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Ann Thorac Surg 1996;61:12-16
© 1996 The Society of Thoracic Surgeons
Medical University of South Carolina, Charleston, South Carolina, Virginia Mason Clinic, Seattle, Washington, Medical College of Pennsylvania, Pittsburgh, Pennsylvania, University of Colorado Health Sciences Center, Denver, Colorado, Washington University School of Medicine, St. Louis, Missouri, Pennsylvania State University, Hershey, Pennsylvania, and University of North Carolina, Chapel Hill, North Carolina
Accepted for publication October 26, 1995.
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A thoracic surgeon who had been in practice for 16 years performed a total of 820 coronary artery bypass procedures during this period, with a 3% operative mortality. Despite these results, his cardiac surgery privileges were suspended because he failed to meet his hospital's new volume standards (50 in 1994, 75 in 1995, 100 in 1996). This surgeon's application to participate in the US Healthcare Network was also denied because he failed to meet their volume requirements (150/year) (personal communication, F. Geoffrey Toonder, MD). These volume standards were reportedly based on previous recommendations of national task forces, managed care groups, and the Pennsylvania State Health Plan. Indeed, the Pennsylvania State Health Plan, which was approved in 1991, states, ``Each cardiac surgeon caring for patients with ischemic heart disease should perform an annual minimum of 150 open heart operations'' [3]. Interestingly, of the 170 surgeons performing cardiac surgery in Pennsylvania in 1990, approximately 85% did not meet these minimal guidelines [4].
Are there data to support the position taken by this and other hospitals and HMOs? A special committee of The Society of Thoracic Surgeons (STS) was established in 1994 to investigate carefully all previous recommendations as well as any other published data that might potentially link individual physician volume to patient outcome. In addition, data from new sources (STS National Cardiac Surgery Database, Northern New England Cardiac Surgery Database, and Veterans Affairs Hospital CABG Database) have also been evaluated.
Hospitals and third-party payers have frequently quoted previous recommendations of national committees to support the establishment of numerical minimums. In 1975, a Report of the Inter Society Commission on Heart Disease Resources recommended that cardiac surgical programs should perform 200 procedures annually. This recommendation was based on the premise of a hospital dedicating one room to cardiac surgery and functioning at 80% capacity (4 cases per week x 52 weeks 
200). No recommendations were made regarding individual surgeon volume [5]. In 1984, the Subcommittee on Cardiac Surgery Standards of the Cardiovascular Committee and the Advisory Council for Cardiothoracic Surgery of the American College of Surgeons stated, ``The performance of at least 150 open heart operations per year by an independent team is desirable to maintain an adequate standard'' [6]. This report was updated in 1991 with the identical recommendation [7]. No recommendations regarding individual surgeon volume were made. In 1991, ``Guidelines and Indications for Coronary Artery Bypass Graft Surgery'' was published by the American College of Cardiology/American Heart Association Task Force Subcommittee on Coronary Artery Bypass Graft Surgery. This report recommended that ``a yearly minimum of 200-300 open heart operations, the majority of which are coronary bypass operations, should be performed in hospitals caring for patients with ischemic heart diseases ... In general, a yearly minimum of 100 to 150 open heart operations ... should be performed by each surgeon ... These recommendations about case load are general and should be applied with the knowledge that several reports attest it is possible for a particular low volume hospital or surgical group or surgeon to have good results.'' For the first time, recommendations were made regarding individual surgeon volumes; however, there were no firm data to support this position [8].
The available data linking outcome to volume for a variety of surgical procedures including CABG have been thoroughly reviewed in the book Hospital Volume, Physician Volume, and Patient Outcomes by Luft and associates [9]. Luft and associates concluded that there are significant data to suggest a linkage between the volume of CABG performed by a hospital and subsequent outcomes. In other words, low-volume hospitals appear to have poorer overall results. However, they were unable to document any consistent relationship between individual surgeon volume and patient mortality or outcome.
In a review of 3883 CABG procedures involving 99 surgeons in 26 hospitals, Kelly and Hellinger [10] concluded that ``our findings indicate that heart patients who undergo a CABG or cardiac catheterization procedure are most likely to survive when their procedures are performed in high volume hospitals. However, there is no statistical relationship between patient outcome and the volume of similar procedures performed by surgeons. In other words, the number of CABG procedures performed by individual surgeons is not related to inhospital mortality.'' Showstack and associates [11] reviewed 18,986 CABG procedures at 77 California hospitals. After adjusting for case mix, high-volume hospitals had lower in-hospital mortality, but this study compared institutions and not individual surgeons. Luft and colleagues [12] reviewed data from 5,172 CABG procedures involving 114 hospitals. In general, outcomes were better in high-volume hospitals than in low-volume hospitals, but individual physician data were not analyzed. Hughes and co-workers [13] analyzed data for 503,662 patients undergoing ten selected surgical procedures. There appeared to be a relationship between surgeon volume and good outcome for all procedures except CABG. Hughes and co-workers explained this by the fact that there were relatively few low-volume surgeons performing CABG and that such procedures were generally done by a team.
In a related area, Hosenpud and colleagues [14] reviewed all patients (7,893) undergoing cardiac transplantation from 1979 to 1991 and concluded that ``the risk of mortality at early and intermediate points were substantially higher in low volume transplant centers'' but then defined a low-volume center as one performing fewer than 9 transplantations per year. No individual surgeon data were available.
The most up-to-date data attempting to link CABG volume to outcome are presented in a series of articles by Hannan and associates in 1989, 1991, and 1995, using data from New York State [15–17]. The first [15], in 1989, was a study of 16 operative procedures analyzing the relationship between surgical volume and mortality rates for operations performed during 1986. For CABG, high- and low-volume surgeons were classified using the median as a split, so that if a surgeon performed more than 116 coronary bypass procedures during the preceding 12 months, he or she was considered a high-volume surgeon, whereas all those performing fewer than that were considered low-volume surgeons. When the data were analyzed with logistic regression analysis, lower individual surgical volume was one of the risk factors predictive of a higher operative mortality for CABG. This study has significant limitations. Risk stratification was suboptimal because of the limited nature of the database. Half of the surgeons were considered low-volume surgeons because of the arbitrary definition of low volume as ``below the median.'' The study covered only a 1-year period and, as its authors note in their discussion, there are probably large variations across calendar years and across areas of the country.
The relationship between in-hospital mortality and surgical volume was reexamined in 1991 after a more sophisticated method of risk adjustment was developed [16]. In this study, 1989 New York State data for CABG procedures were used for 12,448 patients operated on by 126 surgeons with 458 deaths (3.68%). On the basis of logistic regression analysis, low surgeon volume and low hospital volume were both predictors of outcome. Surgical volume was broken down into four groups: 1 to 54, 55 to 89, 90 to 259, and 260 or more procedures. The risk-adjusted operative mortality for each of these four groups was 8.14%, 5.56%, 3.61%, and 2.43%, respectively. Similarly, the risk-adjusted mortality according to hospital volume was broken down into hospitals with a volume of 1 to 199, 200 to 889, and 890 or more procedures, with adjusted mortality of 7.25%, 4.23%, and 2.89%, respectively. When the data are carefully examined, it becomes obvious that there is considerable scatter and variation in risk-adjusted mortality when compared with volume. For example, there is less variation in risk-adjusted mortality in hospitals with higher volumes and more scatter (greater variation) in those with lower volume. There are, however, a number of hospitals that have excellent risk-adjusted mortality in the lower volume group. A threshold analysis reveals a statistically significant threshold at approximately 125 cases annually. That is, hospitals with volumes less than 125 cases/year had a statistically significant higher adjusted mortality than did hospitals with volumes greater than 125 cases/year.
This group's most recent report (1995) describes decreasing operative mortality for CABG in New York State over the period 1989 to 1992 [17]. The relationship between individual surgeon volume and in-hospital mortality rate and its change over time was documented for each of four volume groups: less than 50, 51 to 100, 101 to 150, and 151 or more procedures. The database was also examined for any changes in case loads of low-volume surgeons and the composition of the low-volume surgeon cohort. During this period of time, the actual in-hospital mortality rate decreased from 3.5% to 2.78%, in spite of increasing severity of illness of the patients. Thus, there was a decrease in risk-adjusted mortality from 4.17% in 1989 to 2.45% in 1992. Of interest is the fact that the relationship that Hannan and associates previously identified between individual surgeon volume and operative mortality was not statistically significant during the more recent years. The 1989 data for low-volume surgeons may have been skewed by the fact that six of the surgeons operating during that year had a combined case load of only 67 patients, with a combined risk-adjusted mortality of 21.5%. In 1991 and 1992, there were no significant differences in risk-adjusted mortality rates among any of the surgeon volume groups. In the most recent year of the study (1992), the risk-adjusted mortality rate for surgeons performing less than or equal to 50 cases was 3.2%; for 51 to 100, 2.5%; 101 to 150, 2.2%; and 151 or more, 2.49%. None of these differences is statistically significant or clinically relevant.
This article, as well as the previous two, examines CABG only as opposed to all cardiac operations requiring cardiopulmonary bypass. Surgical expertise is probably equivalent for all types of cardiopulmonary bypass cases and not just CABG. Accordingly, total major operative volume should have been considered, as opposed to CABG only. It would have been useful if a threshold analysis of variance had been performed to see if indeed there is a surgeon-specific caseload that might be statistically significant. No aggregate analysis for the entire 4-year period was performed. Because three of the four 1-year periods did not indicate a statistical relationship, it is unlikely that an aggregate relationship would be found. This study does demonstrate, however, that low-volume providers have greater variation in their risk-adjusted mortality rates than do high-volume providers. Clearly there are many variables in addition to volume that play an important role in outcome. Equally clearly, many surgeons in hospitals in the low-volume group have acceptable operative mortality.
This database and the conclusions have recently been critically evaluated by Green and Wintfield [18]. They found the predictive accuracy of the model to be very low. The results would have been markedly different if the definition of the outcome had been changed slightly. The prevalence of several risk factors increased significantly over the course of the study despite no real change in the population being studied, suggesting a change in the method of reporting. As a result, Green and Wintfield suggest that further improvement in the database is necessary if truly meaningful conclusions regarding outcomes are to be made.
As part of the Veterans Affairs Continuous Improvement in Cardiac Surgery Project, Shroyer and colleagues [19] have reviewed the potential relationship between volume of cardiac surgery performed and operative mortality among the 43 cardiac surgery programs in the Department of Veterans Affairs Hospitals. Data were collected for 24,394 CABG patients from 1987 to 1992. Shroyer and colleagues concluded that with no risk adjustment or age adjustment only, there appears to be a significant relationship between volume and mortality. However, with a comprehensive adjustment for patient risk, no evidence of a volume/mortality relationship could be found. It was recognized that, in many cases, those patients operated on at Veterans Affairs hospitals represented only a part of the individual surgeon's workload, because the surgeons also operated at affiliated university hospitals. The fact that there was a correlation without risk adjustment and no correlation with risk adjustment illustrates how mortality data, when not properly analyzed, can lead to potentially erroneous conclusions. Low-volume hospitals had a greater variation in risk-adjusted mortality rates than did high-volume hospitals. A number of low-volume hospitals performed equally well or better than high-volume hospitals. It would appear that there are important processes and structures of care other than volume alone that affect outcome. Based on these data, the Veterans Affairs Cardiac Surgery Consultant's Committee has taken the stance of reviewing only hospitals with volumes consistently less than 100 cases per year. More importantly, the quality of work based on risk-adjusted outcomes rather than on volume is the primary indicator of the level of performance.
The Northern New England Cardiovascular Disease Study Group Database contains information on 18,000 patients operated on at five institutions by 28 different surgeons (personal communication, William Nugent, MD). This database includes every isolated CABG performed in northern New England since 1987. Unfortunately, the small number of surgeons represented in this database does not allow sufficient degrees of freedom to draw definitive conclusions about volume and outcome.
Data from the National Cardiac Data Base (STS) have been examined by Clark and associates [20] in an attempt either to substantiate or refute the hypothesis that a low number of annual CABG operations per surgeon predictably results in poor outcome. In the years 1991 to 1993, 180 practices (groups) performed CABG on more than 50 patients in a 6-month interval and contributed a total of 120,377 patients. Surgeon-specific volume could not be determined because the STS Database does not capture surgeon, hospital or patient identifiers, and, accordingly, the data are analyzed on a group basis. The analysis was used to look for a volume threshold that might have an influence on mortality. Except for the lowest (<100) and highest (>900) volumes, the observed mortalities were in a range of 3.0% to 3.5%. Sixty-three percent of the total caseload was done by groups performing fewer than 600 cases per year. There were no break points or thresholds of statistical significance except for the lowest volume group (<100/year), which had the highest observed and expected mortalities and the highest observed/expected ratio. Linear regression and logistic regression analyses demonstrated weak to very weak inverse correlation of volume to operative mortality. From a statistical viewpoint, larger group volumes did favor improved outcome, but the strength of the relationship was weak because of the wide variability of risk and mortality in the lower group volume practices.
This analysis was hindered by the inability to study surgeon-specific data, but that information is not available from the STS Database. In addition, there may be other variables that are important besides the individual surgeon and his or her volume, such as the role of ``the process of care.'' The process of care for patients with coronary artery disease is complex and involves multiple groups of talented, highly trained individuals, and it is clear that this process of care may have an influence on an individual surgeon's or group's overall outcome. These data from the STS Database, although not surgeon- or hospital-specific, demonstrate a weak statistical correlation of volume to mortality after CABG that is not clinically relevant. The vast majority of practice groups perform fewer than 600 cases per year. There are no meaningful differences in terms of outcome except at the lowest (<100/group per year) and the highest (>600/group per year) extremes. It is clear, however, that outcomes have a much wider range of variability in groups performing lower volumes of surgery than in those groups performing a high volume of surgery, where the outcome varies within a narrow range.
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Comment |
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Previous recommendations of national committees are frequently quoted as the basis for these numeric requirements; yet, with one exception, all of these previous recommendations were intended for hospitals or programs and not for individual surgeons. Even that exception recognized that it is possible for a low-volume individual surgeon to achieve good results.
There do appear to be data to support a significant relationship between hospital volume (of CABG) and patient outcome, thus justifying a possible recommendation for a minimum volume of CABG to support a cardiac surgical program. Even this conclusion presents some difficulties, however, because these volumes might be difficult to achieve in some areas. In a large but sparsely populated state with four programs doing 150 cases per year, it would perhaps be better theoretically to consolidate all of the heart surgery into one program doing 500 to 600 cases per year, thus perhaps decreasing the mortality rate slightly. On the other hand, how many patients would be denied access to care on a geographic basis because of this regionalization and thus die of their disease without the potential benefit of operation?
Although there are data linking hospital volume to outcome, is the same true for individual physicians? Until recently, the conclusion in essentially every study was that no such relationship exists. The most recent article by Hannan and associates [17] would appear to draw a relatively weak link between the two, but a closer examination of the data questions this conclusion [18]. Unfortunately, many databases, including the STS/Summit Database, record data by physician groups and not individuals. In some in which physician-specific data are available, the numbers are not yet large enough to allow definite conclusions. The lack of risk-adjusted data may also invalidate the conclusions of some of the previous studies.
If, in fact, it could be shown conclusively that a relationship exists between individual physician volume and outcome, it would still need to be determined whether this is because of the individual physician's skill or because he or she works in an environment that is conducive to good outcome. In other words, what is the role played by other factors (eg, intensive care unit/anesthesia/ancillary care/nursing) on patient outcome?
The recent American Association for Thoracic Surgery/STS Workforce Survey indicates that the mean number of cardiac procedures done in 1993 by members of these two organizations was 151 (median, 134) fs21]. This number included all types of cardiac procedures and was not limited to CABG. Fifty-seven percent did fewer than 150 per year and would thus be immediately disenfranchised by any organization requiring 150 CABGs per year. Is the cardiothoracic surgeon who does 50 valve replacements, 50 CABGs, and 50 congenital heart procedures measurably different than the one who does 150 CABGs only? Or what about the surgeon who does 50 CABGs, 50 major vascular reconstructive procedures, and 50 pulmonary resections? Is it the total volume of cases or the volume of each specific procedure that is important? What about the influence of assisting in these procedures? Most CABGs are done by two surgeons (one primary, one assistant). Is assisting on 100 CABGs equivalent to performing 25, 50, or some other number? If a minimum of 150 per year is necessary, how does a new surgeon get started, given that relatively few new graduates are referred more than 150 cases in their first year? Will all new surgeons be excluded, or what criteria will be used for them? Is there no role for the older surgeon with documented outstanding results but who now perhaps wants to slow down? Will he or she be immediately excluded as soon as his or her volume dips below 150/year? Is there not something to be said for the accumulated wisdom of 25 years of practice as opposed to simply a numeric volume of 150 cases/year?
It is apparent that a relationship exists between hospital volume and outcome for CABG. It is equally apparent that at this point, few data exist to document a similar relationship between individual surgeon volume and outcome. It is also clear that recommendations of previous committees have been quoted out of context and applied inappropriately to individual surgeons. Thus, it seems that the new minimum volume requirements being imposed for credentialing and participation in provider networks are unrealistic at this point. A much more important factor than volume is documented quality. Few surgeons could object to being compared with their peers by objective criteria that included risk-adjusted mortality and number of complications. It is the arbitrary and undocumented linkage of these criteria to volume that is objectionable.
It is therefore imperative that these volume requirements be rolled back and that each surgeon be judged on his or her own outcomes. For this to occur, each surgeon must participate in a database and be certain that the data provided to it are completely accurate. Participation in such a program would allow each surgeon to be compared with his or her peers and judged on his or her own outcomes, and eventually would provide an answer to the question regarding any linkage between individual volume and overall outcome.
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