SPECIFICATIONArtificial heart valveThe present invention relates to bioprosthetic heart valves suitable for, for example, natural aortic, pulmonary, mitral or tricuspid valve replacement.
Considerable effort has been expended in recent years to provide a bioprosthetic heart valve the performance and efficiency of which approaches those of a natural valve, and in the present invention notable progress has been made in meeting these requirements while atthe same time providing a valve which is also relatively cheap to produce.
until now many bioprosthetic heart valves have consisted essentially of a fabric-covered cylindrical supportframeorstenttowhich is attached, for example, a porcine heart valve. Avariety of such valves are at present commercially available and while some have had success in terms of extending the life of the recipient, they have all involved a method of construction in which the cylindrical wall of tissue of the "new" valve is located concentrically within the supporting frame, i.e. the cylindrical walls ofthe valvulartissue and the supporting frame are concentrically arranged with respect to one another.
Thus, when such a valve is implanted it effectively narrows the valve orifice through which blood must pass, which is generally considered to be an undesirable feature.
In the present invention this undesirable characteristic of known valves has been substantially eliminated by a novel method of construction.
Thus the present invention provides a bioprosthetic heartvalve comprising substantially cylindrical valvulartissue and a substantially cylindrical support frame therefor in which the respective cylindrical walls of the valve and frame are substantially coincidentally arranged.
The present invention also provides a method of manufacturing a bioprosthetic heart valve in which substantially cylindrical valvulartissue is supported on a substantially cylindrical frame with a waisted circumferential ring having at least two commissure support loops in the circumferential plane so that tissue located within the frame lies substantially coincidentallytherewith.
Preferably, the support loops are closed and integra with the skirt portion of the cylindrical frame.
Such an agreementofvalvulartissueand support frame has been found to offer noticeably reduced resistanceto the flow of blood therethroughwhen compared with earlier "concentrically" arranged valves.
Preferably, the supportframe is machined from a waisted, hollow cylinder of plastics material, such as anacetal homo-orcopolymer,forexample, as soldunderthe registered trade marks DELRIN and HOSTAFORM respectively. The circumferential skirt portion oftheframebelowthewaistconfers rigidityonthe frameasawholewhileatthesametime permitting limitedflexibilityin orderthattheframecan be "naturally" accommodated bythe recipient vessel.
The valve commissure support loops of the support frame are usually slightly asymmetrical arranged in order to readily accommodate the natural asymmetry of, e.g. a porcine valve.
So that the tissue of the valve may be secured to the supportframe, the frame is covered, usually completely, with a biologically compatible textile fabric or film material which is stiched tothe frame, for example, a polyester or P.T.F.E. fibre velour, which also incorporates a "conventional" integral sewing ring. The integral seam ofthe sewing ring mayIncorporate a radio-opaque silicon rubbermarkerfor X-ray identification.
The material covering gives the supportframe a smooth liquid flow profile and facilitates tissue ingrowth as well as providing means for attaching valvulartissue.
The valve which has previously been fullyfixed (for example glutaraldehyde treated) is trimmed to fit insidethecylindrical coveredsupportframeandthe inflow aspect of the valve (below the waist of the frame) is sewn to the covering material in the customaryway.
The outflow aspect of the valve is sewn into the closed loops of the support frame through an inverted annulus or glycerol treated pericardium tissue, preferablyfrom the same animal species as the valvular tissue. Using pericardium tissue in this way has the advantage of spreading the load of the stitches and by sewing through an inverted annulusthe stitches and the cut edge of the valvularwall are hidden when the annulus is everted and the free edge of the pericardium is trimmed and oversewn to the edge of the frame. Thus, the use of an annulus of pericardium avoids any joints or seams and contributes to an efficient flow profile.
Afterfinal sewing the complete valve is fixed by immersion in a buffered solution ofglutaraldehyde, sterilized and stored in a 4% solution of buffered formaldehyde.
The invention may be further described, byway of example only (for a three cusp or leaflet valve suitable for implanting into the aortic position) by reference to the accompanying drawings in which:~Figure 1 is a side perspective view of a waisted supportframe showing the location of valvulartissue within the commissure support loops (for claritythe supporting fabric (Figure 4) has not been shown);Figure 2 is a plan view of Figure 1, illustrating the coincident arrangement of the frame and valve parts;Figure 3 is a plan view similar to that of Figure 2 showing a conventional valve in which the same parts are arranged concentrically; andFigure 4 is a cross-sectional side view of a valve according to the present invention showing the position of the constituent parts including the everted pericardium.
Figure 1 shows the position of cylindrical valvular tissue 1 which has been trimmed to fit inside the closed, inverted, substantially U-shaped upstanding (commissure support) loops 2 ofthe support frame generally indicated at 3. The lower circumferential edge ofthe valvulartissue being located inside the waisted portion 4 of the skirt 5 of the frame. The position of the valve cusps within the support frame is shown at6.
A plan view of the valve shown in Figure 1 is illustrated in Figure 2 where the coincident arrangement of valvulartissue 1 and supportframe 3 is clearly shown. Figure 3 is a similarviewofa conventional valvewherethevalvulartissue 1 and frame 3 are arranged concentrically.
In Figure 4the parts are shown in a slightly exploded arrangementso asto clearly identify their relative positions. Thus, the valvular wall 1 is shown separated from its supporting textile fabric 7 which overlays the waisted supportframe 3 and to which it is sewn. A sewing ring 8 containing, for example, a semi-rigid expanded P.T.F.E. foam material is also provided for assisting in implanting the valve in position in a recipient heart. An everted pericardium annulus 9 overlies the top circumferential edge ofthe valve to give the valve a neat and aesthetic appearance.
Typically,the inflow and outflow aspects ofthe valve are sewn to the adjacent textile fabric with a (compatible) white braided polyesterthread using a modified blanket stitch.
In an extensive in vitro assessment of aortic replacementvalves made according to the present invention, the hydro-dynamic performance of the valveintermsof(1)the pressure drop across the valves and (2) the volume of fluid refluxed by the valves per cycle (gross regurgitation) was found to besatisfactory. Also, a high speed cine film taken of the valves in action (at 75 beats per minute) showed nocusp instability and thatthe valve opened well to givea good orifice area. The in vitro assessment gaveevery indication that the valves would perform well when implanted clinically.
Seven animals successfully underwent implantation of valves made according to the present invention in the mitral position, and were long term survivors.
Mean survival to elective sacrifice was 5.47 months.
Haemodynamic results in terms of atrial pressure and cardiac output were good and the condition of the explanted valves was found to be excellent. There were no cusp tears or perforations and there was no evidence of endocarditis or of calcification. Paravalve fibrous tissue ingrowth was also found to be satisfactory and did not extend beyond the polyesterfabric.