Abstract:
A shunt rivet for implantation in the aorta and inferior vena cava to treat chronic obstructive pulmonary disease, and a method of treating chronic obstructive pulmonary disease.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 11/696,635 filed Apr. 4, 2007 which is a continuation-in-part of U.S. application Ser. No. 11/356,876 filed Feb. 17, 2006, which is a continuation-in-part of U.S. application Ser. No, 10/927,704 filed Aug. 27, 2004, each of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The inventions described below relate to treatments for pulmonary hypertension and vascular surgery. 
       BACKGROUND OF THE INVENTION 
       [0003]    Chronic obstructive pulmonary disease (COPD), chronic hypoxia, hypertension, and left ventricular hypertrophy and pulmonary hypertension are diseases of the cardiopulmonary system. Chronic obstructive pulmonary disease (COPD), which includes chronic bronchitis and emphysema, is a slowly progressive lung disease caused primarily by smoking. In COPD, the lungs are damaged and the airways are partly obstructed, making it difficult to breath and leading to a gradual loss of lung function. Symptoms of COPD include chronic cough, excessive sputum production, low blood oxygen levels and severe disabling shortness of breath, COPD represents the fourth leading cause of death in the United States. Chronic hypoxia (reduction of oxygen supply to the body despite adequate blood flow through the body), hypertension, and left ventricular hypertrophy are related conditions which may be symptomatic of COPD or coincident with COPD. 
         [0004]    These serious conditions affect many people, and the primary treatments are merely ameliorative. The primary treatments for COPD include avoidance of irritants such as tobacco smoke and breathing supplemental oxygen. In advanced cases of COPD, lung reduction surgery is sometimes performed, but it is not clear that it helps. There is no known cure for COPD. 
         [0005]    An aortocaval fistula (ACF) is a rare clinical condition that can be either spontaneous (80% of the cases), related to abdominal aortic aneurysm, or the result of some trauma such as lumbar disk surgery. It is currently seen as a defect that should be cured with surgery and, possibly, stem-graft implantation in the aorta. 
         [0006]    Contrary to this understanding, an intentionally formed aortocaval fistula appears to be a viable treatment for COPD. Recently, in our co-pending U.S. patent application Ser. No. 10/820,169 fried Apr. 6, 2004, entitled Implantable Arteriovenous Shunt Device and listing John L. Faul, Toshihiko Nishimura, Peter N. Kao &amp; Ronald G. Pearl as inventors (the entirety of which is hereby incorporated by reference), we propose creation of an artificial aortocaval fistula as a treatment for COPD, and we disclose the method of creating the fistula and an implantable shunt for maintaining the aortocaval fistula. 
         [0007]    Shunts or stents for connecting blood vessels have been proposed for the treatment of coronary artery disease. Makower, Device, System And Method For Interstitial Transvascular Intervention, U.S. Pat. No. 6,746,464 (Jun. 8, 2004) (filed Oct. 28, 1998) discloses a stent with a short tubular section spanning the thickness of a coronary artery and an adjacent parallel coronary vein. This stent includes “clovers” on either end of the stent, and these clovers fold radially outwardly to obstruct movement of the stent through the vessel walls. Two clovers on the proximal end of the stent are orthogonal (relative to the radial cross section of the stent) to two clovers on the distal end of the stent, and the interconnecting wires are parallel to the longitudinal axis of the device. 
       SUMMARY OF THE INVENTION 
       [0008]    The devices and methods described below provide for treatment of COPD, hypertension (e.g., pulmonary hypertension, cardiac hypertension, etc.), and left, ventricular hypertrophy, and chronic hypoxia. A vascular shunt rivet is disclosed which serves to hold contiguous points of the patient&#39;s aorta and inferior vena cava (or other arteries and there associated veins, such as the femoral artery and femoral vein, or the carotid artery and the carotid vein) together and maintain an open flow path from the aorta to the vena cava. The device functions as a rivet holding the two vessel walls in close proximity, and as a shunt, permitting and maintaining flow from one blood vessel to the other. The device is implanted, between the aorta and inferior vena cava, as a treatment for pulmonary hypertension, COPD and chronic hypoxia. 
         [0009]    The shunt rivet is provided in the form of an expandable wire frame structure adapted for transcutaneous delivery and deposit at the desired implantation site. The wire frame structure may be compressed into a small diameter configuration to fit within the distal tip of a delivery catheter. Upon expulsion from the catheter, the wire frame structure resiliently or pseudoelastically expands into a flow-through rivet comprising a tube with expanded heads at either end. When the rivet is released within an artificial fistula formed through the aorta and vena cava walls, it expands to trap the walls between the two expanded heads. The tubular section between the two expanded head may resiliently expand, and may also be balloon-expanded or otherwise plastically deformed to enlarge the flow-through lumen of the tubular section. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  illustrates the method of installing the shunt rivet to create and maintain an artificial aortocaval fistula. 
           [0011]      FIG. 2  illustrates an aortocaval shunt rivet in its restrained condition. 
           [0012]      FIG. 3  illustrates the aortocaval shunt rivet of  FIG. 2  in a resiliently expanded configuration. 
           [0013]      FIG. 4  is a perspective view of the aortocaval shunt rivet of  FIG. 2  in a resiliently expanded configuration. 
           [0014]      FIG. 5  illustrates the aortocaval shunt rivet of  FIG. 2  in a fully expanded configuration. 
           [0015]      FIGS. 6 through 11  illustrate the deployment of the aortocaval shunt rivet of  FIG. 2 . 
           [0016]      FIG. 12  illustrates an aortocaval shunt rivet with asymmetrically shaped distal and proximal flanges. 
           [0017]      FIG. 13  illustrates an aortocaval shunt rivet with asymmetrically shaped distal and proximal flanges. 
           [0018]      FIGS. 14 ,  15  and  16  illustrate an aortocaval shunt rivet with strut members that form diamond-shaped cells in the central section upon expansion. 
           [0019]      FIGS. 17 and 18  illustrates an aortocaval shunt rivet formed with a single wired wrapped to form the device. 
           [0020]      FIG. 19  shows a detail of the clinch member, illustrating radiopaque markers on the shunt rivet. 
           [0021]      FIGS. 20 and 21  illustrate a mandrel useful for forming and training/heat setting the shunt rivets. 
           [0022]      FIG. 22  is a perspective view of a shunt rivet in which the clinch members are biased to provide a pair of clinch members biased to close upon contiguous parallel portions of adjacent vessels while exerting slight pressure on circumferentially spaced points on the side walls of the adjacent blood vessels. 
           [0023]      FIG. 23  is a side view of the shunt rivet  22  showing the substantial closure of longitudinally oriented clinch members. 
           [0024]      FIG. 24  is a side view of the shunt rivet  22  showing the preferred angle of the transversely oriented clinch members relative to the axis of the device. 
           [0025]      FIG. 25  is a side view of the shunt rivet of  FIG. 22  showing transversely oriented clinches. 
           [0026]      FIG. 26  shows the shunt rivet of  FIGS. 22 through 25  installed between an artery and a vein, illustrating the construction of the device relative to the environment of use. 
           [0027]      FIG. 27  shows another variation of shunt rivet which may include varying lengths of the respective clinch members. 
           [0028]      FIG. 28  shows a partial cross-sectional view of another variation of a shunt rivet as deployed having clinch members of differing lengths. 
           [0029]      FIG. 29  shows a top view of another variation of a shunt rivet having an angled connector between the clinch members, which may also have differing lengths. 
           [0030]      FIG. 30  shows a partial cross-sectional view of yet another variation of a shunt rivet having an angled connector which may also be tapered along its length. 
           [0031]      FIG. 31  shows a partial cross-sectional view of yet another variation of a shunt rivet having hinges or flanges between the clinch members and the connector to adjust or change an angle between the shunt rivet and the vessels. 
           [0032]      FIG. 32  shows a top view of another variation of a shunt rivet having one or more break-away or frangible segments which may be integrated with the shunt rivet along a periphery of the connector. 
           [0033]      FIG. 33  shows a top view of another variation of a shunt rivet having one or more plastically deformable sections which may be integrated along the periphery of the connector. 
           [0034]      FIG. 34  shows a partial cross-sectional view of yet another variation of a shunt rivet having plastically deformable, elastically deformable, or break-away segments or portions along a length of the connector to adjust a length of the lumen through which blood is shunted. 
           [0035]      FIG. 35A  shows a partial cross-sectional view of yet another variation of a shunt rivet illustrating an example of an ordered sequence in which the clinch members may be deployed. 
           [0036]      FIGS. 35B and 35C  illustrate side views, respectively, of clinch members of a shunt rivet being deployed entirely within a vessel. 
           [0037]      FIG. 36  shows a side view of one variation of an instrument which may be used to adjust a length of the connector lumen. 
           [0038]      FIG. 37  shows a side view of another variation of an instrument which may be used to adjust an angle of the shunt rivet with respect to the vessels. 
           [0039]      FIG. 38  shows side and end views, respectively, of another variation of an instrument having an inflatable balloon which may be used to adjust a cross-sectional area of the shunt rivet to adjust the flow rate between the vessels. 
           [0040]      FIG. 39  shows a top view of a shunt rivet having an oval cross-sectional area which may be optionally adjusted. 
           [0041]      FIG. 40  shows a top view of another shunt rivet having a rectangular cross-sectional area. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]      FIG. 1  illustrates the method of installing the shout rivet to create and maintain an artificial aortocaval fistula. The patient  1  is shown with a delivery catheter  2  inserted into the left femoral artery/external femoral artery  3 L and pushed upwardly through the left common iliac artery  4 L to a point just above the aortic/iliac bifurcation in the distal abdominal aorta  5 . The inferior vena cava  6  runs parallel to the aorta, and typically is contiguous with the aorta. As shown in the illustration, the left femoral artery provides a nearly straight pathway to a suitable site of the artificial aortocaval fistula  7  within the abdominal aorta (the right femoral vein  9 R also provides a straight pathway to the same site on the vena cava side, and may be also be used as an access pathway). The fistula is created by forming a small hole or slit through the walls of both the aorta and the vena cava at immediately adjacent sites, and is maintained by inserting the shunt rivet  8  described below. The device may also be implanted via a route through the left femoral vein  9 L, or through the right femoral artery  3 R and/or right common iliac artery  4 R, though these pathways are not expected to be so readily navigable. The shunt rivet may also be installed in an artificial arterio-venous fistula formed between the femoral, vein and femoral artery on either side of the body, indicated as items  10 R and  10 L, or between the iliac artery and the femoral vein, and at locations in the aorta above the renal arteries. 
         [0043]      FIG. 2  illustrates the aortocaval shunt rivet  8  in its restrained condition, while  FIG. 3  illustrates the aortocaval shunt rivet of  FIG. 2  in its resiliency expanded configuration. The shunt rivet may be formed from a single tube  11  of resilient material, such as nitinol, spring steel, glass or carbon composites or polymers, or pseudoelastic (at body temperature) material such as nitinol or comparable alloys and polymers, by laser cutting several closed-ended slots  12  along the length of the tube (leaving the extreme distal and proximal edges of the tube intact) and cutting open-ended slots  13  from the longitudinal center of the tube through the distal and proximal edges of the tube. The open-ended slots are cut between each pair of closed-end slots to form a number of loops  14  joined at the center section by waist segments  15 . Though the shunt rivet illustrated in these figures can be made of several loops of wire welded together at the waist section, and many other fabrication techniques, manufacture from a single tube as illustrated has been convenient. 
         [0044]    After the tube is cut as described above, it is formed into its eventual resiliently expanded configuration illustrated in  FIG. 3 . In this configuration, the loops turn radially outwardly from the center section, and evert toward the center plane of the center section, thus forming clinch members  16  in the form of arcuate, everted, petaloid frames at either end of the loop, extending from the generally tubular center section formed by the waist segments  15 . For clarity, the term everted is used here to mean that the arc over which the petaloid frame runs is such that the inside surface of the device as configured in  FIG. 2  faces radially outwardly from the cylinder established by the tube.  FIG. 4  is a perspective view of the shunt rivet in the resiliency expanded configuration illustrated in  FIG. 3 , more clearly illustrating the relationship between the several petaloid frames at each end of the shunt rivet. 
         [0045]      FIG. 5  shows a side view of the aortocaval shunt rivet of  FIG. 2  in a fully expanded configuration. Even after the device has resiliently expanded to the extent possible given its impingement upon the walls of the aorta and the vena cava, the center section may be further expanded by plastic deformation. This may be accomplished by inflating a balloon within the center section, inflating the balloon, and expanding the center section beyond its elastic or super/elastic deformation range. By plastically deforming the center section of the shunt rivet, the center section becomes more rigid and able to withstand the compressive force of the walls of the aorta and vena cava. 
         [0046]    As illustrated, the construction provides several pairs of longitudinally opposed (that is, they bend to come into close proximity to each other, and perhaps but not necessarily, touch) and aligned (they are disposed along the same longitudinal line) distal and proximal petaloids. Overall, the petaloid frames of the distal section form a “corolla” (analogous to the corolla of a flower) flange or rivet clinch, which impinges on the vena cava wall and prevents expulsion into the aorta, and the petaloid frames of the proximal section form a corolla, flange or rivet clinch (this clinch would be analogous to a rivet head, but it is formed like the clinch after insertion of the rivet), which impinges on the aorta wall and prevents the expulsion of the shunt rivet into the vena cava, and the central section  17  forms a short length of rigid tubing to keep the fistula open. The resilient apposition of the two distal and proximal flanges or corollas so formed will securely hold the shunt rivet in place by resiliently clamping the walls of the aorta and vena cava (even over a considerable range of wall thickness or “grip range”). 
         [0047]    Referring to  FIGS. 2 through 5 , the shunt rivet may be manufactured with an overall initial length L of about 8 to 10 mm to obtain a grip range G of about 3 mm (given a typical aortic wall thickness of 2 mm and a typical inferior vena cava wall thickness of 1 mm at the target site), a clinch allowance C of at least about 3 mm (the clinch allowance is the distally protruding portion of a rivet that is turned over, curled or flattened to form the formed head), a formed or blind head allowance A of about 10-16 mm (we use the term blind head to refer to the distal head, which is the head that is formed on the blind side of the joint), a head diameter H of 5-16 mm, an initial shank diameter D 1  of 3-8 mm (in the resiliently expanded configuration, prior to plastic deformation), a final shank diameter D 2  of 5-12 mm to create a flow through lumen of about 5-10 mm diameter. The grip strength of the shunt rivet should provide for a slight compressive force exerted by the opposing clinch members on the intervening blood vessel walls. Thus, the shunt rivet is formed such that, in the resiliently expanded configuration, produces a grip strength in the range of 0.1 to 1.5 oz (about 3 to 45 gram-force) per clinch member upon the intervening blood vessels of the expected thickness. 
         [0048]      FIGS. 6 through 11  illustrate the method of releasing the shunt rivet so that the distal clinch members are released within the vena cava and the proximal clinch members are released within the aorta. Prior to insertion of the delivery catheter, the surgeon performing the implantation will image the aorta and inferior vena cava with appropriate fluoroscopic, ultrasonic, or other imaging methods, and create a pilot hole in the vessel walls with a crossing catheter. As shown in  FIG. 6 , the shunt rivet is housed within the distal tip of a delivery catheter  23 , and is entirely restrained within the delivery catheter. The delivery catheter includes an outer sheath  24 , a shaft  25  which is longitudinally slidable within the outer sheath, and a tapered or rounded tip  26  disposed on the shaft. The tapered may be mounted on a separate shaft, slidably disposed within the shaft  25 , so that it may be pushed through the prepared aperture while holding the remainder of the device steady within the aorta. The distal edge of the outer sheath may also be rounded or tapered, as shown. A distally facing shoulder  27  on the shaft, just proximal to the shunt rivet, serves to keep the shunt rivet in place longitudinally as the outer sheath is withdrawn. A guide wire lumen  28  may be provided in the shaft for use with a guide wire  29 , and may extend to the proximal end of the shaft for over-the-wire operation or may exit the shaft just proximal to the shunt rivet holding segment for monorail guidewire operation, and other guide wire configurations may also be used. A balloon  30  may be disposed on the shaft (and a suitable balloon inflation lumen provided in the shaft, and a suitable inflation pressure source in fluid communication with the lumen). 
         [0049]    As shown in  FIG. 7 , the distal tip of the delivery catheter is pushed through a small aperture in the walls of the aorta and vena cava (items  31  and  32 ) (the aperture is made by the operator, using a separate or integral punch, needle or lance) to create the artificial aortocaval fistula. After the distal tip has entered the vena cava, the outer sheath is pulled proximally to release the distal petaloids, as shown in  FIG. 8 . After the distal petaloids have reverted to their unrestrained configuration, the entire device is polled proximally to seat the distal petaloids against the inner wall of the vena cava. Prior to complete release of the shunt rivet, the operator should confirm that its location is acceptable (any suitable imaging technique may be used). To allow retraction in case the shunt rivet must be repositioned, a hook  33  protrudes radially from the shaft  25  and passes through a loop of the shunt rivet. This traps and secures the shunt rivet within the outer sheath  24  until the outer sheath is moved proximally to release the proximal clinch members, so that the operator may pull the shunt rivet back into the outer sheath in case its location, as visualized prior to complete release of the shunt rivet, is undesirable. Any other retaining means, such as a resilient or spring-loaded detent, a retractable pawl which engages a loop of the shunt rivet, of a retractable hook extending inwardly from the outer sheath, may be used in place of the illustrated hook. 
         [0050]    Then the outer sheath is pulled further proximally to release the proximal petaloids, as shown in  FIG. 9 . With the shunt rivet securely set in the artificial fistula, the center section may then be expanded by inflating the balloon as shown in  FIG. 10 . Upon withdrawal of the shaft, the shunt rivet remains in place to hold the two perforations in the blood vessel wall in apposition to each other to maintain the fistula, and to maintain an open shunt pathway between the aorta and vena cava, as shown in  FIG. 11 . 
         [0051]    The final form of the shunt rivet is, according to the above description, accomplished with the method that includes forming the generally tubular structure having a central section with a first diameter, a proximal clinch section defined by one or more clinch members, and a distal clinch section defined by one or more clinch members, training the proximal and distal clinch members to make them resiliently biased to bend radially outwardly from the central section; then resiliently compressing the tubular structure to maintain a generally tubular shape and restraining the compressed tubular structure in a compressed configuration suitable for percutaneous insertion into the body; inserting the structure through apposing apertures in the aorta wall and vena cava wall of a patient such that the distal clinch members protrude into the vena cava of the patient and the central section is disposed within the apertures; and then releasing the distal clinch members to permit resilient expansion of the distal clinch members followed by expanding the central section through plastic deformation to larger diameter and releasing the proximal clinch members to permit resilient expansion of the proximal clinch members (the proximal clinch members may be released before or after expansion of the central section). 
         [0052]    The shunt rivet illustrated above may be modified as shown in  FIGS. 12 and 13 , which show an aortocaval shunt rivet with asymmetrically shaped distal and proximal flanges. In  FIG. 12 , the shunt rivet  35  is similar to the shunt rivet of  FIGS. 2 through 4 , and includes the central section, the distal flange comprised of multiple petaloid wire-frame members  16   d , and the proximal flange comprised of multiple petaloid wire-frame members  16   p . In this embodiment, the distal corolla is horn-shaped, “salverform” or “funnelform” (as those terms are used in botany), with the petaloids arcing outwardly without everting (without a substantial arc in the proximal direction), while the proximal corolla is perianth-like, arcing outwardly and everting with a substantial arc in the distal direction. Each petaloid is significantly reflexed, like the perianth of a narcissus cyclamineus.  FIG. 13  illustrates another embodiment of the aortocaval shunt rivet with asymmetrically shaped distal and proximal flanges. In  FIG. 13 , the proximal petaloids are highly reflexed, and evert to form pigtails with an arc of over 180°, and preferably, as illustrated, an arc in excess of about 270°, such that the proximal petaloids bend radially inwardly toward the tips  36  to present a length of wire  37 , rather than the tip of the petaloids, for impingement on the blood vessel wall. One or both of the distal or proximal petaloids/clinch members may be modified to form the pigtails illustrated in  FIG. 13 . In the embodiments shown, the petaloids are gamopetalous (with the petals united by their margins, at least at the base, as in  FIG. 2  et seq.), but they may also be polypetalous as shown below  FIGS. 14 ,  15  and  16 . The embodiments shown are also actinomorphic, though they may be constructed in zygomorphic fashion with asymmetrical petaloids. 
         [0053]      FIGS. 14 ,  15  and  16  illustrate an aortocaval shunt rivet  8  with diamond shaped strut members in the central section. This shunt rivet provides a central section  17  with a series of expandable loops joined by circumferentially oriented struts  38 .  FIG. 14  illustrates a tube  11  with numerous slots cut into it to form the shunt rivet shown in  FIG. 16 . Slots  12  are closed-end slots, leaving a loop  14  extending from the central section  17  to form a clinch member cell  39 . Slots  40  are open or closed-end slots extending from the center of the device, leaving small circumferential struts  41  connecting adjacent cells of the device. Slots  42  are open or closed-end slots extending from the center section of the device, leaving larger waist sections  43  connecting the circumferential struts with adjacent clinch member cells of the device. Slots  44  are closed-end slots extending through the waist sections. As shown in  FIG. 15 , some waste area (segments intended to be removed)  46  shown in  FIG. 14  are cut away and discarded, leaving expandable waist section cells  47  and clinch cells  39 , interconnected by the circumferential struts  38 . Though the device is illustrated with three clinch members on each end, the number of clinch members formed in the shunt rivet may be varied. The waist section cells and clinch member cells, can, as shown at  48  share struts which define contiguous cells. As shown in  FIG. 16  the waist section cells, when expanded, form the diamond shaped cells of the central section. The clinch member cells comprise petaloid cells which may be described as lanceolate (narrow and tapering to an apex (though the apex is preferably blunt)), or ovate (having a broad base and narrow tip) rather than reniform or orbicular. The tip of the petaloid is preferably obtuse, rounded or blunt. As can be appreciated from  FIG. 16  the clinch members may also be described as longitudinally extending wires which connect the longitudinally tips of adjacent waist section cells. 
         [0054]      FIGS. 17 and 18  illustrate an aortocaval shunt rivet  51  formed with a single wired wrapped to form the device. In this device, a single wire has been wrapped around a specially formed mandrel to form a number of clinch members  52  on one end of the device and a number of clinch members  53  on the other end of the device. As illustrated, each clinch member is slanted relative to the radius of the device, and the wires forming the waist segment of the device are also oblique to the longitude of the device. As viewed from the top, each cinch member comprises a substantially circular arc, and the wire continues from the arc longitudinally toward the opposite end of the device, forming straight waist segment  54  where it runs substantially parallel to the long axis of the device until it arcs circumferentially away from the previous arc to form the clinch member on the opposite end, whereafter it loops around to extend retrograde relative to the circumference, forming waist segment  55  running obliquely relative to the long axis, and back toward the first end of the device until it curves again circumferentially forward to form the loop of the next clinch member circumferentially adjacent the first loop and longitudinally in line with the immediate previously formed clinch member on the opposite end of the shunt rivet, and continues in this fashion until the entire tubular structure of the device is achieved. In tracing its path, the wire may cross over one or more other portions of the wire. 
         [0055]      FIG. 19  shows a detail of the clinch member, illustrating radiopaque markers on the shunt rivet. A radiopaque marker may be provided in the form of a radiopaque rivet  61  disposed near the tip of the clinch member  16 , or it may be provided in the form of a wrapped coil of radiopaque wire or thread  62 . The radiopaque markers may be comprised of platinum, iridium, tantalum, barium sulfate or other radiopaque materials. Similar markers may also be applied to the waist section. The marker material may also be selected to enhance visibility under ultrasound imaging, magnetic resonance imaging, or other suitable imaging techniques. 
         [0056]      FIGS. 20 and 21  illustrate mandrels or dies useful for forming and training/heat setting the shunt rivets. As shown in  FIG. 20 , a two-part mandrel comprises a distal mandrel portion  63  and a proximal mandrel portion  64 . Each mandrel is shaped to correspond to the desired final shape of the shunt rivet and its clinch members. The mandrel portions are inserted into the tube, after it has been cut, so as to deform the device. Where the device is formed from a pseudoelastic material that must be heat set or trained, the mandrels are dimensioned to deform the device to its desired open configuration. Where the device is formed of spring steel or the like, the mandrel is dimensioned to bend the clinch members beyond the desired final configuration. Thus, the mandrel of  FIG. 20  and the mandrel of  FIG. 21 , though shaped differently, may be used to form quite similar shapes for devices made of nitinol and spring steel. The mandrel shapes may be modified as desired to achieve various clinch member shapes, such as the asymmetrical shapes shown in  FIGS. 12 and 13 . 
         [0057]    The shunt rivet may be modified as shown in  FIGS. 22 through 25 .  FIG. 22  is a perspective view of a shunt rivet  65  in which the clinch members are biased to provide pairs of clinch members  66   a  and  66   v  biased to close upon contiguous parallel portions of adjacent vessels and a pair of clinch members  67   a  and  67   v  biased to exert slight pressure, and establish slight compliance mismatch, on circumferentially spaced points on the side walls of the adjacent blood vessels. Each clinch member is slit down the center to allow radially expansion of the device through radial deformation of the clinch member. 
         [0058]      FIG. 23  is a side view of a shunt rivet of  FIG. 22  showing the substantial closure of longitudinally oriented clinch members  66   a  and  66   v . These clinch members are formed to evert, such that the tips of opposing clinch members  66   a  and  66   v  are closely proximate each other when released (in the expanded configuration shown). A short segment at the distal tip of each clinch member is turned away from the transverse midline  68  of the device to form an atraumatic bearing surface for impingement on the blood vessels walls. As illustrated, the clinch members  66   a  and  66   v  comprise a continuously formed clip, with no intervening waist segment between the arterial portion of the clip and the venous portion of the clip. The clip resembles a tool clip, as that term is used in other arts. Preferably the clinch members making up the tool clip are joined directly together, without an intervening rectilinear base (though a rectilinear base may be incorporated if desired to accommodate the anatomy of the arterio-venous fistula in a particular site), to create a smoothly arcuate transition from the distal clinch member to the proximal clinch member.  FIG. 24  is a side view of the shunt rivet  22  showing the preferred angle of the transversely oriented clinch members  67   a  and  67   v  relative to the axis  70  of the device. In this embodiment, the transversely oriented clinch members  67   a  and  67   v  (both the near and far pairs are visible in this view) are set at a small angle from axis  70 . In the unrestrained configuration, the clinch members  67   a  on the arterial side of the device (typically the first side of the device to be released from the catheter given the preference for transvenous delivery) are inclined toward the upstream or retrograde direction. Clinch members  67   v  on the venous side of the device are inclined toward the upstream or retrograde direction within the vein. This configuration facilitates release of the device from the small delivery catheter used to insert it into a fistula. 
         [0059]      FIG. 25  is a side view of the shunt rivet of  FIGS. 22 through 24  showing transversely oriented clinch members  67   a  and  67   b  with substantial spacing between the tips of the clinch members (in the expanded configuration shown). Also, clinch members  67   a  and  67   b  constitute a continuously formed tension spring (shaped substantially like the tension spring used in window frames, having an arcuate or bow shape, with the ends arcing outwardly from the axial centerline  70  of the device and adapted to impinge upon or exert force on the blood vessels and the middle of the arch adapted to exert force on the remainder of the shunt rivet to which it is fixed), with no intervening waist segment between the arterial portion of the tension spring and the venous portion of the tension spring, and the tension spring formed to impinge on the sidewall of the artery or vein at a point circumferentially displaced from the center of the rivet without deforming the artery and/or vein walls to bring the opposite tips  69   a  and  69   v  into apposition such as that achieved by the tips of the tool clips. A short segment at the distal tip of each clinch member is turned away from the axial centerline  70  of the device to form an atraumatic bearing surface for impingement on the blood vessel walls. 
         [0060]    The device may thus be described, in their open and unconstrained conditions, as comprising two parallel tool clips secured at their closed ends to two parallel tension springs, at the midpoints of the tension springs, to create an orthogonal or cruciform grouping of alternating spring clips and tension springs. Adopting the botanical language used for other embodiments, each side of the device comprises a pair of petaloids arcing outwardly from the axial centerline of the device without everting (without a substantial arc in the proximal direction), and a pair of petaloids arcing outwardly and everting with a substantial arc in the distal direction, with corresponding petaloid structures being joined at their proximal ends without an intervening waist segment. Each petaloid is formed in an open frame V-shape. Though illustrated with a pair of clips and a pair of tension springs, the device may be formed with additional tension springs or clips, as dictated by the local anatomy of a particular installation. In appropriate anatomical conditions, the device may comprise four clips in the tool clip configuration, or the comparable everting petaloid pairs (in which all clinch members evert substantially to close upon the vessel wall), arranged orthogonally, where the tool clips are arranged in a circular arrangement with the closed end of each clip being secured to the closed and of an adjacent clip, such that the open end of each tool clip is directed outwardly from the circular arrangement. The device may also include additional arcuate tension springs and/or tool clip portions, thus departing from the cruciform configuration shown while achieving the benefit of substantial spacing of the vessel contacting tips from the arterio-venous fistula. 
         [0061]      FIG. 26  shows the shunt rivet of  FIGS. 22 through 25  installed between an artery  71  and vein  72 , in order to illustrate the construction of the device relative to the environment of use. The tips of the “tool clip” portion of the device ( 66   a  and  66   b ) close upon points in the respective vessels  73   a  and  73   v  which are longitudinally spaced (relative to the blood vessels) from the arterio-venous fistula formed in which the device is placed. The points of impingement are significantly spaced from the fistula, as illustrated. The tips of the tension spring portion ( 67   a  and  67   v ) of the device impinge on circumferentially spaced points  74   a  and  74   v . As shown in  FIG. 26 , the circumferential points of impingement are significantly spaced from the fistula. The circumferential spacing is preferably 30° to 90°, but may be adjusted to fit local anatomy. In this manner, the shunt rivet avoids engagement of the blood vessels adjacent the fistula. As shown in  FIG. 26 , the ultimate shape of the installed shunt rivet may vary from the unrestrained shape due to the remaining constraint of the blood vessel walls, though the device is biased to resiliently or superelastically return to the unrestrained shapes of  FIGS. 22 through 25 . After installation, the shunt rivet holds the adjacent artery and vein together and maintains an open flow path through opening defined by the roughly circular arrangement of the clips and tension springs. Should the arrangement appear to be somewhat squared or angular, pentagonal, hexagonal, etc., given the particular geometries of the various parts, it is intended that such departures from perfect circular arrangement be included under the description of a circular arrangement. 
         [0062]    Yet another variation for the shunt rivet may include varying a length of the respective clinch members. As illustrated in the perspective view of  FIG. 27 , shunt rivet  80  may include the longitudinally oriented clinch members  81   a ,  81   a ′ and  81   v ,  81   v ′ positioned opposite to one another and transversely oriented clinch members  82   a  and  82   v  positioned transverse relative to an axial centerline of shunt rivet  80 , as described above. In this variation, clinch members  81   a  and  81   v ′ may be sized to have a length which is less than clinch members  81   a ′ and  81   v , as described in further detail below. The respective lengths of clinch members  81   a ,  81   v ′ relative to  81   a ′,  81   v  may be variably sized to maximize or optimize the stability of shunt rivet  80  with respect to the vessels when deployed between adjacent vessels. 
         [0063]    Moreover, varying the lengths of the respective clinch members may further provide additional advantages. For instance, the clinch members which are shortened in length may facilitate the positioning and securement of the shunt rivet between the vessels by allowing for the relatively shorter member to swing into position within the vessel lumen during deployment, as described in further detail below. Moreover, a shorter member may provide for a minimized implant size when placed against the vessel interior wall for securement as well as a mitigating any physiologic reaction to the implant, e.g., a reduction in thrombosis, etc. Additionally, clinch members which are lengthened relative to other members may provide for increased shunt stability by increase the amount of force applied against the tissue walls. 
         [0064]    Moreover, clinch members having different lengths may additionally place the adjacent vessels in tension such that the vessel walls are drawn towards one another and the clinch members  81   a ,  81   a ′ and  81   v ,  81   v ′ contact the vessel luminal walls to stabilize not only the shunt rivet within the vessels but also the vessels with respect to one another. Additionally, having one or more clinch members  81   a ,  81   v ′ sized to have a length shorter than its respective apposed clinch member may also facilitate the deployment and/or positioning of the clinch members  81   a ,  81   v ′ within the vessel since the shorter length clinch members can more easily “swing” through an arc within the vessel lumen without contacting the interior walls. Clinch members with differing lengths may further be configured to align along different planes when deployed to facilitate vessel separation, if so desired. 
         [0065]    As above, each of the clinch members may be formed without an intervening waist segment between the arterial portion of the shunt rivet  81   a ,  81   a ′ and the venous portion of the shunt rivet  81   v ,  81   v ′. As also previously described, the clinch members may be joined directly together, without an intervening rectilinear base (though a rectilinear base may be incorporated if desired to accommodate the anatomy of the arterio-venous fistula in a particular site), to create a smoothly arcuate transition from the distal clinch member to the proximal clinch member. 
         [0066]    Aside from the variable length clinch members, shunt rivet  80  may further define one or more slots  83  along the length of the clinch members, such as at the terminal ends of each clinch member. The one or more slots  83  may be formed or cut, e.g., by a laser, to provide a region through which a radio-opaque marker or wire, such as tantalum wire or any other radio-opaque material as described herein, may be passed through to facilitate imaging during deployment. Shunt rivet  80  may also further include an optional radio-opaque center marking band about the center of rivet  80  to indicate the center, e.g., when viewed under fluoroscopy or any other imaging modality. Additionally, one or more of each clinch member may also optionally include a slot  84  defined along a length of the individual respective clinch member struts, as shown, to further function as a stress-relieving slot. 
         [0067]    Although shunt rivet may be formed without an intervening waist member, it may be optionally included. As shown in the illustrative partial cross-sectional view of FIG.  28 , another variation of shunt rivet  93  may be seen deployed between two respective vessels, artery  90  and vein  91 . Clip connector  99  may extend between sets of clinch members  95 ,  96  and  97 ,  98  while defining lumen  94 . Although the transverse clinch members have been omitted from the illustration for clarity, they may be optionally omitted from the shunt rivet entirely, if so desired. In its deployed configuration when placed through fistula  92  defined between vessels  90 ,  91 , lumen  94  may define a flow path between the vessels, as described above. In this variation, clinch members  96 ,  97  are shortened in length relative to the lengths of clinch members  95 ,  98 . The shortened clinch members  96 ,  97  may be configured to be deployed on opposite ends of the shunt rivet such that shortened clinch member  96  is disposed within artery  90  while shortened clinch member  97  is disposed within vein  91  and extends in a direction opposite to that of clinch member  96 . Shortened clinch members  96 ,  97  may be similar in length and configuration or they may be varied in length relative to one another. 
         [0068]    Likewise, clinch member  95  may be disposed in artery  90  while clinch member  98  is disposed in vein  91  such that they extend in opposing directions and are positioned opposite to their respective shortened clinch members. Like their shortened counterpart members, clinch members  95 ,  98  may be similar in length and configuration or they may also be varied in length relative to one another. Clinch members with differing lengths may be utilized in any of the variations described herein in combination with additional features, as described. 
         [0069]    In addition to having clinch members of different lengths, the connector member itself may be modified such that its extends between the respective clinch members at an angle relative to a centerline of the shunt rivet, as illustrated by angled connector  100  in the top view of  FIG. 29 . The angled connector  100  may be configured over a number of various angles such that the blood flow between the vessels  90 ,  91  through angled connector  100  avoids a 90° turn. 
         [0070]    In yet another variation, angled connector  110  may be further modified such that the cross-section of the connector is tapered along its length, as shown in the partial cross-sectional view of  FIG. 30 . Accordingly, in addition to having clinch members of various lengths and an angled connector, the connector  110  and/or connector lumen  94  may be tapered or it may define a non-constant cross-sectional area along its length. For instance, the connector lumen  94  may be tapered such that the cross-sectional area increases as the connector  110  extends from the arterial vessel  90  to the venous vessel  91 , as shown. Alternatively, the cross-sectional area may decrease as the connector  110  extends away from the arterial vessel  90 . 
         [0071]    In yet a further variation, the shunt rivet may optionally include a hinge or flange  111  connecting one or more of the clinch members to the connector  110 , as shown in  FIG. 31 . Such a hinge or flange may be adjustable to change an angle at which connector  110  extends between the clinch members and may utilize any number of hinging mechanisms. For instance, hinge or flange  111  may simply comprise a plastically deformable portion of the shunt rivet or it may be a mechanically hinged mechanism, e.g., which provides for frictional engagement between the clinch members and the connector  110  to maintain its position yet also allows for adjustment. The hinge or flange  111  may be adjusted prior to deploying the shunt rivet such that the clinch members extend at their predetermined angle when deployed. Alternatively, hinge or flange  111  may be adjusted during deployment or after the shunt rivet has been placed between the vessels  90 ,  91  by using an inflatable balloon instrument or other expandable tool. In yet another alternative, the hinge or flange  111  may be adjusted both before deployment and during or post deployment into the vessels. For example, post deployment adjustments may be accomplished anytime, e.g., within one hour of shunt deployment, or alternatively in a subsequent procedure, e.g., prior to or after thirty days of deployment within a patient. 
         [0072]    Another variation may utilize one or more break-away or frangible segments  120  which may be integrated with the shunt rivet along a periphery of connector  99 , as illustrated in the top view of  FIG. 32 . In this example, two break-away segments  120  may be integrated on either side of connector  99  such that when the shunt rivet has been positioned or during positioning into the vessels, connector  99  may be adjusted in size, e.g., by expanding the opening via a balloon instrument, to allow for a greater flow through the shunt rivet. The break-away segments  120  may be comprised of a number of different biocompatible materials which may be dissolved into the blood or they may be configured as opposing portions of connector  99  which are overlapped or otherwise held temporarily to one another. 
         [0073]    Alternatively, segments  120  may be comprised of plastically deformable bands which break apart to adjust or allow for the adjustment of the cross-sectional area of the connector  99 . The adjustability of the connector cross-section may allow for the shunt rivet to change from a circular cross-sectional area to an oval cross-sectional area. In the same manner, the cross-sectional area may be changed from an oval area to a round area. The adjustment of the cross-sectional area utilizing the break-away segments  120  may be performed pre-implantation, during implantation, or post implantation of the shunt rivet into the vessels. 
         [0074]    Another variation is shown in the top view of a shunt rivet in  FIG. 33  which utilizes one or more plastically deformable sections  130  which may be integrated along the periphery of connector  99 . As shown, the plastically deformable sections  130  may be plastically deformed, e.g., via an inflatable balloon, either prior to, during, or post deployment to adjust the cross-sectional area of connector  99 . Moreover, plastically deformable sections  130  may be integrated into connector  99  such that when connector  99  is expanded or deformed, sections  130  plastically deform and retain their deformed configuration when a deforming force is removed. 
         [0075]    Aside from variations in adjusting the cross-sectional flow area of the shunt rivets, other optional variations may be incorporated in any of the shunt rivets described herein. For instance,  FIG. 34  shows a partial cross-sectional side view of yet another variation which may utilize plastically deformable, elastically deformable, or break-away segments or portions  140  along a length of connector  110  to adjust a length of the lumen through which blood is shunted. The portions  140  may be utilized along a length of connector  110  to allow for adjustment of the distance between the vessels  90 ,  91  and they may be utilized with a connector length which is uniform in diameter or which is tapered or narrowed, as described above. 
         [0076]    In utilizing a shunt rivet having different clinch member lengths, shorter length clinch members can more easily “swing” through an arc within the vessel lumen without contacting the interior walls, as mentioned above. Accordingly, such a shunt rivet may be implanted such that the clinch members are deployed in an ordered sequence. In one example, once a needle has been passed through the tissue wall to cross between vessels  90 ,  91 , guidewire  30  may be advanced intravascularly through the needle which may then be removed leaving guidewire  30  passing through vessels  90 ,  91 . Shaft  25  and/or outer sheath  24  may be advanced through vessel  90  over or along guidewire  30  to follow guidewire  30  into vessel  91 , as shown in  FIG. 35A . Shaft  25 , described above, may be fabricated with a stiffened tip, such as polyimide, to facilitate crossing between vessels. Once properly positioned within vessel  91 , outer sheath  24  may be pulled proximally while tracking its distal end visually via a marker band  152  until clinch member  98  is first released from the constraints of outer sheath  24  and allowed to reconfigure itself into its angled configuration, relative to a longitudinal axis of the shunt rivet. The individual clinch members of the shunt rivet may be optionally retained via anchoring pins  153  integrated with shaft  25  which may hold the clinch members in place as outer sheath  24  is retracted. These anchoring pins  153  may also serve to prevent or limit the motion of the shunt rivet itself until outer sheath  24  has been fully retracted. This particular configuration may be utilized in situations where a clinician may wish to re-sheath the shunt rivet, e.g., for abandoning a procedure or for repositioning the shunt rivet, etc. 
         [0077]    With outer sheath  24  pulled further proximally, shortened clinch member  97  may be subsequently released. With its shortened length, relative to clinch member  98 , clinch member  97  may fully deploy and arc  151  entirely within vessel  91  without interfering or contacting the distal region of the vessel wall until clinch member  97  comes into contact against the proximal region of the vessel wall. With clinch members  97 ,  98  fully deployed within vessel  91 , outer sheath  24  may be further withdrawn relative to shaft  25  to subsequently release shortened clinch member  96 , which may then arc  150  entirely within adjacent vessel  90  to contact the tissue surrounding the fistula. Subsequently, outer sheath  24  may be fully retracted to release clinch member  95  to allow it to come into contact against the tissue wall within vessel  90 , thereby fully deploying the shunt rivet between vessels  90 ,  91 . The shunt rivet may be partially deployed from shaft  25  and optionally removed and/or re-positioned and re-deployed elsewhere within the body. 
         [0078]    Another example is illustrated in  FIGS. 35B and 35C , which illustrate side views of clinch members of a shunt rivet being deployed entirely within a vessel. As shown in  FIG. 35B , once the assembly has been advanced intravascularly through vessel  90 , e.g., an artery, and the needle and guidewire advanced from within vessel  90  and into adjacent vessel  91 , e.g., a vein, shaft  25  carrying the shunt rivet may be advanced at least partially from outer sheath  24  (alternatively, outer sheath  24  may be retracted relative to shaft  25 ) to expose the transversely oriented clinch members  154 ,  156  and the clinch members  97 ,  98  for expansion and/or reconfiguration within vessel  91 . As shown, clinch member  97  may reconfigure from a low profile configuration where clinch member  97  is positioned to extend distally along shaft  25  during delivery to a configuration where member  97  swings proximally within vessel  91 , as shown, to a securement configuration. 
         [0079]    One or more members can be deployed and by advancing the outer sheath  24  (and/or retracting shaft  25  relative to outer sheath  24 ), the members can be recaptured and at least partially re-sheathed to allow for removal and/or repositioning of the shunt rivet. Once desirably repositioned, the clinch members may be fully deployed into position. 
         [0080]    As further illustrated in this example, the lengthened clinch member  98  may engage against the vessel wall within vessel  91  during deployment. If excessive pull force is applied to the shunt rivet, member  98  can deform and straighten while deflected by the walls of vessel  91 , as illustrated by deformed tissue  158 , so as to prevent or inhibit damage to the surrounding tissue. A clinician can visually assess, e.g., via fluoroscopy or ultrasound, the wall-to-shunt engagement by gauging the amount of deflection indicated by the lengthened clinch member  98 . Along with the tactile feedback perceived by the clinician, the visual indication of the clinch member deformation may further aid in confirming suitable shunt rivet positioning. 
         [0081]    Once the position of the shunt rivet has been confirmed within vessel  91 , clinch member  98  may be fully deployed and clinch member  97  may be fully deployed to swing proximally into its securement position within vessel  91 , as shown in  FIG. 35C . The remaining clinch members may be subsequently released from outer sheath  24  and shaft  25  to be deployed within vessel  90 . 
         [0082]    In delivering and configuring the shunt rivets described above, additional delivery instruments may be utilized to facilitate adjustment of the shunt rivets to a desirable configuration. For instance, adjusting the cross-sectional area of the connector portion of the shunt rivet or adjusting a length of the connector lumen between the clinch members, or adjusting an angle of the shunt rivet and clinch members with respect to the vessel lumens, etc., may be accomplished with instruments as shown in  FIGS. 36 to 38 . 
         [0083]      FIG. 36  illustrates one variation of an instrument  160  which may be used to adjust a length of the connector lumen. Instrument  160  may generally comprise an outer sheath  161  having an inflatable balloon or expandable member  162  disposed around a distal portion of the sheath  161 . Inner sliding core  163 , upon which the shunt rivet may be disposed upon or over, may be slidingly disposed within outer sheath  161  and may also have an inflatable balloon or expandable member  164  also disposed around a distal portion of core  163 . As mentioned above, with the shunt rivet disposed upon sliding core  163 , inflatable members  162 ,  164  may be expanded to temporarily engage the respective clinch members to lengthen or extend the connector length between the clinch members of the shunt rivet, e.g., to accommodate vessel separation distances, either prior to, during, or post implantation of the shunt rivet within the vessels. Alternatively, with members  162 ,  164  expanded, the connector length may be shortened between the clinch members. 
         [0084]    The ability to adjust the length of the connector may allow for not only accommodating for the distance between the vessels, but also to “fine-tune” a flow rate of the blood through the shunt rivet to achieve a desired therapeutic result and/or to mitigate any side effects of the fistula. Moreover, although the adjustment to the shunt rivet may be done intra-operatively in vivo, adjustments may also be performed prior to insertion within the patient body. Moreover, an electronic or mechanical gauge or markers (such as visual markings or radio-opaque markers) may be integrated with the instrument  160  to provide feedback to the user as to the length that the shunt rivet is shortened or lengthened. 
         [0085]    An example of another instrument which may be used to adjust an angle of the shunt rivet with respect to the vessels is shown in  FIG. 37 . As above, an inner sliding core  163  may be translatably positioned within outer sheath  161 . A pull wire  165  may have a fixation point  166  near or at a distal end of sheath  161  and may be routed through outer sheath  161  and articulated  169  to adjust an angle of outer sheath  161  with respect to a longitudinal axis of sheath  161 . Likewise, inner core  163  may also have a separate pullwire  167  with a fixation point  168  near or at a distal end of inner core  163  to adjust  169 ′ its angle with respect to a longitudinal axis of inner core  163 . Sliding core  163  and outer sheath  161  may both be articulated independently of one another to create multiple bending configurations. In this manner, a shunt rivet disposed within outer sheath  161  and/or upon sliding core  163  may be bent or curved into various configurations by the forces imparted upon the shunt rivet to adjust its angle with respect to the clinch members and vessels. 
         [0086]    Such an instrument may be utilized to adjust not only an angle of, e.g., connectors between the clinch members, but also the hinge or flange  111  as well as other portions of the shunt rivet variations described herein. Moreover, the instrument may be utilized to plastically deform the portions of the shunt rivet. One or more radio-opaque markers may be included on the instrument to visually indicate an angle of the instrument. An additional and/or alternative variation may further include an instrument which is used to deform the tissue neighboring the fistula site in the same manner as adjusting angles, distances, etc., of the shunt rivet. The shunt rivet may also be plastically deformed or it may be simply elastically deformed to accommodate the tissue shape changes. Additionally, the instrument may further include a mechanical or electronic gauge to indicate the degree of force imparted on the shunt rivet as well as relaying other information during or post deployment. 
         [0087]    Yet another feature of a deployment instrument is shown in the partial cross-sectional side and end views, respectively, in  FIG. 38 . As shown, inflatable end effector  170  may include an inflation balloon  173  in fluid communication with an inflation lumen  172  which is disposed near or at a distal end of a delivery shaft  171 . A shunt rivet may be disposed proximate to, upon, or distal to inflation balloon  173  in its deflated state for delivery into the vessels. Prior to, during, or post deployment of the shunt rivet into the vessels, inflation balloon  173  may be inflated to adjust a cross-sectional area of the shunt rivet to adjust the flow rate between the vessels, e.g., up to 5 mm or more diameter and as described above in the shunt rivet variations. Inflation balloon  173  may be configured to have a circular cross-sectional area such that expansion within the shunt rivet may adjust the shunt to have a corresponding circular cross-sectional area. Alternative variations of the inflation balloon  173  may include balloons having non-circular cross sections, e.g., such as an oval cross section with adjustable major and/or minor axes, as shown in the end view of  FIG. 38 , to optionally adjust a shunt rivet cross section accordingly. Other non-circular cross-sectional areas may be utilized, e.g., polygon, trapezoid, triangle, rhombus, rectangle, square, parallelogram, etc., to optimize a flow through the fistula and to vary or optimize an effective flow diameter through the shunt rivet and between the interconnected vessels. 
         [0088]      FIG. 39  illustrates a top view of a shunt rivet having an example of an oval cross-sectional area  180 , which may be optionally adjusted via the one or more instruments above. Another non-circular cross-sectional area is illustrated in  FIG. 40 , which shows a top view of a shunt rivet having a rectangular cross-sectional area  181 . As mentioned, other non-circular cross-sectional areas (e.g., polygon, trapezoid, triangle, rhombus, rectangle, square, parallelogram, etc.) may be utilized to optimize flow conditions and/or therapeutic results for implantation between the vessels, as desired. 
         [0089]    The devices described above may be provided with coatings or additional structures which serve as matrices for various therapeutic compounds. Drug eluting coatings, additional drug eluting strut members, drug eluting membranes surrounding the central section or drug eluting masses filling the cells of the device may be added to the devices. For the aortocaval application and the arterio-venous application, therapeutic agents such as heparin and other anti-coagulants and paclitaxol, rapamycin (Sirolumis™), everolimus and other anti-stenotic compounds can be applied to the stent in polymer matrices which permit elution of these drugs over a period of time ranging from several hours to several months after implantation. Polymers such as polyurethane can be used as the matrix. 
         [0090]    While the preferred embodiments of the devices and methods have been described in reference to the environment in which they were developed, they are merely illustrative of the principles of the inventions. Modification or combinations of the above-described assemblies, other embodiments, configurations, and methods for carrying out the invention, and variations of aspects of the invention that are obvious to those of skill in the art are intended to be within the scope of the claims.