Page 101 - Biomedical Engineering and Design Handbook Volume 2, Applications

80 MEDICAL DEVICE DESIGN

therapy and placement of an extra-anatomic bypass, graft infections result in amputation rates up to
30 percent and a mortality rate approaching 40 percent (Bandyk and Esses, 1994).

3.5.5 Future Trends
Much current research is focused on improving the performance of existing vascular graft designs
(Kapadia et al., 2008). Novel coatings have been developed to steer the biological response to an
implanted vascular graft toward endothelialization and improved biocompatibility, while moving
away from the proliferation of cellular components linked to vessel narrowing and failure (Greisler,
1996). Various approaches to improve graft performance, healing, and long-term patency include
endothelial cell seeding (Park et al., 1990; Williams et al., 1992) and the use of antithrombotic and
antibiotic coatings (Devine et al., 2001). Clinical experiences with some of these approaches
(endotheliaiization) have shown promising results (Deutsch et al., 1999) compared to traditional
grafts, while others, such as antibiotic coatings, lack definitive outcomes (Earnshaw, 2000).
One area of vascular graft research with significant future promise is the development of tissue-
engineered blood vessels (TEBV). Early efforts to create TEBV (L’Heureux et al., 1998; Niklason
et al., 1999) have matured and begun to bear fruit, with tissue-engineered vascular conduits currently
undergoing clinical trials in patients with congenital heart defects (Shin’oka et al., 2005), and for
high-pressure applications such as arteriovenous bypass for hemodialysis and arterial revasculariza-
tion (L’Heureux et al., 2007a; L’Heureux et al., 2007b). Initial results indicate that complex host-
graft interactions such as size changes with flow volume (Shin’oka et al., 2005) and improved
compliance (L’Heureux et al., 2007b) are present in these constructs. Although long-term results are
needed, the potential applications for TEBV are broad and could result in solutions for problems that
have plaqued cardiovascular surgeons since the field’s beginning.

3.6 ARTIFICIAL KIDNEY

3.6.1 Market Size
According to the U.S. Renal Data System, the number of Americans in end-stage renal disease
(ESRD) is expected to rise from around 485,000 current patients in 2005 to almost 800,000 by 2020
(US Renal Data System, 2007). The aggregate cost for treating ESRD is staggering, with approxi-
mately 32 billion USD expended in 2005, of which more than 21 billion USD came from Medicare
(US Renal Data System, 2007). Treatment modalities used to replace the failing kidney are few,
including hemodialysis with an artificial kidney or hemodialyzer, peritoneal dialysis, or kidney
transplantation. With about 341,000 ESRD patients supported in 2005, hemodialysis remains the
dominant therapy for ESRD in the United States and is predicted to expand to over half a million
patients by 2020 (US Renal Data System, 2007). Hemodialysis is also the dominant therapy world-
wide, with only a few countries showing high rates of peritoneal dialysis use (US Renal Data System,
2007). In the United States, kidney transplantation follows hemodialysis in number of patients treated,
but a limited donor pool prevents dramatic expansion of this option. The percentage of U.S. ESRD
patients receiving peritoneal dialysis continues to decline, falling from 13.5 percent in 1998 (US Renal
Data System, 2000) to less than 6 percent in 2005 (US Renal Data System, 2007). The present section
focuses on the use of hemodialyzers as a cardiovascular device addressing kidney failure.

3.6.2 Indications
Two physical processes, diffusion and convection, are in widespread use as methods to mimic the
excretion functions of the native kidney. Therapies utilizing predominantly convective transport are
accurately termed hemofiltration, while diffusion-dependent methods are grouped under hemodialysis.
Both hemodialysis and hemofiltration are used in the inpatient and outpatient setting to treat renal and
nonrenal diseases. In general, the purpose of these therapies is to either remove toxins circulating in the
blood or reduce the blood volume by removal of water.