Blood clots in the lungs are called pulmonary emboli. They commonly originate in the leg or pelvic veins, where they can fragment and travel to the lungs via the inferior vena cava (IVC, large vein which carries blood from the lower body to the heart). Further emboli are usually prevented by blood thinning medications (anticoagulants). In some instances (approximately 4% of cases), anticoagulation fails, or it is too dangerous to give anticoagulation.
Vena caval filters are metal alloy devices inserted within the IVC to trap blood clots. Modern filters are 'retrievable,' allowing their removal once they are no longer required. However, a number of retrievable filters are not removed. The long-term safety profile of these devices is not known. The aim of this review was to assess the effectiveness and safety of vena caval filters. The review authors looked for studies comparing filters with no filter, and studies comparing different filter designs.
Study characteristics and key results
We included six trials with a total of 1388 participants in the review (current until 10 September 2019). There were too many differences between these studies so we could not combine the results.
Two trials were applicable in current clinical settings. One trial showed there is no clear benefit in receiving a retrievable filter for the first three months after an acute PE, for those who can receive anticoagulation, in terms of recurrent pulmonary embolism (PE), deep vein thrombosis (DVT), death or bleeding. Not all filters could be removed. Only minor complications from the filters were noted at six months.
Another study of people who had sustained multiple traumatic injuries did not show any benefit of inserting a filter three days after injury to prevent PE, or reduce deaths. Preventive anticoagulation and calf compression devices were administered to participants when possible.
We are unable to draw any conclusions from the remaining four included studies. This is because three studies are no longer clinically relevant because they utilised permanent filters which are seldom used now, or they did not use routine preventative anticoagulation which is current standard practice. One study showed an increased rate of lower-extremity DVT in the permanent filter group. The fourth study compared two filter types and was stopped prematurely and thus did not generate sufficient data.
Reliability of the evidence
Two studies were relevant in current clinical contexts. The evidence presented by both of these studies is of moderate certainty. We reached this assessment because we were not able to combine the data from the studies, and because of the low numbers of participants and events involved. The studies differed in type of participants and clinical situations. There is a further need for trials evaluating the effectiveness of caval filters in people who cannot receive anticoagulation, or when PE occurs despite adequate anticoagulation.
Two of the six identified studies were relevant for current clinical settings. One showed no evidence of a benefit of retrievable filters in acute PE for the outcomes of PE, death, DVT and bleeding during the initial three months in people who can receive anticoagulation (moderate-certainty evidence). The other study did not show any benefit for prophylactic filter insertion in people who sustained multiple traumatic injuries, with respect to symptomatic PE, mortality, or lower extremity venous thrombosis (moderate-certainty evidence). We can draw no firm conclusions regarding filter efficacy in the prevention of PE from the remaining four RCTs identified in this review. Further trials are needed to assess vena caval filter effectiveness and safety, and clinical differences between various filter types.
Pulmonary emboli (PE), or blood clots in the lungs,can be potentially fatal. Anticoagulation is the first line therapy to prevent PE. In some instances anticoagulation fails to prevent more emboli, or cannot be given because the person has a high risk of bleeding. Inferior vena caval filters (VCFs) are metal alloy devices that mechanically trap fragmented emboli from the deep leg veins en route to the pulmonary circulation. Retrievable filters are designed to be introduced and removed percutaneously. Although their deployment seems of theoretical benefit, their clinical efficacy and adverse event profile is unclear. This is the third update of a Cochrane Review first published in 2007.
To assess the evidence for the effectiveness and safety of vena caval filters (VCFs) in preventing pulmonary embolism (PE).
For this review update, the Cochrane Vascular Information Specialist (CIS) searched the Specialised Register (last searched 10 September 2019) and the Cochrane Register of Controlled Trials (CENTRAL) (2019, Issue 8) via the Cochrane Register of Studies Online. The CIS also searched MEDLINE Ovid, EMBASE Ovid, CINAHL, and AMED (1 January 2017 to 10 September 2019) and trials registries to 10 September 2019.
We included randomised controlled trials (RCTs) and controlled clinical trials (CCTs) that examined the efficacy of VCFs in preventing PE.
For this update, studies were assessed and data extracted independently. We assessed study quality with Cochrane's 'Risk of bias' tool and used the GRADE approach to assess the overall certainty of the evidence. The outcomes of interest were PE, mortality, lower limb venous thrombosis, filter-related complications and major bleeding.
We identified four new studies for this update, bringing the total to six included studies involving 1388 participants. The six studies were clinically heterogeneous and we were unable to carry out meta-analysis. Only two studies were considered to be both applicable in current clinical settings and of good methodological quality.
One was a randomised open-label trial studying the effect of a retrievable inferior vena caval filter plus anticoagulation versus anticoagulation alone on risk of recurrent pulmonary embolism (PE) in 399 participants over three months. There was no evidence of a difference in the rates of PE, death, lower extremity deep vein thrombosis (DVT), or bleeding at three and six months after the intervention (moderate-certainty evidence). A filter was inserted in 193 people, but could only be successfully retrieved from 153. Minor filter complications were noted at six months.
The second clinically relevant study was a randomised open-label trial of 240 participants who had sustained multiple traumatic injuries, allocated to a filter or no filter, three days after injury, in conjunction with anticoagulation and intermittent pneumatic compression. Prophylactic anticoagulation was initiated in both groups when it was thought safe to do so. There was no evidence of a difference in symptomatic PE, death, or lower limb venous thrombosis rates (moderate-certainty evidence). The only major filter complication was that one person required surgical removal of the filter.
We are unable to draw any conclusions from the remaining four included studies. One study showed an increased incidence of long-term lower extremity DVT at eight years. Three studies are no longer clinically applicable because they utilised permanent filters which are seldom used now, or they did not use routine prophylactic anticoagulation which is current standard practice. The fourth study compared two filter types and was terminated prematurely as one filter group had a higher rate of thrombosis compared to the other filter type.