Vena cava filter

A vena cava filter and placement set includes features which can be visualized solely utilizing sonography. A method of implanting a vena cava filter employs sonography to enable the surgeon to direct the filter to a desired location and ensure that the filter is properly deployed.

BACKGROUND 
1. Field of the Invention 
The present invention relates generally to implantable blood filters. More 
particularly, the invention relates to caval filters having 
sonographically conspicuous features. 
2. Related Art 
Advances in many surgical specialties have saved the lives of many patients 
suffering serious illness or injury, and have improved the quality of life 
of countless others. However, such surgical repair of organs and tissues 
can disrupt the body's plumbing, e.g., the circulatory system, 
sufficiently to give rise to new risks. For this reason, minimally 
invasive techniques have been developed, for example wherein highly 
specialized surgical tools are manipulated from outside a patient's body 
through a catheter or tube inserted through a tiny incision or puncture 
and guided to a surgical site. Yet, both invasive and minimally invasive 
procedures disturb circulation sufficiently so that arterial plaques can 
become dislodged or clots can form in the bloodstream and move with the 
circulation with the body. Such debris, moving along with normal 
circulation, can become lodged in and partially or completely block 
vessels supplying blood and oxygen to critical organs, such as the heart, 
lungs and brain. 
Medication is often used to reduce the likelihood of blood clot formation 
during and after surgery, however, post-operative thrombosis, as such 
blood clot formation is called, remains an important problem to be solved. 
Therefore, filters implantable in a patient's body using minimally 
invasive techniques have been developed. By appropriately positioning such 
filters, dangerous blood clots can be removed from circulation and held in 
a safe location until they can be dissolved by medication or extracted, 
again using minimally invasive techniques. Thus, there has been a 
significant reduction in the incidence of morbidity and mortality due to 
post-operative embolism which occurs when a thrombolus moves from its site 
of formation to block a vessel, becoming an embolus. 
Conventional implantable blood filters employing a variety of geometries 
are known. Many are generally basket or cone shaped, in order to provide 
adequate clot-trapping area while permitting sufficient blood flow. Also 
known are filters formed of various loops of wire, including some designed 
to partially deform the vessel wall in which they are implanted. 
Along with their many functional shapes, conventional filters may include 
other features. For example, peripheral loops or arms may be provided to 
perform a centering function so that a filter is accurately axially 
aligned with the vessel in which it is implanted. In order to prevent 
migration under the pressure induced by normal circulation, many filters 
have anchoring features. Such anchoring features may include sharp points, 
ridges, etc. Finally, conventional filters are known which have specific 
features for facilitating implanting and extracting using catheterization. 
Thus, a surgeon can select from a variety of conventional filters, to 
optimize one or another parameter of interest, and implant or extract that 
filter using minimally invasive techniques. 
The minimally invasive techniques mentioned above require that a surgeon 
guide a catheter to a precise location within a patient's body. The 
precise location within the body is visualized using conventional x-ray 
imaging and marked on the patient's body with marker or using x-ray 
fluoroscopy during surgery. The position of the catheter or other 
instrument within the body is visualized using similar techniques. As is 
well-known, x-rays, a form of ionizing radiation, produce an image showing 
by variations in image density corresponding variations in transmission 
density indicative of the position of various anatomical structures and of 
the instrument introduced into the body by the surgeon. In order to 
improve the fluoroscopic image of soft tissues, such as blood vessels, 
contrast media are sometimes introduced into a vessel to be imaged. An 
instrument which might otherwise be radiologically transparent may also be 
given a radiopaque tip or other feature. However, exposure to ionizing 
radiation or contrast media is contraindicated for a significant number of 
patients, such as pregnant women or patients exhibiting anaphylactic 
reactions to contrast media. 
SUMMARY OF THE INVENTION 
What is desired is a filter which is implantable in vivo in a human blood 
vessel, without the problems or disadvantages noted above. 
In one embodiment, the invention may be realized in a filter, implantable 
in a blood vessel solely by sonographic visualization. Such a filter may 
include one or more wires arranged to trap blood clots without 
substantially interfering with normal blood flow; and an echogenic feature 
on at least one wire located in a position which during deployment of the 
filter remains fixed relative to one end thereof, so the filter position 
can be accurately determined solely by sonographic visualization. In such 
a filter, the echogenic feature may, for example, be a bead or a tube. 
When the feature is a tube, a marker wire may pass through the echogenic 
tube. The marker wire may include a plurality of echogenic markers, 
whereby correct visualization in a sonogram of a true longitudinal slice 
along the filter axis is readily ascertainable by presence in the sonogram 
of the tube in its entirety and each of the plurality of echogenic 
markers. Alternatively, depending on the design of the tube and its 
appearance in a sonogram, correct visualization in a sonogram of a true 
longitudinal slice along the filter axis is readily ascertainable by 
presence in the sonogram of the tube in its entirety. 
Filters embodying the invention may be characterized by several different 
geometries. For example, in one geometry, one or more wires are arranged 
to define a cone-shaped basket attached to the echogenic feature at a 
vertex. The wire may be further arranged to define a substantially 
coplanar flower, the echogenic feature attached thereto at a center 
thereof. The cone and flower geometries may be combined, being joined by 
an outer ring of wire connecting a base of the cone to an outer position 
of the flower. The basket may be defined by substantially radially 
extending wires or by a mesh of wires extending in both radial directions 
and directions transverse the radial directions. Visibility using 
sonography of various geometries using the cone may be further enhanced by 
a plurality of echogenic markers substantially at a periphery of a base of 
the cone. 
Filters embodying the invention may include other enhancements, as well. 
For example, the filter may include a plurality of loops of wire at a 
periphery of the basket, whereby the basket is axially aligned thereby 
during deployment thereof. The filter may also include a plurality of 
echogenic markers substantially at a periphery of the filter, whereby 
deployment thereof can be visualized. 
According to another aspect of the invention, there is a method of 
implanting a blood filter in a blood vessel, comprising steps of: moving a 
blood filter having an echogenic feature located in a position which 
during deployment of the filter remains fixed relative to one end thereof 
through the blood vessel to an implantation site in the blood vessel; and 
during the step of moving, visualizing the echogenic feature of the filter 
and the implantation site sonographically. This method may be enhanced by 
adjusting a sonographic transducer to correctly visualize in a sonogram a 
true longitudinal slice along the filter axis, which is readily 
ascertainable by presence in the sonogram of at least one echogenic 
feature of the filter. 
According to yet another aspect of the invention, a caval filter placement 
set includes a guide wire, a dilator, a sheath for introducing the filter, 
a caval filter including one or more wires arranged to trap blood clots 
without substantially interfering with normal blood flow, and an echogenic 
feature on at least one wire located in a position which during deployment 
of the filter remains fixed relative to one end thereof, so the filter 
position can be accurately determined solely by sonographic visualization. 
Moreover, the guide wire may be bent to facilitate locating a renal vein.

DESCRIPTION 
The present invention will be better understood upon reading the following 
description of several embodiments thereof, in connection with the 
drawings. 
The present invention may be embodied in a set of devices for delivery of a 
caval filter to a desired position in a patient's body, preferably absent 
the use of fluoroscopic guidance. A caval filter embodying aspects of the 
invention is constructed of nitinol wire, in a form which is compact at 
room ambient temperature, but which expands to an operating configuration 
at a patient's body temperature. Alternatively, stainless steel, titanium, 
or other known materials implantable in the human body can be used. The 
filter includes a sonographically conspicuous feature at a distal, leading 
end thereof, by which the filter can be guided into position solely by use 
of sonographic imaging. As used herein, a sonographically conspicuous 
feature is one whose sonographic image, density, shape, etc. contrasts 
significantly with the image produced by other features. In order to 
improve the accuracy of positioning using the sonographically conspicuous 
feature at the distal end of the filter, the filter geometry and 
deployment apparatus is defined in such a way as to avoid foreshortening 
or other movement of the distal end of the filter during deployment. Also, 
sonographically conspicuous features may be included on the guide wire by 
which the filter is inserted and positioned in the patient's body. 
Filters embodying the invention can also include sonographically 
conspicuous features located at the periphery of the filter. These 
features may be located at or near either end of the filter, depending on 
the design of the filter. Placing sonographically conspicuous features at 
the peripheral ends of the filter struts allows the opening of the filter 
to be observed using sonographic imaging only, if so desired. These 
peripheral, sonographically conspicuous features can mark the ends of 
filter struts or crossbars, as well as the ends of the filter. The ends of 
the filter can also be marked separately, as described above. 
One type of sonographically conspicuous feature is one which is highly 
echogenic. A highly echogenic feature reflects a substantial portion of 
the ultrasound energy directed at it at a frequency of interest. A rough 
surface finish, having numerous acoustic interfaces, is more echogenic 
than a smoother surface finish. Also, a larger surface is more echogenic 
than a smaller one having a similar surface finish. Conventional bent-wire 
designs have been found not to be sonographically apparent. 
Filters according to the principles of the present invention may be 
permanently implantable, e.g., having hooks to keep the filter in place, 
or may be removable, e.g., relying solely on pressure to keep the filter 
in place. Such filters can also have a variety of shapes suitable for 
their filtering purpose, as is known in the art. One preferred filter has 
an inverted cone-shaped basket at the proximal end thereof, for filtering. 
Some illustrative embodiments are now described in connection with the 
accompanying Figures. 
The side view of FIG. 1 and the top view of FIG. 2 show one filter 101 
having an echogenic feature located as described above. Three continuous 
strands of nitinol wire 103, 105, 107 form the illustrated filter 101. The 
upper strand 103 is coiled into an upper flower form having five loops 
111. The lower strand 107 is coiled into a lower cone form, also having 
five loops 115. Finally, the middle strand 105 is coiled into an outer 
ring having ten loops, five directed up 109 and five directed down 113. 
The upper loops 109 of the middle strand 105 join the loops 111 of the 
upper strand 103; the lower loops 113 of the middle strand 105 join the 
loops 115 of the lower strand 107. The lower cone formed by the lower 
strand 107 has a vertex 117 joined to the center 119 of the upper flower 
formed by the upper strand 103 at weld 121. Weld 121 defines an echogenic 
bead 123. Preferably weld 121 has a diameter of 2 mm, although the 
diameter may also be as small as about 1 mm or even as large as about 3 
mm. The diameter should be sufficient to define the echogenic bead 123 so 
as to provide adequate sonographic conspicuousness. 
Filter 101 of this embodiment is preferably about 3 cm long and about 3 cm 
in diameter when fully expanded. In use, the filter may not expand fully, 
slight compression holding it in place where the surgeon desires. 
Various alternative filter design features are now described in connection 
with FIGS. 3-7. These figures show various features which can be combined 
in numerous other permutations and combinations than shown, as will be 
apparent to those skilled in this art. 
The side view of FIG. 3 shows alternative configurations for an echogenic 
feature and for centering features. In this alternative configuration, an 
echogenic tube 301 supports individual wire strands 303 arranged as a cone 
filter. Wire strands 303 further support individual centering loops 305. 
The components are welded or brazed together using conventional 
techniques. Echogenic properties are imparted to the tube 301 by virtue of 
its size and surface finish, as described above. In preferred 
configurations using an echogenic tube 301, the tube 301 has an inside 
diameter large enough to freely pass a 0.035 inch (0.89 mm) diameter 
guidewire. Also preferred in some configurations, the tube 301 has a 
flared, dilated or contracted feature 307 at one end to facilitate 
gripping of the tube 301 for purposes of removing the filter after it has 
served its purpose. 
Although FIGS. 1-3 show filter wires (e.g., 107, 303) which are generally 
straight or smooth curves, as shown in FIG. 4, bent filter wires 401 can 
reduce the size of openings between wires, resulting in a more effective 
filter, without blocking the free flow of normal blood. 
FIGS. 5 and 6, side and top views, respectively, illustrate a configuration 
employing an echogenic tube 301, as described above, to which straight 
filter wires 501 are conventionally welded or brazed. Instead of centering 
loops 305, this configuration has a split side ring 503 to prevent filter 
migration, once placed where desired by the surgeon. Spring pressure 
pushes the split side ring 503 into the vessel wall when the filter is in 
the position desired, thus anchoring the filter against the pressure of 
blood flow through the filter. Gaps 505 between segments of the split side 
ring 503 allow the filter to be compressed to a smaller diameter than when 
deployed for insertion, removal or positioning within a vessel of a 
patient's body. Instead of straight filter wires 501, this configuration 
can use bent filter wires 401, as in the configuration of FIG. 4. The 
echogenic tube 301 can have the characteristics and features described 
above in connection with FIG. 3, if desired. 
A modification of a conventional filter design made by MediTech, Boston 
Scientific (Natick, Mass.) is illustrated in the side view of FIG. 7. In 
this configuration, filter 701 includes an echogenic tube 301 not found in 
the conventional MediTech design. The echogenic tube 301 of this 
configuration also may include the features described above, such as the 
extraction feature 307. 
Yet another filter configuration is shown in FIGS. 8 and 9 in side and top 
view, respectively. The filter 800 shown has a central echogenic tube 301, 
supporting a plurality of bent wires 801, 803. Bent wires 801 are arranged 
as one filter cone, while bent wires 803 are arranged as a second filter 
cone. Bent wires 801 and 803 are connected together through straight wires 
805 and a split side ring 503, similar to that described above in 
connection with FIGS. 5 and 6. 
Finally, a simple filter is shown in FIGS. 10 and 11 in side and top view, 
respectively. This filter 1001 also has a central echogenic tube 301, 
supporting a plurality of wire legs 1003. Wire legs 1003 each have a 
radially extending portion 1005, arranged to form a filter cone, and a 
longitudinally extending portion 1007. The longitudinally extending 
portion 1007 centers and axially aligns the filter with the axis of the 
vessel in which it is to be implanted, similar to the action of centering 
loops 305 and the middle strand 105 of the above described embodiments. 
The longitudinally extending portions 1007 end in loops 1009 and points 
1011, which securely anchor the filter in the vessel in which it is 
implanted. 
Filters embodying aspects of the invention are generally used in the same 
manner as other filters used in this art. However, the imaging used in 
connection with embodiments of the invention can be sonographic, rather 
than fluoroscopic. 
The invention can be embodied in a complete set using any of the 
above-described filters or variations. The set employs a #7 or #8 French 
dilator having an echogenic tip. The set further includes a #8 French 
sheath, also having an echogenic tip, of flexible plastic. Finally, the 
set is guided by a 0.035 inch (0.89 mm) diameter guide or marker wire 
having one or more, preferably three, echogenic sites at or near the 
distal end thereof. The guide or marker wire may be a straight wire or a 
curved wire, as described below in connection with FIGS. 12-14. The set 
thus described can be used to implant a filter using only external 
sonography or a combination of external sonography with other techniques, 
as described below. Although components of the set may be separately 
available, it is expect that the dilator, sheath, guide wire end filter 
will be supplied as a complete set also, for convenience and consistency. 
Delivery and deployment of a filter, e.g., FIG. 1, 101, having an 
echochenic lead 123, below the renal veins proceeds as follows. 
The locations of the renal veins are determined and marked on the skin with 
marker, using any conventional means. This may include the techniques for 
internal sonography described by Bonn et al., Intravascular Ultrasound as 
an Alternative to Positive-contrast Vena Cavography prior to Filter 
Placement, Journal of Vascular Interventional Radiology, Vol. 10, No. 7, 
pp. 843-849, 1999. Alternatively, external sonography can also be used to 
locate the renal veins. 
As shown in FIGS. 12-14, a curved guide wire 1201 having a series of 
sonographically conspicuous features 1203, for example 3-12 features 
spaced about 1 cm apart, is then inserted to a point just past the 
confluence 1205 of the renal veins 1207 forming the vena cava 1209, and 
then withdrawn to engage one renal vein 1207. Once the wire 1201 engages 
the renal vein 1207, slight advancement anchors the renal vein 1207. The 
progress of the curved, sonographically conspicuous wire 1201 is easily 
followed using external sonography. 
The guide wire, particularly the curved guide wire 1201 introduced as 
described above, and the echogenic features of the guide wire make 
possible very precise identification using external sonography of the 
desired placement location. The dilator is then introduced over the guide 
wire and the echogenic tip thereof is also followed by external sonography 
to the filter placement location. Finally, the filter, contained in the 
end of the sheath, is guided over the guide wire, inside of the dilator, 
to the placement location and deployed. The precise location of the filter 
is followed using external sonography. 
The filters described above are very flexible, having low profiles, i.e., 
small undeployed diameters and short lengths, permitting access to the 
patient's venous system through various conventional locations, including 
the jugular vein, the groin vein, etc. Moreover, because the filter 
features avoid foreshortening of the filter relative to the distal end 
thereof, highly accurate tracking and placement is achieved by simply 
visualizing the distal end of the filter as it progresses to the placement 
location. 
Delivery and deployment of a filter below the renal veins with an echogenic 
tube 301 instead of or in addition to an echogenic lead 123 proceeds in 
similar fashion, but with some advantages now described in connection with 
FIGS. 15-17. Principally, when using a filter configured with an echogenic 
tube 301, or other configuration of sonographically conspicuous features 
which are aligned to show the axial alignment and position of the filter, 
the filter can be slid over a marker wire 1501, as generally shown in FIG. 
15 while the true longitudinal position and axial orientation of the 
filter is correctly visualized. 
As shown in the sonographic image sketched in FIG. 16, when the sonographic 
beam is aligned with the tube 301 and the marker wire 1501, a true image 
1601 is obtained, and the true longitudinal and axial position can be 
determined from the angle of incidence of the beam and the depth of the 
tube 301 and wire 1501 within the true image 1601. The true image 1601 of 
FIG. 16 is contrasted with the oblique image 1701 of FIG. 17, in which 
incomplete information is available. In this oblique image 1701, the angle 
of incidence of the beam is turned slightly, so that only a portion of 
tube 301 and marker wire 1501 are each visible in the oblique image 1701. 
When such an oblique image 1701 presents itself, the surgeon guiding the 
filter including tube 301 can determine whether the visible portion of 
tube 301 represents the distal or proximal end of the filter and either 
guide the filter accordingly or adjust the sonogram beam or the filter 
position to obtain a true image 1601. 
It is expected that struts, and other filter parts which do not 
specifically include sonographically conspicuous features will be 
difficult or impossible to see, while the tube 301 will be easily seen. 
Thus, the tube 301 and marker wire 1501 combine in the image to permit 
optimum sonographic imaging to be arranged. 
The overall filter and placement set and procedures described above permit 
filter placement by a physician at bedside, in an office (as compared to 
hospital) setting, or in a sonographic suite. Thus, the need for a 
fluoroscopic suite or equipment is obviated by the inventive filters, 
placement sets and methods. The inventive filters, placement sets and 
methods are also appropriate for intraoperative placement where 
fluoroscopy is not available, cumbersome, inappropriate or otherwise 
untenable due to operating room facilities, availability, required 
procedures or contraindication of any other kind. 
When desired, the filters and methods of the present invention can be 
assisted by magneto-resonant (MR) and computed tomography (CT) 
visualization methods. Such methods are particularly suitable when the 
filter is constructed of nitinol or another material which shows up in an 
MR or CT image. Fluoroscopic assistance can also be used with nitinol 
filter structures, as well as other materials which show up in a 
fluoroscopic image. A highly visible marker wire may also be helpful to 
these methods. 
The invention has now been shown and described in connection with an 
embodiment thereof and some variations, but the invention is not limited 
thereto. Other variations should now be evident to those skilled in this 
art, and are contemplated as falling within the scope of the invention 
which is limited only by the following claims and equivalents thereto.