Studies in adult cardiac surgery have concluded that the observed insitutional volume/ mortality association was largely mediated by individual surgeon volume [25]. It has even been suggested in adult vascular surgery that a means to improve one’s chances of survival would be to select a surgeon who performs a specific operation frequently [26, 27]. In pediatric cardiac surgery, this issue was investigated by 4 groups. In 1998, Hannan et al. showed, using a New-York State clinical database, that surgeons with pediatric cardiac surgical cases volumes of less than 75 per year had significantly higher mortality rates (8.77 %) than surgeons with surgical volumes more than 75 cases per year (5.90 %) [10]. But this result was not confirmed by 2 subsequent studies that addressed this issue in the specific population of patients that required a Norwood procedure. Indeed, Checcia et al. found using a large administrative database that surgeon volume was not associated with patient outcomes after a Norwood procedure [23]. In 2010, the CHSS study by Karamlou et al. showed that neither center nor surgeon volume were associated alone with Norwood outcomes [21]. The results of these two studies might have been limited by the use of administrative data and the methodology used for calculating surgical volume.

More recently, Hornik et al. evaluated the relative impact of surgeon and center volume on mortality in a large Norwood cohort, using the Society of Thoracic Surgeons Congenital Heart Surgery Database [28]. They showed that, when analyzed individually, both lower center and surgeon volumes were associated with higher in-hospital mortality (odds ratio for surgeons with 0–5 cases versus surgeons with more than 10 cases per year = 1.60). This surgeon volume/mortality association after Norwood procedure was true in all center volume strata: lower volume surgeons had higher adjusted in-hospital mortality rates across low, medium, and high volume centers. A low-volume surgeon’s outcomes were worse regardless of center volume, but the surgeons’s results were mitigated by a large center volume. These results have been reproduced most potently in a recent analysis of the Single Ventricle Reconstruction trial, which also showed a significant survival advantage for high-volume surgeons [29]. This association can be easily understood as it has been shown that surgical technical performance improves outcomes irrespective of preoperative physiologic status or case complexity in the Stage 1 norwood procedure [30] and in other neonatal cardiac surgical procedures [31, 32]. These data could lead to the development of regional collaboration and centralization policies within and across centers through enhanced mentoring program by the highest-volume surgeons. Nevertheless, this impact of surgeon volume on Norwood mortality demonstrated by Hornik et al. was less strong when compared to the impact of surgeon volume in adult cardiac surgery [25]. This could be explained by the key role played by other providers, human factors and hospital-related factors impacting on the preoperative and postoperative management of complex single-ventricle physiology, thus decreasing the direct consequences of the impact of surgeon volume in pediatric vs adult cardiac surgery.[33]

Recent studies have demonstrated that a volume-independent center-effect seems to contribute substantially to the between-center variability in outcomes. This center effect was has been demonstrated after orthotopic heart transplantation [34]: Kilic et al. demonstrated that institutional volume alone only accounted for 16.7 % of the variability in mortality between centers, and that a significant between-center variability persisted after adjusting for this factor (P < 0.001). This finding was confirmed in pediatric cardiac surgery in 2013 by Vincour et al. [35]. Vinocur et al. aimed at characterizing the relative contribution of patient factors, center surgical volume, and a volume-independent center effect on early postoperative mortality in a retrospective cohort study of North American centers in the Pediatric Cardiac Care Consortium. Although the center volume was inversely associated with outcome in all age groups and risk categories (except the lowest one), a volume-independent center effect contributed substantially more to the risk model than did the volume.

Another group revealed the impact of prior hospital performance on the current outcomes after surgery for congenital heart disease [36]. They demonstrated using the Pediatric Health Information Systems database, that prior hospital postoperative mortality was significantly associated with mortality across all risk strata of congenital heart surgery, whereas, prior hospital surgical volume tended to be associated with improved mortality after only higher-risk operations. These intriguing recent results suggest that center-specific variation in outcomes after pediatric cardiac surgery is only partially explained by operative volume and that other factors have yet to be clearly identified.

The results of the most recent previously mentioned studies demonstrate that a relationship between case volume and mortality should be interpreted with caution. The trend for lower mortality at larger centers is not universal: all larger programs do not perform better than all smaller programs. Morevover, it has been shown that the volume accounted for only a small proportion of the overall between-center variation in outcome [24, 35, 36]. The lack of long-term follow-up (beyond 30 days) in most of these studies also limits the evaluation to the very early mortality. This serious challenge prevents the authors from addressing the long-term mortality, morbidity, functional status, and neurologic status which is quite significant in single vessel pathologies even after repair [18]. Thus, the center or surgeon volume alone may not be reliable enough to measure and compare center outcomes. The use of center-specific risk adjusted outcome as a proxy tool for quality assessment may be more reliable than relying upon volume alone [17, 37]. Such an adjustment should consider at the minimum both surgical case complexity and patient specific factors [24]. Indeed, a patient’s risk factors and their level of disease severity may play a more important role in determining their individual outcome than the impact of the program’s volume.

The true mechanism of the volume/outcome association remains controversial. Higher volume centers probably have other organizational, logistical, technical and/or human characteristics that at least partially explain this relationship. These factors include the availability of highly equipped operating rooms and cath labs, better management of health resources, ergonimic design and deployment of new technologies, composition of the care team, advanced training programs, improved preoperative and intraoperative care, multidisciplinary discussions, the use of standardized management protocols, and better resilience and timely recognition and treatment of complication [18, 19, 28, 38–41]. That suggests that higher center volume may be a surrogate for other aspects of care that are more likely to be provided at larger centers. These process measures and structural characteristics of systems that lead to better outcomes are not currently captured in available databases. These aspects including the role of human factors, team training and debriefing and non technical team skills should be extensively studied to determine their respective roles in outcomes after pediatric cardiac surgery [33, 42, 43]. The increasing mobility of skillful and experienced surgical, anesthesia and ICU staff should also be taken into account when studying the volume/outcome relationship [44]. Finally, we could also wonder whether high case volumes may lead to the improvement of outcomes thanks to an increased practice or better results attract more referrals, thus leading to higher volumes [11]. The relation between high volume and better outcomes remains strong and persistent in the field of pediatric cardiac surgery. [24, 28, 29], What then should policy makers do? what should parents and healthcare mangers do? and in view of the results of the latest studies [45, 46].