Why are implantable cardioverter defibrillator outcomes in practice different from clinical trials?

Data from implantable cardioverter-defibrillator (ICD) registries offer access to information about the real-world patient as opposed to randomized control data, which tend to study selected patients in ideal care environments. This article discusses similarities and differences between registries and randomized control trials on ICDs. Registries in the United States, Canada, and elsewhere are discussed. The role of registry data for evaluation of complications and for examining accessibility and patterns of ICD care delivery are examined. In addition, the article discusses data from registries for monitoring quality and appropriateness of ICD care.