Area socioeconomic status and mortality after coronary artery bypass graft surgery: the role of hospital volume.

BACKGROUND Individuals of low socioeconomic status (SES) have reduced access to coronary artery bypass graft surgery (CABG). It is unknown if low-SES CABG patients have reduced access to hospitals with better outcomes. METHODS We conducted a retrospective cohort analysis of the California CABG Mortality Reporting Program, consisting of individuals with zip code information who underwent CABG at participating hospitals in 1999-2000 (n = 18,961). Primary outcome measures were inhospital mortality after CABG; primary independent variables of interest were area-level SES, clinical risk factors, and hospital volume. We used 2-level hierarchical random-effects logit models to estimate the relationship between explanatory variables and inhospital mortality. RESULTS Within high-volume hospitals, patients of low-SES areas had greater mortality than those of mid- and high-SES areas (2.5% vs 1.5% vs 1.8%, P = .024). However, there was no relationship between SES and mortality in lower-volume hospitals. Contrary to expectations, individuals of high-SES areas (42%) underwent surgery at low-volume hospitals more often than patients of low-SES areas (28%, P < .001), although mortality at low-volume hospitals was greater than that at high-volume facilities (P < .001). Discrepancies were not explained by distance traveled. CONCLUSIONS Mortality after CABG is modified by both SES and hospital volume. Within high-volume hospitals, patients of low-SES areas fared worse than patients of higher-SES areas. Patients of high SES tended to have CABG surgery at low-volume hospitals where mortality was greater and therefore had higher mortality than expected.