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The Aristotle Complexity Score Project is the first of its kind whose impetus was born out of a growing frustration among pediatric cardiac surgeons over the fact that their surgical performance was being evaluated based on hospital mortality without regard for the complexity of the operations performed. A working group of Congenital Heart Surgeons from Europe and the United States decided to develop a risk-stratification method which could be adapted to our specialty. Aristotle Institute Incorporated was created as a collaboration mechanism which could collect and and analyze detailed information concerning the complexity of congenital heart surgery.

When starting this project in 1999, two difficulties were encountered: 1) Multi-institutional databases were just starting and there was no reliable data yet available. 2) Due to the absence of risk stratification, the more prominent centers dealing with the sickest patients and potentially having a significant mortality were very reluctant to send their data. It was, therefore, necessary to base this risk-adjustment on an evaluation that was largely subjective. Following many discussions, it was concluded that a subjective probability approach based on the consensus of a panel of experts was valid, provided that the risk-adjustment score is subsequently validated based on collected outcome data.

A group of 50 internationally accepted experts worked for more than five years on a new method to evaluate the quality of care in Congenital Heart Surgery (CHS) that is called Aristotle. Senior, experienced congenital heart surgeons considered the possible risk factors for each procedure and assigned scores based on potential for mortality, potential for morbidity, and anticipated surgical difficulty.

The Aristotle system, electronically available, has been introduced by both the European Association for Cardio-Thoracic Surgery (EACTS) and Society of Thoracic Surgeons (STS) as an original method to compare the performance of Congenital Heart Surgery (CHS) centers. Pediatric cardiologists joined the project and have been developing a complexity score for interventional cardiology procedures 
The Aristotle system is based on an original concept of complexity which is a constant for a given patient all over the world. Many variables can affect patient care performance, and most are difficult to define and are not constant among care providers or institutions. Complexity of a given patient with a specific medical condition undergoing a surgical procedure at a given time, however, is a constant regardless of the location around the globe. Defining complexity based on surgical procedures and the factors that may significantly modify the clinical outcomes of those procedures can provide the quantitative basis for evaluation of performance. The complexity issue has recently generated more than 31 publications, 15 Lectures and 13 Studies representing various scientific societies including:  European Congenital Heart Surgeons Association (ECHSA), Association of European Pediatric Cardiology (AEPC), Congenital Heart Surgeons Society (CHSS), Association for Thoracic Surgery (AATS), Society of Thoracic Surgeons (STS), and European Association for Cardio-Thoracic Surgery (EACTS).
The project was named Aristotle, to support an approach based upon expert opinion, with reference to Aristotle’s writing (Rhetoric, Book I, 350 BC); “When there is no scientific answer available, the opinion (Doxa) perceived and admitted by the majority has value of truth.” 
The Aristotle adjustment method objectively rates the projected complexity of the surgical procedures performed. The complexity score is based on three subjective determinations; potential for mortality, potential for morbidity, and anticipated surgical difficulty. Complexity is calculated in two phases. First, the basic complexity of the procedure involved is scored. Scores range from 0.5 to 15.0. The Basic Aristotle Complexity Score rates only the simplest form of the procedure and does not take into consideration factors which can significantly alter the projected complexity and outcome of the operation. Second, specific value is added, based on a precise analysis of the associated pathology along with any co-morbid conditions potentially present.

The Comprehensive Aristotle Complexity Score considers other significant complicating factors which will impact on the eventual outcome. Procedure dependent factors include anatomical variations, associated procedures, and patient age, and can add a maximum of 5 points to the basic score. Procedure independent factors include patient characteristics which are more general, but have the potential to significantly affect the outcome. Procedure independent factors can add up to an additional 5 points. The mechanism used to assign scores defining the level of added complexity was same process that resulted in the Basic Aristotle Complexity Score.