Surgery – Instruments – Electrical application
Reexamination Certificate
2001-07-11
2002-12-24
Gibson, Roy D. (Department: 3739)
Surgery
Instruments
Electrical application
C606S049000, C307S101000
Reexamination Certificate
active
06497705
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to an apparatus for creating a virtual electrode. More particularly, the present invention relates to an apparatus for the creation of a virtual electrode that is useful for the ablation of soft tissue and neoplasms.
BACKGROUND OF THE PRESENT INVENTION
The utilization of an electric current to produce an ameliorative effect on a bodily tissue has a long history, reportedly extending back to the ancient Greeks. The effects on bodily tissue from an applied electric current, and thus the dividing line between harmful and curative effects, will vary depending upon the voltage levels, current levels, the length of time the current is applied, and the tissue involved. One such effect resulting from the passage of an electric current through tissue is heat generation.
Body tissue, like all non-superconducting materials, conducts current with some degree of resistance. This resistance creates localized heating of the tissue through which the current is being conducted. The amount of heat generated will vary with the power P deposited in the tissue, which is a function of the product of the square of the current I and the resistance R of the tissue to the passage of the current through it (P=I
2
R.).
As current is applied to tissue, then, heat is generated due to the inherent resistance of the tissue. Deleterious effects in the cells making up the tissue begin to occur at about 42° Celsius. As the temperature of the tissue increases because of the heat generated by the tissue's resistance, the tissue will undergo profound changes and eventually, as the temperature becomes high enough, that is, generally greater than 45° C., the cells will die. The zone of cell death is known as a lesion and the procedure followed to create the lesion is commonly called an ablation. As the temperature increases beyond cell death temperature, complete disintegration of the cell walls and cells caused by boiling off of the tissue's water can occur. Cell death temperatures can vary somewhat with the type of tissue to which the power is being applied, but generally will begin to occur within the range of 45° to 60° C., though actual cell death of certain tissue cells may occur at a higher temperature.
In recent times, electric current has found advantageous use in surgery, with the development of a variety of surgical instruments for cutting tissue or for coagulating blood. Still more recently, the use of alternating electric current to ablate, that is, kill, various tissues has been explored. Typically, current having a frequency from about 3 kilohertz to about 300 gigahertz, which is generally known as radiofrequency or radiofrequency (RF) current, is used for this procedure. Destruction, that is, killing, of tissue using an RF current is commonly known as radiofrequency ablation. Often radiofrequency ablation is performed as a minimally invasive procedure and is thus known as radiofrequency catheter ablation because the procedure is performed through and with the use of a catheter. By way of example, radiofrequency catheter ablation has been used to ablate cardiac tissue responsible for irregular heartbeats or arrhythmias.
The prior art applications of current to tissue have typically involved applying the current using a “dry” electrode. That is, a metal electrode is applied to the tissue desired to be affected and a generated electric current is passed through the electrode to the tissue. A commonly known example of an instrument having such an operating characteristic is an electrosurgical instrument known as a “bovie” knife. This instrument includes a cutting/coagulating blade electrically attached to a current generator. The blade is applied to the tissue of a patient and the current passes through the blade into the tissue and through the patient's body to a metal base electrode or ground plate usually placed underneath and in electrical contact with the patient. The base electrode is in turn electrically connected to the current generator so as to provide a complete circuit.
As the current from the bovie knife passes from the blade into the tissue, the resistance provided by the tissue creates heat. In the cutting mode, a sufficient application of power through the bovie knife to the tissue causes the fluid within the cell to turn to steam, creating a sufficient overpressure so as to burst the cell walls. The cells then dry up, desiccate, and carbonize, resulting in localized shrinking and an opening in the tissue. Alternatively, the bovie knife can be applied to bleeding vessels to heat and coagulate the blood flowing therefrom and thus stop the bleeding.
As previously noted, another use for electrical instruments in the treatment of the body is in the ablation of tissue. To expand further on the brief description given earlier of the ablation of cardiac tissue, it has long been known that a certain kind of heart tissue known as sino-atrial and atrio-ventricular nodes spontaneously generate an electrical signal that is propagated throughout the heart along conductive pathways to cause it to beat. Occasionally, certain heart tissue will “misfire”, causing the heart to beat irregularly. If the errant electrical pathways can be determined, the tissue pathways can be ablated and the irregular heartbeat remedied. In such a procedure, an electrode is placed via a catheter into contact with the tissue and then current is applied to the tissue via the electrode from a generator of RF current. The applied current will cause the tissue in contact with the electrode to heat. Power will continue to be applied until the tissue reaches a temperature where the heart tissue dies, thereby destroying the errant electrical pathway and the cause of the irregular heartbeat.
Another procedure using RF ablation is transurethral needle ablation, or TUNA, which is used to create a lesion in the prostate gland for the treatment of benign prostatic hypertrophy (BPH) or the enlargement of the prostate gland. In a TUNA procedure, a needle having an exposed conductive tip is inserted into the prostate gland and current is applied to the prostate gland via the needle. As noted previously, the tissue of the prostate gland heats locally surrounding the needle tip as the current passes from the needle to the base electrode. A lesion is created as the tissue heats and the destroyed cells may be reabsorbed by the body, infiltrated with scar tissue, or just become non-functional.
While there are advantages and uses for such “dry” electrode instruments, there are also several notable disadvantages. One of these disadvantages is that during a procedure, coagulum—dried blood cells and tissue cells—will form on the electrode engaging the tissue. Coagulum acts as an insulator and effectively functions to prevent current transfer from the blade to the tissue. This coagulum “insulation” can be overcome with more voltage so as to keep the current flowing, but only at the risk of arcing and injuring the patient. Thus, during surgery when the tissue is cut with an electrosurgical scalpel, a build-up of coagulated blood and desiccated tissue will occur on the blade, requiring the blade to be cleaned before further use. Typically, cleaning an electrode/scalpel used in this manner will involve simply scraping the dried tissue from the electrode/scalpel by rubbing the scalpel across an abrasive pad to remove the coagulum. This is a tedious procedure for the surgeon and the operating staff since it requires the “real” work of the surgery to be discontinued while the cleaning operation occurs. This procedure can be avoided with the use of specially coated blades that resist the build up of coagulum. Such specialty blades are costly, however.
A second disadvantage of the dry electrode approach is that the electrical heating of the tissue creates smoke that is now known to include cancer-causing agents. Thus, preferred uses of such equipment will include appropriate ventilation systems, which can themselves become quite elaborate and quite expensive.
A further, and
Czaja, Esq. Timothy A.
Gibson Roy D.
Latham, Esq. Daniel W.
Medtronic Inc.
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