Patent Publication Number: US-6216043-B1

Title: Asymmetric multiple electrode support structures

Description:
RELATED APPLICATION 
     This application is a continuation-in-part of application Ser. No. 08/557,790, filed Nov. 13, 1995 and entitled “Multiple Electrode Support Structures Having Optimal Bio-Mechanical Characteristics,” now U.S. Pat. No. 5,904,680, which is itself a continuation-in-part of copending application Ser. No. 08/206,414, filed Mar. 4, 1994 and entitled “Multiple Electrode Support Structures.” (now abandoned). 
    
    
     FIELD OF THE INVENTION 
     The invention relates to multiple electrode structures deployed in interior regions of the heart for diagnosis and treatment of cardiac conditions. 
     BACKGROUND OF THE INVENTION 
     Physicians make use of catheters today in medical procedures to gain access into interior regions of the body to ablate targeted tissue areas. It is important for the physician to be able to precisely locate the catheter and control its emission of energy within the body during tissue ablation procedures. 
     The need for precise control over the catheter is especially critical during procedures that ablate endocardial tissue within the heart. These procedures, called electrophysiological therapy, are use to treat cardiac rhythm disturbances. 
     During these procedures, a physician steers a catheter through a main vein or artery into the interior region of the heart that is to be treated. The physician then further manipulates a steering mechanism to place the electrode carried on the distal tip of the catheter into direct contact with the endocardial tissue that is to be ablated. The physician directs energy from the electrode through tissue either to an indifferent electrode (in a uni-polar electrode arrangement) or to an adjacent electrode (in a bi-polar electrode arrangement) to ablate the tissue and form a lesion. 
     Physicians examine the propagation of electrical impulses in heart tissue to locate aberrant conductive pathways and to identify foci, which are ablated. The techniques used to analyze these pathways and locate foci are commonly called “mapping.” 
     Conventional cardiac tissue mapping techniques introduce several linear electrode arrays into the heart through vein or arterial accesses. There remains a need for improved endocardial mapping, impedance sensing, or ablation techniques using three dimensional, multiple electrode structures. 
     SUMMARY OF THE INVENTION 
     The invention provides asymmetric support structures and associated methods of deploying these structures in interior body regions. The structures are capable of supporting diagnostic or therapeutic elements, such as, for example, electrodes for sensing electrical events to map tissue or for sensing an electrical characteristic (such as impedance) of the tissue, or other types of therapeutic techniques. 
     One aspect of the invention provides a radially asymmetric support structure. In a preferred embodiment, the structure includes spline elements that extend between a hub and a base. The spline elements are circumferentially spaced apart about the hub axis to define angular intervals between adjacent spline elements. According to this aspect of the invention, two of the angular intervals are different by at least 20° to create a radially asymmetric geometry about the hub axis. Because of the radial asymmetry, the structure has a first region where adjacent spline elements are located radially closer together than in a second region. The radially asymmetric structure varies circumferential spacing between spline elements, thereby making it possible to vary the density of diagnostic or therapeutic elements about the periphery of the structure. 
     An associated method deploys the radially asymmetric structure in an interior body region. Contact between tissue and the second region of the structure also supports and stabilizes contact between tissue and the first region of the structure, where the greater density of diagnostic or therapeutic elements exists. 
     Another aspect of the invention provides a structure for deployment within an interior body cavity comprising a distal hub having an axis, a proximal base, and spline elements extending between the hub and the base. The spline elements exist in a circumferentially spaced relationship about the hub axis defining angular intervals between adjacent spline elements. The spline elements are adapted to contact tissue within the interior body cavity. The structure includes a mechanism to variably adjust the angular interval between at least two adjacent spline elements. An associated method deploys the variably adjustable structure in an interior body region. 
     Another aspect of the invention provides an axially asymmetric support structure. In a preferred embodiment, the structure comprises a spline element extending between a hub and a base along an elongated axis. The spline element includes a geometric midpoint between the hub and the base. According to this aspect of the invention, the spline element has a preformed memory normally biasing the spline element into a shape along the elongated axis that is asymmetric about the geometric midpoint. The spline element thereby possesses an axially asymmetric geometry along the elongated axis. An associated method deploys the axially asymmetric structure in an interior body region, which is preferable also axially asymmetric. The axially asymmetric structure makes it possible to position one or more diagnostic or therapeutic elements in conforming contact with tissue within asymmetric body cavities, such as a heart chamber. 
     Another aspect of the invention provides a support structure that is both radially and axially asymmetric. In a preferred embodiment, the support structure comprises spline elements that are both radially and axially asymmetric, as above described. The dual asymmetry of the structure makes it possible to provide localized density of diagnostic or therapeutic elements, while also closely conforming to the irregular contours of an interior body cavity, such as the heart. 
     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 side view of a multiple electrode probe having an electrode support assembly that is both axially and radially symmetric when in its deployed condition; 
     FIG. 2 is an end view of the electrode support assembly shown in FIG. 1, showing its radial symmetry; 
     FIG. 3 is an enlarged view, with parts broken away and in cross section, of the distal end of the probe shown in FIG. 1, showing the associated electrode support assembly in a collapsed condition within a sliding outer sleeve; 
     FIG. 4 is a side view of a multiple electrode probe having an electrode support assembly that is axially symmetric but radially asymmetric when in its deployed condition; 
     FIG. 5 is an end view of the electrode support assembly shown in FIG. 4, showing its radial asymmetry; 
     FIG. 6 is an end view of another electrode support assembly that is radially asymmetric; 
     FIG. 7 a side view of a multiple electrode probe having an electrode support assembly that is radially symmetric but axially asymmetric when in its deployed condition; 
     FIG. 8 is an end view of the electrode support assembly shown in FIG. 7, showing its radial symmetry; 
     FIG. 9 is a side view of another electrode support assembly that is axially asymmetric; 
     FIG. 10 a side view of a multiple electrode probe having an electrode support assembly that is both axially and radially asymmetric when in its deployed condition; 
     FIG. 11 is an end view of the electrode support assembly shown in FIG. 10, showing its radial asymmetry; 
     FIG. 12 is a side view of a hoop-like spline body having two spline elements that are axially asymmetric; 
     FIG. 13 is a top cross-sectional view of an end cap used in association with the spline body shown in FIG. 12, the end cap providing a radially asymmetric pattern of spline elements; 
     FIG. 14 is a side cross-sectional view of the end cap shown in FIG. 13, with a spline body attached, taken generally along line  14 — 14  in FIG. 13; 
     FIG. 15 is an exterior side view of the end cap shown in FIG. 13, with three spline bodies attached in a radially asymmetric pattern; 
     FIG. 16 is an exploded, perspective view of a multiple electrode assembly formed from three axially asymmetric spline bodies in a radially asymmetric geometry; 
     FIG. 17 is a perspective view of a base that is used in association with the end cap shown in FIGS. 13 to  15  to form the multiple electrode assembly shown in FIG. 16; 
     FIG. 18 is a side cross-sectional view of the end cap shown in FIGS. 13 to  15 , demonstrating the preferred angular relationship between the spline elements and the end cap; 
     FIG. 19 is a side view of the multiple electrode assembly shown in FIG. 16 in contact with tissue; 
     FIG. 20 is a diagrammatic view of a system that comprises a family of electrode support structures of various symmetric and asymmetric geometries, together with criteria suggesting their selection and use by a physician according to functional and physiological factors; 
     FIG. 21 an end view of a multiple electrode probe having an electrode support assembly that is radially asymmetric when in its deployed condition, and which also possesses asymmetric mechanical properties; 
     FIG. 22 is a side view of the electrode support assembly shown in FIG. 21; 
     FIG. 23 is a perspective side view of a distal hub assembly for joining together the distal regions of two flexible spline elements, which are held in woven registration by a length of flexible tubing; 
     FIGS. 24 to  26  are perspective side views of the assembly of the distal hub assembly shown in FIG. 23; 
     FIG. 27 is a perspective side view of a distal hub assembly for joining together the distal regions of two flexible spline elements, which are threaded through a length of flexible tubing encapsulated within a resilient sealing material; 
     FIG. 28 is a perspective side view of the assembly of the distal hub assembly shown in FIG. 27; 
     FIG. 29 is a perspective side view of an integral, radially asymmetric, axially symmetric support assembly, which possesses asymmetric mechanical properties and which has been cut from a single sheet of material; 
     FIGS. 30A and 30B are top views showing the manufacture of the support assembly shown in FIG. 29 by cutting a single sheet of material; 
     FIG. 31 is a perspective view of the interior portion of a heart, which appears in somewhat diagrammatic form for the purpose of illustration, showing a transeptal deployment of a radially asymmetric and axially symmetric multiple electrode support assembly in the left atrium for the purpose of creating long lesion patterns; 
     FIG. 32 is a diagrammatic representation of a long lesion pattern in tissue, which the electrodes carried by the support assembly shown in FIG. 31 create by additive heating effects; 
     FIG. 33 is a diagrammatic representation of a segmented lesion pattern in tissue, which multiple electrodes create in the absence of additive heating effects; 
     FIG. 34 is a diagrammatic representation of a complex long lesion pattern in tissue, which the electrodes carried by the support assembly shown in FIG. 31 create by additive heating effects; 
     FIG. 35 is a diagrammatic representation of a large lesion pattern in tissue; 
     FIG. 36 is a perspective view of the interior portion of a heart, which appears in somewhat diagrammatic form for the purpose of illustration, showing deployment of a radially asymmetric and axially symmetric multiple electrode support assembly in the left ventricle for the purpose of creating a large lesion pattern; 
     FIG. 37 is a diagrammatic representation of a large lesion pattern in tissue, which the electrodes carried by the support assembly shown in FIG. 36 create by additive heating effects; 
     FIGS. 38A and 38B are side sectional views, somewhat diagrammatic for the purpose of illustration, showing the deployment of an asymmetric multiple electrode structure within a body region, which is shown as a heart chamber; 
     FIG. 39 is a diagrammatic view of a system for identifying the characteristics of a multiple electrode support structure using a machine-readable code, which uniquely identifies the individual physical, mechanical, and functional characteristics of the structure; 
     FIG. 40 is a diagrammatic view of one implementation of the machine-readable code used to identify the individual physical, mechanical, and functional characteristics of the support structure shown in FIG. 39; 
     FIG. 41 is a diagrammatic view of another implementation of the machine-readable code used to identify the individual physical, mechanical, and functional characteristics of the support structure shown in FIG. 39; 
     FIGS. 42 to  44  are side views of a structure for supporting electrodes, which includes a slidable memory wire to vary the geometry of the structure from radially symmetric (FIG. 42) to different radially asymmetric geometries (FIGS.  43  and  44 ); 
     FIG. 45 is a diagrammatic view of a support spline usable in the structure shown in FIG. 42, which includes temperature-activated memory wire to vary the geometry of the structure from radially symmetric (FIG. 42) to different radially asymmetric geometries (FIGS.  43  and  44 ); 
     FIG. 46 is a side section view, largely diagrammatic, showing a structure for supporting electrodes, which includes an array of sliding plates to vary the geometry of the structure from radially symmetric to different radially asymmetric geometries; 
     FIG. 47 is a top perspective view of the structure shown in FIG. 46, with the plates spread apart to create a radially symmetric geometry; 
     FIG. 48 is a top perspective view of the structure shown in FIG. 46, with the plates stacked together to create a radially asymmetric geometry; 
     FIG. 49 is a top perspective view showing a structure for supporting electrodes, which includes an elastic joint and a movable array of wedges to vary the geometry of the structure from radially symmetric to different radially asymmetric geometries, the structure being shown with the movable wedges fully advanced near the elastic joint to create a radially symmetric geometry; 
     FIG. 50 is a top perspective view of the structure shown in FIG. 49, with the movable wedges fully retracted from the elastic joint to create a radially asymmetric geometry; and 
     FIG. 51 is a side view, largely diagrammatic, showing a structure for supporting electrodes comprising spline elements arranged in a radially asymmetric geometry in one region of the structure, the other region being free of spline elements. 
    
    
     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 
     I. Radially and Axially Symmetric Multiple Electrode Probe 
     FIG. 1 shows a multiple electrode probe  10 ( 1 ). The probe  10 ( 1 ) includes a flexible catheter tube  12  with a proximal end  14  and a distal end  16 . The proximal end  14  carries an attached handle  18 . The distal end  16  carries an electrode support assembly  20 ( 1 ), shown in side view in FIG.  1  and in end view in FIG.  2 . 
     As FIGS. 1 and 2 show, the support assembly  20 ( 1 ) comprises an array of flexible spline elements  22 ( 1 ). Each spline element  22 ( 1 ) preferably comprises a flexible body made from resilient, inert wire or plastic. Elastic memory material such as nickel titanium (commercially available as NITINOL™ material) can be used. Resilient injection molded plastic or stainless steel can also be used. 
     The spline elements  22 ( 1 ) extend longitudinally between a distal hub  24  and a base  26 . The base  26  is carried by the distal end  16  of the catheter tube  12 . As FIGS. 1 and 2 show, each spline  22 ( 1 ) is preformed with a convex bias, creating a normally open three dimensional basket structure expanded about a main center axis  36 . 
     The probe  10 ( 1 ) also includes an electrode circuit assembly  28 , one for each spline  22 ( 1 ). Each circuit assembly  28  comprises an array of multiple electrodes  30 . The electrodes  30  are electrically coupled by signal wires  34 , which extend through the catheter tube  12 , to external the connector  32 , which the handle  18  carries (see FIG.  1 ). Further details of the construction of the electrode circuit assemblies are shown in pending U.S. application Ser. No. 08/206,414, filed Mar. 4, 1994, which is incorporated herein by reference. 
     In the probe  10 ( 1 ), the geometry of flexible spline elements  22 ( 1 ) is radially symmetric about the main axis  36 . That is, when viewed from distal hub  24 , as FIG. 2 shows, the spline elements  22  uniformly radiate from the main axis  36  at generally equal arcuate, or radial, intervals. 
     In FIGS. 1 and 2, there are eight, radially symmetric spline elements  22 ( 1 ), each circumferentially separated by about 45°. This uniform, equal circumferential spacing of the spline elements  22 ( 1 ) completely about 360° forms a structure that this Specification calls radially symmetric. 
     The geometry of flexible spline elements  22 ( 1 ) of the probe  10 ( 1 ) is also axially symmetric along the main axis  36 . That is, when viewed from the side, as FIG. 1 shows, the proximal region  38  and the distal region  40  of each spline assembly  22 ( 1 ) occupied by the electrodes  30  have essentially the same curvilinear geometry along the main axis  36 . Thus, if bent upon itself at its geometric midpoint  42  along the main axis  36 , the proximal and distal regions  38  and  40  of the spline assembly  22 ( 1 ) would essentially overlie each other. This degree of symmetry between the proximal and distal electrode-bearing regions  38  and  40  of the spline elements  22  forms a structure that this Specification calls axially symmetric. 
     As FIG. 3 shows, in the illustrated and preferred embodiment, the probe  10 ( 1 ) includes an outer sheath  44  carried about the catheter tube  12 . The sheath  44  has an inner diameter that is greater than the outer diameter of the catheter tube  12 . As a result, the sheath  44  slides along the catheter tube  12 . 
     As FIG. 3 shows, forward movement (arrow  41  in FIG. 3) advances the slidable sheath  44  completely over the electrode support assembly  20 ( 1 ). In this position, the slidable sheath  44  compresses and collapses the support assembly  20 ( 1 ) into a low profile for introduction through a vein or artery to the intended treatment site within the body. 
     As FIG. 1 shows, rearward movement (arrow  43  in FIG. 3) retracts the slidable sheath  44  away from the support assembly  20 ( 1 ). This removes the compression force. The freed support assembly  20 ( 1 ) opens and assumes its three dimensional shape. 
     When deployed for use (as FIG. 1 shows)—which, in a preferred embodiment, is inside a heart chamber—the support assembly  20 ( 1 ) of the probe  10 ( 1 ) holds the electrodes  30  in contact against the endocardium. Due to its radial symmetry, the pattern density of electrodes  30  is generally the same wherever electrode-tissue contact occurs. Thus, the number of electrodes per unit area of endocardium contacted by the electrodes  30  is generally equal throughout the chamber. 
     II. Axially Symmetric/Radially Asymmetric Multiple Electrode Probe 
     FIGS. 4 and 5 show a multiple electrode support assembly  20 ( 2 ), which can be attached to the distal end  16  of a catheter tube  12  in the manner support assembly  20 ( 1 ) shown in FIG.  1 . Like the support assembly  20 ( 1 ), the support assembly  20 ( 2 ) includes an array of flexible spline elements  22 ( 2 ) extending longitudinally between a distal hub  24  and a base  26 . 
     For reasons that will be discussed later, due to the radial asymmetry of the assembly  20 ( 2 ), not all the spline elements  22 ( 2 ) need to carry electrodes  30 . Therefore, as FIGS. 4 and 5 show, electrode circuit assemblies  28 ( 2 ) are not present on all the spline elements  22 ( 2 ). Signal wires  34  electrically couple the electrodes  30  that are present to the external connectors  32 . 
     As FIG. 4 shows, the geometry of flexible spline elements  22 ( 2 ) of the assembly  20 ( 2 ) is symmetric in an axial sense for the same reasons that the array of spline elements  22 ( 1 ) shown in FIG. 1 is axially symmetric. FIG. 4 shows the proximal region  38  and the distal region  40  of each spline assembly  22 ( 2 ) being or capable of being occupied by electrodes  30  to have essentially the same curvilinear geometry along the main axis  36 . 
     However, unlike the assembly  20 ( 1 ), the geometry of flexible spline elements  22 ( 2 ) of assembly  20 ( 2 ) is asymmetric in a radial sense. That is, when viewed from distal hub  24 , as FIG. 5 shows, the spline elements  22 ( 2 ) do not radiate from the main axis  36  at generally equal circumferential intervals. Instead (as FIG. 5 shows), there are at least some adjacent spline elements  22 ( 2 ) that are circumferentially spaced apart more than other adjacent spline elements  22 ( 2 ). As described in this Specification, an assembly of spline elements is defined as being “radially asymmetric” when the largest angle measured between two adjacent spline elements in the assembly (designated angle α in FIG. 5) exceeds the smallest angle measured between two other adjacent spline elements (designated angle β in FIG. 5) is greater than 20°. 
     The particular arrangement shown in FIG. 5 includes ten spline elements  22 ( 2 ), designated S 1  to S 10 . The asymmetric arrangement shown in FIG. 5 comprises a first discrete group  46  of five adjacent spline elements  22 ( 2 )(S 1  to S 5 ) and a second discrete group  48  of five adjacent spline elements  22 ( 2 )(S 6  to S 10 ). The groups  46  and  48  are shown to be diametrically arranged, and each group  46  and  48  occupies an arc of about 90°. Within each group  46  and  48 , the adjacent spline elements S 1  to S 5  and S 6  to S 10  are circumferentially spaced apart in equal intervals of about 22° (which comprises angle β). However, the spline elements S/S 10  and S 5 /S 6 , marking the boundaries between the groups  46  and  48 , are circumferentially spaced farther apart, at about 90° (which comprises angle α). This non-uniform circumferential spacing of the spline elements  22 ( 2 )—in which angle α minus angle β is about 68° (that is, exceeds 20°)—exemplifies one type of structure that this Specification calls radially asymmetric. In the particular radial asymmetric geometry shown in FIG. 4, the splines S 1  to S 5  carry electrodes  30 , whereas the splines S 6  to S 10  do not. 
     Other types of structures can also be radially asymmetric. For example, FIG. 6 shows eight spline assemblies S 1  to S 8  arranged in a radially asymmetric geometry that differs from the one shown in FIG.  5 . In FIG. 6, the spline assemblies S 1  to S 3  (group  46 ) and S 5  to S 7  (group  48 ) are each generally circumferentially spaced apart at equal 30° intervals through an arc of about 60°. However, adjacent spline assemblies S 3 /S 4 ; S 4 /S 5 ; S 7 /S 8 ; and S 1 /S 8  are each circumferentially spaced apart at greater intervals than about 60°. In FIG. 6, the spline assemblies S 1  to S 3  carry electrodes  30 , whereas the remaining spline assemblies S 4  to S 8  do not. 
     It should also be appreciated that the groups  46  and  48  of spline assemblies  22 ( 2 ) need not be diametrically spaced apart (as FIGS. 5 and 6 show), nor do the spline assemblies  22 ( 2 ) within any given group  46  and  48  need to be equally spaced apart. Radially asymmetric structures are formed whenever the arcuate spacing between any two spline element differs significantly from the arcuate spacing between any two other spline elements. Furthermore, the mounting of electrodes  30  on all or some of the spline assemblies can vary. The particular functional requirements for the assembly  20 ( 2 ) dictate the particular radial asymmetric geometry selected for the spline elements  22 ( 2 ), as well as the particular placement of electrode  30  on all or some of the spline elements  22 ( 2 ). 
     By way of further example, FIG. 51 shows a spline assembly  264  which is radially asymmetric. The spline assembly  264  includes an array of spline elements  266  arranged in a closely spaced relationship in one region of the assembly  264 . The spline elements  266  carry electrodes  268 . The remainder of the assembly  264  is free of spline elements and, thus, free of electrodes. 
     In this arrangement, the spline elements  264  include elastic memory that bias the spline elements  264  toward an outwardly bowed condition. The elastic memory thus presents an outward force against tissue, facilitating intimate contact. 
     Alternatively, or in combination with elastic memory, the assembly  264  can include a pull wire  272  attached to the distal hub  270 , from which all the spline elements  266  radiate. Pulling on the wire  272  bows the spline elements  266  outward, toward tissue, creating an force against tissue contacting the spline elements  266 . 
     A. Structures Having Variable Radial Asymmetry 
     FIGS. 42 to  44  show a support assembly  190 , which allows the circumferential spacing of the spline elements (designated  192 ( 1 ),  192 ( 2 ), and  192 ( 3 )) to be changed by the physician either before or during deployment. The radial geometry of the support assembly  190  is therefore adjustable before and during deployment from a radially symmetric geometry (shown in FIG. 42) to various different asymmetric geometries (shown in FIGS. 43  and  44 ). 
     There are various ways to provide variable radial geometries. In the embodiment shown in FIGS. 42 to  44 , at least one spline element (designated  192 ( 1 )) is enclosed by an exterior sleeve  196  that includes an interior lumen  200 . The sleeve  196  extends through the catheter tube  12 . The lumen  200  accommodates a sliding wire  194  (see FIG. 42) having elastic memory at its distal end that defines a curve  198 . 
     When confined within the catheter tube  12 , the curved distal wire end  198  is urged into a generally straight geometry. When advanced in the lumen  200  beyond the catheter tube  12  and along the spline element  192 ( 1 ), the elastic memory of the distal wire end  198  bends the spline element  192 ( 1 ) along the curve  198 , as FIG. 43 shows. 
     The wire  194  can also be rotated within the lumen  200 . Rotation of the wire  194  within the lumen  200  shifts the orientation of the curve  198 , thereby altering the direction of the bend along the spline element  192 ( 1 ), as a comparison of FIGS. 43 and 44 show. By adjusting the curve  198  to bend the spline element  192 ( 1 ) orthogonal to the axis of the structure  190  toward the spline element  192 ( 2 ) (see FIG.  43 ), the circumferential spacing between the spline element  192 ( 1 ) and its neighboring spline element  192 ( 2 ) is altered. Conversely, by adjusting the curve  198  to bend the spline element  192 ( 1 ) orthogonal to the axis of the structure  190  toward the spline element  192 ( 3 ) (see FIG.  44 ), the circumferential spacing between the spline element  192 ( 1 ) and its neighboring spline element  192 ( 3 ) is altered. 
     A circumferential pattern of spline elements  192 ( 1 ),  192 ( 2 ), and  192 ( 3 ) that was radially symmetric before introduction of the wire  194  (see FIG.  42 ), thus becomes radially asymmetric after the introduction and rotation of the wire  194  within the spline element  192 ( 1 ). Rotating the wire  192 ( 1 ) to shift the orthogonal orientation of the curve  198  (see FIGS. 43 and 44) also shifts the nature of the radial asymmetry of the structure  190 . 
     As FIG. 45 shows, formation of the curve  198  can be electrically accomplished in situ by providing two temperature activated memory elements  202 A and  202 B within one or more of the spline elements  192  (FIG. 45 shows the elements  202 A and  202 B in spline element  192 ( 1 ) of the structure shown in FIG.  42 ). 
     The elements  202 A and B can be formed, for example, from wires or flat strips of nickel titanium alloy. The elements  202 A and B are each annealed to a preset, curved shape. The elements  202 A and B are cooled and straightened to a shape that conforms to the normal geometry of the spline element. 
     The elements  202  are coupled to a source  205  of electric current. Current flow through a selected one of the elements  202 A or  202 B heats the selected element  202 A or  202 B, causing it to return to its annealed curved shape. Interruption of the current flow allows the element  202 A and B to cool and return to its cooled, straightened geometry. A joystick control  204  directs current flow to a selected one of the elements  202 A and B. 
     Further details of the use of electrically controlled temperature-activated memory elements to steer tubular bodies, like catheters, are discussed in McCoy U.S. Pat. No. 4,543,090, which is incorporated herein by reference. 
     As before described, a circumferential pattern of spline elements  192 ( 1 ),  192 ( 2 ), and  192 ( 3 ) that was radially symmetric before conduction of current by the element  202 A becomes radially asymmetric after the element  202 A is heated by current flow to bend and reorient the associated spline element  192 ( 1 ) in one direction orthogonal to the axis of the structure (as FIG. 43 shows). Conduction of current by the element  202 B bends and reorients the associated spline element  192 ( 1 ) in another direction orthogonal to the axis of the structure (as FIG. 44 shows) Use of the joystick control  204  selects which one of the elements  202 A or  202 B is heated, so that the nature of the radial asymmetry of the structure  190  can be adjusted accordingly. 
     FIGS. 46 to  48  show another alternative way of creating a support assembly  206  having a variable radial asymmetry. In this embodiment, the support assembly  206  includes a base  208  attached to the distal end  16  of the catheter tube  12 . The base  208  includes an array of movable plates  210 ,  212 ,  214 ,  216 . The plates  210 ,  212 ,  212 ,  214 , and  216  are preferably made from stainless steel or other chemically inert metal. The movement of the plates is such that the plate  216  is slidable over the adjacent plate  214 ; the plate  214  is slidable over the next adjacent plate  212 ; and the plate  212  is slidable over the next adjacent plate  210 . The plate  210  is secured to the base  216  and does not move. 
     The plates  210 ,  212 ,  214 , and  216  are coupled to an actuator  218 , which rotates about an axle  220 . Rotation of the actuator  218  moves the plates  212 ,  214 , and  216  relative to the stationary plate  210 . 
     More particularly, counterclockwise rotation of the actuator  218  causes the movable plates  212 ,  214 , and  216  to slide, one over the other, toward the stationary plate  210 . This movement reduces the circumferential spacing between each plate, as FIG. 48 shows, as the plates move together, stacking up one atop the other. 
     Clockwise rotation of the actuator  218  causes the movable plates  212 ,  214 , and  216  to slide, one over the other, away from the stationary plate  210 . This movement enlarges the circumferential spacing between each plate, as FIG. 47 shows, as the plates move apart. 
     In the preferred embodiment, spring elements  222  couple the stationary plate  210  to each of the movable plates  212 ,  214 , and  216 . The spring elements  22  normally urge the plates  212 ,  214 , and  216  toward the stationary plate  210 . The spring elements  22  thereby make the movement of the plates  212 ,  214 , and  216  toward and away from the plate  210  more uniform in response to the actuator  218 . 
     The actuator  218  includes a bevel gear surface  228 . The gear surface  228  meshes with a bevel gear surface  230  on a second actuator  232 , which is carried for rotation about an axle  234 . The axle  234  is generally perpendicular to the axle  220 . 
     Wires  236  couple the second actuator  232  to a control element  238 , intended to be carried within the proximal handle  18  of the catheter tube  12 . Rotation of the control element  238  by the physician clockwise or counterclockwise pulls on the wires  236 . Wire tension rotates the second actuator  232  in the same direction as the control element  238  about the axle  234 . The gear surfaces  228  and  230  transfer rotation of the second actuator  232  into rotation of the actuator  218  about its axle  220 , thereby affecting movement of the plates  210 ,  212 ,  214 , and  216 , as before described, depending upon the direction of rotation. 
     A spline element  224  is attached to the periphery of each  210 ,  212 ,  214 , and  216  plate. Other spline elements  226  are secured to the base  208 . The spline elements  224  extend from the plates  210 ,  212 ,  214 , and  216  to a distal hub  226  (as FIG. 46 shows). 
     As shown in FIG. 47, when the plates  210 ,  212 ,  214 , and  216  are in their fully expanded condition, the structure  206  possesses a radially symmetric geometry. Movement of the plates  212 ,  214 , and  216  toward the plate  210  in response to counterclockwise rotation of the actuator  218  decreases circumferential spacing between the splines  224 , without altering the circumferential spacing between the spline  226 . As shown in FIG. 48, when the plates  210 ,  212 ,  214 , and  216  are moved from their fully expanded condition toward their fully retracted condition, the structure  206  possesses a radially asymmetric geometry. 
     As before described, the structure  206  exemplifies a radially symmetric pattern of spline elements  224  and  226 , which can be caused to become radially asymmetric in a variable way by the physician&#39;s operation of the actuator  218 . 
     FIGS. 49 and 50 show another alternative embodiment of a support assembly  240  possessing variable radial asymmetry. The support assembly  240  includes a base  242  attached to the distal end  16  of the catheter tube  12 . The base  242  includes first and second arrays of splines  244  and  245 , which radiate from the base to a distal hub (not shown), in the manner shown in FIG.  46 . 
     The proximal ends of the splines  245  are secured in a stationary fashion to the base  242 . However, the proximal ends of each spline  244  are mounted for elastic movement orthogonal to the spline axis. In the illustrated embodiment, the proximal ends of the splines  244  are joined to arms  250 , which radiate from an elastic center joint  252  supported within the base  242 . The elastic joint  252  can be made from nickel titanium, stainless steel, or an elastic polymer. The elastic joint  252  biases the splines  244  toward a first, circumferentially spaced relationship, as FIG. 50 shows. 
     The first, circumferentially spaced relationship of the movable splines  244  is closer together than the fixed circumferentially spaced relationship of the other splines  245 . The support assembly  240  thereby presents a radially asymmetric geometry. 
     An array of wedges  254  are mounted on an axially movable actuator  256  within the base  242 . Each wedge  254  includes oppositely spaced, tapered wedge surfaces  262 . The surfaces  262  are preferably coated with a lubricious coating, such as TEFLON™ plastic material. 
     The actuator  256  is attached to a control shaft  258 . The shaft  258  extends through the catheter tube  12  and is coupled to a push-pull control lever  260  housed within the proximal handle  18  carried by the catheter tube  12 . Pushing the control level  260  advances the actuator  256  within the base  242  toward the array of splines. Pulling the control lever  260  retracts the actuator  256  within the base  242  away from the array of splines. 
     As FIG. 49 shows, advancement of the actuator  256  toward the spline array moves the wedges  254  as a unit progressively into the spaces between adjacent splines  244 . The tapered wedge surfaces  262  push against adjacent splines  244 , overcoming the elasticity of the joint  252 . The wedge surfaces  262  progressively push the splines  244  apart. As shown in FIG. 49, the progressively advanced actuator  256  thereby establishes a range of circumferential spacing between the splines  244 , which is greater than the normal first circumferential spacing. Advancement of the actuator  256  is stopped when a desired circumferentially spaced relationship within the range is established. 
     Advancement of the actuator  256  does not affect the circumferential spacing between the other splines  245 . When the actuator  256  is fully advanced (see FIG.  49 ), the splines  244  are circumferentially spaced apart at generally the same distance than the splines  245 . A radially symmetric geometry is thereby established. 
     Retraction of the actuator  256  away from the spline array moves the wedges  254  as a unit progressively out of the space between adjacent splines. The elasticity of the joint  252  urges adjacent splines  244  further together in a range of decreasing circumferential spacing, until the first circumferential spacing established by the joint  252  is reached, as FIG. 50 shows. 
     As in the other previously described embodiments, the assembly  240  demonstrates how a radially symmetric pattern of spline elements  244  and  245  can be caused to become variably radially asymmetric by operation of an actuator  256 . 
     B. Use of Radially Asymmetric Structures 
     When deployed, for example, inside a heart chamber, the radially asymmetric support assembly  20 ( 2 ) holds the electrodes  30  in contact against the endocardium with a varying electrode pattern density. That is, the number of electrodes  30  per unit area of endocardium contacted by electrodes  30  is denser where the group  46  contacts tissue than in other regions of the heart chamber (where there are no electrodes  30  contacting tissue). 
     In the preferred arrangement shown in FIGS. 4 and 5, the assembly  20 ( 2 ) provides high density, unidirectional sensing by associating multiple electrodes  30  with only one discrete group  46  of spline assemblies  22 ( 2 ). In this arrangement, the remaining spline assemblies  22 ( 2 ), being free of electrodes  30 , serve to support and stabilize the electrodes  30  of the group  46  contacting tissue. 
     Radially asymmetric structures make possible high density mapping, or derivation of an electrical characteristic in localized regions, or pacing in localized regions, without unduly increasing the total number of splines elements  22  or electrode signal wires  34 . Systems and methods for deriving an electrical characteristic of tissue, such as tissue impedance, are disclosed, for example, in Panescu et al U.S. Pat. No. 5,494,042, which is incorporated herein by reference. An electrical characteristic is derived by transmitting electrical energy from one or more electrodes into tissue and sensing the resulting flow of electrical energy through the tissue. 
     III. Radially Symmetric/Axially Asymmetric Multiple Electrode Probe 
     FIGS. 7 and 8 show a multiple electrode support assembly  20 ( 3 ), which is radially symmetric, but axially asymmetric. The assembly  20 ( 3 )can be attached to the distal end  16  of a catheter tube  12  in the manner support of assembly  20 ( 1 ), shown in FIG.  1 . 
     The electrode support assembly  20 ( 3 ) includes an array of flexible spline elements  22 ( 3 ), which extend longitudinally between the distal hub  24  and the base  26 . The spline elements  22 ( 3 ) carry electrode circuit assemblies  28 ( 3 ), each with an array of multiple electrodes  30  coupled by signal wires to the external connectors  32 , as already described with reference to FIG.  1 . 
     The geometry of flexible spline elements  22 ( 3 ) shown in FIGS. 7 and 8 is radially symmetric for the same reasons that the array of spline elements  22 ( 1 ) of the assembly  20 ( 1 ) are radially symmetric. As FIG. 8 shows, the spline elements  22  uniformly radiate from the main axis  36  at generally equal arcuate, or circumferential, intervals. In FIGS. 7 and 8, there are eight, radially symmetric spline elements  22 ( 3 ), each circumferentially separated by about 45°. 
     However, unlike the assemblies  20 ( 1 ) and  20 ( 2 ), the geometry of flexible spline elements  22 ( 3 ) of the assembly  20 ( 3 ) is asymmetric in an axial sense. When viewed from the side, as FIG. 7 shows, the proximal electrode-bearing region  38  is not generally symmetric to the distal electrode-bearing region  40 . In the arrangement shown in FIG. 7, the spline elements  22 ( 3 ) flare outward in a substantially perpendicular direction from the base  26 , providing a bowl-like proximal region  38 . In contrast, the spline elements  22 ( 3 ) extend outward from the distal hub  24  at a significantly smaller acute angle, providing more of a tapered, conical distal region  40  with a smaller average diameter than the proximal region  38 . Thus, if bent upon itself at its geometric midpoint  42  along the main axis  36 , the proximal and distal regions  38  and  40  of a given spline assembly  22 ( 3 ) would not overlie each other. This lack of symmetry between the electrode-bearing regions  38  and  40  along the main axis  36  of the spline elements  22 ( 3 ) forms a structure that this Specification calls axially asymmetric. 
     Many other axially asymmetric structures can be formed. For example, FIG. 9 shows spline elements  22  ( 3 ), which are J-shaped. Diametrically opposite pairs of the J-shaped spline elements  52  extend from the distal hub  24 , with one end  54  of each J-shape element  52  facing the other end  56  of another J-shape element  52 . This reverse positioning of J-shape elements  52  forms an electrode support assembly  58  having an elongated, asymmetric bulge along a secondary axis  50 , which extends at a non-perpendicular angle across the main axis  36 . The reverse positioning of the elements  52  also creates an axial asymmetry that differs among the spline elements. The axial asymmetry of the spline elements  52  shown as occupying the bottom portion of FIG. 9 differs from the axial asymmetry of the spline elements  52  shown as occupying the top portion of FIG.  9 . 
     Axially asymmetric spline elements  22 ( 3 ) can be preformed from memory elastic materials to assume any desired normally biased, curvilinear shape. Preferably, the axially asymmetric geometry for the assembly  20 ( 3 ) is selected to best conform to the expected interior contour of the body chamber that the assembly  20 ( 3 ) will, in use, occupy. 
     The use of axial asymmetric geometries is particular well suited for deployment for multiple electrode structures within the heart. This is because the interior contour of a heart ventricle differs from the interior contour of a heart atrium. Furthermore, neither atrium nor ventricle is axially symmetric. The ability to provide electrode support assemblies with differing axially asymmetric shapes makes it possible to provide one discrete configuration tailored for atrial use and another discrete configuration tailored for ventricular use. 
     To assure that the axially asymmetric support assembly  20 ( 3 ) (or, for that matter, any normally open, preformed support assembly of the type described in this Specification) will uniformly collapse, when desired (for example, by use of the sliding sheath  44 ), the linear length of each spline element forming the structure must be essentially equal. 
     When deployed, for example, inside a heart chamber, the axially asymmetric support assembly  20 ( 3 ) of the probe  10 ( 3 ) holds the electrodes  30  in intimate contact against the endocardium. Since the support assembly  20 ( 3 ) is radially symmetric, and each spline assembly  22 ( 3 ) carries electrodes  30 , it establishes a uniform electrode pattern density throughout the chamber. Furthermore, since the axial asymmetry of the support assembly  20 ( 3 ) is purposely fashioned to generally match the expected interior asymmetric contour of the chamber, the support assembly  20 ( 3 ) conforms better to the chamber. The axially asymmetric assembly  20 ( 3 ) provides more stable and more uniformly aligned contact between electrodes  30  and tissue. The axially asymmetric assembly  20 ( 3 ) is less prone to shift or slide within the chamber in response to the natural contractions, expansions, and twisting forces imposed against it within the dynamic environment of a beating heart. 
     IV. Both Radially and Axially Asymmetric Multiple Electrode Probe 
     FIGS. 10 and 11 show a multiple electrode support assembly  20 ( 4 ), which is both radially and axially asymmetric. The electrode support assembly  20 ( 4 ) can be carried at the distal end  16  of the catheter tube  12 , just like the assembly  20 ( 1 ) shown in FIG.  1 . 
     The assembly  20 ( 4 ) includes an array of flexible spline elements  22 ( 4 ) extending longitudinally between the distal hub  24  and the base  26 . The spline elements  22 ( 4 ) provide an array of multiple electrodes  30  coupled by signal wires to the external connectors on the handle  18 . 
     The geometry of the flexible spline elements  22 ( 4 ) shown in FIGS. 10 and 11 is radially asymmetric for the same reasons that the array of spline elements  22 ( 2 ) (see FIG. 5) are radially asymmetric. As FIG. 11 shows, eight spline assemblies S 1  to S 8  are arranged in two discrete groups  46  and  48  of four spline assemblies each. 
     Each group  46  and  48  spans an arc of about 90°, with the splines in each group  46  and  48  equally circumferentially separated by about 30° each (which corresponds to the smallest angle β). The groups  46  and  48  themselves are circumferentially separated by about 90° (which corresponds to the largest angle α). The radial asymmetric criteria is met, since angle α minus angle β is about 60°, i.e., greater than 20°. 
     As FIG. 11 further shows, only the splines S 1  to S 4  of the group  46  carry electrodes  30 . The splines S 5  to S 8  of the group  48  are free of electrodes  30  and serve a support function, as previously described. Still, it should be appreciated that electrodes  30  can be mounted on one or more additional splines according to the electrode sensing functions required during use. 
     The geometry of flexible spline elements  22 ( 4 ) shown in FIGS. 10 and 11 is also axially asymmetric for the same reasons that the geometries of spline elements shown in FIGS. 8 and 9 are axially asymmetric. 
     When deployed, for example, inside a heart chamber, the support assembly  20 ( 4 ) of the probe  10 ( 3 ) establishes a non-uniform electrode pattern density throughout the chamber. The assembly  20 ( 4 ) therefore provides a localized high electrode density at the electrodes  30  in the group  46 , for mapping, or derivation of an electrical characteristic in localized regions, or pacing in localized regions, while other spline assemblies, free of electrodes (i.e., the group  48 ), provides support and stabilization. The localized high density achieves better signal resolution and results in less need to interpolate for electrical events that happen to fall between spline assemblies, as the spline assemblies are closer together. 
     In addition, the axial asymmetry of the support assembly  20 ( 4 ) better matches the expected interior asymmetric contour of the chamber. The axially asymmetric support assembly  20 ( 4 ) thereby helps to maintain stable and uniformly aligned contact between the high density electrodes  30  and tissue. Loss of contact between tissue and electrodes, which can produce motion artifacts and a breakdown of intended function, is thereby minimized. Because the contact is more stationary, the physician can be more certain that information obtained from one location during a beat comes from the same location in the next beat. 
     The ability of the axially asymmetric structure  20 ( 4 ), and other axially asymmetric structures matched to the expected contour of the targeted site, to maintain intimate contact also minimizes the risk of trauma. Repeated movement and sliding of an electrode support structure across and against the endocardium and interior trabecula and tendonae can lead to perforation or tamponade if the trauma is severe enough. Less severe trauma can still locally injure tissue, increasing the likelihood of clot formation and potential emboli. 
     V. Criteria for Use 
     As FIG. 20 shows a system  96  that is based upon the different symmetries of the various support structures  20 ( 1 ) to  20 ( 4 ). The system  96  includes a family  98  of multiple electrode structures. In the illustrated embodiment, the family  98  comprises a representative of each of the four geometries of support structures  100 ( 1 ) to  100 ( 4 ) described above; namely, (i) an axially and radially symmetric structure  100 ( 1 ) (exemplified by structure  20 ( 1 ) shown in FIGS.  1  and  2 ); (ii) an axially symmetric and radially asymmetric structure  100 ( 2 ) (exemplified by structure  20 ( 2 ) shown in FIGS.  4  and  5 ); (iii) a radially symmetric and axially asymmetric structure  100 ( 3 ) (exemplified by structure  20 ( 3 ) shown in FIGS.  7  and  8 ); and (iv) an axially asymmetric and radially asymmetric structure  100 ( 4 ) (exemplified by structure  20 ( 4 ) in FIGS. 10 and  11 ). 
     As FIG. 20 shows, each support structure  100 ( 1 ) to  100 ( 4 ) is carried at the distal end of a flexible catheter tube  12 , in the manner shown in FIG.  1 . Each structure  100 ( 1 ) to  100 ( 4 ) is individually adapted for selection by a user. 
     As FIG. 20 further shows, the system  96  also includes an established set of criteria  102 . The criteria  102  suggests selection by the user of one support structure  100 ( 1 ) to  100 ( 4 ) within the family  98 , by correlating use of a given structure  100 ( 1 ) to  100 ( 4 ) with an anatomical region, or a disease state, or other diagnostic or therapeutic circumstance. 
     The criteria  102  can be established in various ways, for example, by the manufacturer(s) of the support structures, the medical community using the support structures, governmental regulatory agencies overseeing licensure of the support structures, or a combination of these. The criteria  102  can be derived from actual and/or predicted functional and physiological requirements, such as the bio-mechanical properties of each support structure; the region of the heart in which the structure will be deployed; the disease state that is to be diagnosed or treated; the type of diagnosis or treatment contemplated; and/or known congenital abnormalities of the patient. The criteria  102  can be based on, for example, empirical data, in vitro or in vivo tests, finite element analysis, anecdotal data, or a combination thereof. The criteria correlates use of one or more geometries of support structures with these functional and/or physiological factors. 
     The criteria  102  can be presented in various formats. It can be in the form of written suggestions to be read by the physician, or in digital form entered in a computer database or look-up table accessible to the physician, or in audio or video form to be listened to or viewed by the physician. 
     The following Table exemplifies one embodiment of the criteria  102  presented in written form: 
     
       
         
           
               
            
               
                   
               
               
                 CRITERIA TABLE 
               
               
                 SUGGESTED GEOMETRY OF ELECTRODE SUPPORT STRUCTURE 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 R-Sym 
                 R-Asym 
                 A-Sym 
                 A-Asym 
                 General 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 By 
                   
                   
                   
                   
                   
               
               
                 Anatomic 
               
               
                 Region 
               
               
                 L Vent 
                 ✓ 
                   
                 ✓ 
               
               
                 Normal 
               
               
                 L Vent 
                   
                 ✓ 
                 ✓ 
               
               
                 Ischemic 
               
               
                 R Vent 
                   
                 ✓ 
                   
                 ✓ 
               
               
                 R Vent 
                 ✓ 
                   
                 ✓ 
               
               
                 Outflow 
               
               
                 Tract 
               
               
                 R Atrium 
                   
                 ✓ 
                   
                 ✓ 
               
               
                 L Atrium 
                   
                 ✓ 
                 ✓ 
               
               
                 By 
               
               
                 Disease 
               
               
                 State 
               
               
                 A Fib(I) 
                   
                 ✓ 
                 ✓-L Atrium 
                 ✓-R Atrium 
               
               
                 Region 
               
               
                 Known 
               
               
                 Anomaly 
                   
                   
                   
                   
                 Based upon 
               
               
                 is 
                   
                   
                   
                   
                 Chamber 
               
               
                 Congenital 
                   
                   
                   
                   
                 Image 
               
               
                 When 
                   
                 ✓ 
                 ✓-L Vent 
                 ✓-R Vent 
               
               
                 Foci 
               
               
                 Region 
               
               
                 Known 
               
               
                   
               
            
           
         
       
     
     VI. Asymmetric Mechanical Properties 
     FIGS. 21 and 22 show a multiple electrode support structure  104 , which is axially symmetric but radially asymmetric for the reasons set forth with respect to the support structure  20 ( 2 ) shown in FIGS. 4 and 5. The particular arrangement shown in FIGS. 21 and 22 includes ten spline elements, designated S 1  to S 10 . The asymmetric arrangement shown in FIG. 21 comprises a first discrete group  106  of eight adjacent spline elements S 1  to S 8  and a second discrete group  108  of two adjacent spline elements S 9  and S 10 . Within the first group  106 , the adjacent spline elements S 1  to S 8  are circumferentially spaced apart in equal intervals of about 22° (which comprises angle β). Within the second group  108 , the adjacent spline elements S 9  and S 10  are spaced apart by about 40°. The two groups  106  and  108  are themselves spaced apart by about 70°. Angle α is therefore about 70°, and the angle α minus angle β difference is thereby greater than 20°, which meets the radial asymmetric definition of this Specification. 
     In the particular radial asymmetric geometry shown in FIGS. 21 and 22, the splines S 1  to S 8  carry electrodes  110 , whereas the splines S 9  and S 10  do not. 
     As further shown in FIGS. 21 and 22, the splines S 1  to S 8  in the first group  106  possess different mechanical properties than the spline S 9  and S 10  in the second group  108 . More particularly, the splines S 9  and S 10  are each wider in their transverse direction than each of the splines S 1  to S 8 . The splines S 9  and S 10  are therefore individually more stiff than the individual splines S 1  to S 8 . 
     The degree of “stiffness” of the splines S 1  to S 10  can be expressed in terms of a spline radial stiffness function S r . S r  expresses the ratio between radial force (F r ) applied to a given spline perpendicular to the axis of the structure  104  and the radial distance (D r ) the given spline deflects toward the axis of the structure  104  in response to the radial force. That is:          S   r     =       F   r       D   r                       
     The spline radial force function S r  for a given spline can be determined by placing the structure  104  in a cylinder which presses against and restrains all but the given spline  22 , which projects through a window in the cylinder. A pin applies force perpendicular to the mid portion of the given spline. A transducer coupled to the pin measures the force F r  exerted against the spline at successive points of radial deflection D r  from the spline&#39;s normal rest position in the structure  104 . Radial forces F r  can be plotted as a function of radial deflections D r  for the given spline. The slope of the resulting plot is the radial stiffness function S r  for the given spline. The function S r  is expressed in terms of units of force (for example, in grams) per unit of deflection (for example, in inches). 
     Lower values of S r  indicate lower radial stiffness values and indicate a better ability to deform and create intimate contact along the contour of the endocardium without damage to tissue. 
     The geometry of the support structure  104  therefore presents the one group  106  of closely spaced spline elements S 1  to S 8 , which are more flexible (i.e., which individually have a lower radial stiffness value S r ) than the other group  108  of less closely spaced spline elements S 9  and S 10  (which individual exhibit a higher radial stiffness value S r  than the spline elements S 1  to S 8 ). 
     The group  106  of more flexible splines S 1  to S 8  carry the electrodes  110  and, due to their greater flexibility, are more conformal to tissue than the group  108  of splines S 9  and S 10 , which do not carry electrodes. On the other hand, the less flexible group  108  of splines S 9  and S 10  individually impart greater force against the tissue, thereby urging the other, more flexible splines S 1  to S 8  and their electrodes  110  toward intimate tissue contact. However, since the tissue contact force (F c ) of the spline elements S 9  and S 10  in the second group  108  is applied over a relatively large surface area (A c ), the tissue pressure function T p  is lessened, where T p  is expressed as follows:          T   p     =       F   c       A   C                       
     The quantity T p  is a determinant of tissue trauma. Trauma caused by contact force exerted on small, localized area can be mediated by distributing the same contact force over a larger contact area, thereby reducing contact pressure. 
     The structure  104  shown in FIGS. 21 and 22 therefore provides asymmetric mechanical properties in different regions of the tissue contact. The asymmetric mechanical properties serve to establish and maintain balanced, intimate contact between a high density of electrodes  110  and tissue in a way that minimizes trauma. 
     VII. Asymmetric Ablation Structures 
     A. Long Lesions 
     As the foregoing Criteria Table shows, radially asymmetric electrode structures are well suited for diagnostic or therapeutic use in the atrial regions of the heart. This is because the location of anatomical obstacles that cause abnormal, irregular heart rhythm, called atrial fibrillation, are known with respect to anatomical landmarks within the left or right atrium. Spline density can thereby be concentrated to contact these known obstacles, so that localized ablation can be performed. 
     In FIG. 31, a transeptal deployment is shown, from the right atrium (RA), through the fossa ovalis at the septum (S), into the left atrium (LA), where a radial asymmetric support structure  142  is located for use. In conformance with the foregoing Criteria Table, the structure  142  occupying the left atrium is axially symmetric. 
     The more closely radially spaced longitudinal splines  154  of the structure  142  carry an array of multiple electrodes  156 . The electrodes  156  serve as transmitters of ablation energy. The less closely radially spaced longitudinal splines  155  do not carry electrodes  156 . 
     The electrodes  156  are preferably operated in a uni-polar mode, in which the radio frequency ablation energy transmitted by the electrodes  156  is returned through an indifferent patch electrode  158  externally attached to the skin of the patient. Alternatively, the electrodes  156  can be operated in a bi-polar mode, in which ablation energy emitted by one or more electrodes  156  is returned an adjacent electrode  158  on the spline  154 . 
     The size and spacing of the electrodes  156  shown in FIG. 31 are purposely set for creating continuous, long lesion patterns in tissue. FIG. 32 shows a representative long, continuous lesion pattern  160  in tissue T, which is suited to treat atrial fibrillation. Continuous, long lesion patterns  160  are formed due to additive heating effects when RF ablation energy is applied in a uni-polar mode simultaneously to the adjacent electrodes  156 , provided the size and spacing requirements are observed. The additive heating effects cause the lesion pattern  160  to span adjacent, spaced apart electrodes  156 , creating the desired elongated, long geometry, shown in FIG.  32 . The additive heating effects will also occur when the electrodes  156  are operated simultaneously in a bipolar mode between electrodes  156 , again provided the size and spacing requirements are observed. 
     The additive heating effects between spaced apart electrodes  156  intensify the desired therapeutic heating of tissue contacted by the electrodes  156 . The additive effects heat the tissue at and between the adjacent electrodes  156  to higher temperatures than the electrodes  156  would otherwise heat the tissue, if conditioned to individually transmit energy to the tissue, or if spaced apart enough to prevent additive heating effects. 
     When the spacing between the electrodes  156  is equal to or less than about 3 times the smallest of the diameters of the electrodes  156 , the simultaneous emission of energy by the electrodes  156 , either bipolar between the segments or unipolar to the indifferent patch electrode, creates the elongated continuous lesion pattern  160  shown in FIG. 32 due to the additive heating effects. Conversely, when the spacing between the electrodes  156  is greater than about 5 times the smallest of the diameters of the electrodes  156 , the simultaneous emission of energy by the electrodes  156 , either bipolar between segments or unipolar to the indifferent patch electrode, does not generate additive heating effects. Instead, the simultaneous emission of energy by the electrodes  28  creates an elongated segmented, or interrupted, lesion pattern  162  in the contacted tissue area T, as shown in FIG.  33 . 
     Alternatively, when the spacing between the electrodes  156  along the contacted tissue area is equal to or less than about 2 times the longest of the lengths of the electrodes  156 , the simultaneous application of energy by the electrodes  156 , either bipolar between electrodes  156  or unipolar to the indifferent patch electrode, also creates an elongated continuous lesion pattern  160  (FIG. 32) due to additive heating effects. Conversely, when the spacing between the electrodes  156  along the contacted tissue area is greater than about 3 times the longest of the lengths of the electrodes  156 , the simultaneous application of energy, either bipolar between electrodes  156  or unipolar to the indifferent patch electrode, creates an elongated segmented, or interrupted, lesion pattern  162  in tissue T (FIG.  33 ). 
     In the embodiment shown in FIG. 31, the radially asymmetric structure  142  also includes periodic bridge splines  164 . The bridge splines  164  are soldered or otherwise fastened to the adjacent longitudinal splines  154 . The bridge splines  164  carry electrodes  166 , or are otherwise made to transmit ablation energy by exposure of electrically conductive material. Upon transmission of ablation energy, the bridge splines  166  create long transverse lesion patterns  168  in tissue T (shown in FIG. 34) that span across the long longitudinal lesion patterns  160  created by the adjacent splines  154 . The transverse lesions  168  link the longitudinal lesions  160  to create complex lesion patterns that emulate the patterns formed by incisions during an open heart, surgical maze procedure. 
     Further details of the creation of complex long lesion patterns in the treatment of atrial fibrillation are found in copending U.S. application Ser. No. 08/566,291, filed Dec. 1, 1995, and entitled “Systems and Methods for Creating Complex Lesion Patterns in Body Tissue,” which is incorporated herein by reference. 
     The electrode elements  156  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 spline elements  154 . The segmented electrodes can each comprise a solid ring of conductive material, like platinum, which is pressure fitted about the spline element  154 . Alternatively, the electrode segments can comprise a conductive material, like platinum-iridium or gold, coated upon the spline element  154  using conventional coating techniques or an ion beam assisted deposition (IBAD) process. In a preferred embodiment, spaced apart lengths of closely wound, spiral coils are wrapped about the spline element  154  to form an array of generally flexible electrodes  156 . The coils are made of electrically conducting material, like copper alloy, platinum, or stainless steel. The electrically conducting material of the coils can be further coated with platinum-iridium or gold to improve its conduction properties and biocompatibility. 
     In another embodiment, the electrodes  156  comprise elongated, porous bodies holding a medium containing ions that is coupled to a source of radio frequency energy. The porous bodies enable ionic transport of the radio frequency energy to tissue, which electrically heats the tissue to cause the desired lesion. The use of porous electrode bodies to create lesions in body tissue is disclosed in greater detail in copending U.S. patent application Ser. No. 08/631,575, filed Apr. 12, 1996 and entitled “Tissue Heating and Ablation Systems and Methods Using Porous Electrode Structures,” which is incorporated herein by reference. 
     B. Large Lesions 
     The elimination of ventricular tachycardia (VT) substrates is thought to require significantly larger lesions, with a penetration depth greater than 1.5 cm, a width of more than 2.0 cm, with a lesion volume of at least 1 cm 3 . There also remains the need to create lesions having relatively large surface areas with shallow depths. FIG. 35 exemplifies the geometry of a typical larger surface area lesion  144  in tissue T. 
     Radially asymmetric electrode structures are also well suited for creating large lesions in ventricle regions of the heart. FIG. 36 shows a representative radial asymmetric support structure  146  located for use within the left ventricle. In conformance with the foregoing Criteria Table, the structure  146  occupying the left ventricle is axially symmetric. 
     The more closely radially spaced longitudinal splines  148  of the structure  146  carry an array of multiple electrodes  150 . The electrodes  150  serve as transmitters of ablation energy. The less closely radially spaced longitudinal splines  149  do not carry the electrodes  150 . 
     Preferably, the electrodes  150  are all simultaneously operated in a uni-polar mode, collectively transmitting radio frequency ablation energy for return through an indifferent patch electrode  166  externally attached to the skin of the patient. 
     The size and spacing of the electrodes  150  shown in FIG. 36 are purposely set in the same relationship manner described in connection with FIG. 31, to create continuous lesion patterns in tissue due to additive heating effects, also as previously described. In the arrangement shown in FIG. 36, the size and spacing relations conducive to additive heating effects are established between adjacent electrodes  150  both longitudinally along each spline  148  as well as radially between each spline  148 . As a result (as FIG. 38 shows), the additive heating effects not only span between adjacent electrodes  150  along each spline  148 , but also between adjacent electrodes on different adjacent splines  148 , thereby creating a continuous large lesion pattern  144  in tissue T, like that shown in FIG.  35 . 
     Preferable (as FIG. 36 shows), the predetermined closely spaced pattern of multiple electrodes  150  for creating large lesions  144  is congregated near the distal hub  24  of the structure  146 . Here, the required close radial spacing between splines  148  (and thus between the electrodes  150 ) can be best maintained. In addition, the splines  148  in this region near the distal hub  24  can be preformed with elastic memory to normally provide a radial bias, which urges the splines  148  toward each other. 
     VIII. Representative Preferred Constructions 
     FIGS. 12 to  17  show a preferred embodiment of an electrode support structure  60  (shown fully assembled in FIG. 16) comprising spline elements  62  arranged in a geometry that is both radially and axially asymmetric. 
     As FIG. 12 shows, the structure  60  includes an integral spline body  64  formed by joining together two axially asymmetric spline elements  62 . Each body  64  includes a mid-section  66  from which the spline elements  62  extend as an opposed pair of legs. In this arrangement, the body  64  is generally shaped like a lopsided hoop (see FIG.  12 ). The mid-section  66  includes a preformed detent, whose function will be described later. 
     The hoop-like body  64  is preferably made from the resilient, inert elastic memory wire, like nickel titanium described above. The body  64  preferably has a rectilinear cross section, to provide increased resistance to twisting about its longitudinal axis. The spline elements  62  are preformed in the desired axially asymmetric shape on opposite sides of the mid-section  66 . The axially asymmetric shape generally conforms to the shape earlier shown and described in FIG.  9 . 
     The distal hub  24  takes the form of an end cap  68  (see FIGS. 13 to  15 ). The end cap  68  has a generally cylindrical side wall  70  and a rounded end wall  72 . A longitudinal bore  74  (see FIGS. 13 and 14) extends through the center of the cap  68 . 
     Slots  76 A;  76 B; and  76 C extend through the cap  68  diametrically across the center bore  74 . In the hub  68 , the slots  76 A-C are generally equally circumferentially spaced within an arcuate segment of about 60°. The axis of each slot  76 A-C extends diametrically through the center bore  74 . This provides two 90° segments  82  and  84  of slots  76 A-C on diametric sides of the cap  68 , the slots being circumferentially separated within each segment  82  and  84  by about 45°. The segments  82  and  84  are separated by about 90°. Of course, the slots  76 A-C can be formed at other non-uniformly spaced circumferential intervals about the end cap  68 . Fewer or more slots can also be provided to achieve the desired asymmetric geometry. 
     The slots  76 A-C are also spaced longitudinally along the bore axis  78 . As FIG. 15 best shows, slot  76 A is closest to the end wall  72 . The slot  76 C is farthest from the end wall  72 . Intermediate slot  76 B is spaced in between the slots  76 A and  76 C. This spacing allows the spline elements to pass through the hub  68  without interference. 
     In the illustrated and preferred embodiment, the cap  68  is made of an inert, machined metal, like stainless steel. The bore  74  and slots  76 A-C are preferably formed by conventional EDM techniques. Still, other metallic or molded plastic materials can be used to form the cap  68  and associated openings. 
     A spline leg  62  of the hoop-like body  64  can be inserted through a slot  76 A-C until the mid-body section  66  enters the bore  74  (see FIG.  14 ). The detent in the midsection  66  snaps into the bore  74 . This locks the body  64  to the end cap  68 , with the opposed pair of asymmetric spline legs  62  radiating free of the respective slot  76 A-C. Sequentially inserting three hoop-like bodies  64  in the three slots  76 A-D orients and locks the spline elements  62  in the radiating pattern shown in FIG.  16 . The three dimension support assembly  60  results (shown in FIG.  16 ), having a geometry that is both radially and axially asymmetric. 
     Multiple electrodes  30  can be attached to one or more of the spline elements  62 , in the manner shown in pending U.S. application Ser. No. 08/206,414, filed Mar. 4, 1994, which is incorporated herein by reference. In the preferred embodiment, electrodes  30  are provided on the spline elements  62  in the segment  82 , but not in the segment  84 , in the manner previously described and shown in FIGS. 10 and 11. 
     In the illustrated and preferred embodiment, the lower surface  86  of the end cap slots  76  is curved (see FIG. 14) The curved lower surface  86  contacts the spline elements  62  (as FIG. 14 shows) when they are bent, or deflected, a prescribed amount. The curvature of the lower slot surface  86  is selected to lend positive support to the spline elements  62  when bent this amount, to prevent spline deflection beyond a minimum bend radius. The bend radius is selected to be above that which failure-mode stresses are most likely to develop in the spline elements  62 , which are most likely to occur when the slidable sheath  44  compresses and collapses the spline elements  62  in the manner shown in FIG.  3 . 
     In the support structure  60 , the base  26  includes an anchor member  88  and a mating lock ring  90  (see FIGS.  16  and  17 ). The anchor member  88  fits with an interference friction fit into the distal end  16  of the catheter tube  12 . The lock ring  90  includes a series of circumferentially spaced grooves  92  into which the free proximal ends of the spline legs  62  fit. The lock ring  90  fits about the anchor member  88  to capture the free ends of the spline legs  62  between the interior surface of the grooves  92  and the outer surface of the anchor member  88  (see FIG.  17 ). 
     The anchor member  88 /lock ring  90  assembly holds the spline elements  62  in their asymmetric radial spaced relationship while their preformed shape holds them in a desired axially asymmetric flexed condition. 
     The hoop-like body  64 , slotted end cap  68 , and anchor member  88 /lock ring  90  assembly provide manufacturing efficiencies, as the number of the components parts required to form the asymmetric electrode support assembly  58  is minimized. The slotted cap  68  circumferentially aligns and stabilizes the spline elements  62 , both circumferentially and longitudinally. The sequential insert and snap lock process of the attaching the bodies  64  to the slotted cap  68  also significantly simplifies the assembly process. 
     The preferred structure  60  creates a relatively large distal surface area and small deflection forces, and thus reduces the overall magnitude of pressure exerted against tissue. As FIG. 18 shows, the spline elements  62  of the preferred embodiment extend through the axis of the cap  68  at an angle χ that is greater than about 45° (as shown by spline boundary line  62 A in FIG.  18 ), but is less than about 110° (as shown by spline boundary line  62 C in FIG.  18 ). Preferably, the angle χ is between about 80° and 100°. In the illustrated preferred embodiment (as shown by spline boundary line  62 B in FIG.  18 ), the angle χ is about 90° (i.e., the spline boundary line  62 C extends generally perpendicular to the axis of the cap  48 ). 
     As FIG. 19 shows, the angle χ that the cap  68  imposes creates a structure  60  having an enlarged, dome-shaped distal surface area  94 . The surface area  94  conforms intimately to endocardial tissue as the heart beats. The slotted structure of the cap  68  makes possible the location of the distal-most spline elements  62  very close to the distal end of the cap  68 . As a result (see FIG.  19 ), when the structure  60  is fully deployed for use, the cap  68  projects only a minimal distance beyond the envelope of the resulting structure  60 . Practically speaking, the cap  68  lies essentially within the envelope of the distal surface area  94 . 
     The distal geometry that the cap  68  permits creates a relatively smooth surface area  94  that is essentially free of major projections that can extend to a significant extent into endocardial tissue. The contour of the surface  94  extends along an essentially constant arc from one spline  62 , across the end cap  68  to an opposite spline  62 . The end cap  68  presents a surface  94  free of outward physiologically significant projections that can poke endocardial tissue to cause blunt tissue trauma. The contoured surface  94  extending about the cap  68  thus minimizes the chances of damage to endocardial tissue during use. 
     The contoured surface  94  permits access to and intimate contact with tissue in the apex of the heart, at the base of the ventricles. About 6 to 8% of infarcted heart tissue is found to lie within the apex. Therefore, providing non-traumatic access to this region offers considerable diagnostic benefit. 
     Furthermore, the alignment of the end cap  68  along this contoured surface  94  makes it possible to use the end-cap  68  itself as an electrode. The contour surface  94  and non-projecting end-cap  68  allow the physician to deploy the structure  60  and obtain electrogram signals from the apex of the heart using the end-cap  68  as an electrode. Again, considerable diagnostic benefits result. 
     Further details of the benefits of the construction shown in FIGS. 16 to  19  are found in copending U.S. application Ser. No. 08/557,790, filed Nov. 13, 1995, and entitled “Multiple Electrode Support Structure Having Optimal Bio-Mechanical Characteristics,” which is incorporated herein by reference. 
     FIGS. 23 to  26  show an alternative embodiment of a distal hub  112  for joining flexible spline wires  114  and  116  together. Instead of using the machine, slotted hub  24  (shown in FIGS. 13 to  15 ), the distal hub  112  comprises a short length of resilient, small diameter plastic tubing  114 , which snugly cinches together the mutually looped ends of two spline wire  116  and  118 . 
     The tubing can be made from any inert plastic material having a resilient memory, which normally urges the tubing bore  115  toward a preset interior diameter. Material made from, for example, polyethylene terepthalate (PET), polyolefin, or composites made from TEFLON™ plastic and KEVLAR™ plastic (for example, a triple laminate of KEVLAR™ plastic sandwiched between two layers of TEFLON™ plastic) can be used. The spline wires  116  and  118  can comprise metal or plastic, as before described. Metal wire made from NITINOL™ material is well suited for this use. 
     The tubing  114  is precut to the desired length. As FIG. 24 shows, the first spline wire  116  is bent upon itself and passed as a loop  120  through the bore  115  of the tubing  114 . The interior diameter of the tubing bore  115  is selected to snugly engage the bent-over wire  116 . The tubing  114  is positioned short of the formed loop  120 . 
     As FIG. 25 shows, the second spline wire  118  is passed, end-first, through the formed loop  120 , without passage through the bore  115  of the tubing  114 . The spline wire  118  is bent upon itself within the loop  120 , forming a second loop  122 , which is thereby engaged or “woven” through the first loop  120 . Addition lengths of spline wire could also be passed through and bent back over the loop  120  in the same fashion, forming a registration of loops mutually woven through the first loop. 
     As FIG. 26 shows, the tubing  114  is then slid, like the knot of a necktie, upward along the looped first spline wire  116  (see arrow  117  in FIG.  26 ). The tubing  144  bears against the woven registration of the loops  120  and  122 . The resilient memory of the tubing  114  exerts a force at its distal end to snug holds the woven registration of the loops  120  and  122  together. The free legs of the spline wires  116  and  118 , which depend from the tubing  114 , can be manually manipulated to achieve the desired radial orientation. These legs, once arranged in the desired orientation, can be connected to the anchor  88  in the manner previously described. Electrodes can be mounted on the free spline legs, also in the way previously described. 
     FIG. 27 shows another alternative embodiment of a distal hub  124 . The hub  124  includes a puncturable material, which is capable of being pierced by threading spline wire  130  end-first through it. 
     In the illustrated and preferred embodiment, the hub  124  is formed from a precut, short length of rigid tubing  126  made, for example, of a rigid polycarbonate material or a metal material. Through-slots  127  are drilled through the tubing  126 , to accommodate passage of spline wires  130 . As FIG. 27 also shows, the tubing  126  is encapsulated by a resilient, elastomeric sealing material  128 , like silicone rubber or a soft urethane material. 
     In one embodiment, when the sealing material  128  has cured, individual lengths of spline wire  130  are punched, end-first, into and through the slots  127  of the encapsulated tubing (as shown by arrows  131  in FIG.  28 ). The spline wire  130  pierces the elastomeric sealing material  128  in passing through the slots  127 . Preferably, the elastomeric sealing material  127  is transparent or semi-transparent, to enabling viewing of the slots  127  through it. 
     Multiple lengths of wire  130  are threaded through the encapsulated material  128  and tubing  126  in the desired orientation to form the desired number of pairs of depending spline legs. Once threaded through, the depending spline legs are secured to the anchor  88  and electrodes attached in the manner previously described. 
     Alternatively, spline wires  130  can be threaded through the slots  127  of the tubing  126  before encapsulation by the material  128 . In this embodiment, the elastomeric material  128  is applied by coating or dipping after the spline wires  130  are threaded through the slots  127 . 
     FIG. 29 shows an alternative embodiment of a support assembly  132 . The support assembly  132  includes spline elements  134  radiating in a circumferentially spaced relationship from a center web  136 , which constitutes the hub  24 . 
     As FIG. 29 shows, the support assembly  132  is of the type previously shown in FIGS. 21 and 22, which is axially symmetric but radially asymmetric. The support assembly  132  also possesses asymmetric mechanical properties, as already described in connection with FIGS. 21 and 22. 
     More particularly, the assembly  132  includes seven spline elements  134 , designated S 1  to S 7 , arranged in two discrete groups  106  and  108  about a central web  136 . The group  106  comprises five adjacent spline elements S 1  to S 5 , and the second group  108  comprises two adjacent spline elements S 6  and S 7 . This provides a radially asymmetric structure, as the difference between the smallest angle β (about 36°) and the largest angle α (about 60°) is greater than 20°. 
     Furthermore (similar to the structure  104  shown in FIGS.  21  and  22 ), the splines S 6  and S 7  (in group  108 ) are each wider in their transverse direction than each of the splines S 1  to S 5  (in group  106 ), and are therefore individually stiffer than the individual splines S 1  to S 5 . This provides the asymmetric of physical properties previously described with reference to the structure  104  in FIGS. 21 and 22. 
     As FIG. 30A shows, the spline elements  134  and web  136  are machined from a single sheet  138  of material. In the illustrated embodiment, the sheet  138  comprises Nickel Titanium stock having a thickness of about 0.004 inch. Other materials, like extruded or molded plastic, or stainless steel can be used for the sheet. 
     As FIG. 30A also shows, circumferentially spaced, pie shaped segments  140  are initially cut from the sheet  138 , leaving behind the spline elements  138  having the desired width and circumferential spacing attached to a peripheral rim  141 . The rim  141  is then cut away, leaving the spline elements as shown in FIG.  30 B. Laser cutting or another accurate, mechanized cutting technique, like EDM, can be used for this purpose. 
     One end of the spline elements  138  are connected to the web  136 , from which they radiate like spokes. The free ends of the spline elements  138  are connected to the anchor  88  and electrodes attached in the manner previously described. 
     IX. Deployment of the Support Assemblies 
     The methodology for deploying each of the symmetric and asymmetric support structures described is generally the same. FIGS. 38A and 38B show a representative deployment technique usable when vascular access to a heart chamber is required. 
     As FIG. 38A shows, the physician uses an introducer  185 , made from inert plastic materials (e.g., polyester), having a skin-piercing cannula  186 . The cannula  186  establishes percutaneous access into, for example, the femoral vein  188 . The exterior end of the introducer  185  includes a conventional hemostatic valve  190  to block the outflow of blood and other fluids from the access. The valve may take the form of a conventional slotted membrane or conventional shutter valve arrangement (not shown). A valve  190  suitable for use may be commercial procured from, for example, B. Braun Company. The introducer  185  includes a flushing port  187  to introduce sterile saline to periodically clean the region of the valve  190 . 
     As FIG. 38A shows, the physician advances a guide sheath  192  through the introducer  185  into the accessed vein  188 . A guide catheter or guide wire (not shown) may be used in association with the guide sheath  192  to aid in directing the guide sheath  192  through the vein  188  toward the heart  194 . It should be noted that the views of the heart  194  and other interior regions of the body in this Specification are not intended to be anatomically accurate in every detail. The Figures show anatomic details in diagrammatic form as necessary to show the features of the invention. 
     The physician observes the advancement of the guide sheath  192  through the vein  188  using fluoroscopic or ultrasound imaging, or the like. The guide sheath  192  can include a radio-opaque compound, such as barium or titanium, for this purpose. Alternatively, a radio-opaque marker can be placed at the distal end of the guide sheath  192 . 
     In this way, the physician maneuvers the guide sheath  192  through the vein  188  into an atrium  196 . The guide sheath  192  establishes a passageway through the vein  188  into the atrium  196 , without an invasive open heart surgical procedure. Further advancement allows entry into the associated underlying ventricle  198  through the intervening valve  199  (as FIG. 38A shows). If access to the other atrium or ventricle is desired (as FIG. 31 shows), a conventional transeptal sheath assembly (not shown) can be used to gain passage through the septum between the left and right atria. 
     As FIG. 38A shows, once the guide sheath  192  is placed in the targeted region, the physician advances the catheter tube  12 , which carries the structure (generally designated by the letter S in FIGS.  38 A and  38 B), with the structure S confined within the slidable sheath  44 , through the guide sheath  192  and into the targeted region. 
     As FIG. 38B shows, pulling back upon the slidable sheath  44  (see arrow  200  in FIG. 38B) allows the structure S to spring open within the targeted region for use. The structure S in FIG. 38B is radially asymmetric and axially symmetric. 
     When deployed for use (as FIG. 38B shows), the three dimensional shape of the support structure S (whether symmetric or asymmetric) holds the spline elements (generally designated by the letter SPL), with associated electrodes (designated by the letter E) in intimate contact against the surrounding tissue mass. 
     X. Automated Structure Identification 
     The differences among the support structures disclosed can be characterized in terms of various physical, mechanical, and functional attributes. These attributes include the physical property of the structure, the physical property of the electrodes, and the functional property of the electrode. 
     The physical property of the structure can include the size of the structure; the shape of the structure; the radial symmetry or asymmetry of the structure; the axial symmetry or asymmetry of the structure; the number of spline elements; or the stiffness value of the spline elements, expressed in terms, for example, of the radial stiffness function S r  discussed above, and whether the stiffness value is symmetric or asymmetric; the recommended criteria for use, as above discussed; or combinations thereof. 
     The physical property of the electrodes can include the total number of electrodes carried by the structure; the number of electrodes carried per spline element; the distance between electrodes on each spline; the distribution or density pattern of multiple electrodes on the structure; or combinations thereof. 
     The functional property of the electrodes can include the functionality of the electrodes in terms of a diagnostic capability, such as mapping, or derivation of an electrical characteristic, or pacing, or a therapeutic capability, such as transmission of electrical energy to form a tissue lesion; the characteristics of lesions formed using the structures, whether segmented, large, or long; or combinations thereof. 
     According to the invention, a family of identification codes is provided for the family  98  of structures. Each identification code uniquely identifies a particular structure in terms of the physical property or properties of the structure or electrode, and in terms of the functional property or properties of the electrodes carried by the structure. An identification element is attached in association with each structure within the family  98  to retain the identification code. The identification element is adapted to provide an output representative of the identification code. 
     In a preferred embodiment (see FIG.  39 ), each structure  20  carries an identification component  170 . The identification component  170  carries the assigned identification code XYZ, which uniquely identifies the individual physical, mechanical, and functional characteristics of the particular structure. 
     In the illustrated embodiment (see FIG.  39 ), the coded component  170  is located within the handle  18  of the probe  10  that carries the structure  20 . However, the component  170  could be located elsewhere on the probe  10 . 
     The coded component  170  is electrically coupled to an external interpreter  178  when the probe  10  is plugged into a control unit  172  for use. The unit  172  can incorporate a signal processor  174  for processing electrical impulses sensed by the electrodes  30  on the structure  20 . The unit  172  can also incorporate, alone or in combination with the signal processor  174 , a generator  176  for supplying ablation energy to the electrodes  30 . 
     The interpreter  178  inputs the code XYZ that the coded component  170  contains. The interpreter  178  electronically compares the input code XYZ to, for example, a preestablished master table  180  of codes contained in memory. The master table  180  lists, for each code XYZ, the physical, mechanical, and functional characteristics of the structure  20 . The interpreter  178  displays for the physician in understandable alpha/numeric format the physical, mechanical, and functional characteristics of the structure  20  that the code XYZ signifies in the table  180 . 
     The control unit  172  can also include functional algorithms  188  coupled to the processor  174  or generator  176 , which set operating parameters based upon the code XYZ. For example, the code XYZ could cause an algorithm to set and control power limits for the generator  176 . As another example, the code XYZ can provide input to tissue mapping algorithms, or electrical characteristic derivation algorithms, or provide interpolation for evaluating electrograms between electrodes, or extrapolate sensed electrical activities to locate potential ablation sites, or create a positioning matrix using the electrodes, to help guide ancillary probes within the structure. Further details of establishing a localized coordinate matrix within a multiple electrode structure for the purpose of locating and guiding a movable electrode within the structure are found in copending patent application Ser. No. 08/320,301, filed Oct. 11, 1994 and entitled “Systems and Methods for Guiding Movable Electrode Elements Within Multiple Electrode Structures.” This application is incorporated herein by reference. 
     The coded component  170  can be variously constructed. It can, for example, take the form of an integrated circuit  184  (see FIG.  40 ), which expresses in digital form the code XYZ for input in ROM chips, EPROM chips, RAM chips, resistors, capacitors, programmed logic devices (PLD&#39;s), or diodes. Examples of catheter identification techniques of this type are shown in Jackson et al. U.S. Pat. No. 5,383,874, which is incorporated herein by reference. 
     Alternatively, the coded component  170  can comprise separate electrical elements  186  (see FIG.  41 ), each one of which expressing an individual characteristic. For example, the electrical elements  186  can comprise resistors (R 1  to R 4 ), comprising different resistance values, coupled in parallel. The interpreter  178  measures the resistance value of each resistor R 1  to R 4 . The resistance value of the first resistor R 1  expresses in preestablished code, for example, the number of electrodes on the structure. The resistance value of the second resistor R 2  expresses in preestablished code, for example, the distribution of electrodes on the structure. The resistance value of the third resistor R 3  expresses in preestablished code, for example, the radial symmetry or asymmetry of the structure. The resistance value of the fourth resistor R 4  expresses in preestablished code, for example, the axial symmetry or asymmetry of the structure. 
     In the preferred embodiment, the code XYZ includes code segments, X and Y and Z. Each code segment represents a physical or functional property, or a group of related physical or functional properties. 
     The segmentation of the code XYZ can, of course, vary. As one example, the X segment can carry identification values representing the shape and size of the structure; the Y segment can carry identification values representing distribution of spline elements and electrodes on the structure; and the Z segment can carry identification values representing the number of splines and the number of electrodes per spline.