Patent Publication Number: US-8114044-B2

Title: Arteriovenous access valve system and process

Description:
RELATED APPLICATIONS 
     The present application claims priority to and is a Continuation-In-Part Application of U.S. patent application Ser. No. 10/456,697 filed on Jun. 6, 2003 now U.S.Pat. No. 7,025,741. 
    
    
     BACKGROUND OF THE INVENTION 
     The function of kidneys, which are glandular organs located in the upper abdominal cavity of vertebrates, is to filter blood and remove waste products. Specifically, kidneys separate water and waste products of metabolism from blood and excrete them as urine through the bladder. Chronic renal failure is a disease of the kidney in which the kidney function breaks down and is no longer able to filter blood and remove waste substances. Should certain toxic waste substances not be removed from the blood, the toxic substances may increase to lethal concentrations within the body. 
     Hemodialysis is a life-sustaining treatment for patients who have renal failure. Hemodialysis is a process whereby the patient&#39;s blood is filtered and toxins are removed using an extracorporeal dialysis machine. For hemodialysis to be effective, large volumes of blood must be removed rapidly from the patient&#39;s body, passed through the dialysis machine, and returned to the patient. A number of operations have been developed to provide access to the circulation system of a patient such that patients may be connected to the dialysis machine. 
     For example, the most commonly performed hemodialysis access operation is a subcutaneous placement of an arteriovenous graft, which is made from a biocompatible tube. The biocompatible tube can be made of, for instance, a fluoropolymer such as polytetrafluoroethylene. One end of the tube is connected to an artery while the other end is connected to a vein. The arteriovenous graft is typically placed either in the leg or arm of a patient. 
     Blood flows from the artery, through the graft and into the vein. To connect the patient to a dialysis machine, two large hypodermic needles are inserted through the skin and into the graft. Blood is removed from the patient through one needle, circulated through the dialysis machine, and returned to the patient through the second needle. Typically, patients undergo hemodialysis approximately four hours a day, three days a week. 
     Various problems, however, have been experienced with the use of an arteriovenous graft. For example, arterial steal occurs when excessive blood flow through the arteriovenous graft “steals” blood from the distal arterial bed. Arterial steal can prevent the proper supply of blood from reaching the extremity of a patient. 
     Various other complications can also occur. For instance, the blood flowing through the arteriovenous graft can often reach turbulent flow rates. This stream of fast moving blood then exits the arteriovenous graft and contacts the vein connected to the graft. This collision between the flow of blood and the vein may cause the development of myointimal hyperplasia which leads to the thickening of the vein walls and a narrowing of the vessel. As the vein narrows, flow through the arteriovenous graft decreases and blood within the graft may ultimately clot. 
     The cessation of blood flow through the graft caused by clot formation is known as graft thrombosis. Numerous techniques and medications have been studied in attempts to block the development of the scar tissue. Graft thrombosis, however, continues to remain a reoccurring complication associated with the use of arteriovenous grafts. 
     In view of the above drawbacks, a need currently exists in the art for an arteriovenous graft that can prevent and minimize arterial steal and graft thrombosis. A process for using an arteriovenous graft in minimizing arterial steal and graft thrombosis is also needed. 
     SUMMARY OF THE INVENTION 
     In general, the present invention is directed to subcutaneous arteriovenous graft systems and to processes for using the arteriovenous graft systems in a manner that eliminates or at least reduces arterial steal and graft thrombosis. In one embodiment, for instance, the system includes an arteriovenous graft having an arterial end and an opposite venous end. The arterial end is configured to be connected to an artery to form an arterial anastomosis, while the venous end is configured to be connected to a vein to form a venous anastomosis. 
     In accordance with the present invention, the system includes at least one valve device positioned at the arterial end of the arteriovenous graft. In one embodiment, for instance, the valve device comprises an inflatable balloon. The inflatable balloon is positioned so as to restrict blood flow through the arteriovenous graft when inflated. In general, the valve device should be positioned at the arterial end of the arteriovenous graft as close as possible to the intersection of the graft with an artery. For example, the valve device may be positioned so as to restrict blood flow through the arteriovenous graft at a location that is less than about 10 mm from the intersection of the arteriovenous graft and an artery. 
     The inflatable balloon of the valve device may have an annular shape that surrounds the arteriovenous graft. The inflatable balloon may also be a separate structure or may be integral with the arteriovenous graft. When integral with the arteriovenous graft, the arteriovenous graft may include a multi-layered segment located at the arterial end. The multi-layered segment may comprise an inner layer and an outer layer. The inner layer constricts the graft when a fluid is fed in between the inner layer and the outer layer. When having an annular shape, the balloon may be surrounded by a rigid collar that serves to assist the balloon in constricting the graft. 
     In an alternative embodiment, the valve device may include an inner sleeve and an outer sleeve. The inner sleeve may be attached to the outer sleeve except for over a discrete area. The discrete area can be in fluid communication with a fluid delivery device. When a fluid is fed to the discrete area, fluid is fed in between the inner sleeve and the outer sleeve causing the discrete area of the inner sleeve to inflate. In this embodiment, the discrete area, instead of surrounding the arteriovenous graft, can be circular or substantially circular in shape. When inflated, the discrete area forms a spherically shaped or a substantially spherically shaped balloon. In one embodiment, for instance, the outer sleeve may be more rigid than the inner sleeve. Thus, when the inner sleeve is inflated, the outer sleeve maintains its shape. In this embodiment, the balloon may be integral with the arteriovenous graft. Alternatively, the arteriovenous graft may be positioned within the inner sleeve. 
     In order to inflate and deflate the balloon, in one embodiment, the valve device can further include an injection port in fluid communication with the inflatable balloon. The injection port defines a diaphragm configured to receive a hypodermic needle for injecting fluid into or withdrawing fluid from the balloon. Of particular advantage, the injection port may also be subcutaneously implanted. 
     In an alternative embodiment, the inflatable balloon may be positioned in operative association with a piston. In this embodiment, when the balloon is inflated, the balloon forces the piston either towards or away from the arteriovenous graft for opening or closing the valve device. 
     When the valve device contains a piston, the valve device can include various configurations. Further, the piston can be used to inflate a balloon as described above or can be used to activate any other suitable structure configured to open and close the arteriovenous graft. In fact, in one embodiment, the piston itself may be used to open and close the graft. 
     In one embodiment, for example, the valve device may comprise a magnetically activated piston. In this embodiment, when a magnetic field is placed in close proximity to the valve device, the piston is moved for either opening or closing the valve device. For example, in one embodiment, placing a magnetic field in close proximity to the valve device opens the device which normally remains closed. 
     In one particular embodiment, the magnetically activated piston may be activated when exposed to a changing magnetic field, such as a pulsing magnetic field. In this embodiment, the valve device may include a coil member configured to convert a changing magnetic field into an electric current. The coil member is in communication with a solenoid. The solenoid is configured to move the piston and open or close the valve device when electric current is received from the coil member. 
     In an alternative embodiment, the valve device may include a piston that is biased towards a closed position. For example, a spring or other structure may apply a biasing force against the piston that maintains the piston in the closed position. In order to move the piston, the piston can be in operative association with a lever arm. When a magnetic field is placed in close proximity to the valve device, the lever arm may be configured to move causing the piston to move and open the valve device. In this embodiment, for instance, the piston may be in fluid communication with an inflatable balloon as described above. When the piston is moved into an open position, a fluid flows out of the balloon for deflating the balloon. When the piston is placed in the closed position, on the other hand, the fluid can be forced into the balloon for inflating the balloon. 
     In one embodiment, the arteriovenous graft system further includes a second valve device positioned at the venous end of the arteriovenous graft. The second valve device may be any suitable valve device as described above. The second valve device, for instance, may be identical to the first valve device or, alternatively, may be different. 
     In general, for most applications, the second valve device is not exposed or subjected to the same fluid pressures that are exerted on the first valve device. In this regard, the first valve device is designed to restrict or stop fluid flow at relatively high pressures. The second valve device, however, may be a low pressure valve device. In one embodiment, for instance, the second valve device may be a check valve positioned at the venous end of the arteriovenous graft. For example, the second valve device may be formed integral with the arteriovenous graft and may be formed from a membrane that allows fluid flow from the arteriovenous graft and into an adjoining vein but prevents fluid flow from the vein into the arteriovenous graft. 
     In an alternative embodiment, the check valve may comprise a pair of opposing and overlapping flaps positioned within the arteriovenous graft. The flaps can be integral with the graft or can be attached to the arteriovenous graft on opposing sides. For instance, the flaps can be attached to the graft using sutures or through a welding process. In order to prevent leakage, the check valve can further include edge seals that are positioned on opposing sides of each flap. The edge seals can create a seal with the radial wall of the arteriovenous graft. 
     The arteriovenous graft of the present invention is used for hemodialysis. During hemodialysis, two hypodermic needles are inserted into the arteriovenous graft. Blood is removed from the graft using one needle, circulated through a dialysis machine, and returned to the arteriovenous graft through the second needle. When hemodialysis is not being conducted, however, the valve devices of the present invention may be activated in order to minimize arterial steal and prevent thrombosis of the graft. 
     For example, in one embodiment of the present invention, when the arteriovenous graft system only includes a single valve device at the arterial end, after hemodialysis has ended, the valve device is closed thus preventing blood flow through the graft. After the valve device is closed, a blood compatible fluid may be injected into the graft using a hypodermic needle. As used herein, a blood compatible fluid refers to any fluid that is biocompatible with the circulation system. For example, in one embodiment, the blood compatible fluid is a heparinized saline solution. The saline solution is used to flush the graft after the valve device is closed in order to remove blood from the graft. 
     In another embodiment, after hemodialysis, the valve device is partially closed to a first position thereby constricting the arteriovenous graft and reducing blood flow through the graft. The patient is then monitored over a period of time, such as days or weeks, and the valve device may be selectively opened or closed from the first position until arterial steal is minimized. In this embodiment, the valve device is closed an amount sufficient to reduce blood flow through the graft without slowing the blood flow to a point where blood clots may form. 
     As described above, in another embodiment of the present invention, the arteriovenous graft system includes a first valve device at the arterial end and a second valve device at the venous end. In this embodiment, after hemodialysis has ended, the first valve device at the arterial end is closed thereby preventing blood flow through the graft. A hypodermic needle then flushes the graft with a blood compatible fluid evacuating all blood from the graft. After the graft has been flushed with the blood compatible fluid, the second valve device is then closed and the hypodermic needle is removed from the graft. 
     Other features and aspects of the present invention are discussed in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention is set forth in the specification with reference to the following figures. 
         FIG. 1  is a side view with cut away portions of a human arm illustrating the placement of an arteriovenous graft; 
         FIGS. 2A ,  2 B and  2 C are perspective views of embodiments of arteriovenous graft systems made in accordance with the present invention; 
         FIG. 3  is a perspective view of one embodiment of a valve device that may be used in the arteriovenous graft system of the present invention; 
         FIG. 4  is a perspective view of another embodiment of a valve device that may be used in the arteriovenous graft system of the present invention; 
         FIG. 5  is a perspective view of still another embodiment of an arteriovenous graft system made in accordance with the present invention; 
         FIG. 6  is an unassembled perspective view of one embodiment of a balloon valve that may be used in accordance with the present disclosure; 
         FIG. 7  is a cross-sectional view of the valve device illustrated in  FIG. 6 ; 
         FIG. 8  is a cross-sectional view taken along line A-A of the valve device shown in  FIG. 7 ; 
         FIG. 9  is a cross-sectional view of the valve device illustrated in  FIG. 6  showing the balloon inflated; 
         FIG. 10  is a perspective view with cut away portions showing another embodiment of a valve device that may be used in accordance with the present disclosure; 
         FIG. 11  is a cross-sectional view of the valve device illustrated in  FIG. 10 ; 
         FIG. 12  is a side view with cut away portions illustrating the valve device shown in  FIG. 10  in association with a balloon; 
         FIG. 13  is a cross-sectional view of the valve device illustrated in  FIG. 12  illustrating the balloon being deflated; 
         FIG. 14  is an unassembled perspective view of one embodiment of a check valve that may be used in accordance with the present disclosure; 
         FIG. 15  is a perspective view with cut away portions of the check valve illustrated in  FIG. 14 ; 
         FIG. 16  is a cross-sectional view of the check valve illustrated in  FIG. 14 ; 
         FIG. 17  is a cross-sectional view taken along line A-A of the check valve illustrated in  FIG. 16 ; and 
         FIG. 18  is an alternative embodiment of a check valve that may be used in accordance with the present disclosure. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features of the invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made to the embodiments of the invention, one or more examples of which are set forth below. Each example is provided by way of explanation of the invention, not as a limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made in the invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment may be used in another embodiment to yield a still further embodiment. For example, an arteriovenous graft system may include combinations of the valve devices described below. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents. It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary construction. 
     In general, the present invention is directed to an implantable arteriovenous graft system that may be used in carrying out hemodialysis treatments. Although the following description will refer to the arteriovenous graft system being implanted into an arm, it should be understood that the system may be implanted in any suitable location of the body. For example, in other embodiments, the arteriovenous graft system may be implanted into a leg. 
     In addition to being well suited for carrying out hemodialysis, the arteriovenous graft system of the present invention also prevents or minimizes arterial steal and graft thrombosis. In particular, the arteriovenous graft system is designed to prevent or minimize blood flow through the graft when hemodialysis is not occurring. 
     Referring to  FIG. 1 , for purposes of explanation, a right arm  10  of a patient is shown. Selected arteries (shown as dotted pathways) are illustrated in conjunction with selected veins (shown as dark pathways). An arteriovenous graft  12  is shown connected at one end to an artery and at an opposite end to a vein. In particular, the arteriovenous graft  12  is connected to the brachial artery  14  and to the cephalic vein  16 . 
     The arteriovenous graft  12  is made from any suitable biocompatible material. For example, in one embodiment, the graft is made from a fluoropolymer, such as polytetrafluoroethylene, which is commercially available as GORTEX™ from the W. L. Gore Company. 
     Referring to  FIGS. 2A and 2B , one embodiment of an arteriovenous graft system made in accordance with the present invention is shown including an arteriovenous graft  12 . As illustrated, the arteriovenous graft  12  is connected to an artery  14  and to a vein  16 . In order to carry out hemodialysis, a first hypodermic needle  18  is inserted through the skin and into the arteriovenous graft  12 . Blood is removed from the arteriovenous graft  12  through the needle and into a dialysis machine  20 . In the dialysis machine, waste materials are removed from the blood. After circulating through the dialysis machine  20 , the blood is then fed back into the arteriovenous graft  12  through a second hypodermic needle  22 . 
     In accordance with the present invention, the arteriovenous graft system as shown in  FIGS. 2A and 2B  further includes at least a first valve device generally  24  positioned at the arterial end of the arteriovenous graft  12 . Optionally, the arteriovenous graft system can further include a second valve device generally  26  positioned at the venous end of the arteriovenous graft. The valve devices  24  and  26  are in an open position during normal hemodialysis as shown in  FIG. 2A . When hemodialysis has ended, however, the valve devices  24  and  26  are moved to a closed position in order to prevent blood flow through the arteriovenous graft. In this manner, arterial steal is either eliminated or reduced. Further, by reducing arterial steal, graft thrombosis is also prevented. 
     In addition to minimizing arterial steal and preventing graft thrombosis, the system and the process of the present invention also offer various other advantages. For example, reducing or stopping blood flow through the arteriovenous graft when hemodialysis is not occurring also prevents the graft from bleeding when the hypodermic needles used to carry out hemodialysis are removed from the graft. Hypodermic needles as shown in  FIG. 2B , for instance, usually have a relatively large diameter or gauge. Thus, when the needles are removed from a graft, bleeding can occur where the needles have previously been. Needle hole bleeding through the graft can result in the formation of scar tissue and graft pseudoaneurisms. These complications, however, may be prevented through the use of the system of the present invention. 
     In the embodiment shown in  FIG. 2A , the valve devices  26  and  24  each include an inflatable balloon  28  and  30 . When inflated, the balloons  28  and  30  constrict the arteriovenous graft  12  for reducing or eliminating blood flow through the graft. 
     As shown in  FIG. 2A , the inflatable balloons  28  and  30 , in this embodiment, have an annular shape that surround the arteriovenous graft  12 . As shown, each of the inflatable balloons  28  and  30  are also surrounded by a rigid collar  32  and  34 . Each collar  32  and  34  may be included in the system in order to maintain each of the balloons  28  and  30  in the proper position. Further, the collars  32  and  34  also serve to bias the balloon towards the arteriovenous graft  12  when inflated. Each collar  32  and  34  may be made from any rigid biocompatible material. For example, the collars  32  and  34  may be made from a metal, such as titanium, or a plastic material. 
     Each annular balloon  28  and  30  may be a separate structure from the arteriovenous graft  12  or may be integral with the graft. When integral with the graft, for instance, the graft may include a multi-layered segment where each of the valve devices is to be located. For example, within the multi-layered segment, the arteriovenous graft  12  may include an outer rigid layer and an inner luminal layer. The balloon  28  and  30  may be formed in between the outer layer and the inner layer. In particular, when a fluid is injected in between the inner and outer layers, the inner layer may expand and constrict the lumen. See  FIG. 2C . 
     In addition to having an annular shape, it should be understood that each balloon  28  and  30  may have any shape sufficient to constrict the arteriovenous graft when inflated. For instance, in another embodiment, each balloon  28  and  30  may be located on one side of the graft  12 . When inflated, the balloons  28  and  30  force opposite sides of the graft together. 
     For example, referring to  FIGS. 6 through 8 , an alternative embodiment of a valve device containing an inflatable balloon is shown. As illustrated in  FIG. 6 , the valve device includes an inner sleeve  110  positioned within an outer sleeve  112 . The inner sleeve can be attached or bonded to the outer sleeve at all locations except over a discrete area  114 . As shown in  FIGS. 7 and 8 , the discrete area  114  is positioned opposite a fluid passageway  116 . The fluid passageway  116  is placed in communication with a fluid delivery device. When fluid is forced through the fluid passageway  116 , the fluid causes the discrete area  114  to inflate and form a balloon as shown in  FIG. 9 . 
     The inner and outer sleeves can be made from various materials and can be formed using various techniques. In one embodiment, for instance, the inner and outer sleeves can be injection molded and bonded together. For example, both the inner sleeve and the outer sleeve may be made from a suitable elastomer, such as a silicone elastomer. The outer sleeve  112  can be made more rigid than the inner sleeve  110  so that the outer sleeve preserves its shape when the discrete area  114  is inflated. The outer sleeve  112  can be made more rigid by having a greater thickness or by being made from a stiffer material, such as a material that has a higher durometer in comparison to the material used to form the inner sleeve. 
     In order to attach the inner sleeve  110  to the outer sleeve  112 , any suitable technique may be used. For example, in one embodiment, an adhesive material, such as an adhesive material containing a silicone elastomer may be used to bond the two layers together. In other embodiments, the two layers may bond together during the molding process. 
     As shown in  FIG. 8 , in one embodiment, the discrete area  114  may have a thickness that is less than the thickness of the remainder of the inner sleeve  110 . For instance, the discrete area  114  may have a thickness of less than about 0.015 inches. As shown in  FIG. 6 , in one embodiment, the discrete area  114  may have a circular or a substantially circular shape. By having a substantially circular shape, the discrete area expands uniformly and inflates evenly across its plane during inflation, thus minimizing stress on the material. Once inflated as shown in  FIG. 9 , the discrete area  114  can have a spherical or substantially spherical shape. The inflated shape can compress the arteriovenous graft and prevent leakage. The substantially spherical shape also allows the balloon to be inflated to a size and pressure which can assure constriction and sealing of a 200 mmHg pressure gradient across the graft. The balloon can also be designed to be overpressurized by greater than about 30% thus serving as a safety factor. Ultimately, the design is free of bulk, pinch points which minimizes patient discomfort. 
     The valve device as shown in  FIGS. 6 through 9  can be integral with the arteriovenous graft or the arteriovenous graft can fit inside the inner sleeve  110 . In one embodiment, the inner and outer sleeves can be slit along the length in order to facilitate installation over a graft. Once installed over a graft, the slit formed in the valve device can be connected together through thermal bonding, clips or sutures. 
     In order to inflate the balloons as shown in the figures, in one embodiment as shown in  FIGS. 2A and 2B , each valve device may further include an injection port  36  and  38 . For example, as shown in  FIG. 2A , injection port  36  may be in fluid communication with the balloon  28  via a tubing  40 . Similarly, injection port  38  may be in fluid communication with the balloon  30  via a tubing  42 . Each injection port  36  and  38  may be configured to be subcutaneously implanted in a patient. 
     In the embodiment illustrated in  FIG. 2A , injection ports  36  and  38  each include a diaphragm  44  and  46  positioned on one side of a housing  48  and  50 . The housings  48  and  50  may be made from any suitable rigid and biocompatible material. For example, each housing may be made from a metal, such as titanium. Each diaphragm  44  and  46 , on the other hand, may be made from a material capable of receiving the tip of a hypodermic needle. For example, each diaphragm  44  and  46  may be made from an elastomeric film, such as a silicone membrane. 
     As shown particularly in  FIG. 2B , in order to inflate or deflate the balloons  28  and  30 , hypodermic needles  52  and  54  may inject a fluid into each of the injection ports  36  and  38  through the diaphragms  44  and  46 . The fluid travels from the injection ports  36  and  38  through the tubing  40  and  42  and into each respective balloon  28  and  30 . Similarly, the hypodermic needles  52  and  54  may also be used to withdraw fluid from the balloons  28  and  30 . 
     As shown in  FIG. 2B , once inflated, the balloons  28  and  30  constrict the arteriovenous graft  12  at the arterial end and at the venous end. The fluid used to inflate the balloons  28  and  30  may vary depending upon the particular application. The fluid may be, for instance, a gas or liquid. In one embodiment, for instance, a saline solution may be injected into the injection ports  36  and  38  for inflating the balloons. In one embodiment, it may take from about 2 ccs to about 6 ccs of fluid to transition each balloon valve  28  and  30  from an open position to a closed position. 
     When closed, each valve device should be capable of maintaining its position when exposed to systolic pressure. For example, systolic pressures in arteries may be greater than about 250 mmHg, such as from about 170 mmHg to about 270 mmHg. 
     In addition to withstanding relatively high fluid pressures, each of the valve devices  24  and  26  should also be constructed so that the valve devices can constrict the arteriovenous graft as close as possible to the intersection of the graft with the artery  14  and the vein  16 . For example, the first valve device  24 , in one embodiment, constricts the arteriovenous graft at a distance of from about mm from the arterial anastomosis, such as no greater than about 20 mm from the arterial anastomosis. The position of the second valve device  26  in relation to the venous anastomosis may also be within the above defined limits. 
     The methods for using the arteriovenous graft system of the present invention will now be discussed in relation to a system that contains a single valve device positioned at the arterial end of the graft and a system that contains two valve devices as shown in  FIGS. 2A and 2B . 
     When the arteriovenous graft system of the present invention contains a single valve device positioned at the arterial end, in one embodiment, the valve device may be positioned so as to constrict blood flow through the graft when hemodialysis is not occurring. In this embodiment, arterial steal is not being completely prevented but is being minimized. In particular, the single valve device constricts the graft so that blood flow through the graft continues without clotting but is at a reduced flow rate. 
     In this embodiment, the patient&#39;s condition may need to be monitored over a period of time, such as days or weeks, and the valve device may be adjusted in order to minimize arterial steal without causing a complete blood stoppage. For instance, over several days or weeks, the arteriovenous graft of the patient may be monitored and the valve device may be adjusted so as to gradually increase or decrease the narrowing of the arteriovenous graft. The ultimate position of the valve will vary depending upon the patient and the location of the arteriovenous graft. 
     In an alternative embodiment, the single valve device may be used to completely close off the arteriovenous graft  12  at the arterial end. In this embodiment, during hemodialysis, the valve device  24  is in the open position and the arteriovenous graft  12  is cannulated with the two dialysis needles  18  and  22  as shown in  FIG. 2A . Upon completion of dialysis, a fluid is injected into the injection port  36  of the first valve device causing the balloon  28  to inflate thereby closing the valve device and eliminating arterial blood flow through the graft. 
     After the valve device is closed, a blood compatible fluid is then injected into the arteriovenous graft  12  through, for instance, a dialysis needle to flush any residual blood out of the graft. The blood compatible fluid can be, for instance, heparinized saline. The residual blood is flushed out of the graft in order to prevent any clotting. 
     In this embodiment, some residual saline remains in the graft until hemodialysis is once again conducted on the patient. This embodiment should only be used when it is determined that substantially no blood from the vein  16  will flow into the graft once valve device  24  is closed. 
     In order to prevent any blood flowing from the vein  16  back into the arteriovenous graft  12  after the first valve device  24  has been closed, in one embodiment of the present invention as shown particularly in  FIGS. 2A and 2B , the arteriovenous graft system can include the second valve device  26 . In this embodiment, the process as described above is repeated. After the arteriovenous graft  12  is flushed with a blood compatible fluid, however, a fluid is injected into the injection port  38  of the second valve device  26  which causes the second valve device to close. 
     In addition to the valve devices as illustrated in  FIGS. 2A and 2B , in other embodiments, other valve devices may also be utilized in the system of the present invention. For example, referring to  FIG. 4 , another embodiment of a valve device generally  60  is shown in communication with an arteriovenous graft  12 . In this embodiment, the valve device  60  includes a fluid chamber  62  in communication with an injection port  64  similar to the injection ports described above. As shown, injection port  64  includes a diaphragm  68  configured to receive fluid from a hypodermic needle  70 . 
     The valve device  60  further includes a piston  72  contained within a housing  74 . The piston  72  is positioned below the fluid chamber  62 . 
     In this embodiment, when a fluid is injected from the needle  70  into the injection port  64 , the fluid is forced into the fluid chamber  62  via a tube  66 . The pressure of the fluid then forces the piston  72  to lower closing the valve and constricting flow through the arteriovenous graft  12 . 
     Valve device  60  as shown in  FIG. 4  may be used in a single valve system of the present invention or in a double valve system of the present invention as illustrated in  FIG. 2A . 
     Referring to  FIG. 3 , another embodiment of a valve device generally  80  that may be used in the arteriovenous graft system of the present invention is illustrated. In this embodiment, the valve device  80  includes a housing  82  containing a magnetically actuated piston  84 . Specifically, the valve device is configured such that the piston  84  moves between an open and closed position when the valve device is contacted with a magnetic field. 
     In this particular embodiment, the valve device  80  includes a coil member  86 . The coil member  86  is configured to convert a pulsating magnetic field into an electric current. As shown, the coil member  86  then supplies the electric current to a solenoid  88 . Solenoid  88  then moves the piston  84  to either open or close the valve device. 
     In order to activate the valve device  80 , a magnetic key  90  is placed close to the skin of a patient. In this embodiment, the magnetic key  90  may be an electromagnet that creates a pulsating magnetic field. As described above, the pulsating magnetic field is then converted into an electric current by the coil member  86 . The magnetic key  90  may be configured either to open or to close the valve device. In one embodiment, for instance, the valve device  80  may normally be found in a closed position blocking off the arteriovenous graft  12 . When the magnetic key  90 , however, is placed adjacent to the patient&#39;s skin, the valve device  80  then opens allowing blood to circulate through the graft. In other embodiments, however, it should be understood that the valve device may be configured to close when placed adjacent to the magnetic key  90 . 
     In addition to the valve device  80  as shown in  FIG. 3 , other magnetically activated valves may be used in the system of the present invention. For example, in another embodiment of the present invention, the valve device may include a piston in operative association with a permanent magnet. A ferrous plate may be positioned on the opposite side of the arteriovenous graft. Thus, the permanent magnet contained in the piston is attracted to the ferrous surface for closing off the arteriovenous graft. When a magnet with opposite polarity, however, is placed adjacent to the valve device, the permanent magnet contained within the piston is attracted to the reverse magnetic field causing the valve to open. 
     Referring to  FIGS. 10 through 13 , still another embodiment of a magnetically activated valve device that may be used in accordance with the present disclosure is shown. In this embodiment, the valve device includes a magnetically activated piston  120  as shown in  FIG. 10 . As illustrated, the piston  120  is contained within a housing  122 . The piston is biased towards a closed position by a spring  124 . In particular, the spring  124  applies a biasing force to the piston  120 . 
     As shown in  FIGS. 10 and 11 , the piston is also attached to a lever arm  126 . The lever arm  126  is attached to a magnet member  128  or a magnetically attractable member  128 . In this embodiment, when an external key comprising a magnet or an electromagnet is placed adjacent to the member  128 , the lever arm  126  moves which in turn causes the piston to move and open or close the valve. 
     In the embodiment shown in the figures, the piston  120  is normally biased in a closed position. When a magnetic key is placed adjacent to the valve device, the lever arm causes the piston  120  to move and open the valve device. It should be understood, however, that in other embodiments the lever arm may be used to close the valve. 
     The piston  120  as shown in  FIG. 10  can be placed in association with an arteriovenous graft in order to open and close the graft. In one particular embodiment as shown in  FIG. 12 , for example, the piston  120  can be placed in communication with a balloon valve such as the one illustrated in  FIGS. 6 through 9 . In this embodiment, the piston  120  is used as a fluid delivery device that delivers fluid to the balloon. 
     For instance, referring to  FIG. 12 , the piston  120  is shown in a closed position caused by a biasing force being placed against the piston by the spring  124 . When in the closed position, the piston  120  forces a fluid through the conduit  130  and in contact against the discrete area  114 , causing the discrete area to inflate and form a substantially spherical shape. When inflated, the discrete area  114  blocks flow through the arteriovenous graft. 
     When it is desired to open the arteriovenous graft for dialysis treatment, for instance, a key comprising a magnet or an electromagnet is placed adjacent to the valve device. Referring to  FIG. 13 , for instance, the magnetic or electromagnetic key is placed adjacent to the magnetic member or magnetically attractable member  128  causing the lever arm  126  to pivot or move. The lever arm  126  is attached to a linking member  132  that is in turn connected to the piston  120 . When the lever arm  126  is pivoted, the linking member  132  causes the piston to retract as shown. Fluid contained within the conduit  130  is thereby drawn out of the discrete area  114  causing the balloon to deflate. In this manner, the valve device is opened for allowing blood flow through the arteriovenous graft. During the dialysis treatment, the external magnetic key can be fixed into position to ensure that the valve device stays open. For instance, the external key can be taped or otherwise attached to the skin of the patient. When the dialysis treatment is concluded, the external magnetic key is removed and the valve device automatically returns to the closed position. 
     The fluid that is contained within the valve device may vary depending upon the particular application and the desired results. In one embodiment, for instance, a saline solution may be contained within the valve device. 
     In the embodiment illustrated in the drawings, the lever arm  126  is moved based upon an attracting magnetic force. It should be understood, however, that magnetic repulsion can also be used to move the lever arm as well. 
     The valve device as shown in  FIGS. 10 through 13  can be designed to be relatively small for being implanted under the skin of a patient. For instance, the housing  122  as shown in  FIG. 10  can have a diameter less than about 3 cm and can have a height of less than about 1 cm. 
     Using a magnetically actuated valve device as shown in  FIGS. 10 through 13  can provide various advantages. For instance, because the valve device is magnetically actuated, the valve device eliminates the need to use hypodermic needles for transferring liquid into and out of a plenum or port. 
     In still another embodiment, the valve device as shown in  FIGS. 10 through 13  may be actuated other than through use of a magnet. For instance, in one embodiment, the valve device may include a pump in communication with a battery. The pump may be turned on and off using wireless telemetry. In fact, wireless telemetry may also deliver real time pressure measurements thereby communicating the status of the valve device. 
     Referring to  FIG. 5 , another embodiment of an arteriovenous graft system made in accordance with the present invention is shown. Like reference numerals have been used in order to identify similar features and elements of other embodiments. As shown, in this embodiment, the arteriovenous graft system includes a first valve device generally  24  at the arterial end of the graft similar to the valve device shown in  FIGS. 2A and 2B . In particular, the first valve device  24  includes a balloon  28  that is inflated or deflated using an injection port  36 . The balloon  28  is for constricting the arteriovenous graft when desired. As explained above, the first valve device  24 , for most applications, is capable of maintaining a closed or constricted position on the graft even when exposed to relatively high fluid pressures. In some embodiments, however, these same pressures are not experienced at the venous end of the graft. 
     In this regard, in this embodiment, the arteriovenous graft  12  includes a second valve device generally  100  that may be described as a low pressure valve device when compared to the first valve device  24 . 
     For example, in one embodiment, the second valve device  100  may be a check valve that allows fluid flow from the graft  12  into the vein  16  but does not permit flow from the vein  16  into the graft  12 . In general, any suitable check valve may be used in accordance with the present invention. 
     In the embodiment shown in  FIG. 5 , the second valve device  100  includes a membrane  102  made from, for instance, a polymeric film that is formed or is connected so as to be integral with the arteriovenous graft  12 . The membrane  102  may be, for instance, a flap that allows fluid flow in one direction from the graft  12  into the vein  16 . The membrane  102  may be formed from a single piece of film or may be formed from multiple segments. For example, in one embodiment, the film can include one or more slits that permit fluid flow in one direction. 
     The arteriovenous graft system in  FIG. 5  provides various advantages. For example, in the embodiment shown in  FIG. 5 , only the first valve device  24  needs to be manually opened or closed. 
     In the embodiment shown in  FIG. 5 , the first valve device is represented as a balloon valve. It should be understood, however, that the first valve device may be any of the other valve devices shown and described above. 
     The second valve device  100  as shown in  FIG. 5  represents one embodiment of a check valve (a valve that allows flow in one direction) that may be used in accordance with the present disclosure. It should be understood, however, that various other check valves may be used. For instance, referring to  FIGS. 14 through 18 , another embodiment of a check valve device  150  is illustrated. As shown in  FIGS. 14 and 15 , for instance, the check valve device  150  includes a pair of overlapping flaps  152  and  154 . The overlapping flaps allow fluid flow only in one direction. As shown, the opposing flaps  152  and  154  are generally planar and parallel. The flaps can be integral with the arteriovenous graft  156  or can be attached to the graft using any suitable technique. For instance, as shown in  FIG. 15 , the arteriovenous graft  156  can include a pair of opposing slits through which the flaps are inserted. The flaps can then be attached to the graft  156  by being welded in place or through the use of a biocompatible adhesive. In an alternative embodiment as shown in  FIG. 18 , sutures  158  can be used in order to attach the flaps to the arteriovenous graft  156 . 
     In addition to the flaps  152  and  154 , the check valve device  150  can further include edge seals  160 ,  162 ,  164  and  166  as shown in  FIGS. 16 and 17 . The edge seals  160 ,  162 ,  164  and  166  are positioned on both sides of each flap and are designed to create a seal with the radial wall of the graft  156 . The edge seals are generally located where the flaps are not connected to the graft  156 . 
     The check valve device  150  can be made from any suitable material. For instance, the flaps and the edge seals can be made from expanded or unexpanded PTFE, polyurethane and/or silicone. The blood contacting surfaces may be treated and/or textured to enhance their formation of a pseudointima, optimize thrombocompatibility and flow characteristics. 
     These and other modifications and variations to the present invention may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present invention, which is more particularly set forth in the appended claims. In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention so further described in such appended claims.