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J Thorac Cardiovasc Surg 2008;135:1254-1260
© 2008 The American Association for Thoracic Surgery

Surgery for Acquired Cardiovascular Disease

The "occasional open heart surgeon" revisited

Joseph S. Carey, MD*, Joseph P. Parker, PhD, Claude Brandeau, BS, Zhongmin Li, PhD

California Society of Thoracic Surgeons, the California Office of Statewide Health Planning and Development (OSHPD), and the University of California, Davis

Received for publication May 16, 2007; revisions received September 28, 2007; accepted for publication October 4, 2007.

* Address for reprints: Joseph S. Carey, MD, 3475 Torrance Blvd Suite B-1, Torrance CA 90503. (Email: careyjs{at}earthlink.net).


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Conclusions
 References
 
Objectives: Case volume in cardiac surgery has been a concern since the term "the occasional open heart surgeon" was used more than 40 years ago, indicating one who performs cardiac surgery infrequently.

Methods: Risk-adjusted operative mortality (in-hospital or 30-day mortality) for isolated coronary artery bypass grafting procedures reported to the California CABG Outcomes Reporting Program for 2003–2004 was determined by surgeon and by hospital. Standard Society of Thoracic Surgeons item definitions were used. A total of 49,421 coronary artery bypass grafting (40,377 isolated) procedures were performed by 302 surgeons at 121 hospitals. Low-volume surgeons (n = 117) were defined as performing a total of less than 1 coronary artery bypass grafting (isolated or nonisolated) procedure per week at all hospitals (mean ± standard deviation, 22 ± 15/y). High-volume surgeons (n = 185) performed a total of 1 or more cases per week (mean ± standard deviation, 120 ± 62/y). Logistic regression and hierarchic analysis were used to compare volume cohorts.

Results: The overall risk-adjusted mortality rate was 3.62% for low-volume and 3.02% for high-volume surgeons. Analysis by surgeon per hospital produced 610 surgeon–hospital pairs. The lowest risk-adjusted mortality rates were found among surgeons performing more than 1 procedure per week at a single hospital (2.70%). When high-volume surgeons performed less than 1 procedure per week at a hospital, their mortality rates were similar to those of low-volume surgeons (3.39%–4.11%). High-volume surgeons performing procedures at multiple sites had higher mortality than high-volume surgeons working at a single institution.

Conclusion: A high-volume surgeon becomes an "occasional open heart surgeon" when working at multiple hospitals and performing a small volume of procedures at some of them. This study suggests that volume is not as important as processes of care in determining outcomes of coronary artery bypass grafting procedures and that system factors might be more important to outcomes than surgeon experience.

Abbreviations and Acronyms CABG = coronary artery bypass grafting; RAMR = risk-adjusted mortality rate


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Conclusions
 References
 
The term "occasional open heart surgeon" was used by Eiseman and Spencer1Go in an editorial in Circulation in 1965. They quoted a survey conducted by the School of Public Health at Johns Hopkins that found that 41% of cardiac surgery teams performed less than 10 procedures per year. Over the ensuing decades, the relationship between volume and outcome has been extensively studied, often producing more questions than answers.

Provider volume has been shown to be a risk factor in a variety of complex surgical procedures.2-4Go In-hospital mortality was found to be lower in higher-volume facilities for abdominal aortic aneurysm resection, esophagectomy, pancreatic resection, and coronary artery bypass grafting (CABG), resulting in calls for volume thresholds of these procedures by purchaser groups, such as Leapfrog. Physician groups and hospitals dismissed these recommendations because of the variable statistical significance of the findings and the feasibility of volume-based referral. Furthermore, it appears that the volume–outcome conundrum is a moving target. Several recent studies have noted a weaker association in CABG5Go and abdominal aortic aneurysm.6Go Reports from the California CABG reporting programs show no statistical significance of hospital volume on risk-adjusted mortality for the years 2003–2004, whereas hospital volume as an independent variable was significant in previous reports. *

Hospitals do not perform surgical procedures. Rather, surgeons perform them, but they perform them with a team of other caregivers. The relationship between these caregivers is critical in any procedure, but especially so in cardiac surgery. The collection of data on CABG surgery for public reporting on California hospitals and surgeons provides an opportunity to observe the relationships between providers and the systems in which they do their work. California has a large number of cardiac surgical programs, most of which are low volume, performing less than 300 "open heart" cases per year. Many of the cardiac surgeons are low volume, performing less than 100 procedures per year. The present study examines these low-volume providers to study the effect of diminishing numbers of CABG procedures in cardiac surgery in California.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Conclusions
 References
 
Data collected through the California CABG Outcomes Reporting Program was reviewed. Mandatory reporting of all CABG procedures was begun in January 2003. Hospital-specific data from 2003 were released to the public in February 2006, and both hospital- and surgeon-specific data for the years 2003–2004 were released in July 2007. Data are collected by using Society of Thoracic Surgeons National Database formats; identical procedure and risk factor definitions were used, but a unique risk model was calculated for isolated CABG procedures. Mortality was determined by using the Society of Thoracic Surgeons National Database definition of "operative mortality": death after surgical intervention in the hospital or within 30 days. Deaths were verified by means of linkage with the California death file. In addition, independent onsite reabstraction of medical charts was performed at 57 hospitals for these data years.

A total of 49,421 CABG (40,377 isolated) procedures were performed by 302 surgeons at 121 hospitals during 2003–2004. Provider total CABG volume and its relation to outcome, as measured by means of operative mortality for isolated CABG, was analyzed by hospital, surgeon, and surgeon per hospital. Low-volume surgeons were defined as those performing a total of less than 1 CABG (isolated or nonisolated) procedure per week over the 2-year period. High-volume surgeons performed 1 or more CABG procedures per week. Very low volume was defined as less than 1 procedure performed per hospital per month. We adopted the state-published risk model and conducted further descriptive analyses based on state-published hospital and surgeon risk-adjusted operative mortality results for 2003–2004. We then used a hospital-patient hierarchical prediction model to test the association between surgeon/hospital volume and risk-adjusted mortality. All data analyses were conducted with SAS 9.1.3 (SAS Institute, Inc, Cary, NC).


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Conclusions
 References
 
Procedure Volumes and Operative Mortality
The number of CABG procedures performed by low-volume and high-volume surgeons during 2003–2004 is shown in Go Table 1. On average, high-volume surgeons performed more than 5 times as many procedures as low-volume surgeons.


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  		Table 1 CABG procedures performed by low- and high-volume surgeons, 2003–2004 (annualized, mean ± SD)
 
Operative mortality for isolated CABG procedures performed by low-volume and high-volume surgeons during 2003–2004 is shown in Go Table 2. High-volume surgeons performed nearly 90% of the procedures. Overall, low-volume surgeons had higher risk-adjusted operative mortality compared with high-volume surgeons.


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  		Table 2 Operative mortality (observed and expected) for isolated CABG, 2003–2004
 
Hospital Volume and Risk-adjusted Mortality Rate
The relationship of individual hospital total CABG volume to isolated CABG risk-adjusted mortality rate (RAMR) for 2003–2004 is shown in Go Figure 1. Only 26 of 121 hospitals in California performed more than 500 CABG procedures (250/y) over the 2-year period. Although the highest RAMRs occurred at lower-volume sites, many low-volume hospitals had outcomes that were better than those of higher-volume programs. Overall, the relationship between volume and mortality for hospitals was not statistically significant (Pearson correlation coefficient, –0.105; P = .253).


Figure 1
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  		Figure 1. Hospital total coronary artery bypass grafting (CABG) volume versus the risk-adjusted isolated CABG operative mortality rate.

 
Surgeon Volume and RAMR
The relationship of individual surgeon total CABG volume to isolated CABG RAMR for 2003–2004 is shown in Go Figure 2. The highest mortality rates occur among the lowest-volume surgeons (<100/y), but many low-volume surgeons have low or zero RAMRs. As a result, individual surgeon volume was not significantly associated with RAMR (Pearson correlation coefficient, –0.096; P = .095).


Figure 2
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  		Figure 2. Surgeon total coronary artery bypass grafting (CABG) volume versus the risk-adjusted isolated CABG operative mortality rate (RAMR).

 
Surgeon Volume per Hospital and RAMR
Most surgeons operated at more than 1 hospital. Low-volume surgeons operated at a mean of 2.4 hospitals (range, 1–6 hospitals), and high-volume surgeons operated at a mean of 3.0 hospitals (range, 1–8 hospitals). This produced 610 surgeon–hospital pairs. These data are shown in Go Table 3. High-volume surgeons had the best risk-adjusted isolated CABG mortality rates when performing more than 1 procedure per week at a hospital. However, when high-volume surgeons performed less than 1 procedure per week at a hospital, their mortality rates were no better than those of low-volume surgeons.


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  		Table 3 Surgeon volume per hospital and RAMR
 
The relationship between surgeon total CABG volume per hospital and RAMRs for isolated CABG is shown in Go Go Figures 3 and 4. The individual surgeon volume–outcome relationship is not significant (Pearson correlation coefficient, –0.017; P = .677). However, when the surgeon volume per hospital was analyzed in categorical groups by using a hierarchic logistic risk model to predict operative mortality outcome, the odds ratio was 1.52 (P = .007) for surgeons performing less than 1 procedure per month and 1.29 (P = .011) for surgeons performing less than 1 procedure per week compared with surgeons performing more than 2 CABG procedures per week.


Figure 3
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  		Figure 3. Individual surgeon total coronary artery bypass grafting (CABG) volume per hospital versus the risk-adjusted isolated CABG operative mortality (RAMR).

 

Figure 4
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  		Figure 4. Categorical surgeon total coronary artery bypass grafting (CABG) volume per hospital versus the isolated CABG risk-adjusted mortality rate (RAMR): odds ratios of 1.52 (P = .007) for less than 1 procedure per month, 1.29 (P = .011) for less than 1 procedure per week, and 1.17 (P = .127) for 1 to 2 procedures per week versus more than 2 procedures per week.

 
Are high-volume surgeons who work at low-volume sites different? High-volume surgeons who work in multiple hospitals, and thus work at low-volume sites, can differ from other high-volume surgeons. Go Table 4 shows a breakdown of high-volume surgeons working only at high-volume sites and high-volume surgeons working at both high- and low-volume sites. High-volume surgeons working only at high-volume sites (group A) had the lowest RAMRs. Patients of high-volume surgeons working at multiple sites had higher mortality rates at both high-volume (group B-1: odds ratio, 1.22; P = .043) and low-volume (group B-2: odds ratio, 1.34; P = .010) sites compared with patients of group A surgeons.


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  		Table 4 High-volume surgeons working only at high-volume sites (group A) compared with high-volume surgeons working at both high-volume (group B-1) and low-volume (group B-2) sites
 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Conclusions
 References
 
These studies illustrate the interaction between surgeons and the hospitals in which they work and the problem of statistical comparison of volume to outcome. Overall, outcomes as measured by operative mortality for isolated CABG were higher at low-volume sites, but neither hospital nor surgeon total CABG volume was significantly related to isolated CABG mortality as an independent variable by means of logistic regression analysis.

We used the total of isolated and nonisolated CABG procedures to determine volume categories for individual surgeons. When surgeon and hospital volumes are combined, certain relationships emerge within volume categories, as noted in Table 3. The difference in RAMRs between low-volume surgeon/hospital combinations compared with high-volume combinations was much greater than the overall difference. For example, high- and low-volume surgeons performing less than 1 procedure per month in a hospital had RAMRs of nearly 4%, more than 40% higher than that of high-volume surgeons working only at high-volume sites (RAMR, 2.79%), whereas the overall difference in isolated CABG mortality between low- and high-volume surgeons was 3.62% versus 3.02%.

The finding that the RAMR for isolated CABG was similar for both low-volume and high-volume surgeons when performing a low volume of procedures at a hospital suggests that system factors might be more important to outcomes than surgeon experience. We also found that high-volume surgeons who work at multiple sites have higher mortality than high-volume surgeons working at one site. This suggests that these surgeons might bring suboptimal processes with them or that the requirements of working at several hospitals negatively affect their ability to care for their patients.

The significance of surgeon volume, as opposed to hospital volume, was studied by Birkmeyer and associates7Go using Medicare data from 1994–1999. They concluded that surgeon volume accounted for a large part of the effect of facility volume on outcomes, varying widely by procedure type. These authors also examined the predictability of historical hospital volume and mortality on future outcomes. They found that for CABG, historical mortality was more predictive of mortality rates in future years than historical volume.8Go

A confounding factor to be considered is the effect of "clustering" on outcomes.9Go Providers with similar volumes might have differing outcomes based on processes of care. Statistical methods have been used to adjust for this effect by several authors in volume–outcome studies.6,8Go Because such methods generally reduce the significance of the effect of volume on outcome, their use might affect the significance of the surgeon volume per hospital data. Differences in outcomes among high-volume surgeons suggest that clustering might account for findings in the data presented here.

The relationship between RAMRs and surgeon and hospital volumes was examined by Hannan and colleagues10Go using data from the New York CABG registry from 1997–1999. They also found that CABG mortality was lowest for high-volume surgeons operating in high-volume hospitals. However, their highest mortality rates (2.67% for surgeons performing less than 125 procedures annually in hospitals with annual volumes of less than 600) are comparable with the best RAMR that we report. In an earlier study comparing outcomes of CABG procedures in New York to California, we found that higher mortality rates in California were related to the large number of low-volume programs because outcomes of higher-volume programs were comparable in the 2 states.11Go In the present study we did not examine our surgeon-based mortality rates in relation to the total volume of the hospital in which the procedures were performed, and Hannan and colleagues' study10Go did not look at surgeon volume per hospital.

Most studies of the relationship between volume and outcome in cardiac surgery have focused on CABG procedures and in-hospital mortality. Gammie and coworkers,12Go using data from the Society of Thoracic Surgeons National Database, identified processes of care that contributed to better outcomes at higher-volume sites for mitral valve surgery. Several studies have found that β-blocker use and the internal thoracic artery graft have contributed to better outcomes in CABG surgeries. Few other key processes have been identified.

Khuri and Henderson13Go and Shahian and Normand14Go have pointed out the pitfalls of using volume as a surrogate for quality. Studies from the National Surgical Quality Improvement Program did not find a relationship between volume and outcome across a variety of surgical specialties. Systems of care were more important in determining quality of care at an institution. Continuous quality improvement programs should seek to emulate the processes of high-volume, high-quality providers.

This study provides further evidence that volume is not as important as processes of care in determining outcomes of CABG procedures. In view of the diminishing number of CABG procedures and the inevitability of low-volume programs, more studies that focus on methodology and best practices in the wider spectrum of cardiac surgical procedures are indicated.


   Conclusions
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusions
References
 
Hospital volume of CABG procedures did not correlate significantly with RAMR in the clinical database collected by the California CABG Outcomes Reporting Program during 2003–2004. Individual surgeon total CABG volume also did not correlate with RAMR. The majority of programs in California are low volume by accepted standards, and most surgeons work at more than 1 hospital. We therefore studied the relationship of surgeon volume per hospital to RAMR.

Surgeon CABG volume per hospital appears to correlate with RAMR. High-volume surgeons who perform a small volume of CABG procedures in a hospital have no better outcomes than low-volume surgeons, suggesting that system factors are more important to outcomes than surgeon experience.

High-volume surgeons who work primarily in 1 hospital have better outcomes than high-volume surgeons who work in multiple hospitals. Processes of care that are associated with these surgeons might account for this difference.


   Footnotes
 
Read at the Eighty-seventh Annual Meeting of The American Association for Thoracic Surgery, Washington, DC, May 5–9, 2007.

* http://www.oshpd.ca.gov/HQAD/Outcomes/Studies/cabg/2003Report/2003Report.pdf Back


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusions
 References
 

  1. Eiseman B, Spencer FC. Editorial: the occasional open-heart surgeon. Circulation 1965;31:161-162.[Free Full Text]
  2. Halm EA, Lee C, Chassin MR. Is volume related to outcome in health care? A systematic review and methodologic critique of the literature. Ann Intern Med 2002;137:511-520.[Abstract/Free Full Text]
  3. Birkmeyer JD, Siewers AE, Finlayson EV, et al. Hospital volume and surgical mortality in the United States. N Engl J Med 2002;346:1128-1137.[Medline]
  4. Davoli M, Amato L, Minozzi S, et al. Volume and health outcomes: an overview of systematic reviews. Epidemiol Prev 2005;29(suppl):3-63.[Medline]
  5. Rathore SS, Epstein AJ, Volpp KG, Krumholz HM. Hospital coronary artery bypass graft surgery volume and patient mortality, 1998-2000. Ann Surg 2004;239:110-117.[Medline]
  6. Henebiens M, van den Broek TA, Vahl AC, Koelemay MJ. Relation between hospital volume and outcome of elective surgery for abdominal aortic aneurysm: a systematic review. Eur J Vasc Endovasc Surg 2007;33:285-292.[Medline]
  7. Birkmeyer JD, Stukel TA, Siewers AE, et al. Surgeon volume and operative mortality in the United States. N Engl J Med 2003;349:2117-2127.[Medline]
  8. Birkmeyer JD, Dimick JB, Staiger DO, et al. Operative mortality and procedure volume as predictors of subsequent hospital performance. Ann Surg 2006;243:411-417.[Medline]
  9. Panageas KS, Schrag D, Riedel E, et al. The effect of clustering of outcomes on the association of procedure volume and surgical outcomes. Ann Intern Med 2003;139:658-665.[Abstract/Free Full Text]
  10. Hannan EL, Wu C, Ryan TJ, et al. Do hospitals and surgeons with higher coronary artery bypass graft surgery volumes still have lower risk-adjusted mortality rates?. Circulation 2003;108:795-801.[Abstract/Free Full Text]
  11. Carey JS, Danielsen B, Gold J, Rossiter S. Procedure rates and outcomes of coronary revascularization procedures in California and New York. J Thorac Cardiovasc Surg 2005;129:1276-1282.[Abstract/Free Full Text]
  12. Gammie JS, O'Brien SM, Griffith BP, et al. Influence of hospital procedural volume on care process and mortality for patients undergoing elective surgery for mitral regurgitation. Circulation 2007;115:881-887.[Abstract/Free Full Text]
  13. Khuri SF, Henderson WG. The case against volume as a measure of quality of surgical care. World J Surg 2005;29:1222-1229.[Medline]
  14. Shahian DM, Normand SL. The volume-outcome relationship: from Luft to Leapfrog. Ann Thorac Surg 2003;75:1048-1058.[Abstract/Free Full Text]

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