Abstract:
A catheter has a stylet formed of a shape-retentive and resilient material having a preformed curved shape at its distal end resulting in the catheter sheath having the preformed curved shape. The catheter sheath has a plurality of electrodes at its distal end for contacting selected biological tissue for imparting ablation energy thereto. The catheter sheath also has an axially mounted tendon for causing deflection of the distal end. The stylet material permits straightening the catheter sheath during insertion into the patient and advancing the electrodes to the target tissue. Upon removal of the straightening forces, such as by entry into a chamber of the heart, the stylet material resumes its preformed curved distal shape thereby forcing the catheter distal end with the electrodes into the same preformed curved shape. The operator may place the curved distal end into contact with the target tissue and axially move the tendon as desired to gain greater control over the bend in the distal end of the catheter sheath to adjust the radius of curvature of the distal end to obtain greater contact of the electrodes with the heart tissue. Preferably, the stylet is formed of nitinol.

Description:
RELATED APPLICATIONS 
     This is a division of co-pending application Ser. No. 09/072,962, filed May 5, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates generally to an electrophysiological (“EP”) apparatus and method for providing energy to biological tissue, and more particularly, to a steerable catheter with a preformed distal shape for positioning the catheter to a desired location in a patient. 
     The heart beat in a healthy human is controlled by the sinoatrial node (“S-A node”) located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node (“A-V node”) which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as “cardiac arrhythmia.” 
     While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed by percutaneous ablation, a procedure in which a catheter is percutaneously introduced into the patient and directed through an artery to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue. 
     In the case of atrial fibrillation (“AF”), a procedure published by Cox et al. and known as the “Maze procedure” involves continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system. 
     There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is placed on the chest, back or other external location of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, electrodes introduced into the heart have different potentials and complete an electrical circuit between themselves. In the bipolar method, the flux traveling between the two electrodes of the catheter enters the tissue to cause ablation. 
     During ablation, the electrodes are placed in intimate contact with the target endocardial tissue. RF energy is applied to the electrodes to raise the temperature of the target tissue to a non-viable state. In general, the temperature boundary between viable and non-viable tissue is approximately 48° Centigrade. Tissue heated to a temperature above 48° C. becomes non-viable and defines the ablation volume. The objective is to elevate the tissue temperature, which is generally at 37° C., fairly uniformly to an ablation temperature above 48° C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100° C. 
     Failure to bring or maintain the electrodes in contact with the target tissue may result in the RF energy not reaching the tissue in sufficient quantities to effect ablation. Only limited electromagnetic flux in a bipolar approach may reach the tissue when the electrode is non-contacting. In a unipolar approach, the RF energy may spread out too much from the non-contacting electrode before reaching the tissue so that a larger surface area is impacted by the flux resulting in each unit volume of tissue receiving that much less energy. In both cases, the process of raising the tissue temperature to the ablation point may require a much greater time period, if it can be performed at all. Where the electrodes have temperature sensors and those sensors are not in contact with the tissue, they may not sense the actual temperature of the tissue as fluids flowing around the non-contacting electrode may lower the temperature of the electrode and the temperature sensed by the sensors. 
     In some procedures, such as where a longer atrial lesion is required, the lesion produced by a single electrode in a unipolar arrangement is not sufficient. To this end ablation catheters have been designed. In one catheter an electrode device having four peripheral electrodes which extend from a retracted mode is used. See U.S. Pat. No. 5,500,011 to Desai. When extended, i. e., fanned out, the four peripheral electrodes and the central electrode form an electrode array that covers a larger surface area of the tissue than a single electrode. However, there remain some difficulties in manipulating such a device so that when expanded, all electrodes are in contact with the endocardium. An “end on” approach is required such that the end of the catheter, on which all five electrodes are mounted, is in intimate contact with the target tissue. 
     The effectiveness of the above-described technique is further limited by the mechanical configuration of the electrode device. When used for ablation, an electrode device is typically part of a catheter system. Accordingly, it is desirable to minimize the diameter of the electrode device during introduction to and withdrawal from the patient to allow for its use within a catheter and to lessen trauma to the patient. However, it is desirable to obtain a relatively large expandable size to obtain a larger ablation size. Therefore, electrode devices having peripheral expandable electrodes must be configured so that the peripheral electrodes are expandable to a large size yet are retractable to as small a size as practical. Such requirements pose design and manufacturing difficulties and an electrode device configured as such is susceptible to malfunction in that the peripheral electrodes may be damaged or break off as they are extended from a retracted mode or vice versa. Further considerations are the undesirable complexity and increased manufacturing cost associated with an expandable catheter. 
     In other techniques, used in the treatment of atrial fibrillation, a plurality of spaced apart electrodes are located at the distal end of the catheter in a linear array. RF energy is applied to the electrodes to produce a longer lesion. With such a linear array, intimate contact between each electrode and the target endocardial tissue is more difficult to maintain in the heart due to the irregular heart surface shape and the constant movement of the heart. The lesion produced may have discontinuities unless steps are taken to maintain contact. These lesions may not be sufficient to stop the irregular signal pathways and arrhythmia may reoccur. In an attempt to ensure intimate contact between the electrode and the target tissue the distal end of the catheter may have a preformed shape. For example, see U.S. Pat. No. 5,617,854 to Munsif, in which the catheter is made of a shaped-memory material, e.g. nitinol, and formed in a specific shape. During use, the catheter is deformed and introduced through an introducer sheath to the heart where ablation is to occur. Once in position, the catheter is reformed into its specific shape when heated to body temperature or when a current is passed through the shaped-memory material. If the shaped memory of the catheter matches the curvature of the biological cavity, there is more intimate contact between the electrode and the tissue and a more continuous lesion is formed. However, this is somewhat unlikely because the heart surface is irregular. If a given shaped catheter does not conform to the shape of the biological site to be ablated a different catheter having a different preformed shape must be used. Requiring a collection of preformed-shaped catheters, as such, is economically inefficient. 
     Hence, those skilled in the art have recognized a need for a structurally stable minimally invasive ablation apparatus that is capable of controlling the flow of current through a biological site so that lesions with controllable surface and depth characteristics may be produced and the ablation volume thereby controlled. Additionally, a need has been recognized for providing a catheter carrying a plurality of electrodes in its distal end region which is capable of conforming to various curvatures of the biological site so that intimate contact may be maintained between the electrodes and the site. The invention fulfills these needs and others. 
     SUMMARY OF THE INVENTION 
     Briefly, and in general terms, the invention is directed to an apparatus and a method for use in applying energy to a biological site using a catheter carrying a plurality of electrodes in its distal end region which is capable of conforming to various curvatures of the biological site so that intimate contact may be maintained between the electrodes and the site. 
     In a first aspect, the invention is a catheter for positioning at least one electrode at a selected biological site and comprises a sheath having a distal end carrying the at least one electrode, a stylet having a preformed shape with at least one bend, the stylet housed within the sheath and formed of a shape-retentive and resilient material such that the stylet changes shape upon the application of force and upon the removal of force, returns to the preformed shape. The stylet is disposed in the sheath such that the bend is located in the distal end of the sheath and causes the sheath to assume the preformed shape. The catheter also includes a tendon housed within the sheath and attached to the distal end of the sheath such that axial displacement of the tendon causes deflection of the distal end of the sheath. 
     In more detailed aspects, the preformed distal shape has a radius of curvature for conforming the distal end of the sheath to the selected biological site. Additionally, the tendon is disposed in the sheath such that pulling the tendon in the proximal direction decreases the radius of curvature of the distal end of the sheath and movement of the tendon in the distal direction increases the radius of curvature of the distal end of the sheath. In a further aspect, the stylet is formed of nitinol. 
     In a further more detailed aspect, the stylet is attached to the distal end of the sheath. In another detailed aspect, a tip component is positioned at the distal tip of the sheath wherein the stylet and tendon are attached to the tip component. A steering member is positioned at the proximal end of the sheath wherein the tendon is attached to the steering member for moving the tendon in an axial direction. 
     In another aspect, a handle is disposed at the proximal end of the sheath and connected thereto such that rotation of the handle results in rotation of the sheath and stylet. In more detailed aspects, there exist a plurality of band electrodes spaced apart at the distal end of the sheath. The plurality of band electrodes are positioned on the sheath in the region of the preformed distal shape. In yet a further detailed aspect, the distal end of the stylet has a plurality of bends. 
     In second aspect, the invention is an ablation system for use in applying energy to heart tissue. The system comprises a catheter sheath having a distal end region and a lumen, a plurality of electrodes located at the distal end region of the catheter sheath, a stylet having a distal end with a preformed shape with at least one bend. The stylet is housed within the catheter sheath and formed of a shape-retentive and resilient material such that the stylet changes shape upon the application of force and upon the removal of the force, returns to the preformed shape. The stylet is disposed in the catheter sheath such that the bend is located in the distal end region of the sheath and causes the sheath to assume the preformed shape. The ablation system also includes a tendon housed within the catheter sheath and attached to the distal end of the sheath such that axial displacement of the tendon causes deflection of the distal end of the sheath, a handle attached to the proximal end of the catheter sheath and the stylet such that rotation of the handle causes rotation in the catheter sheath and the stylet, and a control member disposed at the handle and connected to the tendon such that a predetermined movement of the control member causes axial movement of the tendon to control deflection of the distal end region of the catheter sheath. 
     In a further detailed aspect of the above, the plurality of electrodes are spaced apart on the distal end region of the catheter to form a linear array. 
     In a third aspect, the invention is a method for applying energy to heart tissue. The method comprises the steps of inserting and advancing into the patient a catheter sheath having a stylet formed of a shape-retentive and resilient material such that the stylet changes shape upon the application of force and upon the removal of the force, returns to a preformed shape. The stylet is mounted in the catheter sheath such that the sheath has the preformed shape at its distal end, the shape having a curvature generally conforming to the curvature of selected heart tissue, the catheter sheath also having a plurality of electrodes, wherein the distal end shape is altered from its preformed shape while being inserted and advanced. Further included are the steps of advancing the catheter sheath into the heart chamber in which the selected heart tissue is located thereby permitting the catheter sheath distal end to resume its preformed curved distal shape, advancing the curved distal end of the catheter sheath into contact with the selected heart tissue, and adjusting the radius of curvature of the distal end region of the catheter such that a plurality of the electrodes contact the selected heart tissue. 
     In more detailed aspects of the method, the step of advancing the catheter into the selected heart chamber comprises the steps of inserting an introducer sheath into the selected heart chamber, guiding the catheter through the introducer sheath, and extending the catheter beyond the distal end of the introducer sheath thereby allowing the distal end region of the catheter to resume its preformed shape. Additionally, in another aspect, the step of adjusting the radius of curvature of the distal end region of the catheter comprises the step of axially moving a tendon attached to the distal end region of the catheter sheath to deflect the distal end of the catheter and thereby decrease the curvature of the distal end region. 
     These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an ablation apparatus including a power control system, electrode device and backplate; 
     FIG. 2 is a diagram of a catheter system including a handle, steering member, and a catheter having a preformed distal end region with a first shape; 
     FIG. 3 is an alternate preformed distal end region of FIG. 2 having a second shape; 
     FIG. 4 is a preformed stylet for shaping the distal end region of the catheter of FIG. 2; 
     FIG. 5 is a sectional view of the proximal region of the catheter of FIG. 3 taken along the line  5 — 5  depicting the catheter sheath with braid, stylet, steering tendon and leads; 
     FIG. 6 is a sectional view of the distal end region of the catheter of FIGS. 2 and 3 taken along the line  6 — 6  depicting the catheter sheath, stylet, steering tendon and leads; 
     FIG. 7 is a representation of the distal end region of the catheter of FIG. 3 proximal biological tissue with the electrodes not in intimate contact with the tissue; and 
     FIG. 8 is a representation of the distal end region of the catheter of FIG. 3 proximal biological tissue with the electrodes in intimate contact with the tissue. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Turning now to the drawings, in which like reference numerals are used to designate like or corresponding elements among the several figures, in FIG. 1 there is shown an ablation apparatus  10  in accordance with aspects of the present invention. The apparatus  10  includes a power control system  12  that provides power or drive  14  to an electrode device  16 . The power control system  12  comprises a power generator  18  that may have any number of output channels through which it provides the power  14 . The operation of the power generator  18  is controlled by a controller  20  which outputs control signals  21  to the power generator  18 . The controller  20  monitors the power  14  provided by the power generator  18 . In addition, the controller  20  also receives temperature signals  22  from the electrode device  16 . Based on the power  14  and temperature signals  22  the controller  20  adjusts the operation of the power generator  18 . A backplate  24  is located proximal to the biological site  26  opposite the site from the electrode device  16 , and is connected by a backplate wire  28  to the power generator  18 . The backplate  24  is set at the reference level to the power provided to the electrodes, as discussed in detail below. 
     In a manual arrangement, the temperature sensed and/or the determined impedance may be displayed to an operator. The operator in response may then manually control the duty cycle or other power parameters such as by rotating a knob mounted on a front panel of an instrument. In the case of a multiple channel instrument and catheter, as discussed below, multiple knobs may be provided in this manual arrangement for control over each channel. 
     The electrode device  16  is typically part of a steerable EP catheter  30  capable of being percutaneously introduced into a biological site  26 , e.g., the atrium or ventricle of the heart. The electrode device  16  is shown in schematic form with the components drawn to more clearly illustrate the relationship between the components and the relationship between the components and the power control system  12 . In this embodiment, the catheter  30  comprises a distal segment  34  and a handle  31  located outside the patient. A preferred embodiment of the electrode device  16  includes twelve band electrodes  32  arranged in a substantially linear array along the distal segment  34  of the catheter  30 . The electrode device  16  may include a tip electrode  36 . (For clarity of illustration, only four band electrodes  32  are shown in the figures although as stated, a preferred embodiment may include many more.) The band electrodes  32  are arranged so that there is space  38  between adjacent electrodes. In one configuration of the electrode device  16 , the width of the band electrodes  32  is 3 mm and the space  38  between the electrodes is 4 mm. The total length of the electrode device  16 , as such, is approximately 8 cm. 
     The arrangement of the band electrodes  32  is not limited to a linear array and may take the form of other patterns. A substantially linear array is preferred for certain therapeutic procedures, such as treatment of atrial fibrillation, in which linear lesions of typically 4 to 8 cm in length are desired. A linear array is more easily carried by the catheter  30  and also lessens the size of the catheter. 
     The band electrodes  32  are formed of a material having a significantly higher thermal conductivity than that of the biological tissue  26 . Possible materials include silver, copper, gold, chromium, aluminum, molybdenum, tungsten, nickel, platinum, and platinum/10% iridium. Because of the difference in thermal conductivity between the electrodes  32  and the tissue  26 , the electrodes  32  cool off more rapidly in the flowing fluids at the biological site. The power supplied to the electrodes  32  may be adjusted during ablation to allow for the cooling of the electrodes while at the same time allowing for the temperature of the tissue to build up so that ablation results. The electrodes  32  are sized so that the surface area available for contact with fluid in the heart, e.g., blood, is sufficient to allow for efficient heat dissipation from the electrodes to the surrounding blood. In a preferred embodiment, the electrodes  32  are 7 French (2.3 mm in diameter) with a length of 3 mm. 
     The thickness of the band electrodes  32  also affects the ability of the electrode to draw thermal energy away from the tissue it contacts. In the present embodiment, the electrodes  32  are kept substantially thin so that the electrodes effectively draw energy away from the tissue without having to unduly increase the outer diameter of the electrode. In a preferred embodiment of the invention, the thickness of the band electrodes is 0.05 to 0.13 mm (0.002 to 0.005 inches). 
     Associated with the electrode device  16  are temperature sensors  40  for monitoring the temperature of the electrode device  16  at various points along its length. In one embodiment, each band electrode  32  has a temperature sensor  40  mounted to it. Each temperature sensor  40  provides a temperature signal  22  to the controller  20  which is indicative of the temperature of the respective band electrode  32  at that sensor. In another embodiment of the electrode device  16  a temperature sensor  40  is mounted on every other band electrode  32 . Thus for a catheter having twelve electrodes, there are temperature sensors on six electrodes. In yet another embodiment of the electrode device  16  every other electrode has two temperature sensors  40 . In FIG. 1, which shows an embodiment having one temperature sensor for each electrode, there is shown a single power lead  15  for each electrode  32  to provide power to each electrode for ablation purposes and two temperature leads  23  for each temperature sensor  40 . 
     In order to ensure a long, continuous lesion the catheter employs a steering tendon and a stylet having a preformed distal shape. As shown in FIG. 2, the catheter  100  includes a distal end region  106  having a preformed distal shape. This distal shape may have any form which generally conforms to the contour of the biological cavity containing the tissue to be ablated. The distal end region  106  of FIG. 2 has been simplified for clarity to depict varying degrees of curvature  108  and  110  obtainable by use of the preformed shape and the steering tendon, as explained below. The distal shape of FIG. 3 is conducive to the treatment of atrial fibrillation in that its shape allows for the distal end region  106  to be easily inserted into the atrium of the heart. The shape, in combination with a steering tendon, also provides a distal end region having a contour which may be adjusted to conform to the contour of the atrium. 
     The catheter  100  also includes a handle  112  and a steering member  114 . A tip component  116  is mounted to a sheath  118  at the very distal tip of the sheath. A shaped-memory wire, i.e., stylet,  104  (FIG. 4) is located in the distal end region  106  and preferably runs the entire length of the sheath  118 . The stylet  104  is attached to the tip component  116  at the axis of the tip component, and is anchored to a fixed position within the handle  112 . The shaped memory wire  104  is formed of an alloy which exhibits a martensitic phase transformation. Such alloys include those which exhibit non-linear superelasticity (typically Ni—Ti with Ni at 49-51.5% atomic) and those which exhibit linear superelasticity (typically Ni—Ti in near equi-atomic composition which has been cold worked). Preferably, the preformed shaped wire  104  is formed of nitinol wire having a diameter in the range of 0.026 to 0.030 mm and a nitinol composition of 49-51.5% Ni. The shape of the distal end region  106  is created by restraining the nitinol wire in the desired shape and heating the wire to approximately 500° C. for about 10 minutes. The nitinol is then allowed to cool. Upon cooling, the wire  104  retains the preformed distal shape. 
     Stress may be applied to the wire to change its shape. For example, the wire  104  may be straightened to negotiate an introducer or various blood vessels on its way to the right or left atrium of the heart. Upon removal of the straightening forces, such as when the distal end enters the left atrium of the heart, the stylet accurately resumes its preformed shape causing the distal end of the catheter sheath surrounding it to likewise take the same shape. Because of the superelasticity of the nitinol, once the stress is removed the wire returns to its original shape. This is distinct from other shape-memory materials which are temperature actuated. 
     Referring now to FIG. 5, the stylet  104  is housed inside a composite sheath  118  constructed of different durometers of Pebax and braided stainless steel ribbon in order to tailor the torsinal and bending stiffness in various locations along the length of the catheter. In the region  130  proximal from the distal end region  106 , as shown in FIG. 3 the sheath  118  is formed of high durometer Pebax outer jacket having an outside diameter of 2.39 mm (0.094 inches) (7 French) and an inside diameter of 1.58 mm (0.062 inches). Imbedded within the sheath  118  are two layers of braid, 0.001×0.006 stainless steel ribbon  120 . The inner lumen  122  has a hollow PTFE tendon sheath  124  bonded to one side  126 . The tendon sheath  124  has an outside diameter of approximately 0.457 mm (0.018 inches). The remaining portion of the tendon sheath  124  is exposed in the inner lumen  122 . The steering tendon  102  is housed within the tendon sheath  124  and is formed of a stainless steel wire having a diameter of approximately 0.23 mm (0.009 inches). At its distal end, the steering tendon  102  is attached to the tip component  116  at a point parallel to the axis of the tip component. In the alternative, the steering tendon  102  may be anchored at a point proximal the tip component  116 . At its proximal end, the tendon  102  is linked to the steering member  114  (FIG. 2) which translates axially along the length of the handle  112 . Also housed within the inner lumen  122  are the leads  128 . Eighteen are depicted in FIG. 4, however, more or fewer may be included depending on the number of electrodes  32  and the configuration of the temperature sensors. 
     In the distal end region  106  (FIGS.  2  and  3 ), as shown in cross section in FIG. 6, the construction of the sheath  118  is generally the same as that of the proximal region  130  except the outer jacket does not include a stainless steel braid  120 . By not including the braid  120 , the distal end region  106  is more flexible than the proximal region  130 . Accordingly, the distal end region  106  is more easily bent for conformance with the biological site. Housed within the sheath  118  and offset from the axis of the sheath is the steering tendon  102 . 
     In operation, the catheter  100  is inserted into the biological cavity containing the tissue to be ablated. In the case of the left atrium where a transseptal approach is used, the catheter may be inserted using an introducer sheath (not shown). The introducer sheath is positioned within the cavity and the catheter  100  is inserted into the introducer sheath. Because of the flexibility of the nitinol stylet  104 , the distal end region  106  of the catheter conforms to the shape of the introducer sheath and follows the tortuous path of the introducer sheath. Once the distal end region  106  of the catheter enters the biological cavity the catheter is either extended beyond the distal tip of the introducer sheath or the introducer sheath is retracted. In either case, the distal end region  106  of the catheter is no longer constrained by the introducer sheath and returns to its original preformed distal shape. Once the preformed distal shape is resumed, the distal end region  106  has a shape more closely following that of the heart. However it may not conform to the shape of the biological site  126  as closely as desired. This situation is shown in FIG.  7 . Accordingly, some or all of the electrodes  32  may not be in intimate contact with the tissue  26  to be ablated. If the electrodes  32  are not in the desired contact with the tissue  26 , the radius of curvature of the distal end region may be adjusted using the steering tendon such that more of the electrodes contact the biological tissue  26 . The dashed lines  108  and  110  of FIG. 2 are examples of how the degree of curvature of the distal end region  106  may be adjusted by use of the steering tendon. This adjustment is performed by axially displacing the steering member  114  in the proximal direction. In doing so, the steering tendon  102  attached to the tip component  116  experiences tension and causes the sheath  118  to compress on the side in which the steering tendon is positioned and to stretch on the opposite side. This causes the radius of curvature of the distal end region to decrease as shown in FIG.  2 . In addition, to further ensure intimate contact between the electrodes  32  and the tissue  26 , the handle  112  may be rotated. Because of the attachment of the sheath  118  to the handle  112  and the construction of the catheter, as previously described in relation to FIGS. 5 and 6, this rotational force at the handle causes the catheter to experience a torquing effect along its length, which may aid in positioning the electrodes against the tissue. 
     Once the distal end region  106  is properly positioned and the electrodes  32  are in intimate contact with the tissue, as shown in FIG. 8, RF energy is applied to the electrodes to ablate the tissue. After applying energy to a first portion of tissue  26  located within the selected biological cavity, the distal end region  106  of the catheter may be repositioned proximal another region of tissue and the curvature of the distal end region adjusted so that the electrode  32  contact the tissue. Thus, the catheter provides for ready adjustment of the electrode carrying region  106  such that a plurality of electrodes aligned in a substantially linear array may be placed in intimate contact with tissue  26  to be ablated. Because of the length of the linear electrode array, the device shown in the drawings and described above is particularly suited for performing the Maze procedure in a minimally invasive way. 
     While certain shapes of the distal end of the catheter are shown in FIGS. 2 and 3, other shapes may be used. The invention is not confined to the shapes shown in these figures. Additionally, the steering tendon may be used by the operator to steer or assist in advancing the catheter distal end through the blood vessels of the patient to the desired target tissue. 
     Thus there has been shown and described a new and useful catheter system having both a preformed distal end and a steering mechanism which greatly increase the chances that a successful ablation can be obtained in a single procedure. 
     It will be apparent from the foregoing that while particular forms of the invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.