Patent Publication Number: US-7591814-B2

Title: Extended treatment zone catheter

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/338,274, filed Jan. 8, 2003 now U.S. Pat. No. 6,899,709, entitled COOLANT INJECTION, which is a continuation of U.S. patent application Ser. No. 09/850,668, filed May 7, 2001, now issued U.S. Pat. No. 6,540,740, entitled CRYOSURGICAL CATHETER, which is a continuation of U.S. patent application Ser. No. 09/201,071, filed Nov. 30, 1998, now issued U.S. Pat. No. 6,235,019, entitled CRYOSURGICAL CATHETER, which is a continuation of U.S. patent application Ser. No. 08/893,825, filed Jul. 11, 1997, now issued U.S. Pat. No. 5,899,899, entitled CRYOSURGICAL LINEAR ABLATION STRUCTURE, which is a continuation-in-part of U.S. patent application Ser. No. 08/807,382, filed Feb. 27, 1997, now issued U.S. Pat. No. 5,899,898, and entitled CRYOSURGICAL LINEAR ABLATION, all of which are incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     n/a 
     FIELD OF THE INVENTION 
     The invention relates to catheters, and more particularly to cryosurgical catheters used for tissue ablation. 
     BACKGROUND OF THE INVENTION 
     Many medical procedures are performed using minimally invasive surgical techniques, wherein one or more slender implements are inserted through one or more small incisions into a patient&#39;s body. With respect to ablation, the surgical implement can include a rigid or flexible structure having an ablation device at or near its distal end that is placed adjacent to the tissue to be ablated. Radio frequency energy, microwave energy, laser energy, extreme heat, and extreme cold can be provided by the ablation device to kill the tissue. 
     With respect to cardiac procedures, a cardiac arrhythmia can be treated through selective ablation of cardiac tissue to eliminate the source of the arrhythmia. A popular minimally invasive procedure, radio frequency (RF) catheter ablation, includes a preliminary step of conventional electrocardiographic mapping followed by the creation of one or more ablated regions (lesions) in the cardiac tissue using RF energy. Multiple lesions are frequently required because the effectiveness of each of the proposed lesion sites cannot be predetermined due to limitations of conventional electrocardiographic mapping. Often, five lesions, and sometimes as many as twenty lesions may be required before a successful result is attained. Usually only one of the lesions is actually effective; the other lesions result in unnecessarily destroyed cardiac tissue. 
     Deficiencies of radio frequency ablation devices and techniques have been overcome by using cold to do zero degree or ice mapping prior to creating lesions, as taught in U.S. Pat. Nos. 5,423,807; and 5,281,213; and 5,281,215. However, even though combined cryogenic mapping and ablation devices permit greater certainty and less tissue damage than RF devices and techniques, both the cryogenic and the RF devices are configured for spot or roughly circular tissue ablation. 
     Spot tissue ablation is acceptable for certain procedures. However, other procedures can be more therapeutically effective if multiple spot lesions along a predetermined line, or a single elongate or linear lesion is created in a single ablative step. Radio frequency ablation devices are known to be able to create linear lesions by dragging the ablation tip along a line while it is active. However, no cryogenic devices are known that are optimized for, or which are even minimally capable of, creating an elongate lesion. 
     Furthermore, when using a catheter to simultaneously perform electrocardiographic mapping as well as to treat tissue being mapped, the treatment zone extending around the catheter is limited. This is especially problematic with regard to bipolar or multipolar electrode catheters having multiple electrocardiogram (ECG) leads disposed at the distal tip. The arrangement of the ECG leads around the distal tip in relation to the tissue treatment element is such that once a tissue node of interest has been mapped, the catheter&#39;s treatment zone extending around the distal tip is generally not large enough or shaped to encompass the tissue node. The catheter must be moved of repositioned to effectuate treatment. It would be desirable therefore, to provide a catheter having the electrocardiographic mapping capabilities as discussed above in addition to an extended treatment zone to as to better perform the twin functions of mapping and treatment or ablation of tissue. 
     SUMMARY OF THE INVENTION 
     The present invention provides a cryogenic catheter having an elongate outer member and a plurality of inner members disposed within the elongate outer member. The inner members have a plurality of controllable openings formed thereon for the selective release of cryogenic fluid. A plurality of electrode members is disposed on an external surface of the outer member. The inner members may be positioned in a staggered configuration or alternatively at least one inner member may be disposed within another inner member. In such a configuration, one of the inner members may be slidable or rotatable to the other. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a schematic illustration of an embodiment of a cryosurgical system in accordance with the invention; 
         FIG. 2  is a schematic depiction of the chambers of the heart showing placement of the catheter of  FIG. 1 ; 
         FIG. 3  illustrates the tip region of one embodiment of the catheter in accordance with the invention; 
         FIG. 4  illustrates an alternative embodiment of the catheter of  FIG. 3 ; 
         FIG. 5  illustrates yet another embodiment of the catheter; 
         FIG. 6  illustrates a deformable tip for a catheter; 
         FIG. 7  illustrates yet another embodiment of the catheter; 
         FIG. 8  is a sectional view of the catheter of  FIG. 7  taken along line  8 - 8 ; 
         FIG. 9  is a sectional view of an alternative embodiment of the linear ablation catheter illustrated in  FIG. 7 ; 
         FIG. 10  illustrates an expansion chamber within a portion of a helical coil; 
         FIG. 11  illustrates a portion of a catheter having an elongate, thermally-transmissive strip; 
         FIG. 12  is a sectional view of the catheter of  FIG. 3  taken along line  12 - 12 ; 
         FIG. 13  is a sectional view of the catheter of  FIG. 3  taken along line  13 - 13 ; 
         FIGS. 14-16  are sectional views of additional catheter embodiments; 
         FIG. 17  illustrates an inner face of a flexible catheter member; 
         FIG. 18  depicts yet another embodiment of a catheter in accordance with the invention; 
         FIG. 19  is a table illustrating cooling performance of a catheter in accordance with the invention; 
         FIG. 20  is a sectional view of another catheter embodiment; 
         FIG. 21  is a sectional view of a portion of the catheter of  FIG. 20 ; 
         FIG. 22  is a detailed view of an area of the catheter portion illustrated in  FIG. 21 ; 
         FIG. 23  is an illustration of yet another catheter embodiment; 
         FIG. 24  depicts still another catheter embodiment; 
         FIG. 25  illustrates yet another embodiment of the catheter; 
         FIG. 26  is a sectional view of the catheter of  FIG. 25  taken along line  26 - 26 ; 
         FIG. 27  illustrates yet still another embodiment of the catheter; 
         FIG. 28  illustrates the catheter of  FIG. 27  in a second configuration; 
         FIG. 29  is a sectional view of the catheter of  FIG. 28  taken along line  29 - 29 ; 
         FIG. 30  is a sectional view of the catheter of  FIG. 28  taken along line  30 - 30 ; 
         FIG. 31  illustrates yet another embodiment of the catheter; 
         FIG. 32  illustrates the catheter of  FIG. 31  in a second configuration; 
         FIG. 33  is a sectional view of the catheter of  FIG. 32  taken along line  33 - 33 ; 
         FIG. 34  is a sectional view of the catheter of  FIG. 32  taken along line  34 - 34 ; 
         FIG. 35  illustrates yet another embodiment of the catheter; 
         FIG. 36  is a sectional view of yet another embodiment of the catheter; 
         FIG. 37  is a sectional view of the catheter of  FIG. 36  after rotation; 
         FIG. 38  illustrates yet another embodiment of the catheter; 
         FIG. 39  illustrates the catheter of  FIG. 38  in a second configuration; 
         FIG. 40  illustrates one embodiment of the catheter proximate a treatment node, with a limited treatment zone shown around the catheter; 
         FIG. 41  illustrates another embodiment of the catheter proximate the treatment node of  FIG. 40 , with an extended treatment zone provided by the catheter; 
         FIG. 42  illustrates an expanded cross-sectional view of the catheter of  FIG. 41 , taken along lines A-A in  FIG. 41 ; 
         FIGS. 43A and 43B  are perspective views of two addition embodiments of the flow conduit of the catheter of  FIG. 42 ; and 
         FIG. 44  illustrates a cross-sectional view of yet another embodiment of the catheter 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic illustration of a cryosurgical system in accordance with the invention. The system includes a supply of cryogenic or cooling fluid  10  in communication with the proximal end  12  of a flexible catheter  14 . A fluid controller  16  is interposed or in-line between the cryogenic fluid supply  10  and the catheter  14  for regulating the flow of cryogenic fluid into the catheter in response to a controller command. Controller commands can include programmed instructions, sensor signals, and manual user input. For example, the fluid controller  16  can be programmed or configured to increase and decrease the pressure of the fluid by predetermined pressure increments over predetermined time intervals. In another exemplary embodiment, the fluid controller  16  can be responsive to input from a foot pedal  18  to permit flow of the cryogenic fluid into the catheter  14 . One or more temperature sensors  20  in electrical communication with the controller  16  can be provided to regulate or terminate the flow of cryogenic fluid into the catheter  14  when a predetermined temperature at a selected point or points on or within the catheter is/are obtained. For example a temperature sensor can be placed at a point proximate the distal end  22  of the catheter and other temperature sensors  20  can be placed at spaced intervals between the distal end of the catheter and another point that is between the distal end and the proximal end. 
     The cryogenic fluid can be in a liquid or a gas state. An extremely low temperature can be achieved within the catheter, and more particularly on the surface of the catheter by cooling the fluid to a predetermined temperature prior to its introduction into the catheter, by allowing a liquid state cryogenic fluid to boil or vaporize, or by allowing a gas state cryogenic fluid to expand. Exemplary liquids include chlorodifluoromethane, polydimethylsiloxane, ethyl alcohol, HFC&#39;s such as AZ-20 (a 50-50 mixture of difluoromethane &amp; pentafluoroethane sold by Allied Signal), and CFC&#39;s such as DuPont&#39;s FREON. Exemplary gasses include nitrous oxide, and carbon dioxide. 
     The catheter  14  includes a flexible member  24  having a thermally-transmissive region  26  and a fluid path through the flexible member to the thermally-transmissive region. A fluid path is also provided from the thermally-transmissive region to a point external to the catheter, such as the proximal end  12 . Although described in greater detail below, exemplary fluid paths can be one or more channels defined by the flexible member  24 , and/or by one or more additional flexible members that are internal to the first flexible member  24 . Also, even though many materials and structures can be thermally conductive or thermally transmissive if chilled to a very low temperature and/or cold soaked, as used herein, a “thermally-transmissive region” is intended to broadly encompass any structure or region of the catheter  14  that readily conducts heat. 
     For example, a metal structure exposed (directly or indirectly) to the cryogenic fluid path is considered a thermally-transmissive region  26  even if an adjacent polymeric or latex catheter portion also permits heat transfer, but to a much lesser extent than the metal. Thus, the thermally-transmissive region  26  can be viewed as a relative term to compare the heat transfer characteristics of different catheter regions or structures. 
     Furthermore, while the thermally-transmissive region  26  can include a single, continuous, and uninterrupted surface or structure, it can also include multiple, discrete, thermally-transmissive structures that collectively define a thermally-transmissive region that is elongate or linear. Depending on the ability of the cryogenic system, or portions thereof, to handle given thermal loads, the ablation of an elongate tissue path can be performed in a single or multiple cycle process without having to relocate the catheter one or more times or drag it across tissue. Additional details of the thermally-transmissive region  26  and the thermal transfer process are described in greater detail below. 
     In exemplary embodiments of the invention, the thermally-transmissive region  26  of the catheter  14  is deformable. An exemplary deformation is from a linear configuration to an arcuate configuration and is accomplished using mechanical and/or electrical devices known to those skilled in the art. For example, a wall portion of the flexible member  24  can include a metal braid to make the catheter torqueable for overall catheter steering and placement. Additionally, a cord, wire or cable can be incorporated with, or inserted into, the catheter for deformation of the thermally transmissive region  26 . 
     The cryogenic system of  FIG. 1  is better understood with reference to its use in an operative procedure as shown in  FIG. 2 . Following the determination of a proposed lesion site within a heart chamber  28 , for example, the catheter  14  is directed through a blood vessel  30  to a region within the heart, such as an atrial or ventricular chamber, where the lesion will be made. The thermally-transmissive region  26  is placed proximate to the tissue to be ablated. The thermally-transmissive region of the catheter may be deformed to conform to the curvature of the tissue before, during, or after placement against the tissue. The controller  16  allows or causes cryogenic fluid to flow from the cryogenic fluid supply  10  to the fluid path in the catheter  14  and thence to the thermally-transmissive region  26  to ablate the desired area or to cold map along the same tissue area. In one embodiment (e.g.,  FIG. 12 ) a first conduit is concentric within a second conduit and cooling fluid travels to a thermally-transmissive region proximate a closed distal end of the catheter through a first conduit (fluid path) and is exhausted from the catheter through the second conduit (fluid path). 
     Having described the function of the cryogenic catheter  14  and its use in a system context, several exemplary embodiments of the thermally-transmissive region  26  of the catheter are now described in greater detail.  FIGS. 3 ,  4 ,  5 ,  12 - 16  and  18  illustrate embodiments of the catheter, or portions thereof, having two or more thermally-transmissive segments in a spaced-apart relationship. Each of the illustrated catheters includes a closed tip  32  that can include a thermally-transmissive material. 
     Referring specifically to the embodiment depicted in  FIG. 3 , multiple thermally-transmissive elements  34  are integral with a distal portion of a catheter. Each of the thermally-transmissive elements  34  includes a first side or face  36  (shown in  FIGS. 12 and 13 ) exposed to a cryogenic fluid path and cryogenic fluid (shown by arrows) and a second side or face  38  exposed to points exterior to the catheter. As shown in  FIG. 13 , the first side  36  and/or second side  38  of any or all of the thermally-transmissive elements  34  can be substantially flush with, recessed below, or protruding from the inner surface  40  and outer surface  42  of a portion of the catheter. The thermally-transmissive elements  34  are separated by flexible portions of material  44  than can range from slightly less thermally-transmissive than the adjacent thermally-transmissive elements to substantially less thermally-transmissive than the adjacent elements. In the illustrated embodiment of  FIG. 3 , the thermally-transmissive elements  34  are annular, cylindrical elements which are made of gold-plated copper, bronze, or stainless steel. Thermocouples  35  can be associated with one or more of the elements  34  and the tip  32 . The thermally-transmissive elements  34  can be completely exposed, embedded, or a combination thereof along the full 360 .degree. of the catheter&#39;s circumference. In certain applications the thermally-transmissive elements traverse or define less than 360 .degree. of the catheter&#39;s circumference as shown in  FIGS. 14-16  and as described below. The longitudinal width of each thermally-transmissive element  34 , the spacing between elements, the material thickness, and the material composition are matched with a selected cryogenic fluid, one or more cryogenic fluid delivery locations within the catheter and fluid delivery pressure to produce overlapping cold regions which produce a linear lesion. 
     The embodiment illustrated in  FIG. 4  is substantially identical to the embodiment of  FIG. 3 , however, at least one of the thermally-transmissive elements  34  includes a first open end  46  that defines a first plane and a second open end  48  that defines a second plane, wherein the first and second planes intersect to give the annular elements a wedge-like appearance. Such a configuration permits adjacent thermally-transmissive elements  34  to be positioned very closely together, but it can limit the possibilities for deforming the thermally-transmissive region  26 , which, in this embodiment, is flexible in the direction indicated by the arrow. 
     With respect to the embodiments shown in both  FIGS. 3 and 4 , the thermally-transmissive elements  34  are substantially rigid and are separated and/or joined by a flexible material  44 . However, in other embodiments the thermally-transmissive elements  34  are flexible and are interdigitated with either rigid or flexible segments.  FIG. 5 , for example, illustrates an embodiment of the cryogenic catheter having three thermally-transmissive elements  34  that are flexible. The flexibility is provided by a folded or bellows-like structure  50 . In addition to being shapable, a metal bellows can have enough stiffness to retain a selected shape after a deforming or bending step. 
     Instead of, or in addition to, flexible, thermally-transmissive elements  34  and/or flexible material  44  between elements, the distal tip  32  (or a portion thereof) can be deformable. For example,  FIG. 6  illustrates a tip  32  having thermally-transmissive, flexible, bellows  50 . 
     Referring now to  FIGS. 7-10 , a different approach is shown for providing multiple thermally-transmissive segments in a spaced-apart relationship.  FIG. 7  illustrates a catheter embodiment having an elongate, thermally-transmissive region  26  that includes a helical coil  52  at least partially embedded in the flexible member  24 . As shown in  FIG. 8 , at least a first portion  54  of the helical coil  52  is exposed to a fluid path within the flexible member  24  and a second portion  56  of the helical coil is exposed to the exterior of the flexible member. As described above with respect to  FIG. 13 , the first portion  54  of the coil can be substantially flush with, recessed below, or protruding from an inner surface  58  of the flexible member  24 . Similarly, the second portion  56  of the coil  52  can be substantially flush with, recessed below, or protruding from an outer surface  60  of the flexible member  24 . 
     In the embodiment of  FIG. 8 , the second portion  56  of the coil  52  is exposed along only a portion of the outer circumference of the flexible member  24  to define a longitudinally-elongate, thermally-transmissive region  26 . This configuration can be provided by eccentrically mating the helical coil  52  to the catheter so that the longitudinal axis of the coil and the longitudinal axis of the catheter are substantially parallel. The eccentric positioning of the coil  52  provides excellent cooling performance because the surface area available for thermal exchange between the first portion  54  of coil and the cryogenic fluid is greater than the surface area available for thermal exchange between the second portion  56  of the coil and adjacent tissue where cooling power is delivered by each exposed coil portion to provide a linear lesion. 
     Referring now to  FIG. 9 , an alternative embodiment is shown wherein a first portion  62  of the coil  52  is exposed around the entire circumference of the flexible member  24 , and a second portion  64  is exposed to a fluid path around the inner surface of the flexible member  24 . This is achieved by having the longitudinal axis of the helical coil  52  co-axial with the longitudinal axis of the catheter. 
     In the embodiments illustrated in  FIGS. 7-9 , the coil  52  is solid. However, in other embodiments the coil can be an elongate, hollow, gas expansion chamber. For example,  FIG. 10  illustrates a portion of a helical coil  52  that includes a passage that defines at least a portion of a fluid path through a flexible member of the catheter. The coil  52  defines a first fluid path diameter at a fluid entry point  66  and a second fluid path diameter that is greater than the first fluid path diameter at a gas expansion or boiling location  68 . Gas escaping from a fluid exit point  70  can be exhausted through an open central region of the coil and/or another passage through the flexible member  24 . 
       FIG. 11  illustrates an embodiment of the catheter wherein a continuous, elongate, thermally-transmissive strip  72  is longitudinally integrated with a flexible member  24 . The strip can include a bellows-like structure. As described above with respect to other embodiments, a first portion of the strip can be substantially flush with, recessed below, or protrude from the outer surface of the flexible member. Similarly, a second portion of the strip can be substantially flush with, recessed below, or protrude from an inner surface of the flexible member. 
     Referring now to  FIG. 12 , an embodiment of the catheter is illustrated having a second or inner flexible member  74  within a lumen of first or outer flexible member  24 , wherein the second flexible member defines a fluid path to the thermally-transmissive region  26 . The inner member  74  can include a single opening  76  at or near the tip  32 . Cryogenic fluid is expelled from the opening  76  and returns to the proximal end of the catheter along a fluid path defined by the outer wall of the inner member  74  and the inner wall of the outer member  24 . This fluid path configuration is also partially illustrated in  FIGS. 8 ,  9 , and  13 . Alternatively, as also shown in  FIG. 12 , the inner member  74  can be provided with multiple openings  78  proximate to and/or aligned with the inner face of one or more thermally-transmissive elements  34  to achieve more uniform cooling across the entire elongate, thermally-transmissive region  26 . 
     Referring now to  FIGS. 14-16 , sectional views of catheter embodiments are illustrated to show alternative configurations for thermally-transmissive elements. The previously described thermally-transmissive elements  34  are arcuate and form complete and continuous 360 degree structures that traverse the complete circumference of the catheter, notwithstanding being flush with, depressed below, or raised above the outermost surface of the flexible member  24 . However, the arcuate elements  34 ′,  34 ″, and  34 ′″ illustrated in  FIGS. 14-16 , respectively, traverse less than 360 degrees of the circumference of the first flexible member and do not form complete loops. For example, in  FIG. 14 , element  34 ′ defines an approximately 270 degree arc. In  FIG. 15  the thermally-transmissive element  34 ″ defines an approximately 180 degree arc; and in  FIG. 16 , the thermally-transmissive element  34 ″′ defines an approximately 90 degree arc. A catheter can include combinations of element types, such as a complete ring or loop element, a 270 degree element and a 180 degree element as desired to define a thermally transmissive region. In addition to the having applicability with respect to rigid thermally-transmissive elements, the bellows-like elements can also be less than 360 degrees. 
     The less than 360 degree arcuate elements provide unique functional benefits with respect to thermal transfer and flexibility of the thermally-transmissive region. For example, because the portion of the catheter between the opposing ends of element  34 ′,  34 ″,  34 ′″ does not include a rigid structure, but rather only the resilient material of flexible member  24 , the thermally-transmissive region of the catheter can be more tightly curved (gap between ends inward and element facing outward) than it could with complete 360 degree structures, especially if the elements are relatively long longitudinally. 
     The inner member  74  can be adapted to direct cooling fluid at only the thermally transmissive element(s) and the shape and/or the number of openings for cooling fluid can be configured differently depending on the length of the arc defined by the thermally-transmissive element(s). For example,  FIG. 14  illustrates an embodiment of the inner member having three openings opposing the thermally transmissive element  34 ′;  FIG. 15  illustrates two openings for a smaller arc; and  FIG. 16  discloses a single opening for an even smaller arc. 
     Another advantage to providing one or more thermally-transmissive elements that have a less than 360 degree configuration is that limiting the span of the elements to a desired lesion width, or somewhat greater than a desired lesion width, reduces the thermal load on the system and/or permits colder temperatures to be achieved than with respect to a complete 360 degree structure. Unnecessary and perhaps undesirable cooling does not occur at any other location along the catheter except at an elongate region of predetermined width. A similar effect can also be achieved by providing a non-circular 360 degree element or by eccentrically mounting a circular 360 degree element with respect to the flexible member, wherein a portion of the 360 degree element is embedded within the wall of the flexible member or otherwise insulated from the cryogenic fluid path in a manner similar to that shown in  FIG. 8 . 
     Referring now to  FIG. 17 , a portion of the inner face of an outer flexible member showing in an exemplary embodiment, thermal transfer pins  80  protruding from the inner face of a thermally-transmissive element  34 . The pins permit thermal transfer through the flexible member  24 . As with the other features of the invention, the pins are equally suitable for complete 360 degree element structures or less than 360 degree structures. Although only pins are shown on any geometric or surface means to increase heat transfer including but not limited to pins, irregularities, channels or surface modifications may be used. Referring now to  FIG. 18 , yet another embodiment of the catheter is shown wherein rigid metal rings  34   a - c  are interdigitated with flexible segments  44   a - c  to define a first flexible member and a thermally-transmissive region approximately one inch in length. A second flexible member is concentric within the first flexible member and has an outlet for cryogenic fluid at its distal end. Thermocouples  82   a - c  can be associated with one or more of the rings  34   a - c.    
     It has been described above how the thermal loading of a cooling system can be reduced by providing thermally-transmissive elements that span less than 360 degrees. However, the thermal loading can also be reduced by sequentially cooling the thermally-transmissive region. One way to sequentially cool is to modulate the pressure of the cooling fluid along the fluid path through the flexible member. This modulation can be performed by the fluid controller which can be programmed to increase and decrease the pressure of the fluid by predetermined pressure increments over predetermined time intervals. When the cryogenic fluid is a liquid that provides cooling by changing phase from liquid to gas, the change of pressure alters the physical location along the fluid path where the phase change takes place and concomitantly changes the point of coldest temperature along the thermally-transmissive region. Thus, varying the pressure of the fluid can provide a moving ice-formation “front” along the catheter, enabling the creation of a linear lesion. 
     Therefore, a method of forming an elongate tissue lesion can include the following steps using any of the above described catheters having an elongate, thermally-transmissive region. In a first step a cryogenic fluid is introduced into the flexible member at a first predetermined pressure. Next, the pressure of the cryogenic fluid is incrementally increased within the flexible member until a second predetermined pressure is achieved. Similarly, the pressure of the cryogenic fluid within the flexible member can be decreased incrementally from the second predetermined pressure to the first predetermined pressure, wherein the steps of incrementally increasing and decreasing the pressure define a thermal cycle. Typically, from one to eight thermal cycles are required to achieve a desired therapeutic effect. In an exemplary method, about ten increments of about five seconds in duration are selected and pressure is increased by about 20 to 40 pounds per square inch in each increment. Thus, using this method an elongate lesion can be created in less than 20 minutes. 
       FIG. 19  is a table that illustrates sequential cooling in a catheter as described above having a thermally-transmissive region that includes a tip and three elements or rings. The table illustrates three tests conducted in a still bath at 37 .degree. C., using AZ-20 as the cryogenic fluid. Alternatively, nitrous oxide could be used as the cryogenic fluid. Associated with each pressure increment are measured temperatures at the tip, first ring, second ring, and third ring. The shaded region illustrates the sequential movement of a target temperature range (upper −40&#39;s to low −50&#39;s or lower) in response to a change in pressure. Although values are only provided for three rings, a similar effect and pattern is obtained with more than three rings or elements. 
     Turning now to  FIG. 20 , a thermally-transmissive portion of another embodiment of a medical device or structure such as a catheter is illustrated in a sectional view. The structure can include an inner passage or lumen as described above with respect to other embodiments, but which is not shown in this illustration for purposes of clarity. Thus, the illustrated portion is the outer passage or lumen that defines an elongate ablation region. Thermally-transmissive elements  84 , such as gold plated copper, are joined to adjacent elements by resilient connecting elements  86 , such as a stainless steel springs welded to the ends of the elements  84 . A resilient bio-compatible material  88  covers the connecting elements  86  and the interstices between adjacent thermally-transmissive elements. In an exemplary embodiment, the material  88  is vulcanized silicone. It should be noted in the illustration that the surface of the elements  84  is contiguous and co-planar with the material  88  to provide a smooth outer surface. 
       FIG. 21  illustrates a single thermally-transmissive element  84  having reduced diameter ends  90  and  92 . The wider central portion  94  provides an expansion chamber for gas (shown by arrows) exiting an apertured inner passage  96 .  FIG. 22  shows additional detail of the end  90  of the element  84 . The end  90  is textured, such as by providing serrations  98 , to provide a good adhesion surface for the material  88 . 
     Referring now to  FIG. 23 , a thermally-transmissive portion of yet another embodiment of a flexible cryogenic structure is illustrated in a sectional view. In this embodiment an inner, apertured structure  100  has a flat wire  102  wrapped around it in a spiral manner. Thermally-transmissive segments  104  are disposed upon the wire  102  in a spaced-apart relationship, and a flexible, bio-compatible material  106  fills the interstices between segments  104 . A thermocouple  108  can be associated with each segment  104 . A wire  109  connects the thermocouple  108  to instrumentation near the proximal end of the structure. The exterior surface of the structure is smooth, and the structure can include 3 to 12 segments  104 . In an exemplary embodiment the inner structure  100  is made of PTFE, the material  106  is 33 D PEBAX, and the wire  102  is stainless steel or Nitinol. An apertured inner passage (similar to that shown in  FIG. 21 ) is placed within the structure. 
       FIG. 24  illustrates still another embodiment of a cryogenic cooling structure that includes a surface or wall  110  including a polymer or elastomer that is thin enough to permit thermal transfer. For example, polyamide, PET, or PTFE having a thickness of a typical angioplasty balloon or less (below 0.006 inches) provides acceptable thermal transfer. However, the thinness of the wall  110  allows it to readily collapse or otherwise deform under vacuum or near vacuum conditions applied to evacuate fluid/gas from the structure. Accordingly, the structure is provided with one or more supporting elements  112  such as a spring. The cooling structure is illustrated in association with a catheter  114  having a closed distal tip  116  and mono or bipolar ECG rings  118 ,  120 ,  122 . The thermally-transmissive region is approximately 30 mm in length and is effective for thermal transfer over its entire circumference. However, as illustrated in  FIG. 11 , the thermally-transmissive region can be confined to specific region(s) of the device&#39;s circumference. 
     Referring now to  FIG. 25 , an embodiment of the catheter is illustrated having three flexible members or injection tubes  210 ,  211  and  212  disposed within a first or outer flexible member  200 . In an exemplary embodiment, the inner flexible members  210 ,  211  and  212  are arranged in a staggered configuration within the outer flexible member  200 . As used herein, term “staggered” may be used to designate both a linearly/axially staggered configuration or alternatively, a rotationally staggered configuration. The flexible members  210 ,  211  and  212  thus define multiple staggered fluid paths within the outer member  200 . In such a configuration, the injection tubes  210 ,  211  and  212  allow for greater aggregate cooling power as well as the creation of a variety of different cooling/freeze zones  201 ,  203  and  205  along the length of the outer flexible member  200 . In an exemplary embodiment, thermocouples  204  disposed along the outer surface of the outer flexible member  200  may be integrated with an internal feedback loop to provide independent and variable regulation of these freeze zones. 
     In an exemplary embodiment, the first inner member  210  includes at least one opening  214  positioned proximate an electrode ring member  207 . Cryogenic fluid is expelled from the opening  214  and returns to the proximal end of the catheter along a fluid path defined by the inner wall  218  of the outer member  200 , as shown in  FIG. 26 . Similarly, the second inner member  211  includes at least one opening  215  positioned proximate a second electrode ring member  208 . Cryogenic fluid is also expelled from the opening  215  and returns to the proximal end of the catheter along the fluid path defined by the inner wall  218  of the outer member  200 . Similarly, the third inner member  212  includes at least one opening  216  positioned proximate a third electrode ring member  209 . 
     Alternatively, the catheter can be provided with only two inner members, or four or more inner members, not shown, disposed within the outer member. The inner members would have one or more openings proximate to and/or aligned with the inner face of one or more transmissive elements, as described earlier herein, to achieve different regions of freeze zones across the entire elongate member. Alternatively, all the staggered inner members may be simultaneously provided with cryogenic fluid to create a linear lesion for selected applications. The flow of cooling fluid along the fluid paths through the flexible members can also be alternated in any number of patterns among the multiple inner members to provide a desired cooling pattern such as a discontinuous or a continuous lesion across the entire catheter. 
     In an exemplary embodiment, a catheter with a plurality of thermally conductive electrode rings would have an underlying injection tube or tubes controlling the release of cryogenic fluid to each electrode. Such a catheter could be placed in the coronary sinus or endocardially along the atrioventricular junction. Once positioned, an electrogram of interest is located using a specific electrode ring on the catheter. Coldmapping may be performed on the selected location to confirm the correctness of the location. Once, confirmed, the area is cryoablated using the same electrode ring. The same embodiments and others described herein are equally suited to other organs besides the heart and/or any body portion that would benefit from the application of thermal energy. 
     Referring now to  FIG. 27 , an embodiment of the catheter is illustrated having an outer member  220  with a fixed injection tube  230  disposed within a slidable sheath or overtube  240  therein. The injection tube and overtube are shown spaced apart for illustrative purposes only. Preferably, the injection tube is sized so that an outer surface of the injection tube engages an inner surface of the overtube while still allowing one member to slide or rotate relative to the other. 
     The fixed injection tube  230  has multiple openings  232 ,  234  formed thereon and the slidable overtube also has multiple openings or ports  242 ,  244  formed thereon. In one configuration shown in  FIG. 27 , opening  232  on the injection tube  230  coincides or is aligned with opening  242  on the slidable overtube  240 . Thus, any fluid exiting the injection tube  230  from opening  232  is able to escape through opening  242 . 
     As the slidable overtube  240  is slid or moved in a first direction as shown by arrow  236  along longitudinal axis  222 , opening  232  is covered or blocked by the surface of overtube  240  as now shown in  FIG. 28 . In a second configuration shown in  FIG. 29 , opening  234  of injection tube  230  is aligned with opening  244  of overtube  240 . In the same configuration, as shown in  FIG. 30 , opening  242  is not aligned with any opening formed on the surface of injection tube  230 . Although only shown in two positions or configurations, the slidable overtube is positionable in any number of positions relative to the fixed injection tube. The overtube may also be used to partially cover the openings on the injection tube to provide for a limited or controlled flow of cryogenic fluid. 
     Depending on which opening of the injection tube is aligned with the openings formed on the overtube, cryogenic fluid is expelled from the opening and returns to the proximal end of the catheter along a fluid path defined by the inner wall  226  of the outer member  220 . The non-aligned opening will not expel fluid since the opening will be blocked. Alternatively, the injection tube and overtube can be provided with three or more openings to achieve multiple cooling/freeze zones along the length of the catheter. 
     Referring now to  FIG. 31 , an embodiment of the catheter is illustrated having a slidable injection tube  260  disposed within a fixed sheath or overtube  270 . As shown in  FIG. 31 , both the injection tube  260  and overtube  270  are disposed within a flexible outer member  250 . The slidable injection tube  260  has multiple openings  262 ,  264  formed thereon which allows for the release of cryogenic fluid. The fixed overtube  270  also has multiple perforations or openings  272 ,  274  formed thereon which allows for the differential release of fluid as described in more detail below. The injection tube may be further provided with a thermistor  254  disposed proximate the distal end of the tube to provide thermistor feedback. In one embodiment, the openings can be controlled by miniaturized means such as micro or nanovalves. 
     In a first configuration shown in  FIG. 31 , opening  262  of the injection tube  260  coincides or is aligned with opening  274  of the fixed overtube  270 . As the slidable injection tube  260  is slid or moved in a first direction as shown by arrow  266 , opening  262  is then aligned with corresponding opening  272  on the overtube  270  in  FIG. 32 . 
     In this second configuration, as shown in  FIGS. 32-34 , openings  262 ,  264  of injection tube  260  are aligned with openings  272 ,  274  of overtube  270 . Although only two configurations for the catheter are shown, the injection tube  260  is positionable in any number of locations relative to the fixed overtube  270 . 
     In operation, cryogenic fluid is expelled from the openings and returns to the proximal end of the catheter along a fluid path defined by an inner wall  256  of the outer member  250 . Alternatively, the injection tube  260  and overtube  270  can be provided with multiple openings proximate to and/or aligned with the inner face of one or more thermally-transmissive elements as described earlier herein to achieve more uniform cooling across the entire elongate, thermally-transmissive region. 
     Referring to  FIG. 35 , an embodiment of the catheter is illustrated having an outer member  280  with an injection tube  290  with multiple opposed openings  292 - 297  formed therein. Either the injection tube  290  or the overtube  300  may be slidable in a longitudinal plane to expose and/or cover one or more of the opposed openings on the injection tube  290 . For example, as shown in  FIG. 35 , openings  294 ,  295  formed on the injection tube  290  are aligned with openings  302 ,  303  formed on the overtube  230 . Furthermore, the injection tube may be positioned in a forwardmost position, not shown, to expose openings on the injection tube proximate the tip of the catheter. In this configuration, the injection tube would provide fluid to cool the area around the tip of the catheter. 
     In the embodiments described and shown above in  FIGS. 32-35 , electrode rings as shown in  FIG. 25  may be provided along the outer surface of any of the outer members. The electrodes would serve both as electrical conductors and as a thermal transmitter at each location. 
     Referring to  FIGS. 36 and 37 , an embodiment of the catheter is illustrated have one or more rotatable members disposed within a flexible outer member  310 . In this embodiment, the catheter includes an overtube member  312  and an injection tube member  314 , one or both of which are rotatable with respect to one another. In an exemplary embodiment as shown in  FIGS. 36 and 37 , the injection tube  314  is rotatable relative to the fixed overtube  312 . The injection tube  314  may be rotatable in either or both a clockwise and counterclockwise direction as indicated by arrows  320  and  322 . As shown in  FIG. 36 , in a first configuration, opening  316  formed on the overtube  312  aligns with an opening  318  formed on the injection tube  314 . As the injection tube  314  is rotated in a counterclockwise direction, the opening  318  on the injection tube  314  is placed out of alignment with the opening  316  formed on overtube  312 , as shown in  FIG. 37 . Alternatively, the injection tube  314  may be fixed in the catheter while the overtube  312  is rotatable. In another embodiment, both the injection tube and overtube may both be rotatable. In yet a further embodiment, the injection tube and/or the overtube are rotatable and slidable within the outer member. 
     In the embodiments shown and described above, the slidable and rotatable inner and outer tubes may have openings so arranged as to allow the fluid releasing openings to be in a variety of open and closed configurations with a minimum of relational movement between the tubes. For example, as shown in  FIG. 38 , an outer member  330  has disposed therein one slidably disposed inner tube  336  which has openings  338  formed thereon in a constant sequence, and a matching slidably disposed outer tube  332  which has openings  334  formed thereon in a constant sequence of slightly different length or intervals. 
     In this configuration, as shown in  FIG. 39 , small linear relational movements bring the openings on the outer tube  332  and the inner tube  336  into an overlapping configuration. 
       FIG. 40  illustrates one embodiment of the catheter proximate a treatment node, with a limited treatment zone shown around the catheter. Only the distal end portion of the catheter is shown, labeled generally as  400 . The catheter  400  is shown positioned proximate a tissue treatment region or node  401 , marked with an “x” in  FIG. 40 . The treatment node  401  may be, for example, the atrioventricular node or any other discrete locus of tissue to be treated and/or mapped. 
     Catheter  400  includes a catheter body  402 , a distal tip  405  and several ECG leads or rings  410 ,  415 , and  420 . Each of the ECG rings is staggered along the length of the catheter body  402  as shown. The ECG rings may be part of a bipolar, tripolar, or quadripolar electrocardiographing mapping apparatus. Each of the ECG rings  410 ,  415 , and  420  are made of an electrically conductive material. 
     In operation, the catheter is coupled to a source of fluid coolant or cryogen at its proximal end portion (not shown). The coolant is directed to flow in an internal injection lumen or tube (not shown) through to the distal tip portion  405 . Generally, the injection tube includes at least one orifice or opening though which the coolant exits the tube and flows into a second return lumen (not shown) back to the proximal end portion of the catheter. The orifice included may be a typical Joule-Thomson gas expansion element, whereupon high pressure coolant in gas phase expands upon exiting the orifice to low pressure, with an attendant drop in temperature. This endothermic process acts to cool the environment immediately around the distal tip  405 , which itself may be thermally conductive or heat-transmitting, so as to transmit the cooling generated by the expansion of gases inside the catheter. The cooling process may also be generated by the change in phase of coolant from liquid to gas, in conjunction with Joule-Thomson expansion. This is generally true for coolant that is supplied at mixed gas-liquid phase. 
     When cooling is thus initiated, the environment around the catheter is cooled to form an iceball, the shape of which is approximately shown by contour  424  in  FIG. 40 . This shape is achieved by a conventional cooling catheter as described above, where only one injection orifice or Joule-Thomson element is employed. However, this shape of cooling contour is generally not desirable when employing the catheter for cardiac mapping in addition to tissue treatment. For cardiac mapping, the catheter is first used to cool tissue so as to locate a treatment node or tissue pathway, as is well-known to those skilled in the art. The treatment node in question is shown in  FIG. 40  as point “X”, or labeled as  401 . For a bipolar ECG catheter, node  401  generally lies at a longitudinal point between the first ECG lead, or distal tip  405 , and the second ECG lead, or ring electrode  410 . To locate the node  401 , the catheter  400  must be positioned and oriented relative to the node  401  as shown in  FIG. 40 . If cooling were initiated the catheter  400  would produce a cooling contour or iceball  424  as shown, which would not cover the treatment node  401 . Thus, the catheter would need to be moved and repositioned to treat or ablate the node  401  after cryomapping such node  401 . 
     The present invention is a device, which provides a cryotreatment catheter having an extended treatment zone or cooling contour. As used herein, the term “cryotreatment” shall mean the application of cooling to tissue of varying degrees, from minor temperature changes effecting reversible alterations in cellular structure to cryosurgical ablation and/or removal of tissue. Also as used herein, the term “tissue mapping” or “cryomapping” shall refer to the method, practice, or process of applying cooling to tissue and measuring the properties of such tissue in response to said cooling. 
     Turning now to  FIG. 41 , the present invention provides the catheter of  FIG. 40 , labeled generally as  400 , and having, instead of a limited treatment zone  424  as shown in  FIG. 40 , an extended treatment zone  426  as shown in  FIG. 41 . The extended treatment zone is not necessarily limited to the shape or contour of zone  426 , but rather may encompass any contour that is relatively larger than contour  424 , extending significantly further away from the distal tip  405 , so as to affect not only the tissue proximate the distal end of the catheter  400 , but also the tissue along the sides of the catheter body  402 . 
       FIG. 42  illustrates an expanded cross-sectional view of the distal end portion of the catheter  400  of  FIG. 41 , taken along lines A-A in  FIG. 41 . The distal end portion of the catheter  400  includes, in addition to distal tip  405  and first ECG ring lead  410 , a fluid conduit  430  defining a return lumen  432 , a first fluid injection port  435 , secondary fluid injection ports  438 , a distal end cap  440 , one or more electrically conductive wires or leads  442 , a thermally transmissive, electrically isolating catheter body section  445 , a thermally conductive body section  450 , and one or more seals  452 . Furthermore, a longitudinal axis  455  and a transverse axis  460  are shown superimposed on the cross-section to illustrate the spatial orientation of the various elements. 
     As shown in  FIG. 42 , catheter  400  includes a fluid injection conduit  430  disposed inside of the catheter body  402  to define a fluid return lumen  432  therebetween. Coolant is supplied to conduit  430  at its proximal end (not shown) and flows along the right-facing arrows as shown in  FIG. 42 . The supplied coolant may be any cryogen or cryogenic fluid capable or stable operation at low temperature and high pressure, such nitrous oxide, nitrogen, AZ20, or any suitable refrigerant. The fluid exits the conduit  430  at two or more points, including a first orifice  435  at the distal end of the conduit  430 , and one or more second orifices  438  defined by the lateral walls of the conduit  430 , at some point less proximate the distal tip  405  than the first orifice  435 . Orifice  435  may be considered a primary injection port for coolant flowing into the return lumen  432 , with orifices  438  being the secondary injection ports. However, the relative flow rates of coolant flowing through the orifices may be of any combination or ratio, depending on the relative size and orientation of the orifices, which may vary considerably. In the embodiment of the present invention shown in  FIG. 42 , two secondary orifices  438  are disposed on opposite sides of the lateral wall of the conduit  430 , such that only one opening is visible in the illustration. 
     The secondary orifices  438  are located anywhere along the conduit  430 , further away from the distal end  405  along longitudinal axis  455 . The orifices  438  may be staggered along such axis, and may have varying normal axes (not shown) that may point in any radial direction perpendicular to the longitudinal axis  455 . As used herein, a “normal axis” shall mean the imaginary straight-line axis in space being perpendicular to and centered on the plane formed by the orifice, pointing in the direction of fluid flow. For example, the normal axis of the first orifice  435  is parallel and coincident with the longitudinal axis, pointing to the right in  FIG. 42 . 
     Further embodiments of the conduit  430  are illustrated in  FIGS. 43A and 43B , showing two perspective views of the flow conduit of the catheter of  FIG. 42 . The each of  FIGS. 43A and 43B , only the conduit  430  with its respective orifices  435  and  438  are shown, including the normal axes for each orifice, shown as arrows. The rest of the catheter is not illustrated for ease of reference. In  FIG. 43A , the secondary orifices  438  are staggered along the longitudinal axis  455 , having normal axes facing away from one another. This arrangement leads to a different shape of a freeze zone or treatment contour around the distal end portion of the catheter.  FIG. 43B  illustrates yet another arrangement of primary orifice  435  with the four secondary orifices. This creates yet another shape of a freeze zone or treatment contour around the distal end portion of the catheter. The present invention thus encompasses any number of arrangements of single or multiple primary and secondary orifices  435  and  438 , so as to create treatment zones of a wide variety, depending on the nature of use intended for the catheter. 
     Referring back to  FIG. 42 , as fluid exits the conduit  430 , it flows along the left-facing arrows of  FIG. 42 . The return lumen  432  may have a pressure that is significantly below the pressure in the flow conduit  430 . The fluid exiting either or all of the orifices  435  and  438  may be transformed under gas expansion and evaporation to gas phase from liquid phase, thereby altering the gas dynamic and thermodynamic properties of the fluid flow. A drop in temperature may create an overall endothermic heat transfer with the environment, thereby cooling any tissue surrounding the device. To effectively transmit this cooling effect, the outer walls of the catheter  400  must be thermally “transmissive”, or readily transmit thermal heat flow. Distal end  405  may thus have a conductive end cap  440  made of a thermally conductive material such as a metal. Other suitably strong, resilient yet thermally transmissive materials may be used. 
     The gas dynamic expansion and evaporation discussed above is generally centered about the orifices  435  and  438 . That is, mixed phase liquid/gas coolant undergoes the most rapid change in pressure and temperature directly at the locus of the orifices  435  and  438 . Since end cap  440  primarily surrounds the first orifice  435 , the cooling effect provided by such orifice is primarily transmitted through such cap to the surroundings. The secondary orifices  438  are separated from the primary orifice  435  along longitudinal axis  455  by a separation length “L” as shown. The portion of the catheter body immediately lateral to orifices  438  consists of catheter body section  445 . Section  445  forms a portion of the outer wall of catheter  400 , and completely surrounds ECG ring electrode  410 . As shown in  FIG. 42 , ECG ring electrode  410  circumscribes the outer surface (the surface not in communication with the return lumen  432 ) of section  445 . An electrical communication wire or lead  442  is coupled to the ring electrode. 410 . Section  445  is made of an electrically isolating but thermally conductive material like boron nitride, such as is manufactured by Advanced Ceramics Corporation, of Cleveland, Ohio. 
     Accordingly, ECG ring electrode  410  is completely isolated from the rest of the device electrically, thereby being able to function as an independent terminal for ECG sensing purposes. The wire  442  couples the electrode  410  to the electrical receiving and monitoring apparatus (not shown). It will be appreciated that the shape and configuration of both the electrode  410  and catheter body section  445  may vary widely. The only requirement is that both elements be separated from the first orifice  435  by some separation length L along the longitudinal axis. Furthermore, it is advantageous, although not entirely necessary, to have the electrode  410  disposed at a longitudinal position directly in line with the second orifices  438 , such that a transverse axis parallel to the axis  460  running through orifices  438  would also run though the ring electrode  410 . 
     Catheter body  402  also includes another section  450  adjacent to section  445 . Body section  450  is also thermally conductive, but need not be electrically isolating. Both of sections  445  and  450  serve to transmit the cooling effect of fluid exiting orifices  438 . A sealing element  452  may be interposed between section  450  and the rest of catheter body  402 . 
     The net result of the cooling effect of orifice  435  and orifices  438  is to create an elongate contour  426  whereby tissue proximate the catheter is treated. This extended treatment zone covers a tissue node  401  as shown. Node  410  is longitudinally positioned at some point along separation length L, having transverse axis  460  running therethough. Separation length L may range anywhere from 1 mm to 50 mm. As shown in  FIG. 42 , this positional relationship may vary. Yet, the object of the invention to provide an extended treatment or freeze zone around the distal tip of catheter  400  is accomplished regardless of the particular position of treatment node  401 , the example used in the drawing figures being merely one of a number of applications for which the present invention may be used. 
       FIG. 44  illustrates a cross-sectional view of yet another embodiment of the catheter discussed in  FIGS. 41 and 42 , and labeled generally as  500 . Catheter  500  has a distal end portion, which includes a thermally-transmissive end cap  505  at its tip. The catheter  500  further includes a first ECG ring electrode  507  disposed between two sections of the catheter shaft  508 . The catheter  500  also includes a central injection tube  510  having injection orifices  511  and  512 . The tube  510  is circumscribed by the shaft  508  to define an expansion chamber  514  proximate the end cap  505  and first orifice  511 , and a fluid return lumen further proximate the second orifice  512 . 
     Cryogen is injected through the tube  510  to flow out into the expansion chamber  514  and return lumen  515 , which both serve as spaces wherein the fluid is expanded and evaporated to cool the structures surrounding it. A source of vacuum may be coupled to the proximal end portion (not shown) of the catheter  500  to provide a negative pressure gradient as the expanded cryogen flows away from the tip. The flow into the return lumen  515  is shown by arrows designated as F in , and represents the flow rate of cryogen into the control volume CV as shown. The flow out of the control volume CV is shown by the arrows designated as F out , and represents the mass or volumetric flow rate of cryogen exiting the control volume CV. The control volume CV effectively covers the entire distal end portion of the catheter  500 , and incorporates the locus of sources of cooling or endothermic heat flow with respect to the environment, schematically represented by the arrow Q as shown. 
     Varying the flow rates Fin and Fout may effect the cooling. Thus, the flow may not always be steady state, where F in =F out . For example, in one application of the invention the flow rates may be equal, where F in =F out  and the fluid entering the expansion chamber  514  and return lumen  515  may be just equal to the amount of fluid exiting the distal end portion or control volume CV. This would be the case of steady state flow. In another application, the initial incoming flow F in  would be much greater than Fout (F in &gt;&gt;F out ), resulting in an unsteady flooding of the tip region, and creating a different cooling profile. The varying flow rates would in turn affect the time and amplitude response characteristics of heat flow Q, depending on the desired cooling profile. 
     Furthermore, catheter  500  includes an electrically isolating, yet thermally conductive, buffer element  525  at the distal end of the catheter shaft  508 . The end cap  505  is attached not to the shaft  508  but to such element  525 , thereby isolating the end cap  505  from the rest of the catheter  500 . This reduces electrical or signal noise during operation of the catheter  500  as described above, such as for cryomapping. 
     A variety of modifications and variations of the present invention are possible in light of the above teachings. Specifically, although many embodiments are illustrated being slender and flexible, other embodiments may be thick and rigid, and introduced into the body directly through incisions or through structures such as trocars. Using nanotechnology and miniaturized valving may also control the opening and closing of the catheter openings. Furthermore, although some of the illustrated devices are particularly well suited for cardiac procedures, the same embodiments and others are equally suited to other organs and/or any body portion that would benefit from the application of thermal energy. For example, the illustrated devices may be used for treating arteries for restenosis or portions of the GI tract to stop bleeding or portions of the GU tract to treat spasm, inflammation, obstruction or malignancy. Thus, the devices as shown are not to be limited to catheters but should be viewed more broadly as cryogenic structures or portions thereof. It is therefore understood that, within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described hereinabove. All references cited herein are expressly incorporated by reference in their entirety. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.