The volume-outcome effect for abdominal aortic surgery: Differences in case-mix or complications?

Hypothesis: Variation in postoperative complications after abdominal aortic surgery contributes to differences in mortality between high- and low-volume hospitals. Background: Hospitals with high surgical volume have been shown to have lower operative mortality rates for complex vascular surgery than those with low volumes. Differences in the rates of complications among hospitals may explain this variation in mortality. Methods: Adult patients who underwent abdominal aortic surgery in Maryland from 1994 to 1996 (N=2987) were included. The primary dependent variable was in hospital mortality and the independent variables included hospital surgical volume, patient case-mix variables, and several specific postoperative complications. Two sequential analyses using multiple logistic regression were performed to determine the relative importance of independent variables in predicting mortality. Results: Hospitals with high surgical volume had a lower mortality rate (5.6%) than those with medium (6.8%) and low (8.7%) volumes (P=.03). In the first multivariate 1 analysis, after adjusting for patient case-mix, having surgery at a high-volume hospital remained associated with a 37% reduction in mortality (odds ratio [OR], 0.63; 95% confidence interval [CI], 0.42-0.92; P=.02). Patients at high-volume hospitals had a decreased relative risk (RR) of several complications: pulmonary failure (RR, 0.45; 95% CI, 0.36-0.55), reintubation (RR, 0.53; 95% CI, 0.44-0.64), pneumonia (RR, 0.74; 95% CI, 0.55-0.99), cardiac complications (RR, 0.63; 95% CI, 0.51-0.78), and shock (RR 0.27; 95% CI, 0.10-0.75). In the second multivariate analysis, which included complications, hospital volume was no longer a significant predictor of mortality. However, several postoperative complications remained significant predictors of mortality. Conclusions: The effect of hospital volume on mortality after abdominal aortic surgery is attributable to differences in postoperative complications and not preoperative differences in case-mix. Efforts to reduce the rates of postoperative complications may reduce mortality rates at low-volume hospitals.