Abstract:
An irrigated ablation catheter provides improved distribution of irrigation fluid across its tip electrode surface resulting in improved cooling and flushing of blood and proteins from the tip region. An axially directed flow of irrigation provides improved heat transfer from the tip electrode to the irrigation fluid allowing for a cooler tip electrode and larger lesions. The irrigation fluid is introduced to the catheter with improved flow by means of a standard constant flow pump. A lumen or tube within a shaft of the catheter transfers the irrigation fluid to a proximal end of the tip electrode where it exits the catheter via a flow directing member mounted on the tip electrode. In one embodiment, the flow directing member is a thin walled tube that surrounds the proximal end of the tip electrode and directs the irrigation fluid distally along an outer surface of the tip electrode.

Description:
FIELD OF INVENTION 
     The present invention relates to ablation catheters, and in particular to irrigated ablation catheters. 
     BACKGROUND 
     Electrode catheters have been in common use in medical practice for many years. They are used to stimulate and map electrical activity in the heart and to ablate sites of aberrant electrical activity. 
     In use, the electrode catheter is inserted into a major vein or artery, e.g., femoral. artery, and then guided into the chamber of the heart which is of concern. Within the heart, the ability to control the exact position and orientation of the catheter tip is critical and largely determines how useful the catheter is. 
     In certain applications, it is desirable to have an irrigated tip catheter in order to cool the tip electrode at the site of ablation and to prevent thrombus. 
     A typical ablation procedure involves the insertion of a catheter having a tip electrode at its distal end into a heart chamber. A reference electrode is provided, generally taped to the skin of the patient. RF (radio frequency) current is applied to the tip electrode, and current flows through the media that surrounds it, i.e., blood and tissue, toward the reference electrode. The distribution of current depends on the amount of electrode surface in contact with the tissue as compared to blood, which has a higher conductivity than the tissue. Heating of the tissue occurs due to its electrical resistance. The tissue is heated sufficiently to cause a lesion. Heating of the electrode results from conduction from the heated tissue. While the blood circulating around the ablation electrode tends to cool it, a stagnant area between the electrode and the tissue may be heated to such a temperature that a thin coating of blood protein forms on the surface of the tip electrode. This can cause an impedance rise and/or a thrombus that could become an embolus. When this occurs, the catheter should be removed and the tip electrode cleaned. 
     When RF current is applied to an ablation electrode in good contact with the endocardium to create a lesion, the amount of power delivered is limited by the heating of the electrode in order to prevent char and thrombus. The resulting lesion tends to be hemispherical, usually about 6 mm in diameter and about 3 to 4 mm deep. 
     When a tip electrode is irrigated, e.g., with room temperature saline, the tip electrode is cooled by the flow of saline through it and the surface of the electrode is flushed. Because the strength of the RF current is no longer limited by the interface temperature, current can be increased. This results in lesions which tend to be larger and more spherical, usually measuring about 10 to 12 mm. 
     Current irrigated catheters utilize either closed or open fluid systems. Open irrigation fluid systems use holes placed in specific locations around the tip electrode to distribute the irrigation fluid. These designs do not provide uniform fluid distribution along the outer surface of the tip electrodes. Additionally, the irrigation fluid is projected far from the tip electrode and does not provide a uniform and complete boundary layer from the surrounding blood. Accordingly, it is desirable to provide an irrigated catheter with a generally complete and uniform boundary layer reducing direct blood contact with the tip electrode during the application of RF energy. With maintained irrigation fluid-to-tip electrode contact, such an improved catheter will provide increased heat loss including convective heat loss resulting in more efficient cooling and thrombus prevention. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an irrigated ablation catheter that provides improved distribution of irrigation fluid across its tip electrode surface resulting in improved cooling and flushing of blood and proteins from the tip region. An axially directed flow of irrigation provides improved heat transfer from the tip electrode to the irrigation fluid allowing for a cooler tip electrode and larger lesions. 
     The irrigation fluid is introduced to the catheter with improved flow by means of a standard constant flow pump. A lumen or tube within a shaft of the catheter transfers the irrigation fluid to a proximal end of the tip electrode where it exits the catheter via a flow directing member mounted on the tip electrode. In one embodiment, the flow directing member is a thin walled tube that surrounds the proximal end of the tip electrode and directs the irrigation fluid distally along an outer surface of the tip electrode. 
     The flow directing member is intended to evenly distribute the irrigation flow over the tip electrode, thus providing several benefits, including improved flushing of blood from the tip electrode surface and improved convective cooling of the tip electrode and tissue-to-tip electrode interface. 
     Improved flushing of blood from the tip electrode can more efficiently reduce or eliminate formation of coagulation on the tip electrode associated with the denaturing of proteins during RF application. By creating a generally uniform boundary layer of irrigation fluid around the tip electrode, blood is kept from the tip electrode or it is diluted such that the quantity of proteins exposed to high heat levels during RF ablation is significantly decreased. Additionally, the generally uniform flow of irrigation fluid along the tip electrode tends to ensure that blood or blood protein contact with heat is momentary. 
     The catheter increases heat loss including convective heat loss by the tip electrode to the irrigation fluid by containing the irrigation fluid along the outer surface of the proximal portion of the tip electrode. By using the outer surface of the trip electrode as the cooling interface rather than internal cooling channels, the convective heat loss can be greatly increased due to the larger surface area provided by the outer surface of the tip electrode. It is also contemplated that the outer surface  63  can be texturized or roughen (see  FIG. 6A ) to increase the surface area for further increased convective heat loss. Heat fins  65  (see  FIG. 6B ) can also be added to the tip electrode for further increased convective heat loss. The flow directing member is configured to also direct the fluid along the distal end of the tip selectrode providing for additional convective heat loss. 
     During RF application, the improved fluid flow along the tip electrode provides more efficient convective heat loss, which could lead to a lower fluid flow rate and thus a decrease in delivery of fluid to the ablation site over the course of the procedure. In one embodiment, the catheter has a catheter body and a tip section adapted for ablation and irrigation. The tip section has a tip electrode and a flow directing member positioned over the tip electrode to direct fluid to flow over an outer surface of the tip electrode. The flow directing member is tubular having an inner surface that is supported away from the outer surface of the tip electrode by ribs extending longitudinally along the inner surface of the flow directing member. Gaps formed by the ribs define fluid channels that guide the fluid to flow over the outer surface of the tip electrode. 
     In a more detailed embodiment, the tip section also has a fluid cavity immediately proximal the tip electrode wherein fluid fed by the catheter body enters the cavity to come in contact with the outer surface of the tip electrode. To protect components extending through the cavity and into the tip electrode, connective tube are provided to bridge the cavity and isolate the components from exposure to the fluid. One or more connective tubes are notched at their distal end to prevent the distal end from cutting into the components, particularly those extending at an angle to the connective tubes. 
     In one embodiment, the catheter includes a temperature sensing means positioned to sense temperature at or near a center of a tip electrode. The temperature sensing means can be positioned to sense temperature omnidirectionally with respect to tissue-contacting surface of the tip electrode. Where the temperature sensing means comprises thermocouple wires, distal ends thereare of are positioned at or near a center of a semi-spherically configured distal end of the tip electrode for omnidirectional temperature sensing. 
     The catheter may be nondeflectable, deflectable by one or more puller wires, or guided by magnetic steering or a deflectable guiding sheath. The catheter may also contain an electromagnetic location sensor. One or more ring electrodes may also be provided distally from the tip electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a side view of an embodiment of the catheter of the present invention. 
         FIG. 2A  is a side cross-sectional view of an embodiment of a catheter according to the invention, including a junction between a catheter body and an intermediate section, taken along a first diameter. 
         FIG. 2B  is a side cross-sectional view of an embodiment of a catheter according to the invention, including the junction between the catheter body and the intermediate section, taken along a second diameter generally perpendicular to the first diameter of  FIG. 2A . 
         FIG. 3A  is a side cross-sectional view of an embodiment of a catheter according to the invention, including a junction between the intermediate section and a tip section, taking along the first diameter. 
         FIG. 3B  is a side cross sectional view of an embodiment of a catheter according to the invention, including a junction between the intermediate section and a tip section, taken along a second diameter generally perpendicular to the first diameter of  FIG. 3A   
         FIG. 4  is a longitudinal cross-sectional view of an embodiment of the tip section of  FIGS. 3A and 3B , taken along line  4 - 4 . 
         FIG. 5  is a longitudinal cross-sectional view of an embodiment of the tip section of  FIGS. 3A and 3B , taken along line  5 - 5 . 
         FIG. 6  is a longitudinal cross-sectional view of an embodiment of the tip section of  FIGS. 3A and 3B , taken along line  6 - 6 . 
         FIG. 6A  is a longitudinal cross-sectional view of another embodiment of a tip electrode. 
         FIG. 6B  is a longitudinal cross-sectional view of yet another embodiment of a tip electrode. 
         FIG. 7  is a perspective view of an embodiment of a flow directing member mounted on a distal end of an intermediate section, with connective tubes. 
         FIG. 8  is a side elevational view of an embodiment of a tip section in contact with tissue during ablation and irrigation, with parts of the flow directing member and ring electrode broken away. 
         FIG. 9A  is a side cross-sectional view of another embodiment of a catheter according to the invention, including a junction between a catheter body and an intermediate section, taken along a first diameter. 
         FIG. 9B  is a side cross-sectional view of an embodiment of a catheter according to the invention, including the junction between the catheter body and the intermediate section, taken along a second diameter generally perpendicular to the first diameter of  FIG. 9A . 
         FIG. 10  is a side cross-sectional view of another embodiment of a catheter according to the invention, including a junction between the intermediate section and a tip section, taking along the first diameter. 
         FIG. 11  is a longitudinal cross-sectional view of an embodiment of the tip section of  FIG. 10 , taken along line  11 - 11 . 
         FIG. 12  is a longitudinal cross-sectional view of an embodiment of the tip section of  FIG. 10 , taken along line  12 - 12 . 
         FIG. 13  is a longitudinal cross-sectional view of an embodiment of the tip section of  FIG. 10 , taken along line  13 - 13 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIGS. 1-8  catheter  10  of the present invention comprises an elongated catheter body  12  having proximal and distal ends, a deflectable (uni- or bi-directionally) intermediate section  14  at the distal end of the catheter body  12 , a tip section  36  at the distal end of the intermediate section, and a control handle  16  at the proximal end of the catheter body  12 . 
     With additional reference to  FIGS. 2A and 2B , the catheter body  12  comprises an elongated tubular construction having a single, axial or central lumen  18 . The catheter body  12  is flexible, i.e., bendable, but substantially non-compressible along its length. The catheter body  12  can be of any suitable construction and made of any suitable material. A construction comprises an outer wall  22  made of an extruded plastic. The outer wall  22  may comprise an imbedded braided mesh of stainless steel or the like to increase torsional stiffness of the catheter body  12  so that, when the control handle  16  is rotated, the catheter body  12 , the intermediate section  14  and the tip section  36  of the catheter  10  will rotate in a corresponding manner. 
     Extending through the single lumen  18  of the catheter body  12  are components, for example, lead wire  40  and thermocouple wires  41 ,  45  protected by a sheath  52 , a first irrigation tube segment  88 , a compression coil  56  through which a puller wire  42  extends. A single lumen catheter body can be preferred over a multi-lumen body because it has been found that the single lumen body permits better tip control when rotating the catheter. The single lumen permits the various components such as the lead wire, thermocouple wires, infusion tube, and the puller wire surrounded by the compression coil to float freely within the catheter body. If such wires, tube and cables were restricted within multiple lumens, they tend to build up energy when the handle is rotated, resulting in the catheter body having a tendency to rotate back if, for example, the handle is released, or if bent around a curve, to flip over, either of which are undesirable performance characteristics. 
     The outer diameter of the catheter body  12  is not critical, but is preferably no more than about 8 french, more preferably 7 french. Likewise the thickness of the outer wall  22  is not critical, but is thin enough so that the central lumen  18  can accommodate the aforementioned components. The inner surface of the outer wall  22  may be lined with a stiffening tube  20 , which can be made of any suitable material, such as polyimide or nylon. The stiffening tube  20 , along with the braided outer wall  22 , provides improved torsional stability while at the same time minimizing the wall thickness of the catheter, thus maximizing the diameter of the central lumen  18 . The outer diameter of the stiffening tube  20  is about the same as or slightly smaller than the inner diameter of the outer wall  22 . Polyimide tubing may be preferred for the stiffening tube  20  because it may be very thin walled while still providing very good stiffness. This maximizes the diameter of the central lumen  18  without sacrificing strength and stiffness. 
     The catheter may have an outer wall  22  with an outer diameter of from about 0.090 inch to about 0.104 inch and an inner diameter of from about 0.061 inch to about 0.075 inch and a polyimide stiffening tube  20  having an outer diameter of from about 0.060 inch to about 0.074 inch and a wall thickness of about 0.001-0.005 inch. 
     Referring also to  FIGS. 3A ,  3 B and  4 , the intermediate section  14  distal of the catheter body  12  comprises a shorter section of tubing  19  having multiple lumens. The tubing  19  is made of a suitable non-toxic material that is preferably more flexible than the catheter body  12 . A suitable material for the tubing  19  is polyurethane braided with a low to medium durometer plastic. The outer diameter of the intermediate section  14 , like that of the catheter body  12 , is preferably no greater than about 8 french, more preferably 7 french. The size and number of the lumens is not critical. In an embodiment, the intermediate section  14  has an outer diameter of about 7 french (0.092 inch). The tubing has four off-axis lumens  30 ,  32 ,  34  and  35  that are generally about the same size, each having a diameter of from about 0.020 inch to about 0.024 inch, preferably 0.022 inch. 
     Referring to  FIGS. 2A and 2B , the catheter body  12  may be attached to the intermediate section  14  comprises an outer circumferential notch  24  configured in the proximal end of the tubing  19  that receives the inner surface of the outer wall  22  of the catheter body  12 . The intermediate section  14  and catheter body  12  are attached by glue or the like. Before the intermediate section  14  and catheter body  12  are attached, the stiffening tube  20  is inserted into the catheter body  12 . The distal end of the stiffening tube  20  is fixedly attached near the distal end of the catheter body  12  by forming a glue joint  23  with polyurethane glue or the like. Preferably a small distance, e.g., about 3 mm, is provided between the distal end of the catheter body  12  and the distal end of the stiffening tube  20  to permit room for the catheter body  12  to receive the notch  24  of the intermediate section  14 . If no compression coil is used, a force is applied to the proximal end of the stiffening tube  20 , and, while the stiffening tube  20  is under compression, a first glue joint (not shown) is made between the stiffening tube  20  and the outer wall  22  by a fast drying glue, e.g. cyanoacrylate. Thereafter a second glue joint  26  is formed between the proximal ends of the stiffening tube  20  and outer wall  22  using a slower drying but stronger glue, e.g., polyurethane. 
     If desired, a spacer can be located within the catheter body between the distal end of the stiffening tube and the proximal end of the tip section. The spacer provides a transition in flexibility at the junction of the catheter body and intermediate section, which allows this junction to bend smoothly without folding or kinking. A catheter having such a spacer is described in U.S. patent application Ser. No. 08/924,616, entitled “Steerable Direct Myocardial Revascularization Catheter”, the entire disclosure of which is incorporated herein by reference. 
     With reference to  FIGS. 1 ,  2 B and  4 , to energize tip and ring electrodes at or near the tip section  36  for RF ablation, lead wires  40  extend through the lumen  34  of intermediate section  14 , the central lumen  18  of the catheter body  12 , and the control handle  16 , and terminates at its proximal end in an input jack (not shown) or connector  77  that may be plugged to an generator or the like (not shown). The portion of the lead wire  40  extending through the central lumen  18  of the catheter body  12 , control handle  16  and distal end of the intermediate section  14  is enclosed within a protective sheath  68 , which can be made of any suitable material, preferably Teflon®. The protective sheath is anchored at its distal end to the proximal end of the intermediate section  14  by gluing it in the lumen  34  with polyurethane glue or the like. 
     A temperature sensing means is provided for the tip section  36  as explained in detail further below. Any conventional temperature sensing means, e.g., a thermocouple or thermistor, may be used. With reference to  FIGS. 2B and 4 , a suitable temperature sensing means for the tip section comprises a thermocouple formed by a wire pair. One wire of the wire pair is a copper wire  41 , e.g., a 40 gauge or similar size copper wire. The other wire of the wire pair is a constantan wire  45 , which gives support and strength to the wire pair. The wires  41  and  45  extend through the lumen  34  in the intermediate section  14 . Within the catheter body  12  the wires  41  and  45  extend through the central lumen  18  within the protective sheath  52 . The wires  41  and  45  then extend out through the control handle  16  and to the connector  77 . Alternatively, the temperature sensing means may be a thermistor. A suitable thermistor for use in the present invention is Model No. AB6N2-GC14KA143T/37C sold by Thermometrics (N.J.). 
     Referring to  FIGS. 2A and 4 , the puller wire  42  for deflecting the intermediate section  14  extends through the lumen  32  of the tubing  19  of the intermediate section and the catheter body  12  and is anchored at its proximal end to the control handle  16 . The puller wire is made of any suitable metal, such as stainless steel or Nitinol, or fiber such as Spectra or Vectran, and is preferably coated with Teflon™ or the like. The coating imparts lubricity to the puller wire. The puller wire preferably has a diameter ranging from about 0.006 to about 0.012 inches. A compression coil  44  is situated within the catheter body  12  in surrounding relation to the puller wire. The compression coil  44  extends from the proximal end of the catheter body  12  to the proximal end of the intermediate section  14 . The compression coil is made of any suitable metal, preferably stainless steel, and is tightly wound on itself to provide flexibility, i.e., bending, but to resist compression. The inner diameter of the compression coil is preferably slightly larger than the diameter of the puller wire  42 . The Teflon® coating on the puller wire allows it to slide freely within the compression coil. If desired, particularly if the lead wire  40  is not enclosed by the protective sheath  52 , the outer surface of the compression coils can be covered by a flexible, non-conductive sheath  99 , e.g., made of polyimide tubing, to prevent contact between the compression coils and any other wires within the catheter body  12 . 
     As shown in  FIG. 2A , the compression coil  44  is anchored at its proximal end to the proximal end of the stiffening tube  20  in the catheter body  12  by glue joint  50  and at its distal end to the intermediate section  14  by glue joint  51 . Both glue joints  50  and  51  preferably comprise polyurethane glue or the like. The glue may be applied by means of a syringe or the like through a hole made between the outer surface of the catheter body  12  and the central lumen  18 . Such a hole may be formed, for example, by a needle or the like that punctures the outer wall  22  of the catheter body  12  and the stiffening tube  20  which is heated sufficiently to form a permanent hole. The glue is then introduced through the hole to the outer surface of the compression coil  44  and wicks around the outer circumference to form a glue joint about the entire circumference of the compression coil. Within the lumen  32  of the intermediate section  14 , the puller wire  42  extends through a plastic, preferably Teflon®, sheath  81 , which prevents the puller wire  42  from cutting into the wall of the intermediate section  14  when the intermediate section is deflected. Longitudinal movement of the puller wire  42  relative to the catheter body  12 , which results in deflection of the tip section  36 , is accomplished by suitable manipulation of the control handle  16 . Suitable control handles are described in U.S. Pat. No. 6,602,242, the entire disclosure of which is hereby incorporated by reference. 
     Extending from the distal end of the intermediate section  14  is the tip section  36  that includes a tip electrode  37  and a fluid directing member  21  as shown in  FIGS. 3A and 3B . Extending between the distal end of the tubing  19  of the intermediate section  14  and the proximal end of the tip electrode  37  are connective tubes  60  that secure the tip electrode. Each connective tube also has an exposed mid-section  61  not covered by either the tubing  19  or the tip electrode  37  such that there is a gap  53  between the distal end of the tubing  19  and the proximal end of the tip electrode  37 . Purposefully separated from each other, the distal end of the tubing  19  and the proximal end of the tip electrode  37  define a cavity  54  for receiving fluid in an otherwise confined tip section. Covering the gap and further defining the cavity is the fluid directing member  21  which is mounted over the distal end of the tubing  19  and the proximal end of the tip electrode. 
     In the illustrated embodiment, the fluid directing member  21  is generally cylindrical (or otherwise being of an atraumatic configuration) and tubular with an inner surface  56  configured with longitudinal ribs  58  that create a circumferential gap  57  between the inner surface  56  and an outer surface  63  of the tip electrode or an outer surface  69  of the tubing  19 . As explained below in further detail, the fluid cavity  54  and the circumferential gap or channel  57  allow fluid fed through the catheter shaft  12  and intermediate section  14  to facilitate convective heat loss in the tip electrode which minimizes contacting tissue damage during ablation. The gap at the proximal end of the member  21  is occluded or otherwise sealed by a plug  71  so that the fluid is directed to exit at the distal end of the member  21 . Suitable materials for sealing the gap include polyurethane or other material used for bonding the member to the tubing  19 . A clasp or outer ring may be used. The bonding may even be accomplished thermally if appropriate materials are chosen. Accordingly, by providing the fluid cavity  54  and a generally continuous fluid flow over the tip electrode exiting the channel  57 , lower tissue resistance can be maintained and larger lesions can be formed at lower powers and/or time settings. In the disclosed embodiment, the member  21  is constructed of a plastic material, e.g., polyetheretherketone (PEEK). However, as understood by one of ordinary skill in the art, the construction may be of any material that can be mounted over the gap  53  and provide sufficient structural strength to provide the channel  57 . In the illustrated embodiment, there are four longitudinal ribs  58  equi-angular about the longitudinal axis of the tip electrode  37 . The cross sectional shape and the plurality of ribs may be varied, so long as the ribs sufficiently support the inner surface  56  of the member  21  from fully contacting the tip electrode. In that regard, there plurality of the ribs may range from at least two or more, preferably from at least three or more, and more preferably from at least four or more. 
     To irrigate the tip section for convective heat loss, the lumen  35  at its distal end opens into the fluid chamber  54  which is generally filled with fluid, e.g., saline, transferred by the lumen  35 . In accordance with a feature of the present invention, heat convention loss occurs between the tip electrode and the irrigation fluid fed through the lumen  35 . The flow directing member  21  provides a means of forcing the fluid to flow across a face  64  and radial surface  66  of the proximal end of the tip electrode creating forced convention between the electrode and fluid. To that end, by increasing the diameter D of the tip electrode and/or the length L of the member  21  covering the tip electrode  37 , a convective heat loss surface A including the face  64  and the radial surface  66  can be increased, which would in turn increase the convective heat loss of the tip electrode to the fluid, as defined by the following equation:
 
 q=hA ( dT/dt )
 
where,
         h=convective heat transfer coefficient   A=(¼)πD 2 +πDL   dT/dt=difference in temperature of tip electrode and fluid with respect to time       

     Advantageously, a proximal edge  70  (circumscribing the proximal face  66 ) of the tip electrode  37  is covered by the flow directing member  21 . As such, the edge  70  (an area often of high energy density) is included in the convective heating loss surface A that is flushed with cooling fluid to minimize edge effect heating that would otherwise cause coagulation. Moreover, as also understood by one of ordinary skill in the art, an exposed length L′ of the tip electrode is generally equal to the diameter D of the tip electrode so that the amount of contacting surface of the distal end with tissue can be generally constant regardless of the angle of contact (see  FIG. 8 ). In that regard, a profile of the tip electrode in the exposed length L′ has a generally straight proximal portion of a length equal to about (R/2) and a curved distal portion equal to about (πr/2), where R is the radius of the tip electrode (or D/2). 
     The tip electrode  37  can have an total length from the proximal face to the distal end ranging between about 2.0 mm and 8.0 mm, and preferably about 3.0 mm and 5.0 mm. In accordance with a feature of the present invention, the flow directing member  21  should cover at least a percentage of the total length of the tip electrode, where the percentage ranges between about 25 and 75, and preferably about 50. The member  21  therefore can have a total length ranging between about 3.0 mm and 7.0 mm, and preferably about 4.5 mm and 6.0 mm. 
     To isolate and protect the various components extending between the distal end of the tubing  19  and the tip electrode from fluid in the fluid chamber  54 , the connective tubes  60  bridge the gap with their proximal ends received in selected lumens of the tubing  19  and their distal ends inserted into blind holes formed in the proximal end of the tip electrode. The connective tubes can be stainless steel hypo-tubes or any other suitable tube segments 
     In the illustrated embodiment, there are three connective tubes  60   a ,  60   b  and  60   c  (see  FIGS. 3A ,  3 B,  6  and  7 . Tube  60   a  extends between the lumen  30  and a blind hole  80   a , isolating thermocouple wires  41  and  45 . Tube  60   b  extends between the lumen  32  and a blind hold  80   b , isolating the puller wire  42 . Tube  60   c  extends between the lumen  34  and a blind hole  80   c , isolating the lead wires  40 . As mentioned, lumen  35  opens into the gap so fluid can fill the gap and flow over the proximal face and proximal radial surface of the tip electrode as directed by the fluid directing member  21 . 
     The blind holes  80  are formed in the tip electrode to allow components to be anchored tip electrode. The lead wire  40  is attached to the tip electrode  37  by any conventional technique. In the illustrated embodiment, connection of the lead wire  40  to the tip electrode  37  is accomplished, by welding the distal end of the lead wire into the blind hole  80   c  ( FIG. 3B ) in the tip electrode  37 . It is noted that a ring electrode  49  is provided in the illustrated embodiment. The ring electrode is mounted over the flow directing member  21  with its lead wire  40  extending through holes provided in the connective tube  60   c  and in the flow directing member  21 . 
     The tip electrode also has a blind hole  80   c  for anchoring the distal end of the puller wire. The blind hole  80   c  is generally longitudinally aligned with the lumen  32  of the tubing  19  of the intermediate section  14 . (As understood by one of ordinary skill in the art, the distal end of the puller wire can also be anchored in the side wall of tubing  19  at the distal end of the intermediate section  14 .) Likewise, the blind hole  80   b  for the lead wires is generally longitudinally aligned with the lumen  34 . A method for anchoring the puller wire  42  within the tip electrode  37  is by crimping metal tubing  46  to the distal end of the puller wire  42  and soldering the metal tubing  46  inside the blind hole  101 . Anchoring the puller wire  42  within the tip electrode  37  provides additional support, reducing the likelihood that the tip electrode  37  will fall off. Alternatively, the puller wire  42  can be attached to the side wall of the tubing  19  at the distal end of the intermediate section  14 . 
     In accordance with a feature of the present invention, the blind hole  80   a  extends deeply into the tip electrode  37  so that the thermocouple wires  41  and  45  can have improved temperature sensing of the tip electrode and hence the tissue in contact with the tip electrode. For omnidirectional temperature sensing with respect to the intended tissue-contacting surface of the distal end of the tip electrode, distal end of the hole  80   a  is positioned generally on the longitudinal axis at a distance R (see  FIG. 3B ) that is equal to about the radius R of the semi-spherical distal end portion of the tip electrode. In the illustrated embodiment, the blind hole  80   a  has a proximal section  81  and an angled distal section  83  that is separately drilled with a smaller drill bit. This angled configuration reduces the stress on the thermocouple wires  41  and  45  during their transition from the off axis lumen  30  in the tubing  19  of the intermediate section  14  to a more centered position at the distal end of the blind hole  80   a . To that end, a notch  79  is formed in the distal end of the connective tube  60   a  so as to avoid the wires  41  and  45  rubbing against a distal edge of the connective tubing. The wires  41  and  45  of the wire pair are electrically isolated from each other except at their distal ends where they contact and are twisted together, covered with a short piece of plastic tubing  63 , e.g., polyimide, and covered with epoxy. The plastic tubing  63  is then attached in the hole  104  of the plug  44 , by epoxy or the like. 
     The irrigation fluid is transferred to the intermediate section  14  by an infusion tube segment  88  ( FIG. 2A ) that extends through the central lumen  18  of the catheter body  12  and terminates in the proximal portion of the lumen  35  of the intermediate section  14 . The proximal end of the infusion tube segment  88  extends through the control handle  16  and terminates in a luer hub  90  ( FIG. 1 ) or the like at a location proximal to the control handle. In practice, fluid may be injected by a pump (not shown) into the infusion tube segment  88  through the luer hub  90 , and flows through the segment  88 , through the lumen  35 , into the fluid cavity  54  in the tip section  36  and exits the distal end of the flow directing member  21  to cool the tip electrode by convention heat loss as described herein. The infusion tube segments may be made of any suitable material, and is preferably made of polyimide tubing. A suitable infusion tube segment has an outer diameter of from about 0.32 inch to about 0.036 inch and an inner diameter of from about 0.14 inch to about 0.032 inch. 
     In accordance with a feature of the present invention, the pump maintains the fluid at a positive pressure differential relative to outside of the tip section so as to provide a constant unimpeded flow or seepage of fluid outwardly from the distal end of the flow directing member  21  which continuously flushes the distal end of the tip electrode (see  FIG. 8 ). 
     In an alternative embodiment, the tip section  36  carries an electromagnetic location sensor  72  situated in a blind hole  80   d , as illustrated in  FIG. 10 . The electromagnetic location sensor  72  is connected to an electromagnetic location sensor cable  74 . As shown in  FIGS. 2A and 5 , the sensor cable  74  extends through the lumen  35  of the tip section  36 , through the central lumen  18  of the catheter body  12 , and into the control handle  16 . The sensor cable  74  then extends out the proximal end of the control handle  16  within an umbilical cord  78  ( FIG. 1 ) to a sensor control module  75  that houses a circuit board (not shown). Alternatively, the circuit board can be housed within the control handle  16 , for example, as described in U.S. patent application Ser. No. 08/924,616, entitled “Steerable Direct Myocardial Revascularization Catheter”, the disclosure of which is incorporated herein by reference. The sensor cable  74  comprises multiple wires encased within a plastic covered sheath. In the sensor control module  75 , the wires of the sensor cable  74  are connected to the circuit board. The circuit board amplifies the signal received from the sensor  72  and transmits it to a computer in a form understandable by the computer by means of the sensor connector  77  at the proximal end of the sensor control module  75 , as shown in  FIG. 1 . Because the catheter can be designed for single use only, the circuit board may contain an EPROM chip which shuts down the circuit board approximately 24 hours after the catheter has been used. This prevents the catheter, or at least the electromagnetic location sensor, from being used twice. An electromagnetic mapping sensor  72  may have a length of from about 6 mm to about 7 mm and a diameter of about 1.3 mm. 
     In this embodiment, the lead wires  40  extend through the lumen  30  along with the thermocouple wires  41  and  45 , so that the sensor cable  74  can occupy the lumen  34  (see  FIGS. 9B and 11 ). As illustrated in  FIG. 10 , the lumen  34  is trepanned at its distal end to accommodate the electromagnetic location sensor  74 , a proximal portion of which extends into the lumen  34  and a distal portion of which extends into an enlarged blind hole  80   d  lined by a larger connective tube  60   d.    
     The preceding description has been presented with reference to presently preferred embodiments of the invention. Workers skilled in the art and technology to which this invention pertains will appreciate that alterations and changes in the described structure may be practiced without meaningfully departing from the principal, spirit and scope of this invention. The instant catheter may be manufactured using materials suitable for use on blood contacting medical devices. Tip electrode size and shape may vary depending on intended catheter size and as required for sufficient cooling, component placement, and ablation effectiveness. The thin walled tube used for flow control may be of any material that is stiff enough to provide the required channeling of fluids while maintaining minimal wall thickness. The catheter may be nondeflectable, or deflectable by means of puller wire(s), magnetic steering or a deflectable guiding sheath. 
     Accordingly, the foregoing description should not be read as pertaining only to the precise structures described and illustrated in the accompanying drawings, but rather should be read consistent with and as support to the following claims which are to have their fullest and fair scope.