Abstract:
An applicator for creating a lesion in tissue, comprising one or more rigid or semi-rigid support members, a compliant material coupled to said support members, at least one passage in communication with the compliant material for infusing a medium to the compliant material and at least one electrode for conducting energy to the tissue. Further, the compliant material or other mechanical linkage may function as means for varying the distance between an ultrasonic transducer element or other ablative energy source and a surface of the tissue.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This Application is a continuation-in-part of U.S. patent application Ser. No. 10/609,692, filed 30 Jun. 2003, now pending, the complete disclosure of which is hereby incorporated by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of Invention  
         [0003]     The invention relates generally to the field of surgical instrumentation, and more particularly, to an applicator for creating linear lesions in living tissue.  
         [0004]     2. Description of Related Art  
         [0005]     Atrial fibrillation is the most common form of cardiac arrhythmia (irregular heartbeat). Irregular heartbeats are caused by abnormal electrical activity of the heart. In atrial fibrillation, the atria, the upper chambers of the heart, beat irregularly and rapidly. The erratic electrical signals may also cause ventricles, the lower chambers of the heart, to beat irregularly and rapidly. This can affect blood flow to the heart muscle and to the rest of the body.  
         [0006]     Treatment for atrial fibrillation includes medication, or cardioversion, electrical stimulation of the heart, to restore normal sinus rhythm. Patients who do not respond to these treatments may be indicated for surgery, including catheter ablation, or more recently developed MAZE techniques.  
         [0007]     In a traditional MAZE procedure, incisions are made in a predetermined pattern in the inter wall of the atria, which are then sutured together. Scar tissue that forms at the incisions inhibits the conduction of electrical impulses in the heart tissue that causes the fibrillation. The electrical impulses are directed along, rather than across, the incisions in a maze-like fashion that leads them to the lower ventricles of the heart.  
         [0008]     Although generally effective, the procedure implicates the risks associated with major heart surgery. The procedure generally takes several hours, during which time the patient must receive cardiopulmonary life support. Even if successful, the procedure is highly invasive and traumatic, with full recovery taking up to six months. Additionally, the procedure requires exacting skill on the part of the surgeon.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     Therefore, an apparatus able to produce lesions of scar tissue in the wall of the heart muscle quickly, reliably, and while minimizing damage to tissue surrounding the lesions would be highly desirable.  
         [0010]     Provided by the present invention is an applicator for creating a lesion in tissue, comprising one or more rigid or semi-rigid support members, a compliant material coupled to said support members, at least one passage in communication with the compliant material for infusing a medium to the compliant material and at least one electrode for conducting energy to the tissue. Further, the compliant material or other mechanical linkage may function as means for varying the distance between an ultrasonic transducer element or other ablative energy source and a surface of the tissue.  
         [0011]     At least one mechanism disclosed herein has the advantage of atraumatically clamping the tissue layer. Consistent loading can help control audible popping due to steam generation during tissue heating. The mechanisms also adapt to a variety of tissue thickness and/or local variations in thickness. Further, the mechanisms offer means to control electrode temperature elevation, avoiding tissue avulsion due to vaporization ablation. Rapid heating of the electrodes can also result in a rapid impedance change and subsequent thrombosis formation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     These and other features, benefits and advantages of the present invention will become apparent with reference to the following specification and accompanying drawing, in which like reference numerals indicate like features across the several views.  
         [0013]      FIG. 1  illustrates an ultrasonic applicator according to a first embodiment of the present invention;  
         [0014]      FIG. 2  illustrates a cross-section of the transducer head of the ultrasonic applicator, taken along the section line  2 - 2  of  FIG. 1 ;  
         [0015]      FIG. 3  is a schematic illustration of a system for creating linear lesions according to the present invention;  
         [0016]      FIGS. 4A through 4G  illustrate various embodiments of transducer heads operative to mechanically alter the depth of focus of the ultrasonic energy;  
         [0017]      FIG. 5  illustrates a clamping mechanism according to the present invention;  
         [0018]      FIG. 6  illustrates a further embodiment of the clamping mechanism of  FIG. 5 ;  
         [0019]      FIGS. 7A and 7B  illustrates another embodiment of a clamping mechanism, in flaccid and turgid states; and  
         [0020]      FIG. 8  illustrates a single-jaw embodiment having a compliant material. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     Referring now to  FIG. 1 , illustrates an ultrasonic applicator, generally  10 , according to the present invention. Ultrasonic applicator  10  has a transducer head  12 , a shaft  14 , and a handle  16 , by which the applicator  10  may be manipulated. Not shown in  FIG. 1  are passages and cables by which power and cooling medium, respectively, are supplied to the applicator  10 . These passages and cables may be either internal or external to the shaft  14 .  
         [0022]     Turning now to  FIG. 2 , a cross section of transducer head  12  is shown, taken along line  2 - 2  of  FIG. 1 . Transducer head  12  is formed with a cavity  14  therein which is open to an acoustic window  16 . The cavity  14  is sealed across the acoustic window  16  by a membrane  18 . Membrane  18  is selected to have a low acoustic impedance and low coefficient of acoustic absorption, for acoustic transparency, such as films of Ultem, PET, or Styron. In one embodiment, 0.001″ thickness PEEK was used.  
         [0023]     Located within cavity  14  is the ultrasonic vibratory element, in this embodiment a piezoelectric crystal  20 . The precise piezoelectric material may be selected from among those known in the art to suit the particular application by minimizing dielectric and motional losses and inefficiencies. Further, the selected crystal may be aged, which is a logarithmic depolarization over time. A suitable aging period can reduce noticeable changes in activity.  
         [0024]     In mounting the crystal  20 , it is desirable to use a compliant mount with minimal damping. For example, epoxy on the back plate and crystal may reduce overall efficiency. In one embodiment, an RTV silicone sealant is used to mount the crystal. It is further desirable to minimize the contact area with the crystal in mounting to reduce crystal loading and heating in the mount. Particularly, elastomers absorb energy thereby reducing overall efficiency.  
         [0025]     Piezoelectric crystal  20  has a curvature illustrated by radius  22  and converges at a focus  24  located in the direction of the acoustic window  16 . The focal length may be varied, and was set to 0.25″ in one exemplary embodiment. Alternately, the transducer head  14  may be provided with a plurality of vibratory elements, either curved or flat, which form some angle with respect to one another. In either case, the ultrasonic energy will converge at some focus.  
         [0026]     Provided in a direction opposite the focal point  24  and acoustic window  16  and adjacent the crystal  20  is an air gap  26 . The air gap  26  acts as an acoustic mirror to reflect all acoustic energy from the adjacent side of the crystal  20  downward towards the acoustic window  16 .  
         [0027]     Also provided in the transducer head  12  are cooling passages  28  and  30 . These cooling passages  28 ,  30  allow for the supply and removal of cooling medium to and from the transducer head  12 . The cooling medium can include, but is not limited to, degassed water or saline. The cooling medium also provides a coupling path for the ultrasonic energy. The flow of cooling medium is determined primarily by the energy losses in crystal  20 . In addition to protecting the physical integrity of the crystal, proper cooling can also minimize frequency drift in the crystal, which could otherwise cause inefficiencies.  
         [0028]     In order to further enhance efficiency, piezoelectric crystal  20  may be provided with an impedance matching coating  32  on the side of the crystal  20  that faces the acoustic window  16 . The coating  32  is shown in exaggerated thickness for illustration, and is typically on the order of one-quarter (¼) of the wavelength of the ultrasonic energy provided by the crystal  20 . The selection of material and its impedance will be well known to those skilled in the art, and need not be explained further. The presence of the coating  32  impacts the cooling needs of the transducer  12 , and adjustment of the coolant flow, in light of the driving power of the crystal  20 , may be necessary.  
         [0029]     Provided on either side of the acoustic window  16  are regions of porous material,  34   a ,  34   b . This porous material  34   a ,  34   b  may be saturated with an ink, so that as the ultrasonic applicator  10  is used to form lesions in the tissue, the area where lesions have been formed will be marked by the ink. Also provided on either side of the acoustic window  16  are electrodes  36   a ,  36   b . The electrodes  36   a ,  36   b , may be used for pacing, i.e., electrically testing of the effectiveness of the lesions formed in inhibiting the propagation of electrical impulses through the tissue.  
         [0030]     Alternately or additionally, electrodes  36   a ,  36   b  may be used to provide RF energy to the tissue to enhance the lesions formed by the ultrasonic energy of crystal  20 . In combination with ultrasound, the RF energy can be used to form a more complete barrier or transmurality in a wider range of tissue thicknesses. This procedure is explained in more detail in U.S. patent application Ser. No. 10/609,694 (attorney Docket No. 16339) entitled Multi-Modality Ablation Device, filed 30 Jun. 2003, the complete disclosure of which is hereby incorporated by reference for all purposes. Electrodes  36   a ,  36   b  may also be adapted to transmit and/or receive ultrasound microwave, cryoablation, radio-frequency (RF), photodynamic, laser, or cautery energy, as will be discussed further, infra.  
         [0031]     The combination of ultrasound and RF energy comprises one means for controlling the depth of the lesion in the tissue. Other means can be mechanical, for example by adjusting the focal length of the applicator. In one embodiment, the ultrasonic applicator has two crystals arranged within the transducer. By altering either or both of the angle and the distance between the two crystals, the depth of focus is adjusted. This aspect is explained further in U.S. patent application Ser. No. 10/609,693 (attorney Docket No. 16335) entitled Ultrasonic Radial Focused Transducer for Pulmonary Vein Ablation, filed 30 Jun. 2003, the complete disclosure of which is hereby incorporated by reference for all purposes.  
         [0032]     Alternately or additionally, the standoff distance between the crystal and the tissue, or between the crystal and the acoustic window, may be adjusted by mechanical means, some of which are illustrated in  FIGS. 4A through 4G .  
         [0033]     Referring to  FIG. 4A , an alternate embodiment of a transducer head, generally  212 , is shown. An articulating cable  214  is operatively connected to and longitudinally articulates one end of linkage  218  within slot  220 . Thereby, an upper portion  212   a  of the transducer head  212  moves vertically with respect to the acoustic window  216 , guided by slots  222   a ,  222   b . Optionally, the space between upper portion  212   a  and acoustic window  216  may be enclosed by a flexible skirt  224 . A similar cable/linkage/slot arrangement may be provided on an opposite side of transducer head  212 .  
         [0034]     Referring to  FIG. 4B , an alternate embodiment of a transducer head, generally  312 , is shown. In this embodiment, an articulating cable  314  is operatively connected to pinions  318   a ,  318   b . The articulating cable  312  rotates the pinions  318   a ,  318   b , which are engaged with racks  320   a ,  320   b , respectively. Upper portion  312   a  of transducer head  312  is thereby moved vertically with respect to acoustic window  316 .  
         [0035]     Referring to  FIG. 4C , an alternate embodiment of a transducer head, generally  412 , is shown. In this embodiment, an articulating cable  414  is operatively connected to band  422 . The articulating cable  412  articulates band  422 , which is engaged with and synchronously rotates nuts  418   a ,  418   b . Nuts  418   a ,  418   b  are engaged with screws  420   a ,  420   b , respectively. As nuts  418   a ,  418   b  rotate, upper portion  412   a  of transducer head  412  is moved vertically with respect to acoustic window  416 .  
         [0036]     Referring to  FIG. 4D , an alternate embodiment of a transducer head, generally  512 , is shown. In this embodiment, an articulating cable  514  is operatively connected to worm gears  522   a ,  5228   b . The articulating cable  512  rotates the worm gears  522   a ,  5228   b , which are engaged with nuts  518   a ,  518   b . Nuts  518   a ,  518   b  are engaged with screws  520   a ,  520   b , respectively. As nuts  518   a ,  518   b  rotate, upper portion  512   a  of transducer head  512  is moved vertically with respect to acoustic window  516 .  
         [0037]     Referring to  FIG. 4E , an alternate embodiment of a transducer head, generally  612 , is shown. An articulating cable  614  is operatively connected to and longitudinally articulates a wedge  618 . Acoustic window  616  may be angled, or may be provided with a sloped flange  620 . A track, a sidewall, a flange, a spring or other similar device may be provided to constrain the movement of wedge  618 . As wedge  618  moved against flange  618 , an upper portion  612   a  of transducer head  612  moved vertically with respect to acoustic window  616 . Upper portion  612   a  is guided by posts  622   a ,  622   b , and corresponding bores  624   a ,  624   b , respectively. A similar cable/wedge/slope arrangement may be provided on an opposite side of transducer head  612 .  
         [0038]     Referring to  FIG. 4F , an alternate embodiment of a transducer head, generally  712 , is shown. A sheathed cable  714  is operatively connected to transducer head  712 . The sheath  714   a  is connected to an upper portion  712   a  of the transducer head  712  at bracket  718 . The core  714   b  is connected to the acoustic window  716  at flange  720 . As the core  714   b  moves within the sheath  714   a , the upper portion  712   a  moves vertically with respect to the acoustic window  716 . This motion is guided by posts  722   a ,  722   b , and corresponding bores  724   a ,  724   b , respectively. A similar cable/flange arrangement may be provided on an opposite side of transducer head  712 .  
         [0039]     Referring to  FIG. 4G , an alternate embodiment of a transducer head, generally  812 , is shown. An articulating cable  814  is operatively connected to transducer head  812 , and to a common pin  816  joining linkages  818  and  820 . As pin  816  moves longitudinally, the upper portion  812   a  moves vertically with respect to the acoustic window  816 . This motion is guided by posts  822   a ,  822   b , and corresponding bores  824   a ,  824   b , respectively. A similar cable/linkage arrangement may be provided on an opposite side of transducer head  812 .  
         [0040]     Further, selection of the frequency of the ultrasonic wave can be used to control the depth and transmurality of the lesion. Lower frequencies are less absorbed by the tissue and provide deeper penetration. The higher frequencies have higher absorption in the tissue and this provides higher rate of heating but lower penetration. Therefore, by selecting or optimizing the frequency of the crystal  20 , the depth of penetration of the ultrasonic energy and the heating rate can be adjusted so that a range of tissue thickness can be ablated, thereby controlling the depth of the lesion. A predetermined target may be established based upon the thickness of the tissue, or a thicker lesion may be formed by adjusting the frequency in process. Control of the ultrasonic frequency comprises yet another means for controlling the depth of the lesion.  
         [0041]     Alternately or additionally, either or both of electrodes  36   a ,  36   b , can be made responsive to ultrasound. These can then be used to receive a lower power inspection ultrasound signal, emitted after the lesion is formed to inspect the physical properties of the lesion.  
         [0042]     Referring now to  FIG. 5 , shown is a clamping mechanism which can be adapted to the present invention. The clamping mechanism, generally  900 , includes a rigid or semi-rigid member  902  coupled to a compliant material  904 . The compliant material can include, but is not limited to, polyethylene terephthalate (PET), flexible polyvinyl chloride (PVC), nylon, polyolefin, polyurethane, latex, silicone, or other elastomers knows to be used in the manufacture of expansible members, used for example for fixation and/or occlusion. The compliant material  904  can transition between a flaccid and turgid states by the infusion of a medium, for example, fluid, gas, gel, rheological material, or other media which affects turgidity. The medium may also consist of a combination of materials, such as a slurry of solid particles suspended in a solvent gas or liquid. In that case, removing the solvent would transition the compliant material  904  from a turgid state to a rigid state.  
         [0043]     Referring now to  FIG. 6 , the clamping mechanism  900  is shown with the rigid member partially formed into the shape of a “P”. A latching mechanism  906  can be provided to hold the distal end  908  in place proximally. Placing the clamping mechanism  900  around tissue to be clamped, by distending the compliant material  904  the tissue is compressed atraumatically. This particular arrangement is particularly well suited for application to hollow tissue structures, for example atrial chambers or pulmonary veins.  
         [0044]     Referring now to  FIG. 7A , shown is another embodiment of a clamping mechanism, generally  1000 . In this embodiment, clamping mechanism  1000  has rigid or semi-rigid jaws  1002   a  and  1002   b , respectively. The jaws as shown are fixed relative to one another, but may articulate in a further embodiment. In this embodiment both jaws are provided with a compliant material  1004   a ,  1004   b , but either one may have a compliant material without the other.  
         [0045]     Also shown are electrodes  1010   a  and  1010   b , which may be provided on the surface of the compliant material  1004   a  and  1004   b , respectively. Electrodes  1010   a  and  1010   b  may consist of a conductive material or an array of conductive surfaces in any geometry. Alternately or additionally, they may comprise conductive elements integrated into the surface of the compliant material. For example, a fiber of carbon or another material conductive of electricity, RF or whichever type of energy the electrode is to be responsive to, may be woven into a bounding surface of compliant material  1004   a ,  1004   b . The electrodes  1010   a  and  1010   b  are operatively connected to an energy source, for example ultrasound, microwave, cryoablation, radio-frequency (RF), photodynamic, laser, or cautery. The four (4) electrodes shown are merely exemplary, and their number may be more or less.  
         [0046]     Alternately or additionally, an ultrasonic vibratory element may be provided in one or both of jaws  1002   a ,  1002   b . Additionally, a reflector may be provided with either or both of jaws  1002   a ,  1002   b  to reflect and/or focus incident energy.  
         [0047]     Referring now to  FIG. 7B , clamping mechanism  1000  is illustrated having compliant material  1004   a ,  1004   b  in a turgid state. The turgid compliant material  1004   a ,  1004   b  is shown compressing and thereby securing a tissue layer  1014 . Once secured, energy can be applied to the electrodes and/or transducer to ablate the tissue and form the desired lesions therein.  
         [0048]     Referring now to  FIG. 8 , shown is a single jaw embodiment  1100 . The single jaw has a rigid or semi-rigid member  1102 , and a compliant material  1104 , in this case shown in a distended or turgid state. A plurality of electrodes  1110  are shown on the surface of the compliant material  1104 . Also shown are passages  1128  and  1130 , which provide for the inflow and outflow of the a medium for altering the turgidity of the compliant material  1104 . In this or other embodiments described above, the surface of the compliant material may be textured to reduce tissue slipping. In this or other embodiments described above, the turgidity inducing medium may be circulated to serve a heat sink for the ablation process.  
         [0049]     Provided in this embodiment is an ultrasonic vibratory element  1120 . In operation, the infusion of a turgidity inducing medium can alter the distance between the ultrasonic vibratory element  1120  and the tissue surface, thereby varying the depth of focus and penetration of the ultrasonic energy. Therefore, embodiments including conforming material as described above will be seen as yet another means of controlling the depth of lesion formed in the tissue.  
         [0050]     Referring now to  FIG. 3 , the system, generally  100 , for creating linear lesions according to the present invention is shown. The ultrasonic applicator  10  is connected to control unit  102  via a conduit  104 . Conduit  104  provides the pathways necessary for electrical, RF, and/or fluid communication with the transducer head  12 .  
         [0051]     Control unit  102  comprises a ultrasonic generator  106 , which supplies power of the appropriate frequency to the crystal  20  for the production of acoustic energy. It would be desirable to provide compensation for the static capacitance of the crystal  20  in order to reduce the capacitive load on the ultrasonic generator  106 . It would also be desirable to match the impedance of the crystal  20  to the ultrasonic generator  106  to minimize reflections from the load. Also, where wire and solder joints are used to connect the crystal  20  to the ultrasonic generator  106 , it would be desirable to use a light wire and small solder joints at the crystal interface. Additional mass of these items can alter the frequency of the crystal. Further, proper solder technique can have an impact, because excess heat caused by poor solder joints can depolarize a ceramic crystal.  
         [0052]     Control unit  102  also provides a coolant control section  108 . Coolant control section  108  can include a pump for the circulation of cooling medium, sensors for monitoring the temperature of the coolant fluid, and in closed cooling systems, a heat exchanger for expelling heat from the coolant fluid before it is recycled back into the transducer.  
         [0053]     Control unit  102  also comprises a lesion monitoring section  110 . In combination with electrodes  36   a ,  36   b , once formed, the lesions created can be tested for effectiveness by electrical pacing, discussed supra, or by monitoring the tissue impedance. Additionally or alternately, other methods of monitoring the effectiveness the lesions, including but not limited to, ultrasound imaging, can be employed to verify the suitability of the lesions formed. Additionally, the control unit may comprise a secondary generator  112 , for applying ultrasound, microwave, cryoablation, radio-frequency (RF), photodynamic, laser, or cautery energy to the tissue at the transducer  12 , as discussed, supra.  
         [0054]     The operation of the system  100 , according to the present invention will now be described. Typically, the surgeon will establish access to the epicardium through sternotomy, thoracotomy, or less invasively, by thorascopic port access. The transducer  12  is placed on the surface of the heart where the lesion is to be formed. A trigger switch, which may be located on the shaft  14  of the applicator  10 , alternately embodied as a foot pedal for the surgeon, or on the control unit  102 , activates the ultrasonic generator  106  to introduce ultrasonic energy to the tissue.  
         [0055]     The ultrasonic generator  106  applies electrical energy to the crystal  20  to induce ultrasonic vibration. In one embodiment, the crystal was tuned to 8.72 Mhz and employed a power setting of 60W. In this exemplary embodiment, acoustic intensity along the focal line including focal point  24  is in a range between 1,000 and 1,500 w/cm 2 , sufficient to coagulate tissue within a short period of time. In vitro testing indicates the transmural lesion in tissue of typical thickness can be made in about 15 to 30 seconds.  
         [0056]     The present invention has been described herein with reference to certain exemplary embodiments. Certain modifications and alterations may be apparent to those skilled in the art without departing from the scope of the present invention. The exemplary embodiments are meant to be illustrative, and not limiting, on the scope of the invention, which is defined by the appended claims.