Vascular Access

Some patients who receive chronic intravenous therapy e.g., hemodialysis require continuous vascular access or a conduit between the skin surface and the implanted device. This represents a special challenge because the skin organ system functions to expel foreign objects by epithelial downgrowth. The epithelium forms an extensive sinus tract around the implant until it is able to reattach, thus expelling the implant. For success, a stable interface must be established between the epithelium and the device surface.

One approach is to encourage collagen ingrowth near the epithelium to provide a contact inhibition layer. This can be seen in Fig. 4 where the concept for a "skin button" has been adapted to a peritoneal dialysis catheter. The textured thermoplastic, aliphatic polyurethane (Tecoflex), used on the implant neck has carefully sized interstices (pores) for collagen ingrowth. The collagen secures the implant in place and also significantly inhibits the epithelial downgrowth by contact inhibition. The reaction products for this device, which are typical for polyurethanes, include methylene bis(cyclohexyl) diisocya-nate, poly(tetramethylene)ether glycol, and 1,4 butane diol chain extender. Aliphatic polyurethanes usually do not discolor with ultraviolet light, oxygen, or age as do aromatic formulations.

drug administration systems

The Medtronic drug administration system consists of an implantable programmable drug pump and delivery catheter.

FfG. 4. "Sktri button" adapted w a peritonea! dialvsis catheter. (ICI Thermo Cardjosy&tems Inc.. Wolnirn, M A Courtesy Victor L. Poirier.1

The pump provides peristaltic drug delivery from a bellows-design reservoir powered by the vapor pressure of a fluid-vapor equilibrium, in addition to the titanium case (ASTM Grade 1), components include a refill septum (ETR silicone—Dow), a suture pad (MDX—Dow), and silicone elastomer catheters. When implanted, the device is sutured in place using a Dacron mesh pouch.

stents

A stent is a device made ol inert materials and designed to serve as a temporary or permanent internal scaffold to maintain or increase the lumen of a vessel (5chatz, 1989). First introduced in the 1960s, this device has received increased attention in recent years because of the high frequency of restenosis (30—50%) following percutaneous transluminal coronary angioplasty (PTCA, balloon angioplasty). Four designs are being investigated: (1) a springlike design with an initially small diameter that expands to a predetermined dimension when a constraint is removed, (2) thermal memory stents in which the "memory metal" Nitinol is used and changes shape upon heating, and (3) balloon-expandable stents, which operate on rhe principle of plastic deformation of a metal beyond its elastic limit (Fig. 5), and stents made of biodegradable polymers, which serve as scaffolds for a finite period and may in addition serve as drug delivery devices as the drug-containing polymers are degraded.

Essentia! stent features include radial and torsional flexibility, biocompatibility, visibility by X-ray, and reliable expandability. Concerns with any stent include the injury to the vessel wall that is caused by its insertion, and the potential consequences of injury, which include acute localized thrombus formation or intimai hyperplasia (leading to restenosis); both these complications have been observed. Results of clinical trials suggest that the incidence of failure caused by thrombosis or restenosis, is lower with stents than with Angioplasty catheters and cannulas

Catheters, cannulas, and related intravascular tubing are placed m virtually every portion of the arterial and venous circulation of patients. They are used for administering fluids (e.g., blood and blood products, nutrients, isotonic saline, glucose, medications, contrast medium for angiography) as well as for obtaining data (e.g., pulmonary artery pressure, gases) and withdrawing blood specimens for chemical analysis. The

FIG. 5, Balloon-expandable coronary artery stent. (Courtesy Richard A. Schatz, Arizona Heart institute Foundation. Phoenix, AZ.)

FIG. 5, Balloon-expandable coronary artery stent. (Courtesy Richard A. Schatz, Arizona Heart institute Foundation. Phoenix, AZ.)

large majority of these procedures are performed without incident. Rarely, catheter-related thrombosis may occur, creating a potential source for thromboembolism.

Catheterization of arteries or veins in the newborn, which is performed with increasing frequency in neonatology, is associated with an especially high risk of thrombosis and vascular occlusion, in part because of the very small diameter of the catheters and consequently the high surface-to-volume ratio, which favors thrombosis. Other factors include the flexibility and chemical composition of the catheter, activation of blood components by the toxic effects of the delivered solution on the vessel wall, and the duration of catheter placement. Polyurethane and silicone elastomer are preferred materials, but controlled clinical studies of the relative incidence of complications with different catheter materials are not available and are difficult to conduct. Injury to the vascular endothelium caused by the catheter or its contents probably plays a major role in initiating thrombosis. Several methods have been developed for binding anticoagulants, especially heparin, to the luminal surface of catheters, directly or by using a spacer between the anticoagulant and the surface to enhance function of the bound molecule. The efficacy of these methods in reducing the risk of thrombosis is not clear.

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