Bovine Valves vs Mechanical - Research Paper Example

?Introduction It was in 1961 that Starr and Edwards described the successful replacement of a diseased valve with a prosthetic valve. Since then, their original design has undergone numerous modifications. Various other manufactures have come out with other designs of mechanical valves. Bioprosthetic valves made from bovine and porcine tissue has also been implanted in numerous patients. Both types have their indications and unique complications. This essay discusses the various types of mechanical and bioprosthetic valves, their complications, advantages and indications. Tissue heart valves (bio-prosthetic heart valves) Compared to mechanical heart valves, tissue heart valves are closer to the design of the natural heart valves. Patients implanted with tissue heart valves have better hemodynamics and less damage to blood cells. They do not require long-term anticoagulation because the risk for clot formation is less. However, valve tissue calcification and degeneration are common problems with tissue heart valves and patients usually require anther heart valve replacement in 10-15 years. There are 2 categories of tissue heart valves: animal and human tissue heart valves (Brown, Bright & Tavris.) Animal tissue heart valves-the 2 common types are porcine and bovine heart valves. Porcine heart valve is harvested from a pig and treated with glutaraldehyde (Brown, Bright & Tavris). Glutaraldehyde fixation of tissues serves multiple purposes. It fixes the protein component of the tissues, making them less prone to enzymatic degeneration, makes valves less antigenic, it sterilizes tissues (Batchelor & Chandrasekaran). Some examples of porcine valves are the Hancock II Porcine (Medtronic) and the Biocor Porcine (St Jude Medical) prostheses (Bloomfield). Some of the porcine valves are stented valves, which are sewn to rigid or flexible stents and cloth sewing cuffs. Others are stentless valves which have excellent hemodynamics (Brown, Bright & Tavris) and have a larger effective orifice area compared to stented valves. However, they are difficult to implant. The durability of a porcine valve is about 10 years (Brown, Bright & Tavris). Bovine heart valves are made out of a cow’s pericardium. They are similar to porcine heart valve in design and sewn into a valvular frame. These valves open more completely and have excellent hemodynamics. Durability is similar or better than porcine valves (Brown, Bright & Tavris). The Ionescue-Shiley pericardial valve has been withdrawn since it was found to be less durable when compared to porcine valves (Bloomfield). The Carpentier-Edwards pericardial valve is fabricated by anchoring the pericardial tissue behind the stents rather than using stitches through the tissue” (Bloomfield) but the long term durability has not been proven (Bloomfield) Mechanical valves are usually composed metal or carbon alloys and are available in 3 types: a. Ball-in-cage (Starr-Edwards) b. Single tilting disc (Bjork-Shiley, Medtronic-Hall or Omniscience) c. Bileaflet tilting disc (St.Jude, Medtronic-Hall) Mechanical valves are very durable, lasting at least 20 years; however, they are thrombogenic and requires lifelong anticoagulation. Ball-in-cage valves are the most thrombogenic, followed by single tilting disc valves. Bileaflet tilting disc valves are the least thrombogenic. The thrombotic potential of these valves is also dependent in part on the position in which they are placed-MV prostheses pose a higher risk of thrombosis than AV valves (Awtry, Jeon & Ware.) Ball-in-cage valves The Starr-Edwards was the original ball valve. It is a silastic ball placed in a sewing ring when closed and moves forward into a cage when open. This original design has undergone subsequent modifications (Bloomfield.) Disc valves The Bjork-Shiley valve comprises of two struts of housing made of stainless steel or titanium. A single graphite disc coated with pyrolite carbon tilts between these struts. Although the original model was modified, there were reports of fracture of the struts in some models, letting the disc to escape with serious complications. Subsequently, the Bjork-Shiley valve was not manufactured. However, other manufacturers like Medtronic-Hall continue producing the single disc valve (Bloomfield.) Bileaflet valves “Bileaflet valves have two semicircular leaflets which open and close creating one central and two peripheral orifices” (Bloomfield). In 1977, the prototype St Jude medical valve was introduced. Currently, this valve along with similar valves manufactured by other manufacturers, are the most common mechanical prosthesis implanted in the world (Bloomfield) Artificial valves (both bioprostheses and mechanical) have some problems and complications; these include the following (Batchelor & Chandrasekaran): Bioprostheses a. uncertain durability, typically only 10-15 years b. Reproducible manufacturing is difficult, since the prostheses are hand made from animal tissues of varying characteristics. c. Calcification and hardening of bioprosthesis after prolonged service. d. Biodegradation from sustained service inside the human body e. Infection risks, from surgery or from the xenograft itself Mechanical valves a. un-natural form and material properties such as elastic modulus b. un-natural flow through the rigid structure of a mechanical valve c. danger of thrombus formation and blood damage due to stagnation areas and high shear stress within the flow field d. lifelong anti-coagulation treatment e. noise and cavitation Studies conducted on different mechanical heart valves Most studies were observational studies of the results of one type of valve replacement. Most of the studies showed good long term valve survival without any difference in the durability of various types of valves. Few randomized controlled trials have been conducted; these compared the outcomes after valve replacement. When compared to the Bjork-Shiley type valve, a higher incidence of thromboembolism has been found to occur with the Starr-Edwards valve. The lowest risk of thromboembolism has been found with bileaflet prostheses like the St Jude valve. Thromboembolic rates are higher after mitral valve replacement than aortic valve replacement (Bloomfield) Studies conducted on different biological heart valves Similar to mechanical valves, most studies of biological valves were observational studies of the results of one type of valve replacement. Many studies have reported porcine valve failure after 7 or more years of surgery, especially in the younger age group (Vongpatanasin et al; Burdon et al.) A non-randomized case–controlled study by David et al., compared results of stentless aortic valve porcine valves with stented aortic valves. The results showed a better durability with the stentless prosthesis. Studies comparing biological heart valves with mechanical heart valves Two large randomized trials (The Department of Veterans Affairs (VA) trial and by Hammermeister and colleagues) have been conducted, which compared the results of biological heart valves with mechanical heart valves. Both the trials used the Bjork-Shiley mechanical valve. The Department of Veterans Affairs (VA) trial randomized 575 male patients who were to receive either a Bjork-Shiley tilting disc prosthesis or a Hancock porcine prosthesis. Of the total, 394 patients had the aortic valve replaced and 181 had the mitral valve replaced. All patients obtaining a Bjork-Shiley valve were treated with anticoagulants. In the Hancock porcine valve group, only a few receiving anticoagulants previously (for other indications) continued receiving them. The mean follow up duration was 15 years. The results showed “a significantly improved survival at 15 years for those who had undergone aortic valve replacement with a Bjork-Shiley prosthesis (79% v 66%), but no significant difference for those who had undergone mitral valve replacement. There was a significantly increased risk of reoperation with the Hancock prosthesis, both for patients who had undergone aortic valve and mitral valve replacement. There was no significant difference in the occurrence of thromboembolism or endocarditis, but there was a significantly greater occurrence of major bleeding with those receiving a Bjork-Shiley prosthesis as a result of the greater use of anticoagulants” (Bloomfield.) The Edinburgh heart valve trial randomized patients undergoing valve replacement with either a Bjork-Shiley or porcine (Hancock or Carpentier-Edwards) prosthesis. The mean follow up period was 20 years. The study “found no difference in patient survival between biological or mechanical valve recipients when all patients were considered together or when the subgroups undergoing aortic valve replacement, mitral valve replacement, and combined aortic and mitral valve replacement were considered separately” (Bloomfield.) The study found “an increased need for reoperation with the porcine prostheses. An actuarial analysis using death or reoperation as combined end points showed a lower event rate and therefore improved valve survival with the Bjork-Shiley prosthesis” (Bloomfield). The study observed a greatly increased risk of bleeding with the Bjork-Shiley prosthesis, as well as a better survival from major complications with the Bjork-Shiley prosthesis (Bloomfield.) Another large non-randomized study by Peterseim and colleagues included a majority of elderly patients undergoing aortic valve replacement, with either a porcine or mechanical prosthesis. The study found that “there was no difference in prosthesis survival between mechanical and porcine prostheses up to 10 years after implantation. Beyond 10 years an increased need for reoperation became apparent in the patients who had received a porcine prosthesis. The risk of bleeding was significantly increased in those who had received a mechanical prosthesis” (Bloomfield.) Recommendations for the choice of prosthetic valve Recommendations for mechanical prosthesis (Bloomfield) 1. Patients in whom a longer life span is expected. 2. Patients with already another mechanical valve at a different location. 3. Patients with hypercalcaemia, on haemodialysis or with renal failure. 4. Patients having risk factors for thromboembolism and requiring warfarin. 5. Patients less than 65 years of age (for atrial valve) and less than 70 years (for mitral valve). Recommendations for bioprosthesis 1. Patients who cannot or will not take warfarin treatment. 2. Patients more than 65 years needing atrial valve replacement, without any risk factors for thromboembolism. 3. Those patients with compliance problems with warfarin treatment. 4. Patients more than 70 years needing mitral valve replacement without any risk factors for thromboembolism. 5. Valve replacement for thrombosed mechanical valve. Conclusion Currently, there are various designs of both mechanical and bioprosthetic valves. On the question of which type is preferable to implant, the studies show no difference in mortality rates between the two, at least in the first 10-years after implantation. The choice depends on various factors. In general, in those aged less than 65 years, having thromboembolic risk factors and other comorbidities, it would be better to implant a mechanical valve. Conversely, those more than 65 years of age, without any risk factors for thromboembolism, and having compliance issues with warfarin etc, will be better off with a bioprosthetic valve. References Awtry, E, Jeon, C, Ware, MG. Blueprints cardiology. Lippincott Williams & Wilkins, 2005. Print. Batchelor, AW, Chandrasekaran, M. Service characteristics of biomedical materials and implants. Imperial College Press, 2004. Print. Bloomfield, P. Choice of heart valve prosthesis. Heart. 2002. 87(6): 583–589. Brown, SL, Bright, RA, Tavris, DR. Medical device epidemiology and surveillance. John Wiley and Sons, 2007. Print. Burdon TA, Miller DC, Oyer PE, Mitchell RS, Stinson EB, Starnes VA, Shumway NE. Durability of porcine valves at fifteen years in a representative North American patient population. J Thorac Cardiovasc Surg. 1992. 103(2):238-51. David TE, Puschmann R, Ivanov J, et al. Aortic valve replacement with stentless and stented porcine valves: a case-match study. J Thorac Cardiovasc Surg. 1998; 116:236–41. Vongpatanasin W, Hillis LD, Lange RA. Prosthetic heart valves. N Engl J Med. 1996. 335(6):407-16. Read More