Essential equipment: Embolization

Essential equipment

Embolotherapy is the deliberate blockage of blood vessels; embolization may be lifesaving when used to stop haemorrhage. Embolization is increasingly used as an adjunct to surgery and in the treatment of a variety of benign and malignant tumours. Many embolic agents are available to block vessels of different sizes, from large arteries to capillaries. The choice of the agent used depends on the individual circumstances of the case.

Embolic agents are usually divided into:

Mechanical occlusion devices: coils and vascular plugs
Particulate agents: polyvinyl alcohol (PVA), Gelfoam and autologous blood clot
Liquid agents: sclerosants, adhesives and Onyx.

Most embolization procedures use permanent embolic material, however there is one ‘temporary’ agent available, ‘Gelfoam’, which can be invaluable in particular circumstances.

Coils and plugs

Purpose

Coils and plugs are permanent embolic agents and are used when you need to block a few feeding vessels or pack small aneurysms.

Action

Coils and plugs have three effects:

They damage the intima leading to release of thrombogenic agents.
They provide a large thrombogenic surface.
They cause mechanical occlusion of the lumen.

The first two factors are the most important; even the tightest-packed coils will not effectively block a vessel without thrombus. Some coils have fibres to increase thrombogenicity.

Coils are pushed through the delivery catheter with a guidewire or a dedicated coil pusher.

Alarm

Friction between the coil and the lumen of the catheter progressively damages the lining and this can cause considerable problems when multiple coils are required. If you notice increasing resistance to coil introduction, change the catheter before a coil jams in the lumen and blocks the catheter leading to loss of your hard-fought-for position!

Units of measurement

There are many types of coil available; all have three size parameters:

Diameter of the coil wire

This is equivalent to guidewire diameter and varies from 0.014 to 0.038 inch diameter. Use a coil that is the correct diameter for your delivery catheter – too big and it will not fit; too small and it may jam with the pusher in the catheter.

Unconstrained length

Measured in centimetres (cm). This varies with the type of coil but, in general, increases with the coiled diameter. Shorter coils are much easier to manage. Very floppy coils often come in very long lengths due to their ability to pack very tightly into almost any shaped space.

Diameter of the formed coil

Measured in millimetres (mm). Coils come in a large range of diameters from 2 mm to over 20 mm. There are even ‘straight’ coils for blocking tiny vessels, e.g. in the colonic mesentery. Coils should be slightly oversized relative to the diameter of the target vessel, as this allows them to grip the vessel wall and to be closely packed ( Fig. 23.1 ).

Fig. 23.1, Embolization of a vertebral artery for renal metastasis.

Know what to ask for

Judging coil size is more often practiced as an art rather than as a science. If possible, and particularly if you are inexperienced, measure from available cross-sectional imaging as a guide. Remember vessel diameters can change due to vasodilation/constriction and the decision at the time of embolization is the key. First, think about the catheter lumen – do you need an 0.018 (microcatheter) or an 0.035 (conventional catheter). Next, decide the target vessel diameter – oversize by at least 1 mm. Softer coils mean that you can oversize a bit more with an added safety margin. Finally, how much space do you have before the next non-target vessel (behind the point of embolization) – this determines the coil length. Eventually, this will all become second nature and you will find you have confidently uttered the words – ‘could I have a 035/6 mm coil and what lengths do we have?’ To complicate matters further however, there are different coil materials and geometry, though these are all far less important than making sure the size is right!

Know how to use it

This is a brief guide relevant only to coils, there is further information below.

Position the catheter

Before even opening that coil, confirm that the catheter position is stable by passing the coil pusher/guidewire to its tip. If the catheter dislodges during this manoeuvre, it will definitely displace if you try to deploy a coil through it!

Introducing coils into the catheter

Coils are held straight in a short cartridge, which is discarded after the coil has been pushed into the catheter. The tip of the cartridge is placed into the hub of the catheter and the coil is pushed into the catheter using the provided pusher or the stiff end of a straight guidewire. Make sure that you hold the cartridge and the catheter tightly together if you do not want the coil to deploy in the catheter hub!

Pushing the coils – standard 0.035 coil

Once the coil is in the catheter, it is normal to use the reverse end of the guidewire to advance it 10–20 cm along the catheter. Coils can be pushed with conventional straight guidewires.

Pushing the coils – 0.018 coil

The guidewires supplied with microcatheters can jam when used to deliver coils. Special microcoil pushers have a flexible plastic fibre tip on a stiff metal shaft. They are packaged separately and have to be requested. Use the back end of the wire until the coil is at least 30 cm into the catheter.

Extruding coils

This is the stage when things are most likely to go wrong. Be careful to hold the catheter in position as the coil is pushed to its tip. Slowly start to push the coil out of the catheter; you may feel slight resistance as it starts to coil. Make sure that the catheter is not pushed back out of the target vessel. Continue slowly until the whole coil is out of the catheter. Following placement of the first coil, the catheter may have to be pulled back a few millimetres to make sufficient space to allow subsequent coils to be deployed.

Tip

Note how much of the pusher remains outside the catheter when the coil is about 15 cm from deployment. Consider marking this position on the drapes. For subsequent coils, do not screen until you reach this stage.

After multiple coils have been deployed, it can be difficult to be certain the last coil has been completely deployed and is clear of the catheter; always use the guidewire against the coil to back the catheter off.

Completion angiography

After a few coils have been deployed, perform an angiogram to assess flow. It is helpful to pull the catheter back a little and to inject gently in order not to dislodge a newly formed clot. When the flow is very slow, wait for a couple of minutes to see if the vessel thromboses. If there is still brisk flow, then more coils are needed. If the flow has stopped, you can stop too.

Vascular plugs

Purpose

Vascular plugs are most often used where multiple coils would be needed or there is high flow and a particular risk of distal embolization. The Amplatzer vascular plug (AVP) is a self-expanding nitinol wire mesh used for occlusion of vessels from approximately 3–16 mm diameter. Microvascular plugs (MVP) are nitinol/polytetrafluoroethylene (PTFE) plugs, which will pass through a standard microcatheter.

Action

The nitinol mesh is mounted on a delivery wire by a screw thread and inserted into a delivery catheter in a compressed form. The tip of the delivery catheter is positioned just beyond the deployment site. When the AVP reaches the target site, the plug is held in position while the delivery catheter is pulled back; this allows the plug to open. Correct sizing can be confirmed before the plug is detached. Thrombosis usually takes 5–10 min to occur.

Know what to ask for

There are a variety of different AVP devices (currently imaginatively numbered 1–4) available with different configurations. The primary decision is to decide the target vessel diameter and then oversize by 30–50%; there is a sizing guide available in the manufacturer's literature. The device lengths vary depending on the particular AVP type.

Know how to use it

Assess the target

As with all embolization, imaging is the key. The size and length of the target vessel must be assessed. If the site is suitable, a device is chosen, which is 30–50% larger than the target vessel diameter.

Suitability

The AVP must be delivered to the target site; this can be a real challenge in very tortuous vessels.

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