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Introduction to Endovascular Surgery: Arterial Access, Guidewires, Catheters, Sheaths, Angioplasty Catheters, and Stents
The core elements of all endovascular procedures include gaining arterial access safely, placing a sheath with hemostasis valve, and navigating to a selected target using shaped diagnostic catheters and guidewires. In most cases, the intervention itself involves use of an angioplasty balloon and deployment of a stent. We will cover this in the current chapter; other, more sophisticated devices will be described elsewhere in this textbook. Some of the more confusing characteristics of endovascular devices are the sizing measurements and device size compatibilities (an overview is presented in Tables 17.1 to 17.3 ).
TABLE 17.1
Calculating French Size
| The French size is three times the diameter in millimeters. A round catheter of 1 French has an external diameter of 1/3 mm; therefore the diameter of a round catheter in millimeters can be determined by dividing the French size by 3: D (mm) = French/3 or French = D (mm) *3 For example, if the French size is 9, the diameter is 3 mm. |
TABLE 17.2
Units for Sizing Endovascular Devices
| Needles | Gauge: thousandths of an inch |
| Wires | Diameter: thousandths of an inch Length: centimeters |
| Dilators | Outer diameter: French (divide by Pi to convert to mm) |
| Sheaths | Inner diameter: French Length: centimeters |
| Catheters | Outer diameter: French Length: centimeters Lumen: thousandths of an inch |
| Balloons | Maximal inflated diameter: millimeters Length: centimeters Shaft diameter: French Corresponding wire system: thousandths of an inch |
| Stents | Expanded size: millimeters Unexpanded size: French Length: centimeters |
TABLE 17.3
Guide to the Sizing of Endoluminal Devices
| Device | Diameter | Length |
|---|---|---|
| Needles | gauge | in/cm |
| Introducer sheath | French (inner diameter) | cm |
| Catheter (diagnostic) | French (outer diameter) | cm |
| Catheter (guide) | French (outer diameter) | cm |
| Balloons | French (shaft diameter) | cm |
Gaining Arterial Access
The femoral route is selected in the overwhelming majority of cases and the Seldinger technique remains largely unchanged from its original description. This approach is convenient for the operator, safe, and associated with the least likelihood of neurologic impairment. Moreover, the access site can be compressed against the underlying femoral head. We use ultrasound-guided access for all arterial and venous procedures that require vascular access. In our experience, this has been associated with a very low rate of groin complications.
The procedure begins by using fluoroscopy to identify the femoral head. Next, ultrasound is used to identify the common femoral artery and its bifurcation. The anterior rim of the femoral head can also be identified in thin patients. The latter is the only situation in which confirmation using fluoroscopy is not required. We use a micropuncture needle (21-G, 7 cm long) introduced under ultrasound guidance to perform a single wall puncture of the midportion of the common femoral artery. Blood return is observed, but this is not as brisk or as pulsatile as it would be through an 18-G access needle. The micropuncture wire is typically 0.018 in in diameter and 40 cm long. The wire must pass easily and freely in its progression up the iliac artery, and this should be observed fluoroscopically. Any resistance to passage of the wire should result in immediate fluoroscopy to ascertain the wire's location. Once the wire has passed into the common iliac artery, the inner dilator and wire are removed. There should now be brisk arterial bleeding. In our practice, a 0.035-inch Bentson wire is introduced and advanced up into the descending thoracic aorta. Once there is adequate wire support inside the arterial system, the remaining micropuncture sheath is removed and, in most cases, a 5-French access sheath is inserted, its dilator removed, and the side port flushed with heparinized saline.
One exception to the use of a micropuncture kit is in the heavily scarred groin. In this situation, a more robust 18-G needle is used for access to permit immediate placement of a 0.035-inch wire. This wire provides the necessary support to drive a sheath through the scarred groin.
Essential Tools
Vessel Dilators
Vessel dilators may be necessary when the groin is scarred, when large-bore sheaths are being placed, or when a combination of these circumstances exists. Although they are rarely necessary, their availability can mean the difference between completing and not completing the procedure.
These devices dilate the fascia and tissue to allow easy insertion of an introducer or catheter. Vessel dilators are measured in French sizes and are usually 10 cm long. They come packaged in a range of sizes. Some physicians do not use the introducer sheath during diagnostic studies but may use vessel dilators to prevent arterial damage when a diagnostic catheter is advanced into the artery. It is important not to overdilate to a diameter larger than that of the catheter to be used. However, since the sheaths' outer diameters are larger than their inner diameters, it may be useful to dilate one size larger than the sheath. For example, a 6-French sheath may need a 7-French dilator to allow easy passage. In our practice, we use hemostatic sheaths on every case.
Introducer Sheaths With Hemostasis Valves
Sheaths allow easy passage of guidewires, catheters, and interventional devices into the artery without vessel damage or excessive blood loss. An introducer sheath with a hemostasis valve should be used during all interventional procedures to protect the vessel and allow ease of passage for devices (balloons, stents, and atherectomy catheters) ( Fig. 17.1 ). Sheaths should also be used for diagnostic studies that require multiple catheter exchanges.
Different manufacturers' valves possess different characteristics, such as a self-centering feature for wires to reduce backbleed. Many devices require insertion tools to help cross the valve. Some sheaths may be wire-braided to reduce kinks or include a radiopaque tip for visualization. Sizes may range from 4 French to 12 French or larger. Some of the new endoluminal devices will require introducers up to 24 French. The larger sizes may require a cutdown to repair the artery postprocedure. Be aware that the French size relates to the inner diameter of the sheath. The outer diameter is usually 1 to 2 French sizes larger if the introducer has wire-braided reinforcement.
Sheaths also come in a variety of lengths and shapes. For example, in placing a stent in the supra-aortic trunks, a 70- or 90-cm sheath may be necessary. This protects the stent during introduction and allows contrast to be injected directly at the site being treated, thereby providing excellent visualization. It also provides a level of protection should the balloon rupture or the stent be displaced.
Guidewires
Two general styles of guidewires are used for vascular access: stainless steel and hydrophilic coated guidewires. Both types are used to gain access, guide catheters into the ostia of vessels, and cross lesions. They have a variety of common characteristics:
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They are available in lengths ranging from 40 to 300 cm.
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Most common working wires are in the 180-cm range.
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Shorter wires are used for Monorail balloons.
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Long wires are used for over-the-wire balloons, thrombectomy catheters, and contralateral tibial interventions.
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“Docking” wires allow an extension to be added to the back end of a wire, thereby “lengthening” the wire.
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Diameter, length, tip configuration, coating, stiffness, and handling properties should be selected for the task at hand.
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They may be used in conjunction with diagnostic catheters to facilitate selective vessel catheterization.
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There are also nonhydrophilic routine access and “working” wires.
In working with any wire, caution is key. If it does not go easily, it should not be forced. When you encounter resistance on removing a wrapped wire, avoid pulling it, as it might unravel. It is better to advance it with catheter support.
Stainless Steel Guidewires
Stainless steel wires may be manufactured as solid steel wires or as wrapped thin steel wires. The wires have a rigid, solid steel core called a mandrel and a flexible tip to help prevent vessel injury. Each guidewire has distinct characteristics that allow passage into the target vessel. Guidewires usually have a floppy tip with a stiff body. The distal tip configuration of the stainless steel guidewires is generally straight, angled, or J -shaped. Diameters are measured in thousandths of an inch, ranging from 0.009 to 0.038. Lengths are measured in centimeters and can range from 80 to 260 cm; however, specialty wires may come in longer lengths. Wires can be broken, but a safety wire built into the construct prevents the components from separating completely.
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