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 . 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 . fig1 a 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 . 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 . to overcome this apparent deficiency in mono - polar ablation techniques , reference is made to fig2 a 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 . 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 . 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 . 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 . 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 . 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 . 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 . for example , fig4 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 . fig5 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 . 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 , fig6 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 . returning to fig3 a 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 . turning to fig7 a 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 . 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 . 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 . fig8 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 . 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 . 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 . in another preferred embodiment according to the present invention , fig9 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 . 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 . 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 . 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 . 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 fig4 – 6 may be adapted for use as the barbs 200 . 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 fig3 a – 6 . 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 . 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 . in yet another preferred embodiment of the present invention , fig1 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 apr . 2003 , vol . 26 , the contents of which are incorporated by reference . 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 . 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 . 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 . 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 . 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 .
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 .