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How Size Matters: The Complex Relationship Between Pediatric Cardiac Surgical Case Volumes and Mortality Rates in a National Clinical Database
Karl F. Welke1, Sean M. O'Brien2, Eric D. Peterson2, Ross M. Ungerleider1, Marshall L. Jacobs3, Jeffery P. Jacobs4; 1Surgery, Oregon Health and Science Univerisity, Portland, OR; 2Outcomes Research and Assessment Group, Duke Clinical Research Institute, Durham, NC; 3St. Christopher Hospital for Children, Philadelphia, PA; 4The Congenital Heart Institute of Florida (CHIF), Saint Petersburg and Tampa, FL

 Comment on this Abstract

Objective: An inverse relationship exists between volume and mortality for a variety of surgical procedures. However, in pediatric cardiac surgery, where such investigations have employed older risk models and state level data, the results have been mixed. We sought to determine the association between hospital pediatric cardiac surgical volume and mortality using sophisticated case-mix adjustment and a national clinical database.
Methods: Patients 18 years of age or less who had a cardiac operation between 2002 and 2006 were identified in the STS National Congenital Heart Surgery Database (32,413 patients from 48 hospitals). After analyzing volume as a continuous variable, hospitals were grouped by yearly pediatric cardiac surgical volume (small <150, medium 150-249, large 250-349, very large >350). Volume categories were created to assure adequate sample size in each group. Logistic regression adjusted mortality rates for volume, surgical case mix (Aristotle and RACHS-1 categories), patient risk factors, and year of surgery.
Results: Overall, raw mortality rates were similar across volume groups (range 3.3% (346/10603) to 4.0% (148/3715), p=0.24). However, the mortality rate for difficult operations (Aristotle technical difficulty component score 4-5) decreased as volume increased, from 17.3% (54/312) at small programs to 9.9% (135/1368) at very large programs, p=0.009. The same was true for the subgroup of patients who underwent Norwood procedures (36.5% (23/63) versus 16.9% (81/479), p<0.0001). After adjustment for surgical case mix and patient risk factors, all groups performed similarly for low difficulty operations. (Table) Conversely, for difficult procedures, small programs performed significantly worse than all other volume groups. For Norwood procedures, very large programs outperformed all other groups.
Conclusion: There was an inverse association between pediatric cardiac surgical volume and mortality that became increasingly important as case complexity increased. Although volume was not associated with mortality for low complexity cases, lower volume programs underperformed larger programs as case complexity increased. For one of the most complex procedures (Norwood), the largest programs had results that were significantly better than all other groups. Since overall, unadjusted, mortality rates do not accurately reflect this complex relationship, institutional comparisons must employ methodology that takes into account both patient risk factors and surgical case mix.

Table: Association Between Annual Case Volume and Mortality
Volume Category N Deaths Adjusted Odds Ratio (95% CI) P-value
All Procedures
350+ 10570 346 (reference) 0.004*
250-349 11978 450 1.05 ( 0.86, 1.29) 0.63
150-249 6051 250 1.14 ( 0.84, 1.55) 0.41
<150 3681 148 1.51 ( 1.19, 1.90) 0.0005
Low Difficulty Procedures
350+ 8663 188 (reference) 0.29*
250-349 10252 295 1.16 ( 0.87, 1.53) 0.31
150-249 5104 148 1.08 ( 0.76, 1.52) 0.68
<150 3229 86 1.21 ( 0.87, 1.69) 0.26
High Difficulty Procedures
350+ 1855 135 (reference) 0.0008*
250-349 1636 138 0.89 ( 0.69, 1.15) 0.38
150-249 894 79 1.22 ( 0.81, 1.84) 0.35
<150 406 54 2.41 ( 1.89, 3.06) <0.0001
Norwood Procedures
350+ 479 81 (reference) <0.0001*
250-349 418 95 1.43 ( 1.06, 1.95) 0.020
150-249 194 47 1.59 ( 1.09, 2.32) 0.016
<150 63 23 2.91 ( 1.98, 4.28) <0.0001

* = P for linear trend
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