore generated, with the output from the differential amplifier 18 as represented by reference numeral 24 being the bioelectric potential difference between electrodes 10 and 12 from the ground defined by the potential of electrode 14.

Difficulties are encountered with the conventional prior art apparatus as shown in FIG. 1, particularly as concerns obtaining clinical quality electrocardiogram tracings due to the large amount of filtering which would be necessary to eliminate ambient noise from two floating sources represented by electrodes 10 and 12. Further, the three-electrode construction of the prior art is quite inconvenient to use since it is necessary to firmly attach the electrodes to the patient's skin, it not being possible for the user of the apparatus to simply hold one electrode in each hand so as to obtain a proper monitoring.

SUMMARY OF THE INVENTION

It is thus apparent that a need exists in this art for an improved electrode assembly of simple construction which is capable of providing clinical grade electrocardiogram signals. It is the primary objective of the instant invention to provide such an improved electrode assembly.

A further objective of the instant invention is the provision of an electrode assembly which does not require three separate surface contacting regions but, instead, produces a high quality bioelectric potential signal from only two surface contacts, thus markedly increasing the convenience to the user thereof.

These objectives, as well as others which will become apparent as the description proceeds, are implemented by the instant inventive electrode assembly which, in its preferred embodiment, constitutes two separate transmitter electrodes adapted to be placed on a subject's skin. Each electrode includes a central conducting surface and an outer conducting surface which is disposed in surrounding relationship about the periphery of the central conducting surface but is spaced therefrom by an intermediate insulating barrier region. The outer conducting surfaces of each electrode are electrically coupled in common to one another and to a common ground of a differential amplifier. The central conducting surface of each of the electrodes are electrically connected to a different respective input of the differential amplifier. As a result, a signal representative of the bioelectric potential is provided at the differential amplifier output, which signal is of high quality and is suitable for complete wave-form analysis.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood, and further features and advantages thereof will become apparent from the following detailed description of a preferred inventive embodiment, such description making reference to the appended sheet of drawings, wherein:

FIG. 1 is a schematic illustration of a prior art electrode transmitter for an electrocardiogram machine;

FIG. 2 is a schematic illustration of the electrode assembly constructed in accordance with the teachings of the instant invention; and

FIG. 3 is an electrical schematic diagram depicting an equivalent circuit of the construction of the instant invention depicted in FIG. 2, for purposes of better explaining the electrical operation of the invention.

DETAILED DESCRIPTION OF A PREFERRED INVENTIVE EMBODIMENT

With specific reference now to FIG. 2 of the application drawings, the construction of the instant inventive electrode assembly will be described. Two electrodes are provided, generally designated by reference numerals 26 and 28, each electrode being contemplated to be disposed over and in contact with the underlying skin of a subject as is shown. Each electrode incorporates three separate and distinct portions.

A central conducting surface or disc 30, preferably constructed of stainless steel, is provided for each of the electrodes. An outer conductive surface or ring 32, also preferably constructed of stainless steel, is disposed in surrounding relationship about the periphery of the central conducting surface or disc 30 but is spaced apart therefrom by an intermediate insulating barrier region 34. The barrier region 34 preferably constitutes an epoxy annulus which provides electrical insulation as well as a structural barrier.

The outer conductive ring 32 of each of the electrodes 26 and 28 is electrically connected in common together via conductor 36 and is further connected to the common ground 16 of differential amplifier 18. The central conducting portions or discs 30 of each of the two electrodes are connected to the respective different inputs 20 and 22 of differential amplifier 18, as is shown. The separate parts or regions of each electrode 26 and 28 are accordingly disposed in a concentric relationship with one another.

In use, the outer conductive surface or ring 32 of each electrode 26 and 28 provides shielding as well as the important ground reference. The palm of the user's hand, or the heel of the hand, when placed on each electrode 26 and 28, will straddle the epoxy insulating annulus 34 so that coupling occurs to both conductors 30 and 32. In this fashion, the body and the electronics will assume the same potential and any differences between electrode leads is cancelled in so-called "common-mode," as is typical. Further, since both outer rings of the electrodes are connected together in electrical common, good contact is assured with either hand of the user for establishing a ground reference.

To analyze the electrical operation of the inventive electrode assembly of FIG. 2, attention is directed to FIG. 3 of the application drawings wherein an equivalent circuit of the electrode assembly is shown. Similar reference letters have been provided on both FIG. 2 and FIG. 3 of the application drawings so as to establish a frame of reference. In this respect, it should be appreciated that the voltage potential E.sub.1 is the electrocardiogram bioelectric potential having a typical magnitude of 1 millivolt. The voltage designated by reference E.sub.2 is the potential difference within the internal conductive tissue between the regions of the electrode ring and disc and is less than 1 microvolt. Accordingly, E.sub.2 for the purposes of the following calculations can be considered to constitute zero.

Z.sub.in is the input impedance of the amplifier, Z.sub.T is tissue impedance, and Z.sub.S is skin-to-electrode impedance. Relative magnitudes among these parameters is expressed by

Z.sub.in >> Z.sub.S >> Z.sub.T

For path AC

V.sub.AC .apprxeq. (E1/2) since Z.sub.S >> Z.sub.T

remembering that E.sub.2 = 0

V .sub.DC .apprxeq. V.sub.AC since Z.sub.in >> Z.sub.S + Z.sub.T

therefore

V.sub.DC .apprxeq. (E1/2)

A similar argument prevails for V.sub.BC and V.sub.EC

so

V.sub.EC .apprxeq. (-E1/2)

The differential amplifier of gain .alpha. subtracts the two phases giving an effective output of .vertline. .alpha. E.sub.1 .vertline.. (Common-mode voltages are cancelled.)

Consider also the case if one electrode does not have the ground path connected. (i.e., Absence of a ground ring or one hand is not making contact with the ring.)

For AC open

V.sub.AC .apprxeq. E.sub.1 since Z.sub.IN >> Z.sub.S + Z.sub.T and V.sub.DC .apprxeq. V.sub.AC for the same reason.

Therefore

V.sub.DC .apprxeq. E.sub.1

For the other leg V.sub.EC .apprxeq. V.sub.BC .apprxeq. 0 because Z.sub.in >> Z.sub.S

These two signals, when combined by the differential amplifier of gain .alpha., also yield an output of .vertline. .alpha. E.sub.1 .vertline..

It should thus be appreciated that the objects initially set forth at the outset of the specification have been successfully achieved, use of the electrode of the instant invention finally enabling the transmission of clinical-grade electrocardiograms from even battery-powered devices with economic advantages necessary for mass screening of the public. Moreover, while there has been shown and described a present preferred embodiment of the invention, it is to be distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

What is claimed is:

1. An electrode assembly for generating electrocardiogram bioelectric potential signals suitable for wave-form analysis, said assembly comprising: two separate transmitter electrodes adapted to be placed on a subject's skin in electrical contact therewith, each electrode comprising a flat exposed disc which includes an electrically conductive central disc member and an outer conducting ring disposed in surrounding relationship about the periphery of said central conductive member, said outer conducting ring being spaced from said central conductive member by an intermediate insulating annulus, said central conductive member, said outer conducting ring, and said insulating annulus being coplanar and concentrically disposed; means for electrically coupling said outer conducting rings of each said electrode in common and to a common ground of a differential amplifier; and means for electrically connecting said central conductive member of each said electrode to a different respective input of the differential amplifier, whereby a signal representative of the bioelectric potential is produced at the differential amplifier output.

2. An electrode assembly as defined in claim 1, wherein said central conductive disc and said outer conducting ring are constructed of stainless steel, and wherein said insulating annulus is of epoxy.