Page 107 - Biomedical Engineering and Design Handbook Volume 2, Applications

86 MEDICAL DEVICE DESIGN

FIGURE 3.13 An implantable vascular access port is shown. The port is
accessed through the skin via needle and is resilient enough to allow hundreds
of punctures and infusions before needing to be replaced.

preferred long-term route, given an infrequent need for vascular access (Namyslowski and Patel,
1999). Figure 3.13 contains an example of a double-ported, fully implantable catheter used for
longer-term access.
As mentioned previously, access for hemodialysis and related therapies is a common indication
for catheter placement. Polysiloxane-tunneled central venous catheters and, more recently, totally
implantable intravascular port devices have been utilized in hemodialysis for long-term vascular
access (Schwab and Beathard, 1999). Temporary central venous catheters for hemodialysis access
are further subdivided into different groups based on catheter flexibility and the length of time the
device will be in use (Canaud et al., 2000). The longer the catheter will be in place, the more sup-
ple the material used for construction. Short-term use catheters possess a high stiffness and are fab-
ricated from polytetrafluoroethylene, polyethylene, polyvinyl chloride, and polyurethane (Schwab
and Beathard, 1999; Canaud et al., 2000), although the use of polyethylene and polyvinyl chloride
is dropping due to concerns over bacterial adherence in vitro (O’Grady et al., 2002). Medium-term
(8 to 30 days) catheters are primarily contructed of polyurethane, while catheters implanted for
longer periods of time are usually based on polysiloxane, although polyurethane can be used
(Canaud et al., 2000).
Catheters have been permeated, coated, or surface-modified with a variety of compounds in an
effort to minimize thrombosis, infection, and friction (Triolo and Andrade, 1983b; Marin et al.,
2000). The ultimate goal is an improvement in catheter-handling characteristics and long-term
performance. Some of the more common strategies for imparting microbial resistance include satu-
rating the catheter material with silver sulfadiazine and chlorhexidine (Maki et al., 1997), coating
the surface with antibiotics (Raad et al., 1997), or bonding heparin to the exterior of the catheter
(Appelgren et al., 1996). Results with antibacterial and antiseptic coatings have been mixed, but a
recent meta-analysis involving several randomized controlled trials has shown a significant reduc-
tion in hospital-acquired infections when catheters modified for bacterial resistance are used