In this nationwide population-based study, low institutional case volume was an independent risk factor of mortality following thoracic aorta replacement surgery. The risk of in-hospital mortality was significantly higher in patients who underwent surgery in low (< 30 cases/year) and medium volume centers (30–60 cases/year), compared to high volume centers (> 60 cases/year). One year and cumulative all-cause mortality rates were both similarly higher in centers with low volume.
First described in the aircraft industry , the positive relationship between higher institutional case volume and improved patient survival have been consistently and repeatedly shown in high risk complex surgical procedures such as hepatectomy , esophagectomy [4, 5], lung resection [5, 15], and pelvic exenteration . Thoracic aorta surgery is also a high-risk surgical procedure which requires complex and skilled surgical technique and immaculate perioperative care for best possible outcome. The reported incidence of operative mortality and major complications including stroke, infection, and renal failure following emergent surgical repair of acute thoracic or thoracoabdominal aortic dissection are exceptionally high, often exceeding 20% and 70%, respectively .
Numerous studies in cardiac surgery have shown that the risk of postoperative death was lower in high volume centers compared to lower volume centers including coronary artery bypass grafting , aortic valve replacement , mitral valve procedures , aortic root replacement , and heart transplantation . A similar volume-outcome relationship have been reported in urgent or emergent abdominal aorta surgery [16, 20], but the relationship was between surgeon case volume, not institutional case volume, and patients outcome. The suggested cutoff was 10 ruptured abdominal aorta repairs and interestingly, there was no relationship between center volume and mortality . Similarly, a previous national study in the United States revealed that the risk of mortality after emergent repair of thoracic or thoracoabdominal aortic dissection doubled in patients operated on by lower volume surgeons and centers (first quartile) compared to the highest volume surgeons . The inverse association between institutional case volume and postoperative mortality was also noted in elective aortic root replacement surgery . Our study included all types of thoracic aorta surgery and showed that the risk of postoperative death decreased significantly as institutional case volume increased.
Regionalization in the medical field is an attempt to concentrate resources to a few specialized health care centers /providers, often with an aim to improve patient outcome . With a few exceptions such as in bariatric surgery , the literature in general tends to favor regionalization as shown in neonatal intensive care units  and designated pediatric trauma centers . One recent relevant example may be the study which showed profound survival benefit in patients with influenza A-related (H1N1) acute respiratory distress syndrome after transfer to centers capable and experienced in extracorporeal membrane oxygenation . A downside of regionalization may be decreased accessibility as shown in a simulated regionalization in pediatric cardiac surgery in the United States by closure of low volume hospitals which reduced postoperative mortality [26, 27]. Considering that previous studies were mostly performed in large countries, regionalization or concentration of high-risk cardiovascular surgeries to a limited number of select centers may be very effective for outcome optimization especially in relatively smaller countries where decreased geographical accessibility is negligible.
There are several limitations in our study that should be considered. First, although all cases of adult thoracic aorta replacement surgery performed during the past 8 years in Korea was included, bias may have been introduced due to the retrospective nature of the study design. Second, potential confounders such as laboratory data or clinical variables could not be obtained since the NHIS database was an administrative database in nature. Third, the information on the severity of thoracic aorta disease was lacking and may have affected postoperative patient outcome. Although a study suggested that the surgical indication for aorta surgery (dissection/ruptured aneurysm vs. intact aneurysms) had little effect on long-term mortality for 30-day survivors , another study suggested that in-hospital mortality seems to be worse in patients with ruptured thoracic aortic aneurysms compared to patients with intact thoracic aortic aneurysms . Fourth, individual surgeon volume was not analyzed. Considering that most centers in Korea, including high volume centers, have a very limited number of surgeons who perform thoracic aorta surgery, the impact of institutional case volume on surgical outcomes may be comparable to that of surgeon volume.