Introduction
The accepted standard of care for patients with venous thromboembolism (VTE) is anticoagulant therapy. Inferior vena caval (IVC) filters are reserved for those patients who fail anticoagulant therapy, or have a complication or contraindication to anticoagulant therapy.¹ Until the early 1970’s, the only method of IVC interruption was surgical, either by clipping, ligation or plication. The first clinical experience of an endoluminally-placed device to interrupt IVC flow was reported by Mobin-Uddin et al. in 1969.² However, it was not until the introduction of a stainless steel umbrella-type filter by Greenfield et al. in 1973 that an effective method of endoluminally trapping emboli while simultaneously preserving IVC flow became possible.³ Indeed, for many years, the Greenfield filter set a benchmark by which newer filters were measured. Early generations of filter were inserted by surgical cut-down and venotomy. Eventually filters were able to be inserted percutaneously: initially through large 24 Fr sheaths, though newer generations of filters are able to be delivered through 6 Fr systems.⁴

IVC Filters Versus Anticoagulant Therapy in VTE
Despite the safety and efficacy of modern day filters, the importance of systemic anticoagulation as the primary treatment of VTE cannot be understated (Table 1). The use of either unfractionated or low molecular weight heparin followed by three months of oral anticoagulation in patients with proximal deep venous thrombosis (DVT) is approximately 94% effective in preventing pulmonary embolism (PE) or recurrent DVT.⁵ The routine placement of IVC filters in addition to anticoagulation in patients with documented DVT was investigated by Decousus et al. in a randomized trial.⁶ This study revealed that the use of a permanent filter in addition to heparin therapy significantly decreased the occurrence of PE within the first 12 days compared to those without a filter. However, no effect was observed on either immediate or long-term mortality, and by 2 years, the initial benefit seen in the group of patients with filters was offset by a significant increase in the rate of recurrent DVT.⁶

Indications for Permanent Filters
Despite the efficacy of anticoagulant therapy in the management of VTE, there are certain situations and conditions in which the benefits of anticoagulation are outweighed by the risks of instituting such a therapy. These include contraindications and complications of anticoagulant therapy. In such circumstances, there may be absolute or relative indications for filter insertion (Table 2).^7,8

Permanent Filter Types
Currently, there are eight different types of permanent caval filters that are FDA approved. These include the Bird’s Nest filter (Cook Incorporated, Bloomington, IN), Vena Tech LGM filter (B. Braun, Bethlehem PA), Vena Tech LP (B. Braun), Simon Nitinol filter (Bard, Covington,GA), Titanium Greenfield filter (Boston Scientific, Natick MA), Over-the-Wire Greenfield filter (Boston Scientific), TrapEase filter (Cordis Corp.) and the Günther Tulip filter (Cook Inc.) (Table 3).

Temporary and Retrievable Filters
Well-founded concerns over the long-term complications of permanent IVC filters, particularly in younger patients in need of PE prophylaxis with a temporary contraindication to anticoagulation, has led to the development of temporary and retrievable filters. Temporary filters remain attached to an accessible transcutaneous catheter or wire. These have been used primarily in Europe for PE prophylaxis during thrombolytic therapy for DVT.¹⁰ Currently these devices are not approved for use in the United States. Retrievable filters are very similar in appearance to permanent filters, but with modifications to the caval attachment sites and/or hooks at one end that can facilitate their removal (Figure 1). Retrievable filters that are currently available in the United States include the Günther Tulip (Cook Inc.), Opt Ease (Cordis Corp.), and Recovery nitinol filters (Bard Peripheral Vascular, Tempe, AZ) (Table 4).¹¹ The time limit of retrievability is in part dependant on the rate of endothelialization of the device, which typically occurs within 2 weeks. However, differences in design may extend the time period in which the filter may be safely retrieved, as has been documented with the Recovery Filter.¹²

Currently no consensus exists as to which patients have an indication for a retrievable filter. However, it is generally accepted that patients at high risk for pulmonary embolism or with documented PE and with a temporary contraindication to anticoagulation are candidates (Table 5).

Placement of Suprarenal IVC Filters
Certain circumstances preclude the placement of a filter in the infrarenal IVC. This includes thrombus extending into the infrarenal IVC, renal vein thrombosis or pregnancy. The safety of suprarenal placement of IVC filters is well documented, with no reported instances of renal dysfunction and no differences in the rates of filter migration, recurrent PE or caval thrombosis.^4,15

Placement of Superior Vena Cava Filters
The rate of upper extremity DVT is on the rise. This is predominantly due to an increasing number of patients having short- and long-term upper extremity central venous access catheters. In one study, 88% of patients found to have an upper extremity DVT had a central venous catheter present at the site of thrombosis at the time of diagnosis or within the previous 2 weeks.¹⁶ Pulmonary embolism may complicate upper extremity DVT in 12–16% of cases.^17,18 In patients who have such a complication or contraindication to anticoagulation, a filter can be safely placed immediately below the confluence of the brachiocephalic veins. However, misplacement of an SVC filter is theoretically more likely than with an IVC filter because of the relatively short target area for deployment.¹⁶

Imaging Techniques for Filter Insertion
The most common imaging modality used for filter insertion is fluoroscopy, performed either in an interventional suite or an operating room. Bedside placement of filters has inherent advantages, particularly for critically ill patients in intensive care settings where transport can be avoided. Portable fluoroscopy, surface duplex ultrasound and intravascular ultrasound (IVUS) have all been used to assist with bedside filter placement.¹⁹ The advantages and disadvantages of these imaging modalities are highlighted in Table 6.

IVC Filters: The Present and the Future
To better achieve the desired performance goals of IVC filters, it is crucially importance to adhere to established criteria in selecting candidates for filter insertion. This is clearer for the use of permanent filters as opposed to retrievable ones. A variety of both permanent and retrievable filters are available in the interventionalists’ armamentarium. It is important to be familiar with the particular characteristics of each device, knowing their comparative advantages and disadvantages.

There are certain design attributes that are desirable to be incorporated into future IVC filters. The ideal filter should be non-thrombogenic, biocompatible, easily inserted and deployable. It should have a high filtering efficiency without affecting flow. In addition, it should offer secure caval fixation with the ability to be easily repositioned or retrieved if necessary. Finally, it should be MRI compatible and be low cost.⁷