Abstract:
A wire housed within a sheath is formed of a shape-retentive and resilient material having a curved shape at its distal-end region resulting in the catheter sheath having the curved shape. The catheter sheath also has an axially oriented tendon for causing deflection of the distal-end region. A movable outer sleeve surrounds the sheath and conforms the portion of the wire positioned within the outer sleeve to the shape of the outer sleeve. Upon relative displacement of the sheath and the outer sleeve a portion of the sheath extends beyond the outer sleeve and resumes its preformed curved distal shape thereby forcing the catheter distal-end region into the same curved shape. The operator may adjust the relative positions of the sheath and outer sleeve to changes the shape of the distal-end region. The operator may also axially move the tendon to adjust the radius or curvature of the distal-end region.

Description:
RELATED APPLICATIONS 
     This is a continuation-in-part of application Ser. No. 09/516,280, filed Mar. 1, 2000, now U.S. Pat. No. 6,270,496, which is a division of application Ser. No. 09/072,962, filed May 5, 1998, now U.S. Pat. No. 6,096,036. 
    
    
     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 and a movable outer sleeve 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 or vein 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 a trial 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 the treatment of atrial fibrillation, a plurality of spaced apart electrodes are located at the distal end of a catheter in a linear array. RF energy is applied to the electrodes to produce a long linear 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 contours 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. Thus the need for catheters having the capability to conform to and maintain intimate contact with various endocardial tissue surface contours is strongly felt by those engaged in the treatment of cardiac arrhythmias, particularly atrial fibrillation. 
     To that end, several catheter systems have been developed in an attempt to ensure intimate contact between the electrodes at the distal end of a catheter and the target tissue. In one such catheter system, described in U.S. Pat. No. 5,617,854 to Munsif, the distal end of the catheter is shaped to conform to a specific region of the heart. 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 sheath is retracted and 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. 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. 
     In another catheter system, described in U.S. Pat. No. 5,882,346, to Pomeranz et al., the catheter system includes a catheter having a lumen extending through it and a plurality of electrodes at the distal-end region. A core wire is insertable into the catheter lumen. The core wire includes a preshaped region that is formed of a superelastic material and which is bent into a predetermined shape. As the core wire is inserted into the catheter, the core wire deforms the distal-end region of the catheter into the predetermined shape of the core wire. If the predetermined shape of the core wire matches the curvature of the biological cavity, there is more intimate contact between the electrodes and the tissue. If, however, a given preshaped core wire does not conform to the shape of the biological site to be ablated a different core wire having a different preformed shape must be used. Thus a collection of different shaped core wires is required. This is also economically inefficient. 
     Hence, those skilled in the art have recognized a need for providing a single 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 related to an apparatus and a method for use in applying energy to a biological site using a catheter carrying at least one electrode 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 related to a catheter including a sheath having a distal-end region and a wire disposed within the sheath and attached to the distal end of the sheath. The wire has a distal-end region having a preformed shape with a radius of curvature. The wire is formed of a shape-retentive and resilient material such that the wire distal-end region changes shape upon the application of force and upon the removal of force, returns to the preformed shape. The wire is disposed in the sheath such that the wire distal-end region is located in the sheath distal-end and causes the sheath to assume the preformed shape. The catheter further includes an outer sleeve having a distal-end region having a distal end. The sleeve surrounds the sheath and is positioned relative thereto for movement between a retracted position and any of a plurality of advanced positions during which a portion of the sleeve distal-end region is coincident with a portion of the wire distal-end region. The sleeve is formed of a material having a stiffness sufficient to change the shape of the wire distal-end region. The catheter also includes a tendon housed within the sheath. The tendon is attached to the distal end of the sheath such that axial displacement of the tendon changes the radius of curvature of the portion of the wire distal-end region extending distal the distal end of the outer sleeve. 
     By providing a catheter having a preshaped steerable catheter sheath and a movable outer sleeve surrounding the sheath and having a stiffness sufficient to change the shape of the sheath, the present invention allows for multifaceted adjustment of the distal-end region shape such that intimate contact between the at least one electrode and the target tissue may be obtained regardless of the curvature of the biological site. For example, the preformed shape of distal-end region may be adjusted using the tendon or the movable outer sleeve or a combination of both. 
     In a more detailed facet of the invention, the outer sleeve has a fully advanced position during which the sleeve distal end is substantially coincident with the wire distal end. In another detailed facet, the outer sleeve has a fully retracted position during which the sleeve distal-end region is proximal the wire distal-end region. In yet another detailed aspect, the outer sleeve distal-end region comprises a preformed shape with a radius of curvature. In still another detailed aspect, the sheath has an axis and the outer sleeve is positioned for rotational movement about the axis of the sheath. In further detailed facets the catheter further includes a locking mechanism for locking the outer sleeve in a position relative to the sheath and a handle having the proximal end of the sheath and the wire connected thereto such that movement of the handle results in movement of the sheath, wire and outer sleeve. In a still another detailed aspect, the tendon is disposed in the sheath such that pulling the tendon in the proximal direction decreases the radius of curvature of the wire distal-end region distal the distal end of the sleeve and subsequent movement of the tendon in the distal direction allows an increase in the radius of curvature of the wire distal-end region distal the distal end of the sleeve. 
     In a second aspect, the invention is related to an ablation catheter for use in applying energy to heart tissue. The system includes a catheter sheath having a distal-end region, at least one electrode located at the sheath distal-end region and a wire disposed within the sheath and attached to the distal end of the catheter sheath. The wire has a distal-end region with a preformed shape with a radius of curvature and is formed of a shape-retentive and resilient material such that the wire distal-end region changes shape upon the application of force and upon the removal of the force, returns to the preformed shape. The wire is disposed in the catheter sheath such that the wire distal-end region is located in the distal-end region of the sheath and causes the sheath to assume the preformed shape. 
     The ablation catheter further includes an outer sleeve having a distal-end region having a distal end. The sleeve surrounds the sheath and is positioned relative thereto for movement between a retracted position and any of a plurality of advanced positions during which a portion of the sleeve distal-end region is coincident with a portion of the wire distal-end region. The sleeve is formed of a material having a stiffness sufficient to change the shape of the wire distal-end region. The system also includes a tendon housed within the sheath. The tendon is attached to the distal end of the sheath such that axial displacement of the tendon changes the radius of curvature of the portion of the wire distal-end region extending distal the distal end of the outer sleeve. The system further includes a locking mechanism for locking the outer sleeve in a position relative to the sheath and a handle attached to the proximal end of the catheter sheath and the wire such that movement of the handle causes movement of the catheter sheath, the wire and the outer sleeve. 
     In a third aspect, the invention involves a method for applying energy to biological tissue within a biological chamber of a patient. The method uses a catheter having a sheath having a distal end-region carrying a plurality of electrodes and a wire disposed within the sheath and attached to the distal end of the sheath. The wire has a distal-end region having a preformed shape with a radius of curvature and is formed of a shape-retentive and resilient material such that the wire distal-end region changes shape upon the application of force and upon the removal of force, returns to the preformed shape. The wire is disposed in the sheath such that the wire distal-end region is located in the sheath distal-end and causes the sheath to assume the preformed shape. The catheter also includes an outer sleeve having a distal-end region having a distal end. The sleeve surrounds the sheath and is positioned relative thereto for movement between a retracted position and any of a plurality of advanced positions during which a portion of the sleeve distal-end region is coincident with a portion of the wire distal-end region. The sleeve is formed of a material having a stiffness sufficient to change the shape of the wire distal-end region. 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 changes the radius of curvature of the portion of the wire distal-end region extending distal the distal end of the outer sleeve. 
     The method of the present invention includes the steps of placing the outer sleeve in an advanced position such that the distal end of the sheath is positioned within the outer sleeve, inserting the catheter into the vasculature of the patient and advancing the catheter into the biological chamber in which the selected tissue is located. The method also includes the steps of retracting the outer sleeve such that a portion of the distal-end region of the sheath is positioned outside of the outer sleeve thereby permitting the wire distal-end region to assume the preformed shape and advancing the distal-end region of the catheter sheath to a position proximal the selected biological tissue. The method further includes the step of adjusting the radius of curvature of the distal-end region of the wire to thereby affect adjustment in the distal-end region of the sheath such that a plurality of the electrodes contact the selected biological tissue. 
     In a detailed facet of the invention, the step of advancing the distal-end region of the catheter sheath to a position proximal the selected heart tissue includes the step of securing the outer sleeve to the sheath and axially displacing the catheter toward the selected biological tissue. In another detailed aspect, the step of advancing the distal-end region of the catheter sheath to a position proximal the selected heart tissue includes the step of maintaining the position of the outer sleeve while axially displacing the catheter sheath toward the selected biological tissue. In yet another detailed facet, the step of adjusting the radius of curvature of the distal-end region of the catheter includes the step of displacing the tendon to deflect the wire distal-end region and thereby decrease the curvature of the wire 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, a substantially linear outer sleeve with a locking mechanism, and a catheter sheath having a preformed distal-end region, with the outer sleeve in a retracted position; 
     FIG. 3 is a diagram of the catheter system of FIG. 2 with the outer sleeve in an advanced position; 
     FIG. 4 is a cross-section of the locking mechanism of FIG. 2 shown in an unlocked state; 
     FIGS. 5A and 5B are cross-sections of exemplary mechanisms for retaining the outer sleeve on the catheter sheath, interior components of the sheath have been removed for clarity; 
     FIG. 6 is a cross-section of the distal end of the outer sleeve depicting a seal positioned between the inner wall of the outer sleeve and the outer wall of the catheter sheath; 
     FIG. 7 is a diagram of a catheter system including a handle, steering member, a curved outer sleeve and a catheter sheath having a preformed distal-end region, with the outer sleeve in an advanced position; 
     FIG. 8 is a diagram of the catheter system of FIG. 4 with the outer sleeve rotated approximately 180 degrees relative to the outer sleeve in FIG. 4; 
     FIG. 9 is a preformed stylet for shaping the distal-end region of the catheter sheaths of FIGS. 2,  3 ,  7  and  8 ; 
     FIG. 10 is a sectional view of the proximal region of the catheter of FIG. 2 taken along the line  10 — 10  depicting the outer sleeve, catheter sheath with braid, stylet, steering tendon and leads; 
     FIG. 11 is a sectional view of the distal-end region of the catheter of FIG. 2 taken along the line  11 — 11  depicting the catheter sheath, stylet, steering tendon and leads; 
     FIG. 12 is a sectional view of the distal-end region of the catheter of FIG. 3 taken along the line  12 — 12  depicting the outer sleeve, catheter sheath, stylet, steering tendon and leads; 
     FIG. 13 is a sectional view of the outer sleeve depicting a tendon for deflecting the distal-end region of the outer sleeve; 
     FIG. 14 is a representation of the distal-end region of the catheter of FIG. 3 with the outer sleeve in an advanced position to straighten the catheter sheath for introduction to a biological site; 
     FIG. 15 is a representation of the distal-end region of the catheter of FIG. 11 with the outer sleeve is retracted to allow the catheter sheath to assume its pre-formed shape; and 
     FIG. 16 is a representation of the distal-end region of the catheter of FIG. 12 with the distal-end region of the catheter sheath having a smaller radius of curvature due to movement of the steering tendon and wherein some of the electrodes are 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  15  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 using controls, e.g, rotatable knobs, pushbuttons or switches, located on a front panel of an instrument. In the case of a multiple channel instrument and catheter, as discussed below, multiple controls 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, such as a circular shape for use in ablation therapy around the pulmonary veins. 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 . 
     With reference to FIG. 2, the catheter  30  includes a sheath  118  having a distal-end region  106  for carrying an electrode device  16  (FIG.  1 ). The catheter  30  employs a shaped-memory wire, i. e., stylet  104 , having a preformed distal shape. The stylet is housed within the sheath  118  such that its preformed distal shape is substantially coincident with the distal-end region  106  and thereby imparts a preformed distal shape to the distal-end region  106 . The preformed distal shape may have any form which generally conforms to the contour of the biological cavity containing the tissue to be ablated. In order to ensure more precise placement of the electrode device within a biological site and intimate contact between the electrode device and the biological site, the catheter  30  further employs a steering tendon  102  and outer sleeve  100 , which may be used individually or collectively to provide enhanced manipulation of the distal-end region  106 . 
     The distal-end region  106  depicted in FIG. 2 has been simplified for clarity to depict varying degrees of curvature  108 ,  110  obtainable by use of the preformed shape and the steering tendon, as explained below. The distal shape of FIG. 2 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  102 , also provides a distal-end region  106  having a contour which may be adjusted to conform to the contour of the atrium. 
     As previously mentioned, the catheter  30  also includes an outer sleeve  100  that surrounds the sheath  118 . The outer sleeve  100  is positioned on the sheath  118  for movement along the length of the sheath and for rotation about the longitudinal axis of the sheath. The outer sleeve  100  includes a handle  134  located at its proximal end for facilitating movement of the sleeve. The sleeve  100  is moveable between a fully retracted position, as shown in FIG. 2, and various advanced positions, one of which is shown in FIG.  3 . 
     With reference to FIG. 4, the handle  134  includes a locking mechanism  136  for locking the outer sleeve to the sheath  118  at a selected location. The locking mechanism  136  includes a lock cap  135  and an expandable rubber seal  137 , each having a through-hole that allows the sheath  118  to pass through. The seal  137  is positioned between the lock cap  135  and the handle  134 . The lock cap  135  engages with the handle  134 , such as by a threaded interface or a snap mechanism. As the lock cap  135  is pressed into the seal  137 , the seal is constrained by the inside wall of the handle  134  and expands. Expansion of the seal  137  causes the seal through-hole to become smaller and to friction grip the sheath  118 . As a result, the locking mechanism  136 , handle  135  and outer sleeve  100  are locked in place relative to the sheath  118 . 
     With reference to FIGS. 5A and 5B, in a preferred embodiment the outer sleeve  100  includes a retaining mechanism for ensuring that the distal end of the outer sleeve does not advance past the distal tip of the catheter sheath  118 . In one retaining mechanism, as shown is FIG. 5A, the tip electrode  116  is configured such that its outer diameter is greater than the inner diameter of the outer sleeve  100 . In an alternate retaining mechanism, as shown in FIG. 5B, the sheath  118  is configured such that the sheath has a narrow section  119  having a diameter less than the inside diameter of the outer sleeve  100 . The section of the sheath  118  both distal and proximal the narrow section  119  have a diameter greater than the inside of the outer sleeve  100 , as such, movement of the sleeve is limited to the narrow section of the sheath. 
     The outer sleeve  100  is formed of a biocompatible material such as Pebax (a Nylon blend) having a stiffness sufficient to change the shape of the preformed distal end, as evident when comparing FIGS. 2 and 3. For example, in one configuration of the catheter the sleeve  100  material has a 63D (Shore D hardness scale) while the distal end of the sheath  106  has a 35 D hardness. In one embodiment, the outer sleeve includes a flush port  132  for flushing out any fluid that enters the space  140  (FIG. 10) between the sheath  118  and the inside surface of the sleeve. 
     With reference to FIG. 6, in another embodiment, the outer sleeve  100  also includes a seal  138 . The seal  138  is a soft rubber short-length tubing or o-ring formed of an elastomeric material (silicon, Santoprene, Viton) and is adhered to the inside diameter of the outer sleeve  100  near the distal end. The seal  138  forms a tight seal against the outer surface of the sheath  118  so as to prevent fluid from entering the space  140  (FIG. 10) between the outer sleeve and the sheath. The seal is pliable enough to allow for movement of the outer sleeve  100  relative to the sheath  118  yet rigid enough to function as a seal. As shown in FIGS. 7 and 8, the outer sleeve  102  may also have a preformed distal shape which in this case is a gradual bend of approximately 30 degrees. 
     The catheter  100  also includes a handle  112  and a steering member  114 . A tip component  116  is mounted to the sheath  118  at the very distal tip of the sheath. The stylet  104  (FIG. 9) is located in the distal-end region  106  and preferably runs the entire length of the sheath  118 . The stylet  104  is offset from the longitudinal axis of the sheath  118 , is attached to the tip component  116  and is anchored to a fixed position within the handle  112 . The stylet  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 stylet  104  retains the preformed distal shape. 
     Stress may be applied to the wire to change its shape. For example, as explained below, the stylet  104  may be straightened by the outer sleeve  100  to negotiate its way through a patient&#39;s vascular system to the right or left atrium of the heart. Upon removal of the straightening forces, such as when the outer sleeve is retracted, the stylet resumes its preformed shape thereby causing the distal end of the catheter sheath  118  surrounding it to likewise take the same shape. Because of the superelasticity of the nitinol, once the stress is removed the stylet  104  returns to its original shape. This is distinct from other shape-memory materials which are temperature actuated. 
     Referring now to FIGS. 10 and 11, 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. The sheath  118 , in turn, is housed inside the outer sleeve  100 . The outer sleeve is formed of biocompatible plastic. In the region  130  proximal from the distal-end region  106  (FIG.  2 ), as shown in the cross section in FIG. 10, 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 FIGS. 8 and 9, 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  (FIG.  2 ), as shown in cross section in FIG. 11, 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 . 
     With reference to FIG. 13, in one embodiment of the catheter, the outer sleeve  100  includes a steering tendon  113  carried by a lumen  111  within the wall of the sleeve. In this configuration of the outer sleeve  100 , the distal region may be made of a lower durometer material to enhance sleeve deflection. The tendon  113  is attached to the distal-end region of the sleeve  100 , runs the length of the sleeve and exits the proximal end of the sleeve where it is attached to a steering mechanism located on the locking mechanism (not shown). The steering mechanism applies axial tension to the steering tendon to axially displace the tendon and to deflect the distal-end region of the outer sleeve, which alters the radius of curvature (FIGS. 7 and 8) of the distal-end region of the sheath. 
     In operation, the catheter  30  is inserted into a patient&#39;s vascular system through an introducer sheath. Prior to being introduced, the outer sleeve  100  is advanced to a position such that substantially all of the distal-end region  106  is within the outer sleeve. With reference to FIG. 14, once introduced into the patient&#39;s vascular system, the catheter is guided to the biological cavity  122  e. g., right atrium, containing the tissue to be ablated. 
     For treatment procedures within the left atrium, the catheter  30  is introduced using a transseptal approach involving a transseptal introducer sheath (not shown). Once the transseptal introducer sheath is positioned between the right and left atria, the catheter  30  is inserted into the introducer sheath. Because of the flexibility of the nitinol stylet  104  and the outer sleeve  100  relative to the stiffness of the transseptal introducer sheath the distal-end region  106  of the sheath  118  and the outer sleeve  100  conform to the shape of the transseptal introducer sheath and follow the tortuous path of the introducer sheath. 
     Referring again to FIG. 14, a catheter  30  is shown being introduced to a biological cavity  142 . The outer sleeve  100  is in an advanced position so that the distal-end region  106  of the catheter sheath  118  has been placed into a desired linear configuration to more efficiently accomplish the introduction process. In FIG. 14, the sleeve  100  has been advanced and is located over the internal stylet  104  (FIG. 12) and is therefore straightening the stylet and the distal-end region  106  of the catheter. The sleeve  100  is also covering all electrodes when advanced. This may or may not occur, depending on the shape needed for the distal-end region  106  of the catheter  30  during the introduction process. 
     With reference to FIG. 15, once the distal-end region of the catheter enters the biological cavity  142 , the catheter sheath  118  is either extended beyond the distal tip of the outer sleeve  100  or the outer sleeve is retracted. In either case, the distal-end region  106  of the sheath  118  is no longer constrained by the outer sleeve  100  and the sheath returns to its original preformed distal shape as dictated by the internal stylet  104 . 
     Once the preformed distal shape is resumed, the distal-end region  106  has a shape more closely following that of the surfaces of the biological cavity  142 . However, it may not immediately conform to the shape of the biological site  126  to be ablated as closely as required. Accordingly, the shape of the distal-end region  106  may not allow for some or all of the electrodes  32  to be placed in intimate contact with the tissue  26  to be ablated. If the electrodes  32  are not in contact with the tissue  26 , the shape of the distal-end region  106  may be adjusted using the sheath steering tendon, the outer sleeve or a combination of both, such that the electrodes contact the biological tissue  26 . 
     With reference to FIG. 16, using the steering tendon  102 , the radius of curvature of the distal-end region  106  may be adjusted. 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. 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. 10 and 11, 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. 
     The position and shape of the distal-end region  106  may also be adjusted by adjusting the position of the outer sleeve  100 . For example, the outer sleeve  100  may be distally advanced relative the distal end of the sheath  118 , as shown in FIG.  3 . Because the outer sleeve  100  is formed of a material having a stiffness greater than the distal-end region  106 , the distal-end region is forced to assume a different shape. Where the outer sleeve  100 , itself includes a preformed distal shape, as shown in FIGS. 7 and 8, the position of the distal-end region  106  of the sheath  118  may also be adjusted by rotating the outer sleeve  100  relative to the catheter sheath. In doing so the shape and position of the distal-end region  106  changes. 
     The distal-end region may also be adjusted by adjusting the position of the outer sleeve  100 , locking the outer sleeve in position relative to the sheath  118  using the locking mechanism  136  and rotating the handle  112  to, in turn, simultaneously rotate both the sleeve and the sheath. Also, for outer sleeves  100  having steering capability, the position of the distal-end region  106  extending beyond the distal end of the outer sleeve may be adjusted by applying tension to the outer-sleeve tendon to deflect the outer sleeve along with the distal-end region in a desired direction. 
     Once the distal-end region  106  is properly positioned and the electrodes  32  are in intimate contact with the tissue, as shown in FIG. 16, 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 electrodes  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  102  and outer sleeve  100  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 a sheath with a preformed distal end and a steering mechanism surrounded by a movable stiffening outer sleeve, the combination of which provide for improved control over the shape of the distal-end region which thereby greatly increases the chances that 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.