NIHR Signal A temporary clot-catching filter inserted after major trauma does not prevent lung clots
Published on 10 September 2019
In adults after major trauma who cannot safely be given anti-clotting drugs, placing a removable metal filter in a major vein to the heart (the inferior vena cava) within 3 days of admission does not reduce their chances of having a clot in their lungs (pulmonary embolus) within 90 days, compared with having no filter. The filters do not affect the risk of bleeding.
The filter aims to catch clots that might develop in the legs and travel to the lungs, until anti-clotting drugs can be safely started.
While these filters are recommended in patients who are known to have leg clots but cannot use anti-clotting drugs, there is currently no national guidance on their preventive use in patients without clots. The findings from this robust trial suggest that routinely using them in trauma patients is not beneficial.
- Respiratory disorders, Surgery, Trauma, Acute and general medicine
Why was this study needed?
About 9 in every 100 patients who have experienced major trauma have blood clots in the big veins of their pelvis or legs, which can break away and damage their lungs (pulmonary embolism). This causes about 12 in every 100 trauma deaths.
Anti-clotting drugs prevent about half these deaths but can’t be used in some patients, such as those with bleeding on the brain, because they can allow bleeding to continue. Mechanical anti-clotting measures can be used in these patients.
One approach is to place a filter into the vein which feeds blood into the heart from the lower part of the body. This aims to stop blood clots travelling from the legs to the lungs. In 2017-2018 English hospital data shows that 1,771 people had vena cava filters inserted.
There have been few high-quality studies of these filters, and this study attempted to provide robust evidence on whether they work in prevention.
What did this study do?
This was a randomised controlled trial — the da Vinci Trial. The researchers recruited 240 adults in Australia with severe injuries, who were not suitable for anti-clotting drugs when first admitted to hospital.
Participants were randomised to either have a filter inserted within 72 hours of admission, or to no filter (control). Anti-clotting drugs were started as soon as it became safe to do so, and the filter removed once these drugs were in use or by 90 days into the study, unless there was a strong reason not to.
All participants had intermittent pneumatic compression of their uninjured legs to help reduce the risk of clots. Anyone with pre-specified signs or symptoms of a possible lung clot had further investigation.
What did it find?
- Early placement of the vena cava filter did not reduce the risk of developing a symptom-causing clot in the lungs or death from any cause within 90 days of being admitted. Overall 13.9% of the filter group experienced this outcome compared with 14.4% of the control group who were managed as usual without a filter (hazard ratio (HR) 0.99, 95% confidence interval (CI) 0.51 to 1.94).
- The filters reduced the risk of symptom-causing lung clots among the subgroup of 80 patients who were still alive seven days into the study but still could not receive anti-clotting drugs by this time. None of these patients (0%) who had a filter developed such a clot between days 8 and 90, compared with five patients (14.7%) in the control group (relative risk (RR) 0, 95% CI 0.00 to 0.55). This was not the main focus of the trial and was in a small number of patients, so should be interpreted with caution.
- There was no difference between the groups in deaths from any cause at 90 days (13.1% with filter vs 9.3% with control; RR 1.41, 95% CI 0.69 to 2.87).
- There was also no difference between the groups in major or non-major bleeding at 90 days.
- In two participants (1.6%) the filter had adhered to the wall of the vein, and needed to be removed surgically in one case. In seven cases (5.7%) more than one attempt was needed to remove the filter.
What does current guidance say on this issue?
NICE guidance from 2018 does not make any recommendations on the use of vena cava filters, and suggests that intermittent pneumatic compression devices should be offered to people who have suffered serious or major trauma until they are mobile.
A British Society for Haematology guideline from 2006 states that vena cava filters are indicated in patients who are known to have clots in their veins (venous thromboembolism) and who cannot have anti-clotting drugs, to prevent migration of clots into the lungs. The guideline does not make recommendations about people who are not known to have clots, nor about trauma patients specifically.
What are the implications?
The results of the current study suggest that early preventive use of removable vena cava filters does not benefit adults after major trauma who have no known clots.
Citation and Funding
Ho KM, Rao S, Honeybul S et al. A multicenter trial of vena cava filters in severely injured patients. N Engl J Med. 2019;381:328-37.
The trial was funded by the Medical Research Foundation of the Royal Perth Hospital in Australia, the Western Australia Department of Health, and the Raine Medical Research Foundation (a bequest received by The University of Western Australia for medical research).
British Committee for Standards in Haematology Writing group, Baglin TP, Brush J, Streiff M. Guidelines on use of vena cava filters. Br J Haematol. 2006;134(6):590-95.
NHS Digital. Hospital admitted patient care activity, 2017-18. Leeds: NHS Digital; 2018.
NICE. Venous thromboembolism in over 16s: reducing the risk of hospital-acquired deep vein thrombosis or pulmonary embolism. NG89. London: National Institute for Health and Care Excellence; 2018.
Preventing pulmonary embolism poses a serious dilemma for clinicians caring for severely injured patients. There is a risk of fatal pulmonary embolism if we don’t prescribe prophylaxis with anticoagulant drugs (e.g. low molecular weight heparin), and a risk of life-threatening bleeding if we do.
Inferior vena cava filters were a possible solution, but insertion is technically difficult and not without risk.
We now know that injured patients who cannot be anti-coagulated do not experience a clinically important reduction in pulmonary embolism when treated with a vena cava filter.
Rupert Pearse, NIHR Research Professor and Consultant in Intensive Care Medicine; Clinical Director for Research and Development, Queen Mary University of London and Barts Health NHS Trust
The commentator is a member of the NIHR HTA General Board
Intermittent pneumatic compression device A device which consists of inflatable cuffs around the legs that fill with air and squeeze the legs. This increases blood flow through the veins of the legs and helps prevent blood clots.
Severe injury Defined in this trial as an Injury Severity Score of more than 15 (score range 0 to 75, with a higher score indicating greater severity). The score assesses the severity of injury across six body regions (head, face, chest, abdomen, extremities, external), and a score of 75 indicates that injuries are not survivable. Participants in the trial had an average score of 27.