Abstract:
A catheter includes a steering system for manipulating the distal-end region of a sheath having a plurality of electrodes into direct contact with difficult-to-reach areas of the human body. The steering system includes a core base, a core distal tip, a stylet, a steering tendon, and a lever. Movement of the lever applies tension to the steering tendon causing the distal end of the sheath to deflect. A positioning mechanism, including a slidable controller, adjusts the position of the steering system relative to the catheter sheath to thereby provide multiple steering profiles.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to steerable catheters. More particularly, the invention relates to a steerable catheter with a movable steering system for deflecting the distal-end region of the catheter in a variety of different profiles.  
           [0003]    2. Description of the Related Art  
           [0004]    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.” 
           [0005]    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.  
           [0006]    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.  
           [0007]    One such EP catheter system, as disclosed in U.S. Pat. Nos. 6,059,778 and 6,096,036, includes a plurality of spaced apart band electrodes located at the distal end of the catheter and arranged in a linear array. The band electrodes are positioned proximal heart tissue. RF energy is applied through the electrodes to the heart tissue to produce a series of long linear lesions similar to those produced by the Maze procedure. The catheters currently used for this procedure are typically flexible at the distal end, and the profile at the distal end is adjustable. However, when using such catheters, it is often difficult to conform the distal end profile to some of the irregular topographies of the interior cavities of the heart. In other instances, it is difficult for a multi-electrode catheter that is designed to produce long linear lesions to access and ablate tissue in regions that require short linear lesions, such as the so-called isthmus region that runs from the tricuspid annulus to the eustachian ridge. Ablation of tissue in this region, and other regions non-conducive to the placement of multi-electrode, long, linear-lesion ablation catheters within them, is best accomplished by delivering RF energy to a tip electrode to produce localized spot lesions or tip-drag lesions.  
           [0008]    Proposed methods of ablating irregular topography areas and regions, such as the isthmus region, use a rigid introducer sheath in combination with a tip-electrode ablation catheter. The introducer sheath is used to position the tip electrode in the proper location. Once positioned, the electrode is either held in place by the sheath to produce a spot lesion or is dragged along the surface of the tissue, by the sheath, to produce a tip-drag lesion. The disadvantage of this system is that it requires the use of two instruments: the introducer sheath and the catheter. The use of an introducer sheath increases both instrument cost and patient trauma.  
           [0009]    Other catheters for producing spot lesions or tip-drag lesions typically comprise a tip ablation electrode and a plurality of mapping band electrodes positioned at the distal end of the catheter. The catheters are steerable in that they are configured to allow the shape of the distal end of the catheter to be manipulated from a location outside the patient&#39;s body. Steerable catheters that produce multiple bending profiles provide a broader range of steerability. However, known steerable catheters such as that disclosed in U.S. Pat. No. 5,195,968 have steering tendons attached to a ribbon, at or near the longitudinal centerline of the catheter. Because these tendons are fixed in place, the catheter is capable of providing only two types of steering profiles. As such, its ability to ablate within a biological site having cavities of various different shapes and sizes is limited.  
           [0010]    Hence, those skilled in the art have identified a need for a catheter having a steerable distal-end region that is not limited to a select few deflection profiles but rather a variety of different profiles to improve access to difficult-to-reach locations of the human body. The present invention fulfills these needs and others.  
         SUMMARY OF THE INVENTION  
         [0011]    Briefly, and in general terms, the present invention is directed to an electrophysiological (“EP”) catheter with a steerable, multi-profile distal-end region for maneuvering through and positioning within irregular topographic and difficult-to-reach locations of the human body.  
           [0012]    In a first aspect, the invention relates to a catheter having a sheath with a proximal end, a distal-end region, and a lumen therebetween. The catheter also includes a steering system for deflecting the distal-end region and a positioning mechanism for adjusting the position of the steering system relative to the sheath. By providing a positioning mechanism that adjusts the position of the steering system, the present invention allows for the catheter flexible distal-end region to assume numerous different profiles to improve accessibility to difficult-to-reach locations of the human body.  
           [0013]    In a detailed aspect of the invention, the steering system includes a core. The core is slidably disposed within the lumen and has a distal end positioned in the distal-end region of the sheath. The steering system also includes a lever located at the proximal end of the sheath and a tendon located within the lumen. The tendon has a first end attached to the core distal end and a second end attached to the lever. In a further detailed aspect, the core includes a distal tip and the first end of the tendon is attached thereto. In yet another detailed aspect, the core further includes a core base proximal the distal tip and a stylet extending between the core base and the core distal tip. In still another detailed facet, the core distal tip is fixed to the distal end of the stylet and the proximal end of the stylet is fixed to the distal end of the core base. In other detailed facets, the positioning mechanism may adjust the steering system to an advanced position to effect deflection at the distal end of the sheath distal-end region, or to a retracted position to effect deflection at the proximal-end region of the sheath distal-end region.  
           [0014]    In another detailed facet, the positioning mechanism includes a handle having a proximal-end region and a distal-end region with the steering system core fixed thereto. The positioning mechanism also includes a cap that is fixed to the proximal end of the sheath. The cap is movable longitudinally along the distal-end region of the handle. In a further detailed facet, the cap includes a locking mechanism for locking the cap in place relative to the handle. In another further detailed facet, the distal-end region of the handle carries a plurality of recesses that interact with the locking mechanism.  
           [0015]    In another detailed facet, the positioning mechanism includes a handle having the proximal end of the sheath fixed thereto. The positioning mechanism also includes a controller that is carried by the handle and attached to the core. The controller is movable longitudinally along the distal-end region of the handle. The positioning mechanism further includes a locking element that is carried by the handle and fixed to the core. The locking element is movable longitudinally along the distal-end region of the handle. The locking element is housed within the handle and is locked and released by a spring-loaded button that can engage in various locking positions. In yet another detailed facet, the distal-end region of the handle carries a plurality of positioning slots that interact with the locking element.  
           [0016]    In a second aspect, the invention relates to a catheter having a sheath with a proximal end, a distal-end region and a lumen therebetween. The catheter also includes a core slidably disposed within the lumen with a distal end positioned in the distal-end region of the sheath. A tendon is located within the lumen and has a first end attached to the core distal end and a second end exiting the proximal end of the sheath. The catheter further includes a handle having the proximal end of the sheath fixed thereto, and a controller with a lever having the proximal end of the tendon attached and movable to effect axial displacement of the tendon. The controller, carried by the handle and attached to the core, is movable longitudinally along the distal-end region of the handle.  
           [0017]    In a detailed aspect of the invention, longitudinal movement of the controller in the distal direction advances the core in the distal direction. In another detailed aspect, longitudinal movement of the controller in the proximal direction retracts the core in the proximal direction.  
           [0018]    In a third aspect, the invention relates to a catheter having a sheath with a proximal end, a distal-end region, and a lumen therebetween. The catheter also includes a core. The core is slidably disposed within the lumen and has a proximal end and a distal end positioned in the distal-end region of the sheath. A tendon is located within the lumen and has a first end attached to the core distal end and a second end exiting the proximal end of the sheath. The catheter further includes a handle with a lever having the proximal end of the tendon attached and movable to effect axial displacement of the tendon. The handle has a proximal-end region and a distal-end region and the core fixed thereto. An adjustable cap is fixed to the proximal end of the sheath with the cap movable longitudinally along the distal-end region of the handle.  
           [0019]    In a detailed aspect of the invention, longitudinal movement of the cap in the distal direction advances the sheath in the distal direction. In another detailed aspect, longitudinal movement of the cap in the proximal direction retracts the sheath in the proximal direction.  
           [0020]    In yet another detailed aspect, the cap includes a locking mechanism for locking the cap in place relative to the handle. In another detailed aspect, the distal-end region of the handle carries a plurality of recesses that interact with the locking mechanism.  
           [0021]    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  
       [0022]    [0022]FIG. 1 is a plan view with a broken-out section of a catheter configured in accordance with the invention depicting the primary components of the catheter including a sheath, a positioning mechanism, and a steering system.  
         [0023]    [0023]FIG. 2A is a section view of the construction of the proximal region of the sheath, taken along the line  2 - 2  from FIG. 1.  
         [0024]    [0024]FIG. 2B is a section view of an alternative construction of the proximal region of the sheath, taken along the line  2 - 2  from FIG. 1.  
         [0025]    [0025]FIG. 3 is a section view of the construction of the distal-end region of the sheath, taken along the line  3 - 3  from FIG. 1.  
         [0026]    [0026]FIG. 4A is a cross section view of the distal portion of the catheter of FIG. 1 depicting detailed components of the catheter steering system in a fully advanced position.  
         [0027]    [0027]FIG. 4B is a downward view of the distal portion of the catheter, taken along the line  4 B- 4 B from FIG. 4A, depicting detailed components of the catheter steering system.  
         [0028]    [0028]FIG. 4C is a cross section view of the distal portion of the catheter of FIG. 1 depicting detailed components of the catheter steering system in a partially retracted position.  
         [0029]    [0029]FIG. 5A is a cross section of the catheter handle of FIG. 1 depicting a fully advanced position of the positioning mechanism and steering system along the length of the handle.  
         [0030]    [0030]FIG. 5B is a cross section of the catheter handle of FIG. 1 depicting a fully retracted position of the positioning mechanism.  
         [0031]    [0031]FIG. 6 is a cross section of an alternate configuration of the positioning mechanism shown in a retracted position and including an adjustable cap and a locking mechanism.  
         [0032]    [0032]FIG. 7 is a cross section of the positioning mechanism of FIG. 6 in an advanced position.  
         [0033]    [0033]FIG. 8 is a schematic depicting the profile created within the distal-end region of the catheter when the steering system is in a fully advanced position such as shown in FIG. 4A.  
         [0034]    [0034]FIG. 9 is a schematic depicting the profile created within the distal-end region of the catheter when the steering system is in a retracted position such as shown in FIG. 4C. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    Referring 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 a catheter  10  configured in accordance with aspects of the present invention. The catheter  10  includes a sheath  12  having a flexible distal-end region  14 , a less flexible proximal-end region  16 , and an inner lumen (not shown) spanning longitudinally throughout the sheath.  
         [0036]    The proximal end  18  of the sheath  12  is secured to a handle  20 . The handle  20  carries a controller  22 , a lever  24 , and a plurality of positioning slots  26 . The handle  20  and controller  22  form a positioning mechanism that is movable along the positioning slots  26 . Movement of the controller  22  within the handle  20  effects the position of the steering system, which in turn, as described further below, effects the steerable profile of the catheter  10 . As also described in detail below, the lever  24  is part of a steering system which further includes a core base  28  and a tendon  30 .  
         [0037]    The distal end  32  of the sheath  12  includes a tip electrode  34  for applying ablation energy to a biological site (not shown). Located proximal from the tip electrode  34  are three band electrodes  36  arranged in a substantially linear array along the distal-end region  14  of the sheath  12 . The band electrodes  36  are arranged so that there is space  38  between adjacent electrodes. In one configuration, the band electrodes  36  are three mm wide and the space  38  between the electrodes is four mm wide. The band electrodes  36  may be used to map the interior surfaces of the heart or to apply ablation energy, or both. The tip electrode  34  may be used to deliver RF energy to the biological site (not shown) to form spot or tip-drag lesions, or for mapping.  
         [0038]    Referring to FIGS. 2A and 2B, the proximal-end region  16  of the sheath is a layered composite. The inner layer  40  is a tube formed of a polymer pocessing a high modulus of elasticity, such as polyetheretherketone (PEEK). The outer layer  42  is formed of a flexible. intermediate-durometer polymer such as polyether block amide, known commercially as Pebax™. In one particular embodiment, the outer layer  42  comprises a 63 D (shore “D” hardness value) hardness scale Pebax™ tube. Positioned between the inner layer  40  and the outer layer  42  is one or more layers of a braided ribbon  44  used to increase the torsional rigidity of the proximal-end region  16 . Only one layer is shown in FIGS. 2A and 2B for clarity of illustration. The three layers  40 ,  42 , and  44  are bonded together by the simultaneous application of heat and pressure, thus creating a flexible tube possessing superior torsional rigidity.  
         [0039]    As shown in FIG. 2A, in one configuration of the catheter, positioned along the circumference on one side of the inner layer  40  is a groove  46 . A separate groove  48  is positioned on the opposite side of the inner layer  40 . The grooves  46 ,  48  span the length of the proximal-end region  16  (FIG. 1) from the handle  20  (FIG. 1) to the distal-end region  14  (FIG. 1). As explained below, the grooves  46 ,  48  are used to carry lead wires to the electrodes  34 ,  36 . A lumen  82  for the steering tendon  30  (FIG. 1) is carried by the core base  28  and positioned perpendicular to the grooves  46 / 48 . The core base  28  is surrounded by the three layer section of the proximal-end region  16  of the sheath  12 .  
         [0040]    Referring to FIG. 2B, in an alternative configuration of the catheter, the core base  29  is fabricated with two grooved indentations  83  on opposite sides which extend longitudinally along the length of the core base. A lumen  82  for the steering tendon  30  (FIG. 1) is offset perpendicular to the grooved indentations  83  within the core base  29 . In both of FIGS. 2A and 2B, the lead wires and steering tendon are not shown for clarity of illustration.  
         [0041]    With reference to FIG. 3, the construction of the distal-end region  14  is essentially the same as that of the proximal-end region  16  (FIG. 2A) except it does not include a middle stainless steel braided ribbon  44  (FIG. 2A) layer. In addition, the outer layer  43  has a thickness greater than the outer layer  42  (FIG. 2A) of the proximal-end region. The inner layer  41  has substantially the same thickness as the inner layer  40  of the proximal-end region. Because the distal-end region  14  does not include a braided ribbon  44  it has more relative flexibility than the proximal-end region  16  enabling it to more easily steer to conform to the selected biological site. Flexibility of the distal-end region  14  may be further increased by using a lower durometer material for the layers  41 ,  43 .  
         [0042]    With continued reference to FIG. 3, positioned along the circumference on one side of the inner layer  41  is a groove  47 . A separate groove  49  is positioned on the opposite side of the inner layer  41 . The grooves  47 ,  49  span the length of the distal-end region  14  and align with the grooves  46 ,  48  in the proximal-end region  16  (FIG. 2) to form continuous grooves that span the entire length of the sheath  12 . As shown in FIGS. 4A and 4C, the grooves  46 / 47 ,  48 / 49  carry individual lead wires  50  and a pair of thermocouple wires  52  to the band electrodes  36  and the bore  54  within the tip electrode, respectively. In order to form one continuous sheath  12  (FIG. 1), the distal-end region  14  and the proximal-end region  16  are bonded together.  
         [0043]    Referring to FIGS.  4 A-C, the tip electrode  34  includes a substantially dome-shaped distal portion  56  and a substantially cylindrical proximal portion  58 . The tip electrode  34  is formed of a biocompatible material having high thermal conductivity properties. Possible materials include silver, gold, chromium, aluminum, molybdenum, tungsten, nickel, platinum, and platinum/10% iridium.  
         [0044]    The band electrodes  36  are also formed of a material having a significantly higher thermal conductivity than that of the biological tissue. Because of the difference in thermal conductivity between the band electrodes  36  and the tissue, the electrodes cool off more rapidly in the flowing fluids at the biological site. The band electrodes  36  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  36  are 7 French (2.3 mm in diameter) with a length of 3 mm.  
         [0045]    Individual lead wires  50 , as shown in FIGS. 4A and 4C, extend from a connector  23  (FIG. 1) at the distal end of the handle  20  to each band electrode  36 . The lead wires  50  are attached to the band electrodes  36  in a way that establishes good electrical contact, such as by welding. The lead wires  50  are grouped together and span throughout one of two grooves  47 ,  49  within the distal-end region  14  and continue into the proximal-end region  16  (FIG. 1) of the sheath  12  through one of two grooves  46 ,  48 . The sheath  60  is formed of a flexible material, such as a thin-walled heat-shrink tubing, so that it may bend as necessary.  
         [0046]    With further reference to FIGS. 4A and 4C, a pair of thermocouple wires  52  run from the handle  20  (FIG. 1) through the sheath  12  to a bore  54  within the tip electrode  34 . Each of the thermocouple wires  52  is separately attached at the distal end of the bore  54  in the tip electrode  34  in a way that maintains good electrical contact, such as by soldering. The attachment of the thermocouple wires  52  to the tip electrode  34  in this manner achieves the thermocouple effect through the tip electrode, and also achieves good thermal contact for a more accurate determination of the temperature of the tip electrode. Subsequent to being attached to the bore  54 , the thermocouple wires  52  are potted into the bore with a resin, such as epoxy. One of the thermocouple wires  52  also serves as a drive wire to transmit ablation energy to the tip electrode  34 . Exemplary configurations of electrodes having combination thermocouple/drive wires are disclosed in U.S. Pat. Nos. 6,049,737 and 6,045,550, the disclosures of which are hereby incorporated by reference. The thermocouple wires  52  are grouped together and span through one of two grooves  47 ,  49  within the distal-end region  14  and continue into the proximal-end region  16  of the sheath  12  through one of two grooves  46 ,  48 .  
         [0047]    With continued reference to FIGS. 4A and 4B, the steering system, in addition to the steering tendon  30  and the core base  28  mentioned above, includes a ribbon stylet  64 , and a core distal tip  66 , all of which are partially or entirely housed within the sheath  12 . The core base  28  and core distal tip  66  are cylindrical and sized to allow for longitudinal movement within the sheath lumen  76 . In a preferred embodiment, the diameter of the core distal tip  66  is the same as the core base  28 . As previously mentioned, the steering system also includes a lever  24  (FIG. 1) positioned external the sheath  12  at the proximal end of the catheter  10 . As shown most clearly in FIG. 4B, the distal end  68  of the tendon  30  is attached to the core distal tip  66 , and is offset from the longitudinal axis of the sheath  12 . The distal end  68  of the tendon  30  is secured to the proximal end of the core distal tip  66  such as by welding, soldering, brazing, adhering, or otherwise attached to the core distal tip  66 . The tendon  30  extends longitudinally through the core base  28  through the lumen  82  to the handle  20  (FIG. 1). As shown in FIG. 1, the proximal end  70  of the steering tendon  30  exits through the proximal end  18  of the sheath  12 , and attaches to the lever  24 . The tendon  30  may be formed from stainless steel wire having a diameter of approximately 0.2 mm.  
         [0048]    Referring again to FIGS. 4A and 4B, the stylet  64  includes a distal end  72  and a proximal end  74 . The distal end  72  of the stylet  64  is attached to the core distal tip  66  at a first end and is secured thereto such as by welding, soldering, brazing or adhering. The stylet  64  extends longitudinally throughout the inner lumen  76  of the sheath  12  and is attached at its proximal end  74  to the core base  28 . In a preferred embodiment, the stylet  64  is formed of a shape-memory alloy element which exhibits martensitic phase transformation. Some examples of alloys with the aforementioned properties 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). It is preferable that the stylet  64  is formed of nitinol having a composition of 49-51.5% Ni. In one embodiment, the stylet  64  is circular in cross-section and has a diameter of 0.030 inches. In another embodiment, the stylet  64  is rectangular in cross-section and has a dimension of 0.008 inches×0.053 inches.  
         [0049]    The core base  28  includes a distal end  78  and a proximal end  80  (FIG. 1), and is preferably a solid cylindrical member made from an extrudable polymer possessing a high elastic modulus. Exemplary of such polymers include polyetheretherketone (PEEK), polyimide, and polyetherimide (Ultem).  
         [0050]    In a preferred embodiment, the core base  28  is sized to fit within the sheath lumen  76  with sufficient clearance to allow for longitudinal movement within the lumen. As previously mentioned, the core base  28  includes a lumen  82  offset from the center axis. The lumen  82  extends through the length of the core base  28  and carries the steering tendon  30 . As shown in FIGS.  4 A-C, the core base  28  is attached to the stylet  64  at its distal end  78 . The distal end  78  of the core base  28  is positioned in the distal-end region  14  of the sheath  12 . As explained further below, the core base  28  is able to move longitudinally throughout the inner lumen  76  of the sheath  12  with its position controlled by corresponding longitudinal movement of the controller  22 . As shown in FIGS. 5A and 5B, the handle  20  has the proximal end  18  of the sheath  12  fixed thereto. The controller  22  is carried by the handle  20  and is attached to the core base  28  at its distal end  112 . The core base  28  extends into the controller  22  and passes through a locking element  110 . The core base  28  terminates just beyond the locking element  110  while the steering tendon  30  carried by the core base extends to the lever  24  where it is attached. The lever  24  is movable about an axis to effect axial displacement of the tendon  30  along the length of the sheath  12 . The controller  22  is positioned between a pair of support plates  25  fixed to the handle  20 . Situated along the exterior of the handle  20  is a series of slots  26  to secure a select position of the controller  22  by engaging the locking element  110  as it moves along the length of the handle  20  when advancing or retracting the steering system. The locking element  110  is positioned at the distal end  112  of the controller  22  and is locked and released by a spring-loaded button  122  that can engage in various locking positions. Although FIGS. 5A and 5B depict a series of four slots  25  positioned along the distal-end region  84  of the handle  20 , the present invention is not limited to such as additional or fewer such slots may be used.  
         [0051]    When the controller  22  is in an advanced position  86  (FIG. 5A), the internal portion of the steering system, i.e., the core base  28 , stylet  64 , and the core distal tip  66 , is positioned as shown in FIG. 4A. When in this advanced position, the steering system, through rotation of the lever  24 , is able to deflect the distal-end region  14  of the sheath  12  to assume the curve  88  as shown in FIG. 8.  
         [0052]    When the controller  22  is in a retracted position  90  (FIG. 5B), the internal portion of the steering system, as mentioned above, is positioned as shown in FIG. 4C. When in this retracted position, the steering system is able to deflect the distal-end region  14  of the sheath  12  to assume a slightly curved shape  92  (FIG. 9) while the portion distal the core distal tip  66  does not curve.  
         [0053]    Referring now to FIGS. 6 and 7, in an alternate configuration of the catheter  10 , the positioning mechanism consists of an adjustable cap  94  which is fixed to the proximal end  18  of the sheath  12 . The cap  94  is movable longitudinally along the length of the distal-end region  84  of the handle  20 . While the cap  94  is movable, the steering system core base  28  is fixed to the handle  20  and hence does not move when the cap  94  is being positioned to a particular setting. The adjustable cap  94  includes one or more locking mechanisms  96  such as set screws, for securing the cap in place relative to the handle  20 . The distal-end region  84  of the handle  20  carries a plurality of annular recesses  98  into which the set screws  96  may be tightened in order to lock the cap  94  in place. Although the figure only shows four recesses, in other configurations, additional recesses maybe included. The steering tendon  30  runs through the entire length of the core base  28  and exits from its proximal end  80 . The tendon  30  continues to extend through the inner lumen  100  of the handle  20  towards a steering lever (not shown in FIGS.  6  or  7 ), where it is attached.  
         [0054]    Retraction of the cap  94  along the length of the handle  20  to a position as shown in FIG. 6, positions the internal portion of the steering system, i.e., the core base  28 , stylet  64 , and the core distal tip  66 , as shown in FIG. 4A. In this position (FIG. 6) the adjustable cap  94  is locked into place in the innermost recess of the distal-end region  84  of the handle  20 .  
         [0055]    Advancement of the cap  94  along the length of the handle  20 , as shown in FIG. 7, positions the internal portion of the steering system as shown in FIG. 4B. In this position (FIG. 7) the adjustable cap  94  is set in a fully advanced position with the set screws  96  locked in the most distal recess  98 . With the advancement of the adjustable cap  94  along the length of the handle  20 , the core base  28  remains in a fixed position in the distal-end region  84  of the handle  20 . As the adjustable cap  94  is set in an advanced position, a space  104  separates the distal-end region  84  of the handle  20  from the proximal-end region  106  of the adjustable cap.  
         [0056]    With further reference to FIG. 6, a lumen  116  extends through the length of the distal-end region  14  of the sheath  12 , the adjustable cap  94 , and the handle  20 , carrying loosely coiled individual lead wires  50  from the electrode bands (not shown) while the cap is set in a fully retracted position. A lumen  117  on an opposite side extends from the distal end  32  of the sheath  12 , through the length of the adjustable cap  94 , and the handle  20 , carrying the set of thermocouple wires  52  from the tip electrode  34  (not shown). The lead wires  50 ,  52  are coiled so as to provide the necessary slack to allow the adjustable cap  94  to move between retracted and advanced positions. As shown in FIG. 7, when the adjustable cap  94  is fully advanced, the wires  50 ,  52  within the lumens  116 ,  117  become taut. As the adjustable cap  94  is retracted, the wires  50 ,  52  assume their coiled shape.  
         [0057]    In operation, as shown in FIGS. 4A and 4C, the profile of the distal-end region  14  of a catheter configured in accordance with the invention can be changed by altering the position of the steering system, particularly the core distal tip  66 , relative to a catheter distal tip  108 , and then applying tension to the tendon  30 . When tension is applied to the tendon  30 , the stylet  64  and core distal tip  66  are deflected toward the inner layer  41  of the sheath  12 . The core distal tip  66  contacts the inner layer  41  and imparts lateral force to the sheath  12 , thereby causing the distal-end region  14  of sheath  12  to curve. The greater the tension applied to the tendon the more pronounced the curve becomes. For example, by positioning the steering system as shown in FIG. 4A and applying tension, the distal-end region  14  of the sheath  12  may be made to assume the curve  88  as shown in FIG. 8. By positioning the steering system as shown in FIG. 4C and applying tension to the tendon  30  with the use of the steering lever  24 , the distal-end region  14  of the sheath  12  may be made to assume a slightly curved shape  92 , while the portion distal the core distal tip  66  does not curve as shown in FIG. 9. FIGS. 8 and 9 represent only two of many possible deflection profiles that may be obtained with the catheter. A variety of different profiles are available by adjusting the position of the steering system.  
         [0058]    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.