Risk Factors

There are three main risk factors which are believed to lead to DVT and PE, and this is known as Virchow’s Triad.

They are:

• slowing of the flow of blood (venous stasis) – the most important risk factor
• increased tendency for the blood to clot
• damage to the lining of the vein

Other risk factors are:

• Prolonged bed rest, or immobilization, such as after a heart attack (10%) or an operation such as prostatectomy - (50% of people who are not treated with preventative heparin develop a DVT), hip operation (70% without heparin), general surgery (12% without heparin), cerebrovascular accident (CVA), neurological trauma, paraplegia, a stay in Intensive Care Unit, or a stay in a General Medical ward ( for longer than 4 days).
• In the period before or especially after a baby is born (pregnancy and the post partum period).
• Wearing a long leg plaster, multiple fractures
• General anaesthesia is said to increase the risk by 500%
• Malignancy and chemotherapy raise the risk
• Inflammatory bowel disease. Pelvic sepsis (from pelvic inflammatory disease) and appendicitis all increase the chance of a thrombosis and they are frequently missed.
• Conditions such as Systemic Lupus Erythematosis (SLE) predispose to DVT (up to 9% can develop it spontaneously)
• Polycythaemia – around 1 in 7 deaths in polycythaemia are due to venous thrombosis. The mechanism causing this is sludging of the blood, producing flow reduction, due to changes in the blood cells. Similarly there is an increased incidence in people with the condition of Sickle Cell Disease.
• Injury to veins by local trauma (internal and external, frequently caused by medical care, such as indwelling catheters), surgery, certain medical conditions such as Buerger’s Disease, and previous clots / DVT (which can make the development of a new DVT 5 times more likely)
• The use of oral oestrogens such as in oral contraceptives and HRT (Hormone Replacement Therapy)
• Increased age (possibly due to increased immobility, particular illnesses, and other lifestyle factors such as obesity, smoking, dehydration)
• DVT can occur in normal, healthy individuals for no obvious reason, and it can also occur in people who sit for long periods, and there has been much publicity and information surrounding the “Economy Class Syndrome”, in which thrombotic episodes occur during or up to weeks after long haul travel.
• Blood coagulation abnormalities, leading to an increased readiness in the blood to clot (“Thrombophilia”)
• Obesity (that is where the person’s weight is greater than 20% over their “norm”). Opinions differ, and some feel there is insufficient evidence to link obesity with DVT.

Amended April 2008