Polyurethane or silicone skin‐tunnelled catheters are available in single‐, double‐ and triple‐lumen versions with a Dacron cuff (Wilkes [472]). The cuff is used to secure the catheter as fibrous tissue grows around it and obliterates part of the subcutaneous tunnel within 1–2 weeks of insertion. As it becomes overgrown with epithelial cells, this also serves to prevent easy passage of micro‐organisms from the skin along the catheter into the vein (Light et al. [270], Wilkes [472]). The cuff is about 1 cm wide (Hadaway [194]) and is usually located about 3–5 cm from the exit site (Stacey et al. [427], Weinstein and Hagle [465]). Knowledge of the distance from the cuff to the bifurcation may assist the practitioner in locating the cuff during removal of the catheter, although the distance will vary according to the brand of catheter.

The tip of the catheter may be valved or non‐valved (Dougherty [123], Ives [225]). The disadvantage of a non‐valved catheter is the problem of blood reflux, which can result in occlusion. Valved catheters can have the valves in a round closed tip (such as in the Groshong catheter) or in the hub. Valves (proximal or distal) close and open the catheter depending on the pressure, so if a catheter is damaged or disconnected, or at the end of an infusion, the low pressure closes the valve, thereby preventing backflow and occlusion. The valve opens with minimal positive pressure for infusion but requires four times as much negative pressure for aspiration (Weinstein and Hagle [465]). This feature reduces the risk of air embolism or bleeding resulting from accidental disconnection; it also eliminates the need for catheter clamping and the need for heparin, and reduces the frequency of flushing (Ives [225], Mussa [341]). The valve is usually a two‐way valve that remains closed at normal vena caval pressure. Application of a vacuum in order to withdraw blood enables the valve to open inwards, whereas positive pressure into the catheter forces the valve to open outwards (Figure 17.39) (Dougherty [123]). When a valve is incorporated within the hub and not the tip, it may reduce the risk of reflux even during periods of raised central venous pressure (Weinstein and Hagle [465]). However, if a thrombus forms around the tip, it can result in malfunction and loss of valve competence (Dougherty [123]).