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  3. Valvular heart diseases are quite common, essentially resulting in impaired blood flow and were very difficult to treat. Some 30

Valvular heart diseases are quite common, essentially resulting in impaired blood flow and were very difficult to treat. Some 30

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问题     Valvular heart diseases are quite common, essentially resulting in impaired blood flow and were very difficult to treat. Some 30 years ago, it became possible to replace .diseased valves with prostheses to impose a greater control over blood flow.
    Early devices were of the mechanical variety, in which devices like ball-in-a-cage or tilting disc would be used to allow blood to flow under near-normal conditions.  Although a few mechanical problems were encountered in the early days, the major difficulty lay with the tendency for any foreign material to initiate a blood clot. So, all valve recipients have to be given anticoagulant therapy.  This is not particularly desirable for the patients, who may develop bleeding problems, and in any case is not always successful.
    Although good results are achieved with these valves, it was considered necessary to develop alternatives and the direction was that of natural tissues. It’s not possible to transplant heart valves untreated because of rejection phenomena, but it became apparent that collagenous tissue could be cross-linked by glutaraldehyde and prepared in the form of a heart valve. Two sources of tissue were considered for this purpose, bovine pericardium ( collagenous tissue derived from the wall of a cow’s heart) and porcine valves (heart valves taken from pigs) and the resulting "bioprosthetic valve" appeared to be very promising. It was particularly important that these patients didn’t need anticoagulation. Unfortunately, these valves have not proved very durable, the cross-linked collagen suffering from slow calcification and deterioration so most of the replacement valves themselves need to be replaced within a decade.
    This would  tend  to  suggest  that the  mechanical  valves  give  superior performance, notwithstanding the anticoagulation problem, and a move back towards their use might have been expected. However, most of the valves in current use incorporate an alloy’( usually Stellite) for
the housing, and a carbon coated occuluder. The complex shapes of some of the housing have required combinations of casting and welding technologies to be used in their construction and serious problems have arisen with a valve design from one manufacturer, where a small number of catastrophic fractures have occurred within the housing. In patients where this valve has been used to treat aortic valve disease, this fracture is usually fatal and although the risks are small, the problem is important to the industry.
    Also, at a time when this dichotomy is exercising the minds of surgeons, scientists and regulatory bodies alike, the emergence of the disease BSE in cattle has placed even further restrictions on the use of animal tissue for this type of application and the whole question of prosthetic heart valves has been turned from a reasonable successful example of reconstructive implant surgery to a very confused area. This serves to highlight some of the very varied problems of facing the use of biomaterials.
What does "this dichotomy" in last paragraph refer to?
选项 A、Cow’s heart wall or pig’s heart valve.
B、Tissues or device.
C、Housing or occuluder.
D、Anticoagulation or calcification.
答案B
解析
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