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Inferior Vena Caval Filters: Key Considerations
Inferior Vena Caval Filters: Key Considerations PAUL J. FAILLA, MD; KEVIN D. REED, MD; WARREN R. SUMMER, MD; GEORGE H. KARAM, MD
ABSTRACT: Surgical interruption of the inferior vena cava (IVC) as a means to prevent pulmonary embolism and its consequences has been entertained since the end of the 19th century. Initial methods were crude, however, but their deficiencies led to the development of newer techniques. Despite increasing indications and use of permanent IVC filters there remains controversy regarding their efficacy and complications. The purpose
of this article is to review the pertinent literature and, it is hoped, aid in the development of a rational approach to the use of IVC filters. The evolving data regarding the retrievable filters are also discussed. KEY INDEXING TERMS: Deep venous thrombosis; Pulmonary emboli; Inferior vena cava filter; Complications; Efficacy. [Am J Med Sci 2005;330(2):82–87.]
Rationale for Use
study, 35 patients who were clinically believed to have had a PE were randomized to receive anticoagulation with heparin and nicoumalone or to receive no anticoagulation. Of 19 patients who did not receive any anticoagulation, 5 died secondary to a PE that was proven at autopsy. Of the 16 who did receive anticoagulation, none died from a PE. The randomization feature of this study was stopped at this point because of ethical concerns relating to the untreated control group. Other, more indirect evidence supporting DVT as a source of PEs and the consequences of a lack of treatment can be gleaned from a review of randomized studies comparing various anticoagulant regimens. In two separate studies by Hull and Dees, the incidence of recurrent venous thrombosis and PE was higher in randomized groups that did not receive adequate initial anticoagulation.8,9 Other studies give more support regarding the need for treatment of venous thrombotic disease and the poor results when therapy is not given.10 –12 This limited direct and more extensive indirect evidence reasonably support DVT as a source of PE and also support benefit from treatment.
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ver the last 30 years, it has been demonstrated repeatedly that it is possible to insert a mechanical filter into the inferior vena cava (IVC) while maintaining patency and blood flow1–5 in an attempt to prevent consequences of embolic events from the lower extremity. An assumption underlying this use is that in a majority of cases, pulmonary emboli are manifestations of venous thrombi arising in the lower extremities, and that the risk of pulmonary embolization is increased when venous thrombosis is left untreated. Is this paradigm correct? Kakkar et al6 assessed the natural history of lower extremity deep venous thrombosis (DVT) by following 132 postoperative patients who were screened with an iodine-labeled fibrinogen scan. Forty of these patients were found to have abnormal scans suggesting the presence of a venous thrombosis, which was confirmed in all but one case by phlebography. In 14 of these patients, the trace counts lasted less than 3 days. But in 26 patients, the counts persisted. Phlebography confirmed that the thrombi had extended into the popliteal or femoral veins in nine of these patients, four of whom were believed to have had a pulmonary embolism (PE) based on signs and symptoms. Interestingly, there is only one study that includes a control group of patients who were not treated,7 although this study did not document the presence of DVT. In this 1960 From the Earl K. Long Medical Center, Baton Rouge, Louisiana (PJF, KDR, GHK) and the LSU Health Sciences Center, New Orleans, Louisiana (WRS). Submitted for publication September 14, 2004; accepted for publication April 18, 2005. Correspondence: Paul Failla, MD, Pulmonary/Critical Care Medicine, Earl K. Long Medical Center, 5825 Airline Highway, LSU Unit, Baton Rouge, LA 70805. (E-mail: [email protected]).
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Indications for Inferior Vena Caval Filter Placement According to the seventh ACCP Consensus Conference on Antithrombotic Therapy13 (Table 1), inferior vena caval filter placement is recommended when there is a contraindication or complication of anticoagulant therapy (including heparin induced thrombocytopenia) in an individual with a proximal vein thrombus or PE. It is also recommended for recurrent thromboembolism that occurs despite adequate anticoagulation, chronic recurrent embolism with pulmonary hypertension, and with the concurrent performance of surgical pulmonary embolecAugust 2005 Volume 330 Number 2
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Table 1. Indications for Inferior Venal Cava Placement 1. Contraindication or complication of anticoagulation in a patient with proximal vein thrombosis of the lower extremity or PE. 2. Recurrent thromboembolism despite adequate anticoagulation. 3. Chronic recurrent pulmonary embolism with pulmonary hypertension. 4. Concurrent performance of surgical pulmonary embolectomy or pulmonary endarterectomy. 5. Heparin-induced thrombocytopenia. From the Seventh ACCP Consensus Conference on Antithrombotic Therapy. Chest 2004;126(Suppl);401s-28s
tomy and pulmonary endarterectomy. There are other indications, including free-floating iliocaval clot, patients with limited cardiopulmonary reserve, and compliance concerns, that are not well defined and not addressed in these guidelines. Is there evidence-based data to guide the use of IVC filters? Insertion of Inferior Vena Caval Filters Insertion techniques have evolved from initial cutdown methods to the present-day percutaneous route using fluoroscopic guidance. Femoral, jugular, and basilic venous access sites have all been used. All sites involve standard venipuncture using the Seldinger technique for insertion of a catheter through which the filter is placed. Most uncomplicated procedures can be completed in less than 30 minutes with minimal fluoroscopy and contrast administration. The mortality rate from placement is very low regardless of which filter is used. In Becker’s review of 2557 patients undergoing filter insertion, only 3 deaths (0.12%) were reported.14 This and other complications are summarized in Table 2.16 Selected complications of filters are discussed in detail later in this paper. Types and Characteristics of Inferior Vena Caval Filters Over the last 30 years, many designs of inferior vena caval filters have been introduced onto the market, although most of the experience has been with the stainless steel Greenfield filter15. Currently, there are at least 11 filters that are approved by the Food and Drug Administration and on the market for use in the United States. Design characteristics have undergone several technological changes and the characteristics of the ideal filter have been summarized.16 These features would include easy percutaneous insertion with a small introducer size and low insertion site thrombosis. Short- and long-term complications would be minimal and all potential emboli above a reasonably defined size would be prevented without impedance of vena caval flow. Additionally, an interruption that THE AMERICAN JOURNAL OF THE MEDICAL SCIENCES
Table 2. Complications Reported with Use of Inferior Vena Caval Filters Complication Pulmonary embolism Fatal pulmonary embolism Death linked to insertion of an IVC filter Complications from insertiona Venous access site thrombosis Migration of the filter Penetration of the IVCb Obstruction of the IVC Venous insufficiencyc Filter fracture Guide wire entrapment
Rate (%) 2–5 0.7 0.12 4–11 2–28 3–69 9–24 6–30 5–59 1 ⬍1
a
Complications from insertion include puncture site complications such as bleeding, infection, pneumothorax, vocal cord paralysis, stroke, delivery system complications; air embolism; and filter malposition, tilting, or incomplete opening. b Penetration of the IVC, which can occur immediately, but is more commonly seen as a delayed sequela. Most often, patients are asymptomatic from such IVC penetrations; however, untoward events have been described by penetration of filter struts into small bowel, aorta, and sympathetic ganglia. c Most reports of IVC filters usually report venous insufficiency rates as less than 10%. When studies are conducted for longer periods of follow-up (as long as 6 years), more than 58.8% of patients may have clinical signs of venous insufficiency.The data are somewhat controversial because as many as 30–45% of patients treated with anticoagulation therapy may experience venous insufficiency after follow-up of 6 years.Reprinted with permission: Kinney TB. Update on IVC filters. J Vasc Interv Radiology 2003; 14:425-40.
could be reversed and magnetic resonance imaging compatibility would be advantageous features. Obviously, none of the current filters possess all of these characteristics, and because of the small numbers of patients involved in various case series and marked differences in study design, it is impossible to compare one filter to another. There are, however, specific mechanical differences among various filters, which have been summarized and which may factor in to individual decisions regarding filter choice14,16 (Table 3). Specifically, the Bird’s Nest filter is relatively larger than the other filters and may be an option in the patient with IVC dimensions up to 40 mm. The Simon-Nitinol, LGM VenaTech, and VenaTech LP filters are relatively shorter than the other filters, which may be helpful when the difference between the iliac bifurcation and the renal veins is limited. Additionally, the small introducer size of the Simon-Nitonol and Trap-Ease filters may expand the number of percutaneous approaches and theoretically could reduce the incidence of insertion site thrombosis, although this benefit has been questioned14. The Simon-Nitinol filter is now approved by the Food and Drug Administration for antecubital insertion. It is also worth noting that the Titanium Greenfield, VenTech LGM, Simon-Nitonol, and Trap-Ease filters are nonferro83
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Table 3. Features of Inferior Vena Caval Filters IVC Diameter to 40 mm Bird’s Nest (40 mm) VenaTech LGM(35mm) Antecubital insertion Simon-Nitinol Trap-Ease OptEase Small introducer size Trap-Ease Compatible with magnetic resonance imaging Simon-Nitinol Trap-Ease VenaTech LGM Removable Recovery Gunther Tulip OptEase
magnetic and thus are compatible with magnetic resonance imaging. Efficacy The most striking observation when reviewing the literature regarding IVC filters is the lack of randomized studies testing their efficacy and complications. Considering the issues raised above regarding the consequences of DVT, a controlled study involving such nontreated patients would certainly raise ethical concerns. There are, however, other significant shortcomings noted when reviewing articles dealing with IVC filters. Issues regarding diagnostic evaluations, appropriate descriptions of patient groups studied, adequate analyses of adverse outcomes, and, among others, unbiased and detailed surveillance of patients have been raised.14 What conclusions then can be drawn from literature regarding the efficacy of these mechanical filters, that is, their ability to prevent PE? Greenfield and Michna,2 in 1988, reported their 12-year clinical experience with the stainless steel Greenfield vena caval filter. This series consists of 469 patients covering a period from 1974 to 1986. The authors reported a fatal embolism rate of less than 5%, but many patients were lost to follow-up. Since ventilation-perfusion scanning was not performed on patients during the follow-up period, it is impossible to estimate the rate of nonfatal PEs. Rohrer et al17 reviewed the records of 260 patients who had Greenfield filters placed during an 11-year time period. They reported an overall recurrent PE rate of 3.4%. Fatal PEs accounted for about one-third of these recurrent events. Recurrent rates of asymptomatic PEs were not determined. Golueke et al18 retrospectively reviewed the records of 88 patients who had a Greenfield filter placed from 1978 to 1985. Follow-up was available on 65 patients, 2 (3.1%) suffered from recurrent PEs. As in most studies, only symptomatic PEs were investigated. 84
With respect to other mechanical filters, Greenfield et al19 reported the results of a multicenter study using the modified hook-titanium Greenfield filter. Patients were followed for 30 days after placement of the titanium filters. Three deaths were believed to be due to recurrent PEs for an overall failure rate of less than 2%. Once again, only suspected PEs were investigated. Lord and Benn20 reported their experience of 61 Bird’s Nest filters placed over a 4-year period. Two patients died in the immediate perioperative period, with PEs contributing or causing their demise. No patients during follow-up were believed to have symptomatic PEs. In a randomized trial conducted by Decousus et al,21 400 patients with proximal DVT who were believed to be at high risk for a PE were randomized to receive anticoagulation plus an inferior vena caval filter versus anticoagulation alone. Fifty-six percent of the patients who received filters received the VenaTech LGM filter, 26.5% received the titanium Greenfield filter, and 15.15% received the Bird’s Nest filter. Within the first 12 days, there was a significant difference in that two patients in the filter group (1.1%) and nine patients (4.8%) from the nonfilter group had developed PEs (symptomatic and asymptomatic). Although four of the emboli in the nonfilter group were believed to be fatal, there was overall no mortality difference, with five patients dying in each group. After 2 years, symptomatic PEs had occurred in six patients in the filter group and 12 patients in the nonfilter group though this difference did not achieve statistical significance. Once again, overall mortality differences after this 2-year period were no different between the two groups. There are recent data presented in abstract form with follow-up to 8 years that shows a significant decrease in symptomatic PEs in the filter group (6.2% versus 15.1%).22 In summary, there are studies suggesting that intravenous IVC filters may be successful in preventing major PEs and their potential fatal consequences early after insertion. These conclusions are predominantly based on studies that lack randomization and have other design flaws. Conclusions regarding long-term efficacy are even more difficult to draw because of the same inadequacies. It is hoped that the recent quality improvement guidelines by Grassi et al will improve our ability to determine outcomes in a more standardized fashion.23 Complications Many of the same problems encountered when attempting to analyze articles commenting on the efficacy of mechanical filters are also found when reviewing data concerning complication rates: problems with study design, drop-out rate outcomes, and so on. With these limitations in mind, an attempt is August 2005 Volume 330 Number 2
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made here to review what is known of the complications related to insertion of IVC filters. Generally speaking, complications with a fatal outcome appear to be infrequent. As mentioned earlier, Becker et al14 combined several studies and estimated a fatal complication rate of 0.12%. Nonfatal complications were divided into five groups by this same group and are as follows. Technical Difficulties During Placement Placement difficulties represent a variety of problems, including hematomas, wound infections, filter misplacement, and so on.14 Greenfield and Michna2 reported a insertion complication rate of approximately 10%. The major procedure in that study involved a cut-down of the vein chosen for access, most commonly the jugular vein. Pais et al24 share their experience of 96 patients who underwent percutaneous insertion of a stainless steel Greenfield filter; they reported an approximately 4% placement complication rate. Lord and Denn20 reported no complications with regards to placement of a Bird’s Nest filter in 61 patients. Greenfield et al19 reported technical difficulties in approximately 9% of 184 filters utilizing the modified-titanium Greenfield filter. Insertion Site Thrombosis Because most present day filters are percutaneously inserted through the femoral veins, rates of insertion-site DVT are of particular importance. Pais et al24 placed 102 stainless steel Greenfield filters, 90 being inserted through the femoral veins and 12 through the right internal jugular vein. They used duplex scanning or portable ultrasonography in 24 patients and found evidence of femoral vein thrombosis in eight of these patients, although only three were symptomatic. Mewissen et al25 specifically looked for this complication using Doppler flow imaging or compression ultrasonography 24 hours after 54 percutaneous Greenfield filters were placed, 47 in the femoral veins and 7 in the right internal jugular veins. Nine thrombi were found in the common femoral vein (19%) and one clot was found in the right internal jugular vein. Of the nine patients with femoral clot, four became symptomatic within 10 days after the procedure. Lord and Benn20 performed duplex imaging in 37 of 61 patients after percutaneous insertion of a Bird’s Nest filter. No femoral vein clot was noted. Interestingly, Simon et al26 reported their experience with 44 patients after percutaneous insertion of the Simon-Nitinol vena caval filter. This filter possessed an introducer kit with an outside diameter smaller than any other filters available at that time. Unfortunately, shortterm follow-up with ultrasonography was done in only 18 of these patients, but 5 were noted to have DVT. Three of these were symptomatic. THE AMERICAN JOURNAL OF THE MEDICAL SCIENCES
Filter Migration Subsequent movement of IVC filters after placement has been reported.5,27–29 Although migration into the pulmonary artery and right ventricle has been observed,30 –32 in the majority of cases migration is minor and does not result in any significant morbidity. Movement has been noted in the cephalad and caudad directions and has been reported with the stainless steel Greenfield filter as well as filters of different technological design. Penetration of the Filter into the Inferior Vena Cava When radiographic studies are used to assess complications related to filter position in the follow-up period, erosion of the Greenfield filter into the inferior vena caval wall has been noted. Messmer and Greenfield29 reported this occurrence in 8 of 23 patients they followed and this phenomenon was noted in 10 of 69 patients studied by Atkins et al.27 Although these authors observed no significant morbidity, severe consequences have been reported.27,33–35 Ferris et al5 studied seven filter designs used in 320 patients and found that 9% of the filters had penetrated the IVC, defined as filter components extending more than 3 mm outside of the wall of the IVC. Two of these filters penetrated the abdominal aorta, one penetrated the iliac artery, and in one the third portion of the duodenum. No patient was believed to have symptoms as a result of these events. Inferior Vena Caval Thrombosis/Lower Extremity Venous Thrombosis It is impossible to come up with accurate frequencies with respect to these complications. In most studies, IVC thrombosis was not analyzed unless the patient had clinical signs or symptoms of recurrent PEs or DVT. Greenfield et al2 studied only 127 of his original 469 patients and found that 10 patients had vena caval filter thrombi, for an overall patency rate of 93%. One hundred six Doppler examinations were performed on the lower extremities and it was believed that a new abnormality was noted in only 5%. In the analysis of the Bird’s Nest filter, Lord and Benn20 performed duplex imaging of the vena cava on 37 of their original 61 patients. They believed that the vena cava was patent in all, but two patients did show small echogenic areas consistent with thrombus. They did not find any evidence of DVT in the femoral, iliac, or other leg veins in the same group of patients. In their analysis of several different filter designs in 320 patients, Ferris et al5 imaged 137 patients in an attempt to diagnose IVC thrombosis. Nineteen percent of the patient population studied were noted to have thrombi in the IVC or the filter. Twelve of these patients had extensive thrombus and were symptomatic with bilateral lower extremity edema. The remaining patients had 85
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smaller thrombi and did not have any lower extremity symptoms. They also performed lower extremity imaging studies on 117 of their patients and found that 22% of them had evidence of a new DVT or an extension of a previous DVT. They did, however, include in this group DVT related to the puncture site. In their study of patients randomized to receive anticoagulation plus IVC filters versus anticoagulation alone, Decousus et al21 found recurrent DVT in 20.8% of the group of patients assigned to receive a filter plus anticoagulation compared with 11.6% in the group receiving anticoagulation alone. These thrombi occurred over a 2-year follow-up period, and only symptomatic patients were investigated with imaging studies. In suspected cases of venous thromboembolism imaging studies were utilized to assess patency of the filter itself. Of the 37 patients who developed symptomatic recurrent venous thromboembolism, a thrombus was noted at the filter site at 16, with 4 of these patients developing concomitant PEs. The follow-up 8-year data revealed a DVT rate of 34.1% with a filter and 27.3% in those without a filter (HR 1.44 [0.96 –2.18],P ⫽ 0.08)22. In summary, technical difficulties during insertion, symptomatic insertion site thrombosis, filter migration, and penetration into the IVC do not appear to be frequent causes of serious morbidity. The data are of more concern regarding IVC and lower extremity thrombosis. The study by Decousus et al21 does raise concerns about long-term thrombotic complications arising after placement of IVC filters. This is certainly an area that should be examined in any future study regarding IVC filters. Removable Filters With the above concerns regarding long-term safety, and realizing that contraindications to anticoagulation may be temporary in certain patients, there has been interest in the development of filters that are or can be removed after insertion. There are two categories of removable filters. The first is temporary, which by design must be removed after insertion as it is tethered to a catheter or wire at the insertion site. Currently no examples of this type are available for use in the United States. The second type is retrievable, which can be safely and intentionally removed after some prespecified time or left in permanently. The devices are configured similarly to the permanent filters with modifications to the caval attachment sites, and, unlike the temporary filters, they do not remain in place with a tether. There are currently three devices of this type available for use in the United States (Table 3). Recently, the results of the Canadian Interventional Radiology Association on the Gunther Tulip Retrievable Filter were reported36. The filter can be placed from either the femoral or jugular access, and re86
trieval is from the right jugular site. In the Canadian registry, 91 filters were placed in 90 patients. The most common reason cited for filter placement was contraindication to anticoagulation. There were 53 attempted retrievals, with 52 being successful, with implantation times ranging from 2 to 25 days (mean, 9 days). In the remaining 39 patients, retrievals were not attempted for various reasons, including ongoing contraindication to anticoagulation (n ⫽ 17) and large trapped emboli within the filter (n ⫽ 10). In approximately 8% of the patients, new permanent filters were reinserted 17 to 167 days after retrieval as a result of bleeding or the need for surgery necessitating the cessation of anticoagulation. Two patients developed filter occlusion, and no other complications were documented. Asch37 reported on his clinical experience with the Recovery Nitonol filter placed in 32 patients. Multiple indications were listed for placement of the filter. In 24 of 24 patients, the filter was successfully retrieved with a jugular approach after a mean implantation time of 53 days (range, 5–134 days). No symptomatic PEs or insertion site DVT was reported in this paper. One filter with large trapped thrombus was found to have migrated 4 cm cephalad at the time of elective removal. The filter was successfully removed with thrombi using a larger sheath, and there was no evidence of PE by computed tomographic angiography. These data offer hope that alternatives to permanent filter placement will be available to patients with reversible contraindications to anticoagulation. This optimism must be tempered by many unresolved issues, including the appropriate time to remove the filter, safety of removal, use of anticoagulation in the periremoval period, and concerns about filters that are left permanently. Conclusion Although randomized trials are limited, there is enough evidence in the literature to support the view that the consequences of untreated DVT/PE are significant and potentially fatal. The data do seem to indicate that there is protection against fatal complications early after insertion with acceptable morbidity but raises concerns about potential long-term complications. Any decision regarding permanent IVC filter placement must be based on an appreciation of these facts, even when following currently accepted indications as listed in Table 1. The marked increase in IVC filter insertion reported by Stein et al, including patients with neither DVT or PE, is very worrisome.38 Considering the lack of literature support, one must exercise great caution in response to the recommendations for ever-expanding indications for the placement of IVC filters.15,39 – 41 We certainly concur with the recent ACCP guidelines strongly recommending against August 2005 Volume 330 Number 2
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the routine use of IVC filters in addition to anticoagulation.13 The retrievable filter may offer an alternative approach to maintaining short-term efficacy while reducing long-term complications. It is hoped that the ongoing trials evaluating retrievable filters will answer these concerns. We join others in this era of expanding options for IVC filters in their call for standardizing outcome measures to help answer these difficult questions. References 1. Mobin-Udden K, Culland AS, Booloki H, et al. Transvenous caval interuptions with umbrella filters. N Engl J Med 1972;286:55. 2. Greenfield LJ, Michna BA. Twelve-year experience with the Greenfield vena caval filter. Surgery 1988;104:706–12. 3. Dorfman GS. Percutaneous inferior venal caval filters. Radiology 1990;174:987–92. 4. Grassi CJ. Inferior vena caval filters: analysis of five currently available devices. AJR Am J Roentgenol 1991;156:813– 21. 5. Ferris EJ, McCowan TC, Carver DR, et al. Percutaneous inferior vena caval filters: Follow-up of seven designs in 320 patients. Radiology 1993;188:851–6. 6. Kakkar VV, Flance C, Howe CT, et al. Natural history of post-operative deep vein thrombosis. Lancet 1969;2:230–3. 7. Barritt DW, Jordan SC. Anticoagulant drugs in the treatment of pulmonary embolism: a controlled trial. Lancet 1960; 1:1309–12. 8. Hull RD, Raskob GE, Hirsh J, et al. Continuous intravenous heparin compared with intermittent subcutaneous heparin in the initial treatment of proximal-vein thrombosis. N Engl J Med 1986;315:1109–14. 9. Dees PM, Heijbeer H, Buller HR, et al. Acenocoumarol and heparin compared with acenocoumarol alone in the initial treatment of proximal-vein thrombosis. N Engl J Med 1992;327:1485–9. 10. Kernohan RJ, Todd C. Heparin therapy in thromboembolic disease. Lancet 966;1:621–3. 11. Alpert JS, Smith R, Carlson CJ, et al. Mortality in patients treated for pulmonary embolism. JAMA 1976;236:1477–80. 12. Kavis JA. Heparin in the treatment of pulmonary thromboembolism. Thromb Haemost 1974;32:517–27. 13. Seventh ACCP Consensus Conference on antithrombotic therapy. Chest 2004;126:401s-28s. 14. Becker DM, Philbrick JT, Selby JB. Inferior vena cava filters: indications, safety, effectiveness. Arch Intern Med 1992;152:1985–94. 15. Kanter B, Moser KM. The Greenfield vena cava filter. Chest 1988;93:170–5. 16. Kinney TB. Update on inferior vena cava filters. J Vasc Interv Radiol 2003;14:425–40. 17. Rohrer MJ, Scheidler MD, Wheeler HB, et al. Extended indications for placement of an inferior vena cava filter. J Vasc Surg 1989;10:44–50. 18. Golueke PJ, Garrett WV, Thompson JE, et al. Interruption of the vena cava by means of the Greenfield filter: expanding the indications. Surgery 1988;103:111–7. 19. Greenfield LJ, Chok J, Proctor M, et al. Results of a multicenter study of the modified hook-titanium Greenfield filter. J Vasc Surg 1991;14:253–7. 20. Lord R, Benn I. . Early and late results after Bird’s Nest filter placement in the inferior vena cava: clinical and duplex ultrasound follow-up. Aust N Z J Surg 1994;64:106–14.
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21. Decousus H, Leizorovicz A, Parent F, et al. A clinical trial of vena caval filters in the prevention of pulmonary embolism in patients with proximal deep-vein thrombosis. N Engl J Med 1998;338:409–15. 22. Decousus H. Eight year follow-up of a randomized trial investigation vena cava filters in the prevention of PE in patients presenting a proximal DVT: The PREPIC trial [abstract]. J Thromb Haemost 2003;1(Suppl 1):OC440. 23. Grassi CJ, Swan TL, Cardella JF, et al. Quality improvement guidelines for percutaneous permanent inferior vena cava filter placement for the prevention of pulmonary embolism. J Vasc Interv Radiol 2001;12:137–41. 24. Pais SO, Tobin RO, Austin CB, et al. Percutaneous insertion of the Greenfield inferior vena cava filter: experience with ninety-six patients. J Vasc Surg 1988;8:460–4. 25. Mewissen MW, Erickson SJ, Foley WD, et al. Thrombosis at venous insertion sites after inferior vena caval filter placement. Radiology 1989;156:155–7. 26. Simon M, Athanasoulis CA, Kim D, et al. Simon nitinol inferior vena cava filters: initial clinical experience. Radiology 1989;172:99–103. 27. Carabasi RA III, Moritz MJ, Jarrell BE. Complications encountered with the use of the Greenfield filter. Am J Surg 1987;154:163–8. 28. Berland LL, Maddison FE, Reginald VM. Radiologic follow-up of vena cava filter devices. AJR Am J Roentgenol 1980;134:1047–52. 29. Messmer JM, Greenfield LJ. Greenfield caval filters: longterm radiographic follow-up study. Radiology 1985;156:613–8. 30. Atkins CW, Thurer RL, Waltman AC, et al. A misplaced caval filter: it’s removal from the heart without cardiopulmonary bypass. Arch Surg 1980;115:1133–5. 31. Castaneda F, Herrara M, Cross AH, et al. Migration of a Kimray-Greenfield filter to the right ventricle. Radiology 1983;149:690–1. 32. Friedell ML, Goldenkranz RJ, Parsonnet V, et al. Migration of a Greenfield filter to the pulmonary artery: a case report. J Vasc Surg 1986;3:929–31. 33. Wingerd M, Bernhard UM, Maddison F, et al. Comparison of caval filters in the management of venous thromboembolism. Arch Surg 1978;113:1264–9. 34. Scurr JH, Jarrett PE, Wastell C. The treatment of recurrent pulmonary embolism-experience with the KimrayGreenfield vena cava filter. Ann R Coll Surg Eng 1983;65: 233–4. 35. Todd GJ, Sanderson J, Nowygrad R, et al. Recent clinical experience with the vena cava filter. Am J Surg 1988;156: 353–8. 36. Millward SF, Oliva VL, Bell SD, et al. Gunther Tulip Retrievable Vena Cava Filter: results from the Registry of the Canadian Interventional Radiology Association. J Vasc Interv Radiol 2001 Sep;12(9):1053–8. 37. Asch MR. Initial experience in humans with a new retrievable inferior vena cava filter. Radiology 2002 Dec;835–44. 38. Stein PD, Kayali F, Olson RE. Twenty-one year trends in the use of inferior vena cava filters. Arch Intern Med 2004; 164:1541–5. 39. Sarasin F, Eckman M. Management and prevention of thromboembolic events in patients with cancer related hypercoagulable states: status. J Gen Int Med 1993;8:476–86. 40. Rogers FB, Shackford SR, Wilson J, et al. Prophylactic vena cava filter insertion in severely injured trauma patients: indications and preliminary results. J Trauma 1993;35:637– 42. 41. Leach TA, Pastena JA, Swan KG, et al. Surgical prophylaxis for pulmonary embolism. Am Surg 1994;60:292–5.
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ABSTRACT: Surgical interruption of the inferior vena cava (IVC) as a means to prevent pulmonary embolism and its consequences has been entertained since the end of the 19th century. Initial methods were crude, however, but their deficiencies led to the development of newer techniques. Despite increasing indications and use of permanent IVC filters there remains controversy regarding their efficacy and complications. The purpose
of this article is to review the pertinent literature and, it is hoped, aid in the development of a rational approach to the use of IVC filters. The evolving data regarding the retrievable filters are also discussed. KEY INDEXING TERMS: Deep venous thrombosis; Pulmonary emboli; Inferior vena cava filter; Complications; Efficacy. [Am J Med Sci 2005;330(2):82–87.]
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study, 35 patients who were clinically believed to have had a PE were randomized to receive anticoagulation with heparin and nicoumalone or to receive no anticoagulation. Of 19 patients who did not receive any anticoagulation, 5 died secondary to a PE that was proven at autopsy. Of the 16 who did receive anticoagulation, none died from a PE. The randomization feature of this study was stopped at this point because of ethical concerns relating to the untreated control group. Other, more indirect evidence supporting DVT as a source of PEs and the consequences of a lack of treatment can be gleaned from a review of randomized studies comparing various anticoagulant regimens. In two separate studies by Hull and Dees, the incidence of recurrent venous thrombosis and PE was higher in randomized groups that did not receive adequate initial anticoagulation.8,9 Other studies give more support regarding the need for treatment of venous thrombotic disease and the poor results when therapy is not given.10 –12 This limited direct and more extensive indirect evidence reasonably support DVT as a source of PE and also support benefit from treatment.
O
ver the last 30 years, it has been demonstrated repeatedly that it is possible to insert a mechanical filter into the inferior vena cava (IVC) while maintaining patency and blood flow1–5 in an attempt to prevent consequences of embolic events from the lower extremity. An assumption underlying this use is that in a majority of cases, pulmonary emboli are manifestations of venous thrombi arising in the lower extremities, and that the risk of pulmonary embolization is increased when venous thrombosis is left untreated. Is this paradigm correct? Kakkar et al6 assessed the natural history of lower extremity deep venous thrombosis (DVT) by following 132 postoperative patients who were screened with an iodine-labeled fibrinogen scan. Forty of these patients were found to have abnormal scans suggesting the presence of a venous thrombosis, which was confirmed in all but one case by phlebography. In 14 of these patients, the trace counts lasted less than 3 days. But in 26 patients, the counts persisted. Phlebography confirmed that the thrombi had extended into the popliteal or femoral veins in nine of these patients, four of whom were believed to have had a pulmonary embolism (PE) based on signs and symptoms. Interestingly, there is only one study that includes a control group of patients who were not treated,7 although this study did not document the presence of DVT. In this 1960 From the Earl K. Long Medical Center, Baton Rouge, Louisiana (PJF, KDR, GHK) and the LSU Health Sciences Center, New Orleans, Louisiana (WRS). Submitted for publication September 14, 2004; accepted for publication April 18, 2005. Correspondence: Paul Failla, MD, Pulmonary/Critical Care Medicine, Earl K. Long Medical Center, 5825 Airline Highway, LSU Unit, Baton Rouge, LA 70805. (E-mail: [email protected]).
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Indications for Inferior Vena Caval Filter Placement According to the seventh ACCP Consensus Conference on Antithrombotic Therapy13 (Table 1), inferior vena caval filter placement is recommended when there is a contraindication or complication of anticoagulant therapy (including heparin induced thrombocytopenia) in an individual with a proximal vein thrombus or PE. It is also recommended for recurrent thromboembolism that occurs despite adequate anticoagulation, chronic recurrent embolism with pulmonary hypertension, and with the concurrent performance of surgical pulmonary embolecAugust 2005 Volume 330 Number 2
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Table 1. Indications for Inferior Venal Cava Placement 1. Contraindication or complication of anticoagulation in a patient with proximal vein thrombosis of the lower extremity or PE. 2. Recurrent thromboembolism despite adequate anticoagulation. 3. Chronic recurrent pulmonary embolism with pulmonary hypertension. 4. Concurrent performance of surgical pulmonary embolectomy or pulmonary endarterectomy. 5. Heparin-induced thrombocytopenia. From the Seventh ACCP Consensus Conference on Antithrombotic Therapy. Chest 2004;126(Suppl);401s-28s
tomy and pulmonary endarterectomy. There are other indications, including free-floating iliocaval clot, patients with limited cardiopulmonary reserve, and compliance concerns, that are not well defined and not addressed in these guidelines. Is there evidence-based data to guide the use of IVC filters? Insertion of Inferior Vena Caval Filters Insertion techniques have evolved from initial cutdown methods to the present-day percutaneous route using fluoroscopic guidance. Femoral, jugular, and basilic venous access sites have all been used. All sites involve standard venipuncture using the Seldinger technique for insertion of a catheter through which the filter is placed. Most uncomplicated procedures can be completed in less than 30 minutes with minimal fluoroscopy and contrast administration. The mortality rate from placement is very low regardless of which filter is used. In Becker’s review of 2557 patients undergoing filter insertion, only 3 deaths (0.12%) were reported.14 This and other complications are summarized in Table 2.16 Selected complications of filters are discussed in detail later in this paper. Types and Characteristics of Inferior Vena Caval Filters Over the last 30 years, many designs of inferior vena caval filters have been introduced onto the market, although most of the experience has been with the stainless steel Greenfield filter15. Currently, there are at least 11 filters that are approved by the Food and Drug Administration and on the market for use in the United States. Design characteristics have undergone several technological changes and the characteristics of the ideal filter have been summarized.16 These features would include easy percutaneous insertion with a small introducer size and low insertion site thrombosis. Short- and long-term complications would be minimal and all potential emboli above a reasonably defined size would be prevented without impedance of vena caval flow. Additionally, an interruption that THE AMERICAN JOURNAL OF THE MEDICAL SCIENCES
Table 2. Complications Reported with Use of Inferior Vena Caval Filters Complication Pulmonary embolism Fatal pulmonary embolism Death linked to insertion of an IVC filter Complications from insertiona Venous access site thrombosis Migration of the filter Penetration of the IVCb Obstruction of the IVC Venous insufficiencyc Filter fracture Guide wire entrapment
Rate (%) 2–5 0.7 0.12 4–11 2–28 3–69 9–24 6–30 5–59 1 ⬍1
a
Complications from insertion include puncture site complications such as bleeding, infection, pneumothorax, vocal cord paralysis, stroke, delivery system complications; air embolism; and filter malposition, tilting, or incomplete opening. b Penetration of the IVC, which can occur immediately, but is more commonly seen as a delayed sequela. Most often, patients are asymptomatic from such IVC penetrations; however, untoward events have been described by penetration of filter struts into small bowel, aorta, and sympathetic ganglia. c Most reports of IVC filters usually report venous insufficiency rates as less than 10%. When studies are conducted for longer periods of follow-up (as long as 6 years), more than 58.8% of patients may have clinical signs of venous insufficiency.The data are somewhat controversial because as many as 30–45% of patients treated with anticoagulation therapy may experience venous insufficiency after follow-up of 6 years.Reprinted with permission: Kinney TB. Update on IVC filters. J Vasc Interv Radiology 2003; 14:425-40.
could be reversed and magnetic resonance imaging compatibility would be advantageous features. Obviously, none of the current filters possess all of these characteristics, and because of the small numbers of patients involved in various case series and marked differences in study design, it is impossible to compare one filter to another. There are, however, specific mechanical differences among various filters, which have been summarized and which may factor in to individual decisions regarding filter choice14,16 (Table 3). Specifically, the Bird’s Nest filter is relatively larger than the other filters and may be an option in the patient with IVC dimensions up to 40 mm. The Simon-Nitinol, LGM VenaTech, and VenaTech LP filters are relatively shorter than the other filters, which may be helpful when the difference between the iliac bifurcation and the renal veins is limited. Additionally, the small introducer size of the Simon-Nitonol and Trap-Ease filters may expand the number of percutaneous approaches and theoretically could reduce the incidence of insertion site thrombosis, although this benefit has been questioned14. The Simon-Nitinol filter is now approved by the Food and Drug Administration for antecubital insertion. It is also worth noting that the Titanium Greenfield, VenTech LGM, Simon-Nitonol, and Trap-Ease filters are nonferro83
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Table 3. Features of Inferior Vena Caval Filters IVC Diameter to 40 mm Bird’s Nest (40 mm) VenaTech LGM(35mm) Antecubital insertion Simon-Nitinol Trap-Ease OptEase Small introducer size Trap-Ease Compatible with magnetic resonance imaging Simon-Nitinol Trap-Ease VenaTech LGM Removable Recovery Gunther Tulip OptEase
magnetic and thus are compatible with magnetic resonance imaging. Efficacy The most striking observation when reviewing the literature regarding IVC filters is the lack of randomized studies testing their efficacy and complications. Considering the issues raised above regarding the consequences of DVT, a controlled study involving such nontreated patients would certainly raise ethical concerns. There are, however, other significant shortcomings noted when reviewing articles dealing with IVC filters. Issues regarding diagnostic evaluations, appropriate descriptions of patient groups studied, adequate analyses of adverse outcomes, and, among others, unbiased and detailed surveillance of patients have been raised.14 What conclusions then can be drawn from literature regarding the efficacy of these mechanical filters, that is, their ability to prevent PE? Greenfield and Michna,2 in 1988, reported their 12-year clinical experience with the stainless steel Greenfield vena caval filter. This series consists of 469 patients covering a period from 1974 to 1986. The authors reported a fatal embolism rate of less than 5%, but many patients were lost to follow-up. Since ventilation-perfusion scanning was not performed on patients during the follow-up period, it is impossible to estimate the rate of nonfatal PEs. Rohrer et al17 reviewed the records of 260 patients who had Greenfield filters placed during an 11-year time period. They reported an overall recurrent PE rate of 3.4%. Fatal PEs accounted for about one-third of these recurrent events. Recurrent rates of asymptomatic PEs were not determined. Golueke et al18 retrospectively reviewed the records of 88 patients who had a Greenfield filter placed from 1978 to 1985. Follow-up was available on 65 patients, 2 (3.1%) suffered from recurrent PEs. As in most studies, only symptomatic PEs were investigated. 84
With respect to other mechanical filters, Greenfield et al19 reported the results of a multicenter study using the modified hook-titanium Greenfield filter. Patients were followed for 30 days after placement of the titanium filters. Three deaths were believed to be due to recurrent PEs for an overall failure rate of less than 2%. Once again, only suspected PEs were investigated. Lord and Benn20 reported their experience of 61 Bird’s Nest filters placed over a 4-year period. Two patients died in the immediate perioperative period, with PEs contributing or causing their demise. No patients during follow-up were believed to have symptomatic PEs. In a randomized trial conducted by Decousus et al,21 400 patients with proximal DVT who were believed to be at high risk for a PE were randomized to receive anticoagulation plus an inferior vena caval filter versus anticoagulation alone. Fifty-six percent of the patients who received filters received the VenaTech LGM filter, 26.5% received the titanium Greenfield filter, and 15.15% received the Bird’s Nest filter. Within the first 12 days, there was a significant difference in that two patients in the filter group (1.1%) and nine patients (4.8%) from the nonfilter group had developed PEs (symptomatic and asymptomatic). Although four of the emboli in the nonfilter group were believed to be fatal, there was overall no mortality difference, with five patients dying in each group. After 2 years, symptomatic PEs had occurred in six patients in the filter group and 12 patients in the nonfilter group though this difference did not achieve statistical significance. Once again, overall mortality differences after this 2-year period were no different between the two groups. There are recent data presented in abstract form with follow-up to 8 years that shows a significant decrease in symptomatic PEs in the filter group (6.2% versus 15.1%).22 In summary, there are studies suggesting that intravenous IVC filters may be successful in preventing major PEs and their potential fatal consequences early after insertion. These conclusions are predominantly based on studies that lack randomization and have other design flaws. Conclusions regarding long-term efficacy are even more difficult to draw because of the same inadequacies. It is hoped that the recent quality improvement guidelines by Grassi et al will improve our ability to determine outcomes in a more standardized fashion.23 Complications Many of the same problems encountered when attempting to analyze articles commenting on the efficacy of mechanical filters are also found when reviewing data concerning complication rates: problems with study design, drop-out rate outcomes, and so on. With these limitations in mind, an attempt is August 2005 Volume 330 Number 2
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made here to review what is known of the complications related to insertion of IVC filters. Generally speaking, complications with a fatal outcome appear to be infrequent. As mentioned earlier, Becker et al14 combined several studies and estimated a fatal complication rate of 0.12%. Nonfatal complications were divided into five groups by this same group and are as follows. Technical Difficulties During Placement Placement difficulties represent a variety of problems, including hematomas, wound infections, filter misplacement, and so on.14 Greenfield and Michna2 reported a insertion complication rate of approximately 10%. The major procedure in that study involved a cut-down of the vein chosen for access, most commonly the jugular vein. Pais et al24 share their experience of 96 patients who underwent percutaneous insertion of a stainless steel Greenfield filter; they reported an approximately 4% placement complication rate. Lord and Denn20 reported no complications with regards to placement of a Bird’s Nest filter in 61 patients. Greenfield et al19 reported technical difficulties in approximately 9% of 184 filters utilizing the modified-titanium Greenfield filter. Insertion Site Thrombosis Because most present day filters are percutaneously inserted through the femoral veins, rates of insertion-site DVT are of particular importance. Pais et al24 placed 102 stainless steel Greenfield filters, 90 being inserted through the femoral veins and 12 through the right internal jugular vein. They used duplex scanning or portable ultrasonography in 24 patients and found evidence of femoral vein thrombosis in eight of these patients, although only three were symptomatic. Mewissen et al25 specifically looked for this complication using Doppler flow imaging or compression ultrasonography 24 hours after 54 percutaneous Greenfield filters were placed, 47 in the femoral veins and 7 in the right internal jugular veins. Nine thrombi were found in the common femoral vein (19%) and one clot was found in the right internal jugular vein. Of the nine patients with femoral clot, four became symptomatic within 10 days after the procedure. Lord and Benn20 performed duplex imaging in 37 of 61 patients after percutaneous insertion of a Bird’s Nest filter. No femoral vein clot was noted. Interestingly, Simon et al26 reported their experience with 44 patients after percutaneous insertion of the Simon-Nitinol vena caval filter. This filter possessed an introducer kit with an outside diameter smaller than any other filters available at that time. Unfortunately, shortterm follow-up with ultrasonography was done in only 18 of these patients, but 5 were noted to have DVT. Three of these were symptomatic. THE AMERICAN JOURNAL OF THE MEDICAL SCIENCES
Filter Migration Subsequent movement of IVC filters after placement has been reported.5,27–29 Although migration into the pulmonary artery and right ventricle has been observed,30 –32 in the majority of cases migration is minor and does not result in any significant morbidity. Movement has been noted in the cephalad and caudad directions and has been reported with the stainless steel Greenfield filter as well as filters of different technological design. Penetration of the Filter into the Inferior Vena Cava When radiographic studies are used to assess complications related to filter position in the follow-up period, erosion of the Greenfield filter into the inferior vena caval wall has been noted. Messmer and Greenfield29 reported this occurrence in 8 of 23 patients they followed and this phenomenon was noted in 10 of 69 patients studied by Atkins et al.27 Although these authors observed no significant morbidity, severe consequences have been reported.27,33–35 Ferris et al5 studied seven filter designs used in 320 patients and found that 9% of the filters had penetrated the IVC, defined as filter components extending more than 3 mm outside of the wall of the IVC. Two of these filters penetrated the abdominal aorta, one penetrated the iliac artery, and in one the third portion of the duodenum. No patient was believed to have symptoms as a result of these events. Inferior Vena Caval Thrombosis/Lower Extremity Venous Thrombosis It is impossible to come up with accurate frequencies with respect to these complications. In most studies, IVC thrombosis was not analyzed unless the patient had clinical signs or symptoms of recurrent PEs or DVT. Greenfield et al2 studied only 127 of his original 469 patients and found that 10 patients had vena caval filter thrombi, for an overall patency rate of 93%. One hundred six Doppler examinations were performed on the lower extremities and it was believed that a new abnormality was noted in only 5%. In the analysis of the Bird’s Nest filter, Lord and Benn20 performed duplex imaging of the vena cava on 37 of their original 61 patients. They believed that the vena cava was patent in all, but two patients did show small echogenic areas consistent with thrombus. They did not find any evidence of DVT in the femoral, iliac, or other leg veins in the same group of patients. In their analysis of several different filter designs in 320 patients, Ferris et al5 imaged 137 patients in an attempt to diagnose IVC thrombosis. Nineteen percent of the patient population studied were noted to have thrombi in the IVC or the filter. Twelve of these patients had extensive thrombus and were symptomatic with bilateral lower extremity edema. The remaining patients had 85
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smaller thrombi and did not have any lower extremity symptoms. They also performed lower extremity imaging studies on 117 of their patients and found that 22% of them had evidence of a new DVT or an extension of a previous DVT. They did, however, include in this group DVT related to the puncture site. In their study of patients randomized to receive anticoagulation plus IVC filters versus anticoagulation alone, Decousus et al21 found recurrent DVT in 20.8% of the group of patients assigned to receive a filter plus anticoagulation compared with 11.6% in the group receiving anticoagulation alone. These thrombi occurred over a 2-year follow-up period, and only symptomatic patients were investigated with imaging studies. In suspected cases of venous thromboembolism imaging studies were utilized to assess patency of the filter itself. Of the 37 patients who developed symptomatic recurrent venous thromboembolism, a thrombus was noted at the filter site at 16, with 4 of these patients developing concomitant PEs. The follow-up 8-year data revealed a DVT rate of 34.1% with a filter and 27.3% in those without a filter (HR 1.44 [0.96 –2.18],P ⫽ 0.08)22. In summary, technical difficulties during insertion, symptomatic insertion site thrombosis, filter migration, and penetration into the IVC do not appear to be frequent causes of serious morbidity. The data are of more concern regarding IVC and lower extremity thrombosis. The study by Decousus et al21 does raise concerns about long-term thrombotic complications arising after placement of IVC filters. This is certainly an area that should be examined in any future study regarding IVC filters. Removable Filters With the above concerns regarding long-term safety, and realizing that contraindications to anticoagulation may be temporary in certain patients, there has been interest in the development of filters that are or can be removed after insertion. There are two categories of removable filters. The first is temporary, which by design must be removed after insertion as it is tethered to a catheter or wire at the insertion site. Currently no examples of this type are available for use in the United States. The second type is retrievable, which can be safely and intentionally removed after some prespecified time or left in permanently. The devices are configured similarly to the permanent filters with modifications to the caval attachment sites, and, unlike the temporary filters, they do not remain in place with a tether. There are currently three devices of this type available for use in the United States (Table 3). Recently, the results of the Canadian Interventional Radiology Association on the Gunther Tulip Retrievable Filter were reported36. The filter can be placed from either the femoral or jugular access, and re86
trieval is from the right jugular site. In the Canadian registry, 91 filters were placed in 90 patients. The most common reason cited for filter placement was contraindication to anticoagulation. There were 53 attempted retrievals, with 52 being successful, with implantation times ranging from 2 to 25 days (mean, 9 days). In the remaining 39 patients, retrievals were not attempted for various reasons, including ongoing contraindication to anticoagulation (n ⫽ 17) and large trapped emboli within the filter (n ⫽ 10). In approximately 8% of the patients, new permanent filters were reinserted 17 to 167 days after retrieval as a result of bleeding or the need for surgery necessitating the cessation of anticoagulation. Two patients developed filter occlusion, and no other complications were documented. Asch37 reported on his clinical experience with the Recovery Nitonol filter placed in 32 patients. Multiple indications were listed for placement of the filter. In 24 of 24 patients, the filter was successfully retrieved with a jugular approach after a mean implantation time of 53 days (range, 5–134 days). No symptomatic PEs or insertion site DVT was reported in this paper. One filter with large trapped thrombus was found to have migrated 4 cm cephalad at the time of elective removal. The filter was successfully removed with thrombi using a larger sheath, and there was no evidence of PE by computed tomographic angiography. These data offer hope that alternatives to permanent filter placement will be available to patients with reversible contraindications to anticoagulation. This optimism must be tempered by many unresolved issues, including the appropriate time to remove the filter, safety of removal, use of anticoagulation in the periremoval period, and concerns about filters that are left permanently. Conclusion Although randomized trials are limited, there is enough evidence in the literature to support the view that the consequences of untreated DVT/PE are significant and potentially fatal. The data do seem to indicate that there is protection against fatal complications early after insertion with acceptable morbidity but raises concerns about potential long-term complications. Any decision regarding permanent IVC filter placement must be based on an appreciation of these facts, even when following currently accepted indications as listed in Table 1. The marked increase in IVC filter insertion reported by Stein et al, including patients with neither DVT or PE, is very worrisome.38 Considering the lack of literature support, one must exercise great caution in response to the recommendations for ever-expanding indications for the placement of IVC filters.15,39 – 41 We certainly concur with the recent ACCP guidelines strongly recommending against August 2005 Volume 330 Number 2
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the routine use of IVC filters in addition to anticoagulation.13 The retrievable filter may offer an alternative approach to maintaining short-term efficacy while reducing long-term complications. It is hoped that the ongoing trials evaluating retrievable filters will answer these concerns. We join others in this era of expanding options for IVC filters in their call for standardizing outcome measures to help answer these difficult questions. References 1. Mobin-Udden K, Culland AS, Booloki H, et al. Transvenous caval interuptions with umbrella filters. N Engl J Med 1972;286:55. 2. Greenfield LJ, Michna BA. Twelve-year experience with the Greenfield vena caval filter. Surgery 1988;104:706–12. 3. Dorfman GS. Percutaneous inferior venal caval filters. Radiology 1990;174:987–92. 4. Grassi CJ. Inferior vena caval filters: analysis of five currently available devices. AJR Am J Roentgenol 1991;156:813– 21. 5. Ferris EJ, McCowan TC, Carver DR, et al. Percutaneous inferior vena caval filters: Follow-up of seven designs in 320 patients. Radiology 1993;188:851–6. 6. Kakkar VV, Flance C, Howe CT, et al. Natural history of post-operative deep vein thrombosis. Lancet 1969;2:230–3. 7. Barritt DW, Jordan SC. Anticoagulant drugs in the treatment of pulmonary embolism: a controlled trial. Lancet 1960; 1:1309–12. 8. Hull RD, Raskob GE, Hirsh J, et al. Continuous intravenous heparin compared with intermittent subcutaneous heparin in the initial treatment of proximal-vein thrombosis. N Engl J Med 1986;315:1109–14. 9. Dees PM, Heijbeer H, Buller HR, et al. Acenocoumarol and heparin compared with acenocoumarol alone in the initial treatment of proximal-vein thrombosis. N Engl J Med 1992;327:1485–9. 10. Kernohan RJ, Todd C. Heparin therapy in thromboembolic disease. Lancet 966;1:621–3. 11. Alpert JS, Smith R, Carlson CJ, et al. Mortality in patients treated for pulmonary embolism. JAMA 1976;236:1477–80. 12. Kavis JA. Heparin in the treatment of pulmonary thromboembolism. Thromb Haemost 1974;32:517–27. 13. Seventh ACCP Consensus Conference on antithrombotic therapy. Chest 2004;126:401s-28s. 14. Becker DM, Philbrick JT, Selby JB. Inferior vena cava filters: indications, safety, effectiveness. Arch Intern Med 1992;152:1985–94. 15. Kanter B, Moser KM. The Greenfield vena cava filter. Chest 1988;93:170–5. 16. Kinney TB. Update on inferior vena cava filters. J Vasc Interv Radiol 2003;14:425–40. 17. Rohrer MJ, Scheidler MD, Wheeler HB, et al. Extended indications for placement of an inferior vena cava filter. J Vasc Surg 1989;10:44–50. 18. Golueke PJ, Garrett WV, Thompson JE, et al. Interruption of the vena cava by means of the Greenfield filter: expanding the indications. Surgery 1988;103:111–7. 19. Greenfield LJ, Chok J, Proctor M, et al. Results of a multicenter study of the modified hook-titanium Greenfield filter. J Vasc Surg 1991;14:253–7. 20. Lord R, Benn I. . Early and late results after Bird’s Nest filter placement in the inferior vena cava: clinical and duplex ultrasound follow-up. Aust N Z J Surg 1994;64:106–14.
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21. Decousus H, Leizorovicz A, Parent F, et al. A clinical trial of vena caval filters in the prevention of pulmonary embolism in patients with proximal deep-vein thrombosis. N Engl J Med 1998;338:409–15. 22. Decousus H. Eight year follow-up of a randomized trial investigation vena cava filters in the prevention of PE in patients presenting a proximal DVT: The PREPIC trial [abstract]. J Thromb Haemost 2003;1(Suppl 1):OC440. 23. Grassi CJ, Swan TL, Cardella JF, et al. Quality improvement guidelines for percutaneous permanent inferior vena cava filter placement for the prevention of pulmonary embolism. J Vasc Interv Radiol 2001;12:137–41. 24. Pais SO, Tobin RO, Austin CB, et al. Percutaneous insertion of the Greenfield inferior vena cava filter: experience with ninety-six patients. J Vasc Surg 1988;8:460–4. 25. Mewissen MW, Erickson SJ, Foley WD, et al. Thrombosis at venous insertion sites after inferior vena caval filter placement. Radiology 1989;156:155–7. 26. Simon M, Athanasoulis CA, Kim D, et al. Simon nitinol inferior vena cava filters: initial clinical experience. Radiology 1989;172:99–103. 27. Carabasi RA III, Moritz MJ, Jarrell BE. Complications encountered with the use of the Greenfield filter. Am J Surg 1987;154:163–8. 28. Berland LL, Maddison FE, Reginald VM. Radiologic follow-up of vena cava filter devices. AJR Am J Roentgenol 1980;134:1047–52. 29. Messmer JM, Greenfield LJ. Greenfield caval filters: longterm radiographic follow-up study. Radiology 1985;156:613–8. 30. Atkins CW, Thurer RL, Waltman AC, et al. A misplaced caval filter: it’s removal from the heart without cardiopulmonary bypass. Arch Surg 1980;115:1133–5. 31. Castaneda F, Herrara M, Cross AH, et al. Migration of a Kimray-Greenfield filter to the right ventricle. Radiology 1983;149:690–1. 32. Friedell ML, Goldenkranz RJ, Parsonnet V, et al. Migration of a Greenfield filter to the pulmonary artery: a case report. J Vasc Surg 1986;3:929–31. 33. Wingerd M, Bernhard UM, Maddison F, et al. Comparison of caval filters in the management of venous thromboembolism. Arch Surg 1978;113:1264–9. 34. Scurr JH, Jarrett PE, Wastell C. The treatment of recurrent pulmonary embolism-experience with the KimrayGreenfield vena cava filter. Ann R Coll Surg Eng 1983;65: 233–4. 35. Todd GJ, Sanderson J, Nowygrad R, et al. Recent clinical experience with the vena cava filter. Am J Surg 1988;156: 353–8. 36. Millward SF, Oliva VL, Bell SD, et al. Gunther Tulip Retrievable Vena Cava Filter: results from the Registry of the Canadian Interventional Radiology Association. J Vasc Interv Radiol 2001 Sep;12(9):1053–8. 37. Asch MR. Initial experience in humans with a new retrievable inferior vena cava filter. Radiology 2002 Dec;835–44. 38. Stein PD, Kayali F, Olson RE. Twenty-one year trends in the use of inferior vena cava filters. Arch Intern Med 2004; 164:1541–5. 39. Sarasin F, Eckman M. Management and prevention of thromboembolic events in patients with cancer related hypercoagulable states: status. J Gen Int Med 1993;8:476–86. 40. Rogers FB, Shackford SR, Wilson J, et al. Prophylactic vena cava filter insertion in severely injured trauma patients: indications and preliminary results. J Trauma 1993;35:637– 42. 41. Leach TA, Pastena JA, Swan KG, et al. Surgical prophylaxis for pulmonary embolism. Am Surg 1994;60:292–5.
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