High capacitance myocardial electrodes

Details High capacitance myocardial electrodes High capacitance myocardial electrodes

1999-08-27

2001-09-18

Getzow, Scott M. (Department: 3762)

Surgery: light, thermal, and electrical application

Light, thermal, and electrical application

Electrical energy applicator

Reexamination Certificate

active

06292704

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to electrodes, and specifically to electrodes intended for implantation in the heart.
BACKGROUND OF THE INVENTION
Implantable electrodes are known in the art to suffer from problems of corrosion, specifically faradic effects, due to, inter alia, high energy transfer across the electrodes. When implantable electrodes are used as myocardial pacing electrodes, the faradic effects manifest themselves as electrode corrosion and dangerous build-up of metal deposits in the myocardial tissue contacting the electrode. Attempts have been made in the prior art to reduce the corrosion problem by using specific inert conductors as a substrate, and covering the substrate with different types of corrosion-resistant or corrosion-reducing coatings.
U.S. Pat. No. 5,654,030, to Munshi and Bonnerup, which is incorporated herein by reference, describes a method for making implantable electrodes for cardiac stimulation. The method comprises coating a cleaned valve metal surface with one or more metal oxides selected from the oxides of ruthenium, iridium, titanium, and tantalum. (Valve metals are titanium, tantalum, niobium, hafnium, zirconium, and tungsten.) The electrodes are intended for permanent implantation, i.e., implantation over a time period of the order of years, and can be removed only by an invasive procedure.
The inventors state that stimulation requires that an electric field of adequate field strength and current density be imposed on the excitable myocardial tissue in the vicinity of the electrode to initiate rhythmic contractions. The minimum electrical pulse necessary to produce such contractions is referred to as the stimulation threshold, and the inventors define an electrode efficiency by saying that the greater the efficiency of the electrode to generate contractions, the smaller is the amplitude and/or duration of the pulse required to exceed the threshold. The inventors further state that in all types of stimulation electrodes, the electrode itself must be both chemically corrosion resistant and mechanically stable enough to withstand chronic application, that it must possess a high “charge capacity,” and that it must also inject a substantial level of electric charge into the tissue to be stimulated.
U.S. Pat. No. 5,385,579, to Helland, which is incorporated herein by reference, describes a myocardial electrode formed from titanium, platinum, or platinum iridium which is intended for permanent implantation. The electrode may be coated with a particle coating intended to enhance electrical efficiency and to help minimize fibrotic tissue growth response. Examples of particle coatings are platinum black, metal oxides, and metal nitrides.
Temporary/removable implantable electrodes, i.e., electrodes which are intended to be implanted in excitable tissue for a time period of the order of days, are also known in the art. These electrodes are commonly used after open heart surgery, for example, and may be removed from the heart tissue without further surgery by pulling them firmly through the chest wall.
PCT patent applications PCT/IL97/00012, PCT/IL97/00235 and PCT/IL97/00236, which are incorporated herein by reference, describe apparatus and methods of excitable tissue control (referred to herein as ETC) of tissue such as cardiac muscle. The term “excitable tissue control,” in the context of the present patent application and in the claims, refers to application of electrical signals that do not induce activation potentials in cardiac muscle cells. Rather, such signals affect the response of the heart muscle to the action potentials, by modulating cell contractility within selected segments of the cardiac muscle. Such signals are also referred to as “non-excitatory” signals.
Typically, ETC signals are of significantly longer duration and have a significantly higher current than excitatory electrical stimulation pulses, and so transfer substantially more energy to cardiac muscle than excitatory electrical stimulation pulses.
Myocardial stimulation, both non-excitatory and excitatory, is typically performed using either a unipolar or a bipolar mode. In the unipolar mode, one cardiac electrode with its associated lead is used as a stimulation electrode, and the current return path does not comprise a second cardiac electrode. For example, a conducting body of an implanted pacemaker could be used for the current return. In the bipolar mode, two separate electrodes, with respective associated leads, are used to provide a stimulation and a return terminal. The unipolar mode has the advantage of only requiring one electrode, but suffers from the disadvantage that because the current path is not well-defined, the method can easily be affected by external changes in conditions, such as a patient moving. The bipolar mode has the advantage of a well-defined current path, at the expense of utilizing two electrodes.
In addition to being used for stimulation, a pacing electrode may also be used as a sensing electrode, in order to measure electric potentials at the site where the electrode has been positioned.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide improved electrodes for use in excitable tissue control (ETC) of the heart.
It is a further object of some aspects of the present invention to provide methods for reducing harm to tissue that is temporarily implanted with removable electrodes.
It is yet a further object of some aspects of the present invention to provide methods for reducing wear of electrodes used in ETC.
In some preferred embodiments of the present invention, a removable electrode for excitable tissue control (ETC) of the heart is positioned intramyocardially, primarily for the purpose of delivering non-excitatory electrical signals to the myocardium of a patient. The ETC signals are typically used to increase the stroke volume of the patient's heart, or otherwise to promote recovery of the heart following surgery or other trauma, as described in the above-mentioned PCT patent applications and in corresponding U.S. patent applications Ser. Nos. 09/101,723, 09/254,902 and 09/254,900, which are assigned to the assignee of the present patent application and are incorporated herein by reference. A distal needle, which is preferably coupled to the electrode by a suture thread, is passed through the myocardium during surgery, so as to embed the electrode in the myocardium. The electrode is connected to an insulated conducting lead, which is pulled out to a position external to the body of the patient. The lead couples the electrode to a control unit used to generate the ETC signals. After the electrode has been used, typically after a period of days, the electrode is removed through the chest wall by pulling the conducting lead attached to the electrode, as is known in the art.
In preferred embodiments of the present invention, the electrode is made from inert material, such as platinum-iridium, coated by an inert, high-capacitance material, such as iridium oxide. In this application and in the claims, the term “high-capacitance material” refers to any material which when coated on an electrode, substantially increases the measured electrical capacitance of the electrode without substantially increasing the measured electrical resistance of the electrode. The increased capacitance of the electrode consequently increases an RC time constant of the electrode compared to electrodes that are presently used intramyocardially. Preferably, the RC time constant is at least 10 ms, and more preferably greater than 50 ms. The high capacitance, while generally impractical for pacing signals, enables the electrode to be used for delivering ETC signals by capacitive energy transfer without harm to the myocardial tissue, and without degradation of the electrode, over the time period during which the electrode is in contact with the myocardium.
In some preferred embodiments of the present invention, a plurality of electrodes, preferably coated as described above, are each connected

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