Multi-lumen catheter with separate distal tips

Description of equivalent WO 2004075962 A2

MULTI-LUMEN CATHETER WITH SEPARATE DISTAL TIPS

FIELD OF THE INVENTION [0001] The present invention relates generally to a multi-lumen catheter having a plurality of separate distal tips and more particularly to an improved split-tip catheter.

BACKGROUND OF THE INVENTION [0002] Multi-lumen catheters are used for the purpose of creating two or more separate fluid pathways, such as in hemodialysis applications. A primary goal of hemodialysis access is to provide a reliable and effective means of dialysis, which means that a sufficient volume of blood over a period of time must be removed from and returned to the patient. Because the contaminated and cleansed blood must be kept separate for an effective dialysis procedure, a dual lumen catheter is generally used. These dual lumen catheters are usually configured so that there is a shorter lumen that aspirates blood from a blood vessel of a patient to a dialysis machine where it is processed for the removal of toxins, and a longer lumen that infuses the purified blood to the patient. The shorter lumen utilized for aspiration is generally referred to as the "arterial lumen," while the longer lumen utilized for infusion is generally referred to as the "venous lumen." The reason for the different lengths is to minimize co-mingling of aspirated and infused blood.

[0003] The primary problems occurring in dual lumen dialysis catheters are blood clotting (thrombosis) and fibrin (the protein formed during normal blood clotting that is the essence of the clot) sheath formation. Thrombus and fibrin sheath formation can occlude distal tips of the dialysis catheter lumens, resulting in loss of catheter function when such an occlusion prevents blood flow. This typically occurs initially in the arterial lumen used for aspiration of blood from a patient. A secondary problem relates to the arterial lumen "sucking" against the vessel wall, in which the arterial lumen openings become fully occluded by the patient's vasculature.

[0004] To specifically address these problems, a new type of dialysis access catheter has been designed that utilizes independent "free floating" distal tip sections that separate at a distal end of the catheter to theoretically reduce the likelihood of potential occlusion and "sucking" during dialysis treatment. U.S. Patent No. 6,001,079 to Pourchez and U.S. Patent Nos. 5,947,953 and 6,190,349 to Ash et al., all incorporated by reference herein, are directed to said new type of catheter, hereinafter referred to as a "split-tip catheter."

[0005] Catheters used for hemodialysis are generally inserted according to standard technique. This includes identifying the location of the central veins in which the catheter is to be inserted, utilizing the Seldinger technique to cannulate the central vein with a guidewire, and passing various instruments over the guidewire until the catheter is in place. In the case of a small diameter catheter made of relatively stiff material (i. e. , a percutaneous catheter for acute access), the catheter itself can be passed over the guidewire and into the central vein. In the case of a larger diameter catheter, the passage to the central vein must be widened through the use of dilator(s), after which the catheter can be passed over the guidewire. Another possibility available to the physician is to use an introducer sheath, which is passed over the guidewire (after dilation or simultaneously therewith) forming a path for the catheter to access the central vein.

[0006] Due to the configuration of the split-tip catheter, standard insertion techniques must be modified. With respect to non-sheath insertions, the separate distal tip sections must each be passed over the guidewire to ensure their arrival at the targeted central vein without attendant problems. Suggested means for accomplishing this (in a dual lumen catheter) is to pass the back end of the guide wire through the distal end hole of one tip section and out a side hole thereof into the distal end hole of the other tip section (Patel et al, J. Vase. Interv. Radiol. (2001), vol. 12, pages 376- 378). However, such a method can become problematic depending on the placement of the side holes and, in fact, may be impossible in the event that no side holes are provided. With respect to sheathed insertions, the separate distal tip sections may tend to flare outward and catch or "snag" on the inner wall of the delivery sheath thereby preventing smooth delivery of the catheter to the central vein. [0007] There is a need for improvements to split-tip catheters and new configurations thereof, which enhance the design and further alleviate concerns with regard to typical problems encountered in hemodialysis catheters. There is also a need for new configurations and new methods for insertion of split-tip catheters to overcome problems associated therewith.

SUMMARY OF THE INVENTION [0008] In accordance with the present invention, several embodiments are described, which may be improvements to prior art split-tip catheters or may be novel catheter embodiments heretofore undisclosed. As used herein, the following terms have the following meanings:

[0009] "Split-tip catheter" refers to a catheter having a body enclosing at least two lumens and a dividing point that separates at least two tip sections from one another distal thereto, each of the tip sections enclosing at least one lumen and being separated or separable from one another along their length.

"Dividing point" refers to a point along the length of the split-tip catheter distal to which at least two tip sections are separated or are separable from one another.

[0011] "Tip section" refers to a portion of the split-tip catheter, enclosing at least one lumen, which is separable or is separated from another tip section along its length distal to a dividing point.

[0012] "Inner surface" refers to a surface on a tip section that is facing or adjacent a companion tip section (a tip section that separates or is separable distal to the same dividing point).

[0013] "Outer surface" refers to a surface on a tip section that is not facing a companion tip section.

[0014] "Separation force" refers to a force required to separate the tip sections from one another. [0015] "Excessive separation force" refers to a force required to separate the tip sections from one another, which is too great to be considered practical for a clinician in a typical setting. By definition, an excessive separation force is required to separate the tip sections of the split tip catheter from one another proximal to the dividing point.

[0016] "Cylindrical" is used according to its ordinary meaning (i.e., a shape having a constant circular cross-section).

[0017] One improvement to prior art split-tip catheters comprises a unique configuration of each tip section to improve flow characteristics, provide ease of delivery and overcome typical problems associated with hemodialysis catheters. In a dual lumen embodiment, the split-tip catheter comprises a shorter arterial tip section comprising a semi-cylindrical shape with a rounded distal end, and a longer venous tip section comprising a semi-cylindrical shape in a proximal portion which smoothly transitions in a transition region to a cylindrical shape in a distal portion, the cross- sectional size of the distal portion being greater than the cross-sectional size of the proximal portion, the distal portion terminating in an inward bevel. Another improvement comprises the strategic placement of openings placed on the tip sections of the split-tip catheter. With particular respect to an arterial tip section in a dual lumen configuration, the openings are offset both longitudinally and circumferentially so that full 360° fluid flow is realized and problems related to "sucking" against the vessel wall are mitigated.

[0018] Another improvement to the prior art split-tip catheters comprises a means for reducing dead space on the surface of the tip sections of a split-tip catheter in the region of separation, which includes, but is not limited to, an opening or openings located on an inner surface of one or more tip sections near a dividing point of the split-tip catheter. In one embodiment of the invention, a hole or opening is positioned on an aspirating lumen tip section. In another embodiment of the invention, a hole or opening is positioned on an infusion lumen tip section. In each of these embodiments, there could be multiple holes or openings on the lumen tip sections. Additionally, the hole(s) or opening(s) could be positioned in many different locations along the length of the tip sections and/or catheter. [0019] An additional improvement to prior art split-tip catheters is related to the delivery thereof either with or without the use of a delivery sheath. With respect to a non-sheathed delivery, the improvement concerns strategic placement of a guidewire opening in the distal end of the venous tip section so that the guidewire is easily passed through the venous tip section and into the arterial tip section. With respect to a sheathed delivery, the improvement relates to a friction-reducing means for reducing friction between the catheter and a delivery sheath. As discussed above, movement of the delivery sheath with respect to the split-tip catheter can result in the inside surface of the delivery sheath engaging the tip sections, which can cause tears in the delivery catheter, problems in the delivery process and other unwanted occurrences.

[0020] Thus, in one embodiment of the invention, a bump or protrusion of material is positioned on the outer surface of one or more tip sections adjacent the inside surface of the delivery sheath. In another embodiment of the invention, a ring of material is positioned around the end of one or more tip sections. In yet another embodiment of the invention, an inflatable material is positioned on the outer surface of the catheter in one or more locations to be inflated during delivery to provide a buffer zone for prevention of direct contact of the tip sections with the inside surface of the delivery sheath. Following delivery of the split-tip catheter, said material would be deflated. In still another embodiment, bumps or protrusions of material are positioned on the inside surface of the sheath.

[0021] With respect to prior art split-tip catheters that are releasably joined or

"splittable," an improvement is described in which tip sections that are initially joined together distal to a dividing point can be separated along a predetermined length thereof upon an application of a particular separation force. In this aspect of the invention, the separation force required to disjoin the tip sections varies along the length of the tip sections (in the described embodiments the separation force increases from the distal end to the proximal end of the catheter) as the bond between the tip sections is varied. [0022] Thus, in one embodiment, the splittable tip sections are releasably joined to one another distal to a dividing point, enabling split and joined configurations to be interchangeable. In another embodiment of the invention, the bond between the splittable tip sections varies in a continuous fashion along their length in such a way that the force required to separate them is progressively increased in a straight line fashion as measured from a distal end of the tip sections to the dividing point. In yet another embodiment of the invention, the required separation force increases in incremental steps as measured from a distal end of the tip sections to the dividing point. In each of these embodiments, while the tip sections are splittable along a predetermined length of the catheter distal to a dividing point, it should be appreciated that said dividing point is a fixed point beyond which separation can not take place without use of excessive separation force.

[0023] These and other embodiments, features and advantages of the present invention will become more apparent to those skilled in the art when taken with reference to the following more detailed description of the invention in conjunction with the accompanying drawings that are first briefly described.

BRIEF DESCRIPTION OF THE DRAWINGS [0024] Fig 1 is a perspective view of a split-tip catheter according to the present invention.

[0025] Fig. 2 is a perspective view of an alternate embodiment of a split-tip catheter according to the present invention.

[0026] Fig. 3 is a close-up view of the tip sections of the split-tip catheter of

Fig. 1 , illustrating the positioning of openings therein.

[0027] Fig. 4 is a similar view to Fig. 3, illustrating an alternate embodiment thereof.

[0028] Fig. 5 is a perspective view of a straight version of a split-tip catheter according to the present invention. [0029] Fig. 6 is a perspective view of a precurved version of a split-tip catheter according to the present invention.

[0030] Fig. 7 is a close-up view of the distal end of the split-tip catheter of

Figs. 5 and 6.

[0031] Figs. 8A-8G are cross-sectional views of the distal end of the split-tip catheter of Fig. 7, taken along lines 8A-8A, 8B-8B, 8C-8C, 8D-8D, 8E-8E, 8F-8F, 8G- 8G, respectively.

[0032] Fig. 9 is a perspective view of Fig. 7 taken along line 9-9.

[0033] Fig. 10 is a cross-sectional view of Fig. 9, taken along line 10-10.

[0034] Figs. 11A-11E are cross-sectional views of Fig. 10, taken along lines

11A-11A, 11B-11B, 11C-11C, 11D-11D, HE- HE, respectively.

[0035] Fig. 12 is an alternate embodiment of the split tip catheter of the present invention, showing a close-up view of a distal end thereof.

[0036] Fig. 13 is a perspective view of Fig. 12 taken along line 13-13.

[0037] Fig. 14 is a cross-sectional view of Fig. 13, taken along line 14-14.

[0038] Fig. 15A is an inside view of an introducer sheath containing a prior art split-tip catheter.

Fig. 15B is a close-up view of the prior art catheter of Fig. 15 A, showing forces acting thereon upon delivery out of the introducer sheath.

[0040] Fig. 16 is a perspective view of the tip sections of a split-tip catheter according to one embodiment of the present invention.

[0041] Fig. 17 is a perspective view of the tip sections of a split-tip catheter according to another embodiment the present invention.

[0042] Fig. 18 is a perspective view of the tip sections of yet another embodiment of a split-tip catheter according to the present invention

[0043] Fig. 19 is an inside view of an introducer sheath according to one embodiment of the present invention.

[0044] Fig. 20 is a cross-sectional view of Fig. 11 taken along line 20-20. [0045] Fig. 21 A is a graph illustrating a continuous separation force F as a function of position P of the tip sections.

[0046] Fig. 2 IB is a graph illustrating an incremental separation force F as a function of position P of the tip sections.

[0047] Fig. 21C is a representative view of a split-tip catheter, showing one embodiment of the present invention regarding varying separation force required to separate the tip sections.

[0048] Fig. 2 ID is a representative view of a split-tip catheter, showing another embodiment of the present invention regarding varying separation force required to separate the tip sections.

[0049] Fig. 2 IE is a representative view of a split-tip catheter, showing another embodiment of the present invention regarding varying separation force required to separate the tip sections.

[0050] Fig. 21 F is a representative view of a split-tip catheter, showing another embodiment of the present invention regarding varying separation force required to separate the tip sections.

[0051] Fig. 21 G is a representative view of a split-tip catheter, showing another embodiment of the present invention regarding varying separation force required to separate the tip sections.

[0052] Fig. 21H is a representative view of a split-tip catheter, showing another embodiment of the present invention regarding varying separation force required to separate the tip sections.

[0053] Fig. 22A is a representative view of a split-tip catheter, illustrating a twisted configuration according to one embodiment of the present invention.

[0054] Fig. 22B is a representative view of a split-tip catheter, illustrating a twisted configuration according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION [0055] The following detailed description should be read with reference to the drawings, in which like elements in different drawings are identically numbered. The drawings, which are not necessarily to scale, depict selected preferred embodiments and are not intended to limit the scope of the invention.

[0056] The detailed description illustrates by way of example, not by way of limitation, the principles of the invention. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what is presently believed to be the best mode of carrying out the invention.

[0057] The present invention is directed to multi-lumen catheters with separate distal tips in the form of improvements over prior art split-tip catheters as well as new configurations and embodiments. While the examples and embodiments of a multi- lumen catheter are discussed herein in terms of a dialysis catheter, it should be appreciated that there are many other possible uses for a multi-lumen catheter of the type presented herein. Also, while a currently preferred material is medical grade polyurethane, it should be appreciated that a variety of suitable materials could be utilized to form the multi-lumen catheter of the present invention. Importantly, the material chosen should possess characteristics such as flexibility, durability and relative softness and conformability when placed within a blood vessel.

[0058] Referring now to Fig. 1, a split-tip catheter 10 is shown having a proximal end 12 and a distal end 14. The proximal end of the catheter 12 is attached to a Y-connector 60, which in turn is connected to extension tubing 62, as is standard in dialysis catheters. The catheter 10 has two lumens defined by an outer wall 16 and a bisecting planar septum 18. The two lumens extend from the proximal end of the catheter 12 to the distal end of the catheter 14 and have a D-shape configuration due to the planar septum 18. Of course, this is not the only possible configuration and certainly the lumens could assume a variety of shapes, including circular, crescent- shaped, oval, etc. The Y-connector 60 and extension tubing 62 fluidly connect the lumens to a blood treatment unit or dialysis machine (not shown). The distal end 14 of the catheter 10 is separated into tip sections 40 and 50 distal to a dividing point 30. While only two tip sections are shown in Fig. 1, it is certainly possible for a plurality of tip sections to branch from the dividing point 30, depending, for example, on the number of lumens within the catheter 10. Thus, it is contemplated that three, four or more tip sections could separate or be separable distal to the dividing point 30, corresponding to three, four or more lumens within the catheter 10. [0059] In addition, although only one dividing point is illustrated in Fig. 1, it is contemplated that two or more could be utilized, which is shown as an alternate embodiment in Fig. 2. A multi-lumen catheter 80 has a first tip section 82 and a second tip section 84 distal to a first dividing point 81 and a third tip section 86 and a fourth tip section 88 distal to a second dividing point 85 distal to the first dividing point 81 along the second tip section 84. Of course, it would be possible to have additional dividing points and tip sections, which could accommodate any number of multi-lumen configurations. Referring back to Fig. 1, the two lumens of the catheter 10 continue from the proximal end 12 to the distal end 14 into respective tip sections 40 and 50 distal to the dividing point 30. The lumens can be of equal size and shape from proximal to distal end or can vary in one or more locations. Thus, for example, if it was desirable to increase the flow of blood, the lumen in tip section 50 could increase in cross-sectional area distal to the dividing point 30 so that blood flowing out of tip section 50 would have a reduced velocity.

[0060] Referring now to Fig. 3, the cross-sectional shape of the lumen within tip section 50 transitions from a D-shape to a circular shape, the transition occurring distal to the end opening 44 of tip section 40. Such a configuration for the tip section 50 is optimal for many reasons, which will be explored below with reference to Figs. 7-14. Importantly, the overall configuration for the distal end 14 of the catheter 10, including the shape of tip sections 40 and 50, has been found to be optimal regarding delivery of the catheter into a body lumen as well as with respect to its performance in vivo. [0061] As shown in Figs. 1 and 3, tip section 40 terminates in a rounded configuration and tip section 50 terminates in a beveled configuration, providing end openings 44 and 54 therethrough. While certainly there are many possible degrees of angles that can be used for the bevel cut according to the present invention, the currently preferred range is between approximately 25° and 75°, while the angle shown is approximately 45°. Of course, there are a multitude of end configurations possible for each tip section in addition to those shown, including blunt, tapered, curved, notched, etc. In particular, a reverse bevel or notched (area removed from the end) configuration could be formed in the tip section 40 to minimize the opportunity for "sucking" against the wall of the superior vena cava or right atrium during a dialysis procedure. As is standard for dialysis catheters, tip section 40 is used to aspirate blood from the patient, while tip section 50 is used to infuse treated blood from a dialysis machine, as discussed above.

[0062] Fig. 3 illustrates one of the stated improvements over the prior art split- tip catheters, namely the presence of an opening in close proximity to the dividing point, by showing the tip sections of Fig. 1 in more detail. Tip sections 40 and 50 distal to dividing point 30 have inner surfaces 45 and 55 and outer surfaces 47 and 57 respectively. In close proximity to the dividing point 30 on inner surface 45 is positioned an opening 46, providing an additional inlet on the tip section 40 for the aspiration of blood. Importantly, opening 46 addresses an apparent problem with certain split-tip catheter designs with respect to clotting and fibrin sheath formation at or near the dividing point. This phenomenon is likely due to the unused surface area proximal to the arterial and venous end openings 44 and 54, which increases the surface area for platelet adhesion. By positioning opening 46 in close proximity to the dividing point 30, blood flow therethrough acts to dislodge any clot formation on the inner surfaces 45 and 55. In addition, the presence of one or more openings in close proximity to the dividing point may permit slow leaching of heparin along the inner surfaces 45 and 55, which also would prevent clot formation as the catheter lumens are flushed between dialysis treatments. As used herein, the term "close proximity" refers to a distance that is nearer the dividing point than the distal end of the subject tip section and can refer to any point along the length of the subject tip section in that range.

[0063] It should be appreciated that even though one hole on inner surface 45 is shown in Fig. 3, a plurality of openings are also possible and may even be preferable, depending on the particular application. In addition, although a small circular opening 46 is shown, certainly other shapes and sizes would be equally within the scope of the invention, including oval shapes, slits, etc. For example, an opening near the dividing point could comprise a valve such as that described in U.S. Patent No. 4,549,879 to Groshong et al., which is incorporated by reference herein. By utilizing a valve, leaching may be controlled, which could be advantageous in certain situations, depending on the patient's disposition. In an alternate embodiment, small mounds of material or protrusions 49 are positioned adjacent the opening 46 on either side thereof as shown in Fig. 4. Of course, while two protrusions 49 are shown, certainly any number of protrusions 49 are contemplated. The protrusions 49 serve at least two purposes, including maintaining separation between the tip sections 40 and 50 and preventing opening 46 from becoming occluded.

[0064] While the embodiment shown in Fig. 3 illustrates opening 46 on inner surface 45, it may instead be preferable to place one or more openings on the inner surface 55 of tip section 50. Alternatively, it may be desirable to place one or more openings on both inner surfaces 45 and 55 at similar positions or offset from one another. Similar arrangements are also contemplated with respect to the presence of an opening or openings near the dividing point in catheters having more than two tip sections or more than one dividing point as discussed above, as well as with respect to the use of protrusions near or around the openings. In a split-tip catheter having tip sections that are releasably joined, an opening or openings could be placed at intervals along the length of at least one of the tip sections on an inner surface thereof.

[0065] Both tip sections 40 and 50 may include side hole(s) 48 and 58 respectively, positioned near their terminal end for enhanced fluid flow into and out of the tip sections 40 and 50. As should be appreciated by one of skill in the art, many embodiments with respect to the side hole(s) are possible, including an embodiment with no side holes. The side hole(s) 48 and 58 can be positioned at various locations around the circumference of the tip sections 40 and 50 and can be one or several in number. As shown in the embodiment of Fig. 3, tip section 50 has one side hole 58 in the side thereof, while tip section 40 has four side holes spaced in different locations around its circumference and offset lengthwise from the end opening 44. This particular configuration will be explored below in reference to Figs. 7-8. [0066] Figs. 5-11 illustrate an embodiment of the split-tip catheter of the present invention similar to that depicted in Fig. 1. Fig. 5 depicts a straight configuration of a split-tip catheter 100, while Fig. 6 depicts a precurved configuration thereof. In each instance the split-tip catheter 100 includes a flexible cylindrical catheter tube 114 that encloses a pair of distinct lumens. In the precurved configuration in Fig. 6, a medial portion of the catheter tube 114 is formed into a bend 116. The distal end 128 of the split-tip catheter 100 is bifurcated into a pair of distal tip sections 124, 126, which are distal to a dividing point as discussed above. Each of the tip sections 124, 126 enclose a lumen that is respectively continued from the pair of distinct lumens enclosed in the catheter tube 114.

[0067] Figs. 7-8 illustrate the distal tip sections 124, 126 in greater detail. As seen in Fig. 8 A, the catheter tube 114 encloses substantially similar D-shaped lumens 120, 122 separated by a planar septum 118. These lumens 120, 122 continue on into the distal tip sections 124, 126 respectively, the lumen 120 and associated tip section 124 being the shorter arterial lumen and tip section that is used to withdraw blood from the cardiovascular system for dialysis treatment outside the body, and the lumen 122 and associated tip section 126 being the longer venous lumen and tip section that is used to return blood to the cardiovascular system following said dialysis treatment. The arterial tip section 124 has a planar surface 130 which faces and is adjacent to a planar surface 132 of the venous tip section 126, each of the planar surfaces 130, 132 being extensions of the planar septum 118 distal to the dividing point. [0068] Similar to the embodiment shown in Fig. 1, the distal arterial tip section

124 terminates in an end opening 134 that assumes a semicircular profile when viewed from the side (Fig. 7). While the lumen 120 is D-shaped and continues through the end opening 134 as such (i.e., the lumen 120 does not taper or enlarge), the side view shows a semicircular profile due to the edges of the distal end of the arterial tip section 124 being rounded. This rounding can be seen in greater detail in Figs. 3 and 4 as well as in Fig. 9, which illustrates a view from the side of the arterial tip section 124 identified by line 9-9 in Fig. 7. As shown, an edge 133 of the distal end of the arterial tip section 124 assumes a scalloped configuration as a result of the rounding process. Such rounding is accomplished by heating with radio frequency (RF) energy, although certainly many other methods of heating could be utilized, but is not limited to the distal end of the arterial tip section 124. Indeed, rounding of the split-tip catheter 100 can be employed on various edges and ends thereof.

[0069] As mentioned, the D-shape of the arterial tip section 124 is unchanged along the full length thereof, having planar surface 130 and a semicircular surface 135 that interconnects rounded corners 136 and 137 at the lateral ends of the arterial tip section 124 (see Figs. 8B-8E) and having an end opening 134 as explained above. Arterial tip section 124 also has four additional openings for withdrawing blood and for maintaining patency of the lumen 120 and arterial tip section 124 in the event that the end opening 134 is somehow blocked, the four openings being strategically placed in a longitudinal and circumferential pattern for maximum effect. Specifically, opening 141 can be seen in Fig. 9 (and Fig. 8E), just proximal to the end opening 134 in the semicircular surface 135, while Figs. 8B-8D show openings 138, 139 and 140.

[0070] In Fig. 8B, opening 138 is through the planar surface 130 of arterial tip section 124, to provide a pathway for blood flow between the arterial and venous tip sections 124, 126, thereby reducing blood stagnation between the tip sections as discussed above in connection with Figs. 3 and 4 (showing openings adjacent the dividing point 30). Opening 138 is positioned at a longitudinal location proximal to opening 141. Figs. 8C and 8D show openings 139 and 140 respectively through the corners 136, 137. These side or corner openings 139 and 140 are offset as can be appreciated from Fig. 7, opening 139 being positioned at a longitudinal location proximal of opening 140, each of which openings are proximal of opening 141. As depicted, the openings are approximately equidistant from one another, or more precisely, the distance between each consecutive opening 138-141 in the arterial tip section 124 is substantially the same. It should be specifically noted that side openings 139 and 140 are multi-planar in nature having portions formed through adjacent sides of the catheter (i.e., planar surface 130 and semicircular surface 135). The multi-planar aspect of the side openings 139 and 140 is particularly advantageous with respect to prevention of occlusion of the arterial lumen due to the remote possibility that either of the openings could be completely obstructed by a vessel wall.

[0071] The positioning of the openings 138-141, as well as the distance therebetween, is very important with respect to the stated goals of facilitating blood flow and maintaining patency of the catheter. By positioning each opening at longitudinal locations offset from one another (and in this case approximately equidistant from one another) as well as at different points around the circumference of the arterial tip section 124, alternative flow through the arterial tip section 124 is permitted, preventing thrombus and fibrin formation as well as "sucking" against the vessel wall. More specifically, the offset positioning on four different sides enables 360° multi-planar flow into the arterial tip section 124 with five effective sides (including the end opening 134) for the receipt of blood, such that total occlusion of the arterial tip section 124 is largely eliminated.

[0072] The distal venous tip section 126 comprises a proximal portion 142, a transition region 143 and a distal portion 144. The proximal portion 142 has a semicircular cross-section similar to the arterial tip section 124 that is unchanged from the dividing point to the transition region 143 as can be appreciated by viewing the cross-sections shown in Figs. 8B-8E. The planar surface 132 of the venous tip section 126 forms a planar outer wall of the proximal portion 142 on the side of the venous tip section 126 adjacent to the arterial tip section 124 as explained above. A semicircular wall 145 interconnects the lateral ends of the venous tip section 126 to complete an enclosure of the venous lumen 122. As with the arterial lumen 120, venous lumen 122 is the same size as its counterpart in the catheter tube 114, although as mentioned above many variations are possible.

[0073] The transition region 143 connects the proximal portion 142 to the distal portion 144 and comprises a transition shoulder 156. The transition region 143 continuously changes in cross-sectional shape along its length in transitioning from the semicircular shape of the proximal portion 142 to the circular shape of the distal portion 144. As shown, the transition shoulder 156 is positioned on a side of the venous tip section 126 adjacent the arterial tip section 124, although certainly other configurations are possible with respect to positioning of the transition shoulder. In addition, configurations including multiple transition shoulders are contemplated herein.

[0074] The shape of the venous lumen 122 transitions from a D-shape in the proximal portion 142 to a circular shape in the distal portion 144, although the actual cross-sectional area of each shape can be manipulated to accommodate the particular application (i.e., the cross-sectional area of the distal portion with respect to the proximal portion can increase, decrease or remain substantially equivalent). In one embodiment, the cross-sectional area for the venous lumen 122 in the distal portion 144 of the venous tip section 126 is standardized to permit all sizes of the catheter to be tunneled subcutaneously during implantation using a single size of tunneling trocar. Thus, the cross-sectional area of the lumen 122 in the proximal portion 142 of the venous tip section 126 (as well as the cross-sectional area of the proximal portion 142 itself) would vary depending on the size of the catheter. One possibility, therefore, would include an increase in cross-sectional area of the venous lumen 122 from the proximal portion 142 to the distal portion 144 of the venous tip section 126. This could have advantages in that the discharge velocity of fluid flowing out of the end 146 would be reduced, translating into less trauma to the vessel and minimized effects related to shear stress. Of course, it should be apparent to one of skill in the art that various combinations of cross-sectional shapes and sizes for the venous lumen 122 are possible along the length thereof, each of which could enjoy certain advantages and benefits. [0075] The distal portion 144 has a cylindrical shape with an open end 146 that is inwardly beveled in the proximal direction toward a plane P]₃₂. End opening 148 through the open end 146 is contained in a plane P₁₄₈ that is shown in Fig. 7 as being oriented at a bevel angle A₁₄₆ to plane P₁₃₂. As a result of the bevel, the end opening 148 is necessarily oval shaped. The bevel angle Aι ₆ is shown as approximately 45°, although as stated above in connection with Figs. 1 and 3, certainly many other angles are possible, the preferred range being between approximately 25° and 75°. The open end 146 of the venous tip section 126 is rounded (represented by the double line in Figs. 7 and 9) similar to the end of the arterial tip section 124. Referring to Figs. 8F and 8G, the distal portion 144 of the venous tip section 126 contains two openings 150 and 152 that are longitudinally offset from one another. As with the various openings described above, openings 150 and 152 can be provided in different shapes and sizes and can be positioned at various points along the venous tip section 126 to provide additional outlets for the flow of blood in the event that a blockage occurs.

[0076] Fig. 9 is a view of the distal end 128 of the split-tip catheter 100 perpendicular to plane Pι₃ identified in Fig. 7 by line 9-9. Fig. 10 is an enlarged longitudinal sectional view of the venous tip section 126 taken along line 10-10 in Fig. 9, illustrating the transition region 143. Various transverse cross-sectional views of venous tip section 126 taken along various lines in Fig. 10 are shown in Figs. 11A- 11E. Taken together, Figs. 9-11 reveal that in transition region 143 a shoulder 156 of continuously longitudinally varying width and height projects outwardly from the planar surface 132 of proximal portion 142. Referring to Fig. 9 along with Figs. 11B- 11D, it can be seen that the transition shoulder 156 has a semicircular cross-sectional aspect with a width and a height above the planar surface 132 that increase from the proximal portion 142 to the distal portion 144 such that shoulder 156 is equal to the outer diameter of the distal portion 144 at the distal end thereof.

[0077] In Fig. 9, shoulder 156 is shown as having linear lateral boundaries 160 at which the exterior surface of the shoulder 156 emerges from the planar surface 132 of venous tip section 126. The lateral boundaries 160 intersect at a proximal end 158 of the shoulder 156 and are symmetrically disposed on opposite sides of the longitudinal axis L₁₂₈ of the distal end 128 of the catheter 100. While a specific configuration for a transition shoulder is described above, it should be understood that other configurations are possible and are contemplated herein, including, for example, a transition shoulder that decreases in width from the proximal portion 142 to the distal portion 144. In any case, the slope of the shoulder 156 should be positive, although it can be formed with varying degrees with respect to plane Pι₃₂. Thus, although a positive slope of approximately 5° is shown in Fig. 10, certainly many other possibilities exist that would be within the scope of the invention. [0078] The shoulder 156 is penetrated obliquely by a somewhat longitudinally- aligned guidewire aperture 162 that communicates between the exterior of the venous tip section 126 and the venous lumen 122 in transition region 143 and distal portion 144. As clearly seen in Fig. 7, the guidewire 164 is shown extending from arterial lumen 120 through the open end 134 of arterial tip section 124 and through a guidewire aperture 162 in the transition shoulder 156 into the venous lumen 122 of the venous tip section 126, thereby seamlessly connecting the two lumens and tip sections. While threading a guidewire through adjacent tip sections has been preliminarily explored in the prior art, the embodiments disclosed herein are advantageous for several important reasons. For example, the configuration of the tip sections with respect to one another in combination with a transition shoulder having a positive slope and having a guidewire therethrough affords a streamlined approach to connecting the tip sections for ease of delivery to a targeted lumen. In addition, the particular configuration of the inner wall of the guidewire aperture imposes minimal binding stresses on an insertion guidewire, making the split-tip catheter freely slidable thereon.

[0079] Although the examples herein with respect to a guidewire aperture are discussed in terms of passage of a guidewire therethrough, it will be appreciated by one of skill in the art that an aperture could also be configured for passage of a stylet or other instrument, such as the applicator discussed in U.S. Patent No. 5,405,341 to Martin, which is incorporated by reference herein. Assuming a larger diameter for a stiffening stylet when compared to a standard guidewire, such an aperture would tend to be greater in size than the described guidewire aperture, although certainly many sizes and shapes are possible, including slits.

[0080] Referring to the transition shoulder 156 as seen in Figs. 9-11, the guidewire aperture 162 is formed therein, which is bound by an encircling interior wall 163 that has an inner periphery 165 and an outer periphery 166. The interior wall 163 defines an open cylindrical space through the transition shoulder 156 and has a longitudinal central axis L₁₆₃. The guidewire aperture 162 is formed through the wall of the transition shoulder 156 at the point through which it is formed at an angle A₁₆₂, which is the measurement between plane P₁₃₂ and longitudinal axis L₁₆₃. This angle facilitates free slidability of the guidewire through the arterial and venous tip sections 124, 126. It is important to note that many different values for angle Aι₆₂ can be provided, which should equally result in the desired slidability with respect to an insertion guidewire. In particular, it has been discovered that angles (as measured between plane P₁₃₂ and longitudinal axis Lj₆₃) within the range of approximately 0° to 80° provide the desired functionality, with the optimum range being approximately 0° to 45°. The guidewire aperture 162 should be large enough to accommodate passage of a standard 0.038 inch guidewire, but not so large as to permit significant back flow of blood therethrough. In particular, with respect to a 0.038 inch guidewire, it has been discovered that an opening having a radius in the range of approximately 0.039 inch to 0.050 inch is sufficient to allow passage while minimizing unwanted back flow, with the optimum range being approximately 0.039 inch to 0.041 inch. Certainly, however, other radius possibilities exist and would be within the scope of the invention.

[0081] In implanting the split-tip catheter 100, the insertion guidewire 164 is threaded into the catheter 100 by first passing the proximal end thereof into end opening 148 of the venous tip section 126 and along the venous lumen 122 within the distal portion 144, through the guidewire aperture 162 and into the end opening 134 of the arterial tip section 124 where it is advanced along the full length of arterial lumen 120 to exit the catheter tube 114 at a proximal end thereof. This course taken by the guidewire 164 is shown in Fig. 7. During implantation into a target vessel, the distal end 128 of the split-tip catheter 100 is slid in a distal direction along guidewire 164 to a desired location, the guidewire having been previously extended thereto. [0082] In another embodiment of the present invention, a guidewire lumen is formed through the distal portion of the venous tip section, but is separate from the venous lumen. Thus, in this embodiment the distal portion of the venous tip section comprises two lumens whereas the proximal portion of the venous tip section comprises only one. This would be possible, for example, by providing a transition region similar to that shown in Fig. 7, where the size of the venous tip section increases from proximal to distal portions. In such a configuration, the venous lumen could remain approximately the same size from the proximal to distal portions with the additional cross-sectional size of the distal portion of the venous tip section being utilized for an additional lumen therethrough. The additional lumen ideally would be positioned at a location in the venous tip section closest in proximity to the arterial tip section (i.e., above the venous lumen in reference to Fig. 7) so that an instrument passing therethrough will have to bend only slightly or not at all when passing out of the arterial tip section and into the venous tip section.

[0083] The presence of an additional lumen permits the use of a stiffening stylet as described above, without having to increase the size of the guidewire aperture (which may be advantageous to avoid excessive loss of blood therethrough). Thus, the described embodiment could be formed with or without a guidewire aperture. In the case that a guidewire aperture was not formed through the transition region, the additional lumen could accommodate a guidewire, stiffening stylet or both (in addition to possible other medical instruments). In the case that a guidewire aperture is formed through the transition region, a guidewire and stiffening stylet could be used simultaneously if so desired.

[0084] An alternate embodiment of the present invention with respect to the configuration of the tip sections is provided in Figs. 12-14. A split-tip catheter 200 is shown having arterial tip section 224 and venous tip section 226 in Fig. 12, the arterial tip section 224 having a planar surface 230 extending from a dividing point and venous tip section 126 having a planar surface 232 in facing relation thereto. Planar surfaces 230, 232 extend from a planar septum 228, which divides a catheter tube 214 into a pair of distinct lumens that continue into the arterial tip section 224 and the venous tip section 226 respectively distal to the dividing point. Arterial tip section 224 has a terminal end that is cut perpendicular to a longitudinal axis thereof through which an end opening 234 is formed. Arterial tip section 224 contains three additional openings spaced approximately 120 degrees from one another around the circumference of the arterial tip section 224, which are offset longitudinally approximately equidistant from one another. Openings 238 and 240 can be seen in Fig. 13, while the third opening is positioned through the planar surface 230 of the arterial tip section 224. [0085] The venous tip section 226 has a proximal portion 242, a transition region 243 and a distal portion 244 and has an end 246 with an opening 248 therethrough (see Fig. 13). Similar to the above-described embodiment, venous tip section 226 has two openings on opposite sides of the distal portion 244, of which opening 252 is shown in Fig. 12. These openings are offset longitudinally along the length of the distal portion 244. The proximal portion 242 has a cross-sectional semicircular shape, while the distal portion 244 has a cross-sectional cylindrical shape. The transition region 243 increases in width and height in a distal direction moving from the proximal portion 242 to the distal portion 244. The transition region 243 includes a pair of longitudinally extending, parallel reinforcing ridges 254, 256 (Fig. 13), which reduce the bending of the transition region 243 in response to forces imposed on the venous tip section 226 during disposition in the cardiovascular system of a patient. Between the reinforcing ridges 254, 256, the surface of the transition region 243 forms a longitudinally extending recess 258 having a floor 260 (Fig. 14) that rises smoothly and continuously from the planar surface 232 of the proximal portion 242 to the curved outer wall of the cylindrical distal portion 244.

[0086] In the vicinity of the curved outer wall of the cylindrical distal portion

244, the floor 260 of the recess 258 is penetrated by a guidewire aperture 262 that communicates between the exterior of the venous tip section 226 and the venous lumen 222 (Fig. 14). The planar surface 232 of venous tip section 226 is extended to a longitudinal point where the floor 260 of the transition region 243 merges with the wall of the cylindrical distal portion 244 so that an overhead view (Fig. 13) does not reveal the guidewire aperture 262. This forms a ledge of sorts that, in combination with parallel reinforcing ridges 254, 256, directs the guidewire 264 and maintains it in position as the split-tip catheter 200 is slid therealong into the cardiovascular system of the patient. To prepare for use, the insertion guidewire 264 is threaded into the split-tip catheter 200 by first passing the proximal end thereof into end opening 248 of the venous tip section 226 and along the venous lumen 222 within the distal portion 244, through the guidewire aperture 262 and into the end opening 234 of the arterial tip section 224 where it is advanced along the full length of arterial lumen 220 to exit the catheter tube 214 at a proximal end thereof.

[0087] While the split-tip catheters 100 and 200 are configured to allow for sheathless delivery into a cardiovasculai" system of a patient due to the presence of guidewire apertures 162 and 262, respectively, an improvement to related to the sheathed delivery of split-tip catheters is illustrated in Figs. 15-20. With respect to Figs. 15A-15B, a prior art split-tip catheter 310 is shown within a delivery sheath 370 to illustrate a potential problem solved by the present invention. Tip sections 340 and 350 are shown distal to dividing point 330. Each tip section 340 and 350 transitions at a distal end from a D-shaped lumen to a lumen that is circular in cross-section and each has a terminal end that is cut perpendicular to a longitudinal axis thereof. As shown in Fig. 15 A, edges 348 and 358 located at the terminal ends of tip sections 340 and 350 abut the inside surface 372 of the sheath 370. Thus, as shown in Fig. 15B, when the catheter 310 is moved out of delivery sheath 370 and into a blood vessel in a direction 390, the opposing friction force 392 forces the tip sections in an outward direction into the sheath 370 as indicated by arrows 394 and 396. This movement of the tip sections 340 and 350 into the inside surface 372 of the delivery sheath 370 causes the edges 348 and 358 to catch or "snag" on the inside surface 372, resulting in difficult insertions or other attendant problems with the delivery process.

[0088] One potential method for alleviating this problem would be to use a larger diameter sheath so that the tip sections will have a greater distance to travel to come in contact with the inside surface of the sheath when the friction forces act on the tip sections, thereby minimizing the chance of a snag. However, this solution is flawed in that snags are still possible and the risk of air embolism is amplified due to the increase in surface area between the catheter and the sheath. Fig. 16 illustrates one embodiment of the present invention that addresses a solution to the sheathed delivery problems described. In particular, a multi-lumen catheter 410 having tip sections 440 and 450 distal to a dividing point 430 is shown, the catheter 410 being positioned within delivery sheath 370 having an inside surface 372. To prevent the terminal ends of the tip sections from catching or "snagging" on the inside surface 372 of the delivery sheath 370, a friction reducing structure is provided.

[0089] In Fig. 16, the friction reducing structure is illustrated as a slight protrusion or "bump" of material 480, which is positioned on the outside surface of tip sections 440 and 450 near the terminal ends thereof. Thus, as friction forces tend to push the tip sections outward from a central axis of the catheter 410 upon removal from the sheath 370 and into the blood vessel (as described above), the protrusion 480 provides a buffer to prevent edges 448 and 458 from coming into direct contact with the inside surface 372 of the delivery sheath 370 and also provides a relatively small surface area for contact. Of course, there are many ways for reducing friction utilizing the protrusions as discussed above, including using a plurality of protrusions on the catheter tip sections in various locations along their length. However, the embodiment shown in Fig. 16 is advantageous due to the small surface area utilized, the ease of manufacture and the wide-ranging applicability for use with a variety of catheter configurations.

[0090] Fig. 17 illustrates an alternate embodiment for reducing friction, wherem each of the tip sections 440 and 450 have a ring of material 482 around an end thereof distal to the dividing point 430. The rings 482, similar to the protrusions 480, distance the sharp edges of the tip sections 440 and 450 from the inside surface of a delivery sheath and therefore reduce friction forces associated with the delivery of a split-tip catheter from a delivery sheath and into a patient's vessel. The rings 480 may also act as a buffer of sorts for the tip sections 440 and 450 while within a patient's vessel by acting to keep the distal ends from coming into direct contact with the walls of the vessel.

[0091] Another embodiment for reducing friction comprises an inflatable material being positioned on one or more points along the catheter. In this embodiment, a small inflation lumen is positioned on the inner or outer of either or both arterial or venous lumens, extending from the proximal end of the catheter to the point on the catheter containing the inflatable material. When the catheter is positioned for delivery within a delivery sheath, air, gas or liquid is transmitted through the inflation lumen to the inflatable material, expanding said material into contact with the inner of the delivery sheath. Preferably, the inflatable material would be positioned close to the distal end of the catheter on the tip sections so that the goal of preventing the edges of the tip sections from coming into contact with the inside surface of the delivery sheath can be realized. Alternatively or in addition to the goal of reducing friction, placement of inflatable material on various areas on a surface of the catheter could act to remove clots and/or fibrin through the inflation thereof (/. e. , by knocking the formations off of the surface upon inflation). Referring now to Fig. 18, the catheter 410 has balloons 444 and 454 positioned near the distal ends of the tip sections 440 and 450, respectively, and are connected to respective inflation lumens 442 and 452. Balloon 444 is shown in its inflated, expanded position for ready contact with the inside surface of a delivery sheath such as delivery sheath 370. Balloon 454 is shown in its deflated, unexpanded position.

[0092] In yet another embodiment of the present invention, a friction reducing structure is provided by positioning a ridge or ridges 380 on the inside surface 372 of delivery sheath 370, as shown in Figs. 19 and 20. As with the embodiments involving protrusions positioned on the surface of the catheter, the ridge(s) 380 prevent direct contact of any sharp edges of a tip section with the inside surface 372 of the delivery sheath 370. The ridges 380 can be formed from a mound or bump of material or could also be formed from balloons as described with reference to Fig. 18. [0093] It is noted that although the delivery problems presented with reference to Figs. 15A and 15B are certainly more likely to be presented in terminal ends such as those shown, other tip configurations could also present problems with snags or tears, which problems can be minimized through any of the friction reducing structures described above.

[0094] Referring now to Figs. 21A-21H, an improvement to prior art split-tip catheters that are releasably joined or "splittable" distal to the dividing point is illustrated, wherein increasing separation force is required to separate the tip sections from one another. An increasing separation force as used herein means that the bond strength between the tip sections of the catheter will be greater as measured from a distal end toward the dividing point. However, the releasably joined or splittable region need not extend over the entire length of the tip sections and need not begin adjacent the dividing point. Thus, while the releasably joined or splittable region could extend the entire distance from a terminal end of one or more tip sections to the dividing point, it could also span a much shorter distance along the tip sections such that the releasably joined or splittable region is not adjacent either the dividing point or a terminal end of one or more tip sections.

[0095] The creation of an increasing separation force between tip sections can be accomplished in a variety of ways, as one of skill in the art should appreciate, some of which ways will be described in detail below (though certainly many others would be within the scope of the present invention). In addition, although the examples herein are described with respect to an increasing separation force, the production of a decreasing separation force is also contemplated, the implementation of which should be apparent to one of skill in the art in light of the discussion regarding an increasing separation force. Further, the discussion with respect to the separation of splittable tip sections herein encompasses both permanent and temporary (i.e., releasably joined) separation.

[0096] The concept of increasing the required separation force to separate tip sections from one another is illustrated by way of example, not limitation, through the graphs in Fig. 21 A and 2 IB. In each of these graphs, the y-axis represents the separation force F required to separate the tip sections from one another and the x-axis represents the position P along the joined portion of the tip sections as measured beginning at a distal end thereof and moving toward the dividing point. The points on the graph, Pi, P₂ and P₃ are points along the joined portion of the tip sections (as further illustrated in Figs. 21C-21H), having corresponding increasing separation forces F\, F₂ and F₃. While three distinct points are shown, certainly any number of points are possible and are contemplated herein. The point P_F corresponds to F_E, which is the excessive separation force as defined above.

[0097] The graphs in Fig. 21 A and 21B illustrate two possibilities for varying the separation force along the length of a split-tip catheter, wherein said force is increasing as a function of position of joined portion of the tip sections when measured beginning at the a distal end thereof and moving in a proximal direction toward the dividing point. Fig. 21 A illustrates a straight line continuous separation force, while Fig. 21B illustrates an incremental separation force accomplished in discreet steps, meaning that the change in force ΔF is too great for spontaneous splitting.

[0098] One embodiment of tip sections having an increasing separation force is illustrated in Fig. 21C, where fusion points are shown on an inner surface 512 of a tip section 510 of a catheter 500 along a predetermined length L_\ (tip section 510 is shown as separated from an adjacent tip section 520 for purposes of illustration). As used herein, the term "fusion point" refers to a fused surface area between a splittable interface and should not be restricted to a "point" per se. Thus, referring to Fig. 21C, the fusion points would be fused surface areas between the inner surface 512 of the tip section 510 and the inner surface 522 of the tip section 520 (even though the tip sections are shown as separate). In this embodiment, the fusion points will preferentially detach upon application of a consistent force relative to the surface area of the fusion.

[0099] As shown in Fig. 21 C, the fusion points of a first zone 530, formed between Pi and P₂, are lighter in shading than the fusion points of a second zone 540, formed between P₂ and P₃, which in turn are lighter in shading than the fusion points of a third zone 550, formed between P₃ and P_F- While the shading is shown to be uniform within each zone, one contemplated variation would be to increase the shading (i.e., fusion strength) within each zone when moving toward a higher number (i.e., Fι to P₂) as shown in Fig. 2 IE. In effect, such a configuration creates subzones or additional fusion zones. The darker shaded fusion point in these embodiments represents a fusion point having a greater bond strength, meaning that the fusion points of the third zone 550 would have a greater bond strength than those of the second zone 540, which in turn would have a greater bond strength than those of a first zone 530. This effect could be accomplished, for example, by utilizing different adhesives or solvents for different zones, using various dilutions of adhesives or solvents for different zones, utilizing varying application times of the adhesive or solvent for different zones or using variable pressure upon bonding (more force on one end than another) for different zones, as will be described in greater depth below.

[00100] In addition to greater bond strength for the fusion points in adjacent zones in Fig. 21C, the number (density) of the fusion points are increased in each successive zone and within each fusion zone, with the third zone 550 having a greater number of fusion points than the second zone 540, in turn having a greater number of fusion points than the first zone 530. With a greater number of fusion points comes an increased surface area for fusion between inner surface 512 of the tip section 510 and inner surface 522 of the tip section 520, thereby increasing the required separation force when moving from the distal end of the catheter 500 to the proximal end of the catheter 500. Also, with the number of fusion points progressively increasing within each fusion zone, a continuous configuration is created. The combination of greater bond strength in fusion points between adjacent fusion zones and increasing number of fusion points within each fusion zone creates a hybrid continuous/stepped configuration, wherein the separation force increases within each zone following a continuous curve (as represented in Fig. 21 A) and increases in a stepped manner with respect to adjacent zones (as represented in Fig. 21B).

[00101] Fig. 21D illustrates an embodiment of the present invention following the stepped configuration represented by Fig. 2 IB, wherein the number of fusion points within each fusion zone is uniform as is the bond strength of the fusion points within each fusion zone, but the bond strength of the fusion points is greater in each successive fusion zone as in Fig. 21C. Fig. 21E follows the continuous configuration represented by Fig. 21 A, wherein the number of fusion points is uniform within each fusion zone, but the bond strength of the fusion zones is progressively increased as between fusion zones and within each fusion zone (i.e., the bond strength of the fusion points in the first fusion zone 530 are progressively increased as are those in the second fusion zone 540 and the third fusion zone 550). Fig. 21F follows a stepped configuration as the bond strength of the fusion points is the same in all fusion zones, but the number of fusion points is increased in each successive fusion zone (the number of fusion points is uniform within each zone). In Fig. 21G, the bond strength of the fusion points is the same in all fusion zones, as is the number of fusion points, but the size of the fusion points is increased in successive fusion points, creating a stepped configuration.

[00102] Yet another embodiment of the present invention with respect to the establishment of an increasing separation force is illustrated in Fig. 21H, where the zones themselves increase in size. A catheter 600 is shown with tip sections 610 and 620, having four discrete and separate fusion zones, indicated by shading (the shading representing fusion points), on inner surface 612. A first zone 630 is smaller and therefore has less surface area for adhesion than a second zone 640, which in turn is smaller than a third zone 650, which in turn is smaller than a fourth zone 660. In this embodiment, there are free zones that do not have fusion points thereon, positioned in between each of the four stated zones. The first, second, third and fourth zones 630- 660 can consist of fusion points as described above, in any combination thereof. Thus, for example, the first zone 630 could have a certain density of fusion points, while the second zone 640 could have a somewhat higher separation force created by a larger size of fusion points.

[00103] A further embodiment of a catheter having a variable separation force is illustrated in Fig. 211. In this figure, a catheter 700 is shown with tip sections 710 and 720, having a continuous fusion zone 730 that increases in size along Li from a distal end to a dividing point. Of course, one of skill in the art will appreciate the myriad of possibilities within the scope of the present invention with respect to the varying of fusion points and establishment of an increasing separation force in light of the embodiments illustrated herein.

[00104] Methods for producing the desired effects with respect to the establishment of an increasing separation force, as described above, include (but are not limited to) utilizing constant separation stress with a variable area and using a variable separation stress with a constant area. Realization of these embodiments can be accomplished through many different bonding techniques, a few of which are described below.

[00105] With respect to constant separation stress over a variable area, a first potential technique is the use of adhesive bonding in creating a varying pattern of adhesive to join the tip sections of a split-tip catheter. Types of adhesive which could be used include epoxy and cyanoacrylates, although certainly other comparable adhesives would also be suitable. This idea has been explored above, for example, with respect to varying the density of the fusion points and the size of the fusion points. A second technique is the use of solvent bonding in creating a varying pattern of solvent to join the tip sections of a split-tip catheter. Examples of the types of solvents that could be employed include Tecoflex^® 1-MP (Thermedics), methyl ethyl ketone (MEK), cyclohexanon and alcohol, though certainly others could be used. A third technique is the use of thermal welding, in which heat can be used to join the tip sections of a split-tip catheter. A fourth technique is the use of RF energy to fuse the tip sections of a split-tip catheter. Finally, a fifth technique is the use of variable extrusion where surface area of a joining web could be increased between two lumens during the extrusion process.

[00106] In reference to variable separation stress over a constant area, the techniques above would be applicable in a different way. Specifically, in an adhesive bonding technique, the concentration of the adhesive could be varied to produce the variable bond strength. This idea has been explored above with respect to varying the type of adhesive used. In a solvent bonding technique, the solvent could be varied or the pressure utilized to join the tip sections could be varied. In a thermal welding technique, the temperature or dwell time could be varied. In an RF fusion technique, variable RF energy could be utilized. Finally, in a variable extrusion technique, pressure, temperature or dwell time could be varied during the extrusion process. [00107] Still another embodiment of the present invention with respect to the establishment of an increasing separation force is illustrated in Figs. 22 A and 22B, where the separation force is created by twisting the catheter upon formation thereof. Fig. 22A illustrates a gradual twist being applied to the catheter, while Fig. 22B illustrates a more abrupt twist over a shorter length. The twisting configuration imparts a variable separation force along the length of the catheter, increasing as measured from the distal end to the proximal end. For example, if a constant separation force is applied linearly to tip sections in a catheter having a 90° twisted septum, the force F a distance L away (i. e. , the distal end of the catheter) would diminish to zero in the direction needed to continue the splitting, because F would be resolved in the tensile direction of the septum if the tube could not rotate. On the other hand, were the tube able to rotate, the force would not diminish to zero but would be reduced by the energy required to rotate the tube 90°.

[00108] The twisting action can be implemented in a number of ways, including employing a mandrel having a desired twist, which when inserted into a still warm tube, imparts the twist to the tube during cooling and utilizing a series of mandrels to force a previously straight tube into a desired twisted pattern, wherein heat is applied to reset the tubing configuration. Another technique would be to employ an extrusion die having a preset desired twist so that the twist is imparted to the catheter during formation thereof. Referring back to Figs. 22A and 22B, the gradual twist 820 of the catheter 810 permits slow increase in resistance by transferring the splitting energy into rotational force if the catheter 810 is sufficiently flexible to twist when split. Differently, the abrupt twist 920 imparted to catheter 910 creates a tactile "positive stop," which notifies a user when a predetermined position along the length of the catheter 910 has been attained. Multiple twists could also be employed to provide a tactile measurement method for the user, and to provide an incremental separation force as described above in Fig. 2 IB. [00109] Although improvements to the functioning and usability of a split-tip catheter have been presented separately herein, the improvements described above (for example, openings positioned near a dividing point of a split tip catheter, positioning of a guidewire aperture in a venous tip section, configuration of arterial and venous tip sections with respect to one another, friction reducing structures provided on a delivery sheath and/or a catheter, varying the separation force required to separate tip sections from one another, etc.) should not be considered exclusive and instead should be considered for use in conjunction with one another.

[00110] The present invention has been described above in terms of certain preferred embodiments so that an understanding of the present invention can be conveyed. However, there are many alternative arrangements for a multi-lumen catheter not specifically described herein but with which the present invention is applicable. Although specific features have been provided, the multi-lumen catheter of the present invention would equally be embodied by other configurations not specifically recited herein. The scope of the present invention should therefore not be limited by the embodiments illustrated, but rather it should be understood that the present invention has wide applicability with respect to catheter systems generally. All modifications, variations, or equivalent elements and implementations that are within the scope of the appended claims should therefore be considered within the scope of the invention.