Expanding use of emergency endovascular repair for ruptured abdominal aortic aneurysms: Disparities in outcomes from a nationwide perspective

Article Outline

• Abstract
• Methods
• Results
  • Trends from 2001 to 2004
  • Detailed analysis of 2003 and 2004 data
• Discussion
• Author contributions
• References
• Copyright

Background

Endovascular repair (EVAR) of abdominal aortic aneurysms (AAA) has become widely accepted in the elective setting but remains controversial for emergency repair of ruptured aneurysms (rAAA). We sought to examine the national trends in use and associated outcomes with EVAR.

Methods

The Nationwide Inpatient Sample (NIS) was used to analyze all admissions for rAAA from 2001 through 2004. Nationwide temporal trends and demographics using weighted samples were evaluated. Focused univariate and multivariate analyses comparing outcomes from open repair and EVAR were done for the years 2003 and 2004.

Results

There were 28,123 admissions for rAAA, with a stepwise decline in admissions from 2001 to 2004. Use of EVAR increased significantly from 6% of all emergency repairs in 2001 to 11% in 2004 (P < .01). Mortality for EVAR declined significantly from 43% to 29% (P < .01), but mortality with open repair showed no change (40% to 43%). From the 2003 to 2004 data set, 949 EVAR and 8982 open repairs were identified. Compared with open repair, the EVAR patients had lower mortality (31% vs 42%), shorter hospital stay (6 vs 9 days), and were more likely to be discharged to home (59% vs 37%, all P < .01). The total hospital charges for EVAR and open repair were similar ($71,428 vs $74,520, P = .59). Mortality for EVAR was significantly higher at nonteaching hospitals compared with teaching centers (55% vs 21%, P < .01) and at nonteaching centers, even exceeding that of open repair (46%). Regression modeling confirmed the overall benefits of EVAR as well as the worse outcomes at nonteaching facilities after adjusting for patient comorbidities, disease severity, and hospital or system covariates.

Conclusions

Endovascular repair is being increasingly used in the emergency management of ruptured AAA, with steadily decreasing mortality during the study period. Endovascular AAA repair is associated with improved mortality and outcomes compared with open repair, but results in nonteaching centers are substantially worse than those in teaching hospitals.

Abdominal aortic aneurysm (AAA) formation is the result of degenerative vascular disease and is associated with high morbidity, mortality, and financial cost. Elective repair of these aneurysms is usually well tolerated and associated with low mortality and morbidity. These outcomes are drastically different for patients presenting with an acutely ruptured or rupturing aneurysm. Ruptured AAA (rAAA) confers a significantly increased risk of adverse outcome and death, and is responsible for >15,000 deaths annually,¹ with an estimated 59% to 83% prehospital mortality and 30% to 80% mortality among those who survive to reach medical care.² The use of endovascular AAA repair (EVAR) has increased drastically since the initial report in 1991 by Parodi et al³ and has been followed by marked improvements in endovascular techniques and equipment.

Although a large amount of prospective controlled data validate this technique for elective AAA repair,⁴, ⁵ there is a paucity of data available for the use of EVAR emergently in the setting of rAAA.⁶ Several case reports and case series have demonstrated acceptable outcomes with EVAR for rAAA,⁷, ⁸, ⁹, ¹⁰ but most are small, retrospective, and limited by the lack of a valid control group. In addition, these may represent a highly selective set of results from large-volume expert centers and may not be truly representative of all centers. The purpose of this study is to examine the demographic patterns and outcomes associated with the use of EVAR for patients presenting with rAAA from a large nationwide sample. Our study compares these factors with those of patients with rAAA who were chosen for open repair.

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Methods 

Data for this study were obtained using the 2001 through 2004 Nationwide Inpatient Sample (NIS), a product of the Health Care Utilization Project, Association for Healthcare Research and Quality. Provided by the Department of Health and Human Services, the NIS is the largest inpatient, all-payer database in the United States (US), with data on >8 million hospital stays per year. The stratified sampling frame and discharge weights contained within the NIS allow for the creation of accurate national estimates from this approximate 20% sample of all nationwide discharges.

During our study period, between 986 and 1004 hospitals from 33 to 37 states were sampled by the NIS. States excluded from each year group were not identical from year to year. Discharge weights are created by the NIS to weight each admission so that it is properly represented within the data set to allow nationwide estimates to be obtained from this 20% subsample. The weights are based on the various demographic and hospital strata created by the NIS design. All analyses performed were weighted analyses. This administrative database contains information on both admission and discharge diagnoses, patient demographics and comorbid conditions, procedures performed, complications, hospital mortality, hospital charges, and discharge destination. In addition, the NIS now contains multiple validated severity-adjustment measures to aid in group comparisons for clinical research.¹¹

The 2001 through 2004 NIS data set was queried for all patients with a primary admission diagnosis of abdominal aortic aneurysm using International Classification of Disease, Ninth Revision, Clinical Modification (ICD-9-CM) codes. All patients with an ICD-9-CM code of 441.3 (AAA, ruptured) or 441.4 (AAA, not ruptured) who were directly admitted to the hospital were included. Our analysis excluded patients who were transferred from outside facilities.

Patients were then classified as undergoing either an open AAA repair (open group) or an endovascular aneurysm repair (EVAR group) by review of all available ICD-9-CM procedure codes. All records with procedure codes for both open repair and EVAR were assumed to be conversions from EVAR to open repair and were grouped as EVAR using an intention-to-treat design. Patients who did not survive to undergo repair or who had no procedure code for a definitive aneurysm repair were excluded.

All data fields were analyzed for completeness and appropriate range of values. Continuous variables with significant deviation from a normal distribution (ie, length of stay) were log-adjusted before statistical analysis. This data set was used to analyze temporal trends in the demographics, types of repair, and unadjusted outcomes during the 4-year period. The two groups have inherent differences at baseline that confer selection bias. The univariate comparisons are presented to describe and compare the groups, not to attribute any causal relation to type of repair or outcome.

For a more detailed analysis of this patient cohort and the associated type of repair, the data from the 2003 and 2004 NIS data sets were combined. The years 2001 and 2002 were excluded from the detailed analysis because they were considered to be early in the EVAR experience. The last 2 years of our study period were chosen for in-depth query to include only the most recently available data and outcomes in detail.

The primary axis of comparison was between the open and EVAR groups. Patient and hospital demographics, hospital charges, and outcomes were compared using unadjusted univariate analysis. A χ² analysis or Fischer exact test was used for categoric variables, and a Mann-Whitney U test or Student t test was used for continuous variables. Select variables were then entered into a standard multivariate logistic regression model to identify any independent factors associated with hospital mortality. Significant interactions between independent variables in the model were assessed using standard collinearity diagnostics, with a cutoff tolerance of < 0.10 or variance inflation factor >10. No significant collinearity interactions were identified in the model.

An adjustment for individual patient disease severity was included in this model, using the predicted mortality score contained in the NIS derived from the proprietary Disease Staging 5.21.3 system (Medstat, Ann Arbor, Mich).¹² The mortality score represents the predicted mortality divided by the overall rate of in-hospital mortality (for the NIS) × 100. It is the best validated variable contained in the data set for comparing the overall illness or disease severity of patients or groups, incorporating baseline variables and all available diagnostic codes. An internal validation analysis of the mortality score was performed for our specific data set and found to be strongly predictive of mortality for the entire data set and for the subgroups of open and EVAR. Results are presented as adjusted odds ratios with 95% confidence intervals (CI), where appropriate. Statistical significance for this study was set at α = 0.05. This study was performed in accordance with the NIS Data User Agreement and approval was obtained through the local Institutional Review Board. All data adjustment and analysis was performed using SPSS 12.0 software (SPSS Inc, Chicago, Ill).

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Results 

Trends from 2001 to 2004 

The use of endovascular repair for all AAA increased during our study period, from 17,775 performed in 2001 to 29,046 in 2004. During this time, the number of patients presenting to US hospitals with the diagnosis of rAAA decreased, from 7749 in 2001 to 6383 in 2004. Fig 1 shows the significant increase in the use of EVAR for rAAA, from 6% of all repairs in 2001 to 11% of repairs in 2004 (P < .01). Although the mortality for all patients admitted with AAA was 2.9% to 3.9%, mortality among all patients presenting with a ruptured aneurysm remained significantly higher, at about 50% (range, 49%-53%). Fig 2 shows the unadjusted hospital mortality rates for all patients with rAAA who underwent open repair or EVAR. Although the mortality for open repair has remained steady at 40% to 45% during the study period, the mortality associated with EVAR has progressively declined, from 43% in 2000 to 29% in 2004 (P < .01). The interaction demonstrated between the year of surgery and type of surgery (open vs EVAR) was also significant (P < .01).

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• Fig 1. 

  Percentage of ruptured abdominal aortic aneurysms (rAAA) that underwent endovascular repair.

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• Fig 2. 

  Mortality rate of open (gray bars) vs endovascular (EVAR, black bars) repair of ruptured abdominal aortic aneurysms (rAAA).

Detailed analysis of 2003 and 2004 data 

All records meeting our inclusion criteria from the 2003 and 2004 NIS data sets were included in the weighted study sample. There were 9931 records identified and included for analysis, of whom 8982 patients underwent open repair and 949 underwent EVAR. The demographics and comorbidities of the two groups are reported in Table I. The two groups were similar with respect to age, sex, and pre-existing comorbid conditions, but patients undergoing open repair had a significantly higher estimated mortality score and underwent more secondary surgical procedures than the EVAR group. The differences in the hospital characteristics between the two groups were also significant, with EVAR being performed more frequently in teaching hospitals and in the Northeast and South geographic distributions.

Table I. Demographics patients with ruptured abdominal aortic aneurysm undergoing open vs endovascular repair and hospital variables

Patient and hospital variables	Open	EVAR	P
Patient, No. (%)	8982(90)	949(10)
Age, mean ± SD years	73.1±8.9	73.9±9.5	.01
Sex, %			.39
Female	22	24
Male	78	76
Race, %			.004
Caucasian	61	66
Non-Caucasian	10	10
Missing	29	24
Surgical procedures, mean ± SD No.	5.3±3.0	4.9±2.6	<.001
Mortality score, mean ± SDa	2015±833	1813±840	<.001
Known comorbid conditions, %
Pulmonary disease	33	32	.838
Diabetes	10	11	.666
Hypertension	47	48	.714
Liver Disease	0.8	2	<.001
Peripheral vascular disease	18	18	.761
Renal failure	7	8	.249
Region of hospital, %			<.001
Northeast	19	28
Midwest	27	21
South	35	42
West	19	9
Location of hospital, %			.89
Urban	90	90
Rural	10	10
Hospital teaching status, %			<.001
Teaching	52	72
Nonteaching	48	28

EVAR, Endovascular aneurysm repair.

Table II compares the unadjusted outcomes between the two groups. The EVAR group had a significantly lower mortality compared with the open repair group. In addition, EVAR was associated with shorter hospital stay and less discharge disability requiring skilled nursing placement. Of note, the median hospital charges between the two group were not significantly different (Table II) at $74,520 (interquartile range, $88,585) for open repair and $71,428 (interquartile range, $66,749) for EVAR for rAAA (P = .58). The number of rAAA performed at nonteaching centers slightly decreased from 2408 in 2003 and 2195 in 2004, and a slight increase was seen at teaching centers, 2405 and 2923, respectively. This difference was statistically significant (P < .01). This sample represented data from 450 hospitals, of which 288 were nonteaching (64%) and 162 were teaching (36%). On further univariate analysis, a significant difference in mortality was noted between patients with rAAA treated at nonteaching hospitals compared with teaching centers (Fig 3). The mortality in both the open and EVAR groups was higher at nonteaching centers compared with teaching hospitals. In addition, at nonteaching hospitals the EVAR associated mortality (55%) was actually higher than that of open repair (46%), which was the reverse of the pattern seen at teaching centers (P < .001).

Table II. Outcome measures for open vs endovascular aneurysm repair

Outcome measure	Open	EVAR	P
Patient, No (%)	8982(90)	949(10)
LOS, median (IRQ) days	9(15)	6(10)	<.001
Total hospital charges, median (IRQ)	$74,520($88,585)	$71,428($66,749)	.58
Discharge status, %
To home	22	41	<.001
To skilled care	42	20	<.001
In-hospital mortality, %	42%	31%	<.001

EVAR, Endovascular aneurysm repair; IQR, interquartile range; LOS, length of stay.

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• Fig 3. 

  Mortality rate of open (gray bars) vs endovascular repair (EVAR, black bars) of ruptured abdominal aortic aneurysms in teaching and nonteaching hospitals.

To further analyze factors associated with mortality and adjust for patient and disease-related factors, all variables of interest from the univariate analysis were entered into a forward stepwise logistic regression model. This included age, sex, race, presence of comorbid conditions, mortality score, type of aortic repair, hospital location (region, hospital type, urban/rural), and teaching status of hospital. Table III lists the results of the multivariate regression. Increased age, higher mortality score, and open repair were all independent factors associated with hospital mortality. Although hospital region or type were not significant factors, the hospital teaching status remained a strong independent predictor as it was on univariate analysis. Adjusted mortality for rAAA was increased at hospitals designated as “nonteaching” centers in the NIS, with an odds ratio of 1.3 (P < .001). Both nonwhite race and female sex approached but did not reach statistical significance as independent predictors of mortality.

Table III. Independent variables associated with hospital mortality on multivariate logistic regression

CI, Confidence interval; OR, odds ratio.

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Discussion 

Our data demonstrated a decrease number of patients presenting with rAAA nationwide. Increased use of AAA screening is likely responsible for this decline. For the general public, the US Preventative Services Task Force recommends a one-time screening ultrasound for all men between 65 and 75 years who smoke or have previously smoked. This represents 70% of all men in this age group.² One prospective study on the effect of AAA screening on the incidence of rAAA cites a 49% reduction during a 7-year period (95% CI, 3%-74%).¹³

Endovascular repair of AAA has increased significantly nationwide since its introduction in the early 1990s. It is considered the procedure of choice for those with severe medical comorbidities that amplify perioperative risks, such as myocardial infarction and multisystem organ failure. The option of repair under local anesthesia provided by EVAR potentially compounds this benefit.⁶, ⁷ A recently published, large retrospective study of high-risk patients receiving care at the Veterans Affairs Administration who underwent EVAR for elective AAA repair cites a significantly lower 30-day and 1-year mortality in these patients.¹⁴

These benefits of EVAR are also demonstrated in patients presenting with rAAA. On univariate analysis, our data collected from the NIS reflects a significant decrease in mortality after EVAR for rAAA, shorter hospital stay, and increased likelihood of returning home after hospital admission. Patients who underwent EVAR required fewer secondary procedures while hospitalized for rAAA, most common of which were tracheostomy placement and exploratory laparotomy. Although differences in outcomes are significantly associated with patients selected for each repair type, we cannot causally attribute them to the procedure due to selection bias, because the groups are clearly not randomly assigned or equally selected.

Our data reflect an increasing role for EVAR in rAAA repair, and this trend is mirrored in emerging literature.³, ¹⁵, ¹⁶ A recent review conducted by the Cochrane collaboration recognizes that although no completed randomized controlled trials currently exist, evidence from both prospective and retrospective reviews demonstrates the feasibility of the use of EVAR for rAAA. They show that this technique may reduce blood loss, length of stay in the intensive care unit, and death in selected patients.⁶

Most studies in the literature are small, single-institution retrospective reviews that lack information on patient selection that would offer more insight into its widespread applicability. Factors that are proposed to influence choice of operative repair are aneurysmal characteristics and patient hemodynamic instability. The percentage of patients presenting with rAAA with favorable anatomy suitable for stent graft placement has been reported with variability in the literature, between 46% and 81%.³, ⁶ This percentage is likely to increase as advances continue in stent graft technology and manufacturing.

The inherent suitability of an aneurysm for endovascular repair, including relative “ease” of repair conferred by its geometry, introduces the potential for selection bias into our data set. Although many variables were included in our multivariate analysis attempting to correct the imbalance between the two groups, anatomic information was not available within the NIS. Anatomic characteristics are not an isolated consideration, because a patient's hemodynamic stability influences the likelihood of obtaining a preoperative computed tomography (CT) scan vs going straight to the operating room. Many retrospective reviews of EVAR use for rAAA have excluded the population deemed too unstable to undergo preoperative CT imaging¹⁵, ¹⁶ for accurate measurement of aneurysm diameter and length. Others include these patients, using techniques such as an aortic occlusion balloon to gain rapid proximal vascular control⁹, ¹⁷, ¹⁸ or stent grafts that fit a variety of common aneurysm sizes.⁹, ¹⁰, ¹⁷

On analysis of mortality rates after EVAR for rAAA at teaching and nonteaching hospitals, our data reflect paradoxic outcomes. Although a significantly favorable mortality benefit is seen with EVAR for rAAA at teaching hospitals, this procedure carries a higher risk of in-hospital death than open repair when performed at a nonteaching facility. This trend was confirmed on multivariate analysis, representing “adjusted” analytic results. Hospital patient volume, availability, and accessibility of resources, surgeon experience, including on-call staff, with endovascular technique and comfort level of performing an EVAR likely contribute to this discrepancy. Other factors, such as a vascular resident available to set up the operating room, measure the AAA, and obtain the endograft, also likely play a role in the difference.

One large retrospective review examining factors associated with morbidity and mortality after repair of rAAA found that high-volume surgeons with subspecialty training in vascular or cardiothoracic surgery conferred a significant survival benefit.¹⁹ As with anything in a procedure-based specialty, volume increases proficiency.

Some analyses of outcomes for patients undergoing repair of rAAA in teaching or university centers vs nonteaching or community centers have cited no significant differences in mortality,⁸, ²⁰ but so far, these studies have been small, have excluded EVAR, or compared data from a single institution.²¹ A large prospective trial would be useful in delineating qualities of both hospital types that contribute to positive and adverse outcomes.

Prehospital care, mobilization of resources, and hospital staff expertise that accompany facilities designated as regional trauma centers have a demonstrated morbidity and mortality benefit in the treatment of rAAA.²² Proficiency in expediting care in many types of emergency situations is part of the phenomenon referred to as the “halo effect” that these facilities confer.²² Hospital staff, including nurses, technicians and vascular surgeons with knowledge of the preoperative preparation, necessary equipment, and experience deploying endoluminal grafts are essential. Institutional capacity to offer EVAR for rAAA also requires access to multiple stent grafts of different sizes or grafts that can fit several aortic diameters.³ After establishing and practicing a protocol for treatment of rAAA with EVAR, one large teaching center demonstrated through a prospective analysis of 40 consecutive patients a mortality rate of only 18%.¹⁷ The study included both stable and unstable patients, with 25% of patients presenting with a systolic blood pressure of <80 mm Hg.

Results of a recent Canadian study of 126 patients substantiated the mortality benefit of protocol establishment for the use of EVAR for rAAA. A prospective analysis of 30-day mortality after its introduction demonstrated a significant reduction in mortality for patients undergoing EVAR for rAAA from 30% before the protocol to 17.9% after the protocol was instituted (odds ratio, 0.385; 95% CI, 0.141-0.981; P = .046).²³ Interestingly, this study showed a trend toward improved mortality for hemodynamically unstable patients who underwent EVAR rather than open repair.²³

Our study does have several significant limitations that should be noted. It is a retrospective review of a large prospectively collected administrative database, which can be affected by missing data fields and inaccurate or miscoded entries. As is common with these types of databases, it is hard to “personalize” all the relevant clinical variables that a physician would consider when deciding on a course of treatment, such as physiologic status of the patient and availability of local resources. The NIS does now contain several proprietary “disease severity” variables that can be used to adjust for severity of illness, but these are based on available diagnostic and procedure codes that may vary by hospital or region.

The time frame of this study is during the relative infancy of the application of EVAR for rAAA, and may not reflect the current or future outcomes associated with this approach as new devices and modifications have been made. Similarly, the differences noted between teaching and nonteaching centers may reflect these centers being at different points on the learning curve for this technology. It may also be the result of differences in patient disease, selection, or timing of intervention that could not be appreciated from the available data. This finding would require further detailed and prospective analysis to confirm.

Hospital and surgeon-specific procedure volumes likely influence patient selection for type of rAAA repair. We believe that including this entirely separate and complex variable could introduce further bias and confounding. We chose to analyze only the clearly stated and defined hospital variables contained in the NIS, such as hospital region and hospital teaching status. Without detailed information on procedure volume, individual surgeon volume, and infrastructure, we are hesitant to undertake this additional analysis, which may be more likely misleading than additive. This topic would be an excellent subject for a completely separate future analysis.

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Author contributions 

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References 

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