The role of vena caval interruption in patients with venous thromboembolism

The Role of Vena Caval I n t e r r u p t i o n in P a t i e n t s With V e n o u s T h r o m b o e m b o l i s m David Bergqvist

E C A U S E D E E P vein thrombosis (DVT)

B may be complicated with pulmonary embolism (PE), which in a certain frequency is fatal, it seems logical that interruption of the femoral or caval veins has been attempted to avoid this serious outcome. After John Hunter attempted this in 1784 to limit thromboembolism, 1 Homans 2 and Allen 3 suggested ligation of the femoral vein as an effective method to prevent PE. However, it was rapidly shown that this distal venous ligature was not effective.4 Nonetheless, it was used even during the 1980s in neurosurgical patients. 5 Ligation of the inferior vena cava was attempted by several surgeons. The first use of this method was ascribed to Hunter, and it was then used by Trendelenburg, John Homans, and Alton Ochsner. 1,6 However, the ligation of a major vein is not an attractive option, and reports on serious sequelae of venous stasis explain why the method has not come into widespread use. Severe edema was reported in at least one third of the patients, and development of leg ulcers was frequent. 7,8 Moreover, ligature was found not to be a guarantee agains t PE because of the development of large collaterals T M and the fact that the ovarian vein may enlarge to the size of a normal vena cava. Collaterals may also develop between veins of the lower extremities and the portal vein with the possibility of hepatic embolization. 16 With the aim of preventing pulmonary embolization while preserving caval patency; various types of plications and caval clips were developed. There was no consensus on which was better, plication or clipping, but insertion of clips is technically more simple with less extensive dissection necessary. Several types of clips have been constructed) 7-2~As a modification or intermediate between plication and clipping, stapling of the vena cava may be considered. 21 Performing plication prophylactically in highrisk patients undergoing aortic and colonic surgery resulted in a higher complication rate than that observed in a nonrandomized control group and was, thus, not recommended by Carmichael and Edwards. 22 Using a similar study design (ie,

historical controls), Fuller and Willbanks 23 concluded that prophylactic teflon clips during laparotomy were safe, but in fact, they did not show a prophylactic effect. However, Korwin et a124 reported that prophylactically applied Clips prevented PE after aortic surgeP2. The frequency of postoperative stasis sequelae were similar with all of these methods for caval interruption.I, 25 Although a few investigators still recommend clips, 26 they are considered by most to belong to the history of medicine. The idea behind caval interruption by clipping, ie, effective filtration with caval patency, was maintained in the development of various types of transvenous filters. As a combination of total caval interruption and percutaneous transvenous technique, various balloon techniques may be considered. 27-3~Before the detachable devices came into use, there were experimental studies of sieves and balloons that were attached to a catheter for later removal.Z7, 31 TYPES OF TRANSVENOUS FILTERS

The various types of transvenous filters are catalogued in Table 1. The ideal type of filter has been discussed by Appleberg and Crozier 32 and should fulfill the criteria listed in Table 2. The fundamental principle must be to trap emboli and at the same time maintain patency. There are many filters on the market (see below), which indicates that we have not yet found the ideal one. There are several reviews on various filters. 33-37 Mobin-Uddin Umbrella Filter This device was developed in the mid1960s 38-41 and is constructed in the shape of an

From the Department of Surgery, University Hospital, Uppsala, Sweden. Supported by the Swedish Medical Research Council Grant No. 00759 and the Swedish Heart & Lung Foundation. Address reprint requests to David Bergqvist MD, PhD, Professor of Vascular Surgery, Department of Surgery, University Hospital, S-751 85 Uppsala, Sweden. Copyright 9 1994 by W..B. Saunders Company 0033-0620 / 94/3701-000355. O0/ 0

Progress in Cardiovascular Diseases, Vol XXXVII, No 1 (July/August), 1994: pp 25-37

25

26

DAVID BERGQVIST Table 1. Characteristics ofTransvenous Caval Filters

Filter Greenfield Greenfield Bird's Nest Vena-Tech (LGM) Simon Nitinol Amplatz Gfinther Mobin-Uddin

Carrier Size (French)

Introducer size (FrenchOD)

Max Caval Diameter Recommended(mm)

24 12 11 10 7 12 10

29 14 14 12 9 14 12

26 28 40 28 28

28

Material

Configuration

Stainless steel Titanium Stainless steel Elgilog Nickel, titanium Stainless steel Stainless steel Stainless steel with silastic cover

Pyramid Pyramid Random tangle Pyramid with lateral struts Pyramid and dome Inverted pyramid with hook Elongated basket with struts Umbrella

Abbreviations: Max, maximum; OD, outer diameter.

inverted umbrella from stainless steel with a thin silastic coverage. Originally, it expanded to 23 mm, but because of frequent migration problems, the expandable dimension was increased to 28 mm. The high frequency of caval occlusion with stasis sequelae, the high rate of recurrent PE and proximal migration have resulted in withdrawal of this filter. 4~ GreenfieM Filter The stainless-steel type was first described in 1973. 42 The shape is conical with six hooks with a base diameter Of 30 ram. The shape is de: signed to allow the cone to be filled to 70% by thrombus material and results in only a 50% reduction in effective cross-sectional area. 42-44 Emboli as small as 2 mm are trapped. 45 Both experimentally and clinically, lysis of entrapped thrombotic material has been shown. 42,46 The original type is inserted through a 24 French (F) system (outer diameter, 29 F ) w h i c h is a main disadvantage. The stainless-steel system has been developed with a small diameter introducer system (12 F). 36 To be able to use smaller introducer size for percutaneous insertion, a titanium version of the Greenfield filter has been developed. 47 This system can be used in a 12 French carrier. It has a broader base than the Table 2. Criteria of an Ideal Caval Filter

Nonthrombogenic, biocompatible, degradation-resistant High filter efficacy Maintenance of blood flow Secure fixation to the caval vein wall Rapid and safe percutaneous insertion No mortality No complications or sequelae Nonferromagnetic (to allow MR imaging) Retrievability Low cost

stainless-steel system (38 v 30 mm). It also differs in flexibility, height, and hook angle. 48 Initially, this system had a high frequency of caval perforation, 49 but by hook modification, this problem has largely been overcome. 5~ Bird's Nest Filter This filter was first tested in 1982, reported in 1984, and consists of a random configuration of stainless-steel wires formed by the operator. 36,51-53 The shape of wires, when applied, looks like a bird's nest. It can be introduced through an 11 French system and used in large caval veins (up to 40 mm in diameter). The frequency of oversized vena cavae is between 2% and 3% and in these patients, other types of filters are less well-suited because of their tendency to migrate. 54 Vena Tech or L. G. Medical (LGM) Filter This filter can be considered a modification of the stainless-steel version of the Greenfield filter. In addition to t h e basic cone-shaped structure, this filter has six mural side rails in parallel to the walls of the vena cava, which should be a guarantee against tilting. The side rails have small hooks. The sheath size is 12 F. It is easy to introduce and has few complications, but the frequency of caval occlusion seems high.55 Simon Nitinol Filter This filter has unique properties in that it is an alloy (nitinol) made of nickel and titanium. The wires are straightened at cooled temperature (4 to 10~ but reform to the filter shape at body temperature. 56-58This filter needs the small-

VENA CAVAL INTERRUPTION

27

est insertion devices (a sheath with a diameter of 9 F). 13.

Amplatz Filter This is another cone-shaped filter made from stainless steel. 59 There is addition of a loop to allow for retrieval or repositioning.6~ Experience to date is limited with this device.

14. 15.

Gi~nther Filter This filter consists of a helix of stainless-steel wires with an inverted cone a b o v e . 62,63 The experience with this filter is also limited.

16.

INDICATIONS

18.

There are four indications for filter application in patients with venous thromboembolism (VTE) on which clinicians generally agree34,36: 1. Contraindication to anticoagulant therapy. 2. Severe bleeding complications on adequate anticoagulant therapy. 3. PE in spite of adequate anticoagulation. 4. Postpulmonary embolectomy. There are several additional indications that have been discussed and recommended by various authors. These can all be considered relative and must be studied in a systematic way before definite conclusions can be made. 5. Extensive, free-floating thrombi in iliac or caval veins.44,64-67 6. Remaining DVT after an initial embo-

17.

tients with previous thromboembolism.44,sl,s2,ss-s7 Thromboembolism in pregnant women as an adjunctive therapy to heparin or alone if heparin is contraindicated.8s-91 Failure of a previous d e v i c e . 59,67,92,93 In patients undergoing pulmonary thrombendarterectomy because of pulmonary hypertension caused by chronic proximal pulmonary artery thrombosis. 94 Patients with renal artery bypass crossing the vena cava anteriorly.8 Adolescents with DVT and PE, especially in trauma situations.95 Patients developing DVT after kidney or heart transplantation.96,97 CONTRAINDICATIONS

Contraindications are relative, and not all are agreed on by different investigators: 1. Severe coagulopathy with risk for bleeding that perhaps is more theoretical in this group of patients. 2. Obstructing thrombi in the vein path needed for filter insertion. 3. Noncompliance with postinsertion bedrest, especially if large introducer systems have been used (24 F). 4. Septic embolization and septicaemia. However, septic PE has also been considered an indication.8~ 5. Metastatic cancer. 98,99

IUS. 68

7. Prepulmonary embolectomy.69 8. As primary treatment for VTE in patients with malignant disease. 7~ 9. Suprarenal placement in patients with renal cancer with renal vein tumor thromb u s 76-78 o r when nontumor thrombi extend to or above the renal veins or are located in the renal veins.79 Suprarenal placement has also been recommended in fertile women. 10. Prophylactically in patients with minimal cardiopulmonary reserve capacity and severe pulmonary hypertension.67,8~ 11. Patients with chronic obstructive pulmonary disease and DVT. 84 12. Prophylactically in high-risk situations (for instance major surgery and fractures in the elderly or spinal cord injury) in pa-

THE POTENTIAL NEED FOR CAVAL FILTERS

Some of the indications for filter insertion seem obvious and acceptable. Moreover, the insertion procedure has been simplified, and both the short- and long-term safety seem reasonable. However, there is almost a complete lack of controlled and prospective studies. On the other hand, patients with the classical indications are rare, making it difficult to perform prospective trials. One danger with new technical innovations is the tendency to be less strict concerning indications, and, before implementing new techniques, this author is convinced that randomized studies should be performed. There are various ways to look at the need for filters. An interesting phenomenon regarding caval filters is the huge difference in their use

28

geographically. In the United States, there are between 30,000 and 40,000 filters inserted annually, 1~176 whereas in Sweden, the number is 20 to 30 (140 v 3 per million inhabitants). Calculations of the need for filters may be based on: 1. Accepting expanding indications as they have been published, which, according to this author, is hardly acceptable. 2. Prospective calculations of accepted indications, which would seem important but have not been made so far. 3. Extrapolations from epidemiological data (see below). 4. Randomized studies, which would be the correct way from a strict scientific standpoint to determine the need for filters, but such studies are not easy to perform and, in fact, almost none exist. ~0~ With the aim to evaluate the potential need of filters an epidemiological survey was performed to register and analyze all patients with VTE during 1 year (1987) in Malta6, Sweden2 ~ Prospective data were obtained from autopsies (autopsy rate, 80%), phlebographies, lung scintigraphies (no pulmonary angiograph was performed), and patients undergoing hip fracture surgery as a high-risk group. Among the 435 patients with thromboembolism at autopsy, there were 141 with lower extremity DVT and serious PE. Of these, 93 had a potentially curable disease, but only 1 had clinical symptoms of DVT premortem. Of the 366 positive phlebographies, 235 had proximal DVT, of whom 8 had a serious embolization. Two of these had a potentially curable disease and both were old (86 and 87 years old, respectively). Of 44 scintigraphies, positive for PE, 2 patients died with serious embolization to the lungs; 1 had bowel gangrene, and the other (89 years old) had no other obvious cause. Of 546 operations for hip fracture, 52 patients died within 3 months, 7 from PE. Of these 7 patients, 3 were younger than 80 years old, but only 1 was otherwise fit and healthy. The other 2 had metastasizing cancer and stroke respectively. If the indications for filter insertion had been proximal DVT, scintigraphically proven PE, and prophylaxis at hip fracture surgery, 825 filters would have to be inserted to prevent 1 fatal PE in patients younger than 80 years old with a curable disease.

DAVID BERGQVIST

PULMONARY EMBOLISM AFTER FILTER INSERTION PE after filter placement is a special concern because the aim of filter insertion is to avoid and prevent PE. Very few investigators have looked systematically for this complication. Murphy et a193found two scintigraphic emboli of 19 investigated patients with LGM filters. A small percentage of symptomatic emboli have been reported. 67,s1,1~176 Fatal PE is rare but has been deseribed.39,55,67,69,sz,100j06-110 Patients with malignant diseases are more prone to develop PE despite seemingly adequate filters. H~ There are several possible mechanisms that can explain pulmonary embolization after filter insertion: 1. Ineffective filtration, a risk that increases with tilting of the filter. 2. Continuous growth of trapped thrombi through the filter. 3. Development of thrombosis on the proximal part of the filter. 59'63'111'112 4. Filter migration to a position where it does not function optimally. 5. Filter retraction from the caval wall at thrombus retention, occurring if some of the hooks have grasped the thrombus, which creates a channel between the filter and the caval wall. 6. Embolization through collaterals that may be lumbar or embolization that may be via the ovarian/spermatic veins. Duplication of the vena cava should also be included in this category. 7. Embolization from thrombi proximal to the filter (arm veins, renal or hepatic veins, the right heart). 8. Incorrect position of the filter. 86 COMPLICATIONS TO CAVAL FILTER

The various types of filters have different risks for complications. Many of the complications are known from single case reports, which indicate very low frequencies. Others occur to a certain extent, but reporting routines vary as do diagnostic methods to detect the complications. Carabassi et al 1~ classified complications in three categories, those because of anatomical variation, those occurring intraoperatively, and those occurring postoperatively.

VENA CAVAL INTERRUPTION

29

Complications During Filter Insertion

Filter-Related Complications

After having passed the learning curve, the overall insertion success rate is usually well above 95 %,81 but there may be practical difficulties in placing the filter in the correct position in u p to 15%. 70314 Reported complications related to filter insertion include: 1. Filter deformation, nl,ns 2. Filter fracture, 59,n6-n8 including fractured struts that may also migrate, a19 3. Premature release or insufficient opening of the filter. 36,93,1~176 4. Improper anatomical placement of the filter: a. the right side of the heart, 46,53,117,122-124 which may cause serious arrhythmias, a24 b. the renal vein. 36,46,59,7~176 c. the hepatic vein. 126 d. the right spermatic veinP 9 e. the iliacvein. 39,41,46,86 f. Iumbal veins. 67 g. suprarenal placement, which can occur with surprisingly little morbidity67 but may sometimes interfere with the kidney function. 78 (As already mentioned, some investigators recommend suprarenal filter insertion in patients with tumor thrombus in the renal vein.) 5. Thrombus formation on the filter, 127which may result in insufficient opening of the filter.

Filter-related complications are as follows: 1. Migration 63,nla12,136 This usually occurs in the distal direction and is rare proximally. 117'137"139However, migration into the pulmonary artery has been reported 39,14~ and seems to occur more commonly with the Mobin-Uddin than the Greenfield filter. In the long-term follow-up of 2,215 patients, Mobin-Uddin et aP 9 described proximal migration to the right side of the heart in 22 of these patients, and of these 22 patients, 4 died immediately. One case of Greenfield filter migration into the right atrium is described where the migration caused an intimal dissection of the posterior descending artery and a subsequent myocardial infarction. 141There are several possible causes of filter migration, including too large a vena cava, inadequate positioning, and massive embolization into the filter with caval dilatation. 2. Tilting and angulation of the filter55,1~ decreases its ability to effectively trap emboli. 3. Caval stenosis. 59,14e 4. Caval occlusion. 55,59,70,103,126,143,144 This has even led to the development of venous gangrene. 1~ It has been claimed that, with regard to caval occlusion, patients with brain tumors belong to a special risk group. 145 Filters that are more effective in trapping emboli may lead to a higher frequency of caval occlusion, as is suggested in a sheep study where L G M and Greenfield filters were compared. 146Filter occlusion is multifactorially related to factors such as filter thrombogenicity, design, and flow patterns. 5. Dislodgement of the filter by gross em-

Complications at Puncture Site Various types have been reported and, in principle, do not differ from complications observed locally with all sorts of catheter techniques. 128 Hematoma formation 67,69,86,129,13~and arteriovenous fistula 1~ have been mentioned. Pneumothorax has been described after jugular insertion, 8m32 as has air embolism. 33,39,125 Of greater concern is the development of DVT at the puncture site. Using routine ultrasonography, this seems to be the case in between 8% to 25%. 50'93'133'134 Dorfman et al I33 found a frequency of 14.3% in the immediate course but normalization at a minimum follow-up time of 182 days in 96.3%. Using routine venography, an even higher frequency (41%) was reported by Kantor et aP 35 5 to 8 days after percutaneous insertion of Greenfield filters; however, the series only contained 17 patients.

boli.lxl,147

6. Erosion of the caval wall. This is fairly common and caused by the sharp anchoring prongs that are present to prevent migration. It may even lead to perforation, which, however, is often asymptomatic. 14s Sometimes it may cause retroperitoneal hemorrhage, 39,63,1~176 possibly of arterial origin, 15~ but is rarely fatal.nsa 51,152 Other organs may be perforated as well, such as the bowel, 32,39,87,115J19,153 the ure-

30

DAVID BERGQVIST

ter, 39 and the liver. 153Perforation may also cause adhesions leading to bowel obstruction.154,15s . Extremity edema in the long run. It may be difficult to establish the cause-effect relationship, because the majority of the patients already had a cause for edema in the form of DVT. Occurrance has been reported in up to one-third of the cases. 125,126 Most are symptom-free, 81 but leg ulcers are not uncommon 156 (3.1% reported by Golueke et al). 81

General Complications The general complications include: 1. Mortality. Many patients belong to a highrisk group and die because of their disease. Death after filter insertion is probably not procedure-related, although fatal PE has been known to occur occasionally.67Johnson et a1117reported a case with filter migration to the right ventricle together with large thrombus masses, PE, and most of the anchoring legs of the Gfinther filter broken. Death occurred at 11 days postoperatively. However, although admittedly arbitrary, it seems relevant to report 30-day mortality because this is the time period for postoperative mortality. It is reasonable to maintain this 30-day mortality period for the sake of comparison. The mortality rates given by different authors vary extremely from 0.5% l~ to almost 20%. 67 One obvious reason for this difference is the postinsertion interval examined. Therefore, in 1981, with a 14-day follow-up, Greenfield reported 4%; in 1984, with a 30-day followup, he reported 14%, and in 1991, also with a 30-day follow-up and in a multicentre setting, he reported 19%. 2. Renal failure. 157 In patients with compromised renal function, the contrast dose must be carefully quantitated and kept as low as possible. It is also important to avoid suprarenal or renal placement of the filter in these patients. The placement of a suprarenal filter per se does not alter kidney function. 158,159 3. Sepsis. a52

FILTER INSERTION

Before insertion, it is important to perform cavography to define venous anomalies (such as duplication of the caval vein), important collaterals, the location of the renal veins, the diameter of the caval vein, and the proximal extension of the thrombus. It was a great step forward when percutaneous introduction of filters was begun. 16~ With today's percutaneous techniques, most filters are inserted with local anesthesia, with general anesthesia used only for specific indications. The right femoral vein is most commonly used for the insertion, followed by the right internal jugular. Using the femoral vein as compared with the jugular vein diminishes the risk for air embolization, pneumothorax, cardiac arrhythmias, and inadvertent release of the filter into the heart or hepatic or renal veins. It is also a more familiar route for the interventional radiologist. 161 If this is not possible (thrombotic occlusion, groin infections, scars, etc), the right internal jugular vein is used. McCowan et a1162 also successfully used the external jugular vein. With Seldinger technique, 163 a flexible guide wire is introduced under sterile conditions and fluoroscopic control, and the location where the filter is to be placed is identified. It is recommended to inject bolus doses of heparinized saline through the carrier system. In case of narrowness or spasm at insertion, heparinized solution may also be used to distend the vein to remove the carrier device. 157 To overcome problems at insertion and to prevent eccentric filter placement, Greenfield et a1164 developed a special guide wire. When there are difficulties in passing the filter to a correct position, it may be possible to grasp it with a Curry loop. 165 In the case of a misplaced filter, it is possible to remove it percutaneously a2L~57,166,~67or even to change the position to an adequate place. 12~ It has been a goal to construct filters and systems for filter retrieval, should the filter be misplaced or no longer necessary; this is now possible. 59a21,157,168 Retrieval becomes impractical and virtually impossible after 2 to 3 weeks. Some investigators recommend the use of oral anticoagulants in all cases where their use is not contraindicated, because the filter is thrombogenica~ however, other investigators do not feel that this is necessary. 169

VENA CAVAL INTERRUPTION

31

COMPARISON OF FILTERS

When considering the criteria of an ideal filter as listed in Table 1, it is obvious that this filter does not yet exist. Moreover, filter technology is still in the developmental phase. Therefore, conclusions made today may not be relevant tomorrow. Katsamouris et a117~performed in vitro studies of clot trapping and flow dynamics of different filters. In their system, it was shown that the efficiency of the Greenfield filter is very sensitive to position of the top, and when excentric, it looses much of its clot-trapping capacity. Bird's nest, Simon nitinol, and G/inther filters were superior when flow and clottrapping properties were considered together. Clinically, no comparative studies have been performed and probably never will be per-

formed between different filters, both for logistic reasons and because large sample sizes would be needed. The most extensive experience has been obtained with the Mobin-Uddin, Greenfield, and bird's nest filters, where also the longest follow-up periods exist. The first one is no longer available on the market. The majority of publications is comprised of small case series, only a few with more than 50 patients and not all of them consecutive. Many of the reported complications are case reports and do not allow for calculations of frequencies. Many investigators have published several reports, each with accumulation of all patients, thus increasing the numbers. In Table 3, studies reporting more than 30 cases are summarized. As one can see, reporting routines, follow-up

Table 3. Summary of Filter Series of More than 30 Patients

Year

Filter

Castellani et al TM Chimochowski et a141 Cohen et a175 Crochet et a1171 Cull et a1112

Author

1987 1980 1992 1988 1991

2612 G G LGM LGM

Denny et al ~72 Dorfman et a1133

1988 1989

G G

Epstein et a159 Fink and Jones TM Fobbe et a163 Goff et al 1~7

1989 Amplatz 1991 G 1988 GL~nther 1988 G

Golueke et al 8~ Gomez et al ~29 Greenfield ~73 Greenfield et a167 Hye et a169 Jarrell et als6 Kolachalam and Jullian ~45 Magnant et al 1~176

1988 1983 1984 1991 1990 1983 1990 1992

Millward et a155 Murphy et a193 Pals et a133 Pomper and Lutchman 84 Ricco et al TM Roehm et a153 Rohrer et a182 Rose et aP~ Schneider et a198 Walsh et a199 Wingerd et al ~~

G G G G(+) G G G G BN 1991 LGM 1991 LGM 1989 G 1991 G 1988 LGM 1988 BN 1989 G 1987 G 1990 GLinther 1987 G 1978 G

No. of No. of Attempted No. of Follow-Up Patients Insertions Patients Time* Followed Up

36 98 52

111 90 43 186

61

101

264

35 32 41 82 4i 49 68

3 > 34 6.4 6 <257 d 1 wk 6

28 1 37 10

Mortality End of Caval Total Fatal Early Follow-Up Occlusiont PE

0 1 1 3 1

0 1 1 0 0

1 0

19 8 7

49 54

11 14 <21

34 78

88 40 260 181 120 209 59 33 42 64 91 96 76 100 568 260 109 34 41 33

16.4 12.8 34 30 d 18

65

2

99

5 6

0

64 3

0 0 0 0 2 1 1 1 2

4 6 37 35 15

1 3 < 10 wk 8

> 6 wk

6 20.6 <60 wk

1 1 76 56 440

34

2 1/22 4/90 0/15

9

52 165 59 97

<34 wk 11

0

1 2 2 9 5 1 0

1 1

18 1 6 3

9/42

13 5

3/40 1/33 3/103 0 2 2/21 9/59

11

42 22 18 16

7/41

14 5/60 1

8 1 2 3 0 0 0

13

3

8 20 5

1/19

Abbreviations: G, Greenfield; BN, bird's nest. *Values shown are given in months unless otherwise indicated. "i'lf there is one value in this column, this means that it is what has been reported for the whole series. If there are two values, that means that only a limited number was followed up in that respect.

32

DAVID BERGQVlST

times, and follow-up compliance vary considerably: This is also true for indications that are not shown in Table 3. Those factors obviously make comparisons between filters difficult. The dominance of the Greenfield filter is also without doubt. Some important clinical characteristics of filters are listed in Table 4. There is obviously no filter Of evident superiority. When suprarenal filtering is contemplated, the Greenfield filter seems to have the lowest eaval-occ!usion risk and, thereby, would seem not to jeopardize kidney function. When there is a short distance between the most distal renal vein and the iliac vein confluence, it maybe best to use a SimonNitinol filter, which is the shortest. Only the bird's nest filter can be used in caval veins wider than 30 mm. CONCLUDING REMARKS AND FUTURE IMPLICATIONS Caval filtration as a means of preventing PE is

one of the methods within the rapidly expanding field of endovascular surgery and is probably here to stay. The problem at present is to identify correct indications and, thereby, the number needed. Although caval filtration is probably effective in som e instances, there is a remarkable lack of controlled and systematic studies to support the use of this procedure. There are several difficulties when it comes to evaluating the preventive effect of filters on PE: 1. There are no controls in randomized studies. 2. Surveillance for PE with some objective diagnostic method has n o t been made systematically after filter insertion. Table 4. Comparison Between Various Caval Filters PE in Spite Significant C a v a l Proximal of Filter Tilting Occlusion Migration Greenfield steel

:t-

Yes

+

+

Greenfield titanium

+

Yes

+/-

+ ++*

Bird's nest

+

No

+++

Vena-Teeh

+

Yes

++

++

Simon-Nitinol

+

No

+++

-/+

Amplatz

?

-

+

Yes Yes

+++

G0nther

++

+

Mobin-Uddin

+

Yes

+++

+

Abbreviations: - , absent; + / - , not reported; +, ++, +++,

increasing degrees of the presence of indicated complication. * N o migration was reported after filter modification.

3. Autopsy rates are rarely reported and probably low. 4. The natural history of patients with potential filter indications is not known, but, at least in patients with PE, it is not good. 5. In patients without contraindication to oral anticoagulation, this therapy is often instituted as well; therefore, in such cases, it is difficult to isolate the filter's effect. Some of the indications seem reasonable (lack of effect or serious problems with adequate anticoagulation and combination with pulmonary embolectomy or thrombendarterectomy, which are all major therapeutic undertakings). Nonetheless, it would be of great value to prospectively establish the frequency of these indications within the population to reach a reasonable calculation on the need for filters. Concerning the other indications, there is a complete lack of knowledge of risks and benefits in the various suggested situations. Rather than diffusing filter technology, the development of prospective randomized studies would seem to be the correct scientific solution to this problem; however, this is not very easy. Therefore, for exarnp!e, prophylactic use in surgery must be compare d with today's effective pharmacologic methods where the risk of fatal PE is extremely low, and where the safety concerning bleeding complications can be kept in the same order of magnitude as when using placebo. In such a situation, aside from providing equal efficacy and safety, the economic implications must be considered. Without having made the proper analysis, the bias is that filters are not costeffective for prophylactic use in high-risk postoperative situations. I t would also seem desirable that reporting routines be defined in future studies so that at least preliminary comparisons can be made. Becker et a137 have suggested methodological standards that may be used. These are (1) adequate diagnostic evaluation, (2) adequate description of patients, (3) adequate description of patient recruitment process, (4) adequate analysis of severity of thromboembolism, (5) adequate description of treatment, (6) adequate documentation of adverse outcome, (7) unbiased surveillance of patients, and (8) complete follow-up. In addition to the complete follow-up regarding number of patients, a well-

VENA CAVAL INTERRUPTION

33

defined time for follow-up should be considered. AS has been mentioned above; the mortality may vary considerably depending on the duration of the postinsertion follow-up.

Besides constructing an optimal retrievable filter, future research will deal with resorbab!e filters and filters releasing active pharmacologic substances.

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