Impact of patient risk on the hospital volume-outcome relationship in coronary artery bypass grafting

Background: The impact of surgical risk on the relationship between hospital volume and outcomes in corenary 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 27355 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.