Patent Publication Number: US-7591832-B2

Title: Expandable guide sheath and apparatus with distal protection and methods for use

Description:
This application claims benefit of provisional application Ser. No. 60/499,823, filed Sep. 3, 2003 for ACU-004, the entire disclosures of which is hereby incorporated herein, by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to apparatus and methods for delivering instruments and/or agents during a medical procedure, and, more particularly, to guide sheaths for accessing body lumens and/or delivering instruments into body lumens of a patient. 
     BACKGROUND 
     Minimally invasive procedures have been implemented in a variety of medical settings, e.g., for vascular interventions, such as angioplasty, stenting, embolic protection, electrical heart stimulation, heart mapping and visualization, and the like. These procedures generally rely on accurately navigating and placing instruments within a patient&#39;s vasculature. 
     During such procedures, a target vessel may be accessed using a guidewire advanced through the intervening vasculature into the target vessel, thereby providing a “railway” to the vessel. One or more instruments, e.g., catheters, sheaths, and the like, may be advanced over the guidewire or “rail” into the vessel. Thus, a diagnostic and/or therapeutic procedure may be performed by advancing one or more instruments over this railway. 
     There are many risks involved with advancing instruments over a guidewire. For example, a catheter or other instrument may skive or otherwise damage a wall of a vessel, particularly as the instrument passes through narrow passages or tortuous anatomy involving sharp bends. Such instruments also risk dislodging embolic material or even perforating the vessel wall. 
     In addition, it is often desirable to access very small vessels deep within the body, e.g., within a patient&#39;s heart, for example, to place a ventricular pacing lead within a coronary vein. However, the instrument(s), e.g., guide sheath, lead, etc., may have a relatively large cross-section and/or may have a relatively blunt distal tip, making it difficult to advance such instruments as deeply as desired into such small vessels. 
     Accordingly, apparatus and methods for delivering instruments into blood vessels or other body lumens and/or for otherwise accessing vessels or other body lumens would be useful. 
     SUMMARY OF THE INVENTION 
     The present invention is directed generally to apparatus and methods for providing access to body lumens and/or for delivering instruments and/or agents into body lumens during a medical procedure. More particularly, the present invention is directed to guide sheaths and methods for using such sheaths to facilitate delivering instruments and/or agents into body lumens of a patient, e.g., within the patient&#39;s coronary, neuro, and/or peripheral vasculature, within the patient&#39;s gastrointestinal tract, urogenital tract, respiratory tract, lymphatic system, and/or within surgically created passages. 
     In accordance with one aspect of the present invention, a sheath apparatus is provided that includes an elongate expandable sheath that may be expandable from a contracted condition to minimize a profile of the sheath, e.g., to allow insertion into a body lumen, to an enlarged condition wherein the sheath at least partially defines a lumen therein. The sheath may be formed from a lubricious material, a polymer, and/or an elastomeric material, preferably having a relatively thin wall, thereby providing a tubular sheath that may be substantially flexible and/or flimsy. For example, the sheath may include a wall thickness between about 0.001-1.25 millimeter, and preferably between about 0.005-0.06 millimeter. 
     Optionally, the sheath may include one or more reinforcing elements extending along the sheath, e.g., axially, helically, and/or circumferentially around the sheath. Such reinforcing elements may support the sheath during delivery and/or may enhance the sheath assuming a desired shape and/or size in the enlarged condition. In addition or alternatively, a stiffening or reinforcing member may be provided for supporting or otherwise carrying the sheath to facilitate its introduction in the contracted condition. Optionally, the sheath and/or stiffening member(s) may be coated, e.g., with an anti-thrombotic agent and/or hydrophilic coating. 
     Preferably, the expandable sheath includes a proximal end, a distal end, and has a length sufficient to extend between an entry site into a patient&#39;s body and a target body lumen, e.g., between about two and three hundred centimeters (2-300 cm), and preferably between about twenty and one hundred fifty centimeters (20-150 cm). In one embodiment, the sheath may be a tubular member, while, in another embodiment, the sheath may include a sheet of material whose edges are attached to a stiffening member such that the sheet and the stiffening member together define the lumen. 
     The distal end of the sheath may include an opening communicating with the lumen such that an instrument inserted through the lumen may be advanced from the opening into a body lumen. Alternatively, the distal end of the sheath may be substantially closed and/or may include a break-away portion for creating an opening such that an instrument inserted through the lumen may be advanced from the opening into a body lumen. Optionally, the sheath may include a fixation device, e.g., an expandable cuff or balloon, on the distal end for substantially securing the apparatus at a location within a body lumen, e.g., to prevent movement during delivery of a treatment device through the sheath. 
     In accordance with another aspect of the present invention, an apparatus is provided for providing access to a body lumen of a patient that may include a flexible stiffening member having a proximal end and a distal end having a size and shape for insertion into a body lumen. An expandable sheath may extend between the proximal and distal ends of the stiffening member, the sheath being expandable from a contracted condition to minimize a profile of the sheath to allow insertion along with the stiffening member into a body lumen, and an enlarged condition wherein the sheath at least partially defines a lumen extending between the proximal and distal ends of the stiffening member. A distal protection element, e.g., a balloon or other occlusion member, a filter, and the like, may be carried on the distal end of the stiffening member beyond a distal end of the sheath. 
     In accordance with still another aspect of the present invention, a method is provided for performing a procedure at a location within a body lumen of a patient. An expandable sheath may be advanced from an entry site into the body lumen with the sheath in a contracted condition. The sheath may be expanded to an enlarged condition, thereby defining a lumen within the sheath, and a distal protection element may be deployed beyond a distal end of the sheath adjacent the location. A diagnostic and/or therapeutic procedure may be performed within the body lumen via the lumen defined by the sheath, the distal protection element preventing embolic material from leaving the location. Upon completing the procedure, the sheath and distal protection element may be removed from the body lumen. 
     In accordance with yet another aspect of the present invention, a method is provided for accessing a body lumen of a patient. The body lumen may be, for example, a vessel or other passage within a patient&#39;s urogenital tract, respiratory tract, gastrointestinal tract, lymphatic system, or vascular system. In addition or alternatively, the body lumen may be a passage surgically created within the patient, e.g., an interstitial space accessed via a surgically-created entry site. 
     Generally, an expandable sheath is advanced from an entry site to a body lumen with the sheath in a contracted condition. The sheath may be advanced over a guide wire or in conjunction with another rail. Preferably, the sheath is advanced from the entry site until a distal end of the sheath is disposed within the body lumen while a proximal end of the sheath remains outside the entry site. 
     Once the sheath reaches the target body lumen, the sheath may be expanded to an enlarged condition, thereby defining a lumen within the sheath, e.g., that extends from the entry site to the target body lumen. The sheath may be expanded to the enlarged condition, e.g., by introducing a fluid into the lumen defined by the sheath, such as saline, contrast, carbon dioxide, oxygen, and/or air, and/or by introducing an instrument into the lumen defined by the sheath. 
     A diagnostic and/or therapeutic procedure may be performed within the body lumen via the lumen defined by the sheath. In one embodiment, the entry site may be a percutaneous site communicating with the patient&#39;s vasculature, and the body lumen may be a blood vessel, e.g., within the patient&#39;s coronary, peripheral, or neuro vasculature. The procedure may include introducing one or more instruments or agents through the lumen defined by the sheath into the target blood vessel, e.g., a catheter, a guidewire, a balloon, a stent, a filter, a pacing lead, an atherectomy device, a thrombectomy device, and/or a medicament (e.g., anti-inflammatory drug, anti-thrombotic agent, inhibitors, and the like). In a preferred embodiment, the target blood vessel may be a stenotic or occluded region within an artery. In another preferred embodiment, the target blood vessel may be a coronary vein, and the one or more instruments may include an electrical lead. 
     Upon completing the procedure, the sheath may be removed from the body lumen. Optionally, the sheath may be at least partially collapsed from the enlarged condition before removing the sheath from the body lumen, e.g., by creating a vacuum within the sheath and/or by withdrawing the sheath into a catheter or other tubular member. Alternatively, the sheath may be split along a single longitudinal seam, or along two or more seams into two or more pieces to facilitate removal from the body lumen. 
     Thus, a sheath in accordance with the present invention may provide a primary access device that is substantially flexible, collapsible, and/or compliant. This primary device may assume a relatively low profile to facilitate advancement through narrow passages and/or tortuous anatomy, e.g., into difficult to access locations deep within a patient&#39;s body. Once a desired location is reached, the primary device may be released and/or expanded, and one or more secondary device(s) may be advanced through the primary device. The primary device may provide a lubricious path, prevent dissection of a vessel wall, and/or otherwise facilitate delivering the secondary device(s) into difficult to access locations. The primary device may be free to expand to accommodate the secondary device(s) and/or to substantially translate axial forces, e.g., caused by pushing the secondary device(s), into radial forces, thereby minimizing risk of damage to the passages through which the secondary device(s) is(are) advanced. 
     In accordance with another aspect of the present invention, an apparatus is provided for delivering an instrument, e.g., a guidewire, into a body lumen of a patient, e.g., across a total occlusion within a blood vessel. The apparatus generally includes a catheter or other elongate and/or tubular member including proximal and distal ends, and an expandable sheath attached to an outer surface of the catheter. The sheath, e.g., a flexible and/or flimsy tubular member and/or sheet, may be expandable from a contracted condition to minimize a profile of the sheath, and an enlarged condition wherein the sheath at least partially defines an accessory lumen extending between proximal and distal ends of the sheath. 
     In accordance with yet another aspect of the present invention, a method is provided for delivering an instrument through an occlusion in a body lumen using a catheter or other elongate member including an expandable sheath extending along an outer surface of the elongate member. A distal end of the catheter may be introduced into the body lumen proximal to the occlusion with the sheath in a contracted condition. The distal end of the catheter may be advanced through the occlusion until a distal end of the sheath is disposed distal to the occlusion. The distal end of the catheter may include one or more elements for facilitating advancing the distal end through the occlusion, e.g., an imaging element, a dissection element, and/or a steering element. 
     An instrument, e.g., a guidewire or other elongate member, may be advanced through the sheath until a distal end of the instrument is disposed distal to the occlusion. The sheath may be expanded before inserting the instrument, e.g., by introducing fluid into the lumen of the sheath, or the sheath may be expanded as the instrument is inserted into the sheath. 
     The catheter and sheath may then be withdrawn from the body lumen, e.g., leaving the guidewire or other elongate member across the occlusion. One or more instruments may be advanced over the guidewire, e.g., after the catheter is withdrawn to observe and/or treat the occlusion. 
     Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are partial cross-sectional side views of a first preferred embodiment of a sheath apparatus, including an expandable sheath and a stiffening member, showing the sheath in contracted and expanded conditions, respectively. 
         FIGS. 2A and 2B  are cross-sections of the apparatus of  FIGS. 1A and 1B  taken along lines  2 A and  2 B, respectively. 
         FIGS. 3A-3C  are alternate cross-sections of a stiffening member that may be incorporated into a sheath apparatus. 
         FIGS. 4A and 4B  are cross-sectional views of an alternative embodiment of a sheath apparatus, including an expandable sheath attached to a stiffening member, such as that shown in  FIG. 3A , showing the sheath in contracted and expanded conditions, respectively. 
         FIG. 5  is a perspective detail of a distal end of a sheath apparatus, including an expandable sheath having a weakened region for creating an opening in a wall thereof. 
         FIG. 6  is a perspective detail of an alternative distal end of a sheath apparatus, including an expandable sheath including a beveled distal end having an opening therein. 
         FIG. 7  is a side view of an alternative embodiment of a sheath apparatus, including an expandable sheath with an inflatable cuff on one end. 
         FIG. 8  is a partial cross-sectional side view of a sheath apparatus including wire loops wrapped around an expandable sheath to constrain the sheath in a contracted condition. 
         FIGS. 9A and 9B  are perspective details of a sheath apparatus, showing an expandable sheath in contracted and expanded conditions, respectively. 
         FIGS. 10A and 10B  are perspective details of another embodiment of a sheath apparatus, showing an expandable sheath in contracted and expanded conditions, respectively. 
         FIGS. 11A-11F  are partial cross-sections of a patient&#39;s vasculature, showing a method for delivering a stent across a stenosis using a sheath apparatus, in accordance with the present invention. 
         FIGS. 12A-12D  are partial cross-sections of a patient&#39;s heart, showing a method for delivering an electrical lead into a coronary vein using a sheath apparatus, in accordance with the present invention. 
         FIGS. 13A and 13B  are side views of another embodiment of a sheath apparatus including a tubular constraint from which the apparatus may be deployed. 
         FIGS. 14A-14C  are partial cross-sectional views of a patient&#39;s colon, showing a method for delivering a colonoscope using a sheath apparatus, in accordance with the present invention. 
         FIGS. 15A-15D  are partial cross-sectional views of a vein, showing a method for delivering a stent through valves of the vein. 
         FIGS. 16A-16C  are perspective details of another embodiment of a sheath apparatus including an outer sheath that may separate to accommodate expansion of an expandable sheath therein. 
         FIGS. 17A-17C  are cross-sectional views of the apparatus shown in  FIGS. 16A-16C , respectively. 
         FIG. 18  is a side view of another embodiment of an apparatus including a catheter and an expandable sheath, in accordance with the present invention. 
         FIGS. 19A-19C  are cross-sections of the apparatus of  FIG. 18 , showing the sheath contracted, expanded, and with a guidewire therein, respectively. 
         FIGS. 20A-20D  are cross-sectional views of a blood vessel, showing a method for delivering a guidewire across an occlusion in the vessel. 
         FIGS. 21A-21F  are cross-sectional views of a blood vessel, showing a method for treating a stenosis therein, in accordance with the present invention. 
         FIGS. 22A-22C  are cross-sectional views of a blood vessel, showing a method for delivering fluid upstream from an occlusion balloon of a sheath apparatus, in accordance with the present invention. 
         FIG. 23  is a cross-sectional view of a blood vessel, showing an alternative method for delivering fluid upstream from an occlusion balloon of a sheath apparatus. 
         FIG. 24  is a perspective view of a distal end of another embodiment of a sheath apparatus including a filter attached to a stiffening member extending from the sheath. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning to the drawings,  FIGS. 1A-2B  show a first preferred embodiment of an apparatus  10  for providing access within a body lumen (not shown) and/or for delivering one or more instruments (also not shown) within a body lumen, such as a vessel within a patient&#39;s vasculature, a passage within a patient&#39;s gastrointestinal tract, urogenital tract, respiratory tract, lymphatic system, and the like. 
     Generally, the apparatus  10  includes a flexible elongate stiffening member  12  providing a “backbone” for the apparatus  10 , and an expandable membrane or sheath  20 . The stiffening member  12  includes a proximal end  14  and a distal end  16  defining a longitudinal axis  18  therebetween. In addition, the stiffening member  12  may have sufficient length to be advanced from a location outside a patient&#39;s body (not shown) through any intervening body passages into a site to be accessed and/or treated. The distal end  16  may have a size and/or shape for insertion into a body lumen, e.g., including a rounded or other substantially atraumatic distal tip  17 , a “J” tip (not shown), and the like. 
     In one embodiment, the stiffening member  12  may be a solid or hollow guidewire, a catheter, a thread, and the like. Preferably, the stiffening member  12  is substantially flexible to facilitate advancement through tortuous anatomy without causing dissection or perforation, yet has sufficient column strength and/or torque-ability that the distal end  16  may be advanced through a body lumen by pushing the proximal end  14  without substantial risk of kinking and/or buckling. The stiffening member  12  may include one or more lumens  19  (shown in  FIGS. 2A and 2B ) extending between the proximal and distal ends  14 ,  16 , e.g., to allow fluids to be delivered therethrough and/or to receive a guide wire or other instrument (not shown) therethrough. 
     The stiffening member  12  may have a substantially symmetrical cross-section, e.g., a cylindrical cross-section, as shown in  FIGS. 2A and 2B , or may have an asymmetrical cross-section, e.g., an arcuate cross-section, as shown in  FIGS. 3A-4B . In the embodiments shown in  FIGS. 3A-4B , the stiffening member  112  may define a groove  113  extending at least partially between its proximal and distal ends (not shown in  FIGS. 3A-4B ). 
     The stiffening member  12  or  112  may be formed from a variety of materials and using various methods known in the art. For example, the stiffening member may be formed from plastic, glass, composite, and/or metal using known methods, such as extrusion and the like, thereby providing a desired combination of flexibility and column strength. As used herein, the terms “backbone,” “backbone member,” or “stiffening member” may include any elongate flexible structure capable of supporting or reinforcing an expandable membrane or other sheath to facilitate introducing the sheath into a body lumen of a patient and/or to facilitate tracking a secondary device along the axis of the sheath and/or over the entire apparatus. The stiffening member  12  may have a diameter or other maximum cross-section between about 0.05-5 millimeters, and preferably between about 0.2-2 millimeters. 
     Returning to  FIGS. 1A and 1B , the expandable membrane or sheath  20  includes a proximal end  22 , a distal end  24 , and one or more side walls extending between the proximal and distal ends  22 ,  24 , thereby at least partially defining a lumen  26 . As used herein, the term “sheath” or “guide sheath” may include any structure that at least partially defines a lumen, whether the structure is substantially tubular or only partially defines the lumen. 
     The sheath  20  may be expandable from a contracted condition, as shown in  FIGS. 1A and 2A , to an enlarged condition, as shown in  FIGS. 1B and 2B . When the sheath  20  is in a contracted condition, the apparatus  10  may assume a low profile to facilitate insertion into a body lumen (not shown). For example, as best seen in  FIG. 2A , the sheath  20  may be folded, twisted, wrapped, or otherwise compressed around or adjacent to the stiffening member  12  (e.g., using an internal vacuum with the lumen  26  of the sheath  20  and/or an external force). In the enlarged condition, the sheath  20  may unfold, untwist, unwrap, or otherwise expand to at least partially define the lumen  26 , e.g., for receiving a fluid (e.g., a medicament, anti-thrombotic agent, and the like) and/or one or more instruments therethrough (not shown). 
     Because the sheath  20  is relatively thin-walled, the apparatus  10  may attain a relatively low profile when the sheath  20  is in its contracted condition, e.g., between about 0.1 and about ten millimeters (0.1-10 mm), and preferably between about 0.2 and about three millimeters (0.2-3 mm). Conversely, a relatively large lumen  26  may be provided when the sheath  20  is expanded to the enlarged condition, e.g., having a diameter or other maximum cross-section between about 0.3 and about one hundred millimeters (0.3-100 mm), and preferably between about 0.3 and about twenty millimeters (0.3-20 mm). 
     The sheath  20  may be formed from relatively thin, flexible material, as compared to the stiffening member  12 . Thus, the sheath  20  may be flimsy, i.e., may have little or no rigidity such that the sheath  20  provides little resistance to expansion and/or contraction, and/or may conform substantially to anatomy within which it is deployed. As used herein, “flimsy” means that the material of the sheath  20  is not biased to assume any particular configuration or shape, and therefore, the sheath  20  may adopt whatever shape and/or configuration that is imposed upon it, e.g., by being folded or otherwise compressed, by being subjected to internal pressure or force, and the like. To achieve this, the sheath  20  may have a relatively thin wall thickness, e.g., between about 0.001-1.25 millimeters, and preferably between about 0.005-0.06 millimeter. 
     The sheath  20  may be constructed of materials that may be fabricated to a relatively thin, flexible configuration, e.g., polytetrafluoroethylene (PTFE), expanded polytetrafluoroethylene (EPTFE), fluorinated ethylenepropylene (FEP), polyethylene teraphathalate (PET), urethane, olefins, polyethylene (PE), silicone, latex, isoprene, chronoprene; and the like. The sheath  20  may be formed from a lubricious material and/or hydrophilically coated with a liquid silicone or other coating, e.g., for facilitating inserting one or more instruments (not shown) through the lumen  26 . Preferably, the sheath  20  is formed from substantially inelastic material, i.e., such that a primary contribution to the sheath  20  expanding and contracting is unfolding or folding the material of the sheath  20 . Thus, a total peripheral length of the sheath  20  may not change substantially between the contracted and enlarged conditions. Alternatively, the sheath  20  may be formed from an elastic material such that a secondary contribution to the sheath  20  expanding and contracting is an elasticity of the material of the sheath  20 , i.e., such that the peripheral length of the sheath  20  may increase as the sheath  20  expands towards the enlarged condition. 
     The sheath  20  may be substantially nonporous. Alternatively, the sheath  20  may be porous, for example, substantially continuously along its length or at one or more locations, e.g., to allow fluid delivered into the lumen  26  to pass through the wall of the sheath  20  in a desired manner, e.g., to deliver fluid to a wall of a vessel (not shown) through which the sheath  20  extends. In a further alternative, the sheath  20  may include one or more discrete openings (not shown) at one or more locations along its length. 
     In addition or alternatively, the sheath  20  may include a thin-walled metal foil. Alternatively, the sheath  20  may include a thin mesh, e.g. a perforated urethane film and the like. In a further alternative, the lubricity of the sheath  20  may be enhanced by providing a lubricious coating, lining, ribbing, and the like (not shown), and/or applying a lubricant, e.g., to the interior surface of the sheath  20 . The sheath  20  may include a single layer or multiple layers of such materials, such that a desired flexibility and lubricity is achieved. Thus, the sheath  20  may easily expand and/or line a body lumen to reduce friction and/or accommodate instruments being advanced through the body lumen, as explained further below. 
     Optionally, the sheath  20  may include one or more reinforcing elements (not shown). For example, a wire, thread, filament, and the like, formed from plastic, glass, composite, and/or metal, may be attached to an outer surface, an inner surface, and/or embedded in a wall of the sheath  20 . In addition or alternatively, the sheath  20  may include relatively thickened regions that may be formed directly from the wall material. The reinforcing element(s) may extend circumferentially and/or helically around the sheath  20 , and/or may extend axially along the sheath  20 , depending upon the reinforcement desired. The reinforcement element(s) may also bias the sheath  20  assume a desired shape or configuration when expanded towards the enlarged condition. 
     In alternative embodiments, the sheath may be a tubular structure or a partially tubular structure of braided material (not shown) that may be attached to a stiffening member to provide a sheath apparatus. For example, the sheath may be formed from one or more plastic and/or metal wires, threads, or other strands that are braided, woven, and/or knit into a tubular body or into a sheet whose edges may be attached to a stiffening member, similar to the embodiments described elsewhere herein. The braided strands may be compressed to the contracted condition by twisting, folding, crimping, and/or wrapping, as described above, and/or by axial translation, e.g., pulling opposite ends of the sheath away from one another to cause the braided strand(s) to compress radially under the axial tension. 
     The proximal end  22  of the sheath  20  may include an annular collar or handle that may facilitate manipulating the sheath  20  and/or inserting one or more instruments into the lumen  26 . In addition or alternatively, the proximal end  22  may include a hemostatic valve (also not shown) that may substantially seal the lumen  26  from proximal flow of fluid, yet may allow instruments to be introduced into the lumen  26 , as is known in the art. For example, as shown in  FIGS. 1A and 1B , a toughy borst valve  23  may be provided on the proximal end  22  of the sheath  20 , e.g., to control passage into the lumen  26 . Alternatively, other structures may be provided, e.g., a slit valve adapter, a removable clamp, an o-ring, a side port, an adjustable fixation device, and the like (not shown), for selectively sealing the lumen  26 , manipulating the sheath  20 , and/or for facilitating introducing instruments into the sheath  20 . 
     The distal end  24  of the sheath  20  may include a variety of configurations. For example, as shown in  FIGS. 1A and 2A , the distal end  24  of the sheath  20  may be offset proximally from the distal tip  17  of the stiffening member  12 . In addition, the distal end  24  may include a rounded shape (e.g., as shown in  FIG. 2A ), a beveled shape (e.g., as shown in  FIG. 6 ), or other tapered shapes that may enhance atraumatic advancement of the apparatus  10 . 
     For example, as shown in  FIG. 6 , the taper  28 ″ of the distal end  24 ″ of the sheath  20 ″ may facilitate tracking the apparatus  10 ″ over a guide wire (not shown) and/or may facilitate advancing the apparatus  10 ″ through a body lumen (also not shown). The taper  28 ″ may minimize risk of the sheath  20 ″ catching and/or may cause the sheath  20 ″ to collapse towards the contracted condition as the apparatus  10 ″ is advanced through a body lumen without needing external constraint. Alternatively, the distal end of the sheath may be substantially blunt (not shown). 
     The distal end  24  of the sheath  20  may be substantially closed or may include one or more openings. For example, as shown in  FIG. 2A , the distal end  24  of the sheath  20  may be attached to the distal end  16  of the stiffening member  12  to substantially close the lumen  26  at the distal end  24 . The distal end  24  of the sheath  20  may be detachable from the stiffening member  12  to release the sheath  20  from the stiffening member  12 . 
     For example, the distal end  24  of the sheath  20  may be attached to the stiffening member  12  by an adhesive, solder, or other bonding materials, yet may be torn free or otherwise separated from the distal end  16  of the stiffening member  12 , e.g., by pulling the sheath  20  proximally relative to the stiffening member  12  or by pushing the stiffening member  12  distally relative to the sheath  20 . Alternatively, an instrument (not shown) may be advanced through the expandable member  20  and pushed against the closed distal end  24  to cause the distal end  24  to separate from the stiffening member  12 . This may create an opening (not shown) in the distal end  24  of the sheath  20  through which fluid and/or one or more instruments (also not shown) may be advanced. 
     Alternatively, a mechanical connector (not shown) may be provided that may secure the distal ends  16 ,  24  together, and that may be actuated to release the distal end  24  of the sheath  20  from the distal end  16  of the stiffening member  12 . In further alternatives, a cinch loop (not shown) may be provided on the distal end  24  that may be selectively opened or closed, or a removable clip (also not shown) may be provided. In addition or alternatively, the distal end  24  of the sheath  20  may be bonded to the distal end  16  of the stiffening member  12 , e.g., using a bioabsorbable adhesive or other material that may dissolve when exposed to bodily fluids to provide an opening (not shown). 
     In a further alternative, shown in  FIG. 5 , the distal end  24 ′ of the sheath  20 ′ may include a break-away or penetrable region, e.g., a relatively thin or otherwise weakened region  30 ′ of the sidewall of the sheath  20 .′ The weakened region  30 ′ may tear along a weakened seam or may be punctured (not shown) when an instrument (also not shown) within the lumen  26 ′ is pushed against the weakened region  30 ′ to create an opening, thereby allowing the instrument to be advanced through the opening into the body lumen. 
     In yet another alternative, shown in  FIG. 6 , the distal end  24 ″ of the sheath  20 ″ may include one or more openings  28 ″ communicating with the lumen  26 ,″ e.g., an axial opening defined by the tapered distal end  24 ,″ such that an instrument (not shown) inserted through the lumen  26 ″ may be advanced from the opening  28 ″ and into the body lumen. 
     Optionally, as shown in  FIGS. 21A-21F , a sheath apparatus  610  may include a distal protection device, e.g., a balloon, or other expandable member  680 , a filter (not shown, see  FIG. 24  and description below), and the like, in addition to a stiffening member  612  and an expandable sheath  620 . The balloon  680  may be disposed on a distal portion  616  of the stiffening member  612 , e.g., adjacent to its distal tip  617 . At least one lumen (not shown) within the stiffening member  612  may communicate with an interior of the balloon  680  for delivering inflation medium into and/or aspirating inflation medium from the interior for expanding and/or collapsing the balloon  680 , as is known to those skilled in the art. 
     The balloon  680  may be used to anchor the apparatus  610  once positioned within a desired body lumen  690 . In addition or alternatively, the balloon  680  may be used to substantially seal the body lumen  690 , e.g., to prevent embolic material (not shown) released during a procedure from traveling downstream, and/or to facilitate isolated infusion, perfusion, agitation, or aspiration distal and/or proximal to the balloon, as explained further below. For example, as shown in  FIGS. 22A-22C , the stiffening member  712  may include a lumen (not shown) therein with one or more ports  719  located distally relative to an occlusion balloon  780 . In yet another alternative, as shown in  FIG. 23 , ports  719 ′ may be provided that are located proximally relative to an occlusion balloon  780 .′ 
     Alternatively, as shown in  FIG. 24 , a filter  880  may be provided on any of a stiffening member  812  of a sheath apparatus  810 . Preferably, the filter is attached to a distal end  816  of the stiffening member  812  distal to a sheath  820 . The stiffening member  812  and sheath  820  may be any of the embodiments described herein. The filter  880  may include an expandable frame  882  and a mesh or porous member  884  attached to the frame  882 . 
     The frame  882  may be biased to expand away from the stiffening member  812 , e.g., to define a hoop that may expand the mesh  884  across a blood vessel or other body lumen (not shown). In addition or alternatively, the frame  882  may include one or more struts, e.g., a plurality of struts (not shown) that may extend generally radially away from the stiffening member  812 . The frame  882  may be formed from an elastic or superelastic material, e.g., Nitinol, or may be selectively expandable and/or collapsible, e.g., including one or more lumens (not shown) into which fluid may be delivered to expand the frame  882 . 
     The mesh  884  may include any material having a appropriate pore size for collecting emboli or other particles larger than a predetermined size. The mesh  884  and frame  882  may be disposed such that the mesh  884  opens towards the sheath  820 , although alternatively, the mesh  884  may open away from the sheath  820  (not shown). 
     The filter  880  may be deployed similar to the balloon  780  described above to capture emboli released during a procedure. For example, the filter  880  may be deployed from a sheath or catheter (not shown) overlying the apparatus  810  and/or may be captured within a sheath or catheter upon completing a procedure during which emboli may be collected within the filter  880 . Alternatively, a filament or other control element may be provided that may be actuated from a proximal end (not shown) of the apparatus  810  for selectively collapsing the filter  880  to retain emboli captured therein. 
     In a further alternative, as shown in  FIG. 7 , the distal end  424  of the sheath  420  may include an expandable cuff  430 . The cuff  430  may be an inflatable annular balloon attached around a periphery of the sheath  420 . One or more lumens (not shown) may be provided within the stiffening member  412  that communicate with the interior of the cuff  430  to expand and/or collapse the cuff  430 . The expanded cuff  430  may be used to fix the sheath  420  within a body lumen (not shown) and/or may serve to isolate a distal segment of a body lumen from a segment proximal to the expanded cuff  430 . 
     For example, the sheath  420  may be delivered into a blood vessel, and the cuff  430  may be expanded to seal the vessel. One or more intravascular manipulations or procedures may be performed within the vessel proximal to the expanded cuff  430 . Thus, if embolic debris is released within the proximal portion of the vessel, the cuff  430  may prevent the debris from passing to the distal portion of the blood vessel. 
     In another alternative, the proximal opening  426  of the sheath  420  may be positioned within a blood vessel (not shown), thereby creating a bypass or shunt channel that may be positioned within a body lumen beginning at the proximal opening  426  and extending through the lumen  426  of the sheath  420  to the distal cuff  430 . 
     In a further alternative, one or more lumens (not shown) may be created within or along a wall of the sheath  420  itself. Optionally, one or more cuffs or pockets (not shown) may be provided at one or more locations along the sheath  420 , e.g., to anchor the sheath  420 , to restrict flow, e.g., for the purpose of retrograde infusion of contrast, and/or to bias the sheath  420  to expand to a particular shape or configuration. 
     Returning to  FIGS. 1A-2B , in one embodiment, the sheath  20  may be a substantially enclosed tubular body that is attached to the stiffening member  12  at one or more locations between the proximal and distal ends of the stiffening member  12 . For example, the sheath  20  may be attached to the stiffening member  12  only at the distal ends  24 ,  16 . Alternatively, the sheath  20  may be attached to the stiffening member  12  at or near the proximal end  22 , the distal end  24  of the sheath  20 , or both, e.g., when the stiffening member  12  is disposed along a wall of the sheath  20 . 
     Optionally, the sheath  20  may be attached to the stiffening member  12  intermittently, e.g., at one or more additional locations between the proximal and distal ends  22 ,  24 . Alternatively, the stiffening member  12  may be attached substantially continuously to the sheath  20  between the proximal and distal ends  22 ,  24  of the sheath  20 . The stiffening member  12  may be attached to an interior of the sheath  20 , i.e., within the lumen  26 , or, alternatively, may be attached to an exterior of the sheath  20 . The sheath  20  and the stiffening member  12  may be attached to one another using any known method, e.g., bonding with adhesive, sonic welding, sutures or other strands (not shown) and the like. 
     Alternatively, the stiffening member  12  may be received in a separate pocket or lumen (not shown) of the sheath  20  that extends between the proximal and distal ends  22 ,  24  of the sheath  20 . The stiffening member  12  may be loose within the pocket or lumen or may be attached to the sheath  20  within the lumen, as described above. 
     In another embodiment, shown in  FIGS. 4A and 4B , the sheath  120  may be a sheet of material whose edges  121  are attached to a stiffening member  112 . Thus, the stiffening member  112  may partially define the lumen  126 , along with the sheet of the sheath  120 . The sheath  120  may be attached substantially continuously along the length of the stiffening member  112 , although, alternatively, the sheath  120  may be attached intermittently to the stiffening member  112 , as described previously. 
     Optionally, any of the sheaths described above, e.g., the sheath  20  shown in  FIGS. 1A-2B  may be configured to split, e.g., along one or more seams (not shown) extending at least partially from the proximal end  22  towards the distal end  24 , and/or to separate from the stiffening member  12 . For example, the sheath  20  may include two or more sets of intermittent perforations and/or relatively thin-walled seams (not shown) that extend between the proximal and distal ends  22 ,  24 . In addition or alternatively, the material of the sheath  20  may be biased to tear preferentially, e.g., along the longitudinal axis  18 , to separate the sheath  20  into two or more longitudinal strips (not shown). These features may facilitate removing the sheath  20  and/or the entire apparatus  10  from a body lumen when an instrument (not shown) is disposed within the lumen  26 , for example, when the instrument is intended to be substantially permanently implanted within the body lumen. 
     The apparatus  10  may also include a constraint for covering and/or protecting the sheath during advancement through a patient&#39;s vasculature or other body passages. For example, as shown in  FIG. 8 , one or more wires, threads, monofilaments, or other strands  50  may be disposed around the sheath  20  to maintain the sheath  20  in its contracted condition. The strands  50  may include a plurality of loops  52  that extend around the sheath  20  and are secured using one or more removable filaments (not shown) fed through the loops  52 . To release the sheath  20 , the removable filament(s) may be removed from the loops  52 , e.g., by pulling the removable filament(s) from the proximal end of the apparatus  10 . The loops  52  may then be released and their strand(s) may be removed. 
     Alternatively, other constraints, e.g., wraps, ties, adhesives, bio-absorbable encapsulating materials, e.g., sucrose, and the like, may be used (not shown). In further alternatives, a catheter, sheath, or other tubular member (also not shown) may be provided. For example, as shown in  FIGS. 13A and 13B , a catheter  80  may be used that may include a lumen  82  within which the apparatus  10  (or any other sheath apparatus described herein) may be disposed when the sheath  20  is in the contracted condition. The apparatus  10  may include a pushing element  84 , e.g., a wire or other elongate member extending from the stiffening member  12  (see  FIGS. 1A-2B , not shown in  FIGS. 13A and 13B ). The pushing element  84  may be advanced distally relative to the catheter  80  to deploy the apparatus  10 , as shown in  FIG. 13B , or may be pulled proximally to retract the apparatus  10  into the catheter  80 , as shown in  FIG. 13A . 
     In another embodiment, shown in  FIGS. 16A-17C , an outer sheath  180  may be provided that is at least partially bonded to a sheath apparatus  110  to maintain an expandable sheath  120  in a contracted condition. The outer sheath  180  may be formed from materials similar to the expandable sheath  120 , e.g., having a wall thickness between about 0.005-0.06 millimeter (mm). Thus, the outer sheath  180  may be substantially flexible and/or conformable to facilitate advancing the apparatus  110  and outer sheath  180  through a body lumen of a patient. 
     The apparatus  110  may include a stiffening member  112 , similar to that described above with reference to  FIGS. 3A-4B , to which the expandable sheath  120  may also be attached, similar to other embodiments described above. The stiffening member  112  may include one or more lumens  119  (three exemplary lumens being shown), and the outer sheath  180  may be bonded or otherwise attached to the stiffening member  112 , e.g., continuously or intermittently along a length of the stiffening member  112 . The lumen(s) in the stiffening member  112  may provide infusion, aspiration, agitation, and/or perfusion through the apparatus  110 , e.g., from the proximal end to the distal end of the apparatus  110 . Alternatively, the outer sheath  180  may be attached directly to the expandable sheath  120 , e.g., at a location around the periphery and/or along a length of the expandable sheath  120  (not shown). 
     As best seen in  FIGS. 16A and 17A , the outer sheath  180  may at least partially define a lumen  182  (either alone or along with the stiffening member  112 ) within which the expandable sheath  120  may be received in its contracted condition. In addition, the outer sheath  180  may include one or more weakened regions  186 , e.g., extending between proximal and distal ends (not shown) of the outer sheath  180 . The weakened region(s)  186  may include intermittent perforations and/or continuous or intermittent thin-walled regions that may extend, e.g., substantially axially along the outer sheath  180 . 
     The weakened region(s)  186  may separate upon exposure to internal pressure to allow the expandable sheath  120  to expand towards its enlarged condition. For example, fluid may be introduced into the lumen  182  of the outer sheath  180  until sufficient pressure develops to cause the weakened region(s)  186  to tear or otherwise separate. Alternatively, fluid or an instrument (not shown) may be introduced into the expandable sheath  120  such that the expandable sheath  120  expands until it pushes radially outwardly against the outer sheath  180  and causes the weakened region(s)  186  to separate. 
     For example, during use, the apparatus  110  and outer sheath  180  may be advanced through a body lumen, e.g., a blood vessel and the like (not shown), with the outer sheath  180  intact and the expandable sheath  120  in its contracted condition, as shown in  FIGS. 16A and 17A . Once a desired location is reached, internal pressure may be applied to separate the weakened region(s)  186  of the outer sheath  180 , as shown in  FIGS. 16B and 17B . The expandable sheath  120  may then be expanded to its enlarged condition, as shown in  FIGS. 16C and 17C . The separated portions  188  of the outer sheath  180  may be substantially flexible such that they do not interfere substantially with the expansion of the expandable sheath  120  and/or may conform substantially to the shape of the expandable sheath  120  as it expands. 
     In a further alternative, with reference to  FIGS. 1A-2B , when a constraint is provided around the sheath  20 , the sheath  20  need not be attached to the stiffening member  12 . The sheath  20 , e.g., a tubular body, may simply be disposed around the stiffening member  12  and folded, twisted, crimped, wrapped, or otherwise compressed into the contracted condition around or adjacent to the stiffening member  12  before being inserted into or otherwise constrained by the constraint. In addition or alternatively, vacuum pressure, either alone or in conjunction with folding, may be used to compress the sheath  20  and/or fluid may be infused into the lumen  26  to expand the sheath  20 , as explained further below. 
     Although only one stiffening member  12  is shown in  FIGS. 1A-2B , it will be appreciated that a sheath apparatus in accordance with the present invention may include more than one stiffening member. For example, as shown in  FIGS. 9A and 9B , an embodiment of a sheath apparatus  210  is shown that includes two stiffening members  212  to which an expandable membrane or other sheath  220  is attached. The sheath  220  may be compressed to a contracted condition, shown in  FIG. 9A , e.g., by twisting the stiffening members  212  about longitudinal axis  218  to wind the sheath  220  around the stiffening members  212 . The sheath  220  may be unwound and expanded to an enlarged condition, shown in  FIG. 9B , by untwisting the stiffening members  212 , thereby providing lumen  226  within the sheath  220 . 
     In addition, although the stiffening members  12 ,  212  shown in  FIGS. 1A-2B ,  9 A, and  9 B extend generally parallel to the longitudinal axis  18 ,  218 , other stiffening members may also be provided in addition or instead of the stiffening member(s)  12 ,  212 . For example, as shown in  FIGS. 10A and 10B , one or more reinforcing members  312  may be provided that extend helically around a tubular sheath  320 . The reinforcing members  312  may be wires or other strands attached to an outer or inner surface of the sheath  320  or embedded within a wall of the sheath  320 . One or more helical reinforcing members may also facilitate expanding and/or collapsing the sheath  320 . For example, ends of the reinforcing member(s)  312  may pulled axially away from one to compress the sheath  320 . In addition or alternatively, the reinforcing member(s)  312  may be biased to expand, e.g., towards the configuration shown in  FIG. 10B , to facilitate expanding the sheath  320  towards the enlarged condition. 
     In addition or alternatively, one or circumferential or peripheral rings or strands (not shown) may be provided around the sheath  320 . Such reinforcing rings or strands may support the sheath  320  and/or bias the sheath  320  towards the enlarged condition, similar to the helical reinforcing member(s) described above. 
     Further, although the apparatus described above include an expandable sheath having a single lumen, it will be appreciated that it may be possible to provide one or more sheaths, each including one or more lumens therein. For example, a sheath may be fabricated to have multiple lumens, e.g. by extrusion. As a further example, a plurality of expandable sheaths, similar to those described above, may be attached to one another and/or to one or more common stiffening members to provide an apparatus with multiple expandable lumens (not shown). The sheaths may be concentric with one another, e.g., disposed loosely within one another, or attached along one or more edges to one another. Alternatively, the sheaths may be disposed adjacent to one another, e.g., attached together or to one or more common stiffening members. Thus, multiple instruments may be advanced through respective lumens of the sheaths in conjunction with one another. 
     Optionally, one or more of the components of the sheath apparatus described above, e.g., stiffening member(s), sheath, and/or constraint, may include one or more radiopaque markers, e.g., bands or other elements made from radiopaque material, radiopaque inks or other depositions, and the like (not shown). Such markers and/or radiopaque material may facilitate imaging and/or monitoring the apparatus during its use, e.g., using fluoroscopy. In addition, markers may be provided that are spaced relative to one another in a predetermined manner, e.g., at known distances from one another, to denote distances and/or facilitate mapping and/or accessing a patient&#39;s vasculature. Alternatively, other external or internal imaging systems and methods may be used, e.g., magnetic resonance imaging (“MRI”), ultrasound, and the like, as is well known in the art. 
     A sheath apparatus in accordance with the present invention may be used to provide access to a variety of body lumens, e.g., to perform a diagnostic and/or therapeutic procedure. Generally, the apparatus, with an expandable sheath in a contracted condition, may be introduced into an entry site, e.g., a natural or created opening in a patient&#39;s body, and advanced into one or more body passages, including created passages, e.g., dissection planes, within the patient&#39;s body. Preferably, the apparatus is advanced from the entry site until a distal end of the sheath is disposed within a target body lumen while a proximal end of the sheath remains outside the entry site. Because of its low profile, the apparatus may be easily advanced through tortuous anatomy until the distal end is disposed within relatively small, difficult to access body lumens. 
     The sheath may then be expanded to an enlarged condition, thereby defining a lumen within the sheath. Thus, the lumen defined by the sheath may extend from the entry site through any intervening body passages to the target body lumen or site to provide a path from the entry site to the target body lumen or site. Optionally, if, as described above, the sheath is maintained and/or covered in the contracted condition by a constraint, the constraint may be at least partially removed from the sheath before the sheath is expanded to the enlarged condition. 
     A diagnostic and/or therapeutic procedure, such as the exemplary procedures described below, may be performed within the body lumen via the lumen defined by the sheath. Upon completing the procedure(s), the sheath may be withdrawn from the body lumen, and preferably from the patient&#39;s body. 
     For example, as shown in  FIGS. 11A-11F , a sheath apparatus  10  may be used to facilitate delivering a stent  60  within a stenosis or occlusion  92  within a blood vessel  90 . The vessel  90  may be an artery or vein within a coronary, carotid, cerebral, or other vessel, e.g., within a patient&#39;s coronary, peripheral, or neuro vasculature. 
     As shown in  FIG. 11A , the apparatus  10 , with the sheath  20  in its contracted condition, may be advanced from a percutaneous entry site (not shown) and introduced into the patient&#39;s vasculature, until the distal end  16  is disposed within the target vessel  90 . For example, a guidewire (not shown) may be previously placed from the percutaneous entry site to the vessel  90  beyond the stenosis  92  using conventional methods. The apparatus  10  may be advanced over the guidewire until the distal end  24  of the sheath  20  is positioned within or beyond the stenosis  92 , as shown in  FIG. 11A . If the apparatus  10  includes a stiffening member  12 , the stiffening member  12  may include a guidewire lumen (not shown) such that the guidewire may be inserted through the guidewire lumen to advance the apparatus  10  over the guidewire. Alternatively, the sheath  20 , in its contracted condition may be advanced without a previously placed guidewire. The distal tip of the sheath  20  or stiffening member  12  may facilitate navigation through the vasculature, e.g., by being substantially flexible or floppy, to avoid trauma to the vasculature during advancement, or by being shaped, shapeable, or deformable to enable steering during advancement. 
     Once the apparatus  10  is positioned within the vessel  90 , any constraint (not shown) maintaining the sheath  20  in the contracted condition or otherwise covering the sheath  20  may be removed, e.g., mechanically, chemically (e.g., bio-absorbable sutures), and/or physiologically (e.g., heat-activated nitinol ties). For example, as described above, this may involve retracting an overlying catheter or sheath (not shown), or removing loops or other strands (also not shown) surrounding the sheath  20 . 
     The sheath  20  may then be expanded from its contracted condition to its enlarged condition. For example, a fluid (not shown) may be introduced into the lumen  26  to expand the sheath  20 , e.g., such that the sheath  20  expands to line or otherwise contact walls of one or more vessels within the patient&#39;s vasculature, e.g., between the entry site and the vessel  90 . Alternatively, the sheath  20  may simply be released such that the sheath  20  is free to expand as one or more instruments, such as the catheter  62  and stent  60 , are advanced through the lumen  26 , as shown in  FIG. 11B . 
     With continued reference to  FIG. 11B , one or more instruments may be introduced through the lumen  26  defined by the sheath  20  into the vessel  90 . For example, a catheter  62  carrying the stent  60  may be introduced into the proximal end  22  (not shown in  FIGS. 1A-11F ) of the sheath  20  and advanced through the lumen  26  until the stent  60  is located within the vessel  90 . The stent  60  may be any known endoprosthesis, e.g., a balloon-expandable or self-expanding stent, and the catheter  62  may include any known delivery apparatus, e.g., including a balloon, sheath, nose cone, and/or other elements. Optionally, other instruments, e.g., thrombectomy devices, agitation devices, balloon catheters, filters, occlusion devices, and the like (not shown) may be introduced through the sheath  20  before or after the catheter  62 , e.g., to dilate the stenosis  92  and/or to provide distal protection, i.e., capture embolic material released during the procedure. 
     The guidewire or other rail over which the apparatus  10  may be advanced may be removed before any instrument(s) are advanced through the sheath  20 , or the guidewire may remain in place across the stenosis  92 . In addition, if the apparatus  10  includes a concentric stiffening member  12  within the sheath  20 , the catheter  62  may be advanced over the stiffening member  12 , i.e., between the stiffening member  12  and the sheath  20  as shown in  FIG. 11B . Alternatively, the stiffening member  12  may be removed from the sheath  20  if the sheath is detachable from the stiffening member  12  (not shown). In a further alternative, such as that shown in  FIGS. 4A and 4B , the instrument(s) (not shown in  FIGS. 4A and 4B ) may be advanced through the lumen  126  adjacent to the stiffening member  12 . Thus, one advantage of delivering a catheter or other instrument within the sheath  20  is that there may be no need for a guidewire lumen in the instrument, which may allow the profile of the instrument to be reduced as compared to instruments that are delivered over a guidewire. 
     With continued reference to  FIG. 11B , the lubricious nature of the sheath  20  may facilitate advancing the catheter  62  through the lumen  26 , and, consequently, through any narrow, tortuous, diseased, and/or friable regions of the patient&#39;s vasculature. For example, the sheath  20  may translate axial forces, e.g., created when an instrument, such as the catheter  62 , is pushed against or into a narrow region, into radial forces that may facilitate dilating the narrow region to accommodate the instrument passing therethrough. This action may be enhanced further by pressurizing the sheath  20  internally, e.g., by introducing a fluid into the lumen  26 . Thus, the sheath  20  may facilitate advancing the stent  60  across the stenosis  92 , as shown in  FIG. 11C . 
     In addition, the sheath  20  may substantially reduce the risk of perforating or otherwise damaging the walls of vessels within the vasculature and/or snagging an instrument, since the sheath  20  may guide any instruments advanced through the sheath  20  along the lubricious path of the lumen  26 . Further, the sheath  20  may minimize the risk of embolic material and the like from being dislodged from the patient&#39;s vasculature, since the sheath  20  may expand to line the walls, thereby restraining plaque, thrombus, and the like between the sheath  20  and the vessels&#39; walls. 
     If the distal end  24  of the sheath  20  is closed, an opening  28  may be created to accommodate advancing one or more instruments through the lumen  26  and out of the sheath  20 . For example, as explained above, the distal end  24  may be separated from the stiffening member  12  when the first instrument, e.g., the catheter  62  carrying the stent  60  is advanced through the lumen  26  and against the distal end  24 , as shown in  FIG. 11D . Alternatively, if the distal end  24  of the sheath  20  is attached to a stiffening member  12 , the distal end  24  may be separated from the stiffening member  12 , e.g., by advancing the stiffening member  12  distally and/or retracting the sheath  20  proximally, to create the opening  28 , e.g., before or after introducing the catheter  62  or other instrument (not shown) into the lumen  26 . 
     In a further alternative, as shown in  FIG. 5  and described above, the sheath  20 ′ may include a weakened region  30 ′ in the distal end  24 ′ that may be used to create an opening (not shown in  FIG. 5 ). For example, a fluid, e.g., saline and/or contrast, may be introduced into the lumen  26 ′ of the sheath  20 ′ until sufficient internal pressure is created to cause the weakened region  30 ′ to rupture or tear. Alternatively, an instrument (e.g., the catheter  62  shown in  FIGS. 11B-11F , a guidewire, and the like, not shown), may be advanced into the lumen  26 ′ and pushed against the weakened region  30 ′ to cause the weakened region  30 ′ to fail and create an opening. In still a further alternative, an apparatus  10 ,″ such as that shown in  FIG. 6 , may be used that includes a sheath  20 ″ with an opening  28 ″ already provided in the distal end  24 .″ 
     Turning to  FIG. 11E , once the stent  60  is positioned across the stenosis  92 , the sheath  20  may be at least partially retracted to expose the stent  60  and/or the catheter  62  within the vessel  90  and/or stenosis  92 . The stent  60  may be expanded, as shown in  FIG. 11F , e.g., using conventional methods to dilate and/or scaffold the stenosis  92 . Once the stent  60  is delivered (and any additional procedures completed), the catheter  62  and the apparatus  10  may be removed from the vessel  90 , and the patient&#39;s vasculature via the entry site. 
     In one embodiment, the catheter  62  or other instruments (not shown) may first be removed into and through the sheath  20 , whereupon the sheath  20  may be removed. If desired, the sheath  20  may be collapsed at least partially from the enlarged condition before removing the sheath  20  from the vessel  90  by aspirating any fluid from within the lumen  26  of the sheath  20 , e.g., by connecting a source of vacuum to the proximal end  22  of the sheath  20 . Alternatively, a catheter or other tubular member (not shown) may be advanced over the sheath  20  to facilitate its removal. In another alternative, the sheath  20  may be removed simultaneously with the catheter  62  or other instrument(s), e.g., by retracting the instrument(s) into the sheath  20 . In yet another alternative, the sheath  20  may be removed from the patient before the catheter  62  or other instrument(s) are removed. It will be appreciated that external imaging, e.g., fluoroscopy, MRI, and/or ultrasound, may be used to monitor the procedure, and that any of the components, e.g., the apparatus  10 , the stent  60 , and/or the catheter  62  may include elements, e.g., radiopaque markers (not shown) for facilitating such imaging during their advancement and/or removal. 
     In a further alternative, shown in  FIGS. 21A-21F , a distal protection device, e.g., balloon  680 , may be provided on a sheath apparatus  610 , which may be similar to any of the embodiments described above. Initially, as shown in  FIG. 21A , the balloon  680  and an expandable sheath  620  of the apparatus  610  may be provided collapsed, and the apparatus  610  may be advanced into a body lumen  690 , e.g., an artery or other blood vessel having a stenosis  692  therein via a percutaneous entry site (not shown). Preferably, a distal end  616  of a stiffening member  612  of the apparatus  610  is advanced through the stenosis  692  until the balloon  680  is located downstream or distal to the stenosis  692 . The apparatus  610  may include a constraint (not shown) for covering the balloon  680  and/or for maintaining the sheath  620  in the contracted condition, as described previously. 
     Once the apparatus  610  is positioned as desired, the balloon  680  may be expanded, as shown in  FIG. 21B , to substantially seal the lumen  690 . Thus, the balloon  680  may prevent blood or other fluid from flowing downstream from the lumen  690 , thereby allowing fluids to be delivered into the proximity of the stenosis  692  and/or preventing embolic material released during the procedure from being carried downstream from the stenosis  692 . 
     As shown in  FIG. 21C , the sheath  620  may be expanded to its enlarged condition, e.g., by introducing fluid and/or instrument(s) into its lumen  626 . If the sheath  620  is maintained in the contracted condition using a constraint (not shown), the constraint may be removed, as described above, before the sheath  620  is expanded. 
     One or more instruments may then be advanced into the lumen  690  via the sheath  620 . For example, as shown in  FIG. 21D , an angioplasty balloon catheter  662  may be advanced through the lumen  626  of the sheath  620  until a balloon  660  thereon is disposed within the stenosis  692 , as shown in  FIG. 21D . The balloon  660  may then be expanded to dilate the stenosis  692 , as is well known to those skilled in the art. After the stenosis  692  is sufficiently dilated, the balloon  660  may be collapsed, and the catheter  662  may be withdrawn through the sheath  620 . If desired, other instruments, e.g., balloon catheters, stents, and the like (not shown) may be advanced through the sheath  620  to treat the stenosis  692 . 
     Once the stenosis  692  is treated, any embolic material (not shown) released during the procedure and/or disposed adjacent the balloon  680  may be removed. For example, a catheter (not shown) may be advanced through the sheath  620  or over the device  610  until its distal end is located adjacent the balloon  680 . A source of vacuum (also not shown) may be coupled to the catheter to aspirate embolic material and/or fluid from the lumen  690  adjacent to the balloon  680 , as is well known to those skilled in the art. Alternatively, an aspiration lumen (not shown) may be provided in the stiffening member  612  that may be used to aspirate material from the lumen  690 . As shown in  FIG. 21F , the balloon  680  may then be collapsed, and the apparatus  610  may then be removed from the lumen  690 . 
     In yet another alternative, turning to  FIGS. 22A-22C , an apparatus  710  is shown that includes a stiffening member  712 , an expandable sheath  720 , and an occlusion balloon  780 , similar to the embodiment shown in  FIGS. 21A-21F . In addition, the apparatus  710  may be used to deliver fluid into a lumen  790 , e.g., distal to the balloon  780 . Specifically, the stiffening member  712  may include a lumen (not shown) that extends between its proximal end (not shown) and its distal end  716 . The stiffening member  712  may include one or more ports  719  located distal to the balloon  780  that communicate with the lumen, such that fluid infused through the lumen may be delivered into the body lumen  790  distal to the balloon  780 , as shown in  FIG. 22B . Alternatively, as shown in  FIG. 23 , one or more ports  719 ′ may be provided in the stiffening member  712 ′ proximal to the balloon  780 ′ for delivering fluid proximal to the balloon  780 .′ 
     Thus, during a procedure, such as the stent delivery or angioplasty procedures described above, the apparatus  710  may be delivered into a body lumen  790  with the sheath  720  in its contracted condition, as shown in  FIG. 22A . The balloon  780  may be expanded to substantially seal the lumen  790 , and then fluid may be delivered via the port(s)  719  into the lumen  790  upstream from the balloon  780 , e.g., to provide retrograde perfusion into the body lumen  790 . 
     In a further alternative, a filter or other distal protection element may be provided, such as the filter  880  shown in  FIG. 24  and described above. Similar to the balloon  780 , the filter  880  may capture embolic material released during a procedure, such as those described elsewhere herein. 
     Turning to  FIGS. 12A-12D , another method is shown that uses a sheath apparatus  10  (or any of the sheath apparatus described above) for providing access to a target vessel within a patient&#39;s vasculature. Specifically, the apparatus  10  may be used to deliver an electrical lead  70 , e.g., for a pacemaker, into a coronary vein  96 , e.g., adjacent to the left ventricle of the heart. As shown in  FIG. 12A , initially, the apparatus  10  may be advanced into the coronary vein  96  with an expandable sheath  20  carried by a stiffening member (not shown) in its contracted condition. 
     For example, with the sheath  20  collapsed, the apparatus  10  may be introduced from a percutaneous entry site (not shown), e.g., a femoral vein or subclavian vein, and advanced through the patient&#39;s venous system into the right atrium of the heart, and into the coronary sinus (not shown) to reach the target coronary vein  96 . The apparatus  10  may be advanced over a guidewire (not shown), similar to the previous methods. Because of its relatively low profile, the apparatus  10  may be able to access smaller coronary veins or be advanced further into a target coronary vein than conventional devices. 
     Once the apparatus  10  is positioned within or near a target vein  96 , fluoroscopy and/or other external imaging may be used to facilitate positioning the apparatus  10 . The apparatus  10 , e.g., the distal end  24  of the sheath  20 , an overlying constraint, and the like (not shown), may include one or more radiopaque markers (not shown) to facilitate such imaging. In addition or alternatively, contrast may be introduced into the vein, e.g., via a fluid lumen in a stiffening member (not shown) of the apparatus  10  and/or through the lumen  26  of the sheath  20 , to facilitate fluoroscopic imaging. Such imaging may be used to identify the location of the sheath  20  relative to nearby structures, e.g., to ensure that the apparatus  10  is advanced as close as possible to a target location. Preferably, the apparatus  10  is advanced such that the distal end  24  of the sheath  20  is disposed within a coronary vein  96  adjacent the left ventricle of the patient&#39;s heart (not shown). 
     As shown in  FIG. 12B , the sheath  20  may be expanded between the entry site and the target vein  96 . If a constraint (not shown) is provided, it may be removed before expanding the sheath  20 . A fluid, e.g., including saline and/or contrast, may be introduced into the sheath  20  to expand the sheath  20  towards its enlarged condition. Contrast delivered into the sheath  20  may also facilitate imaging the vein  96 . For example, if the distal end  24  of the sheath  20  is initially closed, contrast delivered into the lumen  26  may cause the sheath  20  to expand and conform to the shape of the surrounding vessels, thereby facilitating imaging of the vessels through which the sheath  20  is deployed. 
     As shown in  FIGS. 12B and 12C , an electrical pacing lead  70  may be advanced through the lumen  26  of the sheath  20  until a tip  72  of the lead  70  is disposed within the target vein  96 . Similar to the methods described above, the sheath  20  may already include an opening  28  in its distal end  24  that communicates with the lumen  26  or an opening may be created in a closed sheath distal end  24  (not shown). The tip  72  may be deployed from the sheath  20 , e.g., by retracting the sheath  20  or advancing the tip  72  through the opening  28  beyond the distal end  24  of the sheath  20 , thereby substantially anchoring the tip  72  within the vein  96 . 
     As shown in  FIG. 12D , the sheath  20  may then be removed from the vein  96  and from the patient&#39;s vasculature. Preferably, the sheath  20  may be split into one, two, or more pieces to allow the sheath  20  to be removed easily from around the lead  70 . Implantation of the lead  70  may then be completed using conventional methods. 
     Turning to  FIGS. 14A-14C , another method is shown for delivering a colonoscope  76  into a patient&#39;s colon  98  using a sheath apparatus  10 , such as any of the embodiments described above. As shown in  FIG. 14A , the apparatus  10  is advanced into the colon  98  with the sheath  20  in its contracted condition. Once advanced to a desired location, the sheath  20  may be expanded to an enlarged condition to define a lumen  26 , as shown in  FIG. 14B . The colonoscope  76  may be advanced through the lumen  26  of the sheath  20 , similar to the previous embodiments. If desired, the sheath  20  may be configured such that the colonoscope  76  may image surrounding tissue within the colon  76  as the colonoscope  76  is advanced through the sheath  20 . For example, the material of the sheath  20  may be substantially transparent to visible light (or other portions of the spectrum) that is used by the colonoscope  76 . Alternatively, in other applications, the sheath  20  may allow other imaging sources, e.g., ultrasound energy, to pass substantially freely through the sheath  20  to allow imaging of the surrounding tissue. The sheath  20  may be removed from around the colonoscope  76 , as shown in  FIG. 14C , or may remain at least partially around the colonoscope  76 , e.g., to facilitate its removal after a procedure. In addition, the sheath  20  may be adapted for introducing fluid, e.g., air, carbon dioxide and the like, into the gastrointestinal lumen. 
     Turning to  FIGS. 15A-15D , a sheath apparatus in accordance with the present invention may facilitate advancement of one or more instruments retrograde into a patient&#39;s venous system. For example, as shown in  FIG. 15A , an exemplary vein  102  is shown that includes a plurality of valves  104 . A sheath apparatus  10  (which may be any of the embodiments described herein) may be advanced into the vein  102  with an expandable sheath  20  in its contracted condition, as shown in  FIG. 15B . Once advanced to a desired position, the sheath  20  may be expanded to its enlarged condition, as shown in  FIG. 15C , thereby opening the valves  104  and provided a relatively large bore lumen  26  through the vein  102 . One or more instruments, such as the stent  60  and catheter  60  shown in  FIG. 14D , may be advanced through the lumen  26  to complete a procedure, similar to the embodiments described above. 
     A sheath apparatus according to the present invention may also be used to perform other diagnostic and/or therapeutic procedures within the vasculature. For example, the apparatus may provide an expandable lumen for delivering an imaging element, e.g., an intravascular ultrasound (“IVUS”) device, endoscope, fiber optic element, and the like, and/or for delivering therapeutic instruments, e.g., angioplasty catheters, stent delivery devices, atherectomy or thrombectomy devices, and the like. 
     In alternative methods, a sheath apparatus in accordance with the present invention may be used to provide access and/or deliver instruments to other body lumens within a patient&#39;s body. For example, the apparatus may be used within a patient&#39;s urogenital tract, respiratory tract, gastrointestinal tract, lymphatic system, or vascular system. In addition, the apparatus may be introduced into surgically created openings and advanced into surgically created passageways, e.g., a passageway created within interstitial space, e.g. to obtain a biopsy. 
     Turning to FIGS.  18  and  19 A- 19 C, another embodiment of a sheath apparatus  510  is shown, in accordance with the present invention. Generally, the apparatus  510  includes a catheter or other elongate member  512  and an expandable sheath  520 . The catheter  512  may be an elongate tubular body formed from uniform or variable flexibility material, e.g., having a substantially flexible distal end  514 . The catheter  512  may include one or more lumens (not shown) extending from its proximal end (also not shown) to the distal end  514 . 
     In a preferred embodiment, the catheter  512  includes one or more diagnostic or therapeutic elements  516  on the distal end  514 . For example, the catheter  512  may include an imaging element, such as an intravascular ultrasound (“IVUS”) device, and the like, and/or a dissection element, such as a cutting or ablation element, e.g., for observing and/or treating an occlusion within a blood vessel (not shown). In addition or alternatively, the catheter  512  may include a steering element (also not shown) for manipulating the distal end  514  within a patient&#39;s body. 
     The sheath  520  may be a substantially flexible, and preferably flimsy, membrane or other structure. For example, the sheath  520  may include any of the materials and/or structures included in any of the embodiments described previously. The sheath  520  may be substantially permanently attached to an outer surface  518  of the catheter  512 . For example, the sheath  520  may include a sheet whose longitudinal edges are attached at one or more locations along a length of the catheter  512 . Alternatively, the sheath  520  may be a tubular member having an outer surface that is attached at one or more locations along the length of the catheter  512 . Thus, the sheath  520  may extend at least partially between the proximal end and the distal end  514  of the catheter  512 . 
     The sheath  520  may be expandable from a contracted condition (shown in  FIG. 19A ), e.g., to minimize a profile of the sheath  520 , and an enlarged condition (shown in  FIGS. 19B and 19C ). In the enlarged condition, the sheath  520  may at least partially define an accessory lumen  522 . The accessory lumen  522  may extend between a proximal end (not shown) and a distal end  524  of the sheath  520 , preferably terminating adjacent the distal end  514  of the catheter  512 . 
     The distal end  524  of the sheath  520  may include an opening (not shown) communicating with the accessory lumen  522 . Alternatively, the distal end  524  may be closed, and may include a break-away portion for creating an opening (not shown) such that an instrument (such as a guidewire  580 , shown in  FIG. 19C ) inserted through the lumen  522  may be advanced from the opening into a body lumen, similar to the previous embodiments. 
     Optionally, a constraint (not shown) may be provided for maintaining the sheath  520  in the contracted condition, e.g., as the apparatus  510  is advanced through a patient&#39;s body. For example, an outer sheath (not shown) may be provided for receiving the catheter  512  and the sheath  520 ) that may be removed before the sheath  520  is expanded. In addition or alternatively, an adhesive may be used to temporarily secure the sheath  520  along the outer surface  518  of the catheter  512 . Thus, the adhesive may retain the sheath  520  substantially flat along the outer surface  518  of the catheter  512 , as shown in  FIG. 19A . The adhesive may release when internal pressure is delivered into the lumen  522  of the sheath  520  or may dissolve within a body lumen to release the sheath  520  to allow the sheath  520  to expand. 
     Turning to  FIGS. 20A-20D , a method is shown for delivering an instrument, e.g., a guidewire  580  through a total occlusion  592 , e.g., within a blood vessel or other body lumen  590 , e.g., using the apparatus  510  shown in  FIG. 18 . Initially, as shown in  FIG. 20A , with the sheath  520  in its contracted condition, the apparatus  510  may be introduced into the vessel  590 , e.g., via a percutaneous entry site (not shown), until the distal end  514  of the catheter  512  is located proximally (e.g., upstream) relative to the occlusion  592 . 
     With reference to  FIG. 20B , the distal end  514  of the catheter  512  may be advanced through the occlusion  592  until the distal end  524  of the sheath  520  is disposed distally (e.g., downstream) relative to the occlusion  592 . The catheter  512  may include one or more elements  516  on the distal end  514  for facilitating advancing the catheter  512  through the occlusion  592 . For example, the catheter  512  may carry an imaging element, e.g., an IVUS device (not shown), on the distal end  514  for imaging the occlusion. In addition or alternatively, a dissection element, e.g., a cutting, core, and/or ablation element (not shown), may be provided on the distal end  514  for creating a passage  594  through the occlusion. Optionally, the catheter  512  may include a steering element (not shown) to facilitate manipulating the distal end  514  of the catheter  512  as it is advanced through the occlusion  592 . 
     Once the sheath  520  extends through the occlusion  592 , an elongate member, e.g., a guidewire  580  or other instrument (not shown), may be inserted through the sheath  520  until a distal end  582  of the guidewire  580  is disposed distal to the occlusion  592 . The sheath  520  may be expanded as the guidewire  580  or other instrument is advanced through the sheath  520 . Alternatively, the sheath  520  may be dilated, e.g., by introducing a fluid into the lumen  522  (see  FIG. 29B ) of the sheath  520 , similar to the embodiments described above. 
     The apparatus  510  may then be withdrawn through the occlusion  592  and/or out of the vessel  590 . One or more instruments (not shown) may then be advanced over the guidewire  592 , e.g., to treat the occlusion  592  and/or to deliver agents or fluids distally beyond the occlusion  592 , as is well known to those skilled in the art. Thus, unlike the previous embodiments, the sheath  520  may provide a lumen for a secondary device, e.g., a guidewire, that may be delivered in cooperation with the catheter  512 . 
     While the invention is susceptible to various modifications, and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the invention is not to be limited to the particular forms or methods disclosed, but to the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims.