Congenital heart surgery: expected versus observed surgical performance according to the Aristotle complexity score.

Background: The Aristotle score quantifies the complexity involved in congenital heart surgery. It defines surgical performance as complexity score times hospital survival. We studied how expected and observed surgical performance evolved over time. Methods: 2312 main procedures carried out between 2006 and 2010 were analyzed. The Aristotle basic score, corresponding hospital survival and related observed surgical performance were estimated. Expected survival was based on the mortality risks published by O'Brien and coauthors. Observed performance divided by expected performance was called the standardized ratio of performance. This should trend towards a figure above 100%. Survival rates and performance are given with 95% confidence intervals. Results: The mean Aristotle basic score was 7.88 +/- 2.68. 51 patients died: observed hospital survival was 97.8% (97.1% - 98.3%). 115 deaths were anticipated: expected survival was 95.2% (93.5% - 96.3%). Observed and expected surgical performance reached 7.71 (7.65-7.75) and 7.49 (7.37-7.59), respectively. Therefore the overall standardized ratio of performance was 102.94%. The ratio increased from 2006 (ratio = 101.60%) to 2009 (103.92%) and was 103.42% in 2010. Performance was high for the repair of congenital corrected transposition of the great arteries and ventricular septal defect (VSD) by atrial switch and Rastelli procedure, the Norwood procedure, repair of truncus arteriosus, aortic arch repair and VSD closure, and the Ross-Konno procedure, with corresponding standardized ratios of 123.30%, 116.83%, 112.99%, 110.86% and 110.38%, respectively. With a ratio of 82.87%, performance was low for repair of Ebstein's anomaly. Conclusion: The standardized ratio of surgical performance integrates three factors into a single value: procedure complexity, postoperative observed survival, and comparison with expected survival. It constitutes an excellent instrument for quality monitoring of congenital heart surgery programs over time. It allows an accurate comparison of surgical performance across institutions with different case mixes.