Pulmonary thromboembolism (PE) is a common condition and
represents a major cause of morbidity and mortality. The true
incidence has not been exactly established, but it is estimated
that between 570,000 and 750,000 cases of PE occur annually in the
United States. The fatality rate resulting solely from PE is
estimated to be approximately 50,000 patients, and PE is thought to
be a contributing factor of mortality in 20,000 to 150,000
Systemic anticoagulation is the therapeutic mainstay for
patients with PE.3,4 However, anticoagulation therapy has been
reported to fail in some patients. In a metaanalysis of patients
treated with anticoagulation, the rate of fatal PE among patients
presenting with DVT was 0.4%, and the rate of fatal PE for patients
presenting with PE was 1.5%.5 The complication rate associated with
anticoagulation also has been reported to be as high as 26%, with a
fatality rate between 5 and 12%.6,7 In addition, contraindications
for anticoagulation exist in certain patients (table 1).
Approximately 80 to 90% of acute PE cases originate from
thrombus migrating from the pelvic or lower extremity veins.
Therefore, inferior vena cava (IVC) filters have been designed to
prevent the migration of thrombus into the lung. It is estimated
that, in the United States, 30,000 to 40,000 IVC filters are being
Indications for filter placement
Inferior vena cava filters are effective in preventing PE.
However, their implantation also results in significant costs, as
well as the potential for serious complications. Therefore, it is
mandatory to establish the presence of deep venous thrombosis (DVT)
or PE and to document the need for an IVC filter. The presence of
DVT in the absence of a contraindication to anticoagulation is not
an indication to place a filter. A recent, randomized trial
compared the usefulness of IVC filtration versus anticoagulation in
patients with DVT who were at risk for PE. This study showed that
patients who had filters placed had an initial beneficial effect in
the prevention of PE, but this was counterbalanced by an excess of
recurrent DVT. In addition, there was no difference in mortality
between the patients who received a filter and those patients who
Absolute indications-The absolute indications for IVC filter
placement are: 1) contraindication to conventional anticoagulation
(table 1); 2) recurrent PE despite adequate anticoagulation; 3)
significant complication of anticoagulation treatment; and 4)
massive PE resulting in hemodynamic compromise.
Relative indications-Relative indications for IVC filter
placement include: 1) large, free-floating thrombus in the IVC or
iliac veins; 2) prophylactic placement in high risk patients; (i.e.
patients status post-pulmonary embolectomy, DVT in patients with
underlying severe cardiopulmonary disease9); 3) presence of any
relative contraindication to anticoagulation; 4) non-compliance
with anticoagulation treatment.
Controversial indications-Some indications for placement of an
IVC filter currently are controversial, including the prophylactic
use of filters in patients without DVT or PE but who are at high
risk due to neurosurgical or orthopedic procedures or trauma,10-13
and patients with incurable malignancy.14,15
Contraindications for placing an IVC filter are not
well-defined. Certainly the risk of filter insertion has to be
balanced with the morbidity and mortality associated with PE.
Because up to 30% of patients with acute PE who do not receive
appropriate treatment die secondary to a complication from
recurrent PE, no absolute contraindication for insertion of an IVC
filter exists. However, potential risks and benefits always have to
be thoroughly considered, especially in the following
Uncorrectable coagulopathy-This can be an occasional finding in
patients after extensive trauma or with liver or multisystem
failure. We generally correct preexisting coagulopathies whenever
possible. In patients where the coagulopathy is not correctable,
the Simon Nitinol filter, which uses the smallest introducer system
(9 F) and allows placement from an antecubital fossa approach, is
the filter that we prefer to use.
Uncontrolled, untreated bacteremia-Although not a preferred
practice, IVC filters can be inserted in patients with uncontrolled
bacteremia. However, the assistance of an infectious disease team
and the immediate institution of antibiotic treatment are mandatory
in this setting. It has been shown in animal experiments that
appropriate antibiotic therapy will sterilize septic emboli caught
in IVC filters.16
Pediatric patients-Indications for filter placement in young
patients should be extremely strict, as the long-term effects of
filter placement, as well as the mechanical durability of the
devices, are not well known.
Types of inferior vena cava filters
There are currently 6 IVC filters that are FDA approved: the
standard 24-F stainless steel Greenfield filters (SGF); the newer,
12-F, over-the-wire stainless steel Greenfield Filter (SGF-OTW);
the titanium Greenfield filter (TGF); the Simon Nitinol filter
(SNF), the Vena Tech filter (VTF) and the bird's nest filter (BNF).
All these devices are designed for permanent implantation.
Naturally, all have individual strengths and weaknesses.
Standard stainless steel Greenfield IVC filter
The stainless steel 24-F Greenfield filter (Meditech, Boston
Scientific Corp., Natick, MA) was introduced in 1973. It was
primarily designed for surgical implantation through a venotomy,
although percutaneous implantation is possible.17 The filter has to
be delivered through a 24-F (29.5-F outer diameter) sheath using a
stiff delivery capsule. The filter is made from stainless steel,
and thus can generate significant artifacts during magnetic
resonance imaging (MRI).
This filter has the longest track record and has been shown to
be fairly reliable and durable. Recurrent PE and IVC patency rates
for this filter range from 2 to 5% and 90 to 98%,
respectively.9,17-19 However, the papers reporting these data
generally lack adequate follow-up, so that the true incidence for
recurrent PE or IVC occlusion remains unknown. Complications have
frequently been reported with this device (figure 1). There is a
high incidence (up to 41%) of insertion site thrombosis, with one
reported case of jugular vein thrombosis resulting in fatal sigmoid
sinus thrombosis and death.17,20 Other complications reported with
this filter include vena cava perforation (in up to 70% of patients
followed with CT) and filter migration (seen in 30 to 49% of
patients, and usually in the caudad direction).21-23 This filter
also has been shown (in vitro) to have decreased clot trapping
capabilities if tilted more than 15 degrees in relation to the long
axis of the IVC.24
Over-the-wire stainless steel Greenfield filters
A newer version of the SGF, the over-the-wire filter, was designed
for percutaneous implantation (figure 2). It requires a 12-F
introducer system (15.6-F outer diameter) and is intended for
placement over a .035" super stiff guidewire. The deployment
mechanism is simple and allows very accurate positioning of the
filter. In order to minimize the chance for clot formation on the
legs of the filter, we infuse saline through the delivery device
during filter insertion. The device can not be used in patients
with an IVC larger than 28 mm in diameter. It is MRI safe, but
causes moderately large artifacts. In vitro studies have shown that
magnetic forces and torque do not cause filter migration. As with
the larger system, tilting beyond 15 degrees adversely affects the
efficacy of the filter (figure 3). Patency and effectiveness rates
are similar to the SGF.25 Compared to the TGF, the SGF-OTW has been
shown to have similar degrees of tilting, but the pattern of the
struts was more uniformly symmetric with the SGF-OTW.26
Titanium Greenfield filter (TGF)-
The TGF (Meditech/Boston Scientific Corp.) device is constructed of
a titanium alloy which is MRI compatible. It has a conical design
that is similar to both of the stainless steel Greenfield filters
(figures 4,5). The TGF is designed for percutaneous insertion and
requires a 12-F (14.3-F outer diameter) introducer sheath. Its
deployment mechanism is simple and allows for precise positioning.
The filter carrying capsule is rigid, and kinking of the introducer
sheath during its insertion in tortuous iliac veins has been
reported (figure 6). As the device is not inserted over a
guidewire, significant tilting within the IVC can occur (figure 5);
this occurs more often when the device is placed from a left common
femoral vein approach. Earlier versions of the device had a high
caval perforation rate (figure 7), but modifications in the design
of the leg hooks have reduced the frequency of this
complication.27-29 Other problems reported in conjunction with this
device include incomplete opening of the filter with or without
crossing of the filter legs at the time of deployment (figure 8A).
When this occurs, careful manipulation of the filter legs can be
attempted using steerable guidewires or catheters in order to
correct this problem (figure 8B). However, care should be taken not
to dislodge the filter. Alternatively, a second filter can be
placed in a more cephalad position. The incidence of incomplete
opening can be reduced if heparinized saline is continuously
infused through the delivery device during filter deployment.
The TGF should be placed when the vena cava is smaller than 28
mm in diameter. The rate of PE recurrence has been reported to be
3.3%. In one series, the caval patency rate at 30 days of follow-up
was reported to be 100%. However, we have seen a number of patients
with IVC thrombosis in the presence of a TGF. Insertion site
thrombosis rate has been reported to be 8.7%.28,30
LGM or Vena Tech filter
-The Vena Tech or LGM filter (B. Braun/Vena Tech, Evanston, IL) was
introduced in France in 1986. The filter is a self-centering device
which is conically-shaped, with stabilizing side rails. The filter
is made of phynox, a non-ferromagnetic alloy, and is MRI
compatible.31 The total filter length is 38 mm and its base
diameter is 30 mm, which restricts its use to caval sizes of 28 mm
or less.32 Placement of this device requires a 12-F introducer
sheath (14.6-F outer diameter). The deployment mechanism is simple
and allows for fairly precise filter placement (figure 9).
Incomplete opening of the device can be seen; however, this problem
can be minimized by constantly infusing heparinized saline through
the lumen of the pusher rod. The hooks on the device prevent
cephalad migration. However, caudal migration can occur, though
this is usually limited to less than one vertebral body height. The
caval patency rate is 92% at one year post-filter placement,33 and
70% at 6 years.34 The recurrent PE rate is 0 to 3.8%.33,34 Tilting
up to 15 degrees has been described in 2 to 9% of filters placed,
but does not seem to be clinically significant; tilting greater
than 15 degrees has not been reported. Incomplete opening of the
device was initially a problem when it was deployed from a jugular
vein approach. However, modifications in the filter design have
made this problem less frequent.23,35 Access site thrombosis has
been reported in 7 to 23% of patients.35 Guidewire entrapment
between the flat-wire struts of this filter can lead to filter
displacement. Therefore, fluoroscopy should be used whenever
placing a central venous line in the presence of this filter.
Bird's nest filter (BNF)-
The Gianturco-Roehm bird's nest vena cava filter (Cook Inc.,
Bloomington, IN) is made from stainless steel and generates the
largest magnetic susceptibility artifact of all the filter devices.
It also has the potential to torque in a high strength magnetic
field. However, the BNF will not get displaced in a 1.5 T
The BNF was introduced for clinical trials in 1982, and is
currently the only FDA approved device for placement in patients
with caval diameters larger than 28 mm (the so-called mega-cavae)
(figure 10).37 It consists of 2 v-shaped struts which are connected
with four 25-cm
long wire strands.38 The device is inserted through a 12-F (14-F
outer diameter) introducer sheath. The delivery mechanism of the
BNF is probably the most complex of all the IVC filters, but it is
still straightforward once the operator is familiar with the
device. Because of the length of the anchoring struts, the filter
requires the longest segment of IVC (approximately 7 cm) for
appropriate insertion. Therefore, it is more difficult to place in
patients who have a short distance between the renal veins and the
iliac vein confluence.
The BNF can be inserted from both a jugular or femoral vein
approach, with the 2 kits only differing in the length of the
delivery catheters (75 vs 40 cm). In its current form, the device
has a push button mechanism attached to a 3-finger handle which
allows separation of the guidewire pusher from the filter. This
filter delivery system also is prone to kinking in patients with
very tortuous iliac veins, especially if placement from the left
common femoral approach is attempted. Phlegmasia cerulea dolens
resulting in death has been reported in association with the BNF.39
The rate of symptomatic IVC occlusion has been described to be
increased in cases where the wires prolapse beyond the level of the
cranial anchoring struts into a suprarenal location.40 Rotating the
delivery sheath 360 degrees 2 or 3 times during deployment of the
filter wires results in a reduction in wire prolapse.41 Several
cases of caval perforation have been described, and access site
thrombosis has been found to occur in about 2% of cases.42,43 In
one study, the IVC occlusion rate was reported to be 2.9%, and the
rate of recurrent PE was 2.7%. However, this study lacks systematic
follow-up and, therefore, the real numbers are unknown.38
Simon nitinol filter (SNF)-
The SNF (Bard Radiology, Covington, GA) was introduced in 1988 and
received FDA-approval in 1990. It is made of a nickel-titanium
alloy that has a thermal memory. At temperatures below 27 degrees
Celsius, this alloy material becomes malleable, thus facilitating
its insertion through tortuous vessels.44,45 The filter is MRI
compatible and causes only minimal artifacts. It consists of a
cloverleaf dome and six conically arranged legs. Both the legs and
the dome provide filtration.
The SNF filter has the smallest introducer sheath (9-F outer
diameter) which, in combination with its flexibility, allows
placement via an antecubital vein approach (figure 11). The filter
measures more than 8 cm in length in the delivery system, but
foreshortens to about 4.5 cm upon deployment. This foreshortening
causes difficulty in precisely positioning this device, as compared
to the other filters. Occasionally, it can take several hours for
the filter to form to its final shape (figure 12). Because the IVC
can enlarge during induction of general anesthesia, it is
recommended that insertion of the SNF be limited to IVCs with a
maximum diameter of 24 mm if the patient is to undergo general
anesthesia within 2 weeks of filter placement. Otherwise, the SNF
can be used if the IVC is 28 mm or less in diameter.
In-vitro studies have shown that this filter has superior clot
trapping performance when compared to the SGF. Tilting does not
affect the efficiency of the SNF.22,45,46 The IVC patency rate is
reported to be between 50 and 91%.23,47 Recurrent PE occurs in 0 to
4.8% of patients with a SNF;48,49 asymptomatic caval penetration by
filter legs has been reported in up to 25% of patients.48,49 Filter
migration has also been encountered, but seems to be rare
-Informed consent should be obtained for all IVC filter placements.
A platelet count, activated partial thromboplastin time (PTT) and
prothrombin time (PT) should be available. Correction of any
coagulation disorders and discontinuation of any heparin use at the
time of the procedure is recommended. The platelet count should be
at least 50,000, and preferably 100,000. Patients should be NPO or
on clear liquids for at least 4 hours prior to the procedure. We
generally perform the procedure using conscious sedation. An IVC
gram also should be obtained prior to the procedure in order to
evaluate for IVC patency, extent of thrombus, diameter of the IVC,
location of the renal veins and iliac confluence, and venous
anomalies. If the patient is allergic to iodinated contrast agents
or has impaired renal function, carbon dioxide can be used as an
alternative contrast agent. If the inflow from the renal veins is
not well delineated or there is concern for an accessory renal vein
(figures 2,9), the renal veins can be selected with a catheter.
Spot films to document the level of the renal veins can then be
taken without the need to perform venography. Without an IVC gram,
either ultrasound or MRI is mandatory in order to document caval
patency or to exclude a venous anomaly.50
The IVC is an elliptical vessel, with its greatest diameter
being in the left to right direction. Therefore, we measure the IVC
diameter on an AP IVC gram. This can be facilitated by the use of a
calibrated pigtail catheter designed for IVC filter placement.
These catheters (PIG-CAVA, Cook Inc., Bloomington, IN) have metal
markers that are 28 mm apart (figure 12A). Alternatively, a
radiopaque ruler can be positioned under the patient. The diameter
of the IVC can vary depending on the hydration status of the
patient and can increase during a Valsalva maneuver. IVC diameters
greater than 28 mm (corrected for magnification) require placement
of a BNF.
Ideally, the filter should be placed immediately below the level
of the renal veins, with the tip of the conical filters (SGF, TGF,
and VTF) extending to the level of the inflow of the renal veins.
Suprarenal placement of an IVC filter has been described in
patients with renal vein thrombosis, those with extensive thrombus
in the infrarenal IVC (figure 4), renal transplant recipients,
pregnant women, and women of child bearing age. In patients with
recurrent PE from upper extremity or internal jugular vein
thrombosis, placement of filters in the superior vena cava can
become necessary and has been described.51
The route preferred by many interventionalists for vena cava
filter insertion is the right common femoral vein, followed by the
right internal jugular vein. The latter offers the advantage of the
straightest access to the IVC and results in the least amount of
tilting of the conical filter devices. If the femoral access is
chosen, care should be taken to enter the vein at the level of the
mid-femoral head, as lower punctures can result in transarterial
puncture of the vein and the development of arteriovenous fistulae.
This is due to the fact that the proximal superficial femoral
artery is frequently positioned anterior to the vein below the
level of the femoral head (figure 13).52 This complication can be
minimized by the use of a single wall puncture needle.
Once the femoral vein is entered, we place a 5-French dilator
and perform a limited iliac venogram to assess venous patency. An
AP IVC gram is then performed using a calibrated pigtail catheter.
Placement of a ruler under the patient facilitates accurate
placement of the device. Alternatively, bony landmarks can be used
as reference points. After defining the level of the renal veins
and the iliac confluence, the appropriate device is chosen and
placed according to the manufacturer's description.
In patients with tortuous iliac veins or a "deep pelvis", it
should be kept in mind that the delivery capsules for the TGF, SGF,
VTF, and BNF are relatively rigid and prone to kinking the delivery
sheath (figure 6). If difficulty is encountered while trying to
advance a filter through a tortuous iliac vein, the operator should
stop. Once the introducer sheath is kinked, the device can not be
delivered. Several options exist at this point: 1) place a bolster
under the patient's buttocks to reduce the curvature of the pelvis;
2) use an Amplatz super stiff wire (Meditech/Boston Scientific
Corp.) adjacent to the introducer sheath to straighten the vein;
and/or 3) advance the sheath and the filter as a unit across the
tortuous bend in the iliac vein (being careful not to force the
sheath through the wall of the IVC). Otherwise, the SNF, which is
the most flexible device, can be used.
A post-placement IVC gram should be performed through the
delivery sheath in order to document filter position. All the
filters allow passage of catheters and guidewires through their
interstices, but dislodgement or displacement of the filtration
device remains a risk. Of the currently available IVC filters, the
VTF and the SGF-OTW seem to carry the greatest risk for guidewire
entrapment and possible displacement when performing transfilter
Timing of filter placement
-Once the diagnosis of PE or DVT is made, timing of filter
insertion often is debated. Jones et al have proposed guidelines
and introduced three urgency classes. Emergent placement (within
the next 12 hours) is indicated in patients with acute PE or
free-floating iliofemoral DVT in whom anticoagulation is
contraindicated or has failed, in patients with cor pulmonale or
other severe cardiorespiratory compromise, and in patients with
septic emboli. Urgent filter placement (the next available slot) is
recommended for patients with a contraindication to anticoagulation
and DVT (not free-floating). Elective placement is reserved for
cases where filters are placed prior to surgical procedures in
high-risk patients.54 Despite these recommendations, except for
elective situations, we attempt to place IVC filters as soon as the
need occurs, as the morbidity from recurrent PE appears to be
greatest within the period immediately after the diagnosis.
Comparison of filters
The SNF has the smallest introducer sheath. It is the most
flexible device and it allows insertion from many access sites,
including the antecubital fossa. The delivery system should be
flushed with heparinized saline during the procedure. However, the
newer model does not require chilled saline, as did the older
system. If the patient is to undergo general anesthesia within the
next 2 weeks, the filter should be restricted to an IVC not larger
than 24 mm in diameter.
The BNF is the only filter that can be used in the so-called
mega-cavae (IVCs larger than 28 mm).37 The alternative to using a
BNF in mega-cavae would be biliac filter placement. The maximum
recommended diameter for the BNF is 40 mm. This device also has
been used as a temporary filter.55
The Greenfield and the VTF filters are the easiest devices to
deploy and they also allow very accurate positioning. These filters
should have saline infused during deployment in order to minimize
clot formation on the legs of the filter (which could result in
incomplete opening of the devices).
Patients with IVC filters can undergo MRI. However, the BNF and
stainless steel Greenfield filters create large artifacts. The
least amount of magnetic susceptibility artifacts can be found with
the TGF, SNF, and VTF filters (table 2).
All filters have shown high efficiency rates in preventing
recurrent PE.24,46,48,56 However, access site and IVC thrombosis
can occur with all of these devices. The true incidence of these
events is unknown due to the lack of well-controlled studies and
follow-up. Wide variations are reported in the literature, but it
is likely safe to say that significant thrombosis at the venous
access site occurs in approximately 5 to 10% for all filters. IVC
thrombus formation seems to occur in approximately 10 to 20% of all
patients; however, only a fraction of these patients become
Other complications such as filter migration, IVC penetration,
and filter fracture have been described for all devices, but their
true incidences are unknown. The extent of tilting seems to affect
the clot-trapping capability of the Greenfield filters in vitro.
has been shown to occur with both the over-the-wire and not
over-the-wire systems.25,26,58 However, the clinical significance
of tilting in vivo is poorly defined.24 Tilting of a conical-shaped
filter can be minimized by placing the filter from a right internal
jugular approach.59 The left common femoral and left internal
jugular vein approaches are known to result in the largest degree
of tilting and should therefore be avoided whenever possible.
Alternatively, other filters should be used if these approaches
Temporary (removable) IVC filters are currently being used in
Europe and are undergoing clinical trials in the United States.
They have been found to be beneficial in patients who are in need
of short term caval filtration due to a temporary risk for
anticoagulation, such as the postoperative or post-trauma state.
Occasionally, these devices have been used in conjunction with
thrombolysis of the IVC or iliac veins in order to reduce the risk
of embolizing large clot volumes into the lungs (figure 14).
Temporary filters need to be removed within 2 to 3 weeks because
after that time neointima formation causes them to adhere to the
vessel wall. Removing a temporary filter may be difficult if it is
filled with thrombus or if a significant risk for PE persists.
Another class of filters currently under development are
retrievable filters. These filters can be left in place permanently
if necessary, but are retrievable during the first 2 or 3
Each filter has its own advantages and disadvantages. Therefore,
radiologists involved with the placement of IVC filters should be
familiar with all of these devices in order to be able to customize
the use of each device for individual patients. For instance, in
patients with a large caliber IVC, only the BNF should be utilized.
In cases where only a short segment of infrarenal IVC is available
or where precise positioning is crucial, the SGF-OTW or VTF are
preferable. For an IVC of less than 18 mm in diameter, the SNF may
not form immediately or may "kink" the IVC during formation (figure
12). For tortuous veins or limited access, the flexibility and
small caliber sheath of the SNF are major advantages.
The systematic use of filters in patients with DVT in the
absence of a contraindication for anticoagulation can not be
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