The complex relationship between center volume and outcome in patients undergoing the Norwood operation

Abstract

Background: Norwood outcomes vary across centers, and a relationship between center volume and outcome has been previously described. It is unclear whether this volume-outcome relationship exists across all levels of patient risk or holds true for all centers. We evaluated the impact of patient risk status on the relationship between center volume and outcome, and the extent to which differences in center volume account for between-center variation in outcome.

Methods: Infants in The Society of Thoracic Surgeons Congenital Heart Surgery Database undergoing the Norwood operation (2000 to 2009) were included. Mortality associated with annual Norwood volume overall and across patient preoperative risk tertiles was evaluated in multivariable analysis. We also estimated the proportion of between-center variation in mortality explained by center volume.

Results: The cohort included 2,557 infants from 53 centers: 34 centers with 0 to 10 Norwood cases per year; 13 centers with 11 to 20 cases per year; and 6 centers with more than 20 cases per year. Unadjusted in-hospital mortality was 22%. In multivariable analysis, lower center volume was associated with higher mortality (odds ratio in low-volume versus high-volume centers 1.54, 95% confidence interval: 1.02 to 2.32, p=0.04). The volume-outcome relationship did not differ across preoperative risk tertiles (p=0.7). Norwood volume explained an estimated 14% of the between-center variation in mortality observed, and significant between-center variation in mortality remained after adjusting for volume (p<0.001).

Conclusions: Center volume is modestly associated with outcome after the Norwood operation independent of patient risk status. However, this relationship explains only a portion of the between-center variation in mortality in this cohort.

Figure 1. Distribution of Center Annual Norwood Volume

Figure 2. Adjusted Mortality Rate Displayed by Increasing Center Volume

The adjusted mortality rate (and 95% confidence interval) for each center is displayed in order of increasing center volume. The overall mortality rate in the entire cohort is represented by the dashed line. As center Norwood volume increases, a greater proportion of center mortality estimates are located to the left of the vertical line (overall mortality rate in entire cohort), indicating lower adjusted mortality rates, or better outcomes. However, this relationship does not hold true for all centers. In particular, there are some middle volume centers (ie. center 31&39) with mortality rates comparable to higher volume centers, and some higher volume centers (ie. center 45&49) with mortality rates similar to those in the lower volume groups.