A HEART VALVE PROSTHESIS

Abstract: An annular valve body (12) having a central passageway (18) for the flow of blood therethrough with two leaflets (14,16) each of which is pivotally supported on an eccentric positioned axis (P,P') in the central passageway (18) for moving between a closed position and an open position. The leaflets (14,16) are curved in a plane normal to the eccentric axis (P,P'') and positionen with the convex side (26, 28) of the leaflets (14,16) facing each other when the leaflets (14,16) are in the open position. Various parameters such as the curvature of the leaflets (14,16), the location of the eccentric axis (P,P'), and the maximum opening angle of the leaflets (14,16) are selected with a view to maximizing the minimum peak velocity through the valve (10), maximizing the net stroke volume, and minimizing the mean forward pressure difference, thereby reducing thrombosis and improving the hemodynamic performance.

Description: This invention relates to a heart valve prosthesis. Various types of heart valve prosthesis have been developed which operate hemodynamically as a result of the pumping action of the heart in which they generally function as check valves. One type of heart valve prosthesis commercially available is the St. Jude valve, which comprises a valve body having a passageway for the flow of blood therethrough from upstream to downstream, and first and second leaflets which are pivotally supported in the passageway and moveable between a closed position inhibiting blood flow through said passageway and an open position allowing blood flow therethrough. The first and second leaflets are flat. Another type of heart valve prosthesis is the Hemex valve which is a bileaflet valve in which the leaflets are curved in a plane parallel to the pivot axis of the leaflets. According to the present invention there is provided a heart valve prosthesis comprising, a valve body having a passageway for the flow of blood therethrough from upstream to downstream, and first and second leaflets individually pivotally supported in said passageway and pivotal between a closed position inhibiting blood flow through said passageway and an open position allowing blood flow therethrough, characterized in that said first and second leaflets are curved in a plane normal to their respective pivot axis and are arranged so that the convex sides of said first and second leaflets face each other when said leaflets are in said open position. Preferably, the concave sides of said first and second leaflets face upstream when in the closed position. Advantageously, the first and second leaflets are pivotally supported approximately two thirds of the width of the leaflets from the outer edges of the leaflets. Preferably, the curvature of said leaflets is of a value to maximize the minimum peak velocity flowing between the leaflets and on the outside of each leaflet. Advantageously, the curvature of said leaflets is circular. More advantageously, the curvature of said leaflets is in the range of approximately .4 to .7 wherein C = d o /r o , and r o = the radius of curvature of the leaflets, and d o = the length of the chord of said leaflets. Preferably, the heart valve prosthesis includes means in said valve body for limiting the maximum angle of opening of said leaflets to approximately ninety degrees from a plane transverse to said passageway. Advantageously, the maximum angle of opening is in the range of approximately eighty-five to ninety degrees. For a better understanding of the present invention and to and to show how the same may be carried into effect, reference will now to made by way of example, to the accompanying drawings in which:- Figure lA is a schematic of a prior art heart valve prosthesis with flat leaflets in the mitral position shown in both the open and closed positions; Figure lB is a view similar to Figure lA but showing a heart valve prosthesis in accordance with the present invention; Figures 2A, 2B and 2C are schematic views of the heart valve prosthesis shown in Figure lB illustrating various parameters. Figure 3 is a graph illustrating the opening angles of the anterior and posterior leaflets of the conventional heart valve prosthesis shown in Figure lA as a function of time; Figure 4A is a graph illustrating the peak velocity in each of the three openings of the conventional heart valve prosthesis shown in Figure lA as a function of opening angle; Figure 4B is a graph similar to that of 4A but with the heart valve prosthesis shown in Figure lB; Figure 5 is a graph illustrating the peak velocity in each of the three openings of the heart valve prosthesis shown in Figure lB as a function of the axis pivot point location; Figure 6A is a graph of the minimum peak velocity as a function of the pivot point location for various leaflet curvatures with a maximum valve opening of 85 degrees; Figure 6B is a graph similar to Figure 6A for a valve opening of 90 degrees; Figures 7A and 7B are enlarged views of parts of Figure 6A and 6B, respectively; Figure 8A is a graph of maximum over pivot point location of minimum peak velocity as a function of leaflet curvature with a maximum valve opening of 85 degrees; Figure 8B is a graph similar to Figure 8A but with a valve maximum opening of 90 degrees; Figure 9A is a graph of stroke volume as a function of pivot point location for various leaflet curvatures with a maximum valve opening of 85 degrees; Figure 9B is a graph similar to that of Figure 9A for a maximum valve opening of 90 degrees; Figure l0A is a graph of mean transvalvular presure difference as a function of pivot point location for various leaflet curvatures with a maximum valve opening of 85 degrees; Figure l0B is a graph similar to that of Figure l0A but with a maximum valve opening of 90 degrees; Figure llA is a graph of the net stroke volume/mean forward pressure difference or hemodynamic benefit/cost ratio as a function of pivot point location for various leaflet curvatures with a maximum valve opening of 85 degrees; Figure llB is a graph similar to that of Figure llA but with a maximum valve opening of 90 degrees; Figure l2A, l2B and l2C are respectively tables I, II and III of various data; Figure l3 is an elevational view, in cross section, of a preferred embodiment of a heart valve prosthesis in accordance with the present invention; and Figure l4 is a top plan view of the embodiment shown in Figure l3. Referring now to the drawings, and particularly to Figures l3 and l4, there is shown a heart valve prosthesis which is generally identified by reference numberal l0. The heart valve prosthesis l0 includes an annular body l2 which supports first and second leaflets l4 and l6 which pivot about axes P, P' respectively to control the flow of blood through a central passageway l8. The normal flow of blood through the valve l0 is in the direction of the arrow 20. Any suitable means may be connected to the annular body 20 for attaching a suturing ring to the annular body l2 to facilitate sewing the valve l0 to the heart tissue as is conventional. The valve body l2 and leaflets l4 and l6 may be made of any suitable material that is biocompatible and non-thrombogenic, for example, isotropic graphite which is suitably coated with pryolitic carbon. The leaflets l4 and l6 are each pivotally supported on respective eccentric positioned axis P and P', The leaflets l4 and l6 are curved in a plane normal to their respective pivot axis. The leaflets l4 and l6 have a concave side 22 and 24, respectively, and a convex side 26 and 28, respectively. The convex sides 26 and 28 face each other when the leaflets l4 and l6 are in the open position and the concave sides 22 and 24 face up­ stream when the valve is in the closed position. Preferably, the curve of the leaflets l4 and l6 is circular and preferably the leaflets l4 and l6 are flat in a plane parallel to the eccentric axis although the leaflets l4 and l6 could also be curved in a plane parallel to the pivot axis. The body l2 may include stop shoulders 30 and 32 positioned in the path of movement of the leaflets l4 and l6, respectively, for limiting the maximum opening angle of the leaflets l4 and l6 as will be more fully discussed hereinafter. The leaflets l4 and l6 are generally circular-arc in cross section and are adapted to engage and seat on a valve seat 34,44 in the body l2 when in the closed position by the outside edges 36 and 38 of the leaflets l4 and l6, respectively. The inside edges 40 and 42 are positioned to engage each other when the valve is in the closed position. Blood flowing in the direction of the arrow 20 from upstream to downstream will open the leaflets l4 and l6 until they reach their shoulder stops 30 and 32, respectively. When the downstream flow of blood is discontinued, the back pressure which is present will cause the leaflets l4 and l6 to swing or pivot around the pivot axes P and P', respectively, towards the closed position. As in the prior art valves using two flat leaflets, the present valve l0 when opened has three separate channels for forward blood flow, that is, two lateral channels and one central channel. However, both physical measurements and computer studies show that the blood flow in the central channel of the prior art flat leaflets tends to be stagnant relative to the flow in the lateral channels of the valve. Stagnant flow is dangerous because it may lead to valve thrombosis. One of the features of the present invention is the reduction of this problem by curving the valve leaflets l4 and l6 as described above to form a central venturi channel which helps open the leaflets l4 and l6 and also funnels flow into the previously stagnant central channel of the valve. In addition, various parameters have been considered to improve the performance of the valve l0 to select an optimal valve that not only maximizes the minimum peak velocity in any of the flow channels, but also maximizes the net stroke volume of blood crossing the valve per heart beat and minimizes the mean pressure difference during forward flow, thereby improving the hemodynamic performance. In order to provide an optimal valve, the parameters considered are the position of the pivot points P and P', the radius of curvature of the leaflets l4 and l6, and the opening angle of the leaflets l4 and l6. A computer was used to define the parameters of a model valve and the performance criteria that are used to select an optimal valve. Referring now to Fig. lA and lB, a prior art heart valve prosthesis shown in Fig. lA having flat leaflets 50 and 52 which are positioned in the mitral annulus between the left ventricle and the left atrium while a valve of the present invention using curved leaflets l4 and l6 is shown in Fig. lB. The open position of the leaflets 50, 52 and l4 and l6 are shown in heavy dotted lines. The initial (closed) position is indicated by the light solid lines. Points M and M' are the extreme anterior and posterior points of the mitral annulus. Points P and P' are the pivot points. As previously indicated, the prior art valve of Fig. lA in tests showed that the blood flow in the central channel between the leaflets 50 and 52 tends to be stagnant relative to the flow in the outside channels which may lead to valve thrombosis. On the other hand, the present design of Fig. lB provides a venturi channel between the leaflets l4 and l6 which not only helps to open the leaflets l4 and l6 but to funnel additional flow into the channel between the leaflets l4 and l6. Referring now to Figs. 2A, 2B and 2C, the geometrical construction of a model valve of the present invention is shown in which the valve is shown in its closed configuration in Fig. 2A and in its open configuration in Fig. 2B and Fig. 2C. The valve is treated as symmetrical despite the asymmetry that is inherent in the mitral position. The valve is constructed starting from the points M and M' that define the mitral ring and the diameter of that ring is d m . The gap distance g separates the ends of the leaflets l4 and l6 from the mitral ring and from each other. Because all boundaries have an effective thickness (of about 2 mesh widths of the computational mesh used in the computer model), these gaps are effectively closed when the valve is in the closed position. The chords AB and A'B' of the leaflets l4 and l6 are constructed in the closed position at an angle ϑ₀ to the line MM' that defines the level of the mitral ring. Throughout, we take ϑ₀ = 25°. This 25 degrees initial angle of the leaflets l4 and l6 relative to the mitral ring was chosen to approximately minimize the volume swept out by the leaflets during their opening or closure movement. This result is independent of the position of the pivot points P and P' and the curvature of the leaflets l4 and l6 and roughly minimizes the volume swept for leaflets having a maximum opening angle in the range between 85 degrees and 90 degrees, which opening angle will be further discussed herein. The leaflets l4 and l6 themselves are the circular arcs APB and A'P'B' with centers of curvature at 0 and 0', respectively, on the upstream (atrial) side of the valve. Let r o = radius of curvature of the leaflets d o = length of AB = chord of the leaflets R = r o /d o C = l/R Then R is the dimensionless radius of curvature, and C, the reciprocal of R, is the dimensionless curvature. The special case of a flat valve is characterized by r o = ∞, R = ∞ and C = 0. In the flat case the leaflet coincides with the chord AB. We shall use the parameter C to characterize the curvature of the valve. This is one of the principal design parameters. Another important parameter is the position of the pivot points such as P (which remains fixed relative to the points M and M' as the leaflets pivot). Its location is defined by measuring arc length from the outer border of the leaflet Let s p = length of the arc AP s o = length of arc APB S P = s P /s o Then S P gives the position of the pivot point in dimensionless form. By definition, O < S p < l, with the larger values of S P corresponding to pivot points which are closer to the center of the valve apparatus. In practice, S P must be chosen, as will be discussed hereafter, in the neighborhood of 2/3 to achieve reasonable performance. If S P < 0.5, the valve does not open at all. The final design parameter is the maximum angle of opening of the valve. This angle, ϑ max , is the angle of the chords AB and A'B' to the plane of the mitral ring, as shown in Fig. 2B. Most of the computations in the current study were conducted with ϑ max = 85° or 90°, as will be more fully discussed hereinafter. Two other of the performance criteria are the net stroke volume (SV) and the mean forward pressure difference (). The net stroke volume is defined as the net volume of blood crossing the plane of the mitral annulus during the time interval when the valve is open. It is the difference between the forward volume and the regurgitant volume associated with the closure movement of the leaflets l4 and l6. The mean forward pressure difference is defined by the formula where p LA (t) and p LV (t) are the left atrial and left ventricular pressures at time t. The time t = 0 is defined as the onset of mitral flow, and t = T is defined as the time of the first zero-crossing of the mitral flow, which occurs during early ventricular systole. The rationale for this choice of times is that it eliminates an inertial component of the pressure difference. These two performance criteria are combined into a single benefit/cost ratio: SV/. This seems appropriate because SV and are not really independent measures of performance. The following performance criterion is concerned with valve thrombosis. Let Q ant (t) be the volume rate of flow crossing the line MP in Fig. 2C, let Q cent (t) be the volume rate of flow crossing the line PP' and let Q post (t) be the volume rate of flow crossing P'M'. Thus Q ant , Q cent , Q post are the anterior 60, central 62 and posterior 64 flows, respectively. Let A ant , A cent , and A post be the cross-sectional areas of the three openings of the valve. That is, let A ant = (length of MP) · L O A cent = (length of PP') · L O A post = (length of P'M')·L O where L O = l.84 cm is a reference length (L O is the diameter of the mitral ring) that is used to convert length in the two-dimensional model to cross-sectional area. Next define v ant (t) = Q ant (t)/A ant v cent (t) = Q cent (t)/A cent v post (t) = Q post (t)/A post The quantities v ant (t), v cent (t), v post (t) have units of velocity; they are the spatially averaged velocity at time t in each of the three openings of the valve. (These velocities are independent of L O because both Q and A contain factors of L O in their definition.) The maximum over time of the velocities v ant , v cent , and v post will be designated by the symbol *. Thus v nt = Mx {v ant (t)} v ent = Mx {v cent (t)} v ost = Mx {v post (t)} These peak velocities are important because a low peak velocity implies stagnation and hence a tendency to thrombosis. To avoid thrombosis altogether, we adopt a worst-case criterion. Let v in = Min {v nt , v ent , v ost } If v in = 30 cm/sec, for example, then a velocity of at least 30 cm/sec is achieved at some time during filling in each opening of the valve. We seek design parameters that make v in as large as possible. In the case of single-disc valves, good performance can be achieved without any built-in constraint on the opening angle. For the curved leaflet valve, however, we were unable to find any combination of pivot point and curvature for which the valve would exhibit good performance without a constraint. The difficulty is related to asymmetrical movement of the leaflets l4 and l6 which, in turn, must be caused by the asymmetry of the mitral position, since the valve is symmetrical. Fig. 3 shows the opening angle as a function of time for a prior art flat, bileaflet valve effectively unconstrained (ϑ max = l35°) pivoted at S P = 0.67. Note that the anterior leaflet opens much faster than the posterior leaflet and that it becomes caught in the open position. This makes the valve incompetent. Competent performance can be achieved by reducing S P , but then the posterior leaflet fails to open sufficiently and the valve is very stenotic. One way out of this dilemma would be to construct an asymmetrical valve in which the asymmetry of the valve compensates for the asymmetry of the mitral position. In the present study, which is restricted to symmetrical valves, the only solution is to impose a constraint on the maximum opening angle. With such a constraint, and with the pivot point and curvature in an appropriate range both leaflets l4 and l6 hit the constraint (albeit at slightly dif­ ferent times) and the open configuration of the valve is symmetrical. Valve performance has not been systematically optimized with respect to maximum opening angle ϑ max . Instead we have chosen two reasonable values of ϑ max and conducted the rest of the study with these values fixed. The motivation for this choice is shown in Fig. 4, in which peak velocity is plotted in each of the three openings 60, 62, 64 of the valve as a function of the opening angle constraint. The results are shown both for the flat valve of Fig. lA (Fig. 4A) and for a curved valve of the present invention of Fig. lB (Fig. 4B). The pivot point in both cases is S P = 0.67. The goal is to make the minimum of the three peak velocities as large as possible. For the flat valve (Fig. 4A) at this pivot point, the minimum peak velocity always occurs in the central opening of the valve. (This has also been observed in vitro.) This minimum peak velocity builds up to its largest value at about a 90 degree opening angle constraint and then levels off. (Competent performance cannot be maintained much beyond 90 degrees, since the anterior leaflet tends to get stuck in the open position and fail to close.) For the curved valve of the present invention, Fig. 4B, the situation is somewhat more complicated, since the smallest peak velocity occurs first in the posterior opening 64, then in the central opening 62, and finally again in the posterior opening 64 as the maximum opening angle increases from 70 degrees to 90 degrees. Here the minimum of the three peak velocities is largest where the center and posterior velocities cross near a maximum opening angle of 85 degrees. These results suggest that opening angle constraints in the range 85 degrees to 90 degrees are most likely to produce good performance. For the rest of this study, we fix the maximum opening angle at ϑ max = 85° or ϑ max = 90°. The figures which follow will show results for these two values of ϑ max side by side. Minimum peak velocity (v in ) varies as a function of pivot point (S P ) and curvature (C). Fig. 5 shows a typical plot of peak velocity in the three openings, 60, 62, and 64, of the valve as a function of pivot point with curvature held fixed (in this case C = .667). Note that the different peak-velocity curves intersect and that the minimum peak velocity (heavy line) is largest at the pivot point where the posterior and central peak velocities are equal. In subsequent figures, we plot only the minimum peak velocity without specifying the opening of the valve in which it occurs. Figs. 6A and 6B plot the minimum peak velocity (v in ) as a function of pivot point (S P ) at various curvatures (C = 0, .333, .4l7, .500, .583, .667, .750). There are two remarkable points. First, at every curvature there is a pivot point that maximizes the minimum peak velocity. These pivot points are clustered around S P = 0.67. Second, the curved valves have substantially higher values of v in (minimum peak velocity) than the flat valves of Fig. lA. In fact, by curving the leaflets l4 and l6, as shown in Fig. lB, one can increase v in by about l0 cm/sec (more than 30%) in comparison with the best flat valve. The optimal combination of pivot point and curvature can be found by expanding Figs. 6A and 6B in the neighborhood of the best valves. The result is shown in Figs. 7A and 7B. For each curvature there is an optimal pivot point, denoted by S* (C) which maximizes v in . The value of v in at this optimal pivot point is denoted by v n (C). Figs. 8A and 8B are plots of v n as a function of C. The values of v n and S* are listed in Table I (Figs. l2A). For each value of ϑ max there is one curvature, denoted by C opt , which results in the largest value of v n . The values of C opt , S* (C opt ) and v n (C opt ) for ϑ max = 85° and 90° are listed in Table II (Fig. l2B). These values define the two best valves in the study. Hemodynamic performance is another important criteria. The foregoing choice of an optimal valve was based on considerations relating to stagnation and thrombosis. Considering the design problem from a hemodynamic (pressure-flow) viewpoint, Figs. 9A and 9B plot the stroke volume (i.e., the net volume of blood crossing the mitral valve during one heart-beat) as a function of pivot point at several curvatures. Figs. l0A and l0B plot the mean pressure difference across the valve in the same way. Figs. llA and llB combine these two performance criteria into a single benefit/cost ratio: the quotient of the net stroke volume and the mean pressure difference across the valve. At each curvature in Figs. llA and llB there is an optimal pivot point. As before these optimal pivot points are clustered in the neighborhood of S P = .667. Starting from the flat valve, C = 0.000, and increasing the curvature, we find that performance (the quotient of stroke volume and mean pressure difference) first improves dramatically and then falls off. Of the valves that we have tested, those with the best hemodynamic performance are listed in Table III (Fig. l2C). In choosing the best overall valve, how much weight should be given to the minimum peak velocity (a criterion related to thrombosis) in comparison with the hemodynamic benefit/cost ratio of the net stroke volume divided by the mean pressure difference? Fortunately, there is no real need to answer this question because the two criteria lead to very similar conclusions (compare Table II and Table III). Moreover, we can adopt either criterion without much reduction in performance as measured by the other criterion. Suppose, for example, that we select the best valve by the criterion of maximizing v in (the minimum peak velocity). While this valve is suboptimal with respect to hemodynamic performance, the reduction in SV/ (the ratio of stroke volume to mean pressure difference) is only 3.8% for the valve with ϑ max = 85° and only 3.5% for the valve with ϑ max = 90°. We are therefore in the fortunate situation of being able to select the valve which will minimize thrombosis while sacrificing very little in terms of hemodynamic performance. In conclusion, a built-in contraint on the maximum angle of opening (ϑ max ) is a necessary design feature of symmetrical butterfly bileaflet valves for the mitral position. The parameter ϑ max should be set in the approximate range of 85 degrees - 90 degrees. With ϑ max = 85°, the valve with least stagnation (technically, the valve with the largest value of v in , the minimum peak velocity) has pivot point S P = 0.67l and curvature C = 0.583. With ϑ max = 90°, the valve with least stagnation has S P = 0.658 and C = 0.500. These valves are also nearly optimal from a hemodynamic viewpoint. That is, they come close (within 4%) to achieving the optimal value of net stroke volume divided by mean forward pressure difference. The most important conclusion is that modest curvature of the leaflets l4 and l6 can make a substantial improvement in the performance of butterfly bileaflet valves. The type of curvature used is in a plane perpendicular to the pivot axes P and P' of the valve l0 and in a direction such that the open valve has a slight waist, like reversed parentheses: )(. When the optimal curvature is selected, the best valve of this type is superior to the best flat valve both with regard to the minimum peak velocity (which should be larger to prevent stagnation and thrombosis) and also with regard to the ratio of net stroke volume to mean forward pressure difference. For the two constraint angles that are considered (85° and 90°) the improvements in minimum peak velocity are 38% and 34%, while the improvements in the hemodynamic benefit/cost ratio are l6% and 20%. These conclusions are predictions made with the help of a computer model to produce the charts and tables of Figs 3-l2.

Claims: 1. A heart valve prosthesis (l0) comprising, a valve body (l2) having a passageway (l8) for the flow of blood therethrough from upstream to downstream, and first and second leaflets (l4,l6) individually pivotally supported in said passageway (l8) and pivotal between a closed position inhibiting blood flow through said passageway (l8) and an open position allowing blood flow therethrough, characterized in that said first and second leaflets (l4, l6) are curved in a plane normal to their respective pivot axis and are arranged so that the convex sides of said first and second leaflets (l4, l6) face each other when said leaflets (l4, l6) are in said open position. 2. A heat valve prosthesis as claimed in Claim l, characterized in that the concave (22, 24) sides of said first and second leaflets (l4,l6) face upstream when in the closed position. 3. A heart valve prosthesis as claimed in Claim l or 2, characterized in that said first and second leaflets (l4,l6) are pivotally supported (P, P') approximately two thirds of the width of the leaflets (l4,l6) from the outer edges (36,38) of said leaflets. 4. A heart valve prosthesis as claimed in Claim l, 2 or 3, characterized in that the curvature of said leaflets (l4, l6) is of a value to maximize the minimum peak velocity flowing between the leaflets (l4, l6) and on the outside of each leaflet (l4, l6). 5. A heart valve prosthesis as claimed in Claim 4, characterized in that the curvature of said leaflets (l4, l6) is circular. 6. A heart valve prosthesis as claimed in Claim 5, characterized in that the curvature C of said leaflets is in the range of approximately .4 to .7 wherein C = do/ro, and ro = the radius of curvature of the leaflets, and do = the length of the chord of said leaflets. 7. A heart valve prosthesis as claimed in any preceeding Claim, characterized in that it includes means (30, 32) in said valve body for limiting the maximum angle of opening of said leaflets to approximately ninety degrees from a plane (M-M') transverse to said passageway. 8. A heart valve prosthesis as claimed in Claim 7, characterized in that the maximum angle of opening is in the range of approximately eighty-five to ninety degrees.

Inventors: MCQUEEN, DAVID M., PESKIN, CHARLES S.
Applicants: MCQUEEN, DAVID M., PESKIN, CHARLES S.
Application year: 1986
Main classification: A61F2
IPC codes: A61F2