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Considerations for Conduit Repair of Vascular Injury
Introduction
In 1949, Jean Kunlin performed the first saphenous vein bypass in the lower extremity of a patient suffering from ischemia. The work was not the result of chance alone as his predecessors in vascular surgery had been working on perfecting the technique of arterial surgery. Individuals such as Alexis Carrel developed the technique of a meticulous anastomosis, as well as experimenting with venous interposition grafts and the use of allografts, and Jay McClean discovered heparin, which was utilized in Kunlin's successful procedure. In the same manner, our current treatment of vascular trauma is based on lessons learned in the civilian sector as well as from military experiences. For example, in World War II (WW II), the majority of vascular injuries were treated with ligation, leading to an amputation rate of 49%. During WW II, vein grafts were employed in a very small number of patients (40), resulting in an amputation rate of 58%. At that time, ligation of vascular injuries was felt to be necessary due to the long transport time required for wounded service personnel. With decreased transport times and knowledge of these past experiences, Rich and colleagues successfully implemented arterial repair in the majority of patients in the Vietnam War and subsequently reported an amputation rate of 13%. In that experience, nearly all interposition grafts were reversed great saphenous vein, and that form of reconstruction was used in 46% of the cases. In the civilian setting in the 1960s and 1970s, the abandonment of ligation as treatment for vascular trauma led to amputation rates that ranged from 2% to 10%. It is these advances, both in the civilian and the military settings, that have led to the current standard of repairing vascular injury—in those that will tolerate repair—with interposition or bypass grafting as needed.
Identification of the Optimal Vascular Conduit
The search for the optimal vascular conduit, in both elective and emergency situations, has been a source of debate and the source of many research projects. The ideal vascular conduit should be durable , able to be incorporated by the host or recipient , resistant to infection , and readily available . In numerous studies of elective peripheral vascular bypass, autologous vein has proven superior to prosthetic modalities in the lower extremities, whereas prosthetic grafts are generally better suited for the larger caliber central arteries. Unlike elective situations, trauma cases differ in the sense that patients are generally younger and have healthy vessels free of atherosclerotic occlusive disease that can complicate repair. The limiting factor in trauma is the fact that many individuals have concomitant orthopedic, soft-tissue, or abdominal injuries that need to be addressed in addition to the vascular injury. Furthermore, although vascular repair is usually feasible, it is the ability to place the repair conduit through a contaminated wound or soft-tissue deficit that often limits success. Specifically, the need to assure adequate soft-tissue coverage to protect the conduit from contamination and disruption often determines ultimate success or failure.
As documented throughout this text, the approach to vascular trauma is generally straightforward. Approaches to the injured vessel include primary repair or restoration of perfusion using an interposition or bypass graft. The technique of patch angioplasty is also a useful approach in select injuries that are less severe. Finally, ligation may be used as a damage control approach in some cases. When considering whether to reconstruct or ligate an arterial injury, one should consider the patient's physiologic condition and other coexisting injuries. Also, one must consider the degree of ischemia likely to result from vessel ligation. If the artery is minimally disrupted, it may be able to be débrided, mobilized, and repaired primarily.
In the situation where the artery cannot be repaired primarily, or cannot be safely ligated there is the need for an interposition or longer bypass graft. As detailed in Chapter 23 , temporary vascular shunts are useful as a bridge to interposition or bypass grafting when ligation is not an option. When considering interposition or bypass grafting, one must address the same technical factors that are important in elective vascular reconstruction as follows: (1) inflow vessel, (2) outflow vessel, and (3) conduit. Although the vascular injury itself may be straightforward, the patient is often not straightforward and may have suffered multiple injuries. The overall injury severity and any hemodynamic instability will impact the choice of conduit and the outcome of the procedure ( Fig. 24.1 ). The ease of availability and necessary length of conduit are also factors to be considered when pursuing this form of reconstruction. It would be nice to imagine that one solution applies to both military and civilian scenarios, but the settings (and the nature of the wounds) are most often different. This chapter will describe the options for selection of the vascular conduit to be used for repair of vascular injury.
Types of Conduit
The use of a conduit in vascular trauma is, in principle, the same as its use for atherosclerotic occlusive or aneurysmal disease. Vascular conduits can be considered in the following categories: (1) autologous vein and artery (i.e., autografts), (2) prosthetics, and (3) biologics. Vascular trauma has a rate of wound contamination that is proportional to the mechanism of injury and degree of soft-tissue injury. The degree of contamination can be minor such as with a single stab wound or a laceration with a piece of glass, or it can be major such as with an open femur fracture with soft-tis-sue wound. More than a decade of war in Afghanistan and Iraq has laid bare the complexities associated with vascular trauma in highly contaminated wounds resulting from improvised explosive devices (IEDs). Traditional teaching has emphasized the use of autologous vein grafts for vascular repair in the setting of contamination. However, due to the complexities of different trauma scenarios such as bilateral lower extremity injury, this conduit (e.g., the great saphenous vein) may not be feasible or appropriate. If autologous vein is not available, vascular hemorrhage can be controlled by ligation, the use of temporary vascular shunts, or reconstruction using a commercially available prosthetic or biologic conduit.
Autologous Conduit
The gold-standard conduit is autologous tissue and most commonly a vein. In rare cases, one may choose to use an arterial conduit for vascular reconstruction. Because the venous system has multiple, redundant outflow tracts there are several choices for vein harvest. The lower extremity has the longest and most commonly used options, including the greater and lesser saphenous veins, the femoral vein, and dorsal foot vein. The cephalic and basilic veins of the upper extremity can be used independently or as a longer single-segment graft. In the neck, the anterior, exterior, and internal jugular veins are options for vascular conduit. The veins of the neck are most commonly used as adjuncts for carotid artery repair because of their proximity.
Use of autologous vein requires adhering to the tenants of safe and effective dissection and procurement. In general, superficial veins may be harvested using a single continuous incision, skip incisions, or a newer minimally invasive technique. The single incision is the most expedient and most commonly described technique for greater saphenous vein harvest. However, this is associated with wound infection and dehiscence in 17% to 44% of patients. In an effort to decrease wound complications, attempts have been made to harvest this vein with multiple, shorter incisions and intervening “skin bridges.” Although this technique may take additional time and familiarity with the approach, it has been shown to decrease wound complications (9.6%) in at least one large series. The least invasive technique for saphenous vein harvesting is the newer endoscopic approach. With this technique, the vein is harvested with electrocautery through several percutaneous incisions. Although risk of wound infection is decreased with the endoscopic technique, this does carry the added risk of thermal injury to the vein. Although it is desirable to reduce wound morbidity associated with saphenous vein harvest, it seems that as the method becomes less invasive, the time needed for the procedure increases, as does the need for expertise with the endoscopic procedure. Because of this, the less-invasive approaches to saphenous vein harvest are not practical in most centers for cases of vascular trauma.
Although rarely used, arterial conduits may provide a better size match for the injured vessel and they do not require lysis of valves. Arterial conduits may also have improved handling characteristics, better compliance match, and even superior patency. The use of autologous arterial conduit is feasible and efficacious, but remains limited in the setting of trauma due to the paucity of harvest sites, their challenging anatomic locations, and the lack of redundancy or length. The internal mammary (internal thoracic) artery is the most commonly used arterial conduit. However, due to its confined location, access is only feasible through a median sternotomy. The gastroepiploic artery has also been used with favorable patency in coronary artery bypass surgery when the internal mammary artery and the saphenous vein are not available. The most commonly explanted autologous artery is the radial artery, which ranges from 2 to 4 mm. The internal iliac artery can be used, but this is infrequent except in select cases of pediatric injury. Klonaris et al. described the benefits of using the internal iliac artery for repair of infected femoral artery pseudoaneurysm resulting from trauma from repeated access during illicit drug use. This report describes the use of internal iliac artery for reconstruction in 9 (5 patch, 4 interposition graft) of 12 patients. At a mean of 19 months after repair, Klonaris et al. reported no complications or instances of limb loss. Finally, the external carotid artery can serve as an autologous conduit in repair of proximal internal carotid artery injuries. In these cases, the external carotid can be transposed onto the mid or distal internal carotid in situations where the proximal portion is injured. Other arteries such as the deep inferior epigastric may be used as a microvascular graft to replace a damaged arterial segment, but these smaller arteries are not typically a consideration in trauma.
Prosthetic Conduits
Since the first prosthetic graft made of woven nylon, a variety of grafts have been developed, including collagen-impregnated, woven nylon (Hemashield Dacron, Maquet Germany), heparin-bonded Dacron, expanded polytetrafluoroethylene (ePTFE), heparin-bonded ePTFE (PROPATEN, Gore Medical, Flagstaff, AZ), hooded PTFE (Distaflo, Bard PV, Tempe, AZ), ring reinforced ePTFE, and even multilayer–hybrid grafts consisting of both woven nylon and ePTFE (Triplex, Vascutek Terumo, Scotland, UK and FUSION Maquet Cardiovascular, Wayne, NJ). Biosynthetic vessels (Omniflow II, LaMaitre Vascular, Burlington, MA) consisting of a woven ovine collagen overlying polyester have been used with some success in infected fields but is unavailable for sale in the United States.
For large vessels such as the aorta and iliac arteries, prosthetic grafts have been used with great success. However, higher rates of thrombosis remain a disadvantage of prosthetic grafts in smaller vessels regardless of conduit composition. In the classic studies of Bergen and Veith, comparing vein to ePTFE for reconstruction of age-related disease, short-term (2-year) patency was comparable between the conduits. When longer-term patency rates of these studies were reported, saphenous vein was found to be superior. Prosthetic grafts are used today for elective bypass procedures, but mainly in the femoral and above-knee location. Adjuncts such as heparin bonding of the luminal surface of the ePTFE have been used with modest or mixed results in attempts to improve patency. The use of prosthetic grafts in trauma has been espoused by some who purport that short segments or lengths of prosthetics are durable and react more favorably than vein in contaminated fields. Figure 24.2 shows a through and through carotid artery injury repaired with a short segment PTFE interposition graft. Some of these studies also point to preservation of the autologous vein for future revascularization as an advantage of using prosthetic conduits as the initial option.
Biologic Conduits
The most modern construct of the vascular conduit is the biologic graft. These may be allografts, xenografts, or those created (i.e., grown) using modern regenerative medicine technologies. Allografts include cryopreserved vein, cryopreserved artery, and preserved treated human umbilical vein (HUV). Dardik began work on HUV as a conduit starting in the 1970s. At 37 to 40 weeks of gestation, the HUV (2- to 3-mm diameter) is of similar caliber to that of small arteries and contains moderate amounts of collagen and elastin to provide elasticity. In a qualitative analysis of the microstructure of HUVs, Li et al. showed that the collagen to elastin ratio in these vessels is similar to an artery of the same caliber. Studies by Li and colleagues also demonstrated that HUV had comparable morphologic and microstructural indices as similar-size arteries. These authors concluded that because of the similarities, HUV may be a substitute for small-caliber arteries such as coronary, brachial, radial, and tibial. In a review of 211 femoral-to-popliteal bypass operations (using the second-generation glutaraldehyde-stabilized HUV grafts), Neufang et al. reported the primary, primary-assisted, secondary patency, and limb salvage after 5 years as 54%, 63%, 76%, and 92%, respectively (with no difference between above-knee and below-knee grafts).
Cryopreserved saphenous vein allografts, also referred to as cadaveric saphenous vein, have been utilized as an alternative conduit. Early results with this conduit demonstrated poor patency. Walker et al. studied 35 patients who underwent lower extremity bypass grafts for symptomatic ischemia. The primary patency was 67% at 1 month, 28% at 12 months, and 14% at 18 months. In an effort to improve patency, Buckley et al. prospectively enrolled patients for femoral-to-below-knee popliteal artery bypass using an anticoagulation protocol. Twenty-four patients with ischemic lower limbs underwent bypass with cryopreserved vein and were treated with aspirin, low-dose heparin, low-molecular-weight dextran 40, dipyridamole, and warfarin. The limb salvage rate in this study was 88% at 6 months and 80% at 24 months. Although this report demonstrated improved patency, it enrolled a small number, and patients required high levels of anticoagulation to obtain the results, an option oftentimes not available to a multiply injured trauma patient.
Cryopreserved, cadaveric arterial allografts have been developed as an alternative to cryopreserved vein. Cryopreserved artery is derived from the descending thoracic and intrarenal aorta, as well as the iliac and femoral arteries of human cadavers. Due to the variety of diameters, one can find an appropriately sized cryopreserved allograft for any vessel in the body. Cryopreserved allografts are commonly used for in-line arterial reconstruction in the treatment of prosthetic graft infections or contaminated wounds such as a mycotic aneurysm or aortoenteric fistula. Although cryopreserved arterial allografts have been anecdotally reported in the repair of vascular trauma with contaminated wounds, there are no large series. Reports on the use of this conduit in infected abdominal and extremity vascular beds suggest that it would be a safe consideration in the setting of resistant or recurrent infection and that it may have applicability in trauma.
Animal-derived conduits (xenografts) include bovine carotid artery (Artegraft, North Brunswick, NJ), bovine pericardium, bovine jugular vein (Contegra, Contegra, Medtronic, Santa Rosa, CA) as well as a porcine pulmonic xenograft. The use of bovine carotid as a hemodialysis graft was initially reported by Chinitz. The patency of bovine carotid has been compared to ePTFE in hemodialysis grafts by Kennealey. Although there was no difference in secondary patency, primary and assisted-primary patency were higher with bovine carotid than with ePTFE (60% versus 10% and 60% versus 21% at 1 year, respectively). Although bovine carotid has not been studied in vascular trauma, experience in lower extremity bypass demonstrates good results for patency in bypasses to the above- and below-knee position as well as in tibial vessels with patency of 87% at one year. Similarly, bovine jugular vein plays a role in reconstruction of the right ventricular outflow tract in congenital heart surgery. Although its use in trauma remains to be defined, this conduit is available in diameters from 12 to 22 mm and would appear to be an appropriate size match for torso vascular structures.
The human acellular vessel (HAV) (Humacyte, Inc., Durham, North Carolina) is a new bioengineered blood vessel or conduit consisting of decellularized (non-antigenic) extracellular matrix originating from arterial smooth muscle cells ( Figs 24.3 and 24.4 ). This product is manufactured using regenerative medicine techniques and results in an “off-the-shelf” conduit of uniform caliber that can be implanted as a patch or as an interposition or bypass graft. Because the conduit is a non-antigenic biologic, evidence suggests that overtime it becomes populated by endothelial cells from the recipient patient. The HAV is not yet cleared by the US Food and Drug Administration (FDA), but pivotal clinical trials designed to assess the safety, efficacy, and durability of the conduit for dialysis access, peripheral arterial disease, and vascular trauma are underway in the US and Europe. The US Military Health System research program has supported the development and clinical study of the HAV in the hopes that this conduit may provide an off-the-shelf option that is well incorporated and resistant to infection for use in the setting of wartime vascular injury.
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