Increasing Complexity in the Open Surgical Repair of Abdominal Aortic Aneurysms

Article Outline

• Abstract
• Introduction
• Methodology
• Results
  • Annual Aneurysm Repair Volumes and Changes in Repair Characteristics
  • Early and Late Survival After Aneurysm Repair
  • Postoperative Complications and Length of Stay
• Discussion
• Acknowledgment
• References
• Copyright

Background

Patient selection and techniques for the operative management of abdominal aortic aneurysms (AAAs) continue to evolve. We sought to examine trends in open surgical repair (OSR) over a 15-year period in which endovascular aneurysm repair (EVAR) has become increasingly prevalent.

Methods

Patients undergoing elective repair of infra- and pararenal AAAs were identified through our center's prospective vascular surgery registry during two time periods: 1995 to 2004 (era 1) and 2004 to 2010 (era 2). Data collected included comorbidities, demographics, and operative characteristics.

Results

A total of 1,188 elective AAAs were repaired during the study period, including 828 (70%) OSRs and 360 (30%) EVARs. The proportion of OSRs requiring suprarenal cross-clamping increased from 14.2% during era 1 to approximately 50% by the end of era 2. Compared with era 1, increases were seen in the unadjusted mortality rates during era 2 for OSR with infrarenal clamping (from 0.62% to 1.73%) and OSR with suprarenal clamping (from 1.22% to 3.98%); after adjusting for other variables, however, no significant temporal trends were seen. Similarly, no significant change in major complication rate was seen after adjusting for other factors. The incidence of major comorbidities among the OSR group was largely unchanged between the two eras.

Conclusions

OSR of AAAs has become increasingly complex, with the increased utilization of EVAR. Despite this complexity, risk-adjusted outcomes may remain good in high-volume centers.

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Introduction 

The advent of endovascular aneurysm repair (EVAR) has had an important impact on the care of patients with abdominal aortic aneurysms (AAAs). Considering that it is as effective as open surgical repair (OSR) in reducing aneurysm-related mortality, has a lower perioperative mortality, and has a shorter length of stay,¹, ², ³, ⁴ the widespread adoption of EVAR might not be unexpected. Candidates for EVAR generally have anatomy that makes for a relatively straightforward OSR (especially the presence of an infrarenal, rather than a pararenal, aneurysm); therefore, as the utilization of EVAR increases, one might expect the complexity of the average OSR to increase. A previous report did document an increase in the complexity of OSR since the introduction of EVAR but did not examine the influence of this complexity on morbidity and mortality.⁵ The objective of this study was to examine changes in the complexity of OSRs that have occurred at a U.S. academic medical center that serves as a high-volume⁶ aortic aneurysm center, and to determine whether these changes in complexity have affected outcome.

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Methodology 

All patients undergoing elective open surgical or endovascular repair of an AAA at the Brigham and Women's Hospital between January 1995 and June 2010 were included in this study. Patients were identified using a prospectively maintained registry of vascular surgery patients, which was created in 1979. Patients undergoing urgent repair for ruptured or symptomatic aneurysms, as well as any patient who required any form of renal artery revascularization (including bypass, endarterectomy, or reimplantation), were excluded. All surgeons performing elective repairs at the Brigham and Women's Hospital offer both EVAR and OSR. EVAR is generally recommended to patients who are candidates for either OSR or EVAR, but only after the benefits and limitations of this procedure are explained.

Prospectively collected variables included demographics (including age, sex, and ethnicity), comorbidities (including history of diabetes, smoking, hypertension, coronary artery disease, and any previous coronary interventions), and operative characteristics (including proximal-most clamp location, site of distal anastomosis, and graft type); these characteristics have been previously described by our group.⁷ Postoperative acute renal failure was defined as a peak postoperative serum creatinine level that was >2.0 mg/dL or doubled compared with baseline, or as the need for new postoperative renal replacement therapy. Glomerular filtration rate was estimated using the Modification of Diet in Renal Disease study group formula.⁸ The two eras studied were defined as 1995 to 2003 and 2004 to 2010, based on when the utilization of EVAR appeared to significantly increase at our institution.

Categorical variables are described using counts and proportions, with Fisher exact test used for all group comparisons. Continuous variables are described using median values and ranges, with the Mann–Whitney U test used for nonparametric comparisons between groups. Time-to-event data are described using Kaplan–Meier survival analysis, and the log-rank test was used to compare the survival of groups. Multivariate analyses of factors impacting survival were performed using Cox proportional hazards regression model. Because of its fundamental importance to the study, the variable denoting era of OSR was kept in the model, regardless of statistical significance, to allow the hazard ratio (HR)/odds ratio (OR) and P value to be observed at every step of the model-building process. Binary variables for each individual surgeon who performed OSR or EVAR during the study period were included to control for any systematic differences that may have affected outcomes in a temporal manner. In a similar manner, logistic regression was used to investigate the factors impacting postoperative complications occurring after aneurysm repair. All statistical analyses were performed using Intercooled Stata 8.2 (StataCorp, College Station, TX). A two-sided P value of <0.05 was considered statistically significant.

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Results 

Annual Aneurysm Repair Volumes and Changes in Repair Characteristics 

Between February 1995 and June 2010, a total of 1,188 aneurysm repairs were performed—828 (69.7%) performed with OSR and 360 (30.3%) performed with EVAR. Median follow-up for these patients was 1,074 days (2.94 years). The annual number of OSRs (with suprarenal and infrarenal proximal cross-clamping) and the proportion of total aneurysm repairs comprising OSR during the study period are shown in Table I. As demonstrated in Table I, the proportion of aneurysm repairs performed with OSR decreased steadily from 1998 (the advent of the use of EVAR in our institution) until 2007. Also notable is the fact that the proportion of OSRs requiring suprarenal cross-clamping increased between 2004 and 2010 (Fig. 1). Based on the frequency of suprarenal clamping that occurred during era 1 (14.2% of cases overall during this period), the frequency of suprarenal cross-clamping seen during years 2007, 2008, and 2009 was significantly higher than expected (P < 0.01 for all, binomial distribution).

Table I. Number and characteristics of aneurysm repairs per year by surgical approach

OSR, open surgical repair.

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• Fig. 1 

  Kaplan–Meier survival plots demonstrating that the long-term survival after open surgical repair (OSR) with infrarenal cross-clamping (solid line) is significantly better than survival after OSR with suprarenal cross-clamping (dashed line; P = 0.04). The numbers below the graph list the number of patients with infrarenal (upper line) and suprarenal (lower line) cross-clamping at risk at the beginning of each time interval.

The preoperative and demographic characteristics of patients undergoing OSR were largely similar (Table II), but some differences did exist. In particular, when compared with patients undergoing OSR in era 1, patients in era 2 were less frequently male (71.7% vs. 79.4% for era 2 vs. era 1; P = 0.02), more frequently Hispanic (1.0% vs. 0.4%; P = 0.01), more frequently had a history of hypertension (79.5% vs. 63.9%; P < 0.001), and had a trend toward higher glomerular filtration rate (68.8 vs. 63.5 mL/min; P = 0.09).

Table II. Comparison of preoperative characteristics of patients undergoing elective OSR of abdominal aneurysm

Parameter	All patients, 1995–2010 (N = 828)	Era 1, 1995–2003 (N = 574)	Era 2, 2004–2010 (N = 254)	P
Median age, number (%)	71 (66–77)	71 (66–77)	71 (66–77)	0.73
Male gender, number (%)	638 (77.1)	456 (79.4)	182 (71.7)	0.02a
Ethnicity, number (%)
Caucasian	790 (95.4)	553 (96.3)	237 (93.3)	0.04a
Black	20 (2.4)	13 (2.3)	7 (2.8)	0.63
Hispanic	8 (1.0)	2 (0.3)	6 (2.4)	0.01a
Asian	10 (1.2)	6 (1.1)	4 (1.6)	0.51
Diabetes mellitus, number (%)	90 (10.9)	58 (10.1)	32 (12.6)	0.33
Hypertension, number (%)	569 (68.7)	367 (63.9)	202 (79.5)	<0.001b
Chronic obstructive pulmonary disease, number (%)	149 (18.0)	92 (16.0)	57 (22.4)	0.03a
Active cigarette smoking, number (%)	280 (33.8)	183 (31.9)	97 (38.2)	0.08
Coronary artery disease, number (%)	419 (50.6)	300 (52.3)	119 (46.9)	0.15
History of coronary artery bypass grafting	177 (21.4)	129 (22.5)	48 (18.9)	0.27
Congestive heart failure, number (%)	54 (6.2)	43 (7.5)	11 (4.3)	0.10
History of stroke, transient ischemic attack, or previous carotid endarterectomy, number (%)	79 (9.5)	56 (9.8)	23 (9.1)	0.80
Median estimated glomerular filtration rate, mL/min (interquartile range)	64.3 (50.6–80.2)	63.5 (49.5–78.8)	68.8 (52.6–83.6)	0.09
Median aneurysm diameter (interquartile range)	5.4 (5.0–6.1)	5.4 (5.0–6.2)	5.4 (4.9–6.0)	0.88

Early and Late Survival After Aneurysm Repair 

The 30-day mortality rate for all OSR patients was 1.2%, with 30-day mortality rates of 0.92% for all OSRs with infrarenal clamping and 2.55% for all OSRs with suprarenal clamping. The unadjusted mortality rates by era were 0.62% and 1.73% for OSR with infrarenal clamping during eras 1 and 2, respectively (P = 0.19, log-rank test), and 1.22% and 3.98% for OSR with suprarenal clamping, respectively (P = 0.28, log-rank test). A multivariate analysis of 30-day mortality of all patients undergoing OSR identified several variables associated with mortality, including black race and Hispanic ethnicity (HR: 30.2 and 29.4, P = 0.007 and 0.01, respectively); increasing age by year (HR: 1.2, P = 0.003); history of previous coronary artery bypass grafting (HR: 7.52, P = 0.02); and aneurysm diameter by centimeter (HR: 1.86, P = 0.02). Factors that did not have any significant relationship with 30-day mortality included the following: era, suprarenal versus infrarenal clamping, the use of a bifurcated versus tube graft, and a distal anastomosis at the femoral level.

The long-term survival rates after OSR were also examined. Overall unadjusted survival rates were 94.1%, 69.1%, and 34.4% at 1, 5, and 10 years after OSR, respectively. Categorized by proximal clamp locations, unadjusted survival rates for OSR with infrarenal clamping were 94.2%, 69.5%, and 36.2% at 1, 5, and 10 years, respectively, and unadjusted survival rates for OSR with suprarenal clamping were 94.2%, 66.7%, and 24.8%, respectively (Fig. 1; P = 0.04 for infrarenal versus infrarenal clamping, log-rank test). Variables that appeared to have a significant impact on long-term survival in a multivariate analysis included the following: congestive heart failure (HR: 2.3, P < 0.001), chronic obstructive pulmonary disease (HR: 1.8, P < 0.001), history of a postoperative complication (HR: 1.7, P < 0.001), age at the time of OSR by year (HR: 1.5, P < 0.001), use of a tube graft versus a bifurcated graft (HR: 1.3, P = 0.04), and aneurysm diameter at the time of OSR by centimeter (HR: 1.2, P < 0.001). Trends toward decreased long-term survival were seen with increasing preoperative serum creatinine (HR: 1.2, P = 0.06) and the performance of a distal anastomosis at the femoral level (HR: 1.5, P = 0.08).

The unadjusted long-term survival for patients undergoing OSR with infrarenal clamping did not differ significantly between era 1 and era 2 (P = 0.36; Fig. 2). Likewise, the unadjusted long-term survival for patients undergoing OSR with suprarenal clamping did not differ significantly between these two eras (P = 0.23; Fig. 3). After adjustment for other important demographic and perioperative variables using a multivariate model of long-term survival, era of repair did not significantly impact survival.

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• Fig. 2 

  Kaplan–Meier survival plots demonstrating long-term survival after OSR with infrarenal cross-clamping during era 1 and era 2 (P = 0.36). Gray area denotes 95% confidence intervals, and number of patients at risk at the beginning of each time interval is listed under the graph.

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• Fig. 3 

  Kaplan–Meier survival plots demonstrating long-term survival after OSR with suprarenal cross-clamping during era 1 and era 2 (P = 0.36). Gray area denotes 95% confidence intervals, and number of patients at risk at the beginning of each time interval is listed under the graph.

Postoperative Complications and Length of Stay 

The unadjusted incidence of major postoperative complications increased from 25.8% overall in era 1 to 31.9% overall in era 2 (P = 0.07, χ² test). Table III lists the postoperative complications that occurred in patients who underwent OSR with infrarenal and suprarenal clamping. Among patients who underwent OSR with infrarenal clamping, the proportion of patients experiencing one or more complications increased from 24.0% to 27.7% from era 1 to era 2 (P = 0.36). A trend was seen toward a higher incidence of myocardial infarction in era 2 (P = 0.07), but no differences reaching statistical significance were seen in the incidence of any other major complications.

Table III. Postoperative complications occurring after OSR during era 1 (1995–2003) and era 2 (2004–2010)

N/A, not applicable.

The rate of having one or more postoperative complications was 39.0% among patients who underwent OSR with suprarenal clamping, significantly higher than the 25.0% rate seen in patients undergoing OSR with infrarenal clamping (P < 0.001, χ² test). Some notable specific complications that occurred at a significantly higher rate among patients undergoing OSR with suprarenal clamping than among patients undergoing OSR with infrarenal clamping included cardiac arrhythmia (13.8% vs. 8.8%; P = 0.07), pneumonia (12.0% vs. 6.7%; P = 0.03), acute renal failure (11.3% vs. 2.4%; P < 0.001), and colon ischemia (3.8% vs. 1.1%; P = 0.01). Only age (OR: 1.06, P < 0.001), suprarenal clamping (OR: 1.8, P = 0.003), and distal anastomoses at the femoral level (OR: 2.07, P = 0.008) were found to be significantly associated with complications in a multivariate regression analysis. Although the unadjusted incidence rates of one or more complications did increase from 36.6% during era 1 to 41.6% for patients with suprarenal cross-clamping during era 2, the era of OSR among suprarenal cross-clamp patients did not appear to be an important factor in the incidence of complications in the multivariate analysis.

The median postoperative length of stay for all OSR patients was 7 days during era 1 (interquartile range of 6–9 days) and era 2 (interquartile range of 6–11 days). Because of the larger proportion of patients with extended length of stay, this difference did reach statistical significance (P = 0.01, Wilcoxon rank-sum test). When adjusted for clamp position and other preoperative variables in a multivariate linear regression analysis, however, no temporal differences were seen in length of stay. The median length of stay for patients with suprarenal cross-clamping was 9 days (interquartile range of 7–13 days), significantly higher than the median length of stay of 7 days (interquartile range of 6–9 days) for patients with infrarenal cross-clamping (P < 0.001, Wilcoxon rank sum test).

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Discussion 

As the utilization of EVAR increases, the complexity of OSR has been increasing as well. With the current generation of aortic stent-graft available in the United States, the main exclusion from EVAR has been an inadequate proximal neck.⁵ Thus, much of the complexity seen in contemporary OSRs would be expected to be related to suprarenal cross-clamping. Chong et al. from our group had previously demonstrated that suprarenal cross-clamping can be done with low mortality (1.8% overall) and acceptable morbidity. The rate of postoperative renal failure was 17% among patients requiring suprarenal cross-clamping, and only 0.6% developed new-onset dialysis.⁷ Although suprarenal cross-clamping is an important technical feature of OSR, we were interested in investigating other characteristics of operative complexity and their potential impact on outcomes.

The findings of this study reinforce the idea that OSR has become more challenging in the contemporary EVAR era. Specifically, we found a significant increase in the need for suprarenal cross-clamping (currently required in approximately 50% of OSR cases at our institution) as well as increased blood loss during these cases. Although other characteristics of technical complexity, including the use of bifurcated prosthetic grafts or the need for at least one femoral distal anastomosis, have not changed, the increased need for suprarenal cross-clamping alone is notable, as it has been consistently associated with increased morbidity⁷, ⁹, ¹⁰, ¹¹ and, in some centers, increased mortality.¹² We found a few differences in the prevalence of systemic comorbidities that were statistically significant; however, none of them seemed to be clinically relevant. In our center, as observed at many other institutions, patients with important systemic comorbidities along with aneurysms that are anatomically amenable are typically treated with EVAR. Thus, from these findings we conclude that aneurysm morphology is typically the most important determinant of surgical approach, and that the increased utilization of EVAR has made OSR more challenging, mainly owing to complex anatomy and not to more frequent comorbidities of the patient population.

After multivariate risk adjustment of outcomes was performed, we found no significant changes in short- or long-term patient survival, major complication rates, or length of stay over the course of the 15-year study period. This suggests that despite the increasing complexity of the OSR patient population, the results of OSR and postoperative management at our institution have remained consistently good. We would point out, however, that condition-specific risk adjustment—which, in this analysis, required the consideration of aortic cross-clamp location, graft type, distal anastomosis, and era—is largely confined to specialty conferences and scientific journals. In the contemporary U.S. health-care environment, however, the lay public is increasingly interested in rankings and comparisons of center and surgeon outcomes.¹³ Even data sources with rich patient-level medical data, such as the American College of Surgeons National Surgical Quality Improvement Project, lack operative variables and, therefore, may be inadequate for the risk adjustment of open aortic surgery; publicly accessible resources, such as HealthGrades,¹⁴ the U.S. News and World Report Best Hospitals rankings,¹⁵ and the U.S. Department of Health and Human Services' “Hospital Compare,”¹⁶ which use administrative data or unadjusted outcomes data would be even less likely to make fair comparisons between hospitals performing open aortic surgery. This should suggest the need for a publicly available condition-specific registry, similar to the New York State Department of Health's Cardiovascular Disease Data and Statistics¹⁷ or the Society for Thoracic Surgery's Adult Cardiac Surgery Database,¹⁸ to report outcomes that are truly representative of a center's performance.

Unadjusted outcomes are, by themselves, important as well. Bundled or partially capitated hospital reimbursements for care of these increasingly challenging patients should be updated using contemporary data and would ideally be categorized by clamp location (if not other additional variables) to avoid undercompensating for OSR and the increasingly frequent prolonged length of stay postoperative complications. Physician compensation should be adjusted as well: in contrast to the 13 different Common Procedural Codes that are used to describe EVAR at its procedural components, only two Common Procedural Codes exist to describe elective (nonruptured) OSR for both pararenal and infrarenal AAAs. As these purportedly reflect the preoperative planning, intraoperative work, and the postoperative care of surgeons,¹⁹, ²⁰ their values should also be updated using contemporary data and should also ideally be categorized by clamp location and other important operative variables.

Finally, these findings also raise the question of whether care of patients and/or the training of future vascular surgeons should be consolidated to high-volume centers with proven outcomes. With the clear volume–outcome relationship in OSR,²¹, ²², ²³ decreasing absolute numbers of OSR imply that fewer centers will have sufficient volume to maintain proficiency in OSR. It may, therefore, be beneficial to both patients²⁴, ²⁵ and institutions¹³ to consolidate care for patients with AAAs requiring OSR to selected medical centers. In the context of increasingly frequent, minimally invasive procedures, such as EVAR, the issue of providing sufficient trainee experience in the traditional open surgical technique may also prove difficult²⁶ and may be exacerbated by the fact that the less technically demanding cases—typically most educational cases for trainees with little relevant experience—are most often repaired with EVAR in the contemporary era. These questions regarding regionalization and training are difficult yet important issues that extend to other specialties beyond vascular surgery.²⁷, ²⁸, ²⁹, ³⁰, ³¹ Answering these questions, however, is beyond the scope of this study.

There are other limitations in this study that we would like to point out. Black race and Hispanic ethnicity was associated with what appeared to be excessive HRs (29.4 and 30.2, respectively). This appears to be an anomaly associated with the small number of black and Hispanic patients in our study. Additionally, the mortality rate of infrarenal OSRs in this series was low (0.9%) but comparable with that reported in the Open Versus Endovascular Repair trial (2.3%).² Our group has previously demonstrated that patient comorbidities³² and the need for renal revascularization and/or suprarenal cross-clamping⁷ negatively impact outcomes. Risk adjustment would have accommodated for the patient characteristics and technical aspects of the cases included in this study, but the low event rate may still have decreased the sensitivity of this study to identify significant temporal trends.

In summary, OSR for AAA has become increasingly challenging, with the increased utilization of EVAR. The challenges appear to be mainly related to anatomic complexity rather than increasing comorbidities. Unadjusted outcomes have changed, but risk-adjusted comparisons of patient survival, major complications, and length of stay at our institution suggest that these outcomes may remain consistently good at high-volume aortic surgery centers. Increasing complexity among these open surgical cases suggests the need for condition-specific risk adjustment models as well as regionalization of care to high-volume centers.

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The authors acknowledge Julie Lombara for assistance with data collection.

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