Patent Publication Number: US-6905498-B2

Title: Transmural ablation device with EKG sensor and pacing electrode

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a division of application Ser. No. 10/038,506, filed Nov. 9, 2001, which is a continuation-in-part of application Ser. No. 10/032,378, filed Oct. 26, 2001, which is a continuation-in-part of application Ser. No. 09/844,225 filed Apr. 27, 2001, now U.S. Pat. No. 6,517,536 which is a continuation-in-part of application Ser. No. 09/747,609, filed Dec. 22, 2000, now U.S. Pat. No. 6,546,935 which claims the benefit of provisional application Ser. No. 60/200,072, filed Apr. 27, 2000. 

   BACKGROUND OF THE INVENTION 
   Atrial fibrillation is the most common heart arrhythmia in the world, affecting over 2.5 million people in the United States alone. Ablation of cardiac tissue, in order to create scar tissue that poses an interruption in the path of the errant electrical impulses in the heart tissue, is a commonly performed procedure to treat cardiac arrhythmias. Such ablation may range from the ablation of a small area of heart tissue to a series of ablations forming a strategic placement of incisions in both atria to stop the conduction and formation of errant impulses. 
   Ablation has been achieved or suggested using a variety of techniques, such as freezing via cryogenic probe, heating via RF energy, surgical cutting and other techniques. As used here, “ablation” means the removal or destruction of the function of a body part, such as cardiac tissue, regardless of the apparatus or process used to carry out the ablation. Also, as used herein, “transmural” means through the wall or thickness, such as through the wall or thickness of a hollow organ or vessel. 
   Ablation of cardiac tissue may be carried out in an open surgical procedure, where the breastbone is divided and the surgeon has direct access to the heart, or through a minimally invasive route, such as between the ribs or via catheter that is introduced through a vein, and into the heart. 
   Prior to any ablation, the heart typically is electronically mapped to locate the point or points of tissue which are causing the arrhythmia. With minimally invasive procedures such as via a catheter, the catheter is directed to the aberrant tissue, and an electrode or cryogenic probe is placed in contact with the endocardial tissue. RF energy is delivered from the electrode to the tissue to heat and ablate the tissue (or the tissue may be frozen by the cryogenic probe), thus eliminating the source of the arrhythmia. 
   Common problems encountered in this procedure are difficulty in precisely locating the aberrant tissue, and complications related to the ablation of the tissue. Locating the area of tissue causing the arrhythmia often involves several hours of electrically “mapping” the inner surface of the heart using a variety of mapping catheters, and once the aberrant tissue is located, it is often difficult to position the catheter and the associated electrode or probe so that it is in contact with the desired tissue. 
   The application of either RF energy or ultra-low temperature freezing to the inside of the heart chamber also carries several risks and difficulties. It is very difficult to determine how much of the catheter electrode or cryogenic probe surface is in contact with the tissue since catheter electrodes and probes are cylindrical and the heart tissue cannot be visualized clearly with existing fluoroscopic technology. Further, because of the cylindrical shape, some of the exposed electrode or probe area will almost always be in contact with blood circulating in the heart, giving rise to a risk of clot formation. 
   Clot formation is almost always associated with RF energy or cryogenic delivery inside the heart because it is difficult to prevent the blood from being exposed to the electrode or probe surface. Some of the RF current flows through the blood between the electrode and the heart tissue and this blood is coagulated, or frozen when a cryogenic probe is used, possibly resulting in clot formation. When RF energy is applied, the temperature of the electrode is typically monitored so as to not exceed a preset level, but temperatures necessary to achieve tissue ablation almost always result in blood coagulum forming on the electrode. 
   Overheating or overcooling of tissue is also a major complication, because the temperature monitoring only gives the temperature of the electrode or probe, which is, respectively, being cooled or warmed on the outside by blood flow. The actual temperature of the tissue being ablated by the electrode or probe is usually considerably higher or lower than the electrode or probe temperature, and this can result in overheating, or even charring, of the tissue in the case of an RF electrode, or freezing of too much tissue by a cryogenic probe. Overheated or charred tissue can act as a locus for thrombus and clot formation, and over freezing can destroy more tissue than necessary. 
   It is also very difficult to achieve ablation of tissue deep within the heart wall. A recent study reported that to achieve a depth of ablation of 5 mm, it was necessary to ablate an area almost 8 mm wide in the endocardium. See, “Mechanism, Localization, and Cure of Atrial Arrhythmias Occurring After a New Intraoperative Endocardial Radiofrequency Ablation Procedure for Atrial Fibrillation,” Thomas, et al.,  J. Am. Coll. Cardiology , Vol. 35, No. 2, 2000. As the depth of penetration increases, the time, power, and temperature requirements increase, thus increasing the risk of thrombus formation. 
   In certain applications, it is desired to obtain a continuous line of ablated tissue in the endocardium. Using a discrete or point electrode or probe, the catheter must be “dragged” from point to point to create a line, and frequently the line is not continuous. Multielectrode catheters have been developed which can be left in place, but continuity can still be difficult to achieve, and the lesions created can be quite wide. 
   Because of the risks of char and thrombus formation, RF energy, or any form of endocardial ablation, is rarely used on the left side of the heart, where a clot could cause a serious problem (e.g., stroke). Because of the physiology of the heart, it is also difficult to access certain areas of the left atrium via an endocardial, catheter-based approach. 
   Recently, epicardial ablation devices have been developed which apply RF energy to the outer wall of the heart to ablate tissue. These devices do not have the same risks concerning thrombus formation. However, it is still difficult to create long, continuous lesions, and it is difficult to achieve good depth of penetration without creating a large area of ablated tissue. 
   As noted above, other forms of energy have been used in ablation procedures, including ultrasound, cryogenic ablation, and microwave technology. When used from an endocardial approach, the limitations of all energy-based ablation technologies to date are the difficulty in achieving continuous transmural lesions, and minimizing unnecessary damage to endocardial tissue. Ultrasonic and RF energy endocardial balloon technology has been developed to create circumferential lesions around the individual pulmonary veins. See e.g., U.S. Pat. No. 6,024,740 to Lesh et al. and U.S. Pat. Nos. 5,938,660 and 5,814,028 to Swartz et al. However, this technology creates rather wide (greater than 5 mm) lesions which could lead to stenosis (narrowing) of the pulmonary veins. See, “Pulmonary Vein Stenosis after Catheter Ablation of Atrial Fibrillation,” Robbins, et al.,  Circulation , Vol. 98, pages 1769-1775, 1998. The large lesion area can also act as a locus point for thrombus formation. Additionally, there is no feedback to determine when full transmural ablation has been achieved. Cryogenic ablation has been attempted both endocardially and epicardially (see e.g., U.S. Pat. No. 5,733,280 to Avitall, U.S. Pat. No. 5,147,355 to Friedman et al., and U.S. Pat. No. 5,423,807 to Milder, and WO 98/17187, the latter disclosing an angled cryogenic probe, one arm of which is inserted into the interior of the heart through an opening in the heart wall that is hemostatically sealed around the arm by means of a suture or staples), but because of the time required to freeze tissue, and the delivery systems used, it is difficult to create a continuous line, and uniform transmurality is difficult to verify. 
   Published PCT applications W0 99/56644 and W0 99/56648 disclose an endocardial ablation catheter with a reference plate located on the epicardium to act as an indifferent electrode or backplate that is maintained at the reference level of the generator. Current flows either between the electrodes located on the catheter, or between the electrodes and the reference plate. It is important to note that this reference plate is essentially a monopolar reference pad. Consequently, there is no energy delivered at the backplate/tissue interface intended to ablate tissue. Instead, the energy is delivered at the electrode/tissue interface within the endocardium, and travels through the heart tissue either to another endocardial electrode, or to the backplate. Tissue ablation proceeds from the electrodes in contact with the endocardium outward to the epicardium. Other references disclose epicardial multielectrode devices that deliver either monopolar or bipolar energy to the outside surface of the heart. 
   It is important to note that all endocardial ablation devices that attempt to ablate tissue through the full thickness of the cardiac wall have a risk associated with damaging structures within or on the outer surface of the cardiac wall. As an example, if a catheter is delivering energy from the inside of the atrium to the outside, and a coronary artery, the esophagus, or other critical structure is in contact with the atrial wall, the structure can be damaged by the transfer of energy from within the heart to the structure. The coronary arteries, esophagus, aorta, pulmonary veins, and pulmonary artery are all structures that are in contact with the outer wall of the atrium, and could be damaged by energy transmitted through the atrial wall. 
   Accordingly, it is the object of the present invention to provide an improved method and device for making transmural ablations to heart tissue. 
   It is a related object to provide a method and device for making transmural ablation in heart tissue that minimizes unnecessary damage to the heart tissue. 
   It is a further object to provide a method and device for making transmural ablation in heart tissue that creates continuous lesions in a single step. 
   SUMMARY OF THE INVENTION 
   These objects, and others which will become apparent upon reference to the following detailed description and attached drawings, are achieved by the use of a clamping and ablating device for use in treating cardiac arrhythmia having first and second handle members, with first and second mating jaw members with opposed mating surfaces associated with the first and second handle members. An elongated electrode extends along the mating surface of each jaw. The electrodes are adapted to be connected to an RF energy source so that, when activated, the electrodes are of opposite polarity. An EKG sensor is located on one of the jaw members intermediate the ends of the electrode. The EKG sensor is spaced away from the electrode a sufficient distance so that it contacts the clamped tissue outside of the line of ablation created by the activated electrodes. In a preferred embodiment, a pacer electrode is located on one of the jaw members on the side of the electrode, and the ablation line created thereby, opposite from the EKG sensor. Thus, if ablation is not complete, i.e., not transmural, the EKG sensor will detect signals generated by the pacing electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view showing a procedure in accordance with the present invention utilizing ablation elements operatively connected to either a source of RF energy or cryogenic fluid. 
       FIG. 2  is a cross-section of an ablation element for use in the present invention taken along lines  2 — 2  of FIG.  1 . 
       FIGS. 3-6  show alternate configurations for the ablation elements of FIG.  2 . 
       FIG. 7  shows a further step in the inventive procedure in which tissue is clamped between the ablation elements. 
       FIGS. 8-12  schematically illustrate the inventive procedure so as to make a transmural lesion that fully circumscribes a pulmonary vein, with  FIG. 9  showing a cross-sectional view of the clamp/ablation element in contact with the atrial tissue to express blood from the clamped area. 
       FIGS. 13-17  show a further method according to the present invention in which transmural lesions are made so as to circumscribe both pulmonary veins. 
       FIGS. 18-22  show a further procedure in which a transmural lesion is made so as to circumscribe a single pulmonary vein. 
       FIGS. 23-27  illustrate a further procedure in which a transmural lesion is made so as to circumscribe both pulmonary veins. 
       FIG. 28  is a perspective view of a further embodiment of a grasper for use in an open chest procedure in accordance with the present invention showing the grasper in its “closed” position. 
       FIG. 29  is a perspective view of the grasper of  FIG. 28  with the grasper in its “open” position. 
       FIG. 30  is an enlarged perspective view of the working position of the grasper of  FIG. 28  with the grasper jaws in the “closed” position. 
       FIG. 31  is an enlarged perspective view of the working portion of the grasper of  FIG. 28  with the grasper jaws in the “open” position. 
       FIG. 32  is an enlarged cross-sectional view of the grasper jaws for the grasper of FIG.  28 . 
       FIG. 33  is a perspective view of a further embodiment of a grasper, which may be used in either an open or a minimally invasive procedure, along with its associated electrosurgical generator. 
       FIG. 34  is a side view of the grasper of  FIG. 33  showing the grasper in its “open” position. 
       FIG. 35  is an exploded perspective view of the grasper of FIG.  33 . 
       FIG. 36  is a side cross-sectional view of the grasper of  FIG. 33  with the grasper jaws in the “open” position. 
       FIG. 37  is a side cross-sectional view of the grasper of  FIG. 33  with the grasper jaws in the “closed” position. 
       FIG. 38  is a cross-sectional view taken along line  38 — 38  of  FIG. 34  showing the grasper jaws in the “open” position. 
       FIG. 39  is a cross-sectional view of the grasper jaws taken along the line  39 — 39  of  FIG. 37  showing the grasper jaws in the “closed” position. 
       FIG. 40  is a cross-sectional view of the graspers taken along line  40 — 40  of FIG.  34 . 
       FIGS. 41-51  show alternate constructions for the electrodes suitable for use in the present invention, with FIGS.  41  and  43 - 51  being cross-sectional views similar to  FIGS. 38 and 39 , and  FIG. 42  being a cross-sectional view taken along line  42 — 42  of FIG.  41 . 
       FIGS. 52A-K  illustrate eleven different ablations to the left and right atrium (as seen from behind in  FIG. 52A ) and the methods for making the lesions (FIGS.  52 B-K). 
       FIG. 53A  is a perspective view of a further embodiment of device for performing transmural ablation according to the present invention. 
       FIG. 53B  is a perspective view of the transmural ablation device of  FIG. 53A  with a portion removed to show detail. 
       FIG. 54  is an exploded perspective view of the transmural ablation device of FIG.  52 . 
       FIG. 55  is a longitudinal cross-sectional view of an obturator tip electrode for use in the device of FIG.  52 . 
       FIG. 56  is a piercing tip electrode for use in the device of FIG.  52 . 
       FIG. 57  is an enlarged side view of the tip of the instrument shown in FIG.  52 . 
       FIGS. 58A-58G  illustrate the use of the instrument of  FIG. 52  to form a transmural ablation. 
       FIG. 59  shows a series of transmural ablations contemplated by the MAZE procedure. 
       FIGS. 60A-60I  illustrate a procedure for performing a circumferential lesion in lumen such as a pulmonary vein. 
       FIGS. 61A-61J  show the use of the instrument of  FIG. 52  for forming a continuous transmural ablation around a pair of pulmonary veins. 
       FIGS. 62A-I  show a further device for performing transmural ablations and the method for making such ablations. 
       FIG. 63  is a perspective view of a further embodiment of a grasper adapted for use in minimally invasive procedures. 
       FIG. 64  is an enlarged plan view of the handle position of the grasper of  FIG. 63 , with portions removed to show detail. 
       FIGS. 65A and 65B  are enlarged plan views of the jaw actuation mechanism for the grasper of FIG.  63 . 
       FIG. 66  is an enlarged fragmentary perspective view of the jaws of grasper of  FIGS. 33-40 . 
       FIG. 67  is an enlarged perspective view of the tip of the fixed jaw shown in FIG.  66 . 
       FIG. 68  shows an anterior plan view of the heart in the chest cavity and an access location for performing ablation where the chest cavity is opened. 
       FIG. 69  shows an anterior plan view of the heart with an opening in the chest cavity. 
       FIG. 70  shows a side view of a bullet dissector according to the present invention. 
       FIG. 71  shows an enlarged side view of the bullet dissector. 
       FIG. 72  shows an enlarged side view of the opened chest cavity including a bullet dissector. 
       FIG. 73  shows an anterior plan view of the chest cavity showing ablation according to the open heart method. 
       FIG. 74  illustrates ablations to the right and left atrium as seen from behind. 
       FIGS. 75-76  illustrates bilateral ablation of the left atrium near the right and left pulmonary veins. 
       FIG. 77  illustrates a schematic diagram of the heart with ablations according to the methods of the present invention. 
       FIG. 78  is an anterior plan view of the heart and access locations for performing ablation according to the intercostal method. 
       FIG. 79  is a top perspective view of the patient showing intercostal insertion of the bullet dissector into the chest cavity. 
       FIG. 80  is an anterior plan view of the heart showing intercostal insertion of the bullet dissector into the intrapericardial space. 
       FIG. 81  is a sectional view along plane  81 — 81  of FIG.  80 . 
       FIG. 82  is a sectional view, similar to  FIG. 81 , showing insufflation of the bullet dissector. 
       FIG. 83  is a sectional view, similar to  FIG. 81 , showing insertion of an ablation device. 
       FIG. 83A  is a plan view of a cardiac grasper similar to the grasper shown in  FIGS. 33-37  with right curved jaw members. 
       FIG. 83B  is a plan view of a cardiac grasper similar to the grasper shown in  FIGS. 33-37  with left curved jaw members. 
       FIG. 84  is a sectional view along plan  84 — 84  of  FIG. 83  with portions of the patient&#39;s body removed. 
       FIGS. 85-86  are sectional views, similar to  FIG. 81 , showing intercostal ablation being performed with the lungs inflated. 
       FIG. 87  is an anterior plan view of the heart and access locations for performing ablation according to the sub-xyphoid method. 
       FIG. 88  is an anterior plan view of the heart showing insertion of the bullet dissector and ablation device according to the sub-xyphoid method. 
       FIG. 89  is an enlarged side view of the heart showing an ablation device inserted according to the sub-xyphoid method. 
       FIG. 90  is an enlarged side view of the heart showing ablation according to the sub-xyphoid method. 
       FIG. 91  is an anterior plan view of the heart showing ablation of the right and left pulmonary veins according to the sub-xyphoid method. 
       FIG. 92  is an anterior plan view of the heart showing an alternate instrument inserted according to the sub-xyphoid method. 
       FIG. 93  is an enlarged side view of the heart showing ablation using the alternate instrument. 
       FIG. 94  is a top plan view of the alternate instrument. 
       FIG. 95  is a side view of the heart and alternate instrument showing ablation according to the intercostal method. 
       FIG. 96  shows an anterior plan view of the heart and access location for performing ablation according to the sub-xyphoid method using a single sub-xyphoid access location. 
       FIG. 97  shows an anterior plan view of the heart during insertion of the bullet dissector through the sub-xyphoid access location. 
       FIGS. 98-99  show an enlarged side view of the heart illustrating insertion of the bullet dissector into the intrapericardial space and dissection by the bullet dissector around the left pulmonary veins. 
       FIG. 100  shows an anterior plan view of the heart similar to  FIG. 97  further including insertion of the ablation device. 
       FIG. 101  shows an enlarged side view of the heart illustrating ablation where sub-xyphoid access of the bullet dissector and ablation device is achieved using a single sub-xyphoid access location. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to the present invention, the compression of the atrial tissue is important because it insures that the exposed electrode surface or cryogenic probe is not in contact with any tissue or blood except the clamped tissue to be ablated. Specifically, the clamping of the tissue between the electrodes or cryogenic probes insures that the conductive or cooled area is only in contact with the clamped tissue. The compressed tissue acts to isolate the electrically active or cryogenically cooled surface, and prevents inadvertent energy delivery to other parts of the heart or blood. The outside temperature of the electrode can easily be monitored to insure that the temperature of the insulation in contact with blood remains below a critical temperature (40° C., for example). 
   In one form of the invention, transmural ablation using RF energy is accomplished by providing an atrial ablation device having a lower “j” clamp/electrode element and placing it on the atrial tissue below the pulmonary veins. 
   Once the pulmonary veins have been isolated, an upper clamp/electrode element is introduced, and the clamp assembly “J” is worked back onto the epicardial atrial tissue. Once the jaws are positioned below the ostia of the pulmonary veins, the tissue is partially clamped, allowing continued flow from the pulmonary veins to the left atrium. Once the clamps are safely away from the pulmonary vein tissue, and onto atrial tissue, the clamps are closed together to compress the tissue. Once the tissue is compressed, bipolar RF energy is used to ablate the clamped atrial tissue. The clamps are then removed, the lesion having been created. Lesions may also be created by inserting one clamp/electrode element through an incision in the heart so as to permit contact with endocardial tissue. This incision may be created with a separate instrument. Alternatively, the tip of one of the jaws may have a piercing structure associated therewith for making the entry incision. Once the clamps are properly located, the tissue is compressed and RF energy is applied. 
   Turning now to the figures of the drawings, a method embodying the present invention is shown schematically in  FIG. 1. A  clamping type device  10  is provided to group the two walls  22 ,  24  of the atrium  20 , and delivers bipolar RF energy through both walls held between the two upper and lower clamp jaws  50 ,  51 .  FIG. 1  shows the upper and lower parallel clamp jaws  50 ,  51  and electrodes  52 ,  53  positioned above and below atrial tissue  22 ,  24 , distal to the pulmonary veins.  FIG. 2 , Section  2 — 2  of  FIG. 1 , shows a cross-section of the clamping member including the insulator  28  and electrode  53 . Alternate configurations of the clamping members are shown in  FIGS. 3-6 .  FIG. 3  shows a cross section of the electrode consisting of an insulating layer  11 , and a conductive strip  12 . The electrode of  FIG. 3  may be constructed of a tungsten wire as the conductive material  12 , with polyamide as the insulating material  11 . The conductive strip is created by exposing a part of the tungsten wire through the polyamide. 
     FIGS. 4 and 5  show an alternate electrode construction consisting of a carbon fiber element  13 , and an insulating material  14 , such as ABS. The conductive strip  15  may be comprised of a copper/gold electrode plated onto the ABS.  FIG. 6  shows a cross section of yet another possible electrode design where the conductive material  16  consists of a stainless steel needle with lumen  17  and insulating material  18 . 
     FIG. 7  shows the parallel jaws  50 ,  51  clamping and ablating the atrial tissue  20  distal to the pulmonary veins  26 . Proximal point A is clamping and ablating the atrial tissue distal to the pulmonary veins. Proximal point A is the most proximal point of ablated tissue on both the upper and lower atrial wall. Distal point B is the most distal point of ablated tissue on both the upper and lower atrial wall. 
     FIGS. 8-12  show the inventive procedure that fully circumscribes a pulmonary vein with transmural lesions.  FIG. 8  shows a top view of the instrument jaws positioned for a 2-step isolation of a single pulmonary vein. The lower jaw is directly beneath the upper jaw, and is not shown. Proximal point A and distal point B correspond to FIG.  7 . 
     FIG. 9  shows a cross-sectional view of the jaws clamping and ablating atrial tissue. Importantly,  FIG. 9  shows that the electrode/clamp configuration provides a clamped zone of tissue that is wider than the zone of ablated tissue. This is achieved by using an electrode width that is narrower than the clamped tissue width, and preferably less than one-third of the clamped tissue width. As shown in  FIG. 9  (and better illustrated in FIG.  26 ), the electrode forms the apex of the triangular clamping member. Other convex shapes are also contemplated. 
   The wider zone of clamped tissue serves several purposes. When the clamping members are closed onto tissue, any blood in the clamped zone is squeezed or expressed out. Further, the distance between the electrodes is minimized, so that the ablation zone remains narrow. It is important to isolate the blood from the ablation zone to avoid creating thrombus. Accordingly, a clamped zone that isolates the ablation zone from the blood minimizes the temperature at the periphery of the ablation zone and will reduce the likelihood of the formation of thrombus by the blood in contact with the clamped zone. 
   Once tissue has been fully ablated with the clamp in the position shown in  FIG. 8 , an ablation line of tissue on both upper and lower atrial walls is created. This is shown as ablation line  60  in FIG.  10 . The clamp is then repositioned to the position shown in  FIG. 10 , so that the distal point D overlaps the ablation line  60 . The tissue is clamped and ablated as shown in  FIGS. 7 and 9 , and a second ablation line  61  ( FIG. 11 ) is formed on both the upper and lower atrial walls. Proximal point C and distal point D correspond to points A and B respectively. The full ablation line is shown in  FIGS. 11 and 12  with points A-D as shown. 
   This “clamping” method and device for creating transmural lesions has a number of advantages. First, using a two step method as shown allows for clamping and ablation of atrial tissue without stopping the blood flow from the pulmonary vein. Secondly, by clamping both walls together, and delivering energy through the clamped tissue, the atrial tissue is not penetrated. Because the atrial tissue is not penetrated, a larger jaw can be used, and the clamping force can be much higher because of the increased stiffness of the jaw. Also, there is no concern of bleeding from an atrial puncture. 
   Another advantage of this method and device is that ablation of tissue within the pulmonary veins is avoided, as recent articles have shown that ablation of tissue within the pulmonary veins can cause pulmonary hypertension and stenosis. Specifically referring to  FIGS. 13-17 , a longer jaw could be used to create an ablation line through atrial tissue which electrically isolates both pulmonary veins using the same method. 
     FIGS. 18-22  show the clamping device in a curved-jaw embodiment that creates a circumferential lesion around the pulmonary vein in one step.  FIGS. 18 and 19  show the clamp jaws positioned around the pulmonary vein.  FIGS. 20 and 21  show the device clamping and ablating atrial tissue distal to the pulmonary vein.  FIG. 22  shows the resulting ablation line  60 . 
     FIGS. 23-27  show the same concept applied to a device and method for creating a lesion around both pulmonary veins. The advantage of this concept is that the entire lesion is created in one step. The disadvantage is that blood flow from the pulmonary vein(s) is cut off during ablation. Using a curved electrode also allows the user to ablate tissue more distal to the pulmonary vein than would be possible with a straight electrode. Note that this curved type electrode could be used with a two step procedure as described above, using “left” and “right” curved devices to create a lesion which was more distal to the pulmonary veins. Note also that this method and device are not limited to use around the pulmonary veins, but could be used anywhere in the atrium that the clamp could be applied. 
   Turning to  FIGS. 28-32 , there is seen a further version of a cardiac grasper  70  suitable for an open chest procedure in accordance with the present invention. The grasper  70  includes two ring handles  72 ,  74  joined together for relative movement by a pivot screw or pin  76 . Each handle  72 ,  74  has a jaw member  78 ,  80  respectively associated therewith, each jaw being curved so that it has a major portion that is substantially perpendicular to the handles. This gives the grasper  70  an L-shaped appearance, with a working portion of the jaws being between approximately 3-8 cm in length. 
   The grasper is made of a rigid material, such as stainless steel, and is substantially encased in a durable insulating material, such as ABS plastic. With reference to  FIG. 32 , which shows the opposed jaw members in cross section, the stainless steel structural support is designated  82 . The structural support  82  is completely encased by insulating members  84 ,  86  and  88 . The tips  78   a ,  80   a  of the jaws may be made of a soft, atraumatic material in order to reduce the likelihood of unintentional injury of tissue by the jaws. 
   In keeping with the invention, the grasper jaws have raised or convex, opposed tissue clamping surfaces,  90 ,  92 , respectively, with each clamping surface,  90 ,  92  centrally supporting an electrode  94 ,  96 , respectively, of opposite polarity. The spacing between the jaws is substantially uniform or constant when in the closed or clamped position. RF energy of opposite polarity is supplied to the electrodes  94 ,  96  through conductors  98 ,  100 , which are connected to an RF generator. As with the previously-described jaw members, this electrode/clamp configuration provides a clamped zone of tissue that is significantly wider than the zone of ablated tissue created by the opposed electrodes. This causes for any blood in the clamp zone to be squeezed or expressed out of the ablation zone, thus reducing the likelihood of thrombus formation, as well as minimizing the distance between the electrodes, so that the ablation zone remains narrow. The clamping also eliminates the cooling effect of circulating blood. 
   With reference to  FIG. 32 , the electrodes  94 ,  96  have a T-shaped cross section, with the cross portion of the T resting on the insulating member  88  and the upright portion of the T protruding through a narrow opening in the insulating member  84 , thus creating an exposed electrode surface that contacts the tissue grasped between the jaws. In practice, the electrodes are preferably made of gold-plated copper and extend along substantially the entire working surface of the jaw members. The exposed portions of the electrode are generally less than 1.25 mm in width, and preferably between approximately 0.12-0.6 mm in width. This insures that most of the jaw surface is insulator, and that the electrode comprises generally less than one-third of the width of the jaw. 
   In keeping with a further aspect of the invention, the graspers may provide feedback that permits the user to gauge the completeness (i.e., degree of transmurality) of the ablation. Specifically, a transmural lesion blocks electrical signals because it is non-conductive scar tissue. Because impedance is simply the inverse of conductivity, the ability of the lesion to block electrical signals is accurately indicated by its impedance, which can be measured simultaneously with the creation of the lesion. During RF energy application to the tissue to be ablated, the current and voltage applied to the tissue are measured, and the impedance calculated and stored. Based upon a function of the impedance (e.g., its value, the change in value, or the rate of change in value)) it is determined whether ablation is complete and transmural. See e.g., U.S. Pat. No. 5,403,312, which is incorporated by reference herein. Indicator lights or other types of signals (e.g., audible may be associated with the grasper to correspond to the degree of ablation determined by the impedance feedback system. For example, once the impedance reaches a certain level for a certain period of time, a red light may be activated to signal that ablation is complete. 
   In keeping with another aspect of the invention, a feedback system for determining the temperature of the ablated tissue is also provided. To this end, the jaws include a series of thermocouples  102  that are supported in the insulating member  84  remote from the associated electrode  94  near the edge of the jaw  78 . The thermocouples  102  protrude slightly through the surface of the insulating member  84  so as to engage any tissue clamped between the jaws  72 ,  74 . Wires  104  are attached to the thermocouples  102  to transmit the information received to a remote location. Again, a visual or other indicator may be provided to alert the user that a certain pre-determined critical temperature (e.g., 40° C.) has been reached, thus permitting the user to avoid undesired thermal spread. 
   Turning to  FIGS. 33-37 , there is a further version of a cardiac grasper  110  suitable for both open and minimally-invasive procedures in accordance with the present invention. As seen in  FIG. 33 , the grasper  110  includes a cord  112  for housing the conductors (not shown) and for plugging into an electrosurgical generator  114  to provide current to the grasper  110 . As discussed above, the generator  114  includes a display  115  to provide a simultaneous visual indication of the degree of conductance of the tissue being ablated. The instrument  110  includes opposed parallel, curved jaw assemblies  116 ,  118  with jaw assembly  116  being fixed and jaw assembly  118  being movable between an open position (as seen in  FIGS. 34 and 36 ) to a closed position (shown in FIG.  37 ), the spacing between the jaws being substantially uniform or constant. The fixed jaw assembly  116  comprises a fixed electrode  120 , a fixed insulator  122  and a fixed jaw cap  124 . The fixed electrode  120  provides an electrical pathway adjacent to the tissue to be ablated and is located on the inside of the fixed jaw assembly  116  (the “inside” being defined as the side that contacts the tissue to be ablated). The fixed insulator  122  surrounds the fixed electrode  120  and forms the inside of the fixed jaw assembly  116 . The fixed jaw cap  124  forms the backside of the fixed jaw assembly  116  (the “backside” being defined as the surface opposite the fixed electrode  120 ). 
   The drive jaw assembly  118  comprises a drive electrode  126 , a drive insulator  128 , and a drive jaw cap  130 . The drive electrode  126  provides a second electrical pathway adjacent the tissue to be ablated and is located on the inside of the drive jaw assembly  118  (“inside” being defined as the side contacting the tissue to be ablated). The drive insulator  128  surrounds the drive electrode  126  and forms the inside of the drive jaw assembly  118 . The drive jaw cap  130  forms the backside of the drive jaw assembly  118  (“backside” being defined as the surface opposite the drive electrode  126 ). 
   Each of the electrodes  120 ,  126  is attached to an electrically conductive means, such as a wire, that runs the length of the extension shaft and through the conductor cord  112  for coupling to the RF generator  114 . 
   Each jaw assembly  116 ,  118  is supported by a two piece extension shaft comprising a right fixed member  132  and left fixed member  134  (for the fixed jaw) and a right drive member  136  and left drive member  138  (for the drive jaw  118 ). A shaft cap  139  covers the coextensive portions of the fixed members  132 ,  134  and the drive members  136 ,  138  (when the jaws are in the open position as seen in FIG.  34 ). The right fixed member  132  and left fixed member  134  combine to form a structure that extends from a handle  140 , through the shaft cap  139 , and then terminating at the distal end of the instrument  110  in the fixed jaw assembly  116  on the right and left sides, respectively, of the instrument. Similarly, the right drive member  136  and left drive member  138  extend from the handle  140 , through the shaft cap  139 , and then terminate in the drive jaw assembly  118  on the right and left sides, respectively, of the instrument. The portions of the fixed members  132 ,  134  co-extensive with the fixed jaw assembly  116  are joined by a fixed bridge  142  along the length of the jaw. Similarly, the portions of the drive members  136 ,  138  co-extensive with the drive jaw assembly  118  are joined together by a drive bridge  144  along the length the drive jaw  118 . 
   The handle  140  comprises two mating halves  140   a ,  140   b  for encapsulating the actuation and force control mechanisms for the grasper, as well as providing for grounding of the shaft components by means of a conductive shaft pin  141 . In order to move the drive jaw assembly  118  between its open and closed positions, the handle  140  includes a lever comprising a pair of lever plates  146  and a lever shroud  148 . The lever is pivotally mounted on a support member  150  extending between the two halves  140   a ,  140   b  of the handle  140 , with a lever spring  151  biasing the lever to its open position (FIG.  34 ). The lever plates  146  are coupled by a lever pin  152  to a carriage  154  that captures the proximal ends of the drive members  136 ,  138 , so as to provide translational motion to these members. 
   The carriage  154  includes a lost motion assembly comprising a carriage spring  156  for controlling the minimum and maximum loads that can be applied to tissues that are to be captured between the jaw assemblies  116 ,  118 . As can be readily appreciated, the thicker the tissue that is grasped between the jaws, the greater the compression of the spring  156 , and the greater the compression force exerted by the jaws on the tissue. (The range of tissue thickness is expected to be between about 1-15 mm.) In other words, the force exerted by the jaws on the tissue held therebetween increases as the distance between the jaws increases. Adjustment of the compression force is accomplished by pre-loading the carriage spring  156  with a load adjustment screw  158 . The lost motion assembly also includes a thumb latch  160  for releasing the clamping pressure and for providing a mechanical stop for the spring-loaded carriage  154 . The thumb latch  160  is pivotally mounted on a latch pin  162  to secure the thumb latch to the handle  140 . Additionally, a latch spring  164  is provided for biasing the thumb latch  160  to its locked position. A latching step on the carriage  154  interfaces with the tip of the thumb latch  160  to provide for the mechanical stop. 
   When the lever is pivoted with respect to the handle  140 , the drive jaw assembly  118  and its drive members  136 ,  138  slide along the longitudinal direction of the shaft to bring the two jaw assemblies  116 ,  118  into contact with the tissue intended to be grasped. 
   In order to ablate a narrow, long region of biological tissue with the instrument  110 , the tissue is first placed between the open instrument jaws  116 ,  118 . The user then grasps the actuation lever comprising the lever plates  146  and lever shroud  148  to apply the force required to drive the drive members  136 ,  138  and drive jaw assembly  118  distally, thus compressing the tissue and automatically engaging the thumb latch  160 . The thumb latch  160  locks the position of the drive members  136 ,  138  and the drive jaw assembly  118  with respect to the handle  140  and the fixed jaw assembly  116 . The amount of jaw force on the tissue is controlled by the lost motion assembly between the lever and the drive members  136 ,  138 . 
   With the jaws closed on the tissue, the operator activates the RF generator  114 . RF energy passes through the tissue between the electrodes  120 ,  126 , thus ablating the tissue between these electrodes. After completion of the ablation cycle, the operator releases the clamping of the tissue by depressing the thumb latch  160 , thus releasing the carriage  154 . With the carriage  154  released, the lever spring  151  drives the drive members  136 ,  138  and the drive jaw assembly  118  proximally to their open positions. The actuation lever, since it is directly coupled to the carriage  154 , also returns to the open position. 
   Turning to  FIGS. 41-51  there is seen in schematic form various configurations for the electrodes  120 ,  126  for use in conjunction with the grasper  110 . Each of FIGS.  41  and  43 - 51  show a cross-section through the instrument jaws as clamped on the tissue to be ablated. Each electrode is formed of a piece of electrically conductive metal that may be plated with a biocompatible material. 
   With reference to  FIGS. 41 and 42 , the electrode geometry consists of a largely rectangular electrode with a window of material removed from the central region. The window area is filled with the insulator material  122 ,  128 . At the clamping surface the electrode insulator material leads away from the electrode on a radius. The electrode material protrudes outside the clamping surface of the insulating material. However, the electrode may also be flush with the clamping surface. 
   With reference to  FIG. 43 , the electrode geometry is largely rectangular and the electrode insulator material leads away from the electrode on a radius. The electrode is flush with the clamping surface of the insulator material. 
   With reference to  FIG. 44 , the electrode is applied to fill a groove in the insulator material by way of a plating process. The electrode geometry is largely rectangular and the electrode insulator material leads away from the electrode on a radius. The electrode plating is largely flush with the clamping surface of the insulator material. 
   With reference to  FIG. 45 , the electrode is formed into a U-shaped element. The electrode insulator material leads away from the electrode on a radius. As shown, the electrode material extends outside the clamping surface of the insulator material. However, the electrode material may also be flush with the insulator clamping surface. 
   With reference to  FIG. 46 , the electrode is applied to fill a groove in the insulator material by way of a plating process, with the electrode geometry being largely rectangular. The electrode insulator material creates a small flat surface perpendicular to the closure plane that is largely flush with the surface of the plate or electrode. As shown, the electrode material is flush with the clamping surface of the insulator material. However, the electrode material may also be applied so that it extends outside the insulator clamping surface. 
   With reference to  FIG. 47 , the electrode geometry is largely rectangular and the electrode insulator material leads away from the electrode on a radius. The electrode material extends outside the clamping surface of the insulator material. 
   With reference to  FIG. 48 , the electrode configuration is again largely rectangular, with the electrode insulator material creating a small flat surface perpendicular to the closure plane that is largely flush with the surface of the plate or electrode. The electrode is flush with the clamping surface of the insulator material and a temperature sensing means, such as a thermocouple  166  (see also FIGS.  35  and  39 ), is positioned in close proximity to the electrode, but electrically isolated from the RF energy. 
   With reference to  FIG. 49 , the electrode is applied to fill a groove in the insulator material by way of a plating process. The electrode geometry is largely rectangular and the electrode insulator material leads away from the electrode on a radius. 
   With reference to  FIG. 50 , the electrode is applied to the surface of the electrode insulator material by way of a plating process. The electrode geometry is largely rectangular with the electrode insulator material leading away from the electrode on a radius. The electrode plating is largely flush with the clamping surface of the insulator material. With reference to  FIG. 51 , the electrode is round wire made from an electrically conductive metal that may be plated with a biocompatible material. The electrode insulator material leads away from the electrode on a radius. As shown, the electrode material extends outside the clamping surface of the insulator material. However, the electrode material may also be flush with the insulator clamping surface. 
   A further embodiment of a grasper according to the present invention is shown in  FIGS. 63-65  and is designated generally  250 . The grasper  250  has jaws  252 ,  254  similar in structure to those described above in connection with the embodiments of  FIGS. 28-32  and  33 - 40 , but includes a different actuation mechanism. Specifically, the jaws  252 ,  254  of the grasper  250  are biased so that they are normally in the closed position, the jaws being moved to the open position by moving the two handle members  256 ,  258  towards each other. This action serves to withdraw a push-rod  260  (FIG.  64 ), which is pivotally connected to the handle members  256 ,  258  by links  262 ,  264 . With reference to FIG.  65 A and FIG.  65 B. The distal end of the push rod  260  includes two pins  266 ,  268  which are captured in slots  270 ,  272  in their respective jaw members  252 ,  254 . When the pins  266 ,  268  are located in the distal ends of the slots  270 ,  272 , the jaws are in the closed position. The jaws  252 ,  254  open as the pins  266 ,  268  move proximally in the slots  270 ,  272  through the withdrawal of the push rod  260  by the closing of the handle members  256 ,  258 . 
   The jaws  252 ,  254  also include a lost motion connection including a spring to bias the jaws toward the closed position. With reference again to FIG.  65 A and  FIG. 65B , the jaws  252  and  254  are pivotally connected to each other by means of a pin  274 . The pin  274  is secured to the jaw member  254 , but is received in an elongated slot  276  in jaw member  252 . The pin  274  is biased to the top of the slot  276 , thus biasing the jaws  252 ,  254  to the closed position, by means of a leaf spring  278  having one end secured to the pin  274  and the other end captured between two studs  280 ,  282  carried on the jaw member  252 . 
     FIGS. 52A-K  illustrate a series of 11 different lesions or ablations that may be made using either an open or a minimally invasive technique with the graspers described above. Turning first to  FIG. 52A , there is seen a view of the heart showing the right and left atriums (as viewed from behind). The heart includes the left atrial appendage (LAA) and the right atrial appendage (RAA). The right pulmonary veins (RPVs) and left pulmonary veins (LPVs) enter into the top of the left atrium. The superior vena cava (SVC) and inferior vena cava (IVC) are also shown. The mitral valve annulus is designated as MVA, while the tricuspid valve annulus designated TVA. In  FIG. 52A , 11 different lesions are indicated by the reference numerals  1 - 11 . A method for making each of these lesions is illustrated in the following  FIGS. 52B-K . It should be appreciated that, depending upon a particular patient&#39;s indications, the lesions  1 - 11  may be created in a variety of combinations. 
   With reference to  FIG. 52B , a method for making lesion  1  to circumscribe the right pulmonary veins (RPVs) is shown. This lesion is made completely epicardially in a manner similar to that illustrated in  FIGS. 23-27 .  FIG. 52C  illustrates lesion  2 , an epicardial ablation that fully circumscribes the left pulmonary veins (LPVs). Again, this lesion may be made in a manner similar to that illustrated in  FIGS. 23-27 . 
     FIG. 52D  illustrates a method for making lesion  3 , which connects lesions  1  and  2 . Lesion  3  is made with only one of the jaws of the graspers being located epicardially. The mating jaw is inserted into the interior of the heart through a small incision which is sealed using a purse-string suture. The incision as illustrated is made interior the lesion  1  encircling the right pulmonary veins (RPVs). 
   In order to be sure that lesions  1  and  2  are fully transmural and completely encircle the RPV&#39;s and LPV&#39;s so as to effectively electrically isolate the RPVs and LPVs from the atrium, one of the jaw members of the grasper may be provided with an EKG sensor intermediate the ends of the jaw. The EKG sensor is located on the jaw so that, when the grasper jaws are closed on the tissue intended to be ablated, the EKG sensor contacts the tissue outside of the ablation line on the pulmonary vein side of the line of ablation. Thus, by monitoring the EKG of the atrial tissue adjacent the pulmonary vein, the surgeon can determine simultaneously with the creation of the ablation line whether the pulmonary veins have been electrically isolated from the atrium. 
   In addition, the grasper jaw may include pacing electrodes intermediate its ends. The pacing electrodes are located on the opposite side of the ablation electrode from the EKG sensor, again outside the line of ablation. Thus, a pacing pulse can be transmitted by the pacing electrodes which will be sensed by the EKG sensors when the jaws of the grasper are closed on the tissue. The pacing and sensing continues as the ablation electrodes are activated to create the lesion. When the lesion is complete, the EKG sensors will no longer be able to detect the pacing signal. 
   When a pacing electrode is paired with the EKG sensor, the EKG sensors may be on either side of the jaw, i.e., on either the pulmonary vein side or the atrial side of the ablation line created by the ablation electrodes. The EKG electrodes and pacing electrodes are best seen in  FIG. 66 , where there is seen a pair of bipolar pacing electrodes  172  and a pair of bipolar EKG electrodes or sensors  174 . The pacing electrodes  172  and the EKG electrodes  174  are connected to a pulse generator and monitor, respectively, in the well-known manner. 
   Lesion  4  connects the lesion  1 , which surrounds the right pulmonary veins, to the mitral valve annulus (MVA). It may be made through the same incision and purse-string suture used for making lesion  3 . With reference again to  FIG. 52D , the jaws of the grasper are merely rotated down so that the distal end of the jaw overlies the mitral valve annulus. 
   When making lesion  4 , care must be exercised in locating the grasper jaws so that the electrodes, when RF energy is applied, do not damage the mitral valve leaflets. It is known that the electrical signals generated by atrial tissue differ from the electrical signals generated by ventricular tissue. Consequently, the distal tip of one of the jaw members of the grasper includes an EKG sensor so that the EKG of the tissue contacted by the tip of the grasper can be monitored. 
   As best seen in  FIGS. 66 and 67 , the distal tip of the fixed jaw  116  includes a pair of laterally-opposed bipolar EKG electrodes or sensors  168  spaced slightly distally from the distal-most end of the electrode  120 . The sensors  168  are connected to conductive leads  170  ( FIG. 33 ) that are adapted to be connected to an EKG monitor (not shown) to provide a display of the EKG. Thus, as the jaws of the grasper are rotated downwardly after making lesion  3 , the surgeon can constantly monitor the EKG, looking for the change from an atrial EKG to a ventricular EKG, to facilitate accurate placement of the jaw tip on the mitral valve annulus, and away from the mitral valve leaflets. 
   It may also be desirable to make a lesion between the superior vena cava (SVC) and the inferior (IVC). This may be created in two steps, in which lesions  5  and  6  are made. With reference to  FIG. 52E , an incision with purse-string suture is made approximately midway between the SVC and IVC, with one of the jaws of the grasper being inserted into the incision so as to have its end adjacent the base of the SVC. The lesion  5  is formed and then the instrument is rotated 180° as shown in  FIG. 52F , to make lesion  6 . Lesion  7  may conveniently be made through the same incision and purse-string suture as lesions  5  and  6 , as shown in FIG.  52 G. Lesion  7  extends from between the SVC and IVC out toward the right atrial appendage (RAA). 
   A lesion  8  is made between the right atrial appendage and the tricuspid valve annulus (TVA) utilizing an incision and purse-string suture made in the RAA, as illustrated in FIG.  52 H. Lesion  8  is made on the opposite side of the right atrium as lesion  7 , and thus is shown in dotted line in  FIG. 52A. A  lesion  9  may also be made circumscribing the right atrial appendage so as to intersect both lesions  7  and  8 . As shown in  FIG. 52I , lesion  9  is made epicardially. A similar epicardial ablation circumscribing the left atrial appendage is designated  10  and illustrated in FIG.  52 J. 
   A final lesion  11  is illustrated that connects lesion  10  circumscribing the left atrial appendage with lesion  2  that circumscribes the left pulmonary veins. As illustrated, the lesion  11  is made utilizing an incision and purse string suture through which the grasper jaw is introduced, the incision being located in the left atrial appendage beyond the lesion  10 . 
   In a further embodiment, the present device consists of two long, linear, wire-type electrodes, which are in parallel relationship to each other, each approximately 1 mm in diameter, and 50 mm long. The electrodes are insulated along their entire surface with a thin layer of high dielectric material such as polyamide, except for a thin strip of electrically conductive material that runs along the length of each electrode, in face-to-face relationship with each other. The electrodes are comprised of a high modulus material, such as tungsten or carbon fiber. 
   One of the electrodes is designed to be introduced into the interior of a hollow organ through a small puncture wound in the wall of the organ. The second electrode is introduced on the opposite side of the hollow organ wall. The device incorporates a mechanism for advancing each electrode individually, or both simultaneously, in parallel relation with each other. The device also includes a clamping mechanism that brings the two electrodes together so that their exposed conductive surfaces are in face-to-face relation and the electrodes exert sufficient pressure to clamp the tissue. Once both electrodes have been advanced to their desired positions, the clamping mechanism is activated which brings the two wires together, and clamps the tissue between the two exposed electrode surfaces. RF energy is then applied between the two electrodes, and the tissue is ablated in a long, continuous, transmural line. A monitoring device measures the voltage, current, impedance, and/or temperature between the two electrodes, and an algorithm determines whether the tissue is fully ablated. 
   This device provides a way to achieve and verify a fully transmural and continuous line of tissue ablation by locating the atrial tissue between two bipolar wire electrodes, and clamping the tissue. The forceps consist of two electrode pads of opposite polarity designed to grasp and clamp tissue. A well-known method of determining the status of the tissue between the electrode pads is to monitor the current, voltage, and impedance of the tissue, as done using the Richard Wolf generator for bipolar forceps. It is well known in the art that the ablative status of tissue clamped between two bipolar electrodes can easily be determined by monitoring the increase in tissue impedance as the tissue desiccates. 
   This device is to be used with an RF generator that monitors current, voltage, and impedance to determine the state of tissue ablation of the tissue compressed between the inner and outer electrodes. The RF generator will be equipped with an indicator which informs the user of the status of the clamped tissue, and when ablation is complete (i.e., transmural along the entire length of the electrodes). 
   This device provides the capability of creating long, transmural lesions through atrial wall tissue of varying thickness because it employs an active bipolar electrode on each side of the atrial wall, and the ablation proceeds from both the inside and outside of the atrial wall. The device is also unique in that the electrodes are used to compress the tissue to be ablated. This compression is critical because the inside and outside surfaces of the atrium can have irregularities, and a high clamping pressure insures that both electrodes are making good contact with tissue along the full length of each electrode. Clamping the tissue also reduces the distance between the electrodes, and makes the ablation more efficient because the electrical energy is more concentrated. Because of this higher concentration of energy, lower powers and temperatures can be used to achieve complete ablation, and the process is considerably faster. 
   As an example, to fully ablate a 5 mm deep lesion, 30 cm long can take several minutes with an endocardial catheter electrode array, and the temperatures can be as high as 80 to 90 degrees centigrade at the tissue surface with the generator power as high as 40 to 50 watts. In benchtop testing of the present invention in animal hearts, a fully transmural 30 mm line through 5 mm of tissue was achieved in 5 seconds at 20 watts. 
   With reference to  FIGS. 53-54 , a further embodiment of the device is shown. The device consists of an inner wire electrode wire electrode  201 , an outer wire electrode  202 , an inner slider button  203 , an outer slider button  204 , and a clamping slider tube  205  and button  206 . The device body  207  houses the wire electrodes, slider tube and buttons, connector wires  207   a  and  208 , and bipolar connector  209 . The device may also include slit needle introducer tip  210 . 
   The operation of the device begins by advancing the inner electrode wire  201  by advancing the slider button  203 . Once the inner electrode  201  is advanced to the desired length, the outer electrode  202  is advanced by advancing slider button  204 . Note that further advancement of slider button  204  also advances slider button  203 , so that both electrodes  201  and  202  advance simultaneously. Because of the bend  202   a  in the electrode wire  202 , and the notch  205   a  in the slider tube assembly  205 , the slider tube advances along with the outer electrode  202 . Once both electrodes are advanced to the desired length, the slider tube  205  is advanced so that the end  205   b  of the slider tube  205  contacts the arcuate wire segment  202   b  of electrode wire  202 . Further advancement of slider tube  205  acts to compress the electrode wires  201  and  202  together along the entire effective length L. 
     FIGS. 55 and 56  show two types of electrode wires, a piercing tip (FIG.  56 ), and an obturator, or blunt tip (FIG.  55 ). The electrodes may be similar in construction to those shown in  FIGS. 2-6 , which are described above.  FIG. 57  shows a side view of the instrument tip. 
     FIG. 58A  shows the instrument used to penetrate the wall of a hollow organ, such as the heart. The slit needle  210  penetrates tissue through the wall of the atrium  218 . In  FIG. 58B , the inner wire electrode  201  is advanced through the puncture wound into the interior of the atrium. In  FIG. 58C , the outer needle  202  is initially advanced onto the external surface of the atrial wall  218 .  FIG. 58D  shows the inner  201  and outer  202  needles as they are simultaneously advanced along the inner and outer surfaces of the atrial wall  218 .  FIG. 58E  shows the pusher tube  205  advanced to compress the tissue of the atrial wall  218  at location  219 . RF energy is then applied between the conductive strips  212  on each electrode to ablate the compressed tissue  219 .  FIG. 58F  shows section B—B of  FIG. 58E , with the inner  201  and outer  202  electrodes compressing the tissue  219 . The area of ablated tissue is shown as  220 . The alternate electrode configuration of  FIG. 5  is shown in FIG.  58 G. Blood cells are represented as  221 . 
   The compression of the tissue is important because it insures that the exposed electrode surface is not in contact with any tissue or blood except the clamped tissue to be ablated. Referring to  FIGS. 58F and 58G  one can see that the clamping of the tissue between the electrodes insures that only the conductive area is in contact with the clamped tissue. Especially important is avoiding any contact between the conductive area of the electrode and blood in the atrium. Contact between an active electrode and blood in the atrium is major cause of thrombus formation in ablation procedures. The compressed tissue acts to isolate the electrically active surface, and prevents inadvertent energy delivery to other parts of the heart or blood. The outside temperature of the electrode can easily be monitored to insure that the temperature of the insulation in contact with blood remains below a critical temperature (40° C., for example). 
     FIG. 59  shows a potential series of continuous transmural ablation lines  222  located around the pulmonary veins  223  in the left atrium  224 . A series of puncture wounds  225  are shown as one means to achieve the pattern of ablation lines (shown in dot-dash lines). 
     FIG. 60A  shows a method for achieving a circumferential lesion in a pulmonary vein  223 . The inner needle  201  is a piercing tip as shown in FIG.  56 . The needle is advanced completely through the wall of the pulmonary vein until it exits the vein. In  FIG. 60B , the outer electrode  2  is advanced parallel to the inner electrode  201 . In  FIG. 60C , the electrodes are compressed, and the compressed vein wall tissue  226  is ablated by applying RF energy between the two electrodes. In  FIG. 60D , the electrodes are released, and the vein wall tissue  226  returns to its original shape.  FIG. 60E  shows the outer electrode  202  retracted back into the instrument body, and the instrument is rotated 180 degrees about the axis of electrode  201 . 
     FIG. 60F  shows the outer electrode  202  advanced along the opposite side of the pulmonary vein from the ablated tissue  220 . In  FIG. 60G , the electrodes are compressed, and the compressed vein wall tissue  227  is ablated by applying RF energy between the electrodes.  FIG. 60H  shows the position of the electrodes with the pusher tube retracted, and the fully circumferential lesion  220 .  FIG. 60I  shows the instrument retracted from the vein, and the circumferential lesion of ablated tissue  220 . 
     FIGS. 61A-61J  show the instrument used in a method to create a circumferential lesion around a pair of pulmonary veins  226  and  227 . In  FIG. 61A  the inner electrode  201  is advanced into the side of the atrial wall  218 , just below the ostium of the pulmonary vein  226  by advancing slider button  203 .  FIG. 61B  shows electrode  201  and slider  203  fully advanced, and exiting the atrial tissue  218  just below the ostium of pulmonary vein  227 .  FIG. 61C  shows outer electrode  202  advanced fully in parallel and to the same length as inner electrode  201  by advancing slider  204 . Note that slider tube button  205  has advanced to its intermediate position. 
     FIG. 61D  shows slider button  205  fully advanced, which clamps electrodes  201  and  202  together just below the ostia of the pulmonary veins on the side of the veins indicated by tissue surface  218   a , and compresses the atrial wall tissue. RF energy is then applied between the two electrodes, and the clamped tissue  219  is ablated. In  FIG. 61E , electrode  202  is retracted by retracting slider button  4 . The line of ablated tissue is shown as  219   a . This line of ablated tissue  219   a  will be completely continuous and transmural, and connect inner needle entry point  229  with inner needle exit point  230  and extend along the side of the atrial wall. 
     FIG. 61F  shows the device body  207  rotated 180 degrees about the axis of the inner electrode  201  so that the atrial surface  218   b  on the opposite side of the pulmonary veins is exposed.  FIG. 61G  shows slider button  204  and outer electrode  202  advanced over the opposite surface of the atrium  218   b .  FIG. 61H  shows slider button  205  advanced, and the electrodes  201  and  202  clamping the tissue  219   b  just below the ostia of the pulmonary veins  226  and  227  along atrial wall  218   b . RF energy is then applied between the electrodes  201  and  202  to ablate the compressed tissue  219   b . In  FIG. 61I  the slider button  205  is retracted, and the electrodes release the tissue  219   b . The outer electrode is then retracted, exposing the tissue  219   b  that is now fully ablated as indicated by the line  219   b .  FIG. 16J  shows a top view of  FIG. 61I  showing the continuous line of ablated tissue surrounding pulmonary veins  226  and  227 , connected by entry point  229  and exit point  230  of internal electrode  201 . The electrode is then retracted, leaving a continuous transmural lesion that electrically isolates the pulmonary veins from the rest of the atrial tissue. 
   In another embodiment of the invention, a penetrating compressive/tensile electrode is used. Once the jaws are positioned below the ostia of the pulmonary veins, the tissue is partially clamped, allowing continued flow from the pulmonary veins to the left atrium. An electrode needle is introduced which enters the left side of the atrial tissue and exits on the right side into a tip securing point on the lower jaw. This point will prevent the tip from moving axially when a needle is pushed. The lower atrial tissue can be compressed by “pushing” on the needle with a force that compresses tissue between the needle electrode and the lower jaw electrode. Bipolar RF energy is then applied between the needle and lower jaw electrodes to ablate a line of tissue from the needle entry to exit point. Once the lower atrial tissue has been ablated, the upper jaw is moved down to contact the tip of the lower jaw. Note that this still provides an open lumen for blood flow from the pulmonary veins to the left atrium. The needle is rotated 180 degrees on its axis so that the electrode surface faces up. The needle is then “pulled” to create tension, and exert a compressive force that compresses tissue between the needle electrode and the upper jaw. Bipolar RF energy is then applied between the needle electrode and upper jaw to ablate the tissue. Note that the partial closing of the upper jaw to contact the tip of the lower jaw could be done prior to compressing the lower atrial tissue. 
   With reference to  FIGS. 62A-62I  the clamping apparatus as generally described above is shown. As illustrated, the device is a “pliers type” apparatus. The device is shown clamped around the atrial tissue below the ostia of the pulmonary veins. In  FIGS. 62B-62D , an electrode needle is advanced through the atrial tissue to contact a receiver at the tip of the device.  FIG. 62E  shows one method of clamping the tissue to a rigid needle electrode, using a non-rigid outer clamping member that flexes either by further motion of the handle as shown or by further extension of the electrode member.  FIG. 62F  shows both sides of the clamping member flexed, and the tissue compressed between.  FIG. 62G  shows the position of the clamping members and electrode prior to tissue clamping.  FIG. 62H  shows these positions during tissue clamping. Bipolar RF energy is applied between the clamping members, and the inner electrode to ablate the atrial tissue, creating a lesion, as shown in FIG.  62 H. Note also, that if the inner electrode had only one exposed electrode surface, the tissue ablation could be carried out first on one side, then the other, without occluding the lumen between the pulmonary veins and the atrium. 
     FIG. 62I  shows another way to achieve tissue compression by advancing a relatively flexible needle electrode which bends as shown to compress the tissue between the electrode and one of the device jaws. 
   A further aspect of this invention provides three methods which may be used to gain access to ablate cardiac tissue. The first method gains access to the heart by opening the patient&#39;s chest or thoracic region. The second method gains access to the heart by intercostal incisions whereby access to the heart is achieve between the ribs. Finally, the third method utilizes a sub-xyphoid approach. In any event, each method is adapted to place the ablations at previously identified locations that require ablation. 
   The first method is illustrated in  FIGS. 68-77 . Access to the patient&#39;s heart is achieved by opening the patient&#39;s chest.  FIGS. 68-77  show a patient&#39;s rib cage R, sternum ST, xyphoid XP, coastal cartilage C, right lung RL and left lung LL. This method requires a gross thoracotomy such as where the ribs or sternum are cut in half with an anterior incision or removal of a portion of the ribs or sternum such as in the form of a median sternotomy. Percutaneous penetration is made by an initial incision into the patient&#39;s chest above the sternum. In  FIG. 68 , percutaneous penetration is made in the longitudinal direction along the patient&#39;s sternum along line  300 . Thereafter, the sternum is cut longitudinally and each side of the sternum pushed apart so as to gain access to the patient&#39;s thoracic or chest cavity  302 , as shown in  FIGS. 69 and 73 . Spreading of the sternum and ribs is achieved using a well known spreading device or the like. If the lungs are not deflated then they will need to be moved aside using clamps  304  or the like to gain access to the heart. Once access to the heart is cleared, all ablation locations must be located and identified. 
   Locating and identifying the ablation locations may be performed by one of several different instruments such as a bullet dissector  306 , which is best shown in  FIGS. 70 and 71 . The bullet dissector  306  identifies a location which requires ablation and creates a viewing and working space from which to ablate the location. The bullet dissector includes a bullet or head  308  and a body  310 . The head  308  is made of a clear or transparent material and has a plurality of irrigation holes  312 . The body  310  includes a tube  314  which supplies saline to the head  308 . As saline is supplied through the tube  314  to the head  308 , it flows out of the irrigation holes  312 , as indicated by the arrows in FIG.  70 . The body  310  of the bullet dissector may be attached to an endoscope  316  or other suitable device, which includes a light or fiberoptic cable  318  to illuminate the site being dissected. The size of the endoscope to which the bullet dissector is attached may vary although the preferred diameter range is between 2 mm to 10 mm. The diameter of the endoscope is approximately equal to or smaller than the diameter of the dissector head. The endoscope is preferably connected to the rear of the dissector head. A portion of the endoscope cable  318  may be inserted into the dissector head  308  while maintaining seal around the cable so that saline flow is unaffected. The endoscope may include a viewing lens or camera which is connected to a video monitor which displays the location which is viewed. 
   The continuous flow of saline from the bullet dissector  306  allows the saline to flush blood and other body debris from the transparent head  308  and endoscope viewing lens so as to provide a clear viewing space. As saline flows out of the head, the flow of saline pushes against the adjacent tissue and aids in dissection by, for example, separating the pericardium from the heart. The continuous flow of saline creates a working space between the dissector and the surrounding tissue and ensures that this space is positively pressurized. 
   In addition, the head  308  of the bullet dissector also may be designed to allow easy dissection around the pulmonary veins or other cardiac tissues. The head  308  is bullet shaped. It has a rounded distal tip  309  which gently separates or dissects tissue. The diameter of the head  307  may be equal to or larger than the body  310 . 
   Once the thoracic cavity is accessible, the bullet dissector  306  is inserted into the patient&#39;s chest cavity adjacent the heart, as shown in FIG.  72 . The bullet dissector  306  may locate the ablation locations over a pericardium P. Alternatively, the pericardium may be pierced so as to contact an epicardium E so as to allow the bullet dissector to directly contact the surface of the heart. The bullet dissector assists the identification and location of ablation locations. Saline exits the irrigation holes  312  of the bullet dissector  306  and insufflates the intrapericardial space around the ablation location. Insufflating the ablation location with saline creates a viewing and working space within the intrapericardial space  318  by which the ablation location may be accessed. Saline also clears blood away from the ablation location and creates a positive pressure in the working space. A positive pressure may be particularly helpful in eliminating the need for complicated hemostatic devices or valves, and/or avoiding or limiting the need to seal off the pericardial incision by a valve, balloon or other inflation device. Insufflating can be done with any compatible biological fluid: saline, contrast medium, CO2, or blood. However, saline or CO2 is preferred because these provide clarity with which to view the space and identify the area to ablate. Excess saline may exit the intrapericardial space and can be removed by a drainage tube which can be inserted into the chest cavity. 
   After the ablation locations are located and identified, the ablation or clamping device is then introduced into the patient&#39;s chest and positioned in contact with the location. The ablation device which is generally used for the open heart procedure is the grasper shown and described in  FIGS. 28-32  although use of other graspers or devices is also contemplated. The ablation device also may have a shape which is particularly suited to the ablation location to be contacted. During insertion of the ablation device, the bullet dissector preferably remains inserted so as to maintain the positively pressurized working space and to assist in visualizing the ablation device. Thereafter, the location is ablated using an RF energy ablation technique as disclosed herein. 
   In  FIG. 73  cardiac tissue is ablated from both the right and left sides.  FIG. 73  shows ablation of the left atrium in the region of the right and left pulmonary veins, RPV and LPV, respectively. The right pulmonary veins RPV are ablated using a grasper  319  such as the one shown and described in  FIGS. 28-32 . The left pulmonary veins LPV are ablated using a similar grasper  321  which is rotated 180 degrees. Jaw members of each grasper  319 ,  321  may be curved and oriented such that the concave portion of the jaw members face the pulmonary veins and the convex portion of the jaw member face away. This orientation is preferred to provide effective clamping and ablation of the atrium near the pulmonary veins. Because the patient&#39;s chest is open, the graspers may be in the form of open scissors such as, for example, the grasper shown in  FIGS. 28-32 , or an ablation device with sliding jaws generally illustrated in  FIGS. 1-27  or of the type shown in  FIGS. 33-37 . It is contemplated that different ablation devices may be used to effectively ablate different cardiac tissues. 
     FIG. 74  shows eleven different locations that may be ablated in treating atrial fibrillation according to any of the methods described herein. By way of example,  FIGS. 75-76  show ablation of an epicardial surface adjacent the left and right pulmonary veins, LPV and RPV, respectively. In  FIGS. 74-77 , the referenced portions of the heart include the right atrial appendage RAA, left atrial appendage LAA, left atrium, superior vena cava SVC, inferior vena cava IVC. 
     FIG. 77  shows bilateral ablation near a pair of pulmonary veins  320  by creating an ablation lesion  322  on the left atrium  324 . Bilateral ablation creates a circumferential ablation lesion on the atrium which isolates the pair of pulmonary veins. 
     FIGS. 78-86  illustrate the second method which achieves access to the heart using a percutaneous intercostal penetration with like parts referenced with like letters and numbers. This method is less invasive than a gross thoracotomy. In  FIG. 78 , one or more percutaneous intercostal penetrations may be made in one or more access locations  336 ,  338  between the ribs. The direction and location of the percutaneous intercostal penetrations will depend on where the cardiac tissue to be treated is located, so that ablation of the cardiac tissue is accessible by an appropriate ablation device. For example, entry through access locations  336  is helpful in achieving access to ablation locations located on the right side of the heart and, likewise, access locations  337  achieve access to the left side of the heart. For most, if not all, intercostal penetrations the lung may be temporarily deflated to allow easier passage of the instrument through the pleural space PS, although such deflation may not be absolutely necessary. One or more suitable access devices may be used to obtain access to the intercostal spaces between the ribs. These devices may be used to protect the incision location and minimize trauma thereto and may include, but are not limited, to trocar sleeves, ports, or other types of percutaneous access cannulae. By way of example, not limitation,  FIGS. 79-86  shows a supporting ring  338  with tensioning members  340 . 
   Once the intercostal penetration is made, the bullet dissector is introduced through the incision, as shown in FIG.  79 . Turning to  FIG. 80 , the pericardium P is preferably pierced or punctured so as to allow the head  308  of the bullet dissector  306  into an intrapericardial space  342 .  FIGS. 81-83  illustrate intercostal ablation where the right lung RL is deflated and the left lung LL remains inflated. As a result, the bullet dissector traverses the pleural space PS on its path to the heart. The bullet dissector insufflates the intrapericardial space  342  with saline and creates a clear working space which is positively pressurized in relation to the pressure outside the pericardium. The dissector is preferably attached to an endoscope or like device which allows for viewing of the ablation location and is inserted into the intrapericardial space  342  until an ablation location is located. Then the ablation device  344  is inserted into the incision and advanced to the ablation location. 
     FIG. 83  shows the ablation device  344  ablating cardiac tissue adjacent the right pulmonary veins RPV. By way of example but not limitation, the ablation device for the intercostal and sub-xyphoid methods may be the graspers shown and described herein in  FIGS. 33-37  or  FIGS. 63-65 . Different ablation devices may be used for reaching different areas of cardiac tissue. The ablation devices may have different curvatures for reaching certain portions of the heart. For example, an ablation device for the left atrium adjacent the left pulmonary veins may have a different curvature than the ablation device for the right atrium adjacent the right pulmonary veins.  FIGS. 83A and 83B  show right and left curved graspers  350  and  352 , respectively, which are similar to the grasper in  FIGS. 33-37  except that the graspers in  FIGS. 83A and 83B  have right and left curved jaw members  354  and  356 , respectively. Both the right and left curved graspers  350  are helpful in ablating cardiac tissue adjacent the right and left pulmonary veins depending on what direction of approaches are used. Likewise, the left curved grasper  352  assists in ablating cardiac tissue adjacent the left pulmonary veins. Other shapes of ablation devices will be apparent to those skilled in the art. It is noted that the intercostal and sub-xyphoid methods preferably will utilize ablation devices having a long handle such that when the instrument reaches the ablation location a gripping portion of the device remains outside the entry location into the patient for control by the doctor. 
   Using the intercostal method, transmural ablations may be performed on other cardiac tissue using RF energy, as previously described herein. Ablation is repeated until all ablation locations have been treated. Ablation devices may be inserted into different access locations  336  depending on which directional approach is preferred for the cardiac tissue which requires ablation. Different ablation devices may be inserted depending on which cardiac tissue requires treatment. Once all areas are treated on the right side in  FIG. 83  with the appropriate ablation device, the right lung is re-inflated and the access location  336  is closed. The method is repeated for the left side if necessary through access location  337 .  FIG. 84  shows ablation of cardiac tissue adjacent the left pulmonary veins LPV. 
     FIGS. 85 and 86  show intercostal ablation which occurs while both lungs are inflated. It may be necessary to use clamps  346  or the like to pull portions of the lung away from the heart and provide a pathway for advancing the bullet dissector  306  and the ablation device  344 . While both lungs remain inflated, it is possible that cardiac tissue may be treated from both sides of the chest, at the same time, with one dissector  306  and one ablation device  344  within the access location  336  and another dissector and ablation device within the access location  337 . 
   In the sub-xyphoid method, the initial incision is made beneath the rib cage R, preferably between the xyphoid XP and the adjacent costal cartilage C. In  FIGS. 87-91 , like parts are again shown with like letter and number.  FIG. 87  shows access locations  358  for the incision. In  FIGS. 88-91 , the ablation device  360  is advanced through the percutaneous sub-xyphoid penetration to the aberrant cardiac tissue which requires ablation. In  FIGS. 88-90 , the bullet dissector  306  may then be inserted through a percutaneous intercostal penetration  362  to identify the ablation location and assist in guiding the ablation device. Insufflation from the dissector  306  creates the positively pressurized and clear working space. Thus, the cardiac tissue may be easy located and ablated with the ablation device  360 , as shown in  FIGS. 89 and 90 . 
   Using the sub-xyphoid approach, it also may be possible to perform ablation on both pairs of pulmonary veins at the same time either through the same incision or through separate incisions.  FIG. 91  shows ablation of the left atrium near the right and left pulmonary veins. Right and left curved ablation devices  350  and  352  are similar to those described herein in  FIGS. 83A and 83B . The ablation devices are inserted through access locations  358 , although it is also possible that both devices could be inserted through one access location  358 . Corresponding bullet dissectors  306  may be inserted through percutaneous intercostal penetrations  362  on the right and left sides, or the bullet dissectors may be inserted through a sub-xyphoid access location  358 . 
   In  FIGS. 92 and 93 , the bullet dissector and ablation device are both inserted using a sub-xyphoid approach. The bullet dissector and ablation device may be separate as previously described or they may be combined into a single instrument for this, as well as the prior methods.  FIGS. 92 and 93  show ablation by a combined dissector and ablation instrument  364 . In  FIG. 94 , the combined instrument  364  includes a proximal end  366  which includes a bullet dissector head  368  and an endoscope  370 . The proximal end  366  is carried by or mounted to an ablation device. Although the ablation device which is shown is similar to the grasper shown in  FIGS. 63-65 , other devices are possible. The ablation device includes grasping jaws  372 , an elongated handle  374  and handle members  376 . A saline irrigation tube  378  is carried by the handle  374  and supplies saline to the bullet dissector head  368 . The combined dissector-ablation instrument  364  may be used for any of the above methods. Use of the instrument  364  in the intercostal method is illustrated by way of example in FIG.  95 . 
     FIGS. 96-101  illustrate using the sub-xyphoid approach where the bullet dissector and ablation device are separate instruments through a single access location  380 . An incision is first made in the sub-xyphoid region at the access location  380  near the tip of the xyphoid, as illustrated. This access location  380  is preferred because it allows access to both sides of the chest for treatment as needed. The size of the incision is about 1 cm. Then the bullet dissector  306  is inserted through the access location  380  and advanced to the pericardium P. The bullet dissector is preferably attached to an endoscope as previously described to aid in visualization. Once the bullet dissector proceeds to the appropriate point on the pericardium, a suitable cutting instrument such as a endoscope knife or scissors is introduced alongside the bullet dissector to create a small incision  382  in the pericardium just large enough for the head of the bullet dissector. The bullet dissector  306  is then inserted into the intrapericardial space  342  and advanced through the intrapericardial space, dissecting the pericardium from the heart as it advances, to the cardiac tissue to be treated. 
   For example, as shown in  FIGS. 98-99  the bullet dissector  306  is advanced within the intrapericardial space to the confluence of the left atrium and the pair of left pulmonary veins LPV. It is expected that minimal dissection of tissue will be performed by the bullet dissector around the left pulmonary veins whereas more tissue dissection will be required around the right pulmonary veins RPV. During dissection the bullet dissector will be advanced both above and below the atrium, the pulmonary veins and the confluence where the pulmonary veins meet the atrium to provide a working space which, as described above, is positively pressurized by insufflation. Creation of the working space allows for insertion of the jaws of the ablation device and aids in visualizing the cardiac tissue which requires treatment. 
   Once dissection is completed, an ablation device  384  may be inserted through the sub-xyphoid access location  380 . The ablation device may be inserted into the intrapericardial space alongside the bullet dissector  306  either by enlarging the pericardial incision using a suitable cutting device or by causing the pericardial incision to stretch or widen to accommodate the ablation device. It is also possible to remove the bullet dissector  306  and then insert the ablation device, although use of the bullet dissector in combination with the ablation device is preferred. The ablation device is advanced to the cardiac tissue to be treated such as, for example, the confluence of the atrium and the pulmonary veins. 
   With the aid of the bullet dissector and endoscope, the tissue to be treated is identified. Within the working space created by the bullet dissector, one jaw of the ablation device is positioned below the pulmonary veins and the other jaw is positioned above the pulmonary veins. The ablation device is properly positioned around the tissue to be treated and then clamped. Verification may be performed to ensure that the clamped tissue between the jaws of the ablation device is the location which requires ablation. The bullet dissector may help visualize and confirm that the appropriate cardiac tissue is being treated. The curved jaw members of the ablation device should be oriented such that the concave portion faces the pulmonary veins and the convex portion faces the atrium. 
   Ablation of the cardiac tissue is performed by using RF energy to create an ablation lesion, as previously described. The sub-xyphoid approach may be used to create ablations as shown in  FIGS. 74-77 . For example,  FIG. 101  shows the ablation device ablating the left atrium near the left pulmonary veins which creates a circumferential ablation lesion similar to the one shown in FIG.  77 . Thereafter the jaws of the ablation device are unclamped and moved to another location which requires ablation. 
   The sub-xyphoid method can be repeated as necessary to create additional ablations. For example, a second circumferential ablation may be created around the right pulmonary veins. When using an ablation device similar to the grasper shown in  FIGS. 63-65 , ablation of a second pair of pulmonary veins may be performed by rotating the grasper 180 degrees so that the concave portion of the jaw members faces the pulmonary veins. In the case of the grasper in  FIGS. 33-37 ,  83 A and  83 B, it may be necessary to use right and left curved ablation devices when using the sub-xyphoid approach so that the concave portion of the jaw members faces the appropriate pair of pulmonary veins. For example, the right curved grasper  350  may be used to treat the atrium near the right pulmonary veins and then withdrawn through the sub-xyphoid access location  380 . Thereafter, the left curved grasper  352  may be inserted through the access location  380  and used to ablate the atrium near the left pulmonary veins. 
   During ablation drainage of excess fluid may be performed by inserting a drainage tube through the sub-xyphoid access location  380 . Once all ablation locations are treated, the ablation device and bullet dissector are withdrawn and the sub-xyphoid access location  380  is closed using well-known techniques. It is not necessary to suture the pericardium incision. 
   Importantly, each method may be performed with a beating heart or with the heart stopped. Where the heart is stopped, it is understood that the patient must be connected to a cardio-bypass machine. The methods may be performed either with the lungs inflated or deflated. If the lung is deflated, then the patient must be connected to a respirator. It is realized that where both the heart is stopped and the lung deflated, then the patient will be connected to a cardiopulmonary bypass system. Where the lung is deflated, it may be preferred to deflate only one lung at a time before proceeding to deflate the other lung. For example, where cardiac tissue adjacent each pair of right and left pulmonary veins requires treatment, either the right lung or the left lung may be deflated, but not both. After all cardiac tissue requiring ablation is treated on the deflated side, the lung is re-inflated and the process is repeated for the other side of the heart. 
   Each method may be used to create transmural ablation as previously described herein for epicardial or endocardial surfaces. For epicardial ablations, an incision is usually made into the pericardium to access the epicardial surface. Although it is possible to perform ablations without penetrating the pericardium, penetration into the intrapericardial space, the space within the pericardial cavity between the pericardium and the epicardium, may allow for better positioning of the ablation device for ablation of the identified cardiac tissue. Where the ablation device is inserted into the intrapericardial space, it is advanced to the location which has been identified as requiring ablation. The ablation device directly contacts the epicardial surface of the heart relating to this location and the location is ablated. Methods of epicardial ablation for creating lesions around the pulmonary veins are disclosed in U.S. Pat. No. 6,161,543, which is incorporated herein by reference. 
   For endocardial ablation, an incision is made in the heart wall by a blade on the tip of the ablation device or by other suitable instrument. The incision in the heart wall may be treated with an appropriate suture such as a purse string suture  327  shown in FIG.  76 . Then the dissector and endoscope can locate and identify the location to be ablated. Once the location is identified, one of the jaws of the ablation device or grasper is inserted into the heart to directly contact the endocardial surface which requires ablation. After ablation, the jaw of the ablation device, is withdrawn back through the incision. The process is repeated until all locations are treated. 
   Thus, it can be seen that a transmural ablation device and method have been provided that overcome the limitations of the prior art. First, current technology involves ablation devices deliver ablation energy to either the inside (endocardium) or outside (epicardium) of the heart. Using these techniques, the tissue ablation proceeds from one wall of the heart through the tissue to the opposite wall. To date, there has been no reliable way to consistently achieve lesions that penetrate the full thickness of the atrial wall (transmural lesions), and there has been no way to determine either continuity or transmurality of these lesions. If the lesion does not penetrate through enough of the atrial wall, conduction can still occur, and the lesion does not fully block the electrical signals that are causing the arrhythmia. Using an endocardial approach, if the lesion penetrates too far through the wall, critical structures such as coronary arteries, veins, or the esophagus can be damaged on the outside of the heart. Using an epicardial approach, if the lesion penetrates too far, blood can be coagulated, or critical structures such as valves, nodes, or connective tissue can be damaged on the inside of the heart. 
   There has also been no reliable and consistent way to safely achieve fully continuous, long (greater than 1 cm) lesions in the atrial wall without a high risk of thrombus, damage to critical structures, or extensive damage to the atrial tissue. 
   The present invention overcomes these shortcomings because the conductive area of each electrode is very narrow compared to the width of the clamped area. As a result, the thermal damage to the tissue is minimal. In contrast, current technology uses catheter electrodes which are typically 1 or 2 mm diameter requiring a lesion width of almost 8 mm to achieve a depth of 5 mm. Using the present invention, a lesion depth of 5 mm with a width of less than 2 mm can be achieved. This aspect of the invention allows for longer linear lesions with less power delivery because less tissue is being heated. There is, therefore, considerably less damage to healthy atrial tissue for a lesion of a given depth and length. Recent efforts in creating linear lesions using endocardial electrodes resulted in ablation of over 20% of the atrial endocardium, and a commensurate decrease in atrial contractility. 
   Another advantage of this device is that ablation can be done on a beating heart. Using a high modulus material such as tungsten or carbon fiber would allow a minimum diameter, and a maximum clamping pressure for a given clamping length. Once the device is clamped onto the atrial wall, the position of the electrodes can be verified by visually inspecting the position of the outer electrode before delivery of RF energy. If the clamping pressure is higher than the atrial pressure, then clamping over a coronary artery would cut off blood flow, and the resulting change in the EKG would act as a warning to the user prior to applying RF energy. The clamping will prevent any movement of the electrodes relative to the heart wall, and RF energy can be applied with confidence that the ablated tissue will be contained completely between the two electrodes. 
   Another important feature of this device is that the energy transfer is limited to the tissue clamped between the two electrodes. The insulated electrodes protect structures on the outside of the heart from being exposed to RF energy. Because of this limitation of current flow, damage to critical structures can be avoided. 
   Another advantage of this device is that it can easily be adapted to a minimally invasive thoracoscopic approach. The device shown has been reduced to a 5 mm diameter device, and can probably be reduced to 3 mm or less. Using video thoracoscopy, the device could be introduced through a small intracostal incision, and used to create fully transmural linear lesions on a beating heart, possibly under local anesthesia on an anesthetized patient. 
   Accordingly, a device and method for performing transmural ablation has been provided that meets all the objects of the present invention. While the invention has been described in terms of certain preferred embodiments, there is no intent to limit the invention to the same. Instead it is to be defined by the scope of the appended claims.