US20070093808A1 - Method and apparatus for creating a bi-polar virtual electrode used for the ablation of tissue - Google Patents

Abstract

A method and apparatus for creating a virtual electrode to ablate bodily tissue. The apparatus includes an outer tube, a first electrode, an inner tube and a second electrode. The outer tube is fluidly connected to a source of conductive fluid and defines a proximal end and a distal end. The distal end includes an opening for delivering conductive fluid from the outer tube. The first electrode is disposed at the distal end of the outer tube for applying a current to conductive fluid delivered from the outer tube. The inner tube is coaxially received within the outer tube and is connected to a source of conductive fluid. The inner tube defines a proximal end and a distal end, with the distal end forming an opening for delivering conductive fluid from the inner tube. Finally, the second electrode is disposed at the distal end of the inner tube for applying a current to conductive fluid delivered from the inner tube. With this configuration, upon final assembly, the distal end of the outer tube is axially spaced from the distal end of the inner tube such that the first electrode is spaced from the second electrode. As a result, a bi-polar virtual electrode can be established.

Description

• This application claims the benefit of U.S. Provisional Application No. 60/091,929, filed on Jul. 7, 1998.
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=I²R.).
• 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 due to 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 heart beats or arrythmias.
• 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 patients 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 perhaps the most significant, disadvantage of dry electrode electrosurgical tools is revealed during cardiac ablation procedures. During such a procedure, an electrode that is otherwise insulated but having an exposed, current carrying tip is inserted into the heart chamber and brought into contact with the inner or endocardial side of the heart wall where the ablation is to occur. The current is initiated and passes from the current generator to the needle tip electrode and from there into the tissue so that a lesion is created. Typically, however, the lesion created by a single insertion is insufficient to cure the irregular heartbeat because the lesion created is of an insufficient size to destroy the errant electrical pathway. Thus, multiple needle insertions and multiple current applications are almost always required to ablate the errant cardiac pathway, prolonging the surgery and thus increasing the potential risk to the patient.
• This foregoing problem is also present in TUNA procedures, which similarly require multiple insertions of the needle electrode into the prostate gland. Failing to do so will result in the failure to create a lesion of sufficient size otherwise required for a beneficial results. As with radiofrequency catheter ablation of cardiac tissue, then, the ability to create a lesion of the necessary size to alleviate BPH symptoms is limited and thus requires multiple insertions of the electrode into the prostate.
• A typical lesion created with a dry electrode using RF current and a single insertion will normally not exceed one centimeter in diameter. This small size—often too small to be of much or any therapeutic benefit—stems from the fact that the tissue surrounding the needle electrode tends to desiccate as the temperature of the tissue increases, leading to the creation of a high resistance to the further passage of current from the needle electrode into the tissue, all as previously noted with regard to the formation of coagulum on an electrosurgical scalpel. This high resistance—more properly termed impedance since typically an alternating current is being used—between the needle electrode and the base electrode is commonly measured by the RF current generator. When the measured impedance reaches a pre-determined level, the generator will discontinue current generation. Discontinuance of the ablation procedure under these circumstances is necessary to avoid injury to the patient.
• Thus, a typical procedure with a dry electrode may involve placing the needle electrode at a first desired location; energizing the electrode to ablate the tissue; continue applying current until the generator measures a high impedance and shuts down; moving the needle to a new location closely adjacent to the first location; and applying current again to the tissue through the needle electrode. This cycle of electrode placement, electrode energization, generator shut down, electrode re-emplacement, and electrode re-energization, will be continued until a lesion of the desired size has been created. As noted, this increases the length of the procedure for the patient. Additionally, multiple insertions increases the risk of at least one of the placements being in the wrong location and, consequently, the risk that healthy tissue may be undesirably affected while diseased tissue may be left untreated. The traditional RF ablation procedure of using a dry ablation therefore includes several patient risk factors that both patient and physician would prefer to reduce or eliminate.
• The therapeutic advantages of RF current could be increased if a larger lesion could be created safely with a single positioning of the current-supplying electrode. A single positioning would allow the procedure to be carried out more expeditiously and more efficiently, reducing the time involved in the procedure. Larger lesions can be created in at least two ways. First, simply continuing to apply current to the patient with sufficiently increasing voltage to overcome the impedance rises will create a larger lesion, though almost always with undesirable results to the patient. Second, a larger lesion can be created if the current density, that is, the applied electrical energy, could be spread more efficiently throughout a larger volume of tissue. Spreading the current density over a larger tissue volume would correspondingly cause a larger volume of tissue to heat in the first instance. That is, by spreading the applied power throughout a larger tissue volume, the tissue would heat more uniformly over a larger volume, which would help to reduce the likelihood of generator shutdown due to high impedance conditions. The applied power, then, will cause the larger volume of tissue to be ablated safely, efficiently, and quickly.
• Research conducted under the auspices of the assignee of the present invention has focused on spreading the current density throughout a larger tissue volume through the creation, maintenance, and control of a virtual electrode within or adjacent to the tissue to be ablated. A virtual electrode can be created by the introduction of a conductive fluid, such as isotonic or hypertonic saline, into or onto the tissue to be ablated. The conductive fluid will facilitate the spread of the current density substantially equally throughout the extent of the flow of the conductive fluid, thus creating an electrode—a virtual electrode—substantially equal in extent to the size of the delivered conductive fluid. RF current can then be passed through the virtual electrode into the tissue.
• A virtual electrode can be substantially larger in volume than the needle tip electrode typically used in RF interstitial ablation procedures and thus can create a larger lesion than can a dry, needle tip electrode. That is, the virtual electrode spreads or conducts the RF current density outward from the RF current source—such as a current carrying needle, forceps or other current delivery device—into or onto a larger volume of tissue than is possible with instruments that rely on the use of a dry electrode. Stated otherwise, the creation of the virtual electrode enables the current to flow with reduced resistance or impedance throughout a larger volume of tissue, thus spreading the resistive heating created by the current flow through a larger volume of tissue and thereby creating a larger lesion than could otherwise be created with a dry electrode.
• While the efficacy of RF current ablation techniques using a virtual electrode has been demonstrated in several studies, the currently available instruments useful in such procedures lags behind the research into and development of hoped-for useful treatment modalities for the ablation of soft tissue and malignancies.
• It would be desirable to have an apparatus capable of creating a virtual electrode for the controlled application of tissue ablating RF electric current to a tissue of interest so as to produce a lesion of desired size and configuration.
SUMMARY OF THE INVENTION
• One aspect of the present invention provides a surgical apparatus for creating a virtual electrode to ablate bodily tissue. The surgical apparatus comprises an outer tube, a first electrode, an inner tube and a second electrode. The outer tube is fluidly connected to a source of conductive fluid and defines a proximal end and a distal end. In this regard, the distal end of the outer tube includes an opening for delivering a conductive fluid from the outer tube. The first electrode is disposed at the distal end of the outer tube and is configured to apply a current to conductive fluid delivered from the outer tube. The inner tube is coaxially received within the outer tube. The inner tube is fluidly connected to a source of conductive fluid and defines a proximal end and a distal end. In this regard, the distal end of the inner tube forms an opening for delivering a conductive fluid from the inner tube. The second electrode is disposed at the distal end of the inner tube. The second electrode is configured to apply a current to conductive fluid delivered from the inner tube. Upon final assembly, the distal end of the outer tube is axially spaced from the distal end of the inner tube such that the first electrode is spaced from the second electrode. With this configuration, then, a bi-polar virtual electrode can be established.
• Another aspect of the present invention provides a surgical system for creating a virtual electrode to ablate bodily tissue. The surgical system includes a fluid source, a current source, and a surgical instrument. The fluid source maintains a supply of conductive fluid. The current source is configured to selectively supply an electrical current. The surgical instrument includes an outer tube, a first electrode, an inner tube and a second electrode. The outer tube is fluidly connected to the fluid source and defines a proximal end and a distal end. The distal end of the outer tube includes an opening for delivering the conductive fluid. The first electrode is disposed at the distal end of the outer tube and is electrically connected to the current source. The inner tube is coaxially received within the outer tube and is fluidly connected to the fluid source. The inner tube defines a proximal end and a distal end, with the distal end of the inner tube forming an opening for delivering the conductive fluid. The second electrode is disposed at the distal end of the inner tube and is electrically connected to the current source. Upon final assembly, the distal end of the outer tube is spaced from the distal end of the inner tube such that the conductive fluid is delivered as a first bolus from the outer tube and a second bolus from the inner tube. Current is applied to the first and second boluses by the first and second electrodes, respectively. With this configuration, a bi-polar virtual electrode can be created.
• Another aspect of the present invention relates to a method for ablating bodily tissue at a target site. The method includes delivering a first bolus of a conductive fluid at the target site. A second bolus of conductive fluid is also delivered at the target site, the second bolus being spaced from the first bolus. Finally, a current is substantially simultaneously applied to each of the first bolus and the second bolus to create a virtual electrode, ablating tissue in contact with the first and second boluses. In one preferred embodiment, tissue between the first and second boluses is collapsed prior to applying the current.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a block diagram of a system for creating a virtual electrode in accordance with the present invention;
• FIG. 2 is a perspective view, with a portion cut away, of a surgical apparatus in accordance with the present invention; and
• FIG. 3 is a schematic view of a portion of the surgical apparatus ofFIG. 2 ablating bodily tissue.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• FIG. 1 illustrates in block form asystem10 for RF ablation useful with the present invention. Thesystem10 includes a current source of radiofrequency alternating electric current12, a fluid source ofRF ablating fluid14, including but not limited to saline and other conductive solutions, and asurgical instrument16 for delivering RF current and ablation fluid to a tissue site (not shown) for ablation purposes. In one preferred embodiment, thesurgical instrument16 is connected to thecurrent source12 and thefluid source14. It will be understood that thecurrent source12 and thefluid source14 may be combined into a single operational structure controlled by an appropriate microprocessor for a controlled delivery of ablating fluid and a controlled application of RF current, both based upon measured parameters such as but not limited to, flow rate, tissue temperature at the ablation site and at areas surrounding the ablation site, impedance, the rate of change of the impedance, the detection of arcing between the surgical instrument and the tissue, the time period during which the ablation procedure has been operating, and additional factors as desired.
• While thesurgical instrument16 is shown as being connected to both thecurrent source12 and thefluid source14, the present system is not so limited but could include separate needles or other instruments useful in RF liquid ablation procedures, that is, for example, a single straight or coiled needle having an exposed end and a fluid flow path there through could be used to deliver both fluid and current to the target tissue for ablation purposes. Alternatively, a separate needle could be used to deliver the current and a separate needle or needles could be used to deliver fluid to the target tissue. In addition, the application of the present system is not limited to the use of straight needles or helical needles as surgical instruments but could find use with any type of instrument wherein a conductive solution is delivered to a tissue and an RF current is applied to the tissue through the conductive fluid. Such instruments thus would include straight needles, helical needles, forceps, roller balls, instruments for the treatment of vascular disorders, and any other instrument.
• In one preferred embodiment, thesystem10 further includes a secondfluid source18 for delivery of tissue protecting fluid, via adelivery instrument20, to a tissue whose ablation is not desired.
• Thesurgical instrument16 may assume a wide variety of forms. One preferred embodiment of asurgical apparatus50 is shown inFIG. 2. Theapparatus50 generally includes a bi-polar electrode that is useful in an ablation procedure using an RF ablating fluid such as, but not limited to, isotonic or hypertonic saline. Further, theapparatus50 generally includes a plurality of coaxial thin walled tubes forming a catheter delivery system for RF ablating fluid and RF ablating current. Thus, theapparatus50 preferably includes a first or outer thinwalled tube52 having a proximally attachedhemostasis valve54. Thevalve54 includes aninlet56 for RF ablating fluid flow as indicated byarrow58 and also anaccess port60 for anelectrical line62, which can be electrically connected to the RF current source12 (FIG. 1). A distal end of thefirst tube52 may be provided with one or more thermocouples orother temperature sensors64. The distal end of thefirst tube52 also includes anelectrode66 which is electrically connected to theline62, thereby providing one of the two bi-polar electrodes envisioned by a preferred embodiment of the present invention. It will be understood that thefirst tube52 is electrically insulated or otherwise non-conductive. Thefirst tube52 provides a flow passage for the RF ablating fluid from the fluid source14 (FIG. 1) to the distal end of thefirst tube52 where it exits thefirst tube52 as indicated byarrows68.
• Theapparatus50 further includes a second or intermediate thinwalled tube70 coaxially disposed within thefirst tube52. Thesecond tube70 has a proximally attachedhemostasis valve72. Thevalve72 includes asuction port74 for directing fluid flow as indicated byarrow76. A distal end of thesecond tube70 may be provided with one ormore vacuum apertures78 through which a suction is applied to surrounding tissue (not shown), via a vacuum formed at thesuction port74. Thus, applying suction to thesuction port74 will draw the surrounding tissue into contact with thevacuum apertures78 and the distal end of thesecond tube70. In addition, with this same process, some RF ablating fluid may be removed via the applied suction as indicated byarrows80.
• Theapparatus50 further includes a third or inner thinwalled tube82 coaxially disposed within thesecond tube70. Thethird tube82 has a proximally attachedhemostasis valve84. Thevalve84 includes aninlet84 for RF ablating fluid flow as indicated byarrow86 and also anaccess port88 for anelectrical line90, which can be electrically connected to the current source12 (FIG. 1). A distal end of thethird tube82 also includes anelectrode92, which is electrically connected to theline90, thereby providing the other one of the two bi-polar electrodes envisioned by a preferred embodiment of the present invention. It will be understood that thethird tube82 is electrically insulated or otherwise non-conductive. THEthird tube82 provides a flow passage for the RF ablating fluid from the fluid source14 (FIG. 1) to the distal end of thethird tube82 where it exits thethird tube82 as indicated byarrows94.
• Thesurgical apparatus50 may further include a probe100 that extends through an interior passage of thethird tube82 and that is freely movable therewithin. The probe100 may include athermocouple102 disposed at a most distal end thereof and may be connected via anelectrical line104 to the RF current source12 (FIG. 1) to provide a temperature measurement at a predetermined distance from theelectrodes66 and92.
• In operation, following delivery to a particular target site, RF ablating fluid will be provided to theports56 and84 from the fluid source14 (FIG. 1). The fluid will exit the distal ends of thefirst tube52 and thethird tube82, respectively, and begin to form boluses of fluid along or within the tissue at the target site. Application of a suction at thevacuum aperture78, via thesuction port74, will cause the tissue surrounding the distal end of thesecond tube70 to be collapsed or pulled into a substantially fluid tight relationship with thesecond tube70, thereby preventing migration of the fluid, in particular via a capillary effect, along thesecond tube70 between the distal ends of the first andthird tubes52,82. RF ablating power can be applied and an ablation procedure can be carried out. By the application of suction to the tissue and the prevention of the flow of fluid along thesecond tube70, shorting of the current between theelectrodes66 and92 can be avoided.
• FIG. 3 represents schematically the fluid flow and current flow achieved during use of the present invention. Thus,FIG. 3 shows atissue110, such as liver, into which a portion of thesurgical apparatus50 has been inserted. Infusion of the RF ablating fluid will initially create two separate boluses of fluid,112 and114. If not for the suction applied to the tissue surroundingvacuum apertures78, fluid would tend to travel along the path created during insertion and placement of theapparatus50 in thetissue110. Applying suction intermediate the release of the fluid, however, prevents such fluid travel and substantially prevents any shortage between theelectrodes66,92 directly therebetween. The applied current can thus spread readily through theboluses112,114 and then travel therebetween as indicated by thelines116. In this way, then, ablation can be accomplished between thebi-polar electrodes66 and92. The lesion created with the use of such a bi-polar structure will essentially be the size of theboluses112,114 at a minimum, though it will be understood that cell-to-cell thermal conduction will occur that will make the lesion in reality larger than theboluses112,114, assuming that power is left on for the appropriate length of time.
• As is well known, when an electro-surgical tool is used, or for that matter, any electrical device, a complete circuit for current flow must be provided. Thus, when a monopolar surgical instrument such as a bovie knife is used, a ground pad connected to the RF current generator is placed under the patient to provide a complete electrical circuit from the generator, to the knife, through the patient to the ground pad, and back to the generator. With the present invention, no ground pad is needed. The current flows from the current source12 (FIG. 1) to one of the twoelectrodes66 or92 on theapparatus50, through the patient to theother electrode66 or92, and then back to thecurrent source12. Such bi-polar structure enables the physician to localize the effect of the current passing through the patient, thereby avoiding certain risks associated with the use of a monopolar device, including but not limited to the risk of burns where the ground pad contacts the patient. Additionally, by controlling the relative spacing between the twoelectrodes66,92 of a bi-polar instrument, the size of the lesion can be controlled. Thus, where theelectrodes66,92 are close to each other, there will be little tissue between them through which the current will travel and the lesion size will be reduced relative to an instrument where the electrodes are spaced relatively farther apart. In addition, the shape of the lesion is somewhat controllable by controlling both the fluid flow and the spacing of theelectrodes66,92. It will be understood that regardless of the device used that the lesion size and shape is a product of many factors, including the tissue composition, the conductivity of the fluid, the amount of applied current, the amount of cell-to-cell thermal conductivity, the time period the current is applied, tissue temperature, and fluid temperature when applied, among others.
• Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims (20)

1. A surgical apparatus for creating a virtual electrode to ablate bodily tissue, the apparatus comprising:

an outer tube fluidly connected to a source of conductive fluid, the other tube defining a proximal end and a distal end, the distal end of the outer tube including an opening for delivering a conductive fluid from the outer tube;

a first electrode disposed at the distal end of the outer tube for applying a current to conductive fluid delivered from the outer tube;

an inner tube coaxially received within the outer tube, the inner tube being fluidly connected to a source of conductive fluid and defining a proximal end and a distal end, the distal end of the inner tube forming an opening for delivering a conductive fluid from the inner tube; and

a second electrode disposed at the distal end of the inner tube for applying a current to conductive fluid delivered from the inner tube;

wherein upon final assembly, the distal end of the outer tube is axially spaced from the distal end of the inner tube such that the first electrode is spaced from the second electrode.

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