Patent Publication Number: US-2019183567-A1

Title: Balloon Catheter with Bulbous Shaped Radiofrequency (RF) Ablation Electrodes

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to medical probes, and particularly to balloon catheters. 
     BACKGROUND OF THE INVENTION 
     Various known catheter designs have an inflatable radiofrequency ablation balloon fitted at their distal end. For example, U.S. Patent Application Publication 2014/033393 describes cardiac ablation catheters and methods of use. In some embodiments the catheter includes at least one camera inside an expandable membrane for visualizing an ablation procedure. In an embodiment, electrodes are disposed over an inflatable balloon. The electrodes are configured and positioned so as to deliver ablative radiofrequency energy to tissue when the balloon is inflated. An electrode or group of electrodes can be visually identified based on their shape. For example, groups of electrodes can be circular, oval, hexagonal, rectangular, square, etc. 
     As another example, U.S. Patent Application Publication 2010/0204560 describes a tissue electrode assembly that includes a membrane configured to form an expandable, conformable body that is deployable in a patient. The assembly further includes a flexible circuit positioned on a surface of the membrane and comprising at least one base substrate layer, at least one insulating layer and at least one planar conducting layer. An electrically-conductive electrode covers at least a portion of the flexible circuit and a portion of the surface of the membrane not covered by the flexible circuit, wherein the electrically-conductive electrode is foldable upon itself with the membrane to a delivery conformation having a diameter suitable for minimally-invasive delivery of the assembly to the patient. The shape and pattern of the electrodes can vary. The surface area, shape and pattern of the electrodes can influence the amount of energy applied and the ablation line created. Various electrode patterns and electrode shapes considered herein including, but not limited to, circular, rectangular, octagonal and polygonal. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides a medical instrument including a shaft, an inflatable balloon and a Radiofrequency (RF) ablation electrode. The shaft is configured for insertion into a body of a patient. The inflatable balloon is coupled to a distal end of the shaft. The RF ablation electrode is disposed on an external surface of the balloon and has a distal edge configured to reduce electric field angular gradients of an RF electric field emitted from the distal edge. 
     In some embodiments, the electrode includes an elongate main section that progressively narrows towards the distal edge, and a distal-end section, which is connected to a distal-most end of the main section and is wider than the main section at the distal-most end. In some embodiments, the distal edge of the electrode has a bulbous shape having at least a given radius of curvature. In an embodiment, by reducing the electric field angular gradients, the distal edge is configured to reduce charring deposits on the electrode. 
     There is additionally provided, in accordance with an embodiment of the present invention, a method for manufacturing a medical instrument, including disposing, on an external surface of an inflatable balloon, a Radiofrequency (RF) ablation electrode having a distal edge configured to reduce electric field angular gradients of an RF electric field emitted from the distal edge. The inflatable balloon is coupled to a shaft configured for insertion into a body of a patient. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, pictorial illustration of a catheter-based ablation system comprising a Radiofrequency (RF) ablation balloon, in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic, pictorial illustration of an ablation balloon comprising bulbous shaped RF ablation electrodes, in accordance with an embodiment of the present invention; and 
         FIG. 3  is a schematic, pictorial illustration comparing electric field gradients generated at distal edges of RF ablation electrodes, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Embodiments of the present invention that are described hereinafter provide an RF ablation balloon comprising electrodes designed to reduce adverse effects of an ablation, such as charring of tissue or blood due to excessive electric field gradients 
     Charring of tissue or blood during ablation is a highly undesired side-effect. One problem encountered in RF ablation is tissue charring around the electrode, which produces a phenomenon called ‘roll-off’ (sudden increase in impedance), in which the tissue surrounding the electrode increases its electrical resistance and with it the circuit impedance. As a result, current circulation stops and no further damage to tissue is produced. 
     Typically, charring would also convert part of the electrode surface from a good electrical conductor to a poor one, which may cause a sharp drop in ablating electrical currents. As noted above, such drop would limit the depth of ablated tissue (also in other subsequent locations, where tissue itself in not charred but the electrode in use is at least partially covered with char), and hence reduce the efficacy of the ablation procedure. Moreover, in rare cases, charring may cause Thromboembolic complications (i.e., thrombosis of blood embolisms) that might subsequently result in a severe clinical outcome, such as a stroke. 
     Despite efforts to eliminate charring, for example by tissue cooling and localized blood dilution using irrigation, charring may still be evident around distal ends (i.e., distal edges) of RF ablation electrodes, where the edges of electrodes (i.e., the electrodes contour) have a small radius of curvature. 
     The cause is estimated in the present disclosure to be systematic: as the conducting edge of such electrode is a line of constant potential, p, the electric field E=−∇φ, should be perpendicular to the electrode edge at every point along the edge. A sharply curved edge of the distal end of an electrode results in the field lines diverging at large angles relative to each other. Thus, large field gradients occur at sharp curves. When passing through tissue or blood, exceedingly large field gradients may result in charring. 
     In some embodiments of the present invention, an ablation balloon fitted at the distal end of a catheter comprises RF ablation electrodes having an enlarged rounded distal edge. Namely, instead of electrodes that are typically continuously narrowing distally and have a pointed distal end, the disclosed electrodes end with a bulbous, circular area, having a relatively large radius of curvature. 
     The disclosed RF ablation balloon, which comprises electrodes having an enlarged rounded distal edge, may reduce tissue charring that may be caused by the high electrical field gradients associated with the small radius of curvature at the electrode distal edge. By reducing tissue charring, the disclosed RF ablation balloon may reduce potentially severe clinical side effects of RF balloon ablation procedures. In addition, avoiding charring may also reduce reliability issues and increase the lifetime of the electrodes. 
     System Description 
       FIG. 1  is a schematic, pictorial illustration of a catheter-based ablation system  20  comprising an RF ablation balloon  40 , in accordance with an embodiment of the present invention. System  20  comprises a catheter  21 , wherein a distal end of shaft  22  of the catheter is inserted through a sheath  23  into a heart  26  of a patient  28  lying on a table  29 . The proximal end of catheter  21  is connected to a control console  24 . In the embodiment described herein, catheter  21  may be used for any suitable therapeutic and/or diagnostic purposes, such as electrical sensing and/or ablation of tissue in heart  26 . 
     Physician  30  navigates the distal end of shaft  22  to a target location in heart  26  by manipulating shaft  22  using a manipulator  32  near the proximal end of the catheter and/or deflection from the sheath  23 . During the insertion of shaft  22 , balloon  40  is maintained in a collapsed configuration by sheath  23 . By containing balloon  40  in a collapsed configuration, sheath  23  also serves to minimize vascular trauma along the way to target location. 
     Control console  24  comprises a processor  41 , typically a general-purpose computer, with suitable front end and interface circuits  38  for receiving signals from catheter  21 , as well as for applying treatment via catheter  21  in heart  26  and for controlling the other components of system  20 . Processor  41  typically comprises a general-purpose computer, which is programmed in software to carry out the functions described herein. The software may be downloaded to the computer in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory. 
     The example configuration shown in  FIG. 1  is chosen purely for the sake of conceptual clarity. The disclosed techniques may similarly be applied using other system components and settings. For example, system  20  may comprise other components and perform non-cardiac ablative treatments. 
     Balloon Catheter with Bulbous Shaped RF Electrodes 
       FIG. 2  is a schematic, pictorial illustration of ablation balloon  40  comprising bulbous shaped RF ablation electrodes  42 , in accordance with an embodiment of the invention. As seen, electrodes  42 , which are disposed over substrates  44 , comprise each of a rounded edge  50 , at their distal end. 
     As seen, rounded (i.e., bulbous) edge  50  increases the radius of curvature of the electrode at its distal end, which otherwise would tend to narrow (as dashed line  52  shows), and to potentially generate high local electrical fields. In other words, a given ablation electrode  42  comprises (i) an elongate main section that progressively narrows towards the distal edge of the electrode, and (ii) a distal-end section, which is connected to the distal-most end of the main section and is wider than the main section at the distal-most end. 
     The example illustration shown in  FIG. 2  is chosen purely for the sake of conceptual clarity. Other designs of curvatures of the distal end are possible, for example elliptical ones, or combinations of various rounded shapes. 
       FIG. 3  is a schematic, pictorial illustration comparing electric field gradients generated at distal edges of RF ablation electrodes, in accordance with an embodiment of the invention. The electrode shown on the left-hand side of the figure is a hypothetical conventional electrode having a relatively pointed end. The electrode shown on the right-hand side of the figure, in accordance with an embodiment of the present invention, comprises an end having an increased radius of curvature. 
     The size electric field gradient between locations  62  (or between locations  60 ) on the edge of electrode  42 , exemplified in  FIG. 3 , is proportional to the angle between normal lines  68  (or  70 ) to the electrode edge at positions  62  (or  60 ), respectively. 
     As seen, angle θ 2  at the bulbous edge is smaller than corresponding angle θ 1  at a common edge. Thus, the increased curvature  50 , relative to curvature  52 , serves to decrease angular electrical field gradients. 
     The example illustration shown in  FIG. 3  is chosen purely for the sake of conceptual clarity. Other designs of curvatures of the distal end are possible for lowering angular electric field gradients at a distal edge of electrode  42 . One alternative example is an elliptical distal edge, or a distal edge having variable curvature so as to, for example, mitigate local excessive charring, i.e., at specific locations over the distal edge of an electrode. 
     Although the embodiments described herein mainly address pulmonary vein isolation, the methods and systems described herein can also be used in other applications, such as otolaryngology or neurology procedures. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and 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 sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.