the present invention may be further understood with reference to the following description and the appended drawings , wherein like elements are referred to with the same reference numerals . the invention is related to medical devices used to introduce a contrast media fluid into a patient , preferably at high pressure and with a large flow rate . specifically , the devices according to the invention may be used to inject the contrast media using a picc . as described above , where repeated access to the vascular system is required , a semi - permanent central venous catheter may be inserted in a vein kept in place for up to two years . a picc typically comprises a flexible elongated portion tunneled from a remote peripheral location ( an arm or leg ) to a location near the heart . the proximal end of the picc may be accessed via a port placed , for example , subcutaneously in the arm or chest of the patient or which may remain outside of the body . as would be understood by those skilled in the art , the pressure exerted by the fluid is a function of the flow rate , the viscosity and the cross sectional flow area of the catheter , among other variables . accordingly , limitations on the fluid pressure and / or flow rate are often specified for various types of catheters to ensure that the catheter will not be damaged during use by excessive strains . however as mentioned above , modern imaging methods often rely on the introduction of contrast fluids at high flow rates . the catheter according to the present invention , may be used for both central venous access and the injection of contrast media decreasing patient discomfort and the time and expense of procedures . the catheter according to this invention , e . g ., a picc venous catheter , is at least partially reinforced to enhance its burst pressure and maximum flow rate to levels suitable for the introduction of contrast media without compromising kink resistance or increasing the cross sectional profile of the catheter , as compared to conventional picc devices . for example , a catheter according to the present invention will withstand a flow rate of about 4 to about 6 cc / sec and a pressure of more than about 300psi typical of power injection devices . a reinforcement is included in the exemplary catheter according to the invention to increase the burst pressure . in one embodiment , both the shaft of the catheter and an extension tube thereof are reinforced , to give a substantially uniform resistance to the increased pressure . alternatively , only the shaft may be reinforced . fig1 shows an exemplary embodiment of a catheter comprising a reinforced portion in accord with the present invention . the exemplary catheter 100 is a dual lumen catheter in which the lumens 110 are separated by a partition 108 extending along a longitudinal axis of the catheter 100 . in the exemplary embodiment , the catheter 100 has a layered construction , in which layers of stronger material are formed near layers of more flexible material to obtain desired mechanical characteristics of an outer wall 102 . for example , an outer layer 104 of a material having a lower durometer value may be used , to retain the flexibility of a conventional catheter . materials such as members of the polyurethane family that are alcohol compatible may be used advantageously in this function . an inner portion 106 of the catheter shaft wall 102 may be made of a material with a higher durometer value , to give strength to the composite assembly . for example , high strength thermoplastic polyurethanes , polyether block - amides and polyolefines may be used . it is often necessary , in the course of a catheterization procedure , to adjust the length of the portion of the catheter inserted into the patient . generally , the surgeon cuts a distal portion of the catheter to a desired length . thus , in the case of a catheter 100 reinforced according to the present invention , the reinforcing material is preferably selected so that it can be easily cut with a blade . the exemplary materials described above fall within this category , so that the reinforced catheter 100 may be cut to a desired length using conventional methods . alternatively , a material that is more difficult to cut may be used and / or a portion of the catheter 100 may be left unreinforced so that it may be cut . for example , the weakest portions of the catheter , such as the portion immediately distal to the suture wing , may be reinforced , leaving a 20 - 40 cm section of the tip of the catheter unreinforced . because the portion immediately distal to the suture wing is one of the weakest and most likely to fail , reinforcement around the weak areas will prevent most failures from occurring . the unreinforced section of the catheter will continue to permit surgeons to easily cut the catheter in conventional manners , such as with a blade . according to the present embodiment , the catheter 100 may be composed of various layers with each layer being formed of a material of different hardness , thereby allowing the catheter 100 to be atraumaticly inserted while exhibiting an improved resistance to the pressures associated with high flow rate power injection . as would be understood by those skilled in the art , the manufacture of the catheter 100 may be accomplished using a co - extrusion or a lamination process . for example , the softer , more flexible outer layer 104 of the shaft wall 102 may be co - extruded with the stiffer , higher durometer inner layer 106 . this configuration provides both the flexible outer portion and the pressure resistant inner portion of the catheter 100 . the co - extrusion process may be carried out with polymers that are either compatible or non - compatible with one another . if non compatible polymers are used , it may be necessary to provide an intermediate tie layer along an interface 112 between the outer layer 104 and the inner layer 106 . in this exemplary embodiment , a soft thermoplastic polyurethane ( tpu ) may be used for the outer layer 104 while a stiff polyester block - amide ( peba ), a stiff polyether block - amide , polyolefin or polytetrafluoroethylene ( ptfe ) may be used for the inner layer 106 . the outer tpu exhibits softening while within the body , giving the desired flexibility , etc ., and allowing atraumatic insertion . however the peba of the inner layer 106 retains its inherent strength and resistance to pressure . fig2 shows a second embodiment of the catheter 120 according to the invention . in this exemplary embodiment , the shaft wall 122 is reinforced by an inner layer 126 of a material with greater durometer values . here , instead of an entire inner portion of the shaft 120 formed of a higher durometer material as in the example of fig1 , both the inner layer 126 and the outer layer 124 are formed of lower durometer , more flexible material . specifically , the outer layer 124 of the wall 122 as well as the inner core 132 of a lumen divider 128 are formed from one piece of the lower durometer material . to this basic catheter shaft is then added a coating of higher durometer material on the inner sides of the two lumens 110 , forming the inner layer 126 of the wall 122 as well as outer portions 130 of the divider 128 . this embodiment provides for a flexible outer surface of the catheter 120 , together with increased mechanical reinforcement of the stiffer lining of the dual lumens 110 . alternatively , the inner layers 126 , 130 may be part of a separate tube of smaller diameter which is inserted into , but not bonded to the shaft of the catheter 120 . a further exemplary embodiment of a catheter shaft according to the invention is shown in fig3 . in this case , the increased resistance to fluid pressure within the lumens 110 is provided by a braid included therewithin . as shown , the catheter 140 includes an outer wall 142 comprising a braid 144 , shown here in cross section . the braid 144 may be formed of any of a variety of materials , depending on the amount of additional pressure resistance desired . the braid 144 may be formed , for example , of a metal or alloy such as nitinol or stainless steel . a material having shape memory properties may be especially well suited for reinforcement braids used in extension tubes of the catheter . in use , the proximal ends of these catheters are clamped shut between uses . thus , the reinforcing braid will preferably be selected so that it will not retain the clamped shape , but will return to the original tubular shape when the clamping force is released . as would be understood by those skilled in the art , for catheters which are to be used in conjunction with mri , the braid 144 is preferably formed of a non - ferro - magnetic material , for example , kevlar , vectran , silk , members of the polyolefin family and other types of polymer or other suitable material . a variation of the braid reinforcement is shown in cross section in fig4 . the exemplary embodiment shown there comprises a braid 154 together with a dual material layered construction of the wall 152 . the catheter shaft 150 includes two lumens with an inner portion 156 of the wall 152 formed of a material having an increased durometer with respect to a material comprising an outer portion 158 thereof . all of the variations in design described above with respect to the embodiments of fig1 - 3 may also be applied to the construction of the exemplary catheter shaft 150 . it will be apparent to those of skill in the art that the radial location of the braid 154 within the wall 152 of the catheter shaft 150 may also be varied . it will also be apparent that the same reinforced construction methods described herein may be used for other components of a catheter , such as extension tubes , or for other medical tubes . in a different embodiment , the reinforcement braid may be disposed on the outside of the catheter body . for example , fig5 shows a dual lumen catheter 160 having an outer wall 162 and a braid 164 disposed outside the surface of the wall 162 . this configuration may provide manufacturing benefits compared to a configuration in which the braid 164 is embedded within the material of the catheter wall . for example , the braid 164 may be added to the assembly after the catheter has been formed by extrusion . the braid 164 may then be bonded to the catheter wall 162 , or may be left free to slide longitudinally relative to the catheter . in this latter embodiment , the user may be allowed to longitudinally move the external braid to a desired position . to further improve the pressure resistance and ultimate hoop strength of the base catheter material , micro particles may be added to the compound forming the catheter wall . the micro particles ( sometimes referred to as nano - particles , depending on their size ) may include clay and fumed silica . fig6 shows an exemplary embodiment , in which a catheter shaft 170 is formed with a wall 172 comprising strengthening particles 174 . the presence of the micro particles 174 increases the radial stiffness of the catheter wall 172 , resulting in a more durable and more pressure resistant base material for the catheter . the distribution of the micro particles 174 both radially and longitudinally along the catheter 170 may be selected to obtain desired mechanical properties of the device . for example , a more pliable section of the catheter may be formed by locally reducing the amount of micro particles 174 added to the material of wall 172 while areas of increased stiffness may be created by increasing the amount of micro particles 174 in a region . as an alternative to introducing strengthening particles into the catheter material , cross linking agents may be incorporated into the base material of the catheter shaft . for example , agents such as silanes , dicumyl peroxide , maleic anhydride and functionalized polymers may be added . these agents are effective in partially cross - linking thermoplastic polymers . activation of the cross linking agents may be accomplished in a conventional manner , for example through secondary exposure to high energy sources such as electron beams to increase the strength of the base material . as indicated above , both the radial and tangential distribution of cross linking agents through the material of the catheter shaft may be selected to obtain desired mechanical properties , as would be understood by those skilled in the art . it will be apparent to those of skill in the art that the various methods described herein to increase the strength of a catheter shaft wall may be applied selectively to certain portions of the catheter in question . for example , fig7 shows a catheter shaft 200 having a reinforced portion 204 and an unreinforced portion 202 . the reinforced portion 204 may comprise any of the reinforcement elements or treatments described above , such as a mesh 206 embedded within wall 208 of the catheter shaft 200 . it will be apparent to those of skill in the art that different types or combinations of reinforcements may be used , such as an external mesh , a layered multi - material composite structure , or the addition of reinforcing particles in the wall material . in the example depicted in fig7 , the shaft wall 208 is altered along its length , in the longitudinal direction . however , for different applications , the variation in structural reinforcement may be carried out in the angular direction or in the radial direction , as was described above . the non - uniform reinforcement construction may be applied to both catheter shaft and to the extension tubes , as needed . in one exemplary application , the longitudinal variation in the strength of catheter wall 208 may be used to allow the user to trim the distal end of the catheter shaft 200 , to provide a better fit in the patient . leaving the unreinforced portion 202 without the reinforcement elements 206 included elsewhere ( i . e ., in reinforced portion 204 ) to increase pressure resistance allows the user to cut the wall 208 more easily , the reinforcement elements 206 may thus be selected to have greater strength , since it is not necessary that the user be able to cut therethrough to trim the catheter shaft 200 to the desired length . in one example , between about 15 cm and 20 cm of the distal end of catheter shaft 200 may form the unreinforced portion 202 . in another exemplary application , the wall of shaft 200 may be composed of varying materials , or may be otherwise reinforced by different amounts along its length to allow for increased strength and durability at specified stress points . these points of increased stress may occur during power injection of a fluid only at certain locations , such as near the injection point or near bends in the catheter . in this manner , the additional material used to strengthen the catheter may be targeted where it is most effective , without having to reinforce the entire catheter . this construction may be simpler and less costly than forming a catheter with reinforcements along its entire length . according to another exemplary embodiment of the invention , the catheter shaft or the extension tube may be constructed with an inherent weak point designed to fail before the rest of the device does . when the catheter experiences excessive pressure , the extension tube will fail and release the pressure , leaving the catheter shaft intact . the extension tube may be formed with a tapered region of lesser strength , or by profiling the wall thickness of the tube to create the designated failure point . as shown in fig8 , a catheter extension tube 250 comprises a reduced thickness portion 252 in which the wall 254 is much thinner and is , at this point , able to withstand a pressure reduced with respect to the rest of the catheter . it will be apparent to those skilled in the art that the wall thickness reduction may be achieved by removing material from the outside of the wall ( as shown ), the inside of the wall or both . in another embodiment , the inherent weak point may be formed by making either or both of the inside and outside diameters of the tube irregular in cross section . the non uniform wall thickness thus created , for example in the extension tube , defines specific sites for failure of the tube . as shown in the example of fig8 , a rectangular inner profile 256 of the extension tube 250 may be placed within a generally circular extension tube 250 . this configuration may be used to define four thin walls 258 at the corners of the profile 256 , which will tend to fail before thicker portions of the wall . in addition , the corners 260 act as stress concentrators , further ensuring that the extension tube 250 will fail at the location of the rectangular profile 256 when subject to excessive pressure . it will be apparent to those skilled in the art that the rectangular profile 256 may be used separately or in conjunction with the reduced thickness portion 252 , as desired in specific applications . the present invention has been described with reference to specific embodiments , and more specifically to a picc catheter used for power injection of contrast media used in ct imaging . however , other embodiments may be devised that are applicable to other medical devices and procedures , without departing from the scope of the invention . accordingly , various modifications and changes may be made to the embodiments , particularly with regard to dimensions and materials , without departing from the broadest spirit and scope of the present invention as set forth in the claims that follow . the specification and drawings are accordingly to be regarded in an illustrative rather than restrictive illustrative rather than restrictive sense .
a catheter for medical procedures comprises a shaft portion having a distal end insertable into a body lumen , the shaft portion having a wall defining a working lumen extending therewithin and a first strengthening element coupled to the wall to increase a burst pressure of the shaft portion , wherein the first strengthening element cooperates with a base material of the wall to define a flexible region of the shaft portion allowing the shaft portion to be atraumatically inserted into the body lumen .