Abstract:
A prosthetic aortic valve intended for native or valve-in-valve within bioprostheses includes an expandable support scaffold and valve leaflets disposed within an upper leaflet portion of the support scaffold. The valve leaflets within the upper portion may be located within the annulus (intravalvular), above the annulus, or above the native or prosthetic leaflets (supravalvular). The valve within a previously implanted degenerated heart valve such that a base or lower portion of the replacement valve is within the previously implanted valve and the upper portion is expanded within the aorta, the internal area of the valve can be increased and the hemodynamics of the valve improved. Alternatively, the valve may include separate upper and lower portions allowing the portions to be implanted sequentially and the length and other characteristics of the valve to be adjusted based on patient anatomy and condition.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to medical devices and methods. More particularly, the present invention relates to devices and methods for implanting a prosthetic aortic valve in a patient. 
         [0003]    Aortic valve replacement procedures have historically been performed in opened chest, stopped heart procedures where the native valve is removed and a synthetic or biologic tissue valve is sutured in its place. Although highly successful, the need to open the chest and the potential side effects of cardiopulmonary bypass needed to sustain the patient while the heart is stopped have spurred the development of less invasive transcatheter and endovascular procedures performed on a beating heart. 
         [0004]    While such transcatheter and endovascular procedures have enjoyed success and are being increasingly utilized, the nature of the valves and limitations of the implantation protocols still limit widespread adoption. In particular, the need to deliver the valves in a compressed configuration stowed in access catheters or other delivery devices has made the transcatheter and endovascular valves less adaptable than are prosthetic valves used in open surgical procedures. In particular, present transcatheter and endovascular valve designs are often not useful in patients having bicuspid aortic valves, degenerated bioprosthetic valves, aortic stenosis associated with aortic aneurysms, aortic dissections, unusual or variant anatomies or in procedures replacing previously implanted prosthetic valves. 
         [0005]    Patients suffering from different valve-related conditions can require valves having quite different geometries. For example, patients suffering from aortic stenoses may have quite different requirements than those having a concomitant aneurysm in the ascending aorta. Patients may also have different anatomies which would in some cases benefit from placement within the valve annulus or in other instances benefit from placement above the annulus and/or above the native or bioprosthetic valve leaflets. Moreover, the replacement of previously implanted prosthetic valves may dictate even different requirements on the prosthetic valve structure, as discussed below. 
         [0006]    The feasibility of transcatheter or endovascular aortic valve implantation within previously implanted bioprosthetic aortic valves has recently been demonstrated. Degeneration of originally implanted bioprosthetic aortic valves can occur through a number of mechanisms, including calcification. Although such “valve-in-valve” (VIV) implantation has shown promise, hemodynamic complications from implantation of currently available transcatheter valves within small bioprosthetic valves can be problematic. A principal problem is the reduction in valve area which occurs when a valve is implanted within the base or frame of the previously implanted valve. Another important problem is the deformation of the implanted valve by the calcified diseased valve. Such deformation can result in leaks around the implanted valve or through the implanted valve leaflets which will not align properly when the stent surrounding the leaflets are squeezed in a smaller size than intended or squeezed away from its intended circular shape. 
         [0007]    Thus, it would be desirable to provide improved transcatheter and endovascular aortic valve designs which can be used in a variety of replacement procedures including both native aortic valve replacement and replacement of previously implanted prosthetic valves. Such aortic valves should be adaptable and configurable to conform to different patient anatomies and to treat patients having different conditions, including aortic stenoses, aortic insufficiency, aortic calcification, aneurysms in the sinus of valsalva or ascending aorta, and any other condition which might require native or prosthetic valve replacement. Such aortic valve designs should be adaptable for use in natural valve replacement as well as for valve-in-valve implantation, where in both cases the valves provide superior hemodynamic performance. The present invention will at least in part meet these objectives. 
         [0008]    2. Description of the Background Art 
         [0009]    Percutaneous aortic valve-in-valve replacement is described in Wenawest et al. (2007) Catheter Cardiovasc Intery 70:760-764. Prosthetic heart valves adapted for supra-annular implantation are described in Turing (2004) Multimedia Manual of Cardiothoracic Surgery doi: 10.1510/mmcts.2004.000646; and Vergnet (2009) Eur J Cardiothoracic Surg 35:970-976 Patents of interest include U.S. Pat. No. 6,582,462; U.S. Pat. No. 5,411,552; U.S. Pat. No. 6,893,460; U.S. Pat. No. 6,730,118; U.S. Pat. No. 7,018,406; U.S. Pat. No. 4,451,936; U.S. Pat. No. 5,489,297; U.S. Pat. No. 5,562,729; U.S. Pat. No. 6,315,791; U.S. Pat. No. 6,440,164; and U.S. Pat. No. 7,025,780. Other publications of interest include Walther et al. (2008) Ann Thorac Surg 85:1072-1073; Azadani et al. (2009) Ann Thorac Surg 88: 1864-1870; Azadani et al. (2010) Interact Cardiovasc Thorac Surg 11:70-77; Azadani et al. (2009) Ann Thorac Surg 88:1857-1863; Dwyer et al. (2009) J Thorac Cardiovasc Surg 138:1227-33. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The present invention provides novel prosthetic aortic heart valve designs and methods for implantation of such heart valves. The heart valves comprise an expandable support scaffold having a lower base portion and an upper valve portion, where the lower base portion and upper valve portion are preferably, but not necessarily separate components which are separately implantable. By providing the prosthetic aortic valves with a two-part modular design, the valve is suitable for treating a variety of patient conditions, including aortic stenoses, aortic insufficiency, aneurysms in the sinus of valsalva, aortic valve disease with concomitant aneurysms of the ascending aorta, as well as placement within previously implanted prosthetic valves which have degenerated. The two component design allows a lower base portion to act as an anchor which can be positioned within the valve annulus (or previously implanted prosthetic valve) while a separate valve portion may be positioned either within the native valve location (intravalvular) or above the native valve location (supravalvular), depending on the patient&#39;s condition and anatomy. Moreover, the two component design allows positioning and repositioning of the valve relative to the base during the procedure so that the implanted configuration can be optimized for the particular patient. Still further, the two-part platform allows the valve component to be removed, leaving the base component in place, should the valve fail for any reason after implantation. A new upper valve portion can then be implanted in the base. The ability to remove the upper valve portion and replace that portion only with a new one while leaving the lower portion implanted is also particularly useful when the upper valve portion degenerates over time and requires replacement years after the initial procedure. 
         [0011]    In a first aspect of the present invention, a method for implanting a prosthetic valve at an aortic valve location comprises providing a replacement aortic prosthetic valve having a lower base portion and an upper valve portion, where the two portions are usually but not necessarily separate structures. The replacement aortic valve is positioned in the aortic valve location so that the lower base portion is located within the valve annulus and the upper valve portion is located within the annulus (intravalvularly) or within the aorta above the valve annulus (supraannular) or within the aorta above the valve leaflets (supravalvularly). A particular advantage of using the separate valve components is that either intravalvular or supravalvular placement can be accommodated. Optionally, the upper valve portion maybe differentially expanded with respect to the lower base portion, particularly when the valve is to be positioned within a previously implanted prosthetic valve. In this way, the base portion can accommodate the restricted diameter of the previously implanted prosthetic valve while the valve portion can be expanded to a larger size in order to improve the hemodynamics of the valve. Depending on the particular valve designs, the valve maybe fully balloon deployable, fully self-expanding, or more typically a combination of balloon expandable and self-expanding components. 
         [0012]    When implanting prosthetic aortic valves having a lower base portion separate from the upper valve portion, the base portion will usually be implanted first, where the base portion has a balloon expandable inner tubular wall and a self-expanding outer tubular wall. With such a design, the base maybe delivered by constraining it within a delivery catheter or other delivery tool and releasing the base from constraint so that the outer tubular wall expands and conforms to the inner well of the valve annulus. Typically, the outer tubular wall will comprise anchors, such as barbs or hooks, on its outer surface in order to penetrate the annulus upon expansion and immobilize the base portion therein. Additionally, the outer tubular wall will usually be covered with a material which seals against the wall of the annulus (or interior of a previously implanted prosthetic valve) to prevent paravalvular leakage. The inner tubular wall will also usually be covered with a material to prevent paravalvular leakage. For placement within native aortic valve leaflets, the self-expanding nature of the outer tubular wall is made to conform to the irregularities of shape created after aortic balloon valvuloplasty and is particularly useful to prevent paravalvular leakage with irregular aortic valve anatomy including bicuspid aortic valves. The balloon expandable inner tubular wall maintains circular orientation to promote hemodynamic flow through the new valve. 
         [0013]    When the upper valve portion is a separate component, it will usually include a lower engagement region and an upper leaflet structure, where the lower engagement region is positionable within the inner tubular wall of the base portion, and both the engagement region and the upper leaflet region are balloon expanded so that the engagement region conforms to the inner tubular wall of the base portion of the valve and the upper leaflet structure assumes a generally circular shape, at least when supravalvular placement is being performed. The lower engagement region may also be self expandable to conform to the inner tubular wall of the base portion of the valve. Usually, the lower engagement region of the upper valve portion is covered to inhibit paravalvular leakage. 
         [0014]    Sequential placement of the two components of the prosthetic aortic valve is particularly advantageous when treating native valves, allowing for significant freedom in placing the components to accommodate patient anatomy and condition. In addition, the upper component may be replaced with a new upper valve component in a procedure using a similar endovascular and transcatheter approach if the first upper valve component degenerates and fails over time. For treating previously implanted degenerated small prosthetic valves, however, it will often be more desirable to use a prosthetic valve structure having an integrated structure where the upper valve portion is fixed to the lower base portion. In such cases, the lower base portion will preferably comprise only a single scaffold structure (i.e., it will not include inner and outer tubular members as described previously) in order to reduce the size of the base portion which is located within the annulus of the previously implanted valve. It will be appreciated that there is only a limited amount of space available within the previously implanted valve particularly when the implanted valve was of small size, so use of inner and outer tubular members in the base portion is contraindicated. 
         [0015]    The present invention further provides prosthetic aortic heart valves comprising an expandable support scaffold including both a lower base portion and an upper valve portion. Valve leaflets are disposed within the upper valve portion of the expandable support, and the lower base portion is adapted to be expanded within the heart valve annulus while the upper leaflet portion is adapted to be expanded either within the annulus itself or above the annulus or above the native or prosthetic valve leaflets. Usually, the expandable support scaffold will have a generally cylindrical geometry where at least the base portion is sufficiently deformable to conform to non-circularities at the implantation site. Optionally, the upper valve portion of the expandable support scaffold will have greater hoop strength than the lower portion in order to maintain circularity despite non-circular expansion of the lower base portion which would have a tendency to deform the upper support scaffold as well. The overall dimensions of the scaffold are typically in the range from 17 millimeters to 30 millimeters in diameter with an axial length in the range from 13 millimeters to 40 millimeters or greater. As discussed above with respect to the method, at least a portion of the base of the expandable support scaffold will typically be covered with a sheath in order to inhibit paravalvular leakage, and the sheath maybe composed of Dacron™, PTFE, ePTFE, and the like. Additionally, the lower base portion may include anchors, such as barbs or hooks, in order to help fix the scaffold within the valve annulus or bioprosthesis and to further reduce paravalvular leakage. 
         [0016]    As discussed above with respect to the methods, the prosthetic aortic valves of the present invention will preferably comprise a separate lower base portion and separate upper valve portion, where the two portions are separately and sequentially positionable within the heart valve annulus. The use of such two-component prosthetic heart valves is particularly useful for implantation in native valves and will usually be less useful for implantation within degraded previously placed prosthetic valves. 
         [0017]    The base portion of the prosthetic aortic valve will typically comprise a balloon expandable inner tubular wall and a self-expanding outer tubular wall. In this way, the base portion can be constrained while being delivered and released from constraint to expand and conform to the valve annulus. The outer tubular wall will typically include anchors, such as hooks or barbs, on its outer surface, where the anchors are adapted to penetrate the annulus upon expansion in order to fix the valve in place. The outer tubular wall of the lower base portion will typically comprise a super elastic metal scaffold, such as Nitinol™ nickel-titanium alloy, covered with a material that inhibits paravalvular leakage, such as the materials listed above with respect to the method. The inner tubular wall will be balloon expanded to maintain a circular geometry for optimal hemodynamics through the upper valve component as well as to provide overlap and anchoring of the upper valve portion. Maintenance of circular geometry of the inner tubular wall is desirable to allow repositioning of the upper valve portion during the procedure or replacement of the upper valve portion if the valve fails acutely or degenerates over time. The inner tubular wall will typically be covered with a material that inhibits paravalvular leakage, such as the materials listed above with respect to the method. 
         [0018]    The upper valve portion of the prosthetic aortic valve of the present invention typically includes a lower engagement region and an upper leaflet structure. The lower engagement region is positionable within the inner tubular wall of the base portion (after the base portion has been deployed) and both the lower engagement region and the upper leaflet region are preferably balloon expandable although the lower engagement region has the option of being self-expandable. The lower engagement region of the upper valve portion will typically also comprise a cover to inhibit paravalvular leakage. 
         [0019]    Specific embodiments of the methods and apparatus of the present invention are intended for treating previously implanted prosthetic aortic valves which have degraded over time. Such previously implanted heart valves may degenerate or otherwise become defective through calcification or other processes over time. Once such valves cease to perform adequately, it becomes necessary to provide a replacement prosthetic aortic valve. Heretofore, it has been proposed to implant generally smaller valves within the previously implanted degenerated heart valve. While such smaller valves can improve valve function, they frequently suffer from a decreased ability to allow blood to flow from the left ventricle into the aorta. In addition to a reduction in diameter, implanted valve-in-valve prosthetics need to maintain circularity at the level of the valve to maintain performance. 
         [0020]    The prosthetic valves of the present invention may also find use in treatment of diseased and degenerated native valves. For example, they may find particular use in treating patients having congenital bicuspid aortic valves which have highly irregular non-circular geometry, and resist prosthetic valve implantation since they will either deform the valve leaflets within the implanted valve or cause significant paravalvular leakage. If implanted valve-in-valve prosthetics in native or bioprosthetic valves do not have sufficient strength to maintain circularity, they face distortion or deformation caused by the calcification within the diseased valve being replaced (or in some instances the native valve). Lack of circularity causes malalignment of the valve leaflets and central leakage. If the prosthetic valve maintains circularity in the face of highly irregular non-circular disease particularly in native bicuspid valves, substantial leakage into the left ventricle can occur around the prosthetic valve (paravalvular leak). The prosthetic valves of the present invention will be usable in the face of irregular noncircular disease since the valve portion of the invention is above the diseased valve and can maintain circularity. 
         [0021]    The present invention provides for repair of previously implanted aortic prosthetic valves in a way which provides improved hemodynamics, particularly providing an increased valvular cross-sectional area to reduce pressure drop and increase blood flow rate. Additionally, moving the new valve outside of the native valve position maintains circularity of the new valve and reduces leakage. The replacement aortic prosthetic valves of the present invention will include an expandable support scaffold having a lower base portion and an upper leaflet portion. The scaffold will typically be formed as a metal lattice, typically using techniques employed with the manufacture of cardiovascular and other medical stents. The expandable support scaffolds may be balloon expandable, typically being formed from stainless steel and alloys thereof, where the scaffolds are delivered using a balloon delivery catheter. Alternatively, the expandable support scaffolds may be self-expanding, typically being formed from a highly elastic metal, such as Nitinol™ or other shape memory metal. Both the balloon-expandable and self-expanding support scaffolds will typically be formed by laser cutting of tubular structures. Other known stent fabrication techniques may also be utilized. 
         [0022]    The aortic valves will further include valve leaflets disposed within an upper leaflet portion of the expandable support. The valve leaflets may be synthetic, for example being formed from Dacron or other suitable fabric, or may be biological, such as being formed from treated animal tissues or may be tissue engineered. The leaflets will be formed into a tricuspid arrangement and secured to an inner surface of the upper leaflet portion of the expandable support, for example by suturing or other known techniques. 
         [0023]    The lower base portion of the expandable support will be adapted to expand within the previously implanted degenerated prosthetic heart valve, while the upper leaflet portion will be adapted to be expanded within the aorta above the degenerated prosthetic heart valve. The lower base portion of the scaffold will preferably be covered with a sheath material such as Dacron, Teflon, or PTFE or formed as a solid wall (optimally folded to permit radial expansion and contraction) to prevent paravalvular leakage and inhibit future stenoses. By positioning the valve leaflets within the region above the degenerated prosthetic heart valve, expandable support may be expanded to provide an increased cross-sectional area so that the valve leaflets can open to maximize the area available for blood flow, thus minimizing the pressure drop across the valve. Some portion of the upper leaflet portion of the scaffold will also preferably be covered to prevent valve leakage. However, the open portion of the stent in which the valve sits will be sized to allow blood flow to bypass the valve to permit blood flow into the coronary arteries which branch off from the aorta. 
         [0024]    The prosthetic aortic heart valve of the present invention will have dimensions suitable for implantation within the previously implanted degenerated prosthetic heart valve. The valve/stent diameter will be wide enough to enable good hemodynamic flow but narrow enough to allow enough space between the upper portion of the device and the walls of the aorta so that blood may flow around the devices and into the coronaries. For example, the prosthetic aortic heart valve of the present invention will typically have a width or diameter, when expanded, in the range from 17 mm to 30 mm and an axial length in the range from 20 mm to 40 mm or greater. The upper portion of the support will typically include about half of the total length, e.g. being in the range from 12 mm to 20 mm, while the lower base portion will have a substantially equal length. 
         [0025]    Methods according to the present invention fowl implanting a prosthetic aortic valve within a previously implanted degenerated prosthetic aortic valve comprise providing a replacement prosthetic aortic valve, generally as described above. The replacement prosthetic aortic valve is positioned within the degenerated prosthetic aortic valve so that the lower base portion is located within the degenerated prosthetic heart valve and the upper leaflet portion is located within the aorta above the degenerated prosthetic heart valve. Positioning may comprise expanding the upper leaflet portion of the replacement aortic valve so that the leaflet portion is wider than the base portion in order to increase the cross-sectional or luminal area available for blood flow through the replacement valve. Expanding the replacement aortic prosthetic valve may comprise inflating a deployment balloon within the replacement heart valve or alternatively, comprise releasing the replacement prosthetic heart valve from constraint so that it self-expands. 
         [0026]    The scaffold (stent) of the present invention will preferably be covered over at least its lower portion to prevent paravalvular leak. The base of the covered scaffold will be designed to allow not only circular expansion but also allow maximum expansion in an irregular fashion to conform to the degenerated leaflets to minimize paravalvular leakage. Shape irregularities over the lower portion of the invention will not affect the function of the fully expanded circular upper portion above where the transcatheter aortic valve leaflets sit. Thus any irregularities in expansion at the base will not diminish valve leaflet adaptation and function above the base. Such loss of function is found within other transcatheter valve designs which require a circular expansion within the diseased leaflets. Additional barbs or hooks, either covered or uncovered may optionally be placed at the base to improve stent fixation within the bioprosthesis and reduce paravalvular leakage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIGS. 1A and 1B  illustrate a prototype replacement prosthetic valve constructed in accordance with the principles of the present invention. 
           [0028]      FIG. 2  is a schematic illustration of the prosthetic aortic valve of  FIG. 1  implanted within a degenerated prosthetic heart valve. 
           [0029]      FIGS. 3 and 4  illustrate an implantation protocol of a replacement prosthetic heart valve in accordance with the principles of the methods of the present invention. 
           [0030]      FIG. 5  illustrates a second embodiment of a prosthetic aortic valve constructed in accordance with the present invention comprising a lower base portion and a separate upper valve portion, where the lower base portion comprises inner and outer tubular members. 
           [0031]      FIGS. 6A through 6D  illustrate implantation of the prosthetic aortic valve of  FIG. 5  in a native valve annulus after the native valve has been removed. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    Referring to  FIGS. 1A and 1B , a replacement prosthetic valve  10  constructed in accordance with the principles of the present invention comprises a stainless steel cylindrical scaffold (stent)  12  comprising six zig-zag rings joined by aligned axial struts having prosthetic valve leaflets secured in its upper end. The scaffold design allows the support to be radially expanded and contracted without foreshortening. Thus, the length of the scaffold will remain the same throughout an implantation procedure. An exemplary length is 30 mm and the exemplary, expanded diameter is 23 mm. Leaflets and an internal Dacron sheet are secured within the scaffold and the valve commissures anchored at the top of the scaffold with stitches. Thus, the scaffold has an upper portion  14  which houses the leaflets and a lower base portion  16  which is generally free from internal structure other than the Dacron sheet lining. 
         [0033]    Referring now to  FIG. 2 , positioning of the prosthetic replacement valve  10  within a degenerated prosthetic aortic valve  20  is schematically illustrated. The replacement valve  10  will be positioned such that the upper leaflet portion  14  of the valve  10  is above the upper extent of the degenerated prosthetic valve  20 . Note that “upper” generally refers to the direction from the left ventricle into the aorta. The base portion  16  of the scaffold  12  is positioned within the interior of the degenerated prosthetic heart valve  20 . 
         [0034]    Referring now to  FIGS. 3 and 4 , a patient heart H initially has the degenerated prosthetic heart valve  20  implanted within the native aortic valve annulus AA as illustrated in  FIG. 3 . The degenerated prosthetic heart valve  20  typically has a limited length, typically about 1 cm, so that it does not extend significantly into the lumen of the aorta A. 
         [0035]    Referring now to  FIG. 4 , the replacement prosthetic valve  10  of the present invention is introduced into the interior of the degenerated prosthetic valve  20 , typically via a percutaneous route, either through the femoral, axillary, subclavian artery to the aorta or via a minimally invasive route via a trans-septal or more commonly transapical approach. Such protocols for delivering aortic valves are well described in the patent and medical literature. 
         [0036]    Once the replacement prosthetic valve  10  has been positioned within the degenerated valve  20 , the replacement valve  10  will be expanded so that the base portion  16  engages and anchors within the interior of the degenerated valve  20 . The upper leaflet portion  14  will be expanded so that it opens to a greater cross-sectional area within the aorta above the degenerated prosthetic valve  20 . In this way, the effective open or luminal area provided by the replacement heart prosthetic valve  10  is increased to provide improved hemodynamic performance. 
         [0037]    A two-component prosthetic aortic valve  50  comprising two physically separate components constructed in accordance with the principles of the present invention is illustrated in  FIG. 5 . The prosthetic valve  50  comprises both a lower body portion  52  and a separate upper valve portion  54 . The lower base portion  52  includes an inner tubular wall  56  and an outer tubular wall  58 . Both the inner tubular wall and the outer tubular wall are typically in the form of expandable metal scaffolds, where the inner tubular wall  56  is balloon-expandable, i.e., is formed from a malleable material which can be expanded and reconfigured by application of an internal force of pressure, typically using a balloon similar to a stent delivery balloon. The outer tubular wall  58  is also typically in the form of a metal scaffold, but the scaffold of the outer tubular wall will typically be formed from a highly elastic material, such as a super elastic alloy, e.g., Nitinol™. The inner and outer tubes will be coupled to each other, typically being connected along their lower edges (toward the bottom of  FIG. 5 ). The outer tubular wall  58  will typically be covered with a generally impermeable material, such as any of the graft materials listed earlier in this application. The cover will help the outer tubular wall  58  seal against the valve annulus when the prosthetic aortic valve is deployed as described below with respect to  FIG. 6A through 6D . The inner tubular wall  56  will also be typically covered with a generally impermeable material, such as any of the graft materials listed earlier in this application. The lower outer tubular wall also preferably include a plurality of barbs  62  or other anchors, such as hooks, which help fix the lower tubular wall in the annulus after the wall expands therein. 
         [0038]    The combination of a self-expanding outer wall and balloon expandable inner wall provides a number of advantages. The self-expanding outer tubular wall is particularly well suited for conforming to irregular annular geometries, such as within bicuspid aortic valves which have irregular orifice shapes. While it is particularly suited for adapting and conforming to irregular shapes, it will also be perfectly well suited for expansion within valves and valve annulus having regular geometries. 
         [0039]    The dimensions of the lower base portion  52  can be selected to conform to different patient anatomies and for different implantation schemes. For example, the height of the inner wall of the base portion maybe made longer when supravalvular placement of the upper valve portion  54  is desired. In contrast, if intravalvular placement is desired the inner tubular wall  56  can be made much shorter. In some instances, such as for treatment of acute dissections, aortic aneurysms, or other conditions present in the ascending aorta, the length of the inner wall  56  can be made quite long. 
         [0040]    The upper valve portion  54  comprises a single expandable scaffold  64 , typically being balloon-expandable, having an upper end which holds the tricuspid valve  66  and a lower end which is surrounded by a cover  68 . The balloon-expandable scaffold is desirable to maintain the circular geometry of the upper valve portion  54  as the scaffold is expanded. Moreover, the lower portion of the scaffold  64  maybe expanded by balloon simultaneously with the inner wall  56  of the lower base portion  52 , allowing those two portions to be fit together very closely. However, the lower portion of the scaffold  64  may also be made of self-expanding scaffold to fit into  56  as desired. The length of the lower portion of the scaffold  64  and the cover  68  can be selected so that it can overlap with the cover  56  on the lower base portion  52 . In this way, good sealing and anchorage of the valve can be achieved. 
         [0041]    The use of the two components allows great adaptability in assembling the prosthetic aortic valve  50  for patients having different conditions and anatomies. Usually, the valve components maybe selected to provide a relatively short valve for replacement of native valves, may have an intermediate length for the repair of the previously implanted prosthetic valve, and maybe quite long when the valve is being placed for the treatment of aortic aneurysms and dissections. 
         [0042]    The two component stent designs also allow for selection between supravalvular positioning, i.e., positioning of the valve above the native valve leaflets and/or a previously implanted prosthetic valve, or intravalvular positioning, i.e., positioning within the native valve annulus, typically for native valve replacement or above the native annulus (supraannular). 
         [0043]    In addition to the adaptability provided by the two-component prosthetic aortic valves of the present invention at the time of implantation, they further facilitate valve repair should they become damaged or their performance degenerate in any way. In particular, it will often be possible to remove the upper valve portion  54  from the lower base portion  52  in a valve  50  which has been implanted in a patient, even after a substantial period of time has passed. Since the outer wall portion  58  of the lower base portion  52  of the valve is firmly implanted in the valve annulus and will be anchored by tissue overgrowth over time while other portions of the valve are less firmly implanted due to stent to stent overlap and lack of tissue overgrowth, it will be possible to remove the upper portion and replace it with a new upper valve portions and procedures which are far easier than removing the entire implanted prosthetic valve. 
         [0044]    Referring now to  FIG. 6A through 6D , an exemplary protocol for delivering a two-component prosthetic aortic valve  50  into a native valve annulus NA will be described. While this description refers to the native valve annulus without presence of the valve leaflets, it will be appreciated that the implantation procedure could be performed either after excision of the native valve leaflets or with the native valve leaflets left in place. 
         [0045]    The valve maybe initially introduced using a constraining sheath  70  which maybe introduced over the aortic arch using a conventional femoral approach, subclavian/axiallary approach or transapical approach. Transapical approach is not illustrated but functions in principle in the opposite direction from ventricular apex to aorta. A pusher  72  maybe employed to eject the prosthetic aortic valve  50  to the desired location within the native annulus NA, as shown in  FIG. 6B . The self-expanding outer tubular wall  58  of the valve  50  will open upon release from constraint and engage the barb  62  into the wall of the annulus. The inner tubular wall  56  will remain less expanded (since it is a balloon expandable element) and available for mating with the upper valve portion  54 , as illustrated in  FIG. 6C . 
         [0046]    The upper valve portion  54  maybe delivered in its unexpanded configuration to the previously implanted lower base portion  52 , as illustrated in  FIG. 6C . The lower end of the expansible scaffold  68  will be introduced through the inner tubular wall  56  until the desired axial alignment of the two portions is achieved. Note that the stent portions will typically have radiopaque markers which permit viewing an alignment of the portions under fluoroscopy in a conventional manner. 
         [0047]    After the proper alignment of the upper valve portion  54  with the lower base portion  52  is achieved, a balloon  72  carried by catheter  74  maybe expanded to open both the scaffold portion which carries the inner valve  66  and the lower portion of the scaffold including cover  68 , as illustrated in  FIG. 6D . Alternatively, the lower end of the upper valve portion  68  may be released as a self expanding scaffold within the inner tubular wall  56 , followed by balloon expansion of the valve portion  54 . The catheter is now fully implanted and the tricuspid valve  66  capable of functioning. 
         [0048]    While the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. Therefore, the above description should not be taken as limiting the scope of the invention which is defined by the appended claims.