Pulmonary embolism is a major cause of hospital morbidity and mortality. Blood clots or deep venous thromboses (DVTs) form in the lower extremities and can occur under a number of different circumstances. Temporary circumstances are prolonged immobility, recent surgery, trauma, pregnancy, or oestrogen therapy. Longer term situations involve people who have cancer, or who have an inherited hypercoagulable tendency. Deep vein thromboses can fragment and travel through the venous system to the lungs causing pulmonary embolism (PE). The major conduit of venous drainage from the lower half of the body is the inferior vena cava.

Deep vein thromboses that extend into the thigh or pelvis are more likely to embolise than those that do not extend beyond the calf of the leg. Case‐series data indicate a rate of between 27 and 60% for the risk of embolism if the clot is situated either within the inferior vena cava, the thigh, or pelvic veins (Norris 1985; Radomski 1987). This sequence of events has long been recognised; the concept of caval interruption was identified as far back as the 1700s. The first surgical vena caval ligation (complete occlusion of the vena cava with sutures or external clips) was successfully performed in 1893. It required general anaesthesia and abdominal surgery, and was associated with considerable mortality. Vena caval ligation in the 1960s carried an operative mortality risk of 14%, and pulmonary embolism still occurred at a rate of 6% (due to the development of a large collateral circulation) with fatal embolism occurring at a rate of 2% (Greenfield 1992). Anticoagulation became the main treatment in the 1950s. Filters which could be inserted percutaneously were developed in the 1970s and are used in increasing numbers.

The treatment for pulmonary embolism is anticoagulation (heparin and vitamin K antagonists). Infrequently, recurrent pulmonary emboli can occur despite therapeutic levels of anticoagulation; Douketis suggested a recurrence rate of 3.8% in a systematic review of the literature (Douketis 1998).

Deep vein thromboses can also occur in the upper limbs because of the increasing use of semi‐permanent venous access devices and catheters. They can embolise to the lungs via the superior vena cava. Vena caval filters may be placed in the inferior or superior vena cava to mechanically trap emboli, interrupting their course before reaching the heart and lungs. These devices most commonly resemble an umbrella in appearance, are made from metal alloys, and can be inserted percutaneously. Inferior vena caval filters are usually inserted below the level of the renal veins. Once deployed, permanent filters are left in situ; they become endothelialised and are eventually incorporated within the blood vessel wall. Temporary or retrievable (also known as optional) filters have been recent developments. These filters can be removed within a certain time interval (specified by the manufacturer) if their use is no longer required. This time period may extend to approximately 12 weeks, depending on the clinical experience of the interventional radiologist. There are currently approximately 12 filter brands, several of which are retrievable. Retrievable filters have potential advantages over the permanent filters; one of which is their subsequent removal if no longer needed, thus avoiding longer‐term sequelae of DVT. They can also be repositioned within the vena cava if significant endothelialisation has not occurred. Despite being called 'retrievable, these filters become permanent implants if their subsequent removal becomes complicated due to endothelialisation; or there is a significant amount of trapped thrombus within the filter such that the filter cannot be retracted back into its sheath, thus preventing percutaneous removal. The majority of retrievable filters have been designed and licensed for temporary use only. Whilst reports have suggested that they can be left in situ longer term (Kirilcuk 2005), the safety of this practice requires more extensive evaluation.

Filters are strongly recommended for two groups of people: those who have a proximal DVT or pulmonary embolism and where it is too dangerous for them to receive anticoagulation; and those who continue to have recurrent emboli despite already receiving appropriate levels of anticoagulation (Buller 2004).

There is controversy in the literature as to whether other groups of people may potentially benefit from having a vena caval filter inserted. These include (Hann 2005; Kinney 2003):

·people with extensive trauma and without established venous thromboembolism;
·people with large free‐floating ilio‐femoral thrombosis who do, or do not, subsequently receive thrombolytic therapy for this;
·people with cancer and concurrent venous thromboembolism;
·pregnant women who have venous thromboembolism.

Interventions examined in this review will assess the efficacy and safety of filters: whether these aspects vary amongst the different filter designs, and with different antithrombotic drugs.
The comparisons in this review are as follows.
1) Filters versus no filters in those for whom anticoagulation is contraindicated.
2) Filters and anticoagulants versus anticoagulants alone.
3) Filters with anticoagulation versus filters with no anticoagulation, to seek to answer the question as to whether long‐term anticoagulation is recommended with permanent filters in situ. There is considerable debate about this (Gomes 2003).
4) Trials of filters with newer antithrombotic drugs are of interest as these newer agents may have greater antithrombotic action or fewer haemorrhagic complications. Both of these effects are relevant to current indications for filters. Fondaparinux appears to have fewer haemorrhagic complications (Yusuf 2006).
5) Permanent versus temporary filters, looking at which type of filter is most effective and at the respective rates of complications.
6) Direct comparisons of filter brands, to see if any one filter is superior to others in terms of filtering efficiency or low rate of complications.

Comparing filters versus no filters also examines the complications and adverse effects from having a filter in situ. Pooled case‐series data indicate a recurrent PE rate of 2 to 5%, with a fatal PE rate of 0.7%, despite the presence of a filter. The mortality rate from complications related to filter insertion is 0.7%. Filter migration has been estimated to occur in up to 69% of recipients, and inferior vena caval perforation in up to 24%; these figures reflect radiological findings and not necessarily clinical events. DVT was reported to occur at rates of up to 45.7%, and post‐thrombotic syndrome at rates up to 59% (Kinney 2003). These problems occurred more frequently with longer durations of follow up. It is controversial whether lower limb complications are the result of having a filter in situ or are part of the intrinsic pro‐thrombotic tendency these people may have.