Patent Publication Number: US-7905856-B2

Title: Introducer sheath with retainer

Description:
This application is a continuation of U.S. application Ser. No. 10/188,403, filed on Jul. 2, 2002, now U.S. Pat. No. 6,884,235 which is a continuation-in-part of U.S. application Ser. No. 09/996,437, filed Nov. 28, 2001, now U.S. Pat. No. 6,764,464, which claims priority from provisional application Ser. No. 60/251,567, filed Dec. 6, 2000. The entire contents of each of these applications are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     This application relates to an introducer sheath and more particularly to a vascular introducer sheath having a retainer to prevent dislodgement during use. 
     2. Background of Related Art 
     An introducer sheath is commonly used in vascular surgery as an access port for surgical instruments. The introducer sheath has a central passageway to accommodate such instrumentation and is inserted through a skin incision and into the vessel wall, such as the renal or femoral artery or vein, so the instruments can access the interior of the vessel. The introducer sheath can also be inserted into dialysis grafts to provide access to the graft. The introducer sheaths have peripheral, cardiac, and neurovascular applications. 
     Once the surgical introducer sheath is placed, various instruments are inserted and withdrawn through the passageway into the vessel interior, depending on the surgical procedure. Examples of such instrumentation include dilators, angioplasty balloon catheters, stent deployment catheters, angiographic instruments, thrombectomy devices and embolization instruments. These instruments typically having an outer diameter close to the internal diameter of the introducer sheath which means they will usually abut the inside wall of the sheath. This relatively tight fit oftentimes results in excessive frictional engagement with the inside wall of the sheath, causing dislodgement of the sheath during instrument withdrawal through the passageway in the sheath. Additionally, surgical instrumentation which include an inflatable balloon, when initially inserted through the sheath have a smaller diameter because the balloon is tightly wrapped around the catheter. However, after the balloon is inflated inside the vessel and then deflated for withdrawal, it is not as tightly wrapped as initial insertion. Thus, when the balloon catheter is withdrawn through the introducer sheath, there is a greater frictional contact with the inside wall of the sheath and therefore a greater likelihood of dislodgement. 
     Dislodgement of the sheath creates numerous problems. If the position of the sheath is altered by removal of an instrument, when the next instrument is inserted, it will not properly be positioned at the surgical site. Thus the surgeon must undertake the time consuming task of repositioning the sheath and instrument within the vessel. The problems with dislodgement become more acute if withdrawal of the instrument actually pulls the introducer sheath out of the vessel wall incision altogether. This can occur if there is sufficient frictional contact with the instrument and introducer sheath, and a sufficient proximal force is applied by the surgeon. Such undesirable removal of the introducer sheath can cause loss of blood, air aspiration which can result in air embolisms possibly causing stroke, and an increased risk of infection and morbidity. Additionally, since the surgeon needs to reintroduce the introducer sheath into the vessel, the surgeon may be unable to locate the exact prior incision site, thereby having to enlarge the incision site or create a second incision, thereby causing additional blood loss and increasing the difficulty of closing the vessel incision(s) at the end of the procedure. Vessel fatigue can also result because re-introduction of the sheath requires insertion of a needle and dilator through the vessel wall. 
     Another disadvantage of complete dislodgement of the sheath is the additional time required to re-introduce the sheath. This time loss can be especially significant if re-introduction is required at a critical time of the procedure. That is, if the sheath is fully withdrawn from the vessel, access to the vessel will be temporarily denied, thereby interrupting the surgical procedure which can mean appropriate instrumentation, perhaps even life-saving instrumentation, cannot be inserted to the surgical site. 
     Due to the concern of dislodgement, surgeons sometimes over-insert the introducer sheath so the tip is spaced further from the incision. This way, if the sheath is inadvertently pulled proximally, it will have some room to move before it is pulled out fully from the incision. However, over-insertion of the introducer sheath can adversely affect surgical access as the surgical site can be blocked by the sheath, especially if the site is adjacent the incision. 
     Therefore, it would be advantageous to provide a mechanism to retain the introducer sheath within the vessel. However, such mechanism needs to be configured so as not to damage the vessel wall. Consequently, a retaining mechanism must effectively strike a balance between sufficient strength to retain the introducer sheath while providing atraumatic contact with the vessel wall. 
     The need therefore exists for an atraumatic introducer sheath which has greater retention capabilities, to thereby minimize the chances of dislodgement. By minimizing the likelihood of dislodgement, the foregoing risks to the patient would advantageously be eliminated. 
     SUMMARY 
     The present invention overcomes the disadvantages and deficiencies of the prior introducer sheaths by advantageously providing an introducer sheath having a retainer that is selectively extendable with respect to the sheath, thereby functioning to retain the sheath within the vessel. Extending the retainer radially from the sheath creates an enlarged diameter region greater than the diameter of the incision into the vessel to prevent withdrawal of the sheath through the incision, and in smaller vessels, enabling the retainer to frictionally engage the vessel wall to restrict sliding movement of the sheath. 
     More specifically, the present invention provides a surgical vascular introducer sheath comprising a first member having a first longitudinally extending lumen configured and dimensioned to receive a surgical instrument therethrough, a second member having a second longitudinally extending lumen configured and dimensioned to receive the first member, and a retainer adjacent a distal portion of the second member movable from a first retracted position to a second extended position, in response to movement of the second member, to limit proximal movement of the introducer sheath. Preferably, rotational movement of the second member in a first direction moves the retainer to the extended position and rotational movement in a second direction moves the retainer to the retracted position. The second member is preferably substantially fixed longitudinally during rotation. 
     Preferably, the retainer comprises a flap having a curved configuration in the extended position and an opening to allow blood flow therethrough. Preferably a first portion of the flap extends from the second member and a second portion of the flap is attached to the first member. 
     The first and second members may each have a plurality of side holes for blood flow for dialysis wherein the side holes are out of alignment when the retainer is in the retracted position and the side holes are moved into alignment when the second member is rotated to move the retainer into the extended position. 
     The introducer sheath may further comprise a locking mechanism for maintaining the retainer in the extended position. The locking mechanism may comprise a locking pin slidable within a locking groove having a narrowed section to retain the pin. Preferably, a proximal portion of the first and second members are positioned within a housing with the locking groove positioned on the housing and the locking pin extending from the second member. The housing may further have an internal keyway slot to receive a key extending from the first member to prevent rotation of the first member. The locking mechanism may alternately comprise a post slidable within a locking groove having a radial region and first and second axial regions. 
     A radiopaque insert can be provided at a distal end of the sheath, preferably blended with a distal portion of the second member during formation of a tip of the sheath. Preferably, the radiopaque insert extends to the distalmost tip of the second member, and after formation has an inner diameter substantially equal to an inner diameter of the first member. 
     The present invention also provides a surgical sheath for providing a passageway for instrumentation into a vessel comprising a tubular member having a lumen extending longitudinally therein dimensioned and configured for receiving surgical instruments therethrough and means spaced proximally from the distalmost tip of the tubular member for limiting proximal movement of the sheath with respect to the vessel. The limiting means is movable from a first position to a second position extending laterally with respect to the tubular member and enabling blood flow therethrough in the second position. 
     The tubular member preferably includes inner and outer tubular members and the limiting means preferably comprises a flap movable to the second position in response to rotational movement of the outer tube. In an alternate embodiment, the limiting means comprises first and second flaps movable to the extended position in response to rotational movement of the outer tubular member. 
     The present invention also provides a surgical sheath for providing a passageway for instrumentation into a vessel comprising an outer tubular member having a first side hole in a sidewall and an inner tubular member disposed within the outer tubular member and having a passageway for receiving surgical instruments therethrough and having a second hole in a sidewall. A retainer at a distal portion of the outer tubular member is movable from a non-blocking position substantially flush with the outer tubular member to a blocking position extending radially outwardly from the outer tubular member. The first and second side holes are moved into alignment when the retainer is moved to its blocking position. 
     A method for retaining a surgical introducer sheath is also provided comprising: 
     inserting an introducer sheath having inner and outer tubular members and a retainer into body tissue; 
     rotating the outer tubular member of the introducer sheath to move the retainer from a retracted position to an extended position so the retainer extends radially outwardly, 
     introducing a surgical instrument into the introducer sheath; 
     performing a surgical step with the surgical instrument; 
     withdrawing the surgical instrument, the retainer limiting proximal movement of the introducer sheath during withdrawal of the surgical instrument; and 
     rotating the outer tubular member of the introducer sheath to return the retainer to its retracted position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiment(s) of the present disclosure are described herein with reference to the drawings wherein: 
         FIG. 1  is a perspective view of a first embodiment of the introducer sheath of the present invention with the retainer in the retracted position; 
         FIG. 2  is a perspective view of the introducer sheath of  FIG. 1  with the retainer in the extended position; 
         FIG. 3  is a longitudinal cross-sectional view of the introducer sheath of  FIG. 2  with the retainer in the extended position; 
         FIG. 4  is a transverse cross-sectional view taken along lines  4 - 4  of  FIG. 3 ; 
         FIG. 5  is a transverse cross-sectional view similar to  FIG. 4  except showing the retainer in the retracted position; 
         FIG. 6  is a perspective view of the distal portion of a second embodiment of the introducer sheath of the present invention having side holes for dialysis and showing the retainer in the retracted position; 
         FIG. 7  is a perspective view of the distal portion of the introducer sheath of  FIG. 6  showing the retainer in the extended position; 
         FIG. 8  is an exploded view of the introducer sheath of  FIG. 6 ; 
         FIG. 8A  is a transverse cross-sectional view showing the orientation of the side port and keyway of the housing; 
         FIG. 8B  is a transverse cross-sectional view showing the dialysis holes of the inner and outer tubular members out of alignment prior to deployment of the retainer; 
         FIG. 9  is a perspective view of the introducer sheath of  FIG. 6  showing the components in phantom inside the housing; 
         FIG. 10  is a perspective view of the introducer sheath of  FIG. 6  showing the tubing extending from the side port for either blood withdrawal or blood return; 
         FIG. 10A  is an enlarged view of the locking pin and groove of  FIG. 10 ; 
         FIG. 11  is a perspective view of the distal portion of an alternate embodiment of the introducer sheath having an angled tip to facilitate insertion; 
         FIG. 12  is a side view of the introducer sheath of  FIG. 11 ; 
         FIG. 13  illustrates the introducer sheath of  FIG. 6  inside a vessel with the retainer in the extended position to prevent withdrawal of the introducer sheath through the incision; 
         FIG. 14  illustrates the distal portion of the introducer sheath of  FIG. 6  inserted into a small vessel wherein the retainer, in its extended position, frictionally engages the vessel wall to limit proximal movement of the introducer sheath; 
         FIG. 15  illustrates a catheter being withdrawn from the introducer sheath of  FIG. 6 , the retainer engaging the vessel wall to limit proximal movement; 
         FIG. 16  is a side perspective view of a third embodiment of the introducer sheath of the present invention; 
         FIG. 17  is an exploded view of the introducer sheath of  FIG. 16 ; 
         FIG. 18A  is a cross-sectional view taken along lines  18 A- 18 A of  FIG. 16 ; 
         FIG. 18B  is a cross-sectional view of the valve of  FIG. 17 ; 
         FIG. 18C  is a cross-sectional view similar to  FIG. 18A  illustrating an alternate embodiment of the radiopaque marker of the introducer sheath of the present invention; 
         FIG. 18D  illustrates the outer tube of the introducer sheath and radiopaque insert prior to formation of the tip of  FIG. 18C ; 
         FIG. 19  is an enlarged perspective view of a portion of the introducer sheath of  FIG. 16  showing the ball lock engaged in the slot; and 
         FIG. 20  is a perspective view of a fourth alternate embodiment of the introducer sheath of the present invention having two retainers; 
         FIG. 21  is a perspective view of a distal portion of a fifth alternate embodiment of the introducer sheath of the present invention having two overlapping flaps; and 
         FIG. 22  is a front view of the introducer sheath of  FIG. 21  showing the overlapping flaps in the extended position. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Introducer sheaths are commonly used in vascular surgery to provide a passageway for instrumentation. The introducer sheath is inserted into a vessel or a graft (e.g. a dialysis graft) and a variety of instruments for performing the specific surgical procedure are introduced therethrough to access the surgical site. Since the instruments are repeatedly inserted and withdrawn during the surgical procedure, and have a diameter closely matching the internal diameter of the sheath, the introducer sheath has the tendency to be dislodged, and perhaps even withdrawn from the vessel incision as discussed above. Additionally, certain instruments, such as balloon catheters, may have a larger diameter during withdrawal then insertion, thereby increasing the chances of dislodgment. The introducer sheaths of the present invention advantageously have a retainer for limiting proximal movement and preventing full dislodgement of the introducer sheath during surgery. That is, the retainer is extendable from the sheath to create an enlarged diameter or circumferential portion exceeding the size of the vessel incision and in smaller vessels, frictionally engaging the vessel wall. 
     Referring now in detail to the drawings where like reference numerals identify similar or like components throughout the several views,  FIGS. 1-5  illustrate a first embodiment of the introducer sheath of the present invention, designated generally by reference numeral  10 . 
     The introducer sheath  10  has a proximal portion  12 , a distal portion  14 , an outer tubular member  20  and an inner tubular member  30  disposed concentrically within the outer tubular member  20 . A retainer  40 , in the form of a curved or U-shaped flap, extends from outer tube  20  and is positioned proximally of the distalmost tip. Retainer  40  is movable from a retracted position where it is substantially flush with the outer surface  24  of outer tube  20  as shown in  FIG. 1 , to an extended (blocking) position where it extends radially outwardly from the outer tube  20  as shown in  FIG. 2 . This radial movement increases the overall circumference or diameter of the outer tube  20 , thereby causing the sheath  10  to engage the vessel wall or wall surrounding the incision in the manner described below. 
     Outer tube  20 , preferably circular in cross section as shown, (although oval or other shapes can be utilized) has a distal portion  21 , a proximal portion  23  and a central longitudinal lumen  22  dimensioned and configured to receive inner tube  30 . A cutout in the outer tube  20  forms flap  40 , integrally extending therefrom at edge  42  and which is attached at edge  43  to outer surface  34  of inner tube  30 . Distal nose  28  of tube  20  is slightly tapered to facilitate insertion through the incision and vessel. 
     Inner tube  30  is also preferably circular in cross section (although oval or other shapes can be utilized) and has a distal portion  31  and a proximal portion  33 . Central lumen  32  extends longitudinally along the entire length of the inner tube  30  and is configured and dimensioned to receive surgical instruments therethrough, as described below. 
     Retainer or flap  40  is deployed by rotation of outer tube  20 . The surgeon grasps the knurled surface  29  of proximal portion  23  and rotates the outer tube  20  in the direction of the arrow. Such rotation causes flap  40  to move to the extended (deployed) position since flap  40  is attached to inner tube  30  (at edge  43 ) which remains stationary. Flap  40  is preferably curved and U-shaped as shown to form an opening  48  to allow blood flow therethrough and extend longitudinally alongside outer tube  60  (“L”). The smooth shaped surface provides atraumatic contact with the vessel wall. 
       FIGS. 6-10  illustrate an alternate embodiment of the introducer sheath of the present invention. Introducer sheath, designated generally by reference numeral  50  is similar to introducer sheath  10  of  FIG. 1  in that it has an outer tube  60 , an inner tube  70  and a flap  80  extending from outer tube  60  and attached at edge  82  to inner tube  70 . Inner tube  70  has a longitudinal lumen  72  for passage of surgical instruments and outer tube  60  has a longitudinal lumen  62  to receive inner tube  70 . As in the first embodiment, flap  80 , in its retracted position is substantially flush with outer tube  60  and in its extended (blocking) position is curved and in a U-shaped configuration to form opening  83  for blood passage. Flap  80  is preferably integrally formed with outer tube  60 , positioned proximally of the distalmost tip of the outer tube, and as shown extends longitudinally alongside a portion of the outer tube  60  as represented by letter “L”. 
     Introducer sheath  50  further includes a retainer locking element to maintain the retainer or flap  80  in the extended position. Turning now to  FIG. 8 , outer tube  60  has a locking pin  85  extending from enlarged head  86  which is received within transverse locking groove  90  of housing  92 . As shown in  FIGS. 10 and 10A , locking groove  90  is slightly arcuate and has two lobes  94 ,  96  at opposite ends and adjacent narrowed regions  93 ,  95 . When the retainer is in the retracted position of  FIG. 6 , locking pin  85  is seated within lobe  96  and is prevented from movement within the groove  90  by narrowed region  95 . To move the retainer to the extended position to limit movement of the introducer sheath  50 , locking pin  85  is grasped by the user and moved within groove  90 . By supplying sufficient force, locking pin  85  is forced through narrowed region  95 , slightly stretching the flexible material around the groove. Movement of locking pin  85  rotates the outer tube  60 , while the inner tube  70  remains stationary, thereby moving retainer  80  to its extended position. This pin and groove arrangement also keeps outer tube  60  fixed longitudinally during rotation. 
     To lock the retainer  80  in its extended position, locking pin  85  is forced through narrowed region  93  into lobe  94 . After being slightly stretched by passage of locking pin  85 , narrowed region  93  returns to its original configuration to block exit of locking pin  85 . Thus, locking pin  85  is prevented from sliding within locking groove  90 , thereby blocking outer tube  60  from rotation and maintaining retainer  80  in the blocking (extended) position. 
     When it is desired to move retainer  60  back to its retracted position, locking pin  85  is forced through narrowed regions  93  and  95 , by movement in the reverse direction, and returned to lobe  96 . During movement of the outer tube  60  in either direction, inner tube  70  remains stationary due to the engagement of key  75  of enlarged head  77  with keyway slot  91  of housing  92 , shown in phantom in  FIG. 8 . Outer tube  60  is prevented from sliding longitudinally by groove  90 . As an alternative to the keyway, an adhesive or any energy welding system, e.g. radiofrequency, ultrasonic, etc., can be utilized to keep the inner tube  70  stationary. 
     Visual indicators could optionally be provided at the lobes to indicate to the user the position of the retainer. For example, an “R” for retracted can be provided adjacent lobe  96  on the housing  92  and an “E” for extended can be provided adjacent lobe  94 . 
     With continued reference to  FIG. 8 , housing  92  has assembly slot  99  communicating with groove  90  for ease of assembly. That is, for assembly, locking pin  95  is slid through slot  99  into groove  90  and locking cap  100  is placed over distal portion  98  of housing  92  effectively closing slot  99  to lock pin  85  within groove  90 . A valve  102  is positioned within housing  92  to prevent outflow of blood through proximal opening  107  of housing  92 . If sheath  50  is used for dialysis as explained below, the valve is preferably a silicone valve to accommodate additional pressure from the vacuum for blood withdrawal. A donut like element  104 , preferably composed of foam and having central opening  105 , is positioned between valve  102  and proximal wall  97  of housing  92 . Donut  104  is preferably laced with a lubricant such as silicone to promote lubricity during insertion of surgical instruments and to prevent valve damage due to friction. 
     Optionally, the introducer sheath can include a plurality of holes for either blood withdrawal or blood return so the introducer sheath can remain in the body for dialysis. As shown in  FIG. 8 , outer tube has side openings or holes  61  formed through its outer wall  64  and inner tube  70  has side openings or holes  71  formed through outer wall  74 . These holes  61 ,  71 , when aligned, allow for passage of blood through lumen  72 , out through side aperture  79  in enlarged head portion  77 , and exiting through side port  101  in housing  92 . Conventional tubing  110 , as shown in  FIG. 10 , is connected to side port  101 . Tubing  110  includes conventional tube clamp  112  and luer fitting  114  which do not form part of this invention and are therefore not further described. If used for dialysis, two introducer sheaths  50  would be provided: one sheath  50  for withdrawal of blood from the vessel for passage to the dialysis machine and a second sheath  50  for return of blood from the dialysis machine to the vessel. Alternatively, if used for dialysis, introducer sheath  50  could be used for blood withdrawal or delivery, and another instrument, such as dialysis needle could be used for opposite blood flow. Also, although three holes are shown, it should be appreciated that various spacings and fewer or greater number of holes could be provided for dialysis or for other procedures. 
     It should be appreciated that it is also contemplated that the sheath need not be provided with any side holes if dialysis or blood flow for other surgical applications is not intended. 
       FIG. 8B  illustrates the interaction of the side holes  61  and  71  of the outer and inner tubes  60 ,  70  respectively. When the retainer  80  is in the retracted position, holes  61  and  71  are out of alignment as shown, thereby preventing blood flow through central lumen  72 . However, when outer tube  60  is rotated to extend retainer  80  to the blocking (extended) position, side holes  61  are rotated into alignment with side holes  71 . Thus when outer tube  60  is locked in the rotated position with locking pin  85  retained in lobe  94 , holes  61  and  71  are in alignment and blood can pass through these holes into central lumen  72 . 
     Proximal opening  107  in housing  92  allows for passage of a guidewire and surgical instruments, the guidewire and surgical instruments passing through opening  102  in donut  104 , and through valve  102  and opening  106  in cap  100  into central lumen  72 . 
       FIGS. 11 and 12  illustrate an alternate embodiment of the introducer sheath having an angled or beveled end to facilitate insertion. Introducer sheath  120 , as shown, has a tip  112  at an angle greater than 90 degrees so that edge  124  will penetrate tissue before edge  126 , thereby reducing the penetration force. Such angled tip can be provided on any of the foregoing introducer sheaths. 
       FIGS. 16-19  illustrate a third embodiment of the locking sheath of the present invention, designated generally by reference numeral  150 . Locking sheath  150  is similar to the foregoing locking sheaths in that it has a retainer  160  in the form of a U-shaped flap that is movable between a retracted substantially flush position to a radially extended position with respect to the outer tube  162 . Locking sheath  150  differs in the locking structure for the retainer  160  and some of the assembly components. 
     More specifically, and with reference to  FIG. 17 , inner tube  170  extends integrally from housing  172 . Side port  179  for mounting conventional tubing as described above (not shown) is shown angled at about 45-degrees to reduce mechanical hemolysis. Inner tube  170  is preferably composed of a dark material, achieved for example by adding carbon black or other particles or by inks or pigments, to absorb laser wavelengths to create heat to laser weld the retainer  160  to the inner tube  170  (see  FIG. 18 ). Other methods of attachment are also contemplated. 
     Seated inside housing  172  is slit valve  174  and end cap  176 . Slit valve  174  is press fit within a tapered inner surface of housing  172  and end cap  176  is fitted with recess  175  of housing  172 . The end cap  176  preferably has a chamfer to direct instruments inserted therethrough towards the center. Gasket  178  is frictionally seated over inner tube  170  to provide a seal between the inner tube  170  and outer tube  162 . 
       FIG. 18B  shows a preferred embodiment of the slit valve which can be utilized in the embodiments of  FIGS. 8 and 17 . For clarity, the valve is designated generally by reference numeral  300 , it being understood that it can correspond to valve  102  or  174 . 
     Valve  300  has a transverse slit  302  extending through a central portion of the valve from a top surface  304  to a bottom surface  306 . As shown, the slit  302  is formed so the walls  302   a ,  302   b  extend slightly outwardly towards the bottom surface  306  as shown. A small hole on top surface  304  provides a lead in to the slit  302 . The top surface  304  of the valve  300  is slightly conical to direct instruments toward the center of the valve. Preferably, this slight conical surface is at an angle of about 80 degrees, such that the height h 1  of the valve  300  at the outer edge is preferably about 0.120 inches and the height h 2  at the center adjacent the hole is preferably about 0.098 inches. 
     Outer tube  162  has a lumen dimensioned to receive the inner tube  170 . Retainer (flap)  160 , extending from outer tube  162  is welded to inner tube  170 . Post  182 , terminating in ball  184 , extends from enlarged cylindrical base  186  and functions to lock the retainer  160  in the retracted position and in the extended position. More specifically, and with reference to  FIG. 17  and  FIG. 19 , a slot  192  is formed in front cap  190 . Front cap  190  is mounted to housing  172  via a U-shaped groove  191  and corresponding tongue arrangement on housing  172 . Slot  192  extends radially along the surface of cap  190  and has two axially extending regions  194 ,  196  at its ends. When post  182  is in region  194 , the retainer  160  is in its retracted position. To rotate the outer tube  162  to move the retainer  160  to the extended position, the user pulls post  182  rearwardly (in the direction of the arrow) along first axis region  194 , into radial region  195 , and moves the post  182  along radial region  195  into second axial region  196  where the outer tube  162  is secured against rotation and the retainer  160  is maintained in the extended position. Note that post  182  in its normal position is seated within the first or second axial regions  194 ,  196  and needs to be flexed proximally to release it and guide it through radial region  195 . Engagement within axial regions  194 ,  196  provides a tactile feel to the user. The locking sheath  150  operates in the manner described above and illustrated in  FIGS. 13-15 . 
       FIG. 20  illustrates an alternate embodiment wherein locking sheath  130  is provided with two retainers  132 , spaced apart as shown. Locking sheath  130  is substantially identical to the locking sheath of  FIG. 6 , in all other respects. Each retainer  132  is attached to inner tube  136  at an edge and is identical to retainer  80  of  FIG. 6 . The retainers  132  are shown in their retracted position, and are deployed simultaneously to their U-shaped configurations upon rotation of the outer tube  134  as described above with respect to the embodiment of  FIGS. 6-10 . 
       FIGS. 21 and 22  illustrate another alternate embodiment of the locking sheath designated by reference numeral  200  (Only the distal portion is shown). Locking sheath  200  has a retainer (flap)  202  extending from the outer tube  204  and a retainer (flap)  206  extending from the inner tube. Retainer  202  is attached, e.g. welded, to retainer  206  so that upon rotation of outer tube  204 , retainer  202  and retainer  206  are moved to the deployed position. The overlapping retainers  202 ,  206  increase the material strength of the flap. 
     The tips or any regions of any of the foregoing introducer sheaths can have radiopaque markers to provide visual indication of the sheath tip location. The markers can take a variety of forms such as a circular marker band wrapped around the outer tube or a radiopaque material attached (e.g. welded) or otherwise applied onto the tip or along other regions of the sheath.  FIG. 18A  shows a marker band  198  formed in the distal tip, composed by way of example, from black tungsten and placed during formation of the tapered tip. 
       FIG. 18C  shows an alternate embodiment of the radiopaque marker insert, extending to the distalmost tip of the sheath  190 ′. Ring shaped insert  198 ′, preferably composed of black tungsten, is placed at the distal end of the outer tube  190 ′. When heated, the insert is blended into the outer tube  190 ′. The inner diameter D is formed so that it is the same as the inner diameter of the inner tube (not shown) to provide a smooth transition for the insertion of instrumentation through the sheath. A small portion of outer tube  190 ′, designated by reference numeral  190   a , typically does not blend with the insert  198 ′. 
     The introducer sheath of the present invention can be made of various dimensions. In a preferred embodiment, the sheath is about 6 French or about 7 French in outer diameter and has a wall thickness of about 0.014 inches (the inner and outer sheath each having a wall thickness of about 0.007 inches). 
     The interior of the inner tubes of any of the foregoing embodiments can have a hydrophilic coating to facilitate instrument insertion through its lumen by reducing frictional contact. The outer surface of the outer tube could also be provided with a hydrophilic coating to reduce frictional contact with the skin and vessel during insertion. To help keep thrombus from forming on the device, an anti-thrombolytic coating can also be provided. 
       FIGS. 13-15  illustrate the introducer sheath of the present invention in use. The introducer sheath  50  of  FIGS. 6-10  is illustrated and described (with the tubing removed for clarity), it being understood however, that any of the aforedescribed introducer sheaths would be utilized in a similar manner. 
       FIG. 13  illustrates introducer sheath  50  positioned inside a vessel “v”, such as a common femoral artery. Sheath  50  is inserted through incision “i” in the vessel wall to gain access to the interior of the vessel. Once positioned as shown, the outer tube  60  is rotated to deploy retainer  80  to an extended position as shown. In this extended position, the sheath  50  cannot fit through the incision “i”. Consequently when surgical instruments such as a balloon catheter shown in  FIG. 15  are inserted and then withdrawn, the retainer  80  will contact the vessel wall around the incision, preventing undesirable withdrawal of the introducer sheath  50  through the incision. Being retained or “locked” inside the vessel, various instruments can be inserted and withdrawn through the sheath lumen  62  without the introducer sheath  50  becoming dislodged and causing the problems associated with such dislodgement discussed above. The pin and groove locking arrangement maintains the retainer  80  in the extended (blocking) position as desired. 
     When the introducer sheath  50  is ready to be removed from the vessel “v”, the outer tube  60  is rotated in the reverse direction, disengaging the pin and groove locking arrangement, to thereby return the retainer to its retracted position substantially flush with the outer surface of the outer tube  60 . Thus, the introducer sheath  50  can be withdrawn through incision “i”. 
     When used in smaller vessels, not only will the retainer  80  prevent full withdrawal from the incision, but it will contact the vessel wall “u” downstream of the incision as shown in  FIG. 14 . This contact results in frictional engagement with the wall, thereby restricting unwanted longitudinal movement of the introducer sheath  50  during withdrawal of surgical instruments, such as the balloon catheter of  FIG. 15 . The curved surface  81  of flap  80  provides atraumatic contact with the vessel wall. 
     The introducer sheaths of the present invention can also be utilized in other minimally invasive catheter procedures, including non-vascular procedures such as genitourinary, biliary, and gastrointestinal procedures which require instrument insertions and withdrawals through introducer sheaths. 
     While the above description contains many specifics, those specifics should not be construed as limitations on the scope of the disclosure, but merely as exemplifications of preferred embodiments thereof. For example, any of the sheath embodiments can optionally be provided with holes for dialysis. Also, if blood flow therethrough is not required, the retainers need not be provided with an opening for blood flow. Although the introducer sheath is preferably composed of Pebax material, other materials such as urethane, nylon, polyethyelene, or polypropylene, or composites with braided components, can be utilized. The sheaths could also be slightly curved or bendable/shapeable. Those skilled in the art will envision many other possible variations within the scope and spirit of the disclosure as defined by the claims appended hereto.