Patent Publication Number: US-6668198-B2

Title: Structures for supporting porous electrode elements

Description:
RELATED APPLICATION DATA 
     This application is a continuation of U.S. patent application Ser. No. 09/538,108, filed on Mar. 29, 2000, now U.S. Pat. No. 6,330,473, which is a continuation of U.S. patent application Ser. No. 08/770,572, filed on Dec. 19, 1996, now U.S. Pat. No. 6,076,012, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates structures for supporting one or more diagnostic or therapeutic elements in contact with body tissue. In a more particular sense, the invention relates to structures well suited for supporting one or more electrode elements within the heart. 
     BACKGROUND OF THE INVENTION 
     The treatment of cardiac arrhythmias requires electrodes capable of creating tissue lesions having a diversity of different geometries and characteristics, depending upon the particular physiology of the arrhythmia to be treated. 
     For example, it is believed the treatment of atrial fibrillation and flutter requires the formation of continuous lesions of different lengths and curvilinear shapes in heart tissue. These lesion patterns require the deployment within the heart of flexible ablating elements having multiple ablating regions. The formation of these lesions by ablation can provide the same therapeutic benefits that the complex incision patterns that the surgical maze procedure presently provides, but without invasive, open heart surgery. 
     By way of another example, small and shallow lesions are desired in the sinus node for sinus node modifications, or along the A-V groove for various accessory pathway ablations, or along the slow zone of the tricuspid isthmus for atrial flutter (AFL) or AV node slow pathways ablations. However, the elimination of ventricular tachycardia (VT) substrates is thought to require significantly larger and deeper lesions. 
     There also remains the need to create lesions having relatively large surface areas with shallow depths. 
     The task is made more difficult because heart chambers vary in size from individual to individual. They also vary according to the condition of the patient. One common effect of heart disease is the enlargement of the heart chambers. For example, in a heart experiencing atrial fibrillation, the size of the atrium can be up to three times that of a normal atrium. 
     A need exists for electrode support structures that can create lesions of different geometries and characteristics, and which can readily adopt to different contours and geometries within a body region, e.g., the heart. 
     SUMMARY OF THE INVENTION 
     The invention provides structures for supporting operative therapeutic or diagnostic elements within an interior body region, like the heart. The structures possess the requisite flexibility and maneuverability permitting safe and easy introduction into the body region. Once deployed in the body region, the structures possess the capability to conform to different tissue contours and geometries to provide intimate contact between the operative elements and tissue. 
     In one embodiment, the invention provides a catheter assembly comprising a elongated, flexible support structure having an axis. The assembly also includes an elongated porous electrode assembly carried by the support structure along the axis for contact with tissue. The elongated porous electrode assembly comprises a wall having an exterior peripherally surrounding an interior area, a lumen to convey a medium containing ions into the interior area, and an element coupling the medium within the interior area to a source of electrical energy. At least a portion of the wall comprising a porous material is sized to allow passage of ions contained in the medium to thereby enable ionic transport of electrical energy through the porous material to the exterior of the wall to form a continuous elongated lesion pattern in tissue contacted by the wall. 
     In one embodiment, the elongated porous electrode assembly comprises an array of electrode, segments formed by segmenting the wall along the axis. 
     In one embodiment, the support structure has a curvilinear geometry, and the elongated porous electrode assembly conforms to the curvilinear geometry. 
     In one embodiment, the support structure is adapted to form a loop geometry, and the elongated porous electrode assembly conforms to the loop geometry. 
     Other features and advantages of the inventions are set forth in the following Description and Drawings, as well as in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a probe, which carries on its distal region a multiple electrode support structure that embodies features of the invention; 
     FIG. 2A is an enlarged side view, with portions broken away and in section, of the distal region of the probe shown in FIG. 1; 
     FIG. 2B is a side view of the multiple electrode structure shown in FIG. 1, in which stiffness is varied using a slidable, tapered spline leg; 
     FIG. 3A is an enlarged side view of the distal region of the probe shown in FIG. 1, showing the multiple electrode structure advanced from the associated sheath to form a loop; 
     FIG. 3B is a perspective end view of an embodiment of the sheath shown in FIG. 3A, in which wires are placed to provide added torsional stiffness; 
     FIG. 3C is an end view of an embodiment of the sheath shown in FIG. 3A, which has been eccentrically extruded to provide added torsional stiffness; 
     FIG. 4A is a side view of the distal region shown in FIG. 3A, in which the catheter tube is stiffer than the sheath, and in which the catheter tube has been rotated within the sheath and flipped over upon itself; 
     FIG. 4B is a side view of the distal region shown in FIG. 3A, in which the catheter tube is not as stiff as the sheath, and in which the catheter tube has been rotated within the sheath to form an orthogonal bend in the loop; 
     FIG. 5 is a side view of an embodiment of the distal region shown in FIG. 3A, in which the size of the slot through which the loop extends can be varied; 
     FIG. 6 is a side view of an embodiment of the distal region shown in FIG. 3A, in which a prestressed spline within the loop structure alters the geometry of the structure; 
     FIGS. 7A,  7 B, and  7 C are top views of different embodiments of the distal region shown in FIG. 3A, in which the slot is shown having different geometries, which affect the geometry of the resulting loop; 
     FIG. 8 is a side view of an embodiment of the distal region shown in FIG. 3A, in which the proximal end of the slot is tapered to facilitate formation of the loop; 
     FIG. 9 is a side view of an embodiment of the distal region shown in FIG. 3A, in which the slot has a helical geometry; 
     FIG. 10 is a side view of the distal region shown in FIG. 9, with the loop support structure deployed through the helical slot; 
     FIG. 11 is a side view of an embodiment of the distal region shown in FIG. 3A, with the catheter tube having a prebent geometry orthogonal to the loop structure; 
     FIG. 12 is a side view of an embodiment of the distal region shown in FIG. 11, with the sheath advanced forward to straighten the prebent geometry; 
     FIG. 13A is a section view of the catheter tube within the sheath, in which the geometries of the sheath and catheter tube are extruded to prevent relative rotation; 
     FIG. 13B is a section view of the catheter tube within the sheath, in which the geometries of the sheath and catheter tube are extruded to permit limited relative rotation; 
     FIG. 14 is an enlarged side view of an alternative embodiment the distal region of the probe shown in FIG. 1; 
     FIG. 15A is a side view of the distal region shown in FIG. 14, showing the multiple electrode structure advanced from the associated sheath to form a loop; 
     FIG. 15B is a side view of an alternative embodiment of the distal region shown in FIG. 14; 
     FIGS. 16A,  16 B, and  16 C are view of the distal region shown in FIG. 14, showing alternative ways to stiffen the flexible junction between the sheath and the catheter tube; 
     FIG. 17A is an enlarged side view of an alternative embodiment the distal region of the probe shown in FIG. 1; 
     FIG. 17B is a section view of an embodiment of the distal region shown in FIG. 17A; 
     FIGS. 18,  19 , and  20 , are side sectional view, largely diagrammatic, showing an embodiment of the distal region shown in FIG. 1, in which the electrode array is movable; 
     FIG. 21 is an enlarged side view of an alternative embodiment of the distal region of the probe shown in FIG. 1, with the associated sheath withdrawn and with no rearward force applied to the associated pull wire; 
     FIG. 22 is an enlarged side view of the distal region of the probe shown in FIG. 21, with the associated sheath advanced; 
     FIG. 23 is an enlarged side view of distal region of the probe shown in FIG. 21, with the associated sheath withdrawn and with rearward force applied to the associated pull wire to form a loop structure; 
     FIG. 24 is an enlarged side view of an alternative embodiment of the distal region shown in FIG. 21, with a pivot connection; 
     FIG. 25 is an enlarged elevation side view of an alternative embodiment of the distal region of the probe shown in FIG. 1, showing a preformed loop structure; 
     FIG. 26 is an enlarged, side section view of the slidable end cap shown in FIG. 25; 
     FIG. 27 is a side view of the distal region shown in FIG. 25, with the interior wire pulled axially to change the geometry of the preformed loop structure; 
     FIG. 28 is a side view of the distal region shown in FIG. 25, with the interior wire bend across its axis to change the geometry of the preformed loop structure; 
     FIG. 29 is a side view of the distal region shown in FIG. 25, with the interior wire rotated about its axis to change the geometry of the preformed loop structure; 
     FIG. 30 is an enlarged, perspective side view of an alternative embodiment of the distal region of the probe shown in FIG. 1, showing a preformed, multiple spline loop structure; 
     FIG. 31 is an enlarged, perspective side view of an alternative embodiment of the distal region of the probe shown in FIG. 30 showing a preformed, multiple spline loop structure with asymmetric mechanical stiffness properties; 
     FIG. 32 is an enlarged, perspective side view of an alternative embodiment of the distal region of the probe shown in FIG. 1, showing a preformed, multiple independent spline loop structures; 
     FIG. 33 is an enlarged elevation side view of an alternative embodiment of the distal region of the probe shown in FIG. 1, showing a preformed loop structure, which, upon rotation, forms an orthogonal bend; 
     FIG. 34 is an enlarged side view of the distal region shown in FIG. 33 with the orthogonal bend formed; 
     FIG. 35 is a section view of the distal region shown in FIG. 33, taken generally along line  35 — 35  in FIG. 33, 
     FIG. 36 is a section view of the distal region shown in FIG. 33, taken generally along line  36 — 36  in FIG. 33 
     FIG. 37 is a section view of the distal region shown in FIG. 34 taken generally along line  37 — 37  in FIG. 34 
     FIG. 38 is an enlarged, perspective side view of an alternative embodiment of the distal region of the probe shown in FIG. 1, showing a pretwisted loop structure, which forms an orthogonal bend; 
     FIG. 39 is a side section views of a portion of the loop structure shown in FIG. 38, taken generally along line  39 — 39  in FIG. 38 
     FIG. 40A is an enlarged side view of an alternative embodiment of the distal region of the probe shown in FIG. 1, showing a preformed loop structure, which, upon rotation, forms an orthogonal bend; 
     FIG. 40B is an enlarged side view of the distal region shown in FIG. 40A, with the orthogonal bend formed; 
     FIG. 41 is an enlarged side perspective view of an alternative embodiment of the distal region of the probe shown in FIG. 1, showing a preformed loop structure, which has a prestressed interior spline forming an orthogonal bend; 
     FIG. 42 is a largely diagrammatic view of the deployment of the distal region of the probe shown in FIG. 1 in the right atrium of a heart; 
     FIG. 43 is a side elevation view of an alternative embodiment of the distal region of the probe shown in FIG. 1, showing a self-anchoring, multiple electrode structure; 
     FIG. 44 is a section view of the self-anchoring structure shown in FIG. 43 
     FIG. 45 is a side elevation view of an embodiment of the distal region shown in FIG. 46 in which the anchoring branch is movable; 
     FIG. 46 is a side elevation view of the distal region of the probe shown in FIG. 43 with the self-anchoring, multiple electrode structure withdrawn within an associated sheath; 
     FIGS. 47,  48 , and  49  show the deployment of the multiple, self-anchoring electrode structure shown in FIG. 43 within a body region; 
     FIGS. 50A and 50B show, in diagrammatic form, the location of regions within the heart in which the self-anchoring structure shown in FIG. 43 can be anchored; 
     FIG. 51 is a side view of an embodiment of the self-anchoring structure shown in FIG. 43 in which the branch carrying electrode elements can be advanced or retracted or rotated along or about its axis; 
     FIG. 52 is a side view of an embodiment of the self-anchoring structure shown in FIG. 43 in which the branch carrying electrode elements can be torqued about the main axis of the structure; 
     FIG. 53 is a side elevation view of an alternative embodiment of the distal region of the probe shown in FIG. 1, showing a self-anchoring, loop structure; 
     FIG. 54 is a side elevation view of an alternative embodiment of the distal region shown in FIG. 52 also showing a type of a self-anchoring, loop structure; 
     FIG. 55 is a side elevation view of an alternative embodiment of the distal region shown in FIG. 43 showing a self-anchoring structure with an active anchoring element; 
     FIG. 56 is a side view of an alternative embodiment of the distal region of the probe shown in FIG. 1, showing a spanning branch structure; 
     FIG. 57 is a side sectional view of the spanning branch structure shown in FIG. 56, with the associated sheath advanced; 
     FIG. 58 is a side view of the spanning branch structure shown in FIG. 56, with the associated sheath retracted and the structure deployed in contact with tissue; 
     FIG. 59 is a side view of an alternative embodiment a spanning branch structure of the type shown in FIG. 56; 
     FIG. 60 is a side view of the spanning branch structure shown in FIG. 59 deployed in contact with tissue; 
     FIG. 61 is a side view of an alternative embodiment of the distal region of the probe shown in FIG. 1, showing a spring-assisted, spanning branch structure; 
     FIG. 62 is a side sectional view of the spring-assisted, spanning branch structure shown in FIG. 61, with the associated sheath advanced; 
     FIGS. 63A and 63B are side views of the deployment in a body region of the spring-assisted, spanning branch structure shown in FIG. 61; 
     FIG. 63C is a side view a spring-assisted, spanning branch structure, like that shown in FIG. 61, with an active tissue anchoring element; 
     FIG. 64 is a representative top view of long, continuous lesion pattern in tissue; 
     FIG. 65 is a representative top view of segmented lesion pattern in tissue; 
     FIG. 66 is a side view of an alternative embodiment of a self-anchoring, loop structure, showing the catheter tube detached from the associated sheath; 
     FIG. 67 is a side view of the self-anchoring, loop structure shown in FIG. 66, with the catheter tube attached to the associated sheath; 
     FIG. 68 is a side view of the self-anchoring, loop structure shown in FIG. 67, showing the catheter tube advanced in an outwardly bowed loop shape from the associated sheath; 
     FIG. 69 is a side section view of a portion of the distal region shown in FIG. 66, showing the inclusion of a bendable spring to steer the self-anchoring loop structure; 
     FIG. 70 is a side view of the self-anchoring, loop structure shown in FIG. 67, showing the structure deployed for use within a body cavity; 
     FIG. 71 is a side view, with parts broken away and in section, of an alternative embodiment of the self-anchoring, loop structure shown in FIG. 67, with an interference fit releasably coupling the catheter tube to the associated sheath; 
     FIG. 72 is a side view, with parts broken away and in section, of an alternative embodiment of the self-anchoring, loop structure shown in FIG. 67, with a releasable snap-fit coupling the catheter tube to the associated sheath; 
     FIG. 73 is a side view of an alternative embodiment of the self-anchoring, loop structure shown in FIG. 67, with a pivoting connection releasably coupling the catheter tube to the associated sheath; 
     FIG. 74 is a side view of a embodiment of a pivoting connection of the type shown in FIG. 73, with the catheter tube released from the associated sheath; 
     FIG. 75 is a side view, with parts broken away and in section, the pivoting connection shown in FIG. 74, with the catheter tube attached to the associated sheath; 
     FIG. 76 is a side perspective view of the pivoting, connection shown in FIG. 75, with the catheter tube pivoting with respect to the associated sheath; 
     FIG. 77A is an exploded, perspective view of an alternative embodiment of a releasable pivoting connection of the type shown in FIG. 73, with the catheter tube detached from the associated sheath; 
     FIG. 77B is an exploded, perspective view of the reverse side of the pivoting connection shown in FIG. 77A, with the catheter tube detached from the associated sheath; 
     FIG. 77C is a top side view of the releasable pivoting connection shown in FIG. 77A, with the catheter tube attached to the associated sheath; 
     FIG. 77D is a top side view of the releasable pivoting connection shown in FIG. 77C, with the catheter tube attached to the associated sheath and pivoted with respect to the sheath; 
     FIG. 78A is an exploded, perspective view of an alternative embodiment of a releasable pivoting connection of the type shown in FIG. 73, with the catheter tube detached from the associated sheath; 
     FIG. 78B is a top view of the releasable pivoting connection shown in FIG. 78A, with the catheter tube attached to the associated sheath; 
     FIG. 78C is a top side view of the releasable pivoting connection shown in FIG. 78B, with the catheter tube attached to the associated sheath and pivoted with respect to the sheath; 
     FIG. 79 shows, in diagrammatic form, sites for anchoring a self-anchoring structure within the left or right atria; 
     FIGS. 80A to  80 D show representative lesion patterns in the left atrium, which rely, at least in part, upon anchoring a structure with respect to a pulmonary vein; 
     FIGS. 81A to  81 C show representative lesion patterns in the right atrium, which rely, at least in part, upon anchoring a structure with respect to the superior vena cava, the inferior vena cava, or the coronary sinus; 
     FIG. 82 shows a loop structure of the type shown in FIG. 34A, which carries a porous ablation element; 
     FIG. 83 is a side section view of the porous ablation element taken generally along line  83 — 83  in FIG. 82; 
     FIG. 84 is a side section view of an alternative embodiment of the porous ablation element, showing segmented ablation regions, taken generally along line- 84 — 84  in FIG. 85; 
     FIG. 85 is an exterior side view of the segmented ablation regions shown in section in FIG. 84; 
     FIG. 86 is a side section view of an alternative embodiment of a porous electrode element of the type shown in FIG. 82; 
     FIG. 87 is a side view of a probe, like that shown in FIG. 1, that includes indicia for marking the extent of movement of the catheter tube relative to the associated sheath; 
     FIG. 88 is a side view of an alternative embodiment of a probe, of the type shown in FIG. 1, showing indicia for marking the extent of movement of the catheter tube relative to the associated sheath; 
     FIG. 89 is a side sectional view of a catheter tube having a movable steering assembly; 
     FIG. 90 is an elevated side view of a preformed loop structure having a movable steering mechanism as shown in FIG. 89; 
     FIG. 91 is a section view of the loop structure shown in FIG. 90, taken generally alone line  91 — 91  in FIG. 90; 
     FIG. 92 is an elevated side view of using the movable steering mechanism shown in FIG. 89 to change the geometry of the loop structure shown in FIG. 90; and 
     FIG. 93 is an elevated side view of using two movable steering mechanisms, as shown in FIG. 89, to change the geometry of a loop structure. 
    
    
     The invention may be embodied in several forms without departing from its spirit or essential characteristics. The scope of the invention is defined in the appended claims, rather than in the specific description preceding them. All embodiments that fall within the meaning and range of equivalency of the claims are therefore intended to be embraced by the claims. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This Specification discloses various multiple electrode structures in the context of catheter-based cardiac ablation. That is because the structures are well suited for use in the field of cardiac ablation. 
     Still, it should be appreciated that the disclosed structures are applicable for use in other applications. For example, the various aspects of the invention have application in procedures requiring access to other regions of the body, such as, for example, the prostrate, brain, gall bladder, and uterus. The structures are also adaptable for use with systems that are not necessarily catheter-based. 
     I. Flexible Loop Structures 
     A. Slotted Jointed Sheath 
     FIG. 1 shows a multiple electrode probe  10  that includes a structure  20  carrying multiple electrode elements  28 . 
     The probe  10  includes a flexible catheter tube  12  with a proximal end  14  and a distal end  16 . The proximal end  14  has an attached handle  18 . The multiple electrode structure  20  is attached to the distal end  16  of the catheter tube  14  (see FIG.  2 A). 
     The electrode elements  28  can serve different purposes. For example, the electrode elements  28  can be used to sense electrical events in heart tissue. Alternatively, or in addition, the electrode elements  28  can serve to transmit electrical pulses to measure the impedance of heart tissue, to pace heart tissue, or to assess tissue contact. In the illustrated embodiment, the principal use of the electrode elements  28  is to transmit electrical energy, and, more particularly, electromagnetic radio frequency energy, to ablate heart tissue. 
     The electrode elements  28  are electrically coupled to individual wires (not shown in FIG. 1, but which will be discussed in greater detail later) to conduct ablating energy to them. The wires from the structure  20  are passed in conventional fashion through a lumen in the catheter tube  12  and into the handle  18 , where they are electrically coupled to a connector  38  (see FIG.  1 ). The connector  38  plugs into a source of RF ablation energy. 
     As FIG. 2A shows, the support structure  20  comprises a flexible spline leg  22  surrounded by a flexible, electrically nonconductive sleeve  32 . The multiple electrodes  28  are carried by the sleeve  32 . 
     The spline leg  22  is preferably made from resilient, inert wire, like Nickel Titanium (commercially available as Nitinol material) or 17-7 stainless steel. However, resilient injection molded inert plastic can also be used. Preferably, the spline leg  22  comprises a thin, rectilinear strip of resilient metal or plastic material. Still, other cross sectional configurations can be used. 
     The spline leg  22  can decrease in cross sectional area in a distal direction, by varying, e.g., thickness or width or diameter (if round), to provide variable stiffness along its length. Variable stiffness can also be imparted by composition changes in materials or by different material processing techniques. 
     As FIG. 2B shows, the stiffness of the support structure  20  can be dynamically varied on the fly by providing a tapered wire  544  slidably movable within a lumen  548  in the structure  20 . Movement of the tapered wire  544  (arrows  546  in FIG. 2B) adjusts the region of stiffness along the support structure  20  during use. 
     The sleeve  32  is made of, for example, a polymeric, electrically nonconductive material, like polyethylene or polyurethane or PEBAX™ material (polyurethane and nylon). The signal wires for the electrodes  28  preferably extend within the sleeve  32 . 
     The electrode elements  28  can be assembled in various ways. They can, for example, comprise multiple, generally rigid electrode elements arranged in a spaced apart, segmented relationship along the sleeve  32 . The segmented electrodes can each comprise solid rings of conductive material, like platinum, which makes an interference fit about the sleeve  32 . Alternatively, the electrode segments can comprise a conductive material, like platinum-iridium or gold, coated upon the sleeve  32  using conventional coating techniques or an ion beam assisted deposition (IBAD) process. 
     Alternatively, the electrode elements  28  can comprise spaced apart lengths of closely wound, spiral coils wrapped about the sleeve  32  to form an array of generally flexible electrode elements  28 . The coils are made of electrically conducting material, like copper alloy, platinum, or stainless steel, or compositions such as drawn-filled tubing. The electrically conducting material of the coils can be further coated with platinum-iridium or gold to improve its conduction properties and biocompatibility. 
     The electrode elements  28  can also comprise porous materials, which transmit ablation energy through transport of an electrified ionic medium. Representative embodiments of porous electrode elements  28  are shown in FIGS. 82 to  85 , and will be described in greater detail later. 
     The electrode elements  28  can be operated in a uni-polar mode, in which the ablation energy emitted by the electrode elements  28  is returned through an indifferent patch electrode  420  (see FIG. 44) externally attached to the skin of the patient. Alternatively, the elements  28  can be operated in a bi-polar mode, in which ablation energy emitted by one or more electrode element  28  is returned through an electrode element  28  on the structure  20  (see FIG.  3 A). 
     The diameter of the support structure  20  (including the electrode elements  28 , flexible sleeve  32 , and the spline leg  22 ) can vary from about 2 French to about 10 French. 
     The support structure  20  must make and maintain intimate contact between the electrode elements  28  and the endocardium. Furthermore, the support structure  20  must be capable of assuming a relatively low profile for steering and introduction into the body. 
     To accomplish these objectives, the probe  10  includes a sheath  26  carried by the catheter tube  12 . The distal section  30  of the sheath  26  extends about the multiple electrode structure  20  (see FIGS.  1  and  2 A). The distal section  30  of the sheath  26  is joined to the end of the multiple electrode structure, e.g. by adhesive or thermal bonding. 
     In the embodiment shown in FIG. 1, the proximal section  34  of the sheath  26  terminates short of the handle  18  and includes a raised gripping surface  36 . The proximal section  34  also includes a hemostatic valve and side port (not shown) for fluid infusion. Preferably the hemostatic valve locks about the catheter tube  12 . 
     The distal section  30  of the sheath  26  (proximal of its connection to the multiple electrode structure  20 ) includes a preformed slot  40 , which extends along the axis of the catheter tube  12  (see FIG.  2 A). A portion of the multiple electrode structure  20  is exposed through the slot  40 . 
     The length and size of the slot  40  can vary, as will be described in greater detail later. The circumferential distance that slot  40  extends about the axis  42  can also vary, but is always less than the outer diameter of the sheath  26 . Thus, a remnant  44  of the sheath  26  underlies the slot  40 . In the illustrated embodiment, the slot  40  extends about 180° about the sheath  26 . 
     The catheter tube  12  is slidable within the sheath in a forward and rearward direction, as indicated by arrows  46  and  48  in FIG.  1 . By grasping the raised gripping surface  36  at the proximal end of the sheath  26 , and pushing the catheter tube  12  in the forward direction (arrow  46 ) through the sheath  26  (see FIG.  3 A), the structure  20 , secured to the catheter tube  12  and to the end  30  of the sheath  26 , bends outwardly from the slot  40 . The sheath remnant  44  forms a flexible joint, keeping the distal end of the structure  20  close to the catheter tube axis  42 , while the element  20  bends into a loop, as FIG. 3A shows. The flexible joint  44  maintains loop stress within the structure  20 , to thereby establish and maintain intimate contact between the electrode elements  28  and tissue. 
     The physician can alter the diameter of the loop structure  20  from large to small, by incrementally moving the catheter tube  12  in the forward and rearward directions (arrows  46  and  48 ) through the sheath  26 . In this way, the physician can manipulate the loop structure  20  to achieve the desired degree of contact between tissue and the electrode elements  28 . 
     If desired, the physician can, while grasping the raised gripping surface  36 , rotate the catheter tube  12  within the sheath  26 . As FIG. 4A shows, when the catheter tube  12  is torsionally stiffer than the sheath  26 , the relative rotation (arrow  50 ) flips the loop structure  20  over upon itself (compare FIGS.  3 A and  4 A), to place the electrode elements  28  in a different orientation for tissue contact. As FIG. 4B shows, when the sheath  26  is torsionally stiffer than the catheter tube  12 , rotation of the catheter tube within the sheath  26  bends the structure  20  generally orthogonally to the axis of the loop. 
     By grasping the raised gripping surface  36  and pulling the catheter tube  12  in the rearward direction (arrow  48 ), the physician draws the multiple electrode structure  20  back into the sheath  26 , as FIG. 2A shows. Housed within the sheath  26 , the multiple electrode structure  20  and sheath  26  form a generally straight, low profile geometry for introduction into and out of a targeted body region. 
     The sheath  26  is made from a material having a greater inherent stiffness (i.e., greater durometer) than the support structure  20  itself. Preferably, the sheath material is relatively thin (e.g., with a wall thickness of about 0.005 inch) so as not to significantly increase the overall diameter of the distal region of the probe  10  itself. The selected material for the sheath  26  is preferably also lubricious, to reduce friction during relative movement of the catheter tube  12  within the sheath  26 . For example, materials made from polytetrafluoroethylene (PTFE) can be used for the sheath  26 . 
     Additional stiffness can be imparted by lining the sheath  26  with a braided material coated with PEBAX™ material (comprising polyurethane and nylon). Increasing the sheath stiffness imparts a more pronounced D-shape geometry to the formed loop structure  20  orthogonal to the axis of the slot  40 . Other compositions made from PTFE braided with a stiff outer layer and other lubricious materials can be used. Steps are taken to keep remnants of braided materials away from the exposed edges of the slot  40 . For example, the pattern of braid can be straightened to run essentially parallel to the axis of the sheath  26  in the region of the slot  40 , so that cutting the slot does not cut across the pattern of the braid. 
     The flexible joint  44  is durable and helps to shape the loop structure. The flexible joint  44  also provides an anchor point for the distal end  16  of the catheter tube  12 . The joint  44  also provides relatively large surface area, to minimize tissue trauma. The geometry of the loop structure  20  can be altered by varying either the stiffness or the length of the flexible joint  44 , or both at the same time. 
     As FIG. 3A shows, a stiffening element  52  can be placed along the joint  44 . For example, the stiffening element  52  can comprise an increased durometer material (e.g., from about 35 D to about 72 D), which is thermally or chemically bonded to the interior of the joint  44 . Examples of increased durometer materials, which will increase joint stiffness, include nylon, tubing materials having metal or nonmetallic braid in the wall, and PEBAX™ material. Alternatively, the stiffening element  52  can comprise memory wire bonded to the interior of the joint  44 . The memory wire can possess variable thickness, increasing in the proximal direction, to impart variable stiffness to the joint  44 , likewise increasing stiffness in the proximal direction. The memory wire can also be preformed with resilient memory, to normally bias the joint  44  in a direction at an angle to the axis of the slot  40 . 
     As FIG. 3B shows, the stiffening element  52  can comprise one or more lumens  546  within the joint  44 , which carry wire material  548 . The lumens  546  and wire material  548  can extend only in the region of the joint  44 , or extend further in a proximal direction into the main body of the sheath  26 , to thereby impart greater stiffness to the sheath  26  as well. 
     As FIG. 3C shows, greater stiffness for the joint  44  can be imparted by extruding the sheath  26  to possess an eccentric wall thickness. In this arrangement, the wall of the sheath  26  has a region  550  of greater thickness in the underbody of the sheath  26 , which becomes the joint  44 , than the region  552  which is cut away to form the slot  40 . As shown in phantom lines in FIG. 3C, one or more of the lumens  546  can be extruded in the thicker region  550 , to receive wire material to further stiffen the region of the joint  44 . 
     Regardless of its particular form, the stiffening element  52  for the joint  44  changes the geometry of the formed loop structure  20 . 
     The geometry of the formed loop structure  20  can also be modified by altering the shape and size of the slot  40 . The slot periphery can have different geometries, e.g., rectangular (see FIG.  7 A), elliptical (see FIG.  7 B), or tapered (see FIG.  7 C), to establish different geometries and loop stresses in the formed structure  20 . 
     The effective axial length of the slot  44  can be adjusted by use of a movable mandrel  54 , controlled by a push-pull stylet member  56  (see FIG. 5) attached to a slider controller  58  in the handle  18 . Axial movement of the mandrel  54  affected by the stylet member  56  enlarges or decreases the effective axial length of the slot  44 . A nominal slot length in the range of 1¼ inch to 1½ inch will provide the D-shape loop structure  20  shown in FIG.  3 A. Shorter slot lengths will provide a less pronounced D-shape, with a smaller radius of curvature. Larger slot lengths will provide a more pronounced D-shape, with a larger radius of curvature. As FIG. 8 shows, the proximal edge  60  of the slot  40  can be tapered distally to guide bending of the structure  20  into the desired loop shape while being advanced through the slot  40 . 
     Instead of extending generally parallel to the catheter tube axis  42 , as FIGS. 1 to  8  show, the slot  40  can extend across the catheter tube axis  42 , as FIG. 9 shows. When advanced from the cross-axis slot  40 , the loop structure  20  extends more orthogonally to the catheter tube axis  42 , as FIG. 10 shows, compared to the more distal extension achieved when the slot  40  is axially aligned with the catheter tube axis  42 , as FIG. 3A generally shows. 
     As FIG. 6 shows, a region  62  of the spline  22  within the structure  20  away from the electrode elements  28  can be preformed with elastic memory to bow radially away from the electrode elements  28  when advanced from the sheath  26 . The radially outward bow of the preformed region  62  forms a more symmetric loop structure  20 ′, in contrast to the more asymmetric D-shaped loop  20  shown in FIG.  3 A. When in contact with tissue, the preformed, outwardly bowed region  62  generates a back pressure that, in combination with the loop stress maintained by the flexible joint  44 , establishes greater contact pressure between electrode elements  28  and tissue. 
     In FIG. 6, the region  62  is preformed with a generally uniform bend in a single plane. The region  62  can be preformed with complex, serpentine bends along a single plane, or with bends that extend in multiple planes. Further details of representative loop structures having complex, curvilinear geometries will be described in greater detail later. 
     Additional tissue contact forces can be generated by mounting a bendable spring  64  in the distal end  16  of the catheter tube (see FIG.  2 A). One or more steering wires  66  are bonded (e.g., soldered, spot welded, etc.) to the bendable spring  64  extend back to a steering mechanism  68  in the handle  18  (see FIG.  1 ). Details of steering mechanisms that can be used for this purpose are shown in Lundquist and Thompson U.S. Pat. No. 5,254,088, which is incorporated into this Specification by reference. Operation of the steering mechanism  68  pulls on the steering wires  66  to apply bending forces to the spring  64 . Bending of the spring  64  bends the distal end  16  of the catheter tube  12 , as shown in phantom lines in FIG.  1 . 
     The plane of bending depends upon the cross section of the spring  64  and the attachment points of the wires  66 . If the spring  64  is generally cylindrical in cross section, bending in different planes is possible. If the spring  64  is generally rectilinear in cross section, anisotropic bending occurs perpendicular to the top and bottom surfaces of the spring  64 , but not perpendicular to the side surfaces of the spring  64 . 
     Alternatively, or in combination with the manually bendable spring  64 , the distal end  16  of the catheter tube  12  can be prebent to form an elbow  70  (see FIG. 11) generally orthogonal or at some other selected angle to the loop structure  20 . In the illustrated embodiment, a preformed wire  72  is secured, e.g., by soldering, spot welding, or with adhesive, to the end  16  of the catheter tube  12 . The preformed wire  72  is biased to normally curve. The preformed wire  72  may be made from stainless steel 17/7, nickel titanium, or other memory elastic material. It may be configured as a wire or as a tube with circular, elliptical, or other cross-sectional geometry. 
     The wire  72  normally imparts its curve to the distal catheter tube end  16 , thereby normally bending the end  16  in the direction of the curve. The direction of the normal bend can vary, according to the functional characteristics desired. In this arrangement, a sheath  74  slides (arrows  76 ) along the exterior of the catheter body  14  between a forward position overlying the wire  72  (FIG. 12) and an aft position away from the wire  72  (FIG.  11 ). In its forward position, the sheath  74  retains the distal catheter end  16  in a straightened configuration against the normal bias of the wire  72 , as FIG. 12 shows. The sheath  74  may include spirally or helically wound fibers to provide enhanced torsional stiffness to the sheath  74 . Upon movement of the sheath  74  to its aft position, as FIG. 11 shows, the distal catheter end  16  yields to the wire  72  and assumes its normally biased bent position. The slidable sheath  74  carries a suitable gripping surface (not shown), like the gripping surface  36  of the sheath  26 , to affect forward and aft movement of the sheath  74  for the purposes described. 
     FIG. 4 shows the loop structure  20  flipped upon itself by rotation of the loop structure  20  within the sheath  26 . The rotation is allowed, because both the loop structure  20  and sheath  26  possess generally cylindrical cross sections. If it is desired to prevent relative rotation of the structure  20  within the sheath  26 , the outer geometry of the structure  20  and the interior geometry of the sheath  26  can be formed as an ellipse, as FIG. 13A shows. The interference (elliptically keyed) arrangement in FIG. 13A prevents rotation of the structure  20  and also provides improved torque response and maintains the electrode elements  28  is a fixed orientation with respect to the sheath  26 . By matching the outer geometry of the structure  20  and the interior geometry of the sheath  26  (see FIG.  13 B), a prescribed range of relative rotation can be allowed before interference occurs. In FIG. 13B, the elliptical sleeve  32  will rotate until it contacts the butterfly shaped keyway within the sheath  26 . The prescribed range allows the loop structure  20  to be flipped over upon itself in the manner shown in FIG. 4, without wrapping the flexible joint  44  about the sheath  26 . Should the flexible joint  44  become wrapped about the sheath  26 , the physician must rotate of the catheter tube  12  to unwrap the joint  44 , before retracting the structure  20  back into the slotted sheath  26 . 
     B. Distal Wire Joint 
     FIGS. 14 and 15 show another structure  100  carrying multiple electrode elements  28 . In many respects, the structure  100  shares structural elements common to the structure  20  shown in FIGS. 2 and 3, as just discussed. For this reason, common reference numerals are assigned. Like the structure  20  shown in FIGS. 2 and 3, the structure  100  is intended, in use, to be carried at the distal end  16  of a flexible catheter tube  12 , as a part of a probe  10 , as shown in FIG.  1 . 
     Like the structure  20  shown in the FIGS. 2 and 3, the support structure  100  comprises a flexible spline leg  22  surrounded by a flexible, electrically nonconductive sleeve  32 . The multiple electrodes  28  are carried by the sleeve  32 . The range of materials usable for the spline leg  22  and the electrodes  28  of the structure  100  are as previously described for the structure  20 . 
     A sheath  102  is carried by the catheter tube  12 . The distal section  104  of the sheath  102  extends about the multiple electrode structure  100 . As FIGS. 14 and 15A show, the distal section  104  of the sheath  102  is joined to the distal end  108  of the multiple electrode structure  100  by a short length of wire  106 . The wire  106  is joined to the two ends  104  and  108 , for example, by adhesive or thermal bonding. The proximal section of the sheath  102  is not shown in FIG. 13, but terminates short of the handle  18  and includes a raised gripping surface  36 , as shown for the probe  10  in FIG.  1 . In FIG. 15A, the wire  106  is joined to the interior of the sheath  102 . Alternatively, as FIG. 15B shows, the wire  106  can be joined to the exterior of the sheath  102 . 
     Like the sheath  26  described in connection with FIGS. 2 and 3A, the sheath  102  is made from a material having a greater inherent stiffness than the support structure  100  itself, e.g., composite materials made from PTFE, braid, and polyimide. The selected material for the sheath  102  is preferably also lubricious. For example, materials made from polytetrafluoroethylene (PTFE) can be used for the sheath  102 . As for the sheath  26  in FIGS. 2 and 3, additional stiffness can be imparted by incorporating a braided material coated with PEBAX™ material. 
     The wire  106  comprises a flexible, inert cable constructed from strands of metal wire material, like Nickel Titanium or 17-7 stainless steel. Alternatively, the wire  106  can comprise a flexible, inert stranded or molded plastic material. The wire  106  in FIG. 14 is shown to be round in cross section, although other cross sectional configurations can be used. The wire  106  may be attached to the sheath  102  by thermal or chemical bonding, or be a continuation of the spline leg  22  that forms the core of the structure  100 . The wire  106  can also extend through the wall of the sheath  102 , in the same way that the stiffening wires  548  are placed within the sheath  26  (shown in FIG.  3 B). The need to provide an additional distal hub component to secure the wire  106  to the remainder of the structure  100 , is thereby eliminated. 
     The catheter tube  12  is slidable within the sheath  102  to deploy the structure  100 . Grasping the raised gripping surface  36  at the proximal end of the sheath  102 , while pushing the catheter tube  12  in the forward direction through the sheath  102  (as shown by arrow  110  in FIG.  15 A), moves the structure  100  outward from the open distal end  112  of the sheath  102 . The wire  106  forms a flexible joint  144 , pulling the distal end  108  of the structure  100  toward the sheath distal section  104 . The structure  100  thereby is bent into a loop, as FIG. 15A shows. 
     The flexible wire joint  106 , like the sheath joint  44  in FIG. 3A, possesses the flexibility and strength to maintain loop stress within the structure  100  during manipulation, to thereby establish and maintain intimate contact between the electrode elements  28  and tissue. The wire  106  presents a relatively short length, thereby minimizing tissue trauma. A representative length for the wire  106  is about 0.5 inch. 
     Like the loop structure  20 , the physician can alter the diameter of the loop structure  100  from large to small, by incrementally moving the catheter tube  12  in the forward direction (arrow  110  in FIG. 15) and rearward direction (arrow  116  in FIG. 15) through the sheath  102 . In this way, the physician can manipulate the loop structure  100  to achieve the desired degree of contact between tissue and the electrode elements  28 . 
     Moving the structure  100  fully in the rearward direction (arrow  116 ) returns the structure  100  into a low profile, generally straightened configuration within the sheath  102  (as FIG. 14 shows), well suited for introduction into the intended body region. 
     The points of attachment of the wire joint  106  (between the distal structure end  108  and the distal sheath section  104 ), coupled with its flexible strength, make it possible to form loops with smaller radii of curvature than with the flexible sheath joint  44  shown in FIG.  3 A. 
     The geometry of the loop structure  100  can be altered by varying either the stiffness or the length of the flexible wire  106 , or both at the same time. As FIG. 16A shows, the flexible wire  106  can be tapered, to provide a cross section that decreases in the distal direction. The tapered cross section provides varying stiffness, which is greatest next to the sheath  102  and decreases with proximity to the distal end  108  of the structure  100 . 
     The stiffness can also be changed by changing the thickness of the wire  106  in a step fashion. FIG. 16B shows the wire  106  attached to the sheath  102  and having the smallest thickness to increase the bending radius. The thickness of the wire  106  increases in a step fashion leading up to its junction with the spline leg  22 . Changing the thickness of the wire can be done by rolling the wire in steps, or by pressing it, or by chemical etching. 
     As FIG. 16C shows, the wire  106  can also be used to impart greater stiffness to the flexible joint  144 , for the reasons described earlier with regard to the flexible joint  44  shown in FIG.  3 A. In FIG. 16C, the wire  106  is thermally or chemically bonded to the flexible joint  144  in a serpentine path of increasing width. The alternative ways of stiffening the flexible joint  44  (shown in FIGS. 3A,  3 B, and  3 C) can also be used to stiffen the flexible joint  144 . 
     In the illustrated embodiment (see FIGS.  15 A and  16 A), the distal sheath section  104  is cut at an angle and tapered in a transverse direction relative to the axis of the sheath  102 . The angled linear cut on the distal sheath section  104  may also be a contoured elongated opening (see FIG. 15B) to make the initiation of the loop formation easier. The angle cut on the sheath  102  helps deploy and minimizes the length of the wire  106 . It is advantageous to cover with the sheath section  104  a significant portion of the wire joint  144 . The sheath section  104  thereby also serves to shield the wire as much as possible from direct surface contact with tissue. The possibility of cutting tissue due to contact with the wire  106  is thereby minimized. 
     As before described in the context of the structure  20 , additional tissue contact forces between the structure  100  and tissue can be generated by mounting a bendable spring  64  in the distal end  16  of the catheter tube (see FIG.  14 ). Alternatively, or in combination with the manually bendable spring  64 , the distal end  16  of the catheter tube  12  can be prebent to form an elbow  70  (as shown in FIG. 11 in association with the structure  20 ) generally orthogonal or at some other selected angle to the loop structure  100 . 
     FIG. 17A shows an alternative embodiment for the structure  100 . In this embodiment, the wire  106  is not attached to the distal sheath section  104 . Instead, the wire  106  extends through the sheath  102  to a stop  118  located proximal to the gripping surface  36  of the sheath  102 . Holding the stop  118  stationary, the physician deploys the loop structure  100  in the manner already described, by advancing the catheter tube  12  through the sheath  102  (arrow  120  in FIG.  17 A). Once the loop structure  100  has been formed, the physician can pull on the wire  106  (arrow  122  in FIG. 17A) to decrease its exposed length beyond the distal sheath section  104 , to minimize tissue trauma. Further adjustments to the loop are made by advancing or retracting the catheter tube  12  within the sheath  102 . The wire  106  unattached to the sheath  102  allows the physician to interchangeably use the structure  100  with any sheath. 
     Alternatively, as FIG. 17B shows, the sheath  102  can include a lumen  107  through which the wire  106  passes. In this embodiment, the presence of the wire  106  within the body of the sheath  102  provides increased torque. Unlike FIG. 17A, however, the sheath and the wire  106  comprise one integrated unit and cannot be interchanged. 
     The embodiment shown in schematic form in FIGS. 18,  19 , and  20  offers additional options for adjusting the nature and extent of contact between the electrode elements  28  and tissue. As FIG. 18 shows, a flexible spline leg  124  extends from an external push-pull control  126  through the catheter tube  12  and is looped back to a point of attachment  128  within the catheter tube  12 . A sheath  130 , made of an electrically insulating material, is slidable along the spline leg  124 , both within and outside the catheter tube  12 . The sheath  130  carries the electrode elements  28 . The proximal end of the sheath  130  is attached to a push pull control  132  exposed outside the catheter tube  12 . 
     By pushing both controls  126  and  132  simultaneously (arrows  134  in FIG.  19 ), both the spline leg  124  and the sheath  130  are deployed beyond the distal end  16  of the catheter tube  12 . Together, the spline leg and sheath  130  form a loop structure  136  to present the electrode elements  28  for contact with tissue, in much the same way that the structure  100  and the structure  20 , previously described, establish contact between the electrode elements  28  and tissue. 
     In addition, by holding the spline leg control  126  stationary while pushing or pulling the sheath control  132  (arrows  134  and  136  in FIG.  20 ), the physician is able to slide the sheath  130 , and thus the electrode elements  28  themselves, along the spline leg  124  (as arrows  138  and  140  in FIG. 20 show). The physician is thereby able to adjustably locate the region and extent of contact between tissue and the electrode elements  28 . 
     Furthermore, by holding the sheath control  132  stationary while pushing or pulling upon the spline leg control  126 , the physician is able to adjust the length of the spline leg  124  exposed beyond the distal end  16  of the catheter tube  12 . The physician is thereby able to incrementally adjust the radius of curvature in generally the same fashion previously described in the context of FIG.  17 . 
     The arrangement in FIGS. 18,  19 , and  20 , thereby provides a wide range of adjustment options for establishing the desired degree of contact between tissue and the electrode elements  28  carried by the loop structure  136 . 
     By pulling both controls  126  and  128  simultaneously (arrows  142  in FIG.  18 ), both the spline leg  124  and the sheath  130  are moved to a position close to or within the distal end  16  of the catheter tube  12  for introduction into a body region. 
     C. Free Pull Wire 
     FIG. 21 shows a multiple electrode support structure  144  formed from a spline leg  146  covered with an electrically insulating sleeve  148 . The electrode elements  28  are carried by the sleeve  148 . 
     The structure  144  is carried at the distal end  16  of a catheter tube  12 , and comprises the distal part of a probe  10 , in the manner shown in FIG.  1 . In this respect, the structure  144  is like the structure  100 , previously described, and the same materials as previously described can be used in making the structure  144 . 
     Unlike the previously described structure 100 , a slidable sheath  150  is intended to be moved along the catheter tube  12  and structure  144  between a forward position, covering the structure  144  for introduction into a body region (shown in FIG.  22 ), and an aft, retracted position, exposing the structure  144  for use (shown in FIGS.  21  and  23 ). Thus, unlike the structure  100 , which is deployed by advancement forward beyond a stationary sheath  102 , the structure  144  is deployed by being held stationary while the associated sheath  150  is moved rearward. 
     A pull wire  152  extends from the distal end  154  of the structure  144 . In the illustrated embodiment, the pull wire  152  is an extension of the spline leg  146 , thereby eliminating the need for an additional distal hub component to join the wire  152  to the distal structure end  154 . 
     Unlike the structure  100 , the pull wire  152  is not attached to the sheath  150 . Instead, the catheter tube  12  includes an interior lumen  156 , which accommodates sliding passage of the pull wire  152 . The pull wire  152  passes through the lumen  156  to an accessible push-pull control  166 , e.g., mounted on a handle  18  as shown in FIG.  1 . When the structure  144  is free of the rearwardly withdrawn sheath  150 , the physician pulls back on the wire  152  (arrow  168  in FIG. 23) to bend the structure  144  into a loop. 
     As FIGS. 21 and 23 show, the wire  152  may include a preformed region  158  adjacent to the distal structure end  154 , wound into one or more loops, forming a spring. The region  158  imparts a spring characteristic to the wire  152  when bending the structure  144  into a loop. The region  158  mediates against extreme bending or buckling of the wire  152  during formation of the loop structure  144 . The region  158  thereby reduces the likelihood of fatigue failure arising after numerous flex cycles. 
     FIG. 24 shows an alternative embodiment for the structure  144 . In this embodiment, the distal structure end  154  includes a slotted passage  160 , which extends across the distal structure end  154 . In FIG. 24, the slotted passage  160  extends transverse of the main axis  162  of the structure  144 . Alternatively, the slottage passage  160  could extend at other angles relative to the main axis  162 . 
     Unlike the embodiment shown in FIGS. 21 to  23 , the wire  152  in FIG. 24 is not an extension of the spline leg  146  of the structure  144 . Instead, the wire  152  comprises a separate element, which carries a ball  164  at its distal end. The ball  164  is engaged for sliding movement within the slotted passage  160 . The ball  164  also allows rotation of the wire  152  relative to the structure  144 . The ball  164  and slotted passage  160  form a sliding joint, which, like the spring region  158  in FIGS. 21 to  23 , reduces the likelihood of fatigue failure arising after numerous flex cycles. 
     As before described in the context of the structure  100 , additional tissue contact forces between the structure  144  and tissue can be generated by mounting a bendable spring  64  in the distal end  16  of the catheter tube (see FIG.  21 ). Alternatively, or in combination with the manually bendable spring  64 , the distal end  16  of the catheter tube  12  can be prebent to form an elbow (like elbow  70  shown in FIG. 11 in association with the structure  20 ) generally orthogonal or at some other selected angle to the loop structure  144 . 
     D. Preformed Loop Structures 
     1. Single Loops 
     FIG. 25 shows an adjustable, preformed loop structure  170 . The structure  170  is carried at the distal end  16  of a catheter tube  12 , which is incorporated into a probe, as shown in FIG.  1 . 
     The structure  170  includes a single, continuous, flexible spline element  172  having two proximal ends  174  and  176 . One proximal end  174  is secured to the distal catheter tube end  16 . The other proximal end  176  slidably passes through a lumen  178  in the catheter tube  12 . The proximal end  176  is attached to an accessible push-pull control  180 , e.g., mounted in the handle  18  shown in FIG.  1 . The flexible spline element  172  is bent into a loop structure, which extends beyond the distal end  16  of the catheter tube  12 . The spline element  172  can be preformed in a normally bowed condition to accentuate the loop shape. 
     The continuous spline element  172  can be formed from resilient, inert wire, like Nickel Titanium or 17-7 stainless steel, or from resilient injection molded inert plastic, or from composites. In the illustrated embodiment, the spline element  172  comprises a thin, rectilinear strip of resilient metal, plastic material, or composite. Still, other cross sectional configurations can be used. 
     As before described in connection with other structures, a sleeve  182  made of, for example, a polymeric, electrically nonconductive material, like polyethylene or polyurethane or PEBAX™ material is secured, e.g., by heat shrinking, adhesives, or thermal bonding about the spline element  172  in a region of the structure  170 . The sleeve  182  carries one or more electrode elements  28 , which can be constructed in manners previously described. 
     The structure  170  includes an interior wire  184 . The interior wire can be made from the same type of materials as the spline element  172 . The distal end of the wire  184  carries a cap  186 , which is secured, e.g., by crimping or soldering or spot welding, to the wire  184 . The cap includes a through passage  188  (see FIG.  26 ), through which the mid portion of the spline element  172  extends. The spline element  172  is slidable within the through passage  188 . It should be appreciated that the wire  184  can be attached to the spline element  172  in other ways to permit relative movement, e.g., by forming a loop or eyelet on the distal end of the wire  184 , through which the spline leg  172  passes. It should also be appreciated that the cap  186  can be secured to the spline leg  172 , if relative movement is not desired. 
     The proximal end of the interior wire  184  slidably passes through a lumen  190  in the catheter tube  12  for attachment to an accessible push-pull control  192 , e.g., also on a handle  18  like that shown in FIG.  1 . 
     As FIG. 27 shows, pushing on the control  180  (arrow  194 ) or pulling on the control  180  (arrow  196 ) moves the spline element  172  to alter the shape and loop stresses of the structure  170 . Likewise, pushing on the control  192  (arrow  198 ) or pulling on the control  192  (arrow  200 ) moves the interior wire  184  in the lumen  190 , which applies force to the cap  186  in the midportion of the structure  172 , and which further alters the shape and loop stresses of the structure  170 . 
     In particular, manipulation of the controls  180  and  192  creates asymmetric geometries for the structure  170 , so that the physician is able to shape the structure  170  to best conform to the interior contours of the body region targeted for contact with the electrode elements. Manipulation of the controls  180  and  192  also changes the back pressures, which urge the electrode elements  28  into more intimate contact with tissue. 
     As FIG. 28 shows, further variations in the shape of and physical forces within the structure  170  can be accomplished by bending the interior wire  184  along its axis. In one embodiment, the wire  184  is made from temperature memory wire, which bends into a preestablished shape in response to exposure to blood (body) temperature, and which straightens in response to exposure to room temperature. Bending the interior wire  184  imparts forces (through the cap  186 ) to bend the spline element  172  into, for example, an orthogonal orientation. This orientation may be required in certain circumstances to better access the body region where the electrode elements  28  are to be located in contact with tissue. 
     Alternatively, one or more steering wires (not shown) can be attached to the interior wire  184 . Coupled to an accessible control (not shown), e.g. on the handle  18 , pulling on the steering wires bends the wire  184 , in generally the same fashion that the steering wires  66  affect bending of the spring  64 , as previously described with reference to FIG.  2 A. 
     As FIG. 29 shows, the control  192  can also be rotated (arrows  222 ) to twist the interior wire  184  about its axis. Twisting the wire  184  imparts (through the cap  186 ) transverse bending forces along the spline element  172 . The transverse bending forces form curvilinear bends along the spline element  172 , and therefore along the electrode elements  28  as well. The loop stresses can also be further adjusted by causing the control  180  to rotate (arrows  224 ) the spline element  172 . 
     As FIG. 30 shows, the through passage cap  186  (see FIG. 26) permits the cap  186  to be repositioned along the spline element  172 . In this way, the point where the wire  184  applies forces (either push-pull, or twisting, or bending, or a combination thereof) can be adjusted to provide a diverse variety of shapes (shown in phantom lines) for and loop stresses within the structure  170 . FIG. 31 shows, by way of example, how changing the position of the cap  186  away from the midregion of the spline element  172  alters the orthogonal bend geometry of the spline element  172 , compared to the bend geometry shown in FIG.  28 . The cap  186  can be moved along the spline element  172 , for example, by connecting steering wires  566  and  568  to the distal region of the interior wire  184  Pulling on a steering wire  566  or  568  will bend the interior wire  184  and slide the cap  186  along the spline element  172 . 
     The single loop structure  170  is introduced into the targeted body region within an advanceable sheath  218 , which is slidably carried about the catheter tube  12  (see FIG.  25 ). Movement of the sheath  218  forward (arrow  226  in FIG. 25) encloses and collapses the loop structure  170  within the sheath  218  (in generally the same fashion that the structure  144  in FIG. 21 is enclosed within the associated sheath  150 ). Movement of the sheath  218  rearward (arrow  230  in FIG. 25) frees the loop structure  170  of the sheath  218 . 
     2. Multiple Loop Assemblies 
     As FIG. 32 shows, the structure  170  can include one or more auxiliary spline elements  202  in regions of the structure  170  spaced away from the electrode elements  28 . In the illustrated embodiment, the auxiliary spline elements  202  slidably extend through the distal cap  186  as before described, and are also coupled to accessible controls  204  in the manner just described. In this way, the shape and loop stresses of the auxiliary spline elements  202  can be adjusted in concert with the spline element  172 , to create further back pressures to urge the electrode  28  toward intimate contact with tissue. The existence of one or more auxiliary spline elements  202  in multiple planes also make it possible to press against and expand a body cavity, as well as provide lateral stability for the structure  170 . 
     As FIG. 33 shows, asymmetric mechanical properties can also be imparted to the structure  170 , to improve contact between tissue and the electrode elements  28 . In FIG. 33 the region of the structure  170  which carries the electrode elements  28  is stiffened by the presence of the closely spaced multiple spline elements  206 A,  206 B, and  206 C. Spaced apart, single spline elements  208  provide a back-support region  210  of the structure  170 . 
     FIG. 34 shows a multiple independent loop structure  220 . The structure  220  includes two or more independent spline elements (three spline elements  212 ,  214 , and  216  are shown), which are not commonly joined by a distal cap. The spline elements  212 ,  214 , and  216  form independent, nested loops, which extend beyond the distal end  16  of the catheter tube  12 . 
     A region  211  on each spline element  212 ,  214 , and  216  carries the electrode elements  28 . The other region  213  of each spline element  212 ,  214 , and  216  is slidable within the catheter tube  12 , being fitted with accessible controls  212 C,  214 C, and  216 C, in the manner just described. Thus, independent adjustment of the shape and loop stresses in each spline element  212 ,  214 , and  216  can be made to achieve desired contact between tissue and the electrode elements  28 . 
     Like the single loop structures shown in FIGS. 25 to  31  the various multiple loop structures shown in FIGS. 32 to  34  can be introduced into the targeted body region in a collapsed condition within a sheath  232  (see FIG.  32 ), which is slidably carried about the catheter tube  12 . As FIG. 32 shows, movement of the sheath  232  away from the loop structure frees the loop structure for use. 
     E. Orthogonal Loop Structures 
     FIGS. 28 and 31 show embodiments of loop structures  170 , which have been bent orthogonally to the main axis of the structure  170 . In these embodiments the orthogonal bending is in response to bending an interior wire  184 . 
     FIGS. 35 and 36 show a loop structure  232  that assumes an orthogonal geometry (in FIG. 36) without requiring an interior wire  184 . The structure  232 , like the structure  170  shown in FIG. 25, is carried at the distal end  16  of a catheter tube  12 , which is incorporated into a probe, as shown in FIG.  1 . 
     Like the structure  170 , the structure  232  comprises a single, continuous, flexible spline element  234 . One proximal end  236  is secured to the distal catheter tube end  16 . The other proximal end  238  passes through a lumen  240  in the catheter tube  12 . The proximal end  238  is attached to an accessible control  242 , e.g., mounted in the handle  18  shown in FIG.  1 . As in the structure  170 , the spline element  234  can be preformed in a normally bowed condition to achieve a desired loop geometry. 
     In FIGS. 35 and 36 the spline element  234  is formed, e.g., from inert wire, like Nickel Titanium or 17-7 stainless steel, or from resilient injection molded inert plastic, with two regions  244  and  246  having different cross section geometries. The region  244 , which comprises the exposed part of the spline element  234  that carries the electrode elements  28 , possesses a generally rectilinear, or flattened cross sectional geometry, as FIG. 37 shows. The region  246 , which comprises the part of the spline element  234  extending within the catheter tube  12  and attached to the control  240 , possesses a generally round cross sectional geometry, as FIG. 38 shows. To provide the two regions  244  and  246 , a single length of round wire can be flattened and annealed at one end to form the rectilinear region  244 . 
     Rotation of the control  242  (attached to the round region  246 ) (arrows  250  in FIG. 35) twists the rectilinear region  244  about the proximal end  236 , which being fixed to the catheter tube  12 , remains stationary. The twisting rectilinear region  244  will reach a transition position, in which the region  244  is twisted generally 90° from its original position (as FIG. 39 shows). In the transition position, the loop structure  232  bends orthogonal to its main axis, as FIG. 36 shows. By stopping rotation of the control  242  once the transition position is reached, the retained twist forces in the loop structure  232  hold the loop structure  232  in the orthogonally bent geometry. 
     FIGS. 42A and 42B show an alternative embodiment, in which each leg  554  and  556  of a loop structure  558  is attached to its own individual control, respectively  560  and  562 . The region  564  of the loop structure  558  carrying the electrode element  28  possesses a generally rectilinear or flattened cross section. The regions of the legs  554  and  556  near the controls  560  and  562  possess generally round cross sections. Counter rotation of the controls  560  and  562  (respectively arrows  561  and  563  in FIG.  42 B), twists the rectilinear region  564  to bend the loop structure  558  generally orthogonal to its axis (as FIG. 42B shows). The counter rotation of the controls  560  and  562  can be accomplished individually or with the inclusion of a gear mechanism. 
     In both embodiments shown in FIGS. 36 and 42B once the orthogonal bend is formed and placed into contact with tissue, controlled untwisting of the spline legs will begin to straighten the orthogonal bend in the direction of tissue contact. Controlled untwisting can thereby be used as a counter force, to increase tissue contact. 
     The characteristics of the orthogonally bent geometry depend upon the width and thickness of the rectilinear region  244 . As the ratio between width and thickness in the region  244  increases, the more pronounced and stable the orthogonal deflection becomes. 
     The diameter of the loop structure  232  also affects the deflection. The smaller the diameter, the more pronounced the deflection. Increases in diameter dampen the deflection effect. Further increases beyond a given maximum loop diameter cause the orthogonal deflection effect to be lost. 
     The characteristics of the electrical insulation sleeve  248 , which carries the electrode elements  28 , also affect the deflection. Generally speaking, as the stiffness of the sleeve  248  increases, the difficulty of twisting the region  244  into the transition position increases. If the sleeve  248  itself is formed with a non-round cross section, e.g. elliptical, in the rectilinear region  244  the orthogonal deflection characteristics are improved. 
     The orthogonal deflection effect that FIGS. 35 and 36 show can also be incorporated into the loop structure of the type previously shown in FIG.  14 . In this embodiment (see FIG.  40 ), the loop structure  252  comprises a spline leg  254  (see FIG. 41 also) enclosed within an electrically conductive sleeve  256 , which carries the electrode elements  28 . The distal end of the structure  252  is attached by a joint wire  260  to a sheath  258 . As previously described, advancing the structure  252  from the sheath  258  forms a loop (as FIG. 40 shows) 
     In the embodiment shown in FIG. 40 the spline leg  254  is rectilinear in cross section (see FIG.  41 ). Furthermore, as FIG. 41 shows, the spline leg  254  is preformed in a normally twisted condition, having two sections  262  and  264 . The section  262  is distal to the section  264  and is attached to the joint wire  260 . The sections  262  and  264  are arranged essentially orthogonally relative to each other, being offset by about 90°. When advanced outside the sheath  258 , the twisted bias of the rectilinear spline leg  254  causes the formed loop structure  252  to bend orthogonally to its main axis, as FIG. 40 shows. 
     In an alternative embodiment (see FIG.  43 ), the structure  252  can include a spline leg  266  preformed to include along its length one or more stressed elbows  268 . The prestressed elbows  268  impart an orthogonal deflection when the structure  252  is free of the constraint of the sheath  270 . When housed within the sheath  270 , the stiffness of the sheath  270  straightens the elbows  268 . 
     F. Deployment of Flexible Loop Structures 
     1. Generally 
     Various access techniques can be used to introduce the previously described multiple electrode structures into a desired region of the body. In the illustrated embodiment (see FIG.  44 ), the body region is the heart, and the multiple electrode structure is generally designated ES. 
     During introduction, the structure ES is enclosed in a straightened condition within its associated outer sheath (generally designated S in FIG. 44 at the end  16  of the catheter tube  12 . To enter the right atrium of the heart, the physician directs the catheter tube  12  through a conventional vascular introducer (designated with a capital-I in FIG. 44 into, e.g., the femoral vein. For entry into the left atrium, the physician can direct the catheter tube  12  through a conventional vascular introducer retrograde through the aortic and mitral valves, or can use a transeptal approach from the right atrium. 
     Once the distal end  16  of the catheter tube  12  is located within the selected chamber, the physician deploys the structure ES in the manners previously described, i.e., either by advancing the structure ES forward through the sheath S (e.g., as in the case of the structures shown in FIG. 3 or  15 ) or by pulling the sheath S rearward to expose the structure ES (e.g., as in the case of the structures shown in FIG. 21 or  25 ). 
     It should be appreciated that the structure ES discussed above in the context of intracardiac use, can also be directly applied to the epicardium through conventional thoracotomy or thoracostomy techniques. 
     2. Loop Structures 
     The various loop structures previously described (shown in FIGS. 1 to  31 , when deployed in the left or right atrium tend to expand the atrium to its largest diameter in a single plane. The loop structure tends to seek the largest diameter and occupy it. The loop structures can also be adapted to be torqued, or rotated, into different planes, and thereby occupy smaller regions. 
     The addition of auxiliary splines, such as shown in FIGS. 32 to  34  serves to expand the atrium in additional planes. The auxiliary splines also make it possible to stabilize the structure against a more rigid anatomic structure, e.g. the mitral valve annulus in the left atrium, while the spline carrying the electrode elements loops upward toward anatomic landmarks marking potential ablation sites, e.g., tissue surrounding the pulmonary veins. 
     The various structures heretofore described, which exhibit compound or orthogonal bends (see, e.g., FIGS. 28,  31 ,  35 ,  40 ,  42 , and  43  (which will be referred to as a group as “Compound Bend Assemblies”) also make it possible to locate the ablation and/or mapping electrode(s) at any location within a complex body cavity, like the heart. With prior conventional catheter designs, various awkward manipulation techniques were required to position the distal region, such as prolapsing the catheter to form a loop within the atrium, or using anatomical barriers such as the atrial appendage or veins to support one end of the catheter while manipulating the other end, or torquing the catheter body. While these techniques can still be used in association with the compound bend assemblies mentioned above, the compound bend assemblies significantly simplify placing electrode(s) at the desired location and thereafter maintaining intimate contact between the electrode(s) and the tissue surface. The compound bend assemblies make it possible to obtain better tissue contact and to access previously unobtainable sites, especially when positioning multiple electrode arrays. 
     Compound bend assemblies which provide a proximal curved section orthogonal to the distal steering or loop geometry plane allow the physician to access sites which are otherwise difficult and often impossible to effectively access with conventional catheter configurations, even when using an anatomic barrier as a support structure. For example, to place electrodes between the tricuspid annulus and the cristae terminalis perpendicular to the inferior vena cava and superior vena cava line, the distal tip of a conventional the catheter must be lodged in the right ventricle while the catheter is torqued and looped to contact the anterior wall of the right atrium. Compound bend assemblies which can provide a proximal curved section orthogonal to the distal steering or loop geometry plane greatly simplify positioning of electrodes in this orientation. Compound bend assemblies which provide a proximal curved section orthogonal to the distal steering or loop geometry plane also maintain intimate contact with tissue in this position, so that therapeutic lesions contiguous in the subepicardial plane and extending the desired length, superiorly and/or inferiorly oriented, can be accomplished to organize and help cure atrial fibrillation. 
     A transeptal approach will most likely be used to create left atrial lesions. In a transeptal approach, an introducing sheath is inserted into the right atrium through the use of a dilator. Once the dilator/sheath combination is placed near the fossa ovalis under fluoroscopic guidance, a needle is inserted through the dilator and is advanced through the fossa ovalis. Once the needle has been confirmed to reside in the left atrium by fluoroscopic observation of radiopaque contrast material injected through the needle lumen, the dilator/sheath combination is advanced over the needle and into the left atrium. At this point, the dilator is removed leaving the sheath in the left atrium. 
     A left atrial lesion proposed to help cure atrial fibrillation originates on the roof of the left atrium, bisects the pulmonary veins left to right and extends posteriorly to the mitral annulus. Since the lesion described above is perpendicular to the transeptal sheath axis, a catheter which can place the distal steering or loop geometry plane perpendicular to the sheath axis and parallel to the axis of the desired lesion greatly enhances the ability to accurately place the ablation and/or mapping element(s) and ensures intimate tissue contact with the element(s). To create such lesions using conventional catheters requires a retrograde procedure. The catheter is advanced through the femoral artery and aorta, past the aortic valve, into the left ventricle, up through the mitral valve, and into the left atrium. This approach orients the catheter up through the mitral valve. The catheter must then be torqued to orient the steering or loop geometry plane parallel to the stated lesion and its distal region must be looped over the roof of the left atrium to position the ablation and/or mapping element(s) bisecting the left and right pulmonary veins and extending to the mitral annulus. 
     Preformed guiding sheaths have also been employed to change catheter steering planes. However, preformed guiding sheaths have been observed to straighten in use, making the resulting angle different than the desired angle, depending on the stiffness of the catheter. Furthermore, a guiding sheath requires a larger puncture site for a separate introducing sheath, if the guiding sheath is going to be continuously inserted and removed. Additional transeptal punctures increase the likelihood for complications, such as pericardial effusion and tamponade. 
     G. Loop Size Marking 
     FIG. 87 shows a probe  524  comprising a catheter tube  526  carrying a slotted sheath  528  of the type previously described and shown, e.g., in FIG.  1 . The catheter tube  526  includes proximal handle  529  and a distal multiple electrode array  530 . The multiple electrode array  530  is deployed as a loop structure from the slotted sheath  528 , in the manner previously described and shown, e.g., in FIG.  3 A. 
     In FIG. 87, the probe  524  includes indicia  532  providing the physician feedback on the size of the formed loop structure. In FIG. 87, the indicia  532  comprises markings  534  on the region of the catheter tube  526  extending through the proximal end of the sheath  528 . The markings  534  indicate how much of the catheter tube  526  has been advanced through the sheath  528 , which thereby indicates the size of the formed loop structure. 
     The markings  534  can be made in various ways. They can, for example, be placed on the catheter tube  526  by laser etching, or by printing on the catheter tube  526  using bio-compatible ink, or by the attachment of one or more premarked, heat shrink bands about the catheter tube  526 . 
     In FIG. 88, the slotted sleeve  528  is attached to the handle  529  of the probe  524 . In this arrangement, the catheter tube  526  is advanced and retracted through the slotted sheath  528  by a push-pull control  536  on the handle  529 . In this embodiment, the indicia  532  providing feedback as to the size of the formed loop structure includes markings  536  on the handle  529 , arranged along the path of travel of the push-pull control  536 . The markings  536  can be applied to the handle  529 , e.g., by laser etching ot printing. As in FIG. 87, the markings  536  indicate how much of the catheter tube  526  has been advanced through the slotted sheath  528 . 
     G. Movable Steering 
     FIG. 89 shows a movable steering assembly  570 . The assembly  570  includes a bendable wire  572  with at least one attached steering wire (two wires  574  and  576  are shown). The steering wires  574  and  576  are attached, e.g. by spot welding or soldering, to the bendable wire  572 . The bendable wire  572  can be formed from resilient, inert wire, like Nickel Titanium or 17-7 stainless steel, or from resilient injection molded inert plastic, or from composites. In the illustrated embodiment, the wire  572  comprises a rectilinear strip of resilient metal, plastic material, or composite. Still, other cross sectional configurations can be used. The distal end  598  of the wire  572  is formed as a ball or another blunt, nontraumatic shape. 
     The steering wires  574  and  576  are attached to an accessible control  584 . The control  584  can take the form, for example, of a rotatable cam wheel mechanism of the type shown in Lundquist and Thompson U.S. Pat. No. 5,254,088, which is already incorporated into this Specification by reference. Pulling on the steering wires  574  and  576  (arrows  600  in FIG.  89 ), e.g., by rotating the control  584 , bends the wire  572  in the direction of the pulling force. 
     The bendable wire  572  is attached by a ferrule  580  to a guide coil  578 . The guide coil  578  provides longitudinal support for the bendable wire  572 . The guide coil  578  acts as the fulcrum about which the steering assembly  570  bends. 
     The assembly  570 , comprising the bendable wire  572 , steering wires  574  and  576 , and guide coil  578 , is carried within an outer flexible tube  582 . Operation of the control  584 , to deflect the wire  572  within the tube  582 , bends the tube  582 . 
     Taking into account the rectilinear shape of the bendable wire  572 , the outer tube  582  is ovalized. The interference between the rectilinear exterior shape of the wire  572  and the oval interior shape of the tube  582  prevents rotation of the wire  572  within the tube  582 . The interference thereby retains a desired orientation of the bendable wire  572  with respect to the tube  582 , and thus the orientation of the applied bending forces. 
     The assembly  570  is attached to an accessible control  582 . Pushing and pulling on the control  570  (arrows  602  and  604  in FIG. 89) axially moves the steering assembly  570  within the tube  582 . Axial movement of the assembly  570  changes the axial position of the bendable wire  572  within the tube  582 . The control  570  thereby adjusts the location where bending forces are applied by the wire  572  along the axis of the tube  582 . 
     FIGS. 90 and 91 show the movable steering assembly  570  incorporated into a loop structure  586  of the type previously disclosed with reference to FIG. 25, except there is no interior wire  184 . The loop structure  586  includes a spline  588  (see FIG.  91 ), which forms a loop. A sleeve  590  surrounds the spline  588 . One or more electrode elements  28  are carried by the sleeve  590 . 
     As FIG. 91 shows, the sleeve  590  includes an ovalized interior lumen  592 , which carries the movable steering assembly  570 . The steering assembly  570 , attached to the accessible control  582 , is movable within the lumen  592  along the spline  588 , in the manner just described with respect to the ovalized tube  582  in FIG.  89 . 
     As FIG. 92 shows, operating the control  584  to actuate the steering wires  574  and  576  exerts a bending force (arrow  604 ) upon the spline  588  (through the bendable wire  572 ). The bending force alters the shape of the loop structure  586  in the plane of the loop, by increasing or decreasing its diameter. Shaping the loop structure  586  using the steering mechanism  570  adjusts the nature and extent of tissue contact. 
     Because the steering mechanism  570  is movable, the physician can selectively adjust the location of the bending force (arrow  604 ) to take into account the contour of the tissue in the particular accessed body region. 
     As FIG. 93 shows, the loop structure  586  can carry more than one movable steering mechanism. In FIG. 93, there are two moveable steering mechanisms, designated  570 ( 1 ) and  570 ( 2 ), one carried on each leg of the structure  586 . A separate steering control designated  584 ( 1 ) and  584 ( 2 ), and a separate axial movement control, designated  582 ( 1 ) and  582 ( 2 ) can also be provided. It is therefore possible to independently adjust the position of each steering mechanism  570 ( 1 ) and  570 ( 2 ) and individually apply different bending forces, designated, respectively, arrows  604 ( 1 ) and  604 ( 2 ). The application of individually adjustable bending forces (arrows  604 ( 1 ) and  604 ( 2 )) allow diverse changes to be made to the shape of the loop structure  586 . 
     It should also be appreciated that the movable steering mechanism  570  can be incorporated into other loop structures including those of the type shown in FIG.  33 . 
     II. Self-Anchoring Multiple Electrode Structures 
     1. Integrated Branched Structures 
     FIGS. 45 and 46 show an integrated branched structure  272 , which comprises an operative branch  274  and an anchoring branch  276  oriented in an angular relationship. The branched structure  274  occupies the distal end  16  of a catheter tube  12 , and forms the distal part of a probe  10 , as shown in FIG.  1 . 
     It should be appreciated that there may be more than one operative branch or more than one anchoring branch. The two branches  274  and  276  are shown and described for the purpose of illustration. 
     The operative branch  274  carries one or more operative elements. The operative elements can take various forms. The operative elements can be used, e.g., to sense physiological conditions in and near tissue, or to transmit energy pulses to tissue for diagnostic or therapeutic purposes. As another example, the operative elements may take the form of one or more tissue imaging devices, such as ultrasound transducers or optical fiber elements. By way of further example, the operative elements can comprise biopsy forceps or similar devices, which, in use, handle tissue. The operative elements can also comprise optical fibers for laser ablation, or a fluorescence spectroscopy device. 
     In the illustrated embodiment, the operative elements take the form of the electrode elements  28  (as previously described). In use, the electrode elements  28  contact tissue to sense electrical events, or to transmit electrical pulses, e.g., to measure the impedance of or to pace heart tissue, or to transmit electrical energy to ablate tissue. 
     In the illustrated embodiment, the operative branch  274  comprises a spline element  282  enclosed within an electrically insulating sleeve  284 . The spline element  282  can be formed, e.g., from resilient, inert wire, like Nickel Titanium or 17-7 stainless steel, or from resilient injection molded inert plastic. In the illustrated embodiment, the spline element  282  comprises a thin, rectilinear strip of resilient metal or plastic material. Still, other cross sectional configurations can be used. Furthermore, more than a single spline element may be used. 
     As before described in the context of other structures, a sleeve  282  made of, for example, a polymeric, electrically nonconductive material, like polyethylene or polyurethane or PEBAX™ material is secured about the spline element  282 . The sleeve  282  carries the electrode elements  28 , which can also be constructed in manners previously described. 
     In the illustrated embodiment, the operative branch  274  extends at about a 45° angle from the anchoring branch  276 . Various other angles between 0° (i.e., parallel) and 90° (i.e., perpendicular) can be used. 
     The angular relationship between the operative branches  274  and the anchoring branch  276  causes the operative branch  274  to inherently exert force against tissue as it is advanced toward it. The spline element  282 , or the sleeve  284 , or both, can be stiffened to bias the operative branch  274  toward the anchoring branch  276 , to thereby enhance the inherent tissue contact force. 
     As FIG. 46 best shows, the anchoring branch  276  comprises a tubular body  286  defining an interior lumen  288 , which extends through the catheter tube  12 . The distal end  290  of the body  286  may be extended outward beyond the distal end  16  of the catheter tube  12 , generally along the same axis  292  as the catheter tube  12 . The proximal end  296  of the body  286  communicates with an accessible inlet  294 , e.g., located on the catheter tube  12  or on the handle  18 . 
     The inlet  294  accommodates passage of a conventional guide wire  306  into and through the lumen  288 . The guide wire  306  includes a blunt distal end  308  for nontraumatic contact with tissue. 
     As FIG. 47 shows, the body  286  can be carried within the catheter tube  12  for sliding movement forward (arrow  298 ) or rearward (arrow  300 ). In this embodiment, an accessible control  302 , e.g., located on the handle  18 , is coupled to the body  286  to guide the movement. In this way, the physician can withdraw the body  286  within the catheter tube  12  during introduction of the structure  272  into the body region. The physician can also adjust the relative length of the body  286  beyond the distal end  16  of the catheter tube  16  to aid in positioning and anchoring the structure  272 , once deployed within the targeted body region. 
     An exterior sheath  304  is slidable along the catheter tube  12  between a forward position (FIG. 48) and a rearward position (FIG.  45 ). In the forward position, the sheath  304  encloses and shields the operative branch  274 , straightening it. When in the forward position, the sheath  304  also encloses and shields the anchoring branch  274 . In the rearward position, the sheath  304  frees both branches  274  and  276  for use. 
     In use within the heart (see FIGS. 49A,  49 B, and  49 C), the physician maneuvers the guide wire  306  through and outwardly of the lumen  288 , with the aid of fluoroscopy or imaging, to a desired anchor site. FIGS. 50A and 50B show candidate anchor sites within the heart, which surround anatomic structures that most commonly develop arrhythmia substrates, such as the superior vena cava SVC; right pulmonary veins (RPV); and left pulmonary veins (LPV); inferior vena cava (IVC); left atrial appendage (LAA); right atrial appendage (RAA); tricuspid annulus (TA); mitral annulus (MA); and transeptal puncture (TP). The physician positions the blunt end portion  308  near tissue at the anchor site (see FIG.  49 A). 
     As FIG. 49B shows, the physician advances the structure  272 , enclosed within the sheath  304 , along the anchored guide wire  306 . When near the anchor site, the physician retracts the sheath  304 , freeing the structure  272 . 
     As FIG. 49C shows, the physician advances the anchoring branch  276  along the guide wire  306  into the anchor site. The anchoring branch  276  provides a support to place the operative branch  274  in contact with tissue in the vicinity of the anchor site. 
     Radiopaque makers  326  can be placed at preestablished positions on the structure  272  for visualization under fluoroscopy, to thereby aid in guiding the structure  272  to the proper location and orientation. 
     FIG. 55 shows an alternative embodiment of the anchoring branch  276 . In this embodiment, the anchoring branch  276  carries an inflatable balloon  346  on its distal end. The balloon  346  is inflated to secure the attachment of the anchoring branch  276  to the targeted vessel or cavity anchor site. The anchoring branch  276  includes a lumen  352  that extends through the balloon  346 , having an inlet  348  at the distal end of the balloon  346  and an outlet  350  at the proximal end of the balloon  346 . The lumen  352  allows blood to flow through the targeted vessel or cavity anchor site, despite the presence of the anchoring balloon  346 . The lumen  306  also allows passage of the guide wire  306  for guiding the anchoring branch  276  into position. 
     As FIG. 46 shows, the operative branch  274  can carry one or more steering wires  310  coupled to a bendable spring  312 . Coupled to an accessible control  314 , e.g. on the handle  18 , pulling on the steering wires  310  bends the spring  312 , in generally the same fashion that the steering wires  66  affect bending of the spring  64 , as previously described with reference to FIG.  2 A. The physician is thereby able to affirmatively bend the operative branch  274  relative to the anchoring branch  276  to enhance site access and tissue contact. The steering wires  310  can be coupled to the spring  312  to affect bending in one plane or in multiple planes, either parallel to the catheter axis  292  or not parallel to the axis  292 . 
     Alternatively, or in combination with the manually bendable spring  312 , the spline element  282  can be prebent to form an elbow (like elbow  70  shown in FIG. 11 in association with the structure  20 ) generally orthogonal or at some other selected angle to the axis  292  of the catheter tube  12 . The spline element  282  can also be prebent into a circular or elliptical configuration. For example, a circular configuration can be used to circumscribe the pulmonary veins in the left atrium. 
     Alternatively, or in combination with a bendable operative branch  274 , the distal end  16  of the catheter tube  12  can include an independent steering mechanism (see FIG. 49C, e.g., including a bendable wire  64  and steering wires  66 , as previously described and as also shown in FIG.  2 A. By steering the entire distal end  16 , the physician orients the branched structure  272  at different angles relative to the targeted anchor site. 
     2. Slotted Branch Structures 
     FIG. 51 shows an embodiment of a branched structure  272 , in which the operative branch  274  can be moved in an outward direction (arrow  316 ) and an inward direction (arrow  318 ) relative to the catheter tube  12 . In this embodiment, the operative branch  274  comprises a tubular body  322 , which slidably extends through a lumen  324  in the catheter tube  12 . An accessible control  328  on the proximal end of the body  322  guides the sliding movement. 
     The spline element  282 , insulating sleeve  284 , and operative elements (i.e., electrode elements  28 ), already described, are carried at the distal end of the slidable body  322 . The catheter tube  12  includes a slot  320  near the distal end  16 , through which the slidable body  322  passes during outward and inward movement  316  and  318 . 
     The ability to move the operative branch  274  outside the catheter tube  12  enables the physician to define the number of electrodes  28  contacting the tissue, and thereby define the length of the resulting lesion. Alternatively, a movable operative branch  274  allows the physician to drag a selected activated electrode element  28  along tissue, while the anchoring branch  276  provides a stationary point of attachment. 
     The slidable body  322  can also be attached and arranged for rotation (arrows  352  in FIG. 51) with respect to the catheter tube  12 , if desired, by making the exterior contour of the slidable body  322  and the interior of the lumen  324  matching and symmetric. Rotation of the slidable body  322  can be prevented or restricted, if desired, by providing an exterior contour for the slidable body  322  that is asymmetric, and sliding the body  322  through a matching asymmetric anchor or lumen within the slot  320  or within the catheter tube  12 . 
     As FIG. 51 shows, radiopaque makers  326  are placed near the slot  320  and near the distal tip of the operative element  274  for visualization under fluoroscopy. The markers  326  can be located at other parts of the structure  274 , as well, to aid in manipulating the operative branch  274  and anchoring branch  276 . 
     The operative branch  274  shown in FIG. 51 can include a steering spring and steering wires in the manner previously shown and described in FIG.  46 . All other mechanisms also previously described to bend the operative branch  274  in planes parallel and not parallel to the catheter axis  292  can also be incorporated in the FIG. 51 embodiment. 
     FIG. 52 shows an embodiment, like FIG. 51, except that the catheter body  12  also carries an accessible control  330  to rotate the slot  320  about the catheter tube axis  292  (arrows  352  in FIG.  52 ). If the operative branch  272  is free to rotated upon itself (as previously described), and if the spline element  282  within the operative branch  274  is circular in cross section, the operative branch  274  will rotate upon itself during rotation of the slot  320 . In this arrangement, rotation of the slot  320  torques the operative branch about the catheter tube axis  292 . 
     On the other hand, if the spline element  282  within the operative branch  274  is rectangular in cross section, the operative branch  274  will rotate upon itself during rotation of the slot  320 , provided that rotation of the operative branch  274  about its axis is not prevented, and provided that the angle (α in FIG. 52) between the axis  332  of the operative branch  274  and the axis  292  of the catheter tube  12  is less than 20°. Otherwise, an operative branch  274  with a rectilinear spline element  282 , will not rotate upon itself during rotation of the slot  320 , and thus can not be torqued by rotation of the slot  320 . 
     FIG. 53 shows an embodiment of the structure  272 , which like FIG. 51, allows movement of the operative branch  274  through a slot  320 . Unlike the embodiment in FIG. 51, the embodiment shown in FIG. 53 includes a pull wire  334  attached to the distal end  336  of the operative branch  274 . The pull wire  334  passes through the exterior sheath  304  or through the catheter tube  12  (previously described) to an accessible stop  336 . Advancing the operative branch  274  forward (arrow  338 ) through the slot  320 , while holding the pull wire  334  stationary, bends the operative branch  274  into a loop, in much the same manner previously described in connection with the FIG. 15A embodiment. Pulling on the wire  334  (arrow  342 ) reduces the amount of exposed length beyond the distal end of the sheath  304 . By advancing the catheter tube (arrow  340 ), the radius of curvature of the looped operative branch  274  can be adjusted, in much the same way previously shown in the FIG. 17A embodiment. 
     FIG. 54 shows an embodiment of the structure  272 , which like FIG. 51, allows movement of the operative branch  274  through a slot  320 . Unlike the embodiment in FIG. 51, the embodiment shown in FIG. 53 includes a flexible joint  344  which joins the distal end  336  of the operative branch  274  to the distal end  16  of the catheter tube  12 . Advancing the operative branch  274  forward (arrow  338 ) through the slot  320 , bends the operative branch  274  into a loop, in much the same manner previously described in connection with the FIGS. 3 and 15 embodiments. The flexible joint  344  can comprise a plastic material or a metal material, as the preceding FIGS. 3 and 15 embodiments demonstrate. 
     3. Spanning Branch Structures 
     FIG. 56 shows a self-anchoring multiple electrode structure  356  comprising multiple operative branches (two operative branches  358  and  360  are shown in FIG.  56 ). Like the operative branch  274  shown in FIG. 45 each operative branch  358  and  360  carries one or more operative elements, which can take various forms, and which in the illustrated embodiment comprise the electrode elements  28 . Each operative branch  358  and  360  likewise comprises a spline element  362  enclosed within an electrically insulating sleeve  364 , on which the electrode elements  28  are carried. 
     In the illustrated embodiment, the operative branches  358  and  360  are jointly carried within a catheter sheath  370 . Each operative branch  358  and  360  is individually slidable within the sheath  370  between a deployed position (FIG. 56) and a retracted position (FIG.  57 ). It should be appreciated that each operative branch  358  and  360  can be deployed and retracted in an individual sheath. 
     Each operative element  358  and  360  includes a distal region, respectively  366  and  368 , which are mutually bent outward in a “bow-legged” orientation, offset from the main axis  372  of the sheath  370 . This outwardly bowed, spaced apart relationship between the regions  366  and  368  can be obtained by prestressing the spline elements  362  into the desired shape, or by providing a spring which is actively steered by steering wires (as described numerous times before), or both. The desired mutual orientation of the branches  358  and  360  can be retained by making at least the proximal portion of the spline elements  362  not round, thereby preventing relative rotation of the branches  358  and  360  within the sheath  370 . 
     In use (see FIG.  58 ), each distal region  366  and  368  is intended to be individually maneuvered into spaced apart anchoring sites, e.g., the pulmonary veins (PV in FIG.  58 ). Once both regions  366  and  368  are suitably anchored, the operative branches  360  and  362  are advanced distally, toward the anchoring sites. The operative branches  360  and  362  bend inward, toward the sheath axis  372 , to place the electrode elements  28  in contact with tissue spanning the anchoring sites. 
     FIG. 59 shows an alternative embodiment of a self-anchoring structure  374 . Like the structure  356  shown in FIG. 56, the structure  374  includes two branches  376  and  378 , which are slidably carried within a sheath  380 . When deployed outside the sheath  380 , the distal ends  384  and  386  of the branches  376  and  378  are located in an outwardly bowed orientation relative to the axis  388  of the sheath  380 . As earlier described in connection with the FIG. 45 embodiment, the branches  376  and  378  can be bent outwardly either by prestressing the associated interior spline elements  380 , located in the branches  376  and  378 , or providing active steering, or both. 
     In FIG. 59, a flexible element  382  spans the distal ends  484  and  386  of the branches  376  and  378 . The flexible element  382  is made of material that is less rigid that the two branches  376  and  378 . In the illustrated embodiment, the flexible element  382  is biased to assume a normally outwardly bowed shape, as FIG. 59 shows. The element  382  carries one or more operative elements, which can vary and which in the illustrated embodiment comprise electrode elements  28 . 
     As FIG. 60 shows, in use, each distal region  384  and  386  is intended to be individually maneuvered into spaced apart anchoring sites, e.g., the pulmonary veins (PV in FIG.  60 ). When the regions  384  and  386  are suitably anchored, the spanning element  382  places the electrode elements  28  in contact with tissue spanning the anchoring sites. If the tissue region between the anchoring sites has a concave contour (and not a convex contour, as FIG. 60 shows), the outwardly bowed bias of the flexible element  382  will conform to the concave contour, just as it conforms to a convex contour. 
     4. Spring-Assisted Branch Structures 
     FIG. 61 shows another embodiment of a spring-assisted multiple electrode structure  390 . The structure  390  includes two operative branches  392  and  394  carried at the distal end  16  of the catheter tube  12 . The catheter tube  12  forms part of a probe  10 , as shown in FIG.  1 . 
     As previously described in connection with the embodiment shown in FIG. 56, each operative branch  392  and  394  comprises a spline element  396  enclosed within an electrically insulating sleeve  398 . Operative elements, for example, electrode elements  28 , are carried by the sleeve  398 . 
     In the FIG. 61 embodiment, the spline elements  396  are preformed to move along the exterior of the distal catheter end  16  and then extend radially outward at an angle of less than 90°. The spline elements  396 , prestressed in this condition, act as spring mechanisms for the operative branches  392  and  394 . The prestressed spline elements  396  hold the branches  392  and  394  in a spaced apart condition (shown in FIG.  61 ), but resisting further or less radial separation of the branches  392  and  394 . 
     A sheath  400  is slidable in a forward direction (arrow  402  in FIG. 62) along the catheter tube  12  to press against and close the radial spacing between the branches  392  and  394 . This low profile geometry (shown in FIG. 62) allows introduction of the structure  390  into the selected body region. Rearward movement of the sheath  400  (arrow  404  in FIG. 61) frees the branches  392  and  394 , which return due to the spring action of the spline elements  396  to a normally spaced apart condition (shown in FIG.  61 ). 
     The catheter tube  12  includes an interior lumen  406 . As FIG. 61 shows, the lumen  406  accommodates passage of a guide wire  408  with a blunt distal end  410 . 
     When deployed in an atrium (as FIG. 63A depicts) the distal end  410  of the guide wire  408  is maneuvered into a selected anchoring site (e.g., a pulmonary vein in the left atrium, or the inferior vena cava in the right atrium). The structure  390 , enclosed within the sheath  400 , is slid over the guide wire  408  to the targeted site (arrow  412  in FIG.  63 A). As FIG. 63B shows, the sheath  400  is moved rearwardly (arrow  414  in FIG. 63B) to free the spring-like operative branches  392  and  394 . Advancing the operative branches  392  and  394  along the guide wire  408  opens the radial spacing between the branches. The spring action of the spline elements  396  resisting this action exerts force against the tissue, assuring intimate contact between the electrode elements  28  and the tissue. The spline elements  396  can also be deployed within an atrium without use of a guide wire  408 . 
     One or more spring-assisted spline elements  396  of the kind shown in FIG. 61 can also be deployed in a ventricle or in contact with the atrial septum for the purpose of making large lesions. As in the atrium, use of the guide wire  408  is optional. However, as shown in FIG. 63C, in these regions, a guide wire  408  can be used, which includes at its distal end a suitable positive tissue fixation element  542 , e.g., a helical screw or vacuum port, to help stabilize the contact between the spline elements  396  and myocardial tissue. Several spline elements  396  can be arranged in a circumferentially spaced star pattern to cover a large surface area and thereby make possible the larger, deeper lesions believed to be beneficial in the treatment of ventricular tachycardia. 
     The spring action (i.e., spring constant) of the spline elements  396  can be varied, e.g., by changing the cross sectional area of the spline elements  396 , or by making material composition or material processing changes. 
     5. Self-Anchoring Loop Structures 
     FIG. 66 shows an assembly  450 , which, in use, creates a self-anchoring loop structure  452  (which is shown in FIG.  68 ). The assembly  450  includes a catheter  486  comprising a flexible catheter tube  454  with a handle  256  on its proximal end, and which carries a multiple electrode array  458  on its distal end  470 . 
     In the illustrated embodiment, the multiple electrode array  458  comprises electrode elements  28  attached to a sleeve  460  (see FIG.  69 ), which is made from an electrically insulating material, as already described. 
     As FIG. 69 best shows, a bendable spring  462  is carried within the sleeve  460  near the distal end  470  of the catheter tube  454 . One or more steering wires  464  are attached to the spring  462  and pass through the catheter tube  454  to a steering controller  468  in the handle. While various steering mechanisms can be used, in the illustrated embodiment, the controller  468  comprises a rotatable cam wheel of the type shown in Lundquist and Thompson U.S. Pat. No. 5,254,088, which is already incorporated into this Specification by reference. 
     Operation of the steering controller  468  pulls on the steering wires  464  to apply bending forces to the spring  462 . Bending of the spring  462  bends (arrows  490  in FIG. 66) the distal end  470  of the catheter tube  454  (shown in phantom lines), to deflect the multiple electrode array  458 . As heretofore described, the catheter  486  can comprise a conventional steerable catheter. 
     The catheter tube  454  carries a sheath  472 . The sheath  472  includes a proximal gripping surface  482  accessible to the physician. The sheath  472  also includes a closed distal end  476 , and a slot  474 , which is cut out proximal to the closed distal end  476 . A region  480  of the sheath remains between the distal edge of the slot  474  and the closed distal catheter tube end  476 . This region  480  peripherally surrounds an interior pocket  478 . 
     The catheter tube  12  is slidable within the sheath  472 . When the distal end  470  occupies the slot  474 , sliding the catheter tube  12  forward inserts the distal end  470  into the pocket  478 , as FIG. 67 shows. The distal end  470  of the catheter tube  454  can be inserted into the pocket  478  either before introduction of the electrode array  458  into the targeted body region, or after introduction, when the electrode array  458  is present within the targeted body region. The pocket  478  is sized to snugly retain the inserted end  470  by friction or interference. 
     By holding the sheath  472  stationary and applying a rearward sliding force on the catheter tube  454 , the physician is able to free the distal catheter tube end  470  from the pocket  478 , as FIG. 66 shows. With the distal end  470  free of the pocket  478 , the physician is able to slide the entire catheter tube  454  from the sheath  472 , if desired, and insert a catheter tube of another catheter in its place. 
     Once the distal catheter tube end  470  is inserted into the pocket  478 , the physician can form the loop structure  452 . More particularly, by gripping the surface  482  to hold the sheath  472  stationary, the physician can slide the catheter tube  454  forward with respect to the sheath  472  (arrow  484  in FIG.  68 ). As FIG. 68 shows, advancement of the catheter tube  454  progressively pushes the multiple electrode array  458  outward through the slot  474 . With the distal end  470  captured within the pocket  478 , the pushed-out portion of the electrode array  458  bends and forms the loop structure  452 . 
     In many respects, the loop structure  452  shown in FIG. 68 shares attributes with the loop structure  20 , shown in FIG.  3 A. The sheath region  488  underlying the slot  474  serves as a flexible joint for the loop structure  452 , just as the flexible joint  44  does for the loop structure  20  in FIG.  3 A. However, unlike the structure  20  in FIG.  3 A, the physician is able to mate with the pocket  478  a catheter of his/her own choosing, since the pocket  478  allows easy insertion and removal of a catheter from the assembly  450 . The physician is thereby given the opportunity to select among different catheter types and styles for use in forming the loop structure  452 . 
     Furthermore, as FIG. 70 shows, the distal end  470  of the catheter tube  454 , when retained within the pocket  478 , can serve to establish contact with an anatomic structure S, while the loop structure  452  contacts nearby tissue T. As FIG. 67 shows, operation of the steering controller  468  serves to deflect the pocket region  480  of the sheath  472  along with the distal catheter tube end  470 , to help maneuver and locate the sheath distal end  470  in association with the anatomic structure S. The distal end  470  of the catheter tube  454 , retained within the pocket  478 , can thereby serve to stabilize the position of the loop structure  452  in contact with tissue T during use. 
     The stiffness of the sheath  472  and the length of the flexible joint region  488  are selected to provide mechanical properties to anchor the loop structure  452  during use. Generally speaking, the sheath  472  is made from a material having a greater inherent stiffness (i.e., greater durometer) than the structure  452  itself. The selected material for the sheath  472  can also be lubricious, to reduce friction during relative movement of the catheter tube  454  within the sheath  472 . For example, materials made from polytetrafluoroethylene (PTFE) can be used for the sheath  452 . The geometry of the loop structure  452  can be altered by varying the stiffness of the sheath  472 , or varying the stiffness or the length of the flexible joint  488 , or one or more of these at the same time. 
     There are various ways to enhance the releasable retention force between the distal catheter tube end  470  and the pocket  478 . For example, FIG. 71 shows a sheath  472  having a pocket region  480  in which the interior walls  500  of the pocket  478  are tapered to provide a releasable interference fit about the distal catheter tube end  470 . As another example, FIG. 72 shows a distal catheter tube end  470 , which includes a ball-nose fixture  502  which makes releasable, snap-fit engagement with a mating cylindrical receiver  504  formed in the pocket  478 . By providing active attachment mechanisms within the pocket  478 , the effective length of the pocket region  480  can be reduced. These preformed regions can be formed by thermal molding. 
     FIG. 73 shows a modification of the self-anchoring loop structure  452  shown in FIG. 68, in which the distal end  470  of the catheter tube  454  forms a pivoting junction  506  with the pocket region  480  of the sheath  472 . FIGS. 74 and 75 show the details of one embodiment of the pivoting junction  506 . 
     As FIG. 74 shows, the pocket region  480  includes an axial groove  508  that opens into the pocket  478 . The distal end  470  of the catheter tube includes a ball joint  510 . As FIG. 75 shows, forward sliding movement of the catheter tube  454  advances the distal end  470 , including the ball joint  510 , into the pocket  478 . As FIG. 76 shows, as further advancement of the catheter tube  454  progressively pushes the multiple electrode array  458  outward through the slot  474 , the ball joint  510  enters the groove  508 . The ball joint  510  pivots within the groove  508 , thereby forming the pivoting junction  506 . The junction  506  allows the distal end  470  to swing with respect to the pocket region  480  (arrows  512  in FIG.  76 ), as the pushed-out portion of the electrode array  458  bends and forms the loop structure  452 , shown in FIG.  73 . 
     FIGS. 77A to  77 D show another embodiment of the pivoting junction  506 . In this embodiment, a separately cast plastic or metal cap  514  is attached to the end of the sheath  472 . The cap  514  includes an interior cavity forming the pocket  478 . Unlike the previously described embodiments, the pocket  478  in the cap  514  includes an interior wall  516  (see FIG.  77 D), which is closed except for a slotted keyway  518 . 
     The cap  514  includes the previously described groove  508 . Unlike the previous embodiments, the groove  508  extends to and joins the slotted keyway  518  (see FIG.  77 A). The groove  508  also extends through the distal end  520  of the cap  514  to an opening  522  (see FIG. 77B) on the side of the cap  514  that faces away from the sheath slot  474 . As FIG. 77B shows, the opening  522  accommodates passage of the ball joint  510  carried at the distal end  470  of the catheter tube  454 . Advancing the ball joint  510  from the opening  522  along the groove  508  locks the ball joint  510  within the pocket  478 . Further advancement brings the ball joint  510  to rest within the slotted keyway  518  (see FIG.  77 C). The slotted keyway  518  retains the ball joint  510 , securing the distal catheter tube end  470  to the cap  514 . The interference between the ball joint  510  and the keyway  518  prevents separation of the distal catheter tube end  470  from the sheath  472  by sliding axial movement of the catheter tube  545  within the sheath  472 . However, as FIG. 77D shows, the ball joint  510  pivots within the groove  508  of the cap  514 , thereby forming the pivoting junction  506 , to allow the distal end  470  to swing with respect to the pocket region  478 . 
     The distal catheter tube end  470  is separated from the cap  514  by sliding the ball joint  510  along the groove  508  into the opening  522 . The ball joint  510  passes through the opening  522 , thereby releasing the catheter tube  454  from the sheath  472 . 
     FIGS. 78A to  78 C show another embodiment of the pivoting junction  506 . In this embodiment, like FIGS. 77A to  77 D, a separately cast plastic or metal cap  606  is attached to the end of the sheath  472 . The cap  606  includes an interior cavity forming the pocket  608 . As FIG. 78A shows, the pocket  608  receives the ball joint  510  (carried by the distal loop structure end  470 ) through the sheath end  612  of the cap  606 , in the manner previously described and shown with reference to FIG.  76 . 
     As FIGS. 78B and 78C show, the ball joint  510  pivots within the pocket  608  through a groove  610  formed in the cap  514 . The pivoting junction  506  is thereby formed, which allows the distal end  470  to swing with respect to the cap  606 . 
     6. Deployment and Use of Self-Anchoring Multiple Electrode Structures 
     1. Left Atrium 
     The self-anchoring multiple electrode structures described above can be deployed into the left atrium to create lesions between the pulmonary veins and the mitral valve annulus. Tissue nearby these anatomic structures are recognized to develop arrhythmia substrates causing atrial fibrillation. Lesions in these tissue regions block reentry paths or destroy active pacemaker sites, and thereby prevent the arrhythmia from occurring. 
     FIG. 79 shows (from outside the heart H) the location of the major anatomic landmarks for lesion formation in the left atrium. The landmarks include the right inferior pulmonary vein (RIPV), the right superior pulmonary vein (RSPV), the left superior pulmonary vein (LSPV), the left inferior pulmonary vein (LIPV); and the mitral valve annulus (MVA). FIG. 80A to FIG. 80D show representative lesion patterns formed inside the left atrium based upon these landmarks. 
     In FIG. 80A, the lesion pattern comprises a first leg L 1  between the right inferior pulmonary vein (RIPV) and the right superior pulmonary vein (RSPV); a second leg L 2  between the RSPV and the left superior pulmonary vein (LSPV); a third leg L 3  between the left superior pulmonary vein (LSPV) and the left inferior pulmonary vein (LIPV); and a fourth leg L 4  leading between the LIPV and the mitral valve annulus (MVA). 
     FIG. 80B shows an intersecting lesion pattern comprising horizontal leg L 1  extending between the RSPV-LSPV on one side and the RIPV-LIPV on the other size, intersected by vertical leg L 2  extending between the RSPV-RIPV on one side and the LSPV-LIPV on the other side. The second leg L 2  also extends to the MVA. 
     FIG. 80C shows a criss-crossing lesion pattern comprising a first leg extending between the RSPV and LIPV; a second leg L 2  extending between the LSPV and RIPV; and a third leg L 3  extending from the LIPV to the MVA. 
     FIG. 80D shows a circular lesion pattern comprising a leg L 1  that extends from the LSPV, and encircles to RSPV, RIPV, and LIPV, leading back to the LSPV. 
     The linear lesion patterns shown in FIGS. 80A,  80 B, and  80 C can be formed, e.g., using the structure  272  shown in FIGS. 45 and 46, by placing the anchoring branch  276  in a selected one of the pulmonary veins to stabilize the position of the operative branch  274 , and then maneuvering the operative branch  274  to sequentially locate it along the desired legs of the lesion pattern. It may be necessary to relocate the anchoring branch  276  in a different pulmonary vein to facilitate maneuvering of the operative branch  274  to establish all legs of the pattern. The branched structures  356  (FIG. 56) or  374  (FIG. 59) can also be used sequentially for the same purpose, in the manner shown in FIG. 58 (for structure  356 ) and FIG. 60 (for structure  374 ). 
     The circular lesion pattern shown in FIG. 80D can be formed, e.g., using an anchored loop structure  458  as shown in FIG. 68 or  73 . Using these structures, the distal end  470  of the catheter tube  454  (enclosed within the pocket  478 ) is located within a selected one of the pulmonary veins (the LSPV in FIG.  80 D), and the loop structure is advanced from the sheath  472  to circumscribe the remaining pulmonary veins. As with other loop structures, the loop structure tend to seek the largest diameter and occupy it. Most of the structures are suitable for being torqued or rotated into other planes and thereby occupy smaller regions. The anchored loop structure  458  is also suited for forming lesion legs that extend from the inferior pulmonary veins to the mitral valve annulus (for example, L 4  in FIG.  80 A and L 3  in FIG.  80 C). 
     To access the left atrium, any of these structures can be introduced in the manner shown in FIG.  44  through the inferior vena cava (IVC) into the right atrium, and then into the left atrium through a conventional transeptal approach. Alternatively, a retrograde approach can be employed through the aorta into the left ventricle, and then through the mitral valve into the left atrium. 
     2. Right Atrium 
     FIG. 79 shows (from outside the heart H) the location of the major anatomic landmarks for lesion formation in the right atrium. These landmarks include the superior vena cava (SVC), the tricuspid valve annulus (TVA), the inferior vena cava (IVC), and the coronary sinus (CS). Tissue nearby these anatomic structures have been identified as developing arrhythmia substrates causing atrial fibrillation. Lesions in these tissue regions block reentry paths or destroy active pacemaker sites and thereby prevent the arrhythmia from occurring. 
     FIGS. 81A to  81 C show representative lesion patterns formed inside the right atrium based upon these landmarks. 
     FIG. 81A shows a representative lesion pattern L that extends between the superior vena cava (SVC) and the tricuspid valve annulus (TVA). 
     FIG. 81B shows a representative lesion pattern that extends between the interior vena cava (IVC) and the TVA. FIG. 81C shows a representative lesion pattern L that extends between the coronary sinus (CS) and the tricuspid valve annulus (TVA). 
     The self-anchoring multiple electrode structures described above can be deployed into the right atrium to create these lesions. For example, the structure  272  shown in FIGS. 45 and 46 can be used, by placing the anchoring branch  276  in the SVC or IVC to stabilize the position of the operative branch  274 , and then maneuvering the operative branch  274  to locate it along the desired path of the lesion pattern. The branched structures  356  (FIG. 56) or  374  (FIG. 59) can also be used sequentially for the same purpose, in the manner shown in FIG. 58 (for structure  356 ) and FIG. 60 (for structure  374 ). 
     Any of these structures can be introduced in the manner shown in FIG.  44  through the inferior vena cava (IVC) into the right atrium. 
     3. Epicardial Use 
     Many of the structures suited for intracardiac deployment, as discussed above, can be directly applied to the epicardium through conventional thoracotomy or thoracostomy techniques. For example, the structures shown in FIGS. 56,  59 ,  61 ,  66 , and  73  are well suited for epicardial application. 
     III. Flexible Electrode Structures 
     A. Spacing of Electrode Elements 
     In the illustrated embodiment, the size and spacing of the electrode elements  28  on the various structures can vary. 
     1. Long Lesion Patterns 
     For example, the electrode elements  28  can be spaced and sized for creating continuous, long lesion patterns in tissue, as exemplified by the lesion pattern  418  in tissue T shown in FIG.  64 . Long, continuous lesion patterns  418  are beneficial to the treatment of atrial fibrillation. The patterns  418  are formed due to additive heating effects, which cause the lesion patterns  418  to span adjacent, spaced apart electrode  28 , creating the desired elongated, long geometry, as FIG. 64 shows. 
     The additive heating effects occur when the electrode elements  28  are operated simultaneously in a bipolar mode between electrode elements  28 . Furthermore, the additive heating effects also arise when the electrode elements  28  are operated simultaneously in a unipolar mode, transmitting energy to an indifferent electrode  420  (shown in FIG.  44 ). 
     More particularly, when the spacing between the electrodes  28  is equal to or less than about 3 times the smallest of the diameters of the electrodes  28 , the simultaneous emission of energy by the electrodes  28 , either bipolar between the segments or unipolar to the indifferent electrode  420 , creates an elongated continuous lesion pattern  58  in the contacted tissue area due to the additive heating effects. 
     Alternatively, when the spacing between the electrodes along the contacted tissue area is equal to or less than about 2 times the longest of the lengths of the electrodes  28 , the simultaneous application of energy by the electrodes  28 , either bipolar between electrodes  28  or unipolar to the indifferent electrode  420 , also creates an elongated continuous lesion pattern  58  in the contacted tissue area due to additive heating effects. 
     Further details of the formation of continuous, long lesion patterns are found in copending U.S. patent application Ser. No. 08/287,192, filed Aug. 8, 1994, entitled “Systems and Methods for Forming Elongated Lesion Patterns in Body Tissue Using Straight or Curvilinear Electrode Elements,” which is incorporated herein by reference. 
     Alternatively, long continuous lesion patterns, like that shown in FIG. 64, can be achieved using an elongated electrode element made from a porous material. By way of illustration, FIG. 82 shows a loop electrode structure  424 , like that shown in FIG.  2 A. The structure  424  includes an electrode body  428 , which includes a porous material  430  to transfer ablation energy by ionic transport. 
     As FIG. 82 shows, the distal end  426  of the electrode body  428  is coupled to a flexible joint  440 , which is part of the slotted sheath  442 , as previously described in connection with FIG.  3 A. Advancement of the electrode body  428  from the slotted sheath  442  creates the loop structure  424 , in the same manner that the loops structure  20  shown in FIG. 3A is formed. 
     As best shown in FIG. 83, the electrode body  428  includes a center support lumen  432  enveloped by the porous material  430 . The lumen  432  carries spaced-apart electrodes  429  along its length. The lumen  432  also includes spaced-apart apertures  434  along its length. 
     The lumen  432  includes a proximal end  430 , which communicates with a source of ionic fluid  438 . The lumen  432  conveys the ionic fluid  438 . The ionic fluid  438  passes through the apertures  434  and fills the space between the lumen  432  and the surrounding porous material  430 . The fluid  438  also serves to expand the diameter of the structure  424 . The structure  424  therefore possesses a low profile geometry, when no liquid  438  is present, for introduction within the targeted body region enclosed within the slotted sheath  442 . Once advanced from the sheath  442  and formed into the loop structure  424 , fluid  438  can be introduced to expand the structure  424  for use. 
     The porous material  430  has pores capable of allowing transport of ions contained in the fluid  438  through the material  430  and into contact with tissue. As FIG. 83 also shows, the electrodes  429  are coupled to a source  444  of radio frequency energy. The electrodes  429  transmit the radio frequency energy into the ionic fluid  438 . The ionic (and, therefore, electrically conductive) fluid  438  establishes an electrically conductive path. The pores of the porous material  430  establish ionic transport of ablation energy from the electrodes  429 , through the fluid  438 , liquid, to tissue outside the electrode body  428 . 
     Preferably, the fluid  438  possesses a low resistivity to decrease ohmic loses, and thus ohmic heating effects, within the body  428 . The composition of the electrically conductive fluid  438  can vary. In the illustrated embodiment, the fluid  438  comprises a hypertonic saline solution, having a sodium chloride concentration at or near saturation, which is about 5% to about 25% weight by volume. Hypertonic saline solution has a low resistivity of only about 5 ohm·cm, compared to blood resistivity of about 150 ohm·cm and myocardial tissue resistivity of about 500 ohm·cm. 
     Alternatively, the composition of the electrically conductive fluid  438  can comprise a hypertonic potassium chloride solution. This medium, while promoting the desired ionic transfer, requires closer monitoring of the rate at which ionic transport occurs through the pores of the material  430 , to prevent potassium overload. When hypertonic potassium chloride solution is used, it is preferred to keep the ionic transport rate below about 10 mEq/min. 
     Regenerated cellulose membrane materials, typically used for blood oxygenation, dialysis, or ultrafiltration, can be used as the porous material  430 . Regenerated cellulose is electrically non-conductive; however, the pores of this material (typically having a diameter smaller than about 0.1 μm) allow effective ionic transport in response to the applied RF field. At the same time, the relatively small pores prevent transfer of macromolecules through the material  430 , so that pressure driven liquid perfusion is less likely to accompany the ionic transport, unless relatively high pressure conditions develop within the body  428 . 
     Other porous materials can be used as the porous material  430 . Candidate materials having pore sizes larger than regenerated cellulous material, such as nylon, polycarbonate, polyvinylidene fluoride (PTFE), polyethersulfone, modified acrylic copolymers, and cellulose acetate, are typically used for blood microfiltration and oxygenation. Porous or microporous materials may also be fabricated by weaving a material (such as nylon, polyester, polyethylene, polypropylene, fluorocarbon, fine diameter stainless steel, or other fiber) into a mesh having the desired pore size and porosity. These materials permit effective passage of ions in response to the applied RF field. However, as many of these materials possess larger pore diameters, pressure driven liquid perfusion, and the attendant transport of macromolecules through the pores, are also more likely to occur at normal inflation pressures for the body  428 . Considerations of overall porosity, perfusion rates, and lodgment of blood cells within the pores of the body  128  must be taken more into account as pore size increase. 
     Low or essentially no liquid perfusion through the porous body  428  is preferred. Limited or essentially no liquid perfusion through the porous body  428  is beneficial for several reasons. First, it limits salt or water overloading, caused by transport of the hypertonic solution into the blood pool. This is especially true, should the hypertonic solution include potassium chloride, as observed above. Furthermore, limited or essentially no liquid perfusion through the porous body  428  allows ionic transport to occur without disruption. When undisturbed by attendant liquid perfusion, ionic transport creates a continuous virtual electrode at the electrode body-tissue interface. The virtual electrode efficiently transfers RF energy without need for an electrically conductive metal surface. 
     FIGS. 84 and 85 show an embodiment of the porous electrode body  428  which includes spaced-apart external rings  446 , which form porous electrode segments. It is believed that, as the expanded dimension of the body  428  approaches the dimension of the interior electrodes  429 , the need to segment the electrode body  428  diminishes. 
     Alternatively, as FIG. 86 shows, instead of a lumen  432  within the body  438 , a foam cylinder  448  coupled in communication with the ionic fluid  438  could be used to carry the electrodes  429  and perfuse the ionic fluid  438 . 
     2. Interrupted Lesion Patterns 
     The electrode elements  28  can be sized and spaced to form interrupted, or segmented lesion patterns, as exemplified by the lesion pattern  422  in tissue T shown in FIG.  65 . Alternatively, spaced-apart electrode elements  28  capable of providing long lesion patterns  418  can be operated with some electrode elements  28  energized and others not, to provide an interrupted lesion pattern  422 , as FIG. 65 exemplifies. 
     When the spacing between the electrodes  28  is greater than about 5 times the smallest of the diameters of the electrodes  28 , the simultaneous emission of energy by the electrodes  28 , either bipolar between segments or unipolar to the indifferent electrode  420 , does not generate additive heating effects. Instead, the simultaneous emission of energy by the electrodes  28  creates an elongated segmented, or interrupted, lesion pattern in the contacted tissue area. 
     Alternatively, when the spacing between the electrodes  28  along the contacted tissue area is greater than about 3 times the longest of the lengths of the electrodes  28 , the simultaneous application of energy, either bipolar between electrodes  28  or unipolar to the indifferent electrode  420 , creates an elongated segmented, or interrupted, lesion pattern. 
     3. Flexibility 
     When the electrode elements  28  are flexible, each element  28  can be as long as 50 mm. Thus, if desired, a single coil electrode element  28  can extend uninterrupted along the entire length of the support structure. However, a segmented pattern of spaced apart, shorter electrode elements  28  is preferred. 
     If rigid electrode elements  28  are used, the length of the each electrode segment can vary from about 2 mm to about 10 mm. Using multiple rigid electrode elements  28  longer than about 10 mm each adversely effects the overall flexibility of the element. Generally speaking, adjacent electrode elements  28  having lengths of less than about 2 mm do not consistently form the desired continuous lesion patterns. 
     4. Temperature Sensing 
     As FIG. 3A shows, each electrode element  28  can carry at least one and, preferably, at least two, temperature sensing elements  540 . The multiple temperature sensing elements  540  measure temperatures along the length of the electrode element  28 . The temperature sensing elements  540  can comprise thermistors or thermocouples. If thermocouples are used, a cold junction  24  (see FIG. 3A) can be carried on the same structure as the electrode elements  28 . 
     An external temperature processing element (not shown) receives and analyses the signals from the multiple temperature sensing elements  540  in prescribed ways to govern the application of ablating energy to the electrode element  28 . The ablating energy is applied to maintain generally uniform temperature conditions along the length of the element  28 . 
     Further details of the use of multiple temperature sensing elements in tissue ablation can be found in copending U.S. patent application Ser. No. 08/286,930, filed Aug. 8, 1994, entitled “Systems and Methods for Controlling Tissue Ablation Using Multiple Temperature Sensing Elements.” 
     Various features of the invention are set forth in the following claims.