A new type of endovascular tool, the mechanical thrombectomy device, has been developed to quickly, effectively, and safely remove thrombus from hemodialysis grafts, representing a new category of "tools of the trade." This article reviews the techniques that interventional radiologists use for restoring blood flow to a thrombosed hemodialysis graft.
The use of mechanical thrombectomy devices to treat thrombosed
Thomas M. Vesely, MD
is Associate Professor of Radiology and Surgery at the
Mallinckrodt Institute of Radiology at Washington University
School of Medicine in St. Louis, MO.
Treatment of an occluded vascular access graft can be a
challenging experience. Throughout the 1990s, there has been a
plethora of published reports describing numerous percutaneous
techniques for the treatment of thrombosed hemodialysis grafts.
These techniques can be grouped into four general categories: 1)
pharmacomechanical thrombolysis using urokinase, 2) pulsespray
thrombolysis using heparinized saline, 3) balloon thrombectomy
techniques, and 4) mechanical thrombectomy devices. No single
method has been proven to be more efficacious than the others.
There is reasonable evidence to support the belief that the
long-term patency of a hemodialysis graft is independent of the
method by hich he thrombus is removed, but is dependent upon
identification and successful treatment of all significant stenoses
and complete removal of the arterial plug.
A new type of endovascular tool, the mechanical hrombectomy
device, has been developed to quickly, effectively, and safely
remove thrombus from hemodialysis grafts. Mechanical Thrombectomy
devices represent a new category of "tools of the trade". As such,
most of us are still in the learning phase of how to best utilize
these devices in our day-to-day practice. This article will review
the techniques that interventional radiologists use for restoring
blood flow to a thrombosed hemodialysis graft.
Mechanical thrombectomy devices are approved for use in
hemodialysis grafts. Although interventionalists are beginning to
utilize these devices for other applications, including treatment
of thrombus in native veins and arteries, it is important to
remember that mechanical thrombectomy devices have not yet received
approval from the United States Food and Drug Administration (FDA)
for these other applications.
There are now seven different mechanical thrombectomy devices
which have been approved for use in polytetrafluoroethylene (PTFE)
hemodialysis grafts (table 1). These include the "Clot Buster"
Amplatz thrombectomy Device (ATD) (Microvena, White Bear Lake, MN)
(figure 1); the AngioJet (Possis Medical, Minneapolis, MN) (figure
2); the Trerotola-Percutaneous Thrombectomy Device (PTD) (Arrow
International, Reading, PA) (figure 3); the Cragg Thrombolytic
Brush and the Castaneda Thrombolytic Brush (Micro Therapeutics,
Inc., San Clemente, CA) (figure 4); the Gelbfish Endovac
(Neovascular Technologies, Brooklyn, NY); the Oasis catheter
(Boston Scientific Corp., Natick, MA) (figure 5); and the
Hydrolyser catheter (Cordis Endovascular, Warren, NJ) (figure
|Summary of currently available
mechanical thrombectomy devices
||7 F (6 F)
Catheter utilizes saline jets to fragment or remove
*New models will be over-the-wire.
(A) The Amplatz Thrombectomy Device catheter and foot pedal. (B)
The distal tip of the ATD, which contains the high-speed impeller
for fragmenting thrombus, is shown here. (Images courtesy of
Microvena, White Bear Lake, MN.)
The distal tip of the AngioJet catheter with its high-pressure
saline jets. (Image courtesy of Possis Medical, Minneapolis,
The Trerotola-Percutaneous Thrombolytic Device (PTD) and handheld
motor drive unit.(B) The distal tip of the PTD is a 9-mm diameter
(A) The Cragg Thrombolytic Brush and motor drive unit. (B) The
distal tips of the Castaneda Thrombolytic Brush (left) and the
Cragg Thrombolytic (right) Brush catheters. (Images courtesy of
Micro Therapeutics, Inc., San Clemente, CA.)
(A) The Oasis catheter. (B) The distal tip of the Oasis catheter
over a guidewire.
The distal tip of the Hydrolyser catheter. (Image courtesy of
Cordis Endovascular, Warren, NJ.)
The above mechanical thrombectomy devices can be divided into
two categories based upon their mechanisms of action (table 2)
. Recirculation-type devices create a hydrodynamic vortex, similar
to a kitchen blender, which homogenizes the thrombus, converting it
into a liquid slurry. The hydrodynamic vortex is created by either
a powerful jet-spray of fluid (saline) or by a rotating high-speed
micropropeller. Depending upon the specific device, the residual
thrombus slurry may be aspirated and removed from the graft, or it
may be allowed to embolize to the pulmonary arterial circulation.
Non-recirculation-type devices, which include the Trerotola-PTD and
the thrombolytic brush catheters (Cragg/Castaneda), utilize a
rapidly spinning wire basket or plastic brush to mechanically
breakup the thrombus into small fragments. Particulate thrombus
which remains within the graft can then be aspirated or dissolved
Categorization of mechanical
| AngioJet (Possis
| Hydrolyser (Cordis
| Oasis (Boston Scientific
| "Clot Buster"
| Endovac (Neovascular
| Trerotola-PTD (Arrow
| Cragg Thrombolytic Brush
(Micro Therapeutics, Inc.)
| Castaneda Thrombolytic
(Micro Therapeutics, Inc.)
There are no universally accepted methods for using these
mechanical thrombectomy devices when treating a thrombosed
hemodialysis graft. To the contrary, a wide variety of styles and
techniques currently are utilized by interventional radiologists
throughout the country. At this point in time, many interventional
radiologists have used one, or maybe two, of the different
mechanical thrombectomy devices, but very few radiologists are
familiar with all seven of the FDA-approved devices. Hopefully, in
the near future, clinical trials will compare these devices to
determine if there are any significant differences in their
performance. We may discover that each device has its own niche and
is best utilized for certain specific situations.
Prior to performing a mechanical thrombectomy procedure, it is
important to determine if the patient has a history of significant
cardiac or pulmonary disease. Studies have revealed that fragments
of thrombus can escape from the graft and travel to the lung as
pulmonary emboli during the thrombectomy procedure. This can occur
during a surgical thrombectomy as well but is more common when
using endovascular techniques. Although most patients are able to
tolerate these small pulmonary emboli without clinical sequelae,
patients with significant pulmonary hypertension are at greater
risk for acute cardiac decompensation.
Patients who have a history of right heart failure, pulmonary
hypertension, or cardiac dysrhythmias also are not good candidates
for mechanical thrombectomy procedures. These patients should
undergo surgical thrombectomy, where venous out-flow can be
occluded (clamped) to prevent any embolization of clot when the
thrombus is removed from the graft.
The basic technique for performing a mechanical thrombectomy is
similar to the method used for standard pulse spray thrombolysis.
Intravenous sedation is frequently administered immediately prior
to the procedure. Some radiologists also give a single dose of an
There are four basic steps to follow when treating a thrombosed
hemodialysis graft using endovascular techniques: 1) an initial
venogram is performed to evaluate the central and peripheral
out-flow veins; 2) the thrombus is removed from the graft; 3) all
significant stenoses are treated utilizing angioplasty,
atherectomy, and/or a vascular stent; and 4) the arterial plug is
dislodged. Each of these steps is further discussed below.
The initial needle puncture should be made in an antegrade
direction into the arterial limb of the graft, approximately 3 cm
from the arterial anastomosis. A standard angiographic catheter is
then advanced into the graft over a guidewire. Leading with the
guidewire, the catheter should be advanced through the venous
anastomosis. A diagnostic venogram is performed to evaluate the
graft and entire native venous out-flow, including that of the
central veins. If a long segment stenosis (>7 cm), multiple
sequential stenoses, or occlusions are identified in the native
veins, the procedure is probably best terminated. It can be
difficult to treat these extensive venous lesions, and even when
they are successfully treated, the results usually are not durable.
When possible, patients with these types of problematic lesions
should undergo a surgical revision to extend the graft above the
diffusely diseased segment. Occasionally, an entirely new graft may
need to be created in a different location. More commonly, the
diagnostic venogram reveals a single, focal stenosis at the venous
anastomosis. This can be easily and effectively treated using
Although there are several different thrombectomy techniques,
many interventional radiologists prefer to leave the venous
anastomotic stenosis intact during thrombus removal. This stenosis
serves to hold the thrombus within the graft, allowing it to be
more readily removed, and prevents embolization of thrombus into
the native venous and pulmonary circulation.
After the venogram, a vascular sheath is inserted into the
graft. This provides access for the mechanical thrombectomy device
and angioplasty balloon catheters. In addition, the sideport of the
vascular sheath can be used to infuse dilute x-ray contrast
material into the graft, which often helps to delineate the
thrombus within the graft. This allows visualization of the
thrombus during the thrombectomy procedure. Before inserting the
mechanical thrombectomy device, heparin (2000 to 5000 units) is
slowly infused into the graft. This prevents the formation of new
thrombus during the procedure. The thrombectomy device is then
inserted through the sheath into the graft. It is activated and
slowly advanced under fluoroscopic observation. The thrombectomy
device can then be appropriately positioned to fragment the
thrombus. Generally, it takes only 2 to 5 minutes to completely
remove the thrombus from the graft.
After removing the thrombus from the venous limb of the graft,
the thrombus in the proximal arterial limb, which is trapped behind
the vascular sheath, will then need to be removed. In order to
reach the arterial limb of the graft, a second retrograde puncture
is performed into the venous limb of the graft. Another vascular
sheath is inserted, and the mechanical thrombectomy device is
advanced into the proximal segment of the arterial limb.
Occasionally, the vascular sheath in the arterial limb will need to
be temporarily removed over a guidewire to facilitate thrombus
removal from this segment of the graft.
At this point, there are two final steps to perform: angioplasty
of the venous stenosis and dislodgement of the arterial plug. As
previously mentioned, the most common location for a stenosis is at
or just beyond the venous anastomosis. These lesions can be very
resistant to dilatation, often requiring the use of high pressure
(20 atm) angioplasty balloons for effective dilatation.
After the venous stenoses have been successfully treated,
attention is directed to the "arterial plug" within the graft
adjacent to the arterial anastomosis. The arterial plug consists of
densely packed red blood cells and fibrin which are formed into a
hard, bullet-shaped plug; this is often adherent to the graft wall.
A Fogarty thrombectomy balloon can be used to dislodge this
Some interventional radiologists favor dislodging the arterial
plug before performing the venous angioplasty. In this method, the
venous stenosis traps the arterial plug within the graft, allowing
it to be fragmented by the thrombectomy device. Other radiologists
prefer to perform angioplasty on the venous anastomosis stenosis
first. Opening the venous outflow channel allows the arterial plug
to quickly exit the graft. This technique prevents the plug from
getting caught up within the graft or at a venous stenosis,
minimizing the risk of graft rethrombosis.
Studies have shown that the volume of the arterial plug is small
(0.3 ml), representing minimal risk if it is allowed to embolize to
After the arterial plug has been dislodged, there should be
brisk blood flow within the graft. A final fistulogram should then
be performed to verify that all of the thrombus has been removed
and the venous stenoses have been effectively dilated.
High flow vascular sheaths can be used if the patient is to
undergo hemodialysis immediately following the procedure. Different
types of high flow hemodialysis sheaths are available from several
companies. The sidearm of a sheath has high blood flow
capabilities, and can be directly connected to thehemodialysis
machine. This type of sheath is advantageous for two reasons: 1)
The radiologist does not have to hold compression on the graft
puncture sites to obtain hemostasis, saving time for both the
radiologist and patient; and 2) the patient does not have to be
re-cannulated with needles for hemodialysis treatment. The nurse
can directly connect the sheaths to the hemodialysis machine.
If the patient does not need hemodialysis immediately following
the thrombectomy procedure, the radiologist may then use a purse
string suture to close the vascular access sites.
This suture technique can substantially reduce the time needed to
achieve hemostasis, particularly when large diameter vascular
access sheaths have been used. The stitch is removed 2 to 3 days
after the thrombectomy procedure, usually at the time of the next
In summary, an interventional radiologist has a wide variety of
tools and techniques for treating thrombosed hemodialysis grafts.
When compared to more traditional methods, the use of mechanical
thrombectomy devices can substantially decrease the time required
to perform thrombectomy procedures. No doubt, these new "tools of
the trade" will also prove useful for other endovascular
applications as well.