首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
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
admin
2013-01-12
59
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.
Which of the following is not mentioned in the passage?
选项
A、The causes of BSE.
B、The quality of the artificial valve.
C、The result of failure of the valves of the heart.
D、The way to make valve with animal tissue.
答案
A
解析
转载请注明原文地址:https://kaotiyun.com/show/yf9O777K
0
考博英语
相关试题推荐
Skippersmustmakeareporttocustomseitherinpersonorbytelephone,iftheyhaveanyduty-freegoodsonboard,orarecarry
ThepoetryofGwendolynBrookshasbeenpraisedfordeepeningthesignificanceofpersonalandsocialexperiencessothatthese
71.Oneofthemajorpleasuresinlifeisappetite,andoneofourmajordutiesshouldbetopreserveit.Appetiteisthekeennes
AnaisNin’sdiariesareoftenscandalous,probablybecauseshedescribesherselfassheisratherthan______.
Forthispart,youarerequiredtowriteacompositiononthetopicMyViewOnJob-Hopping(跳槽)innolessthan150words.Remem
Iwashalfwaybacktothecottagewheremymotherlived______Susancaughtupwithme.
Andrew,mymother’selderbrother,willnotbeatthefamilyparty,______tothefamily’sdisappointment.
Largelyduetotheuniversitytraditionandthecurrentacademicmilieu,everycollegestudenthereworks______.
InarecentbookentitledThePsychicLifeofInsects,ProfessorBouviersaysthatwemustbecarefulnottocreditthelittlew
Theworldisfacingthe______ofwatershortagescausedbypopulationgrowth,unevensuppliesofwater,pollution,andotherfact
随机试题
我国金融监管与货币政策的协调内容主要有()。(2009年真题)
下列物质久置空气中会变质的是()
丹参所没有的药理作用是
秦代君主或皇帝针对一时之事而以命令形式发布的法律文件是:()。
在图示xy坐标系下,单元体的最大主应力σ1大致指向()。
国有资产的特征包括()。
工商行政管理机关是发票的主管机关,负责发票印制、领购、开具、取得、保管、缴销的管理和监督。()
某企业采用先进先出法计算发出甲材料的成本,2007年2月1日,结存甲材料200公斤,每公斤实际成本100元;2月10日购入甲材料300公斤,每公斤实际成本110元;2月15日发出甲材料400公斤。2月末,库存甲材料的实际成本为()元。
内蒙古森林旅游产品有哪些特点?
GregFocker,playedbyBenStiller,representsagenerationofAmericankids(1)_____inthe1980sonthephilosophythatanyac
最新回复
(
0
)