Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Heart valve – Having rigid or semirigid pivoting occluder
Reexamination Certificate
2000-09-19
2003-10-28
Snow, Bruce (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Heart valve
Having rigid or semirigid pivoting occluder
Reexamination Certificate
active
06638303
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a heart valve prosthesis, used to replace diseased natural heart valves, and more particularly to a mechanical heart valve prosthesis that uses one or more naturally-operating pivoting members.
Various types of heart valve prostheses have been developed which operate hemodynamically as a result of the pumping action of the heart in which they generally function as check valves. These prosthetic valves fall generally into two categories: “mechanical” valves, comprising relatively rigid leaflets formed of a stiff, biocompatible substance such as pyrolitic carbon; and “bioprosthetic” valves, comprising flexible leaflets often formed of a biological material such as bovine pericardial tissue. One popular design for a mechanical heart valve prosthesis includes an annular valve body in which a pair of opposed leaflet occluders are pivotally mounted. The occluders move between a closed, mated position, blocking blood flow in an upstream direction, thereby minimizing regurgitation, and an open position, allowing blood flow in a downstream direction.
Typically, after receiving a mechanical heart valve prosthesis, the recipient must take chronic anticoagulation treatment for the rest of their life to prevent blood clots. These blood clots are referred to as thrombosis if they adhere to the heart valve. If the clots float away from the valve where they can occlude blood flow to another part of the body, the clots are referred to as thromboembolism. Thrombosis, thromboembolism, and bleeding that results from the drugs that are used to reduce cases of thrombosis and thromboembolism are the most serious problems faced by heart valve recipients. In contrast, after receiving a biological heart valve, the recipient typically only needs the chronic anticoagulation therapy for about ninety days while the sewing ring heals over.
To explain why blood clots occur more frequently in mechanical valve prostheses, scientists often look to the teachings of Verchow, an important nineteenth century scientist, who believed that clotting of the blood, or thrombosis, was the result of three interacting variables. These variables, often referred to as Verchow's triad, include: (1) the susceptibility of a patient's blood to clot formation; (2) foreign material response; and (3) flow conditions.
The susceptibility of a patient's blood to clot formation can be diagnosed to a limited extent. Patients with particular susceptibility to clot formation may require anticoagulation even without the added risk of a prosthetic heart valve to avoid atrial and peripheral thrombosis. However, in the absence of anticoagulation treatment, mechanical valves generally give rise to greater clotting problems than bioprosthetic valves.
According to Verchow, areas of stasis, or low flow, are the most likely accumulation points for clots. For this reason, clots in prosthetic valves usually occur at the point where the sewing ring joins other valve members, or in low flow regions near struts and pivot points. Considerable effort has been directed at eliminating these low flow regions as discussed in U.S. Pat. Nos. 5,147,390 and 5,192,313. Although such attention to flow is likely to reduce the chance of valvular thrombosis, it does not explain the cause for heightened sensitivity to clot formation. Most tissue valves have many areas of blood stasis that are much worse than the pivots and struts in mechanical heart valves yet thrombosis is much less likely to occur in tissue valves than in mechanical valves, in the absence of anticoagulation therapy.
Since low flow does not explain the clotting risk difference between tissue and mechanical valves, a great deal of work has gone into investigating high flow rate effects on blood. High flow rates have been shown to cause damage to blood and to activate clotting mechanisms. In particular, high wall shear stress occurs where a rapidly moving flow meets an immobile boundary, like the walls of a heart valve or blood vessel. This high wall shear stress can cause blood damage. Michael T. H. Brodeur, M. D., et al., Red Blood Cell Survival In Patients With Aortic Valvular Disease and Ball-Valve Prostheses, XXXII Circulation 570 (1995); Richard M. Rubinson, M. D., et al., Mechanical Destruction of Erythrocytes By Incompetent Aortic Valvular Prostheses-Clinical, Hemodynamic, And Hematologic Findings, Am. Heart J. 179 (February 1966); Charles G. Nevaril, et al., Erythrocyte Damage And Destruction Induced By Shearing Stress, J. Lab & Clin. Med. 784 (May 1968).
Damage to the blood can also be caused by a rapid change in blood velocity, even in the absence of an impinging wall, particularly if the flow is turbulent. A. R. Williams, Viscoelasticity Of The Human Erythrocyte Membrane, 10 Biorheology 313-19 (1973); Paul D. Stein and Hani N. Sabbah, Measured Turbulence And Its Effects On Thrombus Formation, 35 Circulation Research 608-14 (1974); R. S. Figliola and T. J. Mueller, On The Hemolytic And Thrombogenic Potential Of Occluder Prosthetic Heart Valves From In-Vitro Measurements, 103 Journal of Biomechanical Eng'g. 83-89 (1981); Ahmed M. Salam and Ned Hwang, Human Red Blood Cell Hemolysis In A Turbulent Shear Flow: Contribution Of Reynolds Shear Stresses, 21 Biorheology 783-797 (1984); L. J. Worzinger, et al., Platelet And Coagulation Parameters Flowing Millisecond Exposure To Laminar Shear Stress, 381-386.
Laser doppler anemometry has been used to measure the forward flow dynamics in various mechanical and tissue valves. D. D. Hanle, et al., Invitro Flow Dynamics Of Four Prosthetic Aortic Valves: A Comparative Analysis, 22 J. Biomechanics 597-607 (1989). The data from these studies has been used to estimate the shear stress related blood damage that occurs during forward flow. M. Giersiepen, et al., Estimation Of Shear Stress-Related Blood Damage In Heart Valve Prostheses-Invitro Comparison Of 25 Aortic Valves, 13 Int'l. J. of Artificial Organs 306-330 (1990). In these reports, the shear stress and estimated damage to blood components with tissue valves is greater than the damage estimated from modern bileaflet mechanical valves during forward flow. The most recent reference even provides clinical comparative data supporting the fact that more hemolysis (risk of thrombosis) is indicated with tissue valves than with mechanical valves. There is also no evidence to suggest small valves are more prone to clotting than large valves, but both wall shear stresses and Reynolds normal stress are much higher in small valves than in large valves. These facts taken together indicate hemolysis is not a good indicator of the increased risk of clot formation with clinical valves, and the differences between the thromboembolic potential of tissue and mechanical valves is not related to forward flow dynamics.
Another possible link between clotting and flow conditions is the existence of high wall stresses or Reynolds normal stress associated with reverse flow leakage after the valve closes. Reynolds stresses on the order of 20,000 to 60,000 dynes/cm2 have been observed within the regurgitate jets occurring through a Bjork-Shiley valve mounted in a Penn-State heart. Baldwin, J. T., et al., Estimation Of Reynolds Stresses Within The Penn State Left Ventricular Assist Device, 36 ASAIO Trans. M274-M278 (1990); Baldwin, J. T., et al., Mean Velocities And Reynolds Stresses Within Regurgitant Jets Produced By Tilting Disk Valves, 37 ASAIO Trans. M351-M353 (1991). The peak Reynolds normal stresses during forward flow are on the order of 1,000 to 4,500 dynes/cm2, much lower than the Reynolds stresses during flow leakage. These higher stresses during leakage flow could be causing activation of the clotting system but high velocity jets are not unique to mechanical heart valves. Biological heart valves are often designed with a small leak in the center of the valve that results in a high velocity jet. Further, as biological valves degrade jets are created at tears and small holes that progress to complete valvular incompetence. If the high velocity l
Carbomedics Inc.
Scott Timothy L.
Snow Bruce
Williams Morgan & Amerson P.C.
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