Abstract:
A spiral cut transition member is disclosed for controlling the transition in stiffness of a catheter from a stiffer more pushable proximal section to a more flexible and trackable distal section and increasing kink resistance. The transition member has a spiral cut provided therein to vary the flexibility of the transition member over its length. The pitch of the spiral cut can be varied to facilitate a gradual transition in flexibility along the catheter. The transition member may be used in conjunction with any type of catheter including single-operator-exchange type catheters, over-the wire type catheters, and/or fixed-wire type catheters.

Description:
This application is a continuation of co-pending application Ser. No. 09/534,870 file date Mar. 24, 2000, which is a divisional of application Ser. No. 09/241,995, filed Feb. 2, 1999, now U.S. Pat. No. 6,048,338; which is a continuation-in-part of application Ser. No. 08/950,864, filed Oct. 15, 1997, now U.S. Pat. No. 5,891,110. 
    
    
     TECHNICAL FIELD 
     This invention relates to the field of intravascular medical devices, and more particularly, to intravascular catheters that use a relatively stiff proximal section and a more flexible distal section for improved pushability, trackability and crossability. 
     BACKGROUND OF THE INVENTION 
     Intravascular diseases are commonly treated by relatively non-invasive techniques such as percutaneous transluminal angioplasty (PTA) and percutaneous transluminal coronary angioplasty (PTCA). These therapeutic techniques are well known in the art and typically involve the use of a balloon catheter with a guide wire, possibly in combination with other intravascular devices. A typical balloon catheter has an elongate shaft with a balloon attached proximate the distal end and a manifold attached to the proximal end. In use, the balloon catheter is advanced over the guide wire such that the balloon is positioned adjacent a restriction in a diseased vessel. The balloon is then inflated and the restriction in the vessel is opened. 
     There are three basic types of intravascular catheters for use in such procedures including fixed-wire catheters, over-the-wire (OTW) catheters and single-operator-exchange (SOE) catheters. The general construction and use of FW, OTW and SOE catheters are all well known in the art. 
     Several characteristics that are important in intravascular catheters include pushability, trackability and crossability. Pushability refers to the ability to transmit force from the proximal end of the catheter to the distal end of the catheter. Trackability refers to the ability to navigate tortuous vasculature. Finally, crossability refers to the ability to navigate the balloon catheter across narrow restrictions in the vasculature. 
     To maximize pushability, some prior art catheters incorporate a stainless steel outer tube (also referred to as a hypotube) on the proximal shaft section and a polymeric distal shaft section. One limitation of such a construction is that hypotubing is often prone to kinking. To reduce the likelihood of kinking, some prior art catheters use a relatively stiff polymer (e.g., composite) or reinforced polymer in the proximal shaft section. 
     The trackability of a particular catheter design is analyzed in terms of the trackability of the distal portion of the catheter, as this portion must track the guidewire through small tortuous vessels to reach the stenosed area to be treated. A more flexible distal portion has been found to improve trackability. Therefore, to maximize pushability, the catheter should have a relatively stiff proximal section. To maximize trackability, the catheter should have a relatively flexible distal section. 
     A limitation of this basic structure is that kinking can occur at the joint between the relatively stiff proximal shaft section and the relatively flexible distal shaft section. To reduce the likelihood of kinking, some prior art catheters use one or more tubular sections of intermediate flexibility between the relatively stiff proximal section and the relatively flexible distal section to provide a more gradual transition in flexibility therebetween. While this approach provides some benefit, the resulting transition in flexibility is often step wise, and can still be susceptible to kinking at the junctions of the various intermediate sections. It would be desirable, therefore, to provide an intravascular catheter that has a more gradual transition in flexibility along its length. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes many of the disadvantages of the prior art by providing a transition member that transitions or varies the stiffness of a catheter from a stiffer more pushable proximal section to a more flexible and trackable distal section, while reducing kinkability in the transition. The transition member preferably has a spiral cut provided therein over at least a portion of its axial length to increase the flexibility of the transition member. The pitch of the spiral cut is varied to facilitate a gradual transition in flexibility along the catheter as supported by the transition member. It is contemplated that the transition member may be used in conjunction with all types of catheters including, but not limited to, single-operator-exchange type catheters, over-the wire type catheters, and/or fixed-wire type catheters. 
     In one illustrative embodiment of the present invention, the transition member is used in conjunction with a catheter or other device that has a relatively stiff proximal section and a relatively flexible distal section. The junction between the stiffer proximal section and the more flexible distal section provides a transition in flexibility along the length of the catheter. Preferably, the transition member is co-axially disposed relative to the catheter shaft or other device, and is longitudinally positioned to bridge, extend across, or overlap at least part of the junction of the stiffer proximal section and the relatively flexible distal section. In a preferred embodiment, the transition member is included on a catheter having an outer tubular member which has a proximal stiff segment and a distal more flexible segment with the transition member extending both distally and proximally from the junction between these members. In a preferred over-the-wire catheter, an inner tubular member extends coaxially with the lumen of the outer tubular member and the transition member is affixed to the inner tubular member at an axial location proximate the junction in outer segments. 
     The flexibility of the transition member preferably increases along its length. This can be accomplished by providing a spiral cut or the like which extends through the side wall of the transition member. The spiral cut provides flexibility to the transition member, and if the pitch of the spiral cut is changed over the length, can provide a relatively smooth transition in flexibility from the relatively stiff proximal section to the relatively flexible distal section of the catheter while providing increased kink resistance. The transition member may be made from a stainless steel hypotube or other metallic tube, such as nitinol, an un-reinforced polymeric tube, a reinforced polymeric tube, or any other suitable material or element. 
     The transition member preferably has a first end region, an intermediate region, and a second end region, wherein only the first end region is secured to the catheter shaft. The intermediate region and the second end region are preferably left floating relative to the catheter shaft. In a preferred embodiment, the intermediate region and/or the second end region are radially spaced from the shaft when the catheter is in a substantially straight configuration, and are in engagement with at least part of the catheter shaft when the catheter is in a bent configuration. 
     The first end region of the transition member is secured to the shaft proximate the transition of flexibility of the shaft, with the intermediate region and the second end region extending distally therefrom. The length of the transition member is preferably sufficient so that the second end region is distal of the transition in flexibility of the shaft. Thus, like above, the transition member may bridge, extend across, or overlap at least part of the transition in flexibility of the catheter shaft. 
     As previously stated, in preferred embodiments, the transition member is part of or affixed to a co-axial type catheter that includes an elongate outer tube having a transition in flexibility and an elongate inner member. In one embodiment, the inner member is co-axially disposed within the lumen of the outer tube to form an annular lumen therebetween. The transition member is then preferably affixed to the inner member so that it extends coaxially therewith. The transition member extends axially from a point at or proximal of the transition in flexibility of the outer tube to a point at or distal of the transition in flexibility of the outer tube. It is contemplated that the inner member may be a inner tubular member having a guide wire lumen extending therethrough. Alternatively, it is contemplated that the inner member may be a guide wire or any other suitable device or structure. It is further recognized that the inner member can include a transition in flexibility. The transition member can be mounted on the inner member, or affixed to the outer member at an axial position so that the transition member proximal end is at or proximal to the transition flexibility and the distal end is at or distal of the transition in flexibility to provide kink resistance for the inner tubular member. 
     Preferably, the transition member is co-axially disposed within the annular lumen between the inner member and outer tube. It is recognized, however, that the transition member may be positioned inside the inner member (if the inner member is tubular having an inner lumen) or outside of the outer tube. The transition member is preferably positioned adjacent to at least a portion of the transition in flexibility of the catheter shaft, and is spiral cut along its length. Also, the pitch of the spiral cut may be varied at a constant or variable rate, depending on the desired flexibility characteristics of the transition member. 
     The outer tube may have a proximal outer section and a distal outer section joined together at a junction, with the distal outer section more flexible than the proximal outer section. The proximal end of the transition member is preferably located proximal of the junction and the distal end is preferably located distal of the junction. That is, the transition member preferably spans or bridges at least part of the transition in flexibility (i.e., junction) of the outer tube. 
     As previously stated, the inner member may also have a transition in flexibility. In one embodiment, the inner member has a proximal portion, an intermediate portion, and a distal portion, wherein the proximal portion has a first outer diameter, the distal portion has a second outer diameter that is smaller than the first outer diameter, and the intermediate portion has an outer diameter that tapers from the first outer diameter to the second outer diameter. The tapered intermediate portion corresponds to the transition in flexibility of the inner member. 
     In a preferred embodiment, the transition member is secured to or proximate to the intermediate portion of the inner member and extends distally therefrom. To help secure the transition member to the inner member, the transition member may have a proximal portion sized so that the transition member can be friction fit over a portion of the tapered portion of the inner tube. An adhesive may also be used to secure the transition member to the intermediate portion of the inner tube. In a preferred embodiment adhesive is applied proximate the proximal end of the transition member only so that the transition member distal of the adhesive is free-floating. As discussed above, the transition member may engage at least part of the inner member and outer tube when the catheter is provided in a bent configuration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein: 
         FIG. 1  is a cross-sectional view of a catheter showing a preferred embodiment of the present invention; 
         FIG. 2  is a partial cross-sectional view of a preferred embodiment distal tip area of the catheter of  FIG. 1 , illustrating the tip formed from the inner; 
         FIG. 3  is a partial cross-sectional view of a second preferred embodiment of distal tip area of the catheter of  FIG. 1 , illustrating the transition between the stiffer distal end of the inner tube and the more flexible distal tip; 
         FIG. 4  is a cross section view of  FIG. 1  taken along line  4 — 4 ; and 
         FIG. 5  is a partial cross sectional view of another embodiment of the present invention, including a spiral cut transition member bridging a transition in flexibility in the catheter shaft. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following detailed description should be read with reference to the drawings in which like elements in different drawings are numbered identically. The drawings, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. 
     Examples of constructions, materials, dimensions and manufacturing processes are provided for selected elements. All other elements employ that which is known to those skilled in the field of the invention. Those skilled in the art will recognize that many of the examples provided have suitable alternatives which may also be utilized. 
     Referring now to the drawings,  FIG. 1  is a cross-sectional view of an over-the-wire balloon catheter showing a preferred embodiment of the present invention. The balloon catheter  20  includes a shaft assembly  22  and a balloon assembly  24  connected proximate its distal end. A conventional OTW-type manifold assembly  26  is connected to the proximal end of the shaft assembly  22 . The shaft assembly  22  includes an inner tube  28  having a proximal end  30  and a distal end  32 . The proximal end of the shaft assembly  21  extends into a manifold assembly  26  adhesively bonded to the shaft assembly  22 . A polyurethane strain relief  23  is snap-fit to the manifold assembly  26 , and the shaft assembly  22  extends into the manifold assembly  26  through the polyurethane strain relief  23 . An outer tube  34  is co-axially disposed about the inner tube  28  to define an annular inflation lumen  37 . 
     The balloon assembly  24  includes a balloon body portion  36  with a proximal balloon waist  38  and a distal balloon waist  40 . The proximal balloon waist  38  is connected to the outer tube  34  near its distal end  42  by means of an adhesive  44 , or alternatively, is thermally bonded. The distal balloon waist  40  is connected to the inner tube  28  near its distal end  32  by means of an adhesive bond  48  or a thermal bond such that the interior of the balloon  46  is in fluid communication with the annular inflation lumen  37 . 
     A radiopaque marker band  50  is adhesively secured with cyanoacrylate to the inner tube  28  at a point underneath the balloon body  36 . Alternatively, the marker band may be swaged onto the outer surface of the inner. The inner tube  28  defines a guide wire lumen  54  which provides a passage for a guide wire (not shown). The outer tube  34  defines an annular inflation lumen  37  which is in fluid communication with the interior of the balloon  46 . 
     As previously stated, the catheter of the present invention includes an outer tube which may have multiple segments including a relatively stiff proximal outer section, a midshaft section of lesser stiffness, and a tapering distal outer section of the least stiffness. The progressive arrangement of more flexible materials as the catheter proceeds distally provides an optimal level of pushability and trackability to navigate tortuous vasculature. The flexibility of the sections of the outer tubular member were tested utilizing a Gurley bending resistance tester, Part No. 4171-DT, as manufactured by Precision Instruments, Troy, N.Y. The apparatus consists of a balanced pendulum or pointer which is center-pivoted and can be weighted at three points below its center. The pointer moves freely in both the left and right directions. A sample of specific size is attached to a clamp, which in turn is located in one of several positions on a motorized arm which also moves left and right. During the test, the sample is moved against the top edge of the vane, moving the pendulum until a sample bends and releases it. The test is run in two steps, first to the left and then to the right. The scale reading is measured in each direction and the results are averaged. The instrument provides a relative flexibility measurement between the components of the outer tubular member as detailed below to achieve improved trackability and pushability. 
     The outer tube  34  has a relatively stiff, proximal outer section  56  with a proximal end  60  and a distal end  62 . The proximal outer tube may be made of nylon, a polyamide, such as DURETHAN available from Bayer, GRILAMID available from EMS-American Grilon, Inc., a DURETHAN, GRILAMID, CRISTAMID or CRISTAMID/VESTAMID blend braid or polyetheretherketone (PEEK) braid. The preferred embodiment of PEEK braid is a variable PIC tube, wherein said PIC varies from about 30 to 100 PIC to give varying flexibility over the length of the proximal outer tube. The PIC preferably varies from about 50 to about 80. The braiding material in the PEEK or DURETHAN (polymer) braid may be made from stainless steel, or Nitinol (nickel titanium alloy). This proximal outer section  56  will have an outside diameter ranging from 0.040 inches to 0.065 inches with a wall thickness ranging from 0.0026 inches to 0.0056 inches. The proximal outer section has a preferred Gurley value of about 700 to about 1300 over its length. A preferred range is about 800 to about 1200.  FIG. 4  illustrates a cross section view of the proximal outer section having PEEK braid material as taken along  4 — 4  of  FIG. 1 . The PEEK braid includes an inner layer, a braid layer and an outer layer. 
     A midshaft section  58  with a proximal end  64  and a distal end  66  extends distally from the distal end  62  of the proximal outer section  56 . The midshaft section  58  has a stiffness less than that of the proximal outer section  56 . The midshaft section  58  is preferably made from a polyamide, such as CRISTAMID available from Elf Atochem, having a durometer of about 81D. A preferred Gurley value for the midsection is about 350 to about 500, with a range of 400 to 450 preferred. This midshaft section  58  will have an outside diameter ranging from 0.040 inches to 0.045 inches with a wall thickness ranging from 0.0028 inches to 0.0044 inches. 
     The distal end of the proximal outer section  62  is joined to the proximal end of the midshaft section  64  with a urethane adhesive bond or a thermal weld. A distal outer section  68  having a proximal end  70  and a distal end  72  extends distally from the distal end of the midshaft section  66  to the distal end of the outer tube  44 . This distal outer section  68  is more flexible or has less stiffness than both the proximal outer section  56  and the midshaft section  58 . The outer diameter of the distal outer section  68  will taper from about 0.045 inches at the proximal end  70  to 0.030 inches at the distal end  72 . This distal outer section  68  is made of polyether block amide (PEBAX) with a durometer of 70D. The tapered distal outer section preferably has a Gurley value of about 70 to about 90 at its proximal end and about 15 to about 40 at its distal end. Thus, the distal end of the distal outer section  72  will exhibit less stiffness than the proximal end of the distal outer section  70 . The distal end of the midshaft section  66  is joined to the proximal end of the distal outer section  70  with a urethane adhesive bond or a thermal weld. 
     A Nitinol braid insert  74  with a length of about 1.0″ is placed within the proximal end of the distal outer section  70  to provide strain relief and reduce kinkability at the midshaft/distal outer section junction. This Nitinol braid  74  has a 0.001″×0.005″ ribbon. 
     The inner tube  28  is made of polyethylene such as Marlex HDPE or a multilayer coextrusion with Marlex interior layer and PEBAX outer layer. At the proximal end of the inner tube  30 , the inner tube  28  has an outside diameter ranging from 0.022 inches to 0.028 inches and preferably about 0.025 inches, with the inner tube  28  having an inside diameter ranging from 0.016 inches to 0.021 inches for a 0.014 inch guide wire for which this lumen is designed to be compatible with. The inner tube  28  has a wall thickness ranging from 0.0024 inches to 0.005 inches and preferably about 0.0032 inches. The outside diameter to wall thickness ratio must be sufficiently small to minimize the propensity of kinking. 
     As the inner tube  28  extends distally through the junction area between the distal end of the proximal outer section  62  and the proximal end of the midshaft section  64  of the outer tube  28 , both the inner and outer diameters of the inner tube  28  will taper from wider diameters to narrower diameters. Likewise, at the distal end of the inner tube  32 , both the inner and outer diameters of the inner tube  28  will once again taper from wider diameters to narrower diameters as the tube extends distally. 
     As illustrated in  FIG. 2 , in one preferred embodiment, a distal tip  76  is formed on the distal end of the inner tube  32  where the inner tube  28  distally tapers from a larger outer diameter to a smaller outer diameter. The distal balloon waist  40  is attached to the distal tip  76  through a urethane adhesive bond or thermal bond at a bonding area. The area just distal of the distal waist bond is backfilled with adhesive  43  to provide a smooth transition. The adhesive coating provides for improved adhesion between dissimilar substrates. 
     The proximal catheter shaft portion is preferably about 35 to 45 inches in length with a preferred length of 42 inches. The midshaft section, if included, can be about 1 to about 3 inches in length with a preferred length of 2 inches. The distal outer section having the most flexibility is preferably about 8 to about 12 inches in length with a preferred length of about 10 inches. 
     In another preferred embodiment, as shown in  FIG. 3 , a polyethylene, polyamide, or block copolymer such as PEBAX distal tip  80  of durometer between about 50D and 70D, preferably about 63D is heat welded or bonded to the distal end of the inner tube  32  with a durometer of about 63–65D, and the distal balloon waist  40  of the balloon is adhesively or thermally bonded to both the inner and the tip extending therefrom. As shown in  FIG. 3 , the joint  41  between the inner and the tip is located under the distal waist of the balloon. The outer diameter of the polyethylene distal tip  80  distally tapers from a larger outer diameter to a smaller outer diameter. 
     In another preferred embodiment, incorporating a soft tip as described above, the last ½ to 1 mm of the tip at its distal end is made of a different material from the tip material to form a tip extension. In particular, the last ½ to 1 mm is made from a material which is more durable relative to the softer tip material. In particular, the more durable material will resist deforming or tearing when in use, such as tracking tortuous anatomy or through a placed stent. For example, this last ½ to 1 mm may be manufactured from Marlex high-density polyethylene having a 63D durometer which improves the integrity of the tip portion at its distal most end  81 . 
     Referring now to  FIG. 5  there is depicted a partial cross section side view of yet another embodiment of the present invention, including a spiral cut transition member that bridges or overlaps a transition in flexibility in the catheter shaft. The transition member may be used to provide a strain relief to a transition in flexibility along a length of an elongated member. More preferably, the transition member may be used to provide a strain relief to a transition in flexibility along a length of a coaxial catheter having an elongated inner member and a co-axially disposed outer member as depicted in  FIG. 1 . As discussed with respect to  FIG. 1 , numerous transitions in flexibility can be incorporated into a catheter design, such that the junction between the proximal outer  56  and the midshaft section  58  or the junction between the midshaft section  58  and the distal outer section  68 . Transitions in flexibility can be included on the inner member also, such as the necking in the diameter of the inner tube  28  in  FIG. 1 . The transition member of the present invention can be used in conjunction with any such transition in flexibility.  FIG. 5  is illustrative of such uses. 
     The illustrative catheter shaft of  FIG. 5  includes an inner member  112  that has a proximal portion  116 , an intermediate portion  114 , and a distal portion  120 . It is contemplated that the inner member  112  may be a guide wire, an inner tubular member, or any other suitable device or structure. In the embodiment shown, the proximal portion  116  has a first outer diameter, and the distal portion  120  has a second outer diameter that is smaller than the first outer diameter. The intermediate portion  114  has an outer diameter that tapers from the first outer diameter to the second outer diameter. The tapered intermediate portion  114  provides a transition in flexibility from the stiffer proximal portion  116  to the more flexible distal portion  120  of the inner member  112 . Preferably, the inner member  112  is an inner tubular member having a guide wire lumen extending therethrough, and is preferably made from a polymeric material, such as polyethylene, or a multilayer extrusion having a polyethylene inner and PEBAX outer layer. It is recognized that the tube may be un-reinforced polymeric tube, a reinforced polymeric tube, or any other suitable material or element. 
     A transition member  124  may be used in conjunction with the inner member  112 , or in conjunction with the inner member  112  and the outer member  200 , as more fully described below. The transition member  124  is shown co-axially disposed relative to the inner member  112 , and longitudinally positioned to bridge or overlap at least part of the transition in flexibility of the inner member  112 . In preferred embodiments, the transition member has a length of about 2 inches. This length can, however, be varied for specific applications, with preferred lengths of about 0.5 inches to about 4 inches, more preferably, about 1.5 inches to about 2.5 inches. 
     To help provide a transition in flexibility, the transition member  124  has a spiral cut  126  or the like in the side wall thereof. The spiral cut preferably extends through the side wall of the transition member  124 . The pitch of the spiral cut  126  may be varied along the length of the transition member  124  to provide a relatively smooth transition from the relatively stiff proximal portion  116  to the more flexible distal portion  120  of the inner member  112 . The pitch may be varied at a constant or variable rate, depending on the desired flexibility characteristics of the transition member  124 . The pitch may be held constant over a portion of the length of the transition member  124  and varied over other portions of the length to achieve desired flexibility for a particular use. In a preferred embodiment, the spiral cut has a pitch at the proximal end of about 0.11 inches, a pitch in the distal end of about 0.03 inches, and a constant rate of change over its length. It is recognized that pitch can be varied depending upon a particular application with a preferred proximal end pitch of up to 0.3 inches and a distal end pitch of down to 0.01 inches. The pitch is defined herein as the axial distance between an adjacent (360°) spiral cut. In general, pitch is selected so that the flexibility of the transition member in combination with shaft provides a smooth transition in flexibility with no abrupt changes. Spiral cut  126  is preferably formed in the transition member  124  using a laser. 
     The transition member  112  has a first end region  128 , an intermediate region  130 , and a second end region  132 , wherein only the first end region  128  is secured to the inner member  112 . The intermediate region  130  and the second end region  132  are left floating relative to the inner member  112 . The intermediate region  130  and the second end region  132  are preferably spaced from the inner member  112  when the catheter is in a substantially straight configuration, but become in contact with at least parts of the inner member  112  when the catheter is in a bent configuration. 
     It is contemplated that the first end region  128 , the second end region  132 , and/or the intermediate region  130  may be secured to the inner member  112 . It is also contemplated that there may be a space between the inner member  112  and the first end region  128 , the second end region  132  and/or the intermediate region  130 . It is also contemplated that there may not be a space between the inner member  12  and the first end region  128 , the second end region  132  and/or the intermediate region  130 . 
     The first end region  128  of the transition member  124  may be secured to the inner member  112  proximate the transition in flexibility of the inner member  112 , with the intermediate region  130  and the second end region  132  extending distally therefrom without attachment to the inner so that these portions are free floating and can flex without restraint. The length of the transition member  124  is preferably selected so that the second end region  132  extends distal of the transition in flexibility in the inner member  112 . In this configuration, the transition member  124  bridges or overlaps at least part of the transition in flexibility of the inner member  112 . Preferably, the transition member  124  is formed from a heat treated stainless steel hypotube, but could be manufactured from another alloy, such as nitinol, or a polymeric material. 
     As shown in  FIG. 5 , the catheter shaft may also include an outer tube  200  having a lumen  201  extending therethrough. The inner tube  112  and outer tube  200  are preferably co-axially disposed forming the annular lumen  201  therebetween. The outer tube  200  has a relatively stiff proximal outer section  202  and a relatively flexible distal outer section  204 . The progressive arrangement of more flexible materials as the catheter proceeds distally provides an optimal level of pushability and trackability to navigate tortuous vasculature. 
     The proximal outer tube  202  may correspond to the proximal outer tube  56  of the embodiment shown in  FIG. 1 . Thus, the proximal outer tube  202  may be made of nylon, a polyamide, such as DURETHAN available from Bayer, a DURETHAN braid, polyetheretherketone (PEEK) braid or any other suitable material or combination of materials. Alternatively, the proximal outer tube  202  may correspond to the midshaft section  58  of the embodiment shown in  FIG. 1 . Thus, the proximal outer tube  202  may be made from a polyamide, such as CRISTAMID available from Elf Atochem, having a durometer of about 81D, and may have an outside diameter ranging from 0.040 inches to 0.045 inches with a wall thickness ranging from 0.0028 inches to 0.0044 inches. 
     The distal outer section  204  may have a proximal end  206  that extends distally from the distal end  208  of the proximal outer section  202 . The distal outer section  204  preferably has a stiffness that is less than that of the proximal outer section  202 . The distal outer section  204  may correspond to the midshaft section  58  of the embodiment shown in  FIG. 1 . Thus, the distal outer section  204  may be made from a polyamide, such as CRISTAMID available from Elf Atochem, having a durometer of about 81D, with an outside diameter ranging from 0.040 inches to 0.045 inches with a wall thickness ranging from 0.0028 inches to 0.0044 inches. Alternatively, the distal outer section  204  may correspond to the distal outer section  68  of the embodiment shown in  FIG. 1 . Thus, the distal outer section  204  may be made from a polyether block amide (PEBAX) having a durometer of about 70D. 
     The distal end  208  of the proximal outer section  202  is preferably joined to the proximal end  206  of the distal outer section  204  at a junction  210  using a urethane adhesive bond or a thermal weld. Because the proximal outer section  202  is preferably stiffer than the distal outer section  204 , there is a transition in flexibility in the outer tube  200  at junction  210 . This transition in flexibility may be rather abrupt, as provided by an adhesive lap-joint or a thermal butt-joint, or may be more gradual, as provided by a heat flow process or an interrupted layer extrusion process. The proximal end of the transition member  124  may be located proximal of the junction  210  in the outer tube  200 , and the distal end may be located distal of the junction  210 . Thus, the transition member  124  may span or bridge at least part of the transition in flexibility (i.e. junction  210 ) of the outer tube  200 . 
     The transition member  124  is preferably co-axially disposed within the annular lumen between the inner member  112  and the outer tube  200 . It is recognized, however, that the transition member may be positioned inside the inner member  112  (if the inner member  112  is tubular having an inner lumen) or outside of the outer tube  200 . The transition member  124  is preferably longitudinally positioned adjacent to at least a portion of the transition in flexibility of the inner member  112 , the outer tube  200 , or both. 
     Like above, the transition member may be secured to the intermediate portion  114  of the inner member  112 . To help secure the transition member  124  to the inner member  112 , the proximal portion may be sized so that the transition member  124  can be friction fit over the intermediate portion  114 . An adhesive may also be used to secure the transition member  124  to the intermediate portion  114  of the inner tube  112 . 
     Finally, a radius cut  280  may be provided in the transition member  124  to remove any sharp edges that may exists at the proximal end and distal end of transition member  124 . It is recognized that spiral cut  126  may result in a sharp edge or point at the proximal end  282  or termination of the spiral cut of the transition member  124 . This sharp edge or point may engage the outer tube  200  when the catheter is bent. To help reduce any damage that may result, a radius cut  280  may be provided to round the edge proximal end  282 . This may be accomplished by cutting, grinding, filing or any other means that provides a smoother less obtrusive edge to the proximal end of the end coil  282 . For similar reasons, a radius cut may also be provided at the distal end of the transition member  112 . 
     Having thus described the preferred embodiments of the present invention, those of skill in the art will readily appreciate that yet other embodiments may be made and used within the scope of the claims hereto attached.