Abstract:
An implanted single- or dual-lumen device for repeated accessing of a vessel within a body, especially for hemodialysis, plasmapheresis, and other fluid exchange therapy treatments. The device is characterized by having: no septum for sealing but uses a resilient material to form a seal; a smooth streamlined flowpath; low flow resistance; substantially no stagnation points, such that the device is easily and completely flushed; and a positive locking mechanism that accepts and retains a matching needle apparatus. The device is joined to a catheter, in most cases, such that fluids can be extracted from or injected into the vessel to be accessed. The device is designed for the high flowrates, on the order of 400 milliliters per minute, associated with hemodialysis, plasmapheresis, and other fluid exchange therapies. A corresponding straight-needle apparatus is designed to mate and lock with the access device, where alignment and open flowpath is ensured. The needle apparatus first penetrates the skin and then the access device via the seal. The access device is flexibly mounted to the body at the attached catheter allowing the access device itself to move under the skin so as to accommodate and align with the needle apparatus.

Description:
This application is a divisional of U.S. patent application Ser. No. 08/485,498, filed Jun. 7, 1995, now U.S. Pat. No. 5,954,691, issued Sep. 21, 1999, and claims the benefit of priority of that application. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to apparatus that allows access to the vascular system of a human (or other animal) for the high-volume fluid flow required in hemodialysis, plasmapheresis, and other fluid exchange therapies. More particularly, the present invention relates to a septum-less subcutaneously implanted access of single- or dual-lumen construct and a mating needle apparatus. 
     BACKGROUND OF THE INVENTION 
     There exists a class of devices for accessing fluid spaces and vessels within a human (or animal) body that are generally referred to as “ports”. Herein, “vessel” is defined as any conduit carrying a fluid within the patient&#39;s body. These prior art devices comprise a chamber having an access opening sealed by means of a septum and having an egress from a second location leading to a catheter disposed within a fluid space or vessel. The septum allows a needle to pass into the chamber, but then closes when the needle is removed, thereby preventing fluid leakage from within a space or vessel and also anything from entering or exiting the chamber. These devices are usually implanted below the skin to prevent infection, other contamination, and mishandling. 
     Ports are designed for relatively infrequent use, perhaps once a week, and, importantly, for flowrates of 50 milliliters per minute or less, as is common during chemotherapeutic treatment. Modification of these devices for hemodialysis, plasma-pheresis, and other fluid exchange therapies, which require much greater flowrates, by simply enlarging the device components, poses several serious drawbacks that effectively limit use in such applications. First, the septum degrades quickly due to the larger gauge needles necessary to accommodate the flowrates required in hemodialysis. Repeated puncturing of the septum by these large needles produces numerous free-floating septum fragments that can find their way into the circulatory system. Accordingly, the useful life of the devices is substantially shortened, thereby defeating one of the purposes of using an implanted subcutaneous device. Second, the flowpath has several stagnation points where clots may form and also is not completely flushable or easily cleaned, thereby providing breeding grounds for infection, once contaminated or a build-up of material which may adversely affect function. Third, the flowpath is not streamlined and contains flowpath obstructions, sharp corners, and abrupt changes in flow area and flow direction. This tends to increase the shear stress and turbulences experienced by blood flowing through the device due to the significantly higher flowrates required in hemodialysis, thereby increasing erythrocyte damage and platelet activation. Also, the tortuous flowpath increases the pressure drop through the devices, which can increase air release and foaming, causing the dialysis machine&#39;s safety alarms to activate. 
     Typical access port apparati are disclosed in U.S. Pat. No. 5,180,365 (Jan. 19, 1993), U.S. Pat. No. 5,226,879 (Jul. 13, 1993), U.S. Pat. No. 5,263,930 (Nov. 23, 1993), and U.S. Pat. No. 5,281,199 (the &#39;199 patent) (Jan. 25, 1994); all entitled “IMPLANTABLE ACCESS DEVICES” and all issued to William D. Ensminger as either the sole or the first-named inventor. Only the &#39;199 patent is assigned, that assignment being to Michigan TransTech Corporation of Ann Arbor, Mich. The following discussion concerns the (assigned) &#39;199 patent; while all of the references are relevant, the &#39;199 patent embodies the most recent material and also incorporates material from each of the earlier patents. 
     The devices described in the &#39;199 patent include a funnel-shaped entrance to an access housing, which is fixed to the surrounding tissue. The housing is connected to an articulated valve, which is in turn joined to a catheter. Several types of valves are disclosed, including one that is a tube fabricated in a flattened shape that is forced open by the insertion of a filament. Other valves disclosed include manually activated types. In these manual valves, manual actuation applied to the skin and translated to the device moves two disks which slide over and in contact with each other to align holes in those disks. A needle may be inserted when the holes are aligned; the disks secure the needle in the housing when the external manual pressure is released. This patent also discloses a curved entry (presumably to allow the needle to enter at a convenient angle to the skin but still align parallel to the vessel). The disclosure of this patent, in column 9, line 53, mentions use in hemodialysis treatment. 
     The Ensminger et al. &#39;199 device has several characteristics which lead to problems. First, in most embodiments the curved needle must be flexible, and as such can kink or otherwise restrict flow. However, when the needle is inserted, no such kink can be seen by the operator, and may not be detected before damage to the patient results. Another drawback of these devices can best be seen by inspection of FIG. 1A of the &#39;199 patent, showing an abrupt change in flow diameter where the catheter 46 is joined to the valve 24. Abrupt changes form space for fluid stagnation to occur and/or eddy currents that promote clot formation. Further, such spaces are not easily flushed due to the lack of a streamlined flowpath. This same problem is shown in FIG. 1A of this patent in the stagnant space around the leaf valves 38. Indeed, such problems exist at nearly every transition point between the various structures and assemblies of the &#39;199 device. 
     A further drawback of the &#39;199 device is the attachment of the housing to the surrounding tissue. Since the housing cannot move to accept a rigid needle, the needle must be closely aligned with the port entrance. Otherwise, the needle must be moved transversely under the skin causing discomfort for the patient. Ensminger et al. required the use of a flexible tube to solve this problem. A still further drawback of the &#39;199 apparatus is shown in FIGS. 41-43. These drawings show needle points where the flow has a radial direction component as it leaves the needle. This change of direction, especially under high flowrates, can severely damage hematocytes and activate platelets. Also, the flexible tube will have a greater flow resistance and higher shear than a rigid straight needle having a similar outside diameter. 
     A general limitation in all relevant prior art devices is the lack of a streamlined flowpath. Without such streamlining, stagnant volumes exist where clots may form and shear stress is higher, tending towards erythrocytic damage. Such locations cannot be flushed or easily cleaned. Blood residue remaining in the devices after flushing may clot and provide breeding grounds for infection, once contaminated. In addition, pressure drops and abrupt flow direction changes may damage blood components. 
     The Ensminger &#39;199 device is still further limited by its lack of effective sealing provisions. There is no force urging the valve to seal. The valve is therefore not fault-tolerant and particles, clots, skin fragments, and imperfections on the inside surface of the valve will cause leakage. In addition, the valve opens in response to very low changes in pressure. Further, the seal is in line with the housing, making the device longer and increasing the changes in pressure experienced by fluids passing through the device. Finally, there is no locking mechanism whereby the needle may be secured to the device. 
     Accordingly, it is an object of this invention to overcome the above illustrated inadequacies and problems of extant devices by providing a totally implantable access means suitable for repeated use in applications (e.g., hemodialysis with blood flowrates of 250 milliliters per minute or more yet with low pressure drops along the flowpath. 
     It is another object of this invention to provide a laminar flowstream, even during flow diameter transitions. 
     It is a further object to provide means where the flowpath is streamlined and provides substantially no stagnation points, and also to provide an apparatus where the entire flowstream is flushable. 
     It is a still further object of this invention to provide apparatus suitable for single- and dual-lumen catheter systems. 
     It is yet another object of this invention to provide an access housing that is less painful during needle insertion and more accommodating during dialysis for the patient. 
     It is a further object to secure the needle within the access housing during the dialysis session. 
     It is another object of the invention, when using dual-lumen catheters to secure both needles to each other. 
     It is a still further object to have lower clotting, stenosis, and infection rates than synthetic grafts. 
     It is yet another object to have lower infection and lumen clotting than percutaneous catheters. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are met by a subcutaneously implantable device for accessing a vessel within a patient&#39;s body, the device including (a) an access guidance means having a through channel and (b) a catheter having a through channel and comprising an access portion, a sealing portion, and a distal portion. A resilient means for sealing is arranged within the sealing portion of the catheter. The resilient means for sealing ordinarily prohibits fluids from passing the seal. But when a mechanical device is inserted percutaneously, and guided to the catheter&#39;s access portion by the access guidance means, the mechanical device passes through the access portion of the catheter, engages the sealing means, and pushes it open. This provides access to the catheter&#39;s distal portion and, ultimately, the vessel lumen, as the distal portion of the catheter, distal from the access guidance means, extends into a vessel lumen. The catheter is attached to the surrounding tissue supporting the catheter, but the access guidance means&#39;s position is not fixed relative to the tissue. 
     The means for sealing includes, in a preferred embodiment, a tube made of a resilient material, which incorporates a valving feature within the tube lumen. The tube is disposed axially along the inner wall of the channel. A spring clamp is provided adjacent to and external of the tube and acts to compress the tube such that the tube&#39;s inner lumen is closed, thereby preventing fluids from passing. 
     The spring clamp is arranged and constructed to close the tube&#39;s lumen such that the longitudinal transition profile from the open to the closed position forms a particular shape. The shape of the valve allows for the conical point of the needle obdurator to open or push apart the rubber valve slit in a wedging action as the needle is pushed through the seal. The needle pushing force overcomes the spring biasing force and the seal&#39;s internal stresses as the needle enters the sealing area without cutting the rubber. Because no cutting occurs, no rubber particles are generated, as seen with septa in ports. Furthermore, the number of penetration cycles to failure is very high, as negligible damage occurs during penetration. 
     The flowpath transitions between the needle, the tube lumen, and a catheter are arranged and constructed to provide for maximum smoothness and continuous flowpaths without abrupt changes in flow diameter and only gentle changes in flow direction. All narrowing and broadening of the flowpath is gradual, with angles of 25 degrees or less. 
     The invention also provides for a hollow needle apparatus that matingly corresponds to the through channel of the access device, and an obdurator that is inserted into the lumen of the needle, filling the lumen. This needle/obdurator combination provides a needle assembly with a pointed end, and an outer surface having smooth transitions, which are formed to puncture tissue easily and to open the valve without damaging it. 
     The needle apparatus further includes a circumferential groove formed into a sector of the needle&#39;s outer surface of approximately 180°. A spring lock is positioned within the access housing channel upstream from the resilient sealing means, engaging the groove to secure the needle to the access device when the needle is in the correct axial position. The groove and spring lock are designed to disengage when the needle is rotated approximately 90°, allowing the needle to be extracted from the housing. 
     Another preferred sealing means includes a fixed axial seating mount affixed to the through hole inner wall distally from said sealing location, the seating mount having passages to allow fluid to pass. A movable valve poppet is upstream from and fixed to said seating mount to prevent radial movement yet allow axial movement of the needle. A compression spring in said seating mount acting on said movable valve poppet provides a force pushing said valve poppet longitudinally against a valve seat. The movable valve poppet is designed with a surface that mates with a surface on the tube&#39;s inner wall (i.e., valve seat). A seal is provided between the mating surfaces of the tube inner wall and the movable valve poppet, such that the spring biasing force pushes the two mating surfaces together and the seal therebetween prevents flow from passing. The movable valve poppet has a proximally facing surface designed to engage the needle. Pushing the needle into the device&#39;s flowpath against the poppet, which overcomes the spring force, moving the poppet away from the sealing surface, thereby opening the valve sealing means and allowing fluid to pass through the access. The needle may be secured in the device by the groove and locking means arrangement, as described earlier. 
     Another sealing means includes a resilient balloon adjacent to the through channel. The balloon has a septum suitable for penetration by a fine needle. A fluid is introduced through the fine needle to inflate the balloon. The inflated balloon traverses the through channel, contacting the opposing side of the through channel, and thereby closing said hole and preventing any liquids from passing. Alternatively, the balloon may be arranged around the circumference of the through channel, and closes the through channel when inflated. 
     The presently claimed access device is suitable for both single-needle and standard hemodialysis, plasma-pheresis, and fluid exchange therapy applications. For standard applications, which require two flowpaths, the housing may be arranged and constructed to engage two needle assemblies, as described above, and include dual-lumen through channels. When two needles are used, a spring-loaded bar may be provided that engages each needle, thereby locking both needles to each other to preclude inadvertent disconnection of only one needle, thereby enhancing patient safety. 
     It is important to note that the primary object of this invention is to provide an implantable, subcutaneous access device suitable for applications requiring flow rates of 250 ml/min or greater, with low pressure drops along a streamlined flowpath having substantially no stagnation point. Low pressure drops and substantial elimination of stagnation points are achieved by having smooth transition points where different elements of the device abut (e.g., the channel-catheter interface) and by having all changes in lumen diameter be of a gradual nature and having straight or nearly straight flowpath without sharp curves or objects protruding into the flowpath and no dead volume. 
     Because such large flowrates are desired with low resistance, it is necessary to have the largest needle outside diameter that patients will accept. Accordingly, rigidity of the puncture needle is desired. A rigid needle allows a greater inner lumen diameter per outer component diameter (i.e., thinner walls) than does a flexible tube. This is important because it allows the needle to be as small as possible, thereby lessening the trauma on the patient&#39;s puncture site, yet still be capable of handling large flowrates. Flexible tubes have a much higher outer diameter to inner diameter aspect ratios. Thus, to accommodate the bloodflows common during hemodialysis, a much larger outer diameter would be required if flexible materials were used. Also, a rigid needle allows a greater force to be transmitted to the seal to overcome the resistant force generated by the spring. Thus, a greater resistant force can be employed, resulting in a more robust, reliable, and fault-tolerant seal. 
     Further, the lack of sharp angles or bends in the flowpath is much less injurious to fragile hematocytes. Since the flowpath from needle to catheter (or vice versa) is substantially straight, the turbulence is minimized, and the shear stresses lessened, resulting in less erythrocyte damage and a lowered tendency toward platelet activation. 
     Finally, it is anticipated that a medically acceptable, waterbased lubricant will be used on the needle exterior, as a diminished device lifespan of 100-150 cycles has been observed when no lubricant is used. Lifespan should be very long when properly lubricated needles are used for each insertion. 
     Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a cross-sectional view of an implanted access device of the present invention; 
     FIG. 2 is a cross-section of a second embodiment of the device of the present invention; 
     FIG. 3 is a cross-section of a third embodiment of the device of the present invention; 
     FIG. 4 is a cross-section of the embodiment shown in FIG. 3 with the needle inserted; 
     FIG. 5 is a cross-section of the valve of the embodiment shown in FIG. 3; 
     FIG. 6 is a cross-section of the locking mount of the embodiment shown in FIG. 3; 
     FIG. 7 is a cross-section of the valve seating mount of the embodiment shown in FIG. 3; 
     FIG. 8 is a cross-section of the distal housing of the embodiment shown in FIG. 3; 
     FIG. 9 is a cross-section of a fourth embodiment of the device of the present invention with a sliding seal and integral friction lock; 
     FIG. 10 is a cross-section of a fifth embodiment of the device of the present invention with a longitudinally sliding seal; 
     FIG. 11 is a cross-section of a sixth embodiment of the device of the present invention with a trumpet valve; 
     FIG. 12 is a cross-section of the embodiment shown in FIG. 11 with the needle inserted; 
     FIG. 13 is a cross-section of a fifth embodiment of the device of the present invention with an inflatable seal; and 
     FIG. 14 is a cross-section of a preferred needle and obdurator assembly. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In its simplest form, as shown in FIG. 1, the present invention comprises a modified catheter  2  (which may be situated subcutaneously, as indicated by skin line  1 ) having an access segment  4 , a distal segment  6 , and an integral valve segment  8  disposed therebetween. Modified catheter  2  has throughout most of its length a standard inner diameter  10  and a standard outer diameter  12 . However, there are several distinct deviations from these values in order to achieve the functional purposes of the invention. 
     Access segment  4  has disposed at its terminal end a raised identification ring  14  that enables an operator to locate the subcutaneous access device entrance  16 . Access segment  4  has an inwardly directed conical access guidance portion  18  and an access alignment portion  20 . Access guidance portion  18  has an initial inner diameter  22  greater than standard valve inner diameter  10  and which gradually tapers inwardly until standard valve inner diameter  10  is achieved. Thus, upon insertion, conical access guidance portion  18  guides the percutaneous mating needle  40  into the access alignment portion  20 , where the needle  40  is aligned with valve slit  28 . Needle  40  has an outer barrel diameter  50 , compatible with standard valve inner diameter  10 , and an inner barrel diameter  54 . Needle  40  is provided with an obdurator  42  having a conical tip for percutaneous insertion of needle  40  into the device without tissue becoming lodged in the lumen of needle  40 . 
     Integral valve segment  8  comprises a tapered valve access portion  24  and a valve portal  26  to further align needle  40  with valve slit  28 . It is important to note that integral valve segment  8  is most preferably molded with a solid valve seal portion  30 , which has valve slit  28  later formed therethrough. This construction results in a more complete seal and requires less sealing force than does a flattened tube, as is used in the art. 
     Integral valve segment  8  further comprises an opposing tapered distal portion  32  and has formed into its exterior, in radial alignment with valve seal portion  30 , a valve sealing means seat  34 , which is a circumferential depression in the segment exterior such that the catheter outer diameter through valve sealing means seat  34  is less than the standard outer diameter  12 , but greater than standard inner diameter  10 . Valve sealing means seat  34  accommodates valve sealing means  36 , which provides a radial biasing force sufficient to close valve seal portion  30 , and keep it closed while the device is not in use. In an alternate embodiment, valve sealing means  36  may have one or more mounting tabs  38  formed therefrom or attached thereto. During implantation, the one or more mounting tabs  38  are attached to surrounding tissue such that catheter  2  is immobilized throughout integral valve segment  8 , but allowing lateral movement of access segment  4  under the skin. 
     Outflow segment  6  is implanted such that its terminal end is disposed within the vessel or space to which access is desired. To begin treatment, an operator first locates access segment  4  through the skin using raised identification ring  14  as a guide. The operator punctures the skin with obdurator  42  disposed within needle  40  such that the needle-obdurator assembly enters access guidance portion  18  and is aligned by access alignment portion  20 . Continuing to be inserted into the device, the needle-obdurator assembly encounters valve access portion  24  and valve portal  26 . As the tip of obdurator  42  enters valve portal  26 , the tapered leading edge of obdurator  42  presses against valve access portion  24 , overcoming the radial biasing force exerted by valve sealing means  36  and thereby opening valve slit  28  such that needle  40  may pass through the valve seal portion  30 . This is accomplished without damage to valve seal portion  30  because needle  40  has already been axially aligned with valve slit  28  by the access alignment portion  20 . It is understood that this process is much smoother and causes less discomfort to the patient when the needle is provided with a medically acceptable, water-based lubricant prior to insertion. 
     It is important to note that because integral valve segment  8  is formed in a closed fashion and valve slit  28  later opened, and also because of the sealing properties of the material from which catheter  2  is made, the valve of the presently claimed invention achieves a complete seal with minimal biasing forces required to be exerted by valve sealing means  36 . Accordingly, the force that must be imparted by the needle/obdurator combination in order to overcome this biasing force to allow entry of the needle/obdurator combination into the valve is substantially less than would be required to close known valves, which are essentially flattened tubes and which never achieve a complete seal, unless substantially greater biasing forces are used. This diminution of force results in less jarring of the device during needle insertion and withdrawal, thereby greatly enhancing patient comfort. 
     In a second embodiment, as shown in FIG. 2, it is contemplated that distal segment  6  is attached to a standard medical catheter  44  by insertion therebetween of adapter  56 . Adapter  46  has a first end, disposed within distal segment  6 , and a second end, disposed within catheter  44 , tapered such that the streamlined flowpath is minimally disturbed. In addition, adapter  56  has formed within its first end a needle seating region  58  having an inner diameter  50 ′ that corresponds with outer barrel diameter  50  of needle  40 . Needle seat  58   a  extends radially inwardly such that its inner diameter  54 ′ corresponds with inner barrel diameter  54  of needle  40 . In this embodiment, when the needle-obdurator assembly is inserted into the device and axially through the seal, needle  40  will seat against needle seat  58   a  such that the streamlined flowpath is minimally disturbed, if at all. (See FIG. 1, not shown in FIG. 2) 
     In a third embodiment, as shown in FIGS. 3-8, an implanted access device  100  rests below the skin line  1 . The access device  100  comprises an assembly of guidance housing  102 , locking mount  104 , valve  106 , valve seating mount  108 , valve sealing means  120 , adapter  220 , catheter  240 , and distal housing  110 , all arranged about axis AA′. 
     Guidance housing  102  is a modified hollow cylinder having a partially closed first end formed into an inwardly directed conical needle guidance surface  122  that defines an axial access lumen  123  sized to accommodate a needle suitable for use in hemodialysis, plasma-pheresis, and fluid exchange therapies. Guidance housing  102  has an open second end provided with a chamfered leading edge  124 . 
     Locking mount  104  defines lumen  143  capable of accommodating a needle suitable for use in hemodialysis, plasmapheresis, and fluid exchange therapies formed therethrough. Locking mount  104  comprises a locking portion  140 , having lock surface  144  with lock lumen  145  formed therein such that lock lumen  145  communicates with lumen  143 , and a valve mounting portion  142 , having formed therein valve seat  146  with cross-sectional diameter  146 ′. Locking portion  140  has attached thereto lock biasing means  152  such that lock biasing means  152  movably covers lock lumen  145 . Locking means  150  is disposed within lock lumen  145  and is biased toward lumen  143  by lock biasing means  152 . When needle  40  is inserted into lumen  143 , the conical tip of obdurator  42  overcomes the biasing force exerted on locking means  150  by lock biasing means  152 , thereby causing locking means  150  to retract as needle  40  in inserted. When needle  40  is fully inserted into needle seat  148 , semicircular locking groove  44  is aligned with locking means  150 . Rotation of needle  40  allows lock biasing means  152  to push locking means  150  into semicircular locking groove  44 A, thereby locking the needle  40  into the access device  100 . To withdraw needle  40  from access device  100 , needle  40  is again rotated so that locking means  150  again retracts and needle  40  is freely removed. 
     Valve  106  has an access segment  160 , a distal segment  164 , and an integral valve segment  162  disposed therebetween. Access segment  160  has disposed at its terminal end a raised seating ring  166  having an outer cross-sectional diameter  166 ′ and defining valve entrance  163 . Integral valve segment  162  comprises a tapered valve access portion  170  and, optionally, a valve portal  172  to further align needle  40  with valve slit  178 . Integral valve segment  162  further comprises an opposing tapered distal portion  174 . It is important to note that integral valve segment  162  is most preferably molded with a solid valve sealing portion  176 , which has valve slit  178  later formed therethrough. This construction results in a more complete seal and requires less sealing force than does a flattened tube, as is used in the art. 
     Valve seating mount  108  is a disk-shaped member having an outer cross-sectional diameter  108 , a first side oriented toward valve access segment  160 , and a second side oriented toward valve distal segment  164 . Valve seating mount  108  defines seating mount lumen  183  having a cross-sectional diameter  183 ′ capable of accommodating valve  106 . The first side of seating mount  108  has a circumferential groove  186  disposed just axially of its outer peripheral edge. The first side of seating mount  108  also has a raised valve seating spacer  182  formed thereon. Valve seating spacer  182  has an outer cross-sectional diameter  182 ′ substantially similar to valve seating ring cross-sectional diameter  166 ′. Thus, when valve  106  is inserted into seating mount lumen  183 , valve seating spacer  182  and valve access ring  166  have substantially the same cross-sectional diameter and matingly fit recessed valve seat  146  in locking mount  104 . This construction further prevents undesirable lateral movement of seating mount  108  relative to locking mount  104 , thereby enhancing the stability of access device  100  and minimizing patient discomfort. The second side of seating mount  108  has disposed about its outer peripheral edge a raised valve sealing means spacer  184  of sufficient axial thickness to optimally position valve sealing means  120  relative to valve sealing portion  176 . 
     Valve sealing means  120 , may be any conventional or suitable sealing means capable of exerting a radial sealing force sufficient to seal valve slit  178 , similar to valve slit  28  of FIG.  1 . 
     Adapter  220 , has a first end, disposed within distal segment  164 , and a second end, disposed within catheter  240 , tapered such that the streamlined flowpath is minimally disturbed. In addition, adapter  220  has formed within its first end a needle seating region  226  having an inner diameter  50 ′ that corresponds with outer barrel diameter  50  of needle  40 . Needle seat  228  extends radially inwardly such that its inner diameter  54 ′ corresponds with inner barrel diameter  54  of needle  40 . In this embodiment, when the needle-obdurator assembly is inserted into the device and axially through the seal, needle  40  will seat against needle seat  228  such that the streamlined flowpath is minimally disturbed, if at all. 
     Catheter  240  may be of a type typical of use in hemodialysis, plasmapheresis, and fluid exchange therapies. 
     Distal housing  110  has a first end with an inner cross-sectional diameter  110 ′ sufficient to accommodate valve seating mount  108  having an outer cross-sectional diameter  108 . In addition, the first end of distal housing  110  has formed therein valve sealing means retainer  112  capable of optimally positioning valve sealing means  120  relative to valve sealing portion  176 . Distal housing  110  further has a second end having formed therethrough a lumen  113  capable of accommodating catheter  240 . 
     The cross-section of the needle  40  includes a locking groove  44 . Upon insertion of needle  40  into device  100 , locking means  150  extends into locking groove  44  to lock the needle  40  in position. The force exerted by lock biasing means  152  on locking means  150  is designed to allow a firm pull to disengage the locking groove  44  from the locking means  150 . In another preferred embodiment, locking groove  44  is discontinuous around the circumference of the needle, and disengagement of locking means  150  from locking groove  44  is accomplished by rotating the needle  40  and then withdrawing the needle  40 . 
     For hemodialysis, plasmapheresis, and other fluid exchange therapy operations where flowrates of 200 to 500 milliliters/per minute are possible, the needle  40  can be from 15 to 17 gauge. In such operation the pressure drop through the needle  40  should not exceed 250 mm Hg. Under these conditions a needle  40  can be made of stainless steel and have a wall thickness of approximately 0.1 mm, thereby providing sufficient strength with high safety factors. In contrast, the use of flexible materials would require a needle wall thickness three to five times greater in order to prevent buckling and collapse during insertion. 
     In the assembled access device  100 , valve  106  is disposed within lumen  183  of valve seating mount  108 , the combination being seated against locking mount  104 , as described above, which combination in turn is entirely disposed within guidance housing  102 . Chamfered leading edge  124  of guidance housing  102  matingly fits circumferential groove  186  disposed just axially of the outer peripheral edge of valve seating mount  108 . Guidance housing  102  is attached to valve seating mount  108  by known means in order to create a fluid-tight seal. Valve sealing means  120  is optimally positioned by valve sealing means spacer  184  and valve sealing means retainer  112  to seal valve sealing portion  176 . Adapter  220  is disposed partly within valve distal segment  144  and partly within catheter  240 , as described above. Adapter  220  has needle seating region  226  that matingly fits with needle  40 , thereby creating a smooth flowpath from the lumen of needle  40  to catheter  240 . Valve  106 , valve seating mount  108 , valve sealing means  120 , adapter  220 , and catheter  240  are all disposed within distal housing  110 . Catheter  240  emerges from distal housing  110  via axial lumen  113  formed therethrough. 
     FIG. 4 shows an assembled access device  100 , with needle  40  inserted and obdurator  42  removed from needle  40 . The needle end  48  is in contact with needle seat  228  of adapter  220 , such that the transition from the inner lumen of needle  40  to the inner lumen of adapter  220  is smooth. The assembly is designed and constructed such that all the flow diameter changes are gradual and continuous. The angles of these transitions are less than 25 degrees, with less than 10 degrees preferred. Herein, flow diameter is defined as the diameter of any conduit with fluid flowing measured normal to the flow. The cross-section of the needle  40  includes a ridge and locking groove  44 . The locking groove  44  is discontinuous around the circumference of the needle, and disengagement of the locking means  150  from the locking groove  44  is accomplished by rotating the needle  40  and then withdrawing the needle  40  from device  100 . In another contemplated embodiment, the locking groove  44  is continuous around the circumference of the needle. The force exerted by lock biasing means  152  on locking means  150  allows the needle  40  to be withdrawn from device  100  with a firm pull to disengage the locking groove  44  from the locking means  150 . 
     In an optional embodiment, catheter  240  has formed therefrom or attached thereto one or more tabs  246  with a through hole  248 . This tab  246  is used to fix the catheter  240 , by tying or suturing, to the surrounding tissue upon implantation of device  100 . The device  100  itself is not fixed to the surrounding tissue. With this arrangement, the device  100  can move underneath the skin enough to align with a needle  40  penetrating the skin without having the needle  40  move transversely to the skin. Adhesions from the tissue to the device  100  are discouraged by treating the housing surface with hyaluronic acid. In addition, to prevent infection, the device may also incorporate or have its exterior surfaces treated with antibacterial material. 
     FIG. 9 shows another contemplated embodiment  300  where there is an integral friction lock to secure the needle  40  within the access device  300 . A sealing plug  304  is disposed within housing assembly  302 / 310  between its access lumen  303  and piston  308 . When the device is not in use, spring  306  biases piston  308  against sealing plug  304 , urging sealing plug  304  against tapered sealing surface  344 , thereby preventing fluid flow through the device. 
     During use, the needle  40  is guided to the access lumen  303  by the conical needle guidance surface  322  of guidance housing  302 , wherein needle  40  contacts sealing plug  304 . As needle  40  is pushed further into the device, the axial force exerted by needle  40  on the sealing plug  304  overcomes the sealing plug biasing force exerted on the biasing force transmission flange  309  of piston  308  by spring  306 . moving sealing plug  304  away from sealing surface  344  and removing the radial compressive forces normally exerted on the sealing plug  304 , sufficiently to allow needle  40  to puncture sealing plug  304 . It is important to note that, unlike septa known in the art, where the needle punctures randomly, which eventually results in fragmentation of the septum, sealing plug  304  consistently is punctured in the same place and direction due to guidance of the needle  40  by the conical needle guidance surface  322  of guidance housing  302 . This feature effectively eliminates sealing plug fragmentation. 
     Once needle  40  punctures sealing plug  304 , needle  40  contacts needle seat  318  of piston  308 , where the needle tip  48  contacts needle seat  318  to form a smooth transition between the needle  40  and the piston  308 . Once needle  40  is inserted, sealing plug  304  provides enough residual pressure onto the needle  40  to effectively lock the needle  40  into the device  300 . An axial pull on the needle  40  tends to pull the sealing plug  304  against the sealing surface  344 , increasing the radial forces exerted on the needle  40 , thereby holding the needle even more securely. A simple twist of needle  40 , however, introduces dynamic friction and allows the needle  40  to be removed from the device. O-rings  312  and  314  seal the needle  40  from the piston  308  and the piston  308  from the sealing housing  310 , respectively. When the device is not in use, spring  306  biases the piston  308  towards the skin line  1 , compressing the sealing plug  304  such that the sealing plug  304  seals itself, closing the passageway formed by insertion of needle  40 . Note that, as the piston  308  slides relative to the catheter  340  and the sealing housing  310 , the transition from the piston  308  and the catheter  340  inner wall and/or the sealing housing  310  inner wall remains smooth. 
     FIG. 10 is yet another contemplated valving for the present invention. In this embodiment, the needle  40  contacts a sliding spring-loaded poppet  404 . As the needle  40  is pushed into the device  400  using conical needle guidance surface  422  the valve structure  408  is biased away from guidance housing  402  (as shown). The O-ring  416  leaves the housing wall  405  allowing fluid to pass through the valve. Spring  406  forces the poppet  404  and O-ring  416  back into contact with the housing wall  405  when the needle  40  is extracted from the device. The poppet  404  does not extend throughout the valve circumference, as it would then interfere with the fluid flow. Instead, the poppet  404  has a plurality of rod-like extensions  418  that provide open areas for fluid to pass through the valve when the needle is inserted. The O-ring  406  provides a seal to prevent leakage around the needle  40 . As piston  408  (valve structure) slides relative to the catheter  440  and the sealing housing  410  the transition from piston  408  and catheter  440  inner wall and/or sealing housing  410  inner wall remains smooth. 
     FIGS. 11 and 12 show another contemplated embodiment  500  wherein the valve sealing means is a trumpet valve  504 . Prior to each treatment session, a fine needle  509  may be percutaneously introduced into lumen  530  and penetrate the septum  512  to open valve  504  by injecting fluid into reservoir  520  sufficient to overcome the biasing force exerted by spring  508 . Needle  40  may then be introduced into the device  500 , in a similiar manner as described above with respect to  300  (FIG. 9 and 400 (FIG.  10 ), and guidance housing  502  having conical needle guidance surface  522  guides needle  40 . When the treatment session is completed, needle  40  is removed from the device and trumpet valve  504  is closed by withdrawing fluid from reservoir  520  via fine needle  509 , which is then removed from the device. As shown there is a sealing housing  510  which cooperates with guidance housing  502 , and there is a seal  504  which seals passageway  503 . 
     FIG. 13 is another contemplated embodiment  600  where an inflatable seal  604  seals passageway  603 . The needle  40  guided by conical needle guidance surface  622  of guidance housing  602 , such that the needle  40  pushes the expandable seal  604  apart when inserted. The needle  40  then hits the stop  608  built into the sealing housing  610 . When the needle  40  is extracted, the seal  604  expands, closing the passageway  603 . As may be needed from time to time, a fine needle  609  may be percutaneously introduced into lumen  630  and penetrate the septum  612  to re-expand the seal  604  by injecting fluid into reservoir  620 , which is in fluid connection with the lumen of seal  604 . As in the other embodiments, the flowpath transition from the sealing housing  610  to the catheter  640  is smooth. 
     FIG. 14 shows a preferred corresponding needle assembly constructed and arranged to mate with the previously described implanted access housings. The needle barrel  40  is of a thin metal material. Thinner material maximizes the actual flow diameter which is a general goal of any hemodialysis needle. The discomfort to the patient is reduced by smaller diameter needles, but such needles restrict flow or provide large pressure drops when high flows are forced through small needles. Low flowrates would require inordinate treatment time for hemodialysis, and high flowrates through narrow needles damages blood. There is a tradeoff and thin needle walls contributes to maximized flow diameters for a given outer needle diameter. An obdurator  42  is fitted within the needle barrel  40 , providing a smooth transition  43  between the outer surface of needle  40  at the needle tip  48  and the obdurator  42 . The obdurator  42  is secured to a housing  68  via threads  62 . The obdurator  42  is necessary since the needle  40  is hollow and cannot be used to penetrate the skin because its large diameter lumen will become plugged. The obdurator  42  exactly fills the hollow face presented to the skin and has a point  45  suitable for penetrating the skin. The housing  68  provides a channel  69  with the threaded fitting  64  for connecting to the hemodialysis equipment. When the obdurator  42  is removed, there is a slit disk valve  66  that closes off the opening used by the obdurator  42 , allowing the hemodialysis to proceed. 
     It will now be apparent to those skilled in the art that other embodiments, improvements, details and uses can be made consistent with the letter and spirit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents.