Developments in cardiovascular valve technology - Biomaterials and Devices for the Circulatory System - page 85

aging-related degenerative calcifi cation. 1,2 The accumulation of calcifi ed
masses on the valve leafl ets lowers their fl exibility and can markedly
increase the pressure gradient between left ventricular lumen and aortic
conus required to eject the stroke volume. When forced to chronically
increase its pressure development, the left ventricle adapts by undergoing
concentric hypertrophy, 3 which decreases left ventricular compliance and
can eventually culminate in congestive heart failure.
The predominant cause of mitral valve stenosis is rheumatic fever, a
consequence of streptococcal infection. 4,5 In rheumatic fever, diffuse infl am-
matory lesions develop throughout the heart, including the mitral and, less
frequently, the tricuspid and aortic valves. Fibrosis and scarring of these
lesions eventually thickens and deforms the valves. In severe cases, increased
resistance across the stenosed mitral valve causes left atrial dilation, pul-
monary hypertension and, eventually, right ventricular hypertrophy. With
improved diagnosis and treatment of infections, and enhanced socioeco-
nomic conditions in general, the impact of rheumatic fever and its sequelae
has fallen dramatically in the industrialized world in recent decades, 1,6 but
rheumatic valvular disease remains a major health problem in many devel-
oping countries. 5,7
Mitral valve prolapse is the leading valvular disease in the industrialized
world. 1 Degeneration of the mitral cusps and chordae tendineae and dilation
of the valve annulus weakens the valves, causing them to bulge into the left
atrium during ventricular systole. 8 Although mitral prolapse is benign in
most patients, a minority develop serious complications, including mitral
insuffi ciency, ventricular and atrial arrhythmias, 9-11 and infarct of brain,
myocardium or kidney due to embolism of mitral valve thrombi. 12,13
Infective endocarditis, commonly of bacterial origin, is the leading cause
of aortic valve insuffi ciency. 14,15 Infection of the leafl ets precipitates devel-
opment of vegetations that invade and perforate the leafl ets. The vegeta-
tions may fragment, releasing emboli which can infarct brain, heart and
other organs and deliver virulent organisms to these tissues.
Valve replacement is the only effective treatment for severe valvular
stenosis or insuffi ciency. Unfortunately, development of new devices to
correct valvular disease is hindered by regulatory constraints and prohibi-
tive costs of clinical testing. Moreover, artifi cial valves incur the risks of clot
formation and increased susceptibility to infection. Thus, improvement and
refi nement of artifi cial heart valves continues to be a focus of cardiothoracic
surgeons and manufacturers of medical devices.
Over the last four decades essentially two types of artifi cial valves have
been utilized in cardiac surgeries: mechanical valve prosthesis and biologi-
cally based bioprosthesis. The four major types of mechanical valves are
caged-ball valves, caged-disc valves, tilting disc valves, and bileafl et valves,
and the three main categories of bioprostheses are homografts, heterografts,

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