Vascular Access for Acute Renal Replacement Therapy

Catheter Characteristics: Material, Geometry, and Design

CVCs are classified into two main categories according to their intended duration of use. Short-term or acute CVCs are used less than 1 week, whereas long-term tunneled or chronic CVCs may be used for extended periods, up to several months or years. The difference between these two types of device relies on the fact that long-term CVCs could be tunneled. Cannulas are made of synthetic polymers (e.g., polyvinyl chloride, polytetrafluoroethylene, polyethylene, polyurethane, silicone elastomer, thermoplastic polyurethane elastomer), which give them their specific characteristics (resistance, softness, hemocompatibility). Several models of large-bore hemodialysis catheters are currently available but are not necessarily equivalent in performance. Design and cannula engineering (inner lumen diameter, cannula thickness, length, tip design, central and/or side holes) affect the performance of the catheter (maximum blood flow, flow resistance, recirculation, dysfunction). Different models of short-term single and double-lumen catheters are shown in Fig. 167.1 . When choosing the catheter material, the physician should know these characteristics and should be aware of CVC-related hazards.

Catheter stiffness depends on polymer nature, plasticizer content, and extrusion mode. Polyurethane and silicone rubber are the materials most widely used for CVCs. Catheter stiffness dictates the procedure for insertion. Rigid and semirigid acute CVCs are easily introduced percutaneously by means of the Seldinger method, over a soft metallic guidewire ( Fig. 167.2 ).

Soft and flexible chronic CVCs (e.g., silicone) require a hybrid percutaneous two-step technique that combines a mini-invasive surgery and catheter introduction using a desilet catheter and a sheath vein dilator (peelable or nonpeelable). Soft CVCs have the following major advantages over rigid ones: (1) they are less traumatic to the host vein and more biocompatible with the blood, (2) they are subcutaneously tunneled, and (3) they have a subcutaneous anchoring system (Dacron cuff, purse-string suture).

Single-lumen or double-lumen cannulas may be used for all forms of extracorporeal blood circuit. Single-lumen catheters have a single port used alternatively for inflow and outflow. Occasionally a single-lumen catheter may be used (rescuing, waiting), but in these cases a specific monitoring device (single-needle module) is required, consisting of a double pump with a specific blood tubing set. Double-lumen catheters, which have one arterial flow port and one venous flow port, now are used almost universally to start RRT in acute renal failure ( Fig. 167.3 ). Designs of the lumen and distal tips vary considerably among catheters and may provide functional advantages. Schematically, one can consider two main types of catheters: one with port sites arranged in a symmetric double-barreled gun fashion (coaxial, double-D, double-O) ( Fig. 167.4 ) and the other with independent or separated port sites (dual catheter or split catheter) or asymmetric tip design ( Figs. 167.5 and 167.6 ).

From a flow perspective, it has been proved that independent catheter lines offer more consistently adequate and high flow than the attached lines. Improved functionality is due to the catheter tip design, which reduces risk of catheter occlusion by arterial wall suction or sidewall contact but also because of handling and best management of dialysis catheters.

Methods of Central Venous Catheters Insertion

Methods of catheter insertion have improved over the last decade, contributing to reduced morbidity associated with CVCs. Ultrasonography- or fluoroscopy-based methods to locate prior insertion and/or to guide the vein cannulation have reduced the incidence of failure, the time spent in catheterization, and the incidence of traumatic complications. Ultrasound-assisted methods of CVC insertion have been shown to be particularly beneficial for inexperienced physicians and in cases of abnormal vein location (20% to 30% of cases).

Acute untunneled CVC placement is derived from the Seldinger method relying on a percutaneous vein cannulation in strict aseptic conditions under local anesthesia. In brief, a direct venipuncture of femoral vein is performed and a souple metallic guidewire is passed through the needle. After the needle is removed, vein dilation is performed by means of a large catheter dilator (or desilet). Large-bore dual-lumen catheter placement then is performed after removing the desilet over the metallic guidewire left in the vein. An alternative choice of venous access sites is discussed in the next paragraph.

Chronic tunneled CVC placement is performed by a hybrid percutaneous method combining the Seldinger method and min-invasive surgery. Catheter insertion is performed in strict aseptic conditions, under local anesthesia, and secured by ultrasound (US) vein tracking and/or fluoroscopic guidance. Depending on the type of catheter (splittable or nonsplittable), there are two ways of placing tunneled CVCs. In case of a splittable catheter (e.g., DualCath, Tesio) placed in the right internal jugular, the first step consists of introducing the two independent cannulas in the vessel. In brief, the jugular vein is punctured with a large-bore needle, and then two metallic guidewires are introduced successively into the vessel. Thereafter, twice in a row a nonpeelable introducer dilator catheter is introduced into the vein over the guidewire, permitting the insertion of the cannulas. After each procedure, the introducer dilator is removed while the catheter is left in the vein and clamped. The second step consists in creating the subcutaneous tunneling. In that case, twice in a row, cannulas are stowed securely on tunneler, passed downward under the skin, and exited 10 to 15 cm from cervical entrance. The third step consists in customizing the catheter length to patient anthropometrics and ensuring subcutaneous anchorage. Briefly, cannula lengths are adjusted and cut to position catheter tips 1 to 2 cm apart at the junction of superior vena cava and right atrium. Thereafter, cannulas and extension pieces are strap stowing, pushed back under skin, and anchored either by purse-string along subcutaneous track or Dacron patch fixed in the subcutaneous tunnel.

In case of a nonsplittable double-lumen catheter (e.g., Ash Split) placed in the right internal jugular vein, the first step consists of puncturing the right internal jugular vein with a large-bore needle and introducing a metallic guidewire into the vessel. A peelable introducer-dilator then is inserted into the vein over the guidewire to prepare dual-lumen catheter introduction; the second step consists of passing upward the double-lumen catheter (bottom-up) under the skin chest by means of a peelable tunneler catheter; the third step consists in grabbing the catheter tip exiting from subcutaneous tunnel at the neck base and then passing the guidewire exiting from the vein (upside down) through the catheter to its hub. The double-lumen catheter then is pushed back through the sheath over the guidewire into the vessel. During this procedure, the step-by-step dialysis catheter is pushed back while the sheath is peeled away progressively and finally removed. The fourth step consists of ensuring subcutaneous anchoring and skin suturing.