Patent Publication Number: US-2019167347-A1

Title: Catheter adapted for direct tissue contact

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of and claims priority to and the benefit of U.S. application Ser. No. 13/224,291 filed Sep. 1, 2011 now issued as U.S. Pat. No. 10,201,385, the entire content of which is incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     This invention relates generally to methods and devices for invasive medical treatment, and specifically to catheters, in particular, irrigated ablation catheters. 
     BACKGROUND 
     Ablation of myocardial tissue is well known as a treatment for cardiac arrhythmias. In radio-frequency (RF) ablation, for example, a catheter is inserted into the heart and brought into contact with tissue at a target location. RF energy is then applied through electrodes on the catheter in order to create a lesion for the purpose of breaking arrhythmogenic current paths in the tissue. 
     Irrigated catheters are now commonly used in ablation procedures. Irrigation provides many benefits including cooling of the electrode and tissue which prevents overheating of tissue that can otherwise cause the formation of char and coagulum and even steam pops. However, because tissue temperature is assessed during an ablation procedure to avoid such adverse occurrences, it is important that the temperature sensed accurately reflects the real temperature of the tissue and not merely the surface temperature of the tissue which can be biased by the cooling irrigation fluid from the catheter. Moreover, deeper tissue contact in general provides more accurate thermal and electrical readings, including improved impedance measurements for purposes including a determination of lesion size. 
     Accordingly, there is a desire for an irrigated ablation catheter with a distal end that can better probe tissue without significantly damaging or breaching the tissue, for more accurate measurements, including temperature sensing and impedance measurements. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an irrigated ablation catheter adapted for direct tissue contact by means of micro-elements (or micro-sensing members) that provide more accurate sensing of tissue, including thermal and electrical properties for temperature and impedance measurements. 
     In one embodiment, the catheter has an elongated body and a distal electrode assembly that has an electrode having a shell configured with an inner fluid chamber. The shell has a wall with at least one aperture formed on the distal portion of the shell which receives a distal end of a micro-element extending through the inner chamber. The distal end of the micro-element extends to at least through the aperture, if not also outside of the outer surface of the wall such that there is an exposed portion adapted to probe the tissue being ablated. 
     In a more detailed embodiment, the micro-element can be configured as a micro-temperature sensor or a micro-electrode, or a micro-element with both capabilities and functions. The micro-element has a guide tube adapted to protect the components in its central lumen against exposure to fluid and trauma, but is sufficiently flexible to adapt to the complex and small confines inside a hollow electrode that is adapted to receive irrigation fluid and pass the fluid outside of the electrode through irrigation apertures. For temperature sensing function, the micro-element includes a pair of temperature sensing wires (e.g., thermistor wires) encased in a suitable sealant. For electrical sensing function, including impedance sensing, the micro-element carries a micro-electrode member configured for direct tissue contact, and a lead wire. For both temperature sensing and electrical sensing functions, the dual-functioning micro-element carries a pair of thermistor wires, a micro-electrode member and a lead wire. The micro-electrode member can be a discrete structure from the thermistor wires, or an electrically-conductive coating applied to the wires. 
     In a more detailed embodiment, the distal electrode assembly include a plurality of micro-elements whose distal ends are arranged in a radial pattern along a circumference of the distal portion of the shell electrode. Exposed distal ends of the micro-elements extend at an angle relative to the longitudinal axis of the shell electrode. The angle may have at least a distal component, if not also a radial component, as a distal end of a catheter often does not approach and make tissue contact with a direct “on-axis” approach. 
     Also, the plurality of micro-electrodes can include one group of micro-thermistors and another group of micro-electrodes, each group being arranged on the same circumference at the distal end of the shell electrode, interspersed with each other, or on a larger circumference and a smaller circumference, respectively. 
     Furthermore, the exposed portion of a micro-element can range between about 0.2 mm and 1.0 mm, preferably between about 0.3 mm and 0.6 mm, and more preferably about 0.5 mm. Each micro-element may have a diameter ranging between about 0.01 inch to 0.03 inch, preferably about 0.0135 inch. 
    
    
     
       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. It is understood that selected structures and features have not been shown in certain drawings so as to provide better viewing of the remaining structures and features. 
         FIG. 1  is a perspective view of a catheter in accordance with an embodiment of the present invention. 
         FIG. 2  is a perspective view of an electrode assembly in accordance with an embodiment of the present invention. 
         FIG. 2A  is a perspective view of an electrode assembly in accordance with another embodiment of the present invention. 
         FIG. 3 . is a side elevational view of the electrode assembly of  FIG. 2  in direct contact with tissue. 
         FIG. 4A  is a side cross-sectional view of a portion of the catheter of  FIG. 1 , including a junction of a catheter body and an intermediate deflectable section, taken along one diameter. 
         FIG. 4B  is a side cross-sectional view of a portion of the catheter of  FIG. 1 , including a junction of a catheter body and an intermediate deflectable section, taken along another diameter. 
         FIG. 4C  is an end cross-sectional view of the portion of the catheter of  FIG. 4B , taken along line C-C. 
         FIG. 5  is a side-cross-sectional view of the electrode assembly of  FIG. 2 . 
         FIG. 5A  is an end-cross-sectional view of the electrode assembly of  FIG. 5 , taken along line A-A. 
         FIG. 6  is an end view of the electrode assembly of  FIG. 2 . 
         FIG. 7A  is a side cross-sectional view of a portion of the catheter of  FIG. 1 , including a connecting portion, taken along one diameter. 
         FIG. 7B  is a side cross-sectional view of the portion of the catheter of  FIG. 7A , taken along another diameter. 
         FIG. 7C  is a distal end cross-sectional view of the portion of  FIG. 7B , taken along line C-C. 
         FIG. 8  is a perspective view of an electrode assembly in accordance with another embodiment of the present invention. 
         FIG. 9  is a side-cross-sectional view of the electrode assembly of  FIG. 8 . 
         FIG. 9A  is an end cross-sectional view of the electrode assembly of  FIG. 9 , taken along line A-A. 
         FIG. 10  is an end view of an electrode assembly in accordance with another alternate embodiment of the present invention. 
         FIG. 11  is an end view of the electrode assembly of  FIG. 8 . 
         FIG. 12A  is a side cross-sectional view of an embodiment of a connection portion and an intermediate deflectable section suitable for the electrode assembly of  FIG. 8 , taken along one diameter. 
         FIG. 12B  is a side cross-sectional view of an embodiment of a connection portion and an intermediate deflectable section suitable for the electrode assembly of  FIG. 8 , taken along another diameter. 
         FIG. 12C  is an end cross-sectional view of the connection portion of  FIG. 12B , taken along line C-C. 
         FIG. 13  is an end cross-sectional view of the intermediate deflectable section (near its proximal end) suitable for the electrode assembly of  FIG. 8 . 
         FIG. 14  is a partially exploded perspective view of an electrode assembly in accordance with yet another embodiment of the present invention. 
         FIG. 15  is a side cross-sectional view of the electrode assembly of  FIG. 14 . 
         FIG. 15A  is an enlarged view of a distal end of a micro-element of  FIG. 15 . 
         FIG. 15B  is an end cross-sectional view of the electrode assembly of  FIG. 15 , taken along line B-B. 
         FIG. 15C  is an end cross-sectional view of the electrode assembly of  FIG. 15 , taken along line C-C. 
         FIG. 15D  is an end cross-sectional view of the electrode assembly of  FIG. 15 , taken along line D-D. 
         FIG. 16A  is a side cross-sectional view of an embodiment of a connection portion and an intermediate deflectable section suitable for the electrode assembly of  FIG. 15 , taken along one diameter. 
         FIG. 16B  is a side cross-sectional view of an embodiment of a connection portion and an intermediate deflectable section suitable for the electrode assembly of  FIG. 15 , taken along another diameter. 
         FIG. 17A  is a side cross-sectional view of an embodiment of a junction between an intermediate deflectable section and a catheter body suitable for the electrode assembly of  FIG. 15 , taken along one diameter. 
         FIG. 17B  is a side cross-sectional view of an embodiment of the junction between an intermediate deflectable section and a catheter body suitable for the electrode assembly of  FIG. 15 , taken along another diameter. 
         FIG. 18  is a side cross-sectional view of a micro-element in accordance with an embodiment of the present invention. 
         FIG. 18A  is an end cross-sectional view of the micro-element of  FIG. 18 , taken along line A-A. 
         FIG. 18B  is a side cross-sectional view of a micro-element in accordance with another embodiment of the present invention. 
         FIG. 19  is a side cross-sectional view of a micro-thermistor in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As illustrated in  FIGS. 1, 2 and 3 , the present invention includes a steerable catheter  10  with a distal tip section  17  that includes an electrode assembly  19  and at least one micro-element  20  having an atraumatic distal end adapted for direct contact with target tissue  22 . As illustrated in  FIGS. 2 and 3 , the distal end may have an external portion that is exposed and protrudes distally of the electrode assembly  19  to deform tissue and create micro-depression  24  where the external portion depresses and/or sinks into the micro-depression so as to be surrounded and buried in the tissue without penetrating, piercing or otherwise breaching the tissue. Alternatively, the distal end of the micro-element  20  may be flush with an outer surface of the electrode assembly  19 , as illustrated in  FIG. 2A . In either embodiment, each micro-element may be configured as a temperature sensor, e.g., thermistor, thermocouple, fluoroptic probe, and the like, or electrode for sensing and/or ablation. Each micro-element can also be configured to provide all afore-mentioned functions, as desired. 
     Referring to  FIG. 1 , the catheter  10  according to the disclosed embodiments comprises an elongated body that may include an insertion shaft or catheter body  12  having a longitudinal axis, and an intermediate section  14  distal of the catheter body that can be uni- or bi-directionally deflectable off-axis from the catheter body. Distal of the intermediate section  14  is the electrode assembly  19  carrying at least one micro-element. Proximal of the catheter body is a control handle  16  that allows an operator to maneuver the catheter, including deflection of the intermediate section  14 . 
     In the depicted embodiment of  FIGS. 4A and 4B , 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 presently preferred construction comprises an outer wall  30  made of polyurethane or PEBAX. The outer wall  30  comprises an imbedded braided mesh of stainless steel or the like, as is generally known in the art, to increase torsional stiffness of the catheter body  12  so that, when the control handle  16  is rotated, the intermediate section  14  and distal section  17  will rotate in a corresponding manner. 
     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  30  is not critical, but is thin enough so that the central lumen  18  can accommodate any desired wires, cables and/or tubes. The inner surface of the outer wall  30  is lined with a stiffening tube  31  to provide improved torsional stability. The outer diameter of the stiffening tube  31  is about the same as or slightly smaller than the inner diameter of the outer wall  30 . The stiffening tube  31  can be made of any suitable material, such as polyimide, which provides very good stiffness and does not soften at body temperature. 
     As illustrated in  FIGS. 4A, 4B and 4C , the deflectable intermediate section  14  comprises a short section of tubing  15  having multiple lumens, each occupied by the various components extending through the intermediate section. In the illustrated embodiment, there are four lumens  30 ,  31 ,  32  and  33  as best seen in  FIG. 4C . Passing through a first lumen  30  are lead wire  40  for the electrode assembly  19 , a thermocouple pair  41 / 42  for each micro-element adapted as a thermistor, and a cable  36  for an electromagnetic position sensor  34 . Passing through a second lumen  31  is a fluid irrigation tubing  38  to supply fluid to the electrode assembly  19 . For at least uni-directional deflection, a first puller wire  44   a  passes through a third, off-axis lumen  32 . For bi-directional deflection, a second puller wire  44   b  passes through a fourth, off-axis lumen  33 . 
     The multi-lumened tubing  15  of the intermediate section  14  is made of a suitable non-toxic material that is preferably more flexible than the catheter body  12 . A suitable material is braided polyurethane or PEBAX, i.e., polyurethane or PEBAX with an embedded mesh of braided stainless steel or the like. The plurality and size of each lumen are not critical, provided there is sufficient room to house the components extending therethrough. Position of each lumen is also not critical, except the positions of the lumens  32 ,  33  for the puller wires  44   a ,  44   b . The lumens  32 ,  33  should be off-axis, and diametrically opposite of each other for bi-directional deflection along a plane. 
     The useful length of the catheter, i.e., that portion that can be inserted into the body can vary as desired. Preferably the useful length ranges from about 110 cm to about 120 cm. The length of the intermediate section  14  is a relatively small portion of the useful length, and preferably ranges from about 3.5 cm to about 10 cm, more preferably from about 5 cm to about 6.5 cm. 
     A preferred means for attaching the catheter body  12  to the intermediate section  14  is illustrated in  FIGS. 4A and 4B . The proximal end of the intermediate section  14  comprises an inner circumferential notch that receives the outer surface of the distal end of the stiffening tube  31  of the catheter body  12 . The intermediate section  14  and catheter body  12  are attached by glue or the like, for example, polyurethane. If desired, a spacer (not shown) can be provided within the catheter body  12  between the distal end of the stiffening tube  31  and the proximal end of the intermediate section  14  to provide a transition in flexibility at the junction of the catheter body  12  and the intermediate section, which allows the junction to bend smoothly without folding or kinking. An example of such a spacer is described in more detail in U.S. Pat. No. 5,964,757, the disclosure of which is incorporated herein by reference. 
     With reference to  FIGS. 5 and 5A , distal the intermediate section  14  is the distal electrode assembly  19  which includes an elongated, generally cylindrical, dome electrode  50  has a thin shell  57  and a plug  58 . The shell  57  has an enlarged distal portion  51  with an atraumatic dome-shaped distal end  52 . The distal portion defines a cavity or fluid chamber  53  that is in communication with an opening  54  at proximal end  55 . Both the distal portion  52  and the proximal portion  55  have a circular cross-section although the diameter of the proximal portion may be slightly lesser than the diameter of the distal portion, and thus, there may be a transitional section  56  in between, forming a “neck”. The shell  57  provides irrigation apertures  60  through which fluid entering and filling the chamber  53  can exit to outside of the dome electrode  50 . In one embodiment, there are 56 irrigation apertures in total, with a greater portion of the apertures formed in radial wall  62 , arranged in offset rows, and a lesser portion of the apertures formed in distal wall  64 . 
     The plug  58  is shaped and sized to fit in and provide a fluid-tight seal of the opening  54  of the shell  57 . In the illustrated embodiment, the plug is disc-shaped. Formed in the proximal face of the plug is a blind hole  72  receiving a lead wire  40 D for the dome electrode  50 . The plug also has a plurality of through-holes to allow passage of components and the like into the fluid chamber  53 . In the illustrated embodiment, the plug has four through-holes  74 ,  75 ,  76 ,  77 . Passing through each of through-holes  74 ,  75 ,  76  is a pair of thermistor wires  41 / 42 . Received in through-hole  77  is the distal end of the irrigation tubing  38  allowing fluid delivered through the tubing  38  to enter the chamber  53 . The plug and shell made be made of any suitable electrically-conductive material, such as palladium, platinum, iridium and combinations and alloys thereof, including, Pd/Pt (e.g., 80% Palladium/20% Platinum) and Pt/Ir (e.g., 90% Platinum/10% Iridium). 
     Advantageously, the wires  41 / 42  are sealed, insulated and protected by a routing guide tube  80  that extends from a proximal face  59  of the plug  58  to a short distance distal or beyond an outer surface of the distal wall  64  of the dome electrode  50 . The guide tube may be made of any suitable material that is fluid-tight, electrically-nonconductive, thermally-insulating, and sufficiently flexible, e.g., polyimide, to form a thin-walled tubing. Accordingly, the wires are protected from corrosive exposure to the fluid entering the chamber  53  and electrically-insulated from the shell  57 . The guide tube offers many advantages including (i) routing components through the hollow dome electrode having a complex curvature, (ii) protecting the components through the hollow dome electrode, and (iii) insulating the components to minimize cooling effects of fluid flowing through chamber. 
     The portion of the wires  41 / 42  extending through the guide tube  80  is potted along the length of the guide tube by a suitable material  84 , e.g., polyurethane or epoxy, which is shaped to form an atraumatic distal end  86 . The material should be corrosive fluid resistant, and be able to provide structural support and prevent large thermal gradients within the guide tubes that may otherwise result from exposure to irrigation fluid in the chamber  53 . No air exists in the guide tube. It is understood that a suitable micro-thermistor may also be constructed using a pre-existing thermistor. As illustrated in  FIG. 19 , a pre-existing thermistor (including wires  41 / 42  previously encased in potting material  85 ) is inserted in guide tube  80  and sealed at the proximal portion with material  84 . 
     As shown in  FIG. 3 , the distal end  86  and most, if not all, of the exposed distal portion of the micro-element  20  come in direct contact with the tissue  22  by forming a micro-depression  24  in the tissue and nesting therein so that at least the distal end if not also the exposed portion of the micro-element  20  is buried, enveloped, encapsulated and/or surrounded by tissue. Such direct contact with and probing of the tissue enables more accurate sensing. 
     The distal portion of each guide tube  80  extends through an aperture  88  formed in the shell  57  of the dome electrode  50 . In the illustrated embodiment, the apertures  88  are generally aligned with the through-holes in the plug  58  and they are formed along the circumferential corner  90  of the dome electrode  50  generally between the radial wall  62  and the distal wall  64  so that the guide tube  80  extends at an angle α of about 45 degrees relative to a longitudinal axis  92  of the dome electrode. The guide tubes can be held in position by adhesive or can sit naturally if designed with a slight interference fit with the apertures  88 . As such, there can be both a distal component and a radial component in the orientation of protrusion of the exposed distal portion of the micro-element  20 . It is understood however that the location and/or angle α may vary as desired. In typical applications, the distal component is greater than the radial component for improved and direct contact with tissue. 
     In one embodiment, the exposed portion of the micro-elements extending outside of the shell has a length D ranging between about 0.2 mm and 1.0 mm, preferably between about 0.3 mm and 0.6 mm, and more preferably about 0.5 mm. Each micro-element may have a diameter ranging between about 0.01 inch to 0.03 inch, preferably about 0.0135 inch. Although the illustrated embodiment has three micro-elements, with their distal ends arranged equi-distance from each other in a radial pattern, at about 0 degrees, 120 degrees and 240 degrees about the longitudinal axis of the dome electrode ( FIG. 6 ), it is understood that the plurality of micro-elements may vary, ranging between about two and six, and the angular position of the micro-elements may vary as well. 
     With reference to  FIGS. 7A, 7B and 7C , extending between the distal end of the intermediate section  14  and the dome electrode  50  is a connection portion  29  comprising a tubing  26 . The tubing can be single-lumened and be made of any biocompatible plastic such as PEEK. The tubing provides space so that the components extending between the intermediate portion  14  and the dome electrode  50  to be reoriented as needed. Moreover, the position sensor  34  is housed within the tubing  26 . 
     All of the wires pass through a common nonconductive protective sheath  45  ( FIG. 4A ), which can be made of any suitable material, e.g., polyimide, in surrounding relationship therewith. The sheath  45  extends from the control handle  16 , through the catheter body  12  and to the intermediate section  14 . 
     The pair of deflection puller wire  44   a ,  44   b  are provided for deflection of the intermediate shaft  14 . The puller wires  44   a ,  44   b  extend through the central lumen  18  of the catheter body  12  and each through a respective one of the lumens  32  and  33  of the intermediate section  14 . They are anchored at their proximal ends in the control handle  16 , and at their distal end to a location at or near the distal end of the intermediate section  14  by means of T-bars  63  ( FIG. 7B ) that are affixed to the sidewall of the tubing  15  by suitable material  65 , e.g., polyurethane, as generally described in U.S. Pat. No. 6,371,955, the entire disclosure of which is incorporated herein by reference. The puller wires are made of any suitable metal, such as stainless steel or Nitinol, and is preferably coated with Teflon® or the like. The coating imparts lubricity to the puller wire. For example, each puller wire has a diameter ranging from about 0.006 to about 0.010 inch. 
     As seen in  FIG. 4B , each puller wire has a respective compression coil  64  in surrounding relation thereto. Each compression coil  67  extends from the proximal end of the catheter body  12  to at or near the proximal end of the intermediate section  14  to enable deflection. The compression coils are made of any suitable metal, preferably stainless steel, and are each tightly wound on itself to provide flexibility, i.e., bending, but to resist compression. The inner diameter of the compression coils is preferably slightly larger than the diameter of a puller wire. The Teflon® coating on the puller wire allows it to slide freely within the compression coil. Within the catheter body  12 , the outer surface of the compression coil is covered by a flexible, nonconductive sheath  66 , e.g., made of polyimide tubing. The compression coils are anchored at their proximal ends to the outer wall  30  of the catheter body  12  by proximal glue joints and to the intermediate section  14  by distal glue joints. 
     Within the lumens  32  and  33  of the intermediate section  14 , the puller wires  44   a ,  44   b  extend through a plastic, preferably Teflon®, puller wire sheath  69  ( FIG. 4B ), which prevents the puller wires from cutting into the wall of the tubing  15  of the intermediate section  14  when the intermediate section  14  is deflected. 
     Longitudinal movement of the puller wires  44   a ,  44   b  relative to the catheter body  12  for bi-directional deflection is accomplished by appropriate manipulation of the control handle  16 . A deflection knob  94  ( FIG. 1 ) is provided on the handle which can be pivoted in a clockwise or counterclockwise direction for deflection in the same direction. Suitable control handles for manipulating more than one wire are described, for example, in U.S. Pat. Nos. 6,468,260, 6,500,167, and 6,522,933 and US Publication No. 2012/0143088, the entire disclosures of which are incorporated herein by reference. 
     The position sensor  48  can be a 3-coil electromagnetic sensor, or an assembly of single axis sensors (“SASs”). The position sensor enables the electrode assembly  19  (including the connection portion  29  housing the sensor) to be viewed under mapping systems manufactured and sold by Biosense Webster, Inc., including the CARTO, CARTO XP and NOGA mapping systems. Suitable SASs are described in U.S. Pat. No. 8,792,962, the entire disclosure of which is incorporated herein by reference. 
     With reference to  FIGS. 8-13 , an alternate embodiment of a catheter with a distal electrode assembly  19 ′ is illustrated. Structural similarities exist between the embodiments disclosed herein. Accordingly, similar structures are identified by similar reference numerals. 
     In the embodiment of  FIGS. 8 and 9 , a distal electrode assembly  19 ′ has a first plurality of micro-elements  20 A configured as thermistors, and a second plurality of micro-elements  20 B configured as micro-electrodes, where each plurality may range between about two and six, and the first and second pluralities may be equal or unequal. In the illustrated embodiment, the first and second pluralities are equal, namely, three each, and the distal ends of micro-thermistor and the micro-electrodes can be interspersed along a common circumference on the distal wall ( FIG. 10 ), or each occupy their own circumference on the distal wall ( FIG. 11 ), with the micro-electrodes occupying an inner circumference and the micro-thermistors occupying an outer circumference. In either case, the distal ends of one group of micro-thermistors are arranged equi-distant from each other, in a radial pattern interspersed between each other, at about 0 degree, 120 degree and 240 degree about the longitudinal axis of the dome electrode, and the distal ends of the other group of micro-electrodes are arranged equi-distant from each other, in a radial pattern at about 60 degree, 180 degree and 300 degree. 
     Each micro-electrode has its respective guide tube  80  and lead wire  40 M. In the illustrated embodiment, micro-electrode member  83  ( FIG. 9 ) of the micro-electrode is a solid, elongated cylindrical member arranged in axial alignment with the dome electrode  50 . The lead wire  40 M is soldered at its distal end to the cylindrical member and extends through the lumen of the guide tube  80 . The cylindrical member is exposed at a distal end  102  of guide tube  80  for direct with tissue. In one embodiment, the lead wire  40 M is a copper wire. In one embodiment, the diameter of the micro-electrode  20 B about 0.011 inch. 
     The distal ends  102  of the micro-electrodes  20 B and the distal ends  86  of the micro-thermistors  20 A come in direct contact with the tissue by forming micro-depressions in the tissue and nesting therein so that the distal ends are buried, enveloped, encapsulated and/or surrounded by tissue. Such direct and probing contact enables more accurate sensing by both the micro-electrodes and the micro-thermistors. However, as illustrated in the alternate embodiment of  FIG. 2A , it is understood that the distal ends  102  and  86  may be flush with an outer surface of the shell of the dome electrode, so that the micro-electrodes  20 A and  20 B have no exposed portions or protrusions beyond the outer surface of the wall of the shell. The proximal ends of tubings  80  may also extend proximally of the proximal face of the plug  58 , as desired or needed. 
     The plug  58 ′ of the dome electrode  50  is configured with through-holes  106  for micro-electrode lead wires  40 M with their guide tubes  80 . Apertures  88  are provided in the shell  57 ′ for these guide tubes  80 . Again, position of the through-holes in the plug  58 ′ is not critical. In the illustrated embodiment, the through-holes  106  are generally axially aligned with respective apertures  88  in the shell  57 ′. 
     With reference to  FIGS. 12A, 12B, 12C and 13 , proximal of the dome electrode  50 ′ and the connection portion  29 ′, the lead wires  40 M (along with the thermistor wires  41 / 42 , the position sensor cable  46  and the lead wire  40 D for the dome electrode) extend through the first lumen  30  of the tubing  15  of the intermediate section  14 , and through the central lumen  18  of the catheter body where they enter the control handle  16 . 
     With reference to  FIGS. 14-18 , another alternate embodiment of a catheter with a distal electrode assembly  19 ″ is illustrated. Structural similarities exist between the various embodiments disclosed herein. Accordingly, similar structures are identified by similar reference numerals. 
     In the embodiment of  FIGS. 14-16 , the distal electrode assembly  19 ″ has a plurality of micro-elements  20 C, each configured to function both as a micro-thermistor and a micro-electrode within a single common guide tube. In the illustrated embodiment, the thermistor wires  41 / 42  extend through the guide tube  80  in a manner as previously described. The electrode member of the micro-element takes the form of a shell cap  110  is mounted on the distal ends of the thermistor wires  41 / 42 . Best shown in  FIG. 15A , the shell cap  110  is cup-shaped with a proximal cylindrical portion  112  defining an opening and a distal portion with a generally U-shaped cross-section. The shell cap can be made of any suitable electrically conductive material, for example, palladium, platinum, iridium and combinations and alloys thereof, including, Pd/Pt (e.g., 80% Palladium/20% Platinum) and Pt/Ir (e.g., 90% Platinum/10% Iridium). The shell cap can have a thickness ranging between about 0.005 inch and 0.001 inch, preferably about 0.002 inch. The length of the proximal portion can vary. The longer the length the more structural support is provided to the micro-element. The length can be about half the length of the shell. The opening of the shell cap sits inside the distal end of the guide tube such that an outer circumferential surface of the opening  112  of the cap interfaces an inner circumferential surface of the distal end of the guide tube  80 . Soldered to a location on the outer or inner circumferential surface of the cap  110  is a distal end of the lead wire  40 M which extends proximally through the lumen of the guide tube  80  along with the thermistor wires  41 / 42 . The lead wire  40 M and the thermistor wires  41 / 42  are isolated from each other by a suitable electrically nonconductive and non-thermally insulative material  84 , e.g., polyurethane or epoxy, that fills the lumen of the guide tube  80 . In the illustrated embodiment, there are three dual-function micro-electrodes  20 C, with their distal ends arranged equi-distant from each other, in a radial pattern at about 0 degree, 120 degree and 240 degree about the longitudinal axis of the dome electrode. It is understood that the plurality and angular position may be varied as desired. The plurality may range between about two and six, preferably about three. 
     The distal end of each micro-element comes in direct contact with the tissue by forming a micro-depression in the tissue and nesting therein so that the distal end is buried, enveloped, encapsulated and/or surrounded by tissue. Such direct and probing contact enables more accurate electrical and thermal sensing. 
     The plug  58 ″ is configured with through holes  74 - 76  for micro-elements  20 C with their guide tubes  80 , through-hole  77  for irrigation tubing  38 , and blind-hole  72  for dome electrode lead wire  40 D. Apertures  88  are provided in the shell  57 ″ wall for the micro-elements  20 C. Again, position of the through-holes is not critical. In the illustrated embodiment, the through holes  74 - 76  in the plug are generally axially aligned with respective apertures  88  in the shell. 
     With reference to  FIGS. 16A, 16B, 17A and 17B , proximal the dome electrode  50 ″ and the connection portion  29 ″, the lead wires  40 M (along with the thermistor wires  41 / 42 , the position sensor cable  46  and the lead wire  40 D for the dome electrode) extend through the first lumen  30  of the tubing  15  of the intermediate section  14 , and through the central lumen  18  of the catheter body where they enter the control handle  16 . 
       FIGS. 18 and 18A  illustrate an alternate embodiment of a dual-function micro-element  20 D. Thermistor wires  41 / 42  are encased in a suitable sealant  84 , e.g., polyurethane or epoxy. The sealed wires are then coated with a coating  120  of electrically conductive material, e.g., gold impregnated epoxy, that serves as the micro-electrode member. Lead wire  40 M is connected to the coating  120 . The sealed and coated wires are further encased in a guide tube  80  to electrically isolate the wires and the coating from the dome electrode. Where the distal ends of micro-element protrudes beyond the outer surface of the wall of the shell, the distal end of the sealed and coated wires are exposed radially and distally ( FIG. 18 ). Where the distal ends of micro-elements are flush with the outer surface of the wall of the shell, the distal end of guide tubes  80  is coextensive with the distal end of the sealed and coated wires, leaving only the distal face exposed ( FIG. 18B ). 
     All of the through-holes in the plug in each embodiment is sealed around the guide tubes with any suitable sealant or adhesive, for example, polyurethane to prevent fluid leakage. The adhesive is first applied to the distal face of the plug prior to being pressed into the shell. After the electrode assembly is constructed, adhesive is applied to the proximal face of the plug for additional confidence in no fluid leakage. Components extending through the guide tubes, including lead wires and thermistor wires, can be anchored proximally in the catheter, for example, in the intermediate section  14 , to provide strain relief. 
     It is also understood that the distal ends of the micro-elements may be flush with the radial and distal walls of the shell. That is, while the aforementioned embodiments provide micro-elements with a distal end that protrudes from the shell, the present invention includes a distal electrode assembly wherein the distal ends of the micro-elements are coextensive with the outer surface of the shell and do not protrude beyond it. After the electrode assembly is constructed, any protruding distal ends of the micro-elements can be buffed away until the distal ends are even with the outer surface of the shell. 
     For the foregoing embodiments, the wire  41  of the wire pair is a copper wire, e.g. a number “40” copper wire and the wire  42  is a constantan wire. The wires of each pair are electrically isolated from each other except at their distal ends where they are twisted together. Moreover, lead wires  40 D and  40 M, thermistor wires  41 / 42 , puller wires  44   a  and  44   b , cable sensor  36  and irrigation tubing  38  extend proximally through the central lumen  18  of the catheter body  12  before entering the control handle where they are anchored or passed through to appropriate connectors or couplers inside the control handle or proximal thereof. 
     In operation, an operator, such as a cardiologist, inserts a guiding sheath through the vascular system of the patient so that the distal end of the guiding sheath enters a chamber of the patient&#39;s heart, for example, the left atrium. Operator then advances the catheter through the guiding sheath. The catheter is fed through the guiding sheath until at least the electrode assembly  19  is past the distal end of the guiding sheath. 
     The operator can advance and retract the catheter in the left atrium and deflect the intermediate portion  14  as appropriate to aim the electrode assembly  19  toward target tissue. The catheter is advanced until the distal end of the dome electrode contacts tissue. RF energy can be applied to the dome electrode to ablate the tissue for forming a lesion. Irrigation fluid is delivered via the irrigation tubing to the dome electrode where it enters the chamber and exits via the irrigation apertures for various purposes, including cool the dome electrode and keeping the surface free of char and coagulum. Additional normal force can be applied to so that the micro-elements depress the tissue and become nested in the tissue for direct contact which allows for more accurate sensing, including more accurate impedance measurement and more accurate temperature sensing. In the latter instance, deeper temperature sensing via the micro-elements provides a more accurate temperature reading of the tissue to avoid adverse effects of tissue overheating such as charring and steam pop, as opposed to merely the tissue surface temperature which can be biased by the cooling temperature of the irrigation fluid. Deeper impedance measurements are provides for more accurate measurements for various purposes including a determination of lesion size. 
     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. Any feature or structure disclosed in one embodiment may be incorporated in lieu of or in addition to other features of any other embodiments, as needed or appropriate. As understood by one of ordinary skill in the art, the drawings are not necessarily to scale. 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.