Abstract:
A tissue cutting device includes a catheter with a window at its distal tip for admitting tissue into a catheter compartment. A cylindrical cartridge in the compartment has a cutting edge that supports an electrically conductive cutting element, e.g. a band or wire. The cutting element and adjacent tissue can be heated to a selected temperature by generating an electrical current through the cutting element. The catheter is maneuverable to position its distal end near the tissue to be cut. The catheter incorporates a vacuum feature that urges tissue into the compartment prior to cutting or severing and assists in holding the tissue during the cutting or severing process. Then, the cartridge is manipulated from the catheter&#39;s proximal end to move the cutting edge across the window, cutting the tissue. According to alternative embodiments, the cartridge is either rotated or moved axially relative to the catheter and, in either event may be capable of closing the catheter window when the cut is complete. Further alternatives involve either placing an indifferent electrode on the patient and providing an RF signal via a single conductor to the cutting element for ohmic heating, or providing an RF (or a DC) current through the cutting element and two separate conductors for direct resistive heating of the cutting element. Additional feature of the device include therapeutic infusion before, during or after the cut.

Description:
This is a Continuation of prior application Ser. No. 09/256,020, filed Feb. 23, 1999 now U.S. Pat. No. 6,214,024, issued Apr. 10, 2001, which is a Divisional of prior application Ser. No. 08/899,490, filed Jul. 19, 1997, now U.S. Pat. No. 5,876,369, issued Mar. 2, 1999, which is a Divisional of original application Ser. No. 08/376,226, filed Jan. 23, 1995, now U.S. Pat. No. 5,665,062, issued Sep. 9, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to devices for removing obstructions from body lumens, and more particularly to catheters for removing atherosclerotic plaque and thrombotic occlusions from blood vessels. 
     Atherectomy catheters are known for their utility in removing atherosclerotic plaque and thrombotic occlusions from arteries. While intended mainly for use in the coronary arteries, such catheters may as well be used in peripheral vessels. Beyond coronary uses, such catheters can have neurological applications, e.g. removal of lesions in carotid arteries, gynecological use in recanalizing fallopian tubes, and a potential urological application for removal of benign prostate hyperplasia. 
     Atherectomy catheters have a variety of designs. According to one design, the catheter has a rounded or bullet shaped tip with an abrasive surface. At the treatment site, the tip is rotated at high speed and burrows through the occlusion. The resulting debris, typically in particulate form, is not captured by the device. Rather, it is allowed to flow to the capillary bed where it is absorbed. The device is most effective in abrading hardened (calcified) occlusions, with the intent being to produce particulate debris no larger than about 7 microns in diameter. When encountering softer occlusions, however, the device presents the risk of an occlusion breaking off during the atherectomy procedure, causing an acute and potentially life-threatening reclosure of the artery. The abrasive tip may inadvertently remove endothelium cells during catheter insertion, use or removal, thereby creating sites for potential lesions. 
     A second type of catheter employs a cartridge housed within the catheter, near the catheter distal tip. When the distal tip region is placed near the occlusion, plaque enters the cartridge through an opening or “cutting window”. A cup-shaped blade then is rotated or oscillated at high speed, and advanced to cut and capture plaque that entered the cartridge. U.S. Pat. No. 5,312,425 (Evans); U.S. Pat. No. 5,087,265 (Summers); and U.S. Pat. No. 5,085,662 (Willard) disclose versions of atherectomy catheters with a movable blade or cartridge within a tissue collection volume near the catheter distal tip. 
     An example of an atherectomy catheter that depends on vaporization of plaque is found in U.S. Pat. No. 5,087,256 (Taylor). A dome-shaped head on the tip can be heated to temperatures in the range of 300-400 degrees C., for disintegrating plaque. U.S. Pat. No. 5,098,431 (Rydell) discloses a catheter in which an RF discharge between two spaced apart annular electrodes, electrosurgically cuts tissue to remove a blockage. 
     Conventional atherectomy catheters are limited principally to an axial cutting direction and subject to smooth cell muscle migration (restonosis) after treatment. Patients treated with conventional atherectomy devices have restonosis rates of 30-40 percent within the six months following treatment. 
     Therefore,it is an object of the present invention to provide an atherectomy catheter with a cutting edge that achieves a finer, more accurate cutting of unwanted tissue, to reduce the risk of acute blockage due to the breaking off of an occlusion. 
     Another object is to provide a tissue cutting element for an atherectomy catheter that is effective in severing calcified and soft occlusions. 
     A further object is to provide a flexible atherectomy catheter that incorporates means for capturing tissue being severed, to ensure against the escape of such tissue into the blood stream. 
     Yet another object is to provide an atherectomy catheter with enhanced versatility, due to the incorporation of longitudinal and transverse (arcuate) tissue cutting motions. 
     Further, it is an object of the present invention to provide an atherectomy catheter and procedure tending to seal the region of the cut, thus tending to reduce the incidence of restonosis. 
     SUMMARY OF THE INVENTION 
     To achieve these and other objects, there is provided a device for removing atheromas from a body lumen. The device includes an elongate catheter having a proximal end and a distal end. The catheter has a wall that defines a compartment within the catheter near its distal end, and a window is formed through the catheter wall to allow entry into the compartment. The catheter incorporates an electrically conductive tissue cutting element. A means is provided for generating an electrical current in the cutting element to heat the cutting element and adjacent tissue at least to a selected temperature above normal body temperature (i.e. 37 degrees C.). A carrier is mounted movably relative to the catheter, for supporting the cutting element proximate the window and for a controlled movement of the cutting element along and adjacent the window. The catheter is flexible and maneuverable to locate its distal tip within a body lumen and to place the window against a tissue wall segment of the body lumen to acquire tissue within the compartment. The cutting element, when heated and when undergoing the controlled movement, severs the acquired tissue. 
     There are several suitable approaches for generating the current necessary for cutting. At present, the most preferred approach involves a biocompatible cutting element (e.g. of platinum) subjected to RF energy in combination with an indifferent plate electrode on the patient&#39;s back. Application of the RF energy causes ohmic heating of tissue near the cutting element as current passes through the tissue. 
     As an alternative, RF energy can be applied to heat an electrically resistive cutting element formed of nickel or a nickel chromium alloy. In this approach, the cutting element is heated to a temperature sufficient to sever a lesion that comes into contact with the element. As a third and presently least preferred alternative, DC power is applied to heat an electrically resistive cutting element, again of nickel or a nickel chromium alloy. For biocompatability, an insulative jacket or coating is applied to the nickel or nickel chromium cutting element. 
     The preferred cutting element is a flat band, having a thickness of about 0.015 inches and a substantially greater width. The band achieves a highly accurate and fine cut of the tissue, considerably reducing the pulling and tearing of tissue as compared to the conventional oscillating or rotating blades. Unwanted tissue is severed more cleanly, reducing patient risk. Further, it is believed that the elevated temperatures of the band and adjacent tissue have a sealing or cauterizing effect along the region of the cut. This is believed to result in a substantial reduction in restonosis in the treated artery. 
     As an alternative to a flat band, the cutting element can be a fine wire having a diameter of about 0.030 inches or less to provide the cutting edge. 
     The carrier preferably includes a cartridge contained within the compartment. The cartridge can have an axially extended cartridge wall, a cartridge opening and a cutting edge along the cartridge opening for supporting the cutting element. A control means is coupled to the cartridge proximal end, and operable to selectively position and move the cartridge within the compartment, to provide the controlled movement of the cutting element. Preferably the cartridge wall is longer axially than the window, so that the cartridge can be positioned to substantially close the window and thus capture severed tissue. With tissue secured in this manner, there is no need for a suction device or other means to withdraw tissue proximally through the catheter immediately after it is cut. With no need to pay attention to a vacuum or suction means, the physician is able to direct more attention to the atherectomy procedure at hand. 
     Conversely, a vacuum means (or alternatively a plunger device and a diaphragm valve) may be provided if desired for removing severed tissue from the compartment, to allow cutting and removal of tissue at several locations during a single procedure. 
     There are several alternatives for supporting the cutting element. For example, the cutting edge can be the leading or distal edge of the cartridge, preferably but not necessarily annular. In this event, the cartridge is moved distally to provide the necessary controlled movement of the cutting element. Alternatively, the cutting edge can be an axial edge of the cartridge opening, in which event the cartridge is rotated about a longitudinal axis to move the cutting element in an arcuate path. 
     A preferred control means includes an elongate drive member, e.g. a rod, coil or tube, attached to the proximal end of the cartridge and extending proximally to a proximal end of the catheter. The member can be rotated* pushed or pulled at the catheter proximal end, to impart like motion to the cartridge. 
     Another aspect of the invention is a process for removing tissue from a body lumen, according to the following steps: 
     a. providing, near a distal end of an elongate catheter, a compartment and a window to the compartment through a catheter wall, and an electrically conductive tissue cutting element mounted to the catheter for a controlled movement of the cutting element adjacent and along the window; 
     b. inserting the catheter into a body lumen, and guiding the catheter to position the window against a tissue wall segment of the body lumen, thus to cause tissue to enter the compartment via the window and occupy the compartment; 
     c. generating an electrical current in the cutting element to heat the cutting element and adjacent tissue at least to a selected temperature above normal body temperature, while causing the cutting element to undergo the controlled movement, thereby to sever the tissue occupying the compartment. 
     After the tissue is severed, the compartment can be closed to prevent egress of the severed tissue. The controlled movement of the cutting element can be one of two alternatives: substantially linear and axial with respect to the catheter, or arcuate about an axis running longitudinally of the catheter. 
     Thus in accordance with the present invention, there is provided an atherectomy device and procedure suitable for treating soft occlusions and calcified occlusions alike, with clean and accurate cutting that reduces the risk of tissue tearing and escaping into the blood vessel under treatment. The catheter is versatile, due to its flexibility for traversing tortuous vessels and for the ability to cut in either an arcuate or axial path. The application of heat is concentrated along the region of the cut, i.e. the cutting element and adjacent tissue, reducing the required cutting force, increasing cutting accuracy and tending to cauterize or seal the region of the cut, thus tending to substantially prevent restonosis. 
    
    
     IN THE DRAWINGS 
     For a further understanding of the above features and advantages, reference is made to the following detailed description and to the drawings, in which: 
     FIG. 1 is an elevation of an atherectomy device constructed in accordance with the present invention; 
     FIG. 2 is a sectional view taken along the line  2 — 2  in FIG. 1; 
     FIGS. 3 and 4 are sectional views taken along the lines  3 — 3  in FIG.  1  and  4 — 4  in FIG. 2; 
     FIG. 5 is a perspective view of a tissue retaining cartridge employed in the device; 
     FIG. 6 is a schematic view illustrating circuitry for providing an electrical current through a mounted heating element on the cartridge; 
     FIGS. 7-9 are diagrammatic views illustrating use of the device; 
     FIG. 10 is an elevational view of the distal end region of an alternative atherectomy device constructed according to the invention; 
     FIG. 11 is a sectional view taken along the line  11 — 11  in FIG. 10; 
     FIG. 12 is a perspective view of a tissue retaining cartridge used in the device of FIG. 10; 
     FIGS. 13 and 14 diagrammatically illustrate use of the device of FIG. 9; 
     FIG. 15 is a three-dimensional illustration of an alternative embodiment cartridge; and 
     FIG. 16 is a schematic view of an alternative circuit for heating the cutting element. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, there is shown in FIG. 1 an atherectomy device  16  for removing unwanted tissue from body lumens, more particularly blood vessels. The device includes a control housing or handle  18 , and an elongate and pliable catheter  20  connected at its proximal end to the handle. 
     Catheter  20  is formed of a biocompatible polymer such as Pebax (brand name) polyether block amides, Pellethane (brand name) polyurethane or polyimide, and can have an outside diameter in the range of 3 Fr. (1 mm) to 8 Fr. (2.7 mm) or larger. Catheter  20  includes several lumens that run axially from handle  18  to a distal end region  22 . A guidewire  24 , contained within one of the lumens, extends proximally beyond handle  18  and distally beyond a tapered distal tip  26  of the catheter. Further lumens are provided in the catheter, for delivery of contrast fluid or treatment fluid to the distal end region, and for balloon inflation. While not shown, a braided or other filament structure can be embedded into the catheter body to enhance torque transmission, if desired. 
     A window  28  is formed through a wall  30  of the catheter along the distal end region. Window  28  is elongate in the axial direction and extends along about forty percent of the circumference of catheter wall  30 . A cartridge or cutting sleeve  32  is contained within catheter  20 . The cartridge is shown adjacent and along window  28 , but is movable axially relative to the catheter. This enables a proximal retraction of the catheter from window  28 , to open the window whereby the catheter can acquire tissue. An elongate drive rod  34 , contained within a lumen of catheter  20 , is coupled to the proximal end of cartridge  32  and extends proximally through the catheter and into handle  18 . 
     Drive rod  34  is formed of an electrically insulative material, e.g. a PTFE coated wire coil or spiral wrapped polyimide. The drive rod has a high degree of bending elasticity to permit its movement (with the remainder of catheter  20 ) along serpentine passages toward an intended treatment site. At the same time, drive rod  34  is sufficiently rigid in the axial direction to permit controlling the cartridge axial position by manipulating the drive rod at its proximal end. Drive rod  34  is constructed of an electrically insulated material. A pair of conductors  36  and  38 , preferably polyimide jacketed copper or copper-clad steel wires, are bonded to the drive rod and electrically isolated from one another. Conductors  36  and  38  extend along the entire catheter length to control housing  18 . 
     A dilatation balloon  40  is formed to extend axially along distal end region  22 , axially aligned with window  28  but also spaced apart angularly 180 degrees, i.e. on the opposite side of the catheter from the window. Balloon  40  is inflated using a fluid supplied to the balloon under pressure via a lumen of catheter  20 . 
     A fluid port  42  admits contrast or treatment fluids in the region just proximally of window  28 , while such fluids can be administered distally of the window via the guidewire lumen. 
     Control housing  18  includes three branches for coupling two sources of fluid (not shown): a branch  44  for balloon inflation, and two branches  46  and  48  for delivery of contrast fluids or treatment fluids such as heparin. At the proximal end of the control housing is a connector  50  with two electrically independent paths coupled to conductors  36  and  38 , respectively. A line  52  (e.g. coaxial) removably electrically couples connector  50  to a power supply  54 . Power supply  54  preferably is an RF source, but can be a DC source as well. Also mounted at the housing proximal end are a fitting  56  and a control knob  58 . Knob  58 , coupled to the proximal end of drive rod  34 , is movable axially relative to fitting  56 , to control the axial position of cartridge  32 . 
     A display  60  (e.g. LED) indicates the temperature of a cutting element mounted to the cartridge. A temperature control  62  adjusts the current through the cutting element, thus to set the cutting element temperature. 
     FIG. 2 shows several lumens running through the catheter, including a guidewire lumen  64  that also can be used to administer contrast and treatment fluids. A control lumen  66  accommodates drive rod  34 . Further lumens include a balloon inflation lumen  68  and an upper infusion lumen  70  for providing fluids to fluid port  42 . 
     As seen in FIGS. 3 and 4, control lumen  66  along the distal end region forms a compartment  72  to accommodate cartridge  32 . The compartment extends distally beyond window  28  a slight distance, and extends proximally of the window a sufficient amount to allow cartridge retraction, i.e. placing the complete cartridge proximally of the window. 
     Cartridge  32  is cylindrical, having an axial wall  74  and a disk-shaped proximal wall  76 . As seen in FIG. 5, cartridge  32  is open at its distal end to provide a circular cutting edge  78 . A tissue cutting element, more particularly a fine, flat band  80 , is mounted to the cutting edge to form an arc. Band  80  need not circumscribe the cutting edge, but should extend a sufficient amount to span window  28 . Band  80  has a thickness of about 0.015 inches, and is formed of nickel or a nickel chromium alloy coated for biocompatability. Alternatively (in an indifferent electrode approach discussed below) the band can be formed of platinum or another electrically conductive and body compatible material. The band is heated when subject to an electrical current. As an alternative to the band, the cutting element can be a wire with a diameter of at most 0.030 inches. 
     The polymer forming cartridge  32  can be blended with barium sulphate, bismuth trioxide or another suitable radiopaque material to facilitate fluoroscopic observation of the cartridge position before and during tissue removal. Radiopaque markers also may be placed along the cartridge cutting edge and on corresponding (distal and proximal, or intermediate) edges of the window. Along with aiding accurate positioning, these markers are useful in confirming when the cartridge is closed after cutting. 
     Conductors  36  and  38  are coupled to opposite ends of cutting element  80 . As indicated in broken lines, conductors  36  and  38  are embedded in axial wall  74  which, like the drive rod, is electrically insulative. A thermocouple  82  can be mounted to the cartridge near cutting edge  78  and preferably bonded to cutting element  80 , to monitor the cutting element temperature. Conductors  84 , embedded in the cartridge and the drive rod, permit the cutting element temperature to be monitored from control housing  18 . Alternatively, thermistors can be used to sense temperature. Further, for a resistive cutting element (i.e. nickel or nickel chromium) a characteristic dependence of resistance upon temperature can be used to monitor the cutting element temperature. 
     Tissue is cut by advancing cartridge  32  distally from the retracted position, with cutting element  80  and adjacent tissue maintained at at least a predetermined temperature above body temperature. A circuit for heating the cutting element and tissue is schematically illustrated in FIG. 6. A controller  86  adjusts power from supply  54  as required, for generating current through cutting element  80  in an amount selected to heat the cutting element to the predetermined temperature. Thermocouple  82  senses the temperature and provides the corresponding temperature reading at display  60 . If the display indicates a need to increase or reduce the temperature, controller  86  is adjusted to alter the current accordingly. A broken line at  88  indicates that if desired, the output of thermocouple  82  can be employed to automatically adjust the cutting element current. 
     The use of device  16  to remove unwanted tissue is shown in FIGS. 7-9, where catheter  20  has been either steered or moved over guidewire  24  to a treatment position near a lesion  90  in a coronary artery. Catheter  20  is manipulated from housing  18  to axially and angularly align the catheter and lesion  90 , so that window  28  faces the lesion (FIG.  7 ). At this point, dilatation fluid is supplied to expand balloon  40  against an arterial wall  92  opposite the lesion (e.g. as in aforementioned U.S. Pat. No. 5,085,662). Balloon dilatation drives the catheter distal end upward as viewed in FIGS. 7-9, forcing the catheter against the arterial wall about lesion  90  and causing at least a portion of the lesion to enter compartment  72  through window  28 . 
     With tissue thus acquired, cartridge  32  is advanced distally from the retracted position, while cutting element  80  is maintained at the selected temperature, from 50-600 degrees C. and more preferably 50-400 degrees C. This heating of the wire and adjacent tissue considerably enhances the cutting of tissue. Moreover, the heating has a cauterizing effect at the region of the cut and thus tends to seal the wound almost immediately after cutting, to significantly reduce smooth cell muscle migration or restonosis after treatment. The minute filament results in a precise, well defined cutting path that minimizes stretching and tearing of tissue. Accordingly, cutting element is effective in severing soft tissue as well as calcified or hardened tissue. This minimizes the risk of tissue fragmenting into particles or pieces that escape into the bloodstream. 
     Cartridge  32  is advanced until cutting edge  78  is distally of window  28  (FIG.  9 ), to completely acquire severed tissue within the cartridge. During or following the cut, a treatment fluid may be administered through lumens  64  and  70 . After balloon deflation, the cartridge can be proximally withdrawn to remove the severed tissue. 
     In addition to the cauterizing/sealing effect, a salient feature of the invention is the continuity of axial wall  74 , which closes window  28  when the cartridge is completely advanced. This insures that severed tissue remains captured within compartment  72  as the catheter is withdrawn, thus to eliminate the risk that severed tissue will enter the bloodstream. 
     FIG. 10 illustrates, in axial (longitudinal) section, the distal end region of an alternative device  96 . This device differs from device  16  in several respects concerning primarily the catheter distal end, but also as to the manner of controlling the cartridge. A catheter  98  of device  96  has flexible band  100  along its distal end in lieu of a dilatation balloon. Band  100  can be flexed radially away from the catheter and, upon contacting an artery, drives the catheter toward the opposite end of the artery. An example of such band and its control is found in U.S. Pat. No. 5,087,265 (Summers). 
     A single, central lumen  102  accommodates the guidewire and is enlarged to provide a compartment  104 . The compartment accommodates a cartridge  106  having an axially extended wall  108 , a proximal end wall  110  and a distal end wall  112 . A drive tube  114 , connected to the proximal end wall; is manipulated at the housing to selectively position cartridge  106 . 
     Drive tube  114  is used to rotate cartridge  106  about a longitudinal axis, rather than to move the cartridge axially. Consequently, while it requires less axial stiffness than drive rod  34 , drive tube  114  must be resistant to torsional bending to apply the necessary torque to rotate cartridge  106 . Also because of the lack of axial movement, compartment  104  need not extend proximally beyond window  116  to accommodate the cartridge length. As seen in FIGS. 11 and 12, cartridge all  108  is open along a portion of its circumference, to provide an arcuate cartridge opening  118  defined by end walls  110  and  112  and axial edges  120  and  122 . Axial edge  120  provides the cutting edge, and supports a linear cutting element  124 . Cutting element  124  is flat and thin (e.g. 0.015 inches thick), with a rectangular profile, to provide a lower profile cutting edge as compared to a wire. 
     FIGS. 13 and 14 illustrate the use of catheter  98  to remove a lesion  126  from an artery  128 . The catheter is positioned to axially align window  116  with lesion  126 , and further to angularly align the window and lesion. A control knob (not shown but similar to knob  58 ) is turned rather than pushed or pulled, to angularly align cartridge  106  such that opening  118  coincides with window  116  (FIG.  13 ). At this point, band  100  is flexed to urge the catheter upwardly against the arterial wall whereby a portion of lesion  126  enters compartment  104 . 
     At this point, the knob is turned to rotate cartridge  106  clockwise as viewed in FIG. 13 while cutting element  124  is heated, thus to progressively cut the lesion. Cartridge rotation continues until cutting edge  120  is carried beyond the clockwise edge of window  116 , which closes the window and captures acquired tissue. Then, catheter  98  is proximally withdrawn to remove captured tissue, or a plunger is used to remove tissue from the cartridge. 
     FIG. 15 illustrates an alternative embodiment cartridge  130  having an axial wall that is open at the distal end and along part of its circumference to define two cutting edges  132  and  134 . A continuous wire  136  (0.030 inch diameter) is mounted to the cartridge and shaped to provide an arcuate segment  138  mounted to cutting edge  132 , and a linear segment  140  along axial cutting edge  134 . Accordingly, cartridge  130  can be used to effect axial cutting and arcuate cutting. Cartridge wall  142  is continuous over at least sixty percent of its circumference to enable axial and angular positioning that closes the associated catheter window, to insure capture of severed tissue. Drive member  144  can be a tube, rod or coil and has sufficient axial and torsional rigidity to impart axial and rotational movement to the cartridge. 
     FIG. 16 illustrates an alternative circuit for heating an arcuate cutting element  146 . A conductor  148  connects the cutting element with an RF power supply  150 . A second conductor  152  couples the power supply and an indifferent electrode  154 . The indifferent electrode preferably is an electrode plate, typically applied to the back of the patient. The power supply provides an RF signal to the cutting element via conductor  148 . The signal returns to the power supply via conductor  152 . Between cutting element  146  and indifferent electrode  154 , current flows through body tissue. Consequently, ohmic heating of tissue is the primary factor in raising the temperature of tissue adjacent the cutting element to the predetermined or desired level. Cutting element  146  preferably is platinum, for high biocompatability and electrical conductivity. Accordingly cutting element heating from the current, while present to a degree, is slight compared to the ohmic heating effect. A controller  156  governs power supply  150  to provide the appropriate current in cutting element  146 . While not shown in FIG. 16, a thermocouple or other sensing element can be mounted at the cutting element, to sense the temperature of the cutting element and adjacent tissue, and provide feedback to controller  156 . As noted above, this approach (highly conductive cutting element and indifferent electrode) is presently the most preferred. 
     While the disclosure has focused on the treatment of coronary arteries, it is to be appreciated that devices in accordance with the present invention can be used to treat peripheral arteries and other vessels. These principles further can be applied in constructing and utilizing devices in neurology for removal of lesions in the carotid arteries, in gynecology for recanalization of Fallopian tubes, and in urology for removal of benign prostrate hyperplasia. The heating of the cutting element and adjacent tissue provides a fine, accurate cut to sever tissue at minimal risk of forming fragments, and provides a sealing action believed to reduce the incidence of restonosis. The cartridge can be either rotated or moved axially of the catheter, for greater versatility in severing unwanted tissue. After the cut, the cartridge completely closes the tissue-admitting window of the catheter, to insure complete capture of severed tissue.