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Impact of Patient Risk on the Hospital Volume–Outcome Relationship in Coronary Artery Bypass Grafting
Brahmajee K. Nallamothu, MD, MPH;
Sanjay Saint, MD, MPH;
Timothy P. Hofer, MD, MSc;
Sandeep Vijan, MD, MSc;
Kim A. Eagle, MD;
Steven J. Bernstein, MD, MPH
Arch Intern Med. 2005;165:333-337.
Background The impact of surgical risk on the relationship between hospital volume and outcomes in coronary artery bypass grafting (CABG) is uncertain. We assessed (1) whether in-hospital mortality rates differ across lower- and higher-volume hospitals by expected surgical risk and (2) whether high-risk patients are more likely to undergo CABG at low-volume centers.
Methods We used clinical data on 27 355 adults who underwent CABG at 68 hospitals in California between 1997 and 1998. Hospitals were divided into low-volume (n = 44), medium-volume (n = 19), and high-volume (n = 5) categories on the basis of tertiles of annual CABG volume. Using hierarchical logistic regression and log-binomial regression models, we assessed for differences in in-hospital mortality rates across hospital volume categories and the likelihood of CABG being performed in each hospital volume category after adjusting for expected surgical risk.
Results Differences in adjusted in-hospital mortality rates between low- and high-volume centers rose as the expected risk of in-hospital death increased: 0.8% vs 0.4% at the 20th risk percentile and 3.8% vs 2.5% at the 80th risk percentile (P<.001 for all comparisons). While a similar trend was seen between medium- and high-volume centers, absolute differences were substantially smaller. The likelihood of patients having surgery at a low-volume center also rose significantly with expected surgical risk (relative risk of undergoing CABG at a low-volume center for patients at 80th vs 20th risk percentile, 1.29 [95% confidence interval, 1.14-1.51; P<.001]).
Conclusion High-risk patients are more likely to undergo CABG at low-volume facilities where their risk of dying is higher.
Author Affiliations: Department of Internal Medicine, University of Michigan Medical School, Ann Arbor (Drs Nallamothu, Saint, Hofer, Vijan, Eagle, and Bernstein); Patient Safety Enhancement Program, University of Michigan Health System, Ann Arbor (Drs Nallamothu, Saint, and Hofer); and Health Services Research and Development Field Program, Veterans Affairs Medical Center, Ann Arbor (Drs Saint, Hofer, Vijan, and Bernstein).
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