Page 84 - Biomedical Engineering and Design Handbook Volume 2, Applications

OVERVIEW OF CARDIOVASCULAR DEVICES 63

FIGURE 3.3 Two tissue-based artificial heart valves are shown above with a U.S. quarter dollar for size comparison.
The valve on the far left is a porcine heart valve, while the other valve is constructed of bovine pericardium. Both valves
are intended for aortic placement.

response, and improves the toughness of the tissue by cross-linking the structural collagen (Bonow
et al., 1998). Some bioprosthetic valves are further treated with surfactants, diphosphonates, ethanol,
or trivalent metal cations to limit the rate of calcification and associated structural deterioration
(Schoen and Levy, 1999).
Porcine heterograft valves can be mounted on a support scaffold with sewing ring, although
unmounted designs have been introduced to improve flow characteristics and structural endurance.
Heterograft valves constructed of bovine pericardium are formed over a scaffold with sewing ring to
mimic an anatomic valve shape. Because the pericardial valves are constructed to design criteria
rather than harvested, the orifice size can be made larger to improve flow characteristics, while the
higher collagen content may allow improved graft resilience when cross-linked (Bonow et al., 1998).
Figure 3.3 shows representative porcine and bovine pericardial heterograft valves.

Design Performance Evaluation. The design of artificial heart valves has benefited from the
advent of computational fluid dynamics and other computationally intensive modeling techniques.
Simulations have been used to predict the performance of both bioprosthetic (Makhijani et al., 1997)
and mechanical (Krafczyk et a1., 1998) valve designs. Results from computer modeling can be com-
pared with findings from experimental studies using such methods as particle image velocimetry
(PIV) (Lim et a1., 1998) and high-speed photography of valve structure motion (De Hart et al.,
1998). Such comparisons provide necessary model validation, revealing inadequacies in the numer-
ical model and capturing phenomena not predicted using existing model assumptions.

3.2.4 Complications and Patient Management
Mechanical and bioprosthetic valves suffer from complications that dictate follow-up care and pre-
ventive measures. Possible complications facing heart valve recipients include thromboembolism,
hemolysis, paravalvular regurgitation, endocarditis, and structural failure of the valve
(Vongpatanasin et al., 1996). Some preventive measures are indicated for both mechanical and bio-
logic valve recipients, such as the use of antibiotics during dental surgery and invasive procedures to
avoid infective endocarditis (Wilson et al., 2007). Other preventive treatments, such as long-term
anticoagulation, are administered differently, depending on the type of valve implanted.
Because of the high incidence of thromboembolic complications associated with mechanical arti-
ficial heart valves, chronic anticoagulation is required. Anticoagulation with warfarin and an
antiplatelet agent, such as aspirin, is indicated for both mechanical and heterograft bioprosthetic
valves for the first 3 months after implantation (Bonow et al., 2006), with the level of anticoagulation