Diagnostic Catheters and Guidewires



Diagnostic Catheters and Guidewires


Curtis W. Bakal and Sebastian Flacke


The basic principles of vascular access and catheter guidewire manipulation for selective and superselective angiography were described over a half century ago. Continued refinements in early techniques and equipment have facilitated the myriad sophisticated interventions that are considered current standard practice. The basic principles of diagnostic catheter and guidewire design and manipulation are nearly universal, and an understanding of these fundamental concepts and techniques is crucial to successful contemporary interventional practice.



Puncture Needles


The first percutaneous arteriograms were performed by direct needle puncture of the target vessel. In 1927, Egas Moniz developed the technique for cerebral angiography by direct puncture of the carotid artery; he is considered the father of cerebral angiography.1 Dos Santos and colleagues described a technique for nonselective aortography in which long needles were inserted via the translumbar approach under the guidance of surface anatomic landmarks because fluoroscopy was not yet available.2,3 Early selective angiography was performed via direct cutdown and catheter insertion. The Seldinger technique for femoral access, first reported in 1953, enabled simple and safe flush aortography.4 Seldinger’s U-shaped catheter extended application of his technique to selective catheterization of first-order aortic branches.5 However, the use of long 16-gauge sheathed needles for direct aortic access continued up to the early 1990s for cases in which femoral artery access was unavailable because the high brachial or axillary artery approach was cumbersome and had higher rates of significant complications.6 With the advent of newer 4-French (4F) catheters, brachial artery access became safer, and translumbar aortography soon became obsolete.


Needle gauges are derived from wire-gauging standards first used in England during the late 1880s (e.g., Stubbs or Birmingham Iron Wire Gauge system). Needle diameter (inches, estimated) = 1/gauge. Smaller gauges indicate larger diameters. The gauge number (G) always indicates the outer diameter of a needle.7 French (F) size refers to the diameter of a catheter in French units (3F = 0.97 mm = 0.038 inch). Larger French sizes indicate wider catheters. Catheter and dilator sizes are designated by the outer diameter, whereas sheaths are typically designated by their inner lumen size, that is, the maximum catheter diameter capacity (e.g., a 6F sheath accepts a 6F catheter). Typically, the outer diameter of a vascular sheath is 1.5F to 2F larger than the inner lumen.


Here are some conversions: A 16G needle has an outer diameter of 1.65 mm, corresponding to the outer diameter of a 5F catheter; a 20G needle has an outer diameter of 0.97 mm, corresponding to 3F.


Brachial access from the left (rather than right) side is normally favored. The left-sided approach should produce fewer neurologic complications because the carotid arteries are not in the catheter-wire path; manipulation of the catheter in the descending thoracic and abdominal aorta and its branches is also easier, especially in older patients with ectatic aortic arches. Direct aortic access via translumbar techniques for embolization of endoleaks and for direct vena cava dialysis and catheter placement has revitalized the use of translumbar access needles as interventional rather than diagnostic tools.8,9 The needles are also used to provide transhepatic access for long-term central venous access.10,11 Early arteriography used direct needle puncture techniques for both vascular access and injection. After initial needle placement, the sharp puncture stylet would be removed, and if vigorous arterial flashback was confirmed, the needle hub would be tipped downward to approximate the axis of the artery, and a blunt stylet inserted to advance the needle into the vessel. The needle was subsequently used for nonselective hand injection.12 These blunt obturators are packaged in some contemporary puncture needle kits, a testament to primordial diagnostic angiography.


Seldinger’s original technique involves the use of a hollow nonbeveled thin-walled cannula that accepts a sharp stylet for puncture. The classic 18G needle will accept a 0.038 inch guidewire, whereas the 19G needle will allow a 0.035 inch guidewire. Because 18G puncture needles have larger cross-sectional diameters than 4F diagnostic catheters do, they have been supplanted by 19G needles as the standard needle in many interventional radiology suites. The hub end of such needles is typically flanged, which assists in stabilizing it during access. The back end is funneled to direct the guidewire into the trailing portion of the needle. The double-wall puncture is made through the superficial anterior and deeper posterior vessel walls. After the stylet is removed, the needle is pulled back through the deep arterial wall into the lumen; when pulsatile flashback is returned, the guidewire is advanced into the vessel.4


Many interventional radiologists now favor single-wall puncture, a variant of Seldinger’s original technique. Needles used for this technique have a hollow nonstylet needle with a beveled cutting edge; the artery is entered on a forward pass through the anterior arterial wall into the lumen. Although these two techniques are often used interchangeably and according to individual operator preference, specific situations may favor the single-wall technique.13 Posterior wall puncture is generally avoided in patients with coagulopathy or those in whom lysis is planned because it increases the risk of bleeding. Single-wall cutting needles also facilitate access through fibrotic postoperative groins. They are favored by many for femoral venous puncture before dilation for placement of inferior vena cava filters to avoid accidental double puncture of the femoral vein and artery. (The vein may lie partially behind the artery with a low puncture.) As a cautionary note, because the single-wall needle tip is beveled, it can be both luminal and intramural and return pulsatile flow. Advancing the guidewire in this circumstance may initiate subintimal dissection. Dissection can also occur with good luminal position of any needle tip because the firmly held stiff needle cannot back away from a constrained advancing wire tip. Guidewire-related dissections can be difficult to recognize and thus may involve relatively long sections of the punctured artery (Fig. e4-1).




The Potts or Potts-Cournand needles are also variants of the original Seldinger design and are favored by neuroradiologists. This needle family was originally designed for direct carotid artery puncture. The outer cannula has a short bevel to minimize trauma during anterior wall entry, and its trailing end protrudes slightly into the hub to minimize the chance of thrombus formation or air trapping. The solid or hollow stylet is also beveled and is thought to facilitate entry into mobile neck arteries during direct puncture. Modifications of this needle type have eliminated the trailing protrusion by substituting a funneled hub for easier insertion of the guidewire.


The use of coaxial micropuncture systems for vascular access has increased in recent years; these systems are based on longer devices originally designed for safe organ access for nephrostomy or biliary drainage. The initial puncture is thought to be relatively error tolerant because it is performed with a 21G (rather than a standard 18G or 19G) needle. After passage of a stiff 0.018-inch guidewire, the needle is exchanged for coaxial Teflon dilators that allow conversion to a 4F or 5F system and subsequent passage of a 0.035-inch standard guidewire. This technique is especially favored for perceived difficult access situations in which multiple passes might leave an abandoned needle puncture (e.g., antegrade femoral puncture, thrombolysis), for dialysis access interventions, for ultrasound-guided punctures through anatomically sensitive regions (e.g., internal jugular vein), and for early resident or fellowship training. Although design improvements have been made, at times these microsystems may be extremely difficult to use in scarred groins or obese patients; during upsizing, the 0.018-inch guidewire support for antegrade passage of the dilator may be insufficient.


Microsystems or large butterfly-type intravenous (IV) needles are often used for access in small children.


Single-wall and double-wall puncture systems engineered to minimize operator exposure to pulsatile blood during vascular access have become popular. These systems consist of a vascular entry needle attached to a blood containment element, typically a short length of transparent valved tubing attached to the hub via a side port. The needle hub also contains a one-way valve that precludes backflow of fluid but allows antegrade passage of a guidewire. Flashback into the clear containment tubing signals lumen entry. The ability to hand-inject contrast material around a guidewire via the side port has helped popularize this type of needle.



Sheath and Dilator Systems


Vascular dilators are stiff short catheters with tapered leading edges that are used to create a soft-tissue tract to facilitate passage of a diagnostic catheter. They are particularly useful in scarred tissue and obese patients. The opening of the dilator tip should be exactly matched to the diameter of the guidewire, which must be very tightly fixed during advancement. A stiff guidewire should be used when advancing a vascular dilator.


Vascular sheaths are thin-walled catheters inserted over tightly matched dilators. After removal of the inner dilator, the sheath is left in place to allow subsequent passage of a catheter. Vascular sheaths generally contain a hemostasis valve at the trailing end, with a side port used for intermittent flushing with heparinized saline or for injection of contrast material. Sheaths are extremely valuable for stabilizing access through scarred tissue, vascular grafts, and in obese patients. The sheath also minimizes the vascular injury at the entry site into the vessel if multiple catheter exchanges are needed. They are used during virtually all interventions. During complex procedures such as embolization, the use of a vascular sheath is mandatory to maintain access and to aid in “rescue” of a kinked or occluded catheter. The use of even a short (11 cm) sheath will greatly aid in the application of pushing force or torque to a diagnostic catheter. Long sheaths can straighten tortuous vascular segments. A wide variety of long sheaths (up to 90 cm) with specialized tips are available; they enable passage of diagnostic catheters and interventional devices through tortuous vessels or branches that have acute angles of origin. These tips can be placed in a proximal selective position and increase stability of vascular access into the target vessel to assist in placement of a superselective diagnostic catheter. Injection of the sheath sidearm will allow proximal opacification of the vascular territory, while injection into the superselective catheter allows visualization of the distal territory. Continued forward flushing through the sheath sidearm is mandatory to prevent clot formation in the space between superselective catheter and sheath. This coaxial use of sheath and catheter has wide acceptance in more complex interventional procedures such as crossover angioplasty or carotid and renal artery stenting.



Angiographic Catheters


The typical interventional radiology suite is stocked with dozens of catheter types. Of historical note, there are still some active senior “angiographers” (e.g., the authors, some of the editors) who remember when every angiography suite had a hot plate and a steam tea kettle. The kettle would be fired up to steam-bend the tips and flare the hub ends of catheters that were cut from long rolls of polyethylene tubing. The current bounty of high-quality commercially available catheter designs have obviated the need to “roll your own.” Advances in materials technology have provided interventional radiologists with preformed diagnostic catheters that are much more user friendly, anatomically adaptable, and safer than the catheters available in the recent past.


Catheters are possibly the most varied products used by the typical interventional radiologist. It is useful to classify them functionally into two broad general categories: flush (nonselective) and selective.


Flush catheters must allow high-flow injections into the aorta or inferior vena cava and uniform dispersal (with minimal recoil) of contrast media via multiple side holes. The tip of a flush catheter is usually designed to help center the shaft in the vessel and preclude engagement and injection into a branch vessel; side-hole placement is typically behind the tip, on the shaft. Flush catheters with such blunt leading contours can be advanced without a leading guidewire. Flush catheters must be composed of material with high wall strength and low friction coefficients.


Selective catheters are designed with rotational stiffness to seek a vessel orifice, but with enough flexibility to pass the catheter far into the vessel. Flow rate and pressure considerations are less important because selective injections are performed at low flow rates that do not approach the hydraulic limits of the catheter.


Catheters can also be characterized by elements of their engineering:



All these characteristics are matched to the intended use of the device, affect specific performance parameters, and should be considered when purchasing and using catheters.



Catheter Shape


The shape of the catheter tip is perhaps its most essential or defining element.13



Flush Catheter


Aortic flush catheters traditionally include circular or “pigtail” protective tips. Flush catheter designs that produce a more compact bolus than traditional pigtail catheters have been of interest to diagnostic angiographers since the widespread adoption of digital acquisition in the late 1980s.14-16 Variants also include flush catheters with angled shafts that facilitate catheterization of the pulmonary arteries (e.g., “Grollman Pigtail” [Cook, Inc., Bloomington, Ind.]) or catheterization of the iliac artery contralateral to the femoral puncture site (Omni Flush [AngioDynamics, Queensbury, N.Y.]).



Selective Catheter


For selective catheterization of vascular branches of the aorta or vena cava, a variety of more complex shapes are needed because of the large size of the trunk vessel relative to the target vessel origin. Here, selection of the ostium, advancement down the vessel, and positional stability during subsequent manipulation and injection all depend on the wall-seeking behavior of the catheter; it must be in contact with the vessel wall opposite the target branch orifice. It is firm contact with the back wall that turns a catheter potentially flopping in midstream into an efficient device that will engage a branch origin and reorient the relatively vertical pushing force applied at the groin down the axis of the target vessel (Fig. e4-2). For selective catheterization, the two factors that most influence the choice of tip shape are the diameter of the trunk vessel and the angle of origin of the target vessel. The primary curve, closest to the tip, is generally chosen to approximate the takeoff angle of the target vessel. The secondary curve shape is chosen to enable passage of the catheter more peripherally into the target vessel (over the guidewire). Thus, the shape and radius of the combination of curves of a catheter should be chosen to maximize “push-off” from the back wall of the aorta.




Some catheter designs use a tertiary curve behind the secondary curve to further enhance wall-seeking ability. Complex catheters in which the primary and secondary curves are in the same direction include the Cobra and Head Hunter families. These catheters can be used right from the package. The Cobra catheter is typically used for catheterization of down-going vessels such as the visceral arteries, but it may not work well if the target vessel is steeply angled. Cobra-type catheters are advanced by pushing and removed by pulling. Waltman and colleagues reported the loop technique for reversing the curve of a braided Cobra catheter to enable catheterization of upgoing vessels such as the left gastric artery.16 Numerous selective catheters have primary and secondary curves of opposite orientation (Fig. e4-3

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Dec 23, 2015 | Posted by in INTERVENTIONAL RADIOLOGY | Comments Off on Diagnostic Catheters and Guidewires
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