in both ionic and nonionic forms; nonionic agents are less osmolar than their ionic counterparts (Singh and Daftary 2008). Severe reactions are reported to occur with similar incidence among all iodinated contrast agents, but mild and moderate contrast reactions occur more commonly with the use of higher osmolality iodinated contrast agents (Singh and Daftary 2008). Nephrotoxicity is a major potential complication associated with the use of iodinated contrast agents and has become the third most common cause of acute renal failure in humans (Akgun et al. 2006). The most commonly used iodinated contrast agents are the nonionic monomers such as iohexol, iopromide, iopamidol, and ioversol (Dickinson and Kam 2008).
Gadolinium‐based contrast agents and CO2 are used most commonly in patients who have had a previous adverse reaction to an iodinated contrast agent and in those patients with an increased risk for development of nephrotoxicity (Moresco et al. 2000; Spinosa et al. 2000, 2001; Dickinson and Kam 2008), although some studies have reported nephrotoxicity in association with gadolinium contrast usage (Akgun et al. 2006; Ergün et al. 2006). Agents such as gadopentetate dimeglumine, gadodiamide, gadoteridol, and gadoversetamide are the most readily available gadolinium‐based contrast agents (Akgun et al. 2006) and are used when previous CO2 usage has resulted in a suboptimal study due to bowel gas artifacts or as a supplement to CO2 angiography (Spinosa et al. 2000, 2001). Gadolinium‐based contrast agents produce less detailed contrast studies as compared with iodinated agents and are therefore less useful for angiography during IR procedures (Spinosa et al. 2000). When using gadolinium‐based contrast agents, digital subtraction angiography is recommended to compensate for the less detailed study that is otherwise obtained (Spinosa et al. 2000).
To outline a hollow viscus such as the esophagus, urethra, and colon, substances such as barium and iodinated contrast agents have been used in veterinary patients (Hume et al. 2006; Weisse et al. 2006; Farese et al. 2008). In a recent study of esophageal tumors in dogs, barium sulfate was found to be useful in identifying mass location (Farese et al. 2008). In dogs, iodinated contrast agents have been used to evaluate urethral obstructions prior to urethral stenting (Weisse et al. 2006). Additionally, iodinated contrast agents have been used prior to colonic and esophageal stenting to delineate obstructions (Hume et al. 2006; Hansen et al. 2012).
Instrumentation and Implants
Access Needles
Traditional hypodermic needles or over‐the‐needle catheters (Figure 3.1) can be used to puncture vessels when obtaining vascular access using the Seldinger technique (Seldinger 1953). The size of the access needle used determines the wire size that can be introduced through the needle and into the vessel. The standard venous access needle is an 18‐gauge needle, which accepts guidewires up to 0.038 inches in diameter (Braun 1997). Needles that are 21‐ to 22‐gauge are considered to be micropuncture needles and allow for introduction of guidewires up to 0.018 inches in diameter (Braun 1997; Valji 2006).
Figure 3.1 Interventional oncology instrumentation. From left to right: (a) 18‐gauge over‐the‐needle catheter (left), 22‐gauge over‐the‐needle catheter (right). (b) 0.035‐inch hydrophilic guidewire. (c) Dilator and vascular access sheath. (d) Catheter with angled‐tip.
Guidewires
Selection of a particular guidewire (Figure 3.1) is dictated by the size of access needle that has been placed, the technique to be performed, and the vessel(s) to be selected. Most guidewires are available in three standard lengths: 150, 180, and 260 cm (Braun 1997). Alternative lengths of 60, 125, and 145 cm have been reported, but these are not readily available (Valji 2006; Kipling et al. 2009). The standard diameters of most guidewires are 0.035 and 0.038 inches. Smaller gauge wires generally ranging from 0.010 to 0.018 inches are used when microcatheters and smaller (micropuncture) vascular access needles are used (Braun 1997; Valji 2006; Kipling et al. 2009).
There are a few primary principles that must be adhered to when using guidewires. First, many guidewires contain a hydrophilic coating made of polytetrafluoroethylene that requires priming with saline to allow for smooth passage through the lumen that has been selected (Braun 1997; Kipling et al. 2009). When sufficiently wet, the guidewire should pass easily through a catheter and allow an increased ability to perform vascular selection (Braun 1997; Kipling et al. 2009). It is essential that the guidewire remains wet during the procedure to improve the function of the guidewire (Kipling et al. 2009). Second, the length of the selected guidewire should be at least twice the length of the catheter that is being used (Braun 1997). Third, if a guidewire is not passing easily through a vascular access needle, the needle may need to be repositioned. The wire should not be forced, as the needle may be subintimal or against a sidewall (Valji 2006). Lastly, a torque device can be placed on the end of a guidewire (approximately 5–10 cm from a catheter hub that has been introduced over the guidewire) to better manipulate and steer the guidewire (Kipling et al. 2009). These torque devices can be invaluable when passing a guidewire into vessels that are difficult to access and when crossing stenotic regions.
Guidewires are also used for nonvascular stenting procedures (Hume et al. 2006; Weisse et al. 2006; Culp et al. 2007; Kipling et al. 2009; Hansen et al. 2012). Stents that are placed through malignant obstructions are introduced over a guidewire, and the stent delivery system tapers down to the guidewire to allow for easier placement. In companion animals, 0.035‐inch hydrophilic guidewires have been used to facilitate stent placement for tracheal, urethral, esophageal, and colonic obstructions (Hume et al. 2006; Weisse et al. 2006; Culp et al. 2007, 2011; Hansen et al. 2012).
Sheaths
The use of IV sheaths (Figure 3.1) is indicated when a procedure involves multiple exchanges into and out of a vessel. The sheath protects the vessel wall from damage and allows for easier passage of different catheter types. Additionally, sheaths protect the vessel from stiff IV devices, balloon catheters, and IV foreign bodies (Braun 1997; Valji 2006; Stavropoulos et al. 2006).
A dilator that tapers down to a guidewire is usually present within a sheath and allows expansion of the previously made hole in the blood vessel. Sheaths contain a valve that prevents blood leakage while allowing entrance of specialized catheters, wires, stents, snares, and biopsy forceps (Snow and O’Connell 2000; Stavropoulos et al. 2006). Additionally, sheaths contain a sidearm that allows for injection of contrast, which can pass around wires and nonocclusive catheters (Snow and O’Connell 2000; Valji 2006). The French gauge of a sheath is determined by the largest gauge catheter that can fit through the sheath and represents the inner diameter of the sheath (Braun 1997; Snow and O’Connell 2000). The French size of a sheath is generally considered to be 2 French gauges smaller than the outer diameter (Valji 2006).
In human medicine, sheaths have also been used for nonvascular procedures such as antegrade ureteric stenting, percutaneous transhepatic biliary drainage, and colonic stenting (Braun 1997; Snow and O’Connell 2000). The use of sheaths in urethral stenting has been described in veterinary medicine (Weisse et al. 2006; Newman et al. 2009). Some sheaths are designed with a peel‐away component that is used during the placement of venous access devices and drainage catheters (Braun 1997). The peel‐away component allows a device to be inserted through the sheath, and the sheath can then be removed while leaving the device in place.
