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a Cardiac Surgery, The Congenital Heart Institute of Florida (CHIF), Cardiac Surgical Associates of Florida (CSAOF), All Children's Hospital and Children's Hospital of Tampa, 603 7th St S, Ste 450, St. Petersburg, FL 33701
b Department of Surgery, Massachusetts General Hospital, Boston, Massachusetts
c Cardiothoracic Surgery, Drexel University College of Medicine, Newtown Square, Pennsylvania
d Pediatric and Congenital Heart Surgery, Cleveland Clinic Lerner College of Medicine, Case Western Reserve University, Cleveland, Ohio
(Email: jeffjacobs{at}msn.com).
The report by Tsang and associates in this issue of The Annals of Thoracic Surgery is another important contribution to surgical performance monitoring by a group whose senior member introduced the concept of cumulative sum (CUSUM) charts into cardiothoracic surgery in 1994 [1]. This resulted in the early identification of a suboptimal trend of performance in arterial switch surgery, and appropriate modifications in technique and management were successfully instituted [2].
Monitoring of performance in pediatric cardiac surgery has historically been challenged by two issues: the large number of unique operative procedures mandated by the diverse spectrum of congenital cardiac malformations, and the relatively small numbers of any particular operation at any given hospital. As a result, development of risk adjustment strategies has been more challenging than in adult cardiac surgery, where coronary artery bypass grafting (CABG) and valve operations are useful index procedures that are performed frequently, validating their use as indicators of performance.
The challenge of true risk modeling in pediatric heart surgery gave rise to the development of systems that stratified operative procedures by complexity, based in part on subjective expert opinion (the Risk Adjustment in Congenital Heart Surgery-1 system and the Aristotle Complexity Score). As the Society of Thoracic Surgeons (STS) and the European Association for Cardio-Thoracic Surgery (EACTS) Congenital Heart Surgery Databases during the last decade have catalogued the outcomes of more than 150,000 operations, the subjective element of these tools is about to be supplanted by true risk modeling based on empirical data.
Still, in pediatric cardiac surgery, even when aggregating data for several years, case numbers for most operations may be inadequate to discriminate among good, bad, and average providers with any degree of statistical certainty. This problem of "small numbers" was elegantly illustrated by Dimick and associates [3]. Among relatively common, complex surgical procedures, including pediatric cardiac surgery, only CABG was performed with sufficient frequency at most hospitals to detect a doubling of the mortality rate using 3-year aggregate data. This problem of small numbers was also the basis for the funnel plot, as described by Spiegelhalter [4], which plots the rates of mortality of institutions or surgeons on a graph in conjunction with 95% and 99% binomial confidence intervals centered around the average population mortality. This graphic technique, adopted in 2008 for use in the Reports of the STS Congenital Database, explicitly demonstrates the substantial random sampling variation that occurs at low volumes and the difficulty in distinguishing levels of performance. As a consequence, low volume surgeons or institutions are protected from inappropriate conclusions about their data.
To monitor performance and to demonstrate definitive differences among providers of rare services such as congenital heart surgery, rates of mortality may have to be collected for many years. The requirement of several years' aggregate data means that any risk model must periodically be recalibrated to avoid obsolescence as quality improvement occurs [5]. As an alternative to such batch processing, deLeval and colleagues applied to congenital heart surgery the concept of CUSUM charts, which had initially been used for industrial quality control. This form of sequential, real-time monitoring of cumulative performance has had multiple iterations, including the Variable Life Adjusted Display (VLAD) used here by Tsang and associates. These authors have further advanced the concept of using CUSUM methodology to monitor the results of pediatric heart surgery. And in the same exercise they have reinforced the importance of regular periodic recalibration of any predictive risk model.
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