Abstract:
Various catheter designs with irrigated tip electrodes are provided for reducing the heating of the tip resulting from RF biasing. The electrode tip of the catheter may be comprised of a highly thermal conductivity layer covered by a biologically compatible material and having irrigation or irrigation channels for removing excess thermal energy from the catheter tip and the surrounding area of the catheter tip. The catheter tip may be designed with multiple irrigation channels, multiple channel pathways, and/or exterior shapes to improve the cooling of the tip. These approaches may be used individually or in combination to produce a catheter tip with improved heat dissipation characteristics.

Description:
TECHNICAL FIELD  
         [0001]    The present invention generally relates to catheters. More specifically, the present invention relates to an improved catheter that may be used in mapping and ablation procedures of biological tissues.  
         BACKGROUND OF THE INVENTION  
         [0002]    For many years, catheters have had widespread application in the medical field. For example, mapping and ablation catheters have been extensively used in the treatment of cardiac arrhythmia. Cardiac arrhythmia treatments help restore the normal operation of the heart in pumping blood to the body. Mapping and ablation catheters play a critical role in these highly delicate treatments.  
           [0003]    Typically, the catheters used in mapping and ablation procedures are steerable electrophysiological (“EP”) catheters that may be precisely positioned anywhere in the heart. These catheters are generally used during two distinct phases of treatment for heart arrhythmia. In one phase of treatment, the catheters are used to map the heart by locating damaged tissue cells. This involves the locating of damaged cells by steering the catheter to selected locations throughout the heart and detecting irregularities in the propagation of electrical wave impulses during contraction of the heart (a procedure commonly referred to as “mapping”). During the other phase of treatment, the same catheters are typically used to create scarring lesions at the location where damaged cells have been found (a procedure commonly referred to as “ablation”).  
           [0004]    Ablation procedures using EP catheters are typically performed using radio frequency (“RF”) energy. In this regard, an EP catheter has one or more ablation electrodes located at its distal end. The physician directs energy from the electrode through myocardial tissue either to an indifferent electrode, such as a large electrode placed on the chest of the patient (in a uni-polar electrode arrangement), or to an adjacent electrode (in a bipolar electrode arrangement) to ablate the tissue. Once a certain temperature has been attained, resistance heating of the tissue located adjacent the one or more electrodes occurs, producing lesions at the targeted tissue.  
           [0005]    Referring to FIG. 1, a conventional catheter that may be used in mapping and ablation procedures is provided. FIG. 1 shows the distal end of a catheter. The catheter distal end comprises a body member  170 , for example, a plastic tubing, and an electrode tip  160 , attached to the distal end of the body member  170 . A RF wire  150  runs through an irrigation channel  110 , or alternatively through a separate lumen formed within the body member  170 , and is connected to the electrode tip  160 . At the distal end of the electrode tip  160  is a sensor  140 , for example, a thermistor or a thermocouple, which is in thermal contact with the electrode tip  160 . A sensor wire  145  extends from the sensor  140  back through the irrigation channel  110 , or alternatively through a separate lumen formed within the body member  170 . Ring electrodes  90  may be mounted around the body member  170 . The electrode tip  160  is used to provide RF energy to heart tissues during ablation procedures. The RF wire connects the electrode tip  160  to a RF power supply (not shown). The ring electrodes  90  may be used together with the electrode tip  160  for mapping procedures.  
           [0006]    Conventional catheters, such as those used for mapping and ablation procedures, are typically made entirely from a biologically compatible material, for example, a platinum iridium alloy (90 percent/10 percent). In general, however, the thermal conductivity of a platinum iridium alloy is not as high as that of other materials, such as copper or gold. One possible approach to increasing the thermal conductivity of an electrode is to form it from a material that is both biologically compatible and highly thermally conductive. However, many materials that are both biologically compatible and have highly thermal conductivity characteristics, such as gold, tend to be expensive.  
           [0007]    As a result, catheter tips made entirely from economically feasible materials may be inefficient at dissipating excess thermal energy, thus creating thermal issues. Specifically, when excessive thermal energy is applied to a catheter electrode during ablation procedures, blood protein and other biological tissue may coagulate on the electrode, creating an embolic hazard. Such build up of coagulant on the electrode also hinders the transmission of RF energy from the electrode into the target tissue, thereby reducing the effectiveness of the ablation procedure. Ideally, it would be preferable to be able to focus the RF energy entirely on the targeted heart tissues without damaging the surrounding tissues or blood cells. That is, it would be highly preferable to be able to generate a good size lesion at a specifically defined area without altering, damaging, or destroying other surrounding tissue or blood.  
           [0008]    In addition, it is generally desirable to be able to minimize the amount of time it takes to complete an ablation procedure. Typically, the longer it takes to complete an ablation procedure, the greater the health risk to the patient. Unfortunately, the time it takes to perform an ablation procedure may be related to how much thermal energy is directed towards the targeted tissue. That is, the greater the thermal energy directed towards the targeted tissue, the quicker the procedure can be performed. However, the amount of thermal energy that may be applied to the targeted tissue may be limited by damage that may potentially occur to the surrounding blood cells and tissues at highly thermal energy levels. For the above reasons, an EP catheter that is able to efficiently dissipate excess heat would be highly desirable.  
         SUMMARY OF THE INVENTION  
         [0009]    The present inventions are directed to medical ablation electrodes that are capable of more efficiently dissipating heat during an ablation procedure.  
           [0010]    In accordance with a first aspect of the present inventions, a medical ablation electrode comprises a biologically compatible outer layer, e.g., platinum iridium alloy, and a thermally conductive inner layer, e.g., copper. An irrigation channel is in contact with the inner layer for channeling cooling fluid. Preferably, the inner layer is in contact with the outer layer, e.g., by plating the outer layer onto the inner layer. In this manner, the conductive inner layer provides a highly conductive medium for increased heat dissipation from the electrode surface and its surrounding space, to an irrigating fluid flowing through the irrigation channel. The irrigating fluid then quickly removes the heat from the electrode during a heating operation, for example, during ablation.  
           [0011]    In accordance with a second aspect of the present inventions, a medical ablation electrode comprises a thermally conductive proximal section having a substantially distally facing wall, and an irrigation channel formed within the proximal section for channeling cooling fluid. The electrode further comprises a thermally conductive distal section and one or more irrigation exit ports that extend through the distally facing wall of the proximal section. Thus, when cooling fluid is conveyed through the irrigation channel, it flows out through the exit ports over the exterior surface of the distal section, dissipating heat from the electrode.  
           [0012]    In accordance with a third aspect of the present inventions, a medical ablation electrode comprises a thermally conductive housing having one or more concave sections and one or more convex sections, an irrigation channel formed within the housing for channeling cooling fluid, and one or more irrigation exit ports adjacent the one or more concave sections of the housing. In this manner, cooling regions are provided between the concave sections and the tissue to be ablated during the ablation process, whereby cooling fluid conveyed out of the exit ports from the irrigation channel enters the cooling areas to cool the tissue.  
           [0013]    In accordance with a fourth aspect of the present inventions, a medical ablation electrode comprises a spiral-shaped thermally conductive irrigation tube having an irrigation channel. In this manner, the area of the irrigation channel exposed to the cooling fluid is maximized. In this case, the housing may form a single unitary structure that can be composed essentially of a biologically compatible material, which otherwise may not be feasible absent the additional cooling of the electrode.  
           [0014]    In accordance with a fifth aspect of the present inventions, a medical ablation electrode comprises a thermally conductive rigid housing and one or more flow-through channels formed by an external surface of the rigid housing for channeling biological fluids over the external surface. As a result, the external surface of the rigid housing is increased by use of the flow-through channels.  
           [0015]    In accordance with a sixth aspect of the present inventions, a medical ablation electrode comprises an inner cylinder having an inner irrigation channel extending therethrough for channeling cooling fluid. The electrode further comprises an outer cap that is mounted in a concentrically overlapping arrangement with the inner cylinder, such that an annular irrigation channel is formed between an inner surface of the outer cap and an outer surface of the inner cylinder for channeling the cooling fluid. In this manner, the cooling fluid flows over the inner and outer surfaces of the inner cylinder, thereby maximizing thermal dissipation of the heat into the cooling fluid. In a preferred embodiment, the inner cylinder and outer cap are composed of a thermally conductive biologically compatible material, and are mounted to each other using a pin. The electrode can further include one or more irrigation exit ports that are in fluid communication with the annular irrigation channel.  
           [0016]    In accordance with a seventh aspect of the present inventions, a medical ablation electrode comprises a thermally conductive housing, and an irrigation channel formed in the housing for channeling cooling fluid. The housing in thin-walled, i.e., the wall of the housing has a thickness that is less than the diameter of the irrigation channel divided by the number 2. In this manner, the heat transfer rate through the housing wall in increased, thereby increasing the amount of thermal energy dissipated into the cooling fluid.  
           [0017]    Thus, as can be seen, that in accordance with the second through seventh aspects of the present inventions, the electrode structure can be composed essentially of a biologically compatible material, which otherwise may not be feasible absent the additional cooling of the electrode and/or tissue. Alternatively, however, the housing can be composed of a highly thermally conductive inner layer and biologically compatible outer layer, or can be composed purely of a highly thermally conductive, but biologically compatible material, such as gold, to further increase the cooling of the electrode. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a cross-sectional view of a conventional catheter tip electrode used for mapping and ablation procedures.  
         [0019]    [0019]FIG. 2 is a cross-sectional view of an internally irrigated catheter tip electrode comprising a biologically compatible outer layer and a thermally conductive inner layer.  
         [0020]    [0020]FIG. 3 is a cross-sectional view of a flushing irrigating catheter tip electrode comprising a biologically compatible outer layer and a thermally conductive inner layer.  
         [0021]    [0021]FIGS. 4A and 4B are cross-sectional views of self-cooling catheter tip electrodes, wherein the external surfaces of the electrodes are cooled by irrigation exit ports.  
         [0022]    [0022]FIG. 5 is a cross-sectional view of a tissue-cooling catheter tip electrode, wherein the external surface comprises concave regions with irrigation exit ports for cooling surrounding tissue.  
         [0023]    [0023]FIG. 6 is a cross-sectional view of a tissue-cooling catheter tip electrode comprising multiple irrigation exit ports.  
         [0024]    [0024]FIG. 7 a side view of a spiral shaped catheter tip electrode.  
         [0025]    [0025]FIG. 8 is a cross-sectional view of a catheter tip electrode with flow-through channels.  
         [0026]    [0026]FIG. 9 is a cross-sectional view of a catheter tip electrode with an outer cap and an inner cylinder forming an annular irrigation channel therebetween.  
         [0027]    [0027]FIG. 10 is a cross-sectional view of a “thin-walled” catheter tip electrode. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    The present invention is directed towards a catheter device having a biologically compatible irrigated tip electrode. Such catheters may be used in, for example, mapping and ablation procedures of the human heart.  
         [0029]    In a preferred embodiment of the present invention a catheter with an efficiently cooled tip is provided. Efficient cooling of a catheter tip electrode is achieved by combining two techniques for improving the cooling efficiency of the electrode. First, the electrode is constructed using components that have highly thermal conductivity and are compatible with biological fluids and tissues. Specifically, the electrode is composed of components that are made from highly conductive but non-biologically compatible materials that are encased or covered with a layer of biologically compatible material, e.g., a composite tip. By using highly thermal conductivity materials to construct the electrode, the thermal energy that may build up on the surface of the electrode and the surrounding area (where blood and tissue may be present) may be quickly dissipated. The electrode is also irrigated such that the heat present at the tip is quickly and efficiently carried away from the tip (two exemplary ways to irrigate an electrode will be discussed below—“internal irrigation” and “flushing irrigation”.  
         [0030]    Referring now to FIG. 2, one embodiment of a catheter incorporating the principles of the present invention is illustrated. The catheter is shown comprising a catheter tip electrode  260  that includes a biologically compatible outer layer  210  and a highly thermal conductivity inner layer  220 . A temperature sensor  240 , for example, a thermistor or a thermocouple, may be located at the distal end of the electrode  260 . Other sensors, such as a 3D sensor (not shown), may also be placed at the distal end of the electrode  260 . The biologically compatible outer layer  210  may be, for example, 90 percent platinum/10 percent iridium alloy. The outer layer  210  may, of course, be made from biologically compatible material other than a platinum iridium alloy. For example, gold and gold alloys, platinum and platinum alloys, titanium, tungsten, stainless steel, etc. may also be used for the outer layer. The highly thermal conductivity inner layer  220  may be, for example, pure copper, silver, or a copper or silver alloy. Other highly thermal conductivity materials other than copper or silver may be used for the inner layer. The proximal end of the inner layer  220  is exposed, such that the distal end of a catheter tube  235  can be bonded thereto.  
         [0031]    The outer layer  210  may be thin, for example, in the range of microns, and may be placed over the inner layer  220  by, for example, plating techniques. Alternative methods of forming the tip may also be used. Preferably, the technique provides a good thermal and electrical connection between the outer layer  210  and inner layer  220 . Details on general composite catheter tip electrodes are disclosed in U.S. Pat. No. 6,099,524 issued to Lipson et al., which is hereby expressly incorporated by reference as if fully set forth herein.  
         [0032]    For purposes of irrigation, an irrigation channel  230  runs through the inner layer  220  to provide a flow path for irrigating fluids such as saline. The irrigation channel  230  is in fluid communication with an irrigation lumen  205 , which extends proximally through the catheter tube  235  to a suitable pump (not shown). The irrigation channel  230  can be formed using any suitable method, such as machining or wax molding. As the irrigating fluid flows through the irrigation channel  230 , it removes the heat being dissipated by the inner layer  220 . For example, when the catheter of FIG. 2 is used in ablation procedures, excess heat may build up at the outer layer surface  215  and in the surrounding space (which may include blood and tissue). The excess heat will readily transfer through the highly conductive inner layer  220  and dissipate into the irrigating fluid flowing through the irrigation channel  230 . The temperature and flow rate of the irrigating fluid can be of any suitable value, e.g., in the range of 30° C.-33° C., or alternatively room temperature, and 30-40 cc/min, or even lower, respectively.  
         [0033]    By using an electrode with a core material (e.g., inner layer  220 ) that has highly thermal conductivity, several advantages may be realized. For example, the use of a highly conductive inner (core) material may result in a more efficient dissipation of heat energy, thus requiring a lower flow rate for the irrigating fluid. A lower flow rate would benefit the system because lower pump pressure would be required. This may thus lower the cost of the fluid pump, which is used to set the flow rate. A low flow rate decreases the chance of catheter failure due to lower pressure, thus making the catheter more safe for the patient. Also, because of improved heat dissipation, an irrigation fluid having higher temperature may be used. This will eliminate or reduce the need to cool the irrigation fluid temperature. For example, under the same power settings during ablation, the flow rate of the irrigating fluid may be lower than a tip made from a less conductive biologically compatible material. Furthermore, the present invention provides an irrigated catheter that delivers more power to the targeted tissue during ablation procedure without the need for increasing the flow rate of the irrigating fluid, thereby improving its efficiency at producing lesions.  
         [0034]    The method of cooling the electrode  260  with an internal irrigation channel illustrated in FIG. 2 is commonly referred to as “internal irrigation”. In internal irrigation, the irrigating fluid will not exit the electrode  260  and flow out of the catheter. Instead, the irrigating fluid stays completely within the catheter and is typically re-circulated. Thus, the same fluid may be used over and over again to cool the electrode  260 .  
         [0035]    Referring now to FIG. 3, another embodiment of the present invention is illustrated. In FIG. 3, a catheter is shown with a “flushing irrigation” catheter tip electrode  300  comprising a biologically compatible outer layer  310  and a thermally conductive inner layer  320  similar to the catheter tip electrode of FIG. 2. Unlike the electrode of FIG. 2, however, no irrigation channel runs through the inner layer  320 . Rather, the irrigation fluid flows through an irrigation channel  335  formed by the inner surface of the inner layer  320  (where the RF wire and steering mechanism is typically situated) and exit out of exit ports  340 , which extend through both the inner layer  320  and outer layer  310 . Thus, the exit ports  340  provide an exit for the irrigation fluid flowing through the irrigation channel  335  via flow path  330 . Again, a sensor  350 , such as a thermistor, may be located at the distal end of the tip and the irrigation channel  335  may contain a 3D sensor (not shown).  
         [0036]    Preferably, the walls of the irrigation channel  335  and/or exit ports  340  are covered by a layer of biologically compatible material, to ensure that there will be no adverse interaction between the highly conductive inner layer  320  (which is not biocompatible) and the cooling fluid, as well as any biological fluids and/or tissues, present in the irrigation channel  335  and exit ports  340 .  
         [0037]    The present invention, as embodied in FIG. 3, allows the excess heat in the outer layer  310  and in the outside space (which typically comprises of blood and tissue) immediately adjacent to the outer layer  310 , to be dissipated through the inner layer  320  and into the irrigating fluid as it flows through the irrigation channel  335  and exits the exit ports  340 . Upon exiting the exit ports  340 , the irrigating fluid will mix with the body fluid (e.g., blood) in the surrounding outside space.  
         [0038]    The electrode  300  described in FIG. 3 is one type of “flushing irrigation” catheter and is commonly referred to as a “showerhead” catheter tip electrode. The highly thermal conductivity inner layer  320  again has the benefits as described in reference to the embodiment of FIG. 2. In this case it is further advantageous for the patient, since the inner layer  320  allows the use of less irrigating fluid, thereby minimizing the fluid that enters the patient&#39;s body.  
         [0039]    Other showerhead tip designs are also contemplated by the present invention and are described below. Referring to FIG. 4A, a catheter tip electrode  425  that “cools itself” is provided. The electrode  425  comprises a housing  410  composed essentially of a biologically compatible material, such as a platinum-iridium alloy. Alternatively, the housing  410  can be formed of a highly thermally conductive inner layer and a biologically compatible outer layer, much like the electrodes shown in FIGS. 2 and 3. Of course, the housing  410  may also be formed of both a highly conductive and biologically compatible (e.g., gold), in which case there would be no need to cover such a material with a biologically compatible material.  
         [0040]    The housing  410  comprises a proximal section  405  and a distal section  435 . The proximal section  405  comprises an irrigation channel  440  and a distally facing surface  455 . The distal section  435  is mushroom-shaped, i.e., it comprises a neck  455  and head  465 . The electrode  400  comprises irrigation exit ports  420 , which extend through the distally facing surface  455  of the proximal section  405 . Thus, an irrigating fluid, for example, a saline solution, flows through the irrigation channel  440  via flow path  450  and exits the electrode  400  through exit ports  420  proximal the distal end  435 . As the irrigating fluid flows through the irrigation channel  440 , the irrigating fluid takes away the thermal energy being dissipated from the housing  410 . As the irrigating fluid exits the electrode  400  it is directed to flow over the head  465  of the distal section  435  to “cool itself.” At the distal end of the electrode  400  is a sensor  430 , for example, a thermistor or a thermocouple, which is in thermal contact with the electrode tip  160 .  
         [0041]    Alternatively, a catheter tip electrode  470  may be shaped such that a distal section  480  is straight rather than mushroom-shaped, as illustrated in FIG. 4B. The electrode  470  comprises a proximal section  475  with an irrigation channel  485  and a substantially distally facing surface  477 . Thus, an irrigating fluid, for example, a saline solution, flows through an irrigation channel  485  via flow path  490  and exits the electrode  470  through exit ports  495  extending through the distally facing surface  477  proximal the distal end  480 . As the irrigating fluid exits the electrode  470 , it is directed to flow over the straight distal end  480  to “cool itself.” 
         [0042]    Referring now to FIG. 5, a catheter tip electrode  500  that “cools the surrounding tissue” is provided. This is achieved by forming the electrode  500  with an undulating outer surface  505 , i.e., alternating between convex sections  525  and concave sections  515 . The electrode  500  comprises a housing  510  composed essentially of a biologically compatible material, such as a platinum-iridium alloy. Alternatively, the housing  510  can be formed of a highly thermally conductive inner layer and a biologically compatible outer layer, much like the electrodes shown in FIGS. 2 and 3. Of course, the housing  510  may also be formed of both a highly conductive and biologically compatible (e.g., gold), in which case there would be no need to cover such a material with a biologically compatible material.  
         [0043]    In any case, the irrigating fluid flows through the irrigation channel  540  via flow paths  550  and exits the electrode  500  through exit ports  520  in the concave sections  515  of the tip housing  510 . As the irrigating fluid exits the electrode  500  it cools the surrounding tissues  530 , e.g., at cooling areas  560 . Thus, cooling fluid that may otherwise be blocked by direct tissue contact when using a level irrigated catheter tip, is delivered to the pertinent tissue substantially unimpeded.  
         [0044]    Referring to FIG. 6, another catheter tip electrode  600  comprises a housing  610  made of a non-conductive material, e.g., plastic, that preferably withstand tissue temperatures without deforming. The housing  610  comprises a large number of irrigation exit ports  620  to provide sufficient cooling. Of course, the tip housing  610  may comprise a composite tip with a highly thermal conductive core (e.g., inner layer) covered by a biological compatible material. This tip housing  610  design, however, may be comprised entirely of biologically compatible material, since sufficient cooling is obtained through the use of a plethora of exit ports  620  in the housing  610 . That is, an irrigating fluid flows through the irrigation channel  650  via flow paths  640  and exits the catheter through the exit ports  620  in the housing  610 . The large number of exit ports  620  allows for reduced irrigation fluid flow and higher irrigation fluid temperature. This embodiment may also include a ring electrode  630  disposed around the housing  610 , so that mapping may occur. Using this catheter tip design, the cooling liquid could be used as the source of RF energy that is applied to the tissue during ablation procedures.  
         [0045]    Referring to FIG. 7, a catheter tip electrode  700  comprises a spiral shape irrigation tube  705  composed essentially of biologically compatible material, such as a platinum-iridium alloy. Alternatively, the irrigation tube  705  can be formed of a highly thermally conductive inner layer and a biologically compatible outer layer, much like the electrodes shown in FIGS. 2 and 3. Of course, the irrigation tube  705  may also be formed of both a highly conductive and biologically compatible (e.g., gold), in which case there would be no need to cover such a material with a biologically compatible material.  
         [0046]    The irrigating fluid flows through a passage  710  in the spiral irrigation tube  705  and exits the electrode  700  out through exit port  730 , thereby cooling the inner surface of the tube  705 . The wall of the spiral irrigation tube  705  is thin, thereby allowing the cooling fluid to run along a path closer to the surface of the electrode  700 , causing greater cooling than an electrode cooled more towards the inner part thereof. Thus, the spherical design of this embodiment provides a larger cooling surface area resulting in improved cooling efficiency.  
         [0047]    Although in FIG. 7 the irrigating fluid is shown to be exiting through the outlet  730 , the fluid does not have to exit the electrode  700 . Rather, the cooling fluid may be re-circulated by directing the outlet  730  back through the catheter tubing to a pump. That is, the electrode  700  may be configured as an internally irrigated tip. Further, the shape of the spiral may vary (e.g., spacing between loops) and the cross-section of the hollow channel can also vary (e.g., circular, oval, etc.). Referring to FIG. 8, another embodiment having a catheter design with improved catheter tip cooling will be described. In this design, an electrode  800  comprises a tip housing  820  with “flow through” channels  810 . Unlike the previously disclosed electrode designs, the flow through channels  810  of the catheter in FIG. 8 are not connected to an internal irrigation channel, but rather is a through channel for the external fluids (e.g., blood) to flow through. As such, the electrode cooling is accomplished in a first instance by the natural circulation of the surrounding fluid (i.e., blood). Alternatively, this embodiment could be combined with one or more of the other embodiments for even more enhanced tip cooling by internally circulating an irrigating fluid. For example, the tip housing  820  may comprise a thermally conductive core covered by a biological compatible material (not shown), such that the thermally conductive core is not in direct contact with the surrounding biological liquid and/or tissue. Of course, if the conductive core is biologically compatible, a biologically compatible material need not be used to cover the core. In any case, the electrode  800  is designed such that the through channels (or passageways) increase the surface area of the electrode  800 , allowing for quicker cooling.  
         [0048]    Referring now to FIG. 9, yet another embodiment for an improved cooling of a catheter tip electrode  900  is provided. In this design, the irrigating fluid exits from an exit port  940 , which is located at the proximal end of the electrode  900 , rather than through exit ports located at the distal end. The electrode  900  comprises an inner cylinder and a concentrically overlapping outer cap  905 , which are attached together by a pin  950  that extends laterally therethrough. Thus, an annular entry port  930  is formed between a distal end  925  of the outer cap  905  and the distal end of the inner cylinder  910 , and an annular channel  920 , which is in fluid communication with the annular channel  920 , is formed between the inner surface of the outer cap  905  and the outer surface of the inner cylinder  910 . It should be noted that the outer cap  905  and inner cylinder  910  may overlap anywhere in region  915 , such that the channel  920  is formed. Following flow path  960 , the irrigating fluid enters the electrode  900  from a proximal irrigation channel  945  within the inner cylinder  910  and flows towards the distal end  925  of the outer cap  905 , where it enters into the annular entry port  930 , through the annular channel  920 , and out the exit ports  940 . As a result, the electrode  900  is cooled.  
         [0049]    The outer cap  905  is composed essentially of biologically compatible material, such as a platinum-iridium alloy. Alternatively, the outer cap  905  is formed of a highly thermally conductive inner layer and a biologically compatible outer layer, much like the electrodes shown in FIGS. 2 and 3. Of course, the outer cap  905  may also be formed of both a highly conductive and biologically compatible (e.g., gold), in which case there would be no need to cover such a material with a biologically compatible material.  
         [0050]    Referring to FIG. 10, yet another embodiment for an improved cooled catheter tip electrode  1000  is shown. In this design, the walls of the electrode  1000  are thinned to improve the cooling rate of the electrode during irrigated procedures. The electrode  1000  includes a housing  1010  having an irrigation channel  1020 . The housing  1010  has an inner diameter (ID) defined by its inner surface, and an outer diameter (OD) defined by its outer surface. Thus, the wall thickness of the housing  1010  can be defined as t=(OD−ID)/2. In the preferred embodiment, the wall of the housing  1010  is considered “thin-walled,” which for the purposes of this specification is an ablation electrode, each wall of which exhibits a thickness t that is less than ID/2. Preferably the wall thickness t of the housing  1010  is less than ID/4 and more preferably less than ID/10. Thus, the cooling effects of irrigating fluid flowing through the electrode  1000  will be greater than that for electrodes that are not “thin-walled.” 
         [0051]    Specifically, the heat rate of conduction through a cylindrical wall can be described by the following equation: q=(2πLk*ΔT)(ln(r 2 /r 1 ), where L=length, k=thermal conductivity, ΔT=difference in temperature across the wall of the cylinder, r 1 =ID/2, and r 2 =OD/2. In comparing the heat rate of conduction between a conventional electrode, which typically has an ID and OD of 0.050 and 0.105 inches, respectively (i.e., r 1   =0.025″ and r   2 =0.0525″) with exemplary ID and OD values of the electrode  1000  of 0.08750 and 0.105 inches, respectively (i.e., r 1 =0.04375″ and r 2 =0.0525), the increase in heat rate between the conventional electrode and the electrode  1000  (equal to [1/ln(r 2 /r 1 )] new /ln(r 2 /r 1 )] conv ) is 4.069 greater. As a result, the electrode  1000  is capable of producing larger volume lesion than conventional electrodes.  
         [0052]    Although particular embodiments of the present invention have been shown and described, it will be understood that it is not intended to limit the invention to the preferred embodiments and it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the invention is intended to cover alternatives, modifications, and equivalents, which may be included within the spirit and scope of the invention as defined by the claims.