Venous thrombus formation and propagation most usually depend on the presence of one or more of the following:
- venous stasis
- blood vessel wall trauma or abnormality
- increased risk of blood coagulation
These elements are known collectively as Virchow’s triad (Figure. 1) and many predisposing factors can alter one or more of these factors.
Figure 1: ViRchow's Triad
Venous stasis occurs when there’s a decrease in movement of blood, causing venous congestion in the lower extremities; this may occur after prolonged immobility or confinement to bed. Venous obstruction can also arise from external compression by enlarged lymph nodes, tumours or intravascular compression by previous thromboses 1.
The endothelium (lining) of a healthy vein is smooth and provides a physical barrier between the circulating blood and the thrombogenic tissues beneath. When the vein becomes injured the lining loses its normal negative charge, becoming rough and provoking platelet aggregation and adhesion. Endothelial injury may be caused by a previous DVT, venous distension, trauma and surgery.
Changes in blood chemistry causing hypercoagulability (increased blood stickiness) can occur as a result of many factors including dehydration, malignancy, surgery or trauma, oestrogen therapy and systemic inflammatory diseases.
The risk of VTE by clinical speciality
Geerts et al (2008) representing the American College of Chest Physicians (ACCP) identified the frequency of DVT in hospitalised patients in the absence of prophylaxis 2.
||Prevalence without prophylaxis
|Major gynaecological surgery
|Hip / knee arthroplasty
|Spinal cord injury
|Critical care patients
Table 1: VTE incidence rates without prophylaxis
Table 1 illustrates the high incidence of DVT and thus the requirement for effective prophylactic measures.
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1 Turpie AG, Chin BS, Lip GY. Venous thromboembolism: pathophysiology, clinical features and prevention. BMJ. 2002; 325: 887-890.
2 Geerts WH, Bergqvist D, Pineo GF et al. Prevention of venous thromboembolism. Chest. 2008; 133 (6): 381S-453S.