Abstract:
A probe including an elongate body defining a distal region adapted to be bent into a loop and an inflatable tissue coagulation body supported on the elongate body distal region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of co-pending U.S. application Ser. No. 09/730,010, filed Dec. 4, 2000. 
    
    
     BACKGROUND OF THE INVENTIONS 
     1. Field of Inventions 
     The present inventions relate generally to medical devices that support therapeutic elements in contact with body tissue and, more particularly, to loop structures that support therapeutic elements in contact with bodily tissue. 
     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. 
     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. 
     Maze-like procedures have also been developed utilizing catheters which can form lesions on the endocardium (the lesions being 1 to 15 cm in length and of varying shape) to effectively create a maze for electrical conduction in a predetermined path. The formation of these lesions by soft tissue coagulation (also referred to as “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. 
     Catheters used to create lesions typically include a relatively long and relatively flexible body portion that has a soft tissue coagulation electrode on its distal end and/or a series of spaced tissue coagulation electrodes near the 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 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 coagulation 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. 
     In some instances, the proximal end of the catheter body is connected to a handle that includes steering controls. Exemplary catheters of this type are disclosed in U.S. Pat. No. 5,582,609. In other instances, the catheter body is inserted into the patient through a sheath and the distal portion of the catheter is bent into loop that extends outwardly from the sheath. This may be accomplished by pivotably securing the distal end of the catheter to the distal end of the sheath, as is illustrated U.S. Pat. No. 6,071,279. The loop is formed as the catheter is pushed in the distal direction. The loop may also be formed by securing a pull wire to the distal end of the catheter that extends back through the sheath, as is illustrated in U.S. Pat. No. 6,048,329. Loop catheters are advantageous in that they tend to conform to different tissue contours and geometries and provide intimate contact between the spaced tissue coagulation electrodes (or other diagnostic or therapeutic elements) and the tissue. 
     One lesion that has proven to be difficult to form with conventional catheters are lesions that are used to isolate the pulmonary vein and cure ectopic atrial fibrillation. Lesions may be formed within the pulmonary vein itself or in the tissue surrounding the pulmonary vein (the “pulmonary vein ostium”) to isolate the pulmonary vein. Conventional steerable catheters and loop catheters have proven to be less than effective with respect to the formation of such circumferential lesions. For example, it can be difficult to achieve the level of tissue contact required to form an effective lesion. 
     A variety of inflatable lesion formation structures have also been proposed. Depending on their size, such structures are typically inflated within the pulmonary vein or inflated and then advanced into contact with the pulmonary vein ostium. Such devices tend to establish better tissue contact than conventional steerable catheters and loop catheters. However, they also act as a plug and occlude the flow of blood through the pulmonary vein. In addition, the inventors herein have determined that it can be difficult to manufacture a balloon that will be large enough when inflated to engage the entire circumference of the pulmonary vein ostium, and small enough when deflated to be advanced through a sheath to the ostium. 
     Accordingly, the inventors herein have determined that a need exists for structures that can be used to create effective lesions within or around the pulmonary vein without occluding blood flow. 
     SUMMARY OF THE INVENTION 
     Accordingly, the general object of the present inventions is to provide a device that avoids, for practical purposes, the aforementioned problems. In particular, one object of the present inventions is to provide a device that can be used to create lesions in or around the pulmonary vein and other bodily orifices in a more effective manner than conventional apparatus. Another object of the present invention is to provide a device that can be used to create lesions in or around the pulmonary vein and other bodily orifices with occluding the flow of blood or other body fluids. Still another object of the present invention is to provide a device including an inflatable structure that can be used to create lesions in or around the pulmonary vein and is also small enough when deflated to be advanced through a sheath. 
     In order to accomplish some of these and other objectives, a probe in accordance with one embodiment of a present invention includes an elongate body defining a distal region adapted to be bent into a loop and an inflatable tissue coagulation body supported on the elongate body distal region. In a preferred implementation, the inflatable tissue coagulation body will be a half-balloon structure located on one side of the loop and spaced inwardly from the apex (or distal end) of the loop. 
     The present probe provides a number of advantages over conventional catheter devices. For example, the present probe provides superior tissue contact at the pulmonary vein ostium. The apex (or distal end) of the loop in the present probe may be inserted into the pulmonary vein to such an extent that the inflatable tissue coagulation body will be adjacent to the pulmonary vein ostium. Such positioning will wedge the inflatable tissue coagulation body into close contact with the ostium and, when the tissue coagulation body is inflated, the level of contact will be increased. After the lesion has been formed, the tissue coagulation body may be deflated and the loop repositioned so that another lesion can be formed. This process will continue until a continuous circumferential lesion has been formed around the ostium. 
     When the apex of the present probe is inserted into the pulmonary vein ostium, the present probe defines an open region that allows blood to pass therethrough. As a result, the present probe facilitates the formation of a circumferential lesion without the occlusion of blood associated with conventional inflatable lesion formation structures. 
     The above described and many other features and attendant advantages of the present inventions will become apparent as the inventions become 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 inventions will be made with reference to the accompanying drawings. 
         FIG. 1  is a side view of a catheter-based probe in accordance with a preferred embodiment of a present invention. 
         FIG. 2  is an enlarged view of the distal portion of the probe illustrated in  FIG. 1 . 
         FIG. 3  is an end view of the distal portion of the probe illustrated in  FIG. 1 . 
         FIG. 4  is a top view of the distal portion of the probe illustrated in  FIG. 1 . 
         FIG. 5  is a top cutaway view of the distal portion of the probe illustrated in  FIG. 1 . 
         FIG. 6  is a section view taken along line  6 — 6  in  FIG. 4 . 
         FIG. 7  is a section view in accordance with another preferred embodiment of a present invention. 
         FIG. 8  is a side view of an expandable tissue heating structure in accordance with another preferred embodiment of a present invention. 
         FIG. 9  is a side view of the distal portion of a catheter-based probe in accordance with another preferred embodiment of a present invention. 
         FIG. 10  is a section view taken along line  10 — 10  in  FIG. 9 . 
         FIG. 11  is a section view in accordance with another preferred embodiment of a present invention. 
         FIG. 12  is a partial section view of the tip region of the probe illustrated in  FIG. 1 . 
         FIG. 13  is a partial section view of the hinge region of the probe illustrated in  FIG. 1 . 
         FIG. 14  is a side view of the distal portion of a catheter-based probe in accordance with another preferred embodiment of a present invention. 
         FIG. 15  is a side view of the tip region of a catheter-based probe in accordance with yet another preferred embodiment of a present invention. 
         FIG. 16   a  is a side view showing the probe illustrated in  FIG. 1  positioned adjacent the pulmonary vein ostium with the expandable electrode in a collapsed state. 
         FIG. 16   b  is a side view showing the probe illustrated in  FIG. 1  positioned adjacent the pulmonary vein ostium with the expandable electrode in an expanded state. 
     
    
    
     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. Introduction   II. Exemplary Catheter and Expandable Lesion Formation Device Structures   III. Exemplary Loop Formation Devices   IV. Methods of Use       

     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 inventions. 
     I. Introduction 
     The present inventions may be used within body lumens, chambers or cavities for diagnostic or therapeutic purposes in those instance where access to interior bodily regions is obtained through, for example, the vascular system or alimentary canal and without complex invasive surgical procedures. For example, the inventions herein have application in the diagnosis and treatment of arrhythmia conditions within the heart. The inventions herein also have application in the diagnosis or treatment of ailments of the gastrointestinal tract, prostrate, brain, gall bladder, uterus, and other regions of the body. 
     With regard to the treatment of conditions within the heart, the present inventions are designed to produce intimate tissue contact with target substrates associated with various arrhythmias, namely atrial fibrillation, atrial flutter, and ventricular tachycardia. For example, the distal portion of a catheter in accordance with a present invention, which may include diagnostic and/or soft tissue coagulation electrodes, can be used to create lesions within or around the pulmonary vein to treat ectopic atrial fibrillation. 
     The structures are also adaptable for use with probes other than catheter-based probes. For example, the structures disclosed herein may be used in conjunction with hand held surgical devices (or “surgical probes”). The distal end of a surgical probe may be placed directly in contact with the targeted tissue area by a physician during a surgical procedure, such as open heart surgery. Here, access may be obtained by way of a thoracotomy, median sternotomy, or thoracostomy. Exemplary surgical probes are disclosed in U.S. Pat. No. 6,071,281, which is incorporated herein by reference. 
     Surgical probe devices in accordance with the present inventions preferably include a handle, a relatively short shaft, and one of the distal assemblies described hereafter in the catheter context. Preferably, the length of the shaft is about 4 inches to about 18 inches. This is relatively short in comparison to the portion of a catheter body that is inserted into the patient (typically from 23 to 55 inches in length) and the additional body portion that remains outside the patient. The shaft is also relatively stiff. In other words, the shaft is either rigid, malleable, or somewhat flexible. A rigid shaft cannot be bent. A malleable shaft is a shaft that can be readily bent by the physician to a desired shape, without springing back when released, so that it will remain in that shape during the surgical procedure. Thus, the stiffness of a malleable shaft must be low enough to allow the shaft to be bent, but high enough to resist bending when the forces associated with a surgical procedure are applied to the shaft. A somewhat flexible shaft will bend and spring back when released. However, the force required to bend the shaft must be substantial. 
     II. Exemplary Catheter and Expandable Lesion Formation Device Structures 
     As illustrated for example in  FIGS. 1–6 , a catheter  10  in accordance with a preferred embodiment of a present invention includes a flexible catheter body  12  that may be formed from a biocompatible thermoplastic material such as braided or unbraided Pebax® (polyether block emide), polyethylene, or polyurethane, and is preferably about 5 French to about 9 French in diameter. Preferably, the catheter body  12  will have a two part construction consisting of a relatively long less flexible proximal member  14  (formed from braided Pebax®) and a relatively short flexible distal member  16  (formed from unbraided Pebax®). The proximal and distal members  14  and  16  may be bonded together with an overlapping thermal bond or adhesive bonded together end to end over a sleeve in what is referred to as a “butt bond.” The proximal end of the catheter body  12  is secured to a handle  18 . An expandable (and collapsible) coagulation body is mounted on the proximal member  14 . As described below, the expandable coagulation body may be either an expandable porous electrode structure  20  (as shown in  FIGS. 1–5 ) or an expandable structure  58  that can be heated to a predetermined temperature (as shown in  FIG. 8 ). 
     The exemplary expandable porous electrode  20 , which is formed from an electrically non-conductive thermoplastic or elastomeric material, includes a porous region  22  having micropores  24  and a non-porous region  26 . Liquid pressure is used to inflate the expandable porous electrode  20  and maintain it in its expanded state. The liquid used to fill the expandable porous electrode  20  is an electrically conductive liquid that establishes an electrically conductive path to convey RF energy from the electrode  20  to tissue. 
     Referring more specifically to  FIGS. 5 and 6 , the conductive liquid is supplied under pressure to the expandable porous electrode  20  by way of an infusion/ventilation port  28  in the handle  18 , an inlet lumen  30  that is connected to the port, and an aperture  32  formed in the catheter body  12 . The inlet lumen extends  30  from the port  28  to the aperture  32 . The pressure should be relatively low (less than 5 psi) and will vary in accordance with the desired level of inflation, strength of materials used and the desired degree of body flexibility. The expandable porous electrode  20  will then expand from its collapsed, low profile state (between about 2 mm and about 4 mm in diameter) to its expanded state (between about 5 mm and about 15 mm in diameter). [Use of the term “diameter” is not intended to imply that the electrode  20  is necessarily circular in cross-section.] The liquid is removed by way of the aperture  34 , which is connected to an outlet lumen  36  that is also connected to the infusion/ventilation port  28 . Preferably, a vacuum force is applied to the outlet lumen  36  at the infusion/ventilation port  28  to remove the liquid. 
     Alternatively, as illustrated for example in  FIG. 7 , the proximal member  14  may replaced by a proximal member  38  that is a multi-lumen extrusion with lumens  40 ,  42  and  44 . Lumen  40 , which is connected to the infusion/ventilation port  28  in the handle  18 , functions as both an inlet lumen and an outlet. The aperture  32  ( FIG. 5 ) is connected to the lumen  40  and aperture  34  may be eliminated. The distal end of the lumen  40  is sealed with an appropriate plug (not shown). Liquid may be supplied to the lumen  40  under pressure and withdrawn by applying a vacuum force at the infusion/ventilation port  28 . 
     It is preferable, although not required, that the expandable porous electrode  20  be placed proximally of the loop apex  76  (discussed below) regardless of the type of fluid lumen employed because the lumens could be pinched, and flow disrupted, at the apex. 
     Although the shape may be varied to suit particular applications, the preferred geometry of the expandable porous electrode  20  is essentially that of an elongate half-balloon. In other words, when expanded, the electrode  20  extends radially outwardly away from the loop  62  (discussed below) in the manner illustrated for example in  FIGS. 1–3 . The length of the exemplary electrode  20  is about 1.0 cm, but may range from 0.5 cm to 2.0 cm, while the width is about 5 mm, but may range from 3 mm to 15 mm. The length of the exemplary porous region  22  is about 3 mm, but may range from 2 mm to 5 mm, while the width is about 1.5 mm, but may range from 0.5 mm to 3 mm. This porous electrode configuration is especially useful for forming relatively deep lesions in the entrance to the pulmonary vein. 
     As illustrated for example in  FIG. 5 , an electrode  46  formed from material with both relatively high electrical conductivity and relatively high thermal conductivity is carried within the expandable porous electrode  20 . Suitable materials include gold, platinum, and platinum/iridium. Noble metals are preferred. The pores  24  establish ionic transport of the tissue coagulating energy from the electrode  46  through the electrically conductive fluid to tissue outside the porous electrode  20 . The liquid preferably possesses a low resistivity to decrease ohmic loses, and thus ohmic heating effects, within the porous electrode  20 . The composition of the electrically conductive liquid can vary. A hypertonic saline solution, having a sodium chloride concentration at or near saturation, which is about 20% weight by volume is preferred. Hypertonic saline solution has a low resistivity of only about 5 ohm∘cm, compared to blood resistivity of about 150 ohm∘cm and myocardial tissue resistivity of about 500 ohm∘cm. Alternatively, the fluid can be a hypertonic potassium chloride solution. This medium, while promoting the desired ionic transfer, requires closer monitoring of the rate at which ionic transport occurs through the pores  24 , to prevent potassium overload. When hypertonic potassium chloride solution is used, it is preferred keep the ionic transport rate below about 1 mEq/min. 
     Ionic contrast solution, which has an inherently low resistivity, can be mixed with the hypertonic sodium or potassium chloride solution. The mixture enables radiographic identification of the porous electrode  20  without diminishing the ionic transfer through the pores  24 . 
     Due largely to mass concentration differentials across the pores  24 , ions in the conductive fluid will pass into the pores because of concentration differential-driven diffusion. Ion diffusion through the pores  24  will continue as long as a concentration gradient is maintained across the porous electrode  20 . The ions contained in the pores  24  provide the means to conduct current across the porous electrode  20 . When RF energy is conveyed from a RF power supply and control apparatus to the electrode  46 , electric current is carried by the ions within the pores  24 . The RF currents provided by the ions result in no net diffusion of ions, as would occur if a DC voltage were applied, although the ions do move slightly back and forth during the RF frequency application. This ionic movement (and current flow) in response to the applied RF field does not require perfusion of liquid through the pores  24 . The ions convey RF energy through the pores  24  into tissue to a return electrode, which is typically an external patch electrode (forming a unipolar arrangement). Alternatively, the transmitted energy can pass through tissue to an adjacent electrode (forming a bipolar arrangement). The RF energy heats tissue (mostly ohmically) to coagulate the tissue and form a lesion. 
     The temperature of the fluid is preferably monitored for power control purposes. To that end, a temperature sensing element, such as the illustrated thermocouple  48 , may mounted on the catheter body  12  within the expandable porous electrode  20 . A reference thermocouple  50  ( FIG. 2 ) may be positioned on the distal member  16 . Alternatively, a thermistor or other temperature sensing element within the electrode  20  may be used in place of the thermocouple and reference thermocouple arrangement. Referring to  FIGS. 1 and 6 , the electrode  46 , thermocouple  48  and reference thermocouple  50  are respectively connected to an electrical connector  52  by electrical conductors  54  which extend through the catheter body. The catheter  10  may be connected to a suitable RF power supply and control apparatus by a connector  56 . Additional information concerning controllers which control power to electrodes based on a sensed temperature is disclosed in U.S. Pat. Nos. 5,456,682, 5,582,609 and 5,755,715. 
     With respect to materials, the porous region  22  of the expandable porous electrode  20  is preferably formed from regenerated cellulose or a microporous elastic polymer. Materials such as nylons (with a softening temperature above 100° C.), PTFE, PEI and PEEK that have micropores created through the use of lasers, electrostatic discharge, ion beam bombardment or other processes may also be used. Such materials would preferably include a hydrophilic coating. The micropores should be about 1 to 5 μm in diameter and occupy about 1% of the surface area of the porous region  22 . A slightly larger pore diameter may be employed. Because the larger pore diameter would result in significant fluid transfer through the porous region, a saline solution having a sodium chloride concentration of about 0.9% weight by volume is preferred. 
     The non-porous regions are preferably formed from relatively elastic materials such as silicone and polyisoprene. However, other less elastic materials, such as Nylon®, Pebax®, polyethylene, polyesterurethane and polyester, may also be used. Here, the expandable porous electrode  20  may be provided with creased regions that facilitate the collapse of the porous electrode. A hydrophilic coating may be applied to the non-porous regions to facilitate movement of the porous electrode  20  in to and out of a sheath. 
     Additional information and examples of expandable and collapsible bodies are disclosed in U.S. patent application Ser. No. 08/984,414, entitled “Devices and Methods for Creating Lesions in Endocardial and Surrounding Tissue to Isolate Arrhythmia Substrates,” U.S. Pat. No. 5,368,591, and U.S. Pat. No. 5,961,513, each of which is incorporated herein by reference. 
     Turning to  FIG. 8 , catheters in accordance with other embodiments of the present inventions may include a heated expandable (and collapsible) coagulation body  58  in place of the porous electrode  20 . The exemplary coagulation body  58 , which is bonded to and disposed around the proximal member  14 , can be inflated with water, hypertonic saline solution, or other biocompatible fluids. The fluid may be supplied under pressure to the coagulation body  58 , and withdrawn therefrom, through the infusion/ventilation port  28  in either of the manner described above with reference to  FIGS. 6 and 7 . Here too, the pressure should be relatively low (less than 5 psi) and will vary in accordance with the desired level of inflation, strength of materials used and the desired degree of body flexibility. 
     A fluid heating element is located within the expandable coagulation body  58 . The fluid heating element is preferably an electrode (not shown) that may be formed from metals such as platinum, gold and stainless steel and mounted on the catheter body within the coagulation body  58 . A bi-polar pair of electrodes may, alternatively, be used to transmit power through a conductive fluid, such as isotonic saline solution, to generate heat. The temperature of the fluid may be heated to about 90° C., thereby raising the temperature of the exterior of the expandable coagulation body  58  to approximately the same temperature for tissue coagulation. It should be noted, however, that the expandable coagulation body  58  tends to produce relatively superficial lesions. 
     Suitable materials for the exemplary expandable coagulation body  58  include relatively elastic thermally conductive biocompatible materials such as silicone and polyisoprene. Other less elastic materials, such as Nylon®, Pebax®, polyethylene and polyester, may also be used. Here, the expandable coagulation body will have to be formed with fold lines. Temperature sensing elements, such as the exemplary thermocouple  48  and reference thermocouple  50  illustrated in  FIGS. 2 and 5 , may also be provided. The heating electrode, thermocouple and reference thermocouple will be connected to the electrical connector  52  by electrical conductors which extend through the catheter body  12  in the manner described above. 
     Another exemplary embodiment, which is generally represented by reference numeral  59 , is illustrated in  FIGS. 9 and 10 . The catheter  59  includes a pair of expandable porous electrode structures  20  that are either substantially identical to, or slight variations of, one another. The electrode structures  20  may be inflated together or separately. The expandable coagulation body  58  ( FIG. 8 ) may be used in place of one or more of the electrode structures  20 . With respect to infusion and ventilation, catheter  59  includes respective pairs of inlet lumens  30  and outlet lumens  36  that operate in the manner described above with reference to  FIGS. 5 and 6 . 
     Infusion and ventilation may, alternatively, be accomplished through the use of the proximal member  38 ′ illustrated for example in  FIG. 11 . The proximal member  38 ′ includes a pair of fluid lumens  40 ′ that each function as an inlet and an outlet for a respective electrode structure  20 . One of the lumens  40 ′ is plugged to prevent fluid from flowing beyond the distal end of the proximal member  38 ′. A similarly configured distal member (not shown), with a single fluid lumen that is connected to the other lumen  40 ′, is connected to the proximal member  38 ′. The distal end of this fluid lumen is also plugged. Liquid will be supplied to and withdrawn from each of the electrode structures  20  by way of the lumens in the manner described above with reference to  FIG. 7 . 
     In another exemplary embodiment, the catheter  59  may be configured such that the proximal electrode structure  20  is infused and ventilated in one of the manners described above with reference to  FIGS. 6 and 7 . The distal electrode structure  20 , on the other hand, will be infused and ventilated through the use of the infusion/ventilation pull device  67  that is described below with reference to  FIG. 15 . 
     III. Exemplary Loop Formation Devices 
     As illustrated for example in  FIGS. 1 and 2 , the exemplary catheter  10  may be used in conjunction with a sheath  60  and configured such that the distal region of the catheter may be deployed in a loop  62 . The sheath  60 , which preferably has a greater inherent stiffness than the portion of the catheter body  12  that forms the loop  62 , should be lubricious to reduce friction during movement of the catheter body. A handle  64 , which is a Toughy Borst connector in the illustrated embodiment that can be used to fix the relative positions of the catheter body  12  and sheath  60 , is mounted on the proximal end of the sheath. With respect to materials, the exemplary sheath  60  is a Pebax® and stainless steel braid composite. Other materials, such as polytetrafluoroethylene (PTFE), can also be used. The wall thickness of the sheath  60  is preferably about 0.013 inch, which will not add significantly to the overall thickness of the catheter body  12 . Also, although the distal end of the sheath  60  is perpendicular to the longitudinal axis of the sheath in the exemplary embodiment, the distal end may also be cut at an angle and tapered in a transverse direction relative to the longitudinal axis of the sheath to effect the shape of the loop  62 . 
     A pull wire  66  extends from the distal end of the catheter body  12  and through the sheath  60 . The proximal end of the pull wire  66  includes an adjustable stop/handle  68 . The pull wire  66  is preferably a flexible, inert cable constructed from strands of metal wire material, such as Nickel Titanium (commercially available under the trade name Nitinol®) or 17-7 stainless steel, that is about 0.012 to 0.018 inch in diameter. Alternatively, the pull wire  66  may be formed from a flexible, inert stranded or molded plastic material. The pull wire  66  is also preferably round in cross-section, although other cross-sectional configurations can be used. 
     Holding the pull wire  66  and handle  68  stationary, the physician deploys the loop  62  by advancing the catheter body  12  through the sheath  60 . Once the loop  62  has been formed, the physician can pull on the pull wire  66  to decrease the exposed length of the pull wire beyond the distal end of the sheath  60 . Further adjustments to the loop  62  may be made by advancing or retracting the catheter body  12  within the sheath  60  or by putting tension on the pull wire  66 . The loop structure  60  may also be rotated by rotating the catheter body  12  with the handle  18 . 
     As illustrated in  FIGS. 12 and 13 , the exemplary catheter body  12  includes a flexible spline (or “core wire”)  70 . The flexible spline  70  is preferably a wire having a diameter of approximately 0.023 inch that is positioned inside of and passes within the length of the catheter body  12 . The flexible spline  70  is fixedly secured to the handle  18  at the proximal end of the catheter body  12  and to a tip member  72  in the manner described below. The tip member  72  is in turn secured to the distal end of the catheter body  12  with adhesive. In the preferred embodiment, the flexible spline  70  is made from resilient, inert wire, such as Nitinol® material or 17-7 stainless steel. Resilient injection molded plastic can also be used. The exemplary spline  70  is round in cross section, although other cross sectional configurations can be used. The flexible spline  70  may, if desired, also have a preset curvature accomplished by thermally presetting the spline at 500° C. for 8 minutes. The super-elastic properties of the material should, however, be maintained. 
     The flexible spline  70  includes a flattened portion  74  that is located within the portion of the distal member  16  that forms the apex  76  of the loop  62 . The flattened portion  74  acts as a hinge and allows the portion of the catheter body  12  distal to the flattened portion to be bent back into a loop with less force than would otherwise be required. The flattened portion  74  also causes the distal member  16  to bend, and the loop  62  to be formed, in a flat loop plane. The placement of the flattened portion  74  in the area that will form the apex  76  of the loop  62  also results in a much sharper bend at the apex, and a more compact loop, than would be obtained otherwise. Specifically, conventional loops often have a flattened portion near the proximal end of the loop and tend to assume a generally circular shape when deployed, while the present loop  62  in the exemplary embodiment has a generally flat, elliptical shape resulting from the location of the flattened portion  74 . 
     The flattened portion  74  should also be thinner than the flattened portion in a conventional loop catheter. In an embodiment where the flexible spline  70  has a diameter of approximately 0.023 inch, the thickness of the flattened portion  74  would be about 0.008 inch, as compared to about 0.018 inch in a conventional catheter. It should be noted, however, the flattened portion should be heat treated at least three times during the flattening process in order to insure the requisite strength. The length of the exemplary flattened portion  74  is about 1.0 inch, but may range from 0.5 inch to 2.0 inches. 
     Preferably, the materials and configurations selected for the catheter body proximal member  14  and distal member  16 , as well as the and the spline  70  and flattened portion  74 , will produce a relatively flat elliptical loop  62  that can be inserted into a pulmonary vein in the manner illustrated in  FIGS. 16   a  and  16   b . In a preferred embodiment, the length (measured along the longitudinal axis of the catheter  10 ) will be about 3.0 cm, but may range from 2.0 cm to 4 cm, while the height (measured transverse to the longitudinal axis) will be about 20 mm, but may range from 15 mm to 30 mm. 
     It should also be noted that, in the exemplary embodiment, the intersection of the proximal and distal members  14  and  16  is located proximal of the flattened portion  74 . This configuration provides at least two advantages. For example, locating the porous electrode  20  on the less flexible proximal member  14  (which also has better torque transmission properties) makes it easier to position the electrode during lesion formation procedures. Additionally, locating the flattened portion  74  within the relatively flexible distal member  16  improves bending at the apex  76  and makes it easier to pull the loop  62  into a compact orientation. 
     The flexible spline  70  may also be used to anchor the pull wire  66 . As illustrated for example in  FIG. 12 , the distal end  78  of the flexible spline  70  is fixedly engaged in an in-line manner to the end  80  of the pull wire  66  with a stainless steel crimp tube  82 . The in-line connection of the flexible spline  70  and pull wire  66  allows for a reduction in the overall diameter of distal portion of the catheter body  12 . This provides a significant clinical advantage over devices having side by side pull wire connections which create a larger diameter device. The pull wire  66  passes through a pull wire bore  84  in the catheter tip member  72  and through a bore  86  in the distal end of the crimp tube  82 . 
     The tip member  72  is preferably formed from platinum and is fixedly engaged with, for example, silver solder, adhesive or spot welding, to the distal end of crimp tube  82 . The flexible spline  70  is preferably electrically insulated with a thin walled polyester heat shrink tube  88  that extends beyond the proximal end of the crimp tube  82 . Other pull wire configurations, other methods of attaching the pull wire to the catheter body, and methods of reducing stress on the pull wire are disclosed in U.S. Pat. No. 6,048,329, which is incorporated herein by reference. 
     It should be noted that the present inventions are also applicable to loop catheters in which the distal portion of catheter body is connected to the distal portion of the sheath. As illustrated for example in  FIG. 14 , the exemplary catheter  90 , which is otherwise identical to catheter  10 , does not include a pull wire and instead is used in combination with a sheath  92  which has a distal member  94  that is connected to the distal end of the catheter  90 . A slot is formed in the distal portion of the sheath  92  and the remnant  96  forms a flexible joint. The loop  98  is formed when the catheter  90  is urged distally relative to the sheath  92 , thereby causing the distal portion of the catheter to bulge outwardly in the manner illustrated in  FIG. 14 . Other loop catheters where the distal portion of catheter body is connected to the distal portion of the sheath may also be used. One example is a catheter wherein the distal end of the catheter body is connected to the distal end of a sheath by a short wire. This and other examples of such loop catheters are disclosed in U.S. Pat. No. 6,071,274, which is incorporated herein by reference. 
     As illustrated for example in  FIG. 15 , an infusion/ventilation pull device  67  may be used in place of the pull wire  66 . The infusion/ventilation pull device  67  is especially useful in those instanced where a porous electrode  20  or coagulation body  58  is located distal of the apex  76  (such as in the exemplary embodiment illustrated in  FIG. 9 ). Nevertheless, the infusion/ventilation pull device  67  may be used in conjunction with any of the embodiments described herein. The infusion/ventilation pull device  67  is preferably formed from a dual lumen braid tube with the outer portion  69  removed in the vicinity of the tip member  72 ′ to expose the braids  71 . One lumen is used for infusion and the other is used for ventilation. Alternatively, a braid tube with the braids on the exterior may be used. 
     In either case, the braids  71  are separated from the remainder of the tube and connected to the core wire  70  with a crimp tube  73 . The inner portion  75  of the braid tube extends through an aperture  77  in the tip member  72 ′ to the porous electrode  20  or coagulation body  58 , where the ends of the dual fluid lumens are plugged. Apertures are formed through the wall of the inner portion  75  to each of the two lumens at positions within the porous electrode  20  or coagulation body  58 . So configured, the infusion/ventilation pull device  67  may be used to perform the pull wire function in a loop catheter (or probe) in addition to the infusion/ventilation function. 
     IV. Methods of Use 
     The exemplary catheter  10  may be used to, for example, form a lesion around a pulmonary vein ostium in the following manner. With the expandable porous electrode  20  in a collapsed state, the sheath  60  and catheter  10  are directed into the left atrium and the loop  62  is then deployed in the manner described above. The loop  62  is compact enough to allow the apex  76  to be wedged into the pulmonary vein in the manner illustrated in  FIG. 16   a . This places core wire  70  in a compressed state and, accordingly, the upper and lower portions of the loop  62  (as oriented in  FIG. 16   a ) push against the corresponding portions of the pulmonary vein ostium. 
     Next, conductive fluid is supplied to the expandable porous electrode  20  to expand the electrode into the state illustrated in  FIG. 16   b . The porous electrode  20  will press against, and conform to, the pulmonary vein ostium. The underlying loop  62  will also be acting as a brace to press the now expanded porous electrode  20  against the ostium. As a result, the porous electrode  20  will achieve the level of contact necessary to form an effective lesion. 
     With the porous electrode  20  urged against the pulmonary vein ostium, power is supplied to the electrode  46  to create a lesion. Once a lesion is created, the liquid will be removed from the porous electrode  20  to return it to its collapsed state. This allows the loop  62  to be rotated by the physician by, for example, rotating the handle  18 . The loop  62  may have to also be withdrawn slightly prior to rotation. The porous electrode  20  will be re-expanded, and a lesion created, when the porous electrode is aligned with the next intended lesion location. This process will continue until a continuous lesion has been formed all the way around the pulmonary vein ostium. 
     After the formation of a lesion around the pulmonary vein is complete, a series of electrodes  100  positioned on the side of the loop  62  opposite the porous electrode structure  20  may be used for pacing and recording purposes to determine whether or not a continuous line of electrical block has been formed through conventional mapping techniques. The electrodes  100 , which are connected to the connector  52  ( FIG. 1 ) by conductors  102  ( FIGS. 6 ,  7 ,  10  and  11 ), are preferably in the form of solid rings of conductive material, like platinum, or can comprise a conductive material, like platinum-iridium or gold, coated upon the device using conventional coating techniques or an ion beam assisted deposition (IBAD) process. For better adherence, an undercoating of nickel or titanium can be applied. The electrodes are also about 4 mm in length. Other types of electrodes, such as wound spiral coils, helical ribbons, and conductive ink compounds that are pad printed onto a non-conductive tubular body, may also be used. 
     Although the present inventions have been described in terms of the preferred embodiments above, numerous modifications and/or additions to the above-described preferred embodiments would be readily apparent to one skilled in the art. By way of example, but not limitation, although the inflatable lesion formation is positioned on the proximal side of the flattened portion prior to formation of the loop, it may be positioned on the distal side if so desired. It is intended that the scope of the present inventions extend to all such modifications and/or additions and that the scope of the present inventions is limited solely by the claims set forth below.