Abstract:
Vascular access systems, devices and methods for facilitating repeated access to a blood vessel. These systems, devices and methods can be used in external blood treatment, such as dialysis, and in intra-venous administration of medicines, such as heparin, for extended periods of time, while avoiding deleterious effects such as those derived from repeated puncturing of the blood vessel tissues or exposure of such tissues to abnormal fluid flows. The vascular access systems comprise an anastomosis graft vessel, an occlusal device, such as an occlusal balloon, and a port device for accessing the occlusal device. Occlusal devices can be self-contained, they can rely on osmosis, and they can serve as the support of an agent to which the blood stream is exposed, either by transport or by mere contact. In addition, occlusal devices can adopt a distended and a collapsed configuration, the latter allowing for blood flow through the anastomosis graft vessel.

Description:
BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to external treatment of blood. In particular, the present invention relates to methods for externally treating blood without having to repeatedly puncture the blood vessels being accessed. 
     2. Present State of the Art 
     Procedures that require the repeated access to blood vessels include dialysis and the delivery of medicines for an extended period of time. The multiple punctures that such repeated access necessitates eventually render the blood vessel unsuitable for further effective injections. In addition, some external blood treatment methods rely on the extraction of blood from an artery and on the subsequent injection of the treated blood into a vein. The characteristics of the fluid flow in an artery are significantly different from the characteristics in the fluid flow in a vein. These fluid flow dissimilarities may lead to additional adverse effects that detrimentally affect the long term accessibility of the blood vessels that must be accessed for the external blood treatment to be effectively performed. For example, in an arterio-venous graft constructed as a vascular access for dialysis, the blood flow and blood pressure characteristic of the arterial circulation are so different from the blood flow and blood pressure in the vein into which the blood of the AV graft flows, that the vein usually develops hyperplasia and stenoses. 
     It is desirable to provide a device that permits multiple access to a blood vessel for the purpose of delivering medicines into the patient&#39;s blood stream in such a way that the receiving blood vessel is not so severely damaged that it becomes unavailable after a few medicine administrations. 
     It is also desirable to provide a device that permits multiple access to a blood vessel for external blood treatment, such as hemodialysis, in such a way that the blood vessel being accessed does not become unavailable for successive dialysis operations. 
     Furthermore, it would be desirable to provide a device that is suitable for multiple vascular access for the purpose of long term medicine delivery into the patient&#39;s blood flow and also for the purpose of effectively practicing hemodialysis for a long period of time. 
     The practical advantages of such device would be considerably enhanced if it could be lodged subcutaneously and if it were reliably attachable to a blood vessel by anastomosis techniques. In addition, such vascular access device would have to be appropriately configured to allow for controlled and selected blood flow through it and to allow for a controlled delivery of physiologically active agents, such as medicines. It is also desirable to provide methods for performing hemodialysis such that blood to be dialyzed can be drawn from a vein and injected into another vein once it has been dialyzed, without having to repeatedly puncture the veins involved in hemodialysis. 
     These goals should be accomplished while minimizing, or avoiding to the maximum extent possible, undesirable adverse effects such as vessel thrombosis, blood stagnation, the formation of undesirable turbulence, and the formation of blood clots. 
     The present invention focuses on objectives described hereinbelow for solving problems which are associated with repeated vascular access, and provides devices and methods with advantageous features for solving such problems. 
     OBJECTS AND BRIEF SUMMARY OF THE INVENTION 
     A blood vessel that is repeatedly accessed, and in particular repeatedly punctured, deteriorates to the point that vascular access becomes increasingly difficult and eventually impossible. When vascular access places the blood vessel under exceptional fluid dynamics conditions or subjects the vascular tissues to the deleterious side effects of certain medications, vascular deterioration can be seriously accelerated. An example of such exceptional fluid dynamics conditions is the blood volume and pressure that a vein is subjected to when it receives the arterial blood flow from an AV (arterio-venous) graft that has been created to provide vascular access for dialysis. 
     Although an occasional vascular access can be performed at any one among a plurality of generally available access sites, the availability of vascular access sites for the intravenous delivery of medicine for a long period of time or for dialysis can be seriously diminished because vascular access under such conditions has to be performed repeatedly. For example, hemodialysis typically requires from about 150 to about 200 vascular access operations per year for a period that typically ranges form about 2 years to about 5 years. 
     It is therefore desirable to provide vascular access devices and systems that can be repeatedly accessed, and in particular repeatedly punctured, while avoiding the deleterious effects on the blood vessel itself. These systems and devices should be biocompatible and in particular they should not significantly perturb the normal blood flow within the blood vessel that is to be accessed. In addition, these systems and devices should be made of readily available materials that can be clinically manipulated according to known techniques. It is also desirable to provide methods for repeatedly accessing blood vessels, and in particular for repeatedly accessing veins in the practice of vein-to-vein hemodialysis, so that vein accessibility is not diminished by the repeated vein access. 
     The general object of this invention is to provide vascular access systems and devices that facilitate repeated vascular access while reducing, or even eliminating, the deleterious effects that the vascular tissue would otherwise be subjected to. More specifically, it is an object of this invention to provide vascular access systems and devices that permit access to the blood stream while avoiding repeated punctures into the blood vessel being accessed. 
     It is another object of this invention to provide vascular access systems and devices that can be attached to a blood vessel by known anastomosis techniques. 
     It is another object of this invention to provide vascular access systems and devices that can be used for the intravenous long term delivery of medicines and also be used in dialysis. 
     It is a further object of this invention to provide methods for external treatment of blood such as hemodialysis methods, and in particular vein-to-vein hemodialysis methods, that enable the practice of hemodialysis between two blood vessels, such as two veins, while avoiding deleterious effects on these vessels. These deleterious effects would otherwise reduce blood vessel accessibility thus rendering the number of feasible hemodialysis operations unacceptably small. 
     These and other objects of this invention are preferably achieved by devices that comprise an occlusal balloon in fluid communication with a port device, and by systems that comprise an occlusal balloon in fluid communication with a port device to be used in conjunction with a graft vessel that in turn is configured to be anastomosed to a blood vessel. 
     The devices and systems of this invention preferably feature materials that are suitable for their subcutaneous disposition. This feature advantageously permits the placement of the vascular access systems and devices at a location that is not directly exposed to external pathogens. 
     The devices and systems of this invention preferably feature materials that can be repeatedly punctured and that are self-sealing. These features advantageously permit multiple injection to and extraction from the vascular access systems and devices of a variety of fluids such as blood samples, biocompatible solutions, medicines, and blood to be dialyzed or to be received from a dialysis apparatus. 
     The devices and systems of this invention preferably incorporate features that facilitate the exposure of the blood stream to desired physiologically effective (or bioactive) agents. This exposure is achieved by contact or by transport phenomena. In any case, these features advantageously permit, inter alia, the delivery into the blood stream of medications at desired and controlled dosages. Another advantage derived from these features is that the blood stream can be exposed to an agent that prevents the formation of blood clots. 
     The methods of this invention focus on repeated vascular access that is facilitated by preferably subcutaneous devices and systems which can be repeatedly punctured while preserving their physical integrity, biocompatibility and operability. These characteristics advantageously permit the practice of hemodialysis according to the methods of this invention for the extended periods of time that are typically needed by patients. 
     These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the manner in which the above-recited and other advantages and objects of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1 is a partial cross sectional view of an embodiment of a vascular access system with an occlusal balloon. 
     FIG. 2 is a partial cross sectional view of another embodiment of a vascular access system with a reinforced graft vessel that has an enlarged portion, and an occlusal balloon with a semipermeable membrane. 
     FIG. 3 is a partial cross sectional view of the port device of the embodiment shown in FIG.  2 . 
     FIG. 4 shows a perspective view of an embodiment of a port device. 
     FIG. 5 is a partial cross sectional view of an embodiment of a vascular access system with two occlusal balloons, two semipermeable membranes, and a graft vessel with an enlarged portion. 
     FIGS. 6A-6D schematically illustrate different configurations of a semipermeable membrane at the delivery end of an occlusal balloon. 
     FIGS. 7A-7B schematically illustrate several steps in a technique to attach a semipermeable membrane to the delivery end of an occlusal balloon. 
     FIG. 8 schematically shows the practice of hemodialysis according to the methods of this invention. 
     FIG. 9 shows the time evolution of the osmotic pressure and the osmotic pressure and heparin transfer for a heparin aqueous solution with no albumin. 
     FIG. 10 shows the time evolution of the osmotic pressure and the osmotic pressure and heparin transfer for a heparin aqueous solution with 1% albumin. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to vascular access systems, devices and methods, and in particular to venous access systems and devices, and methods for external treatment of blood, particularly vein-to-vein hemodialysis, that permit access to the blood flow while avoiding repeated punctures into the blood vessel being accessed. To this end, an exemplary embodiment of the system of the present invention includes the following components: a graft vessel that is adapted for being anastomosed to the blood vessel that is to be repeatedly accessed, an occlusal balloon, a port device, and a semipermeable membrane that permits the selective and controlled exposure of the blood flow to an agent such as a physiologically active agent. 
     Selective and controlled exposure to a physiologically active agent is, in some preferred embodiments, provided by letting such agent migrate from the interior of an occlusal balloon into the blood in the vessel being accessed. This migration of a physiologically active agent is preferably realized by diffusion across a semipermeable membrane of adequately chosen porosity. In addition, preferred embodiments of the semipermeable membrane function according to the present invention by letting the migration of an aqueous fluid from the blood stream in the vessel being accessed into the interior of an embodiment of an occlusal balloon. This migration of aqueous fluid is preferably realized by permeation across a semipermeable membrane of adequately chosen porosity. By migrating from the blood stream into the interior of an embodiment of an occlusal balloon, this aqueous fluid keeps the occlusal balloon in a distended configuration by osmosis, thus preventing the invasion of the anastomosed graft by blood from the accessed vessel. 
     FIG. 1 schematically and generally shows in a cross sectional view relevant features of this invention as illustrated by an exemplary embodiment. Blood vessel  10  in this exemplary embodiment is accessed with the aid of graft vessel  20  that is anastomosed to blood vessel  10  at anastomosis site  21 . Graft vessel  20  houses, in this particular embodiment, occlusal balloon  40  with a delivery end  42  and an access end  44 . 
     The anastomosed graft of this invention provides a passage for establishing fluid communication with the lumen of the vessel, for example a vein, being accessed. Methods, systems and devices for anastomosing a graft vessel to a blood vessel have been disclosed, for example, in copending U.S. patent application Ser. No. 09/293,336, now abandoned, which is entitled Methods, Systems and Apparatus for Intraluminally Directed Vascular Anastomosis, filed on Apr. 16, 1999, and U.S. application Ser. No. 09/293,617, now U.S. Pat. No. 6,248,117, which is entitled Anastomosis Apparatus for Use in Intraluminally Directed Vascular Anastomosis, filed on Apr. 16, 1999. Both applications, including Ser. No. 09/293,336 and Ser. No. 09/293,617, are herein incorporated by reference in their entirety. The present invention, however, does not require a specific anastomosis technique for its implementation. 
     A plurality of factors may cause blood flowing in lumen  12  of blood vessel  10  to coagulate in the region near anastomosis site  21  resulting in vessel thrombosis. These factors include the presence of foreign bodies used in the anastomosis procedure, irregularities at anastomosis site  21 , and disrupted intima at anastomosis site  21 . To prevent this formation of blood clots, blood flowing in lumen  12  is exposed in the region near to delivery end  42  to an anticoagulant agent that is provided with the aid of occlusal balloon  40 . 
     In addition to coagulation, blood flow stagnation in the region near anastomosis site  21  should be minimized and preferably avoided. To this end, occlusal balloon  40  is so configured as to be able to seal the anastomosis site in a way such that no significant cavity is formed at anastomosis site  21 ; the presence of a cavity or substantially recessed space in this region would lead to blood flow stagnation or to a clot. In addition, a cavity or substantially recessed space can lead to the formation of unacceptably turbulent blood flow when the device is anastomosed to an artery. 
     Graft vessel  20  is shown in FIG. 1 as being anastomosed to blood vessel  10  which is accessed at anastomosis site  21 . Graft vessel  20  has port end  24  which is opposite to anastomosis end  22 ; port end  24  is in this exemplary embodiment connected to port device  50 . Graft vessel  20  and port device  50  are typically located subcutaneously and graft vessel  20  is made of a material such as polytetrafluoroethylene (PTFE) or some other biocompatible self sealing material that can be punctured as schematically shown in FIG. 1 by hypodermic needle  58  or by any other medical device that is ordinarily used to inject fluids in or to draw fluids from a cavity. As illustrated by the embodiment shown in FIG. 1, delivery end  42  of occlusal balloon  40  generally corresponds with anastomosis end  22  of graft vessel  20  in the sense that both ends are generally located in the region of the anastomosis site  21 . 
     The exemplary embodiment of port device  50  shown in FIG. 1 comprises conduit  52  that is connected in fluid communication at one of its ends with occlusal balloon  40  at access end  44 . The opposite end of conduit  52  can be externally accessed through a self-sealing aperture  54 . This self-sealing aperture can be penetrated by a hypodermic needle or any other medical instrument that is typically used to inject fluid into or to draw fluid from a cavity. Embodiments of the self-sealing aperture according to this invention are preferably made of silicone rubber. Port devices such as port device  50  are common medical devices and hence no further detailed description of the structure of the connection of such a port device to access end  44  is herein sketched. Commercially available port devices for vascular access include devices that are marketed under the trademarks OmegaPort, TitanPort, and Vortex, by Horizon Medical Products, Manchester, Georgia, and under the trademarks P.A.S. Port and P.A.S. Port II by Smiths Industries Medical Systems, or SIMS Deltec, Inc., Saint Paul, Minn. 
     As shown in the example depicted in FIG. 1, port end  24  of graft vessel  20  is detachably connected to port device  50  by a pressure device  51  that exerts sufficient pressure to maintain the leak proof attachment of graft vessel  20  to port device  50 . Pressure device  51  can in particular be embodied by an O-ring or by any other device that exerts sufficient pressure to maintain the leak proof attachment of graft vessel  20  to port device  50 . This leak proof attachment can be accomplished in other embodiments of this invention by a threaded engagement, a snap joint engagement, a bound engagement, an adhesive bound engagement, or by any type of leak proof engagement that is well known in the art. Embodiments of the port device are preferably made of stainless steel or titanium, although other biocompatible materials can also be used, particularly other biocompatible materials that are preferably resistant to the abrasion of sharp needle tips. 
     Occlusal balloon  40  can be inflated with fluid provided thereto through port device  50 , in which case occlusal balloon  40  prevents the flow of blood into graft vessel  20  by occluding and effectively sealing anastomosis site  21 . Occlusal balloon  40  can be selectively deflated by drawing its fluid content through port device  50 , in which case blood flow from blood vessel  10  invades the interior of graft vessel  20  through anastomosis site  21 . Embodiments of inflatable balloons according to the present invention, are made of any elastic biocompatible material, such as rubber, PTFE, latex, and combinations of these materials. When the embodiment of the inflatable balloon comprises a membrane that is attached to the balloon with an adhesive, the balloon material is preferably gluable, such as silicone rubber. 
     When blood flow from blood vessel  10  reaches the interior of graft vessel  20  because occlusal balloon  40  is in a deflated configuration, graft vessel  20  can be punctured by a needle to perform, for example a hemodialysis. When the dialysis session is finished, occlusal balloon  40  can be inflated again by injecting an appropriate fluid through port device  50  and any remaining blood left near access end  44  can be drawn out of this space and replaced with a fluid such as saline solution or any other appropriate biocompatible fluid. 
     In its inflated configuration, occlusal balloon  40  is filled with a fluid that causes, or in some embodiments contributorily causes, the expansion within elastic compliance limits of such balloon. In addition, delivery end  42  of occlusal balloon  40  is preferably manufactured to expose the blood flow in blood vessel  10  near anastomosis site  21  to a physiologically active agent such as an anticoagulant agent. 
     A preferred embodiment of this invention comprises an occlusal balloon which can be repeatedly inflated and deflated within its elastic compliance limits. The occlusal balloon in its inflated configuration effectively seals the graft vessel at the anastomosis site. 
     In some embodiments of this invention, the fluid injected into the occlusal balloon cannot diffuse out of the occlusal balloon, in which case it is this fluid that directly causes the inflation of the occlusal balloon. An occlusal balloon of this type is herein described as a self-contained occlusal balloon. 
     In other preferred embodiments of this invention, the occlusal balloon is configured with a semipermeable membrane that allows for fluid transport out of and into the interior of the occlusal balloon, in which case the fluid injected into the occlusal balloon contributorily causes its initial inflation, with other phenomena, such as osmosis, causing the occlusal balloon to remain in an inflated configuration. 
     The exposure of the blood in the vessel being accessed to a physiologically active agent can be accomplished, in particular, by delivering into the interior of occlusal balloon  40  a fluid that contains a physiologically active agent, particularly anticoagulants such as heparin at the appropriate dosage, and allowing this heparin to be transported into luminal space  12  of blood vessel  10  across a semipermeable membrane of the adequate porosity that is part of delivery end  42  of occlusal balloon  40 . These features and elements of a vascular access device according to this invention function to provide a selective and controlled exposure, and more specifically, to provide a selective and controlled transport. 
     In this specific exemplary embodiment, this semipermeable membrane preferably allows the flow of aqueous fluid from the blood flow in blood vessel  10  into the interior space of occlusal balloon  40  by osmotic pressure. Osmosis can be accomplished by delivering into the interior of occlusal balloon  40  a fluid that contains a preferably biocompatible substance that cannot permeate across the membrane through which heparin is delivered. An example of such substance is albumin. The fluid within occlusal balloon  40  thus contributes in providing the adequate conditions for osmosis to take place and hence to the maintenance of occlusal balloon  40  in an inflated configuration as heparin, or some other substance, diffuses from the interior of occlusal balloon  40  into the blood flow in blood vessel  10 . 
     In addition to, or instead of, heparin or another anticoagulant, occlusal balloon  40  of the present invention can be used to deliver a medication, and in particular a medication for a long term treatment of a chronic disease. This medication can also be delivered by letting it diffuse across a permeable membrane at delivery end  42  of occlusal balloon  40 . In the exemplary embodiment shown in FIG. 1, heparin and any other substance that diffuses through a semipermeable membrane at delivery end  42  can be periodically supplied to the interior space of occlusal balloon  40  by injection through port device  50 . 
     In the practice of hemodialysis and also in the prolonged delivery of medicine for the treatment of a chronic disease, the occlusal balloon of this invention typically contains an aqueous solution that includes a high molecular weight substance that cannot diffuse through the pores of the chosen semipermeable membrane and at least one physiologically active agent of a smaller molecular weight that can diffuse through the pores of the chosen semipermeable membrane. As indicated in the discussion of the embodiment shown in FIG. 1, the preferred high molecular weight substance is albumin and the preferred physiologically active agent is typically heparin. 
     In some of the embodiments of this invention, heparin is the physiologically active agent and also the solute whose concentration gradient gives rise to the osmotic pressure that keeps the occlusal balloon inflated. The occlusal balloon holds in these embodiments a relatively large volume of solution so that the concentration of heparin does not decrease too rapidly as a consequence of its diffusion rate across the properly chosen semipermeable membrane. 
     The aqueous solution of albumin and heparin provides the concentration gradient driving the osmotic process which in turn keeps the occlusal balloon in an inflated configuration. Osmosis in this context involves the diffusion of aqueous fluid from the blood in the blood vessel being accessed into the interior of the occlusal balloon through the pores of an appropriately selected semipermeable membrane that is in contact with the blood flow at the anastomosis site. Albumin used in this invention is preferably human albumin with a molecular weight of approximately 65000. 
     Heparin diffuses through the pores of such semipermeable membrane into the blood in the blood vessel which is being accessed, thus preventing the coagulation of blood that might otherwise take place as a consequence of a variety of factors that are associated with the features of the anastomosed structures. The molecular weight of the heparin preferably used in embodiments of the present invention ranges from about 500 to about 18000. Heparin inhibits reactions that lead to the clotting of blood and the formation of fibrin clots both in vitro and in vivo. The clinical pharmacology of heparin is that of a substance that acts at multiple sites in the normal coagulation system. In particular, small amounts of heparin in combination with antithrombin III (heparin cofactor) can inhibit thrombosis by inactivating activated Factor X and inhibiting the conversion of prothrombin to thrombin. Once active thrombosis has developed, larger amounts of heparin can inhibit further coagulation by inactivating thrombin and preventing the conversion of fibrinogen to fibrin. It is reported that heparin also prevents the formation of a stable fibrin clot by inhibiting the activation of the fibrin stabilizing factor. 
     In choosing the appropriate concentrations of albumin and heparin, however, a variety of determining factors have to be taken into consideration. Heparin and albumin associate to some extent. This association leads to the effective sequestering of heparin that is not available to diffuse into the blood stream. In addition, some of the albumin can be adsorbed on the semipermeable membrane, thus decreasing the effective concentration of albumin that influences osmosis. 
     The concentration of albumin is accordingly determined so that the osmotic pressure is comparable to and slightly greater than the vascular pressure in the blood vessel being accessed. For example, venous pressure is typically in the approximate range of about 5 mmHg to about 15 mmHg, and rarely exceeds 30 mmHg, in which case a venous vascular access according to this invention should preferably provide an albumin solution in the occlusal balloon at an osmotic pressure slightly greater than 30 mmHg, such as in the approximate range of about 35 mmHg to about 45 mmHg. 
     “Nominal molecular weight pore size membrane” in this context characterizes a semipermeable membrane whose pore size is such that particles whose molecular weight is less than the given nominal molecular weight are able to diffuse through the membrane&#39;s pores, whereas substances whose molecular weight is greater than or about equal to the given nominal molecular weight cannot diffuse through the membrane&#39;s pores. Unless otherwise indicated, molecular weights given herein are expressed in Daltons; albumin concentration units given herein are expressed as a percentage that refers to mass in grams of albumin in 100 ml of solution, and heparin concentration units are expressed as International Units (IU) heparin per ml of solution. 
     The preferred membrane used in embodiments of this invention is formed from polyethersulfone and is most preferably the semipermeable material sold as Biomax from Millipore. This semipermeable membrane is available in several nominal molecular weight pore sizes in the range from about 5000 to about 50000. Preferred membranes for embodiments of this invention are characterized by a pore size in the range from about 30000 to about 50000 nominal molecular weight. Among these types of semipermeable membrane, a more preferred type is a membrane with a nominal molecular weight pore size of about 50000. 
     In general, preferred membranes for embodiments of this invention are ultrafiltration membrane materials. In addition to the Biomax membrane, Millipore provides other membranes such as regenerated cellulose membranes sold as Amicon 4M which has a nominal molecular weight pore size of about 1000 to about 100000, and hydrophilic polysulfone membrane sold as Amicon Zm which has a nominal molecular weight pore size of about 500 to about 500000. 
     Generally, semipermeable membrane base materials include polymeric materials such as polytetrafluoroethylene, polysulfone, polyamide, polyacrylonitrile, and cuprophane of the adequate pore size, although the hydrophobicity of some polymers requires the treatment of the base material prior to its use as a semipermeable membrane. 
     Clinical dialyzer materials that can be used in the context of this invention include a cuprophane material sold as CF 15.11 from Baxter Health Care Corp., Deerfield, Ill.; cellulose acetate material sold as COAK 4000 and saponified cellulose ester sold as SCE from Cordis Dow Medical, Miami Lalles, Fla.; polymethylmethacrylate Filtryzer membrane from Toray, Tokyo, Japan; cuprammonium material sold as Rayon from Terumo, Japan; and cuprophane material sold as Hemoflow D3 and polysulfone material sold as Hemoflow 60 from Fresenius A. G., Germany. 
     Semipermeable membranes used in different embodiments of this invention can be attached to the delivery end of the occlusal balloon with or without a backing that provides structural support, depending on the type of membrane being used. Also, the occlusal balloon material at the delivery end can in some embodiments provide structural support to the semipermeable membrane. 
     The polyethersulfone membrane used in embodiments of this invention are preferably conditioned prior to its use by immersing it in an albumin solution. For example, by immersing it in a 10% albumin aqueous solution for about one week. Once conditioned, the membrane can be repeatedly used as long as it is not allowed to substantially dehydrate. 
     FIG. 2 shows another exemplary embodiment of the present invention in which an occlusal balloon is used for sealing the end of the graft vessel at the anastomosis site. Blood vessel  110  is being accessed with the aid of graft vessel  120  that is shown as being anastomosed to blood vessel  110  at anastomosis site  121 . Graft vessel  120  houses, in this particular embodiment, occlusal balloon  140  with a delivery end  142  and an access end  144 . 
     To prevent the formation of blood clots, blood flowing in lumen  112  is exposed in the region near to delivery end  142  to an anticoagulant agent that is provided with the aid of occlusal balloon  140 . Instead of, or in addition to, an anticoagulant agent, blood flowing in lumen  112  can be exposed to other substances. In this particular embodiment, the substances to which blood is exposed are initially contained in the interior of occlusal balloon  140  and they are delivered into the blood stream at delivery end  142  which is so configured as to allow diffusion transport into the blood stream of substances, and in particular diffusion of an anticoagulant agent. 
     Delivery end  142  is specifically configured in the embodiment of this invention shown in FIG. 2 with the occlusal balloon material at delivery end  142  being perforated and adjacent to a suitable semipermeable membrane  143 . In this configuration, a substance that is to be delivered into the blood stream can pass through the perforations at delivery end  142 , reach semipermeable membrane  143 , and diffuse into the blood stream through the pores of semipermeable membrane  143 . Instead of perforations, occlusal balloon material can have at delivery end  142  any other feature that performs the same function that is performed by perforations, namely allowing for the passage of fluid from and towards semipermeable membrane  143 . 
     In addition to coagulation, blood flow stagnation in the region near anastomosis site  121  must be minimized and it is preferably avoided. To this end, occlusal balloon  140  is so configured as to be able to seal the anastomosis site in a way such that no significant cavity is formed at anastomosis site  121 . As indicated regarding the embodiment shown in FIG. 1, the presence of a cavity or substantially recessed space in this region may lead to blood flow stagnation, clot formation, and, in arteries, formation of unacceptably turbulent blood flow. 
     Graft vessel  120  is shown in FIG. 2 as being anastomosed to blood vessel  110  which is accessed at anastomosis site  121 . Graft vessel  120  has port end  124  which is opposite to anastomosis end  122 ; port end  124  is in this exemplary embodiment connected to port device  150 . Graft vessel  120  and port device  150  are typically located subcutaneously and graft vessel  120  is made of a material such as polytetrafluoroethylene (PTFE) or some other biocompatible self sealing material that can be punctured as schematically shown in FIG. 2 by hypodermic needle  158  or by any other medical device that is ordinarily used to inject fluids in or to draw fluids from a cavity. 
     Graft vessel  120  as shown in the embodiment depicted in FIG. 2 is preferably provided with an enlarged portion  125  near anastomosis end  122 . This enlarged portion provides a recessed space into which delivery end  142  and semipermeable membrane  143  collapse when occlusal balloon  140  is deflated. 
     The exemplary embodiment of port device  150  shown in FIG. 2 comprises conduit  152  that is connected in fluid communication at one of its ends, access end  144 , with occlusal balloon  140 . The opposite end of conduit  152  can be externally accessed through a self-sealing aperture  154 . This self-sealing aperture can be penetrated by a hypodermic needle or any other medical instrument that is typically used to inject fluid into or to draw fluid from a cavity. As indicated regarding the embodiment shown in FIG. 1, port devices such as port device  150  are common medical devices and hence no detailed structure of the connection of such port device to access end  144  is herein sketched. 
     As shown in the example depicted in FIG. 2, port end  124  of graft vessel  120  is detachably connected to port device  150  by a pressure device  151 . As indicated in the discussion of the embodiment depicted in FIG. 1, pressure device  151  can be embodied by any device that exerts sufficient pressure to maintain the leak proof attachment of graft vessel  120  to port device  150 . This leak proof attachment can be accomplished in other embodiments of this invention by other engagements as disclosed in the preceding discussion of the embodiment shown in FIG.  1 . 
     The graft vessel of this invention can be provided with a reinforcement structure in its entire length or in part of its length. An exemplary embodiment of such reinforcement structure is schematically shown in the embodiment depicted in FIG. 2 by reinforcement rings  123 . These rings are preferably embedded into the material, for example PTFE, of which graft vessel  120  is made, but they can also be partially embedded or externally disposed on graft vessel  120  and attached thereto. Instead of rings, these reinforcement structures can be embodied by a helical coil, longitudinal features aligned with the longitudinal axis of occlusal balloon  140 , longitudinal features that present any one amongst a variety of possible chiral configurations, crisscross stripes, or any other reinforcement pattern that is known to provide structural reinforcement to a flexible, generally cylindrical body. Embodiments of these reinforcement structures are preferably made of plastic. Reinforced PTFE graft material that can be used as graft vessel  120  is sold under the name IMPRA by Bard, under the name MEDOX from Boston Scientific and by W. L. Gore, of Phoenix, Ariz. 
     Typical embodiments of this invention are configured to be adapted to an anastomosis fenestra of about 4 mm, in which case the internal diameter of the graft vessel is about 6 mm. Embodiments of the graft vessel that are provided with an enlarged portion such as enlarged portion  125  in FIG. 2 are configured so that the internal diameter of the graft vessel&#39;s enlarged portion is between about 8 mm and about 9 mm. A typical length of embodiments of the occlusal balloon from its delivery end to this access end is preferably about 2 cm. The length of the graft vessel is preferably chosen so that it provides a plurality of puncture sites. 
     Operations such as inflation, deflation, and use of the embodiment schematically depicted in FIG. 2 are generally performed as described with regard to the embodiment shown in FIG.  1 . 
     Embodiments of this invention that are provided with an occlusal balloon are preferably configured in a way such that the access end of the occlusal balloon and the port device are separated by several centimeters. In some embodiments, however, the inflated occlusal balloon can extend up to and be in contact with the port device. 
     FIG. 3 schematically shows a cross-sectional view along plane  5 - 5 ′ in FIG. 2 of an embodiment of the port device therein depicted. Elements in the cross sectional view are labelled with the same numbers as the corresponding elements are labelled in FIG.  2 . 
     FIG. 4 shows an exploded perspective view of another embodiment of a port device comprising a body  186 , a self sealing plug  182  and means for keeping self sealing plug  182  within body  186  effectively sealing cavity  184 . In some embodiments of this invention, self sealing plug  182  is embodied by a sheet of silicone. This means for keeping self sealing plug  182  is embodied in the example shown in FIG. 4 by compression ring  180 . Body  186  is provided with connector  188  to establish leak proof fluid communication between cavity  184  and the interior of an occlusal balloon. A passage, such as passage  190 , is configured for effectively establishing fluid communication between cavity  184  and the interior of an occlusal balloon attached to connector  188 . This connector is in some embodiments provided with features such as flanges or ridge  192  for assisting in establishing leak proof communication with an occlusal balloon. 
     Some embodiments of this invention may be provided with more than one occlusal balloon. FIG. 5 shows another exemplary embodiment of the present invention which is provided with two occlusal balloons  340  and  341 . 
     Blood vessel  310  is being accessed with the aid of graft vessel  320  that is shown in FIG. 5 as being anastomosed to blood vessel  310  at anastomosis site  321 . Graft vessel  320  houses in this particular embodiment first occlusal balloon  340  with first delivery end  345  and first access end  344 , and second occlusal balloon  341  with second delivery end  343  and second access end  346 . 
     Blood flowing in lumen  312  is exposed in the region near to delivery ends  343  and  345  to agents that are provided with the aid of occlusal balloons  340  and  341 . When more than one agent is to be provided, the range of molecular weights of such agents may be so broad that a single membrane might not be adequate for the diffusion of the different agents into the blood stream. Even if a single membrane were adequate, conditions to be satisfied regarding the replacement, mixing and compatibility of the agents might require that they be kept in different occlusal balloons. In the arrangement shown in FIG. 5, for example, occlusal balloon  340  may contain an aqueous solution of albumin and heparin. Heparin would be delivered into the blood stream by diffusion across a semipermeable membrane at delivery end  345  and the balloon would be kept inflated by osmotic pressure due to the diffusion of an aqueous fluid across the same membrane into the interior of occlusal balloon  340 . Occlusal balloon  341  could contain a solution of one or more physiologically active agents, such as medications, that would be delivered into the blood stream by diffusion across a semipermeable membrane at delivery end  343 . 
     The embodiment shown in FIG.  5  and equivalents thereof are preferred embodiments for long term peripheral vascular access, particularly for venous access for parenteral medication. Membrane  343  in these embodiments is suitable for allowing slow diffusion of small molecular weight solutes, such as medication that requires parenteral administration, including antibiotics, small peptides, and hormones. 
     Delivery ends  343  and  345  of occlusal balloons  340  and  341  are so configured as to be able to seal the anastomosis site in a way such that no significant cavity is formed at anastomosis site  321 . As indicated regarding the embodiment shown in FIGS. 1 and 2, the presence of a cavity or substantially recessed space in this region would lead to blood flow stagnation or to the formation of unacceptably turbulent blood flow, both of which would be expected to predispose to thrombosis. 
     Graft vessel  320  is shown in FIG. 5 as being anastomosed to blood vessel  310  which is accessed at anastomosis site  321 . Graft vessel  320  has port end  324  which is opposite to anastomosis end  322 ; port end  324  is in this exemplary embodiment is connected to port device  350 . Graft vessel  320  and port device  350  are typically located subcutaneously and graft vessel  320  is made of a material such as polytetrafluoroethylene (PTFE) or some other biocompatible self sealing material that can be punctured as schematically shown in FIG. 5 by hypodermic needle  358  or by any other medical device that is ordinarily used to inject fluids in or to draw fluids from a cavity. 
     As indicated in the discussion regarding the exemplary embodiment shown in FIG. 2, graft vessel  320  may have an enlarged portion near anastomosis end  322  like enlarged portion  125  shown in the embodiment depicted in FIG.  2 . Such an enlarged portion provides a recessed space for accommodating collapsing delivery ends  343  and  345  as occlusal balloon  340  is deflated. Deflation of occlusal balloon  340  is accompanied when necessary by deflation of occlusal balloon  341 . 
     The exemplary embodiment of port device  350  shown in FIG. 5 comprises conduits  352  and  353 . One of the ends of conduit  352  is in fluid communication with occlusal balloon  340  and the opposite end can be externally accessed through self-sealing aperture  354 . Analogously, one of the ends of conduit  353  is in fluid communication with occlusal balloon  341  and the opposite end can be externally accessed through self-sealing aperture  355 . Self-sealing apertures  354  and  355  can be penetrated by a hypodermic needle or any other medical instrument that is typically used to inject fluid into or to draw fluid from a cavity. Self-sealing apertures  354  and  355  may be arranged relative to each other in port device  350  in a variety of ways. For example, they can be located next to each other and aligned on the same side of port device  350  as shown in FIG. 5, or they can be located at any desired angle relative to each other and facing along different axial directions. As indicated regarding the embodiments shown in FIGS. 1 and 2, port devices such as port device  350  are common medical devices and hence no detailed structure of the connection of such port device to access ends  344  and  346  is herein sketched. 
     It is understood that configurations of balloons  340  and  341  that depart from that shown in FIG. 5 while including the basic elements therein shown are within the scope of this invention. For example, access ends  344  and  346  can in some embodiments be flush with respect to each other, or balloon  341  can in some embodiments be contained within balloon  340 . In other embodiments, balloons  340  and  341  are placed within the graft vessel essentially next to each other, in which case the balloon that is located closer to the port device preferably has an elongated delivery end that extends substantially up to the anastomosis site. Either one of access ends  344  or  346 , or both access ends, can in some embodiments extend back to port device  350  and be in contact engagement with such port device, particularly in the inflated configuration. In still other embodiments, balloon  341  can be predominantly located in the space between access end  344  and port device  350 , a configuration that would be preferred if balloon  341  had to be subject to, for example, special pressure conditions. In this latter case, an additional conduit, not shown in FIG. 4, would establish fluid communication between the delivery end of balloon  341  and membrane  343 . Whether including only one occlusal balloon or a plurality of balloons, the foregoing features describe a variety of embodiments of this invention. In addition, the access end of one or the access ends of several balloons of some embodiments of this invention can be directly connected to port device  350  either in the form of an integral attachment, a detachable connection, or by bonding, such as by adhesive bonding. These embodiments are particularly suitable when any portion of the conduit connecting an access end of a balloon with a self-sealing aperture in the port device has to be eliminated. 
     Port devices according to this invention can also be embodied by port devices that have additional ports for conventional uses, such as ports that are configured to operate probes, sampling devices, imaging devices and imaging device elements, or medical intervention assisting devices. 
     As shown in the example depicted in FIG. 5, port end  324  of graft vessel  320  is detachably connected to port device  350  by a pressure device  351  that exerts sufficient pressure to maintain the attachment of graft vessel  320  to port device  350  leak proof. The pressure device  351  is preferably an embodied by an O-ring. As noted in the description of the embodiments shown in FIGS. 1 and 2, this leak proof attachment can be embodied by threaded engagement, a snap joint engagement, a bound engagement, in particular an adhesive bound engagement, or by any type of leak proof engagement that is well known in the art. 
     The semipermeable membrane in embodiments of this invention that allow for diffusion of matter into the blood stream can be attached to an occlusal balloon in a variety of configurations. Three examples of such configurations are shown in FIGS. 6A-6C. FIG. 6A depicts a configuration like that of the embodiment shown in FIG. 1, where occlusal balloon material at the delivery end  401   a  is punctured, perforated, or its structure is such that it allows for the passage of fluid across it so that it is the semipermeable membrane  402   a  that determines which species diffuse across it into and from the blood stream. In a preferred configuration shown in FIG. 6B, occlusal balloon material at delivery end  401   b  is provided with a window that generally corresponds with a functional portion of semipermeable membrane  402   b.  In another configuration shown in FIG. 6C, occlusal balloon material at delivery end  401   c  is provided with features  409  that brace the edges of semipermeable membrane  402   c.    
     The interior of the occlusal balloon of this invention is in fluid communication with the self-sealing aperture in the port device through an essentially leak-proof connection. This connection is achieved in any of the forms known in the art. Consequently, the detailed features shown in the accompanying Figures pertaining to this connection are merely exemplary and they are not to be regarded as descriptive of a unique way of achieving such connection. 
     Some embodiments of this invention are provided with a graft vessel that has enlarged portion  125  for housing the collapsed balloon as shown by the phantom lines in FIG.  2 . This configuration, or any other equivalent configuration, reduces any possible impediment to the blood flow through the graft vessel that could otherwise be caused by the deflated balloon. Although enlarged portion  125  is not shown for the sake of clarity in the illustrations of the embodiments shown in FIGS. 1 and 5, these and any other embodiment of this invention, can optionally be provided with such enlarged portion of the graft vessel. 
     FIG. 6D schematically shows a portion of an embodiment of an occlusal balloon in which semipermeable membrane  402   d  is integrally formed in occlusal balloon material at delivery end  401   d.  In one embodiment of this invention, the occlusal balloon is made of, for example, PTFE that is impermeable to the solvent and solute or solutes in the occlusal balloon, and the delivery end of the balloon is made of porous PTFE that embodies semipermeable membrane  402   d.    
     In addition to single layer and bi-layer configurations described above for the disposition of the semipermeable membrane at the delivery end of the occlusal balloon, other configurations are also possible. These additional configurations include a tri-layer configuration and configurations in which the semipermeable membrane is sandwiched between two layers of material, one at each side of the membrane, that allow for the passage of fluid from and to the membrane. 
     Preferably, the shape of the functional portion of the semipermeable membrane used in some embodiments of this invention is generally circular, in which case corresponding features at the delivery end of the occlusal balloon are also generally circular. These shapes, however, are not unique or determinative of the characteristics and functions of the vascular access device of this invention, and other geometrical shapes can also be used, particularly when the base materials or manufacturing tools can more efficiently be used with noncircular membranes. 
     The occlusal balloon of specific embodiments of this invention at its delivery end and the membrane or membranes therein located present a generally curved surface that slightly protrudes out of the occlusal balloon&#39;s body. This generally curved surface is preferably convex on the side exposed to the blood stream of the blood vessel being accessed. This preferred shape is consistent with the slightly greater pressure within the occlusal balloon relative to the vascular pressure in the blood vessel being accessed by an embodiment of a device according to this invention. 
     Although a variety of techniques can be relied on to attach a semipermeable membrane to the delivery end of an occlusal balloon as shown in FIG. 6B, a preferred technique comprises the steps of placing a protective material  510  between occlusal balloon delivery end  512  and semipermeable membrane and bonding, preferably with a biocompatible adhesive, contour  512  of semipermeable membrane to the terminal end of the occlusal balloon as schematically shown in FIG.  7 A. Occlusal balloon material  518 , which may be formed from expandable material such as silicone or latex, is subsequently cut as indicated by broken arrows A-A′, thus obtaining the type of configuration shown in FIG.  7 B, where functional region  520  of the semipermeable membrane is typically surrounded by small non-functional portions  522  bound to the occlusal balloon material. 
     Although preferred embodiments of occlusal balloons according to this invention include a semipermeable membrane that allows for transport and is part of the osmosis that keeps the occlusal balloon inflated, other embodiments of the occlusal balloon do not include any semipermeable membrane. For example, some embodiments of the occlusal balloon are inflated by the injection of a fluid that is kept within the balloon while it is inflated, with no osmosis contributing to its distension. These embodiments are configured so that the exposure to a physiologically active agent of the blood in the vessel being accessed is accomplished by merely subjecting the blood stream to contact with the agent rather than by relying on diffusion across a membrane and subsequent diffusion in the blood stream. The effects of this contact are predominantly in situ or local effects. These type of occlusal balloons are self-contained occlusal balloons. 
     When the physiologically active agent is heparin, in situ prevention of clot formation is preferably achieved by subjecting the blood stream to contact with heparin in a heparin immobilizing biocompatible material at the delivery end of the self-contained occlusal balloon. Heparin immobilizing materials include polyvinyl alcohol; surface-modified polymeric biomaterials with poly(ethylene oxide), albumin, and heparin; derivatized dextrins; polymers with hydrophilic spacers; vinyl-pyridine-grafted styrene-butadiene-styrene triblock copolymer; and dimethyl-amino-ethyl-methacrylate-grafted styrene-butadiene-styrene triblock copolymer. 
     Furthermore, a multifunctional thrombo-resistant coating can be incorporated on the delivery end of an occlusal balloon. This coating includes a siloxane surface onto which a plurality of amine functional groups have been bonded. Covalently bonded to the amine functional groups are a plurality of poly(ethylene oxide) chains, such that a single poly(elthylene oxide) chain is bonded to a single amine functional group. A plurality of different bioactive molecules, designed to counteract specific blood-material incompatibility reactions, are covalently bonded to poly(ethylene oxide) chains, such that a single bioactive molecule is coupled to a single poly(ethylene oxide) chain. Methods of manufacturing these materials have been previously described. See, for example, International Patent Applications Nos. PCT/US89/01853 and PCT/US91/02415, which are herein incorporated by reference in their entirety. The resulting siloxane that is so manufactured contains a plurality of different bioactive molecules capable of reacting with blood components which come in proximity to the siloxane surface in order to resist blood-material incompatibility reactions. 
     In the preferred embodiments of the occlusal balloon of this invention with a semipermeable membrane, the physiologically active agent is effective at the release site, namely in situ. The dosage can be regulated so that the active agent is effective systemically because the active agent circulates with the blood stream. This type of sources of physiologically active agents are herein described as permeating sources of physiologically active agents, and they include embodiments such as those shown in FIGS. 6A-6D. The dose required to achieve the anticoagulant effect locally is much less than a systemically therapeutic dose, thus the long term risk associated with in situ effects is less than the risk associated with full systemic anticoagulation. 
     When the physiologically active agent is provided by immobilizing it on a self-contained occlusal balloon, the active agent is predominantly effective in situ, at or near the contact site. Such sources of physiologically active agents are herein described as in-situ sources of physiologically active agents. They include embodiments of the delivery end of an occlusal balloon on which the physiologically active agent is attached at the outer surface that is exposed to the blood flow. 
     In addition, other embodiments of this invention incorporate a self-contained balloon that provides a source of at least one physiologically active agent whose effects are manifested in situ and systemically without transport across a semipermeable membrane. In these embodiments, the physiologically active agent is typically released by a substance that is incorporated on the delivery end of the occlusal balloon that is exposed to the blood flow. This type of sources of physiologically active agents are herein described as nonpermeating sources of physiologically active agents. For example, when the physiologically active agent is an anticoagulant, nitrogen oxide releasing polymers can be incorporated on the delivery end of the occlusal balloon so that NO is released into the blood stream. Examples of NO-releasing polymers include diazeniumdiolates added to plastics such as polyvinylchloride and polyurethane. In this case, diazeniumdiolates include specific compounds such as sodium 1-(N,N-diethylamino)diazen-1-ium-1,2-diolate, disodium 1-[2(S)-carboxylatopyrrolidin-1-yl]diazen-1-ium-1,2-diolate, sodium 1-(piperazin-1-yl)diazen-1-ium-1,2-diolate, and 1-{N-methyl-N-[6-(N-methylammonio)hexyl]amino}diazen-1-ium-1,2-diolate. 
     The features of each one of the herein described embodiments of the occlusal balloon are not meant to be exclusive of features of other embodiments that can be incorporated in the same occlusal balloon to render a functional combination. For example, an occlusal balloon with a semipermeable membrane can also incorporate a source of a physiologically active agent for predominantly in-situ effects, and/or incorporate a source of a physiologically active agent for in situ and systemic effects of the type described in relation to embodiments of self-contained occlusal balloons. 
     A vascular access with a system according to this invention is preferably created by first performing a vascular anastomosis to attach a graft vessel to the blood vessel that is being accessed, and then placing an occlusal balloon within the graft vessel. This occlusal balloon may be provided with a port device already attached to it, or the port device may be subsequently attached to the occlusal balloon by conventional techniques. Once a vascular access system according to this invention is placed at the access site, the entire system preferably remains subcutaneously placed for its use in procedures such as dialysis, in particular hemodialysis, and drug delivery. 
     It is understood that elements of any embodiment of the vascular access system according to this invention may be provided with suitable radio-opaque markings so that its location or particular configuration can be externally observed. This markings can be particularly useful when incorporated in the vascular graft or in the occlusal balloon. 
     FIG. 8 schematically illustrates an embodiment of a method for externally treating blood according to this invention. In the example shown in FIG. 8, blood is extracted through an extraction vascular access apparatus such as the embodiment of extraction vascular access apparatus  5 , and delivered through a delivery vascular access apparatus such as the embodiment of delivery vascular access apparatus  5 ′. Because the vascular access apparatus of this invention permits multiple vascular access, whether any given vascular access apparatus is employed in any specific treatment episode as an extraction or a delivery apparatus is a matter of convenience and choice. In addition, once blood has been extracted through an extraction vascular access apparatus and there is an available delivery vascular access apparatus to return the blood flow to a blood vessel, such extracted blood can be subjected to a hemodialysis or to any other blood treatment. Consequently, the term “hemodialysis” in the context of this invention is understood to broadly refer to external treatment of blood, including an actual hemodialysis treatment, and any other treatment of blood that is performed outside a patient&#39;s body, and which requires the extraction, treatment and subsequent delivery of the treated blood to the patient. 
     Blood vessels  90  and  90 ′ represent the blood vessels involved in the treatment process. When blood is extracted from blood vessel  90 , it is subjected to treatment, and it is subsequently returned to blood vessel  90 ′, blood vessel  90  is referred to as the extraction blood vessel and blood vessel  90 ′ is referred to as the delivery blood vessel. Although the apparatus, systems and methods of this invention are suitable for the practice of a variety of external treatments of blood, they are particularly suitable for the practice of vein-to-vein hemodialysis. In this case, blood vessels  90  and  90 ′ would represent the vein from which blood is extracted and the vein to which dialyzed blood is injected, respectively. 
     An embodiment of an apparatus or system according to this invention is attached to each one of blood vessels  90  and  90 ′ as schematically shown in FIG. 8 by embodiments and  5 ′, respectively. These embodiments are anastomosed at sites  65  and  65 ′, and they can be embodiments of any of the foregoing vascular access devices and systems of this invention and combinations thereof. For the practice of a preferred method according to this invention, these embodiments comprise occlusal balloons  60  and  60 ′, port devices  30  and  30 ′, and conduits  63  and  63 ′. 
     In some embodiments of this invention, port devices  30  and  30 ′, together with occlusal balloons  60  and  60 ′, define respective chambers  15  and  15 ′. Occlusal balloons  60  and  60 ′ are preferably disposed within graft vessels  66  and  66 ′ in these embodiments so that chambers  15  and  15 ′ allow for the injection therein of a biocompatible fluid, such as isotonic saline solution. In other embodiments of this invention, no significant chamber or significant volume is left between occlusal balloons  60  and  60 ′ and respective port devices  30  and  30 ′. The volume occupied by distended occlusal balloon  60 , plus optionally the volume of chamber  15 , defines an occludable interior in vascular access apparatus  5  that is configured for receiving a fluid such as blood. Similarly, the volume occupied by distended occlusal balloon  60 ′, plus optionally the volume of chamber  15  ′, defines an occludable interior in vascular access apparatus  5 ′ that is configured for receiving a fluid such as blood. 
     As indicated in the description of preferred embodiments of this invention, graft vessels  66  and  66 ′ and port devices  30  and  30 ′ are self-sealing, so they can be repeatedly accessed without requiring replacement or additional sealing procedures after each treatment episode. Access is preferably performed by appropriately puncturing the graft vessels and port devices as desired. The interior of the occlusal balloon is preferably accessed through the corresponding port device, while the interior of the graft vessel is preferably accessed by directly puncturing the graft vessel wall. 
     Occlusal balloons that selectively and controllably expose the blood flow in vessels  90  and  90 ′ to a physiologically active agent are preferred for the practice of the methods of this invention. This can be achieved by any of the occlusal balloon embodiments to this effect that have been described hereinabove and their equivalent devices. Vascular access apparatus  5  and  5 ′ preferably remain subcutaneously placed during the practice of hemodialysis and also during the intervening periods between hemodialysis episodes. 
     Hemodialysis, or any other external blood treatment, is preferably performed according to methods of this invention by extracting blood from blood vessel  90  through vascular access apparatus  5 , having this extracted blood dialyzed, and returning it by injecting it into blood vessel  90 ′ through vascular access apparatus  5 ′. Extraction of blood is preferably performed with occlusal balloon  60  in a deflated configuration. Similarly, injection of blood is preferably performed with occlusal balloon  60 ′ in a deflated configuration. Because the walls of graft vessels  66  and  66 ′ are repeatedly punctured in repeated hemodialysis episodes, blood vessels  90  and  90 ′ remain viable and unaffected by the repeated access. This procedure facilitates vein-to-vein hemodialysis because the number of venous sites that are available for extended periods of time for the practice of hemodialysis is very limited. Furthermore, the practice of vein-to-vein hemodialysis is a desirable dialysis practice because AV (arterio-venous) graft hemodialysis typically leads to venous hyperplasia and stenosis. 
     Blood flow into the interior of vascular access apparatus  5  or  5 ′ is achieved by deflating occlusal balloon  60  or  60 ′, respectively. This can be achieved by drawing the fluid that keeps occlusal balloon  60  in a distended configuration through port device  30 , which is maintained in fluid communication with occlusal balloon  60  through conduit  63 . An analogous operation can be performed to achieve blood flow into the interior of vascular access apparatus  5 ′, which involves the deflation of balloon  60 ′ by drawing fluid through conduit  63 ′ and port device  30 ′. When blood flow into graft vessels  66  and  66 ′ has been allowed, hemodialysis or any other external blood treatment can proceed in a conventional manner. For this purpose, graft vessels  66  and  66 ′ are punctured and blood is allowed to flow from vessel  66  to vessel  66 ′. When necessary, blood flow is forced with an appropriate pump. 
     Depending on the specific treatment to which the blood is subjected externally, the device that provides such treatment is part of the fluid communication between the extraction vascular access apparatus and the delivery vascular access apparatus. In certain treatments, such as irradiation, the blood flow is exposed to the treating effects without actually being in fluid communication with the device that provides such effects. Since the blood flow must interact in some external manner with the device that provides the treatment, it is said that the fluid communication between the extraction vascular access apparatus and the delivery vascular access apparatus encompasses communication with a blood treating device. 
     Examples of external blood treatments that can be performed with the present invention include plasmapheresis, cytopheresis, hemodialysis, apheresis, hemoperfusion, and hemofiltration. 
     In some of these treatments, such as plasmapheresis—also known as plasma separation or plasma exchange—, whole blood is removed from the body, the bloods cellular components are separated in a blood treatment device, and subsequently reinfused in a saline solution or some other plasma substitute, thus depleting the body&#39;s own plasma without depleting its blood cells. In this case, the external treatment of blood is typically performed with a cell separator. Plasmapheresis is currently widely accepted for the treatment of myasthenia gravis, Lambert-Eaton syndrome, Guillain-Barré syndrome, and chronic demyelinating polyneuropathy. An average course of plasma exchanges is six to ten treatments over two to ten weeks, with some centers performing one plasmapheresis session per week and other centers performing more than one session per week. Patients undergoing plasmapheresis are typically administered blood anticoagulant medications, and the blood treatment device includes a plasmapheresis separator. Plasmapheresis and cytopheresis are specific instances of the more general apheresis, which is the withdrawal of whole blood from the body, separation of one or more components, and return by transfusion of the remaining blood to the donor. 
     Hemoperfusion is the technique of passing blood extracted from the body through an extracorporeal sorbent column for the purpose of removing harmful substances. In one practice of hemoperfusion, blood is passed through a blood treatment device that comprises a biocompatible hemoperfusion cartridge that contains activated carbon adsorbent coated with an antithrombogenic heparin-hydrogel. This technique permits the removal of a variety of toxins in the blood, and it is used in the treatment of drug overdoses, hepatic failure, encephalopathy, and removal of chelated aluminum from hemodialysis patients. 
     Hemodialysis is one of the more common forms of dialysis used in the United States. In hemodialysis, a hemodialyzer, or artificial kidney, takes the place of failed kidneys which may have lost up to 80 or even 90% of their functions. Patients with chronic kidney or renal failure need dialysis to remove excess urea, fluid, electrolytes, minerals, and other wastes form the blood stream since the kidneys cannot perform this cleansing. In this case, the external treatment of blood is typically performed with a hemodialyzer as a blood treatment device. An ultrafiltration hemodialyzer is a hemodialyzer that uses fluid pressure differentials to typically bring about loss of protein-free fluid from the blood to the bath, as in certain edematous conditions. 
     With hemofiltration, patients have fluid and waste products removed from the blood at a constant rate, twenty-four hours a day, for as long as necessary, with the aid of a blood treatment device that comprises a hemofiltration cartridge. This technique is typically used on patients for whom hemodialysis is not considered safe, and also to treat conditions such as uremia, acute renal failure, refractory fluid overload, and massive edema. 
     Embodiments of this invention that are provided with chambers  15  and  15 ′ would in principle permit the puncturing of the corresponding graft vessels prior to the deflation of the corresponding balloons. However, as indicated above, occlusal balloons  60  and  60 ′ are preferably deflated prior to the puncturing of the respective graft vessels  66  and  66 ′. Similarly, any puncturing device inserted through the walls of graft vessels  66  and  66 ′ is preferably removed prior to the distension of the respective occlusal balloons  60  and  60 ′. 
     When a treatment episode is completed, occlusal balloons  60  and  60 ′ are brought back to their distended configurations by inflating them. Inflation is preferably achieved by injecting a fluid through the respective port devices  30  and  30 ′. These configurations of occlusal balloons  60  and  60 ′ prevent blood flow into the interior of graft vessels  66  and  66 ′, respectively. With the aid of the same needle that has been used in the practice of hemodialysis or with a different puncturing device, the interior of graft vessels  66  and  66 ′ can be washed and their contents replaced by a biocompatible fluid such as isotonic saline solution. 
     As indicated above, embodiments  5  and  5 ′ preferably remain subcutaneously placed and ready to be accessed again in another treatment session. As also indicated in the foregoing description of preferred embodiments of this invention, blood flow in vessels  90  and  90 ′ can preferably be exposed to an appropriate physiologically active agent, such as heparin. This exposure is preferably provided during the periods between hemodialysis sessions, so that the formation of blood clots, or any other coagulation-related phenomenon, near the anastomosis sites is significantly reduced. 
     As indicated in the foregoing description of preferred embodiments of this invention, embodiments  5  and  5 ′ can be additionally used to intravenously deliver medication while at least one of them is anastomosed, and this goal can be achieved too when both embodiments  5  and  5 ′ are anastomosed for hemodialysis. 
     The sequence of steps related to the expansion/contraction of the occlusal balloons, the replacement of any fluid within graft vessels  66  and  66 ′, and the optional intravenous delivery of medicine can be performed according to the methods of this invention in any desired biocompatible order. 
     Examples of Use of Occlusal Balloons 
     FIGS. 9 and 10 show pressure and heparin transfer as a function of time as experienced by an embodiment of a permeable balloon according to the present invention. In these experiments, solutions of heparin were used at concentrations up to about 20000 IU. Solutions of albumin were used at concentrations of up to about 5%. Commercial availability of the respective solutions of heparin and albumin determined the choice of these upper concentration limits. 
     Osmotic pressures were measured with a pressure transducer. The use of this device instead of liquid column height measurements reduces or even avoids errors that are associated with the use of liquid columns. These errors are typically associated with factors such as solute concentration inhomogeneity problems or frictional problems that can lead to incorrect pressure readings. 
     Solutions with heparin at different concentrations were used in successive experiments with embodiments of a permeable balloon. In some experiments, the solutions contained albumin, whereas in other experiments no albumin was present. FIGS. 9 and 10 illustrate results obtained in experiments performed with a heparin solution with no albumin (FIG.  9 ), and with a heparin solution with 1% albumin (FIG.  10 ). In embodiments with solutions that contained heparin only, the osmotic pressure is due to the heparin that remains in the balloon, whereas in embodiments with heparin and albumin solution, the osmotic pressure is due to the albumin and to the heparin that remains in the balloon. The data shown in FIGS. 9 and 10 were obtained with Millipore  50  as semipermeable membrane, and with 20000 IU/ml heparin solutions. 
     As shown in FIG. 9, osmotic pressure of almost 80 mmHg was measured shortly after the heparin solution was placed in a permeable balloon. The pressure remained above 80 mmHg for over 150 h, and remained at or about 100 mmHg for at least 120 h. Heparin transfer rates were about 4 IU ml −1  h −1  one day after the solution was placed in the balloon, and about 7.7 IU ml −1  h −1  about 170 h after the solution was placed in the balloon. 
     FIG. 10 shows that a peak pressure of over 120 mmHg was obtained with a solution that contained heparin and 1% albumin. This observation should be expected because in this case albumin, which does not significantly permeate through the membrane, causes osmotic pressure in addition to the heparin that remains within the balloon. Heparin transfer rates were about 5 IU ml −1  h −1  one day after the solution was placed in the balloon, and almost 9 IU ml −1  h −1  about 150 h after the solution was placed in the balloon. 
     These heparin transfer rates are adequate in light of desirable transport rates in the range of about 5 IU ml −1  h −1  to about 10 IU ml −1  h −1 . These transport rates from a balloon whose volume is about 2 ml lead to the intravenous administration of not more than 500 IU heparin per day, or to the administration of not more than 5000 IU heparin in a ten-day period. 
     Safety measures, in addition to practical factors, determine the preferred size of embodiments of permeable balloons of this invention. The administration of at most about 20000 IU heparin in a single administration is currently regarded as an acceptable risk. The amount of heparin that would be suddenly delivered upon rupture of a 5 ml balloon right after having been filled with heparin solution at a concentration of 20000 IU/ml would be about 100000 IU. Instead, this amount would be about 20000 IU if a 2 ml balloon were filled with 10000 IU/ml heparin. Consequently, a 2-ml balloon is preferred in most embodiments of permeable balloons. 
     The transport rates shown in FIGS. 9-10 also indicate each supply of heparin within the balloon can intravenously provide heparin for at least a ten-day period before the balloon is recharged with a fresh supply of heparin. The pressure data shown in the same Figures show that sufficiently high pressure can be achieved with embodiments of the present invention because even in unusual conditions the venous pressure does not rise above 50 mmHg. 
     The foregoing procedure to determine osmotic pressure and concentrations of substances in the fluid filling of an embodiment of an occlusal balloon according to this invention can be properly adapted with ordinary skill in the art to analogously determine the osmotic pressure and adequate concentrations of other substances in the same or in a different type of vascular access. 
     SUMMARY OF PREFERRED EMBODIMENTS 
     The elements of the embodiments of this invention disclosed hereinabove, equivalents thereof, and their functionalities can be expressed as means for performing specified functions as described hereinbelow. 
     Many examples are provided herein of a means for selectively occluding an opening in a blood vessel. Examples of means for selectively occluding an opening according to this invention include: occlusal balloons such as self-contained occlusal balloons, occlusal balloons with a semipermeable membrane such as balloon  140  in FIG. 3, occlusal balloons with radio-opaque markings, occlusal balloons that are inflated with a liquid, occlusal balloons that are inflated with a gas, and occlusal balloons that are configured to operate in conjunction with or in the presence of at least another occlusal balloon, such as occlusal balloons  340 ,  341  in FIG.  5 . Occlusal balloon  40  in FIG. 1 illustrates an exemplary embodiment of a self-contained occlusal balloon when delivery end  42  does not essentially allow for significant matter transport. Occlusal balloon  40  in FIG. 1 illustrates an exemplary embodiment of an occlusal balloon with a semipermeable membrane when delivery end  42  allows for selective matter transport. 
     Note that each embodiment of the means for selectively occluding an opening according to this invention has a delivery end that is generally located in the region near the anastomosis site and an opposite access end that is typically provided with a connection to the means for selectively providing access to a means for selectively occluding an opening. Each embodiment of a means for selectively occluding an opening in a blood vessel functions according to this invention by adopting a variety of configurations such as a distended configuration and a contracted configuration. In particular, the distended configuration can be an inflated configuration, and the contracted configuration can be a collapsed configuration. Preferably, the distended configuration is adopted when an embodiment of a means for selectively occluding an opening is filled with a liquid, although the fluid filling some of such embodiments can also be a gas. Blood from the accessed vessel cannot infiltrate into the anastomosed graft vessel when the embodiment of the means for selectively occluding an opening is in its distended configuration, whereas fluid communication from the interior of the anastomosed graft vessel into the lumen of the accessed blood vessel is allowed in the contracted configuration of the same embodiment. Any of such specific embodiments is manufactured so that it can change from any one of these particular configurations to the other and vice-versa a plurality of times. The number of times which these changes in configuration are experienced by embodiments of the means for selectively occluding an opening according to this invention can be of the order of the number of injections that a blood vessel would typically be subjected to during a long term treatment of a chronic affliction or during dialysis treatment. 
     Many examples are also provided herein of a means for selectively providing access to a means for selectively occluding an opening in a blood vessel. Examples of means for selectively providing access to a means for selectively occluding an opening in a blood vessel include: port devices such as a port device with one self-sealing access cavity, such as port devices  50  shown in FIG.  1  and port device  150  shown in FIGS. 2-3, a port device with a plurality of self-sealing access cavities, such as port device  350  shown in FIG. 5, and a port device that includes ports for providing conduits to operate probes, sampling devices, imaging devices and imaging device elements, or medical intervention assisting devices. 
     Each embodiment of the means for selectively providing access to a means for selectively occluding an opening facilitates the external introduction into or the extraction from a specific embodiment of the means for selectively occluding an opening of fluid therein contained. In particular, an embodiment of the means for selectively providing access to a means for selectively occluding an opening is adapted for a subcutaneous placement and it is provided with at least one self-sealing cavity for selectively allowing fluid communication through a conduit into the access end of an embodiment of a means for selectively occluding an opening in a blood vessel. 
     Examples are also provided herein of a means for selectively and controllably exposing blood flow to an agent in a vascular access. Means for selectively and controllably exposing blood flow to an agent according to this invention is embodied by means for selectively effectuating transport of an agent in a vascular access, and by means for selectively subjecting blood flow to contact with an agent. Exemplary embodiments of each one of these means are enumerated in turn below. 
     Means for selectively effectuating transport of an agent in a vascular access according to this invention is embodied by permeating sources of agents such as physiologically active agents. These permeating sources are more specifically embodied by sources such as a semipermeable membrane, exemplified by semipermeable membranes  143  shown in FIG. 3,  402   a  shown in FIG. 6A,  402   b  shown in FIG. 6B,  402   c  shown in FIG. 6C, and  402   d  shown in FIG. 6D; sources that include a plurality of semipermeable membranes, exemplified by semipermeable membranes  343  and  345  shown in FIG. 5, semipermeable membranes in any of a mono-layer, bi-layer, tri- or generally multi-layer and sandwiched configurations; sources that include at least a semipermeable membrane with a backing (membrane mounting by backing); sources that include at least a semipermeable membrane braced to an embodiment of means for selectively occluding an opening (membrane mounting by bracing), exemplified by the embodiment shown in FIG. 6C; sources that include at least a semipermeable membrane bonded to an embodiment of means for selectively occluding an opening (membrane mounting by bonding), exemplified by the embodiment shown in FIG. 6B; sources that include at least a semipermeable membrane that is backed by material of an embodiment of means for selectively occluding an opening (membrane mounting by backing), exemplified by the embodiment shown in FIG. 6A; and sources that include a semipermeable region of material of an embodiment of means for selectively occluding an opening, exemplified by the embodiment shown in FIG.  6 D. 
     Each embodiment of a means for selectively and controllably exposing blood flow to an agent in a vascular access is integrally formed in or attached to the delivery end of an embodiment of a means for selectively occluding an opening according to this invention. The means for selectively and controllably exposing blood flow to an agent in a vascular access functions according to the present invention by exposing the blood flow at the anastomosis site to at least one physiologically active agent, such as a substance that will prevent the formation of blood clots. Means for selectively and controllably subjecting blood flow to contact with an agent according to this invention is embodied by in-situ sources of physiologically active agents and by nonpermeating sources of physiologically active agents. 
     The anastomosed graft of this invention provides physical support to a particular embodiment of the means for selectively occluding an opening and to a particular embodiment of the means for selectively providing access to a means for selectively occluding an opening. In preferred embodiments, this support is provided by a housing such that the anastomosed graft contains in its interior an embodiment of a means for selectively occluding an opening which has attached thereto an embodiment of a means for selectively exposing blood flow to an agent in the vascular access. 
     One of the ends of the anastomosed graft of this invention is anastomosed to the vessel being accessed. The opposite end of the anastomosed graft is integrally or detachably connected to an embodiment of means for providing access to a means for selectively occluding an opening. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.