Page 85 - Biomedical Engineering and Design Handbook Volume 2, Applications

64 MEDICAL DEVICE DESIGN

depending on the valve type and anatomic location. After that time warfarin is discontinued for het-
erograft bioprosthetic valves, unless the patient possesses a risk factor that increases their suscepti-
bility to thromboembolic complications (Bonow et al., 2006). Despite the use of chronic anticoagulant
therapy, between 0.4 and 6.5 percent of mechanical heart valve recipients will experience a throm-
boembolic event per year, a range that is dependent upon valve type, number, placement, and other
risk factors (Salem et al., 2004). The long-term risk of thromboembolism in bioprosthetic heart valve
recipients is comparatively low, ranging from 0.2 to 5.5 percent per year (Salem et al., 2004).
In contrast to the anticoagulation requirement associated with mechanical valves, the largest
problem facing patients with bioprosthetic valves is progressive structural deterioration due to calci-
fication, which can eventually require valve replacement and the risks of a reoperation (Schoen and
Levy, 1999; Hammermeister et al., 2000). Heterograft bioprosthetic valves and homograft (allograft)
valves exhibit accelerated deterioration in younger patients, which promotes the use of mechanical
valves in this age group (Bonow et al., 2006).
Although the literature is rife with comparisons between valve designs in regard to their compli-
cation rates, the general lack of large randomized trials using standardized methods makes valid
comparisons problematic (Horstkotte, 1996). To reduce some of the confusion surrounding outcomes
reporting in heart valve studies, the Society of Thoracic Surgeons and the American Association of
Thoracic Surgery developed guidelines for reporting common surgical and nonsurgical artificial
valve complications (Edmunds et al., 1996). The guidelines distinguish between structural and non-
structural valve deterioration, thrombosis, embolism, bleeding events, and infection. In addition, var-
ious types of neurologic events are graded, and methods of statistical data analysis are suggested
based on the type of data being collected and analyzed. Adherence to such guidelines should allow
valid comparisons to be made between manuscripts reporting on outcomes with different valve types
and clinical approaches (Edmunds et al., 1996).

3.2.5 Future Trends
There are two major areas of development in artificial heart valves that are poised to change the field
in the near term: percutaneous or minimally invasive valve implantation and valvular tissue engi-
neering. Recent developments in both areas will be reviewed below.
Percutaneous valve implantation represents a natural progression toward minimal invasive thera-
pies wherever lesion characteristics permit such an approach or the patient’s clinical condition
demands it. Percutaneous valvular prostheses have been implanted in the aortic (Cribier et a1., 2002;
Grube et al., 2005) position within the native valve and in the analogous pulmonic position
(Bonhoeffer et al., 2000) in right ventricular outflow tracts (RVOT) used in congenital heart repair.
These devices and other novel designs are undergoing extensive trials at the time of writing and con-
tinue to advance at a rapid rate (Babaliaros and Block, 2007). An example of a percutaneous valvu-
lar prosthesis is shown in Fig. 3.4.
Tissue engineering holds great promise for the development of replacement valves with improved
long-term function and viability. As reviewed in a recent article, approaches under investigation
involve the use of synthetic scaffolds or decellurized tissue with cell seeding (Mendelson and
Schoen, 2006). Efforts to produce artificial valves de novo using harvested endothelial cells and
fibroblasts grown on a biodegradable scaffold have resulted in functional pulmonary valve leaflets
in a lamb model (Shinoka et al., 1996). A biohybrid trileaflet pulmonary valve has also been suc-
cessfully tested in lambs for a limited period (Sodian et al., 2000). More recently, decellurized valves
seeded with autologous cells and matured in a bioreactor have been used as a pulmonic valve
replacement in humans with good midterm results (Dohmen et al., 2007). These efforts show much
promise, although challenges in creating valves with ideal performance characteristics remain.
Successful development of a viable autologous biologic valve capable of long-term stability and
growth would likely revolutionize the management of heart valve disease through a reduction of
implant-related morbidity and mortality, with particular applicability to pediatric patients.
In addition to efforts to grow artificial valves, researchers have attempted to stimulate the growth
of endothelial cells on existing bioprosthetic valves to limit valve degradation and thromboembolic