Management of Abdominal Aortic Aneurysms

Rupture of an abdominal aortic aneurysm (AAA) remains a common vascular catastrophe in all emergency departments. Currently, the natural history of AAAs indicates that risk of rupture increases considerably when the aneurysm is greater than 5 cm in diameter. Appropriate management of aneurysms is elective repair for patients with a good operative risk whose aneurysm is between 5 and 6 cm. For patients with a serious medical comorbidity, the threshold for AAA repair is usually 6 cm. Surgical management is generally safe with extraordinarily durable results. Another current option is an investigational endovascular stent graft, but the long-term outcome for these new devices remains unknown. In addition, optimal medical management should include careful control of hypertension and smoking cessation. The current prognosis for healthy patients who undergo elective aneurysm repair is excellent. Rupture of an abdominal aortic aneurysm (AAA) remains a common vascular catastrophe in all emergency departments. Currently, the natural history of AAAs indicates that risk of rupture increases considerably when the aneurysm is greater than 5 cm in diameter. Appropriate management of aneurysms is elective repair for patients with a good operative risk whose aneurysm is between 5 and 6 cm. For patients with a serious medical comorbidity, the threshold for AAA repair is usually 6 cm. Surgical management is generally safe with extraordinarily durable results. Another current option is an investigational endovascular stent graft, but the long-term outcome for these new devices remains unknown. In addition, optimal medical management should include careful control of hypertension and smoking cessation. The current prognosis for healthy patients who undergo elective aneurysm repair is excellent. Rupture of an abdominal aortic aneurysm (AAA) is one of the leading 15 causes of death among elderly persons in the United States.1Halleit Jr, JW Abdominal aortic aneurysm: natural history and treatment.Heart Dis Stroke. 1992; 1: 303-308PubMed Google Scholar Approximately 3% to 4% of adults older than 65 years harbor an AAA. Fortunately, many of these aneurysms are small and never rupture. Nonetheless, the risk of rupture has been a controversial topic for years. Only recently have population-based studies and a randomized clinical trial provided better evidence for clinical management. During the past 30 years, the incidence of AAAs has tripled.1Halleit Jr, JW Abdominal aortic aneurysm: natural history and treatment.Heart Dis Stroke. 1992; 1: 303-308PubMed Google Scholar This change is primarily due to the incidental detection of more aneurysms by ultrasonography and computed tomography. For example, the incidence of small aneurysms (<5 cm in diameter) has increased l0-fold, whereas the incidence of medium-sized (5–7 cm) and large (>7 cm) aneurysms has increased by a factor of only 2 to 3. Small aneurysms now account for approximately 50% of all clinically recognized AAAs. That last-mentioned observation is an important epidemiological finding because many physicians are uncertain about the appropriate management of small aneurysms. At least 1 million Americans have a clinically recognized AAA, but only 70,000 to 80,000 surgical repairs are performed annually. Many of these patients are older than 70 years and have other serious medical comorbidities. Consequently, their operative risk is increased. Treatment of aneurysms is expensive, and Medicare reimbursement is declining. All these factors continue to drive clinical trials examining the risk and benefits of observation vs early repair of small aneurysms. Although mass screening of elderly persons is not costeffective, certain patients are at increased risk of developing an aneurysm: those with a family history of AAAs, male patients older than 70 years, long-term smokers, and those with systemic hypertension. Such patients older than 50 years should undergo screening with an abdominal examination and ultrasonography. Primary care physicians should be aware that 1 cause of chronic abdominal or back pain and sometimes ureteral obstruction is the entity of inflammatory AAA. The cause of the intense retroperitoneal inflammation is unknown. Computed tomography is the diagnostic modality of choice. The inflammatory process generally does not abate until after AAA repair. At least 5 population-based studies have found consistently that risk of rupture increases when an AAA exceeds 5 cm in diameter.1Halleit Jr, JW Abdominal aortic aneurysm: natural history and treatment.Heart Dis Stroke. 1992; 1: 303-308PubMed Google Scholar, 2Nevitt MP Ballard DJ Hallett JW Jr. Prognosis of abdominal aortic aneurysms: a population-based study.N Engl J Med. 1989; 321: 1009-1014Crossref PubMed Scopus (421) Google Scholar These studies have also shown that expansion rates vary considerably among aneurysms. Only 20% of aneurysms expand at a rate greater than 0.4 cm/y. The vast majority of aneurysms expand slowly at a rate of 0.2 to 0.3 cm/y. Based on Laplace law, the most important clinical factor affecting aneurysm expansion and wall tension is blood pressure. Patients with chronic obstructive pulmonary disease may also have an increased risk of expansion and rupture of an AAA. This increased risk may be due to lack of α1-antitrypsin, which inhibits elastases that weaken arterial connective tissues. In routine clinical practice, the most useful factor for decision making is the maximal diameter of the AAA on the most recent ultrasound study. The diameter enables the physician to inform the patient about the potential risk of rupture in the following year (Figure 1). In a recent population-based study from Rochester, Minn, the estimated risk of rupture based on the latest ultrasound study was 0% per year when the AAA diameter was less than 4 cm, 1% per year for 4.0 to 4.9 cm, 11% per year for 5.0 to 5.9 cm, and 25% per year for aneurysms greater than 6 cm.3Reed WW HalleTT Jr, JW Damiano MA Ballard DJ Learning from the last ultrasound: a population-based study of patients with abdominal aortic aneurysm.Arch intern Med. 1997; 157: 2064-2068Crossref PubMed Google Scholar Management of small aneurysms remains problematic.4Katz DA Littenberg B Cronenwett JL Management of small abdominal aortic aneurysms: early surgery vs watchful waiting.JAMA. 1992; 268: 2678-2686Crossref PubMed Scopus (114) Google Scholar, 5Hallet Jr, JW Naessens JM Ballard DJ Early and late outcome of surgical repair for small abdominal aortic aneurysms: a population-based analysis.J Vase Surg. 1993; 18: 684-691Abstract Full Text Full Text PDF PubMed Scopus (92) Google Scholar Recently, the United Kingdom Small Aneurysm Trial analyzed ultrasonographic surveillance vs early surgery for small AAAs (4.0–5.5 cm in diameter).6UK Small Aneurysm Trial Participants Mortality results for randomised controlled trial of early elective surgery or ultra-sonographic surveillance for small abdominal aortic aneurysms.Lancet. 1998; 352: 1649-1655Abstract Full Text Full Text PDF PubMed Scopus (1092) Google Scholar This study concluded that early surgery provided no long-term survival advantage over ultrasonographic surveillance at 6-month intervals, unless the aneurysm exceeded 5.5 cm in diameter (Figure 2). However, over a 5-year follow-up, 61% of observed patients underwent elective surgical repair. This finding emphasizes the importance of careful surveillance for all patients with recognized AAAs. Elective repair is generally appropriate for healthy patients with AAAs in the 5- to 6-cm range. In the future, the natural history of aneurysms may be altered by new medical therapies. Current clinical trials are investigating whether β-blockers retard growth of small aneurysms. Recent evidence indicates that cigarette smoking remains a risk factor for progression of size of aneurysm and eventual rupture. In addition, upcoming clinical trials will investigate whether tetracycline-related drugs can impede proteolytic enzyme activity in AAAs and slow or stabilize their growth. Other therapies that focus on the basic pathogenic mechanisms will likely be developed. Coronary artery disease is the most important underlying medical illness contributing to morbidity and mortality in the perioperative period. In recent years, the American College of Cardiology and American Heart Association have published guidelines to aid the physician in evaluating a patient's cardiovascular risks before noncardiac surgery.7Eagle KA Brundage BH Chaitman BR et al.Guidelines for perioperalive cardiovascular evaluation for noncardiac surgery: report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee on Peri-operative Cardiovascular Evaluation for Noncardiac Surgery).J Am Coll Cardiol. 1996; 27: 910-948Abstract Full Text PDF PubMed Scopus (333) Google Scholar Clinical predictors and risk of surgical procedure are important background information. Surgical AAA repair is considered high-risk surgery. Therefore, only patients with minor or no clinical predictors and moderate or excellent functional capacity (ability to achieve >4 metabolic equivalents of exertion) should undergo surgery without further cardiac evaluation. In all other situations, these.guidelines suggest more detailed preoperative cardiac assessment (usually with noninvasive testing) to stratify cardiac risks before a decision is made to proceed with AAA repair. Other medical comorbidities may double or triple the usual low risk (3%-5%) of aneurysm repair. These conditions include chronic renal failure (serum creatinine level, >3 mg/dL. or hemodialysis), chronic lung disease (forced expiratory volume, <1 L), and liver cirrhosis with portal hypertension. For such high-risk patients, the 3-day mortality rate is commonly in the 8% to 10% range, even at experienced medical centers. Less than 10 years ago, Parodi et al8Parodi JC Palmaz JC Barone HD Transfemoral intraluminal graft implantation for abdominal aortic aneurysms.Ann Vase Surg. 1991; 5: 491-499Abstract Full Text PDF PubMed Scopus (2977) Google Scholar from Argentina reported the first successful stent graft for an AAA. Usually, a stent graft is passed into the aneurysm from a catheter delivery system inserted via the femoral artery (Figure 3). Subsequently, other surgeons have been key players in the design and clinical testing of other aneurysm stent grafts. Several clinical trials of various devices are currently under way.9May J Woodburn K White G Endovascular treatment of infrarenal abdominal aortic aneurysms.Ann Vase Surg. 1998; 12: 391-395Abstract Full Text PDF PubMed Scopus (30) Google Scholar Stent grafts will definitely have some role in future aneurysm management. Currently, most endovascular teams involve various combinations of surgeons, interventional radiologists, and cardiologists. The Food and Drug Administration emphasizes that endografts for aneurysm repair have 3 essential components: (1) graft material, (2) a stent or attachment system, and (3) a delivery system. From an engineering and safety standpoint, each one of these components must be designed and tested carefully. There have been problems with each one of these components in clinical trials. In a few cases, the graft fabric has torn or leaked over time. In addition to Dacron polyester, polytetrafluoroethylene grafts are being investigated. Problems associated with metallic stents include metal fatigue and fracture, breakage of sutures connecting stent rings, and late graft migration. In addition, the delivery systems are relatively large and stiff. Negotiating tortuous iliac arteries can be difficult and can result in iliac artery laceration. Large holes in the femoral artery increase the risk of femoral artery complications. All, these problems must be considered before the use of endografts becomes widespread. Various endograft products are currently available.9May J Woodburn K White G Endovascular treatment of infrarenal abdominal aortic aneurysms.Ann Vase Surg. 1998; 12: 391-395Abstract Full Text PDF PubMed Scopus (30) Google Scholar Most studies report similar early results, but late outcomes at 5 and 10 years in large numbers of patients will not be available for several years. Initial successful deployment of a stent graft has been reported in approximately 95% to 97% of cases, with the need for immediate surgical conversion in approximately 3% to 5%. The most common early complications are groin hematoma (6%-7%), arterial thrombosis (2%-3%), iliac artery rupture (1%-1.5%), and thromboemboli (1%-2%). Because the complication rate associated with the femoral artery is approximately 10%, many investigators are now recommending surgical exposure of the femoral arteries for introduction and removal of the delivery device. Future miniaturization of the delivery systems may favor percutaneous delivery. The 30-day mortality rate is approximately 3% with stent grafts. Mortality has been 2% to 3% for patients with a good operative profile and 10% to 13% for those at high risk. These mortality rates do not differ significantly from those associated with standard surgical repair. Long-term follow-up is relatively limited for all stent grafts (mean, 1.5–3 years). The most common long-term problem is an endoleak. An endoleak represents persistent filling of the aneurysm from either an anastomotic site or other collateral blood vessels that enter and leave the aneurysm sac (eg, lumbar arteries). To date, approximately 10% to 20% of endografts have an endoleak at last follow-up. Endoleaks close spontaneously in approximately 40% to 50% of patients and by secondary endovascular procedures in another 8% to 10%. Endoleaks have been observed in approximately one third of cases, and surgical intervention has been used to treat approximately 10% to 15% of long-term endoleaks. Aneurysm rupture after stent grafts is rare but occurs in approximately 1% of patients within 1 to 2 years. The other common late complications are severe graft kinking (2%), graft migration (2%), and graft thrombosis (3%). These late problems associated with the current stent grafts emphasize the need for continual, serial, and long-term imaging of the devices. Patients remain at risk for an endoleak into the AAA sac and rupture (about 1% per year). Follow-up is facilitated by a system that schedules evaluations and ensures compliance. Physicians and surgeons should inform patients of the early and late results of both endografts and standard surgical repair (Figure 4).10Hallett Jr, JW Marshall DM Petterson TM et al.Graft-related complications after abdominal aortic aneurysm repair: reassurance from a 36-year population-based experience.J Vase Surg. 1997; 25: 277-284Abstract Full Text Full Text PDF PubMed Scopus (336) Google Scholar, 11May J White GH Waugh R et al.Endovascular treatment of abdominal aortic aneurysms.Cardiovasc Surg. 1999; 7: 484-490Crossref PubMed Scopus (21) Google Scholar The 30-day mortality rate for surgical repair is 3% to 5%. Early surgical complications of arterial thrombosis, anastomotic rupture or bleeding, peripheral emboli, and limb loss are rare at experienced centers (1%-3%). Obviously, an endoleak is not a long-term problem with surgical repair. In a recent 36-year population-based study of late graft complications, 5-year survival free of graft complications was 93%.10Hallett Jr, JW Marshall DM Petterson TM et al.Graft-related complications after abdominal aortic aneurysm repair: reassurance from a 36-year population-based experience.J Vase Surg. 1997; 25: 277-284Abstract Full Text Full Text PDF PubMed Scopus (336) Google Scholar The most common long-term complications of a surgical graft included pseudoaneurysm (3%), thrombosis of a graft limb (2%), graft infection (1.3%), and aortoenteric fistula (1.6%) (Table 1).Table 1Early and Late Complications of Abdominal Aortic Aneurysm RepairEndografts*Data from May et al.9 (%)Standard surgical repair†Data from Hallett et al.10 (%)30-Day mortality rate33–5Early complications Thrombosis2–3<1 Arterial rupture1–2<1 Postoperative bleeding6.51 Emboli1<1 Loss of limb1<1Late complications Endoleak10–20‡Not a problem. Abdominal aortic aneurysm rupture1<1 Pseudoaneurysm‡Not a problem.3 Thrombosis32 Graft infectionNo data1.3 Graft kink2<1 Graft migration2‡Not a problem. Aortoenteric fistulaNo data1.6Survival free of endoleak80 (18 mo)Not applicableSurvival free of graft complications67 (2 y)93 (5 y)* Data from May et al.9May J Woodburn K White G Endovascular treatment of infrarenal abdominal aortic aneurysms.Ann Vase Surg. 1998; 12: 391-395Abstract Full Text PDF PubMed Scopus (30) Google Scholar† Data from Hallett et al.10Hallett Jr, JW Marshall DM Petterson TM et al.Graft-related complications after abdominal aortic aneurysm repair: reassurance from a 36-year population-based experience.J Vase Surg. 1997; 25: 277-284Abstract Full Text Full Text PDF PubMed Scopus (336) Google Scholar‡ Not a problem. Open table in a new tab Whether endografts will change the indication for AAA repair is debatable. However, a recent randomized clinical trial of small aneurysms in the United Kingdom indicated that ultrasonographic surveillance is preferable to early surgery for small AAAs (4.0–5.5 cm). Consequently, third-party payers will not be enthusiastic about accelerated repair of small AAAs by stent grafts. In addition, a recent Markov decision analysis model addressed the following question: Should endovascular surgery lower the threshold for repair of AAAs?12Finlayson SRG Birkmeyer JD Fillinger MF Cronenwett JL Should endovascular surgery lower the threshold for repair of abdominal aortic aneurysms?/.Vase Surg. 1999; 29: 973-985Abstract Full Text Full Text PDF Scopus (78) Google Scholar The assumptions for this model included (1) annual risk of rupture is a continuous function of AAA diameter (0% for those <4 cm, 1% for 4.5 cm, 11% for 5.5 cm, and 26% for 6.5 cm), (2) operative mortality is 1% for endovascular repair and 3.5% for surgical repair for patients 70 years old, and (3) immediate endovascular-to-open conversion risk is 5%, with a late conversion rate of 1% per year. With these assumptions, this model did not justify changing the indications for AAA repair in most patients. Only older patients in poor health seemed to benefit from endovascular repair compared to surgical intervention.