Endovascular treatment of ruptured abdominal aortic aneurysms in the United States (2001-2006): A significant survival benefit over open repair is independently associated with increased institutional volume
Article Outline
- Abstract
- Methods
- Results
- Overall patient characteristics
- Independent determinants of EVAR usage in patients with RAAA
- Postoperative mortality following RAAA repair evaluated according to elective open AAA repair volume, elective EVAR volume, and total RAAA volume
- Elective open AAA repair volume
- Elective EVAR volume
- Total RAAA volume
- Independent determinants of mortality after RAAA repair
- Resource utilization
- Discussion
- Conclusion
- Author contributions
- References
- Copyright
Objective
Endovascular aortic repair (EVAR) has gained wide acceptance for the elective treatment of abdominal aortic aneurysms (AAA), leading to interest in similar treatment of ruptured abdominal aortic aneurysms (RAAA). The purpose of this study was to evaluate national outcomes after EVAR for RAAA and to assess the effect of institutional volume metrics.
Methods
The Nationwide Inpatient Sample was used to identify patients treated with open or EVAR for RAAA, 2001-2006. Procedure volume was determined for each institution categorizing hospitals as low-, medium-, and high-volume. The primary outcome was in-hospital mortality. Secondary outcomes related to resource utilization. Multivariable logistic regression models were used to determine independent predictors of EVAR usage and mortality.
Results
From 2001 to 2006, an estimated 27,750 hospital discharges for RAAA occurred; 11.5% were treated with EVAR. EVAR utilization increased over time (5.9% in 2001 to 18.9% in 2006, P < .0001) while overall RAAA rates remained constant. EVAR had a lower overall in-hospital mortality than open repair (31.7% vs 40.7%, P < .0001), an effect which amplified when stratified by institutional volume. On multivariable regression, open repair independently predicted mortality (odds ratio [OR] 1.56; 95% confidence interval [CI] 1.29-1.89). EVAR usage for RAAA increased with age (>80 years) (OR 1.58; 95% CI 1.30-1.93), high elective EVAR volume (>40/y) vs medium (19-40/y) (OR 2.65; 95% CI 1.86-3.78) and low (<19/y) (OR 5.37; 95% CI 3.60-8.0). EVAR had a shorter length of stay (11.1 vs 13.8 days, P < .0001), higher discharges to home (65.1% vs 53.9%, P < .0001), and lower charges ($108,672 vs $114,784, P < .0001).
Conclusions
In the United States, for RAAA, EVAR had a lower postoperative mortality than open repair. Higher elective open repair as well as RAAA volume increased this mortality advantage for EVAR. These results support regionalization of RAAA repair to high volume centers whenever possible and a wider adoption of endovascular repair of RAAA nationwide.
Since endovascular aortic repair (EVAR) was initially reported by Parodi in 1991,1 the use of EVAR for elective abdominal aortic aneurysm (AAA) repair has gained increasing popularity. The minimally invasive nature of this technique, as well as the demonstrated reduction in 30 day2, 3, 4 morbidity and mortality rates, have contributed to the rapid adoption of this new technology. The successful application of EVAR for elective AAA repair has led to an increasing interest in expanding its role in patients presenting with ruptured abdominal aortic aneurysms (RAAA).
Rupture of an AAA portends a dismal prognosis. Most patients do not survive long enough to reach medical care4 and, for those who do survive and undergo traditional open repair, the reported mortality rate continues to exceed 40%.5, 6, 7 Given these poor outcomes after open RAAA repair, as well as the reduction in perioperative morbidity and mortality seen after elective EVAR, some centers have established protocols to use EVAR for patients with RAAA. Recently, Mehta and colleagues demonstrated excellent results after implementation of such a protocol, with reported EVAR mortality rates for RAAA as low as 18%.8 Mastracci and colleagues, who report an aggregate 21% mortality rate for RAAA treated with EVAR, have corroborated these results in a systematic review.9
We therefore sought to determine the frequency with which EVAR is used to repair RAAA on a national level and to investigate three primary endpoints: (1) predictors of EVAR use for the treatment of RAAA, (2) postoperative mortality rates for EVAR and open repair, and (3) resource utilization. We also evaluated the relationship between hospital aortic aneurysm repair volume and the RAAA mortality rate at individual centers. We hypothesize that EVAR of ruptured aortic aneurysms is performed with lower mortality and lower resource utilization than open repair.
Methods
The Nationwide Inpatient Sample (NIS) from 2001 to 200610 was utilized to evaluate operative outcomes associated with RAAA at the national level. The NIS is the largest representative database of inpatient hospital discharges, derived from an annual national survey of 20% of nonfederal United States hospitals. It contains all-payer data and is supported by the Healthcare Cost and Utilization Project (HCUP). The NIS provides a validated weighting strategy in order to allow estimates to be drawn at the national level. Because the statistical calculations are based on these weighted frequencies, the data reported in this study are provided in weighted format; this technique is consistent with prior reports.11, 12
We assembled a study cohort of patients undergoing RAAA repair using diagnostic and procedural codes from the International Classification of Diseases Ninth Revision, Clinical Modification (ICD9-CM).13 Patients were identified by linking the diagnostic codes for RAAA (441.3; 441.30) with the procedure codes for either open aortic repair (38.44; 39.25) or endovascular aortic repair (39.71). Patients who did not undergo either open or endovascular surgical intervention were excluded. We intentionally excluded all years prior to 2001 because the specific ICD9-CM code for EVAR was not available until October of 2000. Additionally, we specifically excluded all patients with an ICD9-CM code for intact AAA (441.4) in order to focus the analysis on RAAA.
Patient level factors analyzed included age, gender, race, insurance type, and comorbid medical conditions (using the Elixhauser comorbid technique, designed for use with administrative datasets14). Hospital level factors were obtained by directly linking the NIS to the American Hospital Association's Annual Survey of Hospitals.10 These factors included hospital size (number of inpatient beds),15 teaching status, and location (rural vs urban).
Hospital volume designations
Three variables were created to describe yearly hospital surgical volume: elective open aneurysm volume, elective EVAR volume, and total RAAA volume (repair by open or EVAR). Annual institutional volume was calculated by dividing the total number of procedures performed at a given institution by the number of years in which the institution was included in the survey. Institutional volume was divided into tertiles in order to maintain similar group numbers for each volume stratum prior to any statistical analysis, as has been previously described in other volume-based analyses.16 For elective open aneurysm repair annual volume, low volume centers performed less than 13 repairs, medium volume centers performed 13 to 29 repairs, and high volume centers performed more than 29 repairs. For elective EVAR annual volume, low volume centers performed less than 19 repairs, medium volume centers performed 19 to 40 repairs, and high volume centers performed more than 40 repairs. For RAAA annual volume (regardless of repair type), low volume centers performed less than three repairs, medium volume hospitals performed three to six repairs, and high volume centers performed more than six RAAA repairs.
Study endpoints
The primary endpoint in this cross-sectional study was in-hospital mortality, defined as a postoperative death from any cause prior to hospital discharge (regardless of time from the index procedure). Secondary outcomes, pertaining to hospital resource utilization, were length of hospital stay, hospital charges, incidence of adjunctive procedures (feeding tube or tracheostomy), and disposition at time of discharge (home vs rehabilitation facility, skilled nursing facility, nursing home, or other).
Statistical analyses were performed using SAS version 9.1 (Cary, NC). Univariate analysis was performed by t test for continuous variables and by Rao-Scott χ2 for categorical variables. Trend analyses were performed using the Mantel-Haenszel χ2 test. Multivariable logistic regression models were utilized to identify independent predictors of RAAA mortality and to identify independent determinants of the procedure type selected for the treatment of RAAA (EVAR vs open). All tests were considered statistically significant at an alpha level of .05 (P = .05, two-tailed).
Results
Overall patient characteristics
The mean age of the entire cohort was 73.1 years; 26% were older than 80 years of age. Overall, 77% of patients were men and 89% of patients were white. Comorbid medical conditions were common in this population, with 68% of patients having at least one documented comorbid condition. The majority of patients (70%) were treated at the largest hospitals in their geographic region.
A comparison of the characteristics between the open and EVAR repair groups revealed significant differences between the two groups included older age in the EVAR group (P < .0001) and a corresponding greater percentage of octogenarians (P < .0001). In addition, the RAAA patients treated with EVAR were more likely to have certain pre-existing comorbidities, specifically, renal failure (P = .01), hypertension (P = .02), and liver disease (P = .02). The two groups were similar in terms of gender, race, the total number of comorbid conditions, and insurance type (P > .05 for all), Table I.
Table I. Comparison of open and endovascular repairs of ruptured abdominal aortic aneurysms in the United States, 2001-2006
| Open | EVAR | Total | P value | |
|---|---|---|---|---|
| Weighted size n (%) | 24,571 (88.5) | 3179(11.5) | 27,750(100) | |
| Characteristics | ||||
| Mean age, y [SEM] | 73.0 [0.13] | 74.3 [0.43] | 73.1 [0.12] | <.0001 |
| >80-y-old (%) | 6020(24.5) | 1065(33.5) | 7085(25.5) | <.0001 |
| Male (%) | 18,958(77.2) | 2436(76.6) | 21,394(77.1) | .76 |
| Race (%) | .12 | |||
| Non-white | 1946(11.0) | 320(13.6) | 2266(11.3) | |
| White | 15,797(89.0) | 2023(86.4) | 17,819(88.7) | |
| Comorbidities | ||||
| Renal failure | 1892(7.7) | 340(10.7) | 2232(8.0) | .01 |
| CHF | 702(2.9) | 66(2.1) | 768(2.8) | .25 |
| DM | 2123(8.6) | 312(9.8) | 2434(8.8) | .39 |
| Chronic lung disease | 8466(34.5) | 1055(33.2) | 9521(34.3) | .54 |
| HTN | 9106(37.1) | 1325(41.7) | 10,431(37.6) | .02 |
| Obesity | 889(3.6) | 119(3.8) | 1008(3.6) | .86 |
| Liver disease | 210(0.9) | 56(1.8) | 266(0.96) | .02 |
| Comorbid conditions | .05 | |||
| No comorbidity | 7978(32.5) | 879(27.7) | 8857(31.9) | |
| 1-3 comorbidities | 15,686(63.9) | 2163(68.0) | 17,849(64.3) | |
| >3 comorbidities | 907(3.7) | 137(4.3) | 1044(3.8) | |
| Insurance type | .81 | |||
| Private/Medicare | 23,108(94.1) | 2996(94.4) | 26,104(94.2) | |
| Medicaid/self-pay | 1438(5.2) | 178(5.6) | 1617(5.8) | |
| Hospital bedsize | .15 | |||
| Small | 1779(7.3) | 255(8.0) | 2034(7.3) | |
| Medium | 5762(23.5) | 585(18.4) | 6347(22.9) | |
| Large | 16,997(69.3) | 2339(73.6) | 19,336(70.0) | |
| Hospital type (%) | <.0001 | |||
| Teaching | 12,329(50.2) | 2196(69.1) | 14,524(52.4) | |
| Non-teaching | 12,209(49.8) | 983(30.9) | 13,193(47.6) | |
| Hospital location (%) | .17 | |||
| Urban | 22,514(91.8) | 3008(94.6) | 25,522(92.1) | |
| Rural | 2024(8.2) | 171(5.4) | 2195(7.9) | |
| Elective open AAA volume | 0.34 | |||
| Low (<13/y) | 11,528(46.9) | 1404(44.2) | 12,932(46.6) | |
| Medium (13-29/y) | 7220(29.4) | 867(27.2) | 8087(29.1) | |
| High (>29/y) | 5822(23.7) | 908(28.6) | 6731(24.3) | |
| Elective EVAR volume | <.0001 | |||
| Low (<19/y) | 17,013(69.2) | 1394(43.8) | 18,407(66.3) | |
| Medium (19-40/y) | 4563(18.6) | 660(20.8) | 5223(18.8) | |
| High (>40/y) | 2995(12.2) | 1125(35.4) | 4120(14.9) | |
| Ruptured AAA volume | .02 | |||
| Low (< 3/y) | 8202(33.4) | 800(25.2) | 9002(32.4) | |
| Medium (3-6/y) | 9934(40.4) | 1306(41.1) | 11,240(40.5) | |
| High (>6/y) | 6434(26.2) | 1073(33.7) | 7507(27.1) |
The choice of procedure type (EVAR vs open) for RAAA repair was evaluated according to elective open AAA repair volume, elective EVAR volume, and total RAAA volume. From 2001 to 2006, an estimated 27,750 patients underwent repair of RAAA. Overall, open aneurysm repair was utilized with far greater frequency than EVAR, representing 88.5% of RAAA repairs. Nonetheless, over the 6-year study period, despite the relatively unchanged incidence of aneurysm rupture, the percentage of patients undergoing EVAR for RAAA increased from 5.9% in 2001 to 18.9% in 2006 (P < .0001 by trend test, Fig 1).
Fig 1.
Procedures performed for ruptured AAA, 2001-2006.
The volume of elective open AAA repairs performed by an institution did not significantly impact utilization rates of EVAR for RAAA by univariate analysis (Fig 2, a). In contrast, higher volume elective EVAR centers treated a significantly higher percentage of RAAA patients with EVAR (27.3 % vs 12.6% vs 7.6%, P < .0001) (Fig 2, b). Similarly, higher volume RAAA centers also treated a significantly higher percentage of RAAA patients with EVAR (14.3% vs 11.6% vs 8.9%, P = .02) (Fig 2, c).
Fig 2.
A, Procedure type performed for RAAA based on elective OPEN AAA institutional volume. B, Procedure type performed for RAAA based on annual elective EVAR institutional volume. C, Procedure type performed for RAAA based on annual RUPTURED AAA institutional volume.
Independent determinants of EVAR usage in patients with RAAA
Multivariable logistic regression models that included patient characteristics, hospital characteristics, volume metrics, and procedure year were constructed to identify independent predictors of EVAR utilization in patients with RAAA (Table II). Adjusting for all other variables in the model, institutions in the high volume tertile (>40/y) for elective EVAR were more likely to use EVAR for RAAA than were the medium tertile (19-40/y) institutions (odds ratio [OR] 2.65; 95% confidence interval [CI] 1.86-3.78) and the low tertile (<19/y) institutions (OR 5.37; 95% CI 3.6-8.0). Other factors that were independently predictive of EVAR usage for RAAA included age >80 years (OR 1.58; 95% CI 1.30-1.93), treatment at a teaching hospital (OR 1.85; 95% CI 1.40-2.45), or being treated in the year 2006 vs 2001 (OR 2.96; 95% CI 2.01-4.36). Conversely, a significant negative predictor by multivariate regression of EVAR usage for RAAA was high volume of open elective AAA repair: the high volume tertile (>29/y) open elective AAA centers were less likely to utilize EVAR for RAAA than the medium tertile (13-29/y) centers (OR .72; 95% CI .50-1.03) and the low tertile (<13/y) centers (OR .36; 95% CI .23-.57).
Table II. Multivariable analysis of the predictors of EVAR for treatment of ruptured AAA
| Factor | Odds ratio | 95% confidence interval | P |
|---|---|---|---|
| Patient characteristics | |||
| Men (vs women) | 1.11 | .91-1.37 | .30 |
| Age group | |||
| >80 (vs <80) | 1.58 | 1.30-1.93 | <.0001 |
| Comorbid conditions | |||
| CHF (vs none) | .63 | .33-1.22 | .17 |
| Hypertension (vs none) | 1.17 | .88-1.57 | .29 |
| Chronic lung disease (vs none) | 1.01 | .77-1.34 | .92 |
| Liver disease (vs none) | 1.73 | .74-4.04 | .21 |
| Renal failure (vs none) | 1.38 | .98-1.94 | .07 |
| Diabetes (vs none) | 1.11 | .77-1.60 | .59 |
| Comorbid groups | .98 | ||
| >3 comorbidities (vs none) | 1.03 | .46-2.33 | |
| >3 comorbidities (vs 1-3) | .99 | .55-1.79 | |
| Insurance type | .99 | ||
| Medicaid/self-pay (vs private) | 1.00 | .69-1.47 | |
| Year of procedure | |||
| 2006 (vs 2001) | 2.96 | 2.01-4.36 | <.0001 |
| Hospital characteristics | |||
| Annual elective open AAA volume | <.0001 | ||
| High (>29/y) vs medium (13-29/y) | .72 | .50-1.03 | |
| High (>29/y) vs low (<13/y) | .36 | .23-.57 | |
| Annual elective EVAR volume | <.0001 | ||
| High (>40/y) vs medium (19-40/y) | 2.65 | 1.86-3.78 | |
| High (>40/y) vs low (<19/y) | 5.37 | 3.60-8.0 | |
| Annual ruptured AAA volume | .83 | ||
| High (>6/y) vs medium (3-6/y) | .92 | .68-1.25 | |
| High (>6/y) vs low (<3/y) | .98 | .67-1.44 | |
| Bed size | .37 | ||
| Large (vs small) | 1.18 | .78-1.78 | |
| Medium (vs small) | .94 | .6-1.46 | |
| Hospital location | .26 | ||
| Urban (vs rural) | 1.3 | .83-2.03 | |
| Teaching status | <.0001 | ||
| Teaching (vs non-teaching) | 1.85 | 1.4-2.45 |
Postoperative mortality following RAAA repair evaluated according to elective open AAA repair volume, elective EVAR volume, and total RAAA volume
The overall crude mortality following operative repair of ruptured AAA was 40.7% for open repair and 31.7% for EVAR, P < .0001 (Fig 3).
Fig 3.
Overall In-hospital mortality by procedure type for RAAA.
Elective open AAA repair volume
The low volume elective open AAA institutions (<13/y) had similar mortality for RAAA for both EVAR and open repair (39.3% vs 43.9%, P = .13). Similarly, at the medium volume elective open AAA institutions (13-29/y), EVAR for ruptured aneurysm repair was associated with a similar mortality to open repair (32.1% vs 38.3%, P = .14). In contrast, at the high volume open elective AAA centers (>29/y), the mortality was significantly lower for EVAR than for open repair (19.7% vs 37.2%, P < .0001) (Fig 4, a).
Fig 4.
A, In-hospital mortality of RAAA by procedure type based on elective open AAA institutional volume. B, in-hospital mortality of RAAA based on elective EVAR institutional volume. C, in-hospital mortality of RAAA based on RAAA annual institutional volume.
Elective EVAR volume
At the centers performing a low annual volume of elective EVAR (<19/y), the mortality after repair of a RAAA was similar for both EVAR and open repair (41.1% vs 41.3%, P = .95). Institutions in the medium volume tertile for the number of elective EVAR procedures (19-40) demonstrated significantly lower RAAA mortality rates with EVAR than with open repair (27% vs 40.6%, P = .003). The difference in postoperative mortality rates was even greater at the centers performing a high annual volume of elective endovascular aneurysm repairs (>40/y), where the postoperative mortality rates after endovascular repair for aneurysm rupture (22.9%) were 14% less than that for open repair (37.1%), P < .0001 (Fig 4, b).
Total RAAA volume
At the institutions performing the lowest annual volume of ruptured AAA repairs (<3/y), the postoperative mortality rate after both EVAR and open repair was similarly in excess of 40%, P = .64. At centers performing a medium annual volume of ruptured AAA repairs (3-6/y), EVAR patients had a lower postoperative mortality rate than open repair (31.2% vs 40.5%, P = .008). Similarly, at the centers performing the highest annual volume of ruptured AAA repairs (>6/y), the postoperative mortality rate after EVAR was significantly lower than after open repair, (22.3% vs 37.4%, P < .0001) (Fig 4, c).
Independent determinants of mortality after RAAA repair
Multivariable logistic regression models that included patient characteristics, hospital characteristics, volume metrics, and procedure year were constructed to identify independent predictors of mortality in patients with RAAA (Table III). Adjusting for all other variables in the model, open repair of RAAA was associated with a significantly higher mortality than EVAR (OR 1.56; 95% CI 1.29-1.89). Likewise, low volume of open elective AAA was associated with higher mortality (OR 1.24; 95% CI 1.01-1.52). Other independent predictors of mortality include female gender (OR 1.41; 95% CI 1.23-1.61), age >80 years (OR 1.95; 95% CI 1.72-2.22) and treatment at a non-teaching hospital (OR 1.24; 95% CI 1.08-1.43).
Table III. Multivariable analysis of the predictors of mortality for treatment of ruptured AAA
| Factor | Odds ratio | 95% confidence interval | P |
|---|---|---|---|
| Patient characteristics | |||
| Women (vs men) | 1.41 | 1.23-1.61 | <.0001 |
| Age group | |||
| >80 (vs <80) | 1.95 | 1.72-2.22 | <.0001 |
| Comorbid conditions | |||
| CHF (vs none) | .83 | .57-1.23 | .35 |
| Hypertension (vs none) | .72 | .6-.87 | .0005 |
| Chronic lung disease (vs none) | .74 | .62-.89 | .001 |
| Liver disease (vs none) | 3.35 | 1.94-5.78 | <.0001 |
| Renal failure (vs none) | 1.07 | .84-1.37 | .6 |
| Diabetes (vs none) | 1.17 | .92-1.48 | .2 |
| Comorbid groups | .02 | ||
| >3 comorbidities (vs none) | .83 | .48-1.44 | |
| >3 comorbidities (vs 1-3) | 1.1 | .73-1.66 | |
| Insurance type | |||
| Medicaid/self-pay (vs private) | .83 | .65-1.1 | .14 |
| Year of procedure | |||
| 2006 (vs 2001) | 1.05 | .85-1.29 | .12 |
| Procedure type | |||
| Open repair (vs EVAR) | 1.56 | 1.29-1.89 | <.0001 |
| Hospital characteristics | |||
| Annual elective open AAA volume | .09 | ||
| Low (<13/y) vs high (>29/y) | 1.24 | 1.01-1.52 | |
| Medium (13-29/y) vs high (>29/y) | 1.09 | .91-1.32 | |
| Annual elective EVAR volume | .19 | ||
| Low (<19/y) vs high (>40/y) | 1.06 | .85-1.32 | |
| Medium (19-40/y) vs high (>40/y) | 1.2 | .96-1.49 | |
| Annual ruptured AAA volume | .95 | ||
| Low (<3/y) vs high (>6/y) | 1.03 | .84-1.25 | |
| Medium (3-6/y) vs high (>6/y) | 1.03 | .86-1.22 | |
| Bed size | .09 | ||
| Large (vs small) | .81 | .64-1.04 | |
| Medium (vs small) | .93 | .73-1.19 | |
| Hospital location | .08 | ||
| Rural (vs urban) | 1.22 | .97-1.54 | |
| Teaching status | .003 | ||
| Non-teaching (vs teaching) | 1.24 | 1.08-1.43 |
Resource utilization
Following the repair of RAAA, patients who underwent EVAR had significantly shorter mean hospital length of stay than those undergoing open repair (11.1 days vs 13.8 days, P < .0001). Similarly, a significantly larger proportion of patients treated with EVAR were discharged directly to home than those undergoing open repair (65.1% vs 53.9%, P < .0001). Open repair of RAAA was associated with an increased percentage of adjunctive procedures during the index hospitalization, including tracheostomy placement (5.2% vs 2.9%, P = .01) and feeding tube placement (4.6% vs 2.7%, P = .03). With regards to mean total hospital charges, EVAR had lower expenditures than open repair ($108,672 vs $114,784, P < .0001) (Table IV).
Table IV. EVAR vs open AAA resource utilization
| Open | EVAR | Overall | P value | |
|---|---|---|---|---|
| Additional surgical procedures | ||||
| Tracheostomy (%) | 5.2 | 2.9 | 4.9 | .01 |
| Feeding tube (%) | 4.6 | 2.7 | 4.4 | .03 |
| Mean length of stay (d) | 13.8 | 11.1 | 13.5 | <.0001 |
| Discharge disposition | <.0001 | |||
| Discharged to home (%) | 53.9 | 65.1 | 55.4 | |
| Discharged to rehab/other (%) | 46.1 | 34.9 | 44.6 | |
| Mean hospital charges | $114,784 | $108,672 | $114,316 | <.0001 |
Discussion
This study shows that while the incidence of RAAA between 2001 and 2006 remained fairly constant, EVAR was used to treat RAAA in an increasing proportion of patients (from 5.9% in 2001 to 18.9% in 2006). Independent predictors of the use of EVAR for the treatment of RAAA included advanced age, high institutional elective EVAR volume, and treatment at a teaching institution. Endovascular repair of RAAA was independently associated with a lower postoperative mortality risk than was open repair (OR 1.56, CI 1.29-1.89), and the reduction in postoperative mortality was strongly related to the institutional volume of elective AAA repairs. Additionally, patients undergoing EVAR demonstrated lower resource utilization than those undergoing open repair.
The finding in the current study that an increasing percentage of patients with RAAA were treated with EVAR during the study period correlates well with previous findings from the NIS and from other population-based datasets. Using statewide data from California, Florida, New Jersey, and New York (approximately one-third of US population), Greco et al found that the percentage of patients undergoing EVAR for RAAA increased from 0.3% to 6.3% between 2000 and 2003.17 L'Esperance et al, in a recent report based on data from the NIS also noted an increasing trend toward EVAR usage from 6% in 2001 to 11% by 2004.18 The current study may more accurately reflect national trends than the regional report of Greco et al. Similarly, the NIS, unlike Medicare data, incorporates all-payer information and therefore included patients less than 65 years of age and uninsured patients (6% of total); it also incorporates more rural regions such as the Midwest and Southern United States. Additionally, this study extends the findings of L'Esperance and colleagues by extending the analysis to 2006 (the most recent available data) and by explicitly investigating predictors of EVAR use, RAAA repair associated mortality, and resource utilization.
We observed a lower mortality rate after EVAR for treatment of RAAA than after open repair on both univariate and multivariable analysis. Previous authors have suggested that annual hospital surgical volume may contribute to outcomes following RAAA repair. L'Esperance et al did not explicitly include a volume analysis, but hypothesized that hospital volume likely contributed to their observed differences between teaching and non-teaching institutions.18 A volume analysis by Greco et al also found that greater elective and non-elective AAA volume was associated with lower mortality following AAA rupture.17 On univariate analysis, we found a clear mortality advantage directly related to all volume measures analyzed: annual elective open repair volume, annual elective EVAR volume, and annual RAAA volume. For each of these metrics, mortality decreased significantly as annual surgical volume increased. The absolute mortality benefit was most pronounced when analyzed according to elective EVAR volume; institutions in the high elective EVAR volume tertile (>40/y) had an absolute mortality decrease of >15% (Fig 4, b). This volume outcome relationship has been clearly demonstrated in the elective AAA setting,19, 20 however, the volume impact on outcomes following repair of RAAA has been heretofore limited. Unfortunately due to the de-identified patient information and administrative nature of this dataset, comprehensive clinical information is unavailable. Conclusions drawn from this work should be made with the understanding that an unmeasured selection bias may exist such that hemodynamically unstable patients may not have been considered for endovascular management. However, depending on the comfort level of the surgeon and institutional experience with EVAR, it is feasible that some unstable patients may still have been treated by endovascular means as was the case in 25% of patients in the EVAR arm of the work from the Albany Medical Center.8
The absence of significant independent effects of EVAR volume and RAAA volume on mortality deserves special comment. This finding may be related to our study design which involved the creation of three distinct volume categories for each procedure examined (elective EVAR, elective open, RAAA). This analytic design assigned three different volume designations to each single institution. For example, it was possible for a single institution to be designated a high volume EVAR institution and a high volume open institution, while also a low volume RAAA institution. Because all volume characteristics were included in the multivariate regression, the effects of a single volume characteristic may be attenuated due to colinearity with another volume characteristic of the same institution. To test for this effect, we performed a separate analysis where each volume metric was included in the regression separately. On these analyses, each low volume designation was independently predictive of increased mortality, confirming a colinearity effect.
Of interest, we found that even after adjustment for hospital surgical volume characteristics, teaching hospitals continued to show lower mortality risks following RAAA repair than non-teaching hospitals. This finding is similar to the conclusions of L'Esperance et al who identified a survival advantage for EVAR at teaching hospitals but did not specifically assess or control for surgical volume measures in their analysis.18 In our study, this overall survival advantage in favor of teaching hospitals persisted after volume adjustment. While we recognize that this survival advantage at teaching hospitals is likely multifactorial, we believe that even low volume centers may benefit from the presence of a specific vascular surgical liaison (resident/fellow) “on the ground,” in order to expeditiously triage and oversee the timely transfer of critically ill patients to the computed tomography (CT) scanner or operating room, depending on the degree of cardiovascular compromise.
Previous authors have shown that establishment of a multidisciplinary ruptured AAA protocol maximizes the ability to perform EVAR in an emergent fashion in both hemodynamically stable and unstable patients.8, 21 In a single institution report from the Albany Medical Center (n = 85), Mehta and colleagues found that following the implementation of a protocol for EVAR for RAAA, the mean time to the operating room from a presumptive diagnosis of RAAA was 20 minutes, with an overall mortality rate of 18%.8 They attributed their success to maintaining an adequate inventory of available stent grafts, surgeon comfort with EVAR in the elective setting, and the practice of “hypotensive hemostasis” (minimizing volume resuscitation and tolerating hypotension as long as the patient maintained a detectable blood pressure only, in an effort to minimize ongoing hemorrhage). Our national findings support their institutional observations, in that high volume elective EVAR centers had a fivefold increase in odds of using EVAR in a ruptured AAA setting, with a mortality rate as low as 20%. This finding may relate to the greater ability of high volume EVAR centers to stock a diverse array of endograft options, thereby facilitating emergency usage. In addition, high volume EVAR centers are more likely to have available an endovascular surgeon with greater technical fluency in endovascular therapies.
In our opinion, the current and previous findings, within the limitations of studies derived from administrative datasets including the potential for selection bias in favor of EVAR, may serve as an impetus for hospitals to establish RAAA protocols to maximize the potential for EVAR usage when appropriate. Institutions at which EVAR protocols for RAAA are not feasible may consider the implementation of systems by which appropriate patients can be rapidly transferred to institutions with EVAR capabilities. This concept was underscored by the Albany medical center group in which more than one-third of the study patients were transferred from an outside institution.8 This suggests that, although critically ill, a substantial number of RAAA patients may be candidates for transfer to higher volume EVAR institutions.
In terms of resource utilization between the two procedures, the statistical significance observed may be of limited clinical or practical significance. Both procedure types were associated with total hospital charges greater than $100,000 and the average length of stay was greater than ten days for both procedures. The detailed characteristics of specific costs of the two procedures would best be evaluated by an in-depth cost analysis between both procedure types. The current dataset is limited to overall hospital charge information and therefore specific cost factors attributable directly to the procedure itself or related complications are unavailable in this type of study.
Our study design faces several of the limitations that are inherent to any work utilizing large administrative datasets.22, 23 First, coding inaccuracies relating to patient factors, hospital characteristics, or outcomes may exist including the apparently protective effect of certain chronic medical conditions. This observation was appreciated by previous authors and found that likely a bias against the coding of chronic medical conditions in patients that die in the hospital accounts for this phenomenon to some degree.24 In this study, these coding errors are expected to equally affect open aneurysm repair patients and EVAR patients, and therefore are unlikely to introduce significant bias in comparing the two procedures.
Second, because the NIS dataset excludes several important clinical factors, we were not able to control for possible selection bias favoring the use of EVAR for more stable patients. These missing factors include, but are not limited to, imaging data pertaining to aortic anatomy, hemodynamic status on arrival to the hospital, laboratory data, and time delay to diagnosis. We attempted to compensate for these limitations by using previously validated comorbidity software,14 and by including all of the comorbidity variables in the multivariable regression models. However, this methodology cannot completely capture the severity of every individual patient's comorbid disease state.
Finally, while the primary study outcome, in-hospital mortality, is an important and reliable endpoint, it by no means captures all of the elements necessary to judge treatment success. The rigorous patient de-identification process employed by the NIS to protect patient confidentiality precludes the analysis of longitudinal clinical data. Therefore, it was impossible to evaluate other equally important outcomes, such as long-term morbidity and mortality, need for secondary interventions, and quality of life indices.
Conclusion
An increasing proportion of patients presenting with RAAA in the United States were treated by EVAR between 2001 and 2006. The strongest predictor of the use of EVAR for RAAA repair is elective EVAR institutional volume. At the national level, within the confines of administrative datasets including a likely unmeasured selection bias, EVAR was found to have a significantly lower national mortality rate than the open repair of RAAA. The regionalization of RAAA to high volume EVAR institutions, when feasible, may serve to decrease the in-hospital mortality rate for this critically ill patient population.
Author contributions
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Competition of interest: none.
PII: S0741-5214(08)01881-8
doi:10.1016/j.jvs.2008.11.002
© 2009 The Society for Vascular Surgery. Published by Elsevier Inc. All rights reserved.
