Circulating blood has the capacity to clot (form a thrombus) at sites of injury, foreign materials or where blood flow is either slowed or irregular (due to stasis or turbulence).

A thrombus can arise on the blood vessel wall or on the surface of a catheter and in time, due to cellular factors being deposited, undergo transformation into a fibrin sheath. A thrombus within the vessel lumen around a catheter tip can cause partial blockage permitting infusion of substances while preventing withdrawal of blood. If extensive, the thrombus may cause total blockage (occlusion). Another risk of thrombosis is thromboembolism. Minimising the potential for thrombus formation can be achieved by:

• Selecting materials with low thrombogenicity to minimise thrombus formation on the catheter surface, such as silicone or polyurethane.
• Using atraumatic surgical technique and suitably flexible ('soft') catheters with low surface friction to minimise vessel trauma.  Vessel trauma can lead to thrombosis around the implant.
• Positioning the catheter tip in a flowing blood stream can help prevent thrombosis due to stasis. Some types of catheter have a tip configuration which acts as a valve preventing ingress of blood and luminal thrombosis (e.g. a Groshong catheter).
• Good catheter use and management will also minimise the possibility of thrombosis. This involves minimising blood residues in the catheter lumen by careful flushing and locking (see below). 
• Be aware that thrombosis risk is greater in arterial catheters due to higher pressure having the potential to force blood into the catheter lumen. Use of such catheters requires careful management.

Catheter locking and access

During catheter access it is important to minimise the time in which blood remains stationary in the lumen.

Immediately after the blood samples have been collected, the catheter lumen should be flushed thoroughly before blood begins to clot. During the time periods when a catheter is not being accessed, the lumen must be filled with a suitable solution to exclude blood and prevent clotting - this process is called locking. Locking technique and solutions are also important in preventing infection. A wide range of lock solutions and techniques are used and have been described (see Table 1) and (Luo et al. 2000). The choice of lock solution should be made to ensure that its composition minimises experimental interference (e.g. heparin may be undesirable if blood clotting mechanisms are being studied and taurolidine is an antibacterial compound). 

Table 1. Catheter lock solutions

Resources and references

• Goossens, G. A. (2015). Flushing and locking of venous catheters: available evidence and evidence deficit. Nursing research and practice
• Luo, Y. S., Luo, Y. L., Ashford, E. B., Morin, R. R., White, W. J., & Fisher, T. F. (2000). Comparison of catheter lock solutions in rats. In 51 st Annual Meeting of the American Association of Laboratory Animal Science
• Cousins, T. R., & O'Donnell, J. M. (2004). Arterial cannulation: a critical review. AANA journal, 72(4)
• Nolan, T. E., & Klein, H. J. (2002). Methods in vascular infusion biotechnology in research with rodents. ILAR journal, 43(3), 175-182
• Morton, D. B., Jennings, M., Buckwell, A., Ewbank, R., Godfrey, C., Holgate, B., & Verschoyle, R. (2001). Refining procedures for the administration of substances. Laboratory animals, 35(1), 1-41

Other resources on vascular catheterisation