Abstract:
A catheter that comprises a sheath that is connected at opposing ends to concentric tubes that move relative to each other in a manner that alternatively covers and exposes a medical device loaded onto the catheter. A portion of the sheath is arranged so as to invert upon itself so that axial movement of one tube relative to the other simultaneously moves the inversion point over or away from the device, alternatively covering or exposing the device.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to an apparatus and method for loading a medical device onto a minimally invasive delivery system, such as a delivery catheter, and deploying the device in situ. 
         [0003]    2. Description of the Related Art 
         [0004]    Percutaneous aortic valve replacement (PAVR) technology is emerging that provides an extremely effective and safe alternative to therapies for aortic stenosis specifically, and aortic disease generally. Historically, aortic valve replacement necessitated surgery with its attendant risks and costs. The replacement of a deficient cardiac valve performed surgically requires first opening the thorax, placing the patient under extracorporeal circulation or peripheral aorto-venous heart assistance, temporarily stopping the heart, exposing and excising the deficient valve, and then implanting a prosthetic valve in its place. This procedure has the disadvantage of requiring prolonged patient hospitalization, as well as extensive and often painful recovery. Although safe and effective, surgical replacement presents advanced complexities and significant costs. For some patients, however, surgery is not an option for one or many possible reasons. As such, a large percentage of patients suffering from aortic disease go untreated. 
         [0005]    To address the risks associated with open-heart implantation, devices and methods for replacing a cardiac valve by less invasive means have been developed. For example, CoreValve, Inc. of Irvine, Calif. has developed a prosthetic valve fixed to a collapsible and expandable support frame that can be loaded into a delivery catheter. Such a prosthesis may be deployed minimally invasively through the vasculature at significantly less patient risk and trauma. A description of the CoreValve bioprosthesis and various embodiments appears in U.S. Pat. Nos. 7,018,406 and 7,329,278, and published Application Nos. 2004/0210304 and 2007/0043435. By using a minimally invasive replacement cardiac valve, patient recovery is greatly accelerated over surgical techniques. In the case of the CoreValve device, the support frame is made from shape memory material such as Nitinol. Other catheter-delivery valve replacement systems use stainless steel, or do not rely upon a rigid frame. 
         [0006]    As demonstrated successfully to date, using a transcatheter procedure, percutaneous aortic valve replacement proceeds by delivering a prosthetic valve to the diseased valve site for deployment, either using a balloon to expand the valve support against the native lumen or exposing a self-expanding support in situ and allowing it to expand into place. With the latter, the self-expanding frame remains sheathed during delivery until the target site is reached. Advantageously, the frame may be secured to the catheter to avoid premature deployment as the sheath is withdrawn. In the CoreValve valve prosthesis, a hub is employed with two lateral buttons around each of which a frame zig may reside during delivery. The internal radial force of the sheath keeps the frame compressed against the catheter, including the frame zigs in place around the lateral buttons. The catheter generally comprises at least two tubes, an inner tube that carries the prosthesis and an outer tube that carries the sheath, permitting the sheath to move relative to the prosthesis. 
         [0007]    As with traditional cardiovascular interventional therapies, transcatheter device deployment may proceed retrograde against normal blood flow, or antegrade, with blood flow. For aortic valve replacement, entry through the femoral arteries proceeds in a retrograde format through the iliac, descending aorta, over the arch and to the native annulus. In some cases, entry has been made closer to the arch; for example through the left subclavian artery. Antegrade procedures have been performed whether delivery takes place through the venous system transeptally to the native aortic annulus. More recently, transapical procedures have been performed whereby a cardiac surgeon delivers a catheter through the left ventricle apex to the target site. 
         [0008]    With retrograde deployment, it is generally desired that the catheter be advanced within the vasculature so that the device is positioned where desired at the annulus site. With some embodiments under development, the desired site is the annulus itself. With the CoreValve device, the desired site extends from the annulus to the ascending aorta, given its relative length. In the transfemoral approach, when the CoreValve device is positioned at the desired site, the sheath is withdrawn to the point where the inflow end of the device (preferably positioned at the native annulus) expands to engage and push radially outwardly the native valve leaflets. The sheath continues to be withdrawn proximally as the prosthesis continues to expand as it is exposed until the sheath covers just the outflow portion of the prosthesis still secured to the hub ears. Any readjustment of the axial position of the device in situ can be made during this process based upon electronic visual feedback during the procedure. Once well positioned, the sheath is fully withdrawn, the device fully expands in place, and the catheter is withdrawn through the center of the device and out through the vasculature. While it would be possible to deploy the prosthetic device such that the sheath could be withdrawn distally so that the outflow end of the prosthesis deploys first, such an arrangement would require advancing distally the outer tube of the catheter connected to the sheath distally. In the case of transfemoral retrograde delivery, that would cause the outer tube to project well into the left ventricle, which is not desirable. In a antegrade approach, for example transapical delivery, the reverse situation exists. There it is more desirable to advance the sheath distally to expose the inflow end of the prosthesis at the native annulus first. The native anatomy can accommodate this distal deployment because the outer tube carrying the sheath is advanced up the ascending aorta towards the arch. Like the retrograde approach, once the valve prosthesis is fully deployed, the catheter may be withdrawn through the center of the prosthesis and removed through the apex of the heart. 
         [0009]    With minimally invasive cardiac valve replacement, as may be appreciated, loading of a self-expanding valved frame into a sheath (or capsule) can be difficult because of the frictional forces that inhibit movement of the frame into and out of the sheath. The radial forces attendant in a self-expanding frame are pushing the frame against the inner wall of the sheath during the axial movement of the frame relative to the sheath. The friction translates into a greater axial force that must be applied to smooth and reliably load and deploy the frame from within the distal sheath. Where precision is demanded, such friction requiring greater axial force to be applied makes accurate deployment more difficult. Accordingly, a need exists for a suitable system and method of loading and deploying a self-expanding valved frame using a delivery catheter that reduces the inhibiting nature of the frictional forces during loading and deployment. 
       SUMMARY OF THE INVENTION 
       [0010]    The invention provided comprises embodiments for minimally invasively delivering a medical device to a patient. The apparatus comprises a sheath that is connected at opposing ends to concentric tubes that move relative to each other in a manner that alternatively covers and exposes the medical device. A portion of the sheath is arranged so as to invert upon itself causing an inversion point. It is contemplated that axial movement of one tube relative to the other simultaneously moves the inversion point over or away from the medical device. In such a manner, there is little frictional engagement between the inversion point and the device (e.g., self-expanding frame). As contemplated, there are several different embodiments that can be made to employ the invention claimed herein, including some with more than one inversion point. These and other features, aspects and advantages of embodiments of the present invention are described in greater detail below in connection with drawings of the apparatus and method, which is intended to illustrate, but not to limit, the embodiments of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIGS. 1A-C  are cross-sectional views of one embodiment of a device delivery system showing sequential axial movement of an internal tube relative to an outer tube. 
           [0012]      FIGS. 2A-C  are cross-sectional views of a second embodiment of a device delivery system showing sequential axial movement of an internal tube relative to an outer tube. 
           [0013]      FIGS. 3A and 3B  are cross-sectional views of another embodiment of a device delivery system showing sequential axial movement of an outer tube relative to an outer tube. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]    Referring to  FIGS. 1A-C , one exemplary embodiment of an improved delivery system  10  for a medical device  12  comprises a catheter  14  having a distal end  16  and a proximal end  18 . In the figures shown, and by way of example, the medical device  12  is a self-expanding frame. 
         [0015]    The catheter  14  further comprises a first inner tube  22  and an outer tube  24 . At the distal end of the outer tube  24  is a cap  26  affixed to the outer tube  24 . The cap  26  is preferably configured to have a smooth rounded surface at its distal most-end. By way of simplifying the description herein,  FIG. 1A  shows the system  10  such that the distal end of inner tube  22  is positioned proximal the distal end of outer tube  24 , whereas  FIG. 1B  shows the inner tube  22  pulled in a proximal direction, with  FIG. 1C  showing it pulled further in the proximal direction. 
         [0016]    The catheter  14  further comprises a sheath  30  preferably made of resilient pliable material, such as those used in the industry. The sheath may comprise in whole or in part a braided, woven, or stitched structure, a polymer, or may comprise an inflatable balloon. A first end  32  of the sheath  30  is affixed to an outer surface of the cap  26  affixed to the distal end of the outer tube  24 . A second end  34  of the sheath  30  is affixed to the outer surface of the distal end of the inner tube  22 . As shown in  FIG. 1A , the sheath  30  is configured to constrain the medical device  12  in a collapsed position for delivery to a target site. 
         [0017]    The sheath  30  is configured so that it overlaps itself on an external surface of the catheter  14  to form an inversion point  36  proximal of the distal end. The sheath  30  is further configured to conform to the smooth rounded distal surface of the cap  26  such that, as the inner tube is pulled in a proximal direction, the sheath smoothly slides over the cap causing the inversion point  36  to move distally.  FIGS. 1A through 1C  show that progression. As the sheath  30  is pulled so that the inversion point  36  moves distally, the medical device  12  is progressively exposed, permitting it to expand as desired. 
         [0018]    With the embodiment shown in  FIGS. 1A-1C , the inner tube  22  does not need to advance distally beyond the distal cap  26  of the outer tube  24 . Indeed, the catheter  14  need not be placed much more distal than the target site of the medical device  12 . Thus, deployment of a medical device using this embodiment may be made translumenally through the vasculature in one of many possible directions. For example, with respect to an aortic valve replacement, where the medical device  12  is an expandable valved frame, the catheter  14  may be directed transfemorally, transapically or through the sub-clavian artery conveniently. The device  12  may be delivered antegrade or retrograde through the arterial or venous system. Once the medical device  12  is deployed, the entire catheter  14  may be withdrawn proximally from the target site. 
         [0019]    It should be appreciated that loading of the medical device  12  onto the outer tube  24  of catheter  14  would entail collapsing the medical device over the outside surface of the outer tube  24  and then moving the inner tube  22  distally relative to the outer tube  24  so as to cause the inversion point  36  to move proximally over the medical device  12 . When the inversion point has reach its proximal-most point, as shown in  FIG. 1A , then catheter  14  may then be used to deliver the medical device  12 . For a self-expanding frame, collapse may be induced by, for example, reducing its temperature. For a balloon expandable frame, the device  12  can be crimped onto the outer tube  24  in one of many known ways. In that case, outer tube  24  would comprise a dilation balloon for in-situ deployment. 
         [0020]    A variation on the embodiment of  FIGS. 1A-1C  is shown in  FIGS. 2A through 2C , where the components are the same. With this embodiment of catheter  114 , the second end  34  of the sheath is attached to the inner tube  22  so as to permit effective advancement of the inner tube  22  in the distal direction, rather than the proximal direction. As the inner tube  22  is directed distally, the inversion point  36  also advances distally, exposing the medical device  12 . While the embodiment of  FIGS. 2A-2C  may be used in a variety of delivery directions, as discussed above with the embodiment of  FIGS. 1A-1C , it is preferred that the target site for the medical device  12  using this catheter embodiment  14  be such that there is sufficient room distal of the target site for effective advancement of the inner tube  22 . 
         [0021]    Referring to  FIGS. 3A and 3B , a third exemplary embodiment is shown. There, a medical device  212  is shown sheathed within catheter  214 , which has similar components to catheters  14  and  114  discussed above, but with a somewhat different arrangement. Catheter  214  has a proximal end  216  and a distal end  218 , and comprises an inner tube  222  and an outer tube  224 , where the outer tube and inner tube are movable relative to each other. A collar  226  is affixed to the outside of inner tube  222 . A sheath  230 , covering medical device  212 , has a first end  232  affixed to collar  226  and a second end  234  affixed to the outside distal end of outer tube  224 . The sheath  230  is arranged so as to create an inversion point  236  at a distal location. As the outer tube  224  is retracted proximally, the inversion point  236  likewise moves proximally, exposing the medical device  212  in the same way as explained with the other embodiments. 
         [0022]    It should be understood that with any of these exemplary embodiments, or any variation on these configurations, the clinician may manipulate the alternative of the inner or outer tubes to expose the medical device, although this would result in the medical device moving toward a target site during its deployment, rather than remaining stationary during deployment. For example, in the first embodiment, instead of pulling the inner tube  22  proximally, the medical device  12  may be exposed by advancing the outer tube  24  distally. The result is the same; the inversion point  36  is advanced distally. Likewise, the outer tube  24  of embodiment  2 A- 2 C could be pulled proximally rather than the other tube advanced distally and the inner tube  222  of embodiment  3 A- 3 B could be pulled distally, rather than the other tube being advanced proximally. 
         [0023]    One advantage of using an inverting sheath to load and deploy a medical device is that the sheath alternatively covers and exposes the medical device by predominantly a rolling motion rather than a sliding motion, which results in less friction between the medical device and the sheath. This reduces the force required to retract the sheath, which enables more control over the deployment position by, for example, reducing the compression and elongation of the delivery catheter. In addition, where the medical device comprises a prosthetic tissue-based heart valve sutured to a self-expandable frame, the patient&#39;s body heat can cause the frame to want to revert to its natural expanded configuration, thereby exerting an outward force against the sheath. During deployment, friction between the sheath and medical device can damage the tissue-based heart valve and sutures. Accordingly, reducing the friction between the sheath and medical device by using a rolling motion rather than a sliding motion can help reduce damage to the medical device and help maintain the condition of the medical device. In other cases, the medical device may be coated with a drug or bioactive material, and the friction caused by sliding the coated stent out of the sheath can result in removal of some of the drug or bioactive material. 
         [0024]    In some embodiments, the surface of the sheath that contacts the medical device may be tacky, which enables the tacky surface to frictionally engage the medical device and reduce sliding between the medical device and sheath. The surface can be made tacky by, for example, application of a polymeric material such as polyurethane or another thermoplastic elastomer to the surface or by fabricating the surface from the tacky material. 
         [0025]    It is contemplated that the surface of the sheath that contacts itself when inverted may be provided with a lubricious coating or can be made of a lubricious material. The lubricious coating or material can be made of, for example, PTFE, ePTFE, a hydrophilic material, or any other substance that reduces the friction as the inverted sheath slides over itself. Where desired, the sheath may be reinforced to minimize elongation of the sheath as tension is applied. For example, axially-oriented tension bands (not shown) having high tensile modulus material such as ultra high weight polyethylene, Kevlar, carbon, steel, titanium, in the form of a monofilament or fiber may be incorporated within or on the sheath. 
         [0026]    In operation, the catheters described are particularly suited for delivery of a heart valve, where precise placement is important. Other critical and less-critical target sites are also contemplated. In the case of a self-expanding aortic valve replacement, the catheter may be delivered transfemorally, transeptally, transapically or through the sub-clavian, among other possible entry ways. In one procedure, the catheter is deployed so that the valved frame is positioned entirely aligned with the target site; e.g., aortic annulus up to ascending aorta. The frame may then be exposed from one end to the other, depending upon the direction of delivery, by either advancing the inner tube relative to the outer tube (for the embodiment of  FIG. 2 ) or vice versa (for the embodiment of  FIG. 1 ), or retraction of the outer shaft (for the embodiment of  FIG. 3 ). As the frame is exposed, it expands outwardly to engage the native intimal lining so placement accuracy is maximized. When the sheath is fully removed and the frame fully expanded, the catheter may then be withdrawn though the functioning prosthetic valve and removed from the patient. 
         [0027]    Although embodiments of this invention have been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the embodiments of the present invention extend beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In particular, while the present loading system and method has been described in the context of particularly preferred embodiments, the skilled artisan will appreciate, in view of the disclosure, that certain advantages, features, and aspects of the system may be realized in a variety of other applications, many of which have been noted above. Additionally, it is contemplated that various aspects and features of the invention described can be practiced separately, combined together, or substituted for one another, and that a variety of combination and subcombinations of the features and aspects can be made and still fall within the scope of the invention. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims.