Interventional radiology (IR) is a growing specialty, which uses the combination of image guidance and device technology to perform a wide range of minimally invasive procedures. There is hardly a single area of hospital medicine in which IR has not had impact on patient management. Interventional radiologists have a wide range of techniques at their disposal and have key roles in treating bleeding, cancer, aneurysms, trauma, peripheral arterial disease and venous thromboembolism.1
Embolisation
Angiography can be used to localise any site of bleeding, which can then be stopped by transcatheter embolisation, whereby various materials or devices are administered or deployed inside the blood vessel, blocking flow and stopping bleeding. Frequently, this obviates the need for major surgery.
Embolisation can also be performed in the elective setting, and this can be used to block problematic blood vessels, such as the iliac arteries prior to aneurysm repair, a gonadal vein causing a testicular varicocoele, arteries which supply fibroids in the uterus (and so avoiding hysterectomy) or arteries which supply tumours in the liver (chemoembolisation).
A variety of embolic materials are available and are selected based on the required clinical effect. These can be temporary or permanent and can be foam, small particles, particles loaded or mixed with chemotherapy or radioactivity to treat tumours, or they can be devices like coils (metallic wires which coil and block the vessels) or plugs.2
Figure: 1 An AVP-4 device being deployed in the splenic artery.
Vascular plugs
There are a variety of plugs, but the concept of a plug is that it is loaded into a delivery system, and when the sheath or sleeve of this delivery system is drawn back, the plug then self-expands toward its native shape and can then be released from its delivery system. A range of different plugs are available and new devices are coming to market. The most used and well-researched plug is the Amplatzer Vascular Plug (St. Jude Medical). This has had a number of unique developments resulting in the current availability of three different plugs: the AVP, AVP-2 and AVP-4. Each has unique characteristics, which are discussed later. Other commercially available plugs include the Cera plug (LifeTech Scientific) and the MVP Microvascular Plug System (Reverse Medical, Irvine, CA, USA). The Amplatzer Septal Occluder is also known as AVP-3, an another form of plug with specific indications in cardiology. Larger forms of plugs are also available as part of endovascular aortic stent graft systems where the common or external iliac arteries may be required to be occluded on occasion.
The most commonly used plugs are the range of Amplatzer Vascular Plugs. Each has different characteristics. The AVP is a one-cell design and so is short. The AVP-2 has a three-cell design to encourage vascular occlusion more readily but can be long. The AVP-4 is ‘low profile’ and has a smaller delivery system, which means it can often be deployed into tortuous vessels or indeed into deeper parts of the circulation, where the branches are smaller.
Plugs have several advantages over coils, the traditional method for blocking medium to large vessels. The main advantages are its mode of deployment and its ability to cause vascular occlusion.3
Firstly, they can be repositioned and their release can be controlled. Coils risk migrating distally or can be lost to other vascular territories (so-called non-target embolisation).
Secondly, they can be cost effective, as multiple coils may be required to block a specific artery, whereas a single plug may suffice.
There are, however, some disadvantages. Despite being designed to cause vascular occlusion, there are cases where a deployed plug has not led to vessel occlusion and the candidate vessel remained patent. The plug delivery wire is somewhat stiff and it can be difficult for this to get around severely tortuous vessels.
The AVPs are not plugs per se that instantly seal the target vessel as there are small holes within the mesh. Furthermore, the device lacks any intrinsic thrombogenic properties other than being a foreign body. Instead, the small holes in the plug reduce flow and intrinsic (contact activated) thrombosis seals the device. Therefore, it takes time to achieve complete occlusion. The AVP occlusion time is highly variable: prolonged occlusion times are seen in cases of underlying coagulation disorders, vessels with large diameters and with high velocity. Often in these cases there may be persistent patency, which needs the utilisation of more or different embolic material.4
Migration and recanalisation of the AVP have also been documented as complications of vascular plugs. AVPs can not be recanalised unless there is persistent patency.5-6
The case for cost savings for the AVP is well established. However, in large vessels with high flow rates multiple embolic agents may be required such as multiple plugs or plugs in conjunction with coils and cost benefits are reduced in these scenarios.4 Furthermore, in smaller vessels the cost savings may also be less significant as fewer coils are required to achieve occlusion.7 The MVP microvascular plug system has a PTFE membrane which may facilitate better vessel occlusion. Cost benefit can be achieved by using gelfoam slurry. A new device “MVP Microvascular Plug System” is a real plug that has PTFE member and can cause immediate cessation of flow after deployment.
Arterial applications
Arterial embolisation has been the main use of vascular plugs; this is primarily due to the superior stability offered by AVP as opposed to coils that can migrate.
Internal iliac artery embolisation is a common indication to prevent endoleaks in patients before endovascular aortic aneurysm repair (EVAR); the procedure has been widely reported to have high success rates and low rates of recanalisation.8 Vascular plugs have been used to successfully treat internal iliac artery aneurysms and this is achieved by occluding the distal and proximal ends of the aneurysm sac. Furthermore, vascular plugs have shown advantages over coils in costs, procedure time and reduction of radiation exposure for the treatment of iliac artery aneurysms.9 Similarly, there is growing use of plugs in association with endovascular repair of thoracic aortic aneurysms (TEVAR), where plugs can be used to embolise the left subclavian artery in order to widen the landing zone in the aortic arch.
Embolisation techniques in the carotid arteries have been a long established practice mainly with the use of detachable coils. Vessel wall compliance makes this procedure difficult for coil-mediated occlusion especially in segments of the carotid that are outside the bony canal. Vascular plugs have been successfully used to treat many conditions including caroticocavernous fistulae, giant cerebral aneurysms, and haemorrhages from a variety of causes including trauma.10
Splenic artery embolisation is an increasingly performed procedure that is offered as an alternative to surgery. It has specifically been used to treat portal hypertension, splenic artery aneurysm and in post-traumatic cases avoiding the need for a splenectomy.11
Other common arterial embolisations that have utilised vascular plugs include: gastroduodenal artery pseudoaneurysms, bleeding angiomyolipomas and renal arteriovenous fistula.12 Case reports have presented a multitude of other arterial structures and end organs benefiting from vascular plug embolisation.
Venous applications
In venous occlusion, vascular plugs have a significant advantage over other embolisation agents in occluding veins; their inherent stability and need for fewer devices is particularly suited to veins which have thin walls and that are of large calibre.
Common venous embolisations are performed for symptomatic varicoceles and to treat infertility in patients with varicoceles. Ovarian vein embolisation for female pelvic congestion syndrome has also become an accepted indication.
Portal vein embolisation as a preoperative adjunct to hepatic resection is an established treatment strategy; embolising the tumour-side portal vein stimulates hypertrophy of the contralateral lobe.13
Vascular anomalies
Abnormal vascular communications refer to pathological connections between arteries and veins. They can be divided into congenital malformations and acquired fistulae. The vessels are often large and have high velocities and these features favour the use of vascular plugs as they can be accurately placed and are less likely to migrate than coils.
Pulmonary arteriovenous malformations (PAVMs)
Transcatheter embolisation is now well established as the preferred treatment for PAVMs, reducing the risk of paradoxical embolisation and improving oxygenation. Most of the cases required only one vascular plug to occlude each feeding vessel.
The AVP has also been used to successfully treat many other congenital conditions including patent ductus arterious (PDA), splenorenal shunt, mesocaval shunt, scimitar vein to left atrium, and congenital coronary artery fistula.14
Vascular plugs have been utilised to reduce complications of haemodialysis arteriovenous fistulas and they have been used to occlude transjugular intrahepatic portosystemic shunts (TIPS) to ameliorate hepatic encephalopathy. Other uses of vascular plugs in acquired arteriovenous fistulae and shunts include the Blalock-Taussig shunt, traumatic arteriovenous fistula, popliteal arteriovenous fistula and aortic-caval fistula.
Nonvascular disease
The use of the AVP has been expanded to nonvascular procedures with promising results. Bronchopulmonary fistulae are associated with high morbidity and mortality. Endobronchial closure using the AVP was recently reported as a safe and effective method for the treatment of small postoperative bronchopulmonary fistulae.15 AVP also has been used for the treatment of oesophagopleural fistulae (assisted with endoscopy), ureter occlusion for vesicovaginal fistulae, intraperitoneal bile leak and sclerosis of biliary cutaneous fistulas.
Conclusion
Interventional radiology is an important medical specialty and embolisation procedures are performed for a wide range of indications across many different organ systems. Embolisation can be performed acutely or electively dependent on the indication. Vascular plugs are one device from an armamentarium of devices and techniques available to block large blood vessels with high flow and other body structures. Plugs have several advantages, their clinical effect is well evidenced and they can be cost effective.
References
1 Department of Health. Interventional Radiology: Guidance for Service Delivery. A Report from the National Imaging Board. London, UK: DOH, 2010.
2 Avinash M et al. A case-based approach to common embolization agents used in vascular interventional radiology. Am J Roentgenol 2014;203(4):699–708.
3 Wang W et al. Gelfoam-assisted Amplatzer Vascular Plug I for achieving rapid occlusion in proximal splenic artery embolization. J Vasc Interv Radiol 2012; 23(3) S:S65–S66.
4 Zhu X et al. Utility of the Amplatzer vascular plug in splenic artery embolization: a comparison study with conventional coil technique. Cardiovasc Intervent Radiol 2011;34(3):522–31.
5 White HA et al. The Amplatzer vascular plug. Cardiovasc Intervent Radiol 2008;31(2):448–9.
6 Dorenberg et al. Recurrent rupture of a hypogastric aneurysm caused by spontaneous recanalization of an Amplatzer vascular plug. J Vasc Interv Radiol 2006;17(6):1037–41.
7 Pech M et al. Embolization of the gastroduodenal artery before selective internal radiotherapy: a prospectively randomized trial comparing platinum-fibered microcoils with the Amplatzer vascular plug II. Cardiovasc Interv Radiol 2009;32(3):455–61.
8 Letourneau-Guillon L et al. Embolization of pulmonary arteriovenous malformations with Amplatzer vascular plugs: safety and midterm effectiveness. J Vasc Interv Radiol 2010;21(5):649–56.
9 Libicher M et al. Occlusion of the internal iliac artery prior EVAR: comparison of coils and plugs. Vasc Endovasc Surg 2012;46(1):34–9.
10 Stratil PG et al. Visceral trauma: principles of management and role of embolotherapy. Semin Intervent Radiol 2008;25(3):271–80.
11 Widlus DM et al. Evaluation of the Amplatzer vascular plug for proximal splenic artery embolization. J Vasc Interv Radiol 2008;19(5):652–6.
12 Shih CH et al. Transcatheter embolization of a huge renal arteriovenous fistula with Amplatzer vascular plug. Heart Vessels 2010;25(4):356–8.
13 Madoff DC et al. Portal vein embolization in preparation for major hepatic resection: evolution of a new standard of care. J Vasc Interv Radiol 2005;16(6):779–90.
14 Fischer G et al. Transcatheter closure of coronary arterial fistulas using the new Amplatzer vascular plug. Cardiol Young 2007;17(3):283–7.
15 Fruchter O et al. Endobronchial closure of bronchopleural fistulas with Amplatzer vascular plug. Eur J Cardiothorac Surg 2012;41(1):46–9.
