Imaging for the Evaluation and Treatment of Vascular Trauma

Indications

Transcatheter angiographic evaluation is a useful tool for trauma patients, both to diagnose and to localize and treat vascular injury. Conventional angiography may be necessary when images on CTA are obscured by artifacts caused by metallic fragments, by soft-tissue air, or streak artifacts. Thus, shotgun or wounds with multiple metallic fragments are often best evaluated with direct catheter angiography ( Fig. 8.1 ). With complex wounding mechanisms, such as high-energy explosives, vascular injuries needing repair may be missed during early phases of care. For combat-injured soldiers evacuated through several echelons of care, physical examination and clinical assessment may be insufficient. Routine arteriography at centers providing higher-level care can identify vascular injuries that may be amenable to endovascular treatment. Thus, arteriography should be considered in cases of complex trauma involving high-energy, penetrating mechanisms or wounds in proximity to named vessels.

Transcatheter angiography is often used as part of a strategy for endovascular therapy of vascular injury. As one example, if angiography demonstrates significant arterial injury, low-pressure inflation of a compliant balloon may be used to occlude flow into the damaged area. In these types of cases, endovascular techniques may be used both to control bleeding and accomplish definitive treatment of the injury.

Preparation

As intravascular contrast agents are used for transcatheter arteriography and CTA and have the potential for nephrotoxicity, it is important to be aware of the risk factors for renal injury. Post-contrast acute kidney injury (PC-AKI) is the general term used for a decline in renal function within 48 hours of intravascular administration of iodinated contrast medium. PC-AKI is a correlative diagnosis. It does not indicate that the contrast was the cause of the observed deterioration in renal function. Contrast-induced nephropathy (CIN) is the specific term used when the cause of decline in renal function was caused by the contrast administration. Factors associated with PC-AKI in trauma patients include hypotension (systolic blood pressure less than 80 mm Hg), heart failure, advanced age (older than 75 years), anemia, diabetes, preexisting renal insufficiency, and increasing volume of contrast. Commonly used criteria for the diagnosis of PC-AKI or CIN include either a greater than 25% increase of serum creatinine or an absolute increase in serum creatinine of 0.5 mg/dL after administration of a contrast agent.

Baseline renal function should be assessed prior to contrast administration, when possible. Patients at risk for allergic reactions to contrast may be premedicated with intravenous (IV) corticosteroids and histamine blockers. Although allergies to shellfish do not predict risk of a contrast reaction, atopy or a history of prior contrast reactions may. Avoiding use of large doses of contrast (as may occur in patients undergoing multiple imaging procedures), avoiding hypovolemia (a practical concern in many trauma patients), and monitoring renal function are recommended. Use of iso-osmolar contrast (e.g., iodoxinol) and reduced volumes of contrast reduce CIN risk. There is limited evidence suggesting that the use of n -acetylcysteine, theophylline, sodium bicarbonate, and statins further reduce the incidence of CIN.

Pitfalls and Danger Points

The risks of contrast angiography include the following:

• ■

  Vascular access site complications, such as vessel injury (e.g., hematoma, pseudoaneurysm, embolization, thrombosis)

• ■

  CIN

• ■

  Anaphylactic response to the contrast agent

• ■

  Technical and time requirements

• ■

  False-negative studies

• ■

  Risks of ionizing radiation

Catheterization for angiography poses a small risk of iatrogenic vascular injury at the site of access. The risk of access site injury is minimized with the use of ultrasound guidance, direct operative exposure, and/or the initial use of small-caliber access needles and wires (i.e., micropuncture devices). Diagnostic yield depends on the angiographic technique used and the size and location of the vessel being imaged. Injuries may be missed in high-flow vessels (such as the aorta) due to rapid washout of contrast, or an injury may be overlooked if imaging is performed in only a single plane. Improper timing or an insufficient contrast bolus may result in poor quality imaging, especially if the contrast is not directly administered into the vessel of interest via selective or subselective catheterization.

There is attendant risk with the use of ionizing radiation (x-rays) for angiography, though the risks to most patients are usually negligible. Surgeons and staff who have regular exposure to x-rays during procedures should have specific training in radiation safety and practices to minimize their own occupational radiation exposures. It is important to note that scatter from the patient is the main source of radiation to which medical personnel are exposed. Practices to maintain exposure to levels as low as reasonably achievable (ALARA) should be mandated. Radiation doses may be affected by factors that cannot be readily modified, including the size of the patient and the part of the body being imaged. Specific actions to reduce radiation doses include reduction of the time of exposure, increase of the distance from the source of the scattered radiation, and effective use of shielding (including lead garments and glasses).

Operative Strategy

There are three tiers of technical sophistication for trauma arteriography, as follows: (1) simple “on table”; (2) portable C-arm; and (3) fixed, floor, or wall-mounted system either in a dedicated suite (i.e., radiology department unit) or in a “hybrid” OR space. Resource availability and clinical circumstances typically dictate which approach is used. Patients who are hemodynamically or physiologically unwell may need to be taken immediately to the OR for management of their injuries. Although modern trauma centers are moving to building fixed imaging systems in many of their trauma or resuscitative ORs, these advanced capabilities may not be available in most centers. As such, on-table angiography using several single-picture x-rays or with a portable C-arm using basic cine loop angiography (with or without digital subtraction) may be required.

Traumatic injuries of the descending thoracic aorta are preferentially managed with thoracic endovascular aortic repair (TEVAR) ( Fig. 8.2 ). Early experience with TEVAR highlighted shortcomings of first-generation graft devices, which were designed primarily to treat aortic aneurysms. However, current devices are available in sizes that better fit a normal-caliber aorta, and that better appose to the distal transverse arch and proximal descending thoracic aorta. Endovascular techniques can be used to manage hemorrhagic shock and certain patterns of vascular trauma, including pelvic fractures with associated retroperitoneal hemorrhage. Temporary deployment of an endovascular balloon can provide proximal occlusion of the aorta for hemorrhage control during open operative resuscitation or surgical management of vascular injuries. In selected cases, transcatheter management can provide definitive therapy ( Fig. 8.3 ).

The yield of angiographic identification of a pelvic source of bleeding ranges from 43% to 78%. Sources of hemorrhage include injuries to major pelvic arterial and venous structures, but a small vessel disrupted by fractures can bleed substantially and transcatheter angiography often directly identifies these sources of bleeding. Endovascular treatment with embolization is effective for management of hemodynamically unstable patients with pelvic fractures, though external fixation and pelvic packing may be better initial therapies.

As detailed in another chapter of this textbook, selective catheterization with flow-directed particulate embolization is one method of controlling bleeding from small arteries at sites of injury. Embolic coils may be deployed to proximally occlude an injured vessel, but temporary occlusion alone may be inadequate for pelvic trauma. As such, catheter-directed use of inexpensive and readily available materials, such as Gelfoam pledgets or slurry into the source vessels (Upjohn, Kalamazoo, MI) can also be effective.

Angiographic findings indicative of extremity vascular injury or disruption include uncontained extravasation of contrast, pseudoaneurysm or contained extravasation of contrast, arteriovenous fistulae, intimal tear, spasm, or occlusion. Covered stents and other endovascular strategies can be used for extremity vascular injuries (although the benefit of peripheral artery endovascular therapy is less obvious than the superiority of endovascular treatment of blunt traumatic aortic injuries). Because injuries in patients who are in shock require immediate attention and because extremity vascular injuries are often associated with skeletal, soft-tissue, or other trauma, open surgical repair remains the more common approach. Still, endovascular techniques may be advantageous when the extremity exposure is difficult or associated with considerable morbidity, as with injuries to the subclavian or axillary arteries.

Operative strategy may vary with the situation. Factors to consider include the patient’s hemodynamic and physiologic status, the level of endovascular expertise, the quality of the available imaging systems, and the inventory. Endovascular maneuvers may be used to temporize or definitively control hemorrhage (e.g., balloon catheter occlusion or embolization). Some endovascular treatments may be safely delayed and not performed for hours, or even days after the original injury, for example, TEVAR for mild or moderate blunt traumatic aortic injury (BTAI) (see Fig. 8.2 ).

Surgeon training and experience (or the availability of an interventional specialist) may determine whether an open surgical therapy, an endovascular treatment, or a mixed or hybrid approach is most practical for a given injury scenario. Simple endovascular maneuvers for arterial access and pressure monitoring, hemorrhage control and resuscitation (i.e., resuscitative endovascular balloon occlusion of the aorta [REBOA]), and arteriography should be in the armamentarium of general and trauma surgeons. Advanced endovascular techniques, subselective catheterization, use of aortic endografts, and other complex interventions require additional training and credentialing. When possible, complex vascular interventions should be performed with optimal imaging equipment.

Angiography and simple endovascular interventions can be performed with a relatively limited inventory of access needles, wires, and sheaths as well as catheters, working wires, balloons, and stents. As more complex interventions are contemplated, sufficient inventory of endovascular devices and supplies is needed to ensure success. Supplies needed for trauma interventions may include aortic endograft systems, large sheaths and compliant aortic balloons, snares, microcatheters, embolic devices and agents, and covered stents. It is also important to have an appropriate range of device sizes to meet a variety of needs. The availability of anticipated implants and supplies must be confirmed before embarking on a plan of endovascular therapy.

Operative Technique for Angiography

On-table angiography does not require advanced skills or specialized equipment. Contrast is injected by hand, and a single radiograph is obtained. This technique may be of practical use during operative management of extremity injuries when the presence, location, or extent of an injury is uncertain ( Fig. 8.4 ). It can also be used to evaluate the technical result of a vascular repair. Vascular access is obtained, either in a percutaneous manner or after open surgical exposure of the vessel. The artery in question is accessed using a hollow-tip needle, a butterfly set, a catheter, or a sheath placed using the Seldinger over-the-wire exchange technique. The imaging plate can be inserted in a sterile wrap and positioned on the surgical field under a limb to be imaged.

Although this simple and useful arteriographic technique is available for use in any situation, it has limitations. First, the delay between contrast injection and imaging must be estimated, and errors in timing the transit of contrast to the area of interest will result in failure to opacify the vascular segment of interest. Second, this technique provides only one image per injection. Each individual image must be processed for evaluation of the adequacy of the technique, the projection, and the field of view. This approach can be time consuming.

The limitations of single-image, on-table angiography can be overcome by the use of a portable C-arm fluoroscopy system with cine loop recording and digital subtraction capabilities. By using cine loop angiography, timing of imaging is less critical. Multiple images can be recorded with each single contrast injection. Digital subtraction angiography (DSA) provides superior definition of vessels as it removes the image of overlying or surrounding structures, including bone, from the vessels of interest. Because of this advantage, DSA generally requires less contrast than nonsubtracted angiography, including that of the aorta and visceral vessels.

Intraoperative use of a C-arm fluoroscopy system can provide the real-time imaging needed for selective catheterization with shaped wires and catheters as well as guidance for interventions, such as placement of an occlusion balloon, therapeutic embolization, or placement of covered stents. In order to use fluoroscopy, the patient must be positioned on a radiolucent operating table. Use of a surgical table designed for endovascular procedures is helpful. The surgeon can move the endovascular table to position the anatomic area of interest in the field of view while the fluoroscopy unit remains stable. Many other tables used for trauma surgery and orthopedic procedures (including the Jackson table) are radiolucent and suffice for basic fluoroscopic imaging and endovascular intervention. Use of a fixed table, however, often requires a radiology or equipment technician to be more actively involved in C-arm positioning to center the field of view.

Fixed imaging systems are standard in larger hospitals with busy vascular surgery and interventional radiology programs. These have wall, ceiling, or floor mounted systems, typically integrated with a contrast management or injection system. Fixed imaging units have table mounted controls for use by the operating surgeon. They are programmed with various image acquisition protocols and have features to facilitate intravascular catheter navigation. Fixed imaging systems provide larger imaging fields and magnification capabilities, which provide high resolution imaging of the vessels of interest ( Fig. 8.5 ). Centers that have invested in fixed imaging suites in the OR (i.e., hybrid ORs) also have a more extensive inventory of catheters, guidewires, and other endovascular supplies, and accessory equipment.