Abstract:
A catheter adapted for insertion into a body of a subject has at least one electrode disposed on its distal section. The electrode is coupled to an energy source to ablate tissue that is placed in contact with the electrode. The electrode has a wall with a plurality of perforations formed therethrough, and has edges defining a peripheral section that is adjacent the edges and a central section remote from the edges, wherein the wall of the peripheral section is thicker than the wall of the central section. A lumen passing through the insertion tube is coupled to deliver a fluid to the tissue via the perforations. In operation, the electrode functions as an effective heat sink.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to medical devices. More particularly, this invention relates to cooling of tissue contacted by an invasive probe within the body. 
     2. Description of the Related Art 
     Cardiac arrhythmia, such as atrial fibrillation, occurs when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm. Important sources of undesired signals are located in the tissue region along the pulmonary veins of the left atrium and in myocardial tissue associated with cardiac ganglionic plexi. In this condition, after unwanted signals are generated in the pulmonary veins or conducted through the pulmonary veins from other sources, they are conducted into the left atrium where they can initiate or continue arrhythmia. 
     Procedures for treating arrhythmia include disrupting the areas causing the arrhythmia by ablation, as well as disrupting the conducting pathway for such signals. Ablation of body tissue using electrical energy is known in the art. The ablation is typically performed by applying alternating currents, for example radiofrequency energy, to the electrodes, at a sufficient power to destroy target tissue. Typically, the electrodes are mounted on the distal tip or portion of an invasive probe or catheter, which is inserted into a subject. The distal tip may be tracked in a number of different ways known in the art, for example by measuring magnetic fields generated at the distal tip by coils external to the subject. 
     A known difficulty in the use of radiofrequency energy for cardiac tissue ablation is controlling local heating of tissue. There are tradeoffs between the desire to create a sufficiently large lesion to effectively ablate an abnormal tissue focus, or block an aberrant conduction pattern, and the undesirable effects of excessive local heating. If the radiofrequency device creates too small a lesion, then the medical procedure could be less effective, or could require too much time. On the other hand, if tissues are heated excessively then there could be local charring effects due to overheating. Such overheated areas can develop high impedance, and may form a functional barrier to the passage of heat. The use of slower heating provides better control of the ablation, but unduly prolongs the procedure. 
     Previous approaches to controlling local heating include the inclusion of thermocouples within the electrode and feedback control, signal modulation, local cooling of the catheter tip, and fluid-assisted techniques, for example irrigation of the target tissue during the energy application, using chilled fluids. Typical of the last approach is Mulier, et al. U.S. Pat. No. 5,807,395. 
     Commonly assigned U.S. Pat. No. 6,997,924, which is herein incorporated by reference, describes a technique of limiting heat generated during ablation by determining a measured temperature of the tissue and a measured power level of the transmitted energy, and controlling the power output level responsively to a function of the measured temperature and the measured power level. 
     More recently, commonly assigned U.S. Patent Application Publication No. 2010/0030209 by Govari et al., which is herein incorporated by reference, describes an insertion tube, having an outer surface with a plurality of perforations through the outer surface, which are typically about 100 μm in diameter, and are distributed circumferentially and longitudinally over the distal tip. A lumen passes through the insertion tube and is coupled to deliver a fluid to the tissue via the perforations. 
     Commonly assigned U.S. Patent Application Publication No. 2010/0168548 by Govari et al., describes a catheter having perforated electrodes that bulge above the outer surface of the catheter to permit an outflow of irrigating fluid. 
     SUMMARY OF THE INVENTION 
     The walls of currently used irrigating electrodes have a constant thickness along their entire length. During ablation, the two edges or peripheral regions of the electrodes may heat up excessively. This may result from poor irrigation, as the edges tend to be spaced apart from the irrigation holes and a higher density of the electric field lines at the peripheral zones, which form sharp conductive edges of the electrode, known as an “edge effect”. 
     In the embodiments presented herein, the wall thickness of the electrodes is increased at the edges, providing a relatively greater thermal mass and providing increased heat absorption capacity compared to the central portion of the electrode. Consequently, during ablation there is a decreased rise in temperature at the ablation site, thereby minimizing the probability for blood coagulation, tissue scorching, and damage to the adjacent catheter structure. 
     There is provided according to embodiments of the invention an insertion tube or catheter, adapted for insertion into a body of a subject. The tube has at least one electrode disposed on its distal section, which is coupled to an energy source to apply energy to tissue that is placed in contact with the electrode. The electrode has a wall with a plurality of perforations formed therethrough, and has edges defining a peripheral section that is adjacent the edges and a central section remote from the edges, wherein the wall of the peripheral section is thicker than the wall of the central section. A lumen passing through the insertion tube is coupled to deliver a fluid to the tissue via the perforations. 
     According to one aspect of the device, the wall continually and gradually thickens from a point on the central section toward the peripheral section. 
     According to aspect of the device, the wall thickens in discrete stages from the central section toward the peripheral section. 
     According to still another aspect of the device, the wall of the central section has a uniform thickness. 
     According to an additional aspect of the device, the perforations of the electrode have diameters between 0.05 mm and 2.5 mm. According to still another aspect of the device, the diameters are between 0.5 and 2.5 mm. According to a further aspect of the device, there are up to sixty perforations. 
     According to yet another aspect of the device, the electrode is round in contour, and the wall thickens from the central section toward the peripheral section in all outward directions. 
     According to one aspect of the device, the electrode is elliptical in contour, and the wall thickens from the central section toward the peripheral section in all outward directions. 
     According to a further aspect of the device, a mass of the peripheral section is between three and five times a mass of the central section. 
     According to another aspect of the method, a mass of the peripheral section exceeds a mass of the central section and is up to twice the mass of the central section. 
     According to yet another aspect of the device, the electrode includes a plurality of ring electrodes linearly arranged on the distal section of the insertion tube. 
     According to an additional aspect of the device, the electrode bulges between 0.05 mm and 1.0 mm above the outer surface of the insertion tube. 
     Other embodiments of the invention provide methods that are carried out by the above-described device. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein like elements are given like reference numerals, and wherein: 
         FIG. 1  is a pictorial illustration of a system for performing ablative procedures on a heart of a living subject, which is constructed and operative in accordance with an embodiment of the invention; 
         FIG. 2  is a detailed view of the distal portion of the catheter shown in  FIG. 1 , showing multiple electrodes distributed in linear arrays, in accordance with an embodiment of the invention; 
         FIG. 3  is a composite view of the distal portion of a catheter, which is constructed in accordance with an alternate embodiment of the invention; 
         FIG. 4  is a sectional view of a segment of the distal portion of a catheter, which is constructed in accordance with an alternate embodiment of the invention; 
         FIG. 5  shows a linear arrangement of multiple electrodes on the distal portion of a catheter, in accordance with an alternate embodiment of the invention; 
         FIG. 6  is a cross-sectional view through an irrigation ring of a catheter in accordance with the embodiments shown in  FIG. 4  and  FIG. 5 ; and 
         FIG. 7  is a detailed longitudinal sectional view through the irrigation ring shown in  FIG. 6 , indicating various typical dimensions. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various principles of the present invention. It will be apparent to one skilled in the art, however, that not all these details are necessarily always needed for practicing the present invention. In this instance, well-known circuits, control logic, and the details of computer program instructions for conventional algorithms and processes have not been shown in detail in order not to obscure the general concepts unnecessarily. 
     EMBODIMENT 1 
     Turning now to the drawings, reference is initially made to  FIG. 1 , which is a pictorial illustration of a system  10  for performing ablative procedures on a heart  12  of a living subject, which is constructed and operative in accordance with an embodiment of the invention. The system comprises a catheter  14 , which is percutaneously inserted by an operator  16  through the patient&#39;s vascular system into a chamber or vascular structure of the heart  12 . The operator  16 , who is typically a physician, brings the catheter&#39;s distal portion or tip  18  into contact with the heart wall at an ablation target site. Optionally, electrical activation maps may then be prepared, according to the methods disclosed in U.S. Pat. Nos. 6,226,542, and 6,301,496, and in commonly assigned U.S. Pat. No. 6,892,091, whose disclosures are herein incorporated by reference. One commercial product embodying elements of the system  10  is available as the CARTO® 3 System, available from Biosense Webster, Inc., 3333 Diamond Canyon Road, Diamond Bar, Calif. 91765. 
     Areas determined to be abnormal, for example by evaluation of the electrical activation maps, can be ablated by application of thermal energy, e.g., by passage of radiofrequency electrical current through wires in the catheter to one or more electrodes at the distal tip  18 , which apply the radiofrequency energy to the myocardium. The energy is absorbed in the tissue, heating it to a point (typically about 50° C.) at which it permanently loses its electrical excitability. When successful, this procedure creates non-conducting lesions in the cardiac tissue, which disrupt the abnormal electrical pathways causing the arrhythmia. The principles of the invention can be applied to different heart chambers to treat many different cardiac arrhythmias. 
     The catheter  14  typically comprises a handle  20 , having suitable controls on the handle to enable the operator  16  to steer, position and orient the distal end of the catheter as desired for the ablation. To aid the operator  16 , the distal portion of the catheter  14  contains position sensors (not shown) that provide signals to a positioning processor  22 , located in a console  24 . 
     Ablation energy and electrical signals can be conveyed to and from the heart  12  through an ablation electrode  32  located at the distal tip  18  via cable  34  to the console  24 . While a single ablation electrode  32  is shown, more than one can be present. Pacing signals and other control signals may be conveyed from the console  24  through the cable  34  and the ablation electrode  32  to the heart  12 . Sensing electrodes  33 , also connected to the console  24  are typically disposed near the ablation electrode  32  and have connections to the cable  34 . 
     Wire connections  35  link the console  24  with body surface electrodes  30  and other components of a positioning sub-system. A temperature sensor (not shown), typically a thermocouple or thermistor, may be mounted on or near the ablation electrode  32  and to electrodes  37 . 
     One or more electrodes  37  are distributed about the shaft of the catheter  14 , generally along the distal segment. The electrodes  37  are adapted for enhanced heat conductance, which is useful when they are employed for ablation. However the electrodes  37  may be used as sensing electrodes or for pacing. The electrodes  37  typically bulge above the surface of the catheter, but may be flush with the surface of the catheter, so long as the wall thickness varies as described in the various embodiments hereinbelow. While a generally curved profile is shown in the example of  FIG. 1 , in some embodiments the electrodes may appear as a flat plateau above the surface of the catheter shaft when viewed in profile. 
     The console  24  typically contains one or more ablation power generators  25 . The catheter  14  may be adapted to conduct ablative energy to the heart using any known ablation technique, e.g., radiofrequency energy, ultrasound energy, and laser-produced light energy. Such methods are disclosed in commonly assigned U.S. Pat. Nos. 6,814,733, 6,997,924, and 7,156,816, which are herein incorporated by reference. 
     The positioning processor  22  is an element of a positioning subsystem of the system  10  that measures location and orientation coordinates of the catheter  14 . 
     In one embodiment, the positioning subsystem comprises a magnetic position tracking arrangement that determines the position and orientation of the catheter  14  by generating magnetic fields in a predefined working volume its vicinity and sensing these fields at the catheter using field generating coils  28  and may include impedance measurement, as taught, for example in commonly assigned U.S. Pat. No. 7,756,576, which is herein incorporated by reference. 
     As noted above, the catheter  14  is coupled to the console  24 , which enables the operator  16  to observe and regulate the functions of the catheter  14 . Console  24  includes a processor, preferably a computer provided with appropriate signal processing circuits. The processor is coupled to drive a monitor  29 . The signal processing circuits typically receive, amplify, filter and digitize signals from the catheter  14 , including signals generated by the above-noted sensors and a plurality of location sensing electrodes (not shown) located distally in the catheter  14 . The digitized signals are received and used by the console  24  and the positioning subsystem to compute the position and orientation of the catheter  14  and to analyze the electrical signals from the electrodes. 
     Typically, the system  10  includes other elements, which are not shown in the figures for the sake of simplicity. For example, the system  10  may include an electrocardiogram (ECG) monitor, coupled to receive signals from one or more body surface electrodes, so as to provide an ECG synchronization signal to the console  24 . As mentioned above, the system  10  typically also includes a reference position sensor, either on an externally applied reference patch attached to the exterior of the subject&#39;s body, or on an internally placed catheter, which is inserted into the heart  12  and maintained in a fixed position relative to the heart  12 . Conventional pumps and lines for circulating liquids through the catheter  14  for cooling the ablation site are provided. 
     Reference is now made to  FIG. 2 , which is a detailed view of the distal portion of the catheter  14 , showing multiple electrodes  37  distributed in linear arrays about its shaft  39 . The electrodes  37  may bulge between about 0.05-0.5 mm above the outer surface of the shaft  39  and have a generally rounded profile, forming a cap on the surface of the shaft. In some embodiments the electrodes  37  may have a larger bulge, up to 1 mm above the surface. The electrodes  37  may extend over 25-70 per cent of the circumference of the surface  41 , as contrasted with a conventional ring electrode, which covers 100% of the circumference. The electrodes  37  may have a circular border contour. Alternatively, they may be elliptical in contour, as further described in commonly assigned application Ser. No. 12/345720, entitled “Dual-Purpose Lasso Catheter with Irrigation”, which is herein incorporated by reference. The electrodes  37  may be 2-5 mm in dimension. These configurations provide substantial contact between the electrodes  37  and the cardiac tissue, lowering electrical resistance as compared with conventional electrodes. When the electrodes  37  are used for ablation, the reduced electrical resistance is particularly advantageous. In one embodiment, two of the electrodes  37  are selected for performing bi-polar ablation, e.g., radiofrequency ablation in which case a cable  43  may include wires individually leading to the electrodes  37 . 
     As shown in a representative view, cross-section  45  has a generally curved contour, forming a bulge on the shaft  39 . The bulge of the electrode increases the surface area that is in contact with the target tissue, and reduces electrical resistance when the electrodes are used for ablation. Wall  47  of the electrodes  37  has a non-uniform thickness. It is relatively thin at a central point  49 , and thickens in all directions toward peripheral regions  51  adjacent the edges. 
     The wall  47  may thicken in a continual, gradual manner toward the edges of the electrodes  37 . Alternatively, the wall  47  may thicken in discrete stages from the center toward the edges. In any case, the non-uniform thickness of the wall  47  provides enhanced heat conductance at the edges of the electrodes, which act as a heat sink. The heat sink helps to prevent thermal damage to the electrodes and to the surrounding areas on the shaft  39 , and reduces the likelihood of undesired blood coagulation at or near the site of ablation. The shaft  39  is typically formed of plastic, and is susceptible to overheating. 
     The wall  47  is fenestrated by multiple small perforations  53  formed therethrough. Typically there are between 1 and 60 perforations having diameters of 0.5-2.5 mm. Alternatively, much smaller diameters of between 0.05-0.4 mm may be used in order to generate turbulent flow, as described in commonly assigned, co-pending application Ser. No. 13/339,782, entitled “Electrode Irrigation Using Micro-Jets”, which is herein incorporated by reference. The perforations  53  are in fluid communication with an irrigating chamber  55 , formed beneath the wall  47 . The chamber  55  in turn is in fluid communication with an irrigation lumen  56  within the shaft  39 , so that the fluid flows outward, through the perforations  53 , as indicated by arrows  41 . There may be a plurality of irrigation lumens in order to more conveniently supply the electrodes  37  when they are distributed about the circumference of the shaft  39 . 
     EMBODIMENT 2 
     Reference is now made to  FIG. 3 , which is a composite view of the distal portion of a catheter  57 , which is constructed in accordance with an alternate embodiment of the invention. The catheter  57  is similar to the catheter  14  ( FIG. 1 ), except that raised, perforated electrodes  43  are elliptical rather than circular. In  FIG. 3  the orientation of the ellipses is such that their major axes are perpendicular to the long axis of the catheter, shown as line  3 A- 3 A. However, the electrodes  43  may be oriented such that their major axes are aligned with or parallel to line  3 A- 3 A. 
       FIG. 3  shows sectional views  59 ,  61  of one of the electrodes  43  along lines  3 A- 3 A and  3 B- 3 B, respectively. In both cases, wall  63  is relatively thin at central point  65  and relatively thick in regions  67 ,  69  near the edges of the electrodes  43 . 
     EMBODIMENT 3 
     Reference is now made to  FIG. 4 , which is a sectional view of a segment of the distal portion of a catheter  71 , which is constructed in accordance with an alternate embodiment of the invention. 
     A perforated electrode  73  bulges above the catheter&#39;s outer surface  75  and may form a ring about the catheter shaft. The electrode  73  comprises relatively thick peripheral sections  77 , a relatively thin central section  79  and an interior chamber  81 . The wall thickness may vary as in the electrodes of the previous embodiments, i.e., gradually from center to edge, or in stepwise increments from center to edge. The chamber  81  is in fluid communication with a lumen  83 , through which irrigating fluid is supplied to the chamber  81  via channels  85 . The irrigating fluid escapes from the chamber  81  through orifices  87  where it cools the ablation site and in particular the central section  79 . The peripheral sections  77  act as an excess heat sink, preventing overheating of the ablation site and the catheter structure itself during ablation. 
     The peripheral sections  77  have a greater thermal mass than the central section  79 . The ratio can be 2:1, but preferably is at least 3:1, and may be as high as 5:1. A current embodiment has a ratio of approximately 3.5:1, in order that the peripheral sections  77  function effectively as improved heat sinks in comparison to electrodes having uniform wall thickness. The electrode  73  and the electrodes shown in other embodiments herein may be made from an electrically conductive biocompatible material. For example, it could be made out of platinum, gold, other noble metals, and alloys thereof in a variety of compositions. 
     Reference is now made to  FIG. 5 , which shows a linear arrangement of the multiple electrodes  73  on the distal portion of the catheter  71 , in accordance with an embodiment of the invention. 
     EMBODIMENT 4 
     Reference is now made to  FIG. 6 , which is a cross sectional view of an irrigation ring electrode  90  in accordance with an alternate embodiment of the invention. Reference is now made to  FIG. 7 , which is a detailed longitudinal sectional view of the irrigation ring electrode shown in  FIG. 6 . Peripheral sections  91  are beveled at an angle A (2×38°). The peripheral sections  91  are more than twice as thick as central section  93  (0.009″ vs. 0.0035″). 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.