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Deep vein thrombosis - aetiology and prevention

VOL: 97, ISSUE: 17, PAGE NO: 34

Sandra Geraghty, BSc, RGN, staff nurse at St James's University Hospital, Leeds

Selina Gilbourne, BSc, RGN, staff nurse at St James's University Hospital, Leeds;Jane Russell, BSc, RGN, is a staff nurse at South Cleveland Hospital, Middlesbrough; andJacqueline Young, BSc, RGN, is a staff nurse at St Alban's City Hospital

Deep vein thrombosis (DVT) of the lower limbs and pulmonary embolism (PE) continue to be significant causes of post-operative morbidity and mortality (Collier, 1999). DVT is also an important indicator for the development of both venous and arterial leg ulcers, costing the NHS about £600m a year in treatment and management (Bosenquet et al, 1993).

Deep vein thrombosis (DVT) of the lower limbs and pulmonary embolism (PE) continue to be significant causes of post-operative morbidity and mortality (Collier, 1999). DVT is also an important indicator for the development of both venous and arterial leg ulcers, costing the NHS about £600m a year in treatment and management (Bosenquet et al, 1993).

DVT is defined as the formation of a blood clot (thrombus) in a deep vein, such as the saphenous, femoral or iliac vein. PE is a life-threatening complication that results when a portion of the thrombus breaks off and travels through the great veins, eventually becoming lodged in the pulmonary circulation (Brunner and Suddarth, 1992).

Aetiology
The occurrence of DVT has been described as the result of an interplay between three predisposing factors commonly referred to as Virchow's triad (Schreiber, 2000):

- Alteration in the blood-clotting mechanism;

- Alteration in the anatomical structure of the lining of a blood vessel;

- Alteration in blood flow.

These factors may result from a number of causes, including injury to the vein due to handling or devices used at the time of surgery (Fitzgerald, 1997). Changes in blood viscosity may follow loss of fluid or dehydration as a result of trauma or preoperative fasting or, more commonly, decreased blood flow as a result of restricted movement after surgery (Autar, 1998).

The use of a risk-assessment guide on surgical wards is essential if the occurrence of DVT is to be reduced. The Autar risk assessment scale is a comprehensive guide covering every risk associated with the formation of a DVT (Autar, 1996; Autar, 1998). See Box 1 for a summary of common risk factors. It is probable that several factors act together to produce thrombosis (Kakkar, 1990).

Signs and symptoms
Many thrombi are asymptomatic or do not obstruct the vein significantly and reduce the venous flow. Up to 50% of DVT cases have no initial observable symptoms (Schreiber, 2000).

The most specific symptom associated with a DVT is thought to be unilateral oedema of the leg. Localised pain due to thrombophlebitis (inflammation of the vessel wall) combined with oedema in the surrounding tissues can result in cramp-like pain (Schreiber, 2000). Erythema may also be present.

Patients who feel discomfort in the calf muscles on dorsiflexon of the foot show signs of DVT (known as Homans' sign). However, a positive Homans' sign, together with pain or leg cramps and swelling, does not provide a definite diagnosis - 70% of patients presenting with symptoms of a possible DVT are found to have alternate diagnoses (Brunner and Suddarth, 1992)

Diagnostic tests
Where DVT is suspected, a positive confirmation can be achieved by performing diagnostic tests. Venography is an invasive technique, where a contrast is injected into a vein in the foot, mixes with the patient's blood and flows proximally, enabling an X-ray image to show the entire deep venous system (Cogo et al, 1998; Hirsh and Hoak, 1996). A thrombus would show up as an intraluminal filling. This technique allows the identification of thrombi in the proximal and calf veins. However, it is expensive and patients have been known to react to the contrast (Cogo et al, 1998; Schreiber, 2000).

Impedance plethysmography (IPG) is a non-invasive technique involving the inflation of a compression cuff around the patient's thigh to obstruct venous return. Two electrodes are then placed on the calf, and the cuff is slowly deflated to allow the electrodes to record the length of time it takes for venous volume to return. A prolonged return time can indicate the presence of a DVT (Hirsh and Hoak, 1996). This technique cannot adequately determine DVT in calf veins, only proximal veins, and is usually used on symptomatic patients (Cogo et al, 1998; Schreiber, 2000).

Compression ultrasonography (CUS) or venous ultrasonography (VUS) is a non-invasive technique that uses a high-resolution colour doppler to create two-dimensional images of the vessel (Hirsh and Hoak, 1996). Pressure is applied to the suspect vein to test for total compressibility. If the vein cannot be successfully compressed, this indicates the presence of a DVT. Again, this method is mainly used to detect DVTs in the proximal veins as it is not sensitive enough for detection in the calf veins (Schreiber, 2000).

Other diagnostic tests include magnetic resonance imaging, which is thought to be a more sensitive non-invasive test for calf vein thrombosis (Hyers, 1999; Schreiber, 2000); a technique where a radionuclide is tagged to a small molecule that localises to a part of a thrombus (Hyers, 1999); and testing for levels of D-dimer - a degradation product of fibrinogen (Hyers, 1999; Wells et al, 1995).

It is recommended that the D-dimer test is combined with either IPG or VUS to counter the high incidence of false positives (6%-17%) recorded after these tests (Wells et al, 1995).

Prevention
In any situation where a patient is assessed as being at risk of DVT, preventive measures should be taken. These include education in the use of anti-embolism stockings and prophylactic low-molecular-weight heparin, if applicable. In addition, patients should be taught leg exercises by a physiotherapist as part of a leg mobilisation regime. If a patient's legs are usually mobile and are then suddenly restricted by bedrest, there is an alteration in the blood flow. Active or passive leg exercises involving the calf muscles activate the calf muscle pump and can increase venous blood flow.

Patients should be taught to observe for the clinical signs of DVT, for example, swelling, pain, erythema and discoloration. These signs may indicate the occurrence of DVT or that anti-embolism stockings have been fitted incorrectly. However, misdiagnosis is common as the clinical signs can be misleading.

Research into the use of anti-embolism stockings includes the work of Flanc et al (1969), Kakkar et al (1970) and Holford (1976). By exerting an evenly distributed pressure over the entire surface of the calves, these stockings reduce the calibre of the superficial veins of the lower extremities, resulting in increased flow in the deeper veins (Brunner and Suddarth, 1992).

Research results vary but it is commonly concluded that a combination of elastic stockings and leg elevation reduces the frequency of postoperative DVT more than threefold. Optimum graded compression is provided by correctly fitting anti-embolism stockings.

Conclusion
Despite research into the prevention of DVT since 1965, a significant deficit remains in the literature on this subject and the incidence of DVT and PE continues to be costly, both financially and in terms of patient morbidity. The education of both staff and patients is essential to ensure effective risk assessment and prevention.

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