Abstract:
Devices, systems and methods are disclosed for the formation of an arteriovenous fistula in the limb of the patient. Embodiments include a device for the creation, modification and maintenance of a fistula that includes an integral fistula creation assembly near its distal end that passes through the skin of the patient, through a first vessel such as an artery, and into a second vessel such as a vein. The fistula creation assembly preferably includes an anastomotic implant that is placed within the fistula to maintain long-term blood flow therethrough. The devices, systems and methods can be used to treat patients with one or more numerous ailments including chronic obstructive pulmonary disease, congestive heart failure, hypertension, hypotension, respiratory failure, pulmonary arterial hypertension, lung fibrosis and adult respiratory distress syndrome.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of PCT/US2006/024858 (Attorney Docket No. 022102-000510PC), filed on Jun. 26, 2006, which claims priority from U.S. Application No. 60/696,319 (Attorney Docket No. 022102-000500US), filed on Jun. 30, 2005, the full disclosures of each which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The present invention relates generally to medical devices and methods. More particularly, the present invention relates to devices and methods for creating or modifying a fistula between a first vessel and a second vessel, such as for the treatment of chronic obstructive pulmonary disease. 
         [0003]    Chronic obstructive pulmonary disease affects millions of patients in the United States alone. The present standard of care is externally supplied oxygen therapy, which requires a patient to remain near a stationary oxygen source or carry a bulky oxygen source when away from home or a treatment facility. It is easy to appreciate that such oxygen therapy has many disadvantages. 
         [0004]    Lung reduction surgery has recently been proposed for treating patients with chronic pulmonary disease. Such surgery, however, is not a panacea. It can be used on only a small percentage of the total patient population, requires long recovery times, and does not always provide a clear patient benefit. Even when successful, patients often continue to require supplemental oxygen therapy. 
         [0005]    There is therefore a need for improved approaches, including both devices and methods, for treating patients suffering from chronic obstructive pulmonary disease. If would be desirable if such devices and methods were also useful for treating patients with other conditions, such as congestive heart failure, hypertension, lung fibrosis, adult respiratory distress syndrome, and the like. Such devices and methods should provide for effective therapy, preferably eliminating the need for supplemental oxygen therapy in the treatment of chronic obstructive pulmonary disease. There is a need for simplified devices and procedural methods that reduce costs to the patient and healthcare system, as well as decrease procedure times and minimize patient risks. Improved devices and procedures must be developed to apply to a broad base of patient populations with a wide range of applicable arteriovenous anatomies. At least some of these objectives will be met by the invention described hereinafter. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    According to a first aspect of the invention, a device for creating a fistula in a patient is disclosed. The device comprises an elongate tubular structure comprising a proximal end and a distal end. The distal end is configured to penetrate or otherwise pass first through the skin of the patient, then through a first vessel, and then into a second vessel. The device includes an integral assembly configured to create a fistula between two neighboring vessels. In a preferred embodiment, the first vessel is an artery and the second vessel is a vein. In an alternative, also preferred embodiment, the first vessel is a vein and the second vessel is an artery. The fistula is typically created at a location wherein the artery and vein vessel walls are within 20 mm of each other. The fistula is created to provide therapeutic benefit, such as for an acute period less than twenty-four hours, a sub-chronic period between twenty-four hours and thirty days, or for a chronic period greater than thirty days. The fistula creation assembly is preferably mounted on a core that is slidable within an outer sheath of the device. The fistula creation assembly can be uncovered from the sheath by an operator through advancement of the core, retraction of the sheath, or a combination of the two movements. In a preferred embodiment, a second slidable core, such as a slidable needle assembly with a sharpened, beveled tip, is slidingly received by the first core. When fully advanced, the needle assembly exits the distal end of both the sheath and the first core, and assists in penetrating first through the skin of the patient and then through the tissue that exists along the trajectory to the proposed fistula. Both the first core and the sheath preferably have tapered ends to assist in penetration and/or advancement through the skin and tissue up to the fistula site. In another preferred embodiment, the needle assembly includes a guidewire lumen for insertion of a guidewire from outside the patient&#39;s body, through the first vessel and into the lumen of the second vessel. In another embodiment, the needle assembly is removable. In a preferred embodiment, the device is rigid along a majority of its length. In an alternative embodiment, the device is flexible along a majority of its length, such as a flexible section that can be advanced down a segment of the first vessel prior to entering the second vessel. In another alternative embodiment, the device includes both flexible segments and rigid segments along its length. 
         [0007]    The fistula creation assembly of the present invention can be configured in numerous forms to produce the desired fistula. In a preferred embodiment, a cone shaped dilator is expanded and/or delivers energy to create the fistula. In another embodiment, an expandable balloon is used to create the fistula. An anastomotic implant is preferably deployed to initially create the fistula and/or to improve the long-term patency of the fistula. The implant can perform numerous functions, and may include self-expanding materials, plastically deformable materials, or a combination of self-expanding or plastically deformable materials. In a preferred embodiment, the anastomotic implant forms the fistula into an oval cross section. In an alternative embodiment, the anastomotic implant forms the fistula into a circular cross section. Numerous forms of energy can be used to create and/or improve the fistula, including energies selected from the group consisting of: electrical energy such as radiofrequency or microwave energy; cryogenic energy; heat; radiation; chemical energy; light such as light delivered to photoreactive agents; and combinations thereof. The fistula creation assembly may deliver an agent to the fistula and its surrounding tissue, such agents selected from the group consisting of: anti-proliferative; anti-biotic; anti-thrombogenic; and combinations thereof. 
         [0008]    In a preferred embodiment, the device is configured to create a fistula to treat a patient suffering from COPD, such as via the fistula decreasing systemic vascular resistance of the patient. In these and other patient populations, the fistula may provide therapy by increasing the oxygen content of venous blood, such as blood supplied to a lung of the patient. The fistula may additionally cause oxygen content in arterial blood to also increase. The fistulas of the present invention are configured to have blood flow through the fistula of at least 5 ml/min, and preferably greater than 50 ml/min. 
         [0009]    In yet another preferred embodiment, the device includes a handle at its proximal end. The handle includes one or more controls, such as controls to advance and/or retract a slidable core. In a preferred embodiment, one or more controls are included to perform a function selected from the group consisting of: initiate or modify the delivery of energy to an intended or existing fistula location; expand a distal portion of the device such as an expandable dilator or inflatable balloon; deploy an implant such as an anastomotic device which applies tension between the two vessels at the fistula location and/or scaffolds the lumen of the fistula; and combinations thereof. 
         [0010]    In yet another preferred embodiment, the device is configured to create a fistula in a limb of a patient, such as between an artery and vein selected from the group consisting of: axillary artery; brachial artery; ulnar artery; radial artery; profundal artery; femoral artery; iliac artery; popliteal artery; carotid artery; saphenous vein; femoral vein; iliac vein; popliteal vein; brachial vein; basilic vein; cephalic vein; medial forearm vein; medial cubital vein; axillary vein; and jugular vein. In an alternative embodiment, the fistula is located in the abdomen or thorax of the patient. 
         [0011]    In yet another preferred embodiment, the fistula creation device further comprises a flow measurement element, such as an ultrasound or Doppler ultrasound element, or a lumen that allows a flow measurement catheter to be inserted into the proximal end of the device and advanced to a location near or beyond the device&#39;s distal end. Flow measurement can be made directly using Doppler technologies and techniques, or indirectly by measuring flow channel geometries. 
         [0012]    In yet another preferred embodiment, the fistula creation device further comprises flow adjustment means mounted to one or more of the outer sheath or an inner core. The flow adjustment means can be activated on demand by an operator and preferably includes: an energy delivery element; an agent delivery element; an inflatable balloon; an expandable dilator; a deployable implant such as a second implant of the fistula creation device; and combinations thereof. 
         [0013]    According to a second aspect of the invention, a system for creating a fistula is disclosed. The system includes one or more of the embodiments of the fistula creation device of the first aspect of the invention, and an ultrasound visualization monitor. The visualization monitor may be configured to display information received from one or more ultrasound crystals integral to the fistula creation device, or may work with separate device such as an external or internal ultrasound probe. 
         [0014]    According to a third aspect of the invention, a system for creating a fistula is disclosed. The system includes one or more of the embodiments of the fistula creation device of the first aspect of the invention, and an apparatus selected from the group consisting of: a balloon catheter; an anastomotic implant deployment catheter; a flow measurement device such as a flow catheter or an external Doppler probe; an angiography catheter; a venography catheter; a guidewire; an introducer; a needle; a biopsy needle; and combinations thereof. In a preferred embodiment, the system further comprises an ultrasound visualization monitor configured to provide an image received from one or more of: an ultrasound element integral to the fistula creation device; an external ultrasound probe; and an internal ultrasound apparatus such as an intravascular ultrasound catheter; and an inserted probe such as a transesophageal probe. 
         [0015]    According to a fourth aspect of the invention, a method of creating a fistula is disclosed. The distal end of a fistula creation device is placed through the skin of the patient. The distal end, which is preferably the distal end of a sharpened, beveled tip needle assembly, such as a removable needle assembly, is advanced through a first vessel, such as a vein or an artery. The distal end is then advanced into a second vessel at an existing-fistula or an intended-fistula location. A fistula is then created, or an existing fistula is maintained, such that a long-term flow of blood is provided between the first vessel and the second vessel. The fistula creation device includes an elongate tubular structure that may be rigid along a majority of its length, may be flexible along a majority of its length, or may include rigid and flexible portions such as two rigid portions attached with a flexible hinge. In a preferred embodiment, blood flows from the first vessel to the second vessel. In an alternative, also preferred embodiment, blood flows from the second vessel to the first vessel. At the intended fistula location of the patient, the vessels may lie in various geometric configurations, such as wherein the first vessel is “on top” of the second vessel such that the lumen of the first vessel lies relatively proximate the shortest line between the lumen of the second vessel at the fistula location and the surface of the patient&#39;s skin. In alternative fistula locations, the vessels may lie in a more “side-to-side” configuration. When inserted, the elongate body of the fistula creation device is positioned to lie relatively in the plane defined by the lumens of the two vessels near the intended fistula location. While maintaining position within this plane, the fistula creation device can be inserted at an angle relatively perpendicular to the surface of the patient&#39;s skin, or at a smaller angle, such as an angle between 20 and 80 degrees. This insertion angle may be chosen by the clinician to affect the fistula angular geometry between the two vessels, such as at a small insertion angle to correspond to a similarly small angle between the lumen of the first vessel and the lumen of the fistula. Such a small angle between the first vessel lumen and the fistula lumen may be desirous to reduce turbulent flow through the fistula. In alternative embodiments, an insertion angle approximating ninety degrees may be chosen in order to minimize the length of the fistula. 
         [0016]    In a preferred embodiment, the method further comprises the step of determining the anatomical location for the fistula. Prior to creating the fistula, the fistula location is determined using one or more of: angiography; venography; extra-vascular ultrasound; intravascular ultrasound; Doppler ultrasound; and MRI. The fistula location is determined based on an analysis of a parameter selected from the group consisting of: first vessel diameter; second vessel diameter; artery diameter; vein diameter; ratio of artery to vein diameter; distance between the artery and vein lumens; geometric relationship between the artery and vein lumens; distance from an arterial side branch; distance from an venous side branch; arterial flow; venous flow; oxygen content in artery; oxygen content in vein; wall thickness of artery; wall thickness of vein; degree of calcification of artery; degree of calcification of vein; geometric relationship between the artery and vein lumens at the fistula site; hemodynamic factors; other parameters; and combinations thereof. 
         [0017]    In another preferred embodiment, the method further comprises the step of performing a blood flow measurement procedure, such as a procedure performed prior to fistula creation, during fistula creation, after fistula creation, and combinations thereof. The information determined during the flow measurement procedure can be used to select the fistula site, modify the fistula such as a balloon dilation fistula modification procedure and/or otherwise treat the fistula. 
         [0018]    In yet another preferred embodiment, an anastomotic implant is placed in the fistula. The anastomotic implant is placed to provide a function selected from the group consisting of: scaffolding an opening between the first vessel and the second vessel; reducing neointimal proliferation into the fistula flow path; preventing tissue from protruding into the fistula flow path; placing a portion of the first vessel wall in tension with the tissue of the second vessel wall; reducing bleeding of the tissue neighboring the fistula; enhancing healing of the tissue neighboring the fistula; and combinations thereof. The anastomotic implant may include one or more coatings, such as anti-bacterial; anti-thrombogenic and anti-prolific coatings. The anastomotic implant may additionally or alternatively include a covered portion, such as a partial covering, such covering materials selected from the group consisting of: polytetrafluoroethylene; Dacron; Nitinol; stainless steel; and combinations thereof. The fistula creation device of the present invention preferably places the anastomotic implant. 
         [0019]    In yet another preferred embodiment, a guidewire is placed between the first and second vessel through the fistula, prior to, during, or after the fistula is created. The guidewire preferably remains in place after the fistula creation device is removed, or partially removed, such that a second catheter device can be placed over that guidewire. The second catheter device can be used to perform a diagnostic event such as a radiographic dye injection catheter inserted to perform angiography or venography, or an ultrasound catheter used to visualize the fistula structure. The second catheter device can be used to modify the fistula such as a balloon catheter inserted to enlarge the fistula or an anastomotic implant deployment catheter inserted to increase or decrease flow through the fistula. 
         [0020]    In yet another preferred embodiment, the fistula is created to provide therapeutic benefit that results from: a decrease is systemic vascular resistance; an increase in the oxygen content in at least a portion of the venous system such as the venous supply to a lung of the patient; and combinations thereof. Blood flow through the fistula is at least 5 ml/min and preferably greater than 50 ml/min. The fistula is preferably created between an artery and a vein at a location in a limb of the patient. The artery is selected from the group consisting of: axillary artery; brachial artery; ulnar artery; radial artery; profundal artery; femoral artery; iliac artery; popliteal artery; and carotid artery. The vein is selected from the group consisting of: saphenous; femoral; iliac; popliteal; brachial; basilic; cephalic; medial forearm; medial cubital; axillary; and jugular. The artery is preferably between 5 mm and 25 mm in diameter at the intended fistula location. The vein is preferably less than 35 mm in diameter at the intended fistula location. The fistula preferably has a non-circular or oval cross section, such that the major axis of the oval is greater than either the vein diameter or the artery diameter. In an alternative embodiment, the fistula has a circular cross section. The geometry of the cross section of the fistula is preferably matched with a similar geometry of an anastomotic implant placed during the disclosed method. 
         [0021]    According to a fifth aspect of the invention, a kit for creating a long-term fistula in a patient for the treatment of COPD is disclosed. The kit includes a first fistula creation device for forming a fistula with a first geometry. The kit further includes a second fistula creation device for forming a fistula with a second geometry. Either the first or the second fistula creation device is used to create the long-term fistula based on an analysis of information gathered during a visualization procedure performed on the patient. The visualization procedure is preferably selected from the group consisting of: ultrasound visualization including intravascular ultrasound and extravascular ultrasound; angiography; venography; MRI; and combinations thereof. The first fistula creation device and the second fistula creation device can be configured to create a first fistula and a second fistula respectively. The first fistula and the second fistula may have two different cross sectional geometries, such as a circular and an oval cross sections, two different circular cross sections or two different oval cross sections. Varied oval cross sections may include ovals with different major axes and different minor axes, and ovals with different major axes with similar minor axes. 
         [0022]    Both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the embodiments of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the present invention, and, together with the description, serve to explain the principles of the invention. In the drawings: 
           [0024]      FIG. 1  illustrates a partial cross sectional side view of a fistula creation apparatus consisting with the present invention; 
           [0025]      FIG. 2   a  illustrates a cross sectional side view of a fistula creation apparatus consistent with the present invention shown with a slidable needle assembly in the advanced position; 
           [0026]      FIG. 2   b  illustrates the fistula creation apparatus of  FIG. 2   a  with the needle assembly removed; 
           [0027]      FIG. 2   c  illustrates the fistula creation apparatus of  FIG. 2   b  with an outer sheath retracted such that an integral anastomotic implant is partially expanded; 
           [0028]      FIG. 2   d  illustrates the fistula creation apparatus of  FIG. 2   c  with the outer sheath further retracted such that the integral anastomotic implant is fully expanded; 
           [0029]      FIG. 3   a  is a cross sectional side view of a device and method for creating a fistula consistent with the present invention shown prior to advancement of the device through the patient&#39;s skin; 
           [0030]      FIG. 3   b  illustrates the device and method of  FIG. 3   a  shown with the distal end of a slidable needle assembly of the device having penetrated the skin and the first wall of an artery; 
           [0031]      FIG. 3   c  illustrates the device and method of  FIG. 3   b  shown with the needle assembly having further penetrated a second wall of the artery and a first wall of a vein, and a guidewire having been advanced down the vein through a lumen of the needle assembly; 
           [0032]      FIG. 3   d  illustrates the device and method of  FIG. 3   c  shown with the needle assembly having been retracted while leaving the guidewire seated in the vein; 
           [0033]      FIG. 3   e  illustrates the device and method of  FIG. 3   d  shown with an anastomotic implant of the device partially deployed in the vein; 
           [0034]      FIG. 3   f  illustrates the device and method of  FIG. 3   e  shown with the anastomotic implant of the device fully deployed in the fistula between the artery and the vein, and the device outer sheath partially retracted from the patient; 
           [0035]      FIG. 4   a  is a cross sectional side view of a device and method for creating a fistula consistent with the present invention shown with advancement of the distal end of the device through the patient&#39;s skin and into an artery and an ultrasound probe located proximate the entry site; 
           [0036]      FIG. 4   b  illustrates the ultrasound image produced by the ultrasound probe of  FIG. 4   a.    
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
         [0038]      FIG. 1  depicts a preferred embodiment of the fistula creation device of the present invention. Device  10  is configured to be inserted by an operator through the skin of a patient to create and/or maintain a fistula that provides a flow of blood between a first vessel and a second vessel, such as a long-term flow of blood to achieve a therapeutic benefit. Device  10  includes an elongate tubular structure with a proximal end and a distal end, the tubular structure comprising multiple tubes that surround or are slidingly received within a separate tube. Each tube may have a rigid, semi-rigid, and/or flexible construction and each tube comprises one or more materials such as: nylon; polyvinyl chloride; polyethylene; polypropylene; polyimide; Pebax™; Hytrel™; polyurethane; silicone; steel; Nitinol™; blends, alloys and copolymers of the preceding, or other biocompatible materials, and may include a structural braid such as a nylon or metal braid commonly used in interventional guide catheters. Each tube may include a tapered, sharpened, beveled, expandable such as balloon expandable, and/or energy emitting distal end, and each tube may include one or more lumens, such as a Teflon-lined or Teflon-coated lumen. The elongate tubular structure may be rigid or flexible along a majority of its length, or may include both rigid and flexible portions such as two rigid portions separated by a flexible hinge portion. Device  10  further includes, on its proximal end, handle  40 , which is grasped by an operator to advance, retract, rotate, control, activate, and/or otherwise manipulate device  10  or a component or sub-assembly of device  10 . Each advanceable and/or retractable tube of device  10  may be attached at its proximal end to one or more advancement and/or retraction controls, such as a tube that is operably attached to a control integral to or proximate to handle  40 . Handle  40  may include additional controls, such as a control to enlarge a dilator, inflate a balloon, deploy an implant, initiate energy or agent delivery, activate a diagnostic device and/or perform another function. 
         [0039]    The outermost tube, outer sheath  30 , which preferably has a tapered distal end and is constructed of a biocompatible plastic, surrounds and slidingly receives a first slidable core, inner core  20 , which includes conically tapered tip  21  and is preferably constructed of biocompatible metal and/or plastic. Fistula creation assembly  25  is mounted near the distal end of inner core  20  and is configured to create a fistula on demand by an operator. Fistula creation assembly  25  may include one or more of various means to create the fistula such as a cone shaped dilator, not shown, such as a dilator that is expandable and/or configured to deliver energy. Fistula creation assembly  25  may include a force-exerting balloon, such as a compliant or non-compliant balloon and/or a balloon to dilate an implant. Alternatively or additionally, fistula creation assembly  25  may include a delivery assembly and an anastomotic implant, such as a vessel-to-vessel tensioning anastomotic clip and/or a fistula scaffolding assembly. Alternatively or additionally, fistula creation assembly  25  may include an energy delivery element such as an element configured to deliver electrical energy such as radiofrequency or microwave energy; cryogenic energy; heat; radiation; chemical energy; light; and/or other forms of energy. Energy may be delivered to ablate tissue, cut tissue and/or coagulate blood and tissue. Alternatively or additionally, fistula creation assembly  25  may include an agent delivery assembly, such as an agent delivery mechanism configured to deliver one or more of anti-proliferatives; anti-biotics; and anti-thrombogenics. In a preferred embodiment, fistula creation assembly  25  further is configured to modify an existing fistula, such as a fistula created by device  10  previously used to create the fistula, or by a separate device  10  or an alternative fistula creation device, such as during the same medical procedure or a previously performed procedure. In order to modify an existing fistula, fistula creation assembly  25  may include an anastomotic implant, such as a second anastomotic implant nested within a first implant, an expandable balloon, an energy or agent delivery element, a tissue removing element such as a forward or pull back atherectomy catheter, or other means. Other fistula modifying events can be performed such as the placement of an implant, which partially covers the fistula from either the venous or arterial side. 
         [0040]    Referring back to  FIG. 1 , fistula creation assembly  25  is operably attached to conduit  26 , a flexible or rigid conduit which travels proximally to handle  40 . Conduit  26  may include one or more of: a power or data transfer conduit such as one or more electrical wires and/or optical fibers, a tube such as an inflation lumen or cryogenic flow tube; and a slidable cable such as a pull wire. Conduit  26  is electrically attached through an electrical switch control, button  42 , to another control on handle  40 , port  41 . Port  41  is an electrical jack that can be attached to an energy delivery unit such as an RF generator, not shown. An operator depresses button  42  to deliver energy to fistula creation assembly  25 . In an alternative or additional embodiment, port  41  is attached to a balloon endoflator that is used to inflate a balloon integral to fistula creation assembly  25 . In another alternative or additional embodiment, port  41  is attached to a drug delivery pump or supply to deliver drugs to fistula creation assembly  25 . Outer sheath  30  is operably attached to a control on handle  40 , sheath retraction knob  32 , which can be slid proximally by an operator to retract outer sheath  30  and subsequently slid distally to advance outer sheath  30 . Outer sheath  30  is retracted to expose fistula creation assembly  25 , such as when fistula creation assembly  25  is an expandable balloon and/or an anastomotic implant delivery assembly such as an assembly including a self-expanding anastomotic implant. In an alternative embodiment, inner core  20  is advanced to expose fistula creation assembly  25 , or a combination of advancing inner core  20  and retracting outer sheath  30  is performed. Inner core  20  is operably attached to core advancement knob  22  of handle  40  such that inner core  20  can be advanced and retracted by sliding core advancement knob  22  forward or back. 
         [0041]    Located in the distal portion of inner core  20  and in proximity to fistula creation element  25  is visualization element  70 , preferably an ultrasound element such as a phased array of ultrasound crystals, signal and power wires not shown, or a rotating ultrasound crystal, rotating shaft and signal and power wires not shown. Visualization element  70  can be electrically connected to an ultrasound monitor, not shown, such that a cross sectional view of the tissue and other structures surrounding the distal portion of device  10  and fistula creation assembly  25  can be visualized. In alternative embodiments, visualization element  70  consists of a visualization marker, such as an ultrasonically reflective surface that can be visualized with an external ultrasound probe, a radiopaque marker that can be visualized under fluoroscopy, a magnetic marker, and other markers compatible with visualization equipment found in hospitals, doctor&#39;s offices and other health care settings. An operator utilizes visualization element  70  during various procedural steps involving device  10 , such as penetration of its distal end through the skin and vessels of the patient and rotational orientation of the device. Visualization element  70  also provides valuable information prior to, during, and after the activation of fistula creation element  25  such as information relating to the inflation of a balloon and/or placement of an anastomotic implant. In an alternative embodiment, device  10  further includes a flow measurement element, not shown, preferably embedded in visualization element  70 , such as a Doppler ultrasound function. In another alternative embodiment, a visualization catheter or flow measurement catheter is inserted in a lumen of device  10 , such as within the lumen of inner core  20  in which needle assembly  50  is inserted, a separate lumen of inner core  20  not shown, or a lumen of outer sheath  30 . In a preferred embodiment, device  10  further includes a visualization and/or flow measurement monitor, such as a Doppler ultrasound monitor. 
         [0042]    A second slidable core, needle assembly  50 , is slidingly received within a lumen of inner core  20 . Needle assembly  50 , which may be rigid or flexible along its length, is preferably constructed of one or more metals such as stainless steel and Nitinol. Needle assembly  50  can be retracted, and completely removed from the lumen of inner core  20  by retraction of yet another control of handle  40 , needle retraction knob  52 . In an alternative embodiment, full removal of needle assembly  50  is prevented by the inclusion of one or more mechanical stops. Needle assembly  50  has a sharpened distal tip  51 , which is preferably sharp and beveled. Needle assembly  50  includes a lumen from its proximal end to its distal end, guidewire lumen  53 , which is configured to allow a standard interventional guidewire to be advanced therethrough, and further configured to allow needle assembly  50  to be removed leaving the previously inserted guidewire to reside within the lumen of inner core  20  previously inhabited by needle assembly  50 . In an alternative embodiment, needle tip  51  may be configured to deliver energy, such as RF energy used to assist in advancement, and/or to cauterize, cut and ablate tissue. 
         [0043]    In a preferred embodiment, a kit is provided for the creation of multiple fistulas, in a single patient or multiple patients, includes multiple fistula creation devices of  FIG. 1  with varied fistula creation elements in each device. An operator selects a specific fistula creation device based on the configuration of the fistula creation element included in that device. In one alternative, a first fistula creation device creates a fistula with a different geometry than a second fistula creation device, such as might be chosen to differentiate a fistula between vessels with a first set of luminal diameters and a second set of fistulas with different luminal diameters. Numerous fistula creation parameters can be varied between a first fistula creation device and a second fistula creation device such as use of energy, fistula diameter, fistula cross section geometry such as circular cross section versus elliptical cross section wherein the major diameter of the ellipse is at least 20 percent larger than the minor diameter of the ellipse. In a preferred embodiment, the major diameter of the fistula is at least twice the minor diameter. In another preferred embodiment, a kit includes a first fistula creation device with a target fistula cross section dimensions having unequal major and minor axes, and a second fistula creation device with a target fistula cross section dimensions have similar minor axis length and greater major axis length. In a preferred method, an operator selects either the first fistula creation device or the second fistula creation device based on a visualization procedure performed on the anatomy of the patient proximate the intended fistula creation site. 
         [0044]    Referring now to  FIGS. 2   a  through  2   d , a preferred embodiment of a fistula creation device of the present invention is shown in various stages of a preferred method of activation. Referring specifically to  FIG. 2   a , device  10  is configured to be inserted by an operator through the skin of a patient to create and/or maintain a fistula that provides a flow of blood between a first vessel and a second vessel, such as a long-term flow of blood to achieve a therapeutic benefit. Device  10  includes an elongate tubular structure with a proximal end and a distal end, the tubular structure comprising multiple tubes that surround or are slidingly received within a separate tube. Each tube may have a rigid, semi-rigid, and/or flexible construction and each tube comprises one or more materials such as: nylon; polyvinyl chloride; polyethylene; polypropylene; polyimide; Pebax™; Hytrel™; polyurethane; silicone; steel; Nitinol™; blends, alloys and copolymers of the preceding, or other biocompatible materials, and may include a structural braid such as a nylon or metal braid commonly used in interventional guide catheters. 
         [0045]    The outermost tube, outer sheath  30 , which preferably has a tapered distal end, surrounds and slidingly receives a first slidable core, inner core  20 , which includes conically tapered tip  21 . Outer sheath  30  includes on its proximal end sheath advancement knob  32 , which is manipulated by an operator to advance and retract outer sheath  30 . Inner core  20  includes on its proximal end, core advancement knob  22 , which is manipulated by an operator to advance and retract inner core  20 . Balloon  25  is mounted near the distal end of inner core  20  and is expandable on demand by an operator, inflation lumen and endoflator attachment port not shown, such as to create the fistula and/or expand an implant placed to maintain the fistula. Balloon  25  may comprise a compliant or non-compliant balloon. Surrounding balloon  25  is an anastomotic implant, clip  60 , which is deployed in the fistula to perform one or more functions including but not limited to: scaffolding an opening between the first vessel and the second vessel; reducing neointimal proliferation into the fistula flow path; preventing tissue from protruding into the fistula flow path; placing a portion of the first vessel wall in tension with the tissue of the second vessel wall; and reducing bleeding of the tissue neighboring the fistula; enhancing healing of the tissue neighboring the fistula. In a preferred embodiment, the anastomotic implant includes an active agent, such as an anti-thrombogenic or anti-proliferative agent, and may also include a covering or partial covering. 
         [0046]    A second slidable core, needle assembly  50 , is slidingly received within a lumen of inner core  20 . Needle assembly  50 , which may be rigid or flexible along its length, is preferably constructed of one or more metals such as stainless steel and Nitinol. Needle assembly  50  can be retracted, and completely removed from the lumen of inner core  20  by retraction of needle retraction knob  52 . In a preferred embodiment, Inner core  20  can also be retracted, and completely removed from the lumen of outer sheath  30 , by retraction of knob  20 . Needle assembly  50  has a sharpened distal tip  51 , which is preferably sharp and beveled. Needle assembly  50  includes a lumen from its proximal end to its distal end, guidewire lumen  53 , which is configured to allow a standard interventional guidewire to be advanced therethrough, and further configured to allow needle assembly  50  to be removed leaving the previously inserted guidewire to reside within the lumen of inner core  20  previously inhabited by needle assembly  50 . In an alternative embodiment, needle assembly  50  is partially retracted but remains within lumen of inner core  20 . In another alternative embodiment, needle tip  51  may be configured to deliver energy, such as RF energy used to assist in advancement, and/or to cauterize, cut and ablate tissue. In a preferred method, device  10  is advanced through the skin, through a first vessel and into the lumen of a second vessel with needle assembly  50  in the fully advanced position. A locking mechanism, not shown, may be engaged to prevent relative motion between needle assembly  50  and outer sheath  30  during insertion and subsequent advancement. A guidewire is then advanced through guidewire lumen  53  and further advanced down the lumen of the second vessel. 
         [0047]    Referring now to  FIG. 2   b , needle assembly  50  has been completely removed from the lumen of inner core  20 , such as when a guidewire has been successfully placed from a location outside the patient&#39;s skin to and into the second vessel. Referring now to  FIG. 2   c , the distal end of clip  60  has been released from being constrained by outer sheath  30 , either by retraction of outer sheath  30 , advancement of inner core  20 , or a combination of both movements. Clip  60  of  FIGS. 2   a  through  2   d  is self-expanding, such as a resiliently biased tubular structure made of Nitinol. In  FIG. 2   d , clip  60  has been fully released from being constrained and is in a fully expanded condition. Balloon  25  has been inflated, inflation lumen and endoflator attachment not shown, to provide additional expansion force to clip  60 . In an alternative embodiment, clip  60  is plastically deformable, or includes plastically deformable portions, such that balloon  25  is required to expand clip  60 . In a preferred embodiment, device  10  of  FIGS. 2   a  through  2   d  is used as a system in conjunction with one or more additional devices to create and/or maintain the fistula. Such additional devices include but are not limited to guidewires and various over-the-wire devices that are placed over the guidewire placed through needle assembly  50 , after needle assembly  50  is removed. These additional over-the-wire devices may be placed within a lumen of inner core  20 , within a lumen of outer sheath  30  with inner core  20  removed, or over the guidewire after device  10  has been completely removed. These over-the-wire devices include but are not limited to: balloon catheters; anastomotic implant delivery devices and implants; flow measurement catheters; angiography catheters; venography catheters; and other over-the wire devices applicable to modifying the fistula, such as modifying the flow of the fistula, or to perform a procedure to otherwise enhance and/or maintain the long term benefit of the fistula. 
         [0048]    Referring now to  FIGS. 3   a  through  3   f , a preferred method of using the fistula creation device of the present invention is shown. A cross sectional view of a patient&#39;s anatomy at a proposed fistula location  111  is depicted wherein artery  130  is directly above vein  120  in relation to skin surface  105 . Intended fistula location  111  may be determined using one or more visualization techniques including but not limited to: angiography; venography; extra-vascular ultrasound; intravascular ultrasound; and MRI. Intended fistula location  111  may be determined using one or more flow measurement techniques such as Doppler ultrasound. The intended fistula location  111  may be selected based on parameters selected from the group consisting of: first vessel diameter; second vessel diameter; artery diameter; vein diameter; ratio of artery to vein diameter; distance between the artery and vein lumens; geometric relationship between the artery and vein lumens; distance from an arterial side branch; distance from an venous side branch; arterial flow; venous flow; oxygen content in artery; oxygen content in vein; wall thickness of artery; wall thickness of vein; degree of calcification of artery; degree of calcification of vein; geometric relationship between the artery and vein lumens at the fistula site; hemodynamic factors; other parameters; and combinations thereof. 
         [0049]    Artery  130  includes, in closest proximity to skin  105 , arterial wall  131 . Vein  120  includes, in closest proximity to artery  130 , venous wall  121 . At the intended fistula location  111  of the patient, the vessels may lie in various geometric configurations, such as the geometry of  FIGS. 3   a  through  3   f  wherein the first vessel is relatively “on top” of the second vessel such that the lumen of the first vessel lies relatively proximate the shortest line between the lumen of the second vessel at the fistula location and the surface of the patient&#39;s skin. In alternative fistula locations, the vessels may lie in a more “side-to-side” configuration. When inserted, the elongate body of outer sheath  30  is positioned to lie relatively in the plane defined by the lumens of the two vessels near the intended fistula location. While maintaining position within this plane, the fistula creation device can be inserted at an angle relatively perpendicular to the surface of the patient&#39;s skin, or at a smaller angle as is shown in  FIG. 3   a , such as an angle between 20 and 80 degrees. This insertion angle may be chosen by the clinician to form the resultant fistula angular geometry between the two vessels, such as at a small insertion angle to correspond to a similarly small angle between the lumen of the first vessel and the lumen of the fistula. Such a small angle between the first vessel lumen and the fistula lumen may be desirous to reduce turbulent flow through the fistula. In alternative embodiments, an insertion angle approximating ninety degrees may be chosen, such as to minimize the length of the resultant fistula. Vein  120  is preferably a vein located in a limb of the patient, such as a vein selected from the group consisting of: saphenous vein; femoral vein; iliac vein; popliteal vein; brachial vein; basilic vein; cephalic vein; medial forearm vein; medial cubital vein; axillary vein; and jugular vein. Artery  130  is preferably an artery in a limb of the patient, such as an artery selected from the group consisting of: axillary artery; brachial artery; ulnar artery; radial artery; profundal artery; femoral artery; iliac artery; popliteal artery; carotid artery. 
         [0050]    Referring now specifically to  FIG. 3   a , a fistula creation device is positioned with its distal end near an intended skin puncture site and includes outer sheath  30  with distal end  31 . Outer sheath  30  is preferably constructed of a biocompatible catheter material, such materials and construction methods described in detail hereabove. Extending beyond distal end  31  is the distal end of a slidable needle assembly including needle tip  51  and guidewire lumen  53 . The needle assembly is preferably constructed of a metal or metal alloy such as stainless steel or Nitinol, and needle tip  51  is a sharpened, beveled tip. 
         [0051]    Referring now to  FIG. 3   b , needle tip  51  has been advanced through skin  105 , through artery wall  31 , and into the lumen of artery  130 . Distal end  31  of outer sheath  30  has also passed through skin  105  without any significant displacement between needle tip  51  and distal end  31 , either by a releasable fixation device, not shown but integral to the fistula creation device, or by stabilization of both the outer sheath  30  and the needle assembly by the operator. Referring now to  FIG. 3   c , needle tip  51  has been advanced out of artery  130 , through venous wall  121  and into the lumen of vein  120 . Similar to the advancement shown in  FIG. 3   b , there has been no relative displacement between needle tip  51  and distal end  31  of outer sheath  30 , such that distal end  31  has also advanced into the lumen of vein  120 . Guidewire  80  has been advanced from the proximal end of the fistula creation device, through guidewire lumen  53  and down the lumen of vein  120 . Referring now to  FIG. 3   d , the needle assembly has been retracted such needle tip  51  has moved proximal to outer sheath  30  distal end  31 , while leaving guidewire  80  deep seated into vein  120 . 
         [0052]    Referring now to  FIG. 3   e , a self-expanding anastomotic implant, clip  60 , is partially deployed out of distal end  31  of outer sheath  30 , such that two tensioning arms  61  and two stabilizing arms  62  have been released from being constrained within outer sheath  30 . Clip  60  has been partially deployed through one or more actions including: pushing clip  60  out of outer sheath  30  via advancement of a core contained within outer sheath  30 ; retracting sheath  30  such as while maintaining the longitudinal position of clip  60 ; or by a combination of these two actions. While clip  60  is partially deployed, the fistula creation device is retracted to a position wherein one or more of tensioning arms  61  are in firm contact with venous wall  121 , position not shown. Clip  60  is then fully deployed such as by retraction of sheath  30 , advancement of clip  60 , or a combination of the two actions. Referring now to  FIG. 3   f , clip  60  has been fully deployed such that fistula  110  is scaffolded by clip  60 , the four tensioning arms  61  placing vein  120  and artery  130  in tension at a location neighboring fistula  110 , and stabilization arms  62  stabilizing clip  60  in the vessels to prevent twisting or other clip  60  migrations. Clip  60  is preferably configured such that fistula  110  has an oval cross-section, with a major axis at least twenty percent greater than the minor axis of the oval. In a preferred embodiment, the major axis has a diameter larger than either vein  120  or artery  130 &#39;s luminal diameter. In an alternative embodiment, clip  60 &#39;s construction geometry causes fistula  110  to have a circular cross section. 
         [0053]    Shown in  FIG. 3   f , the fistula creation device is being withdrawn, such that distal end  31  of outer sheath  30  is almost removed from entering skin  105 . Guidewire  80  remains in place, such that one or more additional devices can be placed over-the-wire and easily access either the artery  130  or venous  120  side of fistula  110 . These subsequent over-the-wire devices, described in detail in reference to  FIG. 2   d , can be used to assess the fistula such as an ultrasound catheter to visualize the fistula, or a Doppler ultrasound catheter to measure fistula flow. The over-the-wire devices can be used to modify the fistula such as to modify the flow rate through the fistula, or to otherwise improve the therapeutic benefit of the fistula such as to increase the long-term patency of the fistula or to minimize adverse side effects of the fistula. In a preferred embodiment, an over-the-wire or other procedure is performed to measure flow through the fistula. If inadequate flow is determined, a flow modification procedure may be performed, such as an over-the-wire flow modification procedure utilizing an inflatable balloon or a tissue removing device to increase fistula flow. In a preferred embodiment, the inflatable balloon has a non-circular geometry which corresponds to a fistula created with a non-circular geometry. The balloon may be integral to the fistula creation device, or a separate over-the-wire catheter, and may be inflated to apply force to clip  60 , or a second implant, all not shown. Other flow modification procedure may also be performed, such as procedures which place implants, within or external to the flow path, to increase or decrease fistula flow to maximize therapeutic benefit and/or reduce adverse side effects. Other flow modification procedures that may be performed include application of an agent such as an anti-biotic, anti-thrombogenic or anti-proliferative agent, or delivery of energy such as radiation delivery to prevent neointimal growth. 
         [0054]    The method and device of  FIGS. 3   a  through  3   f  are used to create a fistula for therapeutic benefit such as to treat a patient with COPD. The fistula can be created for various purposes such as: increasing the oxygen content of venous blood supplying a lung of the patient, increase the oxygen content of arterial blood; and/or decreasing systemic vascular resistance. The fistula can be created for an acute period less than twenty-four hours, a sub-chronic period between twenty-four hours and thirty days, as well as for a chronic period longer than thirty days. The fistula preferably provides a flow of blood from the arterial system to the venous system of greater than 5 ml/min, typically greater than 50 ml/min. Clip  60  provides one or more functions including but not limited to: scaffolding an opening between the first vessel and the second vessel; reducing neointimal proliferation into the fistula flow path; preventing tissue from protruding into the fistula flow path; placing a portion of the first vessel wall in tension with the tissue of the second vessel wall; reducing bleeding of the tissue neighboring the fistula; enhancing healing of the tissue neighboring the fistula; and combinations thereof. While clip  60  has been described as a self-expanding device such as a resiliently biased Nitinol component, anastomotic implants that are plastically deformable or include both self-expanding sections and balloon expandable portions are also preferred. In an alternative embodiment, clip  60  includes a covering, such as a covering that surrounds the interior of the tissue within the fistula between the artery and vein lumens. The covering is a biocompatible material such as polytetrafluoroethylene; Dacron; Nitinol; stainless steel; or combinations thereof. In another alternative embodiment, clip  60  includes an agent, such as an agent that is eluded over time including anti-bacterial, anti-thrombogenic and/or anti-prolific agents. While, the method of  FIGS. 3   a  through  3   f  illustrate an artery to vein connection method, a vein to artery approach is also a preferred method of this invention. 
         [0055]    The device and method of  FIGS. 3   a  through  3   f  create an initial puncture through the skin of the patient, subsequently penetrating into and through the first vessel, and into the lumen of the second vessel. In an alternative, also preferred embodiment, not shown, a standard vessel introducer and sheath is utilized, making the initial puncture through the skin of the patient and into the first vessel. The fistula creation device distal end is then inserted into the sheath, passing through the skin and into the first vessel. The fistula creation device distal end exits the end of the sheath and further exits the lumen of the first vessel at the intended fistula location site by penetrating through the first vessel wall, and enters the lumen of the second vessel by penetrating through the second vessel wall. The intended fistula location may be proximate the site that the vessel introducer entered the first vessel, or at a location remote from this site, such as at a location greater than 20 mm from the first vessel entry site. Fistulas may be created remote from the first vessel entry site by intraluminally advancing the introducer sheath and/or the fistula creation device down the lumen of the first vessel prior to the distal end of the fistula creation device exiting the first vessel lumen and penetrating into the second vessel. 
         [0056]    Referring now to  FIGS. 4   a  and  4   b , a method and system for creating a fistula is shown in which an operator utilizes a fistula creation device and ultrasound visualization system in combination. A cross sectional view of a patient&#39;s anatomy at a proposed fistula location  111  is depicted wherein artery  130  is directly above vein  120  in relation to skin surface  105 . Intended fistula location  111  may be determined using one or more visualization techniques including but not limited to: angiography; venography; extra-vascular ultrasound; intravascular ultrasound; and MRI. Intended fistula location  111  may be determined using one or more flow measurement techniques such as Doppler ultrasound. The intended fistula location  111  may be selected based on parameters selected from the group consisting of: first vessel diameter; second vessel diameter; artery diameter; vein diameter; ratio of artery to vein diameter; distance between the artery and vein lumens; geometric relationship between the artery and vein lumens; distance from an arterial side branch; distance from an venous side branch; arterial flow; venous flow; oxygen content in artery; oxygen content in vein; wall thickness of artery; wall thickness of vein; degree of calcification of artery; degree of calcification of vein; geometric relationship between the artery and vein lumens at the fistula site; hemodynamic factors; other parameters; and combinations thereof. 
         [0057]    Artery  130  includes, in closest proximity to skin  105 , arterial wall  131 . Vein  120  includes, in closest proximity to artery  130 , venous wall  121 . While vein  120  and artery  130  are shown in a line relatively perpendicular to skin  105 , adjusting the orientation of outer sheath  30  can not only vary insertion angles, but also accommodate anatomies with vessels in a relatively side-by-side configuration (relatively equidistant from skin surface  105 ) as has been described hereabove in reference to  FIGS. 3   a  through  3   f . In a preferred method, the fistula creation device enters the skin at an angle relatively perpendicular to skin  105 . In another preferred embodiment, the fistula creation device penetrated the skin at an angle between 20 and 80 degrees relative to the surface of the skin  105 . Vein  120  is preferably a vein located in a limb of the patient, such as a vein selected from the group consisting of: saphenous vein; femoral vein; iliac vein; popliteal vein; brachial vein; basilic vein; cephalic vein; medial forearm vein; medial cubital vein; axillary vein; and jugular vein. Artery  130  is preferably an artery in a limb of the patient, such as an artery selected from the group consisting of: axillary artery; brachial artery; ulnar artery; radial artery; profundal artery; femoral artery; iliac artery; popliteal artery; carotid artery. 
         [0058]    Referring now to specifically to  FIG. 4   a , a fistula creation device is positioned with its distal end at an intended fistula site  111 . The fistula creation device includes outer sheath  30 , which is preferably constructed of a biocompatible catheter material, such materials and construction methods described in detail hereabove. Extending beyond distal end  31  of outer sheath  30  is the distal end of a slidable needle assembly including needle tip  51  and guidewire lumen  53 . The needle assembly is preferably constructed of a metal or metal alloy such as stainless steel or Nitinol, and needle tip  51  is a sharpened, beveled tip. Needle tip  51  has been advanced through skin  105 , through artery wall  31 , and into the lumen of artery  130  residing near intended fistula site  111 . Distal end  31  of outer sheath  30  has also passed through skin  105  without any significant displacement between needle tip  51  and distal end  31 , either by a releasable fixation device, not shown but integral to the fistula creation device, or by stabilization of both the outer sheath  30  and the needle assembly by the operator. 
         [0059]    Also shown in  FIG. 4   a  is ultrasound probe  71 , which is positioned relatively orthogonal to the surface of skin  105 , and preferably held by an operator, operator not shown, to provide a visual cross sectional image of intended fistula site  111 , as well as artery  130 , vein  120 , and any devices crossing through the imaging plane of ultrasound probe  71 . Ultrasonic coupling gel  74 , common to external ultrasound probe use, is first placed on the skin, in the area to be visualized, to enhance the image produced by ultrasound probe  71  via improved acoustic coupling between ultrasound probe  71  and skin  105 . Referring additionally to  FIG. 4   b , ultrasound probe  71  is attached to an ultrasound generator, not shown, as well as ultrasound monitor  72  which displays on imaging area  73  the cross sectional image associated with the imaging plane of  FIG. 4   a . Manipulation of probe  71 , either in the position of contact with skin  105  and/or the relative angle made with the surface of skin  105 , modifies the location of the imaging plane and the associated image displayed on monitor  72 . Ultrasound probe  71  and monitor  72  are used and manipulated by an operator as a system including the fistula creation device of the present invention. This system is used to assess and pre-determine the location of intended fistula location  111 . The visualization equipment is also used to view and confirm advancements of the percutaneous devices such as catheter, sheath, inner tube and guidewire advancements; confirm device locations such as device distal end (tip) locations; and assist in other preferred fistula creation steps of the present invention in which real-time visualization of the procedure can be made available to an operator. In an alternative, also preferred embodiment, as an alternative to or in conjunction with ultrasound probe  71 , an internal ultrasound probe provides an image to monitor  72 . The internal probe, not shown, is selected from the group consisting of: an ultrasound catheter such as a rotational or phased array intravascular ultrasound catheter; an inserted probe such as a transesophageal probe; and combinations thereof. 
         [0060]    In subsequent steps, not shown but similar to steps  3   c  through  3   f  hereabove, needle tip  51  will be advanced out of artery  130 , through venous wall  121  and into the lumen of vein  120 . A guidewire, not shown, will be advanced from the proximal end of the fistula creation device, through guidewire lumen  53  and down the lumen of vein  120 . The needle assembly will then be retracted such that needle tip  51  will be retracted proximal to distal end  31  of outer sheath  30 , while leaving the guidewire deep seated into vein  120 . An anastomotic implant, not shown, is deployed such as by retraction of sheath  30  while maintaining the position of the anastomotic implant. The anastomotic implant is configured such that the resultant fistula has an oval cross-section, with a major axis at least twenty percent greater than the minor axis of the oval. The fistula creation device is then withdrawn, such that distal end  31  of outer sheath  30  is almost removed from entering skin  105 . The guidewire preferably remains in place, such as through a standard vessel introducer, not shown. In a preferred embodiment, outer sheath  30  performs as the vessel introducer. Leaving the guidewire in place allows one or more additional devices to be placed over-the-wire and easily access either the venous or arterial side of the fistula. These subsequent over-the-wire devices, described in detail in reference to  FIG. 2   d , can be used to assess the fistula such as an ultrasound catheter to visualize the fistula, or a Doppler ultrasound catheter to measure fistula flow. The over-the-wire devices can be used to modify the fistula such as to modify the flow rate through the fistula, or to otherwise improve the therapeutic benefit of the fistula such as to increase the long-term patency of the fistula or to minimize adverse side effects of the fistula. In a preferred embodiment, an over-the-wire or other procedure is performed to measure flow through the fistula. If inadequate flow is determined, a flow modification procedure may be performed, such as an over-the-wire flow modification procedure utilizing an inflatable balloon or a tissue-removing device to increase fistula flow. In a preferred embodiment, the inflatable balloon has a non-circular geometry that corresponds to a fistula created with a non-circular geometry. The balloon may be integral to the fistula creation device, or a separate over-the-wire catheter, and may be inflated to apply force to clip  60 , or a second implant, all not shown. Other flow modification procedure may also be performed, such as procedures that place implants, within or external to the flow path, to increase or decrease fistula flow to maximize therapeutic benefit and/or reduce adverse side effects. 
         [0061]    Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.