Blood clots in the lungs are called pulmonary emboli. They originate in the legs, fragment and travel to the lungs via the inferior vena cava. Vena caval filters are metal alloy devices inserted within the inferior vena cava to trap blood clots and thus prevent pulmonary emboli. Further emboli are usually prevented by blood thinning medications (anticoagulants).
In some instances (approximately 4% of cases), anticoagulation alone is insufficient to prevent more emboli or it is too dangerous for anticoagulation to be given because the person has a high risk of bleeding. Blood clots are known to occur as a result of certain types of surgery or injuries, and are more likely to fragment if they extend into the thigh or pelvis.
Vena caval filters have been in use since the 1970s and were designed to be left permanently within the inferior vena cava. The latest generation of filters are temporary or 'retrievable'. They can be removed at the manufacturer's recommendation between two to 12 weeks, if their use is no longer required. However, despite being called retrievable, a number of retrievable filters cannot be removed because of complications. The long-term safety profile of these devices left inside the body remains to be seen. The authors looked for articles comparing permanent and temporary (or retrievable) filters and comparisons between the different filter designs.
Two trials were included in the review involving a total of 529 people. No recommendations can be made regarding filter efficacy in preventing pulmonary embolism. One trial which was conducted in 1972 showed a reduction in pulmonary embolism rates but not deaths in a group of people who suffered traumatic hip fractures and who had a filter inserted. No preventive DVT treatment was given as this was controversial at the time. Outcomes were given at 34 days.The trial participants were inadequately randomised, had a higher proportion of people who were not able to undergo surgical fixation in the control group, and outcome assessors were not blinded.
In the PREPIC trial, caval filters were associated with an increased risk of blood clot formation in the legs following their insertion. This study did not demonstrate any difference in the death rates between the two groups; the participants were older (average age 73 years) with co-existing medical conditions and the majority of people died from cancer-related causes or heart problems. No details were recorded of adverse events of filters, but the numbers in this trial were not of sufficient size to detect them.
There is a lack of information on the effectiveness of caval filters in other clinical scenarios, especially in the two situations where they are used most frequently and thought to be the most advantageous. These are when patients cannot be anticoagulated, or when pulmonary embolism occurs despite adequate anticoagulation. Vena caval filter use is increasing and more trials are needed to confirm their benefit and accurately assess their safety.