Page 98 - Biomedical Engineering and Design Handbook Volume 2, Applications

OVERVIEW OF CARDIOVASCULAR DEVICES 77

“Fistula First” initiative, the Centers for Medicaid and Medicare (CMS) has a near-term goal to raise
AVF use from around 40 percent at the end of 2005 to 66 percent of prevalent HD patients, and ulti-
mately to between 70 and 80 percent (Centers for Medicare and Medicaid Services, 2005). Although
treatment guidelines from the National Kidney Foundation still advocate the use and placement of
an artificial graft for permanent vascular access when a native fistula cannot be created (Besarab
et al., 2006), it is clear that this access mode is in decline.

3.5.2 Indications
A number of situations can necessitate the placement of an artificial vascular graft. Vascular grafts
can be placed for primary diseases of the vasculature, such as aneurysms or severe atherosclerotic
narrowing; for secondary causes, such as trauma; or for chronic vascular access issues, such as
hemodialysis. In the case of primary disease or injury, the indications for placement are dependent
upon the level of the vascular tree in which the lesion or damage is located, as well as the etiology
of the illness. The choice of interventional therapy also varies, depending on whether the target vas-
cular lesion is cardiac, thoracic, carotid, or peripheral. Replacement or bypass of a native vascular
conduit is a procedure that lies on one end of the interventional spectrum for treatment of vascular
insufficiency. The initial approach to addressing a case of vascular insufficiency (stenosis, occlusion,
etc.) is usually a percutaneous intervention, such as balloon angioplasty and stenting, followed by
surgical repair or replacement of the native vessel, if necessary. Practice guidelines have been devel-
oped that outline the preferred approach to repair (excisional vs. endovascular) in view of current
medical outcomes and lesion characteristics (Norgren et al., 2007). Even when an excisional
approach is indicated, some vascular patients with severe coexisting conditions are unable to toler-
ate the profound sedation required to directly excise and replace a damaged blood vessel. For these
patients, long tunneled extra-anatomic grafts placed outside the path of the existing diseased vessel
are the preferred therapy, despite poorer long-term outcomes and patency rates compared to tradi-
tional excisional grafting (Connolly et al., 1984; Foster et al., 1986; Biancari and Lapantalo, 1998).

3.5.3 Current Graft Designs
Vascular grafts or prostheses can be classified as originating primarily from biologic or synthetic
sources. Biologic grafts can be harvested from other species (xenograft), from other humans
(allograft), or the vasculature of the patient (autograft). As the focus of the present section is on the
engineering aspects of vascular grafts, the following discussion will be limited to conduits of synthetic
origin. Tissue-engineered biologic grafts will be discussed in the section covering future trends.
The ideal synthetic vascular graft would possess a number of characteristics designed to mimic native
conduits, ease surgical placement, and limit manufacturing complexity and cost (Brewster, 2000).
A number of features supporting these design goals are listed in Table 3.2. Initially, vascular graft
research focused upon the development of completely passive conduits that would not elicit a biological
response when exposed to blood. More recent research has focused on the development of grafts that
generate a favorable biological response from the body as it has been recognized that a perfectly inert
surface may be an unattainable goal (Greisler, 1991), and that a complex set of interactions between graft
and host exists, along with a limited window for healing (Zilla et al., 2007).

TABLE 3.2 Desirable Characteristics of Prosthetic Vascular Grafts
Biologic features Handling and implant features
Biocompatible Range of sizes
Infection-resistant Ease of sterilization
Antithrombotic Flexible without kinking
Compliant No blood leakage
Durable Good suturability
Source: Adapted from Brewster (2000).