Patent Publication Number: US-2011066146-A1

Title: Suction Stabilized Epicardial Ablation Devices

Description:
FIELD OF THE INVENTION 
     This invention relates to ablation devices that are used to create lesions in tissue. More particularly, this invention relates to ablation devices that use vacuum or suction force to hold the tissue in a manner that creates linear lesions. 
     BACKGROUND OF THE INVENTION 
     The action of the heart is known to depend on electrical signals within the heart tissue. Occasionally, these electrical signals do not function properly. Ablation of cardiac conduction pathways in the region of tissue where the signals are malfunctioning has been found to eliminate such faulty signals. Ablation is also used therapeutically with other organ tissue, such as the liver, prostate and uterus. Ablation may also be used in treatment of disorders such as tumors, cancers or undesirable growth. 
     Currently, electrophysiology (EP) ablation devices generally have one or more electrodes at their tips. These devices may be used for both diagnosis and therapy. In one instance, electrodes at the tips of EP ablation devices allow the physician to measure electrical signals along the surface of the heart. This is called mapping. When necessary, in another instance, the physician can also ablate certain tissues using, typically, radio frequency (RF) energy conducted to one or more ablation electrodes. 
     Sometimes ablation is necessary only at discrete positions along the tissue is the case, for example, when ablating accessory pathways, such as in Wolff-Parkinson-White syndrome or AV nodal reentrant tachycardias. At other times, however, ablation is desired along a line, called linear ablation. This is the case for atrial fibrillation, where the aim is to reduce the total mass of electrically connected atrial tissue below a threshold believed to be critical for sustaining multiple reentry wavelets. Linear lesions are created between electrically non-conductive anatomic landmarks to reduce the contiguous atrial mass. 
     Linear ablation is currently accomplished in one of several ways. One way is to position the tip portion of the ablation device so that an ablation electrode is located at one end of the target site. Then energy is applied to the electrode to ablate the tissue adjacent to the electrode. The tip portion of the electrode is then slid along the tissue to a new position and then the ablation process is repeated. This is sometimes referred to as the burn-drag-burn technique. This technique is time-consuming (which is not good for the patient) and requires multiple accurate placements of the electrode (which may be difficult for the physician). Furthermore, even if the ablation process creates a continuously linear line along the top surface of the target tissue, it is not assured that the tissue is continuously and completely ablated through further layers of the target tissue (i.e. it is not assured that transmurality is achieved.) 
     A second way of accomplishing linear ablation is to use an ablation device having a series of spaced-apart band or coil electrodes which, after the electrode portion of the ablation device has been properly positioned, are energized simultaneously or one at a time to create the desired lesion. If the electrodes are close enough together the lesions run together sufficiently to create a continuous linear lesion. While this technique eliminates some of the problems associated with the burn-drag-burn technique, some repositioning of the ablation device may be required to create an adequately long lesion. In addition, it may be difficult to obtain adequate tissue contact pressure for each electrode in a multi-electrode ablation device. Also, the use of multiple electrodes to create the linear lesion tends to make the tip portion more expensive to make, more bulky and may cause the tip portion to be stiffer than with a single electrodes. 
     Another ablation-related problem results from the delivery of RF energy to muscular tissue, such as the heart. Ablation of such tissue using conventional ablation devices has a tendency to char or burn the blood or tissue with which the electrodes are in contact if the temperatures exceed a certain threshold (for example, greater than 50° C.). This increases the difficulty of the ablation process because it is necessary to clean the tip portion after a series of burns. Moreover, overheating the blood in the vicinity of the target site can desiccate the blood and can cause overburning. 
     It would be desirable to have an ablation device which is easy to position in relation to the target tissue and which stays stable in position in relation to the target tissue. 
     It would further be desirable to have an ablation device which, when positioned, is capable of easily creating a linear, transmural lesion. 
     It would further be desirable to have an ablation device that is able to monitor tissue temperature in order to avoid burning the tissue. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention provides a suction assisted ablation device. The device includes a support surface, having a first and a second face, a plurality of suction elements disposed adjacent the support surface on the first face, at least one conductive element disposed adjacent the support surface on the first face; and at least one suction conduit operatively connected with the suction elements. The device may also include a maneuvering apparatus, such as a pull wire assembly. The device may also include at least one thermocouple element. The device may include one conductive element on a first support surface and a separate conductive element on a second support surface. The conductive element may be a plurality of needle electrodes. The device may also include at least one fluid opening, which may be located within the conductive element. The conductive element may also be made of a material capable of releasing fluid. 
     Another aspect of the invention provides a method of ablating tissue. A suction assisted ablation device comprising a support surface, having a first and a second face, a plurality of suction elements disposed adjacent the support surface on the first face, and at least one conductive element disposed adjacent the support surface on the first face is provided. The first face of the device is placed adjacent an area of tissue. Suction is conducted to the suction elements via the suction conduit. The tissue is grasped with the suction and ablated. At least one fluid outlet may be provided adjacent the support surface and fluid may be released via the fluid outlet. The fluid outlet may be located within the conductive element. The device may be placed using a maneuvering apparatus. At least one thermocouple element may be placed in communication with at least one suction element and a thermal environment of the suction element may be measured using the thermocouple element. The tissue may be ablated until the measurement of the thermal environment reaches a given level. A second support surface having a second conductive element disposed adjacent a first face of the second support surface may also be provided. The first face of the second support surface may be placed in line with the first support surface to complete a circuit. The tissue is ablated. 
     Another aspect of the invention provides a tissue ablation system. The system comprises at least two support surfaces, each support surface having a first and a second face, a plurality of suction elements disposed adjacent the support surface on the first face, at least one conductive element disposed adjacent the support surface on the first face, at least one suction conduit operatively connected with the suction elements, and at least one maneuvering apparatus, such as a pull wire assembly. The support surfaces may be disposed consecutively to each other in a linear manner along the maneuvering apparatus so that a continuous ablation lesion is achieved. The system may also include a fluid delivery system, which may incorporate at least one fluid opening disposed adjacent the support surface, a fluid conduit, a conductive element including fluid openings or a conductive element made of a material that releases fluid. 
     Another aspect of the invention provides a method of mapping the heart. A suction assisted ablation device comprising a support surface, having a first and a second face, a plurality of suction elements disposed adjacent the support surface on the first face, at least one electrode disposed adjacent the support surface on the first face and at least one suction conduit operatively connected with the suction elements is provided. The first face of the device is placed adjacent an area of tissue. Suction is conducted to the suction elements via the suction conduit. The tissue is grasped with the suction. A signal is sent through a first electrode. The signal is received through a second electrode. The distance is mapped based on the signal from the first electrode to the second electrode. 
     Another aspect of the invention provides a method of pacing a heart. A suction assisted ablation device comprising a support surface, having a first and a second face, a plurality of suction elements disposed adjacent the support surface on the first face, at least one electrode disposed adjacent the support surface on the first face and at least one suction conduit operatively connected with the suction elements. The first face of the device is placed adjacent an area of tissue. Suction is conducted to the suction elements via the suction conduit. The tissue is grasped with the suction. Electrical impulses are sent through the electrode at regular interval and the heart is paced to beat with the impulses. 
     Another aspect of the invention provides a method of ablating tissue. A suction assisted ablation device comprising a support surface, having a first and a second face, a plurality of suction elements disposed adjacent the support surface on the first face, at least one needle electrode disposed adjacent the support surface on the first face and at least one suction conduit operatively connected with the suction elements. The first face of the device is placed adjacent an area of tissue. The tissue is penetrated with the needle electrode: Suction is conducted to the suction elements via the suction conduit. The tissue is grasped with the suction; and ablated. 
     The foregoing, and other, features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims in equivalence thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of the suction assisted ablation device in accordance with the present invention shown within a system for ablating tissue; 
         FIG. 2  is a bottom view of one embodiment of the suction assisted ablation device of the present invention, showing a first configuration of the suction elements and of the ablation electrodes; 
         FIG. 3  is a cross-sectional view of one embodiment of the suction assisted ablation device of the present invention, showing suction activity and ablation pattern at one suction site; 
         FIG. 4  is a bottom view of a second embodiment of the suction assisted ablation device of the present invention, showing a second configuration of the suction elements and of the ablation electrodes; and 
         FIG. 5  is a bottom view of another embodiment of the suction assisted ablation device of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
       FIG. 1  shows one embodiment of system  10  for ablating tissue, such as organic tissue, in accordance with the present invention. Typically the tissue to be ablated will be located within the body cavity, such as the endocardial or epicardial tissue of the heart. Other body organ tissue, such as the liver, lungs or kidney, may also be ablated using the present invention. Tissue types that may be ablated include skin, muscle or even cancerous tissue or abnormal tissue growth. 
     System  10  may include an ablation device  12  that comprises at least one conductive element  22 , such as an electrode, and a connection  28  to a power source  30 . Ablation device  12  may also include one or more suction elements  44  and a suction conduit  34  that provides suction from a suction source  20 . System  10  also may include a conduit  26  to an irrigation source  40  that provides irrigation fluid to the ablation site. System  10  may also include temperature-sensitive elements  36 , which may have the same power source  30  as the electrodes or may have their own power source. 
     System  10  may also include an indifferent (non-ablating) electrode  23  which may serve as a return plate for energy transmitted through electrode  22 . Electrode  23  may be placed elsewhere on the patient&#39;s body other than the ablation site. For example, electrode  23  may be placed on the patient&#39;s back, thigh or shoulder. 
     Ablation device  12  may be any suitable ablation tool such as, for example, a catheter, an electrocautery device, an electrosurgical device, a suction-assisted ablation tool, an ablation pod, an ablation paddle, an ablation hemostat or an ablation wire. Ablation device  12  and its components are preferably made of a biocompatible material such as stainless steel, biocompatible epoxy or biocompatible plastic. Preferably, a biocompatible material prompts little allergenic response from the patient&#39;s body and is resistant to corrosion from being placed within the patient&#39;s body. Furthermore, the biocompatible material preferably does not cause any additional stress to the patient&#39;s body, for example, it does not scrape detrimentally against any elements within the surgical cavity. Alternatively, the biocompatibility of a material may be enhanced by coating the material with a biocompatible coating. 
     Preferably, ablation device  12  may be permanently or removably attached to a maneuvering apparatus for manipulating device  12  onto a tissue surface. For example, ablation device  12  may be attached to a handle  72  such as shown in  FIG. 1 . Ablation device  12  may also be located on one or more of the jaws of a hemostat-like device. Ablation device  12  may also be used in conjunction with a traditional catheter, for example, in a closed heart ablation procedure. Ablation device  12  may also be maneuvered with a leash or pull-wire assembly. Ablation device may also be positioned on a pen-like maneuvering apparatus such as the Sprinkler pen available from Medtronic, Inc. Alternatively any appropriate flexible, malleable or rigid handle could be used as a maneuvering apparatus. Alternatively, any appropriate endoscopic or thoroscopic-maneuvering apparatus may also be used with device  12 . 
     Device  12  also preferably includes a connection  28  suitable for conducting energy to device  12 , particularly to conductive element  22  from a power source. 
     The conductive element  22  of ablation device  12  may preferably be an electrode. This electrode  22  may be positioned in any suitable place on device  12 . Preferably electrode  22  is placed near an end of the device  12 , away from the user, to be more easily manipulated against the tissue  60  to be ablated. 
       FIG. 2  shows one embodiment of a device  12  for ablating organic tissue in accordance with system  10  of the present invention. Suction assisted ablation device  12  may comprise at least one face  15  that may conform to the surface of the target tissue  60 . The face  15  may be any configuration that conforms to the surface of the target tissue, such as the slightly curved or arcuate configuration of  FIG. 1 . Suction device  12  may also include a suction conduit  34  that may be connected to least one suction port  44  containing a suction opening  54 . Suction device may also have at least one conductive element  22  disposed adjacent face  15 . For example, two conductive elements  22 ,  42  are shown in  FIG. 2 . Preferably, the conductive element  22 ,  42  may be an electrode. Alternatively, suction device  12  may be made of a conductive polymer and may serve as a conductive element. The distal end of device  12  may be positioned near the ablation site and the proximal end may be positioned towards the surgeon. 
     Preferably, when face  15  of suction device  12  is positioned against the target tissue, face  15  is adapted to conform to the surface of the tissue. This may be accomplished by making suction device  12  from a flexible material, such as, for example, a pliable polymer, biocompatible rubber, thermoplastic elastomer or PVC. Alternatively, suction device  12  may be made of a more rigid material covered with an elastic over face  15 . Suction force being applied through device  12  may cause device  12  to conform more closely to the shape of the target tissue. Device  12  may also be made of a malleable stainless steel or other material that is shapeable but not necessary flexible. Device  12  may also be made of a conductive polymer. 
     Ablation device  12  may also be permanently or removably attached to a suction tube  24 . Suction conduit  34  may be located within tube  24 . Conduit  34  may communicate suction to the target tissue surface via the suction openings  54  of suction ports  44  in device  12 . 
     The suction ports  44  may be arranged three to six ports in a row, although the specific number of ports and their position may vary. Preferably, for a linear lesion to result from the ablation process, the ports are arranged linearly. Device  12  may be covered with a covering during insertion to prevent blood or tissue from clogging the ports  44 , although this is not necessary. Such coverings may include coverings of biocompatible material that would cover device  12 . Alternatively, coverings may be placed over ports  44 , such as, for example, mesh coverings or ribbed coverings. 
     Each suction port  44  has a suction opening  54 , which may be located in the center or at a position slightly off-center of suction port  44 . Although the openings  54  are circular in  FIG. 2 , other shapes may be used. The suction ports  44  may also be any suitable shape. For example, in the embodiment of  FIG. 2 , the ports  44  are rectangular. Additionally, suction openings  54  may be covered with a covering such as described above to prevent blood or tissue from clogging the openings  54 . 
     Preferably, each suction opening  54  has a smaller diameter than the area of suction port  44 . This creates a high resistance pathway between suction port  44  and suction conduit  34 . Because of this, loss of a tissue-to-port seal in one suction port (and thus loss of fixation of the suction port to the tissue) should not cause a precipitous pressure drop in the remainder of the suction ports. 
     Ablation device  12  may be permanently or removably attached to at least one connection  28  for conveying energy to electrodes  22 ,  42  from power source  30 . This energy is typically electrical, such as radiofrequency (RF) energy. However, it may also be any appropriate type of energy such as, for example, microwave or ultrasound energy. Preferably, electrode  22  runs the length of one side of device  12  and electrode  42  runs the length of the opposite side of device  12 . Electrode  22  may be maneuvered into contact with the target tissue to ablate the tissue. In the embodiment of  FIG. 2 , two electrodes are shown in a bipolar arrangement. In such a bipolar arrangement, electrode  42  may also be maneuvered into contact with target tissue  60  to ablate the tissue. 
     Ablation device  12  may be permanently or removably attached to at least one fluid conduit  26  for irrigating the ablation site with a fluid. Alternatively, ablation site may not be irrigated. Fluid is conveyed to the site via fluid openings  46  which are preferably integrated into electrodes  22 ,  42 . However, fluid may be delivered to the site via a separate irrigation mechanism, such as an irrigation pump (not shown). Moreover, fluid openings  46  may be disposed in any appropriate manner on device  12 . 
     Suction ablation device  12  may be colored so that it can be easily visible against the target tissue. Alternatively, it may be clear to provide less distraction to the surgeon or to provide viewing of blood or other material being suctioned. Suction tube  24  may be a flexible tube constructed of a soft plastic which could be clear or colored. Suction ports  44  may be constructed of biocompatible rubber or epoxy, which could be clear or colored. 
     Electrodes  22 ,  42  may be constructed of stainless steel, platinum, other alloys, or a conductive polymer. If device  12  is made of a more flexible material, electrodes  22 ,  42  may be made of materials that would flex with the device  12 . Such flexible electrodes may be, for example, made in a coil or spring configuration. Flexible electrodes  22 ,  42  may also be made from a gel, such as a hydrogel. Furthermore, electrodes  22 ,  42  may also be an electrode designed to deliver fluid, such as, for example, a microporous electrode, a “weeping” electrode, or an electrode made of a hydrogel. 
     A source  20  for creating suction may be attached to suction tube  24  at the proximal end, preferably by a standard connector. This suction source  20  may be the standard suction available in the operating room and may be coupled to the device  12  with a buffer flask (not shown). Suction is provided at a negative pressure of between 200-600 mm Hg with 400 mm Hg preferred. 
     System  10  may include at least one temperature-sensitive element  36 . The temperature-sensitive element  36  is positioned to communicate with at least one of suction ports  44 . Preferably, an element  36  is positioned to communicate with each suction port  44 . These elements may be, for example, thermocouple wires, thermisters or thermochromatic inks. These thermocouple elements allow temperature to be measured. Such monitoring of temperature may crucial. Too high a temperature will char the tissue or cause the blood at the ablation site to coagulate. Preferably, the elements  36  may be adhered within suction ports  44  so as to contact the tissue when it is suctioned into the ports. Thermocouple elements that may be used are 30 gauge type T thermocouple wire from Dodge Phelps Company. One type of conductive epoxy which may be used to adhere the elements is epoxy no. BA-2902, available from Trecon. 
     A separate temperature sensitive element  36  may be adhered or mounted within each suction port  44 . Alternatively, a temperature sensitive element may be incorporated to run through all of the suctions ports  44 . 
     As ablation occurs, it is sometimes desirable to irrigate the ablation site with irrigation fluid, which may be, for example, any suitable fluid such as saline, an ionic fluid that is conductive or another conductive fluid. The irrigating fluid may cool the electrode  22  of ablation device  12 . Irrigated ablation is also known to create deeper lesions that are more likely to be transmural. Transmurality is achieved when the full thickness of the target tissue is ablated. The application of fluid to an ablation site may also prevent electrodes, particularly metal electrodes, from contacting the target tissue. Direct contact of electrodes to the target tissue may char or burn the tissue, which may clog the device. Furthermore, continuous fluid flow may keep the ablation device surface temperature below the threshold for blood coagulation, which may also clog the device. Use of irrigating fluid may therefore reduce the need to remove a clogged ablation device for cleaning or replacement. The presence of an ionic fluid layer between electrode  22  and the tissue to be ablated may also ensure that an ionic fluid layer conforming to the tissue contours is created. In one preferred embodiment, saline solution is used. Alternatively, other energy-conducting liquids, such as Ringer&#39;s solution, ionic contrast, or even blood, may be used. Diagnostic or therapeutic agents, such as Lidocaine, CA ++  blockers, or gene therapy agents may also be delivered before, with or after the delivery of the irrigating fluid. Irrigation source  40  may be any suitable source of irrigation fluid such as, for example, a standard irrigation pump (not shown). This pump may also be connected to power source  30  or may have its own source of power. Preferably, device  12  also includes a conduit  26  for delivering irrigation to the ablation site from irrigation source  40 . 
     In the embodiment of  FIG. 1 , fluid openings  46  may be located within the electrode  22  itself. These openings may be holes machined into the electrode  22 . These openings may deliver fluid to the ablation site as described above. Furthermore, electrode  22  may also be an electrode designed to deliver fluid, such as, for example, a microporous electrode, a “weeping” electrode, an electrode made of a microporous polymer or an electrode made of a hydrogel. 
     Referring now to  FIG. 3 , a close-up cross section is shown, taken along line A-A of  FIG. 1 . In use, the embodiment of device  12  shown in  FIGS. 1 and 2  is placed against target tissue  360  so that when a suction force is applied through openings  354 , the target tissue is pulled into the suction port  344 . Fluid flows from openings  46  towards the target tissue as indicated by the arrows. Openings  46  are preferably angled at about 30 degrees. Openings  46  preferably face towards suction ports  344 . Ablation may begin at point  300  of the tissue and spread in the direction indicated by the dotted arrows. If left over time, the entire piece of tissue suctioned into the ports  344  may be ablated. 
     Electrodes  322 ,  342  are brought to a temperature sufficient to ablate the tissue within the ports  44 . Thermocouple elements  336  may be used to monitor the temperature and when a given threshold temperature is reached, the surgeon may end ablation. This configuration of device  12  is especially useful because it gives an accurate measurement of the tissue temperature since the tissue  360  is in direct contact with the thermocouple elements  336  located near ports  344 . Thus the temperature of the tissue may be measured by thermocouple elements rather than the temperature of the electrode  322  being measured. The temperature of the tissue may also be determined based on ablation time. 
     The resulting lesion may be transmural. If the tissue is allowed to heat until the elements  336  indicate a temperature that usually indicates cell death (such as, for example, 15 seconds, at 55°), this may indicate that all the tissue has reached this temperature. In turn, this may indicate that the lesion is transmural. 
     The ablation resulting from the arrangement of electrodes in  FIGS. 2 and 3  is linear. The width of the resulting ablation lesion may be determined by the space between electrodes  22 ,  42 . The width of the resulting ablation lesion may also be determined by the depth of the suction port  44  and the amount of the tissue suctioned into port  44 . The depth of the lesion may be controlled by the depth of the suction port  44  and the amount of suction force applied. The depth of the lesion may also be determined by the power applied to the conductive element and the length of ablation time. The lesion resulting from the suction port  344  of  FIG. 3  will be repeated at each subsequent corresponding suction port along the length of device  12 . It is contemplated that for a longer lesion, a longer pod could be used or a series of pods could be strung together. A single pod could also be used to create a longer lesion by ablating to create a first lesion and then being moved to create a second lesion in line with the first lesion. 
       FIG. 4  shows another embodiment of the invention shown in  FIG. 3 . In this embodiment, electrodes  422 ,  423  are arranged in a unipolar arrangement. Electrode  422  is placed on the device  12  while another electrode  423  acts as a ground patch (indifferent, or non-ablating electrode) and is placed separately from the device  12 . For example, electrode  422  on device  12  could be placed on a surface of the heart. Then corresponding electrode  423 , which could be on a separate support surface, could be placed on the back of the patient to complete the circuit. Although the suction ports  444  may be arranged in a linear manner, ports  444  may be arranged in any other appropriate configuration, including for example, in an arcuate or radial arrangement. Although suction openings  454  may be circular, they may also be any appropriate shape to deliver suction. The lesions created by this sort of unipolar arrangement tend to be wider than those created by a bipolar arrangement. 
     In the unipolar arrangement of  FIG. 4 , suction ports  444  are used to grasp target tissue (not shown) but do not pull the tissue into the ports for ablating. Fluid would flow from openings in the electrode  423  or in device  12  in the same manner as described above. Ablation would occur in a similar manner to that described above although the device  12  remains uniformly on the surface of the target tissue rather than pulling the tissue into the ports for ablation. 
     It is contemplated that the electrodes used in the present invention could include any appropriate electrodes for performing ablation such as, for example, metal electrodes, braided metal electrodes or needle electrodes 
       FIG. 5  shows another embodiment of the suction ablation device  512  of the present invention, in which the conductive element may be a series of needle electrodes  522 . A unipolar arrangement of the electrodes  522  is shown. Alternatively, the electrodes may be arranged in a bipolar configuration similar to the arrangement of  FIG. 2 . In a bipolar arrangement, one series of needle electrodes may be arranged down the length of one side of the suction ports  544  and another series of electrodes  522  arranged down the length of the other side. Needle electrodes may be used to poke through fatty tissue covering the target tissue. They may then be used to poke into the target tissue. Suction may then be applied as described above to hold electrodes in place. Ablation may then occur as described above. Additionally, device  512  shows suction conduit  534  which may provide suction to ports  544  and pull wire  572  that serves as a maneuvering apparatus for device  512 . 
     The device  12  may also be used in electrical mapping functions. For example, electrode  22  may be placed on one area of the heart and an appropriate signal sent through it. Then the electrode  42  may receive the signal from electrode  22 . From the strength of the signal, the distance of electrode  22  from electrode  42  may be determined. Conduction delay or block can help determine transmurality of lesions. 
     Device  12  may also be used in pacing functions. For example, device  12  may grasp the heart as described above. Then energy may be sent through electrodes,  22 ,  42  at regular intervals. This energy may cause the heart to beat simultaneously to the signals sent through electrodes  22 ,  42 . The device  12  may thus pace the heart at an appropriate beating rate, thereby serving as a pacemaker. This may be used, for example, during a surgical procedure when it might be necessary to regulate the heart&#39;s beating temporarily. 
     It should be appreciated that the embodiments described above are to be considered in all respects only illustrative and not restrictive. The scope of the invention is indicated by the following claims rather than by the foregoing description. All changes that come within the meaning and range of equivalents are to be embraced within their scope.