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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3869_Библиотеки_им_академика_М_И_Перельмана.pdf
16 D. Tuite and K. O’Regan
The first two of these are the most important and are closely interre-
lated. Although it is important to create a tightly packed nest of coils, physical occlusion of the vessel by the coils themselves will not occur without
thrombosis occurring. Hence coils may be ineffective if clotting is deranged.
Embolization coils can be utilized outside of the vascular system in certain circumstances; however, in these circumstances they work by a different mechanism
(Fig. 2.1).
ab
Fig. 2.1 (a) Microcatheter access into the thoracic duct demonstrates source of chyle leak post-
thoracic surgery. (b) Coil embolization was successful in treating this. Two points are noteworthy.
First embolization in this setting takes approximately 1 week to work as no thrombus can form.
The coils cause an inflammatory reaction in the thoracic duct which leads to fibrosis of the duct.
Second, coils can be used outside of the vascular system
Intimal Injury and Thrombus Formation Are Interrelated
A properly deployed device damages the endothelial surface causing
• Platelet adhesion (platelet glycoprotein 1b binding with von Willebrand factor in
the subendothelial matrix) and activation (PDGF, TGF-β, and thromboxane-A
• Exposure of tissue factor to the blood stream and the initiation of the coagulation
cascade resulting in the formation of a fibrin-rich thrombus.
• Thrombin is generated in large amounts at the site of injury. Arterial wallassociated thrombin activity remains elevated for at least 48 h and returns to
baseline after 1 week.
2
).
2 Coils, Occluders, and Plugs 17
A tightly packed nest of coils
• Has a large thrombogenic surface area.
• This surface area is further increased by the addition of fibers.
Physical Properties of Embolization Coils and Plugs
Materials
• Manufactured from metals; platinum, tungsten, gold, tantalum, and stainless
steel.
• They are all radiopaque (platinum coils are more opaque than stainless steel
coils).
• Coil conspicuity can be further increased by the use of various radiopaque fillers
such as tantalum, tungsten, barium sulfate, bismuth oxide, or bismuth carbonate.
• Nitinol (nickel/titanium alloy) is used in some of the newer plug devices.
• Stainless steel coils maintain greater radial force than platinum coils.
• Some coils have very little radial force These coils come in long lengths and can
be packed very tightly, e.g., liquid–metal coils (Berenstein Liquid Coil, Target
Therapeutics, Fremont, CA, USA), Nestor coils (Cook UK Ltd, Letchworth,
UK). The diameter of these coils is nominal, since the coil will instead simply
fill any space into which it is injected.
• Nestor coils will extrude from the catheter as a straight wire; they must engage
something within the lumen of the vessels to start forming curves and causing
occlusion of the vessel. For this reason, it is often advantageous to place a standard coil as a “backstop” against which the Nestor can start to reform its shape
and cause occlusion.
Novel Materials
Examples of these include the following:
• Bioabsorbable polymeric GDC coils (Boston Scientific, MA, USA) which aim
to obtain a more rapid and controlled wound healing at the site of aneurysmal
repair.
• Hydrophilic hydrogel-coated platinum coils (Microvention, CA, USA) in which
a hydrogel polymer expands from 0.014 to 0.027 inch in the presence of blood.
• Silk suture has been used as an embolic material, typically for intracranial
aneurysms or high-flow AVMs. Silk suture is highly thrombogenic but has the
disadvantage of being radiolucent. Up to 24% of patients have a transient fever
after the intravascular administration of silk. This is usually maximal at days 5–8
post-embolization and is not associated with infection.
18 D. Tuite and K. O’Regan
Material Issues
Nickel allergy. Nickel allergy has been documented as a complication of nitinol
device placement. This device syndrome results in local irritation and mild leukocytosis which is maximal 2–3 days post-procedure. It is rare and can be treated with
systemic prednisolone. In the setting of embolization, these symptoms are obscured.
MRI compatibility. Modern stainless steel coils are MRI safe but may generate
artifact.
Thrombogenicity
• Coils vary in their thrombogenicity. Untreated tungsten microcoils have superior clot-promoting activity compared to untreated stainless steel or platinum
microcoils.
• Various strategies have been employed to further improve thrombogenicity:
◦ Coatings: thrombin, gelatine, and polyurethane.
◦ Fibers (Fig. 2.2): typically individual fibers or bundles of 5–100 fibers
are used. Suitable fibers include synthetic materials Dacron, polyesters,
polyamides, and natural fibers such as silk and cotton.
Fig. 2.2 An array of Nestor coils (Cook Medical Incorporated, Bloomington, NJ, USA) showing
the typical helical design with fibers (Image courtesy of Cook Medical Inc.)
◦ The fibers are looped in a generally serpentine manner along the coil. The
fibrous loops are affixed to (or looped through) the coil at spaced intervals
along the coil.
2 Coils, Occluders, and Plugs 19
◦ Adding filaments to the exterior of the coil also increases the friction of the
fibrous coil against the catheter lumen. The ideal balance of these two
factors is difficult to achieve.
• Use non-heparinized saline while performing coil embolization or you will
inhibit thrombus formation.
Coils Shapes and Sizes
Coils have a predetermined shape that they adopt after release from the deployment
device. This predetermined shape can be achieved by either crimping the coil at
various intervals or heat-treating the coil on a mandril during the manufacturing
process. For this reason, an alloy of platinum with a small amount of tungsten is
often used as tungsten reduces the ductility of the resultant coil.
Coils come in a variety of shapes which are intended for slightly different
indications.
Helical coils: these are the most commonly used and designed for deployment
within non-aneurysmal long vessels (Fig. 2.2).
Spiral coils: such as the tornado coil (Cook). The spiral design maximizes coil
exposure to cross-section of the lumen for greater disruption of blood flow. In
addition they are more suited for embolization of tapered vessels.
More complex shapes are used for more complex aneurysmal vessels (Fig. 2.3).
The most complex shapes are typically used in intracranial berry aneurysms.
Straight coils are also available for the embolization of small vessels such as
those in the colonic mesentery. These are often injected out the end of a catheter
rather than pushed out with a wire or coil-pusher.
Fig. 2.3 More complex-shaped microcoils for use in the embolization of aneurysms. The cor-
rugated surface of the coil is designed to injure the endothelium (Image courtesy of Cook
Medical Inc.)
20 D. Tuite and K. O’Regan
Coil Size
Sizes are described by three parameters:
Coil diameter. This is the diameter of the outstretched coil (equivalent to the
guidewire size) that you are using, e.g., 0.014–0.038-inch diameter. This will
determine with which catheters and pushers the coil can be used. The coil diameter
• Must match delivery system. An oversized coil will not fit in the catheter. An
undersized coil will try to reform in your catheter and may block it.
• Must match the coil pusher size to the coil and both must be compatible with
the delivery system. An undersized coil pusher will jam alongside the coil in the
catheter.
Terminology
◦ Coils for use in a 0.035- or 0.038-inch delivery system are referred to as standard
coils.
◦ Coils used in a 0.014- or 0.018-inch delivery system are referred to as microcoils.
Unconstrained coil length. This is the length of the individual coil when held out
straight. Length varies considerably depending on the type of coil being used, but in
general it increases with increasing coil diameter.
Reformed coil diameter. This is the diameter of the coil when it reforms to its
predetermined shape.
• This must be larger than the target vessel.
• The coil should not adopt its predetermined shape. If it does, then it is undersized.
• Undersized coils
◦ Are at risk of migration and non-target embolization
◦ Fail to injure the vessel wall and thus not induce thrombosis
• Oversized coils will act like a guidewire and pass further distally into the vessel.
Coil size is typically given as a series of those three numbers. A 35-4-8 coil will
pass through a 0.035-inch inner lumen catheter, will measure 4 cm in length, and
have an 8-mm coil diameter.
Coil Deployment
Five steps are taken to ensure appropriate embolization with coils:
1. Establishing Stable Catheter Position
• The target vessel is catheterized in the conventional manner.
2 Coils, Occluders, and Plugs 21
• Once the intended deployment position is reached, catheter stability is checked
by passing the coil pusher through to the end of the catheter. If the catheter
position moves, then it is too unstable to deploy a coil.
• Additional stability can be obtained by using
◦ Co-axial catheter systems
◦ A more stable catheter shape, e.g., reverse curve catheter
• When the coil pusher is approximately 15 cm from the catheter tip, the position of the back end of the pusher should be marked on the table. During coil
deployment, it is not necessary to use fluoroscopy until this point is reached.
This significantly reduces radiation dose.
2. Introducing the coil
• The coil comes preloaded in a cartridge when opened from the packaging.
• The cartridge is firmly engaged in the deployment catheter hub and held in
position.
• Some microcoil cartridges are screwed onto the delivery catheter hub and come
with a small stiff wire to push the coil from the cartridge.
• The back (stiff) end of the coil pusher is used to push the coil out of the cartridge
and into the catheter for a distance of approximately 30 cm.
• At this point, the coil pusher is turned around and inserted front end first.
• The cartridge is snapped or kinked prior to being discarded so that there
is no confusion as to which cartridges have been used and which still
contain coils.
Coil pushers
• For standard catheters, a straight guidewire can be used as a coil pusher. A
wire with a long taper may decrease the risk of catheter dislodgement.
• For microcatheters, a special coil pusher is needed as microcatheter
guidewires will not work.
• Microcoil pushers have a soft flexible tip with a stiff shaft.
• Remember to use the pusher to establish catheter stability before deploying
a coil
3. Extruding the Coil
• Good fluoroscopy is essential at this point.
• The coil is extruded by advancing the pusher, as the coil deploys the catheter will
tend to migrate backward.
22 D. Tuite and K. O’Regan
• It is essential to have control of any guide catheter, delivery catheter, and also
the coil pusher at this stage. Gentle forward pressure on the delivery catheter is
helpful to stabilize the delivery system.
• If the coil is correctly sized, then it will begin to form loops. The number of
loops is approximately the coil length divided by the coil diameter multiplied by
π (length/[diameter × π]).
• A correctly sized coil will form an irregular shape as it is constrained by the
vessel wall.
• Once the first coil is deployed further coils can be placed to build up a tightly
packed nest of coils.
• As the nest becomes bigger it may be necessary to allow the catheter to migrate
backward to allow coils to be extruded.
• Variant techniques
◦ If the catheter system is in a strong stable position, then some coils may
be hydrostatically injected into position. Once the coil is loaded into the
catheter, a 1-ml Luer-lock syringe filled with saline is forcefully injected
pushing the coil into position.
◦ Other methods include introducing multiple coils into the catheter before
pushing them into position with the coil pusher. This method is used when it
is obvious that multiple coils will be needed for a large target vessel.
◦ Liquid–metal coils (Berenstein Liquid Coil, Target Therapeutics, Fremont,
CA, USA) are long, soft coils which are delivered by a slow saline infusion.
This slow stream is sufficient to drive the coil to the catheter tip with minimal
friction between coil and catheter. This makes it particularly useful for long
tortuous vessels. They come in sizes up to 30 cm and this long length makes
embolization of large vessel quicker and more efficient.
◦ Interlocking detachable coils greater than 10 cm are available, but friction
between coil and microcatheter may limit their use. These coils come in
two sizes and can be used in both conventional microcatheters and flowdirected microcatheters. They allow a very dense coil pack to be created.
The main benefit of detachable coils is that they can be placed into the vessel
to be embolized, and detached only if the operator deems placement to be
appropriate.
4. Anchoring Techniques
• Coaxial technique. This is the standard technique used for the placement of
embolization coils and is detailed above.
• Scaffold technique. This is the use of a large coil initially to create a scaffold into
which smaller coils can be placed to build up the coil nest. The initial coil should
ideally have a high radial force so as to secure the scaffold position. As such a
stainless steel coil is a good choice. The subsequent coils such as soft, fibered
platinum coils are a suitable choice here.
• Anchor technique. The initial coil is initially deployed in a small side branch of
the target vessel. As the coil is deployed, the catheter is withdrawn into the larger
2 Coils, Occluders, and Plugs 23
target vessel. This helps to secure the first coil in position in the target vessel. The
subsequent coils are deployed as normal (Fig. 2.4).
• Stent-supported embolization technique. This technique is best used for wide-
necked aneurysms, fusiform aneurysms and pseudoaneurysms, where unsupported coil deployment is not possible. In this technique, a stent is placed across
the aneurysm. A microcatheter is then passed through the interstices of the stent
and embolization can now proceed without fear of coil migration or malposition
(Fig. 2.5).
Fig. 2.4 Anchor technique. A large left pulmonary arteriovenous malformation (both arterial and
venous components arrowed) is shown in the first panel. A Nestor coil is initially anchored in
a side branch and then fully deployed in the arterial limb of the malformation. A second coil is
placed in a similar position to stabilize the coil nest before a further six coils are added to achieve
embolization
Fig. 2.5 Stent-supported embolization technique in the treatment of basilar tip aneurysm. The
posterior cerebral circulation has been accessed via each of the vertebral arteries. A stent has been
placed extending from the basilar artery into the left posterior cerebral artery (small arrows denote
the stent ends). A balloon has been inflated at the origin of the right posterior cerebral artery (large
arrow). A microcatheter has been passed through the stent into the basilar tip aneurysm and coils
have been used to embolize the aneurysm. Completion angiography shows successful exclusion of
the aneurysm
24 D. Tuite and K. O’Regan
• Why not use a covered stent? Covered stents are much stiffer than uncovered stents and it may not be possible to position the stent graft. Also stent
grafts will cover small perforators, which in intracranial intervention may prove
catastrophic.
5. Endpoint Of Deployment
• Coils are added until the nest looks sufficient to occlude the vessel or the coils
reach a vessel intended to be spared.
• Post-embolization angiography is performed. The catheter should be withdrawn slightly to avoid dislodging newly formed thrombus so that the real flow
dynamics are observed.
• Brisk flow through the coils indicates the need for further embolization.
• If slow flow is noted, wait a few minutes to allow thrombus to form and check
again.
• If necessary, continue adding to the nest until occlusion is obtained. If there is not
much room for additional coils, or if the patient is coagulopathic, consider using
some gelfoam.
Additional Techniques
Most coils are deployed as described above. In certain circumstances additional
control is desirable, for which several devices and techniques exist including the
following.
Detachable Coils
• These come in all shapes, sizes, and materials.
• The coil is attached to the delivery wire allowing it to be positioned and
withdrawn if necessary prior to deployment.
• There are several systems for detaching the coil
◦ Mechanical: via a microscrew or via the core mandril (Fig. 2.6).
◦ Electrolytic and heat mechanisms: Guglielmi detachable coils are platinum
coils soldered to a stainless steel delivery wire. Once the coil has been
placed into the target vessel and the position confirmed, a 1-mA current is
applied to the delivery wire. The coil then detaches by electrolysis and the
wire can be removed. Heat mechanisms work by burning a filament that
attaches the coil to the delivery wire.
Amplatzer vascular plug (AVP) (AGA Medical)
• The AVP is a self-expanding nitinol wire mesh (Fig. 2.7). The second-generation
AVP has two waists within it making it a tri-lobed structure.
2 Coils, Occluders, and Plugs 25
Fig. 2.6 Simple mechanical detachable coil system. This is the Interlock system from Boston
Scientific Corporation. More complex systems are available where detachment of the delivery
wire from the coil is achieved by either heat or electrolysis (Image courtesy of Boston Scientific
Corporation)
• The AVP plug comes in sizes ranging from 4 to 16 mm in 2-mm increments (AVP
II ranges from 4 to 22 mm). Plugs should be oversized by 30–50% (e.g., an 8-mm
vessel should be embolized with a 10- or 12-mm AVP).
• A 5Fr guide catheter is needed for 4–8 mm devices, 6Fr for 10–12 mm devices,
and 8Fr for 14–16 mm plugs.
• The AVP is deployed through a guide catheter attached to the end of a 135-cm
long nitinol wire by a microscrew.
Fig. 2.7 Amplatzer vascular plug (AVP). A single AVP is used to occlude a TIPS in a patient with
intractable encephalopathy post TIPS creation. In the first panel, the plug has been uncovered from
the delivery sheath; however, it remains attached to the delivery wire. It is repositioned prior to
complete deployment