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Home | Alpha Telephone | Domain Names | Web Hosting | Get Traffic | xrEvidence | xrSoccer United States Patent
Surgical method and apparatus for positioning a diagnostic or therapeutic element within the body A surgical method and apparatus for positioning a diagnostic or therapeutic element within the body. The apparatus may be catheter-based or a probe including a relatively short shaft.
Primary Examiner: Cohen; Lee S. Attorney, Agent or Firm: CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation of U.S. application Ser. No. 10/160,960, filed May 30, 2002, now U.S. Pat. No. 6,786,905, which is a continuation of U.S. application Ser. No. 09/644,847, filed Aug. 22, 2000, now U.S. Pat. No. 6,425,895, which is a continuation of U.S. application Ser. No. 09/072,872, filed May 5, 1998, now U.S. Pat. No. 6,142,994, which is a continuation-in-part of U.S. application Ser. No. 08/321,424, filed Oct. 11, 1994, now U.S. Pat. No. 5,885,278, which is a continuation-in-part of U.S. application Ser. No. 08/320,198, filed Oct. 7,1994, now abandoned. U.S. application Ser. No. 09/072,872 is also continuation-in-part of U.S. application Ser. No. 08/321,092, filed Oct. 11, 1994, now U.S. Pat. No. 5,836,947; and U.S. application Ser. No. 08/949,117, now U.S. Pat. No. 6,152,920, U.S. application Ser. No. 08/949,083, now abandoned, U.S. application Ser. No. 08/948,729, now abandoned, and U.S. application Ser. No. 08/949,084, now abandoned, each filed Oct. 10, 1997. The specification and claims of each of these applications are incorporated herein by reference. We claim: 1. A method of treating an atrium having an endocardial surface and an epicardial surface, comprising the steps of: engaging the atrial appendage from the epicardial surface; positioning a lasso around the epicardial surface side of the atrial appendage; forming an aperture in an atrial appendage; inserting a surgical probe through the aperture in the atrial appendage; and forming a lesion in the endocardial surface with the surgical probe. 2. A method as claimed in claim 1, further comprising the steps of: inserting an introducer into the atrium through the aperture; and tightening the lasso around the introducer. 3. A method as claimed in claim 2, further comprising the steps of: withdrawing the introducer after the lesion has been formed in the endocardial surface; and tightening the lasso to isolate the atrial appendage. 4. A method as claimed in claim 1, wherein the step of inserting a surgical probe comprises inserting a surgical probe with a collapsible portion in a collapsed state through the aperture in the atrial appendage and then expanding the collapsible portion within the atrium. 5. A method as claimed in claim 1, wherein the step of inserting a surgical probe comprises inserting a surgical probe with a loop portion in a retracted state through the aperture in the atrial appendage and then forming a loop within the atrium. 6. A method as claimed in claim 1, wherein the step of inserting a surgical probe comprises inserting a surgical probe including a malleable shaft through the aperture in the atrial appendage. 7. A method as claimed in claim 1, wherein the step of forming a lesion comprises forming a lesion in the endocardial surface by transmitting electrical energy through atrial tissue with the surgical probe. 8. A method as claimed in claim 1, further comprising the step of: creating a lesion from the atrial appendage to an anatomic barrier. 9. A method of treating an atrium having an endocardial surface and an epicardial surface, comprising the step of: forming an aperture in an atrial appendage; inserting a surgical probe through the aperture in the atrial appendage; forming a lesion in the endocardial surface with the surgical probe; and creating a lesion from the atrial appendage to the mitral valve annulus. 10. A method as claimed in claim 9, wherein the step of forming an aperture comprises forming an aperture in an atrial appendage from the epicardial surface. 11. A method as claimed in claim 9, further comprising the steps of: inserting an introducer into the atrium through the aperture; and tightening sutures around the introducer. 12. A method as claimed in claim 11, further comprising the steps of: withdrawing the introducer after the lesion has been formed in the endocardial surface; and tightening the sutures to isolate the atrial appendage. 13. A method as claimed in claim 9, wherein the step of inserting a surgical probe comprises inserting a surgical probe with a collapsible portion in a collapsed state through the aperture in the atrial appendage and then expanding the collapsible portion within the atrium. 14. A method as claimed in claim 11, wherein the step of inserting a surgical probe comprises inserting a surgical probe with a loop portion in a retracted state through the aperture in the atrial appendage and then forming a loop within the atrium. 15. A method as claimed in claim 9, wherein the step of inserting a surgical probe comprises inserting a surgical probe including a malleable shaft through the aperture in the atrial appendage. 16. A method as claimed in claim 9, wherein the step of forming a lesion comprises forming a lesion in the endocardial surface by transmitting electrical energy through atrial tissue with the surgical probe. BACKGROUND OF THE INVENTIONS 1. Field of Invention The present inventions relate generally to structures for positioning one or more diagnostic or therapeutic elements within the body and, more particularly, to devices which are particularly well suited for treatment of cardiac conditions. 2. Description of the Related Art There are many instances where diagnostic and therapeutic elements must be inserted into the body. One instance involves the treatment of cardiac conditions such as atrial fibrillation and atrial flutter which lead to an unpleasant, irregular heart beat, called arrhythmia. Normal sinus rhythm of the heart begins with the sinoatrial node (or "SA node") generating an electrical impulse. The impulse usually propagates uniformly across the right and left atria and the atrial septum to the atrioventricular node (or "AV node"). This propagation causes the atria to contract in an organized way to transport blood from the atria to the ventricles, and to provide timed stimulation of the ventricles. The AV node regulates the propagation delay to the atrioventricular bundle (or "HIS" bundle). This coordination of the electrical activity of the heart causes atrial systole during ventricular diastole. This, in turn, improves the mechanical function of the heart. Atrial fibrillation occurs when anatomical obstacles in the heart disrupt the normally uniform propagation of electrical impulses in the atria. These anatomical obstacles (called "conduction blocks") can cause the electrical impulse to degenerate into several circular wavelets that circulate about the obstacles. These wavelets, called "reentry circuits," disrupt the normally uniform activation of the left and right atria. Because of a loss of atrioventricular synchrony, the people who suffer from atrial fibrillation and flutter also suffer the consequences of impaired hemodynamics and loss of cardiac efficiency. They are also at greater risk of stroke and other thromboembolic complications because of loss of effective contraction and atrial stasis. Although pharmacological treatment is available for atrial fibrillation and flutter, the treatment is far from perfect. For example, certain antiarrhythmic drugs, like quinidine and procainamide, can reduce both the incidence and the duration of atrial fibrillation episodes. Yet, these drugs often fail to maintain sinus rhythm in the patient. Cardioactive drugs, like digitalis, Beta blockers, and calcium channel blockers, can also be given to control the ventricular response. However, many people are intolerant to such drugs. Anticoagulant therapy also combats thromboembolic complications, but does not eliminate them. Unfortunately, pharmacological remedies often do not remedy the subjective symptoms associated with an irregular heartbeat. They also do not restore cardiac hemodynamics to normal and remove the risk of thromboembolism. Many believe that the only way to really treat all three detrimental results of atrial fibrillation and flutter is to actively interrupt all of the potential pathways for atrial reentry circuits. One surgical method of treating atrial fibrillation by interrupting pathways for reentry circuits is the so-called "maze procedure" which relies on a prescribed pattern of incisions to anatomically create a convoluted path, or maze, for electrical propagation within the left and right atria. The incisions direct the electrical impulse from the SA node along a specified route through all regions of both atria, causing uniform contraction required for normal atrial transport function. The incisions finally direct the impulse to the AV node to activate the ventricles, restoring normal atrioventricular synchrony. The incisions are also carefully placed to interrupt the conduction routes of the most common reentry circuits. The maze procedure has been found very effective in curing atrial fibrillation. However, the maze procedure is technically difficult to do. It also requires open heart surgery and is very expensive. Thus, despite its considerable clinical success, only a few maze procedures are done each year. More recently, maze-like procedures have been developed utilizing catheters which can form lesions on the endocardium to effectively create a maze for electrical conduction in a predetermined path. Exemplary catheters are disclosed in commonly assigned U.S. Pat. No. 5,582,609. Typically, the lesions are formed by ablating tissue with an electrode carried by the catheter. Electromagnetic radio frequency ("RF") energy applied by the electrode heats, and eventually kills (i.e. "ablates"), the tissue to form a lesion. During the ablation of soft tissue (i.e. tissue other than blood, bone and connective tissue), tissue coagulation occurs and it is the coagulation that kills the tissue. Thus, references to the ablation of soft tissue are necessarily references to soft tissue coagulation. "Tissue coagulation" is the process of cross-linking proteins in tissue to cause the tissue to jell. In soft tissue, it is the fluid within the tissue cell membranes that jells to kill the cells, thereby killing the tissue. Catheters used to create lesions (the lesions being 3 to 15 cm in length) typically include a relatively long and relatively flexible body portion that has an ablation electrode on its distal end. The portion of the catheter body portion that is inserted into the patient is typically from 23 to 55 inches in length and there may be another 8 to 15 inches, including a handle, outside the patient. The proximal end of the catheter body is connected to the handle which includes steering controls. The length and flexibility of the catheter body allow the catheter to be inserted into a main vein or artery (typically the femoral artery), directed into the interior of the heart, and then manipulated such that the ablation electrode contacts the tissue that is to be ablated. Fluoroscopic imaging is used to provide the physician with a visual indication of the location of the catheter. Atrial appendages are primary potential sources of thrombus formation. The atrial appendages are especially important in the transport of blood because they have a sack-like geometry with a neck potentially more narrow than the pouch. In this case, contraction of the appendage is essential to maintain an average absolute blood velocity high enough to eliminate potential stasis regions which may lead to thrombus formation. In the maze procedure performed through open heart surgery, the typical access points into the interior of the atria are the atrial appendages. Therefore, at the conclusion of the surgical procedure, the region occupied by the atrial appendages is eliminated by surgically removing the appendages. This mitigates subsequent problems resulting from blood stasis in the atrial appendages as well as from electrical isolation of the appendages from the rest of the atria. However, as noted above, open heart surgery is very expensive and the incision based maze procedure is difficult to perform. Although catheter-based procedures do not admit themselves to surgical removal of the appendages, catheter-based procedures and apparatus have been recently developed which reposition the atrial appendages, affix them in an altered position and/or fuse the walls of the appendages to one another to isolate the appendages, reduce stasis regions and ultimately thrombus formation. Such procedures and apparatus are disclosed in commonly assigned U.S. application Ser. No. 08/880,711, filed Jun. 23, 1997, which is a File Wrapper Continuation of U.S. application Ser. No. 08/480,200, filed Jun. 7, 1995, entitled "Atrial Appendage Stasis Reduction Procedures and Devices" and incorporated herein by reference. One of these procedures involves the use of a catheter having a lasso which is tightened around the appendage. RF energy is then transmitted to the appendage by way of the lasso to thermally fuse the walls of the appendage to one another, thereby isolating the appendage. It is believed the treatment of atrial fibrillation and flutter requires the formation of long, thin lesions of different lengths and curvilinear shapes in heart tissue. Such long, curvilinear lesion patterns require the deployment within the heart of flexible ablating elements having multiple ablating regions. The formation of these lesions by ablation can provide the same therapeutic benefits that the complex incision patterns that the surgical maze procedure presently provides, but without invasive, open heart surgery. With larger and/or longer multiple electrode elements comes the demand for more precise control of the ablating process. The delivery of ablating energy must be governed to avoid incidences of tissue damage and coagulum formation. The delivery of ablating energy must also be carefully controlled to assure the formation of uniform and continuous lesions, without hot spots and gaps forming in the ablated tissue. The task is made more difficult because heart chambers vary in size from individual to individual. They also vary according to the condition of the patient. One common effect of heart disease is the enlargement of the heart chambers. For example, in a heart experiencing atrial fibrillation, the size of the atrium can be up to three times that of a normal atrium. Catheter-based ablation and atrial appendage isolation have proven to be a significant advance over the conventional open heart surgery based approaches. Nevertheless, the inventors herein have determined that further improvements are possible. For example, and with respect to ablation procedures in particular, the inventors herein have determined that it can be quite difficult to accurately position an ablation electrode on the endocardium surface by manipulating the distal end of a relatively long catheter body from a remote handle. This is especially true with respect to left atrial sites. The present inventors have also determined that fluoroscopy is a somewhat inaccurate method of visualizing the ablation electrodes during positioning and when determining whether the electrodes are in proper contact with tissue. Additionally, a primary goal of any ablation procedure is to create contiguous lesions (often long, curvilinear lesions) without over-heating tissue and causing coagulum and charring. Tissue ablation occurs at 50.degree. C., while over-heating occurs at 100.degree. C. The present inventors have further determined that it can be difficult to produce tissue contact that will accomplish this result with an electrode mounted on the distal end of a relatively long catheter. This is especially true in those procedures where an electrode on the distal tip of the catheter is dragged along the tissue. Such dragging also makes accurate placement of the electrode very difficult. Other shortcomings identified by the present inventors concern the convective cooling effects of the blood pool on the electrodes. For example, the system power requirements must be high enough to compensate for the heat losses due to convective cooling. One proposed method of solving the over-heating problems associated with conventional ablation catheters is the so-called "cooled tip" approach. Here, the tissue surface is cooled with a saline solution. Although the saline is somewhat useful in keeping the surface temperature below the over-heating temperature, the sub-surface tissue temperature can still rise well above 100.degree. C. Such temperatures will cause gas within the sub-surface tissue to expand. Ultimately, the tissue will tear or pop, which will result in perforations of the epicardial surface and/or the dislodging of chunks of tissue that can cause strokes. Turning to atrial appendage isolation, the present inventors have determined that catheter-based procedures suffer from many of the same disadvantages discussed above, such as those concerning positioning and visualization. Additionally, the inventors herein have determined that the lasso can bunch up the tissue when the lasso is tightened and that tissue fusion would be improved if this bunching could be avoided. With respect to energy control, conventional ablation devices include controls that are located either on the RF energy source, or on a foot pedal. The inventors herein have determined that such arrangements are inconvenient and can make it difficult to control power during a surgical procedure. Turning to surgical procedures in general, one problem associated with many surgical procedures is excessive bleeding. For example, a high level of bleeding is often associated with the removal of liver lobes and certain cancerous tumors. The inventors herein have determined that present surgical methods could be improved in the area of blood loss. SUMMARY OF THE INVENTIONS Accordingly, the general object of the present inventions is to provide an apparatus for positioning an operative element (such as an ablation electrode) within the body which avoids, for practical purposes, the aforementioned problems. In particular, one object of the inventions is to provide tissue ablation systems and methods providing beneficial therapeutic results without requiring highly invasive surgical procedures. Another objective of the inventions is to provide systems and methods that simplify the creation of complex lesions patterns in soft tissue, such as myocardial tissue in the heart. In order to accomplish these and other objectives, certain embodiments of one of the present inventions include an electrode support structure carried at the distal end of a guide body. The support structure includes a bendable stylet extending along an axis outside the distal end of the guide body. The structure also includes at least one flexible spline leg having a near end attached to the distal end of the guide body and a far end extending beyond the distal end of the guide body and attached to the bendable stylet. The spline leg is normally flexed between the distal guide body end and the bendable stylet in a first direction that extends along and radially outward of the axis of the stylet. At least one electrode element is on the flexible spline. The structure further includes a control element to apply tension to the stylet. The tension bends the stylet, thereby flexing the spline leg in a second direction. The flexure of the spline leg in the first direction facilitates intimate contact between the electrode element and tissue. The additional flexure by the stylet of the spline leg in the second direction makes possible the creation of a diverse number of additional shapes and tissue contact forces. In accordance with another embodiment of one of this invention, an electrode support structure is provided that, in addition to bending the stylet, includes another control element that moves the stylet along its axis to increase or decrease flexure of the spline leg in the first direction. This additional control over the flexure of the spline leg further enhances intimate contact against tissue, regardless of variations in the dimensions of the surrounding tissue region. In accordance with another embodiment of this invention, an electrode support structure is provided that includes a malleable stylet. The physician imparts a desired flexure to the spline leg in the second direction by bending the malleable stylet. Alternatively, an electrode support structure is provided in which the spline leg itself is malleable. Structures that embody the features of this invention make possible the creation of diverse number of shapes and contact forces to reliably achieve the type and degree of contact desired between electrode elements and targeted tissue areas, despite physiologic differences among patients. Another aspect of this invention is associated with structures and methods for ablating tissue in a heart. The structures and methods include a probe for deployment within the heart. The probe carries at least one elongated flexible ablation element to which a bendable stylet is attached. The structures and method apply tension to bend the stylet. The bending of the stylet flexes the ablation element into a curvilinear shape along the contacted tissue region. By transmitting ablation energy to the ablation electrode while flexed in the curvilinear shape and in contact with the tissue region, the structures and methods make possible the formation of curvilinear lesion patterns in heart tissue. In order to accomplish the above-described and other objectives, a surgical device in accordance with one embodiment of another one of the present inventions includes a relatively short shaft, a bendable spline assembly associated with the distal end of the shaft and having a predetermined configuration, the spline assembly being adapted to collapse in response to external forces and expand when the forces are removed, and an operative element associated with the bendable spline. Optionally, a substantially tubular member may be positioned around the shaft. Movement of the substantially tubular member over the spline assembly will cause the spline assembly to collapse, while the spline assembly will expand to the predetermined configuration in response to a retraction of the substantially tubular member. In order to accomplish above-described and other objectives, an soft tissue coagulation probe in accordance with one embodiment of one of the inventions includes a relatively short shaft defining a distal end and a proximal end, a handle associated with the proximal end of the shaft, and at least one soft tissue coagulation electrode associated with the shaft and located in spaced relation to the handle. In order to accomplish above-described and other objectives, a surgical device in accordance with another embodiment of this invention includes a relatively stiff shaft, a handle associated with the proximal end of the shaft, and a distal tip assembly associated with the distal end of the shaft, the distal tip assembly including a distal member, which is flexible and/or malleable, and an operative element carried by the distal member. In order to accomplish this and other objectives, a surgical device in accordance with another embodiment of this invention includes a shaft, a relatively stiff tubular member positioned around a predetermined portion of the shaft and movable relative thereto, a distal tip assembly associated with the distal end of the shaft and including a flexible distal member and an operative element carried by the distal member, and a pivot assembly associated with the distal end of the tubular member and a distal portion of the tip assembly. There are many advantages associated with these inventions. For example, the above-described embodiments of this invention may be used in a method of treating atrial fibrillation wherein access to the heart is obtained by way of a thoracostomy. Here, the operative element is an ablation electrode. Such a method may also be used to treat atrial fibrillation during mitral valve surgery wherein access to the heart is obtained through a thoracostomy, thoracotomy or median sternotomy. The relatively short shaft and manner of insertion allows the ablation electrode to be easily inserted into the atrium and visually guided to the desired location. Thus, the ablation electrodes in the present device do not have to be guided by manipulating the relatively long shaft of an endovascular catheter. This makes the positioning of the electrodes within the heart easier and more accurate. Endocardial visualization is also improved because surgical methods employing the present device allow the endocardium to be viewed directly with the naked eye, a fiberoptic camera or other imaging modalities. This eliminates the need for fluoroscopic images and reduces the amount of radiation required, as compared to catheter-based procedures. Moreover, the shaft in the present device can be relatively stiff, as compared to a catheter shaft, because the present shaft does not have to travel through the tortuous vascular path to the heart. Along with the relatively short length of the present shaft, the additional stiffness enhances torque transmission and provides superior and more reliable electrode-endocardium contact force. Surgical devices in accordance with this invention may also be used during procedures, such as valve replacement where the patient is on cardiopulmonary bypass, to create tissue lesions. During bypass, the electrodes elements will not be in contact with the blood pool and, accordingly, will not be affected by the convective cooling. Patients can only be on bypass for a period of approximately four hours. Long bypass times are associated with increased morbidity and mortality. Thus, all procedures performed during bypass must be rapidly completed. Surgical devices in accordance with the present invention may include a series of temperature controlled electrodes that allow a long lesion to be created in rapid fashion, i.e. in approximately 30 to 120 seconds. The ability of the present surgical devices and techniques to create lesions rapidly allows procedures to be performed during bypass that, heretofore, could not due to the time constraints. For example, a conventional surgical maze procedure takes approximately 12 hours to complete (note that a portion of the procedure is performed while the patient is not on bypass), while such a procedure may be completed in approximately 5 to 15 minutes with the present devices and methods. In accordance with another advantageous aspect of this invention, the shaft and/or sheath (if present) may be formed from a malleable material that a physician can bend into a desired configuration and remain in that configuration when released. Although malleable, the stiffness of such material must be at least such that the shaft and/or sheath (if present) will not bend under the forces applied thereto during a surgical procedure. Alternatively, or in addition, the distal end of the device may also be malleable, thereby allowing the physician to bend the distal end of the device into a shape corresponding to the bodily structure to be acted upon. This is particularly important in endocardial applications because the endocardial surface is typically non-uniform with ridges and trabeculae residing in the right and left atria. There are also dramatic differences between endocardial surface morphology from patient to patient and from lesion location to lesion location. To create contiguous lesions with a surgical approach, the device must either distend the atria to flatten out the non-uniformities, or the probe must be configured to conform to the atrial surface. There are, however, some regions where the atria cannot be distended to a flat state because of trabeculae, orifices, and ridges. A surgeon can observe the atrial surface and bend the present malleable device so as to conform thereto. The distal end may, instead, be spring-like or even rigid if the application so requires. In order to accomplish the above-identified and other objectives, a surgical device in accordance with one embodiment of another one of the present inventions includes a handle having at least one movable handle member, first and second support members operably connected to the handle, at least one of the support members being movable with respect to the other support member in response to movement of the at least one movable handle member, and at least one ablation electrode associated with the first support member. There are many advantages associated with this invention. By way of example, this invention is especially useful in a method of isolating an atrial appendage. Access to the atrium may be obtained by, for example, a thoracostomy and the appendage may be captured between the support members. RF energy is then applied to the captured portion of the appendage to thermally fuse the walls of the appendage to one another. This method provides better heating and fusing than the lasso catheter-based approach because the tissue is not bunched up when captured between the support members, as it is when the lasso is tightened. Additionally, the disadvantages associated with the use of catheters in general are also avoided. A surgical clamp in accordance with one embodiment of another of the present inventions includes first and second clamp members, and at least one electrode associated with at least one of the clamp members. The clamp may be used to isolate an atrial appendage in a manner similar to that described in the preceding paragraph with the same advantageous results. Thereafter, the clamp may be either removed or left in place. A surgical device in accordance one embodiment of another of the present inventions includes an energy source, at least one energy transmission device, and a handle including an energy control device coupled to the energy source and to the at least one energy transmission device. The energy control device is adapted to selectively control the transmission of energy from the energy source to the at least one energy transmission device. Because the energy control device is located on the handle, which is necessarily grasped by the physician during surgical procedures, the present surgical device provides more convenient energy control than that found in conventional devices. Alternatively, and in accordance with one embodiment of another of the present inventions, energy control may be accomplished through the use of a remote energy control device that is connected to power unit, but located in close proximity to the patient or otherwise within the sterile zone of an operating room. Such an arrangement also provides more convenient energy control than that found in conventional devices. Additionally, whether the power control interface is located on the handle of a surgical probe or on a remote control device, the power control aspect of the overall electrophysiological system can be more conveniently brought into the sterile zone because both the present surgical probe and remote control device are both readily sterilizable. Conventional power control interfaces, on the other hand, are part of a power control unit that is not readily sterilizable. To further improve tissue contact, a pressure application probe in accordance with one embodiment of another of the present inventions may be used in conjunction with a probe having an energy transmission device on a support member. The pressure application probe includes an elongate main body portion and an engagement device adapted to releasably engage the support member. The pressure application probe can be used by the physician to insure that sufficient tissue contact is realized prior to energy transmission. A coupling device in accordance with another of the present inventions can also be used in conjunction with a probe having an energy transmission device on a support member. One embodiment of the coupling device includes a base member adapted to be removably secured to a first portion of the probe's flexible support member and an engagement device connected to the base member and adapted to be removably secured to a second portion of the flexible support member. The coupling device enables a physician to form a distal loop in the support member when desired, thereby increasing the flexibility of the probe. In order to reduce the blood loss associated certain surgical procedures, a surgical method in accordance with another of the present inventions includes the steps of coagulating soft tissue and then forming an incision is the coagulated tissue. If the incision is no deeper than the coagulation, the incision will not result in significant bleeding. This process can be repeated until an incision of the desired depth is achieved. The above described and many other features and attendant advantages of the present invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Detailed description of preferred embodiments of the invention will be made with reference to the accompanying drawings. FIG. 1 is a plan view of an ablation probe having a full-loop structure for supporting multiple ablation elements. FIG. 2 is an elevation view of a spline used to form the loop structure shown in FIG. 1. FIG. 3 is an elevation view of the distal hub used to form the loop structure shown in FIG. 1. FIG. 4 is a side section view of the hub shown in FIG. 3. FIG. 5 is a perspective, partially exploded view of the spline, distal hub, and base assembly used to form the loop structure shown in FIG. 1. FIG. 6a is an enlarged perspective view of the base assembly shown in FIG. 5. FIG. 6b is a side section view of an alternative base assembly for the loop structure shown in FIG. 1. FIG. 7 is an elevation view of a half-loop structure for supporting multiple electrodes. FIG. 8 is an elevation view of a composite loop structure for supporting multiple electrodes comprising two circumferentially spaced half-loop structures. FIG. 9 is an elevation view of a composite loop structure comprising two full-loop structures positioned ninety degrees apart. FIG. 10 is an elevation view, with parts broken away, of multiple electrode elements comprising segmented rings carried by a loop support structure. FIG. 11a is an enlarged view, with parts broken away, of multiple electrode elements comprising wrapped coils carried by a loop support structure. FIG. 11b is an elevation view, with parts broken away, of multiple electrode elements comprising wrapped coils carried by a loop support structure. FIG. 12 is a top view of a steering mechanism used to deflect the distal end of the probe shown in FIG. 1. FIG. 13 is a plan view of a full-loop structure for supporting multiple electrode elements having an associated center stylet attached to a remote control knob for movement to extend and distend the full-loop structure. FIG. 14 is a side section view of the remote control knob for the center stylet shown in FIG. 13. FIG. 15 is a plan view of the full-loop structure shown in FIG. 13, with the control knob moved to extend the full-loop structure. FIG. 16 is a plan view of a full-loop structure shown in FIG. 13, with the control handle moves to distend the full-loop structure. FIG. 17 is a plan view of a half-loop structure for supporting multiple electrode elements having an associated center stylet attached to a remote control knob for movement to extend and distend the half-loop structure. FIG. 18 is a plan view of the half-loop structure shown in FIG. 17, with the control knob moved to extend the half-loop structure. FIG. 19 is a plan view of a half-loop structure shown in FIG. 17, with the control handle moves to distend the half-loop structure. FIG. 20 is a plan view of a full-loop structure for supporting multiple electrode elements having an associated center stylet attached to a remote control knob for movement to extend and distend the full-loop structure, and also having a remotely controlled steering mechanism to flex the center stylet to bend the full-loop structure into a curvilinear shape. FIG. 21 is a side elevation view of the full-loop structure shown in FIG. 20. FIG. 22 is an enlarged sectional view, generally taken along line 22--22 in FIG. 20, showing the steering wires attached to the center stylet to flex it. FIGS. 23a and 23b are side elevation views showing the operation of the steering mechanism in bending the full-loop structure, respectively, to the left and to the right. FIG. 24 is a largely diagrammatic, perspective view of the full-loop structure bent to the right, as also shown in side elevation in FIG. 23b. FIG. 25 is a plan view of the full-loop structure shown in FIG. 20 and the associated remote control knob for extending and distending as well as bending the full-loop structure. FIG. 26 is a side section view, taken generally along lines 26--26 in FIG. 25, of the control knob for extending and distending as well as bending the full-loop structure. FIG. 27 is a largely diagrammatic, perspective view of the full-loop structure when distended and bent to the right. FIG. 28 is a largely diagrammatic, perspective view of a half-loop structure with steerable center stylet bent to the right. FIG. 29 is a plan, partially diagrammatic, view of a full-loop structure for supporting multiple electrode elements having a movable spline leg attached to a remote control knob for movement to extend and distend the full-loop structure. FIG. 30a is a section view, taken generally along line 30a 30a in FIG. 29, of the interior of the catheter body lumen, through which the movable spline leg passes. FIG. 30b is a side section view of an alternative way of securing the full-loop structure shown in FIG. 29 to the distal end of the catheter tube. FIG. 31 is a plan, partially diagrammatic view of the full-loop structure shown in FIG. 29 being extended by pulling the movable spline leg inward. FIGS. 32 and 33 are plan, partially diagrammatic views of the full-loop structure shown in FIG. 29 being distended by pushing the movable spline leg outward. FIGS. 34 and 35 are largely diagrammatic views of the full-loop structure shown in FIG. 29 being distended by pushing the movable spline leg outward while deployed in the atrium of a heart. FIGS. 36, 37, and 38 are plan, partially diagrammatic views of a full-loop structure for supporting multiple electrode elements having two movable spline legs attached to remote control knobs for coordinated movement to extend and distend the full-loop structure. FIG. 39a is a plan view of a full-loop structure for support multiple electrode elements having a smaller, secondary loop structure formed in one spline leg. FIG. 39b is a side view of the full-loop structure shown in FIG. 39a, showing the smaller, secondary loop structure. FIG. 40a is a perspective view of a modified full-loop structure for supporting multiple electrode elements having an odd number of three or more spline legs. FIG. 40b is a top section view of the base of the full-loop structure shown in FIG. 40a. FIGS. 41, 42, and 43 are plan, partially diagrammatic, views of a bifurcated full-loop-structure for supporting multiple electrode elements having movable half-loop structures to extend and distend the bifurcated full-loop structure. FIGS. 44 and 45 are plan, partially diagrammatic, views of an alternative form of a bifurcated full-loop structure for supporting multiple electrode elements having movable center ring to extend and distend the bifurcated full-loop structure. FIG. 46 is a plan, partially diagrammatic, views of an alternative form of a bifurcated full-loop structure for supporting multiple electrode elements having both a movable center ring and movable spline legs to extend and distend the bifurcated full-loop structure. FIGS. 47, 48, and 49 are plan, partially diagrammatic, views of another alternative form of a bifurcated full-loop structure for supporting multiple electrode elements having movable half-loop structures to extend and distend the bifurcated full-loop structure. FIG. 50 is a plan view of a full-loop structure for supporting and guiding a movable electrode element. FIG. 51 is a side elevation view of the full-loop structure and movable electrode element shown in FIG. 50. FIG. 52 is an enlarged view of the movable electrode supported and guided by the structure shown in FIG. 50, comprising wound coils wrapped about a core body. FIG. 53 is an enlarged view of another movable electrode that can be supported and guided by the structure shown in FIG. 50, comprising bipolar pairs of electrodes. FIG. 54 is a largely diagrammatic view of the full-loop structure and movable electrode element shown in FIG. 50 in use within the atrium of a heart. FIG. 55 is a perspective, elevation view of a bundled loop structure for supporting multiple electrode elements, comprising an array of individual spline legs structures, each having a movable portion that independently extends and distends the individual structures to shape and flex the overall bundled loop structure. FIG. 56 is a top view of the bundled loop structure shown in FIG. 55. FIG. 57 is a perspective elevation view of the bundled loop structure shown in FIG. 55 with some of the independently movable spline legs extended and distended to change the flexure of the bundled loop structure. FIG. 58 is a top view of the bundled loop structure shown in FIG. 57. FIGS. 59a and 59b are, respectively, top and side views of a bundled loop structure like that shown in FIG. 55 in position within an atrium, out of contact with the surrounding atrial wall. FIGS. 60a and 60b are, respectively, top and side views of a bundled loop structure like that shown in FIG. 57, with some of the independently movable spline legs extended and distended to change the flexure of the bundled loop structure, to bring it into contact with the surrounding atrial wall. FIG. 61 is a top section view of the base of the bundled loop structure shown in FIG. 55. FIG. 62 is a side, partial section view of a surgical device for positioning an operative element within a patient in accordance with a preferred embodiment of one of the present inventions. FIG. 63 is an end view of the surgical device shown in FIG. 62. FIG. 64a is a side view of a surgical device for positioning an operative element within a patient in accordance with another preferred embodiment of one of the present inventions. FIG. 64b is a partial side view of a portion of the surgical device shown in FIG. 64a. FIG. 65 is a side, partial section view of a portion of the surgical device shown in FIG. 64a. FIG. 66 is a side view of a surgical device for positioning an operative element within a patient in accordance with still another preferred embodiment of one of the present inventions. FIG. 67a is a partial side, cutaway view of a surgical device for positioning an operative element within a patient in accordance with yet another preferred embodiment of one of the present inventions. FIG. 67b is a section view taken along line 67b--67b in FIG. 67a. FIG. 68 is a section view showing an operative element coated with regenerated cellulose. FIG. 69a is a section view showing a partially masked operative element. FIG. 69b is a section view showing an alternative operative element configuration. FIGS. 70a 70c are front views of a spline assembly in accordance with an embodiment of one of the present inventions. FIG. 70d is a side view of the spline assembly shown in FIGS. 70a 70c. FIG. 70e is a section view taken along line 70e--70e in FIG. 70a. FIG. 70f is a partial front, partial section view of a surgical device for positioning an operative element within a patient in accordance with yet another preferred embodiment of one of the present inventions. FIG. 71a is a side view of a surgical device for positioning an operative element within a patient in accordance with a preferred embodiment of one of the present inventions. FIG. 71b is a side, partial section view of an alternate tip that may be used in conjunction with the device shown in FIG. 71a. FIG. 71c is a side, section view of another alternate tip that may be used in conjunction with the device shown in FIG. 71a. FIG. 71d is a perspective view of a probe handle in accordance with a present invention. FIG. 71e is a perspective view of a probe handle in accordance with another embodiment of present invention. FIG. 71f is an exploded perspective view of a probe in accordance with one embodiment of a present invention. FIG. 71g is an enlarged view of a portion of the probe shown in FIG. 71f. FIG. 71h is a plan view of an electrophysiology system in accordance with one embodiment of a present invention. FIG. 71i is an enlarged view of the remote power control unit shown in FIG. 71h. FIG. 72a is a section view of the distal portion of the device shown in FIG. 71a taken along line 72a--72a in FIG. 71a. FIG. 72b a section view of an alternate distal portion for the device shown in FIG. 71a. FIG. 72c is a side, partial section view of another alternative distal portion for the device shown in FIG. 71a. FIG. 73 is a section view taken along line 73--73 in FIG. 71a. FIG. 74 is a side view of a surgical device for positioning an operative element within a patient in accordance with another preferred embodiment of one of the present inventions. FIG. 75 is a side view of a surgical device for positioning an operative element within a patient in accordance with yet another preferred embodiment of one of the present inventions. FIG. 76 is a perspective view of a portion of the device shown in FIG. 75. FIG. 77 is a side view of a surgical device for positioning an operative element within a patient in accordance with still another preferred embodiment of one of the present inventions. FIG. 78 is a side view of a damp in accordance with a preferred embodiment of one of the present inventions. FIG. 79 is a section view taken along line 79--79 in FIG. 78. FIG. 80 is a top view of the clamp illustrated in FIG. 78. FIG. 81 is a side view of a surgical device for positioning an operative element within a patient and applying a clamping force to a bodily structure in accordance with a preferred embodiment of one of the present inventions. FIG. 82 is a side view of a surgical device for positioning an operative element within a patient and applying a clamping force to a bodily structure in accordance with another preferred embodiment of one of the present inventions. FIG. 83 is a side view of a surgical device for positioning an operative element within a patient and applying a clamping force to a bodily structure in accordance with still another preferred embodiment of one of the present inventions. FIG. 84 is a top view of the operative element supporting member of the surgical device shown in FIG. 83. FIG. 85a is a top view of another operative element supporting member. FIG. 85b is a top view of still another operative element supporting member. FIG. 86 is a side view of a surgical device for positioning an operative element within a patient and applying a clamping force to a bodily structure in accordance with yet another preferred embodiment of one of the present invention. FIG. 87 is a side, partial section view of an exemplary procedure involving the surgical device shown in FIG. 81. FIG. 88 is a side, partial section view of an exemplary procedure involving a surgical device having an alternate support member configuration. FIGS. 89 and 90 are schematic views of a system for controlling the application of ablating energy to multiple electrodes using multiple temperature sensing inputs. FIG. 91 is a schematic flow chart showing an implementation of the temperature feedback controller shown in FIGS. 89 and 90, using individual amplitude control with collective duty cycle control. FIG. 92 is a schematic view of a neural network predictor, which receives as input the temperatures sensed by multiple sensing elements at a given electrode region and outputs a predicted temperature of the hottest tissue region. FIG. 93 is a fragmentary side view showing the use of a grabbing catheter in conjunction with a lasso catheter for maintaining the walls of the inverted appendage together. FIG. 94 is a fragmentary view of the combination shown in FIG. 93 illustrating further steps of tying an appendage in an inverted orientation. FIG. 95 is a perspective view of a pressure application probe in accordance with a preferred embodiment of a present invention secured to an operative element supporting probe. FIG. 96 is an enlarged perspective view of the pressure application probe shown in FIG. 95. FIG. 97 is a partial perspective view of a pressure application probe in accordance with another preferred embodiment of a present invention. FIG. 98 is a perspective view of a coupling device in accordance with a preferred embodiment of a present invention. FIG. 99 is a perspective view showing a pressure application probe and the coupling device shown in FIG. 98 being used in combination with the surgical device shown in FIG. 71a. FIG. 100 is a perspective view showing the coupling device shown in FIG. 98 being used in combination with the surgical device shown in FIG. 71a. FIG. 101 is a perspective view of a coupling device in accordance with another preferred embodiment of a present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following is a detailed description of the best presently known modes of carrying out the inventions. This description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the inventions. The detailed description of the preferred embodiments is organized as follows: I. Bi-Directional Flexible Structures II. Probe-Type Apparatus III. Operative Elements IV. Epicardial Applications of Probe-Type Apparatus V. Endocardial Applications of Probe-Type Apparatus VI. Other Surgical Applications VII. Apparatus that Apply a Clamping Force VIII. Applications of Apparatus that Apply a Clamping Force IX. Power Control The section titles and overall organization of the present detailed description are for the purpose of convenience only and are not intended to limit the present invention. This specification discloses a number of electrode structures, mainly in the context of cardiac ablation, because the structures are well suited for use with myocardial tissue. Nevertheless, it should be appreciated that the structures are applicable for use in therapies involving other types of soft tissue. For example, various aspects of the present inventions have applications in procedures concerning other regions of the body such as the prostate, liver, brain, gall bladder, uterus and other solid organs. I. Bi-Directional Flexible Structures The exemplary structures, systems, and techniques illustrated in this Section are discussed in the context of catheter-based cardiac ablation. Nevertheless, it should be appreciated that the structures, systems, and techniques are applicable for use in other tissue ablation applications, including those that are not necessarily catheter-based. A. Loop Support Structures for Multiple Electrodes FIG. 1 shows a multiple electrode probe 10 that includes a loop structure 20 carrying multiple electrode elements 28. Instead of, or in addition to the electrode elements, the loop structure can carry one or more of the other operative elements discussed in Section III below. The probe 10 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 a loop structure 20 that supports multiple electrodes. In FIG. 1, the loop support structure 20 comprises two flexible spline legs 22 spaced diametrically opposite each other. The dual leg loop structure 20 shown in FIG. 1 will be called a "full-loop" structure. The far ends of the spline legs 22 radiate from a distal hub 24. The near ends of the spline legs 22 radiate from a base 26 attached to the distal end 16 of the catheter tube 12. The multiple electrode elements 28 are arranged along each spline leg 22. In one implementation, the two spline legs 22 of the structure 20 are paired together in an integral loop body 42 (see FIG. 2). Each body 42 includes a mid-section 44 from which the spline elements 22 extend as an opposed pair of legs. As FIG. 2 shows, the mid-section 44 includes a preformed notch or detent 46, whose function will be described later. The loop body 42 is preferably made from resilient, inert wire, like Nickel Titanium (commercially available as Nitinol material). However, resilient injection molded inert plastic or stainless steel can also be used. Preferably, the spline legs 22 comprise thin, rectilinear strips of resilient metal or plastic material. Still, other cross sectional configurations can be used. In this implementation (see FIGS. 3 and 4), the distal hub 24 has a generally cylindrical side wall 50 and a rounded end wall 52. A longitudinal slot 56 extends through the hub 24, diametrically across the center bore 54. In the illustrated embodiment, the hub 24 is made of an inert, machined metal, like stainless steel. The bore 54 and slot 56 can be formed by conventional EDM techniques. Still, inert molded plastic materials can be used to form the hub 24 and associated openings. In this implementation, to assemble the structure 20 (see FIGS. 4 and 5), a spline leg 22 of the hoop-like body 42 is inserted through the slot 56 until the mid-body section 44 enters the bore 54. The detent 46 snaps into the bore 54 (see FIG. 4) to lock the body 42 to the hub 24, with the opposed pair of spline legs 22 on the body 42 radiating free of the slot 56 (see FIG. 5). In the illustrated embodiment (see FIGS. 5 and 6a), the base 26 includes an anchor member 62 and a mating lock ring 64. The anchor member 62 fits with an interference friction fit into the distal end 16 of the catheter tube 12. The lock ring 64 includes a series of circumferentially spaced grooves 66 into which the free ends of the spline legs 22 fit. The lock ring 64 fits about the anchor member 62 to capture with an interference fit the free ends of the spline legs 22 between the interior surface of the grooves 66 and the outer surface of the anchor member 62 (see FIG. 6). The anchor member 62/lock ring 64 assembly holds the spline elements 22 in a desired flexed condition. In an alternative construction (see FIG. 6b), the base 26 can comprise a slotted anchor 63 carried by the distal end 16 of the catheter tube 12. The slotted anchor 63 is made of an inert machined metal or molded plastic material. The slotted anchor 63 includes an outer ring 65 and a concentric slotted inner wall 67. The interior of the anchor 63 defines an open lumen 226 to accommodate passage of wires and the like between the catheter tube bore 36 and the support structure 20 (as will be described in greater detail later). The inner wall 67 includes horizontal and vertical slots 69 and 71 for receiving the free ends of the spline legs 22. The free ends pass through the horizontal slots 69 and are doubled back upon themselves and wedged within the vertical slots 71 between the outer ring 65 and the inner wall 67, thereby securing the spline legs 22 to the anchor 63. There are other alternative ways of securing the spline legs 22 to the distal end 16 of the catheter tube 12, which will be described later. Preferably, the full-loop structure 20 shown in FIG. 1 does not include a hub 24 like that shown in FIGS. 1 and 3, and, in addition, does not incorporate a detented integral loop body 42 like that shown in FIG. 2. Any single full-loop structure without a center stiffener or stylet (as will be described later) preferably comprises a single length of resilient inert wire (like Nickel Titanium) bent back upon itself and preformed with resilient memory to form the desired full loop shape. Structure 112 in FIG. 29 (which will be described in greater detail later) exemplifies the use of a preshaped doubled-back wire to form a loop, without the use of a hub 24 or detented loop body 42. FIG. 7 shows an alternative loop structure 20(1) that includes a single spline leg 22(1) carrying multiple electrode elements 28. This single leg loop structure will be called a "half-loop" structure, in contrast to the dual leg loop structure 20 (i.e., the "full-loop structure) shown in FIG. 1. In assembling the half-loop structure 20(1) shown in FIG. 7, the hoop-like body 42 shown in FIG. 2 is cut on one side of the detent 46 to form the single spline leg 22(1). The single spline leg 22(1) is snap-fitted into the hub 24 and captured with an interference fit by the anchor member 62/lock ring 64 assembly of the base 26 in the manner just described (shown in FIGS. 5 and 6a). Alternatively, the single spline leg 22(1) can be wedged within the base anchor ring 63 shown in FIG. 6b. In FIG. 7, the half-loop structure 20(1) also includes a center stiffener 40 passing through the base 26 and to the bore 54 of the hub 24. The stiffener 40 can be made of a flexible plastic like PEEK, or from a hollow tube like hypo-tubing or braid plastic tubing. It should be appreciated that other loop-type configurations besides the full-loop structure 20 and half-loop structure 20(1) are possible. For example, two half-loop structures 20(1), one or both carrying electrode elements 28, can be situated in circumferentially spaced apart positions with a center stiffener 40, as FIG. 8 shows. As another example, four half-loop structures, or two full-loop structures can be assembled to form a three-dimensional, basket-like structure 60 (without using a center stiffener 40), like that shown in FIG. 9. Regardless of the configuration, the loop structure provides the resilient support necessary to establish and maintain contact between the electrode elements 28 and tissue within the body. The electrode elements 28 can serve different purposes. For example, the electrode elements 28 can be used to sense electrical events in heart tissue. In the illustrated and preferred embodiments, the principal use of the electrode elements 28 is to emit electrical energy to ablate tissue. In the preferred embodiments, the electrode elements 28 are conditioned to emit electromagnetic radio frequency energy. As described in greater detail in Section III below, the electrode elements 28 can be assembled in various ways. In one preferred embodiment (see FIG. 10), the elements comprise multiple, generally rigid ring electrode elements 30 arranged in a spaced apart, segmented relationship upon a flexible, electrically nonconductive sleeve 32 which surrounds the underlying spline leg 22. The sleeve 32 is made a polymeric, electrically nonconductive material, like polyethylene or polyurethane. The electrode rings 30 are pressure fitted about the sleeve 32. The flexible portions of the sleeve 32 between the rings 30 comprise electrically nonconductive regions. Alternatively, the electrode segments 30 can comprise a conductive material coated upon the sleeve 32. The electrode coating can be applied either as discrete, closely spaced segments or in a single elongated section. In a more preferred embodiment (see FIGS. 11a and 11b), spaced apart lengths of closely wound, spiral coils are wrapped about the sleeve 32 to form an array of segmented, generally flexible electrodes 34. The inherent flexible nature of a coiled electrode structures 34 also makes possible the construction of a continuous flexible ablating element comprising an elongated, closely wound, spiral coil wrapped about all or a substantial length of the flexible sleeve 32. The electrode elements 28 can be present on all spline legs 22, as FIG. 1 shows, or merely on a selected number of the spline legs 22, with the remaining spline legs serving to add structural strength and integrity to the structure. Various access techniques can be used to introduce the probe 10 and its loop support structure 20 into the desired region of the heart. For example, to enter the right atrium, the physician can direct the probe 10 through a conventional vascular introducer through the femoral vein. For entry into the left atrium, the physician can direct the probe 10 through a conventional vascular introducer retrograde through the aortic and mitral valves. Alternatively, the physician can use the delivery system shown in U.S. Pat. No. 5,636,634 entitled "Systems and Methods Using Guide Sheaths for Introducing, Deploying, and Stabilizing Cardiac Mapping and Ablation Probes." In the illustrated and preferred embodiments (see FIGS. 10 and 11a/b), each flexible ablation element carries at least one and, preferably, at least two, temperature sensing elements 68. The multiple temperature sensing elements 68 measure temperatures along the length of the electrode element 28. The temperature sensing elements 68, which can comprise thermistors or thermocouples, can be located on the ablation elements in the manner shown in FIGS. 10 and 11a/b. Preferably, the temperature sensing elements 68 can be located on one or both of the longitudinal end edges of the ablation elements, as shown in U.S. patent application Ser. No. 08/788,782, entitled .sup.mSystems and Methods for Controlling Ablation Using Multiple Temperature Sensing Elements," which is incorporated herein by reference. An external temperature processing element, such as that discussed below in Section IX, receives and analyses the signals from the multiple temperature sensing elements 68 in prescribed ways to govern the application of ablating energy to the flexible ablation element. The ablating energy is applied to maintain generally uniform temperature conditions along the length of the element. Additionally, further details of the use of multiple temperature sensing elements in tissue ablation can be found in co-pending U.S. application Ser. No. 08/638,989, filed Apr. 24, 1996, which is File Wrapper Continuation of U.S. application Ser. No. 08/286,930, filed Aug. 8, 1994, entitled "Systems and Methods for Controlling Tissue Ablation Using Multiple Temperature Sensing Elements." To aid in locating the structure 20 within the body, the handle 16 and catheter body 12 preferably carry a steering mechanism 70 (see FIGS. 1 and 12) for selectively bending or flexing the distal end 16 of the catheter body 12. The steering mechanism 18 can vary. In the illustrated embodiment (see FIG. 12), the steering mechanism 70 includes a rotating cam wheel 72 with an external steering lever 74 (see FIG. 1). As FIG. 12 shows, the cam wheel 72 holds the proximal ends of right and left steering wires 76. The steering wires 76, like the signal wires 58, pass through the catheter body lumen 36. The steering wires 76 connect to the left and right sides of a resilient bendable wire or spring (not shown) enclosed within the distal end 16 of the catheter body 12. Forward movement of the steering lever 74 flexes or curves the distal end 16 down. Rearward movement of the steering lever 74 flexes or curves the distal end 16 up. Further details of this and other types of steering mechanisms are shown in Lundquist and Thompson U.S. Pat. No. 5,254,088, which is incorporated into this Specification by reference. B. Variable Shape Loop Support Structures To uniformly create long, thin lesions having the desired therapeutic effect, the loop support structure 20 or 20(1) must make and maintain intimate contact between the electrode elements 28 and the endocardium. This invention provides loop support structures that the physician can adjust to adapt to differing physiologic environments. 1. Distended Loop Structures The adjustable loop structure 78 shown in FIG. 13 is in many respects similar to the full-loop structure 20 shown in FIG. 1. The adjustable full-loop structure 78 includes the pair of diametrically opposite spline legs 22 that radiate from the base 26 and hub 24. In addition, the adjustable full-loop structure 78 includes a flexible stylet 80 attached at its distal end to the hub bore 54. The stylet 80 can be made from a flexible plastic material, like PEEK, or from a hollow tube, like hypo-tubing or braid plastic tubing. The stylet 80 extends along the axis of the structure 78, through the base 26 and catheter body lumen 36, and into the handle 18. In this arrangement, the stylet 80 is free to slide fore and aft along the axis of the catheter body 12. The proximal end of the stylet 80 attaches to a control knob 82 in the handle 18 (as FIG. 13 shows). The control knob 82 moves within a groove 84 (see FIGS. 13 and 14) in the handle 18 to impart fore and aft movement to the stylet 80. Stylet movement changes the flexure of the structure 78. Forward movement of the stylet 80 (i.e., toward the distal end 16) pushes the hub 24 away from the base 26 (see FIG. 15). The loop structure 78 elongates as the spline legs 22 straighten and move radially inward, to the extent permitted by the resilience of the spline legs 22. With the spline legs 22 straightened, the loop structure 78 presents a relatively compact profile to facilitate vascular introduction. Rearward movement of the stylet 80 (i.e., toward the distal end 16) pulls the hub 24 toward the base 26 (see FIG. 16). The spline legs 22 bend inward in the vicinity of the hub 24, while the remainder of the splines, constrained by the base, distend. The loop structure 78 bows radially out to assume what can be called a "heart" shape. When the structure 78 is positioned within the atrium 88 of a heart in the condition shown in FIG. 16, the stylet 80 compresses the spline legs 22, making them expand or bow radially. The expansion presses the distended midportion of the spline legs 22 (and the electrode elements 28 they carry) symmetrically against opposite walls 86 of the atrium 88. The symmetric expansion of the outwardly bowed spline legs 22 presses the opposite atrial walls 86 apart (as FIG. 16 shows), as the radial dimension of the loop structure 78 expands to span the atrium 88. The symmetric expansion presses the electrode elements 28 into intimate surface contact against the endocardium. The symmetric expansion stabilizes the position of the loop structure 78 within the atrium 88. The resilience of the spline legs 22, further compressed by the pulled-back stylet 80, maintains intimate contact between the electrode elements 28 and atrial tissue, without trauma, as the heart expands and contracts. As FIGS. 17 to 19 show, the push-pull stylet 80 can also be used in association with a half-loop structure 90, like that previously shown and discussed in FIG. 7. In this arrangement, pushing the stylet 80 forward (as FIG. 18 shows) elongates the half-loop structure 90 for vascular introduction. Pulling the stylet 80 rearward (as FIG. 19 shows) bows the single spline leg 22 of the structure outward, expanding it so that more secure contact can be achieved against the atrial wall 86, or wherever tissue contact is desired. 2. Curvilinear Loop Structures FIGS. 20 and 21 show a full-loop structure 92 that includes a center stylet 94, which can be flexed. The flexing of the center stylet 94 bends the spline legs 22 in a second direction different than the radial direction in which they are normally flexed. In the illustrated embodiment, this second direction is generally perpendicular to the axes of the spline legs 22, as FIGS. 23a/b and 24 show, although acute bends that are not generally perpendicular can also be made. The bending of the spline legs 22 in this fashion makes possible the formation of long, thin curvilinear lesions using a full-loop structure 92, or (as will be described later) in a half-loop structure 110, as well. The stylet 94 itself can be either fixed in position between the hub 24 and the base 26, or movable along the axis of the loop structure 92 to extend and distend the radial dimensions of the spline legs 22 in the manner already described (see FIGS. 15 and 16). In the illustrated and preferred embodiment, the stylet 94 slides to alter the radial dimensions of the structure. In one implementation, as FIG. 22 best shows, the stylet 94 is made from a metal material, for example stainless steel 17-7, Elgiloy.TM. material, or Nickel Titanium material. A pair of left and right steering wires, respectively 96(R) and 96(L) is attached to opposite side surfaces of the stylet 94 near the hub 24, by adhesive, soldering, or by suitable mechanical means. The steering wires 96(R) and 96(L) are attached to the stylet side surfaces in a diametric opposite orientation that is at right angles to the radial orientation of the spline legs 22 relative to the stylet 94. The steering wires 96(R) and 96(L) extend along the stylet 94, through the base 26 and catheter body lumen 36, and into the handle 18 (see FIG. 25). Preferably, as FIG. 22 best shows, a tube 98 surrounds the stylet 94 and steering wires 96(R) and 96(L), at least along the distal, exposed part of the stylet 94 within the structure 92, keeping them in a close relationship. The tube 98 can be heat shrunk to fit closely about the stylet 94 and steering wires 96(R) and 96(L). As FIGS. 25 and 26 show, a groove 100 in the handle carries a control assembly 102. The stylet 94 is attached to the control assembly 102, in the manner already described with respect to the control knob 82 in FIGS. 13 and 14. Sliding movement of the control assembly 102 within the groove 100 imparts fore and aft movement to the stylet 94, thereby distending or extending the loop structure 92. The control assembly 102 further includes a cam wheel 104 (see FIG. 26) rotatable about an axle on the control assembly 102 in response to force applied to an external steering lever 108. The cam wheel 104 holds the proximal ends of the steering wires 96(R) and 96(L), in the manner disclosed in Lundquist and Thompson U.S. Pat. No. 5,254,088, already discussed, which is incorporated herein by reference. Twisting the steering lever 108 counterclockwise applies tension to the left steering wire 96(L), bending the loop structure 92 to the left (as FIG. 23a shows). The electrode elements 28 (which in FIGS. 20 to 27 comprises a continuous coil electrode 34, described earlier) likewise bend to the left. Similarly, twisting the steering lever 108 clockwise applies tension to the right steering wire 96(R), bending the loop structure 92 to the right (as FIGS. 23b and 24 show). The electrode elements 28 likewise bend to the right. The bent electrode elements 28, conforming to the bent spline legs 22, assume different curvilinear shapes, depending upon amount of tension applied by the steering wires 96(R) and 96(L). When contacting tissue, the bent electrode elements 28 form long, thin lesions in curvilinear patterns. In an altemative implementation, the stylet 94 is instead made of a malleable metal material, like annealed stainless steel. In this arrangement, before deployment in the body, the physician applies external pressure to manually bend the stylet 94 into a desired shape, thereby imparting a desired curvilinear shape to the electrode elements of the associated loop structure. The malleable material of the stylet 94 retains the preformed shape, until the associated loop structure is withdrawn from the body and sufficient external pressure is again applied by the physician to alter the stylet shape. In addition to having a malleable stylet 94, the splines 22 themselves can also be made of a malleable material, like annealed stainless steel, or untreated stainless steel 17-7, or untreated Nickel Titanium. In one implementation, the most distal parts of the malleable splines 22 are heat treated to maintain their shape and not collapse during introduction and deployment in the vascular system. This will also give the overall structure greater stiffness for better contact with the tissue. It also gives the physician the opportunity to bend the structure to form long, thin, lesions in prescribed curvilinear patterns set by the malleable splines. Whether flexible and remotely flexed during deployment, or malleable and manually flexed before deployment, by further adjusting the fore-and-aft position of the stylet 94, the physician can also control the radial dimensions of the loop structure 94 in concert with controlling the curvilinear shape of the loop structure 92, as FIG. 27 shows. A diverse array of radial sizes and curvilinear shapes is thereby available. As FIG. 28 shows, a half-loop structure 110 can also include a fixed or movable stylet 94 with steering wires 96(R) and 96(L). The use of the same handle-mounted control assembly 102/rotatable cam 104 assembly shown in FIGS. 25 and 26 in association with the half-loop structure 110 makes possible the creation of diverse curvilinear shapes of variable radii. Alternatively, a malleable stylet 94 and malleable splines can be used. 3. Loop Structures with Movable Spline Legs FIGS. 29 to 35 show a full-loop structure 112 in which only one spline leg 114 is attached to the base 26. The fixed spline leg 114 is preformed with resilient memory to assume a curve of a selected maximum radius (shown in FIG. 33). The other spline leg 116, located diametrically opposed to the fixed spline leg 114, extends through the base 26 and catheter body lumen 36 (see FIGS. 30a and 30b) into the handle 18. The spline leg 116 slides fore and aft with respect to the base 26. Movement of the spline leg 116 changes the flexure of the structure 112. The full-loop structure 112 shown in FIGS. 29 to 35 need not include a hub 24 like that shown in FIGS. 1 and 3, and, in addition, need not incorporate a detented integral loop body 42 like that shown in FIG. 2. Any single full-loop structure without a center stiffener or stylet, like the structure 112 in FIG. 29, can comprise a single length of wire bent back upon itself and preformed with resilient memory to form the desired full loop shape. For the same reason, the single full-loop structure 20 shown in FIG. 1 can, in an alternative construction, be made without a hub 24 and a detented loop body 42, and instead employ a preshaped doubled-back wire to form a loop, like the structure 20. FIG. 30b shows an alternative way of securing the fixed spline leg 114 to the distal end 16 of the catheter tube 12, without using a base 26. In this embodiment, the free end of the fixed spline leg 114 lies against the interior of the tube 12. The leg 114 passes through a slit 115 formed in the catheter tube 12. The leg 114 is bent back upon itself in a u-shape to lie against the exterior of the tube 12, wedging the tube 12 within the u-shape bend 117. A sleeve 119 is heat shrunk about the exterior of the tube 12 over the region where the u-shape bend 117 of the spline leg 114 lies, securing it to the tube 12. Alternatively, a metallic ring (not shown) can be used to secure the spline leg 114 to the tube 12. The movable spline leg 116 and wires 58 pass through the interior bore 36 of the catheter tube 12, as before described. The proximal end of the spline leg 116 (see FIG. 29) is attached to a movable control knob 82 carried in a groove 84 on the handle 18, like that shown in FIG. 13. Movement of the control knob 82 within the groove 84 thereby imparts fore-and-aft movement to the spline leg 116. In the illustrated embodiment, the fixed spline leg 114 carries electrode elements 28 in the manner already described. The movable spline leg 116 is free of electrode elements 28. Still, it should be appreciated that the movable spline leg 116 could carry one or more electrode elements 28, too. As FIGS. 31 to 33 show, moving the control knob 82 forward slides the movable spline leg 116 outward, and vice versa. The movable spline leg 116 applies a counter force against the resilient memory of the fixed spline leg 114, changing the flexure and shape of the loop structure 112 for vascular introduction and deployment in contact with tissue. By pulling the movable spline leg 116 inward (as FIG. 31 shows), the counter force contracts the radius of curvature of the fixed spline leg 114 against its resilient memory. Pushing the movable spline leg 116 outward (as FIGS. 32 and 33 show) allows the resilient memory of the fixed spline leg 114 to expand the radius of curvature until the selected maximum radius is achieved. The counter force applied changes the flexure and shapes the fixed spline leg 114 and the electrode elements 28 it carries to establish and maintain more secure, intimate contact against atrial tissue. The magnitude (designated V in FIGS. 31 to 33) of the counter force, and the resulting flexure and shape of the loop structure 112, varies according to extent of outward extension of the movable spline leg 116. Pulling the movable spline leg 116 progressively inward (thereby shortening its exposed length) (as FIG. 31 shows) contracts the loop structure 112, lessening its diameter and directing the counter force progressively toward the distal end of the structure. Pushing the movable spline leg 116 progressively outward (thereby lengthening its exposed length) (as FIGS. 32 and 33 show) progressively expands the loop structure 112 in response to the resilient memory of the fixed spline leg 114, increasing its diameter and directing the counter force progressively away from the distal end of the structure. As FIGS. 34 and 35 show, by manipulating the movable spline leg 116, the physician can adjust the flexure and shape of the loop structure 112 within the atrium 88 from one that fails to make sufficient surface contact between the electrode element 28 and the atrial wall 86 (as FIG. 34 shows) to one that creates an extended region of surface contact with the atrial wall 86 (as FIG. 35 shows). FIGS. 36 to 38 show a full-loop structure 118 in which each spline leg 120 and 122 is independently movable fore and aft with respect to the base 26. In the illustrated embodiment, both spline legs 120 and 122 carry electrode elements 28 in the manner already described. In this arrangement, the handle 18 includes two independently operable, sliding control knobs 124 and 126 (shown diagrammatically in FIGS. 36 to 38), each one attached to a movable spline leg 120/122, to impart independent movement to the spline legs 120/122 (as shown by arrows in FIGS. 36 to 38). Each spline leg 120/122 is preformed with resilient memory to achieve a desired radius of curvature, thereby imparting a resilient curvature or shape to the full-loop structure 118 itself. Coordinated opposed movement of both spline legs 120/122 (as FIGS. 37 and 38 show) using the control knobs 124/126 allows the physician to elongate the curvature of the loop structure 118 into more of an oval shape, compared to more circular loop structures 112 formed using a single movable leg 116, as FIGS. 31 to 33 show. FIGS. 39a and 39b show an alternative full-loop structure 128 having one spline leg 130 that is fixed to the base 26 and another spline leg 132, located diametrically opposed to the fixed spline 130, that is movable fore and aft with respect to the base 26 in the manner already described. The movable spline leg 132 can carry electrode elements 28 (as FIG. 39a shows), or be free of electrode elements, depending upon the preference of the physician. In the structure shown in FIGS. 39a and 39b, the fixed spline leg 130 branches in its midportion to form a smaller, secondary full-loop structure 134 that carries electrode elements 28. In the embodiment shown in FIGS. 39a and 39b, the secondary loop structure 134 lies in a plane that is generally perpendicular to the plane of the main full-loop structure 128. The smaller, secondary full-loop structure 134 makes possible the formation of annular or circumferential lesion patterns encircling, for example, accessory pathways, atrial appendages, and the pulmonary vein within the heart. In the illustrated embodiment, the movable spline leg 132 compresses the secondary full-loop structure 134, urging and maintaining it in intimate contact with the targeted tissue area. FIGS. 39a and 39b therefore show a compound flexible support for electrode elements. While the primary support structure 128 and the secondary support structure 134 are shown as full loops, it should be appreciated that other arcuate or non-arcuate shapes can be incorporated into a compound structure. The compound primary structure 128 integrated with a secondary structure 134 need not include a movable spline leg, or, if desired, both spline legs can be movable. Furthermore, a center stylet to contract and distend the main structure 128 can also be incorporated, with or without a stylet steering mechanism. FIGS. 40a and 40b show a modified full-loop structure 216 having an odd number of spline legs 218, 220, and 222. The structure 216 includes two spline legs 218 and 220 that, in the illustrated embodiment, are fixed to the base 26 about 120.degree. apart from each other. As FIG. 40b shows, the base 26 is generally like that shown in FIG. 6b, with the slotted anchor 63 in which the near ends of the legs 218 and 220 are doubled back and wedged. The structure 216 also includes a third spline leg 222 that, in the illustrated embodiment, is spaced about 120.degree. from the fixed spline legs 218/220. As FIG. 40b shows, the near end of the third spline leg 222 is not attached to the base 26, but passes through the inner lumen 226 into the lumen 36 of the catheter tube 12. The third spline leg 222 is thereby movable fore and aft with respect to the base 26 in the manner already described. Alternatively, all spline legs 218, 220, and 222 can be fixed to the base 26, or more than one spline leg can be made moveable. A hub 24 like that shown in FIGS. 3 and 4 includes circumferentially spaced slots 56 to accommodate the attachment of the three splines 218, 220, and 222. The fixed splines 218 and 220 carry electrode elements 28 (as FIG. 40a shows), while the movable spline 22 is free of electrode elements. As FIG. 40b show, the wires 58 coupled to the electrode elements 28 pass through the anchor lumen 226 for transit through the catheter tube bore 36. The orientation of the fixed splines 218 and 220 relative to the movable spline 222 thereby presents an ablation loop 224, like the secondary loop structure 134 shown in FIGS. 39a/b, that lies in a plane that is generally transverse of the plane of the movable spline 222. Of course, other orientations of an odd number of three or more spline legs can be used. The movable spline leg 222 extends and compresses the secondary structure 134 to urge and maintain it in intimate contact with the targeted tissue area. Of course, a center stylet to further contract and distend the ablation loop 224 can also be incorporated, with or without a stylet steering mechanism. 4. Bifurcated Loop Structures FIGS. 41, 42, and 43 show a variation of a loop structure, which will be called a bifurcated full-loop structure 136. The structure 136 (see FIG. 41) includes two oppositely spaced splines legs 138 and 140, each carrying one or more electrode elements 28. The near end of each spline leg 138/140 is attached to the base 26. The far end of each spline leg 138/140 is attached a stylet 142 and 144. Each spline leg 138/140 is preformed with resilient memory to achieve a desired maximum radius of curvature (which FIG. 41 shows). The spline leg stylets 142/144 are joined through a junction 146 to a common control stylet 148. The common control stylet 148 passes through the catheter body lumen 36 to a suitable slidable control knob 150 in the handle 18, as already described. By sliding, the control knob 150 moves the control stylet 148 to change the flexure of the spline legs 138/140. When the control stylet 148 is fully withdrawn, as FIG. 41 shows, the junction 146 is located near the base 26 of the structure 136, and the spline legs 138/140 assume their preformed maximum radii of curvatures. The spline legs 138/140 form individual half-loop structures (like shown in FIG. 7) that together emulate a full-loop structure (like that shown in FIG. 1), except for the presence of a connecting, distal hub 24. Forward movement of the control stylet 148 first moves the junction 146 within the confines of the structure 136, as FIG. 42 shows. The forward movement of the control stylet 148 is translated by the spline leg stylets 142/144 to urge the spline legs 138/140 apart. The distal end of the bifurcated structure 136 opens like a clam shell. As the spline legs 138/140 separate, they distend. The control stylet 150 thus compresses the splines legs 138/140 to press them into contact with the tissue area along opposite sides of the structure 136. In this way, the bifurcated structure 136 emulates the full-loop structure 78, when distended (as FIG. 16 shows). Continued forward movement of the control stylet 150 (as FIG. 43 shows) moves the junction 146 and attached spline leg stylets 142/146 out beyond the confines of the structure 136. This continued forward movement extends the spline legs 136/140, while moving them radially inward. This, in effect, collapses the bifurcated structure 136 into a relatively low profile configuration for vascular introduction. In this way, the bifurcated structure 136 emulates the full-loop structure 78, when elongated (as FIG. 15 shows). FIGS. 44 and 45 show an alternative embodiment of a bifurcated full-loop structure 152. The structure 152 includes two oppositely spaced spline legs 154/156, each carrying one or more electrode elements 28, like the structure 136 shown in FIGS. 41 to 43. Each spline leg 154/156 is preformed with a resilient memory to assume a desired maximum radius of curvature (which FIG. 44 shows). Unlike the structure 136 shown in FIGS. 41 to 43, the structure 152 shown in FIGS. 44 and 45 fixes both ends of the spline legs 154/156 to the base 26. The spline legs 154/156 thereby form stationary, side-by-side half-loop structures, each with an inner portion 158 and an outer portion 160. Together, the stationary half-loop structures create the bifurcated full-loop structure 152. In this arrangement, a center stylet 162 is attached to a ring 164 that commonly encircles the inner portions 158 of the spline legs 154/156 along the center of the structure 152. Movement of the stylet 162 slides the ring 164 along the inner leg portions 158. The stylet 162 passes through the catheter body lumen 36 to a suitable control in the handle (not shown), as already described. Forward movement of the ring 164 (as FIG. 45 shows) jointly extends the spline legs 154/156, creating a low profile for vascular introduction. Rearward movement of the ring 164 (as FIG. 44 shows) allows the resilient memory of the preformed spline legs 154/156 to bow the legs 154/156 outward into the desired loop shape. FIG. 46 shows another alternative embodiment of a bifurcated full-loop structure 166. This structure 166 has two oppositely spaced spline legs 168 and 170, each carrying one or more electrode elements 28. Each spline leg 168/170 is preformed with a resilient memory to assume a maximum radius of curvature (which FIG. 46 shows). The near end of each spline leg 168/170 is attached to the base 26. The far end of each spline leg 168/170 is individually attached to its own stylet 172/174. Instead of joining a common junction (as in the structure 136 shown in FIGS. 41 to 43), the spline stylets 172/174 of the structure 166 individually pass through the catheter body lumen 36 to suitable control knobs (not shown) in the handle 18. Like the embodiment shown in FIGS. 44 and 45, a third stylet 176 is attached to a ring 178 that encircles the spline stylets 172 and 174. The third stylet 176 passes through the guide tube lumen 36 to its own suitable control knob (not shown) in the handle 18. The embodiment shown in FIG. 46 allows the physician to move the ring 178 up and down along the spline stylets 172 and 174 to shape and change the flexure of the structure 166 in the manner shown in FIGS. 44 and 45. Independent of this, the physician can also individually move the spline stylets 172 and 174 to further shape and change the flexure of each spline leg 168 and 170, as in the case of the movable spline legs 120/122 shown in FIGS. 36 to 38. This structure 166 thus gives the physician latitude in shaping the loop structure to achieve the desired contact with the atrial wall. Another alternative embodiment of a bifurcated full-loop structure 180 is shown in FIGS. 47 to 49. In this embodiment, the structure 180 includes two oppositely spaced spline legs 182 and 184, each carrying one or more electrode elements 28. Each spline leg 182/184 is preformed with a resilient memory to assume a desired maximum radius of curvature (which FIG. 49 shows). The inner portion 186 of each spline leg 182/184 is attached to the base 26. A stationary ring 190 encircles the inner portions 186 near the distal end of the structure 180, holding them together. The outer portion 188 of each spline leg 182/184 is free of attachment to the base 26 and is resiliently biased away from the base 26. Each outer portion 188 is individually attached to its own stylet 192 and 194. The spline stylets 192 and 194 individually pass through the catheter body lumen 36 to suitable control knobs (not shown) in the handle 18. Pulling the spline legs stylets 192/194 rearward pulls the outer portion 188 of the attached spline leg 182/184 radially toward the base 26, against their resilient memories, creating a low profile suitable for vascular access (as FIG. 47 shows). Pushing the spline stylets 192/194 forward pushes the outer portion 188 of the attached spline leg 182/184, aided by the resilient memory of the spline leg 182/184, outward (as FIGS. 48 and 49 show). The spline stylets 192/194 can be manipulated together or individually to achieve the shape and flexure desired. 5. Loop Support Structures for Movable Electrodes FIGS. 50 and 51 show a full-loop structure 196 which supports a movable ablation element 198. The structure 196 includes a pair of spline legs 200 secured at their distal ends to the hub 24 and at their proximal ends to the base 26, in the manner described in association with the structure shown in FIG. 1. A center stiffener 202 extends between the base 26 and the hub 24 to lend further strength. The ablation element 198 (see FIG. 52) comprises a core body 204 made of an electrically insulating material. The body 204 includes a central lumen 206, through which one of the spline legs 200 passes. The core body 204 slides along the spline leg 200 (as shown by arrows in FIGS. 50 to 52). In the illustrated and preferred embodiment (see FIG. 52), a coil electrode element 34 (as already described) is wound about the core body 204. Alternatively, the core body 204 can be coated with an electrically conducting material or have an electrically conducting metal band fastened to ft. As shown in FIG. 53, the ablation element can also comprise a composite structure 198(1) (see FIG. 53) of two bi-polar electrodes 208 separated by an electrically insulating material 210. The core body 204 of the electrode can range in diameter from 3 Fr to 8 Fr and in length from 3 mm to 10 mm. A guide sire 212 is attached to at least one end of the ablation electrode 198 (see FIGS. 50 and 52). The guide wire 212 extends from the handle 18 through the catheter body lumen 36, along the center stiffener 202 and through the hub 24 for attachment to the ablation element 198. A signal wire 214 also extends in common along the guide wire 212 (see FIG. 52) to supply ablation energy to the electrode 198. The proximal end of the guide wire 212 is attached to a suitable control knob (not shown) in the handle 18. Movement of the guide wire 212 forward pushes the ablation element 198 along the spline leg 200 from the distal end of the structure 196 to the proximal end. Two guide wires (212 and 213) may be used (as FIG. 52 shows), which are attached to opposite ends of the ablation element 198. Pulling on one guide wire 212 advances the electrode 198 toward the distal end of the structure 196, while pulling on the other guide wire 213 advances the electrode 198 in the opposite direction toward the proximal end of the structure 196. In an alternative implementation (not shown), the distal tip of a second catheter body can be detachably coupled either magnetically or mechanically to the movable electrode 198. In this implementation, the physician manipulates the distal end of the second catheter body into attachment with the electrode 198, and then uses the second catheter body to drag the electrode 198 along the structure 196. In use (as FIG. 54 shows), once satisfactory contact has been established with the atrial wall 86, sliding the ablation electrode 198 along the spline leg 200 while applying ablation energy creates a long and thin lesion pattern. The ablation can be accomplished by either moving the electrode 198 sequentially to closely spaced locations and making a single lesion at each location, or by making one continuous lesion by dragging the electrode 198 along the tissue while ablating. One or both spline legs 200 can also be movable with respect to the base, as before described, to assure intimate contact between the ablation element 198 and the endocardium. 6. Bundled Loop Structures The assembly of bundled, independently adjustable loop structures to form a dynamic three dimensional electrode support structure 228, like that shown in FIGS. 55 to 58, are also possible. The structure 228 shown in FIGS. 55 to 58 comprises four spline legs (designated L1, L2, L3, and L4) circumferentially spaced ninety degrees apart. Each spline leg L1, L2, L3, and L4 is generally like that shown in FIG. 29. Each leg L1, L2, L3, and L4 is preformed with resilient memory to assume a curve of selected maximum radius. In the illustrated embodiment, each leg L1 to L4 carries at least one electrode element 28, although one or more of the legs L1 to L4 could be free of electrode elements 28. The outer portions 230 of each spline leg L1 to L4 are attached to the structure base 26. As FIG. 61 shows, the base 26 is similar to that shown in FIG. 30b, having an outer ring 236 and a concentric slotted inner element 238, through which the near ends of the outer spline leg portions 230 extend. The near ends are doubled back upon themselves and wedged in the space 240 between the outer ring 236 and inner element 238, as earlier shown in FIG. 6b. The inner portions 232 of each spline leg L1, L2, L3, and L4 are not attached to the base 26. They pass through lumens 242 in the inner element 238 of the base 26 (see FIG. 61) and into catheter body lumen 36 for individual attachment to control knobs 234 on the handle 18 (see FIG. 55). Wires 58 associated with the electrode elements 28 carried by each leg L1 to L4 pass throug |