In brief
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In Brief An abdominal aortic aneurysm is defined as a focal dilation of this blood vessel to a diameter 1.5 times that of its normal size. It is estimated that approximately 5% of the male population above ages 65 to 75 harbor an aortic aneurysm. The complications of aneurysms include rupture and, less frequently, distal embolization. The risk of rupture is closely associated with the diameter of the aneurysm, and efforts have been directed at the identification of aneurysms before rupture, when the operative risks are quite low. This is in sharp contrast to the extraordi- narily high mortality rate once rupture occurs. The definitive repair of abdominal aortic aneurysms began at the mid point of the 20th century, with homograft replacement of the involved segment of the aorta. Soon after, repair with a fabric conduit became the standard treatment; the graft was sewn from within the aneurysm sac to minimize dissection-related injury to surrounding structures. By the end of the century, 50,000 aortic procedures were performed in the United States alone. Although the morbidity and mortality rates that were associated with open surgical repair decreased to remarkably low levels, the procedure continued to be associated with perioperative blood transfusion, a day or more in the intensive care unit, and a hospital stay that averaged more than 1 week. Furthermore, as a consequence of the large incision, patients are often unable to resume normal activities for 1 to 2 months after the operation. The open surgical repair of an abdominal aortic aneurysm can be formidable in patients with significant medical comorbidities, with perioperative mortality rates that greatly exceed those in more healthy individuals. In this context, efforts were directed at a less invasive technique for the repair of aortic aneurysms. In 1991, the first successful treatment of aneurysms with the use of a polyester tube graft that was affixed to a balloon-expandable stent and compacted into a sheath was reported. The devices were placed through a transfemoral approach, which greatly minimized the magnitude of the operation. In the following decade a host of different devices were developed, 2 of which were approved for clinical use in late 1999. Despite variability in the intricacies of design, there are 2 basic configurations of stent grafts. The first is a modular design, in which an aortic main body component is placed initially, followed by the insertion of a contralateral iliac limb to create a bifurcated Curr Probl Surg, March 2002 237
Transcript of In brief
In Brief
An abdominal aortic aneurysm is defined as a focal dilation of thisblood vessel to a diameter 1.5 times that of its normal size. It is estimatedthat approximately 5% of the male population above ages 65 to 75 harboran aortic aneurysm. The complications of aneurysms include rupture and,less frequently, distal embolization. The risk of rupture is closelyassociated with the diameter of the aneurysm, and efforts have beendirected at the identification of aneurysms before rupture, when theoperative risks are quite low. This is in sharp contrast to the extraordi-narily high mortality rate once rupture occurs.
The definitive repair of abdominal aortic aneurysms began at the midpoint of the 20th century, with homograft replacement of the involvedsegment of the aorta. Soon after, repair with a fabric conduit became thestandard treatment; the graft was sewn from within the aneurysm sac tominimize dissection-related injury to surrounding structures. By the endof the century, 50,000 aortic procedures were performed in the UnitedStates alone. Although the morbidity and mortality rates that wereassociated with open surgical repair decreased to remarkably low levels,the procedure continued to be associated with perioperative bloodtransfusion, a day or more in the intensive care unit, and a hospital staythat averaged more than 1 week. Furthermore, as a consequence of thelarge incision, patients are often unable to resume normal activities for 1to 2 months after the operation. The open surgical repair of an abdominalaortic aneurysm can be formidable in patients with significant medicalcomorbidities, with perioperative mortality rates that greatly exceed thosein more healthy individuals.
In this context, efforts were directed at a less invasive technique for therepair of aortic aneurysms. In 1991, the first successful treatment ofaneurysms with the use of a polyester tube graft that was affixed to aballoon-expandable stent and compacted into a sheath was reported. Thedevices were placed through a transfemoral approach, which greatlyminimized the magnitude of the operation. In the following decade a hostof different devices were developed, 2 of which were approved forclinical use in late 1999. Despite variability in the intricacies of design,there are 2 basic configurations of stent grafts. The first is a modulardesign, in which an aortic main body component is placed initially,followed by the insertion of a contralateral iliac limb to create a bifurcated
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system. The second is a “unibody” design, in which the bifurcated graftis inserted through 1 femoral artery and the contralateral limb is carriedinto the contralateral iliac artery with guide wire traction. For the mostpart, tube grafts have been abandoned because of a failure to providedependable long-term sealing at the distal aortic attachment site.
Myriad anecdotal descriptions of endovascular technique and resultsappeared in the literature, which sprouted a new fund of knowledge aboutthe hemodynamics of endoleaks, wall tension, and device fatigue.Engineering principles attained importance that was equal to the clinicalaspects of patient care, and an understanding of both became a prereq-uisite to the practice of endovascular surgical procedures. Severalmulticenter clinical trials were undertaken that compared the clinicaloutcomes in patients who were treated with endovascular devices to theoutcomes of contemporaneously treated control subjects who underwentstandard open surgical aneurysm repair. Although the clinical program of1 device was abandoned after the poor long-term durability was noted, 2other devices were approved for clinical release in the United States inlate 1999.
Endograft repair is not possible in every patient with an aortic aneurysm,however. Currently available endograft devices have limitations of size,flexibility, and configuration. The exact requirements for the patient’s arterialanatomy depend on the particular brand of device. In general, the limitationsof endovascular repair fall into 2 areas: those that are dependent on the iliacartery anatomy and those that pertain to the proximal aortic neck. The iliacarteries must be adequate to accommodate the relatively large deliverysystems of present devices. Tortuosity and calcification are 2 factors thatmust be considered in conjunction with iliac diameter. So too, adequatehypogastric artery perfusion must be maintained to prevent symptoms ofbuttock ischemia, colon ischemia, and paraplegia. The aortic neck should belong enough to provide an adequate zone for sealing and fixation, althoughsome devices rely on the presence of a suprarenal component with a barestent, hooks, or barbs to provide security from migration. These consider-ations define the eligibility criteria for endovascular aneurysm repair-criteriathat are not fixed but change with the development of newer devices andtechniques of insertion.
Endoleaks (defined as the persistent transmission of blood flow orpressure into the aneurysm sac after endograft implantation) have beenconsidered the Achilles heel of endovascular aneurysm repair. Endoleakshave been correlated with a failure of the procedure to protect the patientagainst aneurysm rupture, although the absence of an endoleak does notimply that the risk of rupture is zero. Endoleaks are categorized into 5
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subgroups, depending on their site of origin. Type I leaks are those leaksthat originate at the intended site of attachment proximally or distally.Type II leaks are leaks that arise from the filling of the aneurysm sac byretrograde branch vessels, such as the lumbar arteries or inferior mesen-teric artery. Type III endoleaks occur as a result of defects in the fabric ofthe graft or at junction zones between the members of modular en-dografts. Type IV leaks occur when diffuse leakage of contrast throughthe interstices of the fabric is noted. Although it is generally assumed thattype I and III endoleaks carry the greatest risk of aneurysm rupture, typeII endoleaks are thought to be more benign. More recently, anothervariety of endoleak (type V) has been described in which the aneurysmsac remains pressurized and the aneurysm enlarges in the absence of ademonstrable leak on imaging studies. The term endotension has beenapplied to this scenario, and it is postulated that the persistent pressur-ization can lead to aneurysm rupture despite the lack of contrast bloodflow outside of the lumen on imaging studies. Endotension, however, mayrepresent nothing more than very slow or intermittent leaks or thrombus-sealed leaks that continue to transmit pressure. The differentiation of atype V leak from other types depends on the sensitivity of the imagingtechnique used.
Although endoleaks, migration, and device durability problems occurwith significant frequency and may be associated with a failure to protectthe patient adequately against aneurysm rupture, the overall results ofendovascular aortic repair are quite acceptable. On average, hospital staysare shorter than those of patients who are treated with conventionalsurgical procedures, and intensive care unit use and blood transfusionrequirements are markedly reduced. Rupture has been reported to occur in0.5% of patients at 3 years by Kaplan-Meier analysis with the commer-cially available AneuRx device (Medtronic/AVE, Santa Rosa, Calif), withwhich, conversion to open surgical repair was avoided in 93% of patients.Similar results have now been reported with other devices, including theAncure (Guidant Cardiac and Vascular Division, Menlo Park, Calif),Talent (World Medical/Medtronic, Sunrise, Fla), and Zenith (Cook,Bloomington, Ind). As data accumulate, patient selection and operativetechnique stand out as 2 issues that are closely linked to satisfactorylong-term results, not unlike the situation for any other operativeprocedure.
Endovascular techniques have also been applied to the treatment of thoracicaortic disease, both for aneurysmal disease and aortic dissection. Unlikedevices for the infrarenal aorta, thoracic stent grafts do not need to address abifurcated anatomy. Nevertheless, a large number of thoracic aneurysms
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extend proximally and require placement of the device partially into the aorticarch, with associated complexities related to the attainment of adequatefixation and sealing in a tortuous vessel. Added to the difficulties that areassociated with the introduction of a large system through the iliac arteries,thoracic stent graft procedures can be technically challenging. The treatmentof aortic dissection can be equally demanding. Adequate treatment involvesthe coverage of the intimal tear, the expansion of the true lumen, and therestoration of branch vessel perfusion.
Despite the limitations of currently available technology, the future ofminimally invasive aortic repair remains bright. Our fund of knowledge isadvancing at a rapid pace and is being applied to the development of nextgeneration devices. Newer devices are likely to broaden therapeutic applica-bility and increase patient eligibility for less invasive treatment modalities.Concurrently, modifications in material and design should dramaticallyimprove device durability. In time, endovascular therapy can be predicted tosupplant open surgical procedures as the standard of care, which will resultin a substantial clinical benefit to patients with aortic disease.
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