Page 82 - Biomedical Engineering and Design Handbook Volume 2, Applications

OVERVIEW OF CARDIOVASCULAR DEVICES 61

FIGURE 3.1 A caged-ball prosthetic heart valve is shown (Starr-Edwards
Model 61.20). The simple and durable design is still in use for new implanta-
tions. (Butany and Collins, 2005.) (Image courtesy of Edwards Lifesciences,
Irvine, CA).

cage to periodically occlude the valve orifice. Single-disc valves possess a central disc occluder that
is held in place by struts projecting from the housing ring. The disc opens through a combination of
tilting and sliding over the struts to reveal a primary and secondary orifice. Bileaflet valves feature
leaflets that are hinged into the housing ring. The opened valve presents three orifices; two along the
housing ring edge and one central orifice between the leaflet mount points. A caged-ball valve is
shown in Fig. 3.1, while Fig. 3.2 demonstrates orifice and profile views of representative tilting disc
and bileaflet mechanical valves.
Mechanical valves are expected to perform flawlessly for decades with minimal patient burden.
Criteria used to evaluate designs during development and clinical use can be divided into structural
and hemodynamic groups, although there is considerable overlap. Structural considerations involve
fatigue and device integrity, valve profile, rotatability, and occluder interference (Akins, l995).
To accommodate the wear associated with operating hundreds of millions of times, current mecha-
nical valves are manufactured with durable metal and carbon alloys (Helmus and Hubbell, 1993;
Vongpatanasin et al., 1996), and include a polymer fabric sewing ring for surgical placement.
Rotatability of the valve is desirable as evidence suggests that optimum orientations minimizing tur-
bulence and microembolic signals exist for mechanical heart valves in vivo (Laas et al., l999; Kleine
et al., 2000). Concerns regarding valve profile and occluder interference focus on possible negative
interactions between the valve, adjacent ventricular structures, native valve remnants, or surgical
suture material. The impingement of these structures into the valve could prevent complete closure
or cause binding of the occluder. It is believed that overgrowth of adjacent tissue in particular is an
underappreciated cause of valve failure (Zilla et al., 2008). Although not a structural requirement,
devices tend to be radiopaque to aid in visualization during radiographic procedures.
Hemodynamic performance factors that should be considered during functional evaluation of a
valve design are the transvalvular pressure gradient, rate and duration of valve opening, dynamic
regurgitant fraction, and static leak rate (Akins, l995). The transvalvular pressure gradient is a func-
tion of the effective orifice area of the valve and the flow regime (turbulent or laminar) encountered.