Abstract:
Technology disclosed herein provides a reduced transition between the edge of a rigid vascular dilator and the distal edge of the accompanying introducer sheath. Disclosed dilators can be segmented into two or more primarily longitudinally extending parts, can have rigid circumferential or semi-circumferential leading shoulders to minimize the transition between the dilator and the sheath edge, and can contain internal recesses to allow sequential retraction of segments once the introducer sheath is delivered to a target chamber. With this technology, vascular introducer sheaths can be introduced percutaneously into a broad range of diseased target vessels and chambers with reduced damage to the wall of the vessel or chamber, and with reduced damage to the distal end of the introducer sheath.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/890,961, filed Oct. 15, 2013, and entitled “VASCULAR DILATORS,” which is incorporated by referenced herein in its entirety. 
     
    
     FIELD 
       [0002]    This application is related to vascular dilators and introducer sheaths. 
       BACKGROUND 
       [0003]    Catheter-based intravascular procedures typically require insertion of a vascular dilator followed by an introducer sheath. A conventional vascular introducer sheath  10 , as illustrated in  FIG. 1 , has a vascular crossing profile, or diameter, necessarily wider than the accompanying vascular dilator  12 , such that the dilator can be later retracted through the introducer sheath. This excess crossing profile of the introducer sheath is known to interfere with smooth advancement of the introducer sheath into a target blood vessel  14  or other chamber, and is known to cause tears in the target blood vessel, more commonly at or near the “trailing edge”  18  of the sheath  10 , which refers to the portion of the distal end of the sheath that enters the vessel last when the device is inserted at an angle to the vessel, as in typical use. The “leading edge”  20  of the sheath  10 , on the other hand, refers to the portion of the distal end of the sheath that enters the vessel first. 
         [0004]      FIG. 2  shows an exemplary torn opening  16  in a vessel  14  caused by unequal crossing profiles of a conventional dilator  12  and introducer sheath  10 , as well as a desired entry opening  22  having a smooth elliptical shape. As shown in  FIG. 3 , similar problems, such as puckering and tearing, can also occur during direct transthoracic or direct transmyocardial entry into the heart through a heart wall  24 , such as for prosthetic heart valve implantation. The excess crossing profile in conventional intravascular systems is driven by the finite thickness of the wall of the introducer sheath, the limited flexibility of the introducer sheath, the required rigidity of the vascular dilator, and the requirement to withdraw the vascular dilator proximally through the sheath after delivery. Large-bore introducer sheaths, such as for structural heart interventions, tend to be necessarily thicker and more rigid and exacerbate these problems. 
       SUMMARY 
       [0005]    Technology disclosed herein provides a reduced transition between a vascular dilator and the distal edge of an accompanying vascular introducer sheath. Disclosed dilators can be segmented into two or more primarily longitudinally extending parts, can have rigid circumferential or semi-circumferential leading shoulders to minimize the transition between the dilator and the distal sheath edge, and can contain internal recesses to allow sequential retraction of segments once the introducer sheath is delivered to a target chamber. Some dilators have radially adjustable shoulders and longitudinally movable mandrels to control the position of the adjustable shoulders. With this technology, vascular introducer sheaths can be introduced percutaneously into a broad range of diseased target vessels and chambers with reduced damage to the wall of the vessel or chamber, and with reduced damage to the distal end of the introducer sheath. 
         [0006]    Some disclosed vascular dilators comprise a first segment and a second segment that are coupled together along an interface that extends primarily in the directions of a longitudinal axis of the dilator (i.e., the proximal and distal directions), such that the first segment and the second segment are slidable longitudinally relative to one another along the interface. Such dilators are configured for use with an introducer sheath, wherein the second segment can be retracted proximally through the introducer sheath alongside a proximal shaft of the first segment during a first stage, and then the first segment can be retracted proximally through the introducer sheath during a subsequent stage. 
         [0007]    The dilators can comprise a tapered distal portion and a generally cylindrical shaft portion proximal to the tapered distal portion, with the tapered distal portion including a shoulder portion that extends at least partially around the circumference of the tapered distal portion to shield the distal end of the sheath. Each longitudinal segment, or some of the segments, can include a part of the shoulder portion. 
         [0008]    In some embodiments, the shoulder extends less than 360° circumferentially around the dilator. For example, a first part of the shoulder portion on the first segment can extend less than 180° circumferentially and a second part of the shoulder portion on the second segment can extend less than 180° circumferentially. In some embodiments, each part of the shoulder extends 90° or less circumferentially and/or the total of all shoulder portions extends 180° or less circumferentially. In other embodiments, only one segment contains a shoulder to be applied by the user to the most “vulnerable” portion of the vascular access target, for example a non-circumferential shoulder that is aligned with the “trailing edge” of the vascular entry target. 
         [0009]    The shaft portion of the first segment is engaged with a shaft portion of the second segment such that the first segment can be moved longitudinally relative to the second segment but non-longitudinal motion between the shaft portions of the first and second segments is restricted. The first segment can comprise a recess located at least partially within a tapered distal portion of the first segment, and the distal end portion of the second segment can be configured to deflect inwardly into the recess of the first segment during retraction of the second segment. 
         [0010]    In some embodiments, a dilator further comprises a third segment coupled to the first and second segments along interfaces that extend primarily longitudinally, and the third segment is slidable longitudinally relative to the first and second segments along the interfaces. In such embodiments, the first, second, and third portions can each comprise a portion of a shoulder, and the shoulder can extend all the way around the dilator. 
         [0011]    In some embodiments, the first segment can comprise a fully annular distal tip, or nosecone, that extends 360° circumferentially and defines a distal portion of a guidewire lumen passing through the dilator. The second segment can comprise a proximal portion of the guidewire lumen. 
         [0012]    The dilators described herein can be used with an introducer sheath comprising a distal end portion that has a generally constant outer radius and at least some part of the distal end portion has an inner radius that increases moving distally toward a distal end of the introducer sheath. 
         [0013]    An exemplary method of using a vascular dilator with an introducer sheath comprises advancing a first segment of the dilator distally relative to a second segment of the dilator and the introducer sheath, the first and second segments being engaged together along an interface that extends primarily in the proximal and distal directions, the introducer sheath being positioned around proximal shaft portions of the first and second segments; then retracting the second segment proximally through the introducer sheath alongside a shaft portion of the first segment; and then retracting the first segment proximally through the introducer sheath. 
         [0014]    In some methods, retracting the second segment comprises causing a shoulder portion of the second segment to contact a distal end of the introducer sheath and thereby cause a distal portion of the second segment to deflect radially inwardly into a recess formed in the first segment, such that the shoulder portion of the second segment moves radially inwardly a sufficient distance to enter the introducer sheath. Similarly, retracting the first segment can comprise causing a shoulder portion of the first segment to contact a distal end of the introducer sheath and thereby cause a distal portion of the first segment to deflect radially inwardly a sufficient distance to enter the introducer sheath. 
         [0015]    Some methods further comprise retracting a third segment proximally through the introducer sheath prior to retracting the first segment, the third segment being engaged to the first and second segments along interfaces that extend primarily in the proximal and distal directions. 
         [0016]    Some methods further comprise initially inserting the vascular dilator and the introducer sheath into a blood vessel through a vessel wall or into a hearth through a heart wall, wherein the inserting is performed with a distal end of the introducer sheath positioned adjacent to a proximal surface of a shoulder of the dilator, the shoulder having a maximum radial dimension about equal to the outer radius of the introducer sheath. In some cases, the dilator and introducer sheath can be used to access an aorta from an inferior vena cava or to access a heart chamber through a heart wall. 
         [0017]    Some exemplary vascular dilators comprise a main body having a tapered distal portion and a generally cylindrical proximal portion, at least one adjustable shoulder portion coupled to the tapered distal portion, and at least one mandrel extending through the main body and configured to affect a radial positioning of the adjustable shoulder portion. The mandrel can be moved proximally and distally relative to the main body and the shoulder portion. For example, when the mandrel is in a distal position, the shoulder portion is in a radially extended position, and when the mandrel is in a proximal position, the shoulder portion is allowed to move to a radially collapsed position. In the radially extended position, the shoulder portion can extend radially beyond the radial extent of the proximal portion of the main body, and in the radially collapsed position, the shoulder may not extend radially beyond the radial extend of the proximal portion of the main body. 
         [0018]    In some embodiments, a biasing mechanisms, such as a spring element, can be positioned under each shoulder portion to bias the shoulder portion toward the radially expanded configuration. The biasing mechanism can be resiliently collapsed under radially inward pressure to allow the shoulder portion to move to the collapsed position. 
         [0019]    Any number of shoulder portions and associated mandrels can be included. 
         [0020]    Associated methods of using a vascular dilator with an introducer sheath comprise: (1) with a shoulder portion of the dilator being in a radially extended position to shield a distal end of the introducer sheath, moving a mandrel of the dilator proximally relative to the shoulder portion of the dilator and relative to the introducer sheath; and then (2) moving the dilator proximally relative to the introducer sheath with the shoulder portion of the dilator in a radially collapsed position such that the shoulder portion fits through the introducer sheath. Moving the mandrel proximally can causes the shoulder portion to move from the radially extended position to the radially collapsed position, and/or moving the dilator proximally relative to the introducer sheath can cause the shoulder portion interact with the distal end of the introducer sheath and thereby move from the radially extended position to the radially collapsed position. 
         [0021]    Some methods further comprise moving the mandrel distally relative to the shoulder portion to cause the shoulder portion to move from the radially collapsed position to the radially extended position. 
         [0022]    Some methods further comprise inserting the dilator and introducer sheath through an anatomical wall with the shoulder portion in the radially extended position to shield the sheath. 
         [0023]    Some methods comprise using the dilator and introducer sheath to access an aorta from an inferior vena cava or to access a heart chamber through a heart wall. 
         [0024]    The foregoing and other objects, features, and advantages of the disclosed technology will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  shows a conventional vascular dilator and introducer sheath entering a blood vessel. 
           [0026]      FIG. 2  illustrates a blood vessel having a torn opening caused by the device of  FIG. 1 , in comparison with a desired opening. 
           [0027]      FIG. 3  shows a conventional dilator and introducer sheath entering the heart through the heart wall. The heart muscle is seen to “pucker” as it is displaced inward by the shoulder-free transition of the dilator ( 12 ) to the sheath ( 10 ). This may interfere with cardiac function during the procedure, risks excessive force during sheath advancement, and risks excessive injury to the heart. 
           [0028]      FIGS. 4 and 5  are perspective views of an exemplary segmented vascular dilator and an introducer sheath. 
           [0029]      FIGS. 6 and 7  are perspective views of one segment of the dilator of  FIG. 4 . 
           [0030]      FIGS. 8 and 9  are perspective views of another segment of the dilator of  FIG. 4 . 
           [0031]      FIG. 10  is a perspective view of another exemplary segmented vascular dilator. 
           [0032]      FIG. 11  is a perspective cross-sectional view of the dilator of  FIG. 10 , taken along its longitudinal axis. 
           [0033]      FIG. 12  is a perspective view of yet another exemplary vascular dilator. 
           [0034]      FIG. 13  is a perspective cross-sectional view of the dilator of  FIG. 12 , taken along its longitudinal axis. 
           [0035]      FIG. 14  is a perspective cross-sectional view of an exemplary introducer sheath, taken along its longitudinal axis. 
           [0036]      FIG. 15  is a perspective view of another exemplary vascular dilator. 
           [0037]      FIG. 16  is a perspective view of the dilator of  FIG. 15 , with the distal portion shown in cross-section. 
           [0038]      FIG. 17  is an enlarged side view of the dilator of  FIG. 15 , with the distal portion shown in cross-section. 
           [0039]      FIG. 18  is a perspective view of the dilator of  FIG. 15 , with the distal portion shown in cross-section, and with an adjustable shoulder portion shown in a radially collapsed configuration. 
           [0040]      FIG. 19  is a perspective view of the dilator of  FIG. 15  together with a vascular introducer sheath, with the distal portion of the dilator shown in cross-section. 
           [0041]      FIG. 20  is a side view of the dilator and sheath of  FIG. 19 , with the distal portion of the dilator shown in cross-section, and with an adjustable shoulder portion of the dilator shown in a radially collapsed configuration. 
       
    
    
     DETAILED DESCRIPTION 
     Exemplary Vascular Dilators 
       [0042]      FIGS. 4 and 5  show an exemplary vascular dilator  100  and an exemplary introducer sheath  102 . The dilator  100  and the sheath  102  are shown in an assembled configuration for insertion into a blood vessel, through a heart wall, or similar procedure. The dilator  100  has a distal end  104  and extends proximally through the sheath  102  toward a proximal end. The dilator  100  can have any length as needed for a particular procedure, and can terminate at a proximal hub, for example. The illustrated embodiment is shown truncated at a proximal end  106 . Similarly, the sheath  102  has a distal end  108  and can extend proximally any length as needed, and can terminate at a proximal hub, for example. The illustrated embodiment is shown truncated at a proximal end  110  distal to the proximal end  106  of the dilator. The dilator  100  can also include a central guidewire lumen  115  extending along the length of the dilator. 
         [0043]    The dilator  100  has a frustoconical shaped distal tapered portion  112  for penetration and dilation of a vessel wall, heart wall, or the like, and a generally tubular shaft  114  extending proximally from the tapered portion through the sheath  110 . The external surface of the tapered portion  112  forms a shoulder  113  adjacent to the distal end  108  of the sheath  102  to shield the distal end of the sheath and create a smooth transition with the outer diameter of the sheath. The maximum outer radius of the shoulder  113  can be about equal to the outer radius of the sheath  102 . In other embodiments, the maximum outer radius of the shoulder  113  can be between the inner radius of the sheath  102  and the outer radius of the sheath  102 . In still other embodiments, the maximum outer radius of the shoulder  113  can be slightly larger than the outer radius of the sheath  102 . 
         [0044]    The shoulder  113  can extend around a portion of the circumference of the dilator, and can be broken into plural shoulder portions. For example, the maximum outer diameter of the dilator  100  at the shoulder  113  can be about equal to the outer diameter of the sheath  102 , while the outer diameter of the dilator at other circumferential portions lacking a shoulder can be about equal to the inner diameter of the sheath  102 . 
         [0045]    The dilator  100  comprises a first segment  116  ( FIGS. 6 and 7 ) and a second segment  118  ( FIGS. 8 and 9 ) that mate together and can be independently moved in the proximal and distal directions (also referred to as the “longitudinal” directions) or moved in unison. The first segment  116  and the second segment  118  can each include part of the distal tapered portion  112  and part of the proximal shaft  114 , such that the dilator  100  is generally longitudinally segmented. 
         [0046]    Segmentation of the dilator  100  allows for independent retraction of each segment through the sheath  102 . The two segments can initially be fixed together during insertion, then moved in the longitudinal directions independently for retraction. For example, during retraction from a vessel, the second segment  118  can be initially retracted through the sheath  102  first, and then second segment  116  can be subsequently retracted through the sheath. 
         [0047]    The distal portion  131  of the first segment  116  includes a nosecone  119  forming the distal-most portion of the guidewire lumen  115 , and an internal recess  120  ( FIG. 6 ) proximal of the nosecone  119 . The recess  120  allows the distal portion  122  of the second segment  118  to deflect radially inwardly into the recess  120  during retraction to reduce its radial dimensions for passage through the sheath  102 . 
         [0048]    The first and second segments  116 ,  118  can be held together via an engagement that allows for at least some degree of longitudinal movement relative to each other, but restricts non-longitudinal movement relative to each other. In some embodiments, a dovetail-type engagement and/or interlocking of complementary components of the first and second components  116 ,  118  can be used that allows for relative longitudinal sliding but prevents non-longitudinal separation. For example, the first segment  116  can include female grooves  130  ( FIG. 6 ) that extend longitudinally along the shaft portion  132  of the first segment, and the second segment  118  can include corresponding male rails  134  ( FIG. 9 ) in the shaft portion  136  of the second segment, such that the rails  134  can slide longitudinally in the grooves  130 . The rails  134  and grooves  130  can interlock, using a dovetail-like configuration, to prevent the two segments from separating apart radially or laterally from each other. In other embodiments, the first segment  116  can include male rails and the second segment can include female grooves, or the first and second segments can include one or more rails and one or more grooves that mate with each other. 
         [0049]    In other embodiments, various alternative types of suitable engagement mechanisms can be included instead of or in addition to the rails and groove described above. For example, pins or pegs can be used to help align the two segments and restrict non-longitudinal motion. As shown in  FIG. 6 , for example, the distal portion  131  of the first segment  116  can include a pin  140  that extends proximally and is configured to mate with a hole (not shown) in the distal tip  142  of the second segment  118  to restrict the second segment from separating or collapsing in the non-longitudinal directions relative to the first segment while they are engaged together in the insertion configuration shown in  FIGS. 4 and 5 . Distal motion of the first segment  116  relative to the second segment  118  causes the pin  140  to disengage from the second segment, allowing the distal portion  122  of the second segment to deflect radially inwardly into the recess  120  of the first segment for retraction of the second segment through the sheath  102 . Any number of such pins or similar registration features can be included in different embodiments. 
         [0050]    The shoulder  113  of the dilator  100  can be segmented into discrete circumferential portions. The first segment  116  can include a first shoulder portion  142  ( FIG. 7 ) and the second segment  118  can include a second shoulder portion  144  ( FIG. 8 ). The first and second shoulder portions  142 ,  144  can extend circumferentially about the dilator  100  less than 180° each, such as less than or equal to 150° each, less than or equal to 120° each, less than or equal to 90° each, and/or less than or equal to 75° each. In some embodiments, the first shoulder portion  142  can have a greater or lesser circumferential dimension than the second shoulder portion  144 . For example, the dilator segment that is configured to be positioned adjacent to the trailing edge  18  of the sheath (see  FIG. 1 ), where maximum damage to the vessel wall is most likely to occur due to an uneven transition between the dilator and the sheath, can comprise a larger shoulder portion (greater circumferential dimension) relative to the shoulder portion of the dilator segment that is configured to be positioned adjacent to the leading edge  20  of the sheath, where the least damage to the vessel wall occurs. Dilator segments having a shoulder portion that extends over less than 180° and/or over less than the maximum circumferential dimension of the particular segment allows each segment to have a maximum dimension perpendicular to the longitudinal directions that is less than the inner diameter of the sheath  102 , such that each of the segment fits through the inner diameter of the sheath  102  during retraction. 
         [0051]    The shoulder portions  144 ,  146  can include peripheral inclined surfaces  150 ,  152 , respectively, on the proximal side of the shoulder that help the segments deflect radially inwardly during retraction to fit into the sheath  102 . The inclined surfaces  150 ,  152  can have various geometries that transition from the greater radial dimension of the shoulder  144 ,  146  to the reduced radial dimension of the shaft portion  132 ,  136  of the respective segment. For example, the inclined surfaces  150 ,  152  can be concave, convex, linear, or a combination such shapes. The angle between the inclined surfaces  150 ,  152  and the outer surfaces of the shafts  132 ,  136 , respectively, is generally greater than 90° to facilitate inward deflection of the segments during retraction when the inclined surfaces are flat. When the second segment  118  is retracted, for example, the shoulder portion  146  initially has a greater radial dimension that the inner lumen of the sheath  102 , such that shoulder portion  146  contacts the distal circumferential end  108  of the sheath  102 . However, the inclined surface  152  of the shoulder portion  146  can contact the distal end  108  of the sheath  102  first when the second segment  118  is retracted, which can cause the distal portion  122  of the second segment  118  to deflect radially inwardly enough to allow the shoulder portion  146  fit into and slide through the inner lumen of the sheath  102  without undue stretching or damage to the sheath. The sheath  102  can further have some degree of elasticity and flexibility to assist in this retraction process. 
         [0052]    The first and second segments  116 ,  118  of the dilator  100  can comprise any suitable material that provides sufficient rigidity and flexibility. Exemplary materials include high density polyethylene (HDPE), high molecular weight polyethylene (HMWPE), high molecular weight polyurethane (HMWPU), and/or polypropylene. In some embodiments, the various portions of the dilator can be coated with silicone and/or other materials to reduce friction and/or serve other purposes. 
         [0053]    An exemplary method of using the dilator  100  is as follows. A guidewire is initially inserted through a wall of a target blood vessel and into the vessel. The dilator  100  and sheath  102 , in the configuration shown in  FIGS. 4 and 5 , can then be threaded over the guidewire, with the guidewire passing through the guidewire lumen  115  of the dilator. As the dilator  100  and sheath  102  advance distally over the guidewire, the distal end  104  of the dilator passes through the wall of the vessel first. The gradually increasing diameter of the distal portion  112  of the dilator causes the opening in the vessel wall to gradually increase in size. As the shoulder  113  of the dilator passes through the wall of the vessel, the smooth transition between the shoulder  113  and the distal end of the sheath  102  allows the distal end of the sheath to enter through the vessel wall without tearing the opening, as shown in  FIG. 2 . The dilator  100  and sheath  102  are then advanced through the vessel as needed to dilate the vessel and/or accomplish additional procedures. 
         [0054]    Once vessel dilation and/or any other procedures are completed with the dilator  100 , the dilator can be retracted proximally out of the vessel through the sheath  102 . The first segment  116  can be advanced distally relative to the second segment  118  and the sheath  102  within the vessel. Such advancement of the first segment can be accomplished with the sliding engagement of the rails  134  within the complimentary interlocking grooves  130 , for example. The distal advancement of the first segment  116  can be sufficient to dis-engage the pin  140  from the corresponding hole in the second segment  118  and sufficient to position the distal portion  122  of the second segment adjacent to the recess  120  in the first segment  116  such that the distal portion  122  can deflect into the recess  120 . 
         [0055]    From this position, the second segment  118  can then be retracted proximally relative to first segment  116  and the sheath  102 . Such retraction of the second segment  118  causes the shoulder portion  146  of the second segment  118  to contact the distal end  108  of the sheath  102  and thereby deflect the distal portion  122  of the second segment radially inwardly, with the assistance of the inclined surface  152 . Once the distal portion  122  of the second segment  118  is deflected sufficiently inward, the shoulder portion  146  can enter the inner lumen of the sheath  102  allowing the second segment  118  to slide proximally along the guidewire through the sheath alongside of the proximal shaft portion  132  of the first segment  116 , and eventually out of the vessel and/or out of the proximal end of the sheath. To provide more room within the sheath  102  for the distal portion  122  of the second segment  118  to slide alongside the shaft portion  132  of the first segment  116 , the shaft portion  132  of the first segment can be minimized in cross-sectional profile, such as at least smaller than the shaft portion  136  of the second segment  118 , as is shown in  FIG. 5 . 
         [0056]    With the second segment  118  thus removed, the first segment  116  can then be retracted proximally relative to the sheath  102 . Such retraction of the first segment  116  causes the shoulder portion  144  of the first segment  116  to contact the distal end  108  of the sheath  102  and thereby deflect the distal portion  131  of the first segment radially inwardly, with the assistance of the inclined surface  150 . For example, the distal portion  131  of the first segment can bend primarily at a necked region  154  ( FIG. 6 ) having a reduced cross-sectional area at the proximal end of the recess  120  adjacent to the shaft portion  132 . Once the distal portion  131  of the first segment  116  is deflected sufficiently inward, the broad shoulder portion  144  can enter the inner lumen of the sheath  102 , allowing the first segment  116  to slide proximally along the guidewire through the sheath and out of the vessel and/or out of the proximal end of the sheath. 
         [0057]    Once the entire dilator  100  is retracted out of the vessel and/or out of the sheath  102 , the sheath can be retracted out of the vessel and the opening in the vessel wall can be repaired. 
         [0058]      FIGS. 10 and 11  show another exemplary segmented dilator  200  having three independently retractable segments that functions similarly to the dilator  100 . The introducer sheath is not shown in these figures for clarity. A first segment  202  includes a distal nosecone  208  and defines the distal end of the dilator  200 . The second segment  204  and third segment  206  can be similar in structure to each other, each terminating at a distal end proximal to the nosecone  208 . Each of the three segments can extend less than 180° circumferentially and all three segments can extend proximally to a hub having actuators for moving the three segments longitudinally independently. 
         [0059]    The outer surface of dilator  200  can include an external shoulder  210  extending up to 360° circumferentially, with each of the three segments forming a portion of the shoulder. The dilator  200  includes a tapered distal portion  212  that includes the shoulder  210 , and a generally cylindrical shaft portion  214  proximal to the shoulder  210 . Each of the three segments  202 ,  204 ,  206  can comprise less than 180° circumferentially of the entire dilator  200 , such as each segment comprising about 120°, or the first segment comprising between 120° and 180° with the second and third segments comprising between 90° and 120°, or other proportions and configurations. 
         [0060]    Though not shown, the three segments  202 ,  204 ,  206  can be engaged with each other along the shaft portions  214  with engagement features that allow for at least some degree of longitudinal movement relative to one another, but restrict non-longitudinal movement relative to one another. In some embodiments, dovetail-type engagement features, like those shown with respect to the dilator  100 , can be employed between each mating pair of surfaces in the shaft portion  214  (e.g., three sets of mating surfaces). Other types of suitable engagement mechanisms can also be used to allow for relative longitudinal sliding of the three segments but prevent non-longitudinal separation. 
         [0061]    In addition, or alternatively, the distal tips of the second and/or third segments  204 ,  206  can be engaged with the proximal surface of the nosecone  208  with pin type engagements, similar to the pin  140  ( FIG. 6 ) of the dilator  100 , that restrict non-longitudinal motion of the distal ends of the second and third segments when engaged. 
         [0062]    The three segments of the dilator  200  can form an enclosed internal recess  220  that extends from the nosecone  208  proximally into the shaft portion  214 , as shown in  FIG. 11 . The recess  220  can be defined by all three of the segments in part. The recess  220  can form part of the guidewire lumen  210  and also allow for inward deflection of the distal ends of the segments during retraction, like with the recess  120  in the dilator  100 . As shown in  FIG. 11 , the recess  220  can have the general shape of an hourglass, with expanded distal and proximal portions  222 ,  224  and a necked intermediate portion  226 . 
         [0063]    The first, second, and third segments  202 ,  204 ,  206  can have relatively thin portions  232 ,  234 ,  236  just proximal to the shoulder  210  at a longitudinal location even with the proximal portion  224  of the recess  220 . The thin portions  232 ,  234 ,  236  allow the distal portion of each segment to flex radially inwardly during retraction through an introducer sheath. Like with the dilator  100 , the shoulder  210  of the dilator  200  can include an inclined proximal surface to help cause radially inward deflection when each segment is retracted through the sheath. 
         [0064]    The first, second, and third segments  202 ,  204 ,  206  of the dilator  200  can comprise any suitable material that provides sufficient rigidity and flexibility. Exemplary materials include HDPE, HMWPE, HMWPU, and polypropylene. In some embodiments, the various portions of the dilator  200  can be coated with silicone or other materials to reduce friction and/or serve other purposes. 
         [0065]    An exemplary method of using the dilator  200  is as follows. A guidewire is initially inserted through a wall of a target blood vessel and into the vessel. The dilator  200 , in the configuration shown in  FIG. 10 , with an introducer sheath covering the shaft  214  and abutting the shoulder  210 , can then be threaded over the guidewire, with the guidewire passing through the guidewire lumen  210  of the dilator. As the dilator  200  and sheath advance distally over the guidewire, the distal end of the nosecone  208  passes through the wall of the vessel first. The gradually increasing diameter of the distal portion  212  of the dilator causes the opening in the vessel wall to gradually increase in size. As the shoulder  210  of the dilator passes through the wall of the vessel, the smooth transition between the shoulder  210  and the distal end of the sheath allows the distal end of the sheath to enter through the vessel wall without tearing the opening, as shown in  FIG. 2 . The dilator  200  and sheath are then advanced through the vessel as needed to dilate the vessel and/or accomplish additional procedures. 
         [0066]    Once vessel dilation and/or any other procedures are completed with the dilator  200 , the dilator can be retracted proximally out of the vessel through the sheath. The first and second segments  202 ,  204  can be initially advanced distally (in unison, or one at a time) relative to the third segment  206  and the sheath. Such advancement of the first and second segments can be accomplished with the sliding engagement of the rails within the grooves along the shaft portion  214 , for example. The distal advancement of the first and second segments  202 ,  204  can be sufficient to dis-engage a pin-and-hole engagement between the distal end of the third segment  206  and the proximal side of the nosecone  208 . The longitudinal advancement of the first and second segments  202 ,  204  can be sufficient to position the distal portion  246  of the third segment  206  adjacent to the distal portion  222  of the recess  220  and to position the shoulder portion  256  of the third segment  206  adjacent to the proximal portion  224  of the recess  220 . 
         [0067]    This small distal advancement of the first and second segments  202 ,  204  allows the distal portion  246  of the third segment  206  to deflect into the distal portion  222  of the recess  220  and allows the shoulder portion  256  of the third segment  206  to deflect into the proximal portion  224  of the recess during subsequent retraction of the third segment  206  relative to the sheath. During retraction of the third segment  206 , the shoulder portion  256  contacts the distal end of the sheath, causing inward deflection sufficient to allow the shoulder portion  256  to enter the lumen of the sheath, with the assistance of the inclined proximal surface of the shoulder, and such that the entire third segment  206  can be retracted out through the sheath. To provide more room within the sheath for the shoulder portion  256  of the third segment  206  to slide alongside the shaft portions  262 ,  264  of the first and second segments  202 ,  204 , the shaft portions  262 ,  264  of the first and second segments can be minimized in cross-sectional profile, such that they are at least smaller than the shaft portion  266  of the third segment  206 . Similarly, the second shaft portion  264  can be smaller than the first shaft portion  262  to allow more room for the shoulder portion  254  of the second segment within the sheath. 
         [0068]    With the third segment  206  thus removed, the second segment  204  can then be retracted proximally relative to the first segment  202 . Such retraction of the second segment  204  can be sufficient to disengage the any pin-and-hole type engagement between the distal end of the second segment and the proximal side of the nosecone  208 . As the shoulder portion  254  of the second segment  204  contacts the distal end of the sheath, the distal portion of the second segment deflects inwardly into the recess  220 , with the distal end  214  deflecting into the distal portion  222  of the recess and the shoulder region  254  deflecting into the proximal portion  224  of the recess. Once the distal portion of the second segment  204  is deflected sufficiently inward, the broad shoulder portion  254  can enter the inner lumen of the sheath, allowing the second segment  204  to slide proximally along the guidewire through the sheath alongside the shaft portion  262  of the first segment  202 . 
         [0069]    With the first and second segments  204 ,  206  thus removed, the first segment  202  can be retracted proximally through the sheath, with the distal portion of the first segment deflecting inwardly as needed as the shoulder portion  252  contacts the distal end of the sheath. Once the entire dilator  200  is retracted through the sheath, the sheath can be retracted out of the vessel and the opening in the vessel wall can be repaired. 
         [0070]      FIGS. 12 and 13  show another exemplary dilator  300  comprising a rigid main body  302 , adjustable shoulder portions  304 , and mandrels  306  for adjusting the shoulder portions  304 . The main body includes a distal portion  308  and a proximal shaft portion  310 . An introducer sheath is not shown in these figures for clarity, though an introducer sheath would be positioned around the shaft portion  310  and abutting the shoulder portions  304 . A guidewire lumen  312  extends through the length of the main body  302 . The dilator  300  can comprise two, three, four, or more shoulder portions  304 , with a corresponding number of mandrels  306 . 
         [0071]    In the embodiment shown in  FIG. 12 , the two shoulder portions  304  are located on opposite sides of the dilator and extend about less than the entire circumference of the dilator. Each shoulder portion can extend circumferentially less than 180°, less than 150°, and/or less than 120°, but have sufficient circumferential size to provide a smooth transition between the dilator and the sheath. 
         [0072]    Between the shoulder portions  304 , the main body  302  of the dilator can have a smooth transition  307  from the cone-like distal portion  308  to the shaft portion  310 , such that no shoulder is present at these transitions  307 . 
         [0073]    The mandrels  306  can slide longitudinally to cause the shoulder portions  304  to move between a radially collapsed position and a radially extended position. For example, an actuator at a proximal hub outside the vessel can control longitudinal motion of the mandrels  306  relative to the rest of the dilator  300 . In the radially collapsed position, the mandrels  306  are slid proximally relative to the rest of the dilator, causing the distal ends  316  of the mandrels to move out from under a flexible proximal portion  305  of the shoulder portions  304 , allowing the proximal portions  305  of the shoulder portions to collapse or bend inwardly. For example, in the radially collapsed position, the maximum radial extent of the shoulder portions  304  can be about equal to, or less than, the radius of the shaft portion  310  of the main body  302 . 
         [0074]    In the radially extended position (shown in  FIGS. 12 and 13 ), the mandrels  306  are slid distally relative to the rest of the dilator  300  such that the distal ends  316  of the mandrels  306  move under the proximal portions  305  of the shoulder portions, causing the proximal portions  305  of the shoulder portions to move or expand radially outwardly beyond the diameter of the shaft portion  310 . 
         [0075]    During insertion into a vessel, the shoulder portions  304  can be positioned in the radially extended position, such that the shoulder portions  304  extend radially beyond the diameter of the shaft portion  310 , such as about even with the outer diameter of the sheath to provide a smooth transition and minimize tearing of the vessel wall during insertion. 
         [0076]    To remove the dilator from the vessel through an introducer sheath, the mandrels  306  can be moved proximally relative to the rest of the dilator  300  to cause the proximal portions  305  of the shoulder portions  304  to move radially inwardly to the radially collapse position. With the shoulder portions  304  in the collapsed position, the dilator  300  has little or no shoulder, presenting a smooth, transition between the tapered distal portion  308  and the shaft portion  310  all the way around the dilator. This allows the dilator  300  to be retracted through the sheath. 
         [0077]    In some embodiments, the distal end of the sheath contacts the shoulder portions  304  and forces them to collapse radially inwardly into voids under the proximal portions  305  of the shoulder portions formed by the absence of the distal ends  316  of the mandrels underneath. In such embodiments, moving the mandrels  306  proximally out from under the shoulder portions may not cause the shoulder portions to collapse inwardly, and instead the shoulder portions remain extending naturally radially outwardly until the distal end of the sheath exerts a radially inward force on them, causing them to deflect inwardly. 
         [0078]    In other embodiments, moving the mandrels  306  proximally directly causes the proximal portions  305  of the shoulder portions to move radially inwardly. For example, the distal ends  316  of the mandrels can be coupled to the shoulder portions  305  to pull them inward as the mandrels move proximally. In other embodiments, the proximal portions  305  of the shoulder portions  304  can be resiliently biased toward the radially collapsed position, such that moving the distal ends  316  of the mandrels out from under them causes them to resiliently flex inwardly toward their natural collapsed position. 
         [0079]    The shoulder portions  304  can comprise the same material as the main body  302 , or different material can be used. For example the shoulder portions  304  can comprise a more flexible, elastic material and the main body  302  can comprise a more rigid material. Similarly, the mandrels  306  can comprise the same or different materials as the main body  302  and the shoulder portions  304 . Exemplary materials include HDPE, HMWPE, HMWPU, and/or polypropylene. In some embodiments, the various portions of the dilator  300  can be coated with silicone and/or other materials to reduce friction and/or serve other purposes. 
         [0080]    In some embodiments, the main body  302  and the shoulder portion  304  of the dilator  300  can be of one-piece unibody construction, such that the shoulder portions  304  are contiguous extensions from the main body rather that a separate piece that is attached to the main body. In such embodiment, the main body and the shoulder portions comprise the same materials. 
         [0081]    In some embodiments, the dilator  300  can further comprise one or more spring mechanisms to deflect associated adjustable shoulder portions radially outwardly when the associated mandrels are not under the shoulder portion. The spring mechanism can comprise a pre-shaped nitinol flat wire positioned under the adjustable shoulder portion that is configured to deflect it outwardly. The wire can be resiliently deformed to a flat configuration when the shoulder portion is in the radially collapsed position. 
         [0082]      FIGS. 15-20  show another exemplary vascular dilator  500  comprising a tapered distal portion  502 , a proximal shaft portion  504 , adjustable shoulder portions  506 , and mandrels  508  for adjusting the shoulder portions  506 . The dilator  500  is configured to be introduced through a vessel or chamber wall with an introducer sheath  520 , as shown  FIG. 19 , positioned around the shaft portion  504  and abutting the shoulder portions  506 . A guidewire lumen  516  extends through the length of the dilator. The dilator  500  can comprise two, three, four, or more shoulder portions  506 , with a corresponding number of mandrels  508 , though only two of each are illustrated. 
         [0083]    In the illustrated embodiment, the two shoulder portions  506  are located on opposite sides of the distal end  502  of the dilator and extend around less than the entire circumference of the dilator. Each shoulder portion  506  can extend circumferentially less than 180°, less than 150°, less than 120°, and/or less than 90°, but have sufficient circumferential size to provide a smooth transition between the dilator and the sheath. 
         [0084]    At the outer surfaces between the shoulder portions  506 , the dilator  500  can have a smooth transition from the frustoconical distal portion  502  to the cylindrical shaft portion  504 , such that no shoulder is present at these smooth transition regions between the shoulder portions  506 . 
         [0085]    Each shoulder portion  506  can include a flexible proximal portion  510  forming the outer surface, and a biasing mechanism that biases the flexible proximal portion  510  radially outwardly. The biasing mechanism can comprise a spring element  512 , as illustrated, or other type of biasing mechanism. The spring element  512  can comprise a resilient material, such as Nitinol or other metals, and can be resiliently compressed under sufficient force to allow the proximal portion  510  of the shoulder to collapses inwardly. 
         [0086]    In some embodiments, the distal end  514  of each mandrel  508  can include a non-linear shape, such as a wavy shape as shown in  FIG. 17 , such that the distal end  514  can keep the spring element  512  spread apart when the mandrel  508  is slid distally. In this configuration, the presence of the distal end  514  between the arms of the spring element  512  prevents or restricts the ability of the spring element arms from resiliently collapsing, and thereby keeps the proximal shoulder portions  510  in the radially outward position, such as during insertion through a wall and during advancement of the dilator. 
         [0087]    The mandrels  508  can slide longitudinally to cause the shoulder portions  506  to move between a radially collapsed position and a radially extended position. For example, an actuator at a proximal hub outside the vessel can control longitudinal motion of the mandrels  508  relative to the rest of the dilator  500 . In the radially collapsed position ( FIGS. 18 and 20 ), the mandrels  508  are slid proximally relative to the rest of the dilator  500 , causing the distal ends  514  of the mandrels to move out from under a flexible proximal portion  510  of the shoulder and the spring element  512 , allowing the proximal portions  510  of the shoulder portions and the outer arms of the spring elements  512  to articulate or bend radially inwardly. When the mandrels  508  are pulled proximally, the non-linear distal end  514  can resiliently flatten out as they are pulled into or through narrow passageways in the shaft portion  504  of the dilator. In the radially collapsed position, the maximum radial extent of the shoulder portions  506  can be about equal to, or less than, the radius of the shaft portion  504  of the dilator and/or less than the inner radius of the sheath  520 . 
         [0088]    In the radially extended position ( FIGS. 15-17, and 19 ), the mandrels  508  are slid distally relative to the rest of the dilator  500 , such that the distal ends  514  of the mandrels  508  move into the spring elements  512  and under the proximal portions  510  of the shoulder portions, causing the proximal portions  510  of the shoulder portions to move to, or remain at, a radial position extending beyond the radius of the shaft portion  504  and/or beyond the inner radius of the sheath  520 , such that at least some of the distal end  522  of the sheath  520  is protected by the shoulder. During insertion into a vessel or chamber, the shoulder portions  506  can be positioned in the radially extended position, such that the shoulder portions  506  extend radially beyond the radius of the shaft portion  504 , such as about even with the outer radius of the sheath  520  to provide a smooth transition and minimize tearing of the vessel wall during insertion. 
         [0089]    To remove the dilator  500  from the vessel or chamber through an introducer sheath  520 , the mandrels  508  can be moved proximally relative to the rest of the dilator  500  to allow the proximal portions  510  of the shoulder portions  506  to move radially inwardly to the radially collapse position. In some embodiments, the natural configuration of the shoulder portions  506  is the collapsed position, such that removal of the distal ends  514  of the mandrels from beneath the shoulder portions causes the shoulder portions to resiliently collapse inward. With the shoulder portions  506  in the collapsed position, the dilator  500  has little or no shoulder, presenting a smooth, transition between the tapered distal portion  502  and the shaft portion  504  all the way around the dilator. This allows the dilator  500  to be retracted through the sheath  520  with minimal contact between the shoulder and the distal end  522  of the sheath  520 , which can reduce damage to the sheath. 
         [0090]    In some embodiments, the distal end  522  of the sheath  520  contacts the shoulder portions  506  during retraction and forces them to collapse radially inwardly into the voids under the proximal portions  510  and between the arms of the spring element  514 . In such embodiments, moving the mandrels  508  proximally out from under the shoulder portions  506  may not immediately cause the shoulder portions to collapse inwardly, and instead the shoulder portions remain extending radially outwardly, such as under the urging of the spring element  514 , until the distal end  522  of the sheath  520  exerts a sufficient radially inward force on them, causing them to deflect inwardly, as shown in  FIG. 20 . 
         [0091]    The shoulder portions  506  can comprise the same material as the rest of the distal portion  502  and the shaft portion  504 , or different material can be used. For example the shoulder portions  506  can comprise a more flexible, elastic material and the rest of the distal portion  502  can comprise a more rigid material. 
         [0092]    In some embodiments, the shoulder portions  506 , the distal portion  502 , and the shaft portion  504  of the dilator  500  can be of one-piece unibody construction, such that the shoulder portions  506  are contiguous extensions from the distal portion  502  rather that a separate piece that is attached. 
         [0093]      FIG. 14  shows a cross-section of an exemplary introducer sheath  400 , such as can be used with the dilators  100 ,  200 , and  300  described above. Various other types of introducer sheaths can also be used with the dilators  100 ,  200 , and  300 . The sheath  400  includes an internally “beveled” distal portion  402  that gradually reduces in thickness moving from a transition point  404  toward the distal end  403  of the sheath. The transition portion  404  is the longitudinal position along the inner surface of the sheath  400  where the thickness of the sheath begins to reduce and/or the inner diameter begins to increase. Moving distally from the transition point  404 , the inner surface  408  of the sheath slopes radially outwardly over a first inclined portion  412 . From the distal end of the first inclined portion  412 , a flat portion  414  begins. The inner surface  408  has a substantially constant inner diameter at the flat portion  414 . Moving distally from the distal end of the flat portion  414 , the inner surface  408  of the sheath slopes radially outwardly again over a second inclined portion  416  and terminates at the distal end  406 , at which point the sheath  400  has a minimum thickness and the inner diameter approaches the outer diameter. The outer diameter of the sheath  400  can be substantially constant along the length of outer surface  410 . 
         [0094]    In other embodiments, the inner surface  408  of the sheath  400  can increases in inner diameter at a generally constant rate from the transition portion  404  to the distal end  406 . The inner surface  408  can have other concave, convex, and/or partially linear slope profiles between the transition portion  404  and the distal end  406 . 
         [0095]    The gradual increasing of the inner diameter of the sheath  400  near the distal end  406  can help cause the shoulder of a segment of a dilator to deflect radially inward when is it retracted against the distal end of the sheath, enabling the dilator segment to slide through the inner lumen of the sheath. The inclined portions  412 ,  416  can act as a ramp to gradually cause the inward deflection of shoulder with proximal motion of the shoulder. 
         [0096]    In addition, the reduced thickness of the sheath  400  at the distal end portion  402  can provide a degree of increases flexibility and deformability of the distal end portion  402  to further help the shoulder of a dilator enter into the sheath with minimal damage to the sheath. 
         [0097]    In some embodiments, the “beveled” distal end portion of the sheath  400  can extend over only certain portions of the inner circumference of the sheath. For example, the “beveled” portions be located at discrete positions that match the circumferential positions of the shoulders of an associated dilator, such as the shoulder portions  304  of the dilator  300  or the shoulder portions  113  of the dilator  100 . The sheath  400  can have a constant thickness and/or other types of tapering at other circumferential positions not aligned with the shoulders of an associated dilator. 
         [0098]    The sheath  400  can comprise any suitable materials as is known in the art. In some embodiments, the distal end portion  402  of the sheath can comprise a different material than the rest of the sheath, such as a more pliant or more elastically deformable material that helps guide the shoulders of a dilator into the sheath and helps avoid damage to the distal end of the sheath. 
         [0099]    The dilators and sheaths disclosed herein can be used in various medical procedures other than simple access into a blood vessel through a vessel wall. For example, the disclosed devices can be used for accessing one blood vessel from another blood vessel. In one such procedure, access to the descending aorta can be made from the inferior vena cava, by passing a disclosed device out through a wall of the inferior vena cave and in through a wall in the aorta, or vice versa, where the two vessels are adjacent to each other. 
         [0100]    For another example, the disclosed devices can be used during direct trans-thoracic assess through a heart wall into the heart. In such procedures, the disclosed devices can reduce dimpling or buckling of the heart wall at the transition between the dilator and the introducer sheath, as shown in  FIG. 3 , reducing unnecessary trauma to the heart wall. The issue of dimpling or buckling can also be reduced during access into blood vessels using the disclosed devices. 
       Explanation of Terms 
       [0101]    Unless otherwise noted, technical terms are used according to conventional usage. In order to facilitate review of the various embodiments of the disclosure, the following explanation of terms is provided: 
         [0102]    The terms “distal” and “distally” refer to a location or direction that is, or a portion of a device that when implanted (for example placed within a blood vessel) is, further downstream or farther away from the point of insertion. The terms “proximal” and “proximally” refer to a location or direction that is, or a portion of a device that when implanted, or placed within the blood vessel, is further upstream or closest to the point of insertion. The term “longitudinal” refers to the axis extending in the distal and proximal directions, or to the longitudinal axis of a cylindrical body or lumen. 
         [0103]    The singular terms “a”, “an”, and “the” include plural referents unless context clearly indicates otherwise. The term “comprises” means “includes without limitation.” The term “coupled” means physically linked and does not exclude intermediate elements between the coupled elements. The term “and/or” means any one or more of the elements listed. Thus, the term “A and/or B” means “A”, “B” or “A and B.” 
         [0104]    Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present technology, only certain suitable methods and materials are described herein. In case of conflict, the present specification, including terms, will control. In addition, the materials, methods, and devices are illustrative only and not intended to be limiting. 
         [0105]    In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting the scope of the disclosure. Rather, the scope of the disclosure is defined as being at least as broad as the following exemplary claims. We therefore reserve the right to claim at least all that comes within the scope of these exemplary claims.