Page 81 - Biomedical Engineering and Design Handbook Volume 2, Applications

60 MEDICAL DEVICE DESIGN

3.2 ARTIFICIAL HEART VALVES AND RELATED TECHNOLOGIES

3.2.1 Market Size
The number of valve replacement operations performed in the United States rose from an estimated
60,000 during 1996 (Vongpatanasin et a1., 1996) to more than 78,000 in 2005 (DeFrances et al.,
2007). Worldwide, over 350,000 valves are implanted each year, with over 50 percent being mechan-
ical, or purely artificial, in design (Butany and Collins, 2005). As access to cardiac surgery becomes
more widespread in developing nations, it is expected that the need for durable, safe, low-cost valves
will grow considerably (Zilla et al., 2008).

3.2.2 Indications

The medical indications for valve replacement are thoroughly described in a report from the American
College of Cardiology/American Heart Association Task Force on Practice Guidelines that addresses
the management of patients with valve disease (Bonow et al., 2006). The etiology of valve disease dif-
fers, depending on the patient group and the valve location, as does the preferred corrective treatment.
In developed nations, the young suffer from congenital valve defects, while older adults exhibit
acquired valve disease. Valve replacement can be performed upon all valves of the heart but most cases
involve the aortic or mitral valves. Common reasons for native valve replacement are severe stenosis
and regurgitation with or without symptoms, which may include chest pain, shortness of breath, and
loss of consciousness. The reduction in effective orifice size associated with a stenotic lesion results in
a large transvalvular pressure gradient that may exceed 50 mmHg in the aortic position for severe cases
(Bonow et al., 1998). In regurgitation, the blood pumped forward into the recipient vessel or ventricle
spills back into the adjacent pumping chamber through an incompetent valve, minimizing forward
movement. The end effects of chronic stenosis or regurgitation are compensatory anatomic changes
that accommodate, for a limited time, the reduced pumping efficiency due to restricted blood move-
ment. In general, heart valve replacement is performed when repair is not possible, as the implantation
of an artificial heart valve brings with it another set of problems. Total replacement and removal of
native valve components in the mitral position is particularly limited, as the mitral valve is anatomi-
cally and functionally integral to the left ventricle (David et al., 1983; Yun et al., 1999). Concomitant
illnesses such as congestive heart failure, atrial fibrillation, and coronary artery disease can alter the
indication for valve replacement, as can the surgical need to correct other cardiac disease.

3.2.3 Current and Historical Device Design

Artificial heart valve design has a long and colorful history, with more than 80 different versions of
valve being introduced since the 1950s (Vongpatanasin et al., 1996). The two general types of
replacement valves, mechanical and biologic, each have their own set of indications, complications,
and performance factors. The mechanical valve can be further categorized into three major design
lines: caged-ball, single-tilting-disc, and bileaflet (Vongpatanasin et al., 1996). Caged-ball valves
have been largely supplanted by the more modern single-tilting-disc and bileaflet valves. Biologic
valves are divided based on the source of the tissue material, with the term bioprosthetic reserved for
valves constructed from nonliving, animal-source tissue. Homograft biologic valves are preserved
human aortic valves, and autografts are pulmonary valves surgically moved to the aortic location
within the same patient (Bonow et al., 2006). Heterograft bioprosthetic valves consist of porcine
heart valves or bovine pericardial tissue formed into a valve over a support structure (Vongpatanasin
et al., 1996). Because mechanical and bioprosthetic valves have different design considerations, the
categories are discussed separately.
Mechanical Valves. The assorted mechanical valve designs use different approaches to achieve the
same functional goal. Caged-ball valves use a free-floating polymeric sphere constrained by a metal