Abstract:
The present invention relates to a method and apparatus for creating transmural ablations in heart tissue which, for example, may include two or more electrodes adapted to be connected to opposite poles of a bipolar RF generator so as to energize the electrodes to ablate cardiac tissue between the electrodes. A first electrode may be inserted into cardiac tissue at a first location and a second electrode may be inserted into cardiac tissue at a second location which is spaced from the first location. At least one of the electrodes may be adapted to provide a sufficiently low current density in the vicinity of the electrode to avoid substantial tissue desiccation when energized. Alternatively, the apparatus may include at least one bipolar ablation electrode which comprises an elongated conductive member of spiral shape having a pitch sufficiently small to provide sufficient surface area to avoid substantial desiccation of tissue when energized by a bipolar RF generator.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a non-provisional application which claims the benefit of provisional application Ser. No. 60/561,937, filed Apr. 14, 2004, which application is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Atrial fibrillation is one of the most common heart arrhythmias in the world, affecting over 2.5 million people in the United States alone. Ablation of cardiac tissue to create scar tissue that interrupts the path of the errant electrical impulses in the heart tissue is one method for treating cardiac arrhythmia. Such ablation may take the form of one or more lines or points of ablation of heart tissue, to a series of ablations that form a strategic placement of ablation lines in, around or in proximity to one or both atria to stop the conduction and/or formation of errant impulses.  
         [0003]     More particularly, the pattern of ablation lines may be similar to the pattern of surgical lines that are created in the so-called “Maze” procedure, which was first developed as an open chest procedure. To reduce the trauma associated with the Maze surgical procedure, substantial effects have been made to achieve similar results via less invasive ablation techniques. Among other things, the Maze procedure includes a series of transmural ablations or lesions to be formed on the atrium in the vicinity of the pulmonary veins. A series of lesions that may be made during the Maze procedure are shown in various prior patents or articles such as U.S. Pat. Nos. 6,517,536 and 6,546,935, both to Hooven and assigned to AtriCure, Inc.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention relates to systems, methods and apparatuses for creating transmural ablations in heart tissue. In general terms, the apparatus may include two or more electrodes adapted to be connected to a bipolar electrosurgical RF generator so as to be of the opposite polarity. In accordance with one aspect of the present invention, the electrode or the distal end of the electrode is preferably fashioned to provide a large electrode surface area to provide relatively low current density in the vicinity of the electrode, thereby reducing the tissue desiccation and resultant high resistance that can occur in proximity to the electrode with prior art electrodes of linear or high-pitch threaded designs.  
         [0005]     An electrode embodying the present invention may take different forms, but in one preferred embodiment, the electrode comprises a relatively low-pitch helical or coil configuration of electrically conductive material that is adapted to be threaded or screwed into the myocardium of the heart. Each such electrode may optionally include at its distal end a mass or slug (also called a “sink”) of material of a highly electrically and thermally conductive material that contacts the endocardium, such as in proximity to the electrode, when the electrode is screwed into the myocardium. For purposes of this application, the term “low-pitch” electrode is intended to refer generally to an electrode that provides a relatively large surface area within the heart tissue as compared to a linear or high pitch electrode and includes, without limitation, a helical, spiral coil, screw or other shape, type or style of electrode. For purposes of this description, “spiral” is intended to include spiral, helical, coil and screw shapes, not limited to “hollow” spiral devices and “coil” and “screw” are used interchangeably and generically.  
         [0006]     In use, each electrode is inserted, such as by screw action or the like depending on the electrode shape, into the heart tissue, with care being taken to ensure that the conductive coil preferably does not completely puncture the heart wall (although the present invention is not limited to a non-puncture to a non-puncture application). The second electrode is similarly inserted into the heart tissue at a location spaced a selected distance, such as from about 1 to 4 cm from the first electrode. When attached to the RF generator, one electrode is positively charge and the other is negatively charged. When the generator is activated, bipolar RF energy flows between the electrodes through the tissue, forming a transmural (through the heart wall) ablation line extending through the thickness of the heart tissue and between the electrodes. One of the electrodes may then be withdrawn and inserted into the cardiac tissue at a selected distance from the other electrode to form another or second segment of an ablation line. After the second segment is formed, the other electrode may be removed and reinserted at a location spaced from the one electrode to form a third ablation line segment. This procedure may be repeated as needed to form the desired ablation line.  
         [0007]     The present invention may be performed epicardially by inserting the electrodes from the outside surface of the heart tissue or endocardially by inserting the electrode from inside the heart, where they may be advanced by a catheter or other instrument. Either way, the present invention lends itself to a minimally invasive procedure for forming ablation lines with comparative little trauma as compared to a Maze surgical procedure.  
         [0008]     The low-pitch electrodes of the present invention use a highly thermally conductive material and present a relatively large amount of surface area that is in contact with the tissue to be ablated. The large surface area in the illustrated embodiment is due to the low pitch coil, and may be enhanced by the use of the mass or slug mentioned earlier. Because the electrode/tissue contact area is large, and preferably maximized, the current density at the electrode/tissue interface is relatively low and, preferably, minimized. Complemented by the mass or slug structure, which serves to remove heat from the tissue surrounding the electrode, undue tissue coagulation is reduced at or near the electrode. Additionally, coagulation remote from the electrodes, that is in the line between the two electrodes, is relatively increased.  
         [0009]     In accordance with another aspect of the present invention, each electrode includes an EKG probe wire or sensor that extends distally from the electrode just beyond the termination of the conductive electrode. The distal-most tip of the EKG wire or sensor is exposed so that as the coil electrode is inserted into the heart tissue, the EKG wire is simultaneously advanced. The EKG wire exits the inner or outer surface of the myocardial tissue (depending on the direction of insertion) just before the coil electrode penetrates through the entire tissue thickness. The cardiologist or electrophysiologist can determine when the exposed tip of the EKG wire exits the myocardium by viewing the EKG signal generated by the tip of the wire. As long as the tip of the wire is within the myocardial tissue, an EKG signal is generated. When the tip exits the tissue, the EKG decreases or disappears abruptly. The cardiologist then knows that the coil of the electrode is properly positioned in the tissue, so that it extends essentially entirely through the tissue thickness without completely puncturing the wall. Alternatively, the sensor may be located on the end of the EKG electrode so that advancement of the electrode through the cardiac tissue is immediately detected with the distal end of the electrode exits the tissue. The EKG sensor may also aid the surgeon in determining the proper placement of the electrode in the heart tissue. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a perspective view of a system employing bipolar ablation electrodes according to the present invention, and more particularly low-pitch spiral or screw-shaped electrodes.  
         [0011]      FIG. 2  is an enlarged cross-sectional view of the distal ends of a pair of electrodes embodying aspects of the present invention showing the electrodes attached to heart tissue.  
         [0012]      FIG. 3  is an enlarged cross-sectional view of a first alternate embodiment of an electrode embodying a plurality of slugs or sinks.  
         [0013]      FIG. 4  is a plan view of an electrode according to the present invention including a hand-piece.  
         [0014]      FIG. 5  is an enlarged cross-sectional view of the distal end of a second alternate embodiment of an electrode employing an EKG sensor according to the present invention showing the electrode attached to heart tissue.  
         [0015]      FIG. 6  is a top view of a human heart showing a series of ablation lines that comprise a type of Maze procedure.  
         [0016]      FIG. 7  is a schematic view of a pair of pulmonary veins illustrating the process of making a continuous ablation line that circumscribes a pair of veins.  
         [0017]      FIG. 8  is a schematic view of two pairs of pulmonary veins illustrating the process for making a series of ablation lines that circumscribe each pair of pulmonary veins and connect the circumscribing ablation lines. 
     
    
     DETAILED DESCRIPTION  
       [0018]     Turning to the figures of the drawings, there is seen in  FIG. 1  an electrosurgical system, generally designated  10 , embodying the present invention. System  10  comprises a pair of substantially identical electrodes, or electrode leads generally at  12 ,  14 , each of which is adapted to be connected to one of the positive terminal  16  or negative terminal  18  of an electrosurgical bipolar RF generator  20 . The connections between the electrodes and the generator, as well as the structure of the generator itself, are well known and are not described in detail herein. Although each electrode in  FIG. 1  is preferably identical, such may not be required in accordance with broader aspects of the invention, and only one electrode embodying the present invention may be employed. There may be circumstances where electrode of different shape may be employed. Similarly, there may be circumstances where more than two electrodes are used simultaneously—for example one (or more) electrode may be connected to one terminal of the generator and two (or more) electrodes connected to the other terminal to form two or more ablation line segments at the same time.  
         [0019]     Each illustrated electrode  12 ,  14  has a proximal end  22 , which is adapted to be connected to one of the positive or negative terminals of the RF generator, and a distal end  24  that terminates in an electrode  26  that generates low current density and that is adapted to be inserted into tissue, such as that of the heart. With reference to  FIG. 2 , each electrode preferably comprises an outer tubular insulating member  28  that extends from the proximal end to the electrode  26  at the distal end. The insulating tube  28  is preferably made of a polymer such as PTFE, nylon or silicon and has an outside diameter of typically 1 mm to 4 mm, although the material and size may vary. A conductive member or wire  30  extends through the insulating tube  28  to conduct RF energy from the generator to the conductive terminal electrode  26 . Alternatively, the elongated conductor may be referred to as a lead or wire and the terminal member  26  referred to as the electrode. The terminal end electrode  26  may be separable from the elongated lead or conductor for replacement as needed.  
         [0020]     The conductive member  30  is preferably in the form of a separate copper wire, but may alternatively comprise a thin conductive film, or other conductive element or means that, for example, may extend along or within the wall of tube  28 .  
         [0021]     The low current density electrode  26  may comprise a low pitch, small diameter coil or spiral wire that is integral with or otherwise in electrical contact with the distal end of the conductive wire  30 . The coil is also preferably made of solid copper, but may also be a plated copper, such as gold plated, stainless steel, or other conductive materials such as aluminum or beryllium-copper of sufficient strength to allow it to be threaded or screwed into the cardiac tissue. To this end, the tip of the electrode may be tapered to a point for piercing the myocardium.  
         [0022]     The diameter of the wire comprising the coil or spiral is preferably about 0.01 inches (0.254 mm), but may range between about 0.005 inches (0.127 mm) and 0.025 inches (0.635 mm), while the diameter of the coil itself may be, for example, between about 0.080 inches (2 mm) and 0.20 inches (5 mm).  
         [0023]     The pitch of the coil electrode is defined as the axial distance between adjacent coils of the wire. For purposes of clarity, it should be noted that, as the number of coils increases per unit of axial length, the pitch (or distance between adjacent coils) decreases. Conversely, as the number of coils decreases per unit of axial length, the pitch (or distance between adjacent coils) increases.  
         [0024]     Low pitch for purposes of this description is preferably lower than about 3 coils per 5 mm, with the lower pitch spiral being preferred. The pitch of the thread of the conductive coil or screw is preferably about 1 coil (360° extent) per 1 mm of the axial extent of the coil or more, but may be about 2 coils per 1 mm. The small diameter of the wire and the low pitch tend to increase and maximize the contact area between the electrode with the tissue to which it is attached.  
         [0025]     Low current density for purposes of this description is about 20 milliamperes per square millimeter (20 ma/mm 2 ) or less. By way of example, and not limitation, a current density of approximately 16.2 ma/mm 2  is provided by an electrode that has a wire diameter of 0.01 inches (0.254 mm), a pitch of about 1 coil/mm, a coil diameter of about 0.20 inches (5 mm), which electrode extends through approximately 0.20 inches (5 mm) of tissue (with about 5 coils located in the tissue) and with a current flow of about 1 ampere (1000 ma). Such an electrode has about 62 mm 2  of surface area in contact with the cardiac tissue.  
         [0026]     The current density is calculated by dividing the current flow by the electrode surface area (or current density=current flow/electrode surface area). For a current flow of 1 ampere (1000 ma) and an electrode surface area of 62 mm 2 , the current density is 1 ampere (1000 ma)/62 mm 2  or about 16.2 ma/mm 2 . Other current densities are possible for other configurations of the electrode and also will depend on the number of electrodes employed. Typically, for electrodes located about 2 cm apart in cardiac tissue, a current flow of about 1 ampere is needed to create an ablation line therebetween.  
         [0027]     In addition, the distal end of each electrode may include a slug  34  or a mass of material (or “sink”) such as copper or a copper/aluminum alloy that surrounds the conductive wire  30 . The distal end  36  of the slug  34  may be coterminus with, or extend slightly beyond, the distal end of the outer insulating member  28  so that when the electrode is screwed into tissue, the surface  36  of the slug  34  contacts the surface of the tissue. Thus, when the electrode is attached to the tissue, the slug or mass  34  optionally serves to even further increase the contact area of the electrode, and to provide a mass of material that acts as a heat sink to limit tissue temperature increases.  
         [0028]     As seen in  FIG. 3 , the electrodes  12  and  14  may include a series of slugs or masses  34   a,    34   b,    34   c  at the distal end  24  connected in series to the conductive wire  30  to provide for a higher rate of thermal dissipation. As illustrated, the masses or slugs  34   a,    34   b,    34   c  are spaced along the wire  30  to provide for flexibility of the electrode. Thus, in conjunction with the low pitch, low diameter wire coil  26 , the slug serves as a “heat sink”, and also reducing the degree of tissue coagulation at the electrode site due to the minimization of current density at the electrode/tissue interface. More particularly, the distal end  36  of the slug  34  may present a surface area of between about 0.01 in and 0.1 in 2 , while the slug  34  may have a mass of about 1 gram to 10 grams.  
         [0029]     With reference to  FIG. 4 , an electrode  46  according to the present invention is shown which includes a hand-piece  48  with an elongated shaft or catheter tubing  50  extending distally therefrom and terminating in a screw-type electrode  52  that extends beyond the distal end of the catheter tubing  50 . The electrode  52  is adapted to be conductively connected to one of the outputs of an RF generator (such as the generator  20  shown in  FIG. 1 ) by means of an insulated conductive cable  54  extending from the proximal end of the hand-piece  48 . The hand-piece  48  includes a thumb wheel  56  rotatably secured to the distal end thereof and fixedly connected to the screw electrode  52  (by means including the conductive wire or other drive member not shown) for rotating the electrode  52  with respect to the catheter tubing, thus facilitating screw attachment of the electrode  52  to the heart wall.  
         [0030]     To facilitate the steering or guiding of the catheter  50 /electrode  52  during a minimally-invasive procedure, any well-known structure may be employed. As illustrated, a wire  58  may be secured to the catheter tubing  50  so as to extend between the distal end of the tubing  50  and a thumb lever  60  rotatably mounted to the hand-piece  48 . Actuation of the thumb lever  60  selectively extends or retracts the wire  58  to steer the distal end of the catheter tubing  50 , as indicated by the double-headed arrow  62 . Thus, a pair of devices such as that shown in  FIG. 4  could be introduced through small incisions and positioned on the heart wall using the thumb lever  60  to steer the electrode  52  to the desired position and the thumb wheel  56  to screw the electrode  52  to the heart wall.  
         [0031]     When used for ablating cardiac tissue, it is desirable that the conductive screw  26  of the electrode penetrate as much of the thickness of the cardiac tissue as possible, without completely puncturing the heart wall and protruding into the interior or exterior of the heart. With reference to  FIG. 5 , to this end, each electrode may also include an EKG wire  38  extending through the outer insulating member  28  and terminating in a tip  40  that extends distally beyond the distal-most portion of the conductive screw  26 . (For purposes of clarity, the slug  34  is not shown in the embodiment of  FIG. 5 . However, the spiral of the conductive coil encircles the slug at the distal end of the insulating member  28  while, in the embodiment employing a slug, the EKG wire  36  may extend through a central hole in the conductive slug  34 .) The structure shown in  FIG. 3  could also be used to provide for a bipolar EKG, in which case wire  38  with its exposed tip  40  would be connected to a first EKG and the coiled wire  30  connected to a second EKG.  
         [0032]     The EKG wire  38  is preferably covered by an insulating material, except for the distal tip  40 , which is free of insulation. This permits the EKG reading to be taken only at the very tip of the EKG wire. Accordingly, as the electrode is screwed into the heart tissue, the electrode and EKG wires advance simultaneously. As the electrode wire passes out of the myocardium and penetrates the epicardium, the EKG reading abruptly decreases or disappears, thus signaling to the cardiologist or electrophysiologist that the conductive screw has penetrated nearly the entire tissue thickness. As noted above, an alternative arrangement provides an EKG sensor on the distal end of the electrode  26  so that the physician can immediately detect piercing of the heart wall by the electrode. Alternatively, more than two electrodes may be used to form two or more ablation segments simultaneously.  
         [0033]     The low-pitch electrodes of the present invention can be used to make the series of lesions commonly associated with the Maze procedure. With reference to  FIG. 6 , there is seen a posterior view of a heart showing a series of lesions  1 - 11  that may be made when performing the Maze procuedure. The electrodes of the present invention may be used to make one or more of these lesions from the interior of the heart, which may be relatively easier to access than the exterior surface. However, the procedure can be performed on the exterior surface as well. The procedure will be described with respect only to the creation of lesion  1 , which encircles the right pair of pulmonary veins, lesion  2 , which encircles the left pulmonary veins, and lesion  3 , which connects lesions  1  and  2 . Each of these lesions may be made in a series or sequence of steps in which the electrodes are attached to the heart and activated to form a first lesion segment. Then one of the electrodes is “unscrewed” and moved step-wise, or “leap-frogged”, with respect to the other electrode. The electrodes are then activated again to form a second lesion segment connected to the first, and the steps repeated again a sufficient number of times to complete the desired lesion. Alternatively more than two electrodes may be used to form two or more ablation segments simultaneously.  
         [0034]     With reference to  FIG. 7 , a schematic representation of the right pulmonary veins  42  is shown. The lesion  1  is created by attaching one of the electrodes  12 ,  14  to location A 1  and the other to location B 1 , and activating the electrode to form a lesion between locations A 1  and B 1 . Typically, the electrodes can be placed from 1 to 4 cm apart to provide a satisfactory transmural lesion between the electrodes. For example, the electrodes may be positioned about 2 cm (20 mm) apart, and a current flow therebetween of about 1 ampere (1000 milliamperes) could be employed to create a line of ablation therebetween. Then the electrode from location A 1  is moved to location C 1 , the electrodes activated, thus forming the lesion between points B 1  and C 1 . Then the electrode from point B 1  is moved to location D 1  and the electrodes activated to form the lesion between points C 1  and D 1 . Then the electrode from location C 1  is moved to location A 1  and the electrodes activated to make the lesion from location D 1  to location A 1 , thus completely encircling the right pulmonary veins with lesion number  1 .  
         [0035]     Then, with reference to  FIG. 8 , lesion  2  may be made in a similar fashion about the other pair of pulmonary veins  44  by moving the electrodes serially from locations A 2 -D 2 , and activating the electrodes in the same manner as described above with respect to the formation of lesion  1 . Once the pulmonary veins  44  are encircled by lesion  2 , the electrode from location D 2  can be moved to location F and then activated to make the lesion A 2 -F. Then the electrode from location A 2  can be moved to location A 1  and then activated to make the lesion connecting F with A 1 , thus completing lesion  3 , and connecting lesions  1  and  2 . As noted above, the other lesions required for the Maze or other cardiac procedures can be made in a similar step-wise fashion.  
         [0036]     With the present invention, it is contemplated that a lesion line of about 100 mm could be made with about seven lead placements. While this may seem laborious, it is a relatively easy procedure to employ, which forms a reliable lesion line even with the movement of a beating heart, and without the trauma, cost and recovery of an open heart Maze procedure.