Background: Recent changes in health care financing have raised the specter of operation-specific, volume credentialing for cardiac surgeons. To meet this challenge, the leadership of The Society of Thoracic Surgeons formed an Ad Hoc Committee to study the question of the relationship of case volume to outcome. One product of the committee's work in this analysis of data from The Society of Thoracic Surgery National Cardiac Database.
Methods: We examined data for all types of coronary artery bypass graft-only operations (n = 124,793) from more than 1,200 surgeons working in more than 600 hospitals for the years 1991 through 1993. All in-hospital and 30-day out-of-hospital mortality, both observed and expected as predicted by The Society of Thoracic Surgeons risk stratification method, was plotted against annualized group practice volume. Both patient-based and practice-based sampling techniques were used.
Results: The data show that observed mortality ranged from 2.0% to 3.6% for practices of more than 100 cases through practices with more than 900 cases per year. Those practices with less than 100 cases (n = 18) had a mean mortality of 5%. Expected mortalities ranged from 2.4% to 3.9% and did not vary as a function of volume. No practice volume category had an observed/expected ratio of less than 0.8 and none had a ratio greater than 1.2, if annual volume was more than 100. Practices of less than 100 cases/year had an observed/expected ratio of 1.6% to 1.7%. There was great variation in observed and expected mortalities in the lower volume categories and less variation when volume was greater (more than 600 cases/year).
Conclusions: Although the data are practice-group-specific only, there was no clinically relevant correlation of volume to outcome except at extremely low annual volume (less than 100 cases per year). Variability of outcome was significant in lower volume practices (less than 600 cases/year) and varied little at more than 600 cases per year. There were no differences in expected mortality regardless of the size of the practice.