Patent Publication Number: US-2020282129-A1

Title: Conduit arrangement

Description:
FIELD OF INVENTION 
     The present invention relates to a conduit arrangement and, more particularly, to a conduit arrangement comprising two tubular lumens for the independent transfer of fluid. 
     BACKGROUND OF THE INVENTION 
     In the field of medicine, it is necessary for fluids essential to life, such as blood, to be transferred independently and, more particularly, to be transferred independently to and from an individual. For example, individuals suffering from kidney failure require regular dialysis treatment. Such dialysis treatment requires the removal of blood from the individual and the cycling of the blood through a dialysis machine that performs the function of the failed kidney. After processing, the blood is then returned to the individual. A dialysis catheter is used to transfer blood to and from an individual during such dialysis treatment. A dialysis catheter comprises a dual lumen to keep separate the blood flowing out of the individual and into the dialysis machine from that flowing out of the dialysis machine and into the individual. One lumen of the dialysis catheter is an outflow lumen, which transfers outgoing blood from the dialysis machine to the individual and the other lumen is an inflow lumen, which transfers incoming blood from the individual to the dialysis machine. A dialysis catheter is typically inserted into a vein in the neck, chest or groin of an individual. A further example of the independent transfer of fluid to and from an individual is multiple lumen intravenous catheters; for example, central venous catheters for fluid administration. 
     WO00/38591 relates to artificial or modified natural blood flow tubing (also known as a “vascular graft”). The blood flow tubing of WO00/38591 seeks to replicate natural blood flow, which occurs in a spiral fashion throughout the body (Stonebridge P A, Brophy C M., 1991, Spiral laminar flow in arteries?; The Lancet. 338 (8779): pp 1360-1). In particular, the blood flow tubing of WO00/38591 has helical-flow inducing means adapted to induce helical flow in such a fashion as to eliminate or reduce turbulence. The tubing has internal helical grooving and/or ridging which induces such helical flow. In a vascular graft, the provision of helical blood flow reduces the turbulence of the blood in the vascular graft which, in turn, reduces the likelihood of plaque formation, reduced flow capacity and thromboses. 
     CN204293665 relates to a three-cavity catheter comprising a main cavity coaxial with the axis of the catheter body and first and second secondary cavities which wrap the periphery of the main cavity. Two partition boards are arranged between the first and second secondary cavities and are spirally distributed along the axis of the catheter body. The first and second secondary cavities have a C-shaped cross-sectional profile. The spiral partition boards function to prevent the first and second secondary cavities from being stressed and deforming. Thus the design of CN204293665 is focused on providing support for the secondary cavities as opposed to seeking to improve flow characteristics. The problem with the three-cavity catheter reported in CN204293665 is that it does not produce spiral laminar flow owing to the C-shaped cross-sectional profiles of the secondary cavities. Therefore, natural blood flow patterns and the associated advantages are not attained by the three-cavity catheter of CN204293665. 
     An entirely different technical field is that of devices for improving the quality of drinking water and other beverages. WO2007/110074 relates to an apparatus for treating a liquid such as water or wine in order to improve its quality. The apparatus comprises at least two pipe segments which are twisted about a common axis. The pipe segments comprise inlet and outlet openings which are connectable to a common inlet and outlet such that the flow of liquid is divided in the pipe segments and then reunited into a single flow of liquid which leaves the apparatus via a common outlet opening. As such, the problem with the apparatus of WO2007/110074 is that it is fundamentally unsuited for carrying bodily fluids such as blood. It is, furthermore, to be noted that the design of the apparatus comprises common inlet and outlet openings meaning that the flow of liquids in the different pipe segments is not kept separate. Instead, the liquid flowing through the different pipe segments is re-mixed. 
     The present invention seeks to alleviate the above problems and arises from the recognition that blood flow in a dialysis catheter and fluid flow in a multiple lumen intravenous catheter is not helical and that it would be desirable for it to be so. 
     In particular, it would be desirable for blood flow in the outflow lumen of the dialysis catheter to occur in a helical fashion such that blood entering the vein of the individual at the site of insertion of the dialysis catheter would replicate natural blood flow patterns. Likewise, it would be desirable to induce such properties in the fluid flowing in the lumen of multiple lumen intravenous catheters. In particular, it would be desirable for blood flow in the lumen of a dialysis catheter or in the lumen of a multiple lumen intravenous catheter to have spiral laminar flow (i.e. to replicate natural blood flow). 
     Additional advantages associated with helical fluid flow, in particular spiral laminar flow, in a dialysis catheter or in a multiple lumen intravenous catheter include: (a) the fluid, such as blood, is subjected to less shear stress when flowing in a helical fashion; and (b) higher volumes of fluid can be transferred in a conduit of a given diameter when the fluid has helical flow as compared to non-helical flow. For these reasons, it would be desirable to induce helical fluid flow, in particular spiral laminar flow, in a dialysis catheter or in a multiple lumen intravenous catheter. 
     There are also a range of clinical benefits to inducing helical fluid flow, such as helical blood flow, in particular spiral laminar flow, in a dialysis catheter or in a multiple lumen intravenous catheter. The helical fluid flow reduces turbulence in the catheter, which in turn; reduces the tendency of thrombosis in the catheter; reduces the tendency of infection in the catheter because there is reduced stagnation (stagnation may lead to infection and the formation of infected “biofilm” that makes infection eradication problematic); and reduces endothelial damage at the end of catheter, which reduces venous or vessel tissue response and narrowing. The helical fluid flow increases the velocity of fluid flowing in the lumen, which reduces stasis or stagnation and so reduces thrombosis. In addition, the helical fluid flow reduces stagnation and/or cellular deposition. If the intravenous catheter is used to transfer an infusate comprising a drug to an individual, such as during chemotherapy, the helical fluid flow improves mixing of the fluid contents in the lumen of the catheter. This affects the distribution of the drug in the infusate and may reduce the toxic level of a drug locally that could affect a vessel wall. 
     These clinical advantages are primarily associated with the induction of helical fluid flow, in particular spiral laminar flow, in the lumen of the catheter that transfers fluid to the individual. However, the induction of helical flow in the lumen of the catheter that transfers fluid away from the individual can also be of benefit; for example, to reduce shear stress on the fluid or to enable higher volumes of fluid to be transferred in a conduit of a given diameter. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention, there is provided a conduit arrangement comprising distal and proximal ends and two tubular lumens, wherein each tubular lumen comprises a longitudinal axis extending between the distal and proximal ends and independently permits fluid communication between the distal and proximal ends and wherein at least a portion of at least one tubular lumen is capable of imparting helical flow on fluid passing through said portion of the tubular lumen. 
     Preferably, the helical flow is spiral laminar flow. 
     Preferably, the portion of the tubular lumen capable of imparting helical flow is 10%, 20%, 25%, 30%, 40%, 50%, 60%, 70%, 75%, 80%, 90% or 100% of the total length of the tubular lumen. 
     Conveniently, the conduit arrangement comprises a tubular conduit and a septum which divides the tubular conduit parallel to its longitudinal axis so as to define the two tubular lumens. 
     Preferably, the septum extends substantially across a diameter of the tubular conduit. 
     Alternatively, the conduit arrangement comprises a tubular conduit and wherein the two tubular lumens are located side-by-side in said tubular conduit. 
     Alternatively, the conduit arrangement comprises a tubular conduit comprising an outer wall, which defines an outer tubular lumen, and an inner wall, which defines an inner tubular lumen, and wherein the outer tubular lumen and the inner tubular lumen extend coaxially between the distal and proximal ends of the conduit arrangement. 
     Advantageously, the inner tubular lumen further comprises a subconduit which passes through the outer wall of the tubular conduit. 
     Preferably, the pathway of each of the two tubular lumens is straight with respect to the longitudinal axis of each tubular lumen. 
     Advantageously, said portion of the tubular lumen comprises an axially extending internal helical protrusion located around the inside of the tubular lumen for imparting helical flow on fluid passing through said portion of the tubular lumen. 
     Alternatively, said portion of the tubular lumen follows a helical pathway. 
     Alternatively, sequential non-circular cross-sections of said portion of the tubular lumen transition rotationally along the longitudinal axis of the tubular lumen. 
     Alternatively, the conduit arrangement comprises first and second tubular conduits fused together, one of the tubular lumens being located in the first tubular conduit and the other of the tubular lumens being located in the second tubular conduit. 
     Advantageously, each of the two tubular lumens independently follows a helical pathway. 
     Preferably, at least one of the tubular lumens comprises an axially extending internal helical protrusion located around the inside of the tubular lumen for imparting helical flow on fluid passing through the tubular lumen. 
     Advantageously, the conduit arrangement is made from polyurethane. 
     Alternatively, the conduit arrangement is made from PVC, PTFE, latex, silicone or carbothane. 
     Conveniently, the conduit arrangement is flexible. 
     Conveniently, the interior of each of the two tubular lumens is made from a biocompatible material. 
     In accordance with a second aspect of the invention, there is provided a catheter comprising the conduit arrangement of the invention. 
     In accordance with a third aspect of the invention, there is provided a method of transferring fluid comprising the use of the conduit arrangement of the invention. 
     Preferably, the fluid is transferred to and/or from an individual. 
     Advantageously, the method comprises the use of the catheter of the invention. 
     Conveniently, the fluid is blood. 
     The terms “helix” and “helix angle” as used herein cover the mathematical definitions of helix and helical and any combination of mathematical definitions of helical and spiral. 
     A “helix angle” referred to herein is the angle between the helix and the axial line about which it is formed, in particular the longitudinal axis of a tubular lumen. 
     The term “outflow lumen” as used herein refers to a tubular lumen that transfers outgoing blood from an external source, such as a dialysis machine, to an individual. 
     The term “inflow lumen” as used herein refers to a tubular lumen that transfers incoming blood from an individual to an external location, such as a dialysis machine. 
     The term “spiral laminar flow” as used herein refers to a linear flow with a rotational vector flow component centred on the axis of the tubular conduit. In other words, in addition to the laminar flow in the direction of tubular conduit, there is a rotation in the plane of the cross-section of the tubular conduit. 
     The presence of spiral laminar flow can be determined using colour Doppler ultrasound in vivo as described by Stonebridge et al. in Clinical Science (1996, 9: 17-21), which is incorporated herein by reference. The presence of spiral laminar flow results in a characteristic “red/blue” split being seen using this technique. In addition, colour Doppler ultrasound can be used with a flow rig in order to determine the presence of spiral laminar flow ex vivo. For example, a flow rig comprising a fluid circuit and a blood mimic at 37° C. having either a constant or pulsatile flow can be used to study blood flow characteristics. 
     In addition, the presence of spiral laminar flow can be determined using phase contrast magnetic resonance imaging (MRI). Houston et al. in Nephrol Dial Transplant. (2004 July; 19(7):1786-91), which is incorporated herein by reference, describes the use of MRI to observe the prevalence of spiral laminar flow in vivo. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a perspective view of a tubular conduit, in accordance with a first embodiment of the present invention, showing views along each of the lines A-A, B-B and C-C. 
         FIG. 2  is a perspective view of a tubular conduit, in accordance with a second embodiment of the present invention, showing views along each of the lines A′-A′, B′-B′ and C′-C′. 
         FIG. 3  is a perspective view of a portion of a tubular conduit, in accordance with a third embodiment of the present invention, with some hidden detail shown in dashed lines. 
         FIG. 4  is a perspective view of a portion of a conduit arrangement, in accordance with a fourth embodiment of the present invention. 
         FIG. 5  is a perspective view of a portion of a conduit arrangement, in accordance with a variant of the fourth embodiment of the present invention. 
         FIG. 6  is a perspective view of a portion of a conduit arrangement, in accordance with a further variant of the fourth embodiment of the present invention. 
         FIG. 7  is a perspective view of a conduit arrangement, in accordance with a further variant of the fourth embodiment of the present invention. 
         FIG. 8  is a perspective view of a conduit arrangement, in accordance with a further variant of the fourth embodiment of the present invention. 
         FIG. 9  is a perspective view of a portion of a tubular conduit, in accordance with a fifth embodiment of the present invention, with some hidden detail shown in dashed lines. 
         FIG. 10A  is a cross-sectional view of an outflow lumen of a tubular conduit through a plane perpendicular to the longitudinal axis of the tubular conduit. 
         FIG. 10B  is an enlarged section showing detail of the base of an axially extending internal helical protrusion as circled in  FIG. 10A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a tubular conduit  1  in accordance with a first embodiment of the present invention is shown. The tubular conduit  1  comprises proximal and distal ends  2 ,  3  and a longitudinal axis  4  therebetween. As can be seen from the views along each of the lines A-A, B-B and C-C, the tubular conduit  1  is defined by an axially extending, elliptical perimeter wall  5  which defines the exterior of the tubular conduit  1 . Thus, the tubular conduit  1  has an elliptical cross-section. The tubular conduit further comprises an axially extending septum  6 , which extends substantially across a diameter of the tubular conduit  1 . The inner surface  7  of the perimeter wall  5  of the tubular conduit  1  and each side  8 ,  9  of the septum  6  define two axially extending tubular lumens: an inflow lumen  10  and an outflow lumen  11 . Each of the inflow lumen  10  and the outflow lumen  11  are D-shaped in cross-section and each independently permits fluid communication between the proximal and distal ends  2 ,  3  of the tubular conduit  1 . 
     In this embodiment, each of the inflow and outflow lumens  10 ,  11  follows a helical pathway, with a helix angle of 20°, between the proximal and distal ends  2 ,  3  of the tubular conduit  1  such that sequential non-circular cross-sections of the tubular conduit  1  transition rotationally along the length of the tubular conduit  1 . The helical pathway of each of the inflow and outflow lumens  10 ,  11  between the proximal and distal ends  2 ,  3  of the tubular conduit  1  consists of one single revolution. That is to say, the pathway of each of the inflow and outflow lumens  10 ,  11  between the proximal and distal ends  2 ,  3  of the tubular conduit  1  makes one complete turn of 360°. 
     In use, the tubular conduit  1  is comprised within a dialysis catheter (not shown) and the distal end  3  of the tubular conduit  1  is connected so as to be in fluid communication with a dialysis machine (not shown) and the proximal end  2  of the tubular conduit  1  is inserted into the vein of an individual who is suffering from kidney failure (not shown). At the distal end  3  of the tubular conduit  1 , the inflow and outflow lumens  10 ,  11  separate into two independent conduits (not shown) so as to permit independently fluid communication with a dialysis machine (not shown). At the proximal end  2  of the tubular conduit  1 , the inflow and outflow lumens  10 ,  11  also separate into two independent conduits so as to prevent mixing of the blood in the vein of the individual (not shown). In alternative embodiments, the proximal end  2  of each of the inflow and outflow lumens  10 ,  11  is staggered with respect to the other so as to prevent mixing of the blood in the vein of the individual (not shown). 
     Once the distal end  3  of the tubular conduit  1  is connected so as to be in fluid communication with the dialysis machine and the proximal end  2  is inserted into the vein of the individual, blood flows independently from the individual to the dialysis machine through the inflow lumen  10  of the tubular conduit  1  and from the dialysis machine to the individual through the outflow lumen  11  of the tubular conduit  1 . As the blood passes through each of the inflow and outflow lumens  10 ,  11 , the helical pathway of said lumens imparts helical flow on the blood, which reduces turbulence in the blood. The helical blood flow continues after the blood exits the outflow lumen  11  and enters the blood vessel of the individual. Thus turbulent blood flow is reduced, or even eliminated, in the blood vessel downstream of the outflow lumen  11 . 
     In a variant of the first embodiment, the inflow and outflow lumens  10 ,  11  each follow a helical pathway along only a portion of the length of the tubular conduit  1 . In this variant of the first embodiment, the inflow and outflow lumens  10 ,  11  each follow a helical pathway, with a helix angle of 20°, between the proximal end  2  of the tubular conduit  1  and a termination point, which is short of the distal end  3  of the tubular conduit  1 . In a further variant of the first embodiment, the inflow and outflow lumens  10 ,  11  each follow a helical pathway, with a helix angle of 20°, between the distal end  3  of the tubular conduit  1  and a termination point, which is short of the proximal end  2  of the tubular conduit  1 . The helical pathway of each of the inflow and outflow lumens  10 ,  11  between the proximal end  2  of the tubular conduit  1  and the termination point or, alternatively, between the distal end  3  of the tubular conduit  1  and the termination point consists of one single revolution. That is to say, the pathway of each of the inflow and outflow lumens  10 ,  11  between the proximal end  2  of the tubular conduit  1  and the termination point or, alternatively, between the distal end  3  of the tubular conduit  1  and the termination point makes one complete turn of 360°. From the termination point to the distal end  3  of the tubular conduit or, alternatively, from the termination point to the proximal end  2  of the tubular conduit, the pathway of each of the inflow and outflow lumens  10 ,  11  is substantially straight with respect to the longitudinal axis  4  of the tubular conduit, subject to any overall curvature of the tubular conduit  1 . It is to be appreciated that the total length of the tubular conduit  1  varies depending on its specific use. However, a dialysis catheter typically comprises a tubular conduit  1  that is between approximately 5 and 55 cm in length. In this variant, the proportion of the length of the tubular conduit  1  in which the inflow and outflow lumens  10 ,  11  follow a helical pathway varies but it is generally less than 50% of the total length of the tubular conduit  1 . In preferred embodiments, the proportion of the length of the tubular conduit  1  in which the inflow and outflow lumens  10 ,  11  follow a helical pathway is less than 25% or less than 15% of the total length of the tubular conduit  1   
     In a further variant of the first embodiment, the length of the tubular conduit  1  in which the inflow and outflow lumens  10 ,  11  follow a helical pathway is selected with reference to the diameter of the lumen of the tubular conduit  1 . This relationship is explained in Table 1 below. For example, for a tubular conduit with a lumen diameter of 2 mm, the minimum length of the tubular conduit in which the inflow and outflow lumens  10 ,  11  follow a helical pathway would be 17.26 mm. In embodiments such as the first embodiment, where each lumen  10 ,  11  of the tubular conduit is D-shaped, the diameter of the lumen is taken as an approximation or average of the diameter of the D-shaped lumen. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Relationship between lumen diameter and minimum 
               
               
                 length of helical flow inducing means 
               
            
           
           
               
               
               
            
               
                   
                   
                 Minimum length of helical flow 
               
               
                   
                 Lumen Diameter (mm) 
                 inducing means (mm) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 1 
                 8.63 
               
               
                   
                 2 
                 17.26 
               
               
                   
                 3 
                 25.89 
               
               
                   
                 4 
                 34.53 
               
               
                   
                 5 
                 43.16 
               
               
                   
                 6 
                 51.79 
               
               
                   
                 7 
                 60.42 
               
               
                   
                 8 
                 69.05 
               
               
                   
                 9 
                 77.68 
               
               
                   
                 10 
                 86.31 
               
               
                   
                 11 
                 94.95 
               
               
                   
                 12 
                 103.58 
               
               
                   
                 13 
                 112.21 
               
               
                   
                 14 
                 120.84 
               
               
                   
                 15 
                 129.47 
               
               
                   
                 16 
                 138.10 
               
               
                   
                 17 
                 146.73 
               
               
                   
                 18 
                 155.37 
               
               
                   
                 19 
                 164.00 
               
               
                   
                 20 
                 172.63 
               
               
                   
                   
               
            
           
         
       
     
     In the first embodiment and the variants of the first embodiment described above, the helical pathway of each of the inflow and outflow lumens  10 ,  11  consists of one single revolution. However, in alternative embodiments, the helical pathway of each of the inflow and outflow lumens  10 ,  11  is either shorter or longer than one single revolution and is, for example, between 50% and 150% of a single revolution. 
     In use, the variants of the first embodiment operate in substantially the same way as the first embodiment. As blood passes through each of the inflow and outflow lumens  10 ,  11  in the portion of the length of the tubular conduit  1  that follows a helical pathway, helical flow is imparted on the blood which reduces turbulence in the blood. Furthermore, the helical flow of the blood in the inflow or outflow lumens  10 ,  11  continues after the blood passes the termination point of the helical pathway. Therefore, turbulent blood flow is likewise reduced, or even eliminated, along the length of the inflow or outflow lumen  10 ,  11 . Similarly, the helical blood flow continues after the blood exits the outflow lumen  11  at the proximal end  2  of the tubular conduit  1  and enters the blood vessel of the individual. Thus turbulent blood flow is reduced, or even eliminated, in the blood vessel downstream of the outflow lumen  11 . 
     Referring to  FIG. 2 , a tubular conduit  1 ′ in accordance with a second embodiment of the present invention is shown. The tubular conduit  1 ′ comprises proximal and distal ends  2 ′,  3 ′ and a longitudinal axis  4 ′ therebetween. As can be seen from the views along each of the lines A′-A′, B′-B′ and C′-C′, the tubular conduit  1 ′ is defined by an axially extending, elliptical perimeter wall  5 ′ which defines the exterior of the tubular conduit  1 ′. Thus, the tubular conduit  1 ′ has an elliptical cross-section. The tubular conduit  1 ′ further comprises an axially extending septum  6 ′, which extends substantially across a diameter of the tubular conduit. The inner surface  7 ′ of the perimeter wall  5 ′ of the tubular conduit  1 ′ and each side  8 ′,  9 ′ of the septum  6 ′ define two axially extending tubular lumens  10 ′,  11 ′: an inflow lumen  10 ′ and an outflow lumen  11 ′. Each of the inflow lumen  10 ′ and the outflow lumen  11 ′ is D-shaped in cross-section and each independently permits fluid communication between the proximal and distal ends  2 ′,  3 ′ of the tubular conduit  1 ′. Located around the inner surface  7 ′ of the perimeter wall  5 ′ and the side  9 ′ of the septum  6 ′ is an axially extending internal helical protrusion  12  which projects radially inwardly into the outflow lumen  11 ′. The helix angle of the axially extending internal helical protrusion  12  is 20°. In this embodiment, the cross-section of the axially extending internal helical protrusion  12  perpendicular to the longitudinal axis  4 ′ of the tubular conduit  1 ′ is of a bell shape. Sequential views along each of the lines A′-A′, B′-B′ and C′-C′ demonstrate the location of the axially extending internal helical protrusion  12  along the length of the tubular conduit  1 ′. In this embodiment, the axially extending internal helical protrusion  12  consists of one single revolution. That is to say, the axially extending internal helical protrusion makes one complete turn of 360° between the proximal and distal ends  2 ′,  3 ′ of the tubular conduit  1 ′. 
     In use, the tubular conduit  1 ′ is comprised within a dialysis catheter (not shown) and the distal end  3 ′ of the tubular conduit  1 ′ of the second embodiment is connected so as to be in fluid communication with a dialysis machine (not shown) and the proximal end  2 ′ of the tubular conduit  1 ′ is inserted into the vein of an individual who is suffering from kidney failure (not shown). The structure of the proximal and distal ends  2 ′,  3 ′ of the tubular conduit  1 ′ can be the same as that described above in relation to the first embodiment. 
     Once the distal end  3 ′ of the tubular conduit  1 ′ is connected so as to be in fluid communication with the dialysis machine and the proximal end  2 ′ is inserted into the vein of the individual, blood flows independently from the individual to the dialysis machine through the inflow lumen  10 ′ of the tubular conduit  1 ′ and from the dialysis machine to the individual through the outflow lumen  11 ′ of the tubular conduit  1 ′. As the blood passes the axially extending internal helical protrusion  12  of the outflow lumen  11 ′, helical flow is imparted on the blood, which reduces turbulence in the blood. Furthermore, the helical blood flow continues after the blood exits the outflow lumen  11 ′ and enters the blood vessel of the individual. Thus turbulent blood flow is reduced, or even eliminated, in the blood vessel downstream of the outflow lumen  11 ′. 
     In a variant of the second embodiment, the axially extending internal helical protrusion  12  extends for only a portion of the length of the tubular conduit  1 ′. In this variant of the second embodiment, the axially extending internal helical protrusion  12  extends around the inner surface  7 ′ of the wall  5 ′ and the side  9 ′ of the septum  6 ′ from the proximal end  2 ′ of the tubular conduit  1 ′ to a termination point, which is short of the distal end  3 ′ of the tubular conduit  1 ′. In a further variant of the first embodiment, the axially extending internal helical protrusion  12  extends around the inner surface  7 ′ of the wall  5 ′ and the side  9 ′ of the septum  6 ′ from the distal end  3 ′ of the tubular conduit  1 ′ to a termination point, which is short of the proximal end  2 ′ of the tubular conduit  1 ′. The helix angle of the axially extending internal helical protrusion  12  is 20°. The axially extending internal helical protrusion  12  consists of one single revolution. That is to say, the axially extending internal helical protrusion  12  makes one complete turn of 360° between the proximal and distal ends  2 ′,  3 ′ of the tubular conduit  1 ′. It is to be appreciated that the total length of the tubular conduit  1 ′ varies depending on its specific use. However, a dialysis catheter typically comprises a tubular conduit  1 ′ that is between approximately and 55 cm in length The proportion of the length of the tubular conduit  1 ′ which comprises an axially extending internal helical protrusion  12  varies but it is generally less than 50% of the total length of the tubular conduit  1 ′. In preferred embodiments, the proportion of the length of the tubular conduit  1 ′ which comprises the axially extending internal helical protrusion  12  is less than 25% or less than 15% of the total length of the tubular conduit  1 . 
     In a further variant of the second embodiment, the length of the axially extending internal helical protrusion  12  is selected with reference to the diameter of the lumen of the tubular conduit  1 ′, as discussed above with reference to the first embodiment and Table 1. 
     In the variants of the second embodiment described above, the axially extending internal helical protrusion  12  consists of one single revolution. However, in alternative variants, the axially extending internal helical protrusion is either shorter or longer than one single revolution and may, for example, be between 50% and 150% of a single revolution. 
     In use, the variants of the second embodiment operate in substantially the same manner as is described for the second embodiment. As the blood passes through the outflow lumen  11 ′ in the portion of the tubular conduit  1 ′ that comprises the axially extending internal helical protrusion  12 , helical flow is imparted on the blood which reduces turbulence in the blood. Furthermore, the helical blood flow continues after the blood passes the termination point of the axially extending internal helical protrusion  12 . Therefore, turbulent flow is reduced, or even eliminated, along the length of the outflow lumen  11 ′ that downstream of the termination point. Similarly, the helical blood flow continues after the blood exits the outflow lumen  11 ′ at the proximal end of the tubular conduit  1 ′ and enters the blood vessel of the individual. Thus turbulent blood flow is reduced, or even eliminated, in the blood vessel downstream of the outflow lumen  11 ′. 
     In the first and second embodiments, the tubular conduit  1 ,  1 ′ has an elliptical cross-section. However, in alternative embodiments, the tubular conduit  1 ,  1 ′ has a circular cross-section or a substantially circular cross section. 
     In the first and second embodiments, the septum  6 ,  6 ′ extends substantially across a diameter of the tubular conduit  1 ,  1 ′. However, in alternative embodiments, the septum  6 ,  6 ′ follows a curved or an S-shaped path perpendicular to the longitudinal axis  4 ,  4 ′ of the tubular conduit  1 ,  1 ′. 
     Referring to  FIG. 3 , a portion of a tubular conduit  13  in accordance with a third embodiment of the present invention is shown. The tubular conduit  13  comprises proximal and distal ends  14 ,  15  and a longitudinal axis  16  therebetween. An axially extending, tubular inner wall  17  defines an inflow lumen  18  of relatively small diameter and an axially extending, tubular outer wall  19  defines an outflow lumen  20  of relatively large diameter. In addition, the axially extending, tubular outer wall  19  defines the exterior of the tubular conduit  13 . The inner wall  17  shares the longitudinal axis  16  of the tubular conduit  13  such that the inner wall  17  and the outer wall  19  extend coaxially between the proximal and distal ends  14 ,  15  of the tubular conduit  13 . The inflow lumen  18  has a circular cross-section and the outflow lumen  20  has a ring-shaped cross-section and each of the inflow and outflow lumens  18 ,  20  independently permits fluid communication between the proximal and distal ends  14 ,  15  of the tubular conduit  13 . The tubular conduit  13  has a circular cross-section. Located around the inner surface  22  of the outer wall  19  is an axially extending, internal helical protrusion  21 , which projects radially inwardly into the outflow lumen  20 . The helix angle of the axially extending internal helical protrusion  21  is 20°. In this embodiment, the cross-section of the axially extending internal helical protrusion  21  perpendicular to the longitudinal axis  16  of the tubular conduit  13  is of a bell shape. The axially extending internal helical protrusion  21  extends between the proximal and distal ends  14 ,  15  of the tubular conduit  13  and consists of one single revolution. That is to say, the axially extending internal helical protrusion  21  makes one complete turn of 360° between the proximal and distal ends  14 ,  15  of the tubular conduit  13 . In alternative embodiments, the axially extending internal helical protrusion  21  is either shorter or longer than one single revolution and may, for example, be between 50% and 150% of a single revolution. 
     In some embodiments, the axially extending internal helical protrusion  21  projects radially inwardly into the outflow lumen  20  to the extent that it contacts the axially extending, tubular inner wall  17 . Thus, the axially extending, tubular inner wall  17  is supported in the centre of the outflow lumen  20  by the axially extending internal helical protrusion  21  (not shown). 
     In some embodiments, at each of the proximal and distal ends  14 ,  15  of the tubular conduit  13 , the inner wall  17  penetrates and passes through the outer wall  19  of the tubular conduit  13  via a subconduit (not shown). At these points, the path of the inflow lumen  18  therefore passes across the path of the outflow lumen  20 , to the exterior of the tubular conduit  13 . In an alternative embodiment, at the proximal end  14  of the tubular conduit  13 , the proximal ends of each of the inner wall  17  and the outer wall  19  are staggered (not shown). Specifically, the proximal end of the inner wall  17  extends beyond the proximal end of the outer wall  19 . Thus the inflow lumen  18  extends beyond the outflow lumen  20  at the proximal end  14  of the tubular conduit. 
     In a variant of the third embodiment, the axially extending internal helical protrusion  21  extends for only a portion of the length of the tubular conduit  13 . The description of the variants of the first and second embodiments are also relevant to this variant of the third embodiment. 
     In use, the tubular conduit  13  is comprised within a dialysis catheter (not shown) and the distal end  15  of the tubular conduit  13  is connected so as to be in fluid communication with a dialysis machine (not shown) and the proximal end  14  of the tubular conduit  13  is inserted into the vein of an individual who is suffering from kidney failure (not shown). The structure of the proximal and distal ends  14 ,  15  of the tubular conduit  13  can be analogous to that described above in relation to the first embodiment. 
     Once the distal end  15  of the tubular conduit  13  is connected so as to be in fluid communication with the dialysis machine and the proximal end  14  is inserted into the vein of the individual, blood flows independently from the individual to the dialysis machine through the inflow lumen  18  of the tubular conduit  13  and from the dialysis machine to the individual through the outflow lumen  20  of the tubular conduit  13 . As the blood passes the axially extending internal helical protrusion  21  of the outflow lumen  20 , helical flow is imparted on the blood, which reduces turbulence in the blood. Furthermore, the helical blood flow continues after the blood exits the outflow lumen  20  and enters the blood vessel of the individual. Thus turbulent blood flow is reduced, or even eliminated, in the blood vessel downstream of the outflow lumen  20 . 
     Referring to  FIG. 4 , a portion of a conduit arrangement  23  in accordance with a fourth embodiment of the present invention is shown. The conduit arrangement  23  comprises first and second tubular conduits  24 ,  25 , which each comprise proximal  26 ,  27  and distal  28 ,  29  ends and a tubular perimeter wall  30 ,  31  that extends therebetween. The tubular perimeter wall  30 ,  31  defines the exterior of the first and second tubular conduits  24 ,  25 . Thus, the first and second tubular conduits have circular cross-sections. The tubular perimeter wall  30 ,  31  of the first and second tubular conduits  24 , further defines first and second tubular lumens  32 ,  33 : an inflow lumen  32  and an outflow lumen  33 . Each of the inflow and outflow lumens  32 ,  33  are circular in cross-section and each independently permits fluid communication between the proximal  26 ,  27  and distal  28 ,  29  ends of the tubular conduits  24 ,  25 . 
     The first and second tubular conduits  24 ,  25  are intertwined such that each of the inflow and outflow lumens  32 ,  33  independently follows a helical pathway, having a helix angle of 20°, between the proximal  26 ,  27  and distal  28 ,  29  ends of the tubular conduits  24 ,  25 . In some embodiments, the outer surface  34 ,  35  of the tubular perimeter wall  30 ,  31  of the first and second tubular conduits  24 ,  25  is fused together along the length of the first and second tubular conduits  24 ,  25  for stabilising the conduit arrangement  23 . In other embodiments, at least a portion of the outer surface  34 ,  35  of the tubular perimeter wall  30 ,  31  of the first and second tubular conduits  24 , is fused together at intersection points (not shown) for stabilising the conduit arrangement  23 . 
     In a variant of the fourth embodiment, the conduit arrangement  23  is stabilised by embedding the first and second tubular conduits  24 ,  25  within a single, relatively larger tubular structure, as shown in  FIG. 5 . Referring to  FIG. 6 , a further variant of the fourth embodiment is shown in which the proximal end  27  of the second tubular conduit extends beyond the proximal end  26  of the first tubular conduit  24 . Thus the outflow lumen  33  extends beyond the inflow lumen  32  at the proximal end  26 ,  27  of the conduit arrangement  23 . 
     In a further variant of the fourth embodiment, the inflow and outflow lumens  32 ,  33  independently follow a helical pathway along only a portion of the conduit arrangement  23 . An example of such a variant is shown in  FIG. 7 . The description of the variants of the first and second embodiments are also relevant to this variant of the fourth embodiment. 
     Referring to  FIG. 8 , yet a further variant of the fourth embodiment is shown in which there is located around the inner surface  51  of the tubular perimeter wall  31  an axially extending, internal helical protrusion  50 , which projects radially inwardly into the outflow lumen  33 . The helix angle of the axially extending internal helical protrusion  50  is 20°. In this embodiment, the cross-section of the axially extending internal helical protrusion  50  perpendicular to the longitudinal axis  52  of the tubular conduit  25  is of a bell shape. The axially extending internal helical protrusion  50  extends between the proximal and distal ends  27 ,  29  of the second tubular conduit  25  and consists of one single revolution. That is to say, the axially extending internal helical protrusion  50  makes one complete turn of 360° between the proximal and distal ends  27 ,  29  of the second tubular conduit  25 . In alternative embodiments, the axially extending internal helical protrusion  50  is either shorter or longer than one single revolution and may, for example, be between 50% and 150% of a single revolution. In yet further variants of the fourth embodiment described above and as shown in  FIGS. 4 to 7 , the internal helical protrusion of the variant shown in  FIG. 8  is provided in the outflow lumen  33  thereof. 
     In use of the fourth embodiment (depicted in  FIGS. 4 to 8 ), the conduit arrangement  23  is comprised within a dialysis catheter (not shown) and the distal ends  28 ,  29  of the first and second tubular conduits  24 ,  25  are connected so as to be in fluid communication with a dialysis machine (not shown) and the proximal ends  26 ,  27  of the first and second tubular conduits  24 ,  25  are inserted into the vein of an individual who is suffering from kidney failure (not shown). The structure of the proximal and distal ends  26 ,  27  of the conduit arrangement  23  can be analogous to that described above in relation to the first embodiment. 
     Once the distal ends  28 ,  29  of the first and second tubular conduits  24 ,  25  are connected so as to be in fluid communication with the dialysis machine and the proximal ends  26 ,  27  are inserted into the vein of the individual, blood flows independently from the individual to the dialysis machine through the inflow lumen  32  of the first tubular conduit  24  and from the dialysis machine to the individual through the outflow lumen  33  of the second tubular conduit  25 . As the blood passes through the two lumens  32 ,  33  the helical pathway of said lumens imparts helical flow on the blood, which reduces turbulence in the blood. The helical blood flow continues after the blood exits the outflow lumen  33  and enters the blood vessel of the individual. Thus turbulent blood flow is reduced, or even eliminated, in the blood vessel downstream of the outflow lumen  33 . 
     Referring to  FIG. 9 , a portion of a tubular conduit  38  in accordance with a fifth embodiment of the present invention is shown. The tubular conduit  38  comprises proximal and distal ends  39 ,  40  and a longitudinal axis therebetween  41 . The tubular conduit  38  is defined by an axially extending tubular wall  42 . Thus, the tubular conduit  38  has a circular cross-section. The tubular conduit further comprises two axially extending tubular lumens  43 ,  44 : an inflow lumen  43  and an outflow lumen  44 . Each of the inflow and outflow lumens  43 ,  44  is defined by an inner surface  45 ,  46  of the axially extending tubular wall  42  and each of the inflow and outflow lumens  43 ,  44  is substantially circular in cross-section. The inflow and outflow lumens  43 ,  44  are located side-by-side within the tubular conduit  38  and each of said lumens follows a pathway which is substantially straight with respect to the longitudinal axis  41  of the tubular conduit  38 , subject to any overall curvature of the tubular conduit  38 . Each of the inflow and outflow lumens  43 ,  44  independently permits fluid communication between the proximal and distal ends  39 ,  40  of the tubular conduit  38 . Located around the inner surface  46  of the axially extending tubular wall  42  which defines the outflow lumen  44  is an axially extending internal helical protrusion  47 , which projects radially inwardly into the outflow lumen  44 . The helix angle of the axially extending internal helical protrusion  47  is 20°. In this embodiment, the cross-section of the axially extending internal helical protrusion  47  perpendicular to the longitudinal axis  41  of the tubular conduit  38  is of a bell shape. The axially extending internal helical protrusion  47  extends between the proximal and distal ends  39 ,  40  of the tubular conduit  38  and consists of one single revolution. That is to say, the axially extending internal helical protrusion  47  makes one complete turn of 360° between the proximal and distal ends  39 ,  40  of the tubular conduit  38 . In alternative embodiments, the axially extending internal helical protrusion  47  is either shorter or longer than one single revolution and may, for example, be between 50% and 150% of a single revolution. 
     In a variant of the fifth embodiment, the axially extending internal helical protrusion  47  extends for only a portion of the length of the tubular conduit  38 . The description of the variants of the first and second embodiments are also relevant to this variant of the fifth embodiment. 
     In use, the distal end  40  of the tubular conduit  38  is connected so as to be in fluid communication with a dialysis machine (not shown) and the proximal end  39  of the tubular conduit  38  is inserted into the vein of an individual who is suffering from kidney failure (not shown). The structure of the proximal and distal ends  39 ,  40  of the tubular conduit  38  can be analogous to that described above in relation to the first embodiment. 
     Once the distal end  40  of the tubular conduit  38  is connected so as to be in fluid communication with the dialysis machine and the proximal end  39  is inserted into the vein of the individual, blood flows independently from the individual to the dialysis machine through the inflow lumen  43  of the tubular conduit  38  and from the dialysis machine to the individual through the outflow lumen  44  of the tubular conduit  38 . As the blood passes the axially extending internal helical protrusion  47  of the outflow lumen  44 , helical flow is imparted on the blood, which reduces turbulence in the blood. Furthermore, the helical blood flow continues after the blood exits the outflow lumen  44  and enters the blood vessel of the individual. Thus turbulent blood flow is reduced, or even eliminated, in the blood vessel downstream of the outflow lumen  44 . 
     In one embodiment, the tubular conduit  1 ,  1 ′,  13 ,  24 ,  25 ,  38  is made from polyurethane. However, in alternative embodiments, the tubular conduit  1 ,  1 ′,  13 ,  24 ,  25 ,  38  is made from PVC, PTFE, latex, silicone or carbothane. The interior of each of the inflow and outflow lumens  10 ,  11 ,  10 ′,  11 ′,  18 ,  20 ,  32 ,  33 ,  43 ,  44  is made from a biocompatible material. In the embodiments described above, the tubular conduit  1 ,  1 ′,  13 ,  24 ,  25 ,  38  is flexible. The conduit arrangement  23  of the fourth embodiment is also flexible. 
     In the embodiments described above, the helical flow imparted on the blood in the tubular lumen is spiral laminar flow such that natural blood flow patterns are replicated. Spiral laminar flow has been observed in vivo in both animals and humans and may be a constant (veins primarily) or pulsatile (arteries) flow waveform. In the tubular lumen of the embodiments of the present invention, spiral laminar flow means that in addition to the laminar flow in the direction of the tubular conduit, there is a rotation in the plane of the cross-section of the tubular conduit, produced by the portion of the tubular lumen that is capable of imparting helical flow. 
     If the tubular conduit of any of the embodiments described above were to comprise a narrowed section or a bifurcation or branch, spiral laminar flow would be maintained through the narrowing of a tubular conduit and beyond the braches or bifurcations of the tubular conduit. 
     In the embodiments described above, the cross-section of the axially extending internal helical protrusion  12 ,  21 ,  47 ,  50  perpendicular to the longitudinal axis  4 ′,  16 ,  41 ,  52  of the tubular conduit  1 ′,  13 ,  38 ,  25  is of a symmetric bell shape. However, in variants of these embodiments, the axially extending internal helical protrusion  12 ,  21 ,  47 ,  50  instead has an asymmetrical cross-section. 
     Referring to  FIG. 10 , a cross-section of an axially extending internal helical protrusion  60  with an asymmetric profile is shown. As shown in  FIG. 10 , the axially extending internal helical protrusion has a first face  61  and a second face  62  coupled together by a curved surface  63 . The main central section of the first face  61  is at an angle of approximately 10° to a diameter  64  of the tubular conduit  65  that intersects the curved surface  63 . The main central section of the second face  62  is at an angle of approximately 38° to a diameter  64  of the tubular conduit  65  that intersects the curved surface  63 . The height  65  of the axially extending internal helical protrusion  60  is half the radius of the tubular conduit  65 . In variants of the second, third, fourth and fifth embodiments of the present invention described above, an axially extending internal helical protrusion  60  having an asymmetric cross-section, as shown in  FIG. 10 , is provided. 
     In use, the tubular conduit  65  comprising the axially extending internal helical protrusion  60  with the asymmetric profile is orientated so that the blood flow is against the first face  61 . 
     In alternative embodiments, the axially extending internal helical protrusion  12 ,  21 ,  47 ,  50  has a different cross-section perpendicular to the longitudinal axis  4 ′,  16 ,  41 ,  52  of the tubular conduit  1 ′,  13 ,  38 ,  25 , instead of a bell shape, such as having a U-shaped cross-section as is described in WO03/045279 or a triangular cross-section such as described in WO2005/004751, each of which are incorporated herein by reference. 
     In the embodiments described above, the helix angle of the helical pathway or the axially extending internal helical protrusion  12 ,  21 ,  47 ,  50  is 20°. However, in other embodiments the helical pathway or the axially extending internal helical protrusion  12 ,  21 ,  47 ,  50  has a helix angle of between 5° and 50°, 5° and 40°, 5° and 30°, 5° and 25° and preferably between 5° and 20°, more preferably between 8° and 20°. 
     In the embodiments described above, the axially extended internal helical protrusion  12 ,  21 ,  47  of the second, third or fifth embodiments; the portion of the tubular conduit  1  that follows a helical pathway in the first embodiment; or the portion of the conduit arrangement  23  in which the inflow and outflow lumens  32 ,  33  independently following a helical pathway in the fourth embodiment is comprised within a portion of the dialysis catheter that is not implanted into the body of an individual. However, in alternative embodiments, the feature is comprised within a portion of the dialysis catheter that is implanted into the body of an individual. 
     The first embodiment described above is manufactured using the following method. The tubular conduit  1  is extruded using standard techniques known in the art. The portion of the tubular conduit  1  is which the inflow and outflow lumens  10 ,  11  follow a helical pathway is manufactured through use of a rotating extrusion die during the extrusion process. In an alternative method of manufacture, the tubular conduit  1  is extruded such that the inflow and outflow lumens  10 ,  11  follow a substantially straight pathway. The helical pathway of the inflow and outflow lumens  10 ,  11  is then introduced post-extrusion using appropriate tools and temperature. 
     The fourth embodiment described above is manufactured using a similar method as for the first embodiment. The first and second tubular conduits  24 ,  25  are extruded using standard techniques known in the art. The first and second tubular conduits  24 ,  25  are intertwined such that the inflow and outflow lumens  32 ,  33  independently follow a helical pathway through use of a rotating extrusion die during the extrusion process. In an alternative method of manufacture, the first and second tubular conduits  24 ,  25  extruded such that the inflow and outflow lumens  32 ,  33  follow a substantially straight pathway. The first and second tubular conduits  24 ,  25  are then intertwined post-extrusion using appropriate tools and temperature. 
     The second, third and fifth embodiments described above are manufactured using the following method. The axially extending internal helical protrusion  12 ,  21 ,  47  is produced by injection moulding and is then connected to rest of the tubular conduit  1 ′,  13 ,  38  by using thermal or radio frequency (RF) fusion or by using solvent bonding or gluing. The tubular conduit  1 ′,  13 ,  38  minus the axially extending internal helical protrusion is made using standard techniques in the field. The axially extending internal helical protrusion  12 ,  21 ,  47  is made from the same material as the rest of the tubular conduit  1 ′,  13 ,  38 . 
     Dual or multi-lumen catheters are used for various medical applications, aside from dialysis catheters, and may transfer fluids other than blood independently to and from an individual. Therefore, although the above described embodiments relate to dialysis catheters, it is to be understood that the present invention is not limited to dialysis catheters. The present invention may be embodied in other types of dual or multi-lumen catheters where it would be desirable to induce helical flow during the transfer of fluids independently to and from an individual, such as in urinary catheters, tracheotomy catheters, angioplasty catheters and central venous catheters.