**2. Central venous catheters**

Central venous catheters are used widely for hemodialysis access. Non-tunneled catheters are used for emergent dialysis access and are common in the intensive care setting. Tunneled catheters are placed for hemodialysis access in urgent, but nonemergent, cases in hospitalized patients. Tunneled CVCs can be functional for over a year, but dysfunction and complications are not uncommon. Although CVCs have been associated with worse outcomes compared to AVFs and AVGs, they are still pervasive because they can be placed easily and utilized immediately. Furthermore, accessing CVCs avoids the needlesticks required for AVFs and AVGs. Despite this, it is generally recommended to use CVCs only as a bridge to AVFs because of the common complications of CVCs, or when individuals have limited life expectancy and the risks of AVF or AVG creation do not outweigh benefits. CVCs can be either non-tunneled (for emergent short-term use) or tunneled through subcutaneous tissue (for longerterm use on the order of months), and both are associated with higher morbidity and mortality compared to AVFs. Over the last several years, the design of CVCs has improved to maximize blood flow rate, minimize endothelial injury, improve catheter function and biocompatibility, and minimize infections [7].

Catheter tip design has evolved from step tip to split tip to symmetric tip, with the goal of preventing thrombosis and recirculation of blood (**Figure 1**) [8]. The difference lies in the end of the catheter. The step tip catheter has its two ends offset by a distance, while the split tip catheter has a Y-shaped end with the two tips diverging. The symmetric tip catheter as the two ends of the catheter adjacent and mirroring each other. One study of 302 patients examined split tip and symmetric tip designs and found no difference in mean primary assisted patency [9]. One study found lower recirculation rates with the symmetric tip (0% for symmetric tip vs. 22.3–39.2% for split tip vs. 8.7–16.3% for step tip) [10]. A study comparing split tip and step tip catheters found that the step tip catheter delivered higher blood flow rates (433 mL/min vs. 414 mL/min), but both types were able to deliver blood flow rates that were well above that recommended by the Dialysis Outcomes Quality Initiative [11]. Overall, there have been mixed results, and it is inconclusive which design is optimal.

In theory, catheter coatings can be utilized externally and internally to prevent biofilm formation and activation of the coagulation cascade, which would prevent infection and thrombosis. Catheters can be coated with heparin to prevent thrombosis and antibiotics to prevent infection. However, a systematic review examining coatings have failed to show significant benefit [12].

*Innovations in Hemodialysis Access DOI: http://dx.doi.org/10.5772/intechopen.110467*

#### **Figure 1.**

*Catheter designs. This illustrates three common types of catheter designs for hemodialysis access. The goal of the evolution from step tip to split tip to symmetric tip was to prevent thrombosis and blood recirculation. Although a few studies have demonstrated certain advantages for each, it is inconclusive which design is optimal overall. Reprinted from: [8] Copyright 2019, with permission from Elsevier. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/).*

Although early catheters were made of silicone, newer polyurethane/polycarbonate polymers are used to increase luminal diameter (thin walls) while maintaining strength, flexibility, and rigidity to prevent luminal collapse at high negative pressures [13]. Large randomized controlled studies comparing catheter materials are lacking.

Catheter lock solutions have been used for the purposes of antisepsis (antimicrobials) and anticoagulation (heparin or citrate). There is weak positive evidence that antibacterial lock solutions decrease the incidence of catheter-related bloodstream infection [14]. To date, there has not been an ideal catheter lock solution that has reliably prevented infection or catheter dysfunction. It is controversial whether we should use antibiotic locks to treat catheter-related blood stream infections, as the evidence supporting this is minimal, and only includes small observational studies [15].
