Overview of Catheter Flushing

Flushing procedures are necessary before and after the administration of intermittent medications through a capped catheter lumen; before and after blood sampling or the infusion of blood products; before and after the administration of incompatible medications; and when converting a catheter from a continuous to an intermittent infusion. The healthcare setting also influences the frequency of flushing. For instance, a hospitalized patient will usually receive flushes after each use and at least every 8 or 12 hours. Patients in home care or an ambulatory infusion clinic will have the catheter flushed daily or after each infusion, which could be less frequent than every day.

The concept of intermittent use of an intravenous (I.V.) catheter began in the early 1970s, when a stopcock was added to plastic tubing on a winged needle infusion set used in patients with cystic fibrosis. A diluted unfractionated heparin (UFH) solution was instilled into the lumen to ensure it remained patent between uses. Within a short time, manufacturers made commercial products with preattached injection caps or separate injection caps that could be added to any catheter.4 The concept of intermittent home parenteral nutrition originated in Seattle in 1970 with the use of a tunneled, cuffed central venous catheter. A report of successful long-term infusion of parenteral nutrition in children was reported in 1979 with the use of silicone central venous catheters that were heparin locked during the daytime.5 By 1978, the intermittent infusion of antibiotics through peripheral heparin-locked catheters had moved into the home.6

UFH, a potent anticoagulant agent, does not break down existing blood clots but allows the body’s natural fibrinolytic system to act. It prevents additional clots from forming and existing clots from growing larger. Pharmaceutical-grade heparin is most often obtained from porcine intestine or bovine lung. Heparin is one of the oldest drugs in clinical use and actually predates the existence of the FDA.

UFH interacts with antithrombin III, causing the inactivation of several normal clotting factors; interacts with platelets and endothelial cells; increases vascular permeability; restrains the proliferation of smooth muscle cells; and promotes bone loss by suppressing osteoblast formation. The average half-life of heparin is between 30 to 150 minutes, with larger doses producing a longer half-life. The reason for this dose-dependent difference is thought to be caused by large amounts of the drug binding to endothelial cell receptors and macrophages.7 The drug’s half-life pertains to the dose entering the bloodstream and is not a factor for the dose residing inside the catheter lumen. The therapeutic effect of heparin is measured by the activated partial thromboplastin time (aPTT); values between 1.5 to 2.5 are considered the traditional therapeutic range.7 However, patients receiving dilute heparin only for catheter patency do not usually have this blood test performed on a regular basis.

Low molecular weight heparins (LMWH) are made from unfractionated heparin and have superior pharmacokinetic properties. European studies have used LMWH for catheter locking. In the United States, these drugs are given by the subcutaneous route, are not labeled for I.V. use, and are not used for any catheter maintenance procedures.

The past events with heparin contamination have caused the US Pharmacopeia to require additional testing of heparin products. Additionally this new standard requires a change from measurement with the USP unit to the international unit (IU), resulting in a possible decrease in potency by about 10%. This is not anticipated to result in any clinical changes for heparin lock solutions, however therapeutic doses may be affected.

Instillation of dilute heparin into catheters prompted many questions from the inception of its use, and we are still struggling with many of these issues today. Improvements in the design of peripheral and central venous catheters and numerous changes to the catheter-capping devices have introduced additional confusion. There is also growing concern about the use of heparin and its ability to produce or contribute to other catheter complications.

Currently, there are no commercially available alternatives in the United States to heparin lock solution. Ongoing clinical research with alternative solutions may bring about practice changes; however, we must understand the current issues with heparin and the factors that must be considered before heparin is eliminated from catheter maintenance protocols.