Central venous catheter
A central venous catheter (CVC), also known as a central line (c-line), central venous line, or central venous access catheter, is a catheter placed into a large vein. It is a form of venous access. Placement of larger catheters in more centrally located veins is often needed in critically ill patients, or in those requiring prolonged intravenous therapies, for more reliable vascular access. These catheters are commonly placed in veins in the neck (internal jugular vein), chest (subclavian vein or axillary vein), groin (femoral vein), or through veins in the arms (also known as a PICC line, or peripherally inserted central catheters).
Central venous catheter
Central lines are used to administer medication or fluids that are unable to be taken by mouth or would harm a smaller peripheral vein, obtain blood tests (specifically the "central venous oxygen saturation"), administer fluid or blood products for large volume resuscitation, and measure central venous pressure.[1][2] The catheters used are commonly 15–30 cm in length, made of silicone or polyurethane, and have single or multiple lumens for infusion.[3]
A central venous catheter secured to the skin with suture
Chest x-ray with catheter in the right subclavian vein
The outline of superior vena cava on a chest X-ray is labeled at left.
Before insertion, the patient is first assessed by reviewing relevant labs and indication for CVC placement, in order to minimize risks and complications of the procedure. Next, the area of skin over the planned insertion site is cleaned. A local anesthetic is applied if necessary. The location of the vein is identified by landmarks or with the use of a small ultrasound device. A hollow needle is advanced through the skin until blood is aspirated. The color of the blood and the rate of its flow help distinguish it from arterial blood (suggesting that an artery has been accidentally punctured). Within North America and Europe, ultrasound use now represents the gold standard for central venous access and skills, with diminishing use of landmark techniques.[32][33] Recent evidence shows that ultrasound-guidance for subclavian vein catheterization leads to a reduction in adverse events.[34][35][36]
The line is then inserted using the Seldinger technique: a blunt guidewire is passed through the needle, then the needle is removed. A dilating device may be passed over the guidewire to expand the tract. Finally, the central line itself is then passed over the guidewire, which is then removed. All the lumens of the line are aspirated (to ensure that they are all positioned inside the vein) and flushed with either saline or heparin.[1] A chest X-ray may be performed afterwards to confirm that the line is positioned inside the superior vena cava and no pneumothorax was caused inadvertently. On anteroposterior X-rays, a catheter tip between 55 and 29 mm below the level of the carina is regarded as acceptable placement.[37] Electromagnetic tracking can be used to verify tip placement and provide guidance during insertion, obviating the need for the X-ray afterwards.
Catheter flow[edit]
Hagen–Poiseuille equation[edit]
The Hagen–Poiseuille equation describes the properties of flow through a rigid tube.[38] The equation is shown below:
The equation shows that flow rate (Q) through a rigid tube is a function of the inner radius (r), the length of the tube (L), and the viscosity of the fluid (μ). The flow is directly related the fourth power of the inner radius of the tube, and inversely related to the length of the tube and viscosity of the fluid. This equation can be used to understand the following vital observations regarding venous catheters: that the inner radius of a catheter has a much greater impact on flow rate than catheter length or fluid viscosity, and that for rapid infusion, a shorter, large bore catheter is optimal because it will provide the greatest flow rate.[3]
