Abstract
Background:
As surgical mortality decreases and endovascular utilization increases, it is unknown whether volume-outcome relationships exist in thoracic aortic dissection repair. We characterized volume-outcome relationships for surgical and endovascular management of thoracic aortic dissection.
Methods:
Patients >18 years undergoing repair of thoracic aortic dissection in the United States between 2010 and 2014 were identified in seven all-payer state inpatient administrative databases. Patients were divided into groups based on type of repair: surgical repair of type A dissection (TAAD), surgical repair of type B dissection (TBAD), and endovascular repair (TEVAR). Hierarchical logistic regression models evaluated the association between hospital volume and in-hospital mortality.
Results:
Overall in-hospital mortality rate was 13.4% (890/6650), highest after TAAD (463/2918, 15.9%), followed by TBAD (270/1934, 14.0%) and TEVAR (157/1798, 8.7%). Volume-outcome relationships for adjusted in-hospital mortality were demonstrated for TAAD and TBAD (p-trend<0.001), but not TEVAR (p-trend=0.11). Adjusted in-hospital mortality differed most for TAAD (<3 cases/year, 21%[CI 18–24] vs. ≥11 cases/year, 12%[CI 8–16]; p<0.001) and TBAD (<2 cases/year, 18%[CI 15–22] vs. ≥11 cases/year, 9%[CI 5–12]; p<0.001), while TEVAR did not differ between quartiles. Adjusted mortality was lower at centers with ≥26 overall annual thoracic dissection repairs, compared to any of the three lower-volume quartiles (p<0.001).
Conclusions:
This study demonstrated lower mortality at high-volume hospitals for overall repair of aortic dissection, persisting separately for surgical repair of TAAD and TBAD, but not TEVAR. As endovascular technology advances and practice patterns consequently change, analyses should focus on understanding the balance between procedural volume, mortality, and access to care for thoracic aortic dissection.
Thoracic aortic dissection may present in an acute or chronic setting and confers significant mortality and morbidity. Acute type A aortic dissection (TAAD) carries a 1% to 2% risk of mortality per hour from time of dissection and over 50–60% mortality in the first 24–48 hours if not repaired [1,2]. Even after emergent surgical repair, the International Registry of Acute Aortic Dissection (IRAD) cites nearly 20% surgical mortality and over 30% in-hospital mortality for TAAD [2]. While type B aortic dissection (TBAD) carries a lower incidence of rupture and sudden death, it also poses significant risk of morbidity including malperfusion, renal failure, and stroke. Standard of care has been emergent surgical repair of acute TAAD and either surgical, endovascular, or medical management of acute and chronic TBAD.
Procedural volume is often associated with improved outcomes in the management of complex cardiac surgery [3–5], but its association with outcomes in surgica l and endovascular treatment of thoracic aortic dissection is unclear. Multiple prior analyses have suggested a volume-outcome relationship for surgical repair of acute type A dissection (TAAD) [6–9], but with changing practice patterns, differences in surgeon- and hospital-level analyses, and the evolving concept of a heart team with multidisciplinary care [10], this relationship requires additional investigation. Even less is known about volume-outcome relationships in surgical repair of TBAD, and existing analyses of thoracic endovascular aortic repair (TEVAR) have mostly examined treatment of aneurysm and have not found a volume-outcome relationship [11,12]. Contemporary series [2] demonstrate operative mortality after repair of acute TAAD has decreased nationwide to as low as 2.8% at some centers [10], while TEVAR is now used regularly for repair of TBAD [13] and even in some TAAD patients with poor surgical candidacy and favorable anatomy [14–16]. In the context of decreasing mortality rates and increased endovascular utilization, it is unknown whether volume-outcome relationships exist for surgical and endovascular repair of thoracic aortic dissection (TAAD and TBAD). The implications of these relationships will be important for both low- and high-volume aortic centers.
We therefore characterized hospital volume-outcome relationships for surgical and endovascular management of thoracic aortic dissection by analyzing inpatient databases from seven geographically diverse states. We hypothesized that a volume-outcome relationship exists for surgical and endovascular management of thoracic aortic dissection and that higher volume centers will have lower in-hospital mortality, especially for TAAD.
PATIENTS AND METHODS
This study was deemed exempt from human subjects protections by the Institutional Review Board of the University of Michigan (IRB number HUM00147507, notice of determination of “Not Regulated” status).
Patient Population and Data Sources
Patients greater than 18 years old undergoing surgical or endovascular repair of thoracic aortic dissection in the United States between January 1, 2010 and December 31, 2014 were identified using discharge data from Arizona, Iowa, Florida, Massachusetts, New Jersey, New York, and Washington State Inpatient Databases (SID), Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality, with an over 95% sample of hospital discharges [17].
Diagnosis and procedure codes were according to the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM). Hospitalizations for aortic dissection were identified through a principal discharge diagnosis of ICD-9-CM code 441.01 (dissection of aorta, thoracic) or 441.03 (dissection of aorta, thoracoabdominal). To differentiate between surgical repairs for dissection involving the ascending aorta (TAAD), descending aorta (TBAD), and endovascular repair (TEVAR), we adopted the published criteria developed by Sachs et al. [18] and utilized in prior analyses [19]. Patients were stratified into TAAD, TBAD, or TEVAR as summarized in Figure 1.
Figure 1.
Definitions of aortic dissection repair procedures by International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) diagnosis and procedure codes.
Patient Characteristics
Patient characteristics were reported based on HCUP coding during hospitalization and included age, sex, and race, as well as comorbidities including peripheral vascular disease, congestive heart failure, chronic lung disease, diabetes, alcohol and drug abuse, depression, hypertension, renal failure, liver disease, and obesity.
Hospital Volume
We grouped patients into equal-sized quartiles according to hospital volume, such that a similar number of patients were included in each quartile. Annual hospital volume was determined by calculating the total number of procedures for each hospital over the study period and dividing by the number of years reported. Quartiles were created separately for each procedure and for overall aortic dissection repair procedures. The association between high-volume surgical and high-volume endovascular hospitals was assessed by combining surgical repairs of TAAD and TBAD and generating a scatter plot of individual hospitals with surgical volume on the x-axis and endovascular volume on the y-axis. An R2 statistic was generated and a trend line fit to characterize the relationship between surgical and endovascular volume. The data were log-transformed due to significant right-skew (Figure 4).
Figure 4.
Log-transformed scatter plot of total surgical volume (repair of type A aortic dissection [TAAD] and type B aortic dissection [TBAD]) and thoracic endovascular aortic repair (TEVAR) volume. Each dot represents one hospital.
Statistical Analysis
The primary outcome measure was in-hospital mortality. Categorical variables were displayed as percentages and continuous variables as mean ± standard deviation for descriptive data and univariate analysis. Parametric two-sample t-tests and chi-square tests were used to test for statistical significance. P-values less than 0.05 (2-tailed) were considered statistically significant.
Clinically-relevant variables and those with p<0.20 by univariate survival analysis were included in a hierarchical logistic regression model for adjustment. The model included age, race, sex, congestive heart failure, chronic lung disease, diabetes, alcohol abuse, drug abuse, depression, hypertension, renal failure, liver failure, obesity, peripheral vascular disease, and annual mean hospital volume quartile. The final model also included hospitals as the random effect to account for clustering of patients within each hospital. In an attempt to minimize the effect of the number of patients with chronic dissection developing aneurysm and undergoing elective repair on the results, a sensitivity analysis was performed excluding all patients with a concurrent diagnosis code for thoracic aortic aneurysm (441.1– 441.9).
Performing separate analyses for each of TAAD, TBAD, and TEVAR, hierarchical logistic regression models were used to evaluate the association between hospital procedural volume and in-hospital mortality with adjusted odds ratios and marginal predicted means. Relationships between hospital procedural volume and adjusted in-hospital mortality were then assessed among all procedures through evaluation of whether overall hospital volume of aortic dissection procedures influenced in-hospital mortality both overall and for each of the three groups.
All statistical analyses were performed with Stata 15 (StataCorp LLC, College Station, TX).
RESULTS
Among 6650 patients across 232 hospitals in 7 states, overall age was 62 ± 14 years and 34% (2228/6650) were female. A total of 2918 underwent surgical repair of TAAD, 1934 surgical repair of TBAD, and 1798 TEVAR. Those undergoing TEVAR had highest rates of peripheral vascular disease (754/1798, 42%) and chronic lung disease (395/1798, 22%), while those undergoing repair of TAAD had the most congestive heart failure (583/2918, 20%) and highest mean Elixhauser comorbidity score (3.96±1.89) [Table 1].
Table 1.
Patient characteristics by aortic repair type. TAAD: type A aortic dissection, TBAD: type B aortic dissection, TEVAR: thoracic endovascular aortic repair.
| Variable | Overall (n=6,650) | Surgical TAAD repair (n=2,918) | Surgical TBAD repair (n=1,934) | TEVAR (n=1,798) | P-value |
|---|---|---|---|---|---|
| Age, years ± standard deviation | 62±14 | 61±14 | 61 ±14 | 64±15 | <0.001 |
| Female, n (%) | 2228 (34) | 904 (31) | 652 (34) | 672 (37) | <0.001 |
| Race, n (%) | |||||
| White | 4228 (66) | 1947 (69) | 1156 (62) | 1125 (64) | |
| Black | 1027 (16) | 356 (13) | 356 (19) | 315 (18) | |
| Hispanic | 584 (9) | 248 (9) | 175 (9) | 161 (9) | <0.001 |
| Asian/Pacific Islander | 162 (3) | 64 (2) | 45 (2) | 53 (3) | |
| Native American | 29 (0) | 7 (0) | 11 (1) | 11 (1) | |
| Other | 418 (6) | 215 (8) | 119 (6) | 84 (5) | |
| Peripheral vascular disease, n (%) | 2369 (36) | 1099 (38) | 516 (27) | 754 (42) | <0.001 |
| Congestive heart failure, n (%) | 1025 (15) | 583 (20) | 256 (13) | 186 (10) | <0.001 |
| Chronic lung disease, n (%) | 1226 (18) | 484 (17) | 347 (18) | 395 (22) | <0.001 |
| Diabetes without chronic complications, n (%) | 810 (12) | 366 (13) | 221 (11) | 223 (12) | 0.48 |
| Diabetes with chronic complications, n (%) | 64 (1) | 22 (1) | 19 (1) | 23 (1) | 0.20 |
| Alcohol abuse, n (%) | 313 (5) | 135 (5) | 98 (5) | 80 (4) | 0.65 |
| Drug abuse, n (%) | 285 (4) | 144 (5) | 84 (4) | 57 (3) | 0.01 |
| Hypertension, n (%) | 5210 (78) | 2172 (74) | 1553 (80) | 1485 (83) | <0.001 |
| Renal failure, n (%) | 966 (15) | 399 (14) | 287 (15) | 280 (16) | 0.18 |
| Liver disease, n (%) | 106 (2) | 39 (1) | 30 (2) | 37 (2) | 0.16 |
| Obesity, n (%) | 954 (14) | 426 (15) | 262 (14) | 266 (15) | 0.48 |
| Elixhauser score, mean ± standard deviation | 3.72 ± 1.87 | 3.96 ± 1.89 | 3.57 ± 1.79 | 3.49 ± 1.86 | <0. 001 |
| In-hospital death, n (%) | 890 (13) | 463 (16) | 270 (14) | 157 (9) | <0.001 |
| 2010 | 194/1185 (16) | 101/561 (18) | 61/352 (17) | 32/272 (12) | |
| 2011 | 162/1236 (13) | 77/534 (14) | 54/354 (15) | 31/348 (9) | |
| 2012 | 148/1280 (12) | 79/562 (14) | 45/392 (12) | 24/326 (7) | |
| 2013 | 186/1369 (14) | 98/592 (17) | 50/381 (13) | 38/396 (10) | |
| 2014 | 200/1578 (13) | 108/667 (16) | 60/455 (13) | 32/456 (7) | |
Overall in-hospital mortality rate was 13.4% (890/6650). Unadjusted rate was highest after surgical repair of TAAD at 15.9% (463/2918), followed by surgical repair of TBAD at 14.0% (270/1934), and TEVAR at 8.7% (157/1798). Univariate analysis found that in-hospital mortalities were older than survivors (65 vs. 61 years, p<0.001), had a higher rate of peripheral vascular disease (387/890, 43% vs. 1981/5758, 34%; p<0.001), and greater incidence of congestive heart failure (158/890, 18% vs. 866/5758, 15%, p<0.001) [Supplemental Table 1].
Stratifying by procedure type, mean annual hospital case volumes by repair approach were: 8.9 (95% confidence interval, CI=8.5–9.2) for TAAD, 7.3 (6.9–7.6) for TBAD, and 7.6 (7.1–8.0) for TEVAR. Multivariable regression analysis was performed separately for each procedure with separate volume quartiles for each procedure generated (Supplemental Table 2). The largest difference in adjusted mean mortality rate by hospital was demonstrated in surgical repair of TAAD (<3 cases/year, 21% [CI 18–24%] vs. ≥11 cases/year, 12% [CI 8–16%]; p<0.001) and TBAD (<2 cases/year, 18% [CI 15–22%] vs. ≥11 cases/year, 9% [CI 5–12%]; p<0.001). Adjusted in-hospital mortality across quartiles indicated a volume-outcome relationship for surgical repair of TAAD (p-trend<0.001) and TBAD (p-trend<0.001), but not TEVAR (p-trend=0.11) [Figure 2]. Adjusted in-hospital mortality was lower in higher-volume hospitals for surgical repair of both TAAD and TBAD, with increased odds of mortality for patients at 3rd and 4th quartile (Q) hospitals compared to the 1st quartile, for both TAAD and TBAD, while odds of in-hospital mortality did not differ by quartile for TEVAR (Figure 3).
Figure 2.
Adjusted in-hospital mortality rates by hospital procedural volume quartile (Q) for aortic dissection repair. Hospital volume quartiles were derived separately for each procedure and adjusted in-hospitality mortality is displayed in marginal predicted means generated by hierarchical logistic regression (surgical type A and type B) and logistic regression (TEVAR) analysis, as appropriate. TEVAR=thoracic endovascular aortic repair.
Figure 3.
Adjusted in-hospital mortality by quartile of hospital volume, stratified by procedure with forest plot of adjusted odds ratios (aOR) for quartiles of surgical repair of type A aortic dissection (TAAD), type B aortic dissection (TBAD), and endovascular repair (TEVAR), with the 1st quartile (Q1) as reference. CI=confidence interval; Q2=2nd quartile; Q3=3rd quartile; Q4=4th quartile.
High surgical volume was associated with high endovascular volume across hospitals (R2=0.501, Figure 4). Quartiles of overall (surgical and endovascular) annual aortic dissection repair procedures were: Q1: <6 (n=1705), Q2: 6–12 (n=1649), Q3: 13–25 (n=1748), and Q4: ≥26 (n=1548). Predicted mean in-hospital mortality was lower at centers with ≥26 overall thoracic dissection procedures per year (Q4: 9% [CI 7–12%] adjusted in-hospital mortality), when compared to any of the three lower-volume quartiles (Q3: 13% [CI 11–15%], Q2: 15% [13–17%], Q1: 16% [CI 14–18%]; p<0.001). Utilizing overall hospital aortic dissection procedural volume quartiles, patients undergoing repair at hospitals in the 4th quartile of all dissection repair procedures had lower adjusted in-hospital mortality overall (OR 0.55 [CI 0.40–0.76], p<0.001) and after TAAD (OR 0.55 [CI 0.33–0.91], p=0.020). Patients at either 3rd (OR 0.65 [CI 0.45–0.94], p=0.024) or 4th (OR 0.34 [CI 0.22–0.52], p<0.001) quartile hospitals (by overall procedures) had lower odds of in-hospital mortality if undergoing TBAD, while overall thoracic aortic dissection procedural volume still did not influence in-hospital mortality after TEVAR [Figure 5].
Figure 5.
Adjusted in-hospital mortality by procedure among overall hospital aortic dissection procedural volume and forest plot of adjusted odds ratios (aOR) for quartiles of overall, surgical repair of type A aortic dissection (TAAD), surgical repair of type B aortic dissection (TBAD), and endovascular repair (TEVAR), with the 1st quartile (Q1) as reference. CI=confidence interval; Q2=2nd quartile; Q3=3rd quartile; Q4=4th quartile.
In sensitivity analysis, 9% (522/6650) of patients (TAAD, n=177/2918 [6%]; TBAD, n=142/1934 [7%]; TEVAR, n=203/1798 [11%]) had at least one diagnosis code for thoracic aneurysm, in addition to dissection. Exclusion of these patients did not qualitatively change our findings.
COMMENT
This study demonstrated a relationship between hospital volume and adjusted in-hospital mortality for patients undergoing thoracic aortic dissection repair procedures. Specifically, we found that higher volume was significantly associated with lower adjusted mortality rates in surgical repair of TAAD and TBAD, but not TEVAR. We also found that surgical and endovascular volume were strongly correlated, and that patients undergoing any type of thoracic aortic dissection repair (surgical or endovascular) at a high-volume hospital had significantly lower odds of adjusted in-hospital mortality, compared to any of the three lower-volume quartiles.
To our knowledge, this is the first study to describe the volume-outcome relationship across all thoracic aortic dissection repair procedures. Previous studies demonstrating volume-outcome relationships have focused on surgical repair of TAAD and found both hospital [6,7] and provider [7,9] level mortality benefits for patients undergoing repair at high-volume hospitals. Consistent with these, we found a strong inverse relationship between hospital volume and in-hospital mortality for repair of TAAD, as well as a continued decrease in mortality rate, to 15.9% during our study period. In contrast to TAAD, previous analyses of TEVAR [11,12] have not found a volume-outcome relationship, and neither did this study. Unlike prior studies, however, we also found that hospital surgical volume (surgical repair of TAAD + TBAD) was associated with endovascular volume (TEVAR) and that higher overall hospital aortic repair volume (surgical and endovascular) was associated with lower in-hospital mortality. Importantly, prior analyses [6,7] using the NIS did not distinguish between TAAD and TBAD, whereas we utilized a published algorithm [18,19] to more accurately categorize patients (Figure 1). Also unlike prior TEVAR analyses which included either a complete [12] or majority [11] thoracic aneurysm population, this study exclusively included those with diagnosis of aortic dissection.
Our study may also provide more accurate measures of hospital volume for thoracic aortic dissection repair procedures. Because we examined overall procedural volume and complete case ascertainment from a hospital, as opposed to a 20% sample in prior studies using the NIS, our study may better capture hospital volume. These differences in data sources may explain why Knipp et al.’s highest-volume tercile included hospitals averaging >2.5 TAAD repairs per year [6], compared to our highest-volume quartile for surgical repair of TAAD being ≥11 per year.
Our findings also provide more contemporary data on the volume-outcome relationship in the context of rapidly changing practice patterns related to thoracic aortic dissection repair procedures. While endovascular utilization has dramatically increased for TBAD [2], several small case series [14,16,20–23] and review articles [15,24,25] have also reported endovascular stent grafting of the ascending aorta for acute, subacute, and chronic TAAD. Due to suboptimal device design, a concerning complication profile (e.g. retrograde dissection, stroke, stent migration, endoleak, death), and uncertain long-term outcomes, TEVAR for repair of acute TAAD is currently limited to patients with poor surgical candidacy and favorable anatomy, at select centers. Branched stent grafting was recently introduced to avoid left subclavian coverage for descending thoracic aneurysm (DTA) and/or TBAD repair and has demonstrated an initial learning curve with reasonable outcomes [26–29], but an aggregate stroke rate of approximately 14% across multiple series [27] and as high as 40% [30]. While both the current and prior studies [11,12] have not shown a volume-outcome relationship for TEVAR, these data primarily include standard straight stent grafting. Branched stent grafts are used in lower frequency and at fewer centers, potentially conferring a higher level of difficulty and a steeper learning curve [26]. As utilization of branched graft TEVAR increases for treating TBAD and DTA, with potential future use for TAAD [14], volume-outcome relationships must be re-investigated to characterize outcomes and generalizability of this procedure for thoracic aortic dissection. As newer data become available, further volume-outcome analyses are especially essential for endovascular techniques, as technology and operator experience continue to change. These analyses may best be performed with detailed, multi-center clinical data.
Finally, our findings can be used to inform health policy around TAAD, TBAD, and TEVAR, including Certificate of Need state designations. Our analysis found and reiterated the strong volume-outcome relationship in TAAD repair and provide new insights into the volume-outcome relationship in TBAD, which has not been well-described to date. Our findings were similar for TAAD and TBAD, indicating that patients undergoing complex surgical repair of thoracic aortic dissection (TAAD or TBAD) at high-volume hospitals are associated with lower in-hospital mortality. However, this patient population demands unique clinical and policy considerations, with unrepaired acute TAAD conferring a 1% to 2% per hour risk of rupture and mortality. In contrast to volume-outcome relationships described for procedures like elective mitral valve repair [5] or aortic root surgery [3], expedient access to repair must be further prioritized and balanced with hospital volume and mortality rates to inform policy for regionalization and optimize outcomes. Future analyses of this population should specifically address the tradeoff between access to timely treatment and undergoing repair at high-volume centers.
This study has several limitations. First, state inpatient databases comprise administrative data and lack clinical detail specific to aortic disease. However, these data capture nearly all discharges for a hospital, which provide better estimates of hospital volume across broad settings. Second, our data are limited to seven states with potential differences in practice patterns, though the data encompass over 95% of discharges in these states and are geographically diverse. Third, we cannot ensure the reliability of our procedure groupings into TAAD, TBAD, and TEVAR based on ICD-9-CM coding, although we utilized a previously published [18,19] algorithm in our stratification (Figure 1). These procedure groupings and patient acuity would be most accurately described through data from a clinical database. Fourth, our data lack post-discharge follow-up to characterize long-term survival and complications. Finally, these available data end in 2014 and thus do not completely capture current practice, highlighting the need for ongoing analyses given the ongoing rapid changes in managing these patients.
In conclusion, patients undergoing complex aortic procedures including surgical repair of TAAD and TBAD were associated with a lower in-hospital mortality at high-volume centers. In contrast, TEVAR did not demonstrate a volume-outcome effect and may be amenable to more widespread implementation. As technology for treating thoracic aortic dissection advances and practice patterns consequently change, the balance between access to care, mortality, and procedural volume for repair of thoracic aortic dissection should be further assessed to optimize patient outcomes.
Supplementary Material
Footnotes
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Oral presentation: Society of Thoracic Surgeons 55th Annual Meeting, San Diego, CA, January 28, 2019.
Classifications: Aorta/aortic, aortic arch; aortic dissection; aortic operation; endovascular procedures/stents, except PCI; Outcomes
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