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.
Catheters
When performing IV procedures, there are many catheter types that are available to the interventional radiologist. Decisions about which catheter is most optimal for a specific procedure are based on experience and the anatomy of the vessel that is to be selected. In human medicine, catheters with an outer diameter of 5 French (1 French = 3 mm) (Silberstein et al. 1992; Valji 2006) are chosen most commonly (Braun 1997). This catheter size has the advantage of having good torque control and high flow rates when contrast is injected (Braun 1997). A larger catheter (6–7 French) affords the user increased control of torque (Wojtowycz 1990a; Braun 1997). Commonly used catheters are made of nylon, Teflon, or polyurethane (Wojtowycz 1990a; Valji 2006).
Catheters are often categorized by the shape of the tip, with the basic catheter tip shapes being straight, pigtail, hook, and angled (Braun 1997; Valji 2006). Straight catheters are commonly used for embolotherapy (delivery of a vascular occlusion agent) and for opacification of a vascular tree; however, care should be taken when injecting through a straight catheter with a single end‐on hole, as injury to the vessel is possible (Braun 1997). Pigtail catheters are superior for opacification, as they allow for the injection of a bolus of contrast through several small holes in the catheter, thus preventing the jet effect that may be seen with straight catheters (Wojtowycz 1990a; Braun 1997; Valji 2006). Pigtail catheters should be removed over a guidewire so that the catheter is straight during removal and therefore less likely to cause vascular damage (Wojtowycz 1990a). Hook catheters, such as the shepherd’s hook and cobra catheters, are used to catheterize vessels that have acute angled branches (Braun 1997). These visceral catheters are advanced over a guidewire to the desired location. The catheters are then allowed to reform (take on the original shape of the catheter) in a large vessel (generally aorta or vena cava) and are then gently pulled back into the lumen of the vessel selected (Braun 1997). Angled‐tip catheters (Figure 3.1) are used in the selection of upward‐branching vessels; as with other catheters, a guidewire is often used to facilitate vessel selection and to maintain position once it has been established (Braun 1997). The Berenstein catheter is an example of an angled‐tip catheter (Braun 1997).
Similar to sheaths and guidewires, catheters can be used for nonvascular techniques. When stenting luminal obstructions, such as in the urethra or trachea, marker catheters are employed to determine the appropriate stent size (Hume et al. 2006; Weisse et al. 2006; Culp et al. 2007; Newman et al. 2009). Another nonvascular oncologic use includes palliation of malignant thoracic and abdominal effusions by placing catheters for percutaneous drainage. This has been described in several human studies (Brooks and Herzog 2006; Fleming et al. 2009). Pigtail catheters are often used for draining malignant effusions since they have multiple fenestrations; a locking loop mechanism that maintains the catheter position in the desired location is also present on some pigtail catheters.
When performing IO techniques, there are certain techniques and principles of catheter usage that are important to understand, namely the coaxial technique, the so‐called rule of 110, and superselective catheterization with guidewire‐microcatheter combination. As the size of a vessel that is being targeted for catheterization decreases, the catheter diameter must also decrease. Placement of a catheter that is too large can result in damage to the vessel wall, blood stasis with subsequent tissue ischemia, difficulty in removing the catheter, and vessel rupture (Stavropoulos et al. 2006). The coaxial technique is commonly employed to introduce progressively smaller catheters and wires to allow for superselection of vessels while minimizing the risk for the development of complications. Simply stated, this technique involves placement of a smaller wire or catheter (depending on the stage of the procedure) into a catheter that is already within the blood vessel. The smaller wires and catheters share the same axis as the indwelling catheter, and thus the term coaxial is used. This technique has been described in human patients undergoing selection of small vessels (Korogi et al. 1995; Tajima et al. 2008).
A guide for vessel selection (particularly celiac and superior mesenteric arteries in humans) has been described and termed the “rule of 110” (Chuang 1981; Nemcek 1996). According to Chuang (1981), this rule states that “both the length and width of the catheter tip should be about 110% of the width of the aorta at the level of the artery (that is to be selected) as it branches.” The width refers to the distance between the tip of the catheter and the proximal straight limb of the catheter (Chuang 1981). If this technique is employed with a catheter of appropriate size and tip, the tip of the catheter should engage the branching vessel (i.e. celiac, cranial mesenteric, or renal artery in dogs and cats) as the catheter is pulled caudally and should subsequently enter the chosen vessel.
A guidewire can be combined with a microcatheter (using the coaxial technique) to allow for superselective catheterization. To accomplish this, the guidewire‐microcatheter combination is passed through a guiding catheter to a vessel branch that is a few orders less than the desired branch (Stavropoulos et al. 2006). The guidewire‐microcatheter combination is then advanced toward the desired branch, and the guidewire is used to select the desired branch. The catheter is then immediately but gently advanced over the guidewire and into the vessel to prevent the guidewire from backing out. This should be performed in a series of steps that are slow and calculated (Stavropoulos et al. 2006). The guidewire can then be advanced further into the vessel or into a smaller branching vessel as needed.
Vascular Closure Devices
A vascular closure device is an instrument used to close the hole in a blood vessel that remains after the removal of a vascular access sheath. The use of arteriotomy closure devices is well documented in human interventional cardiologic medicine; however, the use of these devices in veterinary medicine is uncommon (Meyerson et al. 2002; Koreny et al. 2004; Nikolsky et al. 2004). Historically, manual compression combined with bed rest was used to control bleeding at an arteriotomy site (Hoffer and Bloch 2003). Concerns over the high rates of bleeding at these high‐pressure sites spurred the development and use of devices that can be used to close vessels after a vascular IO procedure has been performed (Dauerman et al. 2007). Benefits seen with the use of closure devices have included decreased time to hemostasis and earlier ambulation (Meyerson et al. 2002; Tron et al. 2002; Hoffer and Bloch 2003; Dauerman et al. 2007). Closure devices are not universally used in human cases, however, and some studies have suggested similar or higher complication rates associated with the use of closure devices as compared to manual compression (Meyerson et al. 2002; Nikolsky et al. 2004; Dauerman et al. 2007).
Vascular closure devices can be divided