Abstract:
A method for transmural ablation using an instrument containing two electrodes or cryogenic probes. A clamping force is exerted on the two electrodes or probes such that the tissue of the hollow organ is clamped therebetween. Bipolar RF energy is then applied between the two electrodes, or the probes are cryogenically cooled, thus ablating the tissue therebetween. A monitoring device measures a suitable parameter, such as impedance or temperature, and indicates when the tissue between the electrodes has been fully ablated.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of provisional application Ser. No. 60/200,072, filed Apr. 27, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    A trial 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.  
           [0003]    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.  
           [0004]    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.  
           [0005]    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.  
           [0006]    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.  
           [0007]    The application to the inside of the heart chamber of either RF energy or ultra-low temperature freezing 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.  
           [0008]    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.  
           [0009]    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.  
           [0010]    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.  
           [0011]    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.  
           [0012]    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.  
           [0013]    Recently, epicardial ablation devices have been developed which apply RF or microwave 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.  
           [0014]    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.  
           [0015]    Published PCT applications WO 99/56644 and WO 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, this backplate is not intended to ablate tissue. Instead, the ablation begins at the electrodes within the endocardium, and travels through the heart tissue either to another endocardial electrode, or to the backplate. Tissue ablation proceeds from 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.  
           [0016]    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. Furthermore, when performing ablation on a beating heart, either endocardially, or epicardially, it is very difficult to stabilize the ablation element in one position relative to the moving heart tissue.  
           [0017]    Accordingly, it is the object of the present invention to provide an improved method for making transmural ablations to heart tissue.  
           [0018]    It is a related object to provide a method for making transmural ablation in heart tissue that minimizes unnecessary damage to the heart tissue.  
           [0019]    It is a further object to provide a method for making transmural ablation in heart tissue that creates continuous lesions in a single step.  
           [0020]    It is a further object of this invention to provide a method to stabilize the ablation element relative to the tissue being ablated during a beating heart procedure.  
           [0021]    It is a further object of this invention to provide an improved method for minimizing the amount of ablated tissue and maximizing current density.  
           [0022]    It is a further object of this invention to provide a method for rapidly creating a lesion around the pulmonary veins without excessive clamp time.  
           [0023]    It is a further object of this invention to provide a method for creating a series of lesions to isolate one or both pulmonary veins without stopping blood flow through the veins.  
         SUMMARY OF THE INVENTION  
         [0024]    These objects, and others which will become apparent upon reference to the following detailed description and attached drawings, are achieved by a method that creates an electrically isolating line of ablated tissue on a hollow organ or vessel that includes the steps of bringing the opposing walls of the organ into contact with each other along a line and passing sufficient electrical current through both walls along the line of contact to form a continuous line of ablated tissue through both walls of the tissue in a single step without bonding the walls together.  
           [0025]    More specifically, the invention comprises a method for at least partially electrically isolating a pulmonary vein from a heart atrium comprising the steps of clamping together the opposed walls of the atrium proximal to the pulmonary vein so that the opposed walls are in contact with each other along a line. The tissue in both walls along the line of contact is then ablated to form a corresponding line of ablated tissue without bonding the walls together. Clamping of the tissue insures that the ablation element is fixed with respect to the tissue intended for ablation. Clamping of the tissue between the RF electrodes minimizes the tissue being ablated because the volume of the tissue in a direct path between the electrode is minimized. The energy is more concentrated because the two poles are close together. These steps are performed a plurality of times, each time partially circumscribing the pulmonary vein, with the steps being performed a sufficient number of times to form a series of lines that substantially fully circumscribe the pulmonary vein. Preferably, the method is performed in a single step, with a continuous lesion being formed that circumscribes one or both of the pulmonary veins. If one or both pulmonary veins are isolated in a single step, the blood flow through the veins is cut off. If the blood flow through the pulmonary veins is cut off for several minutes, severe complications can result as a result of the blood stasis, including clotting within the veins. The delivery of RF energy through the clamped tissue is extremely rapid, typically less than 30 seconds, because the volume of ablated tissue is minimized, and the energy concentration is maximized. If a sequence of overlapping lesions is created to isolate the pulmonary veins, then the blood flow is not cut off, and the problems associated with the blood stasis are avoided.  
           [0026]    In a still further method of the invention, a clamping device with opposing jaws moveable between open and closed positions is provided, and either the atrium or pulmonary vein is positioned between the opposing jaws of the clamping device. The jaws are then closed to form a clamping area that has a width so that the opposed walls of the atrium or pulmonary vein come into contact along the clamping area, with the clamping area being expressed of blood. The tissue clamped between the jaws is then ablated along a line that is substantially narrower than the clamping area to create a line of ablated tissue through both of the walls of the atrium or pulmonary vein without bonding them together. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]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.  
         [0028]    [0028]FIG. 2 is a cross-section of an ablation element for use in the present invention taken along lines  2 - 2  of FIG. 1.  
         [0029]    FIGS.  3 - 6  show alternate configurations for the ablation elements of FIG. 2.  
         [0030]    [0030]FIG. 7 shows a further step in the inventive procedure in which tissue is clamped between the ablation elements.  
         [0031]    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.  
         [0032]    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.  
         [0033]    FIGS.  18 - 22  show a further procedure in which a transmural lesion is made so as to circumscribe a single pulmonary vein.  
         [0034]    FIGS.  23 - 27  illustrate a further procedure in which a transmural lesion is made so as to circumscribe both pulmonary veins.  
         [0035]    [0035]FIG. 28 is a perspective view of a further embodiment of a grasper in accordance with the present invention showing the grasper in its “closed” position.  
         [0036]    [0036]FIG. 29 is a perspective view of the grasper of FIG. 28 with the grasper in its “open” position.  
         [0037]    [0037]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.  
         [0038]    [0038]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.  
         [0039]    [0039]FIG. 32 is an enlarged cross-sectional view of the grasper jaws for the grasper of FIG. 28. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]    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 jaws 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).  
         [0041]    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.  
         [0042]    Once the lower jaw is positioned to span across both pulmonary veins, an upper clamp/electrode element is introduced, and the clamp assembly “J” is worked back onto the 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.  
         [0043]    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 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  and  32 . 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 .  
         [0044]    [0044]FIG. 7 shows the 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.  
         [0045]    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.  
         [0046]    [0046]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. 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.  
         [0047]    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.  
         [0048]    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.  
         [0049]    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.  
         [0050]    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.  
         [0051]    FIGS.  18 - 22  show the clamping device in a curved 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 proximal to the pulmonary vein. FIG. 22 shows the resulting ablation line  60 .  
         [0052]    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 proximal to the pulmonary veins 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.  
         [0053]    Turning to FIGS.  28 - 32 , there is seen a further version of a cardiac grasper  70  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.  
         [0054]    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.  
         [0055]    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. 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.  
         [0056]    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 between approximately 0.12-0.6 mm in width. 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. During RF energy application to the tissue to be ablated, the current and voltage applied to the tissue are measured, and the impedence calculated and stored. Based upon a function of the impedence (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. 5,503,312, 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 impedence feedback system. For example, once the impedence reaches a certain level for a certain period of time, a red light may be activated to signal that ablation is complete.  
         [0057]    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  and protrude slightly therethrough 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.)  
         [0058]    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.  
         [0059]    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.  
         [0060]    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.  
         [0061]    Another advantage of this device is that ablation can be done on a beating heart. 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. The clamping insures the electrodes will not move relative to the atrial tissue as the ablation proceeds. 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.  
         [0062]    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.  
         [0063]    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 10 mm diameter device. 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.  
         [0064]    Accordingly, a 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 methods, there is no intent to limit the invention to the same. Instead it is to be defined by the scope of the appended claims.