Abstract:
A cylindrical elongate body having proximal and distal ends and enclosing two similar longitudinally extending lumens separated by a septum and a further lumen within the septum. A convergently tapered tip extending from the distal end of the body defines part of the further lumen that extends distally beyond the longitudinally extending lumens, while a connector at the proximal end of the body couples plural access tubes in fluid communication with respective individual of the lumens. Openings providing access to each of the longitudinally extending lumens are spaced from one another longitudinally along the body. The further lumen terminates at an opening at the distal end of the tip. The longitudinally extending lumens are blocked immediately distally of the openings.

Description:
This application is a continuation application of U.S. patent application Ser. No. 09/139,705 that was filed on Aug. 25, 1998(hereinafter “the Parent Application”), and that issued as U.S. Pat. No. 6,206,849 on Mar. 27, 2001. The Parent Application is a continuation application of U.S. patent application Ser. No. 08/481,169 that was filed on Jun. 7, 1995 (hereinafter “the Grandparent Application”), and that issued as U.S. Pat. No. 5,797,869 on Aug. 25, 1998. The Grandparent Application is a continuation application of U.S. patent application Ser. No. 08/205,331 that was filed on Mar. 3, 1994 (hereinafter “the Great-Grandparent Application”), and that issued as U.S. Pat. No. 5,472,417 on Dec. 5, 1995 . The Great-Grandparent Application is a continuation application of U.S. patent application Ser. No. 07/785,351 that was filed on Oct. 30, 1991 (hereinafter “the Great-Great-Grandparent Application”), and that is now abandoned. The Great-Great-Grandparent Application is a continuation application of U.S. patent application Ser. No. 07/288,364 that was filed on Dec. 22, 1988 (hereinafter “the Great-Great-Great-Grandparent Application”), and that issued as U.S. Pat. No. 5,195,962 on Mar 23, 1993. This application is also related to U.S. patent application Ser. No. 699,421 that was filed on May 31, 1991, as a divisional application of the Great-Great-Great-Grandparent Application, and that issued as U.S. Pat. No. 5,135,599 on Aug. 4, 1992. 
    
    
     BACKGROUND  
     This invention relates to a multiple lumen catheter and more particularly to such a catheter for insertion into a vein of a patient to be used in haemodialysis treatments. The invention also relates to methods for manufacturing the multiple lumen catheter. 
     Multiple lumen catheters have been available for many years for a variety of medical purposes. It is only in recent years, however, that such catheters have been developed for use in haemodialysis. The general form of multiple lumen catheters goes back to as early as 1882 when Pfarre patented such a catheter in the United States under Ser. No. 256,590. This patent teaches a flexible dual lumen catheter which is used primarily for cleaning and drainage of, for example, the bladder, rectum, stomach and ear. In this type of catheterization, the catheter is introduced into an existing body orifice without the use of any puncturing needle or guidewire. 
     More recently, a catheter was developed and patented by Blake et al under U.S. Pat. No. 3,634,924. This 1972 patent teaches a double lumen cardiac balloon catheter which is introduced into a large vein and the balloons inflated to control the flow in the vein. The catheter can in fact be placed by using the balloon as sails to move with the blood into or through the heart to a position where the catheter takes up its intended function. This patent uses two lumens and teaches a method of making a tip which involves the use of a plug and a wire which retains the shape of one of the lumens during formation of the tip in a molding technique. 
     Further patents which teach multiple lumen catheters for general use include the following U.S. Pat. Nos. 701,075; 2,175,726; 2,819,718; 4,072,146; 4,098,275; 4,134,402; 4,406,656 and 4,180,068. 
     Vascular catheter access by surgical cut-down techniques has been known to the medical profession for many years and, in fact, can be traced back to the 17th century. However, it was only with the introduction of the Seldinger technique in 1953 or thereabouts that a new approach could be used to improve vascular access. This technique was taught in an article published by Seldinger resulting from a presentation made at the Congress of the Northern Association of Medical Radiology at Helsinki in June of 1952. The technique essentially involves the use of a hollow needle to make an initial puncture and then a wire is entered through the needle and positioned in the vessel. The needle is withdrawn and the catheter is entered percutaneously over the wire which is later withdrawn. With this technique it became possible to make less traumatic vascular access and has now become the accepted method of performing access in numerous medical techniques. One of these techniques which has been the subject of much research and development, is haemodialysis. 
     Haemodialysis can be defined as the temporary removal of blood from a patient for the purpose of extracting or separating toxins therefrom and the return of the cleansed blood to the same patient. Haemodialysis is indicated in patients where renal impairment or failure exists, that is, in cases where the blood is not being properly or sufficiently cleansed, (particularly to remove water) by the kidneys. 
     In the case of chronic renal impairment or failure, haemodialysis has to be carried out on a repetitive basis. For example, in end stage kidney disease where transplantation of kidneys is not possible or for medical reasons is contraindicated, the patient will have to be dialysed about 100 to 150 times per year. This can result in several thousand accesses to the blood stream to enable the active haemodialysis to be performed over the remaining life of the patient. 
     Towards the end of 1960, Dr. Stanley Shaldon and colleagues developed, in the Royal Free Hospital in London, England, a technique for haemodialysis by percutaneous catheterization of deep blood vessels, specifically the femoral artery and vein. The technique was described in an article published by Dr. Shaldon and his associates in the Oct. 14, 1961 edition of The Lancet at pages 857 to 859. Dr. Shaldon and his associates developed single lumen catheters having tapered tips for entry over a Seldinger wire to be used in haemodialysis. Subsequently, Dr. Shaldon and his colleagues began to insert both inlet and outlet catheters in the femoral vein and this was reported in the British Medical Journal of Jun. 19, 1963. The purpose of providing both inlet and outlet catheters in the femoral vein was to explore the possibility of a “self-service” approach to dialysis. Dr. Shaldon was subsequently successful in doing this and patients were able to operate reasonably normally while carrying implanted catheters which could be connected to haemodialysis equipment periodically. 
     Some use was made of a flexible dual lumen catheter inserted by surgical cutdown as early as 1959. An example of such a catheter is that of McIntosh and colleagues which is described in the Journal of the American Medical Association of Feb. 21, 1959 at pages 137 to 138. In this publication, a dual lumen catheter is made of non-toxic vinyl plastic and described as being inserted by cut-down technique into the saphenous vein to the inferior vena cava. 
     The advantages of dual lumen catheters in haemodialysis is that only one vein access need be affected to establish continued dialysis of the blood, because one lumen serves as the conduit for blood flowing from the patient to the dialysis unit and the other lumen serves as a conduit for blood returning from the dialysis unit to the patient. This contrasts with prior systems where either two insertions were necessary to place the two catheters as was done by Dr. Shaldon, or a single catheter was used with a complicated dialysis machine which alternatively removed blood and returned cleansed blood. 
     The success of Dr. Shaldon in placing catheters which will remain in place for periodic haemodialysis caused further work to be done with different sites. Dr. Shaldon used the femoral vein and in about 1977 Dr. Uldall began clinical testing of a subclavian catheter that would remain in place. An article describing this was published by Dr. Uldall and others in Dialysis and Transplantation, Volume 8, No. 10, in October 1979. Subsequently Dr. Uldall began experimenting with a coaxial dual lumen catheter for subclavian insertion and this resulted in Canadian Patent No. 1,092,927 which issued on Jan. 6, 1981. Although this particular form of catheter has not achieved significant success in the market-place, it was the forerunner of dual lumen catheters implanted in the subclavian vein for periodic haemodialysis. 
     The next significant step in the development of a dual lumen catheter for haemodialysis is U.S. Pat. No. 1,150,122 to Martin who produced a catheter which achieved some commercial success. The catheter avoided the disadvantages of the Uldall structure. 
     A subsequent development is shown in U.S. Pat. No. 4,451,252 also to Martin. This utilizes the well known dual lumen configuration in which the lumens are arranged side-by-side separated by a diametric septum. The structure shown in this patent provides for a tip making it possible to enter a Seldinger wire through one of the lumens and to use this wire as a guide for inserting the catheter percutaneously. Patents to this type of structure followed and include European Patent Application to Edelman published under No. 0 079 719, U.S. Pat. Nos. 4,619,643; 4,583,968; 4,568,329 and U.S. Design Pat. No. 272,651. 
     All of the above examples of haemodialysis catheters suffer from the disadvantages that they can not be used readily for intravenous injection of liquid medication. A person who is using haemodialysis therapy with a dual lumen catheter will have to receive a needle for intravenous injection when medication of this kind is required. It would be desirable that the catheter not only perform the function of haemodialysis, but also provide a facility for intravenous injection without further puncturing of the patient&#39;s veins. It is one of the objects of the present invention to provide such a catheter. 
     The foregoing problems associated with haemodialysis catheters may on some instances be specific to the treatment. However, the catheter of the present invention, in overcoming the disadvantages of the prior art of renal dialysis catheters, provides a catheter which has utility in other procedures. Accordingly, although the present description is directed to haemodialysis, such use is exemplary and it will be evident that catheters according to the invention may be used for other procedures. 
    
    
     
       These and other aspects of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic view of a triple lumen catheter according to a preferred embodiment of the present invention, inserted into the subclavian vein of a patient; 
         FIG. 2  is a diagrammatic perspective view of the catheter drawn to a larger scale than that used in  FIG. 1 ; 
         FIG. 3  is an enlarged sectional view of the distal end of the catheter of  FIG. 1  drawn on line  3 - 3  of  FIG. 2 ; 
         FIGS. 4 and 5  are enlarged sectional views taken on the lines  4 - 4 ,  5 - 5  of  FIG. 3 , respectively, and showing complete sections; 
         FIG. 6  is an end view of the catheter in the direction generally of arrow of  FIG. 3 ; 
         FIG. 7  is a sectional view of a trident-shaped branching connector seen at the proximal end of the catheter in  FIG. 2  and drawn to a larger scale; 
         FIGS. 8 ,  9 ,  10  and  11  are diagrammatic perspective views of an end of the catheter showing the various steps in the manufacture of the trident-shaped branching connector and associated parts; 
         FIG. 12  is a sectional view of the connector after assembly; 
         FIG. 13  is a view similar to  FIG. 3  of the distal end of another embodiment of the present invention; 
         FIG. 14  is a sectional view taken on line  14 - 14  of  FIG. 13 ; 
         FIG. 15  is a sectional view of a further embodiment of the catheter; 
         FIG. 16  is a perspective view of a plug for use in making yet another embodiment of the catheter; and 
         FIG. 17  is a sectional view of still another embodiment of the catheter and using a separate bonded tip. 
         FIG. 18  is a sectional view illustrating an alternative method of manufacturing a tip according to the invention; and 
         FIG. 19  (drawn adjacent  FIG. 15 ) is a side view of a tip made using the method illustrated in  FIG. 18 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention will be described in detail with reference to a preferred embodiment to be used for haemodialysis. However the drawings and description are exemplary of the invention and unless otherwise stated, are not intended to be limited by its restraints of size and properties dictated by haemodialysis procedures. 
     Reference is made first to  FIG. 1  of the drawings which illustrates a triple lumen catheter, indicated generally by reference numeral  20 , according to a preferred embodiment of the present invention, and showing by way of example, a patient receiving the catheter in the subclavian vein using a Seldinger wire  21 . The catheter is to be used for haemodialysis treatment and could of course also be entered in a similar fashion in the femoral vein. 
     The catheter  20  is secured to a conventional dressing  22  by an attachment fitting  23  having wing tabs  24 , and the dressing  22 , in turn, is secured to the skin of the patient. As shown, the catheter  20  passes through the dressing  22  and, as can be seen in broken outline, an elongate and flexible cylindrical body  26 , formed of a polyurethane extrusion, is inserted through the skin and into the subclavian vein in the downstream direction. The catheter  20  has at its distal end  28  a conical tapered tip  29  which is described in greater detail below. The other end of the body  26  is a generally trident-shaped branching connector  30 , which protrudes outwardly from and is secured by dressing  22 . Cylindrical blood extraction and return tubes  32 ,  34  and an intravenous (IV) tube  35  are attached to the trident-shaped branching connector  30 , a full description of which is provided below. For the moment it is sufficient to state that these tubes are connected to lumens running through the body  26 . 
       FIG. 2  shows the catheter  20  in greater detail. The body  26  has at its proximal end the connector  30  for receiving the blood extraction and return tubes  32 ,  34 . These tubes terminate at their outer ends in respective female luer fittings  36 ,  37  for connection to complementary male luer fittings (not shown) leading to a dialysis machine, and carry closure clamps  38  (one of which is shown) to selectively close the tubes. 
     The IV tube  35  terminated at its outer end in a luer lock fitting  39  for receiving a syringe or male luer lock connector. 
     The wing tabs  24 , sometimes known as suture wings, are formed integrally with a central tubular portion  40  which can rotate on the body  26  and is retained in place by a shoulder on the end of the connector  30  and a second shoulder in a reinforcing portion  42  so that the catheter  20  can be rotated relative to the tabs  24 . This rotation is sometimes necessary after insertion of the catheter  20  to re-orientate intake side apertures in the distal end  28  if the apertures happen to be occluded by engagement with the wall of the vein. Details of the apertures are provided below. 
     As will be described, the reinforcing portion  42  is blended into the body  26  over the length of the portion and assists in strengthening the catheter to minimize the likelihood of kinking. Also, the portion  42  assists in sealing the puncture site where the catheter enters the patient. 
     As will be described in more detail with reference to subsequent views, the tube  35  is aligned with a central lumen to permit the Seldinger wire  21  to pass through the catheter. The wires exists at tip  29  which is essentially conical so that the catheter can slide over the wire and into the patient during insertion. The extraction and return tubes  32 ,  34  are linked at connector  30  with lumens in the body  26  to connect with respective groups of side apertures  44 ,  45  (some of which can be seen in this view. one of apertures  44  may be described as a first aperture and one of apertures  45  may be described as a second aperture) near the distal end of the catheter  28 . As a result, when inserted and in use, blood can be removed and returned in a closed loop with a haemodialysis machine using the tubes  32 ,  34 . Between treatments the tube  35  is available for intravenous infusion of liquid medicaments. 
     Reference is next made to  FIGS. 3 to 6  of the drawings which illustrate the distal end  28  including tip  29 . The body  26  comprises an outer wall  46  (i.e., an outer tube) and an integral septum  48  (which may be optionally described as comprising an inner tube  49 A, a first septum  49 B, and a second septum  49 C) extending diametrically across the body  26 , and defining an extraction lumen  50  (which may be optionally described as a first lumen and a return lumen  52  (which may be optionally described as a second lumen), both lumens being generally C-shaped in cross-section and extending from the proximal end towards the distal end. As best seen in  FIG. 4 , a bulbous middle portion  53  of the septum  48  projects into the lumens  50 ,  52  and contains the intravenous (IV) lumen  54  which extends along the longitudinal axis of the body portion  26  from the proximal end to the distal end. This lumen is an extension of the IV tube  35  and is proportioned in this embodiment to receive a 0.038 inch diameter Seldinger wire. 
     The extraction lumen  50  is blocked short of the tip  29  by a first insert  56  which is formed of polyurethane and bonded in place using a suitable solvent such as cyclohexanane. Extraction apertures  44  are provided in the outer wall  46  of the cylindrical portion  26 ,just short of the insert  56 , to permit blood to flow from the patient&#39;s vein into the extraction lumen  50  and thus through the connector  30  to the extraction tube  32  and the dialysis machine. It should be noted that the apertures  44  are conveniently circular but may be of any suitable shape or size including scaphoid. Also, further extraction apertures may be provided around the lumen  50  as required consistent with the aperture nearest the tip being immediately adjacent the insert  56  to minimize dead spaces. 
     The return lumen  52  is similarly blocked by a second insert  60  immediately adjacent the last of several return apertures  45 . This last aperture is positioned closer to the tip  29  than is the last of the intake apertures  44  in the extraction lumen  50  to minimize the risk of cross flow as returning blood finds its way back into the lumen  50 . Although some cross-flow is not critical, excess cross-flow will extend the time needed for haemodialysis. 
     As can be seen in  FIGS. 3 and 6 , the tip  29  is smoothly rounded at the end  28  of the catheter and tapered slightly gently to facilitate insertion of the catheter  20  into a patient. As mentioned previously, the catheter is intended to be used with a Seldinger wire. It is, therefore, clearly desirable that the tapered tip  29  be concentric with the axis of the body  26  and of the lumen  54 . Accordingly, the centrally located IV lumen  54  extends to the tip  29  and terminates at a circular IV aperture  64 . 
     The catheter  20  is made from a length of cylindrical polyurethane extrusion forming the cylindrical body  26 . The extrusion is cut to the required length and the ends formed by further operations. The formation of the tapered tip  29  will be described with reference firstly to  FIG. 3 , followed by a description of the formation of the connector  30 . 
     Before shaping the tapered tip  29 , the inserts  56 ,  60  are positioned and affixed in the respective lumens  50 ,  52  as shown in  FIG. 3 . The inserts are shaped to the cross-section of the lumens and affixed as previously described. A cylindrical wire  66  (shown in chain dotted outline), of corresponding diameter to that of the guide wire  21  ( FIG. 2 ), is inserted through the IV lumen  54  to extend from the distal end of the tubing which is then located in a conical tapered mold  68  (shown in chain-dotted outline). The extrusion is heated by R.F. and as it softens it is pushed into the mold such that the outer wall  46  and the septum  48  merge at the tip  29 . The end of the body assumes a conical tapered shape with a radiused end and the material masses in the lumens  50 ,  52  forming ends  70 ,  72 . The IV lumen  54  retains its internal shape because it is supported on the wire  66 . The now tapered tip is cooled to some extent and then removed from the mold  68  and allowed to cool further and harden. 
     The deformation of the tip results in a thickening of the outer wall  46  and septum  48  to provide a concentration of material substantially exceeding the concentration of material in the main catheter body, and this has the effect of stiffening the tip, which facilitates insertion of the catheter. 
     Because the wire  66  is not deflected at any time from its normal straight condition during the molding operation, there is no energy stored in the wire and consequently there is no tendency for the wire to deflect the tip from the desired orientation after removal from the mold  68 . 
     The wire can therefore be left inside the tip during cooling. The apertures  44 ,  45  are then cut or otherwise formed in the outer wall  46  of the body  26 . Also, because the extrusion is symmetrical about the wire, the deformed material at the tip will move evenly to each side of the central septum. The resulting similar masses at ends  70 ,  72  of the lumens will cool and shrink equally so that the tip will remain concentric about the central or IV lumen  54 . This will result in a well formed tapered tip. 
     The method of manufacture of the trident-shaped branching connector  30  and reinforcing portion  42  will next be described with reference to  FIGS. 7 to 12 . The figures are arranged in order of the steps used in the manufacture and it will be seen in  FIG. 7  that the extruded body  26  has received a short sleeve  71  of polyurethane and preferably the same color as that used for the body. The sleeve  71  is a snug fit on the cylindrical body  26  and after positioning on the body, the assembly is moved into a heated molded  73  which has a frustro-conical interior wall  75  designed to deform the sleeve  71  to create the blended reinforcing portion  42  shown in  FIG. 2 . If preferred, suitable shaped mandrels can be placed inside the lumens of the body  26  to ensure that the lumens are not deformed while the collar is shaped in the mold  73 . The sleeve  71  is heated and the body pushed into the heated mold  73  so that material flows to the desired shape. 
     The upper edge of the sleeve  71  (as drawn) forms a shoulder and is positioned for engagement with the attachment fitting  23  shown in  FIG. 2  to locate this fitting longitudinally on the body. 
     After completing the process illustrated in  FIG. 7 , the fitting  23  is slipped over the end of the body  26  and into engagement with the sleeve  71 . The fitting is a loose fit so that it can rotate freely on the body  26 . The positioning can be seen in  FIG. 8  which also shows the completed reinforcing portion  42  and how it blends into the body  26 . 
     Next another sleeve  74  is engaged over the end of the tube  26  and, if the first sleeve has been positioned correctly, the sleeve  74  will be positioned so that its trailing end becomes flush with the end of the body  26  as shown in  FIG. 9 . The sleeve  74  should not be pushed tightly against the attachment fitting  23  in order to provide clearance of free movement of the fitting. With the sleeve in position, a set of deforming mandrels are brought into play as seen in  FIG. 9 . There are three mandrels, one for each of the lumens. The two outer mandrels  76 ,  78  are mirror images of one another and positioned about a central mandrel  80 . The intent of the mandrels is to form the corresponding lumens to have conical outer portions for receiving shaped ends of the tubes  32 ,  34  and  35  ( FIG. 2 ) as will be described with reference to  FIG. 12 . 
     The mandrels  76 ,  78  have respective leading ends  82 ,  84  which are proportioned simply to provide location as they enter corresponding lumens  50 ,  52  and similarly, a leading portion  86  on the mandrel  80  is proportioned to engage the central lumen  54 . The leading portions  82 ,  84  and  86  blend into respective conical portions  88 ,  90  and  92  which are arranged to complement one another so that the cones will flair outwardly to receive the tubes. Of course for simplicity of engagement, each of the mandrels is supported from shanks which are arranged in parallel so that the mandrels can be brought into the extrusion longitudinally and deformation will take place simply because the conical portions are larger than the lumens and the material around the lumens will be forced outwardly under the influence of heat provided by heating the mandrels. The second sleeve  74  supports the extrusion which is itself insufficient to support this deformation as the size is increased. 
     It will be seen in  FIG. 10 , that after the mandrels are engaged, the sleeve and contained portion of the extrusion are expanded and, after completion, the appearance will be as shown in  FIG. 11 . 
     Reference is next made to  FIG. 12  which shows the engagement of the tubes  32 ,  34  and  35  in the connector  30 . These tubes have their engagement ends deformed to thin the wall and this is done by conical deformations so that the outer surface of the tubes are slightly conical to engage the corresponding internal cones  94 ,  96  and  98  shown in  FIG. 11  and formed by the use of the mandrels. It will be seen in  FIG. 12  that the result in assembly is compact, and provides a relatively smooth internal surface to minimize the risk of blood damage caused by turbulence as blood flows through the tubes and associated lumens  32 ,  34  and  50 ,  52 . Similarly, the tube  35  is engaged so that there is no interference with the Seldinger wire which will slide freely through this tube and lumen  54 . 
     The tubes are attached in the connector  30  using a suitable solvent in a similar fashion to the attachment of the plugs  56 ,  60  described with reference to  FIG. 3 . 
     It is of course possible to make the assembly starting with the trident shaped structure and then add the fitting  23  from the distal end of the body before ending by adding and forming sleeve  71 . 
     In use, as mentioned above, the catheter  20  is inserted such that it points downstream in the patient&#39;s vein, that is, the extraction aperture  44  are upstream of the return apertures  45 , which, in turn, are upstream of the IV tip aperture  64 . When a treatment is in progress the extraction tubes  32 ,  34  are connected to a dialysis machine which draws blood through the extraction lumen  50  and returns it through return lumen  52  in a similar manner to a conventional dual lumen cannula. Between blood treatments the lumens may be filled with a heparin solution to prevent them from being filled with clotted blood. However, if the patient requires medication or is required to give blood between treatments, the IV lumen  54  may be used. This avoids the trauma and discomfort of inserting a further needle or catheter into the patient and does not disturb the heparin lock. 
     Between uses the third lumen may be filled with a relatively small volume of heparin or may be occupied by cylindrical solid and flexible patency obturator, similar to guide wire  21 . This obturator prevents the entrance of blood into the lumen and thus eliminates the need for heparin in the third lumen. Generally, it will be easier to keep the third lumen free of blood due to its smaller cross-section, regular shape and absence of side holes. 
     In addition to this advantage the centrally located lumen offers considerable advantages for insertion and removal of the catheter. As there are no side holes in the lumen, “J” ended guide wires may be used without the possibility that the guidewire will exit through a side hole, rather than the end aperture. In addition, because it is easier to keep the smaller lumen free of clotted blood, it should be possible to use a guidewire to replace a catheter which has clotted blood in the blood lumens without dislodging any blood clots which may have accumulated in the blood lumens. This would be done by first entering the Seldinger wire into the third lumen of the catheter in place in the vein, withdrawing this catheter over the wire leaving the wire in place, and then using the wire to guide a replacement catheter over the guide wire. 
     The exemplary catheter described with reference to the drawings does not have the proportions of a haemodialysis catheter. As mentioned previously, the description is exemplary and in practice, if the catheter is to be used in the subclavian vein it will have proportions as follows. The central lumen will have a diameter of about 0.04 inches to receive a Seldinger wire of diameter 0.038 inches or 0.036 inches. The walls about the central lumen and forming the septum will be about 0.010 inches in thickness and will blend into the outer wall which is about 0.013 inches in thickness. The outer diameter of the body  26  will be 0.149 inches and this will give an area available for blood flow in the lumens of about 0.0048 square inches. The flow rate will be approximately 237 milliliters per minute using accepted pressures to drive the blood through the lumens. 
     Clearly catheters can be made with a variety of proportions depending upon the use and structures defined by the claims and incorporating the description are within the scope of the invention. 
     The distal tip structure shown in  FIG. 3  can be made in a number of ways. An alternative is shown in  FIGS. 13 and 14 . For ease of reference the reference numerals used in relation to these figures correspond to those used above in relation to  FIG. 3 , except that as used in relation to the catheter  120  illustrated in  FIGS. 13 and 14 , those reference numerals are prefixed with the numeral  1 . The distal end  128  of catheter body  126  and the distal tip  129  of catheter  120  enclose inserts  156 ,  160  that fill the unused distal portions of the extraction lumen  150  and the return lumen  152 , respectively. The inserts  165 ,  160  are initially entered into position in the lumens  150 ,  152  and may be affixed therein by a solvent. Then, with a supporting wire  166  filling the distal tip portion of IV lumen  154  within bulbous middle portion  153  of septum  148  and extending distally out of the resulting assembly through circular IV aperture  164 , distal end  128  of catheter body  126  is heated in the conical tapered mould  168 . As a result, the inserts  156 ,  160  are softened and deformed, and the outer wall  146  of catheter body  126  collapses to merge with the septum  148  at and on both sides of bulbous middle portion  153  thereof. The distal end  157  of insert  156  and the distal end  161  of insert  160 , as represented by the ghost outlines in  FIGS. 13 and 14 , also merge in this process with septum  148  at and on both sides of bulbous middle portion  153  thereof. The resulting catheter  120  has an appearance similar to that of catheter  20  described above relative to  FIG. 3 . but catheter  120  of  FIGS. 13 and 14  has a stiffer leading end and distal tip structure. 
     Explaining in further detail, outer wall  146  of catheter body  126  illustrated in  FIGS. 13 and 14  may be described as an outer tube. Bulbous middle portion  153  of septum  148  illustrated in  FIGS. 13 and 14  may be described as an inner tube. The portion of septum  148  on the right side of bulbous middle portion  153  as shown in  FIG. 14  may be described as a first septum extending between the inner and outer tube, and the portion of septum  148  shown in  FIG. 14  on the left side of bulbous middle portion  153  may be described as a second septum extending between the inner tube and the outer tube. Additionally, IV lumen  154  may be described as a first lumen, extraction lumen  150  may be described as a second lumen, and return lumen  152  may be described as a third lumen. Further, circular IV aperture  164  may be described as a first aperture, extraction apertures  144  may be described as a plurality of second apertures, and return apertures  145  may be described as a plurality of third apertures. 
     It will be evident that the form of the inserts  156 ,  160  can vary. For instance distal end  157  of insert  156  and distal end  161  of insert  160  that are to be positioned originally within lumens  150 ,  152  near the portion of distal end  128  catheter body  120  to be formed into distal tip  129  could be thinned to allow for easier deformation of the extrusion into the shape shown in  FIG. 13 . 
     The catheters illustrated and described above feature septums having a bulbous middle portion to accommodate the IV lumen. However, the catheter of the invention is not limited to this particular cross-section and  FIG. 15  shows an alternative cross-section. For ease of reference the numerals used in relation to this figure correspond to those used to describe the preferred embodiment prefixed with the numeral  2 . The catheter illustrated has a septum  248  with planar sides such that the extraction and return lumens  250 ,  252  have a D-shaped cross-section. This thicker septum  248  requires the use of more material to form the catheter and also reduces the ratio between the cross-sectional area of the extraction and return lumens and the cross-sectional area of the catheter. However, there may be uses above where this cross-section is advantageous, for instance, where the outer diameter of the catheter body is less critical than it is when used in a vein for haemodialysis. 
     Reference is now made to  FIG. 16  to describe a molded plug of polyurethane for use in making tips. This plug has end pieces  200 ,  202  shaped to fit snugly in the lumens  50 ,  52  ( FIG. 3 ). The end pieces are attached to respective spacers  204 ,  206  which depend from a hub  208  at respective weakened joints  210 ,  212 . The hub has a central opening  214  matching the third lumen  54  so that the wire used in molding can be used to locate the hub centrally. 
     The procedure, when using the plug of  FIG. 16 , is to first bend the spacers  204 ,  206  about the joints  210 ,  212  so that the end pieces  200 ,  202  come together for insertion in the end of the extruded body  26 . The pieces are pushed home with solvent until the hub  208  meets the end of the body. The pieces  200 ,  202  will then automatically be in the required positions controlled by the lengths of the spacers  204 ,  206 . Molding then proceeds as before so that the hub and adjacent parts of the spacers will become integral portions of the tip. A further embodiment is shown in  FIG. 17 . This structure includes a separate molded tip  216  preferably of polyurethane, which is engaged in and bonded to the end of the extrusion. The tip  216  has an outer conical form and defines a central opening  218  forming a continuation of the third lumen  220 . A pair of extensions  222 ,  224  are shaped to fit in the respective lumens  226 ,  228  and have lengths to match the positions of the apertures  230 ,  232  in the side wall of the lumens. The ends of the extensions are preferably shaped to meet the apertures and complement the natural flow patterns so that dead spaces will be minimized, if not eliminated. 
     The structure shown in  FIG. 17  can also be partly formed by heating in a mold to blend the joint between the tip and the extrusion. This technique can also be used to part form the assembly to improve the tip, if necessary. 
     The method of shaping the end is described as utilizing radio frequency heating devices to soften the plastic material. This is intended to be illustrative of a softening technique, and other techniques, for example, the use of electrical heating elements, are equally effective. 
     The third method of manufacturing the tip is illustrated in  FIG. 18 . Numerals corresponding to those used in  FIG. 3  will be repeated with a prefix “3”. 
     In this embodiment, a body  326  receives an extension piece  400  shaped to fit roughly on the end of the body and having a projection  402  of the shape needed as a continuation of the central aperture or third lumen. The parts are located relative to one another by a central rod  404  within two halves  406 ,  408  of a heated dye shaped to correspond to the tip shape shown in  FIG. 3 . This shape can of course be modified to provide varying ends on the catheter depending upon the desired configuration. 
     The body  326  receives first and second mandrels  410 ,  412  shaped to fit within the lumens  350 ,  352  and positioned so that material flowing under the influence of the heat will engage with the ends of the mandrels in a fashion corresponding to the plugs  56 ,  60  shown in  FIG. 3 . The result will be continuous material from the distal end of the catheter to the ends of the mandrels  410 ,  412 . The shaping can be seen in  FIG. 3  but without the spacing between the plugs  56 ,  60  and the solid end of the catheter. 
     Under the influence of heat, the material of the body  326  and extension  400  will now and be shaped by the closing dye halves  406 ,  408 . The necessary quantity of material required to complete the shape can be augmented by the provision of plugs in the lumens  350 ,  352  of a material which will also flow under the influence of heat. However, with some care in design, it is possible to complete the tip without the use of these plugs. 
     The structure shown in  FIG. 18  has the advantage that the extension  400  can be of any durometer hardness required, consistent of course with the material matching that of the body  326 . Consequently, it is possible to create a distal end on the tip having different characteristics from the main body. The very end of the catheter can be quite soft so that, when it is inserted, it will have minimal strength and therefore reduce the risk of damage to the wall of the vein after insertion. Such a tip may well make it possible to leave the catheter in place for longer periods than would be possible with a tip having a stiff end. 
     Reference is now made to  FIG. 19  which illustrates a further embodiment of tip made using the method of  FIG. 18 . As mentioned, the mold house can be of different shapes and the shapes chosen to make the structure in  FIG. 19  provide a cylindrical central extension  414  made from a part similar to that identified as  402  in  FIG. 18 . There is a transition zone defining a shoulder  416  where the extension  414  blends smoothly into the body  326 . In this embodiment, to provide sufficient material to block the lumens  350 ,  352 , plugs  418  and  420  are provided and these flow into the material around them as indicated by the broken outline at the shoulder  416 . With a suitable selection of material, it is possible to provide the extension  414  with significantly different physical characteristics from the body, notably it can be made of soft material which will have very little effect on the inner wall surface of a vein. Similarly, the strength at the shoulder can be changed by using inserts of soft material or even providing plugs rather than inserts in the manner described with reference to  FIG. 3 . 
     Structures such as shown in  FIGS. 3 ,  13 , and  19  are exemplary of tips which are tapered. Some are frustro-conical whereas others tend to have a shoulder such as that shown in  FIG. 19 . However, functionally they are all tapered since they will dilate tissue as they are moved along a Seldinger wire into position in a patient. For this reason, in the terminology of this application, the word “tapered” is intended to include any structure at the end which is capable of such dilation. 
     It will be appreciated that various other modifications may be made to the catheter, and to the processes for making parts of the catheter as described, without departing from the scope of the invention, for example, the material used to form the tube and inserts may be any suitable medical grade thermoplastic. Also, the positioning of the apertures and the number of apertures is to some extent a matter of choice. Also the length of the conical tip can be varied to include apertures in the wall of the tip. While such a structure is more complicated to make, the flow pattern would be advantageous. 
     Although the catheter has been described in use in haemodialysis in a subclavian vein, it would also be appreciated that it can be used in both femoral and jugular veins, and can also be used in other blood treatments including apheresis, haemoperfusion and non-blood related treatments involving nutrition and drug therapies.