Patent Publication Number: US-2003236455-A1

Title: Probe assembly for mapping and ablating pulmonary vein tissue and method of using same

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates, in general, to electrode probe assemblies and methods for mapping and/or ablating body tissue, and, in particular, to electrode probe assemblies and methods for mapping and/or ablating pulmonary vein tissue.  
       BACKGROUND OF THE INVENTION  
       [0002] Aberrant conductive pathways can develop in heart tissue and the surrounding tissue, disrupting the normal path of the heart&#39;s electrical impulses. For example, anatomical obstacles, called “conduction blocks,” can cause the electrical impulse to degenerate into several circular wavelets that circulate about the obstacles. These wavelets disrupt the normal activation of the atria or ventricles. The aberrant conductive pathways create abnormal, irregular, and sometimes life-threatening heart rhythms called arrhythmias. An arrhythmia can take place in the atria, for example, as in atrial tachycardia (“AT”) or atrial fibrillation (“AF”). The arrhythmia can also take place in the ventricle, for example, as in ventricular tachycardia (“VT”).  
       [0003] In treating arrhythmias, it is sometimes essential that the location of the sources of the aberrant pathways (called focal arrhythmia substrates) be located. Once located, the focal arrhythmia substrate can be destroyed, or ablated, e.g., by surgical cutting or the application of heat. In particular, ablation can remove the aberrant conductive pathway, thereby restoring normal myocardial contraction. An example of such an ablation procedure is described in U.S. Pat. No. 5,471,982 issued to Edwards et al.  
       [0004] Alternatively, arrhythmias may be treated by actively interrupting all of the potential pathways for atrial reentry circuits by creating complex lesion patterns on the myocardial tissue. An example of such a procedure is described in U.S. Pat. No. 5,575,810, issued to Swanson et al.  
       [0005] Frequently, an arrhythmia aberration resides at the base, or within one or more pulmonary veins, wherein the atrial tissue extends. To treat such an aberration, physicians use multiple catheters to gain access into interior regions of the pulmonary vein tissue for mapping and ablating targeted tissue areas. A physician must carefully and precisely control the ablation procedure, especially during procedures that map and ablate tissue within the pulmonary vein. During such a procedure, the physician may introduce a mapping catheter to map the aberrant conductive pathway within the pulmonary vein. The physician introduces the mapping catheter through a main vein, typically the femoral vein, and into the interior region of the pulmonary vein that is to be treated.  
       [0006] Placement of the mapping catheter within the vasculature of the patient is typically facilitated with the aid of an introducer guide sheath or guide wire. The introducer guide sheath is introduced into the left atrium of the heart using a conventional retrograde approach, i.e., through the respective aortic and mitral valves of the heart. Alternatively, the introducer guide sheath may be introduced into the left atrium using a transeptal approach, i.e., through the atrial septum. In either method, the catheter is introduced through the introducer guide sheath until a probe assembly at a distal portion of the catheter resides within the left atrium. A detailed description of methods for introducing a catheter into the left atrium via a transeptal approach is disclosed in U.S. Pat. No. 5,575,810, issued to Swanson et al., which is fully and expressly incorporated herein by reference. Once inside the left atrium, the physician may deliver the probe assembly into a desired pulmonary vein by employing a steering mechanism on the catheter handle. The physician situates the probe assembly within a selected tissue region in the interior of the pulmonary vein, adjacent to the opening into the left atrium, and maps electrical activity in the pulmonary vein tissue using one or more electrodes of the probe assembly.  
       [0007] After mapping, the physician introduces a second catheter to ablate the aberrant pulmonary vein tissue. The physician further manipulates a steering mechanism to place an ablation electrode carried on the distal tip of the ablation catheter within the selected tissue region in the interior of the pulmonary vein. The ablation electrode is placed in direct contact with the tissue that is to be ablated. The physician directs radio frequency energy from the ablation electrode through tissue to an electrode to ablate the tissue and form a lesion.  
       [0008] Problems with this approach include the possibility of misdirecting or misplacing the ablation electrode and inadvertently ablating non-aberrant, i.e., healthy, pulmonary vein tissue. Further, this approach is time-consuming because the physician has to introduce and remove two catheters. This leads to more patient discomfort and room for physician error. Poorly controlled ablation in the pulmonary vein can result in pulmonary vein stenosis. The pulmonary vein stenosis can lead to pulmonary hypertension, pulmonary edema, necrosis of lung tissue, and even complete pulmonary failure of a lung or lung lobe. In severe and rare cases, the only treatment may be a lung transplant.  
       SUMMARY OF THE INVENTION  
       [0009] The present invention includes the following three main aspects that solve the problems with separate mapping catheters and ablation catheters for mapping electrical activity in pulmonary vein tissue and ablating the pulmonary vein tissue: 1) a probe assembly with a microporous ablation body used with a basket assembly for mapping and ablating pulmonary vein tissue; 2) a probe assembly with a basket assembly for mapping and ablating pulmonary vein tissue; and 3) a probe assembly with an expandable body used with a basket assembly for mapping and ablating pulmonary vein tissue. Each of these aspects is summarized in turn below.  
       [0010] 1. Probe Assembly with an Expandable Body used with a Basket Assembly for Mapping and Ablating Pulmonary Vein Tissue:  
       [0011] A first aspect of the invention includes a probe assembly for mapping and ablating pulmonary vein tissue. The probe assembly includes an expandable and collapsible basket assembly including multiple splines, one or more of the splines carrying one or more electrodes adapted to sense electrical activity in the pulmonary vein tissue, the basket assembly defining an interior, a microporous expandable and collapsible body disposed in the interior of the basket assembly and defining an interior adapted to receive a medium containing ions, an internal electrode disposed within the interior of the body and adapted to transmit electrical energy to the medium containing ions, the body including at least one microporous region having a plurality of micropores therein sized to pass ions contained in the medium without substantial medium perfusion therethrough, to thereby enable ionic transport of electrical energy from the internal electrode, through the ion-containing medium to an exterior of the body to ablate pulmonary vein tissue. In an exemplary implementation of the first aspect, the microporous expandable and collapsible body is adapted to be maintained in an expanded condition at a substantially constant pressure by a continuous flow of the medium through the body, providing a cooling effect in the microporous body and the pulmonary vein tissue.  
       [0012] 2. Probe Assembly with a Basket Assembly for Mapping and Ablating Pulmonary Vein Tissue:  
       [0013] A second aspect of the invention involves a probe assembly for mapping and ablating pulmonary vein tissue. The probe assembly includes an expandable and collapsible basket assembly including multiple splines, one or more of the splines carrying one or more electrodes, and at least one of the one or more electrodes adapted to sense electrical activity in the pulmonary vein tissue and ablate the pulmonary vein tissue.  
       [0014] 3. Probe Assembly with an Expandable Body used with a Basket Assembly for Mapping and Ablating Pulmonary Vein Tissue:  
       [0015] A third aspect of the invention includes a probe assembly for mapping and ablating pulmonary vein tissue. The probe assembly includes an expandable and collapsible basket assembly including multiple splines, one or more of the splines carrying one or more electrodes adapted to sense electrical activity in the pulmonary vein tissue, the basket assembly defining an interior, and a non-porous expandable and collapsible body disposed in the interior of the basket assembly and defining an interior adapted to receive a fluid medium for expanding the expandable and collapsible body to exclude blood from the electrodes. In an exemplary implementation of the third aspect, the non-porous expandable and collapsible body is adapted to be maintained in an expanded condition at a substantially constant pressure by a continuous flow of the medium through the body, providing a cooling effect in the body and the pulmonary vein tissue.  
       [0016] Other and further objects, features, aspects, and advantages of the present inventions will become better understood with the following detailed description of the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
     [0017] The drawings illustrate both the design and utility of preferred embodiments of the present invention, in which like elements are referred to with common reference numerals.  
     [0018]FIG. 1 is a schematic illustration of a RF mapping and ablation catheter system including a probe assembly constructed in accordance with a first aspect of the invention.  
     [0019]FIG. 2 is an enlarged elevational view of the probe assembly illustrated in FIG. 1, taken in the region of  2 - 2  of FIG. 1.  
     [0020]FIG. 3A is an enlarged side view of an alternative embodiment of a probe assembly with a fewer number of splines than that depicted in FIGS. 1 and 3.  
     [0021]FIG. 3B is an enlarged cross sectional view of one of the splines of FIG. 3A taken along line  3 B- 3 B.  
     [0022]FIG. 4 is an enlarged side elevational view of a portion of the catheter, taken in the region of  4 - 4  of FIG. 2.  
     [0023]FIG. 5 is an enlarged cross sectional view of the probe assembly, taken along line  5 - 5  of FIG. 2.  
     [0024]FIG. 6 is an enlarged side elevational view of a distal portion of the catheter illustrated in FIG. 1, with a portion of the catheter body removed to show the probe assembly in a collapsed condition.  
     [0025]FIG. 7 is an enlarged side elevational view of an alternate embodiment of the probe assembly.  
     [0026]FIG. 8 is an enlarged side elevational view of a further embodiment of the probe assembly.  
     [0027]FIG. 9 is an enlarged side elevational view of a probe assembly constructed in accordance with a second aspect of the invention.  
     [0028] FIGS.  10 A- 10 C are cross sectional views of the probe assembly illustrated in FIG. 9, and depict alternative embodiments of lesion creating techniques.  
     [0029]FIG. 11 is an enlarged side elevational view of the probe assembly illustrated in FIG. 9 placed at the ostium of a pulmonary vein.  
     [0030]FIG. 12 is an enlarged side elevational view of a probe assembly constructed in accordance with a third aspect of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0031] The present invention involves a mapping and ablation probe assembly for a catheter that solves the problems described above associated with a separate mapping catheter for mapping electrical activity in pulmonary vein tissue and ablation catheter for ablating the pulmonary vein tissue. Three main aspects of the probe assembly are described below. The first aspect is a probe assembly with a microporous ablation body used with a basket assembly for mapping and ablating pulmonary vein tissue. Along with a description of this aspect of the probe assembly, an exemplary catheter system that is applicable to all three main aspects will also be described. The second aspect is a probe assembly with a basket assembly for mapping and ablating pulmonary vein tissue. The third aspect is a probe assembly with an expandable body used with a basket assembly for mapping and ablating pulmonary vein tissue. Each of these aspects will now be described in turn.  
     [0032] 1. Probe Assembly with an Expandable Body used with a Basket Assembly for Mapping and Ablating Pulmonary Vein Tissue:  
     [0033] With reference to FIGS. 1 and 2, a catheter  10  including a probe assembly  14  for mapping and ablating pulmonary vein tissue and constructed in accordance with a first aspect of the invention will now be described. Although the probe assembly  14  and alternative probe assembly embodiments described further below are described in conjunction with mapping and ablating pulmonary vein tissue, it will be readily apparent to those skilled in the art that the probe assemblies may be used to map and ablate other body tissues such as, but not by way of limitation, myocardial tissue. Further, it should be noted, the probe assembly  14  and catheter  10  illustrated in drawings are not necessarily drawn to scale. The probe assembly  14  will first be described, followed by a description of the rest of the catheter system and a method of using the probe assembly.  
     [0034] A. Probe Assembly:  
     [0035] With reference to FIG. 2, the probe assembly  14  may include an expandable and collapsible basket  18  and a microporous body  22  located in an interior region  26  of the basket  18 .  
     [0036] The geometry of the microporous body  22  may be altered between a collapsed geometry (FIG. 6) and enlarged expanded geometry (FIGS. 2, 5) by injecting and removing a pressurized and conductive inflation medium  30  into and from an interior  36  of the microporous body  22 . The pressurized inflation medium  30  also maintains the microporous body  22  in the expanded geometry. The inflation medium  30  is composed of an electrically conductive liquid that establishes an electrically conductive path from a ring electrode  40  to the surface of the microporous body  22 . Preferably, the electrically conductive medium  30  possesses a low resistivity to decrease ohmic losses and, thus, ohmic heating effects, within the microporous body  22 . The composition of the electrically conductive medium  30  can vary. In the illustrated embodiment, the electrically conductive medium  30  comprises a hypertonic saline solution having a sodium chloride concentration at or about 100% weight by volume. The medium may include a 70:30 mixture of 10% saline and radio-opaque solution. An exemplary radio-opaque solution that may be used is sold as Omnipaque® by Nycomed Amersham Imaging of Princeton, N.J. A medium  30  with a radio-opaque solution allows the body  22  to be visualized using fluoroscopy.  
     [0037] The ring electrode  40  is located within the interior region  36  of the microporous body  22 . The ring electrode  40  transmits RF energy that is delivered to pulmonary vein tissue via ionic transport through the conductive inflation medium  30  and micropores in the microporous body  22 . In this regard, the ring electrode  40  is composed of a material having both a relatively high electrical conductivity and a relatively high thermal conductivity, e.g., gold, platinum, or platinum/iridium.  
     [0038] It should be noted that the ring-like structure of the electrode  40  provides a relatively large circumferential exterior surface in communication with the inflation medium  30  in the interior region  36  of the microporous body  22 , providing an efficient means of energizing the inflation medium  30 . Although the electrode  40  is described as a ring, the electrode  40  can take the form of any suitable structure that can contact the inflation medium  30 . The length of the electrode  40  can be accordingly varied to increase or decrease the amount of RF energy delivered to the inflation medium  30 . The location of the electrode  40  can also be varied.  
     [0039] Although in the embodiment shown and described, the operative ablative element is a RF electrode  40  and tissue is ablated through the delivery of RF energy, in alternative embodiments, the ablative element may be adapted to ablate body tissue using an ultrasound transmitter, a laser, a cryogenic mechanism, or other similar means. For example, the body  22  may be adapted to receive a cryogenic medium to thereby enable cryogenic ablation of pulmonary vein tissue via said cryogenic medium and said body  22 .  
     [0040] The microporous body  22  is preferably made of an electrically nonconductive material including micropores in at least a portion of the body  22 . The micropores are preferably 0.0001 to about 0.1 microns in diameter. The microporous structure of the microporous body  22  acts as the energy-emitting surface, establishing ionic transport of RF energy from the RF electrode  40 , through the inflation medium  30 , and into the tissue outside of the microporous body  22 , thereby creating a lesion.  
     [0041] The geometry of the energy-emitting surface of the microporous body  22  can be customized to more efficiently produce the desired lesion characteristics. In particular, the delivery of RF energy from the electrode  40  to the microporous body  22  can be concentrated in certain regions of the microporous body  22 . For example, the microporous body  22  may include a microporous region  32  that runs around a central circumferential portion of the microporous body  22 . Additionally or alternatively, the microporous region  32  may run along another portion of the body  22  such as adjacent to a proximal base of the body  22  or adjacent to a distal tip of the body  22 . One way to concentrate the delivery of RF energy from one or more regions of the microporous body  22  is by masking the micropores of the microporous body  22  in the regions where RF energy delivery is not desired.  
     [0042] The electrical resistivity of the microporous body  22  has a significant influence on the tissue lesion geometry and controllability. Ablation with a low-resistivity microporous body  22  enables more RF power to be transmitted to the tissue and results in deeper lesions. On the other hand, ablation with a high-resistivity microporous body  22  generates more uniform heating, therefore improving the controllability of the lesion. Generally speaking, lower resistivity values for the microporous body  22  (below about 500 ohm-cm) result in deeper lesion geometries, while higher resistivity values for the microporous body  22  (above about 500 ohm-cm) result in shallower lesion geometries.  
     [0043] The electrical resistivity of the microporous body  22  can be controlled by specifying the pore size of the material, the porosity of the material (space on the body that does not contain material), and the water absorption characteristics (hydrophilic versus hydrophobic) of the material. In general, the greater the pore size and porosity, the lower the resistivity of the microporous body  22 . In contrast, the lesser the pore size and porosity, the greater the resistivity of the microporous body  22 . The size of the pores is selected such that little or no liquid perfusion through the pores results, assuming a maximum liquid pressure within the interior region of the microporous body  22 . Thus, the pores are sized to pass ions contained in the medium without substantial medium perfusion therethrough to thereby enable ionic transport of electrical energy from the ion-containing medium  30  to an exterior of the body  22  to ablate pulmonary vein tissue.  
     [0044] In general, hydrophilic materials possess a greater capacity to provide ionic transfer of radio frequency energy without significant perfusion of liquid through the microporous body  22  than do hydrophobic materials. Additionally, hydrophilic materials generally have lower coefficients of friction with body tissues than have hydrophobic materials, facilitating routing of the catheter through the vasculature of the patient. Exemplary materials that can be used to make the microporous body  22  include, but not by way of limitation, regenerated cellulose, nylon, nylon 6, nylon 6/6, polycarbonate, polyethersulfone, modified acrylic polymers, cellulose acetate, poly(vinylidene fluoride), poly(vinylpyrrolidone), and a poly(vinylidene fluoride) and poly(vinylpyrrolidone) combination. A microporous body made of a poly(vinylidene fluoride) and poly(vinylpyrrolidone) combination is disclosed in Hegde, et al., U.S. Application No. ______ (Unknown) entitled “POROUS MEMBRANES”, filed on May 22, 2000, the specification of which is fully and expressly incorporated herein by reference. Also, further details concerning the manufacture of the microporous body  22 , including the specification of the material, pore size, porosity, and water absorption characteristics of the material, are disclosed in Swanson, et al., U.S. Pat. No. 5,797,903, the specification of which is fully and expressly incorporated herein by reference.  
     [0045] The basket  18  includes multiple flexible splines  44 . Each of the splines  44  is preferably made of a resilient inert material such as Nitinol metal or silicone rubber; however, other materials may be used. Multiple electrodes  48  are located on each sphine  44 . Connected to each mapping electrode  48  are signal wires  52  made from a highly conductive metal such as copper. The signal wires  52  preferably extend through each sphine  44  and into catheter body  80 . The splines  44  are connected to a base member  56  and an end member  60 . The sphines  44  extend circumferentially between the base member  56  and the end member  60  when in the expanded geometry. Plastic tubing may be used to cover the splines  44  and contain the signal wires  52  running from the electrodes  48 .  
     [0046] Although the electrodes  48  are described below as mapping electrodes, in alternative embodiments, the electrodes  48  may be multi-functional electrodes used for mapping, pacing, and/or ablating body tissue. In a further embodiment, the splines  44  may not include any electrodes. Any or all of the embodiments described below may also include splines  44  having multi-functional electrodes  48  or no electrodes.  
     [0047] The basket  18  is shown with specific number of splines  44  and electrodes  48  for each spline  44 , i.e.,  8 ; however, it will be readily apparent to those skilled in the art that the number of splines  44  and/or the number of electrodes  48  per spline  44  may vary. For example, FIG. 3A depicts a basket structure with six splines  44  (two splines  44  are hidden from view), with some of the splines  44  having nine electrodes  48  and other splines  44  having ten electrodes  48 . Further, the shape of the splines  44  and electrodes  48  may vary.  
     [0048] Because the electrodes  48  in this embodiment are mounted on flexible splines  44 , when the basket  18  is expanded in the vasculature of a patient, the splines  44  conform to a large range of different vein sizes and shapes. The flexibility and resiliency of the splines  44  also allows for the basket structure to push outward on the tissue. This increases the friction between the electrodes  48  and the vein and thereby anchors the probe assembly  14  in position, yielding a more precise ablation location.  
     [0049] The splines  44  may carry one or more temperature sensors  50  that may take the form of thermistors, thermocouples, or the equivalent, and are in thermal conductive contact with the exterior of the probe assembly  14  to sense conditions in tissue outside the probe assembly  14  during ablation. The temperature sensors  50  may be located on the splines  44  such that when the splines  44  are expanded, the temperature sensors  50  are located at or near the largest diameter of the probe assembly  14 . Although the basket  18  in FIG. 2 is shown with two temperature sensors  50  for each spline  44 , it will be readily apparent to those skilled in the art that the number of temperature sensors  50  per spline  44  may vary.  
     [0050] With reference to FIGS. 3A and 3B, in an alternative embodiment, the electrodes  48  may comprise rings that surround the temperature sensors  50 , splines  44 , and signal wires  52 .  
     [0051] With reference to FIG. 5, the microporous body  22  may include a construction that, when inflated, has a larger volume than the volume V defined by the expanded basket  18 , causing the body  22  to extend or bulge between and beyond the circumferential region or volume V defined by the basket assembly  18  when the basket assembly  18  and the body  22  are in an expanded state. This may help put the microporous body  22  in more direct contact with the targeted pulmonary vein tissue, improving ablation treatment of the tissue. This may also cause the delivery of RF energy from the microporous body  22  to be concentrated in the bulging regions of the microporous body  22 , which may be desirable depending on the targeted tissue that needs ablating. Additionally, the microporous body  22  restricts blood flow to the ablation area, which reduces the possibility of coagulated blood embolus. Finally, restricting blood flow renders the relationship between ablation parameters (power, time, and temperature) and lesion characteristics more predictable, since the important lesion parameters of energy loss attributable to the convective losses and to energy delivery are more predictable.  
     [0052] B. Catheter System:  
     [0053] With reference generally to FIGS.  1 - 4  and  6 , the remaining components of the catheter system will now be described.  
     [0054] The catheter  10  can be functionally divided into four regions: the operative distal probe assembly region  64 , a deflectable catheter region  68 , a main catheter region  72 , and a proximal catheter handle region  76 . A handle assembly  77  including a handle  78  is attached to the proximal catheter handle region  76  of the catheter  10 . With reference to FIG. 6, the catheter  10  also includes a catheter body  80  that may include first and second tubular elements  84  and  86 , which form, in conjunction, the structure of the distal probe assembly region  64 ; a third tubular element  90 , which forms the structure of the deflectable catheter region  68 ; and a fourth tubular element  94 , which forms the structure of the main catheter region  72 . It should be noted, however, that the catheter body  80  may include any number of tubular elements required to provide the desired functionality to the catheter. The addition of metal in the form of a braided mesh layer sandwiched in between layers of the plastic tubing may be used, greatly increasing the rotational stiffness of the catheter. This may be beneficial to practice one or more lesion creation techniques described in more detail below.  
     [0055] With reference to FIG. 2, the operative distal probe assembly region  64  includes the probe assembly  14 . The catheter  10  may also include a sheath  98  that, when moved distally over the basket  18 , collapses the basket  18  (FIG. 6). In a preferred embodiment, the microporous body  22  is collapsed (by the removal of the inflation medium  30  therefrom) before the basket  18  is collapsed; however, in an alternative embodiment, collapsing the basket  18  may cause fluid to be removed from the microporous body  22  and, thus, the microporous body  22  to collapse. Conversely, retracting the sheath  98  or moving the sheath  98  proximally away from the probe assembly  14  may deploy the basket  18 . This removes the compression force causing the basket  18  to open to a prescribed three-dimensional shape. Moving the sheath  98  distally in the direction indicated by arrow  106  causes the sheath  98  to apply a compressive force, thus, collapsing the basket  18 . Moving the sheath  98  proximally in the direction indicated by the arrow  110  removes the compressive force of the sheath  98 , thus, allowing the basket  18  to expand.  
     [0056] With reference to FIGS. 1, 2 and  6 , the deflectable catheter region  68  is the steerable portion of the catheter  10 , which allows the probe assembly  14  to be accurately placed adjacent the targeted tissue region. A steering wire (not shown) may be slidably disposed within the catheter body  80  and may include a distal end attached between the second tubular element  86  and the third tubular element  90  and a proximal end suitably mounted within the handle  78 . The handle assembly  77  may include a steering member such as a rotating steering knob  114  that is rotatably mounted to the handle  78 . Rotational movement of the steering knob  114  counter clockwise relative to the handle  78 , in the direction indicated by the arrow  118 , may cause a steering wire to move proximally relative to the catheter body  80  which, in turn, tensions the steering wire, thus pulling and bending the catheter deflectable region  68  into an arc (shown by broken lines in FIG. 1). On the contrary, rotational movement of the steering knob  114  clockwise relative to the handle  78 , in the direction indicated by the arrow  122 , may cause the steering wire to move distally relative to the catheter body  80  which, in turn, relaxes the steering wire, thus allowing the resiliency of the third tubular element  90  to place the catheter deflectable region  68  of the catheter back into a rectilinear configuration. To assist in the deflection of the catheter, the deflectable catheter region  68  is preferably made of a lower durometer plastic than the main catheter region  72 .  
     [0057] The catheter  10  may be coupled to a RF generator  126  such as that described in Jackson et al., U.S. Pat. No. 5,383,874, the specification of which is fully and expressly incorporated herein by reference. The RF generator  126  provides the catheter  10  with a source of RF ablation energy. The RF generator  126  includes a RF source  130  for generating the RF energy and a controller  134  that controls the amplitude of, and time during, which the RF source  130  outputs RF energy. The RF generator  126  is electrically coupled to the catheter  10  via a cable  138 . One or more signal wires  140  are routed through an ablation wire tubular member  142  (FIG. 2, 4) in the catheter body  80  and couple the ring electrode  40  to the cable  138 . Operation of the RF generator  126  provides RF energy to the ring electrode  40 , which in turn is ionically transferred through the inflation medium  30 , and out through the pores of the microporous body  22 , into the targeted tissue region. Thus, when operated, the RF generator  126  allows the physician to ablate body tissue such as pulmonary vein tissue in a controlled manner, resulting in a tissue lesion with the desired characteristics.  
     [0058] A mapping signal processor  146  may also be coupled to the catheter  10 , allowing a physician to map the electrical activity in the target tissue site before, during and/or subsequent to the ablation process. The mapping processor  146  may be part of the controller  134 . The mapping processor  146  is in electrical communication with the mapping electrodes  48  via a mapping cable  150  and the signal wires  52 . The signal wires  52  are preferably routed through a mapping wire tubular member  156  (FIG. 2, 4) in the catheter body  80 .  
     [0059] An inflation medium reservoir and pump  160  may be coupled to the catheter  10  for supplying the microporous body  22  with the inflation medium  30 . The reservoir and pump  160  may supply ionic fluid at room temperature or may include a chiller for supplying cool ionic fluid. A constant flow of ionic cooling fluid such as a 10% saline solution may be circulated through the microporous body  22  to cool the microporous body  22  and supply the ionic fluid necessary to allow ionic transfer through the body for ablation. An inlet lumen  354  and an outlet lumen  356  are adapted to communicate at proximal ends, inlet port  355  and outlet port  357 , with the reservoir and pump  160  and at distal ends with the mouth or interior of the microporous body  22 . Preferably, the fluid lumens  354 ,  356  have the same length and internal diameters, resulting in a microporous body pressure that is approximately half of that at the inlet port  355 . The pressures at the inlet port  355  and outlet port  357  may be measured with respective inlet and outlet pressure sensors,  358 ,  360 . Thus, the microporous body pressure may be estimated/controlled using the pressure measured at the inlet sensor  358 .  
     [0060] The fluid is preferably circulated at a rate and pressure that maintains the fluid pressure in the microporous body  22  at a predetermined pressure. Alternatively, the microporous body pressure may be controlled by injecting the fluid into the inlet port  355  at a known, controlled rate.  
     [0061] The pump  160  may impart the pressure necessary to circulate the fluid through the microporous body  22  and the fluid may passively flow out of the outlet port  357 . Alternatively, the pump  160  may apply a vacuum pressure to the outlet port  357  to increase the allowable flow rate through the microporous body  22 .  
     [0062] An inlet control valve  362 , e.g., pop-off valve, and/or outlet control valve  364  at the inlet  355  and/or outlet  357  may be used to prevent the microporous body  22  from being inflated above the body&#39;s burst pressure or a lower predefined pressure to prevent over-inflation or bursting, ensuring patient safety. A control valve set to a low pressure value may also be used to ensure that the body  22  remains inflated even when flow to the body  22  is stopped, if the pressure value exceeds that required to maintain body inflation.  
     [0063] A continuous flow of ionic fluid maintains the microporous body  22  and ablation site at a cooler temperature, allowing for more power delivery to the target tissue to make deeper lesions. The continuous flow also enables the use of a smaller RF electrode within the microporous body  22  because heat generated near the electrode can be convected away from that electrode. Finally, the continuous flow reduces the possibility that non-targeted adjacent tissue will be damaged, thereby increasing patient safety.  
     [0064] An auxiliary member  172  may be coupled to the catheter  10  via an external connector  176  and further coupled to the probe assembly  14  via an internal connector or carrier  180  (FIG. 4) in the catheter body  80 . The one or more temperature sensors  50  on one or more of the splines  44  of the basket  18  may be connected to one or more temperature sensor wires guided through the internal connector or carrier  180  of the catheter body  80 . The auxiliary member  172  may be a controller that is coupled to the one or more temperature sensor wires via the external connector  176 . If the auxiliary member  172  is a controller, it is preferably the same as the controller  134  of the RF generator  126 .  
     [0065] Temperatures sensed by the temperature sensors  50  are processed by the controller  172 . Based upon temperature input, the controller  172  adjusts the time and power level of radio frequency energy transmissions by the RF generator  126 , and consequently the ring electrode  40 , to achieve the desired lesion patterns and other ablation objectives and to avoid undesired tissue necrosis caused by overheating.  
     [0066] Temperature sensing and controlling using the one or more temperature sensors  50  of the splines  44  will now be described in more detail. The controller  172  may include an input  182  for receiving from the physician a desired therapeutic result in terms of (i) the extent to which the desired lesion should extend beneath the tissue-electrode interface to a boundary depth between viable and nonviable tissue and/or (ii) a maximum tissue temperature developed within the lesion between the tissue-electrode interface and the boundary depth. The controller  172  may also include a processing element  184  that retains a function that correlates an observed relationship among lesion boundary depth, ablation power level, ablation time, actual sub-surface tissue temperature, and electrode temperature. The processing element  184  compares the desired therapeutic result to the function and selects an operating condition based upon the comparison to achieve the desired therapeutic result without exceeding a prescribed actual or predicted sub-surface tissue temperature.  
     [0067] The operating condition selected by the processing element  184  can control various aspects of the ablation procedure such as controlling the ablation power level, limiting the ablation time to a selected targeted ablation time, limiting the ablation power level subject to a prescribed maximum ablation power level, and/or the orientation of the microporous region  32  of the body  22 , including prescribing a desired percentage contact between the region  32  and tissue.  
     [0068] If the ablating electrode(s) is the microporous body  22  or conventional metal electrode(s) where an expandable body is used to restrict blood flow around the electrode(s), the processing element  184  may rely upon the temperature sensors  50  to sense actual maximum tissue temperature because the body  22  restricts blood flow to the ablation site, minimizing convective cooling of the tissue-electrode interface by the surrounding blood flow. As a result, the region of maximum temperature is located at or close to the interface between the tissue and the microporous body  22 . The temperature conditions sensed by the temperature sensors  50  closely reflect actual maximum tissue temperature.  
     [0069] If the ablating electrode(s) is a conventional metal electrode(s) and blood is free to flow over the electrode(s), the processing element  302  may predict maximum tissue temperature based upon the temperature sensed by the temperature sensors  50  at the tissue-electrode interface. When using a conventional metal electrode(s) to ablate tissue, the tissue-electrode interface is convectively cooled by surrounding blood flow. Due to these convective cooling effects, the region of maximum tissue temperature is located deeper in the tissue. As a result, the temperature conditions sensed by the temperature sensors  50  associated with metal electrode elements do not directly reflect actual maximum tissue temperature. In this situation, maximum tissue temperature conditions must be inferred or predicted by the processor  184  from actual sensed temperatures.  
     [0070] In a preferred embodiment, the one or more temperature sensors  50  are used to sense instantaneous localized temperatures (Ti) of the thermal mass corresponding to the region  32 . The temperature Ti at any given time is a function of the power supplied to the electrode  40  by the generator  126 .  
     [0071] The characteristic of a lesion can be expressed in terms of the depth below the tissue surface of the 50 degree C. isothermal region, which will be called D.sub.50C. The depth D.sub.50C is a function of the physical characteristics of the microporous region  32  (that is, its electrical and thermal conductivities, resistivities, and size); the percentage of contact between the tissue and the microporous region  32 ; the localized temperature Ti of the thermal mass of the region  32 ; the magnitude of RF power (P) transmitted by the interior electrode  40 , and the time (t) the tissue is exposed to the RF power.  
     [0072] For a desired lesion depth D.sub.50C, additional considerations of safety constrain the selection of an optimal operating condition among the operating conditions listed above. The principal safety constraints are the maximum tissue temperature TMAX and maximum power level PMAX.  
     [0073] The maximum temperature condition TMAX lies within a range of temperatures that are high enough to provide deep and wide lesions (typically between about 50 degree C. and 60 degree C.), but are safely below about 65 degree C., the temperature at which pulmonary stenosis is known to occur. It is recognized that TMAX will occur somewhere between the electrode-tissue interface and D.sub.50C. As discussed above, if the ablating electrode is the microporous body  22  or a conventional electrode(s) and an expandable body is used to restrict blood flow at the ablation site, TMAX will be closer to the interface because of the lack of convective cooling by the blood flow. If the ablating electrode is a conventional metal electrode(s) and nothing restricts blood flow to the ablation site, TMAX will be deeper in the tissue because of the convective cooling of the electrode(s) by the blood flow.  
     [0074] The maximum power level PMAX takes into account the physical characteristics of the interior electrode  40  and the power generation capacity of the RF generator  126 . The D.sub.50C function for a given porous region  32  can be expressed in terms of a matrix listing all or some of the foregoing values and their relationship derived from empirical data and/or computer modeling. The processing element  184  includes in memory this matrix of operating conditions defining the D.sub.50C temperature boundary function for multiple arrays of operating conditions.  
     [0075] The physician also uses the input  182  to identify the characteristics of the structure  22 , using a prescribed identification code; set a desired maximum RF power level PMAX; a desired time t; and a desired maximum tissue temperature TMAX.  
     [0076] Based upon these inputs, the processing element  184  compares the desired therapeutic result to the function defined in the matrix, and selects an operating condition to achieve the desired therapeutic result without exceeding the prescribed TMAX by controlling the function variables.  
     [0077] Using the microporous body  22 , typical ablation conditions are to control to sensed temperatures of 65 degree C. and apply RF power for one minute.  
     [0078] With reference back to FIG. 4, the internal carrier  180  (or an internal carrier similar to the internal carrier  18 ) may be used as a transport lumen for drug delivery via the body  22  (if the pores were large enough and/or the drug molecules small enough) or other means. The internal carrier  180  may terminate in the handle assembly  77 , where a physician may inject the medicine into the internal carrier  180  or the medicine may be supplied by the auxiliary member  172 . The medicine may travel through the internal carrier  180  to the body  22 . Additional or fewer auxiliary components may be used depending on the application.  
     [0079] C. Method of Use  
     [0080] With reference to FIGS.  1 - 6 , a method of using the catheter  10  and probe assembly  14  will now be described. Before the catheter  10  can be introduced into a patient&#39;s body, the probe assembly  14  must be in a collapsed condition (FIG. 6). If the catheter  10  is not already in this condition, the probe assembly  14  can be collapsed by moving the sheath  98  forward, towards the distal end of the catheter  10  (in the direction indicated by the arrow  106 ).  
     [0081] Placement of the catheter  10  within the vasculature of the patient is typically facilitated with the aid of an introducer guide sheath or guide wire, which was previously inserted into the patient&#39;s vasculature, e.g., femoral vein. The introducer guide sheath is introduced into the left atrium of the heart using a conventional retrograde approach, i.e., through the respective aortic and mitral valves of the heart. One or more well-known visualization devices and techniques, e.g., ultrasound, fluoroscopy, etc., may be used to assist in navigating and directing the catheter  10  to and from the targeted region. Alternatively, the introducer guide sheath may be introduced into the left atrium using a conventional transeptal approach, i.e., through the vena cava and atrial septum of the heart. A detailed description of methods for introducing a catheter into the left atrium via a transeptal approach is disclosed in U.S. Pat. No. 5,575,810, issued to Swanson et al., which is fully and expressly incorporated herein by reference.  
     [0082] In either method (conventional retrograde approach or transeptal approach), the catheter  10  is introduced through the introducer guide sheath until the probe assembly  14  resides within the left atrium. Once inside the left atrium, the physician may deliver the probe assembly  14  into a desired pulmonary vein through rotational movement of the steering knob  114  on the catheter handle  78 .  
     [0083] The physician situates the probe assembly  14  within a selected tissue region in the interior of the pulmonary vein, adjacent to the opening into the left atrium. The basket  18  is deployed by moving the sheath  98  proximally in the direction indicated by the arrow  110 , causing the sheath  98  to slide away from the basket  18  and removing the compression force thereon. The basket  18  then expands, allowing one or more of the mapping electrodes  48  to contact the pulmonary vein tissue.  
     [0084] The mapping electrodes  48  are used to sense electrical activity in the pulmonary vein tissue, and may be used to pace pulmonary vein tissue as well.  
     [0085] Mapping data received and interpreted by the mapping signal processor  146  is displayed for use by the physician to locate aberrant pulmonary vein tissue. The probe assembly  14  may be moved one or more times which may require collapsing and deploying the probe assembly  14  one or more times, in an effort to locate aberrant pulmonary vein tissue.  
     [0086] When the physician has determined that the aberrant pulmonary vein tissue has been located (basket  18  is deployed), the physician may then expand the microporous body  22  by filling the microporous body  22  with the inflation medium  30  to contact the targeted pulmonary vein tissue. The pump  160  may be activated to introduce the ionic fluid through the inlet lumen  354  and into the microporous body  22  at a constant pressure, inflating the body  22 . The ionic fluid circulated may be cool or at room temperature. The ionic fluid exits the microporous body  22  and flows through the outlet lumen  356  to the outlet  357 . The fluid may passively drip or flow out of the outlet lumen  356 , or may be drawn out of the outlet lumen  356  with vacuum pressure from the pump  160 . Inflating or maintaining the microporous body  22  at less than full pressure is desirable because a non-turgid microporous body  22  better conforms to the tissue surface.  
     [0087] Once the physician has determined that the microporous body  22  is effectively inflated and in contact with the pulmonary vein tissue, the physician may begin ablating the targeted tissue. RF energy is preferably supplied to the ring electrode  40 , which is located within the microporous body  22  and surrounded by inflation medium  30 . Through ionic transport, the electrical energy from the electrode  40  is transported through the inflation medium  30  and through the pores of the microporous body  22 , to the exterior of the microporous body  22 , into and through at least a portion of the pulmonary vein tissue so as to ablate the targeted pulmonary vein tissue, and to a return electrode.  
     [0088] If the electrodes  48  are also (or alternatively) used to ablate the pulmonary vein tissue and saline or a fluid having similar heat transfer characteristics is used to deploy the body  22 , thermal transfer within the body may enable contiguous lesion formation between the electrodes  48  to be created more consistently.  
     [0089] Throughout this process the physician may monitor the temperatures of the tissue region using the temperature sensors  50  to more accurately ablate the target tissue.  
     [0090] Once ablation is completed, or in between ablation treatments, electrical activity in the pulmonary vein tissue may be mapped using the mapping electrodes  48  to confirm effective ablation treatment.  
     [0091] To collapse the probe assembly  14 , the inflation medium  30  in microporous body  106  is removed, but no longer supplied, causing the microporous body  106  to deflate. The basket  18  is also collapsed by moving the sheath  98  forward, towards the distal end of the catheter  10  (in the direction indicated by the arrow  106 ). The catheter  10  is then removed from the patient&#39;s body or moved to a different location for additional diagnosis and/or treatment.  
     [0092] Thus, the probe assembly  14  and method described above are advantageous because they allow the physician to map and ablate the targeted pulmonary vein region with a single probe assembly positioning. Prior to the present invention, the physician would introduce the mapping electrode and map the aberrant region of the pulmonary vein, then remove that mapping electrode, and follow with the ablation electrode to ablate the aberrant region. Problems with the prior approach include the possibility of misdirecting or misplacing the ablating electrode and inadvertently ablating non-aberrant, i.e., healthy, pulmonary vein tissue, and the excessive time-consumption because the physician had to introduce and remove two catheters. This leads to more patient discomfort and room for physician error. Further, the apparatuses and methods of the present invention incorporate all the advantages of an expandable and collapsible microporous body with those of a mapping basket assembly.  
     [0093] With reference to FIG. 7, in an alternative embodiment, a probe assembly  201  is comprised of elements from separate catheters, namely, a microporous body  22  from an ablation catheter  202  and a basket  18  from a main catheter  203 . The basket  18  may include electrodes  48  that are adapted to map, pace, and/or ablate pulmonary vein tissue.  
     [0094] The ablation catheter  202  is slidably removable with respect to the main catheter  203  for positioning the microporous body  22  within or removing it from the basket  18 . The catheter body  203  may include an additional lumen  200  through which the ablation catheter  202  may be slidably disposed.  
     [0095] Both the distal portions of the ablation catheter  202  and the main catheter  203  are preferably steerably controllable in a manner similar to that described above with respect to the catheter  10 .  
     [0096] The microporous body  22  may range in size in the expanded state from the size of one of the electrodes  48  to just larger than the diameter of the basket  18 . The active band  32  of the body  22  is preferably relatively large to better ensure lesion creation. In one embodiment, the body  22 , when expanded, is large enough to create a circumferential lesion in the vein or around the ostium.  
     [0097] However, placement of lesion around the entire circumference is often not required to electrically isolate the pulmonary veins in atrial fibrillation patients. Therefore, in another exemplary embodiment, the expanded microporous body  22  is smaller than the pulmonary vein diameter or vein orifice to create one or more ablation sectors of the pulmonary vein, decreasing the probability of creating clinically significant pulmonary stenosis compared to a complete circumferential lesion. Additionally, a smaller microporous body  22  enables blood flow in pulmonary veins to continue during ablation.  
     [0098] A method of using the probe assembly  201  is similar to that described above with respect to the probe assembly  14 , except the main catheter  203  and ablation catheter  202  may be introduced separately to the targeted site. The ablation catheter  202  may be introduced into the lumen  200  of the main catheter  203  via the handle  78  and snaked through the lumen  200  until the collapsed microporous body  22  is located within the basket  18 . The physician may then inflate the microporous body  22  and steer the body  22  so that it contacts the targeted pulmonary vein tissue. As discussed above, inflation of the microporous body  22  at a pressure corresponding to a less than fully expanded state may be desirable because a non-turgid body  22  better conforms to the tissue surface than a turgid body  22 . The microporous body  22  may be maintained in an expanded state by continuously circulating a fluid medium through the body  22  as described above or by inflating the body  22  with the medium and preventing the medium from exiting the catheter.  
     [0099] For sectional ablation (i.e., non-circumferential ablation), a relative small, expanded microporous body  22  such as that illustrated in FIG. 7 may be used to ablate one or more targeted areas. Additionally or alternatively, the electrodes  48  may be used to ablate one or more targeted areas. If the electrodes  48  are used to ablate tissue, the body  22  may be used to restrict blood flow from the ablation area.  
     [0100] For circumferential ablation, a larger, expanded microporous body  22  such as that illustrated in FIGS. 2 and 5 may be used. A larger, expanded microporous body  22  restricts blood flow to the ablation site, increasing the efficiency of the ablation since RF currents flow substantially into the tissue only, and not into the blood. Restricting blood flow also reduces the possibility of coagulated blood embolus and renders the relationship between ablation parameters (power, time and temperature) and lesion characteristics more predictable since fewer uncontrolled variables exists (mostly attributable to convective losses and to energy delivery to tissue). Further, if the electrodes  48  are also used to ablate the pulmonary vein tissue and saline or a fluid having similar heat transfer characteristics is used to deploy the body  22 , thermal transfer within the body  22  may enable contiguous lesion formation between the electrodes  48  to be created more consistently. Also, the microporous body  22  may create a lossy electrical connection between the electrodes  48  that may enable contiguous lesion formation between the electrodes  48  to be created more consistently.  
     [0101] With reference to FIG. 8, in a further embodiment, a probe assembly  310  includes a basket  18  located at a distal end of a catheter  312  and a microporous body  22  integrated with the basket  18 . The microporous body  22  may be located at the distal end of a steerable member  314  that is steerable in a manner similar to that described above with respect to catheter  10 . The probe assembly  310  is similar to the probe assembly  201  described above with respect to FIG. 7, except the microporous body  22  and steerable member  314  are not removable from the catheter  202 . The catheter  312  is also steerable in a manner similar to that described with respect to catheter  10 . The microporous body  22 , when expanded, can range in size from the size of a single spline electrode  48  to a large body that will be large enough to fill the entire inner cavity of the basket  18 .  
     [0102] The method of using the probe assembly  310  is similar to that described above with respect to probe assembly  201 , except a separate ablation catheter is not snaked through a main catheter or removed therefrom because the microporous body  22  and steerable member  314  are integrated with the basket  18 .  
     [0103] 2. Probe Assembly with a Basket Assembly for Mapping and Ablating Pulmonary Vein Tissue:  
     [0104] With reference to FIGS. 9 and 10A- 10 D, a second aspect of a probe assembly  300  of a mapping and ablation catheter  302  will now be described. Unlike the prior embodiments, the probe assembly  300  does not include a microporous body. Instead, the probe assembly  300  includes a basket  18  with a plurality of multi-functional electrodes  48  adapted to map and ablate body tissue. The catheter  302  is preferably steerable in a manner similar to that described above with respect to catheter  10 .  
     [0105] The number of electrodes  48  that each spline  44  carries, the spacing between the electrodes  48 , and the length of the electrodes  48  may vary according to the particular objectives of the ablating procedure: These structural features influence the characteristics of the lesion patterns formed.  
     [0106] Segmented electrodes  48  may be well suited for creating continuous, elongated lesion patterns provided that the electrodes  48  are adjacently spaced close enough together to create additive heating effects when ablating energy is transmitted simultaneously to the adjacent electrodes  48 . The additive heating effects between close, adjacent electrodes  48  intensify the desired therapeutic heating of tissue contacted by the electrodes  48 . The additive effects heat the tissue at and between the adjacent electrode  48  to higher temperatures than the electrode  48  would otherwise heat the tissue, if conditioned to individually emit energy to the tissue. The additive heating effects occur when the electrodes  48  are operated simultaneously in a bipolar mode between electrodes. Furthermore, the additive heating effects also arise when the electrodes are operated simultaneously in a unipolar mode, transmitting energy to an indifferent electrode.  
     [0107] Conversely, when the electrodes  48  are spaced sufficiently far apart from each other, the electrodes  48  create elongated lesion segments.  
     [0108] The length of each electrode  48  may also be varied. If the electrode  48  is too long, the ability of the splines  44  to conform to the anatomy of the pulmonary vein may be compromised. Also, long electrodes may be subject to “hot spots” during ablation caused by differences in current density along the electrode. Another approach is to use multiple short electrodes  48  on each spline  44  to cover a large effective ablating length and avoid hot spots. An electrode approximately 3 mm in length or less makes an adequate lesion without hot spots, although other lengths may also work.  
     [0109] Ablating energy can be selectively applied individually to just one or a selected group of electrodes, when desired, to further vary the size and characteristics of the lesion pattern.  
     [0110] A basket  18  including eight splines  44  should be adequate for ablating in pulmonary veins of 10 to 15 mm in diameter; however, the basket  18  may have a greater or lesser number of splines  44 , depending on the size of the target anatomy. A small vein may require fewer splines  44  than a larger vein to form a continuous circular lesion around the circumference of the vein.  
     [0111] The method of using the probe assembly  300  is similar to that described above for the probe assembly  14 , except once the basket  18  is at the appropriate location, the physician may begin ablation using the same electrodes  48  that were used to map electrical activity in the pulmonary vein tissue. Should the physician decide that only one section or certain sections of the vein  304  needs ablation, the physician may activate RF energy to select electrodes  44  corresponding to the section or sections of the vein  304 .  
     [0112] With reference additionally to FIG. 10A, if the physician decides that the entire circumference of the pulmonary vein  304  needs treatment and the vein  304  is relatively small relative to the number of splines  44  of the probe assembly  300 , the physician may simply activate RF energy once to all the electrodes  48  or to certain circumferential electrodes  44 .  
     [0113] With reference to FIG. 10B, in an alternate lesion-making technique, where a single ablation step such as that described above with respect to FIG. 10A proves insufficient to form an unbroken tesion line in larger veins  304 , the catheter  302  may be rotated slightly, and a second ablation may be performed. One or more successive rotations and ablations with the probe assembly  300  may be necessary in order to make a contiguous lesion  305 .  
     [0114] With reference to FIG. 10C, in a further lesion-making technique, the catheter  302  may be rotated while simultaneously ablating the pulmonary vein  304 . The handle  76  (FIG. 1) of the catheter  302  may be rotated slowly until the lesion  305  made by one spline  44  begins to overlap the lesion  305  started by an adjacent spline  44 .  
     [0115] After a first round of ablation, the physician may then take further electrode  44  readings, retract the basket  18 , and reposition the catheter  302  for further ablation procedures or, if done, remove the catheter  302  from the patient&#39;s vasculature.  
     [0116] With reference to FIG. 11, an advantage to this aspect of the invention is that the probe assembly  300  does not include a structure likely to block significant blood flow  306  or otherwise occlude the vein  304 . Sufficient blockage can cause hemodynamic compromise in some patients. In addition, blood flow  306  has a beneficial cooling effect that allows the probe assembly  300  to create deeper lesions at lower temperatures and inhibit damaging non-target adjacent tissue. Finally, this embodiment contains separate electrodes  48  that can create lesions at selected sections of the vein  304  or around the entire circumference by one of the lesion-creating techniques described above.  
     [0117] 3. Probe Assembly with an Expandable Body used with a Basket Assembly for Mapping and Ablating Pulmonary Vein Tissue:  
     [0118] With reference to FIG. 12, a probe assembly  400  constructed in accordance with a further aspect of the invention will now be described. The probe assembly  400  is located at a distal end of a catheter  402  that is preferably steerably controlled in a manner similar to that described above with respect to catheter  10 .  
     [0119] The probe assembly  400  is similar to probe assembly  300  described above with respect to FIG. 9, i.e., includes multi-functional electrodes  48  that may map, pace and/or ablate, except the probe assembly  400  further includes a non-porous, non-electrically conducting expandable balloon  404 . The nonporous, non-electrically conducting balloon  404  includes the following two primary functions: (1) to assist in maintaining the position of the basket structure  18  by placing some force against the vein walls, and (2) to restrict blood flow to the ablation area.  
     [0120] A method of using the probe assembly  400  will now be described. A physician may guide the catheter  402  to the appropriate location and deploy the basket  18 . Electrical activity in the pulmonary vein may be mapped using the multi-function electrodes  48  on the splines  44 . The physician may interpret the resulting electrical activity data, and determine the proper position of the probe assembly  400  for ablation.  
     [0121] Once satisfied that the position is accurate, the physician may inflate the non-electrically conducting body  404  with a fluid such as saline or CO 2  and perform ablation of the targeted tissue with the electrodes  48 . As described above with respect to body  22 , the fluid may be constantly circulated though the body  404 .  
     [0122] The expanded body  404  restricts blood flow to the ablation site, increasing the efficiency of the ablation since RF currents flow substantially into the tissue only, and not into the blood. Restricting blood flow also reduces the possibility of coagulated blood embolus and renders the relationship between ablation parameters (power, time and temperature) and lesion characteristics more predictable since fewer uncontrolled variables exist (mostly attributable to convective losses and to energy delivery to tissue). If saline or a fluid having similar heat transfer characteristics is used to deploy the body  404 , thermal transfer within the body  404  may enable contiguous lesion formation between the electrodes  48  to be created more consistently.  
     [0123] While preferred methods and embodiments have been shown and described, it will be apparent to one of ordinary skill in the art that numerous alterations may be made without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited except in accordance with the following claims.