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Editor's Choice – High Annual Hospital Volume is Associated with Decreased in Hospital Mortality and Complication Rates Following Treatment of Abdominal Aortic Aneurysms: Secondary Data Analysis of the Nationwide German DRG Statistics from 2005 to 2013

  • Author Footnotes
    c M. Trenner and A. Kuehnl contributed equally to this article.
    Matthias Trenner
    Footnotes
    c M. Trenner and A. Kuehnl contributed equally to this article.
    Affiliations
    Department of Vascular and Endovascular Surgery, Klinikum rechts der Isar, Technical University of Munich, Munich Aortic Centre (MAC), Munich, Germany
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  • Author Footnotes
    c M. Trenner and A. Kuehnl contributed equally to this article.
    Andreas Kuehnl
    Footnotes
    c M. Trenner and A. Kuehnl contributed equally to this article.
    Affiliations
    Department of Vascular and Endovascular Surgery, Klinikum rechts der Isar, Technical University of Munich, Munich Aortic Centre (MAC), Munich, Germany
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  • Michael Salvermoser
    Affiliations
    Department of Vascular and Endovascular Surgery, Klinikum rechts der Isar, Technical University of Munich, Munich Aortic Centre (MAC), Munich, Germany
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  • Benedikt Reutersberg
    Affiliations
    Department of Vascular and Endovascular Surgery, Klinikum rechts der Isar, Technical University of Munich, Munich Aortic Centre (MAC), Munich, Germany
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  • Sarah Geisbuesch
    Affiliations
    Department of Vascular and Endovascular Surgery, Klinikum rechts der Isar, Technical University of Munich, Munich Aortic Centre (MAC), Munich, Germany
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  • Volker Schmid
    Affiliations
    Department of Statistics, Ludwig-Maximilians-Universität München, Munich, Germany
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  • Hans-Henning Eckstein
    Correspondence
    Corresponding author. Department of Vascular and Endovascular Surgery, Klinikum rechts der Isar der Technischen Universität München, Ismaninger Str. 22, 81675 Munich, Germany.
    Affiliations
    Department of Vascular and Endovascular Surgery, Klinikum rechts der Isar, Technical University of Munich, Munich Aortic Centre (MAC), Munich, Germany
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  • Author Footnotes
    c M. Trenner and A. Kuehnl contributed equally to this article.
Open ArchivePublished:December 27, 2017DOI:https://doi.org/10.1016/j.ejvs.2017.11.016

      Objectives

      The aim of this study was to analyse the association between annual hospital procedural volume and post-operative outcomes following repair of abdominal aortic aneurysms (AAA) in Germany.

      Methods

      Data were extracted from nationwide Diagnosis Related Group (DRG) statistics provided by the German Federal Statistical Office. Cases with a diagnosis of AAA (ICD-10 GM I71.3, I71.4) and procedure codes for endovascular aortic repair (EVAR; OPS 5–38a.1*) or open aortic repair (OAR; OPS 5–38.45, 5–38.47) treated between 2005 and 2013 were included. Hospitals were empirically grouped to quartiles depending on the overall annual volume of AAA procedures. A multilevel multivariable regression model was applied to adjust for sex, medical risk, type of procedure, and type of admission. Primary outcome was in hospital mortality. Secondary outcomes were complications, use of blood products, and length of stay (LOS). The association between AAA volume and in hospital mortality was also estimated as a function of continuous volume.

      Results

      A total of 96,426 cases, of which 11,795 (12.6%) presented as ruptured (r)AAA, were treated in >700 hospitals (annual median: 501). The crude in hospital mortality was 3.3% after intact (i)AAA repair (OAR 5.3%; EVAR 1.7%). Volume was inversely associated with mortality after OAR and EVAR. Complication rates, LOS, and use of blood products were lower in high volume hospitals. After rAAA repair, crude mortality was 40.4% (OAR 43.2%; EVAR 27.4%). An inverse association between mortality and volume was shown for rAAA repair; the same accounts for the use of blood products. When considering volume as a continuous variate, an annual caseload of 75–100 elective cases was associated with the lowest mortality risk.

      Conclusions

      In hospital mortality and complication rates following AAA repair are inversely associated with annual hospital volume. The use of blood products and the LOS are lower in high volume hospitals. A minimum annual case threshold for AAA procedures might improve post-operative results.

      Keywords

      This nationwide study has shown a significant association between high annual hospital volume and low post-operative mortality after AAA repair in German hospitals. Furthermore, it shows lower complication rates after intact AAA repair in high volume hospitals. High hospital volume is also associated with a decreased use of blood products and shorter length of stay after intact and ruptured AAA repair. These real world data support the hypothesis that centralisation of these procedures might lower complication rates. A minimum annual caseload of 75 AAA procedures seems reasonable.

      Introduction

      The practice guidelines of the European Society for Vascular Surgery (ESVS) for the treatment of abdominal aortic aneurysms (AAA) recommend a threshold of 50 elective operations per annum,
      • Moll F.L.
      • Powell J.T.
      • Fraedrich G.
      • Verzini F.
      • Haulon S.
      • Waltham M.
      • et al.
      Management of abdominal aortic aneurysms clinical practice guidelines of the European society for vascular surgery.
      whereas the Society of Vascular Surgery (SVS) states that open aortic repair (OAR) should be performed at centres with a documented in hospital mortality <5%.
      • Chaikof E.L.
      • Brewster D.C.
      • Dalman R.L.
      • Makaroun M.S.
      • Illig K.A.
      • Sicard G.A.
      • et al.
      SVS practice guidelines for the care of patients with an abdominal aortic aneurysm: executive summary.
      In Germany, several statutory regulations were implemented for AAA surgery by the Federal Joint Committee (Gemeinsamer Bundesausschuss; GB-A), for example operations have to be performed by specialist vascular surgeons and the treating hospitals have to offer vascular services 24/7. Apart from this, neither centralisation of vascular services is regulated by law nor is there a statutory quality assurance registry. After a recent decision of the Federal Joint Committee, screening for AAA in Germany is now being rolled out. As a result, optimal treatment for patients has to be ensured and possible structural weaknesses have to be identified.
      To date, available data on a volume outcome relationships for AAA repair in Germany are based on a voluntary registry by the German Vascular Society (Deutsche Gesellschaft für Gefäßchirurgie und Gefäßmedizin; DGG). In two separate analyses of these data, an association between annual hospital volume and post-operative mortality was shown for OAR, but not for EVAR, as is the case in many other countries.
      • Trenner M.
      • Haller B.
      • Söllner H.
      • Storck M.
      • Umscheid T.
      • Niedermeier H.
      • et al.
      Twelve years of the quality assurance registry on ruptured and non-ruptured abdominal aortic aneurysms of the German Vascular Society (DGG)- #art 3: predictors of perioperative outcome with a focus on annual caseload (English Version).
      • Eckstein H.H.
      • Bruckner T.
      • Heider P.
      • Wolf O.
      • Hanke M.
      • Niedermeier H.P.
      • et al.
      The relationship between volume and outcome following elective open repair of abdominal aortic aneurysms (AAA) in 131 German hospitals.
      As that registry gathers only about 40% of the AAA cases treated in Germany, about 120 of >500 hospitals which treat AAA participated. In addition, patient sex was not documented, and external validation was lacking; thus, those registry data are viewed with scepticism and might not be representative of the situation of AAA care in Germany.
      • Venermo M.
      • Mani K.
      • Kolh P.
      The quality of a registry based study depends on the quality of the data – without validation, it is questionable.
      • Trenner M.
      • Haller B.
      • Storck M.
      • Reutersberg B.
      • Kallmayer M.A.
      • Eckstein H.H.
      Trends in patient safety of intact abdominal aortic aneurysm repair: German registry data on 36,594 procedures.
      Another analysis of nationwide data was limited to the year 2007 and only included intact AAA.
      • Hentschker C.
      • Mennicken R.
      The volume-outcome relationship and minimum volume standards–empirical evidence for Germany.
      In a study published in 2017, an inverse relationship between hospital volume, morbidity, and mortality could be shown for pancreatic surgery in Germany using nationwide administrative inpatient data collected by the Federal Statistical Office.
      • Krautz C.
      • Nimptsch U.
      • Weber G.F.
      • Mansky T.
      • Grützmann R.
      Effect of hospital volume on in-hospital morbidity and mortality following pancreatic surgery in Germany.
      Furthermore, a meta-analysis on the volume outcome relationship after AAA repair in European hospitals pointed out that there is a lack of information on secondary outcomes following AAA repair.
      • Phillips P.
      • Poku E.
      • Essat M.
      • Woods H.B.
      • Goka E.A.
      • Kaltenthaler E.C.
      • et al.
      Procedure volume and the association with short-term mortality following abdominal aortic aneurysm repair in European populations: a systematic review.
      Thus, the purpose of the present study was to evaluate the association between annual hospital volume and in hospital mortality after ruptured (r) and intact (i) AAA repair using routinely collected nationwide data. In addition, the relationship between volume and secondary outcomes (length of hospital stay, LOS; complications; use of blood products) was analysed.

      Methods

       Data source

      For the present analysis all cases treated for iAAA and rAAA in Germany between January 1, 2005 and December 31, 2013 were identified in a nationwide cohort, based on hospital episode data. Anonymous data are statutorily collected by the Institute for the Hospital Remuneration System (“Institut fuer das Entgeltsystem im Krankenhaus”) and then transferred to the German Federal Statistical Office (GFSO) according to §21 of the Hospital Reimbursement Act (“Krankenhausentgeldgesetz”, KHEntG). Inpatient episode reporting is mandatory and therefore this study can be considered a full nationwide survey (except military and psychiatry services). Methods of how these administrative data can be used and analysed by controlled remote data processing have been established and were recently described in detail.
      • Krautz C.
      • Nimptsch U.
      • Weber G.F.
      • Mansky T.
      • Grützmann R.
      Effect of hospital volume on in-hospital morbidity and mortality following pancreatic surgery in Germany.
      • Nimptsch U.
      • Mansky T.
      Stroke unit care and trends of in-hospital mortality for stroke in Germany 2005-2010.
      • Nimptsch U.
      • Mansk T.
      Deaths following cholecystectomy and herniotomy: an analysis of nationwide German hospital discharge data from 2009 to 2013.
      • Nimptsch U.
      • Krautz C.
      • Weber G.F.
      • Mansky T.
      • Grutzmann R.
      Nationwide in-hospital mortality following pancreatic surgery in Germany is higher than anticipated.
      • Kühnl A.
      • Erk A.
      • Trenner M.
      • Salvermoser M.
      • Schmid V.
      • Eckstein H.
      Incidence, treatment and mortality in abdominal aortic aneurysms- a secondary analysis of diagnosis-related groups (DRG) data from 2005–2014.
      Study ethics were approved by the local Ethics Committee of the Medical Faculty, Technical University of Munich (Reference 21/16 S). The analysis was conducted according to Good Practice of Secondary Data Analysis guidelines
      • Swart E.
      • Gothe H.
      • Geyer S.
      • Jaunzeme J.
      • Maier B.
      • Grobe T.G.
      • et al.
      Good practice of secondary data analysis (GPS): guidelines and recommendations.
      and reporting follows the STROSA2 guideline (modification of the RECORD guideline that addresses specific characteristics of the German healthcare system).
      • Swart E.
      • Bitzer E.M.
      • Gothe H.
      • Harling M.
      • Hoffmann F.
      • Horenkamp-Sonntag D.
      • et al.
      STandardisierte BerichtsROutine für Sekundärdaten Analysen (STROSA) – ein konsentierter Berichtsstandard für Deutschland, version 2.
      • Benchimol E.I.
      • Smeeth L.
      • Guttmann A.
      • Harron K.
      • Moher D.
      • Petersen I.
      • et al.
      The REporting of studies Conducted using Observational Routinely-collected health Data (RECORD) statement.
      Data have been saved and are available on servers of the German Federal Statistical Office. They were accessed using controlled remote data processing, which means individual patient data or institutional identifiers for hospitals were not available to the authors. A protocol for the study was submitted to the German Federal Statistical Office during the application process, but has not been published separately.

       Case selection and population

      An in hospital episode starts on admission and ends at discharge of a patient. Therefore, in this study only cases, rather than individual patients, are described. All cases with a principal or secondary diagnosis of intact or ruptured AAA (ICD-10 GM I71.3/4) specific treatment (defined as procedure codes for EVAR or infrarenal OAR; Table S1) between 2005 and 2013 were included. Cases with codes for both EVAR and OAR were excluded from this analysis, as the exact strategy of therapy (i.e. hybrid procedures, conversions) cannot be identified using this dataset, and cases with procedure codes for fenestrated, branched, or chimney grafts were also excluded. As there is no individual patient identifier, it is not possible to determine whether a patient received operative treatment in more than one hospital, but the number of these cases was considered negligible. For a detailed overview of case selection see Fig. S1; detailed ICD-10 and OPS codes are available in Table S1.

       Hospital volume

      For each year of observation, the volume of AAA repair (rAAA + iAAA) performed per hospital [identified by institutional code (IC), as well as site code] was calculated. IC and site codes were harmonised over the years by employees of the GFSO. With the aim of generating four comparable groups, hospitals were empirically categorised in volume quartiles by overall AAA volume [Q1 1–5 cases (median annual volume three cases); Q2 6–14 (10); Q3 15–30 (22); Q4 >30 (57) AAA cases/year]. Unlike in other studies, the groups were designed to be equally sized regarding the object of investigation (hospitals, second level of data structure) and not regarding the total number of cases/patients (first level of data).

       Patient characteristics and treatment

      Sex, age, type of admission, indication for repair (rAAA versus iAAA), type of surgery (EVAR/OAR), and length of hospital stay were described for all patients. Possible types of admission were “referred by a physician,” “not referred by a physician/emergency admission,” and “transferred from another hospital.” In the German healthcare system, patient self referrals are possible, and by that do not necessarily indicate emergencies. Comorbidities were evaluated using coding for primary or secondary diagnosis, as described in Table S1. The following comorbidities were considered relevant for the study cohort: hypertension, chronic pulmonary disease, chronic ischemic heart disease (cIHD), peripheral arterial disease (PAD), diabetes, renal disease, and malignant disease. As a measure of comorbidity and for later multivariable adjustment, the modified Elixhauser Score was calculated.
      • van Walraven C.
      • Austin P.C.
      • Jennings A.
      • Quan H.
      • Forster A.J.
      A modification of the Elixhauser comorbidity measures into a point system for hospital death using administrative data.
      Not all comorbidities are relevant for remuneration, therefore under reporting is a potential bias.

       Study outcomes

      In hospital mortality (defined as death before discharge) was the primary outcome measure for all rAAA and iAAA cases. If a patient dies during the hospital stay, this has to be reported as “death as reason for discharge,” therefore data on this measure can be regarded as accurate. Secondary outcomes were administrative procedure codes (“Operationen- und Prozedurenschlüssel,” OPS) for blood transfusions of 1–5 and >5 units, transfusion of thrombocytes, required bowel resection, or major amputation of lower limb; DRG codes for acute myocardial infarction, peripheral arterial thrombosis and embolism, mesenteric thrombosis and embolism, renal artery thrombosis, and embolism. The DRG and OPS codes used are listed in Table S1. Secondary outcomes might be confounded by under reporting, as not all codes are relevant for remuneration of cases.

       Statistical analysis

      Categorical variables were shown as absolute numbers and percentages. Continuous, non-normal variables were presented as the median with first (Q 0.25) and third (Q 0.75) quartiles. Differences between the volume quartiles were assessed using the Cochran–Armitage test for trend.
      To estimate the independent association between AAA volume on in hospital mortality after iAAA and rAAA repair, stratification of risk adjusted mortality was conducted for volume quartiles. To adjust for comorbidities, the Elixhauser Score was used.
      • van Walraven C.
      • Austin P.C.
      • Jennings A.
      • Quan H.
      • Forster A.J.
      A modification of the Elixhauser comorbidity measures into a point system for hospital death using administrative data.
      • Sharabiani M.T.
      • Aylin P.
      • Bottle A.
      Systematic review of comorbidity indices for administrative data.
      • Elixhauser A.
      • Steiner C.
      • Harris D.R.
      • Coffey R.M.
      Comorbidity measures for use with administrative data.
      A multilevel multivariable regression model with robust error variance was applied, adjusting for type of procedure (EVAR or OAR), sex, patient age, Elixhauser Score (to adjust for comorbidities), type of diagnosis (principal versus secondary), and type of admission (referral, not referred or emergency, and transferred from other hospital). Adjusted relative risks were calculated. To evaluate the eventual association of annual iAAA and rAAA volume and mortality after either iAAA or rAAA repair, hospitals were also grouped in quartiles by volume of iAAA and rAAA cases.
      Furthermore, ORs for in hospital mortality after iAAA and rAAA repair were calculated as a function of the continuous volume (on the basis of an exploratory approach). To account for confounding and clustering of patients within hospitals, two multilevel multivariable regression models (GLMM) were applied with volume as a third order polynomial independent predictor. Additional predictor variables such as age, sex, Elixhauser Score, type of admission, type of diagnosis, and type of procedure were added to both models as fixed effects. The hospital identifier (IC plus site code) was entered as a random effect (random intercept only). The variables used for modeling the outcome (in hospital death among iAAA and rAAA, respectively) were selected a priori, according to a pre-specified analysis plan that was developed with regard to the literature and theoretical considerations. The model fit was assessed by a series of Wald tests and plots of the predicted probabilities both times. As the database can be considered complete at a national level, calculation of CIs or p values was not necessary or meaningful, but these were calculated to estimate possible variability in future years.
      For controlled remote data processing and statistical analysis, SAS software (version 9.2, Microsoft Windows, 2015 SAS Institute Inc., Cary, NC, USA) was used. Graphic processing of the data was performed using Microsoft Excel and R (Version 3.2.1; The R Foundation, www.r-project.org). For all tests, a two tailed level of significance of α = 5% was applied.

      Results

       Patient characteristics

      In the period from 2005 to 2013, a total of 96,426 cases that had received operative treatment for AAA in >700 hospitals (with an annual median of 501 hospitals) was identified. In all, 11,795 (12.6%) of these cases presented with ruptures. Within hospital quartile 1 (Q1), hospitals performed a median number of three AAA operations per year. Within Q4, hospitals operated on a median of 57 AAA per year. The proportion of rAAA cases was higher in lower volume hospitals. The frequency of coexisting conditions did not differ significantly in the volume quartiles, except for hypertension, which was more frequent in high volume hospitals, and renal disease, which was more frequent in low volume hospitals. High volume hospitals treated more referred cases and cases transferred from other hospitals. EVAR was used more frequently in high volume hospitals for both iAAA and rAAA (Table 1).
      Table 1Characteristics of all cases (N = 84,631) receiving treatment of abdominal aortic aneurysms in this admission.
      OverallQ1Q2Q3Q4
      No. of hospitals
      Median (2005–2013) number of unique centres providing AAA treatment.
      501141120122130
      No. of all AAA cases96,426314410,22823,59459,460
      No. of AAA cases per hospital (median, Q0.25–Q0.75)38 (22–66)3 (2–5)10 (8–13)23 (19–27)57 (41–82)
      No. of rAAA cases (%)11,795 (13)886 (28)1565 (15)2875 (12)6469 (11)
      Age (mean, SD)71.7 ± 8.571.9 ± 8.971.8 ± 8.571.8 ± 8.471.6 ± 8.5
      Female sex (n, %)11,893 (12)450 (14)1271 (12)2976 (13)7196 (12)
      Elixhauser Score (median, Q0.25–Q0.75)5 (0–11)5 (1–11)5 (0–10)5 (0–10)5 (0–10)
      Comorbidities (n, %)
      cIHD32,544 (34)960 (31)3447 (34)8025 (34)20,112 (34)
      Peripheral arterial disease29,702 (31)1035 (33)3238 (32)7355 (31)18,074 (30)
      Hypertension68,171 (71)2007 (64)7068 (69)16,827 (71)42,269 (71)
      Chronic pulmonary disease12,318 (18)575 (18)1940 (19)4365 (19)10,899 (18)
      Diabetes16,306 (17)501 (16)1753 (17)4122 (18)9930 (17)
      Renal disease19,408 (20)576 (18)1970 (19)4820 (14)12,042 (14)
      Any malignancy3394 (3.5)101 (3.2)407 (4.0)858 (3.6)2028 (3.4)
      Type of admission (n, %)
      Referral71,630 (74)2008 (64)7382 (72)17,902 (76)44,338 (75)
      Not referred20,228 (21)1060 (34)2557 (25)4783 (20)11,828 (20)
      Transferred from other hospital4568 (4.7)76 (2.4)289 (2.8)909 (3.9)3294 (5.5)
      Endovascular therapy (n, %)
      EVAR all47,646 (49)653 (21)3887 (38)11,444 (49)31,622 (53)
      EVAR intact AAA45,608 (54)608 (27)3797 (44)11,054 (53)30,149 (57)
      EVAR ruptured AAA2038 (17)45 (5.1)90 (5.8)390 (14)1513 (23)
      Length of hospital stay (d; median, Q0.25–Q0.75)
      Intact AAA11 (8–16)14 (10–20)13 (9–19)12 (8–17)11 (8–16)
      Ruptured AAA14 (5–25)12 (1–26)14 (3–26)14 (5–25)14 (6–25)
      Hospitals are grouped by annual volume (Q1 = 1–5; Q2 = 6–14; Q3 15–30; Q4 >30 AAA cases/year). AAA = abdominal aortic aneurysm; cIHD = chronic ischemic heart disease; d = days; EVAR = endovascular aneurysm repair; rAAA = ruptured abdominal aortic aneurysm.
      a Median (2005–2013) number of unique centres providing AAA treatment.

       In hospital mortality

      The crude overall in hospital mortality for iAAA was 3.3% (OAR 5.3%; EVAR 1.7%). The in hospital mortality of iAAA repair was inversely associated with hospital volume quartiles. When considering only cases treated by EVAR, this trend was still significant. The same accounts for OAR (Table 2; Fig. 1A).
      Table 2Mortality of patients with intact AAA and ruptured AAA.
      OverallQ1Q2Q3Q4Trendp-value
      Intact AAA
      No. of cases.84,6312258866320,71952,991
      Overall mortality (n, %)2828 (3.3)144 (6.4)404 (4.7)764 (3.7)1516 (2.9)<.005
      EVAR mortality (n, %)771 (1.7)18 (3.0)75 (2.0)191 (1.7)487 (1.6).011
      OAR mortality (n, %)2057 (5.3)126 (7.6)329 (6.8)573 (5.9)1029 (4.5)<.005
      Ruptured AAA
      No. of cases11,795886156528756469
      Overall mortality (n, %)4770 (40)459 (52)760 (48)1223 (43)2328 (36)<.005
      EVAR mortality (n, %)559 (27)18 (40)31 (34)102 (26)408 (27).067
      OAR mortality (n, %)4211 (43)441 (52)729 (49)1121 (45)1920 (39)<.005
      Trend and p value evaluated by two sided Cochran–Armitage test for trend. ↘ = decreasing trend, → = no significant trend. Hospitals are grouped by annual volume (Q1 = 1–5; Q2 = 6–14; Q3 = 15–30; Q4 >30 AAA cases/year). AAA = abdominal aortic aneurysm; EVAR = endovascular aneurysm repair; ICU = intensive care unit; OAR = open aortic repair.
      Figure thumbnail gr1
      Figure 1Mortality of iAAA (A) and rAAA (B) in hospital volume quartiles (whiskers show the 95% CI). Hospitals are grouped by annual volume (Q1 = 1–5; Q2 = 6–14; Q3 15–30; Q4 >30 AAA cases/year). EVAR = endovascular aortic repair; iAAA = intact abdominal aortic aneurysm; OAR = open aortic repair; rAAA = ruptured abdominal aortic aneurysm.
      For rAAA the crude in hospital mortality was 40.4% and it was also inversely associated with hospital volume. The volume relationship was significant for OAR, but not for EVAR (Table 2; Fig. 1B).
      After multivariable adjustment, each lower volume category was associated with a higher risk of in hospital mortality after repair of iAAA and rAAA, when compared with Q4. The adjusted relative risk for in hospital mortality was highest in Q1 for both iAAA and rAAA (Fig. 2).
      Figure thumbnail gr2
      Figure 2Relative risk of in hospital mortality for intact (A) and ruptured AAA (B) by hospital volume quartiles for overall number of cases. RR adjusted for sex, age, comorbidity (Elixhauser Score), type of admission, type of diagnosis, and treatment modality (open aortic repair/endovascular aortic repair). AAA = abdominal aortic aneurysm; RR = relative risk.
      The interaction between the annual volume of iAAA and rAAA repair with the respective in hospital mortality is shown in Fig. S2A, B. These figures demonstrate that high volume of iAAA repair is significantly associated with a lower risk of in hospital mortality after iAAA and rAAA repair. The volume of rAAA repair alone did not show a significant association with mortality for either indication.
      When analysing hospital volume as a continuous variable in an exploratory polynomial model, the optimal fit was achieved when volume was modeled as third degree. The results are shown in Fig. 3A for iAAA and Fig. 3B for rAAA.
      Figure thumbnail gr3
      Figure 3Association between hospital volume as continuous variable and the risk of in hospital mortality for patients with intact AAA (A) and ruptured AAA (B). Estimated risk ratios (compared with hospitals with a caseload of 50 per annum) with point wise confidence intervals adjusted for sex, age, comorbidity (Elixhauser Score), type of admission, type of diagnosis and treatment modality (open aortic repair/endovascular aortic repair). AAA = abdominal aortic aneurysm.

       Secondary outcomes

      For iAAA repair, a higher annual case load was significantly associated with less need of intensive care treatment, peripheral arterial occlusions, resection of bowel, major amputation of lower limbs, and blood transfusion. No significant trend was seen for the rate of myocardial infarctions or stroke as complications of treatment. Renal artery thrombosis was more frequent in high volume hospitals (Table 3).
      Table 3Rates of secondary outcomes for cases with intact AAA.
      OverallQ1Q2Q3Q4Trendp value
      No. of cases84,6312258866320,71952,991
      Secondary outcomes (n, %)
      ICU stay26,738 (32)846 (38)3415 (39)6944 (34)15,533 (29)<.005
      Acute myocardial infarction1417 (1.7)38 (1.7)146 (1.7)343 (1.7)890 (1.7).961
      Acute stroke588 (0.7)20 (0.9)71 (0.8)135 (0.7)362 (0.7).176
      Peripheral arterial thrombosis and embolism3341 (3.9)137 (6.1)391 (4.5)802 (3.9)2011 (3.8)<.005
      Mesenteric thrombosis and embolism955 (1.1)29 (1.3)114 (1.3)231 (1.1)581 (1.1).094
      Renal artery thrombosis and embolism466 (0.6)10 (0.4)32 (0.4)107 (0.5)317 (0.6).006
      Resection of bowel1188 (1.4)47 (2.1)166 (1.9)305 (1.5)670 (1.3)<.005
      Major amputation lower limb220 (0.3)7 (0.3)46 (0.5)55 (0.3)112 (0.2)<.005
      Blood transfusion 1–5 units19,060 (23)722 (32)2271 (26)4607 (22)11,460 (22)<.005
      Blood transfusion >5 units6033 (7)249 (11)754 (8.7)1420 (6.9)3610 (6.8)<.005
      Transfusion of thrombocytes2517 (3.0)94 (4.2)226 (2.6)546 (2.6)1651 (3.1).171
      Trend and p value evaluated by two sided Cochran–Armitage test for trend. Hospitals are grouped by annual volume (Q1 = 1–5; Q2 = 6–14; Q3 = 15–30; Q4 >30 AAA cases/year). ↘ = decreasing trend, ↗ = increasing trend, → = no significant trend. AAA = abdominal aortic aneurysm; ICU = intensive care unit.
      rAAA cases, treated in high volume hospitals, received significantly fewer blood transfusions. On the other hand, patients in these hospitals had to be treated more often in an ICU, very probably because of the higher survival rate, and more frequently developed mesenteric artery occlusions. Rates of other complications did not show any significant trends (Table 4).
      Table 4Rates of secondary outcomes for cases with ruptured AAA.
      OverallQ1Q2Q3Q4Trendp value
      No. of cases11,795886156528756469
      Secondary outcomes (n, %)
      ICU stay5249 (45)331 (37)717 (45)1255 (44)2946 (46)<.005
      Acute myocardial infarction571 (4.8)35 (4.0)69 (4.4)134 (4.7)333 (5.1).057
      Acute stroke213 (1.8)18 (2.0)27 (1.7)51 (1.8)117 (1.8).853
      Peripheral arterial thrombosis and embolism971 (8.2)78 (8.8)130 (8.3)220 (7.7)543 (8.4).977
      Mesenteric thrombosis and embolism938 (8.0)64 (7.2)98 (6.3)198 (6.9)578 (8.9)<.005
      Renal artery thrombosis and embolism109 (0.9)10 (1.1)11 (0.7)22 (0.8)66 (1.0).542
      Resection of bowel887 (7.5)68 (7.7)110 (7.0)193 (6.7)516 (8.0).207
      Major amputation lower limb148 (1.3)14 (1.6)18 (1.2)50 (1.7)66 (1.0).094
      Blood transfusion 1–5 units2663 (23)156 (18)323 (21)645 (22)1539 (24)<.005
      Blood transfusion >5 units7403 (63)627 (71)1061 (68)1788 (62)3927 (61)<.005
      Transfusion of thrombocytes4106 (35)329 (37)515 (33)940 (33)2322 (36).235
      Trend and p-value evaluated by two-sided Cochran–Armitage test for trend. Hospitals are grouped by annual volume (Q1 = 1–5; Q2 = 6–14; Q3 = 15–30; Q4 >30 AAA cases/year). ↘ = decreasing trend, ↗ = increasing trend, → = no significant trend. AAA = abdominal aortic aneurysm; ICU = intensive care unit.

      Discussion

      While causes for mortality after AAA repair are diverse, an inverse relationship between high annual volume of open AAA repair and in hospital mortality has been shown by a large number of studies.
      • Phillips P.
      • Poku E.
      • Essat M.
      • Woods H.B.
      • Goka E.A.
      • Kaltenthaler E.C.
      • et al.
      Procedure volume and the association with short-term mortality following abdominal aortic aneurysm repair in European populations: a systematic review.
      • McPhee J.
      • Eslami M.H.
      • Arous E.J.
      • Messina L.M.
      • Schanzer A.
      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.
      • Marlow N.E.
      • Barraclough B.
      • Collier N.A.
      • Dickinson I.C.
      • Fawcett J.
      • Graham J.C.
      • et al.
      Effect of hospital and surgeon volume on patient outcomes following treatment of abdominal aortic aneurysms: a systematic review.
      • Landon B.E.
      • O'Malley A.J.
      • Giles K.
      • Cotterill P.
      • Schermerhorn M.L.
      Volume-outcome relationships and abdominal aortic aneurysm repair.
      • Karthikesalingam A.
      • Hinchliffe R.J.
      • Loftus I.M.
      • Thompson M.M.
      • Holt P.J.
      Volume-outcome relationships in vascular surgery: the current status.
      • Holt P.J.
      • Poloniecki J.D.
      • Loftus I.M.
      • Michaels J.A.
      • Thompson M.M.
      Epidemiological study of the relationship between volume and outcome after abdominal aortic aneurysm surgery in the UK from 2000 to 2005.
      • Holt P.J.
      • Poloniecki J.D.
      • Gerrard D.
      • Loftus I.M.
      • Thompson M.M.
      Meta-analysis and systematic review of the relationship between volume and outcome in abdominal aortic aneurysm surgery.
      • Holt P.J.
      • Karthikesalingam A.
      • Hofman D.
      • Poloniecki J.D.
      • Hinchliffe R.J.
      • Loftus I.M.
      • et al.
      Provider volume and long-term outcome after elective abdominal aortic aneurysm repair.
      • Hafez H.
      National vascular database analysis: the relationship between AAA repair volume and outcome.
      • Dimick J.B.
      • Upchurch Jr., G.R.
      Endovascular technology, hospital volume, and mortality with abdominal aortic aneurysm surgery.
      In Germany, however, this association has only been shown in a selected cohort of hospitals participating in a quality assurance registry run by the DGG
      • Trenner M.
      • Haller B.
      • Söllner H.
      • Storck M.
      • Umscheid T.
      • Niedermeier H.
      • et al.
      Twelve years of the quality assurance registry on ruptured and non-ruptured abdominal aortic aneurysms of the German Vascular Society (DGG)- #art 3: predictors of perioperative outcome with a focus on annual caseload (English Version).
      • Eckstein H.H.
      • Bruckner T.
      • Heider P.
      • Wolf O.
      • Hanke M.
      • Niedermeier H.P.
      • et al.
      The relationship between volume and outcome following elective open repair of abdominal aortic aneurysms (AAA) in 131 German hospitals.
      and in one nationwide study, but limited to intact AAA treated in 2007.
      • Hentschker C.
      • Mennicken R.
      The volume-outcome relationship and minimum volume standards–empirical evidence for Germany.
      The present study therefore aimed to analyse the association between annual volume of AAA repair and post-operative mortality on a nationwide level.
      A significantly lower mortality for cases treated in hospitals grouped to the highest volume quartile (Q4; annual volume of >30 AAA repairs) was shown. This association remained after multivariable adjustment for age, sex, comorbidities, method of treatment, type of diagnosis, and type of referral. Furthermore, the rate of most complications showed a decreasing trend with increasing hospital volume after iAAA repair. Also with increasing annual caseload the use of blood transfusion was declining after both iAAA and rAAA repair. After iAAA repair, the LOS was shorter in high volume hospitals. When analysing the hospital volume as a continuous variable in a polynomial model, the annual case load of 75 elective AAA repairs seems to be a valuable threshold for both treatment of iAAA and rAAA.
      This is the first study that has shown an inverse relationship between hospital volume and short-term mortality on a nationwide level in Germany for endovascular and open, elective, and emergency AAA procedures. Until now most other studies distinguished between open and endovascular AAA repair and found a significant relation for OAR, but not for EVAR.
      • Dimick J.B.
      • Upchurch Jr., G.R.
      Endovascular technology, hospital volume, and mortality with abdominal aortic aneurysm surgery.
      • Holt P.J.
      • Poloniecki J.D.
      • Khalid U.
      • Hinchliffe R.J.
      • Loftus I.M.
      • Thompson M.M.
      Effect of endovascular aneurysm repair on the volume-outcome relationship in aneurysm repair.
      It is often discussed that volume is a surrogate marker of a latent construct such as “routine” and reasons for a possible routine effect on outcomes are diverse. In Germany, however, it was clearly shown that high volume hospitals are more likely to use endovascular treatment. Still, after multivariable adjustment for the treatment method, the association between volume and in hospital mortality remained. In higher volume hospitals, EVAR seems to be used more deliberately, but with the used database indications for EVAR and OAR cannot be reproduced.
      A meta-analysis on volume and outcome relationships of AAA repair in European hospitals showed a relationship between higher hospital volume and lower short-term mortality after AAA repair, but it was concluded that there is a lack of evidence for other outcomes.
      • Phillips P.
      • Poku E.
      • Essat M.
      • Woods H.B.
      • Goka E.A.
      • Kaltenthaler E.C.
      • et al.
      Procedure volume and the association with short-term mortality following abdominal aortic aneurysm repair in European populations: a systematic review.
      The present study analysed several secondary outcomes and it was shown that in high volume hospitals procedure related complications (peripheral arterial embolism, resection of bowel, major amputations) occurred significantly less often after iAAA repair. Also the use of blood products was less in higher volume hospitals. For severe complications such as myocardial infarction or acute stroke, no difference between the volume groups was shown. For pancreatic surgery, it has been shown that the volume outcome association is linked with a “failure to rescue” patients experiencing severe complications in low volume hospitals.
      • Krautz C.
      • Nimptsch U.
      • Weber G.F.
      • Mansky T.
      • Grützmann R.
      Effect of hospital volume on in-hospital morbidity and mortality following pancreatic surgery in Germany.
      For AAA surgery, possible complications are diverse, but overall coded complication rates seemed to be low. It did not seem to be feasible to conduct a “failure to rescue” analysis for AAA data.
      A possible effect of centralisation was not the subject of the present study, but the results may imply that AAA therapy should be concentrated in high volume hospitals. Centralisation should only be considered, as long as emergency care of rAAA can still be provided sufficiently. Therefore, further studies need to be conducted, for example on spatial distribution of AAA in Germany with the aim of providing a scientific background for possible strategies of centralisation.
      In Germany, the adherence to the guidelines of the ESVS proved to be very low in terms
      • Moll F.L.
      • Powell J.T.
      • Fraedrich G.
      • Verzini F.
      • Haulon S.
      • Waltham M.
      • et al.
      Management of abdominal aortic aneurysms clinical practice guidelines of the European society for vascular surgery.
      of the recommended minimum annual case threshold of 50 AAA operations per annum. When volume was analysed as a continuous variable in a polynomial model, a threshold of 75 cases was shown to be potentially reasonable for German hospitals, even if the mortality risk appeared to increase again in very high volume centres (Fig. 3A and B). The same was found for carotid endarterectomy in German hospitals.
      • Kuehnl A.
      • Tsantilas P.
      • Knappich C.
      • Schmid S.
      • Konig T.
      • Breitkreuz T.
      • et al.
      Significant association of annual hospital volume with the risk of inhospital stroke or death following carotid endarterectomy but likely not after carotid stenting: secondary data analysis of the statutory German carotid quality assurance database.
      Possible reasons would be a ceiling effect limiting the effect of routine to a certain annual volume, but also higher risk patients (with risk factors not represented in the DRG database, like challenging morphology) might be treated predominantly at the very high volume hospitals. These hospitals receive more patient transfers from other hospitals, which is reflected in the presented data. It is very likely that this group of transferred patients are at higher risk overall for surgical or medical reasons. Furthermore, high volume hospitals are typically centres of training, and procedures will be carried out by a larger number of surgeons. While it has been shown that patients operated on by surgeons in training do not have worse outcomes after AAA repair,
      • DiDato S.
      • Farber A.
      • Rybin D.
      • Kalish J.A.
      • Eslami M.H.
      • Moreira C.C.
      • et al.
      The effect of trainee involvement on perioperative outcomes of abdominal aortic aneurysm repair.
      individual surgeon volume seems to be an important factor, especially for OAR.
      • Hafez H.
      National vascular database analysis: the relationship between AAA repair volume and outcome.
      • McPhee J.T.
      • Robinson 3rd, W.P.
      • Eslami M.H.
      • Arous E.J.
      • Messina L.M.
      • Schanzer A.
      Surgeon case volume, not institution case volume, is the primary determinant of in-hospital mortality after elective open abdominal aortic aneurysm repair.
      • Zettervall S.L.
      • Schermerhorn M.L.
      • Soden P.A.
      • McCallum J.C.
      • Shean K.E.
      • Deery S.E.
      • et al.
      The effect of surgeon and hospital volume on mortality after open and endovascular repair of abdominal aortic aneurysms.
      Using the present data, it is not possible to analyse a possible association between surgeon volume (rather than institutional volume) and mortality or complications. After all, the complex nature of AAA operations, whether in the elective or emergency setting, requires a team approach involving all facets of patient care. Favourable results by one individual surgeon will only be achieved with an adequately trained team of vascular specialists, anesthetists, and nurses.
      SVS guidelines recommend that OAR only be performed in hospitals with in hospital mortality <5%.
      • Chaikof E.L.
      • Brewster D.C.
      • Dalman R.L.
      • Makaroun M.S.
      • Illig K.A.
      • Sicard G.A.
      • et al.
      SVS practice guidelines for the care of patients with an abdominal aortic aneurysm: executive summary.
      In Germany, only hospitals with an annual AAA volume of >30 reached this threshold (cumulated mortality 4.3%), while all other volume groups had a higher mortality. In addition, the overall mortality was 5.3%, which is above the recommended goal. Apart from centralisation, a statutory quality assurance registry should be discussed for AAA operations to identify hospitals that perform within the guidelines, but do not reach the actual threshold. These hospitals could potentially benefit from centralisation, as they could receive more referrals.
      The main limitations of remote controlled data processing and use of routinely collected DRG data have been discussed in detail.
      • Kuhnl A.
      • Erk A.
      • Trenner M.
      • Salvermoser M.
      • Schmid V.
      • Eckstein H.H.
      Incidence, treatment and mortality in patients with abdominal aortic aneurysms.
      The most important limitation is the observational nature of the study. Therefore, only associations rather than causal relationships can be inferred. Other issues are potential miscoding of administrative data, which might lead to biased case selection and case grouping. An under reporting of secondary diagnoses and OPS codes may be assumed, as not all secondary diagnoses are relevant for reimbursement. Upcoding, on the other hand, is unlikely, as coding is regularly monitored by the Health Insurance Medical Service (Medizinischer Dienst der Krankenversicherungen; MDK). Lacking from the data are aneurysm specific details, such as diameter or angulation of access vessels, as well as any information on the haemodynamic presentation of a patient in the case of rAAA. Therefore, multivariable adjustment was only possible for age, sex, comorbidities (Elixhauser Score), type of procedure, and type of referral. Adjusting administrative data for comorbidities using Elixhauser Score is a well evaluated tool.
      • Sharabiani M.T.
      • Aylin P.
      • Bottle A.
      Systematic review of comorbidity indices for administrative data.
      Records in this database are case-bound and end at discharge from the hospital. A long-term survival benefit of high annual hospital volume has been shown for AAA repair for the time period of 3 years after the initial operation.
      • Holt P.J.
      • Karthikesalingam A.
      • Hofman D.
      • Poloniecki J.D.
      • Hinchliffe R.J.
      • Loftus I.M.
      • et al.
      Provider volume and long-term outcome after elective abdominal aortic aneurysm repair.
      With the present data, the association between volume and long-term outcomes cannot be further evaluated.

      Conclusion

      In Germany, hospital volume is inversely related to in hospital mortality after repair of iAAA and rAAA. This association is independent of the type of repair. Furthermore, complication rates are lower in high volume hospitals after iAAA repair. The same accounts for the use of resources (blood products, length of stay). These results support the hypothesis that AAA procedures should be performed only in high volume hospitals. rAAA cases should be considered for transfer to a centre of expertise if the patient is sufficiently stable. Future guidelines should take these results into account. A threshold of 75 elective cases per year might be reasonable for Germany.

      Acknowledgements

      We would like to thank Ms. Melanie Scheller from the Research Data Centre of the German Federal Statistical Office for her competent and helpful assistance during planning and performing the data analysis. Additionally, we would like to thank Ms. Jutta Spindler and Ms. Sabine Nemitz (German Federal Statistical Office) for supporting our research.

      Appendix A. Supplementary data

      Figure thumbnail figs1
      Figure thumbnail figs2
      Figure thumbnail figs3

      Conflicts of interest

      None.

      Funding

      None.

      References

        • Moll F.L.
        • Powell J.T.
        • Fraedrich G.
        • Verzini F.
        • Haulon S.
        • Waltham M.
        • et al.
        Management of abdominal aortic aneurysms clinical practice guidelines of the European society for vascular surgery.
        Eur J Vasc Endovasc Surg. 2011; 41: S1-S58
        • Chaikof E.L.
        • Brewster D.C.
        • Dalman R.L.
        • Makaroun M.S.
        • Illig K.A.
        • Sicard G.A.
        • et al.
        SVS practice guidelines for the care of patients with an abdominal aortic aneurysm: executive summary.
        J Vasc Surg. 2009; 50: 880-896
        • Trenner M.
        • Haller B.
        • Söllner H.
        • Storck M.
        • Umscheid T.
        • Niedermeier H.
        • et al.
        Twelve years of the quality assurance registry on ruptured and non-ruptured abdominal aortic aneurysms of the German Vascular Society (DGG)- #art 3: predictors of perioperative outcome with a focus on annual caseload (English Version).
        Gefässchirurgie. 2015; 20: 32-44
        • Eckstein H.H.
        • Bruckner T.
        • Heider P.
        • Wolf O.
        • Hanke M.
        • Niedermeier H.P.
        • et al.
        The relationship between volume and outcome following elective open repair of abdominal aortic aneurysms (AAA) in 131 German hospitals.
        Eur J Vasc Endovasc Surg. 2007; 34: 260-266
        • Venermo M.
        • Mani K.
        • Kolh P.
        The quality of a registry based study depends on the quality of the data – without validation, it is questionable.
        Eur J Vasc Endovasc Surg. 2017; 53: 611-612
        • Trenner M.
        • Haller B.
        • Storck M.
        • Reutersberg B.
        • Kallmayer M.A.
        • Eckstein H.H.
        Trends in patient safety of intact abdominal aortic aneurysm repair: German registry data on 36,594 procedures.
        Eur J Vasc Endovasc Surg. 2017; 53: 641-647
        • Hentschker C.
        • Mennicken R.
        The volume-outcome relationship and minimum volume standards–empirical evidence for Germany.
        Health Econ. 2015; 24: 644-658
        • Krautz C.
        • Nimptsch U.
        • Weber G.F.
        • Mansky T.
        • Grützmann R.
        Effect of hospital volume on in-hospital morbidity and mortality following pancreatic surgery in Germany.
        Ann Surg. 2017 Apr 4; ([Epub ahead of print])https://doi.org/10.1097/SLA.0000000000002248
        • Phillips P.
        • Poku E.
        • Essat M.
        • Woods H.B.
        • Goka E.A.
        • Kaltenthaler E.C.
        • et al.
        Procedure volume and the association with short-term mortality following abdominal aortic aneurysm repair in European populations: a systematic review.
        Eur J Vasc Endovasc Surg. 2017; 53: 77-88
        • Nimptsch U.
        • Mansky T.
        Stroke unit care and trends of in-hospital mortality for stroke in Germany 2005-2010.
        Int J Stroke. 2014; 9: 260-265
        • Nimptsch U.
        • Mansk T.
        Deaths following cholecystectomy and herniotomy: an analysis of nationwide German hospital discharge data from 2009 to 2013.
        Dtsch Arztebl Int. 2015; 112: 535-543
        • Nimptsch U.
        • Krautz C.
        • Weber G.F.
        • Mansky T.
        • Grutzmann R.
        Nationwide in-hospital mortality following pancreatic surgery in Germany is higher than anticipated.
        Ann Surg. 2016; 264: 1082-1090
        • Kühnl A.
        • Erk A.
        • Trenner M.
        • Salvermoser M.
        • Schmid V.
        • Eckstein H.
        Incidence, treatment and mortality in abdominal aortic aneurysms- a secondary analysis of diagnosis-related groups (DRG) data from 2005–2014.
        Dtsch Arztebl Int. 2017; 114: 391-398
        • Swart E.
        • Gothe H.
        • Geyer S.
        • Jaunzeme J.
        • Maier B.
        • Grobe T.G.
        • et al.
        Good practice of secondary data analysis (GPS): guidelines and recommendations.
        Gesundheitswesen. 2015; 77: 120-126
        • Swart E.
        • Bitzer E.M.
        • Gothe H.
        • Harling M.
        • Hoffmann F.
        • Horenkamp-Sonntag D.
        • et al.
        STandardisierte BerichtsROutine für Sekundärdaten Analysen (STROSA) – ein konsentierter Berichtsstandard für Deutschland, version 2.
        Das Gesundheitswesen. 2016; 78: 145-160
        • Benchimol E.I.
        • Smeeth L.
        • Guttmann A.
        • Harron K.
        • Moher D.
        • Petersen I.
        • et al.
        The REporting of studies Conducted using Observational Routinely-collected health Data (RECORD) statement.
        PLoS Med. 2015; 12e1001885
        • van Walraven C.
        • Austin P.C.
        • Jennings A.
        • Quan H.
        • Forster A.J.
        A modification of the Elixhauser comorbidity measures into a point system for hospital death using administrative data.
        Med Care. 2009; 47: 626-633
        • Sharabiani M.T.
        • Aylin P.
        • Bottle A.
        Systematic review of comorbidity indices for administrative data.
        Med Care. 2012; 50: 1109-1118
        • Elixhauser A.
        • Steiner C.
        • Harris D.R.
        • Coffey R.M.
        Comorbidity measures for use with administrative data.
        Med Care. 1998; 36: 8-27
        • McPhee J.
        • Eslami M.H.
        • Arous E.J.
        • Messina L.M.
        • Schanzer A.
        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.
        J Vasc Surg. 2009; 49: 817-826
        • Marlow N.E.
        • Barraclough B.
        • Collier N.A.
        • Dickinson I.C.
        • Fawcett J.
        • Graham J.C.
        • et al.
        Effect of hospital and surgeon volume on patient outcomes following treatment of abdominal aortic aneurysms: a systematic review.
        Eur J Vasc Endovasc Surg. 2010; 40: 572-579
        • Landon B.E.
        • O'Malley A.J.
        • Giles K.
        • Cotterill P.
        • Schermerhorn M.L.
        Volume-outcome relationships and abdominal aortic aneurysm repair.
        Circulation. 2010; 122: 1290-1297
        • Karthikesalingam A.
        • Hinchliffe R.J.
        • Loftus I.M.
        • Thompson M.M.
        • Holt P.J.
        Volume-outcome relationships in vascular surgery: the current status.
        J Endovasc Ther. 2010; 17: 356-365
        • Holt P.J.
        • Poloniecki J.D.
        • Loftus I.M.
        • Michaels J.A.
        • Thompson M.M.
        Epidemiological study of the relationship between volume and outcome after abdominal aortic aneurysm surgery in the UK from 2000 to 2005.
        Br J Surg. 2007; 94: 441-448
        • Holt P.J.
        • Poloniecki J.D.
        • Gerrard D.
        • Loftus I.M.
        • Thompson M.M.
        Meta-analysis and systematic review of the relationship between volume and outcome in abdominal aortic aneurysm surgery.
        Br J Surg. 2007; 94: 395-403
        • Holt P.J.
        • Karthikesalingam A.
        • Hofman D.
        • Poloniecki J.D.
        • Hinchliffe R.J.
        • Loftus I.M.
        • et al.
        Provider volume and long-term outcome after elective abdominal aortic aneurysm repair.
        Br J Surg. 2012; 99: 666-672
        • Hafez H.
        National vascular database analysis: the relationship between AAA repair volume and outcome.
        Br J Surg. 2012; 99: 4
        • Dimick J.B.
        • Upchurch Jr., G.R.
        Endovascular technology, hospital volume, and mortality with abdominal aortic aneurysm surgery.
        J Vasc Surg. 2008; 47: 1150-1154
        • Holt P.J.
        • Poloniecki J.D.
        • Khalid U.
        • Hinchliffe R.J.
        • Loftus I.M.
        • Thompson M.M.
        Effect of endovascular aneurysm repair on the volume-outcome relationship in aneurysm repair.
        Circ Cardiovasc Qual Outcomes. 2009; 2: 624-632
        • Kuehnl A.
        • Tsantilas P.
        • Knappich C.
        • Schmid S.
        • Konig T.
        • Breitkreuz T.
        • et al.
        Significant association of annual hospital volume with the risk of inhospital stroke or death following carotid endarterectomy but likely not after carotid stenting: secondary data analysis of the statutory German carotid quality assurance database.
        Circ Cardiovasc Interv. 2016; 9
        • DiDato S.
        • Farber A.
        • Rybin D.
        • Kalish J.A.
        • Eslami M.H.
        • Moreira C.C.
        • et al.
        The effect of trainee involvement on perioperative outcomes of abdominal aortic aneurysm repair.
        J Vasc Surg. 2016; 63: 16-22
        • McPhee J.T.
        • Robinson 3rd, W.P.
        • Eslami M.H.
        • Arous E.J.
        • Messina L.M.
        • Schanzer A.
        Surgeon case volume, not institution case volume, is the primary determinant of in-hospital mortality after elective open abdominal aortic aneurysm repair.
        J Vasc Surg. 2011; 53 (e2): 591-599
        • Zettervall S.L.
        • Schermerhorn M.L.
        • Soden P.A.
        • McCallum J.C.
        • Shean K.E.
        • Deery S.E.
        • et al.
        The effect of surgeon and hospital volume on mortality after open and endovascular repair of abdominal aortic aneurysms.
        J Vasc Surg. 2017; 65: 626-634
        • Kuhnl A.
        • Erk A.
        • Trenner M.
        • Salvermoser M.
        • Schmid V.
        • Eckstein H.H.
        Incidence, treatment and mortality in patients with abdominal aortic aneurysms.
        Dtsch Arztebl Int. 2017; 114: 391-398