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Vascular and Interventional Radiology |
1 From the Departments of Radiology I (B.V.C., P.W., A.H.D., R.J.B., W.R.J.) and Vascular Surgery (G.F., K.E.R., R.P.), Leopold-Franzens Medical School and University Hospital Innsbruck, Anichstrasse 35, 6020 Innsbruck, Austria. Received October 28, 1999; revision requested November 24; revision received January 21, 2000; accepted February 1. Address correspondence to B.V.C. (e-mail: benedikt.czermak@uibk.ac.at).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Seven patients underwent endovascular stent-graft placement for type B aortic dissection. Five patients had acute and two had chronic dissection. In five patients, the proximal entry tear was within 2 cm of the origin of the left subclavian artery, and in two patients it was beyond this site. In three patients, the noncovered proximal portion of the stent-graft was placed across the origin of the left subclavian artery. The efficacy of the procedure was assessed at follow-up studies 3, 6, 12, and 24 months after intervention.
RESULTS: The procedure was technically and clinically successful in six patients (86%). The left subclavian artery remained patent in all patients. In two patients with involvement of aortic branches, endovascular stent-graft placement restored adequate blood flow to the compromised branches. One patient was readmitted 1 month later because the dissection extended into the ascending aorta. In all but this patient, closure of the entry tear and thrombosis of the false lumen along the stent-graft were achieved. All false lumina shrank considerably. The mean follow-up time was 14 months (range, 1–25 months).
CONCLUSION: Type B aortic dissections within and beyond 2 cm of the origin of the left subclavian artery can be treated safely and effectively by means of endovascular stent-graft placement.
Index terms: Aorta, CT, 942.12915, 942.12916, 943.12915, 943.12916 • Aorta, dissection, 942.743, 943.743 • Interventional procedures, 942.1268, 943.1268 • Stents and prostheses, 942.1268, 943.1268
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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The Stanford classification system proposed by Daily et al (2) has gained wide acceptance. This system designates all dissections involving the ascending aorta as type A (proximal), regardless of the site of the intimal tear or the distal extent of the dissection. All other dissections are called type B (distal). Dissections are categorized as acute if the patient presents within 2 weeks of onset or as chronic if more than 2 weeks have elapsed (3).
The most important predisposing factor for aortic dissection is hypertension. It coexists in 70%–90% in most series and is more common in distal than in proximal dissections. Other predisposing factors for aortic dissection are the congenital disorders of the connective tissue, in particular Marfan syndrome and, to a lesser extent, Ehlers-Danlos syndrome (1). Dissections predominate in male patients by a ratio of 3:1 (4).
Surgery for aortic dissection involving the aortic arch continues to result in high mortality rates (5%–26%) (5–7). Therefore, it has been argued that for type B dissection, medical therapy is superior to surgical treatment. However, 20%–50% of those patients who survive the acute stage of aortic dissection develop aneurysms within 1–5 years after onset (8,9). Furthermore, chronic type B dissection frequently causes a distinct enlargement of the false and a narrowing of the true lumen (10). In some acute cases, ischemic complications can necessitate immediate treatment (11,12).
Ever since Parodi et al (13) described their first clinical experience with use of a stent-graft to treat an abdominal aortic aneurysm, endoluminal stent-graft placement has been emerging as a less invasive alternative to conventional surgery in patients with aortic aneurysms or aortic dissections and with serious coexisting illnesses (14–16). Dake et al (17) reported excellent initial clinical results with transcatheter stent-graft treatment of descending thoracic aortic aneurysms in patients at high risk. Results of several experimental and clinical studies (14,18–25) have shown successful endoluminal closure of entry sites in type B dissections.
New endovascular techniques and devices recently have been developed for the management of aortic dissection. These devices are designed to remodel the inner dissected wall of the aortic true lumen (10,18–20). The purpose of our study, which included seven patients, was to evaluate the use of endovascular stent-graft placement for acute and chronic type B dissections and to verify the feasibility and safety of this method.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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The other 11 patients who presented with Stanford type B aortic dissection during the same period of time were not eligible for endovascular stent-graft placement. Five of these patients had to undergo surgical treatment for various reasons, such as extreme kinking of the aorta or pelvic vessels, aortic diameter greater than 42 cm, extension of the dissection proximal to or into the left subclavian artery, nonavailability of appropriate stent-graft, or no serious coexisting illnesses. The other six patients were asymptomatic; therefore, medical therapy and follow-up were the preferred treatment.
The dissection was considered acute when the patient presented within 14 days of onset and chronic if more than 14 days had elapsed. Of the seven patients who were selected for stent-graft placement, five presented with acute dissection that caused severe pain in the chest, abdomen, or both.
Two of the five had angiographic evidence of dynamic compromise (26) of abdominal aortic branches. In patient 4, the celiac artery, the superior mesenteric artery, and the right renal artery were involved. Angiography showed the floating viscera sign. This patient also presented with ischemia of the left lower extremity. The reentry site of the dissection was located in the distal portion of the left external iliac artery. In patient 5, we observed severe reduction of blood flow in the right renal artery. In none of the patients was static compromise (26) of the abdominal aortic branches observed.
Patient 5 had undergone explorative laparotomy at another hospital because of severe abdominal pain. One day later, the diagnosis of aortic dissection was established by means of contrast material–enhanced computed tomography (CT). The patient then was referred immediately to our hospital for evaluation and treatment. At admission, sedatives and mechanical ventilation were administered.
In the group with chronic dissection, which included only two patients, one patient had severe chest and back pain. Other possible reasons for these symptoms were excluded prior to intervention. The other patient was asymptomatic. In this patient, the indication for treatment was expansion of the false lumen, as confirmed by means of sequential imaging studies.
Three of the seven patients presented with acute clinical symptoms and had to be treated immediately after diagnostic angiographic work-up with standard stent-grafts from the emergency kit. In the other four patients, who presented with less acute symptoms, the intervention was performed later by using individually designed stent-grafts.
Complete written informed consent was obtained from all patients, with the exception of patient 5, who underwent intubation. This patient had to be treated on an emergency basis. Since the patient was a tourist traveling on his own, it was not possible to obtain informed consent from his family.
Preprocedure Work-up
Posteroanterior and lateral chest radiography, spiral CT of the chest, and multiplanar angiography were performed in each patient. At our hospital, CT is performed routinely in patients with Stanford type B dissection to confirm the diagnosis and to exclude any organ infarcts. Multiplanar angiography is necessary for exact preoperative work-up and measurements. CT and multiplanar angiography were performed in all patients to determine the levels of entry and reentry sites.
Preoperative spiral CT examinations were performed by using a scanner (HiSpeed Advantage; GE Medical Systems, Milwaukee, Wis) with a standardized protocol. After obtaining an anteroposterior scout view, a volume of interest was defined that extended from the level of the carotid bifurcation to the level of the external iliac artery. Spiral CT was performed by using 120 mL of iopromide (Ultravist; Schering, Berlin, Germany) administered intravenously at a concentration of 300 mg of iodine per milliliter, a flow rate of 3 mL/sec, and a scanning delay of 25 seconds. A collimation of 5.0 mm, a table speed of 10 mm/sec (pitch, 2.0), and a tube current of 120 kV and 230–250 mA were used.
Angiographic images were obtained by using a digital subtraction angiography unit (Integris 3000; Philips Medical Systems, Hamburg, Germany). Arterial access was gained through the common femoral artery. A graduated 5-F pigtail catheter with radiopaque 1-cm increments (Cook, Bloomington, Ind) was used for injection of iodixanol (Visipaque 270; Nycomed Amersham Imaging, Oslo, Norway) at a rate of 10–15 mL/sec and a volume of 35 mL. Images were obtained after sequential selective catheterization of both the true and the false lumina. If it was not possible to achieve access to both of the lumina through a single puncture site on one side, access was obtained through the contralateral artery. This technique permitted us to determine the exact anatomy of the dissection flap, to locate entry and reentry sites, and to document which of the major branch vessels were supplied by the false lumen and which of them by the true lumen.
Exact intraluminal measurements of the diameters of the aortic segments proximal and distal to the dissection were obtained. In cases in which we planned to cover only part of the dissection with the stent-graft, the diameters of the true and false lumina were measured at the site where the distal end of the stent was intended to be placed.
Stent-Grafts
The dimensions of stent-grafts were determined on the basis of contrast-enhanced spiral CT scans and angiograms. One tubular system (Talent; World Medical, Sunrise, Fla) was used in six cases, and another (Vanguard; Boston Scientific, Natick, Mass) was used in one case.
The Talent stent-graft is made of a 0.079-inch-thick superelastic nitinol monofilament wire (nickel-titanium shape memory alloy with titanium oxide coating). The wire is bent in a zigzag configuration and is welded together after one turn to form a z-shaped stent. The length of a single stent body is 1.5 cm. Depending on the length of the stent-graft, different numbers of stent bodies are used. The stent bodies are connected on two sides with a straight nitinol wire to prevent kinking.
Woven 0.3-mm-thick Dacron is used as graft material. The porosity of the Dacron mesh is less than 200 mL/min/cm2. The Dacron mesh is sewn tightly onto the outside of the stent. The Talent stent-graft system has no barbs or anchoring mechanism other than its self-expanding properties.
Three different designs of the upper end of the stent-graft are available. In the open-web version, the Dacron is cut out in alignment with the zigzag configuration of the uppermost stent body and is sewn tightly onto the struts of the stent. In the bare-spring version, the stent has a double body at its top, and the proximal one is not covered. The noncovered portion of the stent-graft can be placed across the left subclavian artery or visceral arteries, if necessary. In the closed-web version, the Dacron covers the end of the stent body without open spaces.
The Vanguard stent-graft is made of a polyester-covered, self-expanding nitinol stent. The proximal end of the stent is noncovered for 12 mm and has small hooks to allow attachment to the aortic wall (27); this device was used in only one case.
Stent-grafts were selected according to aortic diameter, length of the lesion, and availability. The stent-grafts were designed individually for each of four patients, whereas standard stent-grafts were used in three patients. If the difference in diameter between the proximal and the distal ends of the stent-graft was more than 20%, a tapered graft was used. The proximal diameters of the endovascular devices were 26–42 mm (mean, 38 mm), their distal diameters were 26–38 mm (mean, 35 mm), and their lengths were 50–120 mm (mean, 98 mm).
Technique
All procedures were performed in a vascular intervention suite by a team of vascular surgeons (G.F., R.P.) and interventional radiologists (P.W., W.R.J.). The interventions were performed with the use of general anesthesia attained by using tracheal intubation and mechanical ventilation. The patient was supine, and the surgical field was prepared and draped. Cardiopulmonary bypass equipment was available should a surgical emergency procedure become necessary. In those patients who were not treated on an emergency basis, preoperative angiographic work-up was repeated.
The arterial access site for introducing the stent-graft was the right (n = 4) or left (n = 1) femoral artery. We chose the less tortuous one of these two arteries or that with the wider lumen. In the patient with ischemia of the left lower extremity, the contralateral artery was used. In two instances, the right external iliac artery had to be used because of the diameter or length of the introducing sheath.
A surgical cutdown was performed to expose the femoral or external iliac arteries, which was followed by a transverse arteriotomy at the site where the stent-graft was inserted through a 20–24-F sheath. After identification of the true lumen and the most appropriate angiographic projection to display the involved aortic segment, metallic markers (25-cm-long needles) were placed on the patient to mark the entry site of the dissection and the level of the left subclavian artery or the visceral arteries, if necessary. The upper and lower ends of the region of stent-graft placement were marked with angiographic guidance.
Next, the delivery system was introduced through the true lumen and positioned at the site of the entry tear with fluoroscopic guidance. Care was taken to position the proximal end of the endoprosthesis above the dissection. In three patients, the origin of the left subclavian artery had to be crossed with the proximal open web of the stent-graft.
The prosthesis was implanted distally into a portion of the aorta that showed a nearly normal lumen. If an intimal flap was still present at this end, the diameter of the prosthesis corresponded to that of the true lumen plus one-half of that of the false lumen.
After correct positioning of the device, the outer sheath was withdrawn slowly to deploy the stent-graft fully. Thereafter, a latex balloon was inflated inside the stent-graft to achieve full expansion and to anchor the stent in the aortic wall. During deployment, we reduced the mean arterial pressure by using vasodilators (<70 mm). Additional technical aspects of the stent-graft deployment procedure have been described previously (17,28,29).
In patient 4, ischemia of the left lower extremity was treated by means of endarterectomy and fixation of the distal intimal flap. This procedure was performed by the vascular surgeons (G.F., R.P.).
In all patients, angiography was performed after stent-graft deployment to evaluate obliteration of the entry site, patency of the left subclavian artery, and blood flow in the visceral arteries.
The introducer delivery system was removed, and the arteriotomy was closed. The number of stent-grafts, extent of coverage, total procedure time, and fluoroscopy time were registered.
At surgery, 5,000 IU of heparin sodium was administered intraarterially, followed by 1,000 IU of heparin sodium intravenously for another 72 hours. Patients then received acetylsalicylic acid at a dosage of 100 mg/day thenceforward.
Follow-up Protocol
CT was performed prior to discharge and at 3, 6, 12, and 24 months after intervention. A panel of blood tests to determine complete blood cell count, C-reactive protein level, electrolyte level, creatinine level, blood urea nitrogen level, prothrombin time, partial thromboplastin time, and erythrocyte sedimentation rate and clinical examinations were performed on the same schedule.
The mean follow-up time was 14 months (range, 1–25 months). Long-term follow-up studies, however, will continue to be performed every 12 months. Hospitalization and short- and long-term complications were registered.
At follow-up, in addition to the standardized preoperative CT protocol, an extra nonenhanced spiral CT scan was obtained. The volume of interest was limited to the stent, and a collimation of 3 mm, table speed of 6 mm/sec (pitch, 2.0), and tube current of 140 kV and 230–250 mA were used. A standard algorithm was used with secondary augmented reconstruction (field of view, 25 cm).
Clot formation in the false lumen along and distal to the stent-graft and patency of the left subclavian artery were evaluated.
The volume of the false lumen was assessed by means of the summation of area technique by using a workstation (Advantage Windows Ultra Spark 10 Work Station 3.1; Sun Microsystems, Mountain View, Calif). With this method, the cross-sectional area of the false lumen is measured on each transverse CT scan by tracing the outline of the lumen with the built-in cursor on the workstation console. The volume is calculated automatically by the computer (30). Measurements were obtained by two experienced radiologists (B.V.C., A.H.D.); discrepancies were resolved by means of consensus. Volumetric measurement of the false lumen was performed along the aorta in the region of the stent-graft, and in cases in which the dissection extended beyond the stent-graft, measurements also were obtained in the segment of the aorta distal to the stent-graft.
After volumetric measurement, three-dimensional reconstruction of the stent-graft was performed by using the shaded-surface rendering technique (31). The threshold value was chosen to optimize the display of the metallic stent, and changes in the shape of the stent-graft were evaluated.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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In patient 5, closure of the entry site was incomplete, although the stent-graft was implanted exactly across the entry site, and in spite of balloon dilation and appropriate sizing of the graft. However, substantial reduction in blood flow in the false lumen and shrinkage of the false lumen were observed. This patient was readmitted with chest and back pain 1 month after discharge. The dissection had propagated into the ascending aorta and required surgery to replace the ascending aorta and the hemiarch; the stent-graft was left in situ and did not interfere with the surgical procedure.
Total procedure time was 60–155 minutes (mean, 121 minutes); total fluoroscopy time was 11–27 minutes (mean, 19 minutes).
Five patients received only one stent-graft each, and two patients received two stent-grafts each (Table 2). In three patients, the dissection was covered completely by stent-grafts, whereas in four patients the distal reentry site was left uncovered.
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Angiography was used to confirm complete entry site closure in six patients. Follow-up CT scans showed clot formation in the false lumina in all patients, with the exception of the patient who had incomplete closure at the entry site. In none of the patients was clot formation observed in the false lumen distal to the stent-graft with blood flow still present.
There were two procedure-related complications. In one case, a rupture of the femoral artery occurred, and in another case dissection of the access vessel was observed. Both complications were recognized and treated by means of surgery during the procedure. The rupture was sutured, and the dissection was repaired by means of endarterectomy.
No neurologic complications, in particular spinal cord ischemia, were observed. After the intervention, the patients were transferred to the intermediate care unit where routine examinations were performed by neurologists to monitor for signs of emboli or infarcts. There was no infection; however, fever, leukocytosis, and elevated C-reactive protein levels were seen in five patients within 2–5 days after treatment. These symptoms disappeared without therapy and were related to the postimplantation syndrome described by Blum et al (32). The stay at the intermediate care unit averaged 24 hours, and all patients were discharged between the 4th and 11th postoperative days (mean, 5th day). For safety reasons, patients routinely stay in our hospital for at least 4 days.
One death (patient 6) occurred 6 weeks after uncomplicated and successful stent-graft implantation and was due to cardiac arrest. All other patients are alive. Follow-up time was 1–25 months (mean, 14 months).
No substantial kinking or migration of the stent-grafts was noted when comparing the three-dimensional reconstruction images of the stent-grafts obtained during follow-up (Fig 2). In all patients, the aortic segment with the stent showed shrinkage of the false lumen and expansion of the true lumen (Table 3). This expansion of the true lumen, noted in the follow-up studies, was due to expansion of the stent-graft (Fig 2).
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In the group with acute dissection, the false lumen was obliterated completely in one patient by inflating the latex balloon at the end of the procedure to expand the stent-graft. In the other four patients with acute dissection, the volume of the false lumen decreased 0%–42% (mean, 20%) compared with the preinterventional volume. Within 6 months, the false lumen shrank at least 54% in all acute cases (Fig 3).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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The primary objective of surgical treatment for type B dissections is to close the entry site. The standard surgical methods include graft replacement (9,33) or repair of the open entry site (34). Nevertheless, especially in acute cases, surgical therapy of type B dissections still is associated with substantial morbidity and mortality, mainly owing to bleeding, heart or renal failure, or spinal cord injury (35,36).
Endoluminal repair of type B dissection can obviate thoracotomy, which may cause respiratory problems, especially in patients with preexisting pulmonary insufficiency. The patients included in this feasibility study were at high surgical risk due to coronary disease, severe chronic obstructive pulmonary disease, or both. After intervention, the patients did not have any cardiac complications, and no distal embolization or infection was registered.
Another advantage of the stent-graft procedure over conventional surgery is the absence of aortic clamping that may contribute to major complications, such as failure of the left side of the heart or paraplegia. It remains debatable whether cross-clamping time alone can be used to predict the relative risk of paraplegia. There is no doubt, however, that cross-clamping is associated with the risk of paraplegia (37). The effect of endoluminal grafting on the vascular supply of the spinal cord is unknown and has not yet been evaluated, but there are some reports (16,38,39).
Dissections affect aortic branches, as well as the aorta itself, and much of the morbidity of type B dissections is due to branch vessel compromise (11).
By deploying the stent-graft within the aortic true lumen proximal to the ostium of the visceral arteries in patients 4 and 5, normal blood flow could be restored in the abdominal aortic branches that had been compromised owing to dynamic narrowing. No additional maneuvers such as fenestration or stent placement had to be performed. However, in patients in whom stent-graft placement in the true lumen does not provide adequate flow to the ischemic vascular beds, additional therapeutic procedures such as fenestration or revascularization are necessary (40–42).
In our experience, endoluminal repair of type B dissection by using stent-grafts appears to be promising. However, several limitations need to be discussed.
The entry site frequently originates just beyond the left subclavian artery where the proximal anchoring length is insufficient to provide safe support of the stent-graft in a healthy segment of the aorta. This was observed three times in this series and was resolved by placing the noncovered part of the stent-graft over the ostium of the left subclavian artery. No ischemia or neurologic complications were observed, and no carotid-to-subclavian bypass had to be created. We do not perform subclavian arterial bypass prophylactically prior to stent-graft insertion.
The effect of the bare springs in the proximal stent-graft portion on arterial blood flow is still unknown, but their safety is supported by the results of at least two studies (43,44). Our initial experience appears to be consistent with these reports.
In most cases, the stent-grafts must be designed individually and custom fabricated to meet the characteristics derived from imaging studies. Since in emergency cases even a 30-hour delay is unacceptable and diminishes the attractiveness of endovascular treatment, we generally have on hand an emergency kit containing several stent-grafts of various diameters and lengths. We used stent-grafts from this kit for the emergency treatment of three acute dissections.
Stent-grafts are extremely difficult to deploy precisely at the target site because of the pulsatile blood flow in the aortic lumen. After deployment, the endoprosthesis cannot be repositioned or retrieved.
The absence of longitudinal flexibility of the stent-graft design is another point of concern. The Talent stent-graft used in our study is semirigid and not flexible enough to apply to the curve of the distal arch.
The resistance of the deployed stent-graft to pressure may be too weak to maintain sufficient stability in patients with large false lumina with high intraluminal pressure (45). Although the stent-graft was implanted exactly across the entry site in patient 5, and despite balloon dilation and appropriate sizing of the graft, it was not possible to attach the intima completely to the aortic wall at the entry site.
Incomplete closure at the entry site and nonclotting of the false lumen seem predictors of a poor prognosis after endoluminal repair. On the other hand, shrinkage of the false lumen is a good indicator of a favorable outcome of the endoluminal repair. In our study, shrinkage of the false lumen (>50% within 6 months) was demonstrated in acute and chronic cases.
In conclusion, the preliminary results of our study are encouraging. In our experience, endovascular stent-graft placement for type B dissections within and beyond 2 cm of the left subclavian arterial ostium can be accomplished successfully by using an interdisciplinary team approach. The technique may represent an adequate and safe alternative to conventional surgery in selected patients who are at high surgical risk. Stent-grafts are an effective tool for closing entry sites. If necessary, the left subclavian arterial ostium can be covered by the noncovered proximal portion of the stent. Closing the entry site promotes clot formation in the false lumen and reduces the size of the false lumen. However, further long-term follow-up is mandatory before recommending this alternative method for wider use.
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FOOTNOTES |
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REFERENCES |
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