Abstract:
In one embodiment, the present invention provides a bipolar ablation device with multiple needle electrodes that penetrate a desired target tissue. These electrodes may be arranged in a variety of therapeutically effective arrangements, such as a comb-like shape, a multi-needle wheel, or an expanding bow design. By contacting and preferably penetrating the cardiac tissue with bipolar electrodes, a user can more precisely create ablation-induced scarring and thus electrical block at desired target locations without causing unwanted damage and related complications.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. Provisional Application Ser. No. 60/514,428 filed Oct. 24, 2003 entitled Methods And Devices For Creating Electrical Block At Specific Sites In Cardiac Tissue With Targeted Tissue Ablation and is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     In the field of electrophysiology, there are numerous methods that seek to create blocks of specific electrical pathways for the treatment of atrial arrhythmias. These methods primarily rely on applying energy to the tissue to ablate the tissue and thereby block a pathway for electrical conduction. These techniques are used most often in the left or right atriums and can be used to create electrical block either at discrete sites or along linear paths. Examples of such techniques can be seen in prior art U.S. Pat. Nos. 6,237,605; 6,314,962; 6,527,769, 6,502,576 all of which are incorporated herein by reference.  
         [0003]     In a usage that is increasing in popularity, the ablation devices used in these methods are introduced percutaneously and advanced into the right atrium via the vena cava and possibly into the left atrium by a trans-septal sheath. These devices are then maneuvered inside the appropriate chamber of the heart by torquing the shaft of the catheter and deflecting the tip to bring the ablation tip in contact with the desired site. Since the atria are relatively large chambers and are moving with the beating of the heart, it is difficult, however, to position these devices accurately.  
         [0004]     The most common method of creating an electrical conduction block is by acutely ablating the tissue at the site where the energy delivery catheter is positioned using RF energy, microwave energy, ultrasonic energy or freezing (i.e., cryoablation). These methods typically involve applying a specified energy to the device resting against the endocardial surface of the tissue for a specified time and then reevaluating the region of the ablation to see if the desired electrical conduction block has been created. The result is the formation of an ablation region that extends from the endocardial surface of the cardiac tissue to a tissue depth that is dependant on the amount of energy that is applied.  
         [0005]     These methods have proven to be not only very time consuming but also have become characterized with uncertainty as to whether the appropriate amount of ablation has been performed or if the desired target location is being ablated. For example, the resulting depth of the ablation may extend beyond the targeted tissue and injure adjacent tissue such as the esophagus, the trachea or the bronchial tubes. This can happen because of a varying wall thickness of the target cardiac tissue and because other variables (e.g., variation in the electrical impedance of the tissue) can alter the “burn” depth even when time and energy are controlled.  
         [0006]     Other possible adverse outcomes of not successfully ablating the desired target site or ablating too much or too little include: not successfully blocking the electrical pathway, disrupting the wrong pathway or creating stenosis in a vessel such as a pulmonary vein.  
         [0007]     Of these adverse outcomes, creating a stenosis is a particularly serious complication. As a result, many doctors try to limit their treatment to the wall of the atrium around the ostium of the pulmonary veins to minimize the risk of creating a stenosis in the pulmonary veins. Stenoses as a result of this type of energy ablation in the pulmonary veins has been reported in a small percentage of cases, however it is uncertain if these stenoses are caused more by excessive ablation, missing the appropriate target site or are an inherent risk for this type of treatment in the pulmonary veins.  
         [0008]     Turning back to how the known ablation techniques are implemented, current ablation systems typically are maneuvered by twisting or pushing the device shaft or deflecting the distal end to bring this distal end in contact with the desired site for ablation while this distal end is free in the space of the atrium. The motion of the heart makes it very difficult to accurately control the position of the device in this way. Other systems have been proposed that are intending to seat into the pulmonary veins and then create a circumferential ablation around the pulmonary vein which has been engaged. Examples of such systems are disclosed in U.S. Pat. Nos. 6,117,101 and 6,605,085, each of which are incorporated herein by reference.  
         [0009]     These concepts address some of the problems with locating the ablation elements relative to the pulmonary vein but are potentially limited in that they presume a round pulmonary vein and are geared towards making a circumferential ablation around the veins. These systems are not optimally suited for creating focal ablations of discrete points or for making linear lesions other than the circumferential lesions around the device location. They also rely upon these same energy delivery mechanisms as the means to create the electrical block.  
         [0010]     There are also systems that use a surgical approach to apply the ablation devices to the endocardial surface of the heart. While these approaches can address some of the limitations discussed above, they require a much more invasive access. They also often are characterized by the same drawbacks discussed above, e.g., determining the correct location, determining the correct depth of burn, etc.  
         [0011]     For at least these reasons, there is a need for a system that provides means to create the desired electrical block in the cardiac tissue by ablating the necessary tissue while minimizing the risk of ablating too much or too little of the cardiac tissue. There is also a need for a system that minimizes the risk of ablating structures beyond the targeted cardiac wall.  
       OBJECTS AND SUMMARY OF THE INVENTION  
       [0012]     In view of the foregoing, it is an object of the present invention to overcome the limitations of the prior art.  
         [0013]     It is a further object of the present invention to provide a more precise ablation device and technique to create electrical block.  
         [0014]     It is yet a further object of the present invention to provide a device which creates focused ablation damage to tissue.  
         [0015]     It is yet a further object of the present invention to provide a device which allows a user to easily create linear ablation scars.  
         [0016]     It is a further object of the present invention to provide a device that more reliably creates electrical block in cardiac tissue.  
         [0017]     In one embodiment, the present invention attempts to achieve these objects by providing a bipolar ablation device with multiple needle electrodes that penetrate a desired target tissue. These electrodes may be arranged in a variety of therapeutically effective arrangements, such as a comb-like shape, a multi-needle wheel, or an expanding bow design. By contacting and preferably penetrating the cardiac tissue with bipolar electrodes, a user can more precisely create ablation-induced scarring and thus electrical block at desired target locations without causing unwanted damage and related complications. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1A  illustrates a side view of a prior art mono polar electrode;  
         [0019]      FIG. 1B  illustrates a top view of the prior art mono polar electrode of  FIG. 1A ;  
         [0020]      FIG. 2A  illustrates a side view of penetrating bipolar electrodes according to the present invention;  
         [0021]      FIG. 2B  illustrates a top view of the penetrating bipolar electrodes of  FIG. 2A ;  
         [0022]      FIG. 3A  illustrates a side view of an ablation device according to the present invention;  
         [0023]      FIG. 3B  illustrates a top view of the ablation device of  FIG. 3A ;  
         [0024]      FIG. 4  illustrates a side view of an ablation device according to the present invention;  
         [0025]      FIG. 5  illustrates a side view of an ablation device according to the present invention;  
         [0026]      FIG. 6  illustrates a side view of an ablation device according to the present invention;  
         [0027]      FIG. 7A  illustrates a side view of an ablation device according to the present invention;  
         [0028]      FIG. 7B  illustrates a front view of the ablation device of  FIG. 7A ;  
         [0029]      FIG. 8  illustrates a front view of an ablation device according to the present invention;  
         [0030]      FIG. 9  illustrates a side view of a percutaneous ablation device according to the present invention; and  
         [0031]      FIG. 10  illustrates a side view of a percutaneous ablation device according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]     As previously described, there are a number of mechanisms used to create electrical block acutely. Some of these mechanisms involve the use of energy delivery such as RF or microwave radiation to ablate a target tissue. In these instances, energy is typically applied to either the endocardial or epicardial surface using mono-polar devices.  
         [0033]     More specifically, the ablation energy is delivered through a small surface area ablation tip and transmitted through the tissue to a large area ground plate in contact with the body at a far removed site. The result is a concentration of the ablative current immediately around the ablation tip that rapidly decreases as the current travels further into the tissue, away from the ablation tip.  
         [0034]      FIGS. 1A and 1B  illustrate this concept, depicting a mono-polar ablation device  100  with an ablation tip  102  contacting the tissue wall  104  of tissue  106 . In most practices, it is believed that it is necessary for the lines of burns to be fully transmural for effective isolation. With the hemispherical shape of the discrete burns, this requires either overlap of the burns or burn depths beyond the targeted wall thickness. When activated, the ablation tip  102  creates an area of ablated tissue  108 , spreading out in a generally radial or spherical pattern. The ablative current immediately near the ablation tip  102  begins relatively strong while spreading out in all directions, yet decreases as the distance from the ablation tip  102  increases. As the strength of the ablative current decreases, so does the amount of ablative damage created by the current.  
         [0035]     Also, the ablated tissue is created in a generally circular or spherical pattern within the tissue wall  104  and tissue  106 . As a result, a mono-polar ablation device is not capable at creating a region of ablated tissue with great precision. Thus, some non-target tissue areas may be accidentally ablated, while other target tissue areas may not be ablated enough to create a desired electrical block.  
         [0036]     To overcome this apparent deficiency in mono-polar ablation techniques, reference is made to  FIGS. 2A and 2B  which illustrate a bipolar ablation device  112  having a positive piercing electrode  110 A and a negative piercing electrode  110 B. When activated, ablative current passes from the negative piercing electrode  110 B to the positive piercing electrode  110 A, following a relatively direct route. Such a short, direct ablative current route provides a more consistent concentration of current within the tissue wall  104  and tissue  106 , and therefore a more consistent and narrow area of ablated tissue  108 . Additionally, the bipolar ablation device  112  requires less energy to achieve a desired ablated tissue  108  result when compared to a typical mono-polar ablation device  100 , since a smaller area is ultimately ablated.  
         [0037]     In this respect, bipolar ablation devices  112  allow a user to more precisely create regions of ablated tissue  108 , and therefore electrical block. This basic technique can improve the uniformity of the ablation and decrease the amount of tissue ablated to create the same electrical isolation. Such an approach can be used in a surgical approach on either the endocardial or epicardial surface either alone or as an adjunctive procedure to other surgical procedures such as mitral valve repair or coronary bypass surgery. The technique can also be used in a percutaneous approach.  
         [0038]     The bipolar ablation technique in accordance with the present invention can also be used in association with other approaches and implants to treat conditions such as atrial arrhythmias. Examples of other approaches in which bipolar ablation in accordance with the present invention may be adjunctive are disclosed in co-pending and commonly owned patent applications including, U.S. patent application Ser. Nos. 10/192,402 entitled Anti-Arrhythmia Devices and Methods of Use filed Jul. 8, 2002; 10/792,111 entitled Electrical Block Positioning Device And Method Of Use Therefor filed Mar. 2, 2004; 10/835,697 entitled Methods And Devices For Creating Electrical Block At Specific Targeted Sites In Cardiac Tissue filed Apr. 30, 2004, each of which is incorporated herein by reference.  
         [0039]     Electrode Comb Ablation Devices  
         [0040]     The previously described bipolar ablation technique is further refined, according to the present invention, by utilizing different therapeutic electrode and device configurations. For example, one preferred embodiment illustrates a comb ablation device  130 , including positive comb  132 A and negative comb  132 B having needle electrodes  134 A and  134 B respectively. The needle electrodes  134 A and  134 B are generally parallel and offset from each other and electrically wired to have a negative polarity, in the case of needle electrodes  134 B, and a positive polarity in the case of the needle electrodes  134 A. The actual current path between the needle electrodes  134 A and  134 B could likely form a zig-zag pattern, but for most cases, the spacing between the combs and the barbs would be such that the entire area between the combs would be ablated.  
         [0041]     The comb ablation device  130  provides a narrow, concentrated area of current within the target area. The needle electrodes  134 A and  134 B penetrate the tissue wall  104  and optionally deeper into the tissue  106 , creating a deeper area of ablated tissue  108  and therefore more effectively block electrical signals.  
         [0042]     In one preferred embodiment, the needle electrodes  134 A and  134 B are spaced about 3 mm apart, having lengths from about 1 mm to 4 mm.  
         [0043]     The needle electrodes  134 A and  134 B have a generally pointed needle shape, suitable for puncturing tissue  104  and  106 . However, other electrode shapes are also preferred, according to the present invention.  
         [0044]     For example,  FIG. 4  illustrates triangular electrodes  142  on comb ablation device  142 . The wider triangular shape of the triangular electrodes  142  may allow for a more shallow tissue penetration depth, resulting in shallow ablation and tissue scarring.  
         [0045]      FIG. 5  illustrates another preferred embodiment of a comb ablation device  144  with narrow electrodes  146 . The narrow electrodes  146  have a slender body terminating in an angled tip  146 A, allowing for deep penetration and ablation of the tissue wall  104  and tissue  106 .  
         [0046]     In addition to varying the shape of the electrode, the conductivity may also be varied to create different ablation characteristics in the target area. For example,  FIG. 6  illustrates a comb ablation device  148  with a nonconductive coating  149  disposed over most of the device  148  except a portion of the triangle electrode tips  150 . This arrangement allows the triangle electrode tips  150  to penetrate with tissue wall  104  and tissue  106  while primarily directing current to an area below the tissue surface. This can be beneficial in minimizing the severity of the burning on the endocardial surface to minimize the proliferative healing response in the blood flow lumen which could result in stenosis of blood conduits such as the pulmonary veins.  
         [0047]     Returning to  FIGS. 3A and 3B , the comb ablation device  130  is operated by piercing the tissue wall  104  at a target site of a patient, such as an ostium of a pulmonary vein, and applying current to the device  130  which travels from the positive needle electrodes  134 A to the offset negative needle electrodes  134 B. The current ablates the tissue wall  104  and possibly the tissue  106 , later creating scarring and blocking electrical signals.  
         [0048]     Electrode Wheel Ablation Devices  
         [0049]     Turning to  FIGS. 7A and 7B , a preferred embodiment of an electrode wheel ablation device  160  is illustrated according to the present invention. The electrode wheel ablation device  160  has an overall shape similar to a “pizza cutter”, including a body  161  with a positive electrode wheel  162  and a negative electrode wheel  166  rotatably connected by axle  170 . The positive electrode wheel  162  includes needle electrodes  164  which are wired to have a positive polarity, while the negative electrode wheel  166  includes needle electrodes  168  which are wired to have a negative polarity.  
         [0050]     As with preferred embodiments described elsewhere in this application, the present preferred embodiment penetrates the tissue wall  104  and possibly the tissue  106  with the electrodes  164  and  168 , creating an area of ablated tissue  108  between the two wheels  162  and  168  as current passes between each. The rotatable nature of the wheels  162  and  168  allows a user to create a line of ablation by merely rolling the device  160  while in an active electrical state. In this respect, a user may create a precise and relatively narrow area of electrical block, allowing for the creation of straight lines, tight turns, or any other difficult line shape.  
         [0051]     In a preferred embodiment, the needle electrodes  134 A and  134 B are spaced about 3 mm apart, having lengths between about 1 mm and 4 mm.  
         [0052]      FIG. 8  illustrates a “pizza cutter” design similar to the previously described embodiment. However, the wheels  162  and  166  of the angled electrode ablation device  180  are rotatably mounted to angled, independently mounted axles  182 A and  182 B. This arrangement configures the wheels  162  and  166  to form a V-shape.  
         [0053]     The electrodes  164  and  168  are angled to move closest to each other near the bottom, increasing in distance towards the upper end. This arrangement results in lower resistance between the tips of electrodes  164  and  168  compared with other areas within the tissue  104 ,  106 . Since the wheels  162 ,  166  and electrodes  164 ,  168  have a greater contact area with the tissue wall  104  near the surface, the resistance is decreased, allowing more current and stronger ablation in that area. The V-shaped arrangement may be configured to substantially offset this effect by decreasing the distance between the tips of the electrodes  164 , and therefore decrease the resistance.  
         [0054]     Note that it may also be desired to have the current flow disproportionately higher on either the epicardial or endocardial side of the tissue wall  104 . These variables allow the current to be focused where desired through the thickness of the tissue wall  104 .  
         [0055]     Bow Electrode Ablation Device  
         [0056]     In another preferred embodiment according to the present invention,  FIG. 9  illustrates a bow electrode ablation device  190 , configured for a percutaneous treatment approach. In this example, a catheter body  194  includes an expandable region of bow electrodes  198  which expand to ablate the ostium  204  or possibly the inside surface of the pulmonary vein  206 .  
         [0057]     Each bow electrode  198  is fixed to a proximal hub  192 A and distal hub  192 B, having a barb  200  directed perpendicularly to pierce and therefore engage the ostium  204 . The bow electrodes  198  expand to a ball or bulb shape by the movement of one of the hubs  192 A or  192 B. For example, the proximal hub  192 A may slide in a distal direction, pushing the bow electrodes  198  outward, or the distal hub  192 B may slide in a proximal direction, pushing the bow electrodes  198  outward. In either case, an inner control wire allows a user to control this expansion or retraction. Thus, the bow electrodes  198  can be advanced in their collapsed state into the ostium  204  of the pulmonary vein  206 . Alternatively, the bow electrodes  198  can be formed such that their relaxed state is in the expanded position and they can be constrained to a smaller diameter for introduction and removal by means of a retractable sleeve.  
         [0058]     The bow electrodes  198  are wired at their proximal ends such that alternating bows  198  have alternating polarity. The bulk of the length of the bow is preferably coated with electrical insulation allowing only the bare metal of barb  200  to be exposed. The bow electrode ablation device  190  is delivered to the ostium  204  of a pulmonary vein  206  by an aortic sheath  196  positioned through an aortic wall. The distal guide portion  202  directs the catheter body  194  into the pulmonary vein  206 , allowing the bow electrodes  198  to be positioned and expanded at a desired target area.  
         [0059]     Once in a desired position, energy can then be applied, with every other electrode  198  acting as a positive pole, while the others act as a negative pole. The current then passes from the electrodes  198  with positive polarity to the electrodes  198  with negative polarity, on either side. As with the previously described examples, the current ablates the tissue of target area, in this case creating a circumferential burn along the ostium  204 .  
         [0060]     It is anticipated that other geometries of the barbs  200  could be used to facilitate piercing of the ostium  204  of the pulmonary vein  206 . For example, the electrode shapes seen in  FIGS. 4-6  may be adapted for use as the barbs  200 .  
         [0061]     It is also anticipated that this same technique could be used with a smaller number of barbs  200  to create a burn line along a segment of tissue without completely surrounding the pulmonary vein  206 . Further, it is also anticipated that an ablation line could be created in a line along the atrial wall not adjacent to the pulmonary veins  206 , by a collapsible configuration of the embodiment illustrated in  FIGS. 3A-6 .  
         [0062]     This basic concept of using bipolar energy ablation with piercing electrodes could potentially be used with any of the wavelengths of energy described earlier since the basic advantage is driven by the electrode geometry and location in the tissue and is essentially independent of the energy wavelength. Though the examples described and shown are often illustrated for only one type of electrode, it is understood that these are only examples and the same techniques can be applied for the other types of electrodes.  
         [0063]     The concept of using bipolar energy ablation has numerous potential applications. In addition to treating atrial arrhythmias as discussed above, another preferred embodiment is in the performance of deep tissue ablation in, for example, the treatment of ventricular arrhythmias where the ventricular wall can be relatively thick. The discrete and precise nature of ablation that is enabled by bipolar ablation in accordance with the present invention provides advantages over mono-polar ablation where the degree of ablation will vary according to proximity of the tissue to the mono-polar ablation electrode.  
         [0064]     Ablation Tissue Targeting  
         [0065]     In yet another preferred embodiment of the present invention,  FIG. 10  illustrates a tissue targeting ablation device  220  that allows a user to “target” specific types of tissue and therefore more precisely ablate an intended target area. Specifically, the tissue targeting ablation device  220  is composed of a main sheath  222  that contains two catheters  224   a  and  224   b.  Each catheter  224   a  and  224   b  includes an electrode needle  226   a  and  226   b,  respectively, having a sharpened electrode tip  228   a,    228   b  and a ring electrode  229   a,    229   b,  separated by a nonconductive polymer ring  230   a,    230   b.  [In one embodiment, one electrode tip  228   a  has a positive polarity and the other electrode tip  228   b  has a negative polarity so that bi-polar ablation can be performed between the two needle tips. However, mono-polar ablation is also contemplated with this design.  
         [0066]     In one embodiment, the needle electrodes  226   a,    226   b  are preferably 0.020″ (25 gage) in diameter, while the polymer rings  230   a,    230   b  are about 0.020″ in length. The electrode tip  228   a,    228   b  and the ring electrode  229   a,    229   b  on each needle  226   a  and  226   b  are independent, electrically isolated electrodes that are coupled to wires that exit at a proximal end of the catheter  224   a,    224   b  and ultimately connect to a device which produces ablative current and also measures impedance.  
         [0067]     Generally speaking, the impedance of the surrounding tissue is measured between the electrodes on each catheter  224   a  and  224   b.  For example, the impedance could be measured between the electrode tip  228   a  and ring electrode  229   a.  Since fat, muscle, and blood all have different impedance values, these measurements can be used to determine if the electrode needles  226   a  and  226   b  are positioned within a desired target tissue.  
         [0068]     This tissue-targeted ablation is particularly useful when ablating a specific body structure while minimizing the damage to adjacent structures. In one specific example, the tissue-targeted ablation could be used to ablate pulmonary vein ganglia which have been found to be located within fat pads  218  on the epicardial surface at the ostia  204  of the pulmonary vein  206 . It is believed that the pulmonary vein ganglia may play a significant role in triggering or sustaining atrial fibrillation and therefore that ablating the ganglia may reduce or eliminate the atrial fibrillation.  
         [0069]     In this regard, fat has a much higher impedance and resistance compared with muscular tissue or blood and thus electrical energy applied to tissue generally often avoids the fat tissue. As a result, in an ablation method where the ablation energy is applied either only partially in the fat pad  218  or is applied outside the fat pad, the tissue in the fat pad  218  is mostly avoided by the current. The current thus causes more damage to the muscle tissue  232  than to the fat pad  218 .  
         [0070]     The electrode needles  226   a  and  226   b  as used in connection with this embodiment of the invention can be placed into a specific tissue location, e.g, into a fat pad  218 , by measuring the impedance between electrodes  228   a,    228   b  and  229   a,    229   b,  thereby indicating when the electrodes are positioned in fat tissue. In this manner, positioning the electrodes into contact with muscle tissue can be avoided. As a result, when electrical energy is applied, the pulmonary vein ganglia in the fat pad  218  is ablated and significant ablation damage to muscle tissue  232  (as well as other non-target tissues) is substantially avoided.  
         [0071]     In operation in another embodiment, the targeting ablation device  220  is introduced trans-septally into a left atrium of a heart by the main sheath  222 . The catheters  224   a  and  224   b  are advanced from the main sheath  222  and steered to press against the muscle tissue  232  of the heart. Preferably, the steering of catheters  224   a  and  224   b  is accomplished by a conventional steerable catheter construction, commonly known in the art, which allows the tip to be flexed.  
         [0072]     Next, the electrode needles  226   a,    226   b  are advanced from the catheters  224   a  and  224   b,  causing the sharp electrode tips  228   a,    228   b  to penetrate the muscle tissue  232  of the ostia  204 . As the electrode needles  226   a,    226   b  are introduced, their impedance readings are monitored. A lower impedance reading may indicate that the electrode needles  226   a,    226   b  are positioned in blood or muscle tissue  232  and a higher impedance reading may indicate a position within fat tissue, such as fat pad  218 . A more detailed example technique for finding the desired target site is described by Plaft et al. in their abstract from the 2003 NASPE meeting published in Pacing and Clinical Electrophysiology April 2003, vol. 26, the contents of which are incorporated by reference.  
         [0073]     Once the needle electrodes  228   a,    228   b  have been determined to be in a desired target location, such as within fat pad  218 , a user may cause ablation to the region. Preferably, bipolar ablation techniques may be used to form an oval shaped burn between some or all of the electrodes  228   a,    228   b,    229   a,    229   b  on each needle electrode  226   a  and  226   b.  However, mono-polar ablation may also be used to achieve a cylinder-shaped therapeutic ablation treatment. Once the desired ablation has been performed, the user may remove the needle electrodes  226   a,    226   b,  the catheters  224   a,    224   b,  and finally the main sheath.  
         [0074]     In this respect, the user can create a highly localized ablation burns with the targeting ablation device  220  while minimizing burn damage to the surrounding non-target tissue. This targeted burn is facilitated even for an epicardial target area without requiring more invasive surgical approaches, ultimately leading to a more precise and less invasive approach.  
         [0075]     Another specific anatomical application for this type of tissue targeted ablation can be to create an ablation of the muscular tissue of the atrial wall while avoiding the esophagus wall which abuts the atrial wall in some locations.  
         [0076]     Note that this impedance targeting approach may be used in conjunction with any of the previously described embodiments of this application, and is especially preferable with percutaneous ablation approaches.  
         [0077]     Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.