Rupture in small abdominal aortic aneurysms

ORIGINAL CLINICAL STUDIES Rupture in small abdominal aortic aneurysms Stephen C. Nicholls, MD, Jon B. Gardner, MD, Mark H. Meissner, MD, and Kaj H. Johansen, MD, PhD, Seattle, Wash Background: The decision of whether to repair small abdominal aortic aneurysms (AAAs), which are those that are less than 5 cm in diameter, remains controversial. Methods: We describe 161 consecutive patients who were seen at a single urban hospital with ruptured AAAs (raaas) and in whom aneurysm size was measured with ultrasound scanning, or rarely computed tomography, en route to the operating room. Eleven patients (6.8%) had AAAs that measured less than 5.0 cm. This group was compared with 150 patients who had raaas that were more than 5 cm. Results: The mortality rates were similar in both of the groups 70% for small raaas versus 66% for large raaas. No significant differences were seen between the patients with small and large ruptured aneurysms with respect to the prevalence rates of hypertension (60% vs 50%) or of cardiac disease (20% vs 22%). However, the prevalence rate of obstructive lung disease was significantly different (64% vs 25%; P =.02) as was the rate of diabetes (28% vs 3%; P =.004). Five aneurysms were measured at exactly 5 cm. This suggests that approximately 10% of all aneurysms that rupture in this series do so at 5 cm or less. Conclusion: In view of the safety of elective repair as compared with the prohibitive risk associated with aneurysm rupture, patients who are at good risk with small AAA (between 4 and 5 cm) should be considered for elective aneurysm resection. For unclear reasons, obstructive lung disease and diabetes are associated with a significantly greater risk for rupture of small AAA. Patients with these risk factors should be given special consideration. (J Vasc Surg 1998;28:884-8.) Although the elective repair of an abdominal aortic aneurysm (AAA) is safe, an AAA rupture is usually lethal. Even when those who survive emergency repair are included, the current literature suggests an overall mortality rate of raaa, which includes those who die at home or before arrival to a medical facility, that approaches 90%, 1 with operative mortality rates ranging from 15% to 70%. 2-5 To a large extent, this variation reflects the hemodynamic status of the patients on arrival at the institution. 6 At our medical center, the operative mortality rate is 70% this includes a large number of patients From the Division of Vascular Surgery, Department of Surgery Harborview Medical Center, University of Washington School of Medicine. Reprint requests: Stephen C. Nicholls, MD, Harborview Medical Center, 325 Ninth Ave, Box 359796, Seattle, WA 98104-2499. Copyright 1998 by The Society for Vascular Surgery and International Society for Cardiovascular Surgery, North American Chapter. 0741-5214/98/$5.00 + 0 24/1/93339 with ongoing cardiopulmonary resuscitation and the elective mortality rate is 2.1%. Although there is general consensus on the merit of elective surgery for the vast majority of those with an AAA that is greater than 5 cm in diameter, much controversy and discussion have attended the elective repair of smaller aneurysms between 4 and 5 cm in diameter. 7 This controversy arises in part from the conflicting data in the literature. In an early autopsy study, Darling et al 8 reported that the incidence rate of rupture was 23.4% for aneurysms that were 4.1 to 5.0 cm in diameter, and Cronenwett et al 9 have recently suggested that the risk of rupture may be as high as 6% per year even for small AAAs. In contrast, a recent population-based study from the Mayo Clinic reported that the risk of rupture of aneurysms that are less than 5 cm was 0% at 5 years and concluded that the repair of small aneurysms was not warranted. 10 Strong supportive evidence for a low incidence rate of rupture in small AAAs comes from the population-based studies from Scandinavia, 11,12 Canada, 13 and Great Britain. 14 884

Volume 28, Number 5 Nicholls et al 885 PATIENTS AND METHODS Since 1982, all patients who were seen at Harborview Medical Center, the regional emergency and trauma center for metropolitan Seattle and the Pacific Northwest (Washington, Alaska, Montana, and Idaho), with presumed raaa have undergone ultrasound scan evaluation as a part of the initial evaluation in the emergency department. 15 Therefore, we have available definite data about AAA size on all patients who were later proven to have ruptured AAAs. Emergency ultrasound scan is not particularly accurate for the definitive diagnosis of an AAA rupture, but it is a practical and reliable method for the detection of an existing AAA. 16 This study was performed by the staff of the Radiology Department with a duplex scanner (ATL Ultramark 3000, Advanced Technology Laboratories, Bothell, Wash) that was kept in the emergency department and that was dedicated to this purpose. The scanning was limited to 5 minutes in duration and documented the following: (1) the presence or the absence of AAA, (2) the largest diameter of the aneurysm if present (anteroposterior or transverse), (3) the size of the suprarenal aorta, (4) the size of the aorta immediately cephalad to the aortic bifurcation, and (5) the size of the proximal iliac arteries. The ultrasound scan study addresses aneurysm presence and size only (Fig 1). No attempt is made to diagnose the rupture, although intraperitoneal or retroperitoneal blood may be seen. The rupture is presumed by the patient s constellation of symptoms and signs (back or flank pain, hypotension, and low hematocrit). The confirmation of the diagnosis is on the basis of the operative finding of extra aortic retroperitoneal or intraperitoneal blood at the time of the operation. In a small number of patients who did not undergo operation, the rupture was confirmed with computed tomography (CT) or with postmortem examination findings. Aortic diameters were classified into the following 3 groups: less than 4.0 cm, 4.0 to 4.9 cm, and equal to or greater than 5 cm. Excluded from the study were patients with iliac aneurysms, aortic dissection, aortic pseudoaneurysm, mycotic aneurysm, aortocaval fistula, or aneurysms of smaller intraabdominal visceral branches. Also excluded were 8 patients with non-diagnostic ultrasound scan examinations that were caused by excessive bowel gas, 3 patients with AAAs that were diagnosed with x-ray alone, 3 patients in whom ultrasound scan was performed but no size was recorded, 2 patients in whom ultrasound scan was performed at an outside institution, and 2 patients for whom CT was performed at an outside institution with a result of no available measurements. Fig 1. Ultrasound scan image of ruptured abdominal aortic aneurysm repair performed in emergency department. Study shows mural thrombus and shadowing from calcification in aortic wall. Aneurysm size is measured onscreen in transverse and anteroposterior projections with integrated software package. We compared the following categories in patients whose AAAs measured less than 5.0 cm with those whose AAAs were greater than or equal to 5.0 cm: demographic data, overall mortality rate, hospital length of stay, and medical conditions, which included hypertension, obstructive pulmonary disease, symptomatic coronary artery disease, and diabetes. The data on preexisting medical conditions were obtained with a chart review. The data were analyzed for statistically significant differences with the use of χ 2 test or with Fisher exact test for small sets of numbers. The significance was assumed for P <.05. RESULTS In a recent 10-year period, 161 patients who were admitted to the Harborview Medical Center emergency department met the study criteria. Of this group of patients, 11 (6.8%) had AAAs that measured less than 5.0 cm. Nine of the AAAs (5.0%) were determined to be between 4.0 to 4.9 cm in the greatest diameter. Of these, 1 AAA was suprarenal in location. An additional 2 of the 161 AAAs (1.2%) that were measured were less than 4.0 cm (3.5 and 3.8 cm in greatest diameter). Five AAAs (3.1%) measured exactly 5.0 cm these were grouped with those greater than 5 cm for all calculations. A total of 150 patients had raaas that were more than 5.0 cm. The mean age in both groups was 73 years, with a male:female ratio of 7:3 for those patients with

886 Nicholls et al November 1998 aneurysms that were less than 5.0 cm and of 7:4 for those with aneurysms that were greater than 5.0 cm. The mortality rates also were comparable for both groups 70% for those seen with the small aneurysms and 66% for those seen with the larger aneurysms (not statistically significant, NSS). The average length of stay was 19.4 days for those patients with raaas that were less than 5 cm. For those patients with raaas that were more than 5 cm, the average was 11.4 days (NSS). Of those patients with raaas that were less than 5 cm, 7 (64%) had known significant obstructive pulmonary disease. Only 38 (25%) of the second group suffered from this condition (P =.02) Of the 11 patients with small aneurysms, 6 (60%) had a history of hypertension as compared with 75 (50%) in the group with medium to large aneurysms (NSS). If only those patients who underwent medical treatment for hypertension were included, these numbers dropped to 2 (20%) for the group with raaas that were less than 5 cm and 56 (37%) for those with raaas that were more than 5 cm (NSS). Two (18%) of those patients with small aneurysms had a significant cardiac history (1 patient with angina, and the other with coronary artery bypass grafting). This was not significantly different from the 33 of 151 patients (22%) with aneurysms that were greater than 5 cm who had cardiac histories. Diabetes was reported in 3 patients (28%) with small raaas. Five (3.3%) of those patients with raaas that were more than 5 cm had diabetes (P =.0004.) DISCUSSION Since the first successful raaa repair by Dubost in 1951, the mortality rates for elective AAA repair have steadily decreased, with recent studies reporting numbers that range from 0.8% to 5.0%. 17-19 At the same time, however, there has been only a marginal decrease in the morbidity and mortality rates that are associated with the repair of a raaa. The current literature has reported that the raaa mortality rates range from 15% to 70%. 2,3 Recent data from our own institution indicate a mortality rate of 70% despite rapid response by specialized paramedics, emergency department evaluation averaging 12 minutes, surgical repair of the raaa under the supervision of vascular surgery specialists, and sophisticated postoperative care. This situation is disturbing because rupture of AAAs ranks as 15th among all of the causes of death for men in the United States 20 and because for the last 3 decades, the age-adjusted and the sex-adjusted incidence rates of AAA have tripled. 21 If AAAs are further subcategorized, it can be seen that the incidence rate of small aneurysms (those <5 cm) has increased 10-fold small aneurysms now account for approximately 50% of all clinically recognized AAAs. 21 A report from Great Britain notes an increase in the incidence rate of raaa from 9.2 to 17.5 per 100,000 patients/year between January 1979 and December 1986. 1 These figures suggest that AAA will become an increasingly important public health issue and that there should be a consensus with regard to the appropriate timing of elective surgical intervention. For a patient to benefit from elective repair of an AAA, the risk of death from the rupture must exceed that of elective surgery. In an autopsy study that involved 473 subjects, Darling et al 8 suggested an alarmingly high risk of rupture of small aneurysms. Among the subjects who had died from ruptured AAAs, 9.5% had diameters that were less than or equal to 4 cm in diameter, and 23.4% measured 4.1 to 5.0 cm in diameter. This contrasts with a more recent population-based study in Olmstead County, Minn, in which no aneurysm that was less than 5 cm in diameter ruptured. 10 Both of these studies, however, have been the subject of criticism. The first represents retrospective autopsy data obtained for many years in which the precision of the AAA measurements at autopsy is unknown. The data from Nevitt et al 10 fail to account for 24% of patients with aneurysms that were less than 5 cm who underwent operative repair as a result of symptoms that could be attributed to either enlargement or rupture. Johansson et al 12 reported 213 patients diagnosed by means of CT and managed selectively depending on the size of the aneurysm, with a mean follow-up of 5 years and 4 months. Aneurysms that were less than 5 cm were followed with repeated examinations and were operated on if an increase in size occurred. Although no operations were performed on 39 patients who had aneurysms of less than 5 cm and no deaths occurred in this group, there were 3 deaths in patients with rupture in whom the aneurysm progressed to more than 5 cm. One of these patients had been scheduled for surgery. Gilmaker et al 22 reported on 187 patients who were diagnosed with ultrasound scans in a 9-year period. The rupture rate in aneurysms that were less than 5 cm was 2.5% at 7 years. Brown et al 13 reported on 268 patients who were prospectively followed with ultrasound scan, CT, or both, with a mean follow-up of 42 months.

Volume 28, Number 5 Nicholls et al 887 Surgery was performed on 114 patients, who were fit for operation, when the aneurysm had reached 5.0 cm or had expanded more than 0.5 cm in a 6- month period or when the patient had significant occlusive disease that necessitated repair. Of the 154 patients who were monitored without operation, 1 AAA ruptured. The authors concluded that the data supported a policy of observation for AAAs that were less than 5 cm. Scott et al 14 screened 8944 patients and found 356 (4%) who had AAAs that were more than 3 cm. Repair was performed if the aneurysm reached 6 cm in size, expanded at a rate at or above 1 cm per year, or caused symptoms. These criteria were met by 124 patients. Of the 8820 patients who did not meet the criteria, 1 death (0.4%) was attributed to rupture. The authors concluded that the surgical repair was unnecessary and possibly detrimental in such patients because the mortality rate was lower than for elective surgery (1% to 8%). Katz et al 20 constructed a Markov decision tree to compare early surgery with watchful waiting in patients with asymptomatic AAAs that were less than 5 cm in diameter with respect to long-term survival rates in quality-adjusted life years. The model defines the potential outcomes as specific health states, and the transitions between these states are modeled iteratively. The authors concluded that in most of the scenarios that they examined, early surgery was preferred to watchful waiting for patients with AAAs that were less than 5 cm in diameter, but watchful waiting generally was favored for patients with low risks of rupture, including those patients with small AAA (ie, <4 cm). As has been noted by Hollier et al, 23 the current reports of low rupture risk for small AAAs may be biased because of prior selective repair of those small aneurysms at greatest risk of rupture. A recent autopsy study confirms a higher than expected incidence rate of raaa as a cause of mortality. 24 Clearly small AAAs, (those <5 cm) can rupture. Furthermore, in this study, 5 aneurysms (3.1%) were measured at exactly 5.0 cm, which suggests that approximately 10% of all ruptured AAAs do so at 5.0 cm or less in diameter. These findings are consistent with those of Cronenwett et al 9 who reported that patients with small aneurysms that were managed non-operatively had a 6% risk of rupture. More recently, Limet et al 25 calculated a 12% rupture risk for AAAs that were 4.0 to 4.9 cm in diameter. The validity of post-rupture measurements of AAA has been questioned previously. There have been suggestions that the diameter of an aneurysm may decrease by as much as 5 mm after aortic clamping. 10 If so, this would have obvious serious implications with respect to the results of our own data and most of the other aforementioned data. However, we examined this question by measuring the AAA transverse diameters at both pre-aortic and post-aortic clamping in 17 normotensive patients who underwent elective AAA repair. Of these 17 patients, 14 (82.3%) had a decrease of less than or equal to 1 mm, and a maximum decrease of 3 mm was found in only 1 patient. With actuarial analysis of several variables, Cronenwett et al 9 found that diastolic hypertension, initial anteroposterior aneurysm diameter, and obstructive pulmonary disease were the only factors that were independently predictive of rupture. In our series, 64% (7 of 11) of those patients with small aneurysms suffered from obstructive lung disease as compared with only 25% (38 of 150) of those patients with raaas that were more than 5.0 cm, which confirmed that the patients with small AAAs and obstructive lung disease were at a significantly greater risk for rupture (P =.02). We were unable to confirm the relationship between hypertension and rupture that was found by the Cronenwett group, however, our data suggests an increased risk of small aneurysm rupture in patients who are diabetic. Our findings, combined with the increasing safety of elective AAA repair, warrant reconsideration of the standard accepted threshold of 5 cm for operative intervention. Although the risk of rupture for small AAAs is low, it is not zero, as proposed by Nevitt et al. 10 Patient selection seems critical. Patients at high risk, such as those patients with recent symptomatic coronary disease, chronic renal failure, or advanced age, should still be observed until their AAAs reach 5, or even 6, cm in diameter. However, younger healthier patients with AAAs with a reasonable life expectancy seem appropriate candidates for the consideration of an elective repair at smaller AAA diameters ( 4.0 cm). Finally, our confirmation that patients with AAAs with obstructive lung disease appear to be at significantly greater risk for rupture of small aneurysms suggests that these patients may be particularly appropriate for the elective repair of small aneurysms. Diabetics may warrant special consideration. Thought should be given to the selective use of ultrasound scan screening in these sub-populations that are at high risk for the rupture of small AAAs. 26 REFERENCES 1. Mealy K, Salman A. The true incidence of ruptured abdominal aortic aneurysms. Eur J Vasc Surg 1988;2:405-8.

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