Patent Publication Number: US-2009228003-A1

Title: Tissue ablation device using radiofrequency and high intensity focused ultrasound

Description:
FIELD OF THE INVENTION 
     The present application relates to devices and medical procedures for ablating tissue, more particularly to devices and procedures for ablating heart tissue. 
     BACKGROUND OF THE INVENTION 
     In certain medical procedures, it is desirable to heat tissue surrounding an anatomical structure such as a blood vessel or a gastrointestinal, urinary, genital, or respiratory structure. Depending upon the condition to be treated, energy may be applied to the tissue constituting the wall of the structure, or to tissue surrounding the wall. Energy may be applied to heat the tissue to a degree sufficient to cause death of the tissue. Heating to this degree is referred to herein as “ablation.” Typically, heating to about 60-80° C. for a short time is sufficient. 
     Ablation of tissue in patients with atrial fibrillation or “AF” has been proposed heretofore. Contraction or “beating” of the heart is controlled by electrical impulses generated at nodes within the heart and transmitted along conductive pathways extending within the wall of the heart. Certain diseases of the heart known as cardiac arrhythmias involve abnormal generation or conduction of the electrical impulses. One such arrhythmia is atrial fibrillation. Certain cardiac arrhythmias can be treated by deliberately damaging the tissue along a path crossing a route of abnormal conduction. This causes formation of a scar extending along the path where disruption occurred. The scar blocks conduction of the electrical impulses. The abnormal electrical impulses can be carried by abnormal structures extending within the wall of a pulmonary vein. Conduction of these abnormal electrical impulses may be blocked by forming a scar in the wall of the pulmonary vein or in the opening or ostium of the pulmonary vein. For example, as described in U.S. Pat. No. 5,575,766, ablation may be performed using a catheter having an ablation element such as an RF electrode at its tip. The physician maneuvers the catheter so that the tip moves along the heart wall while the electrode is active to trace the desired scar on the heart wall. This approach manifestly requires a difficult series of manipulations by the physician. U.S. Pat. No. 5,971,983 depicts an elongated ablation catheter having numerous ablation elements, as for example, RF electrodes, arranged along its length so that, at least in theory, an elongated lesion can be formed by positioning the catheter against an elongated region of the heart wall and actuating the ablation elements. U.S. Pat. No. 6,254,599 recites an ablation device carried on the tip of a catheter and adapted for insertion into a pulmonary vein. The ablation device is assertedly capable of forming a ring-like lesion encircling the vein. Certain embodiments of the &#39;599 patent show such a ring-forming device mounted at the distal end of an elongated catheter with numerous additional ablation elements arrayed along its length so that a linear lesion can be formed in conjunction with the ring-like region. 
     Commonly assigned U.S. Pat. No. 6,635,054, the disclosure of which is incorporated by reference herein, teaches, inter alia, an ablation device using an ultrasonic emitter and a reflector formed by a balloon structure to focus ultrasonic energy from the emitter into a ring-like focal region. As discussed in the &#39;054 patent, such a device can be used to form a ring-like lesion in the heart wall, encircling the ostium of a pulmonary vein. Commonly assigned U.S. Patent Publication No. 2004/0176757 discloses, inter alia, a similar ablation device which is mounted on a steerable catheter. As taught in the &#39;757 publication, such a steerable balloon device can be positioned in the desired relationship to the heart wall readily, even where the pulmonary veins lie at unusual angles to the heart wall or have other irregular features. As also taught in certain embodiments of the &#39;757 publication, the steerable ablation device can be used to form linear or spot lesions by turning the device to lie at an appropriate orientation relative to the heart wall. The preferred apparatus and methods in accordance with the &#39;054 patent and &#39;757 publication can provide effective therapy for arrhythmias such as AF. However, still further improvement would be desirable. 
     SUMMARY OF THE INVENTION 
     An ablation device according to one aspect of the present invention includes a catheter having a first ablation element secured to the catheter. A second ablation element is secured to the catheter distal to the first ablation element. The second ablation element&#39;s mode of operation is different from the first ablation element. For example, the first ablation element may be an ultrasonic ablation element, whereas the second ablation element may be an electrode for application of RF or other electrical energy. The catheter may be steered to position at least one of the first ablation element or the second ablation element in a desired location relative to a tissue to be ablated. 
     One aspect of the invention provides apparatus for cardiac treatment. The apparatus according to this aspect of the invention desirably includes a probe having proximal and distal ends and a first ablation element secured to the probe at or adjacent the distal end thereof. The first ablation element may include an expansible balloon structure and an ultrasonic transducer mounted within the balloon structure, the balloon structure having a distal end and a proximal end, the ultrasonic transducer and balloon structure being constructed and arranged so that ultrasonic energy emitted by the ultrasonic transducer will be directed through the balloon structure. The apparatus according to this aspect of the invention most preferably also includes an additional ablation element secured to the probe and located distal to the ultrasonic transducer and at least partially outside the balloon structure. The first ablation element may be arranged to form an arcuate or loop-like lesion, whereas the second ablation element may be arranged to form a spot lesion. 
     A further aspect of the invention provides methods of ablating cardiac tissue to impede flow of abnormal electrical signals. A method according to this aspect of the invention desirably includes the steps of: inserting an elongated probe so that a distal end of the probe and an first ablation element carried on the probe is disposed in a chamber of the heart, ablating tissue using the first ablation element to form a lesion, positioning an additional ablation element by steering the probe, and ablating tissue using the additional ablation element. For example, the first ablation element may be used to form a loop-like lesion, and the additional ablation element may be used to ablate spots at gaps in the loop-like lesion, to form linear lesions, or both. 
     Other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view depicting an ablation device according to an embodiment of the invention in conjunction with cardiac structures. 
         FIG. 2  is a fragmentary schematic view of depicting a portion of the ablation device of  FIG. 1  with certain elements omitted for clarity of illustration. 
         FIG. 3  is view similar to  FIG. 2 , but depicting the ablation device in a different stage of operation. 
         FIG. 4  is a view similar to  FIG. 2  depicting a portion of an ablation device according to a still further embodiment of the invention. 
         FIG. 5  is a schematic view depicting an ablation device according to yet another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows one exemplary embodiment of the ablation device of the invention. As used in this disclosure with reference to structures which are advanced into the body of a subject, the “distal” end of such a structure should be taken as the end which is inserted first into the body and which penetrates to the greatest depth within the body, whereas the proximal end is the end of the structure opposite to the distal end. 
     The ablation device of  FIGS. 1-3  includes a first ablation element  11  which incorporates an inflatable balloon structure  13  and an ultrasonic transducer  20  disposed within the balloon structure  13 . As best seen in  FIG. 2 , the first ablation element  11  is mounted at the distal end  14  of an elongated probe  10 . The probe structure also has a proximal end  12 . A portion of a probe structure  10  between the proximal and distal ends is omitted in  FIG. 2  for clarity of illustration. The probe structure is includes a first catheter  16  defining a plurality of lumens including a lumen  18 . 
     Transducer  20  is generally in the form of a hollow, cylindrical tube of piezoelectric material having electrically conductive layers (not shown) on its interior and exterior surfaces. As best seen in  FIG. 2 , a generally tubular strain relief barrel  81  is mounted on the distal end of catheter  16 . Barrel  81  may be made from brass or any other suitable material. Barrel  81  has projections  82  and  84  at its distal and proximal ends. The surfaces of projections  82  and  84  form a surface for mounting transducer  20 . The conductive coating on the outer surface  86  of transducer  20  is electrically connected to the shield of a HIFU coaxial cable  88  which extends within a lumen of the catheter  16  and which is connected to a source  78  of electrical excitation signals through a connector  22  at or near the proximal end of the probe. The central conductor  90  of coaxial cable  88  is also connected to source  78  of electrical excitation signals through connector  22 . The central conductor  90  is electrically connected to barrel  81  and thus electrically connected to the coating on the inside surface of transducer  20 . 
     The first catheter  16  and transducer  20  define a central axis  24  adjacent the distal end of the probe structure. A first balloon  28 , also referred to herein as a “structural balloon” is mounted to the distal end of catheter  16 , and communicates with a first inflation port  29  near the proximal end of the probe. First balloon  28  includes an active wall  32  formed from a film which is flexible but which can form a substantially noncompliant balloon structure when inflated. The first balloon also includes a forward wall  30 , which may be generally conical or dome-shaped and may project forwardly from its juncture with active wall  32 . Active wall  32  joins the wall of catheter  16  proximally of transducer  20 . Thus, transducer  20  is disposed inside of first balloon  28 . 
     A second balloon  50 , also referred to herein as the “reflector balloon,” is carried on the distal end of catheter  16 , and communicates with a second inflation port  51  adjacent the proximal end of the catheter. The interior spaces within the first balloon  28  and second balloon  50  do not communicate with one another. The active wall  32  of the first balloon also serves as a wall of the second balloon. When both first and second balloons  28  and  50 , respectively, are in a deflated position, second balloon  50  is collapsed inwardly, toward central axis  24  so that second balloon  50  in a deflated condition closely overlies deflated first balloon. In the inflated, operative condition depicted in  FIG. 2 , the first balloon  28  is filled with a liquid, as for example, an aqueous liquid such as saline solution, whereas the second balloon  50  is filled with a gas such as carbon dioxide. Because of the difference in acoustic impedance between the liquid in the first balloon  28  and the gas in second balloon  50 , the boundary between the first and second balloons, at active wall  30 , is highly reflective to ultrasound. The catheter  16  and the mounting of the transducer  20  within the catheter may be constructed and arranged so that a liquid can be circulated into and out of the balloon while balloon  28  is maintained inflated by the liquid, and so that the circulating liquid passes over transducer  20  to cool it. 
     As discussed above, transducer  20  is connected to a source  78  of electrical excitation signals through connector  22 . Source  78  is adapted to provide electrical excitation. Thus, source  78  can provide continuous excitation for a predetermined period of time and then turn the electrical excitation off for a predetermined period of time. The electrical excitation may be turned on and off as required. The electrical excitation actuates transducer  20  to produce ultrasonic waves. The ultrasonic waves propagate substantially radially outwardly as indicated by arrows  80  in  FIG. 2 . Stated another way, cylindrical transducer  20  produces substantially cylindrical wave fronts which propagate generally radially outwardly. These waves are reflected by the interface at active region  32 . Because the interface has a parabolic shape, the waves striking any region of the interface will be reflected substantially to focus  44  defined by the surface of revolution, i.e., into a substantially annular or ring-like focal region at focus  44 . The ring-like focal region surrounds axis  24  and lies just forward or distal to the forward wall  30  of balloon  28 . 
     The probe  10  includes a bendable section  91  disposed proximal to the first ablation element  11  and thus proximal to the balloons  28  and  50  and ultrasonic transducer  10 . The bendable section  91  is controlled by a steering control mechanism  93  so that the bendable section can be selectively bent so as to change the orientation of the first ablation element  11  and the orientation of axis  24 . Merely by way of example, the catheter  16  may be provided with one or more pull wires attached to the steering control  93 . Other ways of selectively controlling the bending may be used, as for example, pneumatic or hydraulic elements linked to the steering control mechanism. 
     The features described above may be generally in accordance with the &#39;054 patent and &#39;757 application. 
     The forward wall  30  of the first balloon  28  is provided with a generally cylindrical extension  35  coaxial with axis  24 . Extension  35  desirably is of relatively small diameter, as for example, about 5-20 mm or less, so that the extension can fit within the pulmonary vein. A polymeric sleeve  31  is disposed within extension  35 , and extension  35  of the balloon  28  is fastened to the sleeve. A metallic, electrically conductive tubular stiffening element  33  is disposed within the first balloon  28 . The stiffening element is mechanically attached to the strain relief barrel  81  and projects distally from the ultrasonic transducer  20 . The stiffening element desirably is electrically insulated from the strain relief barrel  81  and ultrasonic transducer  20 . The distal end of the stiffening element extends through sleeve  31 . An additional ablation element in the form of an electrode  17  is mounted to the stiffening element and sleeve so that the electrode is disposed at the distal extremity of the extension  35  of the first balloon, and the electrode projects slightly beyond the balloon. Thus, the electrode or additional ablation element is disposed distal to the first ablation element  10 , and distal to the balloons and ultrasonic transducer. The electrode has a hole or port  95  which communicates with the bore  96  of the stiffening element. The bore  96  of the stiffening element in turn communicates with lumen  18  of catheter  16 , so that the lumen  18  and bore  96  cooperatively define a continuous passageway extending from adjacent the proximal end of probe  10  to the distal end of the balloon structure, and communicating with the exterior of the balloon structure on the distal side of the balloon structure. 
     The stiffening element  33  and electrode  17  are electrically connected to an RF excitation conductor  97  which extends within catheter  16  to adjacent the proximal end of  12  of the probe, where the conductor  97  is electrically connected to an RF excitation source  99 . For example, conductor  97  may be a conductor of a coaxial cable. 
     A sensing element  15  is mounted on the exterior of the device, at or distal to the distal end of the balloon structure  13 . For example, sensing element  15  may be a conductive electrode disposed on the exterior of sleeve  31  or on the exterior surface of the extension of the balloon where the extension  35  surrounds the sleeve. The sensing element is connected by one or more conductors (not shown) extending within catheter  16  to a sensing device (not shown) so that the sensing element can be used to detect electrical signals. 
     In a method according to one embodiment of the invention, the apparatus of  FIGS. 1 and 2  can be used to treat atrial fibrillation. With balloons  28  and  50  deflated, the distal end  14  of the probe is advanced into the left atrium of the patient&#39;s heart. To facilitate threading, a guide wire may be threaded into the heart and the guide wire may be threaded through the continuous passageway defined by the bore  96  of the stiffening element and the associated lumen  18  of the catheter. Also, the probe may be threaded through one or more sheaths which have previously been threaded into the heart through the vascular system. 
     With the first ablation element  11  disposed in the left atrium of the heart, the balloons  28  and  50  are inflated with a liquid and gas, respectively. The first ablation element is positioned generally as shown in  FIG. 2 , with the axis  24  of the first ablation element extending generally perpendicular to the wall  70  of the atrium and with the axis aligned with the ostium of a pulmonary vein  72 . As discussed in the &#39;757 publication, the steering arrangement  93  may be used to control the orientation of the axis  24 . As also discussed in the &#39;757 publication, the continuous passageway extending through the probe and opening to the distal side of the balloon assembly may be used to introduce a contrast medium through the port  95 , so that the contrast medium flows back through the pulmonary vein into the atrium  70 . The contrast medium can be used to confirm proper placement of the first ablation element  11 . 
     With the first ablation element in this position, the ring-like focal region  44  is disposed within the heart tissue, near the surface of the heart wall, and encircles the ostium of the pulmonary vein. In this position, the extension  35  of the balloon structure, and the additional ablation element  17  may be disposed within the pulmonary vein or ostium. While the first ablation element is in this position, the ultrasonic transducer  20  is actuated to emit ultrasonic waves. The ultrasonic waves are concentrated in focal region  44 . The heart wall tissue located in the focal region is heated rapidly. The rapid heating of the target tissue to the target temperature effectively ablates or kills the tissue at the focal region so that a wall of non-conductive scar tissue forms in the focal region and in neighboring tissue. The time required for ablation will vary with the power applied, but for emitted ultrasonic power on the order of 50 watts, on the order of a few seconds to a few minutes, sonication will form a substantial lesion. 
     If a complete transmural lesion is formed entirely around the ostium, the tissue within the ostium will be electrically isolated from the remainder of the heart wall. Sensing element  15  may be used to detect electrical signals within the pulmonary vein and ostium, as for example, by moving or steering the probe until the sensing element contacts the wall of the ostium or the wall of the pulmonary vein. 
     Additional ablation can be performed using the second ablation element  17 . For example, if the results of the sensing step indicate that the lesion formed by the first ablation element did not fully block conduction of abnormal electrical signals, additional ablation can be performed at one or more locations on the heart wall so as to complete formation of a ring-like lesion fully encircling an ostium. Alternatively or additionally, the second ablation element can be used to form one or more linear lesions. 
     As shown in  FIG. 3 , the probe is retracted proximally and the second ablation element  17  is positioned at a desired location on the wall of the atrium by using the steering mechanism  93  ( FIG. 1 ) to bend the catheter as needed. With the second ablation element in contact with the heart wall at a location where additional ablation is desired, the RF source  99  ( FIG. 1 ) is actuated to apply RF power to the second ablation element  17 . The second ablation element heats tissue in a small spot at and immediately surrounding the point of contact. To form a linear lesion, the second ablation element can be moved continuously or stepwise while repeating the RF actuation. 
     In this embodiment, the mode of operation of the second ablation element  17  is different from that of the first ablation element  11 ; the second ablation element  17  ablates the tissue by delivering RF energy to the tissue, whereas the first ablation element ablates using ultrasonic ablation. The ablation device of  FIGS. 1-3 , therefore, provides two means for ablating tissue. Moreover, the first ablation element  11  is arranged to form a ring-like lesion in each actuation, whereas the second ablation element  17  is arranged to form a localized, spot ablation in each actuation. Both ablation elements are carried into the heart on the same probe, and both can be positioned using the same steering mechanism. Also, as mentioned above, a liquid such as saline solution can be circulated within balloon  28  to cool the ultrasonic transducer. The same circulating liquid also serves to cool electrode  17  of the additional ablation element. 
     In a variant, the two ablation elements may have the same mode of operation. For example, the RF spot ablation element can be replaced by a spot ultrasonic transducer disposed at the distal end of the balloon structure, i.e., at the location occupied by electrode  17  in the embodiment discussed above. 
     In a further variant, the sensing element  15  may be omitted. A separate sensing probe may be inserted into through the lumen of the catheter and positioned in the pulmonary vein in the manner described in PCT publication WO 2005/102199, the disclosure of which is hereby incorporated by reference herein. 
     The stiffening element or tube  33  may be made of steel. However, it is desirable for the stiffening tube  33  to be a good electrical conductor. In one embodiment the stiffening tube is coated with a highly conductive material such as copper, silver, gold or combinations thereof. Such a coating may be in the form of a plated layer or a discrete foil layer covering the outside of the tube. In another embodiment seen in  FIG. 4 , a distal portion of the stiffening tube  33  is wrapped with a conductive wire  19  to enhance the electrical conduction by the stiffening tube  33 . In yet another variant, the stiffening tube  33  is slidable relative to the ultrasonic transducer. For example, the stiffening tube may be arranged to slide proximally relative to the ultrasonic transducer as the balloons are inflated, and may be spring-biased to move distally as the balloons are deflated so as to facilitate collapse of the balloons during deflation. Appropriate flexible or slidable electrical connections between the stiffening tube and the RF conductor in the catheter. In yet another variant, the stiffening tube may be electrically connected to the ultrasonic transducer, as for example, by electrically connecting the stiffening tube to the strain relief barrel  81 . In this case, the conductor which transmits electrical excitation signals to the ultrasonic transducer may also carry the RF power to the electrode  17 . In a still further variant, the stiffening element may be omitted and the additional ablation element  17  may be supported at the distal end of the balloon assembly constituting the first ablation element. In yet another variant, the port  95  of the distal ablation element may be omitted. 
       FIG. 6  shows another exemplary embodiment of the ablation device  200 . This embodiment includes an insertable structure incorporating an elongated catheter  120  having a proximal end which remains outside of the body, and a distal end  160  adapted for insertion into the body of the subject. The insertable structure also includes a first ablation element  180  mounted to the catheter adjacent distal end  160 . Ablation element  180  incorporates a reflector balloon and a structural balloon having a common wall. A cylindrical ultrasonic emitter  230  is mounted within the structural balloon. A lumen  300  is formed within catheter  120 . Lumen  300  extends to from the distal end to the proximal end of the catheter  120 . As also shown in  FIG. 6 , positioning of the ablation device  200  within the heart desirably includes selectively controlling the disposition of the forward-to-rearward axis  240  of the device relative to the patient&#39;s heart. That is, the position of the forward-to-rearward axis desirably can be controlled by the physician to at least some degree. For example, the device may be arranged so that the physician can selectively reorient the forward-to-rearward axis  240  of the ablation device through a range of motion, as for example, through the range between disposition indicated in solid lines by axis  240  and the disposition indicated in broken lines by axis  2401 . To that end, the assembly can be provided with one or more devices for selectively varying the curvature of a bendable region  600  of the catheter just proximal to the ablation device. 
     In this embodiment, the second or additional ablation element  170  is carried on an additional probe element  190  in the form of an elongated stylet bearing the additional ablation element  170  at or near its distal end. Probe element or stylet  190  may be threaded through the lumen  300  so as to form the assembly shown in  FIG. 6 . In this assembly, the additional ablation element  170  is also arranged to form a local or spot lesion, whereas the first ablation element  180  is arranged to form a loop. Here again, when the additional probe element  190  and additional ablation element  170  are in place, the additional ablation element  190  and the catheter  120  form a composite probe bearing both the first ablation element  180  and the additional ablation element  170 . In this embodiment as well, the additional ablation element  170  may be steered using the same steering mechanism that is used to steer the first ablation element  180 . A sensing element  172  may be secured to the second additional probe element  190  proximal to the ablation element  170 . The sensing element also will be moved by steering the catheter  120 . In this embodiment as well, the ablation element  170  may be a RF transducer or other spot-forming element. 
     The ablation device of  FIG. 6  can be used in a manner similar to the device discussed with reference to  FIGS. 1-4 . The additional probe element  190  bearing the additional ablation element  170  and sensing element  172  can be assembled with the catheter  120  before or after operating the first ablation element  180 . In a further variant, a separate sensing probe can be inserted into the lumen  300  of catheter  120  and then removed and replaced by the additional probe element  190 . 
     As these and other variations and combinations of the features discussed above can be employed, the foregoing description of the preferred embodiments should be taken by way of illustration rather than by way of limitation of the invention.