Patent Publication Number: US-7901379-B2

Title: Passive hemostatic sheath valve

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 37 C.F.R. §1.78 and is a continuation-in-part of U.S. patent application Ser. No. 10/636,871, filed on Aug. 6, 2003, the full disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to medical devices and methods. More particularly, the present invention relates to vascular introducer sheaths incorporating improved passive valves that can maintain hemostasis around medical instruments. 
     Vascular introducer sheaths are well known components of vascular access systems that are used in a wide variety of diagnostic and therapeutic vascular procedures. For example, introducer sheaths are used in a number of minimally invasive procedures such as endovascular procedures and laparoscopic surgery. Of specific interest relative to the present invention is the endovascular placement of grafts, stents, stent-grafts, and other endoluminal prostheses. Such procedures are often performed through vascular access systems that include an introducer sheath to provide access to a body lumen (e.g., blood vessels), cavity, or organ. A sealing valve typically is located at a proximal portion of the introducer sheath. When medical instruments or tools are inserted into or withdrawn from the sealing valve, the valve generally prevents fluid from inadvertently leaving (e.g., back bleeding) or entering the body lumen through the introducer sheath. 
     Current introducer sheath valves generally fall into two basic categories: passive and active. A passive valve generally relies on the deformation of a resilient sealing body by the medical instrument inserted through the valve to form the desired fluid tight seal. Active medical valves include a mechanism that moves a sealing body into contact with the traversing medical instrument. 
     A wide variety of passive and active sealing valve structures for use with introducer sheaths have been proposed. These structures provide different hemostatic valve designs that typically vary in terms of the valve shape, aperture or slit geometry, aperture or slit position, and other design aspects. While these structures have met with varying degrees of success and acceptance, they generally still suffer from disadvantages of leakage around the sealing valve. 
     For these and other reasons, it is desirable to provide improved introducer sheath hemostatic valves for use in endovascular, laparoscopic, and other medical procedures. It would be particularly desirable if these improved valves provide enhanced hemostasis sealing (i.e., preventing leakage of fluid from or into the introducer sheath), maximize safety, and do not require any actuation in the various modes of operation. It would be further desirable if these improved valve structures were better adapted for passing instruments both distally and proximally therethrough as well as for providing a fluid-tight seal across a wide range of medical instrument diameters. 
     2. Description of the Background Art 
     Valve sealing structures for sheaths, catheters, cannulas, trocar assemblies, sheaths, and other medical devices are described in U.S. Pat. Nos. 4,978,341; 5,350,364; 5,391,154; 5,456,284; 5,603,702; 5,722,958; 5,865,807; 5,916,198; 5,935,112; 5,989,233; 6,276,661 B1; and 6,520,939 B2, in U.S. Publication No. US 2003/0014015 A1, and in PCT Publication No. WO 03/008035 A1. 
     The full disclosures of each of the above references are incorporated herein by reference. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides vascular introducer sheaths incorporating improved passive valves which can maintain hemostasis around medical instruments. In particular, the hemostatic valves of the present invention provide secure and reliable sealing so at to minimize leakage of fluid from (e.g., back bleeding) or into the introducer sheath when various instruments or tools are inserted into or withdrawn through the sealing valve. Further, these hemostatic valves provide easy loading of instruments both distally and proximally therethrough, a fluid-tight seal across a wide range of medical instrument diameters, and do not require any actuation mechanisms for sealing. 
     In one aspect of the present invention, a passive valve for creating a hemostatic seal around a medical instrument is provided. The valve is configured to be removably housed within an introducer sheath. The valve comprises a valve body having an upstream end, a downstream end, and a lumen having an inner and outer surface therethrough. The valve body defines a sealing aperture or hole that is configured nominally to remain open. The upstream end includes an upstream annulus region defining at least one opening and the downstream end includes a downstream annulus region. Each annulus region and lumen outer surface define at least one chamber. The at least one opening transmits pressure from the upstream end through the upstream annulus region so as to allow a fluid (e.g., blood) to enter the at least one chamber as a greater pressure is generally on the upstream side. 
     The opening advantageously allows for pressure equalization on the upstream and downstream ends of the valve body so that the valve body is less likely to inflate and leak fluids from or into the introducer sheath when medical tools (e.g., guidewires, catheters, snares, balloons, or other interventional instruments) are inserted into or withdrawn from the sealing valve. Moreover, filling of the chamber via the opening enhances the fluid tight hemostatic seal around the medical instrument. The sealing aperture facilitates easy loading of instruments both distally and proximally. The term “opening” as used herein is defined to include a slit, slot, hole, aperture, access, passage, or like opening which facilitates fluid flow into the chamber so as to maintain the integrity of the valve while preventing pressure build up on either ends of the valve. It will therefore be appreciated that the at least one opening may take on a variety of shapes, sizes, number, dimensions, and/or configurations from those described herein. In one exemplary embodiment of the present invention, the upstream annulus region defines four equally spaced pressure equalization slots having slot widths in a range from about 0.030 inch or less to about 0.075 inch or greater, as for example 0.045 inch, although any shape (e.g., circular shape, S-shape, diamond shape, etc.), number (e.g., one, two, three, four, five, six, etc.), and/or configuration (e.g., irregularly or symmetrically spaced) of openings may be utilized. 
     The sealing aperture comprises a variable orifice for sealing around a wide range of medical instrument diameters. The variable orifice typically has a nominal diameter (e.g., diameter without any instrument passing therethrough) in a range from about 0.015 inch or less to about 0.220 inch or greater, as for example 0.030 inch. Other useful ranges for the orifice nominal diameter include from about 0.020 inch to about 0.220 inch, from about 0.015 inch to about 0.050 inch, from about 0.025 inch to about 0.035 inch, and from about 0.0275 inch to about 0.0325 inch. The sealing aperture provides an expansion coefficient in a range from about 500% to about 1200% or higher of the nominal diameter; more particularly from about 500% to about 700% of the nominal diameter. The variable orifice may be expansible from a first reduced diameter when no medical instrument traverses the sealing aperture (e.g., before or after instrument insertion) to a second enlarged diameter when the medical instrument traverses the sealing aperture (e.g., during instrument insertion, placement or withdrawal). The variable orifice may be capable of sealingly receiving therethrough one or more instruments having a diameter ranging from about the same or slightly larger diameter as the nominal diameter of the aperture to about twelve or more times the nominal diameter of the aperture. 
     In one embodiment of the present invention, the lumen may comprise an hourglass configuration having a funnel-shaped upstream lumen that converges inwardly and a funnel-shaped downstream lumen that diverges outwardly. The upstream and downstream lumens intersect at the sealing aperture. This hourglass configuration has many advantages. For example, when a medical instrument is being withdrawn through the aperture, the funnel shape of the upstream lumen reduces the possibility that any sharp edges present on the instrument will damage the valve body, thus improving the likelihood that the sealing aperture will provide a desirable fluid-tight seal around the instrument. Further, this hour glass shape has been found to help maintain the shape of the passive valve, particularly at the sealing aperture. Still further, this hour glass configuration promotes pressure normalization on either ends of the valve so as to minimize wear, tear, or further damage of the valve. It will be appreciated that the funnel-shaped upstream and downstream lumens may be symmetric or non-symmetric (e.g., different lead-in angles, length, etc.) as desired. In another embodiment of the present invention, the lumen comprises a conical configuration having a funnel-shaped downstream lumen, wherein the sealing aperture is positioned adjacent the upstream annulus region. 
     The valve embodiments of the present invention may generally be formed from silicone, rubber, a combination thereof, or like medical device grade materials that can accommodate a variety of medical instrument diameters without plastic deformation or tearing. A useful material for the valve body of the present invention comprises 40 or 50 durometer silicone rubber. The medical instrument may comprise a guidewire, catheter, or like device. 
     The present invention may further comprise at least one reinforcing rib extending, for example, from the valve upstream annulus region to the valve downstream annulus region. Such reinforcing ribs or like reinforcements help to maintain the axial integrity of the variable orifice. In some embodiments, a valve cartridge may further be disposed between the valve upstream annulus region and the valve downstream annulus region. The valve cartridge further inhibits leakage of fluid from or into the introducer sheath by forming an additional seal between the annulus regions. The valve cartridge may comprise any biocompatible polymer (such as polycarbonate or nylon) or like materials and may be mechanically fastened, welded, or otherwise affixed into place. 
     In another aspect of the present invention, a passive valve for creating a hemostatic seal around a medical instrument is provided. The valve is configured to be removably housed within an introducer sheath. The valve comprises a valve body having an upstream end and a downstream end. The valve body defines a lumen having inner and outer surfaces therethrough. The lumen may comprise an hourglass configuration having a funnel-shaped upstream lumen that converges inwardly and a funnel-shaped downstream lumen that diverges outwardly. The upstream and downstream lumens intersect at a sealing aperture configured nominally to remain open, even when no instrument traverses the valve aperture. The upstream end includes an upstream annulus region defining at least one slot or slit and the downstream end includes a downstream annulus region. Each annulus region and lumen outer surface defines at least one chamber. The at least one slot or slit transmits pressure from the upstream end through the upstream annulus region. 
     In a further aspect of the present invention, yet another passive valve for creating a hemostatic seal around a medical instrument is provided. The valve is removably housed within an introducer sheath. The valve comprises a valve body having an upstream end and a downstream end. The valve body defines a lumen having an inner and outer surface therethrough. The lumen comprises a conical configuration having a funnel-shaped downstream lumen. The upstream end includes an upstream annulus region defining at least one slot or slit, wherein a sealing aperture is positioned between the upstream annulus region and the downstream lumen. The downstream end includes a downstream annulus region. Each annulus region and lumen outer surface defines at least one chamber. The at least one slot or slit transmits pressure from the upstream end through the upstream annulus region. 
     In yet another aspect of the present invention, a bifurcated medical introducer sheath is provided comprising a tubular shaft having two proximal ends and one distal end (e.g., Y-shaped configuration). Two hemostasis valves are removably positioned within each of the proximal ends of the tubular shaft and two valve cartridges are removably positioned within each proximal end of the tubular shaft. This proximal dual valve Y-shaped hub advantageously allows for multiple instruments (e.g., two or more) to be inserted, placed, and/or withdrawn within the one unitary structure while preventing fluid leakage. 
     The bifurcated sheath may incorporate a variety of hemostasis valves. For example, at least one valve may comprise a passive hemostatic valve having a valve body including an upstream end and a downstream end. The valve body defines a lumen having an inner and outer surface therethrough and a sealing aperture configured nominally to remain open. The upstream end includes an upstream annulus region defining at least one slot or slit and the downstream end includes a downstream annulus region. Each annulus region and lumen outer surface defines at least one chamber. The at least one slot or slit transmits pressure from the upstream end through the upstream annulus region so as to allow a fluid to enter the at least one chamber to equalize pressure on the upstream and downstream ends of the valve body and to enhance a hemostatic seal around an instrument. In such an embodiment, at least one valve cartridge may be disposed between the upstream annulus region and the downstream annulus region. Typically, each annulus region of the valve engages an inner lumen of the tubular body. The sealing aperture of the valve comprises a variable orifice which is expansible from a first reduced diameter to a second enlarged diameter and which has a nominal diameter range from about 0.020 inch to about 0.220 inch. 
     Alternatively, the bifurcated sheath may incorporate any conventional duckbill valve to be removably received within one end (or both ends) of the bifurcated sheath to prevent any fluid leakage from or into the shaft when no instrument is traversing the valve. Still further, the bifurcated sheath may incorporate any conventional hemostatic valve to be removably received within one end (or both ends) of the bifurcated sheath. For example, conventional hemostasis valves are commercially available from Thomas Medical Products, Inc. of Malvern, Pa. It will be appreciated that any number of valve permutations may be incorporated within the bifurcated sheath. For example, the bifurcated sheath may incorporate two of the passive hemostatic valves described above, one passive hemostatic valve and one conventional valve (e.g., duckbill valve), or two conventional valves. 
     In a further aspect of the present invention, methods for creating a hemostatic seal around a medical instrument are provided. One method comprises providing a passive valve removably housed within an introducer sheath. The valve includes a valve body having an upstream end and a downstream end. The valve body defines a lumen having an inner and outer surface therethrough. The lumen comprises an hourglass configuration having a funnel-shaped upstream lumen that converges inwardly and a funnel-shaped downstream lumen that diverges outwardly. The upstream and downstream lumens intersect at a sealing aperture configured nominally to remain open. The upstream end includes an upstream annulus region having at least one slot or slit and the downstream end includes a downstream annulus region. Each annulus region and lumen outer surface define at least one chamber. As the instrument traverses the valve, the at least one slot or slit transmits pressure from the upstream end through the upstream annulus region so as to allow a fluid to enter the at least one chamber to equalize pressure on the upstream and downstream ends of the valve body and to enhance the hemostatic seal around the instrument. 
     In still a further aspect of the present invention, a method for assembling a medical device is provided. An introducer sheath is provided within which a passive valve is housed. The valve includes a valve body having an upstream end and a downstream end. The valve body defines a lumen having an inner and outer surface therethrough. The lumen comprises an hourglass configuration having a funnel-shaped upstream lumen that converges inwardly and a funnel-shaped downstream lumen that diverges outwardly. The upstream and downstream lumens intersect at a sealing aperture configured nominally to remain open. The upstream end includes an upstream annulus region having at least one slot or slit and the downstream end includes a downstream annulus region. Each annulus region and lumen outer surface defines at least one chamber. A medical instrument is further provided for positioning through the valve. The at least one slot or slit transmits pressure from the upstream end through the upstream annulus region so as to allow a fluid to enter the at least one chamber to equalize pressure on the upstream and downstream ends of the valve body and to enhance the hemostatic seal around the instrument. 
     A further understanding of the nature and advantages of the present invention will become apparent by reference to the remaining portions of the specification and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings should be read with reference to the detailed description. Like numbers in different drawings refer to like elements. The drawings, which are not necessarily to scale, illustratively depict embodiments of the present invention and are not intended to limit the scope of the invention. 
         FIGS. 1A and 1B  are perspective illustrations of an introducer sheath with which various embodiments of the present invention may be used. 
         FIGS. 2A-2F  are perspective, plan, and cross-sectional illustrations of a passive hemostatic sheath valve in accordance with one embodiment of the present invention. 
         FIGS. 3A-3F  are perspective, plan, and cross-sectional illustrations of a passive hemostatic sheath valve in accordance with another embodiment of the present invention. 
         FIGS. 4A-4C  are cross-sectional views of an introducer sheath housing the passive hemostatic sheath valve of  FIGS. 2A-2F . 
         FIGS. 5A-5C  are cross-sectional views illustrating a method for creating a hemostatic seal around a medical instrument in accordance with methods of the present invention. 
         FIGS. 6A-6D  are plan, side, and cross-sectional illustrations of a bifurcated introducer sheath with which various embodiments of the present invention may be used. 
         FIG. 7  is a perspective illustration of another introducer sheath with which various embodiments of the present invention may be used. 
         FIGS. 8A-8F  are perspective, plan, and cross-sectional illustrations of an exemplary passive hemostatic sheath valve in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides vascular introducer sheaths incorporating improved hemostatic passive valves for use in endovascular, laparoscopic, and other medical procedures. Vascular sheaths generally provide access to a body lumen (e.g., a blood vessel), cavity, or organ for the delivery of medical instruments, such as guidewires, catheters, etc. that may further contain devices such as stents, stent-grafts, balloons, etc. The hemostatic valve may be releasable from the introducer sheath or be an integral part of the introducer sheath. The hemostatic valves may further be incorporated into any portion of the sheath (e.g., hub or shaft), at a common position or at a different position. The hemostatic valves of the present invention provide secure and reliable sealing so as to minimize leakage of fluid from (e.g., back bleeding) or into the introducer sheath when medical instruments or tools are inserted into or withdrawn from the sealing valve. Further, the valves of the present invention are adapted to seal about a wide variety of medical instruments having a wide range of instrument diameters. It will be appreciated that the hemostatic valves of the present invention may also be incorporated into a number of different of medical devices, such as bifurcated sheaths, catheters, cannulas, trocar assemblies, and other tubular medical devices, and are not limited to incorporation with an introducer sheath. 
     Referring now to  FIGS. 1A and 1B , an exemplary introducer sheath  10  with which various embodiments of the present invention may be used is illustrated. The introducer sheath  10  generally comprises a tubular shaft  12  having a proximal end  13  and a distal end  15  and a two-piece hub  14  that is adapted for coupling to the proximal end  13  of shaft  12 . Hub  14  comprises hub main body  14 A and cap  14 B which may be coupled to one another by any appropriate means, such as by threading or, as shown in  FIGS. 1A and 1B , by being press fit or mechanically snapped into place. The sheath  10  may be formed from a variety of medical grade materials. Particularly useful for the present invention is a composite sheath comprising an inner liner, a braid, and an outer layer. The inner liner is made of biocompatible lubricious polymers such as fluoropolymers (e.g., polytetrafluoroethylene or PTFE), fluorinated ethylene propylene (FEP), or high density polyethylene. The liner may also be filled with a radiopaque material such as barium sulfate or the like. The braid may be any number of materials such as stainless steel, etc. and may be in a ribbon form. The outer layer is made of a bondable polymer such as nylon or fluoropolymers such as PEBAX®. Other sheath materials and configurations may be used. 
     As shown in  FIG. 1B , a passive hemostatic valve  16  as described in more detail below may removably be positioned at a proximal portion  13  of the introducer sheath  10  within the hub  14 . As noted above, the valve  16  may alternatively be integrally formed within the sheath  10  and positioned at any desired portion of the sheath  10 . As shown in  FIGS. 1B ,  4 B and  4 C, a valve cartridge  18  may also be removably positioned integral with valve  16  as will be described in more detail below. The valve cartridge  18  may be formed from a variety of biocompatible materials including polymers (such as polycarbonate or nylon) and is removably positioned into place. It will be appreciated that the above depictions are for illustrative purposes only and do not necessarily reflect the actual shape, size, or dimensions of the sheath  10 , valve  16 , or cartridge  18 . For example, the shaft  12  has a greater length than illustrated in  FIGS. 1A and 1B . This applies to all depictions herein. 
     Referring now to  FIGS. 2A through 2F , a passive hemostatic sheath valve  16  in accordance with one embodiment of the present invention is illustrated. The valve  16  for creating a hemostatic seal around a medical instrument comprises a valve body  20  having an upstream end  22  and a downstream end  24 . The valve body  20  defines a lumen  26  having an inner surface  28  and an outer surface  30  therethrough. The lumen  26  comprises an hourglass configuration having a funnel-or conical-shaped upstream lumen  32  that converges inwardly and a funnel-shaped downstream lumen  34  that diverges outwardly. The upstream and downstream lumens  32 ,  34  intersect at a sealing aperture or hole  36  configured nominally to remain open (i.e., prior to, during, and subsequent to device insertion). The upstream end  22  includes an upstream annulus region  38  defining at least one opening  40  and the downstream end  24  includes a downstream annulus region  42 . 
     As shown in  FIG. 2B , the valve  16  of this embodiment includes four openings  40  within the upstream end  22  that are symmetrically spaced so as to promote equal stress distribution within the valve body. However, it will be appreciated that any number of openings  40  may be utilized (e.g., from two to six or more) on either end  22 ,  24  of the valve body  20  to achieve the desired pressure equalization, in any configuration, shape, and/or size, as described herein. The circular shaped holes  40  of this embodiments will have a diameter in a range from about 0.005 inch to about 0.080 inch; more particularly from about 0.030 inch to about 0.055 inch. 
     As shown in the view of  FIG. 2D , the sealing aperture  36  comprises an orifice capable of accommodating and providing a fluid-tight seal around a wide range of medical instrument diameters. The variable orifice typically has a nominal diameter (e.g., diameter without any instrument passing therethrough) in a range from about 0.015 inch to about 0.220 inch, as for example 0.030 inch. Other useful ranges for the orifice nominal diameter include from about 0.020 inch to about 0.220 inch, from about 0.015 inch to about 0.050 inch, from about 0.025 inch to about 0.035 inch, and from about 0.0275 inch to about 0.0325 inch. The sealing aperture provides an expansion coefficient in a range from about 500% to about 1200% or higher of the nominal diameter; more particularly from about 500% to about 700% of the nominal diameter. The variable orifice may be expansible from a first reduced diameter when no medical instrument traverses the sealing aperture (e.g., before or after instrument insertion) to a second enlarged diameter when the medical instrument traverses the sealing aperture (e.g., during instrument insertion, placement or withdrawal). 
     Referring now to  FIGS. 3A through 3F , a passive hemostatic sheath valve  16  in accordance with another embodiment of the present invention is illustrated. The passive valve  16  for creating a hemostatic seal around a medical instrument comprises a valve body  20  having an upstream end  22  and a downstream end  24 . The valve body  20  defines a lumen  44  having an inner surface  46  and an outer surface  48  therethrough. The lumen  44  comprises a conical or funnel configuration having a funnel-shaped downstream lumen  50 . The upstream end  22  includes an upstream annulus region  38  defining at least one opening  40 , wherein a sealing aperture  36  is positioned between the upstream annulus region  38  and the downstream lumen  50 . The downstream end  24  includes a downstream annulus region  42 . 
     As shown in  FIGS. 3E ,  3 F,  2 E, and  2 F, the valves  16  of the present invention may further comprise at least one reinforcing rib  52  extending from the upstream annulus region  38  to the downstream annulus region  42 . The valve  16  comprises four reinforcing ribs  52 , however, it will be appreciated that any desired number of reinforcing ribs  52  may be utilized (e.g., two to six or more). Such reinforcing ribs  52  or the like help maintain the axial integrity of orifice  36 . The valve embodiments  16  of the present invention are generally formed from silicone, rubber, a combination thereof, or like medical device grade materials that can accommodate a variety of medical instrument diameters without plastic deformation or tearing. A useful material of the valve body  20  comprises  40  or  50  durometer silicone rubber. 
     Referring now to  FIGS. 4A through 4C , views of an introducer sheath  10  housing the passive hemostatic sheath valve  16  of  FIGS. 2A-2F  are illustrated. As can be seen in the cross sectional views of  FIGS. 4B and 4C , each annulus region  38 ,  42  and lumen outer surface  30  define a chamber  54 . The openings  40  transmit pressure from the upstream end  22  via the upstream annulus region  38  so as to allow a fluid (e.g., blood or air) to enter the chamber  54  as a greater pressure is generally on the upstream side  22 . Pressure from the fluid entering the chamber  54  symmetrically seals the walls of aperture  36  around the instrument. Hence, the opening  40  advantageously allows for pressure equalization on the upstream  22  and downstream  24  ends of the valve body  20  so that the valve body is less likely to inflate and leak fluids from or into the introducer sheath  10  when medical tools are inserted into or withdrawn from the sealing aperture  36 . In this illustration, the valve cartridge  18  is disposed between the upstream annulus region  38  and the downstream annulus region  42  and within the hub  14  so as to form a boundary of the chamber  54 . As such, the cartridge  18  further helps create a seal around the aperture  36  that inhibits leakage of fluid from or into the introducer sheath  10 , particularly in the case of instruments having a diameter which is the same or nearly the same as the orifice nominal diameter. In particular, the valve cartridge  18  helps maintain the integrity of orifice  36  by redistributing forces away from the aperture so that it remains intact. It will be appreciated that the hub  14  may also form a boundary of the chamber  54  in the absence of a valve cartridge  18 . 
     Referring now to  FIGS. 5A through 5C , cross-sectional views of an exemplary method for creating a hemostatic seal around a medical instrument  56  are illustrated. This method comprises providing a passive valve  16  as previously described above in detail. For purposes of illustration only, the valve embodiment of  FIGS. 2A through 2F  are shown, although the valve embodiment of  FIGS. 3A through 3F  may be used as well. As the instrument  56  traverses the valve  16 , as seen in  FIG. 5B , the at least one opening  40  transmits pressure from the upstream end  22  through the upstream annulus region  38  so as to allow a fluid to enter the chamber  54  ( FIGS. 4B and 4C ) to equalize pressure on the upstream  22  and downstream  24  ends of the valve body  20  and to enhance the hemostatic seal around the instrument  56 . The variable orifice  36  may be capable of sealingly receiving therethrough one or more instruments  56  having a diameter ranging from about  0 . 005  inch greater than the nominal diameter of orifice  36  ( FIGS. 5A  and SC) to about eight or more times the nominal diameter of the orifice ( FIG. 5B ). A hemostatic seal may be achieved even when instrument  56  diameter is the same or nearly the same as the nominal diameter of orifice  36 . It is believed that the design features of the present invention, including the fluid pressurization of chamber  54  via opening or openings  40  during use allows aperture  36  to retain its axial integrity and seal around the instrument  56 . The instrument  56  may comprise a variety of tools as small as a guidewire and up to large delivery catheters. Typically, during a medical procedure, the guidewire remains traversed through the aperture  36 . 
       FIGS. 6A-6D  are plan, side, and cross-sectional illustrations of a bifurcated introducer sheath  58  with which various embodiments of the present invention may be used. In this embodiment, the sheath  58  may be bifurcated comprising at least two hubs  60 ,  62  coupleable to a proximal end of the shaft  12 . As shown in  FIG. 6D , a first passive hemostatic valve  64  is incorporated into to the first hub  60  and a second passive hemostatic valve  66  is incorporated into to the second hub  62 . Each annulus region of the valves  64 ,  66  is removably received within an inner lumen of the respective hubs  60 ,  62 . The valves  64 ,  66  may comprise any of embodiments described herein. Additionally, a conventional duckbill valve  68  may be removably received within one end (or both ends) of the bifurcated sheath  58  to prevent any fluid leakage from or into the shaft  12  when no instrument is traversing the valve  64 . 
     Referring now to  FIG. 7 , an introducer sheath  10  in accordance with another embodiment of the present invention is illustrated. The introducer sheath  10  generally comprises a tubular shaft  12  having a proximal end  13  and a distal end  15 , a two-piece hub  14 A,  14 B that is adapted for coupling to the proximal end  13  of shaft  12 . The hub comprises hub main body or housing  14 A and cap  14 B which may be coupled to one another by any appropriate means, such as by threading as shown in  FIG. 7  or by being press fit or mechanically snapped into place. A front seal  17  is disposed between the housing  14 A and a connector ferrule  19  at the proximal end  13  of the shaft  12 . The sheath  10  may be formed from any of the medical grade materials already described above. 
     As further illustrated in  FIG. 7 , an exemplary passive hemostatic valve  16  as described in more detail below may removably be positioned at a proximal portion of the introducer sheath  10  within the hub  14 A,  14 B and between the connector ferrule  19  and a connector isolator  21 . As noted above, the valve  16  alternatively may be integrally formed within the sheath  10  and positioned at any desired portion of the sheath  10 . A valve cartridge or spacer  18  also may be removably positioned integral with valve  16  as described above. 
     Referring now to  FIGS. 8A through 8F , the passive hemostatic sheath valve  16  of  FIG. 7  is illustrated in further detail. The valve  16  for creating a hemostatic seal around a medical instrument comprises a valve body  20  having an upstream end  22  and a downstream end  24 , as shown in  FIG. 8F . The valve body  20  defines a lumen  26  having an inner surface  28  and an outer surface  30  therethrough, as shown in  FIGS. 8B and 8C . The lumen  26  comprises an hourglass configuration having a funnel-or conical-shaped upstream lumen  32  that converges inwardly and a funnel-shaped downstream lumen  34  that diverges outwardly. The upstream and downstream lumens  32 ,  34  intersect at a sealing aperture or hole  36  configured nominally to remain open (i.e., prior to, during, and subsequent to device insertion). In this embodiment, the funnel-shaped upstream and downstream lumens  32 ,  34  are non-symmetric in that the upstream lead-in angle α is more obtuse that the downstream lead-in angle β as measured with respect to a central axis as shown in  FIG. 8C . However, it will be appreciated that the funnel-shaped upstream and downstream lumens  32 ,  34  may be symmetric, have equal (or even non-existent) lead-in angles, or still further equal or different lengths. 
     As shown in  FIGS. 8D and 8E , the upstream end  22  includes an upstream annulus region  38  defining at least one opening  40  and the downstream end  24  includes a downstream annulus region  42 . In this embodiment, the upstream end  20  includes four equally spaced pressure equalization slots  40  so as to promote equal stress distribution within the valve body. It will be appreciated that slots  40  may comprise any alternative shape (e.g., circular shape, S-shape, diamond shape, etc.), number (e.g., one, two, three, four, five, six, etc.), and/or configuration (e.g., irregularly or symmetrically spaced) to achieve the desired pressure equalization. The slots  40  typically will have slot widths in a range from about 0.030 inch or less to about 0.075 inch or greater, as for example 0.045 inch. 
     As shown in the view of  FIG. 8F , the sealing aperture  36  comprises a variable orifice capable of accommodating and providing a fluid-tight seal around a wide range of medical instrument diameters. As described above, the valve  16  may further comprise at least one reinforcing rib  52 , typically four ribs  52 , extending from the upstream annulus region  38  to the downstream annulus region  42 , as shown in  FIGS. 8B and 8C . The valve  16  may be formed from any of the medical device grade materials described above. 
     In operation, the slot openings  40  function similar to the hole openings illustrated in  FIGS. 2A  though  3 F by transmitting pressure from the upstream end  22  via the upstream annulus region  38  so as to allow a fluid (e.g., blood, air, etc.) to enter the chamber  54  as a greater pressure is generally on the upstream side  22 . Pressure from the fluid entering the chamber  54  is exerted against the outer lumen surface wall  30  which symmetrically seals the aperture  36  around the instrument. Hence, the slot opening  40  advantageously allows for pressure normalization on the upstream  22  and downstream  24  ends of the valve body  20  so that the valve body is less likely to inflate and leak fluids from or into the introducer sheath  10  when medical tools are inserted into or withdrawn from the sealing aperture  36 . It is also believed that the fluid pressurization of chamber  54  via slot openings  40  during use allows aperture  36  to retain its axial integrity and seal around the instrument  56 . 
     Although certain exemplary embodiments and methods have been described in some detail, for clarity of understanding and by way of example, it will be apparent from the foregoing disclosure to those skilled in the art that variations, modifications, changes, and adaptations of such embodiments and methods may be made without departing from the true spirit and scope of the invention. Therefore, the above description should not be taken as limiting the scope of the invention which is defined by the appended claims.