Ekg Amplifying Electrode Pickup

Abstract

An amplifying electrode pickup for an electrocardiograph (EKG.) device consisting broadly of an impedance converter integrated amplifier circuit potted in a metal shell. The integrated circuit is biased by a nanoamp electrical current. The EKG. signal is sensed by a small sensor plate or disc positioned in a boot or cover and is electrically insulated from the shell. The circuit has a very high input impedance to minimize the effect of skin contact resistance changes, and a very low output impedance to minimize noise pickup by the signal line between the electrode and the EKG. electronics. Electronic circuitry in the potted metal shell also includes a diode voltage clamp and decoupling RC circuits. A multiconductor cable provides shielding, conducts power to the electrode, and transmits the signal from the electrode through an integral coaxial cable. Because of the low (nanoamp) biasing current, signal traces of numerous separate electrodes can be simultaneously recorded without exceeding the maximum permissible current through the human body as established by the American Heart Association.

Description

The invention described herein was made in the course of, or under, Contract No. W-7405-ENG-48, with the U.S. Atomic Energy Commission.

The maximum permissible current which may be conducted through a live human body for diagnostic purposes is one microamp, this limit being set by the American Heart Association. A typical electrocardiograph (EKG) device has at least five to ten leads connected to the human body. Each lead typically requires relatively large biasing currents (on the order of 0.1 to 1 microamp). Hence, often only one or two signal traces can be recorded simultaneously.

Moreover, a typical EKG lead consists simply of a conductive plate at the end of a shielded single conductor cable. Consequently, these leads pick up considerable extraneous electronic noise (signals) from their environment. Also, changes in skin contact resistance will degrade the quality of the resultant signal traces. In order to minimize noise pickup, the input impedance into the EKG electronics must be kept as low as possible. However, in order to minimize the effects of changes in skin contact resistance, the input impedance into the EKG electronics must be relatively high. Hence, minimizing one effect increases the other.

While various prior art efforts have been directed to the solution of the above impedance problems in EKG electrodes, none are known which incorporate the amplifier electronics into the electrode and which have a very high input impedance to minimize the effect of skin resistance changes, and a very low output impedance to minimize noise pickup. Thus, there has long been a need, for diagnostic purposes, for electrodes which would enable the simultaneous recording of signal traces from every lead; ten leads usually being required for a complete set of EKG traces.

The invention is an amplifying electrode for an electrocardiograph device which solves the above-cited problems of the prior devices. It consists of an integrated impedance converter amplifier circuit having a high input impedance and a low output impedance, potted in a metal shell for electrostatic and electromagnetic shielding. The amplifier circuit is driven by a nanoamp current signal sensed by a small conductive plate which caps, but is insulated from, the conetic metal shell. The high input impedance of the integrated circuit minimizes the effect of skin contact resistance, while the low output impedance minimizes noise pickup in the signal line to the remainder of the EKG electronics. Most importantly, the integrated impedance converter amplifier circuit requires only about a 10-nanoamp biasing current for operation. Hence, signal traces of up to 100 separate electrodes can be simultaneously recorded without exceeding the maximum permissible current through the human body.

Therefore, it is an object of this invention to provide an EKG amplifying electrode pickup.

A further object of the invention is to provide an amplifying electrode having a high input impedance and a low output impedance.

Another object of the invention is to provide an amplifying electrode pickup for an EKG device which utilizes an integrated impedance converter amplifier circuit potted in a metal shell and driven by a nanoamp current signal sensed by a small conductive plate which caps, but is insulated from, the metal shell.

Other objects of the invention will become readily apparent from the following description and accompanying drawings.

FIG. 1 is a greatly enlarged view, partially in cross section, of an embodiment of the inventive amplifying electrode;

FIG. 2 is a simplified schematic of the inventive electrode circuitry;

FIG. 3 is a schematic view showing a plurality of the inventive electrodes operatively connected individually to an electrocardiograph device, tape recorder, etc.; and

FIG. 4 is a schematic view of a pair of the inventive electrodes operatively connected to provide a difference output signal.

DESCRIPTION OF THE INVENTION

Referring now to the drawings, an embodiment of an electrode made in accordance with the invention is illustrated in FIGS. 1 and 2. Generally, the electrode assembly comprises a shell or container 10 constructed of metal or other suitable material, for electrostatic and electromagnetic shielding, with various electronic components, described in detail hereinafter, secured therein by a suitable potting material 11, such as polyurethane, as known in the art. A cover or boot 12 is positioned over the open end of shell 10 and may be made of vinyl or other suitable insulating material, boot 12 having a centrally located aperture 13 within which is positioned a flat conductor or sensor plate 14 constructed, for example, of stainless steel. A multiconductor cable, described in detail hereinbelow, is secured at one end thereof to shell 10 via an internally positioned ferrule 15 and an external metal clamp-type collar 16, and is operatively connected at the opposite end to a plug assembly generally indicated at 17.

The electronic components and circuitry comprising the inventive electrode are illustrated schematically in FIG. 2 with a portion of same being physically illustrated in FIG. 1. The multiconductor cable has an outer insulating cover 18 of Suflex (insulated tubing) or the like, an outer grounded shield 19 of copper braid, for example, a positive voltage DC powerline 20, a negative voltage DC powerline 21, a ground lead 22, a central output signal line or conductor 23, and an inner grounded shield 24 coaxially positioned about output line 23. The outer cover 18 of the cable is secured between ferrule 15 and clamplike collar 16, ferrule 15 being connected to ground as indicated at 25 via inner shield 24; while outer cover 18 and each of the cable components 19-24 are each secured to the plug assembly 17.

The electrode electronics generally indicated at 26 in FIG. 2 is electrically connected to sensor plate 14, positive and negative voltage DC powerlines 20 and 21, and central output signal line 23, as well as the necessary ground connections required for operation. The sensor plate 14 is connected to an integrated impedance converter amplifier circuit 27 via lead 28 and is isolated from the amplifier power supply voltage by a large resistor or other current limiting means 29, which may be a 1 M-ohm resistor. Resistor 29 is not essential to the operation of the electrode but is inserted for safety purposes to isolate the patient from the power supply. The converter amplifier circuit 27 may, for example, be a commercially available LM-302 integrated circuit manufactured by the National Semiconductor Corporation and having a high input impedance of up to 10,000 meg. ohms, for example, and a low output impedance of 1 ohm, for example. Noise in the positive and negative power supply lines 20 and 21, respectively, is decoupled by two simple RC circuits, operatively connected to lines 20 and 21, generally indicated at 28 and 29, and comprising resistors R.sub.1 and R.sub.2 and capacitors C.sub.1 and C.sub.2, the capacitors being connected to ground. For example, R.sub.1 and R.sub.2 may be 300 ohms with C.sub.1 and C.sub.2 being a 2.2 microfarad tantalum capacitor. Two diodes D.sub.1 and D.sub.2 are connected between the central output signal line 23 and the power supply lines 20 and 21 via leads 30 and 31, respectively, and clamp the amplified signal voltage between the voltage of the positive line of +15 volts, for example, and the voltage on the negative line of -15 volts, for example. The amplifier circuit 27 provides a gain of at least 0.999 and requires only a 10-nanoamp biasing current. The voltage clamp provided by diodes D.sub.1 and D.sub.2 isolates the sensitive electronics of an associated EKG device, for example. This feature allows high-voltage shocks to be applied to the patient to stimulate heart action, for example, without disconnecting the EKG apparatus. Structurally, as partly seen in FIG. 1, sensor plate 14 is positively insulated by an insulator 32, while an insulator 33 is positioned between shell 10 and amplifier circuit 27. For example, the insulators 32 and 33 may be made of Dacron fiber tape. The various resistors and capacitors as well as the amplifier circuit and the sensor plate illustrated in FIG. 1 are electrically interconnected via a printed circuit board assembly 34 by leads as partially shown, and as readily known in the art. With the novel arrangement described hereinabove, the amplifying electronics of the remainder of the EKG device, or other apparatus, need not be as sophisticated for purposes of noise-rejection amplification, etc., as presently utilized.

The high input impedance of the integrated converter amplifier circuit 27 minimizes the effect of skin contact resistance, while the low output impedance thereof minimizes noise pickup in the signal line 23 to the remainder of the associated EKG electronics. Most importantly, the amplifier circuit requires only a 10-nanoamp biasing current for operation. Hence, signal traces of up to 100 separate electrodes can be simultaneously recorded without exceeding the maximum permissible current through the human body.

FIG. 3 illustrates the application of a plurality of the inventive electrodes connected to an EKG device or other mechanism whereby the individual signal traces are recorded. While the components of FIG. 1 are utilized in each of the electrodes generally indicated at A, B...N in FIG. 3, the electrodes in FIG. 3 are illustrated very schematically for purposes of simplicity with each respectively comprising a sensor plate 14.sub.A, 14.sub.B and 14.sub.N, a resistor 29.sub.A, 29.sub.B and 29.sub.N, an amplifier 27.sub.A, 27.sub.B and 27.sub.N, and output signal line 23.sub.A, 23.sub.B and 23.sub.N connected to plug assemblies 17.sub.A, 17.sub.B and 17.sub.N. Each of plug assemblies 17.sub.A, 17.sub.B and 17.sub.N comprises a male and female section 35 and 36 with the plug sections 36 connected to separate amplifiers 38.sub.A, 38.sub. B and 38.sub.N, respectively, the outputs thereof, indicated at 39.sub.A, 39.sub.B and 39.sub.N, being directed to a tape recorder or other mechanism as indicated by legend. Generally the plug assembly sections 36 and amplifiers 38.sub.A, 38.sub.B and 38.sub.N are located within a housing 40 or the like, depending on the mechanism associated therewith. The details of the amplifiers 38.sub.A-N do not constitute part of this invention but may, for example, be a commercially produced differential amplifier with a gain of 1,000 such as several cascaded stages made up of UA709C integrated circuits manufactured by Fairchild Camera.

FIG. 4 very schematically illustrates the connection of a pair of the inventive electrode assemblies to provide a difference reading therebetween, again each of the electrode assemblies being constructed in accordance with the FIG. 1 embodiment, though simplified in this illustration. The two electrodes are generally indicated at A and B and consist of sensor plates 14' and 14", large resistors 29' and 29", integrated impedance converter amplifier circuits 27' and 27", central output signal transmission lines or conductors 23' and 23" connected to plug assemblies 17' and 17" each consisting of plug sections 35'-36' and 35"-36". The output signals from plug assemblies 17' and 17", as indicated at 41 and 42, respectively are directed to an amplifier circuit 43 which, for example, may be a commercially produced differential amplifier with a gain of 1,000, such as the UA709C. An output signal indicated at 44 from amplifier 43 is a difference signal of electrode A and B and thus legended "Signal (A-B)," signal 44 being transmitted to a strip chart recorder or other mechanism or point of use.

It has thus been shown that the present invention provides an amplifying electrode which is particularly adapted as a pickup electrode for an electrocardiograph (EKG) device, this being accomplished by an integrated amplifier circuit potted in the electrode shell which is biased by a nanoamp electrical signal, and has a very high input impedance to minimize the effect of skin resistance changes, and a very low output impedance to minimize noise pickup by the output signal line between the electrode and the EKG electronics.

If desired, the electrode can be modified by attaching a stud to the metal shell to aid in attaching the electrode to a patient. Also, a suction hose or bulb along with a channel or pipe through the electrode can be provided to aid in attaching the electrode to the torso of the patient.

While the impedance converter has been set forth as having a gain nearly equal to 1, other values can be used provided all electrodes being used at one time have equal values of gain to within three significant figures, for example, or some form of dynamic calibration. This would be accomplished by a selection process to obtain the desired gain.

Although the power source as described herein is external to the electrode assembly, batteries can be incorporated into the individual electrodes. Also, the multiconductor cable can be attached to the electrode shell by means of a plug-type connector, or the like, such that the cable can be easily repaired or replaced without disturbing the electrode assembly.

Although a particular embodiment of the invention has been illustrated and described, modifications will become apparent to those skilled in the art, and it is intended to cover in the appended claims all such modifications as come within the spirit and scope of the invention.

Claims

What we claim is:

1. An amplifying electrode pickup, particularly adapted for an electrocardiograph device comprising: container means having an open end portion; a cover means for said open end portion of said container means, said cover means being provided with an aperture therethrough; an electrically conductive sensor plate means supported in said aperture of said cover means; electronic means secured in said container means and electrically insulated from said container means and from said cover means; a multiconductor cable means secured in an aperture of said container means and electrically connected to said electronic means; said electronic means including an integrated impedance converter amplifier circuit means electrically connected to said conductive plate means, at least one voltage line constituting certain of the conductors of said multiconductor cable means being connected to said amplifier circuit means, a pair of resistor-capacitor circuit means connected to positive and negative voltage lines of said multiconductor cable means, an output signal line connected to said amplifier circuit means and constituting another conductor of said multiconductor cable means, a diode means connected between said positive voltage line and said output signal line, a second diode means connected between said negative voltage line and said output signal line; said multiconductor cable means additionally including electrical shielding means about said output signal line and about said positive and negative voltage lines.

2. The amplifying electrode pickup defined in claim 1, wherein said container means is constructed of metal and said cover means is constructed of a nonconductive material.

3. The amplifying electrode pickup defined in claim 1, additionally including means for securing said multiconductor cable means in said container means aperture, said securing means comprising a ferrulelike means located internally of said container means and a clamplike collar means located externally of said container means.

4. The amplifying electrode pickup defined in claim 1, additionally including a pluglike assembly means having one end of said multiconductor cable means secured thereto.

5. The amplifying electrode pickup defined in claim 1, wherein said multiconductor cable means comprises an outer insulating cover, an outer grounded shield constituting said electrical shielding means about said positive and negative voltage lines, said positive voltage line, said negative voltage line, a grounded lead, said output signal line, and an inner grounded shield constituting said electrical shielding means about said output signal line, said outer and inner grounded shields being connected for ground purposes to said grounded lead.

6. The amplifying electrode pickup defined in claim 1, wherein said electronic means is secured in said container means by appropriate potting material.

7. The amplifying electrode pickup defined in claim 1, wherein said sensor plate means is positively insulated by an insulator member secured to said container means.

8. The amplifying electrode pickup defined in claim 1, additionally including a resistor means positioned intermediate said sensor plate means and said amplifier circuit means.

9. The amplifying electrode pickup defined in claim 1, wherein said amplifier circuit means has an input impedance of about 10,000 meg. ohms and an output impedance of about 1 ohm and is constructed to be biased by about a 10 nanoamp current.

10. The amplifying electrode pickup defined in claim 1, wherein said container means is constructed of metal, said sensor plate means is constructed of stainless steel, and said cover means is constructed of a insulating material.