We based this study on national Medicare Provider Analysis and Review (MEDPAR) files, which contain all hospital discharge abstracts for fee-for-service, acute care hospitalizations for Medicare recipients. Using appropriate procedure codes from the International Classification of Diseases, Ninth Revision (ICD-9),11 (link) we identified all patients from 65 to 99 years of age who underwent one of the following eight cancer and cardiovascular operations from 1999 through 2008: esophagectomy, pancreatectomy, lung resection, cystectomy, repair of abdominal aortic aneurysm (AAA), coronary-artery bypass grafting (CABG), carotid endarterectomy, and aortic-valve replacement (for a full list of ICD-9 codes, see the Supplementary Appendix , available with the full text of this article at NEJM.org). Six of these procedures have been targeted for volume-based referral by the Leapfrog Group.4 (link),12 We also included lung resection and cystectomy, two procedures that have been cited as potential candidates for regionalization.13 (link)–15 (link)Each year, hospitals were ranked according to the volume of Medicare patients for each procedure, adjusting for the proportion of Medicare patients covered by fee-for-service plans. In assessing changes in hospital volumes over time, we sought to distinguish between the effects of “volume creep” (which occurs when more patients who undergo these high-risk procedures are distributed among the same hospitals) and market concentration (which occurs when patients are redistributed to a smaller number of higher-volume hospitals). To quantify market concentration, we determined the proportion of Medicare patients undergoing one of the eight procedures in the top decile and top quintile of hospitals by volume for each year.
Operative mortality, determined from the Medicare eligibility file, was defined as death before discharge or within 30 days after the operation. In creating cohorts for analysis of operative mortality, we used several limitations to enhance the homogeneity of our study cohorts and reduce confounding due to changes in case mix over time. For cancer resections, we excluded patients without an accompanying diagnosis code for cancer. Patients who underwent AAA repair were excluded if there was a diagnosis code or procedure code indicating rupture of the aneurysm, the presence of a thoracoabdominal aneurysm, or both. For patients who underwent CABG, we excluded those who had simultaneous valve replacement or repair.
Operative mortality, determined from the Medicare eligibility file, was defined as death before discharge or within 30 days after the operation. In creating cohorts for analysis of operative mortality, we used several limitations to enhance the homogeneity of our study cohorts and reduce confounding due to changes in case mix over time. For cancer resections, we excluded patients without an accompanying diagnosis code for cancer. Patients who underwent AAA repair were excluded if there was a diagnosis code or procedure code indicating rupture of the aneurysm, the presence of a thoracoabdominal aneurysm, or both. For patients who underwent CABG, we excluded those who had simultaneous valve replacement or repair.
