Institutional factors beyond procedural volume significantly impact center variability in outcomes after orthotopic heart transplantation

OBJECTIVE: To evaluate the contribution of institutional volume and other unmeasured institutional factors beyond volume to the between-center variability in outcomes after orthotopic heart transplantation (OHT). BACKGROUND: It is unclear if institutional factors beyond volume have a significant impact on OHT outcomes. METHODS: The United Network for Organ Sharing registry was used to identify OHTs performed between 2000 and 2010. Separate mixed-effect logistic regression models were constructed, with the primary endpoint being post-OHT mortality. Model A included only individual centers, model B added validated recipient and donor risk indices as well as the year of transplantation, and model C added institutional volume as a continuous variable to model B. The reduction in between-center variability in mortality between models B and C was used to define the contribution of institutional volume. Kaplan-Meier survival curves were also compared after stratifying patients into equal-size tertiles based on center volume. RESULTS: A total of 119 centers performed OHT in 19,156 patients. After adjusting for transplantation year and differences in recipient and donor risk, decreasing center volume was associated with an increased risk of 1-year mortality (P < 0.001). However, procedural volume only accounted for 16.7% of the variability in mortality between centers, and significant between-center variability persisted after adjusting for institutional volume (P<0.001). In Kaplan-Meier analysis, there was significant variability in 1-year survival between centers within each volume category: low-volume (66.7%-96.6%), intermediate-volume (80.7%-97.3%), and high-volume (83.8%-93.9%). These trends were also observed with 5-year mortality. CONCLUSIONS: This large-cohort analysis demonstrates that although institutional volume is a significant predictor of post-OHT outcomes, there are other unmeasured institutional factors that contribute substantially to the between-center variability in outcomes. Institutional volume should therefore not be the sole indicator of "center quality" in OHT.