Page 105 - Biomedical Engineering and Design Handbook Volume 2, Applications

84 MEDICAL DEVICE DESIGN

for toxin removal, and improved patient management techniques will allow further maturation of
hemodialysis and hemofiltration therapy. For example, considerable benefits could be realized from
selective toxin removal without concomitant elimination of beneficial proteins. It has been suggested
that future devices might utilize the absorption removal pathway with affinity methods as a primary
technique to eliminate specific uremic toxins (Klinkmann and Vienken, 1995).
The promise of a true revolution in artificial kidney design comes from the area of tissue engi-
neering. The living kidney performs a number of important metabolic, endocrine, and active trans-
port functions that are not replaced with current hemofiltration and hemodialysis therapy. An
artificial kidney that successfully replaces these functions could be a significant improvement when
used in conjuction with existing therapies. Researchers have developed a bioartificial renal tubule
assist device that successfully reproduces many of the homeostatic functions of the native kidney
during in vitro studies, and that responds in the proper manner to known physiologic regulators of
the various homeostatic functions (Humes et al., 1999). Efforts have progressed through animal stud-
ies (Humes et al., 2002a; Humes et al., 2002b) to a recently completed phase II clinical trial with
promising results (Tumlin et al., 2008). Although a larger phase III clinical trial is needed, tissue-
engineered artificial kidneys offer the hope of an eventual end to traditional dialysis methods.

3.7 INDWELLING VASCULAR CATHETERS AND PORTS

3.7.1 Market Size
Catheters, in their simplest form, are merely tubes inserted into a body cavity for the purpose of fluid
removal, injection, or both (Thomas, 1989), The term catheter has been expanded to include a num-
ber of tubing-based percutaneous interventional devices used for tasks such as stent delivery and
deployment, clot-removal, atherectomy, radiofrequency ablation, and intra-aortic balloon cardiac
support. Because of their prevalence and representative uses, the present section will be limited to
vascular infusion catheters and access ports. Stenting and cardiac support applications utilizing
catheter-based techniques are discussed elsewhere in this chapter.
In 1991 it was estimated that more than 150 million intravascular catheters were being procured in
the United States each year (Maki and Mermel, 1998). Of this number, more than 5 million were cen-
tral venous catheters (Maki and Mermel, 1998). Catheters have a critical role in modern health care and
are used in increasing numbers for central access of the major arteries and veins, as well as for an ever-
expanding array of invasive procedures (Crump and Collignon, 2000). Given the ubiquitous nature of
catheters, even minor design improvements can have a broad clinical and market impact.

3.7.2 Indications
Catheters are placed when there is a clinical need for repeated sampling, injection, or vascular
access, usually on a temporary basis. In kidney failure, catheters allow emergent blood access for
hemodialysis and hemofiltration (Canaud et al., 2000), and provide temporary access as more per-
manent sites such as arteriovenous fistulas or grafts mature (Trerotola, 2000). Placement of a catheter
or access port is routine for the administration of chemotherapeutic agents and intravenous nutri-
tional supplements. Catheters are often placed when frequent, repeated doses of medication are to be
injected, blood samples are to be taken, and for monitoring of hemodynamic performance in criti-
cally ill patients (Pearson, 1996).
The anatomic location for temporary central venous catheter (CVC) insertion and placement can
be dictated by certain patient or disease restrictions, but the most common sites are the internal jugular
vein (neck), the femoral vein (groin), and the subclavian position (upper chest). The internal jugular
approach is the first choice for placement of a hemodialysis CVC, while femoral placement is
favored when rapid insertion is essential (Canaud et al., 2000). Subclavian vein access has fallen
from favor due to a higher incidence of thrombosis and stenosis associated with this site, which can
ultimately prevent use of the veins in the downstream vascular tree for high-flow applications such