The common goal of catheter-based embolization procedures is to cease the flow of blood mechanically in a particular vessel or vascular territory using either a temporary or permanent agent. In the history of interventional radiology, embolization has an established key role as a frequently used, clinically valuable, and widely applicable endovascular procedure. This is partly a result of its elegance and simplicity as a minimally invasive technique capable of achieving impressive life-saving results.
Untreated or unapparent arterial hemorrhage constitutes one of the most preventable causes of death in abdominal and pelvic trauma. Recent advances in noninvasive imaging and interventional angiography have enabled earlier identification of critical arterial hemorrhage to facilitate prompt and precise endovascular treatments that yield improved outcomes compared with historical results from past decades. Now it is not only identified early, but also treated promptly and with great precision.
Technological advances of computed tomography (CT), CT angiography (CTA), have indispensably replaced the conventional diagnostic angiography to evaluate the cause and pinpoint the site of vascular injury. Currently, laparotomy has been replaced from the first line of management for hemodynamically stable patients, because CTA and nonoperative management have shown favorable benefits. Arterial embolization is now an irreplaceable component of primary resuscitation, more so with further breakthroughs in endovascular techniques.
The key for successful embolization lies in mastering a thorough knowledge of the available equipment/materials and applying it appropriately. ( Fig. 29.1 lists the various embolization materials available currently.)
In this chapter, we present the necessary inventory currently used for delivery of different embolization materials, the relevant tricks of the trade for their optimal use and tips to avoid adverse effects. Most importantly, we shall discuss how to recover once a mistake or complication occurs during the course of an embolization procedure.
Particle Embolization (Gelfoam and Polyvinyl Alcohol)
Procedure Recommendations: The Dos and Don’ts
Ideally, it is best to prepare a separate embolization table distinct from the standard procedural set-up for catheters, guidewires, etc., with two distinct bowls of saline and contrast. Use small syringes (<5 mL) for embolization and specifically mark them as “embolization” or “particles”. After selective catheterization, always ensure optimal catheter positioning prior to embolization by performing a test injection of contrast to assess the potential for reflux of embolic material into nontarget areas.
Magnify the field of view to focus on the target region, collimate the borders, and under constant fluoroscopic guidance begin to slowly inject particles in a series of small puff-like injections, using the same pressure for each. Commonly, particles and Gelfoam have a tendency to aggregate and to form a clump at the junction of the tip of the syringe and the hub of the catheter. This may form a plug and result in catheter occlusion. Plug formation can be prevented by flushing the catheter after each syringe of particles injected to ensure a purged or clean catheter that is free from any residual embolization material. The key to this technique is to continue flushing normal saline until the operator no longer visualizes the opacified agent and there is free flow of saline with minimal injection force.
Occasionally, operators may encounter sudden and/or increased resistance during embolization. In this situation, a common mistake is to forcefully flush large volumes of saline with increasingly higher pressure in a vigorous attempt to clear the blocked catheter. This forceful maneuver may lead to an explosive and uncontrolled dislodgement of embolization agent into nontarget area(s) as the obstruction is relieved. Encountering such resistance is common, and it can be better addressed by injecting small aliquots of saline using a 1-mL syringe.
It is important to remember that as embolization with particles or Gelfoam progresses, the speed and dynamics of vascular flow change. Peripheral flow in distal small vessels becomes progressively stagnant, and further embolization can lead to reflux into nontarget vessels, proximal to the catheter tip. Approaching the end of the embolization process, it is critical to recognize that the catheter system is still full of the embolization agent and large puffs of contrast or saline, even during catheter withdrawal, can cause accidental nontarget embolization of other organs.
Special care should be taken while performing postembolization arteriography to avoid any inadvertent use of embolization syringes, because residual particles can transiently adhere to the wall and/or hubs of syringes. Similarly, it is a good precautionary practice following embolization to aspirate and to discard one syringe of blood before performing a completion arteriogram.
Temporary Agent—Gelfoam—Properties and Uses
Gelfoam is a water-soluble, readily available, temporary embolic agent supplied as a powder or pad that can be easily prepared for transcatheter use. Postembolization, it is completely absorbed by the body with an arterial vascular recanalization time of 2–3 weeks. Gelfoam is available in powder form with a particle size of 40 to 60 μm, which is preferable for distal capillary-level embolization. Alternatively, a slurry of Gelfoam is prepared by scrapping the sponge in a bowl of contrast or chopping out 1- to 2-mm cubes with scissors from a larger pad and then mixing them with contrast to attain a size variation of 500 and 2000 μm. A microcatheter with a relatively large lumen of 0.027–0.028″ is generally recommended for delivery of the agent is this form.
A single large piece or “torpedo” of Gelfoam can be used for proximal embolization, but difficult catheter loading and catheter occlusion are potential disadvantages, especially when using catheters with an internal diameter of less than 0.035″. The most distinctive characteristic of Gelfoam is its rapid absorbability by macrophages, thus restoring vascular patency in a relatively short timeframe of a few days to weeks.
Semipermanent Agents: Properties and Uses
The most commonly used embolic particles include polyvinyl alcohol (PVA), tris-acryl gelatin microspheres (TAGM), and hydrogel particles. These work on the same principle as Gelfoam, with the main difference being the uniformity of particle size and the property to cause permanent or semipermanent vessel occlusion. They can lead to occlusion either by mechanical obstruction or by clinging to the vessel wall and inciting an inflammatory response. It is not uncommon during embolization to develop a conglomerate or clustered mixture of particles within the catheter hub that impedes further injection. This can be avoided by preparing the agent with the use of a ≥50% contrast mix with saline and constant agitation of the injection syringes with frequent resuspension of particles. Of note, over-diluting contrast with too much saline will make fluoroscopic visualization more difficult, especially in obese patients or when motion artefacts are encountered.
PVA particles (Contour SE Boston Scientific, Natick, Massachusetts) are uniformly-sized particles available in calibrated vials of specific size ranges, with the smallest spanning from 100 to 300 μm and the largest from 900 to 1200 μm.
TAGM spheres or Embospheres (Biosphere Medical, Rockland, Massachusetts) are uniformly spherical-shaped spheres produced from acrylic polymer matrix impregnated with porcine gelatin and coated with a hydrophilic surface, which prevents aggregation.
Bead block microspheres (Biocompatibles, distributed by Terumo, Somerset, New Jersey) are made of biocompatible microporous PVA hydrogel and behave more like the TAGM particles than PVA particles. They exhibit good flexibility and compressibility, which facilitates smooth injection and allows them to be tightly packed as they are capable of undergoing 20% to 30% deformation.
Embozene microspheres (CeloNova Biosciences, Newnan, Georgia) are highly compressible hydrogel spheres coated with Polyzene to provide a lubricious outer covering. These newer agents are color-coded and available in relatively narrow size distributions, with an average diameter range of 50 μm within each size offering. They are available over a wide range of diameters starting at 40, 100, 250, 400 μm up to the largest size of 900 μm.
QuadraSpheres (Biosphere Medical, Rockland, Massachusetts) are polyvinyl alcohol sodium acrylate spheres that feature the distinct property of size expansion when the spheres contact blood, nonionic contrast, or saline. They have an expansion capacity of up to four times their original dry state nominal diameter within a short span of about 10 minutes, resulting in increased surface area. This enables tighter packing between spheres and facilitates near-total luminal occlusion. At present, these are predominantly used for drug delivery in hepatocellular carcinoma and uterine fibroids.
Multiple studies have described an increased complication rate of rebleeding following Gelfoam embolization of traumatic abdominal and pelvic hemorrhage, despite arteriographic documentation of hemostasis upon completion of the procedure. Alternatively, the opposite effect can occur with overly aggressive embolization when a Gelfoam slurry occludes an entire segment of a mesenteric vessel and prevents any re-entry of blood through proximal or distal collateral pathways leading to target organ infarction. Additionally, on occasion it is possible for aerobic organisms to multiply in the retained air bubbles, with promoted infection and abscess formation. This complication is typically managed by administration of appropriate antibiotic therapy.
Procedure Recommendations: The Dos and Don’ts
The first step begins with choosing the desired diameter of the wire element composing the coil and a compatible catheter system. The selection is based on a number of factors including size and location of vessel at the site of coil deposition, but paying special attention to correctly matching the chosen coil with a delivery catheter is critical to prevent complications, such as getting a coil stuck within the catheter. Standard coils require a 0.035″ or 0.038″ inner diameter catheter lumen while microcoils require anywhere from a 0.010″, 0.014″, 0.018″, to a 0.021″ microcatheter lumen for deployment.
It is always standard policy to use an end-hole catheter rather than a catheter with side holes, because a coil may wedge a side hole and prevent advancement beyond the catheter tip. Of note, coils will not deploy through side holes.
Difficult coil delivery, failed deployment, or deployment in an unwanted area can occur when a mismatched delivery catheter is used with a sufficiently oversized luminal diameter for the selected coil platform diameter. As a result, the wire or pusher used to advance the coil can override the proximal end of the coil, resulting in jamming within the oversized catheter lumen. In cases when such an inadvertent event occurs and a coil becomes stuck in the catheter, it is always best to withdraw the “coil in catheter” system together and re-engage with an appropriate fresh catheter via an introducer sheath. As a rule, angiographic introducer sheaths should always be used in embolization procedures to maintain arterial access to the branch with smaller catheters to the site for embolization, in the event of an untoward event, such as a partial or failed coil deployment. This avoids loss of access site and wasted time.
It is crucial to size the vessel correctly at the point where the coil is to be placed. Coils should be sized according to the vessel size. Soft coils can be oversized by 20%–30%, but stiff coils allow less than 1 mm of oversizing, otherwise they tend to remain elongated in a sinusoidal form once deployed. A very large or oversized coil can kick back the catheter during deployment, whereas a coil that is too small and undersized will fly off and migrate distally.
Upon loading a coil into the delivery catheter, it is necessary to ensure an end-to-end snug approximation of the catheter hub with the coil-loading cartridge to avoid inadvertent premature coil deployment in the catheter hub that typically causes catheter occlusion and coil damage. It is important to use coil pushers that are specifically provided for delivery of compatible microcoils through recommended microcatheters. Another commonly encountered problem is recoil of the catheter during coil deployment that results in unintended nontarget deployment. The potential for this may be assessed using the wire test method. This simply involves advancing a few centimeters of the guidewire or pusher selected for coil advancement and watching for push-back of the catheter tip. If no tip displacement is observed, it suggests a stable catheter position that will support accurate coil placement. If a kick-back of the catheter tip is noted during the wire test, it is a good idea to obtain a more stable catheter position by advancing distally or by switching to a co-axial catheter system to insure precise deployment accuracy and the support to obtain dense packing of coils, if desired. If the coil is still kicking back the parent system, use a firm grip to hold the system in place and use a stiffer wire pusher for deployment. If the problem persists, the key lies in using a shorter and smaller coil to resolve the issue. If catheter stability is questionable in critical areas where pinpoint accuracy is essential, detachable coils can be used to assure accurate deployment.
The complication of recanalization of previously coiled vessels can be prevented by creating a compact packing or nesting of coils. This can be achieved by using the scaffold technique or the anchor technique. In the scaffold technique, a large stiff coil is placed initially, followed by a dense packing with softer coils within the larger framing coil. The anchor technique involves placing the initial segment of a coil in a small branch vessel to provide good anchorage to the coil and a foundation to secure subsequent coils to form a dense coil nest. Usually it takes less than 5 minutes to attain complete thrombosis once a tight coil mass has been created. Sometimes, operators are concerned when persistent flow is observed through the embolized vessel immediately after coil placement, but predictably with patience and time, a focal coil pack will induce thrombosis and vessel occlusion within a few minutes.
Experience and expertise are necessary for deploying coils that are pushed effectively and without mishap, as they cannot be withdrawn or repositioned easily like detachable coils. If these coils migrate or are misplaced, it may be necessary to retrieve them promptly using a snare or similar retrieval device. If inadvertently a coil is partially left inside the catheter with the most of its length already deployed, then, as an initial maneuver, the coil–catheter combination can be pushed forward as a system to try to re-engage the target site and complete deployment. Detachable coil systems are ideal for controlled release of coils and for accurate embolization in complex anatomical situations. These coils are designed to address the concerns associated with coils that can be pushed and to enable testing coil formations and placement accuracy within the target site prior to release. Although considerably more expensive than coils that can be pushed, detachable coil systems provide interventionalists with the confidence of knowing that a coil may be tested, retrieved, and retested as many times as necessary in order to achieve the desired configuration, and only then is a commitment to deployment made.
The most commonly encountered complication of coil embolization for abdominal/pelvic hemorrhage is rebleeding secondary to coil migration. Distal migration of a coil, for example, in cases of splenic artery embolization, may also be complicated by resultant ischemia, infarction, and infection. This is decreased by the correct sizing of coils. Occasionally, if unresponsive to antibiotic administration, this can necessitate splenectomy.
Liquid Embolization Agents
Absolute alcohol or other sclerosing agents are not used in mesenteric embolization because they result in tissue necrosis. Successful embolization of abdominal or pelvic hemorrhage, irrespective of a patient’s coagulation status, can be achieved by using N-butyl cyanoacrylate or glue (Trufill NBCA, Cordis Neurovascular, Fremont, California) or ethylene vinyl alcohol copolymer or Onyx (Medtronic Inc., Minneapolis, Minnesota).
N-butyl cyanoacrylate or glue is a fast-acting, life-saving, effective, and safe embolic material with multiple benefits. It is mixed with an iodized oil such as lipiodol in varying rations of 1:1 to 1:3 depending on the viscosity desired to account most effectively for the blood flow and position of the catheter tip relative to the intended site of vessel occlusion. When in contact with blood, it undergoes fast polymerization and leads to complete hemostasis. Catheter occlusion is a challenge that many interventionalists face during their first few experiences with glue embolization. The risk of this complication can be mitigated by using a meticulous technique and by following basic guidelines such as careful flushing of all catheter system components with 5% dextrose solution prior to glue injection to prevent premature polymerization of NBCA in the catheter.
Ethylene vinyl alcohol copolymer (EVOH) or Onyx is a nonadhesive liquid embolic agent dissolved in dimethyl sulfoxide (DMSO) and suspended in micronized tantalum powder to provide contrast for better fluoroscopic visualization. The kit contains a 1.5-mL vial of Onyx, 1.5-mL vial of DMSO, and three 1-mL Onyx delivery syringes. A DMSO compatible delivery microcatheter is required (Marathon, Rebar, or Ultraflow HPC catheters) to gain access to the embolization site. Two forms of Onyx are available: Onyx 18 and Onyx 34. Onyx 18 is less viscous and is therefore capable of traveling more distally from the catheter tip to an intended site of occlusion.
Slow injection under fluoroscopic control through a microcatheter is required to allow maximum control during the infusion. Unlike glue, it does not have adhesive properties when in contact with arterial walls but instead relies upon filling up the lumen to achieve its effect. Following embolization, it does not adhere to the vascular endothelium and thus, as opposed to glue, reflux along the catheter during injection does not cause permanent adhesion of the catheter to the vessel.
If a long segment of microcatheter is within a tortuous artery leading to the embolization site, it could make catheter removal difficult. This situation results as a consequence of mechanical trapping rather than adhesion of the catheter, and slow gradual traction is the key to safe microcatheter withdrawal. If further difficulties exist with withdrawal, use of a wire in the catheter is also advised to decrease the risk of avulsion. Occasionally vasospasm occurs as a result of overuse of DMSO to prepare the microcatheter preliminarily and purge its lumen. It is always best to inject only the volume of DMSO calibrated to match the specified dead space of the microcatheter lumen. This is performed in a slow, controlled manner, never to exceed a maximum dose of 200 mg/kg. The injection rate should not exceed 0.3 mL/min. Vigilance is always needed throughout the course of Onyx embolization to avoid its passage into nontarget areas.
High cost and the need for comprehensive expertise in handling complications related to the flow of liquid embolic agents into nontarget areas are the major limitations restricting more widespread use of glue and onyx for embolization.
Amplatz vascular plugs (AVPs) are thrombogenic occlusion devices, consisting of bare-metal nitinol mesh configurations, typically oversized by 30%–50% of the luminal diameter when placed at the desired site of occlusion. After placing a guiding sheath or catheter at the target location, these plugs are easily deployed by unsheathing and then releasing a screw attachment mechanism. One notable advantage is that they can be resheathed if not in an ideal position after initial unsheathing, and then redeployed and simply detached by counter-clockwise rotation of the delivery cable once in the preferred position. These devices are recommended for the occlusion of appropriate medium and larger vessels, such as renal or splenic arteries, where standard coils would pose the risk of accidental misplacement, unwanted migration, or ineffective embolization. Although a plug is much more expensive than a coil, its use as a solitary embolization device has made it handy and a favored choice for instantaneous use when compared with the cost and relative inefficiency of deploying multiple coils. One major challenge to the application of vascular plugs in cases of hemorrhage is the inability to navigate a relatively stiff device through tortuous vasculature to the desired site of embolization. These vascular plugs are available in various shapes and a wide variety of sizes to suit a range of vascular anatomies.
MVP Microvascular Plug System (Medtronic Inc, Bloomington, Minnesota) is a thrombogenic occlusion device that can be advanced using a 5-French catheter. Similar to the Amplatzer plug, these are more expensive than coils and can be used for similar applications.
Covered stents, unlike other embolization agents, maintain the flow pathway while providing hemostasis when there has been a traumatic injury to a relatively large (>5 mm diameter), straight, nonbranched proximal arterial segment. Their use has already been established in hepatic, splenic, renal, and iliac arteries to effectively manage traumatic hemorrhage, pseudoaneurysm, and fistula and iatrogenic traumatic injuries in the abdomen and pelvis.
Standard complications associated with the use of a metal stent, such as kinking, deformation, dislodgement, and occlusion, can all occur. In addition, it is critical to choose appropriate anatomy when considering a covered stent to avoid graft occlusion of vital uninvolved side branches.
Various embolization agents such as coils, Gelfoam pledgets, liquid embolics, vascular plugs, or a combination of these can be selected to endovascularly treat most causes of mesenteric hemorrhage, traumatic solid organ injury and pelvic bleeding. These agents provide good proximal occlusion effectively and may also be used in sandwich technique, typically in regions of potential collateralization, to secure a successful outcome. As an extremely versatile endovascular procedure, the benefits of transcatheter embolization techniques are well-established.
Unfortunately, with the variety of applications and innovations in embolization comes a wide range of potential complications that are difficult to summarize and easily classify. A strong foundation in arterial anatomy, good knowledge of angiographic techniques, access to a wide variety of the available embolic agents, and sound procedural judgment are all important prerequisites to ensure interventionalists are optimally prepared to face the various challenges that occur during embolization of abdominal/pelvic bleeding, to avoid the recognized complications, and to manage any untoward outcomes thus limiting clinical harm.