Abstract
Inferior vena cava (IVC) filters are placed to prevent pulmonary embolism. Despite their widespread and increasing use, there remains little firm evidence to support this. This chapter explores the evidence for their use, describes the types of filter available, the methods for deployment and retrieval and complications associated with filters.
1 Introduction
Pulmonary embolism (PE) is a major cause of mortality, accounting for 30,000–40,000 deaths per year in the United Kingdom, exceeded only by coronary artery disease and malignancy. The mainstay of treatment is with anticoagulation, usually with intravenous unfractionated heparin or subcutaneous low molecular weight heparin followed by warfarin. This treatment is usually very successful in preventing further pulmonary emboli, and allowing the body’s endogenous thrombolytic mechanisms to disperse them. In a minority of patients, further intervention is required.
2 History
Just under two decades after Virchow in 1846 hypothesised that pulmonary embolism was the result of migration of clot from the lower limb veins, Trousseau in 1865 made the suggestion that embolisation to the pulmonary arteries could be prevented by interrupting the vessels providing the route. This led in the 1930s to the development of the technique of femoral vein ligation, with or without clot removal from the femoral vein. However, ligation of the femoral vein was associated with a high incidence of lower limb swelling, and obviously was ineffectual at preventing more central or contralateral deep venous thrombosis migrating to the lungs. For these reasons, in the 1940s the focus switched to ligation of the inferior vena cava (IVC). At around the same time, heparin and then warfarin became available, which were used in conjunction with these techniques. However, ligation of the IVC resulted in an immediate significant drop in cardiac output of around 50% had a significant incidence of bilateral lower limb swelling of around 15% and also had a significant mortality of up to 15%. In addition, further pulmonary emboli could occur from migration of thrombus above the level of ligation, a risk minimised by ligating the IVC just below the level of the renal veins, or from embolism through the collateral veins that invariably developed following IVC ligation and could approach the diameter of the original IVC. Alternatives to ligation of the IVC were therefore developed, including surgical plication of the IVC and surgical clipping, allowing partial interruption of blood flow. While demonstrating an advantage over ligation, these procedures still necessitated laparotomy.
The late 1960s saw the introduction of the first IVC filter, the Mobin Uddin filter device, consisting of six struts placed with the apex inferiorly supporting a silastic membrane. This device necessitated open femoral venotomy for insertion. However, it suffered from problems with migration and caval occlusion as well as persisting pulmonary emboli.
The next significant development was the release of the Greenfield filter. Initially also requiring surgical venotomy, since its release it has undergone modifications to allow percutaneous implantation and to reduce complications of insertion site thrombosis and caval penetration by filter struts.
3 Efficacy of IVC Filters
The vast majority of the literature on IVC filters consists of observational studies, usually retrospective, of various devices. A number of reviews have been published, critically evaluating the evidence for IVC filters (Kinney 2003; Hann and Streiff 2005). The only prospective randomised controlled study to investigate the effectiveness of caval filters is the PREPIC study (PREPIC study group 2005). In this study, 400 patients with proven deep venous thorombosis (DVT) seen on venography were assigned to receive either anticoagulation alone, in the form of unfractionated heparin or enoxaparin followed by oral anticoagulants for a minimum of 3 months, or else anticoagulation and an IVC filter. At 10 days, the incidence of PE was significantly lower in the filter group, but there was no significant difference in the incidence of fatal PE. No significant difference was seen in overall mortality between the groups at 10 days or at follow up of 2 years, but at 2 years there was a significant increase in the number of DVTs seen in the filter group. After 8 years of follow-up, there was a significantly reduced incidence of PE in the filter group but an increased incidence of DVT and no significant difference in survival or incidence of post-thrombotic syndrome (Table 1).
4 Indications and Contraindications
4.1 Accepted Indications
The firmest and commonest indication for IVC filter insertion is pulmonary embolism occurring in the presence of a contraindication to anticoagulation or severe complication of anticoagulation. Examples would include pulmonary embolism occurring in a patient with recent intracranial haemorrhage or active gastrointestinal bleeding (Table 2).
The other generally accepted indications for an IVC filter are pulmonary embolism despite adequate anticoagulation or propagation of DVT despite adequate anticoagulation. The diagnosis of pulmonary embolism while on treatment should be confirmed by investigation, since only a minority of symptomatic patients will have objective evidence of further emboli. If confirmed, before filter placement is contemplated, other measures first require consideration, such as ensuring compliance with anticoagulation, increasing oral anticoagulation to a higher target therapeutic ratio or transferring to a different anticoagulant. True failure of anticoagulation is rare when it is fully optimised.
Propagation of thrombus during anticoagulation occurs in 5–10% of patients. However, the link between propagation of thrombus and embolization to the lungs is not firmly established. Moreover, the placement of a filter does not prevent, and may exacerbate thrombus propagation. Therefore, the initial treatment strategy in this scenario should be to optimise anticoagulant therapy.
4.2 Uncertain Indications
The remaining proposed indications for IVC placement are more contentious, often described as “relative” or “possible” indications.
Some authors have advocated the presence of “free-floating” thrombus in the proximal lower limb veins or IVC as an indication for IVC filter, citing a very high incidence of PE of up to 60%. However, in some of the studies the diagnosis of free-floating thrombus was made after PE had occurred and another prospective study (Pacouret et al. 1997) showed no increased risk of PE in this patient group.
Similarly there are advocates of filter placement to prevent PE when performing thrombolysis or mechanical thrombectomy of proximal venous thrombus, while others consider it safe to perform these procedures without protection.
4.2.1 Prophylactic Filters
Use of IVC filters has been recommended in certain high-risk groups of patients, including trauma patients, patients undergoing orthopaedic procedures and those having bariatric surgery for morbid obesity.
In patients with polytrauma, there is a high incidence of venous thrombo-embolic disease, with observed rates of DVT of up to 58% and PE in up to 4%. There is some evidence that heparin prophylaxis is not completely effective in this group and studies comparing filter use with historical controls or low dose heparin suggest an advantage to filter use. However, the level of evidence is not sufficient to recommend their routine use. Instead, they should be considered in very high-risk groups, such as complex orthopaedic lower limb injury and spinal injury.
In orthopaedic procedures, there is an incidence of venous thromboembolism of up to 20% without prophylaxis, particularly following total hip or knee replacement. However, that risk can be substantially lowered by the correct prescribing of low molecular weight heparin, such that routine IVC filters are not indicated prophylactically.
An incidence of PE of up to 3% is seen in patients undergoing bariatric surgery, but with a high associated mortality rate, most likely because the underlying morbid obesity results in reduced cardiopulmonary reserve. This has led to some surgeons using prophylactic IVC filters, but there is no evidence for their use in this group and, as for high-risk orthopaedic procedures, heparin prophylaxis probably remains a more logical choice.
4.3 Supra-Renal and Superior Caval Filters
The commonest indication for supra-renal placement of a filter is thrombus extending up into the IVC or into the renal veins, such that there is no room for infra-renal placement. If a filter is required in a pregnant woman, then the supra-renal location should also be used. Some workers have extended this argument to advocate supra-renal placement in all women with the capacity for future pregnancy, though this is less important if a non-permanent filter is used. Theoretically, infra-renal IVC filters will not trap pelvic thrombi migrating via the gonadal veins.
The SVC territory accounts for a minority of pulmonary emboli (<5%). IVC filters have been placed in the SVC; the indications are broadly the same as for IVC placement but with a known source of thrombus in the SVC territory. However, less is known about potential complications such as migration.
5 Choice of Filter
5.1 The Ideal Filter
The concept of the “ideal filter” is useful as a comparison for the currently available devices. The ideal filter is cheap and easy to insert and reposition through a small introducer system, so as never to cause entry point thrombosis. It has a 100% capture rate of emboli without causing impedance to flow and is atraumatic to the IVC with a 0% incidence of caval occlusion. It does not migrate and may be retrieved easily following an indefinite period. It is biocompatible and MRI compatible.
5.2 Currently Available Filters
5.2.1 Permanent Filters
Permanent filters are designed to remain in situ indefinitely following placement. The devices vary in their appearance, deployment mechanisms and construction materials but there is no evidence to show that any one device is superior to the others. The greatest volume of data exists for the Greenfield filter, obtained mainly from registries. The Bird’s Nest filter is perhaps more complex to deploy than the others, but is the filter of choice for patients with an abnormally large diameter caval lumen (up to 40 mm).
5.2.2 Temporary Filters
As stated above, the major indication for an IVC filter is a contraindication to anticoagulation. In many instances, the contraindication to anticoagulation may be temporary: for example the requirement for surgery. From the PREPIC study there is evidence of an increased incidence of DVT in patients with a filter in situ. Temporary filters were developed as an attractive means of avoiding the long-term complications of filters. The disadvantage of these pure temporary filters is that the introducing catheter remains in place, which risks the catheter and filter becoming displaced, restricts movement of the patient and also exposes the patient to the risk of introducing infection.
Recent years have, for these reasons, seen the withdrawal of most purely temporary filters in favour of retrievable filters.
5.2.3 Retrievable Filters
Retrievable filters offer the advantages of temporary filters, without the disadvantages mentioned above (Imberti et al. 2012). Their design usually involves a hook or cylinder at one end that can be snared or grasped and then retrieved into an intravascular sheath. There has been an increase in the use of retrievable filters in recent years. However, reported retrieval rates are highly variable, suggesting that some retrievable filters are placed with the intention of leaving them in the IVC indefinitely. Other reasons may be difficulty with the retrieval process (see below) or poor communication between referring clinicians and radiologists. Dedicated follow-up by the interventional radiology team may improve retrieval rates. Although all retrievable filters are licensed for permanent use, there is much less safety and efficacy data with retrievable filters left in long term. There are a number of small series, totalling approximately 1,000 patients, but the length of follow-up is small; in comparison approximately 10,000 patients have been reported with permanent devices inserted and with follow-up data up to 8 years. Therefore, it seems sensible to use retrievable filters only for temporary indications and to plan their retrieval in an appropriate time at the time of insertion.
6 Insertion Technique
6.1 Pre-Procedural Imaging
The IVC is formed by the union of the common iliac veins at the L4/5 level. It ascends to the right of the aorta, receiving tributaries in the form of lumbar veins, renal veins and hepatic veins before draining into the right atrium (Fig. 1).
The most usual site for insertion of an IVC filter is in the infra-renal IVC. Many devices are recommended to be placed ideally with the cranial tip at the level of the renal veins, where there is increased flow, to minimise the risk of caval occlusion.
Placement is normally monitored using venography and fluoroscopy, although grey-scale ultrasound, duplex scanning and intravascular ultrasound have all been reported. The pre-procedural evaluation of anatomy has to assess the diameter of the IVC and accurately localise the number and levels of the renal veins. Anomalies of the venous anatomy are relatively common. Duplication of the IVC (0.2–3%) may require two filter devices to be used, or at least the selection of the correct IVC. A retro-aortic left renal vein (1.8–2.4%) usually enters more inferiorly and requires lower placement of the filter. Multiple renal veins may communicate at the renal hilum, offering a bypass route for emboli; this is particularly true in the case of circumaortic left renal vein (incidence 1.5–8.7%).
In addition, imaging also serves to assess the extent of thrombus in the venous system and allow for planning of site of delivery access and desired position of the filter. For example, thrombus extending into the iliac veins is an indication for jugular placement, and thrombus extending into the IVC up to the renal veins is an indication for supra-renal placement of the filter.
6.2 Filter Placement
The deployment of the filter is usually straightforward. The manufacturers’ instructions are always comprehensive but can be somewhat technical and confusing; when using a device that is unfamiliar it is useful to have present an experienced person to help and advise.
6.3 Retrieval Technique
Prior to removal of a retrievable filter a cavogram is required to assess filter position and exclude significant thrombus within it. Bi-planar imaging can be useful. If thrombus is present, a further period of anticoagulation may be required or thrombolysis may even be considered, accepting the inherent risks. Retrieval into a large sheath is appropriate for small trapped clots, again with consideration of the risks for the patient of inadvertent emboli, depending on the patient’s cardiopulmonary reserve. All retrievable filters may be left in situ as permanent devices; however, there is a paucity of data on their long-term safety (Berczi et al. 2007).
The retrieval technique itself is essentially for foreign body retrieval and should be performed according to the manufacturers’ instructions. There is increasing evidence that the success rate for retrieval decreases with time and the potential for complications increases, often related to limb endotheliasation, limb fractures, caval perforation and filter tilting. In the UK, the Medicines and Healthcare products Regulatory Agency (MHRA) reported in 2007 receiving an increasing number of notifications of complications in attempted retrievals beyond 3 months after insertion. Registry data from the British Society of Interventional Radiology (BSIR 2011) showed a decrease in success rates for retrieval and an increase in complication rates for retrievals attempted more than 9 weeks after insertion.
7 Complications
Complications if IVC filter placement can be divided into immediate and longer term complications. Immediate complications include access site complications associated with central venous puncture, such as inadvertent arterial puncture or pneumothorax. With early devices, access site thrombosis was a common problem, but this appears to occur less frequently with modern small delivery profile devices. A number of operator error complications have been described, including incorrect sizing of the IVC for the device, deployment of a femoral device via the jugular route and deployment of devices into renal veins, gonadal veins, iliac veins and the aorta. These complications can be easily avoided by careful choice of device and appropriate imaging.
Filter migration, defined as 2 cm movement of the device, was similarly seen with early examples, but is now rare (<1%) with modern filters, mostly as a result of hooks being incorporated into them. The downside of this is an increased incidence of caval penetration by the hooks. This is seen commonly in patients followed up on CT or venography, though clinically significant perforation into the aorta, duodenum or other retroperitoneal structures is fortunately rare (Figs. 2 and 3).
IVC thrombosis is an important long-term complication of IVC filters. It is generally reported with an incidence of around 2–10%, although one study found an incidence as high as 33% at 9 months follow-up. About half of patients who have IVC thrombosis develop symptoms, such as lower limb swelling. IVC thrombosis may involve the renal veins, though this in itself may not alter renal function.
From the PREPIC study, we know that DVT occurs more frequently in patients with IVC filters. Long-term sequelae of such DVTs include post-phlebitic limb, with deep venous incompetence, limb swelling and ulceration.
8 Conclusion
There is evidence that IVC filters reduce the risk of pulmonary embolism in the short term in patients with proximal deep venous thrombosis. They should not be considered a substitute for anticoagulation: unlike anticoagulants, they do not prevent propagation of DVTs, they do not help prevent long-term sequelae of DVT such as post-phlebitic limb and they increase rather than decrease the risk of recurrent DVT. The best accepted indication for their use is pulmonary embolism with a contraindication to anticoagulation. These contraindications are often temporary, such as a bleeding peptic ulcer or surgery. The development of retrievable filters that may be left in situ for a number of weeks before removal should avoid the long-term complications of filters, such as recurrent DVT and caval occlusion. However, there is an increasing recognition that the retrieval procedure may be associated with complications, particularly with extended dwell times. Smaller delivery devices should reduce the incidence of access site complications. The continued improvements in filter design will no doubt be paralleled by advances in anticoagulant medication, such that the place for IVC filters will have to be continually re-evaluated.
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© 2012 Springer-Verlag Berlin Heidelberg
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Matson, M. (2012). Inferior Veno-Caval Filters. In: Cowling, M. (eds) Vascular Interventional Radiology. Medical Radiology(). Springer, Berlin, Heidelberg. https://doi.org/10.1007/174_2012_536
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DOI: https://doi.org/10.1007/174_2012_536
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