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Contrast Agents
Anaphylaxis and renal failure are the most common potential adverse reactions associated with the administration of iodinated contrast agents [19]. True, life-threatening anaphylaxis following the administration of iodinated contrast agents is rare, occurring in approximately 1 per 40 000 to 1 to 170 000 patients [10]. Mild allergic reactions, such as urticaria, occur more commonly. Anaphylaxis may be distinguished from a vasovagal response by tachycardia and respiratory distress. It usually occurs shortly after contrast administration. Anaphylaxis must be promptly identified and treated aggressively. Patients who have a history of a contrast allergy should receive another contrast agent or be referred to an allergy specialist in case of severe reaction. Prophylaxis with steroids is not recommended for moderate allergic reactions[19]. As many symptoms experienced during prior contrast administration, such as nausea, may be labelled as a contrast “allergy”, it is important to always obtain a detailed history from patients as to the symptoms previously experienced upon contrast administration [10,20].
Diabetes and pre-existing renal insufficiency may predispose patients to renal failure following contrast administration. Despite the use of low osmolar contrast agents and the provision of other protective measures, such as pre-hydration, up to 15% of patients with diabetes and pre-existing renal insufficiency may experience renal failure following contrast administration. Patients may present with an increase in serum creatinine 24 to 48 hours following contrast administration, which may peak at 72 to 96 hours following contrast administration. Patients are usually oliguric but may become anuric. Management is often expectant as renal function generally returns to baseline in 7 to 14 days [10]. While metformin use does not increase the risk of renal failure following contrast administration, patients who undergo interventional radiology procedures that use contrast agents are instructed to hold this medication and wait 48 hours following the procedure before resuming this medication as they may experience lactic acidosis should they develop renal failure [10,14].
Possible adverse reactions to the administration of iodinated contrast agents have led to the use of alternative contrast agents, particularly in patients with a history of true anaphylaxis to iodinated contrast agents or poor renal function in certain circumstances. Gadolinium and carbon dioxide gas are alternative contrast agents that may be occasionally used [20].
Contrast may be administered by manual or mechanical injection during angiographic interventional radiology procedures. Manual, or “hand” contrast injection, is useful during the initial steps of a procedure to facilitate low volume and low flow rate angiography of small calibre vessels or catheters that may be insecurely positioned. Mechanical injection using a power injector is useful to facilitate high volume and high flow rate angiography of large calibre vessels and reduces the radiation exposure of members of the healthcare team. To prevent contamination, the formation of air bubbles, or disconnection during contrast injection, connection of a catheter to a power injection should be done carefully [10].
Needles
Micropuncture systems may be used when obtaining vascular access is challenging, such as during the access of a small calibre vessel or in the context of thrombolysis. Micropuncture systems use a 21 G needle that accepts a 0.018 inch guidewire. The 21 G needle is then removed over the guidewire and exchanged for a coaxial dilator, which accepts a 0.035 inch guidewire over which a larger sheath or catheter may be inserted [7].
Though there is significant variability in the type of vascular access needles available for various percutaneous angiography procedures, all vascular access needles have a central channel through which a guidewire may be inserted [10]. Vascular access needles may be either a one-piece needle with a beveled-tip or a co-axial system with a stylet housed within an outer metal cannula, such as a trocar needle or a Chiba needle [10,24]. One-piece needles are generally used for venous and arterial vascular access. One-piece needles have a sharp beveled tip, which is favorable for puncturing small or low-pressure vessels or vessels that may be mobile. A guidewire may be inserted into a one-piece needle once the tip is fully within the lumen of the vessel.
Co-axial systems with a stylet housed within an outer metal cannula are generally used for arterial vascular access. A trocar needle has either a beveled or a non-beveled outer cannula that contains a removable inner needle that is sharp and three-sided, which allows the outer cannula to be left in situ in the accessed blood vessel for exchange. A Chiba needle has both a beveled outer cannula and an inner needle. Co-axial systems with a stylet housed within an outer metal cannula must have the stylet removed before a guidewire may be inserted [10,24].
Guidewires
The diameter of a guidewire is measured in inches and it may range from 0.010 to 0.038 inches though 0.035 inch, 0.018 inch, and 0.014 inch guidewires are most common. A 0.035 inch guidewire is considered to be the standard diameter of a guidewire whereas 0.018 inch and 0.014 inch guidewires are considered to be microwires [10,23,24]. The diameter of a guidewire should be comparable to the inner diameter of the instrument or device into which it is inserted. If the diameter of a guidewire is too large as compared to the inner diameter of the instrument or device into which it is inserted then the guidewire will not be able to be inserted or it will become stuck. If the diameter of a guidewire is much smaller than the inner diameter of the instrument or device into which it is inserted then there will be an abrupt transition in size that creates a gap that may trap tissue and cause the instrument or device to become stuck on the guidewire. Such an abrupt transition in size may also cause vascular injury or cause an instrument or device to become stuck at vessel branches or on plaques while being advanced along the guidewire [10]. The length of a guidewire is measured in centimeters (cm) and may range from 80 to 300 cm [23].
Hydrophilic guidewires are coated with a hydrophilic material that is “adhesive when dry” and “slippery when wet” [23]. Hydrophilic guidewires are useful in fluid environments where less resistance is desired and facilitate easier navigation in small calibre or tortuous vessels to pass through blood vessel stenoses or occlusions. Use of a torque device may also facilitate easier manipulation of a hydrophilic guidewire and may assist in pinning and securing a hydrophilic wire as an instrument or device is advanced [7,10].
The stiffness of a guidewire corresponds to its intended use. Access guidewires are inserted through vascular access needles to establish vascular access. Access guidewires are the least stiff and have floppy leading ends to prevent trauma to blood vessels or tissues. Maneuver wires are used for navigation through blood vessels or tissues. Maneuver wires are intermediately stiff and they may have various additional properties to assist in navigating in various situations, such as the length of their floppy leading end. Rail wires are used for instrument or device exchanges and are the most stiff [7,10]. Some such wires have a stiff body and a floppy leading end. They are used for the advancement of large introducer sheaths, devices, angioplasty and stent and stent-graft deployment. Other types of wires have a J-shaped tip. These are ideal for vascular procedures in vessels with several bifurcations as the J-shaped tip is blunt and is therefore atraumatic and avoids inadvertent selection of vessel branches [7,10].
Catheters
Broadly, catheters can be divided into non-selective and selective catheters [23]. Non-selective, or “flush” catheters, are generally thick-walled and facilitate large volume, high pressure injections. They usually have multiple side holes. Straight and pigtail catheters are often used as non-selective catheters. Straight catheters should only be advanced over a guidewire whereas pigtail catheters can be safely advanced once the pigtail has been reformed [10,23]. Selective catheters are generally thin-walled and facilitate small volume and low pressure injections. The majority of selective catheters have a single end hole to direct contrast in a specific direction. Side holes may also be present in selective catheters to reduce the end hole jet effect, which refers to the injection pressure that pushes back the catheter and may dislodge the catheter or cause vascular injury. However, they should not be used for embolization as this may result in non-targeted embolization [10].
Various tip shapes of selective catheters are available and the choice of a selective catheter will depend on the size, orientation and shape of the targeted blood vessel and the technique for selecting the targeted blood vessel [10]. Catheters should usually be advanced or removed over a guidewire to decrease the risk of vascular injury. Aggressive probing with a catheter or advancing the catheter into a target vessel without leading with at least 1 to 2 cm of guidewire may result in vascular injury. It is also important to consider the tip shape and length of a catheter to ensure that once the targeted blood vessel is selected, the catheter will not dislodge [10].
Microcatheters facilitate the catheterization of small calibre and tortuous blood vessels while limiting vascular trauma and potential thrombosis. Selection of small calibre blood vessels is important for many interventional radiology procedures, such as chemo- and radioembolization as well as vascular embolization, as these procedures must be performed with a high selectivity to avoid non-targeted embolization. They may also be used for sampling or small volume injections [7,10]. Microcatheters are generally floppy and measure 2 to 3 F in size. Microcatheters fit coaxially within a standard 4 or 5 French diagnostic catheter through which they are advanced. The diagnostic catheter is usually positioned proximally in the proximal portion of the target vessel and the microcatheter is then advanced in tandem with a selective micro guidewire, which is 0.014 to 0.018 inches in diameter, through the lumen of the standard catheter. Guiding catheters or sheaths facilitate the positioning and stabilization of diagnostic catheters and other instruments or devices [7]. A triaxial catherization combines a guiding catheter, a diagnostic catheter and a microcatheter.
Lines
There are several important considerations for the insertion of CVCs. For CVCs, the internal jugular vein is the preferred site of access, given its large calibre, superficial position, and proximity to the superior vena cava. The location of the tip of a CVC that is intended to be used long term should generally be positioned high in the right atrium [10,27,28]. Catheter tips that are located in the superior vena cava or brachiocephalic vein have a higher incidence of dysfunction secondary to the formation of fibrin sheaths and catheter-related central venous stenosis and thrombosis [10]. CVCs are generally inserted with a patient being supine. When a patient sits or stands upright, the chest wall tissues may drop and the mediastinum may lengthen causing the catheter tip to withdraw a few cm. As such, it is common for the catheter tip to be placed a few cm lower into the right atrium than its intended location to compensate. While many conventional catheters may be trimmed to a required length, catheters that are used for special purposes, such as dialysis catheters, should not be trimmed given their specialized tip configurations [10,27,28].
CVCs are usually inserted with local anesthesia and conscious sedation may also be used. If a tunneled or implantable CVC is inserted, it is recommended not to tunnel or to create a subcutaneous pocket until vascular access has been successfully obtained. Skin that is infected or that has been irradiated as well as breast tissue should be avoided during peripheral or CVC insertion [10].
Balloon Catheters
Balloons may be either compliant or non-compliant. Compliant balloons are made from elastic material, such as polyurethane or latex. They have high elasticity and increased flexibility, allowing them to conform to the lumen of a blood vessel. Compliant balloons are only able to tolerate low pressures and are most often used to facilitate blood vessel occlusion or stent placement. Non-compliant balloons are made from more rigid material, such as nylon and polyethylene terephthalate. They have low elasticity and decreased flexibility, allowing them to inflate to a predictable size and shape even with increasing pressures. Most often non-compliant balloons are used for angioplasty as they are able to tolerate high pressures [10,29]. Nominal and burst pressures for each balloon are included on its packaging and on a sterile card found inside the balloon packaging that may be referred to during interventional radiology procedures. Non-compliant balloons that are rated for very high pressures are preferred for venous angioplasty, angioplasty of hemodialysis access or arterial angioplasty in calcified vessels [10,29]].
Various sizes of balloons are available, ranging from millimeters to larger than 4 cm in diameter and less than 1 cm to greater than 20 cm in length. The characteristics of the vascular bed and the size of the lesion will influence the choice of balloon. Angioplasty balloons are mounted on catheters that have two lumens. One lumen is used for a guidewire and the other lumen is used to facilitate balloon inflation and deflation. In a coaxial balloon, both lumens are at the hub of the catheter and the guidewire lumen can be used for contrast injection while the other lumen is for balloon inflation. Monorail balloons have a shorter lumen for guidewire rapid exchange with an insertion site on the side of the catheter and only one hub for balloon inflation. There are several enhanced variants of angioplasty balloons. Cutting balloons have small blades on the balloon surface while sculpting balloons have a metallic cage on the outside of the balloon to enhance the fracturing of plaques. Drug eluting balloons have a drug coating usually made of sirolimus, everolimus or paclitaxel to prevent restenosis [10].
Inflation of a balloon should be performed gradually as rapid inflation may result in greater trauma to the adjacent normal blood vessel. Softer lesions may dilate with minimal pressure whereas fibrotic or calcified lesions may require increased pressure and may not fully dilate [10]. Patients should be asked about pain during balloon inflation. Sudden severe or sharp pain, particularly persisting after balloon deflation, suggests possible blood vessel dissection or rupture. Hypotension and tachycardia may also be present in the setting of blood vessel dissection or rupture. Balloons should always remain mounted on the guidewire while imaging is subsequently performed to assess for any potential vascular injury. If blood vessel dissection or rupture is suspected, the balloon should be reinflated proximal to or across the lesion to prevent hemorrhage and stent graft placement or surgical repair may be required for blood vessel repair [10].
Stents
Stent insertion and deployment will vary by device though there are several basic principles. Stents should be inserted over a guidewire to maintain vascular access after deployment. Pre-dilation of a lesion with an angioplasty balloon ensures that it is able to be treated with a stent and facilitates easier positioning of the stent though primary stent placement limits overall manipulation of the lesion and may minimize embolic complications [10]. The length of a stent should adequately cover a lesion in its entirety with minimal extension into the normal blood vessel. Adequate visualization of the stent under fluoroscopy is imperative to its appropriate positioning and deployment. Balloon-expandable stents generally deploy from both outer ends toward the middle of the stent. They should be approximately 5 to 10% larger in diameter than the normal lumen of the blood vessel. Self-expandable stents generally deploy from the distal to proximal end of the lesion. They should be approximately 10 to 20% larger in diameter than the normal lumen of the blood vessel as their fixation depends on the apposition of the stent to the blood vessel wall [10]. Complications arising from stent insertion and deployment are similar to that of balloon angioplasty though additional complications, such as stent embolization post deployment, poor lesion expansion, and stent fracture may arise. The incidence of infection is rare and may result in pseudoaneurysm formation. Anticoagulation should be administered during stent insertion and deployment and a regimen of post-procedural antiplatelet therapy is often prescribed [10].
Embolic Protection Devices
Various devices may be employed to minimize the risk of particulate embolization, including filters and occlusion balloons. Filters are placed distal to a lesion while occlusion balloons may be placed proximally or distally to a lesion. Filters most often have a pore size between 80 to 200 micrometers and thus while they allow the antegrade flow of blood they may not fully prevent microembolization. Occlusion balloons placed proximally or distally to a lesion prevent the antegrade flow of blood when fully inflated. Any embolic material must be aspirated prior to deflation for occlusion balloons placed distally while occlusion balloons placed proximally usually rely on blood flow reversal for the removal of embolic material [10,30].
Inferior vena cava filters prevent thrombotic pulmonary emboli by preventing the transit of thrombus to the pulmonary circulation. Inferior vena cava filters do not prevent the formation of thrombus, improve anticoagulation, or treat pulmonary emboli that have already occurred. Patients with deep venous thromboembolism who cannot be anticoagulated or patients at increased risk for the development of deep venous thromboembolism who are not able to be appropriately monitored or receive prophylaxis should be considered for inferior vena cava filters. Inferior vena cava filters may be either permanent or retrievable. Some retrievable filters may only be temporarily placed while some retrievable filters may be left in place permanently, if required [10,22].
Inferior vena cava filters may be placed from the femoral vein or internal jugular vein. An initial cavogram is performed to assess the anatomy and patency of the inferior vena cava. The most inferior renal vein should be identified. A filter delivery sheath is inserted over a guidewire into the inferior vena cava and the filter is deployed. The location of deployment is generally infrarenal though several considerations, such as pregnancy, thrombus, or a short inferior vena cava, may warrant suprarenal placement. Upon deployment, a repeat cavogram is performed to confirm the position of the filter [10,22].
Embolic Agents
Coils
Embolization coils mechanically obstruct blood flow. They are made from various materials and may be coated with additional materials, such as synthetic fibers to promote thrombus formation, or polymers that expand when exposed to liquids. Embolization coils are available in a variety of sizes, lengths, and shapes [32,33]. Careful consideration must be made of these specifications when selecting the appropriate device for a procedure [10]. Coils are supplied straightened in plastic or metal tubing, which is placed in a catheter to advance the coil to its target location with the assistance of a guidewire or coil pusher. The coil diameter should approximate the catheter diameter to prevent the coil from reforming or becoming stuck within the catheter [32,33]. Catheters with side holes should never be used to prevent non-targeted embolization or the coil from becoming stuck within the side holes upon deployment [10]. Detachable coils are preferred in some situations for their precise deployment as they remain connected to the guidewire or coil pusher until an electrical, mechanical, is applied [10,32,33].
Vascular Plugs
Most commonly used vascular plugs are composed of fine nitinol metallic mesh. The nitinol metallic mesh mechanically obstructs blood flow and acts as a scaffolding for rapid thrombus formation that facilitates the occlusion of blood flow. When non-constrained, vascular plugs assume a shape that is preformed but may be lengthened to fit within a catheter. Larger plugs are usually detachable and allow for their repositioning, which is favorable in large calibre, high-flow blood vessels as the stability of the device may be determined before completion of the procedure, whereas small plugs are not detachable and may not be repositioned [10,33].
Microspheres
Microspheres are small spherically-shaped embolic agents, which may be made from various materials, including resins, polymers, glass, acrylics and hydrogels. In addition to their embolic properties, some microspheres may also be drug-eluting and administer drugs locally [10,32]. Microspheres are available in a variety of sizes, ranging from 40 to 1200 micrometers. Some microspheres may expand when hydrated or they may be compressed by approximately 20 to 30% upon deployment, which are important considerations when selecting a size of microsphere. Microspheres are suspended in a contrast agent as they are not radiopaque to facilitate fluoroscopic visualization. They are flow-delivered by a catheter to the site of targeted embolization. Care should be taken to prevent overly forceful injection, which may result in the reflux of the particulate embolic agent into a non-targeted vessel [10,32].
Gelfoam
Gelfoam is a temporary embolic agent that is absorbed 4 to 6 weeks after administration. It may be provided as a powder or as a solid in the form of a gelfoam sponge. Pieces of the gelfoam sponge can be cut to various sizes, ranging from a few millimeters to a centimeter (“torpedo”), which may be used to occlude a single blood vessel, or the gelfoam sponge can be cut and dissolved into a slurry and then used for embolization. Similar to microspheres, gel foam is usually mixed with a contrast agent as it is not radiopaque to facilitate fluoroscopic visualization. Gelfoam “torpedos” may be used with embolization coils to create a gelfoam “sandwich” in which gelfoam is sandwiched between coils to facilitate embolization [10,31,32].
Liquid Embolics
Liquid embolics are agents that are injected as a liquid and that will polymerize upon contact with blood. The main liquid embolics used are the cyanocrylate and the Ethylene vinyl alcohol (EVOH) copolymers. Cyanocrylate is a glue and needs to be diluted with lipiodol, which is an oily radio-opaque agent. The degree of dilution in lipiodol will influence the polymerization speed with a high dilution resulting in slower polymerization. The microcatheter must be flushed with D5% before injection. EVOH embolics are mixed with tantalum powder to be radio-opaque and DMSO as a solvent. The microcatheter must be flushed slowly with DMSO before injection. Different viscosities are available depending on the proportion of DMSO in the mixture [10].