Given the task definition and input, reply with output. Given an abstract, generate a keyword (a noun phrase) that best describes the focus or contribution of the paper. Such keywords can be directly from the given abstract or outside it.

Abstract: The conduction system and the entire heart were examined histologically in four patients with congenital heart disease who had surgery many years before they died suddenly. All were asymptomatic, in sinus rhythm and living normally. The sudden death may have been related to alterations in the conduction system, previously present, or introduced by the previous surgery, or related to inflammatory phenomena, an outgrowth of the previous surgery.
Adult